Rajagopalan, Dilip
ABSTRACT
We discuss mathematical models of capillary flow in complex geometries representative of the void spaces formed between fibers in a textile yam. Moisture transport in textile yams and fabrics is an important factor affecting physiological comfort. We have extended an existing analytical model for capillary flow in circular tubes to more complex geometries. We validate this model using detailed computational fluid dynamics simulations of this flow. These models are used to understand the effect of geometric and material parameters on moisture transport. In vertical wicking in a bundle of filaments, the model predicts that as the nonroundness of the filaments increases, or the void area between the filaments decreases, the maximum liquid height increases while the initial rate of penetration decreases.
Moisture transport in textile fabrics is one of the critical factors affecting physiological comfort. Fabrics that rapidly transport moisture away from the human body make wearers feel more comfortable by keeping them dry. This enhanced moisture transport may also help wearers feel cool as the body provides latent heat to evaporate sweat at an enhanced rate. The comfort afforded by textile fabrics can be improved by understanding the key geometric and material parameters that contribute to moisture transport. Mathematical modeling of surface-tension-driven flow in yarns and fabrics could provide a way to develop such an understanding.
Capillary flow in yams and fabrics has been extensively studied (e.g., [5-7]), and the subject was reviewed by Kissa [9]. Fabrics are typically constructed by knitting or weaving textile yams, which are essentially bundles of several fibers or filaments. For movement of liquid in a fabric, the liquid must wet the fabric surface before being transported by capillary action through the fabric pores formed between fibers and yams. This capillary action is determined by the interaction of the liquid and the fabric material, by liquid properties such as viscosity and surface tension, and by the geometric structure of the pores. The size and shape of the fibers, as well as their alignment, determine the geometry of the void spaces or pores through which the liquid is transported. However, the complexity of a fabric structure makes it impossible to predict pore structure, and very difficult to arrive at a detailed structure experimentally. Furthermore, movement of liquid through the pores can cause shifting of fibers and changes in the pore structure. For some materials, the fibers can absorb liquid and swell considerably, thereby changing the pore structure even more. Thus, a detailed mathematical or computational model of capillary flow in fabric structures is not a reasonable goal.
Literature Cited
1. Adler, M. M., and Walsh, W. K., Mechanisms of Transient Moisture Transport Between Fabrics, Textile Res. J. 54(5), 334-342 (1984).
2. Burgeni, A. A., and Kapur, C., Capillary Sorption Equilibria in Fiber Masses, Textile Res. J. 37(5), 356-366 (1967).
3. Denn, M. M., "Process Fluid Mechanics," Prentice Hall, Englewood Cliffs, NJ, 1980.
4. Fortin, M., Old and New Finite Elements for Incompressible Flows, Int. J. Num. Meth. Fluids 1, 347-364 (1981).
5. Hollies, N. R., Kaessinger, M. M., and Bogaty, H., Water Transport Mechanisms in Textile Materials, Part I: The Role of Yam Roughness in Capillary-type Penetration, Textile Res. J. 26(11), 829-835 (1956).
6. Hollies, N. R., Kaessinger, M. M., Watson, B. S., and Bogaty, H., Water Transport Mechanisms in Textile Materials, Part II: Capillary Type Penetration in Yams and Fabrics, Textile Res. J. 27(1), 8-13 (1957).
7. Hsieh, Y. L., Liquid Transport in Fabric Structures, Textile Res. J. 65(5), 299-307 (1995).
8. Kamath, Y. K., Hornby, S. B., Weigmann, H.-D., and Wilde, M. F., Wicking of Spin Finishes and Related Liquids into Continuous Filament Yarns, Textile Res. J. 64(1), 33-40 (1994).
9. Kissa, E., Wetting and Wicking, Textile Res. J. 66(10), 660-668 (1996).
10. Kistler, S. F., and Scriven, L. E., Coating Flows, in, "Computational Analysis of Polymer Processing," J. R. A. Pearson and S. M. Richardson, Eds., Applied Science Publishers, NY, 1983, pp. 243-299.
11. Laughlin, R. D., and Davies, J. E., Some Aspects of Capillary Adsorption in Fibrous Textile Wicking, Textile Res. J. 31(10), 904-910 (1961).
12. Levine, S., Lowndes, J., Watson, E. J., and Neale, G., A Theory of Capillary Rise of a Liquid in a Vertical Cylindrical Tube and in a Parallel Plate Channel, J. Colloid Interface Sci. 73(1), 136-151 (1980).
13. Lucas, R., Ueber das zeitgesetz des kapillaren aufstiegs von flussigkeiten, Kolloid Z. 23(1), 15-22 (1918).
14. Miller, C., Predicting Non-Newtonian Flow Behavior in Ducts of Unusual Cross Section, Ind. Eng. Chem. Fundam. 11(4), 524-528 (1972).
15. Minor, F. W., Schwartz, A. M., Wilkow, E. A., and Buckles, L. C., The Migration of Liquids in Textile Assemblies, Part II: The Wicking of Liquids in Yams, Textile Res. J. 31(12), 931-939 (1961).
16. Reed, C. M., and Wilson, N., The Fundamentals of Absorbency of Fibres, Textile Structures and Polymers, I: The Rate of Rise of a Liquid in Glass Capillaries, J. Phys. D Appl. Phys. 26(9), 1378-1381 (1993).
17. Ruschak, K. J., A Three-dimensional Linear Stability Analysis for Two-dimensional Free Boundary Flows by the Finite Element Method, Comp. Fluids 11(4), 391-401 (1983).
18. Washburn, E. W., The Dynamics of Capillary Flow, Phys. Rev. 17(3), 273-283 (1921).
19. Woodcock, A. H., Moisture Transfer in Textile Systems, Part II, Textile Res. J. 32(9), 719-722 (1962).
Manuscript received October 5, 2000; accepted December 17, 2000.
DILIP RAJAGOPALAN AND ARUN P. ANEJA
E. I. du Pont de Nemours and Company, Wilmington, Delaware 19880, U.S.A.
JEAN-MARIE MARCHAL
Fluent Inc., B-1348 Louvain-la-Neuve, Belgium
Monday, February 5, 2007
Warm-cool feeling relative to tribological properties of fabrics
Pac, Marie Jose
ABSTRACT
When the human hand touches a garment that is at a different temperature than the skin, heat exchange occurs between the hand and the fabric, and the warm-cool feeling is the very first sensation. This transient transfer of energy depends on the contact interface between the skin and the fabric, and the contact interface depends on many morphological and structural parameters like fiber morphology or yam and fabric structure. This paper describes a new experimental device for measuring heat absorption of textile materials in a transient state. The link between the transient thermal behavior and the tribological properties of fabrics is then made to show the influence of contact interface and therefore the influence of morphological and structural parameters on heat transfer. This investigation involves two cotton varieties (Pima of Morocco and Kaba S of Benin), two yam structures (single and two-ply yams), and three stitch lengths of jersey fabrics.
Because the prime function of a garment is its insulating ability, heat transfer through fabrics has been an important interest since studies of textile properties began [3, 91. Thus, the literature has focused on devices and methods for measuring fabric thermal properties in the steady state [4, 6, 8].
More recently, during the seventies and eighties, what people feel when touching cloth became a significant selling point. Textile manufacturers tried to meet consumer requirements, and a new field of research appeared, called the "hand" of a fabric. The aim was to find ways to quantify and qualify the tactile feeling of fabrics from a mechanical and thermal point of view.
Tribological and transient thermal properties are very important to fabric handle. Kawabata et al. were the first to separately study these two properties of tactile feeling, but they did not establish the link [7, 11, 12].
Kawabata developed an apparatus known as the Thermolabo to evaluate the warm-cool feeling of fabric touch. A mathematical model of a two-layered body assumed as a solid predicted that the maximum heat flowing from the warmer material to the other is achieved 0.2 seconds after contact [11]. From the model results, the Thermolabo was processed by a differential circuit of temperature signals to approximate heat flow and a first-order integral circuit with 0.2 seconds of time constant to introduce a time lag. This device give a parameter denoted Amax, related to warm-cool feelings of human skin.
Starting from the ideas of Kawabata, and from the model considering the ideal contact between two homogeneous semi-infinite solids, Hes [5] introduced another parameter called "thermal absorptivity b" to evaluate the warm-cool feeling. He developed equipment to measure and calculate this parameter with a microcomputer.
Schneider et al. [11] suggested that the sensation of warm-cool feeling was related to the surface hairiness of wool fabrics with an appropriate finishing technique. They didn't consider the influence of fiber types and of the conventional and Sirospun processes.
The aim of this paper is to show the influence of fiber morphology and yarn and fabric structure on transient thermal properties and friction behavior. Here, the variables of this study are at the three scale levels of a fabric, i.e., cotton variety (microscopic variable), yarn structure (mesoscopic variable), and stitch length of knitted fabrics (macroscopic variable). After describing the experimental devices used, we report the results of the transient thermal and tribological behaviors of some fabrics. Finally, we establish the link between these behaviors and the warm-cool touch.
Experimental
ASSESSING THERMAL ENERGY ABSORBED By FABRIC
The warm-cool feeling was previously evaluated using different apparatus [5, 11] based on mathematical models. Here, we develop a simpler device, which allows us to directly obtain the signal without electronic processing and the parameters without mathematical correlation.
Apparatus
When the human hand touches a fabric that is at a lower temperature than the skin surface, heat flows from the hand to the fabric. The warm-cool feeling is mainly due to a transient heat transfer in which heat conduction makes the most important contribution [10, 11]. Heat conduction is the transfer of thermal energy as a result of molecular interactions in a nonhomogeneous temperature distribution.
The warm-cool feeling is perceived just after the skin touches a fabric that is at a different temperature than it, but the feeling can only be perceived at the very first moments. Let's assume that the fabrics tested are continuous media. For homogeneous materials, Equation 1 (Fourier's equation) shows that at a given temperature gradient, heat flow increases with the thermal conductivity of the material. The more a material absorbs thermal energy, the more it is a thermal conductor and the cooler it seems at the very first moments of contact with a warmer body:
Conclusions
Both the surface roughness and warm-cool feeling of a fabric depend on the chosen fibers, the yam spinning method, and the fabric construction processes. Therefore, to produce a fabric with precise tactile properties, it is necessary to study simultaneously the influence of fiber, yam, and fabric construction on these two properties.
Here, we have developed a thermal device based on a hot guarded plate to measure the thermal energy absorbed during the initial time of contact of the plate to a fabric. From these measures, we can estimate the warm-- cool feeling, then experiments with two other devices allow us to evaluate surface fabric roughness and hairiness properties.
The morphological and structural parameters we have studied in this paper are the cotton variety, the kind of yam, and the stitch length of the knitted fabrics. Fabrics seem all the cooler when made from fine fibers. Fabrics made from two-ply yams are cooler than those from single yams. The lower the stitch length of the knitted fabric, the cooler the fabric seems to the initial touch.
Thermal results linked to the surface state results in terms of roughness and hairiness first show that a rougher fabric has a smaller contact surface and so seems warmer. Second, a hairier fabric encapsulates more air on its surface and so seems warmer. However, it is difficult to independently identify the exact roles hairiness and structural roughness play in their influence on thermal behavior, and more experiments are in progress to ponder those roles.
ACKNOWLEDGMENT
We express appreciation for the interest shown in this study by Dr J.-F. Le Magnen, associate professor, Ecole Nationale Superieure des Industries Textiles de Mulhouse (France), and for the cooperation of M. El Fatihi in yam spinning, Ecole Superieure des Industries du Textile et de l'Habillement, Casablanca (Morocco).
Literature Cited
1. Bueno, M. A., Renner, M., Viallier, P., Durand, B., and Larry, B., Instrumental Measurement and Macroscopical Study of Sanding and Raising, Textile Res. J. 67, 779-787 (1997).
2. Bueno, M. A., Durand, B., and Renner, M., Optical Characterization of the State of the Fabric Surfaces, Optic. Eng. 39, 1697-1703 (2000).
3. Clulow, E. E., and Rees, W. H., The Transmission of Heat Through Textile Fabrics, Part III: A New Thermal Transmission Apparatus, J. Textile Inst. 59, 285-294 (1968).
4. Farnworth, B., Mechanisms of Heat Flow Through Clothing Insulation, Textile Res. J. 53, 717-725 (1983).
5. Hes, L., and Dolezal, I., New Method and Equipment for Measuring Thermal Properties of Textiles, J. Textile Mach. Soc. Jpn. 8, T124-T128 (1989).
6. Holmer, L, Heat Exchange and Thermal Insulation Compared in Woolen and Nylon Garments during Wear Trials, Textile Res. J. 55, 511-518 (1985).
7. Kawabata, S., "The Standardisation and Analysis of Hand Evaluation," Textile Machinery Society of Japan, Osaka, 1980.
8. Morris, M. A., Thermal Insulation of Single and Multiple Layers of Fabrics, Textile Res. J. 25, 766-773 (1955).
9. Rees, W. H., The Transmission of Heat Through Textile Fabrics, J. Textile Inst. Trans. 50, T149-T165 (1941). 10. Schneider, A. M., Holcombe, B. V., Properties Influencing
Coolness to Touch of Fabrics, Textile Res. J. 61, 488-494 (1991).
11. Yoneda, M., and Kawabata, S., "A Theoretical Consideration on the Objective Measurement of Warm/Cool Feeling," The Textile Machinery Society of Japan, 1982, pp. 393-406.
12. Yoneda, M., and Kawabata, S., Analysis of Transient Heat Conduction and Its Applications, Part II, J. Textile Mach. Soc. Jpn. 31, 73-81 (1985).
Manuscript received July 24, 2000, accepted December 12, 2000.
MARIE JOSE PAC, MARIE-ANGE BUENO, AND MARC RENNER
Ecole Nationale Suprieure des Industries Textiles de Mulhouse University of A(Whousf, France
SAID EL KASMI
Ecole Superieure des Industries du Textile et de l'Habillement, Casablanca, Morocco
ABSTRACT
When the human hand touches a garment that is at a different temperature than the skin, heat exchange occurs between the hand and the fabric, and the warm-cool feeling is the very first sensation. This transient transfer of energy depends on the contact interface between the skin and the fabric, and the contact interface depends on many morphological and structural parameters like fiber morphology or yam and fabric structure. This paper describes a new experimental device for measuring heat absorption of textile materials in a transient state. The link between the transient thermal behavior and the tribological properties of fabrics is then made to show the influence of contact interface and therefore the influence of morphological and structural parameters on heat transfer. This investigation involves two cotton varieties (Pima of Morocco and Kaba S of Benin), two yam structures (single and two-ply yams), and three stitch lengths of jersey fabrics.
Because the prime function of a garment is its insulating ability, heat transfer through fabrics has been an important interest since studies of textile properties began [3, 91. Thus, the literature has focused on devices and methods for measuring fabric thermal properties in the steady state [4, 6, 8].
More recently, during the seventies and eighties, what people feel when touching cloth became a significant selling point. Textile manufacturers tried to meet consumer requirements, and a new field of research appeared, called the "hand" of a fabric. The aim was to find ways to quantify and qualify the tactile feeling of fabrics from a mechanical and thermal point of view.
Tribological and transient thermal properties are very important to fabric handle. Kawabata et al. were the first to separately study these two properties of tactile feeling, but they did not establish the link [7, 11, 12].
Kawabata developed an apparatus known as the Thermolabo to evaluate the warm-cool feeling of fabric touch. A mathematical model of a two-layered body assumed as a solid predicted that the maximum heat flowing from the warmer material to the other is achieved 0.2 seconds after contact [11]. From the model results, the Thermolabo was processed by a differential circuit of temperature signals to approximate heat flow and a first-order integral circuit with 0.2 seconds of time constant to introduce a time lag. This device give a parameter denoted Amax, related to warm-cool feelings of human skin.
Starting from the ideas of Kawabata, and from the model considering the ideal contact between two homogeneous semi-infinite solids, Hes [5] introduced another parameter called "thermal absorptivity b" to evaluate the warm-cool feeling. He developed equipment to measure and calculate this parameter with a microcomputer.
Schneider et al. [11] suggested that the sensation of warm-cool feeling was related to the surface hairiness of wool fabrics with an appropriate finishing technique. They didn't consider the influence of fiber types and of the conventional and Sirospun processes.
The aim of this paper is to show the influence of fiber morphology and yarn and fabric structure on transient thermal properties and friction behavior. Here, the variables of this study are at the three scale levels of a fabric, i.e., cotton variety (microscopic variable), yarn structure (mesoscopic variable), and stitch length of knitted fabrics (macroscopic variable). After describing the experimental devices used, we report the results of the transient thermal and tribological behaviors of some fabrics. Finally, we establish the link between these behaviors and the warm-cool touch.
Experimental
ASSESSING THERMAL ENERGY ABSORBED By FABRIC
The warm-cool feeling was previously evaluated using different apparatus [5, 11] based on mathematical models. Here, we develop a simpler device, which allows us to directly obtain the signal without electronic processing and the parameters without mathematical correlation.
Apparatus
When the human hand touches a fabric that is at a lower temperature than the skin surface, heat flows from the hand to the fabric. The warm-cool feeling is mainly due to a transient heat transfer in which heat conduction makes the most important contribution [10, 11]. Heat conduction is the transfer of thermal energy as a result of molecular interactions in a nonhomogeneous temperature distribution.
The warm-cool feeling is perceived just after the skin touches a fabric that is at a different temperature than it, but the feeling can only be perceived at the very first moments. Let's assume that the fabrics tested are continuous media. For homogeneous materials, Equation 1 (Fourier's equation) shows that at a given temperature gradient, heat flow increases with the thermal conductivity of the material. The more a material absorbs thermal energy, the more it is a thermal conductor and the cooler it seems at the very first moments of contact with a warmer body:
Conclusions
Both the surface roughness and warm-cool feeling of a fabric depend on the chosen fibers, the yam spinning method, and the fabric construction processes. Therefore, to produce a fabric with precise tactile properties, it is necessary to study simultaneously the influence of fiber, yam, and fabric construction on these two properties.
Here, we have developed a thermal device based on a hot guarded plate to measure the thermal energy absorbed during the initial time of contact of the plate to a fabric. From these measures, we can estimate the warm-- cool feeling, then experiments with two other devices allow us to evaluate surface fabric roughness and hairiness properties.
The morphological and structural parameters we have studied in this paper are the cotton variety, the kind of yam, and the stitch length of the knitted fabrics. Fabrics seem all the cooler when made from fine fibers. Fabrics made from two-ply yams are cooler than those from single yams. The lower the stitch length of the knitted fabric, the cooler the fabric seems to the initial touch.
Thermal results linked to the surface state results in terms of roughness and hairiness first show that a rougher fabric has a smaller contact surface and so seems warmer. Second, a hairier fabric encapsulates more air on its surface and so seems warmer. However, it is difficult to independently identify the exact roles hairiness and structural roughness play in their influence on thermal behavior, and more experiments are in progress to ponder those roles.
ACKNOWLEDGMENT
We express appreciation for the interest shown in this study by Dr J.-F. Le Magnen, associate professor, Ecole Nationale Superieure des Industries Textiles de Mulhouse (France), and for the cooperation of M. El Fatihi in yam spinning, Ecole Superieure des Industries du Textile et de l'Habillement, Casablanca (Morocco).
Literature Cited
1. Bueno, M. A., Renner, M., Viallier, P., Durand, B., and Larry, B., Instrumental Measurement and Macroscopical Study of Sanding and Raising, Textile Res. J. 67, 779-787 (1997).
2. Bueno, M. A., Durand, B., and Renner, M., Optical Characterization of the State of the Fabric Surfaces, Optic. Eng. 39, 1697-1703 (2000).
3. Clulow, E. E., and Rees, W. H., The Transmission of Heat Through Textile Fabrics, Part III: A New Thermal Transmission Apparatus, J. Textile Inst. 59, 285-294 (1968).
4. Farnworth, B., Mechanisms of Heat Flow Through Clothing Insulation, Textile Res. J. 53, 717-725 (1983).
5. Hes, L., and Dolezal, I., New Method and Equipment for Measuring Thermal Properties of Textiles, J. Textile Mach. Soc. Jpn. 8, T124-T128 (1989).
6. Holmer, L, Heat Exchange and Thermal Insulation Compared in Woolen and Nylon Garments during Wear Trials, Textile Res. J. 55, 511-518 (1985).
7. Kawabata, S., "The Standardisation and Analysis of Hand Evaluation," Textile Machinery Society of Japan, Osaka, 1980.
8. Morris, M. A., Thermal Insulation of Single and Multiple Layers of Fabrics, Textile Res. J. 25, 766-773 (1955).
9. Rees, W. H., The Transmission of Heat Through Textile Fabrics, J. Textile Inst. Trans. 50, T149-T165 (1941). 10. Schneider, A. M., Holcombe, B. V., Properties Influencing
Coolness to Touch of Fabrics, Textile Res. J. 61, 488-494 (1991).
11. Yoneda, M., and Kawabata, S., "A Theoretical Consideration on the Objective Measurement of Warm/Cool Feeling," The Textile Machinery Society of Japan, 1982, pp. 393-406.
12. Yoneda, M., and Kawabata, S., Analysis of Transient Heat Conduction and Its Applications, Part II, J. Textile Mach. Soc. Jpn. 31, 73-81 (1985).
Manuscript received July 24, 2000, accepted December 12, 2000.
MARIE JOSE PAC, MARIE-ANGE BUENO, AND MARC RENNER
Ecole Nationale Suprieure des Industries Textiles de Mulhouse University of A(Whousf, France
SAID EL KASMI
Ecole Superieure des Industries du Textile et de l'Habillement, Casablanca, Morocco
Applying a nonformaldehyde crosslinking agent to improve the washing durability of fabric water repellency
Xu, Weilin
ABSTRACT
1,2,3,4-Butanetetracarboxylic acid (BTCA) is confirmed to be an effective crosslinking agent with sodium hypophosphate (SHP) catalyst for washing durability improvement of cotton fabrics treated with fluorocarbon resin. By FTIR analysis, hydroxyl groups (--OH) are confirmed as a water repellency agent, so the resin can be theoretically crosslinked with the surface of the cotton fibers. The water repellency of the sample treated with fluorocarbon resin and 8% BTCA is much higher than the sample treated only with fluorocarbon resin. This kind of difference can be seen especially after fifty washing cycles and subsequent heat treatments. ESCA analysis confirms that the F/O ratio on the fabric surface changes dramatically after fifty washing cycles and subsequent heat treatment. The F/O of the sample treated with fluorocarbon resin and 8% BTCA is almost twice that of the fabric treated with fluorocarbon resin only. Crosslinks can restrict F loss and transfer into the inner part of the fibers. At the same time, this kind of treatment can effectively improve the crease resistance of cotton fabrics.
Water repellency is an important property for some functional fabrics, and fluorocarbon resin is the most effective treating agent. To improve the washing durability of water repellency, some crosslinking agents are usually used along with the repellency agents. Crosslinking agents are usually small molecules containing several functional groups capable of reacting with some active groups in the polymer, such as hydroxyl groups in cellulose.
Traditional crosslinking agents used in cellulose are N-methylol resins or their derivatives. Some of these are prohibited by some governments because the treated fabric will emit formaldehyde during use. In 1988, Welch [4], reported that tetracarboxylic acids, 1,2,3,4-- butanetetracarboxylic acid (BTCA) in particular, are able to form effective crosslinks in cotton fabrics when salts of certain phosphorus-containing acids are used as catalysts [5, 6]. Polycarboxylic acids have been confirmed as the most promising formaldehyde-free crosslinking agents for cotton cellulose among the various new reagents investigated [1, 4-8]. It is now clear that cellulose esterfication with a polycarboxylic acid proceeds first to form a cyclic anhydride, and then to form an ester with the -OH group in the cellulose macromolecule. Polycarboxylic acids have also been used as crosslinking agents for wood pulp cellulose to improve the wet strength and dimensional stability of paper [9].
Sato [2] used some traditional crosslinking agents in the fluorocarbon resin treatment of fabrics. The water repellency of the fabrics treated with fluorocarbon resin decreases significantly with washing, but recovers with subsequent heat treatment. The reason for the decrease with washing is thought to be mostly due to the rotation of the hydrophobic fluoroalkyl groups into the polymer substrate to repel the hydrophobic washing conditions. When crosslinks are formed between the fiber surface and the water repellency film formed by the agents, they can restrain the rotation of the fluoroakyl groups into inner the part of the fibers during washing, which improves washing durability. The objectives of the work we report here are to analyze the effect of BTCA as a new kind of nonformaldehyde crosslinking agent in improving the washing durability of fabrics treated for water repellency by fluorocarbon resins.
Experimental
1,2,3,4-Butanetetracarboxylic acid (BTCA) was purchased from Aldrich Chemical Company. Sodium hypophospfite (SHP) was analytical grade. Fluorocarbon resin TG-490 was supplied by Dakin Company, Japan. Undyed 100% plain cotton fabric (142.0 g/m^sup 2^) was desized, scoured, and bleached by the supplier. The treated samples for the waterproof test were 25 X 25 cm.
BTCA and SHP concentrations are expressed according to the weight of the agent in the water solution. The fabric was treated in the solution comprising 8% fluorocarbon resin and different concentrations of BTCA and SHP, giving a range of concentrations in the treatment solutions. The fabric was then passed through squeeze rolls, again wet with treating solution, an again passed through squeeze rolls to give a specified wet pickup (approximately 80%). The fabric was predried at 85 deg C for 10 minutes and cured in a second oven for 2 minutes at 180 deg C.
A Nicolet FTIR 20 SXB was used to analyze the spectrum of the agent. Resolution for the infrared spectra was 4 cm^sup -1^, and there were thirty-two scans for each spectrum. Standard methods were used to measure the conditioned wrinkle recovery angle (ASTM-1295-67); the WRA of the control sample was 124 deg (w + f). Water repellency of the samples was evaluated according to a spray test method (JIS L-1092 5.2): 250 ml water was sprayed on the fabric fitted on a 20 cm diameter circular frame inclined 45 deg to the horizontal. In order to quantify water repellency changes in the samples, water repellency was evaluated according to the water weight gain (WWG) of a water absorbent paper, which was located directly under the test specimens. The paper had excellent water absorbing properties: when water was transmitted by the fabric, the paper absorbed it very quickly. The higher the WWG of the paper, the poorer the water repellency of the specimen.
Results and Discussion
HYDROXYL GROUP CONFIRMATION IN FLUOROCARBON RESIN
FTIR analysis of the fluorocarbon resin revealed whether there were hydroxyl groups in the fluorocarbon resin used for the water repellency treatment. Before analysis, the fluorocarbon resin was dried in an oven under low pressure at 50 deg C for 3 days, placed over CaCl^sub 2^ for one week, then quickly analyzed by FTIR The results are shown in Figure 1. The absorbing intensity around 3300 cm^sup -1^ is very strong, indicating the presence of active hydroxyl and other similar groups in the agent. Due to the existence of these hydroxyls in the agent, the hydroxyl groups can be crosslinked by BTCA with the hydroxyl in the fibers and can also form effective crosslinks between the fiber surface and the repellency film formed by the fluorocarbon resin during heat treatment.
EFFECT OF BTCA ON WASHING DURABILITY
Water repellency of the samples treated by the fluorocarbon resin and different concentrations of BTCA and SHP are expressed by the water weight gain (WWA) of the water absorbing paper. The results are shown in Table I. There is almost no effect of BTCA on the water repellency before washing; all the WWG values of the water absorbent paper are around 0.4 g, indicating that all the fabrics have excellent water repellency properties. However, even after one washing (according to the AATCC washing standard) and air drying, the water repellency expressed by WWG shows a great difference due to the effect of the crosslinking agent. For example, the wwc of fabric treated without BTCA is 16.5 g, but the wwG of fabric of treated with 8% BTCA is only 4.7 g. When the samples are heat treated at 160 deg C after washing, the water repellency of the samples treated with BTCA in the solution are better than that of the fabric treated without BTCA. After the fabric is washed fifteen times (Table II) and then heat treated at 160 deg C, the WWG for the sample treated with 8% fluorocarbon resin only (fabric 1) is 13.6 g, but for fabric 7 treated with 8% BTCA and 4% SHP, the tested WWG is only 4.4 g. This effect is more evident as the washing times increase. When the samples are washed fifty times and then heat treated, the control sample (fabric 1) shows a WWG of around 20.3 g, but sample 7 shows a WWG of around 4.8 g, indicating that the water repellency is greatly improved when the crosslinking agent is applied together with the water repellency agent.
WRA IMPROVEMENT OF TREATED FABRIC
Since BTCA and the catalyst can also penetrate into the fibers, crosslinking can take place, thereby improving the crease recovery of the fabric. Data for fabrics treated with different concentrations of BTCA are shown in Table III. The WRA values of the fabrics treated at low concentrations of BTCA increase very slowly, which is probably due to some crosslinks being formed in the repellency film and between the film and the fiber surface. As washing times increase, the WRA values of the treated fabrics decrease to some degree.
ESCA ANALYSIS
In order to analyze the surface chemical composition, we made ESCA measurements (Table IV). The results show that N content is almost zero, and the main component elements are F and C, with small amounts of O. More F in the fabric surface indicates better water repellency, and more O in the surface indicates a hydrophilic character of the fabric. So the ratio of F to O (F/O) can be used to express the water repellency of the fabric. From Table IV, we see that before washing, the F/O for the sample treated along with 8% BTCA (sample 7) is only slightly higher than that of sample 1, treated without BTCA, in agreement with the water repellency results in Table I. When the fabric had been washed fifty times and air dried, the F/O for the fabric treated with BTCA (sample 7) is only a little higher than that of sample 1, but after a 160 deg C X 3 min treatment, the F/O for sample 7 is much higher than that for sample 1. Although consistent with water repellency data, there is no linear relation between F/O and water repellency values. A slight increase in F/O will lead to large increase in water repellency. After adding BTCA and the catalyst to the fluorocarbon resin solution, a water repellency film formed during the heat treatment is closely attached onto the fiber surface. It has been suggested that decreased water repellency is mostly due to mechanical action when the fiber is rinsed in water, inducing the water repellency film to loosen from the fiber surface [2, 3]. At the same time, during washing, F can also diffuse into the inner part of the fiber, decreasing surface water repellency. However, apparently this kind of transfer can be reversed during the high temperature treatment (160 deg C x 3 min) (see Table IV). Crosslinks formed by BTCA can restrict F diffusion and transfer into the fiber, improving the film strength and adhesion to the fiber surface.
Conclusions
We have evaluated the effect of polycarboxylic acid (BTCA) crosslinking on the washing durability of water repellent fabrics treated by fluorocarbon resin. We have appraised the water repellency of the treated samples according to the water weight gain (WWG) of water absorbing paper located under the treated samples during water repellency testing. Washing durability of water repellency can be greatly improved when the fabric is treated with 8% fluorocarbon resin in the presence of a certain quantity of BTCA and SHP. By ESCA analysis, the F/O in the surface of the samples treated with 8% BTCA and the catalyst is almost twice that of the samples treated only with fluorocarbon resin. This indicates a crosslinking agent in the treatment can crosslink the water repellency film onto the fiber surface effectively and restrict F diffusion and transfer into the inner part of the fiber.
Literature Cited
1. Schramm, C., Rinderer, B., and Bobleter, O., Quantitative Determination of BTCA Bound to Cellulosic Material by Means of Isocratic HPLC, Textile Res. J. 68, 821-827 (1998).
2. Sato, Yukihiro, Wakida, Tomiji, and Tokino, Seiji, Effect of Crosslinking Agents on Water Repellency of Cotton Fabrics Treated with Fluorocarbon Resin, Textile Res. J. 64, 316-321 (1994).
3. Wakida, T., Li, H., and Sato, Y., The Effect of Washing and Heat Treatment on the Surface Characteristics of Fluorocarbon Resin-treated Polymer, J. Soc. Dyers Color 109, 292-296 (1993).
4. Welch, C. M., Tetracarboxylic Acids as Formaldehyde-- Free Durable Press Finishing Agents, Textile Res. J. 58, 480-486 (1988).
5. Welch, C. M., Formaldehyde-Free Durable Press Finishes, Rev. Prog. Color. 22, 32-41 (1992).
6. Welch, C. M., and Kottes Andrews, B. A., Catalysts for Processes for Formaldehyde-Free Durable Press Finishing of Cotton Textiles with Polycarboxylic Acid, U.S. patent 4,820,307, April 11, 1989.
7. Xu, W. L., and Li, Y., Crosslinking Analysis of Polycarboxylic Acid Durable Press Finishing on Cotton Fabrics and Strength Retention Improvement, Textile Res. J. 70(7), 588-592 (2000).
8. Yang, C. Q., Wang, X., and Kang, I., Ester Crosslinking of Cotton Fabric by Polycarboxylic Acid, Textile Res. J. 67, 334-342 (1997).
9. Zhou, Y. J., Luner, P., Caluwe, P., and Tekin, B., Products of papermaking, U.S. patent 5,759-210 (1993).
Manuscript received August 29, 2000; accepted December 12, 2000.
WEILIN XU
Wuhan Institute of Science & Technology, Wuhan, 430073, People's Republic of China
TIENWEI SHYR
Department of Textile Engineering, Feng Chia University, Taichung, Taiwan
ABSTRACT
1,2,3,4-Butanetetracarboxylic acid (BTCA) is confirmed to be an effective crosslinking agent with sodium hypophosphate (SHP) catalyst for washing durability improvement of cotton fabrics treated with fluorocarbon resin. By FTIR analysis, hydroxyl groups (--OH) are confirmed as a water repellency agent, so the resin can be theoretically crosslinked with the surface of the cotton fibers. The water repellency of the sample treated with fluorocarbon resin and 8% BTCA is much higher than the sample treated only with fluorocarbon resin. This kind of difference can be seen especially after fifty washing cycles and subsequent heat treatments. ESCA analysis confirms that the F/O ratio on the fabric surface changes dramatically after fifty washing cycles and subsequent heat treatment. The F/O of the sample treated with fluorocarbon resin and 8% BTCA is almost twice that of the fabric treated with fluorocarbon resin only. Crosslinks can restrict F loss and transfer into the inner part of the fibers. At the same time, this kind of treatment can effectively improve the crease resistance of cotton fabrics.
Water repellency is an important property for some functional fabrics, and fluorocarbon resin is the most effective treating agent. To improve the washing durability of water repellency, some crosslinking agents are usually used along with the repellency agents. Crosslinking agents are usually small molecules containing several functional groups capable of reacting with some active groups in the polymer, such as hydroxyl groups in cellulose.
Traditional crosslinking agents used in cellulose are N-methylol resins or their derivatives. Some of these are prohibited by some governments because the treated fabric will emit formaldehyde during use. In 1988, Welch [4], reported that tetracarboxylic acids, 1,2,3,4-- butanetetracarboxylic acid (BTCA) in particular, are able to form effective crosslinks in cotton fabrics when salts of certain phosphorus-containing acids are used as catalysts [5, 6]. Polycarboxylic acids have been confirmed as the most promising formaldehyde-free crosslinking agents for cotton cellulose among the various new reagents investigated [1, 4-8]. It is now clear that cellulose esterfication with a polycarboxylic acid proceeds first to form a cyclic anhydride, and then to form an ester with the -OH group in the cellulose macromolecule. Polycarboxylic acids have also been used as crosslinking agents for wood pulp cellulose to improve the wet strength and dimensional stability of paper [9].
Sato [2] used some traditional crosslinking agents in the fluorocarbon resin treatment of fabrics. The water repellency of the fabrics treated with fluorocarbon resin decreases significantly with washing, but recovers with subsequent heat treatment. The reason for the decrease with washing is thought to be mostly due to the rotation of the hydrophobic fluoroalkyl groups into the polymer substrate to repel the hydrophobic washing conditions. When crosslinks are formed between the fiber surface and the water repellency film formed by the agents, they can restrain the rotation of the fluoroakyl groups into inner the part of the fibers during washing, which improves washing durability. The objectives of the work we report here are to analyze the effect of BTCA as a new kind of nonformaldehyde crosslinking agent in improving the washing durability of fabrics treated for water repellency by fluorocarbon resins.
Experimental
1,2,3,4-Butanetetracarboxylic acid (BTCA) was purchased from Aldrich Chemical Company. Sodium hypophospfite (SHP) was analytical grade. Fluorocarbon resin TG-490 was supplied by Dakin Company, Japan. Undyed 100% plain cotton fabric (142.0 g/m^sup 2^) was desized, scoured, and bleached by the supplier. The treated samples for the waterproof test were 25 X 25 cm.
BTCA and SHP concentrations are expressed according to the weight of the agent in the water solution. The fabric was treated in the solution comprising 8% fluorocarbon resin and different concentrations of BTCA and SHP, giving a range of concentrations in the treatment solutions. The fabric was then passed through squeeze rolls, again wet with treating solution, an again passed through squeeze rolls to give a specified wet pickup (approximately 80%). The fabric was predried at 85 deg C for 10 minutes and cured in a second oven for 2 minutes at 180 deg C.
A Nicolet FTIR 20 SXB was used to analyze the spectrum of the agent. Resolution for the infrared spectra was 4 cm^sup -1^, and there were thirty-two scans for each spectrum. Standard methods were used to measure the conditioned wrinkle recovery angle (ASTM-1295-67); the WRA of the control sample was 124 deg (w + f). Water repellency of the samples was evaluated according to a spray test method (JIS L-1092 5.2): 250 ml water was sprayed on the fabric fitted on a 20 cm diameter circular frame inclined 45 deg to the horizontal. In order to quantify water repellency changes in the samples, water repellency was evaluated according to the water weight gain (WWG) of a water absorbent paper, which was located directly under the test specimens. The paper had excellent water absorbing properties: when water was transmitted by the fabric, the paper absorbed it very quickly. The higher the WWG of the paper, the poorer the water repellency of the specimen.
Results and Discussion
HYDROXYL GROUP CONFIRMATION IN FLUOROCARBON RESIN
FTIR analysis of the fluorocarbon resin revealed whether there were hydroxyl groups in the fluorocarbon resin used for the water repellency treatment. Before analysis, the fluorocarbon resin was dried in an oven under low pressure at 50 deg C for 3 days, placed over CaCl^sub 2^ for one week, then quickly analyzed by FTIR The results are shown in Figure 1. The absorbing intensity around 3300 cm^sup -1^ is very strong, indicating the presence of active hydroxyl and other similar groups in the agent. Due to the existence of these hydroxyls in the agent, the hydroxyl groups can be crosslinked by BTCA with the hydroxyl in the fibers and can also form effective crosslinks between the fiber surface and the repellency film formed by the fluorocarbon resin during heat treatment.
EFFECT OF BTCA ON WASHING DURABILITY
Water repellency of the samples treated by the fluorocarbon resin and different concentrations of BTCA and SHP are expressed by the water weight gain (WWA) of the water absorbing paper. The results are shown in Table I. There is almost no effect of BTCA on the water repellency before washing; all the WWG values of the water absorbent paper are around 0.4 g, indicating that all the fabrics have excellent water repellency properties. However, even after one washing (according to the AATCC washing standard) and air drying, the water repellency expressed by WWG shows a great difference due to the effect of the crosslinking agent. For example, the wwc of fabric treated without BTCA is 16.5 g, but the wwG of fabric of treated with 8% BTCA is only 4.7 g. When the samples are heat treated at 160 deg C after washing, the water repellency of the samples treated with BTCA in the solution are better than that of the fabric treated without BTCA. After the fabric is washed fifteen times (Table II) and then heat treated at 160 deg C, the WWG for the sample treated with 8% fluorocarbon resin only (fabric 1) is 13.6 g, but for fabric 7 treated with 8% BTCA and 4% SHP, the tested WWG is only 4.4 g. This effect is more evident as the washing times increase. When the samples are washed fifty times and then heat treated, the control sample (fabric 1) shows a WWG of around 20.3 g, but sample 7 shows a WWG of around 4.8 g, indicating that the water repellency is greatly improved when the crosslinking agent is applied together with the water repellency agent.
WRA IMPROVEMENT OF TREATED FABRIC
Since BTCA and the catalyst can also penetrate into the fibers, crosslinking can take place, thereby improving the crease recovery of the fabric. Data for fabrics treated with different concentrations of BTCA are shown in Table III. The WRA values of the fabrics treated at low concentrations of BTCA increase very slowly, which is probably due to some crosslinks being formed in the repellency film and between the film and the fiber surface. As washing times increase, the WRA values of the treated fabrics decrease to some degree.
ESCA ANALYSIS
In order to analyze the surface chemical composition, we made ESCA measurements (Table IV). The results show that N content is almost zero, and the main component elements are F and C, with small amounts of O. More F in the fabric surface indicates better water repellency, and more O in the surface indicates a hydrophilic character of the fabric. So the ratio of F to O (F/O) can be used to express the water repellency of the fabric. From Table IV, we see that before washing, the F/O for the sample treated along with 8% BTCA (sample 7) is only slightly higher than that of sample 1, treated without BTCA, in agreement with the water repellency results in Table I. When the fabric had been washed fifty times and air dried, the F/O for the fabric treated with BTCA (sample 7) is only a little higher than that of sample 1, but after a 160 deg C X 3 min treatment, the F/O for sample 7 is much higher than that for sample 1. Although consistent with water repellency data, there is no linear relation between F/O and water repellency values. A slight increase in F/O will lead to large increase in water repellency. After adding BTCA and the catalyst to the fluorocarbon resin solution, a water repellency film formed during the heat treatment is closely attached onto the fiber surface. It has been suggested that decreased water repellency is mostly due to mechanical action when the fiber is rinsed in water, inducing the water repellency film to loosen from the fiber surface [2, 3]. At the same time, during washing, F can also diffuse into the inner part of the fiber, decreasing surface water repellency. However, apparently this kind of transfer can be reversed during the high temperature treatment (160 deg C x 3 min) (see Table IV). Crosslinks formed by BTCA can restrict F diffusion and transfer into the fiber, improving the film strength and adhesion to the fiber surface.
Conclusions
We have evaluated the effect of polycarboxylic acid (BTCA) crosslinking on the washing durability of water repellent fabrics treated by fluorocarbon resin. We have appraised the water repellency of the treated samples according to the water weight gain (WWG) of water absorbing paper located under the treated samples during water repellency testing. Washing durability of water repellency can be greatly improved when the fabric is treated with 8% fluorocarbon resin in the presence of a certain quantity of BTCA and SHP. By ESCA analysis, the F/O in the surface of the samples treated with 8% BTCA and the catalyst is almost twice that of the samples treated only with fluorocarbon resin. This indicates a crosslinking agent in the treatment can crosslink the water repellency film onto the fiber surface effectively and restrict F diffusion and transfer into the inner part of the fiber.
Literature Cited
1. Schramm, C., Rinderer, B., and Bobleter, O., Quantitative Determination of BTCA Bound to Cellulosic Material by Means of Isocratic HPLC, Textile Res. J. 68, 821-827 (1998).
2. Sato, Yukihiro, Wakida, Tomiji, and Tokino, Seiji, Effect of Crosslinking Agents on Water Repellency of Cotton Fabrics Treated with Fluorocarbon Resin, Textile Res. J. 64, 316-321 (1994).
3. Wakida, T., Li, H., and Sato, Y., The Effect of Washing and Heat Treatment on the Surface Characteristics of Fluorocarbon Resin-treated Polymer, J. Soc. Dyers Color 109, 292-296 (1993).
4. Welch, C. M., Tetracarboxylic Acids as Formaldehyde-- Free Durable Press Finishing Agents, Textile Res. J. 58, 480-486 (1988).
5. Welch, C. M., Formaldehyde-Free Durable Press Finishes, Rev. Prog. Color. 22, 32-41 (1992).
6. Welch, C. M., and Kottes Andrews, B. A., Catalysts for Processes for Formaldehyde-Free Durable Press Finishing of Cotton Textiles with Polycarboxylic Acid, U.S. patent 4,820,307, April 11, 1989.
7. Xu, W. L., and Li, Y., Crosslinking Analysis of Polycarboxylic Acid Durable Press Finishing on Cotton Fabrics and Strength Retention Improvement, Textile Res. J. 70(7), 588-592 (2000).
8. Yang, C. Q., Wang, X., and Kang, I., Ester Crosslinking of Cotton Fabric by Polycarboxylic Acid, Textile Res. J. 67, 334-342 (1997).
9. Zhou, Y. J., Luner, P., Caluwe, P., and Tekin, B., Products of papermaking, U.S. patent 5,759-210 (1993).
Manuscript received August 29, 2000; accepted December 12, 2000.
WEILIN XU
Wuhan Institute of Science & Technology, Wuhan, 430073, People's Republic of China
TIENWEI SHYR
Department of Textile Engineering, Feng Chia University, Taichung, Taiwan
A numerical analysis of transonic/supersonic flows in the axisymmetric main nozzle of an air-jet loom
Oh, T H
ABSTRACT
This paper reports a numerical analysis of transonic flows in the axisymmetric backward-facing step main nozzle of an air-jet loom. To obtain basic design data for the optimum main nozzle shape of an air-jet loom and to predict transonic/supersonic internal flows, a characteristic-based, upwind flux difference-splitting, compressible Navier-- Stokes method is used. Wall static pressure and flow velocity distributions in the nozzle are analyzed by changing air tank pressures and acceleration tube lengths. The flow inside the nozzle experiences double choking, first at the needle tip and then at the acceleration tube exit at air tank pressures near 4 kg^sub f^/cm^sup 2^. The air tank pressure that leads to critical conditions depends on acceleration tube length, i.e., higher air tank pressures for longer acceleration tubes. The air pressure required to bring the acceleration tube exit to sonic conditions is nearly constant regardless of acceleration tube length. The round needle tip shape could lead to less total pressure loss when compared with step shape.
An air-jet loom inserts the weft into the warp by using high pressure air-jet thrust force and skin friction force along the yarn. The air-jet loom is popular in texturing industries because of its high productivity, convenient controllability, and wide variety of textured fabrics: silk, cotton, wool, and spun textures. It causes no air pollution since it uses air as the yarn carrying medium. The air-jet loom is also capable of texturing spun or cellulose filament fabrics, which cannot be woven by water-jet looms. However, since the density of air is too low (about 1/1000 of water) compared with that of water, the compressed air jet diffuses rapidly into the atmosphere after discharging from the acceleration tube. Also, the viscosity of air is about 1/50 that of water, so air consumption becomes critical in an air-jet loom.
Due to this large consumption of air and compressor electricity, increased manufacturing costs are one of the loom's disadvantages. Main nozzle shape, exit shape of subnozzles, response time of the solenoid valve, body shape, subnozzle locations, and control methods have been studied intensively to reduce air consumption in air-jet looms.
Fundamental research in main nozzle flows is especially necessary to design optimum shapes of main nozzles, which push the weft to fly through the warp. The air jet from the main nozzle is easily diffused, and it is hard to control the flow direction and velocity, thus increasing air and compressor electricity consumption. Reduced air-jet diffusion and effective control of flow direction and velocity are therefore very important in air-jet loom design.
Due to the recent development of high speed air-jet looms, studies of transonic/supersonic flows in main nozzles have become important for performance enhancement of air jets and optimum nozzle design. It is very difficult to measure the flow field near the nozzle throat region experimentally due to its small cross-sectional area; however, the most important flow phenomena, such as shock waves and flow separations, occur frequently inside this nozzle throat area. Therefore, a computational analysis of the flow field inside the air-jet main nozzle is necessary.
Air flow inside the main nozzle shows subsonic, transonic, and supersonic flow characteristics. There are some difficulties in computational analysis due to the complexity of the flow nature, i.e., turbulence and shock wave/boundary layer interaction. The governing Navier-- Stokes equations are mixed elliptic/hyperbolic partial differential equations [4].
In previous studies of the air-jet loom, Duxbury and Lord [2] derived air-jet velocity distribution exposed to free atmosphere from the main nozzle. Lyubovitskii [7] measured supersonic pulsed jet flow from the nozzle exit. Uno and Ishida [11] experimentally measured air-jet velocity and weft flying distance using various acceleration tube prototypes. Mohamed and Salama [8] studied the effects of the diameter and length of acceleration tubes on flow velocity. Researchers investigated the converging-diverging nozzle (Kim [4], Mohamed and Salama [8]) flows both experimentally and numerically, but there has not been much air-jet nozzle flow analysis. Ishida and Okajima [3] measured pressure inside the main nozzle experimentally and obtained qualitatively similar results in the acceleration tube by using a one-- dimensional Fanno flow assumption. In their case, they did not obtain detailed information. If we use the full Navier-Stokes equation to analyze the internal transonic/ supersonic flow, we may be able to explain the complex flow phenomena of shock waves and flow separations in detail.
Since the flow inside the main nozzle is typically an axisymmetric, compressible, viscous, and transonic/supersonic flow, a computational fluid dynamics (CFD) approach using the full Navier-Stokes equations is necessary. This approach is especially important in the early design stage, because cFD can be used to analyze the main nozzle flow effectively with minimum costs and efforts before making experimental scale models.
We use the CSCM upwind compressible Navier-Stokes method of Lombard et al. [6] in this study. This method has the merits of an upwind scheme, ease in applying characteristic boundary conditions, and a fast flow solver using a diagonally dominant ADI. Kwon et al. [5] and Song et al. [10] analyzed the transonic/supersonic compressor cascade flow and the performance of transonic centrifugal compressor diffusers, respectively, using this same method.
In this study, we analyze transonic/supersonic viscous flows inside the nozzle using the inlet nozzle flow conditions from the experiments. We also analyze the effect of air tank pressure, acceleration tube length, and circular arc radius change in the backward-facing step on the flow field. Finally, we investigate flow physics, including choking phenomena at the nozzle throat and at the exit of the acceleration tube, and optimum nozzle shapes for proper weft insertion.
Numerical Analysis
GRID SYSTEM AND BOUNDARY CONDITIONS
A 180 X 70 H-type grid system was generated by an elliptic PDE grid generator. Grids were packed near the wall by a stretching function (Figure 2), and then in order to observe free jet flow outside the tube exit, a 20 X 120 grid system was attached to the internal grid system.
As wall boundary conditions, a no-slip boundary condition was prescribed at the walls, ie., u = 0 and v = 0. The inlet and exit boundary conditions depended on flow conditions, i.e., whether flow was subsonic or supersonic.
For supersonic inflow conditions, all the upstream conditions, i.e., flow direction, velocity, total enthalpy, and entropy, were specified. At the supersonic exit, all flow variables were extrapolated from the inside numerically. Meanwhile, inflow direction, entropy, and total enthalpy for subsonic inlet flow and exit static pressure for subsonic outflow were specified. The exit static pressure was specified as atmospheric pressure (101.3kPa) in this study. An adiabatic wall temperature boundary condition was used at the wall.
Results and Discussion
Flow passage in the main nozzle of the air-jet loom is divided into three regions. The first region includes an air tank, a two-way solenoid valve, an air-jet main nozzle inlet, an inclined flow passage, and a minimum cross-- sectional area at the leading edge of the needle. The flow is accelerating in this convergent nozzle region.
Sudden expansion in the nozzle cross-sectional area causes strong flow expansion, and the accelerated flow pulls the weft into the acceleration tube in this second flow region. A massive flow separation right behind the backward-facing step can be observed.
In the acceleration tube, the cross-sectional area is constant, and due to wall friction, the boundary layer develops continuously and eventually the flow with sufficient air pressure is accelerated to Mach I at the exit of the acceleration tube in the third flow region, the "Fanno flow" region. The air jet along with the weft releases freely from the exit of the acceleration tube to the atmosphere and the weft flies into the warp in the air-jet loom.
AIR-JET FLOWS AND GRID CONVERGENCE TESTS
We have performed grid convergence tests by varying the number of grids extensively. As a typical test condition, the air tank stagnation pressure and temperature are 2 kg^sub f^/cm^sup 2^ and 296K, respectively. The inlet Mach number is 0.43, and inlet pressure and the speed of sound are computed from a perfect gas equation of state and isentropic flow conditions. The Reynolds number, based on nozzle inlet velocity and the radius of the acceleration tube (R = 2.0mm), is 3.57 X 10^sup 4^. Thus, the flow inside the nozzle is fully turbulent. Detailed flow conditions are given in Table I.
Figure 1 shows a schematic diagram of the main nozzle system of the air-jet loom used in our study. Major specifications are the inner diameter of the needle (d^sub i^ = 2.8 mm), acceleration tube length (L = 270 mm), and tube diameter (D = 4.0 mm).
Pulsed air from the solenoid valve is accelerated through a narrow stabilizer and flows into the main nozzle. In this analysis, we assume the air flows steadily into the nozzle instead of pulsing. Computation begins 13 mm upstream of the nozzle throat where the flow is parallel to the axial direction. The main nozzle has a hollow needle (or yarn tube) in the middle for weft insertion, and the co-axial surrounding flow passage is quite different from traditional nozzles.
Figure 2 shows the computational grid system. Since we assume an axisymmetric nozzle, we have constructed the upper half grid system only. We treat the hollow needle as a solid one to simplify the complex flow field in a way similar to Mohamed and Salama [8]. Thus, we assume the main nozzle system is an axisymmetric backward-facing step with the acceleration tube apart. All the lengths are nondimensionalized by the radius of the acceleration tube (R = 2.0mm).
Figure 3 shows the grid convergence test results: pressure distribution along the center line at a reservoir stagnation air pressure of 2 kg^sub f^/cm^sup 2^. We use 140 X 70, 150 X 70, 160 x 70, 170 X 70, 180 x 70, and 190 X 70 grids to study the grid effect. The center-line pressure distributions from various grids are almost identical. As shown in Figure 4, separation lengths based on the reattachment point change only 2% when the grid numbers are doubled. The number of grid points under consideration has no significant effect on the computational resuits, therefore, we used the 180 X 70 grid system for all computations.
EFFECT OF AIR TANK PRESSURE
Figure 5(a-e) shows pressure contours in the sudden expansion zone near the backward-facing step using Ishida and Okajima's test conditions [3] at air tank pressures of 2-6 kg^sub f^/cm^sup 2^, respectively. The flow can be accelerated from subsonic to supersonic at the throat of the convergent-- divergent nozzle. The portion behind the nozzle throat is a sudden expansion zone where complex flow patterns such as turbulent air jets and recirculating flows exist. Pressure decreases rapidly right after the throat, and subsequently increases before the acceleration tube inlet. Thus, pressure contours in this region are also extremely complex. Due to the sudden expansion behind the nozzle throat, there are low supersonic flow regions at 2 and 3 kg^sub f^/cm^sup 2^ air tank pressures; however, flow choking (the maximum mass flow rate possible through the nozzle throat, which could occur at sonic speed) does not occur at the throat. At air tank pressures over 4 kg^sub f^/cm^sup 2^ there is sonic flow (M = 1) at the throat, and it shows similar flow patterns in pressure contours near the backward-facing step zone.
Figure 6 shows Mach contours along the center line at tank pressures from 2 to 6 kg^sub f^/cm^sup 2^. Air jet velocities at the acceleration tube exit are subsonic at 2 and 3 kg^sub f^/cm^sup 2^ and sonic, i.e., choked flow, over 4 kg^sub f^/cm^sup 2^. Due to the choked flow at the acceleration tube exit, the flow patterns, i.e., Mach number distributions, do not change as the tank pressures increase over 4 kg^sub f^/cm^sup 2^.
Static wall pressure distributions nondimensionalized by inlet static pressure at various air tank pressures are shown in Figure 7. Nondimensional wall pressure (p/ p^sub ref^) decreases gradually at 2 kg^sub f^/cm^sup 2^ and reaches nearly choked flow conditions at 3 kg^sub f^/cm^sup 2^ and choked flow conditions at 4 kg^sub f^/cm^sup 2^ or higher, thus showing almost identical values from inlet to exit. The acceleration tube exit pressure reaches atmospheric pressure at 2 and 3 kg^sub f^/cm^sup 2^; however, at 4 kg^sub f^/cm^sup 2^ or higher air tank pressures, exit velocities become sonic and exit pressures are higher than atmospheric pressure.
If we assume the optimum air tank pressure as one that achieves sonic velocity at the acceleration tube exit with minimum air tank pressure, then the optimum pressure in the main nozzle system is close to 4 kg^sub f^/cm^sup 2^. Pressure higher than 4 kg^sub f^/cm^sup 2^ is not necessary, and lower pressure than this does not accelerate the flow effectively.
Figure 8 shows Mach number distribution along the center line in the full computational domain. The center line Mach number decreases rapidly as air-jet flows out of tube exit into the surrounding air.
Figure 9 shows a comparison of a few nondimensional velocity profiles along the path of the free jet at 3 and 5 kg^sub f^/cm^sup 2^ to a Gaussian distribution function. Y and V are nondimensionalized by the jet radius, where the magnitude of velocity is a quarter of the maximum velocity and the maximum velocity at each position, respectively. Velocity profiles along the direction normal to the nozzle axis are self-similar over X/D = 40. These normalized velocity profiles are similar to a Gaussian function, so we can assume that our numerical results are valid when compared with the analytic method.
EFFECT OF ACCELERATION TUBE LENGTH
We have studied the effect of acceleration tube length on the flow field by varying tube lengths (L = 180, 240, and 270 mm) at various air tank pressures. Figure 10 shows the Mach number distribution along the center line at a tank pressure of 3 kg^sub f^/cm^sup 2^. The change in tube length does not cause any significant flow pattern changes near the nozzle throat. Mach numbers increase slowly along the tube and at the tube exit rise slightly as tube length increases, but the changes in exit Mach numbers are very small. Figure 11 shows pressure distributions along the center line with various lengths (L = 180, 240, 270 mm) at an air tank pressure of 3 kg^sub f^/cm^sup 2^. Regardless of tube lengths, the exit pressures are similar to each other. When the tube is long, the mechanical energy loss inside the tube becomes large compared with a short tube. Since dimensionless exit pressures are almost the same, the dimensionless tube inlet pressure (10
The thrust force pulling the yam is generally proportional to yarn-flow contact distance and flow velocity. The longer the acceleration tube, the better flow stability and thrust force. In this regard, the optimum length should be properly considered.
EFFECT OF NOzzLE SHAPE
We used a square backward-facing step in the computations above. In this section, we changed the square backward-facing step to circular arc shapes of different radii to see how flow changes in the nozzle system. The radii (r/R) of the circular arcs considered in this study are 0.0625, 0.125, and 0.1875.
Figure 12 shows pressure distributions along the center line at 5 kg^sub f^/cm^sup 2^. The circular arc shape nozzles show higher static pressures compared with the square shaped nozzle. The largest radius of a circular arc, r/R = 0.1875, has the largest static pressure. Therefore, we know that shape changes inside the nozzle using a circular arc sustain higher static pressures inside the acceleration tube. By rounding the square edge of the backward-facing step, we can reduce total pressure loss inside the nozzle. Thus, we can obtain sonic conditions at the tube exit at slightly lower than 4 kg^sub f^/cm^sup 2^, which reduces air consumption by a few percent. The separation zone lengths, i.e., the distance from the backward-facing step to the reattachment point, are shown for all cases in Table II. As we increase the tank pressure, the separation zone length becomes longer. The separation zone length of a round needle end is smaller than that of a square needle end. Therefore proper changes in the nozzle shape can reduce separation zone length and thus pressure losses by expanding low supersonic flow smoothly after the nozzle throat.
Conclusions
We have studied axisymmetric transonic/supersonic flow fields in the main nozzle of an air-jet-loom using the compressible, upwind flux, difference-splitting Navier-Stokes method. At air tank pressures of 4 kg^sub f^/ cm^sup 2^ or higher, a choking phenomenon occurs, not only at the nozzle throat but also at the acceleration tube exit. Flow velocity distribution near the main nozzle throat and at the acceleration tube exit is similar at air tank pressures of 4 kg^sub f^/cm^sup 2^ or higher. Due to flow choking at the tube exit near air pressures of 4 kg^sub f^/ cm^sup 2^, the optimum air tank pressure is about 4 kg^sub f^/cm^sup 2^ in the main nozzle system we have considered. Even though the change in acceleration tube length does not change flow characteristics much, the tube length seems to be related more to weft stability and thrust force. The change in nozzle shape from the square shaped needle end to the circular arc causes static pressure to rise in the tube and reduces total pressure losses in the nozzle, i.e., we can reduce air consumption slightly.
ACKNOWLEDGMENT
This work was partly supported by the Brain Korea 21 project.
Literature Cited
1. Baldwin, B. S., and Lomax, H., Thin Layer Approximation and Algebraic Model for Separated Turbulent Flows, AIAA paper no. 78-257, 1978.
2. Duxbury, V., and Lord, P. R., A Study of Some Factors Involved in Pneumatic Weft Propulsion, J. Textile Inst. 50(10), 558 (1959).
3. Ishida, M., and Okajima, A., Flow Characteristic of the Main Nozzle in an Air-Jet Loom, Textile Res. J. 64(1), 10-20 (1994).
4. Kim, T. H., A Numerical Study of 2-Dimensional Axisymmetric Rocket Nozzle Flow, Master's Thesis, Seoul National University, 1990.
5. Kwon, C. O., Song, D. J., and Kang, S. H., Compressor
Cascade Flow Analysis Using the Upwind Flux Difference Splitting Method, KSME J. 18(3), 653-661 (1994).
6. Lombard, C. K., Bardina, J., Venkatapathy, E., and Oliger, J., Multi-dimensional Formulation of CSCM-An Upwind Flux Difference Eigenvector Split Method for the Compressible Navier-Stokes Equations, AIAA paper no. 831859, 1983.
7. Lyuboviskii, V. P., Analysis of the Pulsed Air Flow on the P-105 Loom, Technol. Textile Ind. USSR(6), 114 (1966).
8. Mohamed, M. H., and Salama, M., Mechanics of a Single Nozzle Air-Jet Filling Insertion System, Part I, Textile Res. J. 56(11), 683 (1986).
9. Oh, T. H., Oh, C. S., and Song, D. J., A Numerical
Analysis of Transonic Flows in an Axisymmetric Main Nozzle of Air-Jet Loom, in Proc. KSME, Fall Annual Meeting B, 1997, pp. 627-632.
10. Song, D. J., Kim, S. D., Kwon, C. O., and Seo, J. I., A Computational Off-Design Performance Analysis of Centrifugal Compressor Diffusers, CFD J. 6(4), 549-560 (1998).
11. Uno, M., and Ishida, T., A Study of Air Jet Looms (in Japanese), J. Textile Mach. Soc. Jpn. 13(9) (1960).
12. Roe, P. L., Approximate Riemann Solvers, Parameter Vectors, and Difference Schemes, J. Comput. Phys. 43, 357372 (1981).
Manuscript received June 26, 2000; accepted December 12, 2000.
T. H. OH, S. D. KIM AND D. J. SONG
School of Mechanical Engineering, Yeungram University, Gyongsan 712-749, South Korea
ABSTRACT
This paper reports a numerical analysis of transonic flows in the axisymmetric backward-facing step main nozzle of an air-jet loom. To obtain basic design data for the optimum main nozzle shape of an air-jet loom and to predict transonic/supersonic internal flows, a characteristic-based, upwind flux difference-splitting, compressible Navier-- Stokes method is used. Wall static pressure and flow velocity distributions in the nozzle are analyzed by changing air tank pressures and acceleration tube lengths. The flow inside the nozzle experiences double choking, first at the needle tip and then at the acceleration tube exit at air tank pressures near 4 kg^sub f^/cm^sup 2^. The air tank pressure that leads to critical conditions depends on acceleration tube length, i.e., higher air tank pressures for longer acceleration tubes. The air pressure required to bring the acceleration tube exit to sonic conditions is nearly constant regardless of acceleration tube length. The round needle tip shape could lead to less total pressure loss when compared with step shape.
An air-jet loom inserts the weft into the warp by using high pressure air-jet thrust force and skin friction force along the yarn. The air-jet loom is popular in texturing industries because of its high productivity, convenient controllability, and wide variety of textured fabrics: silk, cotton, wool, and spun textures. It causes no air pollution since it uses air as the yarn carrying medium. The air-jet loom is also capable of texturing spun or cellulose filament fabrics, which cannot be woven by water-jet looms. However, since the density of air is too low (about 1/1000 of water) compared with that of water, the compressed air jet diffuses rapidly into the atmosphere after discharging from the acceleration tube. Also, the viscosity of air is about 1/50 that of water, so air consumption becomes critical in an air-jet loom.
Due to this large consumption of air and compressor electricity, increased manufacturing costs are one of the loom's disadvantages. Main nozzle shape, exit shape of subnozzles, response time of the solenoid valve, body shape, subnozzle locations, and control methods have been studied intensively to reduce air consumption in air-jet looms.
Fundamental research in main nozzle flows is especially necessary to design optimum shapes of main nozzles, which push the weft to fly through the warp. The air jet from the main nozzle is easily diffused, and it is hard to control the flow direction and velocity, thus increasing air and compressor electricity consumption. Reduced air-jet diffusion and effective control of flow direction and velocity are therefore very important in air-jet loom design.
Due to the recent development of high speed air-jet looms, studies of transonic/supersonic flows in main nozzles have become important for performance enhancement of air jets and optimum nozzle design. It is very difficult to measure the flow field near the nozzle throat region experimentally due to its small cross-sectional area; however, the most important flow phenomena, such as shock waves and flow separations, occur frequently inside this nozzle throat area. Therefore, a computational analysis of the flow field inside the air-jet main nozzle is necessary.
Air flow inside the main nozzle shows subsonic, transonic, and supersonic flow characteristics. There are some difficulties in computational analysis due to the complexity of the flow nature, i.e., turbulence and shock wave/boundary layer interaction. The governing Navier-- Stokes equations are mixed elliptic/hyperbolic partial differential equations [4].
In previous studies of the air-jet loom, Duxbury and Lord [2] derived air-jet velocity distribution exposed to free atmosphere from the main nozzle. Lyubovitskii [7] measured supersonic pulsed jet flow from the nozzle exit. Uno and Ishida [11] experimentally measured air-jet velocity and weft flying distance using various acceleration tube prototypes. Mohamed and Salama [8] studied the effects of the diameter and length of acceleration tubes on flow velocity. Researchers investigated the converging-diverging nozzle (Kim [4], Mohamed and Salama [8]) flows both experimentally and numerically, but there has not been much air-jet nozzle flow analysis. Ishida and Okajima [3] measured pressure inside the main nozzle experimentally and obtained qualitatively similar results in the acceleration tube by using a one-- dimensional Fanno flow assumption. In their case, they did not obtain detailed information. If we use the full Navier-Stokes equation to analyze the internal transonic/ supersonic flow, we may be able to explain the complex flow phenomena of shock waves and flow separations in detail.
Since the flow inside the main nozzle is typically an axisymmetric, compressible, viscous, and transonic/supersonic flow, a computational fluid dynamics (CFD) approach using the full Navier-Stokes equations is necessary. This approach is especially important in the early design stage, because cFD can be used to analyze the main nozzle flow effectively with minimum costs and efforts before making experimental scale models.
We use the CSCM upwind compressible Navier-Stokes method of Lombard et al. [6] in this study. This method has the merits of an upwind scheme, ease in applying characteristic boundary conditions, and a fast flow solver using a diagonally dominant ADI. Kwon et al. [5] and Song et al. [10] analyzed the transonic/supersonic compressor cascade flow and the performance of transonic centrifugal compressor diffusers, respectively, using this same method.
In this study, we analyze transonic/supersonic viscous flows inside the nozzle using the inlet nozzle flow conditions from the experiments. We also analyze the effect of air tank pressure, acceleration tube length, and circular arc radius change in the backward-facing step on the flow field. Finally, we investigate flow physics, including choking phenomena at the nozzle throat and at the exit of the acceleration tube, and optimum nozzle shapes for proper weft insertion.
Numerical Analysis
GRID SYSTEM AND BOUNDARY CONDITIONS
A 180 X 70 H-type grid system was generated by an elliptic PDE grid generator. Grids were packed near the wall by a stretching function (Figure 2), and then in order to observe free jet flow outside the tube exit, a 20 X 120 grid system was attached to the internal grid system.
As wall boundary conditions, a no-slip boundary condition was prescribed at the walls, ie., u = 0 and v = 0. The inlet and exit boundary conditions depended on flow conditions, i.e., whether flow was subsonic or supersonic.
For supersonic inflow conditions, all the upstream conditions, i.e., flow direction, velocity, total enthalpy, and entropy, were specified. At the supersonic exit, all flow variables were extrapolated from the inside numerically. Meanwhile, inflow direction, entropy, and total enthalpy for subsonic inlet flow and exit static pressure for subsonic outflow were specified. The exit static pressure was specified as atmospheric pressure (101.3kPa) in this study. An adiabatic wall temperature boundary condition was used at the wall.
Results and Discussion
Flow passage in the main nozzle of the air-jet loom is divided into three regions. The first region includes an air tank, a two-way solenoid valve, an air-jet main nozzle inlet, an inclined flow passage, and a minimum cross-- sectional area at the leading edge of the needle. The flow is accelerating in this convergent nozzle region.
Sudden expansion in the nozzle cross-sectional area causes strong flow expansion, and the accelerated flow pulls the weft into the acceleration tube in this second flow region. A massive flow separation right behind the backward-facing step can be observed.
In the acceleration tube, the cross-sectional area is constant, and due to wall friction, the boundary layer develops continuously and eventually the flow with sufficient air pressure is accelerated to Mach I at the exit of the acceleration tube in the third flow region, the "Fanno flow" region. The air jet along with the weft releases freely from the exit of the acceleration tube to the atmosphere and the weft flies into the warp in the air-jet loom.
AIR-JET FLOWS AND GRID CONVERGENCE TESTS
We have performed grid convergence tests by varying the number of grids extensively. As a typical test condition, the air tank stagnation pressure and temperature are 2 kg^sub f^/cm^sup 2^ and 296K, respectively. The inlet Mach number is 0.43, and inlet pressure and the speed of sound are computed from a perfect gas equation of state and isentropic flow conditions. The Reynolds number, based on nozzle inlet velocity and the radius of the acceleration tube (R = 2.0mm), is 3.57 X 10^sup 4^. Thus, the flow inside the nozzle is fully turbulent. Detailed flow conditions are given in Table I.
Figure 1 shows a schematic diagram of the main nozzle system of the air-jet loom used in our study. Major specifications are the inner diameter of the needle (d^sub i^ = 2.8 mm), acceleration tube length (L = 270 mm), and tube diameter (D = 4.0 mm).
Pulsed air from the solenoid valve is accelerated through a narrow stabilizer and flows into the main nozzle. In this analysis, we assume the air flows steadily into the nozzle instead of pulsing. Computation begins 13 mm upstream of the nozzle throat where the flow is parallel to the axial direction. The main nozzle has a hollow needle (or yarn tube) in the middle for weft insertion, and the co-axial surrounding flow passage is quite different from traditional nozzles.
Figure 2 shows the computational grid system. Since we assume an axisymmetric nozzle, we have constructed the upper half grid system only. We treat the hollow needle as a solid one to simplify the complex flow field in a way similar to Mohamed and Salama [8]. Thus, we assume the main nozzle system is an axisymmetric backward-facing step with the acceleration tube apart. All the lengths are nondimensionalized by the radius of the acceleration tube (R = 2.0mm).
Figure 3 shows the grid convergence test results: pressure distribution along the center line at a reservoir stagnation air pressure of 2 kg^sub f^/cm^sup 2^. We use 140 X 70, 150 X 70, 160 x 70, 170 X 70, 180 x 70, and 190 X 70 grids to study the grid effect. The center-line pressure distributions from various grids are almost identical. As shown in Figure 4, separation lengths based on the reattachment point change only 2% when the grid numbers are doubled. The number of grid points under consideration has no significant effect on the computational resuits, therefore, we used the 180 X 70 grid system for all computations.
EFFECT OF AIR TANK PRESSURE
Figure 5(a-e) shows pressure contours in the sudden expansion zone near the backward-facing step using Ishida and Okajima's test conditions [3] at air tank pressures of 2-6 kg^sub f^/cm^sup 2^, respectively. The flow can be accelerated from subsonic to supersonic at the throat of the convergent-- divergent nozzle. The portion behind the nozzle throat is a sudden expansion zone where complex flow patterns such as turbulent air jets and recirculating flows exist. Pressure decreases rapidly right after the throat, and subsequently increases before the acceleration tube inlet. Thus, pressure contours in this region are also extremely complex. Due to the sudden expansion behind the nozzle throat, there are low supersonic flow regions at 2 and 3 kg^sub f^/cm^sup 2^ air tank pressures; however, flow choking (the maximum mass flow rate possible through the nozzle throat, which could occur at sonic speed) does not occur at the throat. At air tank pressures over 4 kg^sub f^/cm^sup 2^ there is sonic flow (M = 1) at the throat, and it shows similar flow patterns in pressure contours near the backward-facing step zone.
Figure 6 shows Mach contours along the center line at tank pressures from 2 to 6 kg^sub f^/cm^sup 2^. Air jet velocities at the acceleration tube exit are subsonic at 2 and 3 kg^sub f^/cm^sup 2^ and sonic, i.e., choked flow, over 4 kg^sub f^/cm^sup 2^. Due to the choked flow at the acceleration tube exit, the flow patterns, i.e., Mach number distributions, do not change as the tank pressures increase over 4 kg^sub f^/cm^sup 2^.
Static wall pressure distributions nondimensionalized by inlet static pressure at various air tank pressures are shown in Figure 7. Nondimensional wall pressure (p/ p^sub ref^) decreases gradually at 2 kg^sub f^/cm^sup 2^ and reaches nearly choked flow conditions at 3 kg^sub f^/cm^sup 2^ and choked flow conditions at 4 kg^sub f^/cm^sup 2^ or higher, thus showing almost identical values from inlet to exit. The acceleration tube exit pressure reaches atmospheric pressure at 2 and 3 kg^sub f^/cm^sup 2^; however, at 4 kg^sub f^/cm^sup 2^ or higher air tank pressures, exit velocities become sonic and exit pressures are higher than atmospheric pressure.
If we assume the optimum air tank pressure as one that achieves sonic velocity at the acceleration tube exit with minimum air tank pressure, then the optimum pressure in the main nozzle system is close to 4 kg^sub f^/cm^sup 2^. Pressure higher than 4 kg^sub f^/cm^sup 2^ is not necessary, and lower pressure than this does not accelerate the flow effectively.
Figure 8 shows Mach number distribution along the center line in the full computational domain. The center line Mach number decreases rapidly as air-jet flows out of tube exit into the surrounding air.
Figure 9 shows a comparison of a few nondimensional velocity profiles along the path of the free jet at 3 and 5 kg^sub f^/cm^sup 2^ to a Gaussian distribution function. Y and V are nondimensionalized by the jet radius, where the magnitude of velocity is a quarter of the maximum velocity and the maximum velocity at each position, respectively. Velocity profiles along the direction normal to the nozzle axis are self-similar over X/D = 40. These normalized velocity profiles are similar to a Gaussian function, so we can assume that our numerical results are valid when compared with the analytic method.
EFFECT OF ACCELERATION TUBE LENGTH
We have studied the effect of acceleration tube length on the flow field by varying tube lengths (L = 180, 240, and 270 mm) at various air tank pressures. Figure 10 shows the Mach number distribution along the center line at a tank pressure of 3 kg^sub f^/cm^sup 2^. The change in tube length does not cause any significant flow pattern changes near the nozzle throat. Mach numbers increase slowly along the tube and at the tube exit rise slightly as tube length increases, but the changes in exit Mach numbers are very small. Figure 11 shows pressure distributions along the center line with various lengths (L = 180, 240, 270 mm) at an air tank pressure of 3 kg^sub f^/cm^sup 2^. Regardless of tube lengths, the exit pressures are similar to each other. When the tube is long, the mechanical energy loss inside the tube becomes large compared with a short tube. Since dimensionless exit pressures are almost the same, the dimensionless tube inlet pressure (10
The thrust force pulling the yam is generally proportional to yarn-flow contact distance and flow velocity. The longer the acceleration tube, the better flow stability and thrust force. In this regard, the optimum length should be properly considered.
EFFECT OF NOzzLE SHAPE
We used a square backward-facing step in the computations above. In this section, we changed the square backward-facing step to circular arc shapes of different radii to see how flow changes in the nozzle system. The radii (r/R) of the circular arcs considered in this study are 0.0625, 0.125, and 0.1875.
Figure 12 shows pressure distributions along the center line at 5 kg^sub f^/cm^sup 2^. The circular arc shape nozzles show higher static pressures compared with the square shaped nozzle. The largest radius of a circular arc, r/R = 0.1875, has the largest static pressure. Therefore, we know that shape changes inside the nozzle using a circular arc sustain higher static pressures inside the acceleration tube. By rounding the square edge of the backward-facing step, we can reduce total pressure loss inside the nozzle. Thus, we can obtain sonic conditions at the tube exit at slightly lower than 4 kg^sub f^/cm^sup 2^, which reduces air consumption by a few percent. The separation zone lengths, i.e., the distance from the backward-facing step to the reattachment point, are shown for all cases in Table II. As we increase the tank pressure, the separation zone length becomes longer. The separation zone length of a round needle end is smaller than that of a square needle end. Therefore proper changes in the nozzle shape can reduce separation zone length and thus pressure losses by expanding low supersonic flow smoothly after the nozzle throat.
Conclusions
We have studied axisymmetric transonic/supersonic flow fields in the main nozzle of an air-jet-loom using the compressible, upwind flux, difference-splitting Navier-Stokes method. At air tank pressures of 4 kg^sub f^/ cm^sup 2^ or higher, a choking phenomenon occurs, not only at the nozzle throat but also at the acceleration tube exit. Flow velocity distribution near the main nozzle throat and at the acceleration tube exit is similar at air tank pressures of 4 kg^sub f^/cm^sup 2^ or higher. Due to flow choking at the tube exit near air pressures of 4 kg^sub f^/ cm^sup 2^, the optimum air tank pressure is about 4 kg^sub f^/cm^sup 2^ in the main nozzle system we have considered. Even though the change in acceleration tube length does not change flow characteristics much, the tube length seems to be related more to weft stability and thrust force. The change in nozzle shape from the square shaped needle end to the circular arc causes static pressure to rise in the tube and reduces total pressure losses in the nozzle, i.e., we can reduce air consumption slightly.
ACKNOWLEDGMENT
This work was partly supported by the Brain Korea 21 project.
Literature Cited
1. Baldwin, B. S., and Lomax, H., Thin Layer Approximation and Algebraic Model for Separated Turbulent Flows, AIAA paper no. 78-257, 1978.
2. Duxbury, V., and Lord, P. R., A Study of Some Factors Involved in Pneumatic Weft Propulsion, J. Textile Inst. 50(10), 558 (1959).
3. Ishida, M., and Okajima, A., Flow Characteristic of the Main Nozzle in an Air-Jet Loom, Textile Res. J. 64(1), 10-20 (1994).
4. Kim, T. H., A Numerical Study of 2-Dimensional Axisymmetric Rocket Nozzle Flow, Master's Thesis, Seoul National University, 1990.
5. Kwon, C. O., Song, D. J., and Kang, S. H., Compressor
Cascade Flow Analysis Using the Upwind Flux Difference Splitting Method, KSME J. 18(3), 653-661 (1994).
6. Lombard, C. K., Bardina, J., Venkatapathy, E., and Oliger, J., Multi-dimensional Formulation of CSCM-An Upwind Flux Difference Eigenvector Split Method for the Compressible Navier-Stokes Equations, AIAA paper no. 831859, 1983.
7. Lyuboviskii, V. P., Analysis of the Pulsed Air Flow on the P-105 Loom, Technol. Textile Ind. USSR(6), 114 (1966).
8. Mohamed, M. H., and Salama, M., Mechanics of a Single Nozzle Air-Jet Filling Insertion System, Part I, Textile Res. J. 56(11), 683 (1986).
9. Oh, T. H., Oh, C. S., and Song, D. J., A Numerical
Analysis of Transonic Flows in an Axisymmetric Main Nozzle of Air-Jet Loom, in Proc. KSME, Fall Annual Meeting B, 1997, pp. 627-632.
10. Song, D. J., Kim, S. D., Kwon, C. O., and Seo, J. I., A Computational Off-Design Performance Analysis of Centrifugal Compressor Diffusers, CFD J. 6(4), 549-560 (1998).
11. Uno, M., and Ishida, T., A Study of Air Jet Looms (in Japanese), J. Textile Mach. Soc. Jpn. 13(9) (1960).
12. Roe, P. L., Approximate Riemann Solvers, Parameter Vectors, and Difference Schemes, J. Comput. Phys. 43, 357372 (1981).
Manuscript received June 26, 2000; accepted December 12, 2000.
T. H. OH, S. D. KIM AND D. J. SONG
School of Mechanical Engineering, Yeungram University, Gyongsan 712-749, South Korea
Bath concentration and add-on control in wet-on-wet padding
Yang, Yiqi
ABSTRACT
One concern associated with wet-on-wet processing is the changing chemical concentration in the pad bath. After processing for several hundred yards or so, chemical add-on considerably decreases, resulting in poor performance properties of finished goods or decreased shade depth. To obtain a constant bath concentration and add-on, an appropriate initial concentration and a reconstitution stream with higher concentration than the initial concentration should be used. The validity of a mathematical model relating parameters in wet-on-wet padding is proven by experimental data from both laboratory and production scales. Using that model, changes in pad bath concentration can be quantitatively described. For given processing conditions and add-on requirements, an initial pad concentration and a different reconstitution concentration can be obtained from the model to assure a constant add-on from the beginning to the end of the wet-on-wet process.
A conventional continuous textile wet process is to apply chemicals to a dry fabric by means of padding after the dry fabric has been wetted in a chemical bath. Such a process is called "wet-on-dry" (WOD). Padding the dry fabric (WOD) allows quick penetration, high add-on, and easy concentration control in the padding bath. "Wet-on-- wet" (wow) is a term used to describe a padding process starting with a wet fabric. This process has been quickly accepted in many textile plants as a padding procedure to substitute for conventional WOD processes from slashing [5, 10] to dyeing [1-4, 6] and finishing [7-9]. Some of the advantages of WOW over WOD are the requirement of smaller amounts of chemicals to obtain the same results as the WOD process, energy savings by eliminating fabric drying between two wet treatments, and decreasing or eliminating wetting agents.
One problem associated with the wow process is varying chemical concentration in the pad bath and chemical add-on of the finished fabric. A common mistake in the WOW practice is to assume that the chemical add-on is based on the net increase of the wet pickup of the fabric after being padded through the chemical pad. For example, if the pickup of the water pad is 50% and the pickup of the chemical pad is 100%, the chemical concentration should be doubled to obtain the same add-on as WOD for a pickup of 100%. If such a calculation is used, the chemical add-on is much higher than the targeted one at the beginning of the process, but decreases gradually during processing, leading to inconsistent performance properties in the finished goods.
When the wet fabric enters the chemical pad and is being nipped, some of the water on the wet fabric goes into the chemical bath. On the one hand, this increases the net pickup from the chemical pad and, on the other hand, dilutes the chemical concentration in the bath over time. Such a water interchange causes a higher chemical add-on in the beginning and a continuous decrease in add-on with processing.
To avoid diluting the chemicals, the interchange of water from the wet fabric into the chemical pad must be considered. Our objective in this work is to determine the quantitative relationship between the change in chemical concentration in the bath and padding conditions, so that constant bath concentration and chemical add-on can be achieved with the wow process.
Conclusions
We have discussed a mathematical model for wet-on-- wet padding for both laboratory and production scales. Using that model, changes in pad bath concentration can be quantitatively described. For given processing conditions and add-on requirements, the model provides an initial pad concentration and a different reconstitution concentration to assure a constant add-on from the beginning to the end of the wet-on-wet process.
The quality of the dyed goods from the wow pad-batch dyeing is the same as fabrics dyed with the conventional WOD technology, including color yield, shade variation, and colorfastness to light, wash, and crock.
A well-controlled dyeing process with constant bath concentration, shade depth, and color quality indicates that the wow technology developed in this work is appropriate for application to all prewet padding processes, including-but not limited to--prewet sizing, wow dyeing, and wow finishing.
ACKNOWLEDGMENTS
We wish to express our appreciation to Neil Stewart and Gary Moore of the Institute of Textile Technology for their assistance throughout this research. We also thank Milliken & Company for providing the machinery, materials, and technical assistance necessary for the study.
Literature Cited
1. Ankeny, M., A Study of Wet-on-wet Pad Batch Dyeing on 100% Cotton Interlock Fabric, report number DF04-96, Cotton Incorporated, NY, 1996.
2. Broadbent, A. D., and Bao, X., Simulating Textile Padding with Vacuum Extraction, Part I: Influence of Process Variables under Ideal Conditions, Textile Res. J. 64(4), 230235 (1994).
3. Broadbent, A. D., Bao, X., Hamoudi, S., and Kong, X., Simulating Textile Padding with Vacuum Extraction, Part II: Perturbation of the Bath Concentration, Textile Res. J. 64(5), 262-269 (1994).
4. Broadbent, A. D., and Kong, X., The Application of Reactive Dyes to Cotton by a Wet-on-wet Cold Pad-batch Method, J. Soc. Dyers Colour. 111(6), 187-190 (1995).
5. Ellis, T., Prewet Sizing, in "Troubleshooting and Innovations in Slashing, Book of Papers, AATCC Warp Sizing Symposium," Athens, GA, March 3-4, 1999.
6. Lavergne, S., Pelletier, G., and Sarabi, P., The Application of Reactive Dyes to Cotton by a Wet-on-wet Padding Technique, "AATCC Book of Papers: 1989 International Conference and Exhibition," October 1989, pp. 75-79.
7. Shah, R. K., and Mittal, R. M., Wet-on-wet Treatments-- Effective Means of Energy Conservation. ATRIA Tech. Digest 21(4), 109-117 (1987).
8. Teli, M. D., Shah, S., and Topiwala, N., Wet-on-wet Finishing Against Conventional Dry-in-wet Finishing, Part 1, Textile Dyer Print. 20(22), 17-22 (1987).
9. Teli, M. D., Shah, S., and Topiwala, N., Wet-on-wet Finishing Against Conventional Dry-in-wet Finishing, Part 2, Textile Dyer Print. 20(24), 17-21 (1987).
10. Trauter, J., Bottle, H., Wunderlich, W., and Vialon, R., Ultrasonic Treatment in the Size Box and Steaming of Raw Yarns for the Purpose of Increasing the Affinity of the Sizing Material to the Fibre, Textil-Prax. Int. 49(7/8), XIII-XIV (1994).
Manuscript received October 13, 1999; accepted December 22, 2000.
YIQI YANG1 AND STEVEN HENSLEY
Institute of Textile Technology, Charlottesville, Virginia 22903, U.S.A.
1 Current address: Dept. of Textiles, Clothing & Design, University of Nebraska-Lincoln, Lincoln, NE 68583-0802.
ABSTRACT
One concern associated with wet-on-wet processing is the changing chemical concentration in the pad bath. After processing for several hundred yards or so, chemical add-on considerably decreases, resulting in poor performance properties of finished goods or decreased shade depth. To obtain a constant bath concentration and add-on, an appropriate initial concentration and a reconstitution stream with higher concentration than the initial concentration should be used. The validity of a mathematical model relating parameters in wet-on-wet padding is proven by experimental data from both laboratory and production scales. Using that model, changes in pad bath concentration can be quantitatively described. For given processing conditions and add-on requirements, an initial pad concentration and a different reconstitution concentration can be obtained from the model to assure a constant add-on from the beginning to the end of the wet-on-wet process.
A conventional continuous textile wet process is to apply chemicals to a dry fabric by means of padding after the dry fabric has been wetted in a chemical bath. Such a process is called "wet-on-dry" (WOD). Padding the dry fabric (WOD) allows quick penetration, high add-on, and easy concentration control in the padding bath. "Wet-on-- wet" (wow) is a term used to describe a padding process starting with a wet fabric. This process has been quickly accepted in many textile plants as a padding procedure to substitute for conventional WOD processes from slashing [5, 10] to dyeing [1-4, 6] and finishing [7-9]. Some of the advantages of WOW over WOD are the requirement of smaller amounts of chemicals to obtain the same results as the WOD process, energy savings by eliminating fabric drying between two wet treatments, and decreasing or eliminating wetting agents.
One problem associated with the wow process is varying chemical concentration in the pad bath and chemical add-on of the finished fabric. A common mistake in the WOW practice is to assume that the chemical add-on is based on the net increase of the wet pickup of the fabric after being padded through the chemical pad. For example, if the pickup of the water pad is 50% and the pickup of the chemical pad is 100%, the chemical concentration should be doubled to obtain the same add-on as WOD for a pickup of 100%. If such a calculation is used, the chemical add-on is much higher than the targeted one at the beginning of the process, but decreases gradually during processing, leading to inconsistent performance properties in the finished goods.
When the wet fabric enters the chemical pad and is being nipped, some of the water on the wet fabric goes into the chemical bath. On the one hand, this increases the net pickup from the chemical pad and, on the other hand, dilutes the chemical concentration in the bath over time. Such a water interchange causes a higher chemical add-on in the beginning and a continuous decrease in add-on with processing.
To avoid diluting the chemicals, the interchange of water from the wet fabric into the chemical pad must be considered. Our objective in this work is to determine the quantitative relationship between the change in chemical concentration in the bath and padding conditions, so that constant bath concentration and chemical add-on can be achieved with the wow process.
Conclusions
We have discussed a mathematical model for wet-on-- wet padding for both laboratory and production scales. Using that model, changes in pad bath concentration can be quantitatively described. For given processing conditions and add-on requirements, the model provides an initial pad concentration and a different reconstitution concentration to assure a constant add-on from the beginning to the end of the wet-on-wet process.
The quality of the dyed goods from the wow pad-batch dyeing is the same as fabrics dyed with the conventional WOD technology, including color yield, shade variation, and colorfastness to light, wash, and crock.
A well-controlled dyeing process with constant bath concentration, shade depth, and color quality indicates that the wow technology developed in this work is appropriate for application to all prewet padding processes, including-but not limited to--prewet sizing, wow dyeing, and wow finishing.
ACKNOWLEDGMENTS
We wish to express our appreciation to Neil Stewart and Gary Moore of the Institute of Textile Technology for their assistance throughout this research. We also thank Milliken & Company for providing the machinery, materials, and technical assistance necessary for the study.
Literature Cited
1. Ankeny, M., A Study of Wet-on-wet Pad Batch Dyeing on 100% Cotton Interlock Fabric, report number DF04-96, Cotton Incorporated, NY, 1996.
2. Broadbent, A. D., and Bao, X., Simulating Textile Padding with Vacuum Extraction, Part I: Influence of Process Variables under Ideal Conditions, Textile Res. J. 64(4), 230235 (1994).
3. Broadbent, A. D., Bao, X., Hamoudi, S., and Kong, X., Simulating Textile Padding with Vacuum Extraction, Part II: Perturbation of the Bath Concentration, Textile Res. J. 64(5), 262-269 (1994).
4. Broadbent, A. D., and Kong, X., The Application of Reactive Dyes to Cotton by a Wet-on-wet Cold Pad-batch Method, J. Soc. Dyers Colour. 111(6), 187-190 (1995).
5. Ellis, T., Prewet Sizing, in "Troubleshooting and Innovations in Slashing, Book of Papers, AATCC Warp Sizing Symposium," Athens, GA, March 3-4, 1999.
6. Lavergne, S., Pelletier, G., and Sarabi, P., The Application of Reactive Dyes to Cotton by a Wet-on-wet Padding Technique, "AATCC Book of Papers: 1989 International Conference and Exhibition," October 1989, pp. 75-79.
7. Shah, R. K., and Mittal, R. M., Wet-on-wet Treatments-- Effective Means of Energy Conservation. ATRIA Tech. Digest 21(4), 109-117 (1987).
8. Teli, M. D., Shah, S., and Topiwala, N., Wet-on-wet Finishing Against Conventional Dry-in-wet Finishing, Part 1, Textile Dyer Print. 20(22), 17-22 (1987).
9. Teli, M. D., Shah, S., and Topiwala, N., Wet-on-wet Finishing Against Conventional Dry-in-wet Finishing, Part 2, Textile Dyer Print. 20(24), 17-21 (1987).
10. Trauter, J., Bottle, H., Wunderlich, W., and Vialon, R., Ultrasonic Treatment in the Size Box and Steaming of Raw Yarns for the Purpose of Increasing the Affinity of the Sizing Material to the Fibre, Textil-Prax. Int. 49(7/8), XIII-XIV (1994).
Manuscript received October 13, 1999; accepted December 22, 2000.
YIQI YANG1 AND STEVEN HENSLEY
Institute of Textile Technology, Charlottesville, Virginia 22903, U.S.A.
1 Current address: Dept. of Textiles, Clothing & Design, University of Nebraska-Lincoln, Lincoln, NE 68583-0802.
A new computerized data acquisition and analysis system for KES-FB instruments
Chen, Yan
A New Computerized Data Acquisition and Analysis System for KES-FB Instruments1
ABSTRACT
The Kawabata evaluation system for fabrics (KES-FB) has been commonly used to measure fabric mechanical behaviors related to hand. However, the data acquisition and analysis technique for this system is obsolete and time-consuming. The purpose of this study is to develop a new data acquisition and analysis system for the KES-FB instruments using LabVIEW(TM) software and corresponding hardware (National Instruments). Windows-- interface programs for each KES test method are created with the LabVIEW system. In order to validate this data acquisition system, twenty apparel fabrics are tested using both the original KES-FB data recording method and the new computerized data processing method. Linear regression and variance analysis are used to compare tested results and evaluate repeatability and variability within the two data acquisition methods. The R^sup 2^ values of most linear regression models are close to 1.00, indicating that the KES data system has been successfully updated.
The instruments of the Kawabata evaluation system for fabrics (KES-FB), including the tensile and shear tester (KES-FB1), bending tester (KES-FB2), compression tester (KES-FB3), and friction and roughness tester (KES-FS4), have been widely used to measure fabric mechanical properties since the 1970s [2]. Other industrial applications of these instruments were also developed [1]. However, the data acquisition method used for the KES-FB instruments has a serious drawback that jeopardizes the ability of KES-FB users to expand industrial applications. The KES-FB instruments provide two options for data acquisition. One option is to use a KES-specific auto data processing unit, which is controlled by a Pc with cpu Intel 486 33MHz or lower and Dos operation system. The software used for this data processing unit produces a special format that cannot be read by other IBM Pcs, and the unit is neither cost-effective nor compatible with general-purpose desktop computers. The other option is to use an X-Y pen recorder to record testing curves and manually calculate Kawabata parameters [3]. This data processing method is time-consuming, and the lack of digitized data hinders research. For example, a large volume of signal noise is produced when the KES-FB bending tester is used to measure very floppy fabrics with high sensitivity. High signal noise makes X- Y chart recording extremely irregular, so it is impossible to infer a fabric responding signal separate from the noise. We have developed a new data acquisition and analysis system (DAQ) to overcome these disadvantages. Existing commercially available hardware and software ensure that the new DAQ is cost-effective and affordable for most KES-FB end-users.
Constructing the Data Acquisition System
The new data acquisition and analysis system consists of a Pentium Pc, a plug-in DAQ interface board, and LabVIEW software version 5.1 [4]. Figure 1 illustrates the structure of this system. Analog signal output from each KS-FB tester, which is usually fed to an X-Y recorder, is input to an interface board through a connector block. This interface board is a product of E series multifunction I/O hardware manufactured by National Instruments, featuring 16 single-ended analog inputs, 16 bits resolution, 100 kS/s sampling rate, and easy plug-in to a PCI slot in the PC. Any brand of IBM desktop PC now available on the market could be used to install and run the DAQ hardware and software.
The LabVIEW software used for DAQ processing control provides a graphic programming environment with the G language, allowing users to build their own customized virtual instruments (VI) for testing, implementation, and control in engineering systems. Because LabVIEW relies on graphic symbols rather than textual languages to compose a program, it is suitable for end-users with little programming experience. There are only two primary steps for LabVIEW programming-design of a block diagram and execution of a virtual instrument. According to a specific end-use application, a block diagram (also called a dataflow diagram) is designed using a series of icons such as functions, structures, control terminals, and indicators, etc., to create a graphic source code for running aVI. After completion of this block diagram, running theVI program will produce a graphic user interface on the front panel of LabVIEW.
As shown in Figure 2, the block diagram programmed in this study produces a windows interface, including a curve display, Kawabata parameter display, filter type adjustment, frequency adjustment, and scan rate selection. This Windows interface enables auto recording and computing when running a KES-FB tester. After completion of a measurement, recorded data are saved in an ASCII-format file that can be conveniently input into spreadsheet software for later analyses.
Results and Discussion
To verify the processing and computing accuracy of the DAQ system, we randomly selected twenty commercial apparel fabrics and measured their mechanical properties using both the KES-FB X- Y recorder method and the computerized DAQ method. We used linear regression to evaluate the strength of the linear correlation between these two data processing methods. R^sup 2^ (Table I) of all linear regression models for tensile properties (LT, WT, RT, and EMT), shear properties (G, 2HG, and 2HG5), bending properties (B and 2HB), and surface properties (MIU, MMD, and SMD) are close to 1.00, reflecting agreement in data recorded by the two methods. Figures 3-5 illustrate the liner correlations for the KES tensile properties.
For the compressive properties, i.e., compressive linearity (LC), compressive energy (WC), compressive resilience (RC), and maximum compressive strain (EMC), the linear regression models obtained indicate low R^sup 2^ values except for WC and To. This creates a need for further analysis of the consistency between the two data acquisition methods in terms of the compression test. First, we use variance analysis [5] to determine if the instrumental data obtained by the computerized DAQ method are significantly different from those obtained by the X-Y recorder. The result of variance analysis, as listed in Table II, indicates that for all the calculated values of each F statistic referred to for each compressive parameter, probabilities of a larger F are all above 0.01. The statistical inference is therefore that there is no significant difference between the two data acquisition methods at the confidence level of 99%.
Table II shows that the variance source of error accounts for a large portion of the grand variance of each parameter. This error may result from non-uniformity of fabric materials, irregularities of fabric surface conditions (such as different hairiness and wrinkling in different locations of a fabric specimen), and other uncontrolled random errors. In this circumstance, we further use variance analysis to estimate repeatability and variability within the X-Y recorder method and the DAQ method. Repeated compression tests are executed by testing six fabric specimens cut from the same piece of polyester suiting fabric. Each specimen is measured with both the X-Y recorder and DAQ system. Variance within each single data acquisition method is listed in Table III. The estimation indicates that when using the DAQ method, the variance of LC and EMC will be two times and RC three times that from the X-Y recorder.
Referring to the X-Y chart recording method, deviation of the compression test using the DAQ method can be estimated by the following equation:
Conclusions
We have used National Instruments' hardware and software to construct a computerized data acquisition system for KES-FB testers. This system provides a user-- interface window that simplifies data recording and allows real-time calculation. Measured instrumental data are automatically stored in a computer and easily output for data manipulation. The signal noise produced by testing low-stiffness fabrics using the KES-FB bending tester with high sensitivity is also eliminated by this DAQ system. Therefore, the testing performance and efficiency of the KES-FB instruments are improved to meet the need for testing diverse textile and nontextile materials using our updated computer techniques that are inexpensive and available commercially. Statistical analysis for the comparative fabric testing shows that no significant difference exists between the X-Y chart recording method and the DAQ method. Repeatability and variability of the DAQ method are estimated by a repeat test of fabric compression on the same fabric. With reference to the X-Y chart recording method, the maximum error rate of instrumental data recorded by the DAQ system is below +/-8.9%.
1 Approved by the Louisiana Agricultural Experiment Station as manuscript no. 00-25-0326.
Literature Cited
1. American Association of Chemists and Colorists, Sueo Kawabata to Receive The Millson Award, Textile Chem. Color. 24(9), 52, 72 (1992).
2. Hearle, J. W. S., Can Fabric Hand Enter the Dataspace? Textile Horizons 6, 16-20 (1993).
3. Kawabata, S., and Niwa, M., Fabric Performance in Clothing Manufacture, J. Textile Inst. 80(1), P19-P50 (1989).
4. National Instruments Corporation, "LabVIEW(TM) User Manual," National Instruments Co., Austin, TX, 1998.
5. Sokal, Robert R., and Rohlf, James F., "Biometry, the Principles and Practice of Statistics in Biological Research," W. H. Freeman and Company, San Francisco, CA, 1969.
Manuscript received August 8, 2000; accepted November 22, 2000.
YAN CHEN AND TAO ZHAO
School of Human Ecology, Louisiana State University Agricultural Center, Baton Rouge, Louisiana, 70803, U.S.A.
BENNY TURNER
Technical Center, Albemarle Corporation, Baton Rouge, Louisiana, 70820, U.S.A.
A New Computerized Data Acquisition and Analysis System for KES-FB Instruments1
ABSTRACT
The Kawabata evaluation system for fabrics (KES-FB) has been commonly used to measure fabric mechanical behaviors related to hand. However, the data acquisition and analysis technique for this system is obsolete and time-consuming. The purpose of this study is to develop a new data acquisition and analysis system for the KES-FB instruments using LabVIEW(TM) software and corresponding hardware (National Instruments). Windows-- interface programs for each KES test method are created with the LabVIEW system. In order to validate this data acquisition system, twenty apparel fabrics are tested using both the original KES-FB data recording method and the new computerized data processing method. Linear regression and variance analysis are used to compare tested results and evaluate repeatability and variability within the two data acquisition methods. The R^sup 2^ values of most linear regression models are close to 1.00, indicating that the KES data system has been successfully updated.
The instruments of the Kawabata evaluation system for fabrics (KES-FB), including the tensile and shear tester (KES-FB1), bending tester (KES-FB2), compression tester (KES-FB3), and friction and roughness tester (KES-FS4), have been widely used to measure fabric mechanical properties since the 1970s [2]. Other industrial applications of these instruments were also developed [1]. However, the data acquisition method used for the KES-FB instruments has a serious drawback that jeopardizes the ability of KES-FB users to expand industrial applications. The KES-FB instruments provide two options for data acquisition. One option is to use a KES-specific auto data processing unit, which is controlled by a Pc with cpu Intel 486 33MHz or lower and Dos operation system. The software used for this data processing unit produces a special format that cannot be read by other IBM Pcs, and the unit is neither cost-effective nor compatible with general-purpose desktop computers. The other option is to use an X-Y pen recorder to record testing curves and manually calculate Kawabata parameters [3]. This data processing method is time-consuming, and the lack of digitized data hinders research. For example, a large volume of signal noise is produced when the KES-FB bending tester is used to measure very floppy fabrics with high sensitivity. High signal noise makes X- Y chart recording extremely irregular, so it is impossible to infer a fabric responding signal separate from the noise. We have developed a new data acquisition and analysis system (DAQ) to overcome these disadvantages. Existing commercially available hardware and software ensure that the new DAQ is cost-effective and affordable for most KES-FB end-users.
Constructing the Data Acquisition System
The new data acquisition and analysis system consists of a Pentium Pc, a plug-in DAQ interface board, and LabVIEW software version 5.1 [4]. Figure 1 illustrates the structure of this system. Analog signal output from each KS-FB tester, which is usually fed to an X-Y recorder, is input to an interface board through a connector block. This interface board is a product of E series multifunction I/O hardware manufactured by National Instruments, featuring 16 single-ended analog inputs, 16 bits resolution, 100 kS/s sampling rate, and easy plug-in to a PCI slot in the PC. Any brand of IBM desktop PC now available on the market could be used to install and run the DAQ hardware and software.
The LabVIEW software used for DAQ processing control provides a graphic programming environment with the G language, allowing users to build their own customized virtual instruments (VI) for testing, implementation, and control in engineering systems. Because LabVIEW relies on graphic symbols rather than textual languages to compose a program, it is suitable for end-users with little programming experience. There are only two primary steps for LabVIEW programming-design of a block diagram and execution of a virtual instrument. According to a specific end-use application, a block diagram (also called a dataflow diagram) is designed using a series of icons such as functions, structures, control terminals, and indicators, etc., to create a graphic source code for running aVI. After completion of this block diagram, running theVI program will produce a graphic user interface on the front panel of LabVIEW.
As shown in Figure 2, the block diagram programmed in this study produces a windows interface, including a curve display, Kawabata parameter display, filter type adjustment, frequency adjustment, and scan rate selection. This Windows interface enables auto recording and computing when running a KES-FB tester. After completion of a measurement, recorded data are saved in an ASCII-format file that can be conveniently input into spreadsheet software for later analyses.
Results and Discussion
To verify the processing and computing accuracy of the DAQ system, we randomly selected twenty commercial apparel fabrics and measured their mechanical properties using both the KES-FB X- Y recorder method and the computerized DAQ method. We used linear regression to evaluate the strength of the linear correlation between these two data processing methods. R^sup 2^ (Table I) of all linear regression models for tensile properties (LT, WT, RT, and EMT), shear properties (G, 2HG, and 2HG5), bending properties (B and 2HB), and surface properties (MIU, MMD, and SMD) are close to 1.00, reflecting agreement in data recorded by the two methods. Figures 3-5 illustrate the liner correlations for the KES tensile properties.
For the compressive properties, i.e., compressive linearity (LC), compressive energy (WC), compressive resilience (RC), and maximum compressive strain (EMC), the linear regression models obtained indicate low R^sup 2^ values except for WC and To. This creates a need for further analysis of the consistency between the two data acquisition methods in terms of the compression test. First, we use variance analysis [5] to determine if the instrumental data obtained by the computerized DAQ method are significantly different from those obtained by the X-Y recorder. The result of variance analysis, as listed in Table II, indicates that for all the calculated values of each F statistic referred to for each compressive parameter, probabilities of a larger F are all above 0.01. The statistical inference is therefore that there is no significant difference between the two data acquisition methods at the confidence level of 99%.
Table II shows that the variance source of error accounts for a large portion of the grand variance of each parameter. This error may result from non-uniformity of fabric materials, irregularities of fabric surface conditions (such as different hairiness and wrinkling in different locations of a fabric specimen), and other uncontrolled random errors. In this circumstance, we further use variance analysis to estimate repeatability and variability within the X-Y recorder method and the DAQ method. Repeated compression tests are executed by testing six fabric specimens cut from the same piece of polyester suiting fabric. Each specimen is measured with both the X-Y recorder and DAQ system. Variance within each single data acquisition method is listed in Table III. The estimation indicates that when using the DAQ method, the variance of LC and EMC will be two times and RC three times that from the X-Y recorder.
Referring to the X-Y chart recording method, deviation of the compression test using the DAQ method can be estimated by the following equation:
Conclusions
We have used National Instruments' hardware and software to construct a computerized data acquisition system for KES-FB testers. This system provides a user-- interface window that simplifies data recording and allows real-time calculation. Measured instrumental data are automatically stored in a computer and easily output for data manipulation. The signal noise produced by testing low-stiffness fabrics using the KES-FB bending tester with high sensitivity is also eliminated by this DAQ system. Therefore, the testing performance and efficiency of the KES-FB instruments are improved to meet the need for testing diverse textile and nontextile materials using our updated computer techniques that are inexpensive and available commercially. Statistical analysis for the comparative fabric testing shows that no significant difference exists between the X-Y chart recording method and the DAQ method. Repeatability and variability of the DAQ method are estimated by a repeat test of fabric compression on the same fabric. With reference to the X-Y chart recording method, the maximum error rate of instrumental data recorded by the DAQ system is below +/-8.9%.
1 Approved by the Louisiana Agricultural Experiment Station as manuscript no. 00-25-0326.
Literature Cited
1. American Association of Chemists and Colorists, Sueo Kawabata to Receive The Millson Award, Textile Chem. Color. 24(9), 52, 72 (1992).
2. Hearle, J. W. S., Can Fabric Hand Enter the Dataspace? Textile Horizons 6, 16-20 (1993).
3. Kawabata, S., and Niwa, M., Fabric Performance in Clothing Manufacture, J. Textile Inst. 80(1), P19-P50 (1989).
4. National Instruments Corporation, "LabVIEW(TM) User Manual," National Instruments Co., Austin, TX, 1998.
5. Sokal, Robert R., and Rohlf, James F., "Biometry, the Principles and Practice of Statistics in Biological Research," W. H. Freeman and Company, San Francisco, CA, 1969.
Manuscript received August 8, 2000; accepted November 22, 2000.
YAN CHEN AND TAO ZHAO
School of Human Ecology, Louisiana State University Agricultural Center, Baton Rouge, Louisiana, 70803, U.S.A.
BENNY TURNER
Technical Center, Albemarle Corporation, Baton Rouge, Louisiana, 70820, U.S.A.
Effect of splitting and finishing on absorption/adsorption properties of split polyester microfiber fabrics
Park, Myung-Ja
ABSTRACT
Nylon/polyester (N/P) conjugate fibers are split by alkaline hydrolysis and then finished with an antimicrobial agent, and the effect of splitting and finishing on the absorption/ adsorption properties of the microfibers is studied. The split microfiber fabrics vary in weight loss and pore structure depending on the various splitting conditions. The absorption behavior of microfiber fabrics is analyzed by the degree of splitting, shrinkage, fabric density, and weight loss. Optimum splitting conditions are investigated for superior absorption rate and capacity. Even and complete splitting produces fine fibers closely packed in a parallel structure, which creates capillary channels that transport water into fabric treated at 140deg C with about 10% weight loss. Values of adsorption, add-on (%), and good durability to repeated laundering and dry cleaning of the agent on the finished rr/P microfiber fabrics are high, in contrast to a conventional fiber fabric. This is most likely due to the high surface area and surface irregularities caused by splitting and hydrolysis. The absorption capacity of the finished fabrics decreases because some pore spaces are filled with the adsorbed agent, while the absorption rate increases due to capillary sorption. The water absorption instrument newly devised for this study is an excellent measurement system. It is possible to measure the amount of water absorption with time, and to distinguish the differences in absorbency of the split rr/P microfiber knitted fabrics, which have pore structures that vary in shape and size, created by and deformed during the splitting and finishing process.
There has been a trend towards finer synthetic filament fibers over recent decades, and consequently various microfibers have been developed with novel fiber spinning techniques to reduce thickness and alter the crosssectional shape. Microfiber fabrics have enhanced drapability, luster, softness, bulkiness, and smoothness, and also high tactile aesthetics and high water absorption and chemical adsorption properties.
Split microfibers are produced by separating the bicomponent conjugate filaments through exposure to alkaline solution in combination with thermal and mechanical treatments [13, 51. This chemical splitting method-- alkaline hydrolysis-is known as a good method for complete splitting and even separation. Many studies [5, 11] of conventional polyester have shown that hydrolysis increases in proportion to treatment time and temperature and to NaOH concentration. For hydrolysis of conventional polyester fibers, the treatment temperature does not exceed 100deg C in a strong alkaline solution. On the other hand, conjugate filaments are treated in a very dilute solution at a higher temperature for greater and more even splitting as well as less hydrolysis. Optimum splitting conditions depend on the intended end-use of the microfiber fabrics, for example, whether they need to be highly absorbent.
Recently, owing to their high water absorption characteristics, microfiber fabrics, especially polyester microfiber knitted pile fabrics, have found practical application in such products as sports towels, dishcloths, and wiping cloths. However, the optimum splitting conditions for maximum water absorption are not known. Moreover, the relationship between the absorption behavior and fabric morphology of these split microfiber fabrics, depending on the extent of splitting, hydrolysis, and pore structure, have rarely been studied.
Generally, water absorption characteristics of hydrophobic polyester fibers are determined by examining the external surface of the fibers [6, 15]. Bright et al. [31 reported that the water taken up by polyester is present on the surface of the fibers, in contrast to hydrophilic fibers. Splitting creates fine, closely packed and aligned capillary columns for water transmission coupled with a large surface area [12]. The extent of splitting and changes in fabric morphology during different splitting processes affects these capillary spaces and, consequently, water absorption into the fabrics [8, 91. Currently, there is no suitable test instrument to determine the rates of water absorption and absorption by the entire capillary process, so the development of such a tester is necessary.
Microfiber fabrics with high liquid water retention also have low rates of moisture loss through evaporation, and may often be susceptible to the growth of microorganisms. Thus, they may need an antimicrobial finish for hygiene and comfort. Antimicrobial finishing of polyester fibers is rare compared with cellulose. Polyester has low reactivity, with no chemical bonding taking place between fiber and agent, so that the treatment is purely a physical coating of the fiber surface [16]. The poor durability of these treatment agents to repeated laundering has been a problem with conventional polyester fibers [10, 14]. The problem might be less serious in microfiber fabrics, since the agent ought to be more effectively adsorbed for the reasons already discussed.
Therefore, we need to determine the optimal splitting and finishing conditions for developing antimicrobial polyester microfiber fabrics with high water absorption through studies on absorption/adsorption related to fabric morphology, fiber surfaces, and pore structures between the microfibers, which were created and modified during the splitting and finishing process. The object of this study is to elucidate the effect of splitting on the water absorption and finishing agent adsorption of split microfibers related to fabric morphology, to examine the effect of finishing on the absorption of the finished split microfibers, and to develop an appropriate absorbency test instrument to measure absorption through capillary action only in fabrics.
Experimental
MATERIALS
A split N/P (nylon/polyester, 25:75 weight %) conjugate fiber (120d/72f multi filaments) pile knit (obtained from Silver Star Ltd.) was used as the material for the splitting process. Ranges of microfiber fabrics produced under various splitting conditions were used as specimens for evaluating absorption properties. One microfiber fabric treated with 0.3% NaOH solution at 135deg C for 40 minutes was used as a material for antimicrobial finishing. The antimicrobial finished fabrics were characterized to evaluate their adsorption properties, and 100% cotton (terry cloth) or 100% conventional polyester (DrY 150d/48f, FDY 250d/48f) pile fabrics were used to compare properties with the polyester microfiber fabrics.
The antimicrobial agent Ultra-Fresh, 300DDN, 1.06% bis(tri-n-butyl tin) oxide as Sn, 1.6% 5-chloro-2(2,4-- dichlorophenoxy) phenol was supplied by Thomson Research Associates. Sodium hydroxide as a splitting chemical, acetic acid as a neutralizer, and all other reagents were reagent grade. Bleach and detergent (multipurpose type) were commercially available.
WATER ABSORPTION MEASUREMENT SYSTEM
To accurately measure the rate of water absorption into the treated fabrics, we devised an instrument based on a conventional method [4, 7]; it is depicted in Figure 1. The round fabric specimen (5 cm in diameter) is positioned on a porous plate in a glass filter, below which there is a water path, a glass tube, leading to a water reservoir on an electronic balance. The electronic balance is connected to a computer that records the weight loss in the reservoir, which is equivalent to the amount of water transferred into the fabric. A reading is recorded every second. A flat round (5 cm in diameter) weight (16g) is placed on the fabric to help ensure that it absorbs water uniformly over the contact area. The levels of the porous plate and water in the reservoir are adjusted to the same height to eliminate any difference in hydrodynamic pressure. These two conditions lead to spontaneous transfer on the basis of capillary sorption and permeability by the fabric structure.
SPLITTING
Microfibers were prepared by splitting split N/P bicomponent conjugate fibers using a chemical method. Appropriate reaction conditions were determined from preliminary experiments. In these, the conjugate fibers treated in a relatively high concentration of NaOH solution below 100deg C were not split evenly, indicating that a longer reaction time was needed. While the conjugate fibers were treated in low concentration over 140deg C, most of the split polyester filaments were dissolved due to the alkaline hydrolysis reaction. Therefore, very dilute NaOH solutions under suitable temperatures were employed to obtain even and proper splitting conditions.
The pile fabric knitted from the conjugate fibers was treated in the NaOH solution in a high-pressure dyeing machine by a batch method under the following treatment conditions: 0.1-0.9% NaOH solutions at 100-- 140deg C for 20-80 minutes at a bath ratio of 50:1. Finally, the treated fabrics were neutralized in acetic acid solution.
ANTIMICROBIAL FINISHING
The antimicrobial finish was applied by the pad-dry method in various bath solutions with antimicrobial agent concentrations of 0.0133-0.1330% (owf) at a bath ratio of 1:15. Fabric padded with the finishing solution was squeezed to 100% wet pickup by two dips-two nips and dried at 50deg C for 30 minutes. Two fabrics, 100% polyester conventional fibers and the NIP microfiber fabric, were used for the finishing (see Figure 2).
CHARACTERIZATION
The weight loss (%), shrinkage (%), and fabric areal density (g/m2) were calculated from the original and final measurements before and after NaOH treatment. The degree of splitting and separation of the conjugate fiber filaments and the fabric surface were examined by scanning electron microscopy (SEM).
The rate of initial water absorption onto the microfiber fabrics was determined from the weight absorbed over 10 seconds, and the water absorption capacity of the fabrics was measured by the maximum weight absorbed. Absorption was represented as two units of percent or gram per mass of absorbed water in a specimen fabric/ mass of dry specimen or unit area of dry specimen, respectively.
Adsorption (% add-on) of antimicrobial agent on the finished fabrics was determined by measuring the Sn content, one of components in the agent, using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Desorption of the agent out of the finished fabrics was determined by the antimicrobial properties.
The antimicrobial properties of the finished and cleaned fabrics were evaluated through AATCC Test Method 100 [1] by measuring the reduction of % Staphylococcus aureus (ATCC No. 6538 gram positive organism).
Durability of the antimicrobial finish to repeated home laundering and to dry cleaning [2] was also evaluated. The finished fabrics were washed with alkaline detergent at 40deg C in an automatic washer set at the permanent press machine cycle and then allowed to line-dry; this procedure was repeated ten times. The finished specimens (15 x 15 cm) were treated in 150 ml dry cleaning solvent consisting of perchloroethylene and hexane with dry cleaning detergents in a Launder-Ometer with ten stainless steel balls at room temperature for 10 minutes. Results and Discussion
FABRIC CHANGES DURING SPLITTING
Weight Loss
The weight loss (%) resulting from the NaOH treatments (Figure 3) appeared to be approximately proportional to treatment temperature, concentration, and time, which is generally similar to the tendency observed in fabrics made from conventional rE r fibers. The ranges of weight loss were narrow at lower temperatures: 1.1 %-3.81 % at 100deg C and 1.44%-15.62% at 1200C. At higher temperatures, weight loss varied from 1.76% to 30.14% at 130deg C and from 2.3% to 47.5% at 140deg C, depending on reaction time and concentration. The rate of weight reduction from the fabric with NaOH treatment at 100-120deg C increased slowly, while at 140deg C it increased rapidly.
Shrinkage
Fabric shrinkage (%) during the splitting process was great: 27.7% in the course direction and 22.4% in the wale direction on average. Shrinkage seems to be positively correlated with NaOH concentration, but hardly affected by either treatment time or temperature. Shrinkage increases the packing density of the fibers and modifies the interfiber capillaries.
Density
Changes in the areal density of the fabric corresponded to shrinkage and weight loss and were thus also susceptible to the treatment conditions, as shown in Table I. Under conditions of 100'C for 20 minutes, fabric density increased markedly. This might be due to the great initial shrinkage and less weight loss at first. After that, the fabric showed almost constant density because shrinkage and weight loss changed little. Results were similar for conditions at 120deg C, except for the 0.9% concentration. With the 0.9% solution, fabric density decreased somewhat with treatment time, suggesting that the effect of weight loss gained predominance. In general, at relatively low treatment temperatures (100 and 120deg C), fabric density values were high and fairly constant due to much shrinkage and little weight loss. But treatments above 0.6% at 130deg C, or in any concentrations at 140deg C, made the fabrics less dense as the reaction time passed. The density of the fabrics treated in 0.6 or 0.9% solution at 140deg C was less than that of the untreated fabric. Splitting occurred in conjugate fiber filaments at the interfaces of polyester and nylon fibers, where weight loss increased with hydrolysis. Alkaline hydrolysis caused dissolution of polyester fibers as well as splitting, so levels of weight loss should be optimized.
SPLITTING CHARACTERISTICS
Pore Structure
The SEM microphotographs shown in Figure 4 present evidence of pore formation depending on the degree of hydrolysis, splitting, and separation after NaOH treatment. Capillary pores (the terms voids, micropores, holes, sites, channels, free volume, accessible space, etc. have often been used to indicate the same thing in the literature) are the spaces between the fibers where liquid molecules are either transported or become lodged. Therefore, pore structure, including the total volume of pores, average pore size, and local distribution, is the greatest structural parameter controlling transport behavior in hydrophobic fibers.
Splitting
The conjugate fibers hydrolyzed below 2% weight loss, shown in Figure 4b, were not entirely split. In Figure 4c, some conjugate filaments were split partially and irregularly. Figures of 4d and e show that all fibers were split and separated evenly and completely to become finer filaments. As a result of compact and even distribution of microfibers and alignment of the spaces, the effective capillary action between the filaments would be expected to lead to good absorbency. In Figure 4f, excessive hydrolysis of polyester fibers decreased the capillary pores and increased the pore sizes. Only nylon cores in NIP conjugate filaments remained, and some spaces between the fibers seemed too big to retain water by capillary sorption.
Surface Area
Examination of the surface morphology of the fabrics and filaments could provide an estimate of the surface area of piles in the fabrics and the shape of the monofilaments after splitting. Figure 4 shows that a large change in the surface area of the filaments occurred through splitting. Complete splitting was accompanied by an increase in the surface area per unit volume of the filament, which led to an increase in capillary walls and total surface areas. We expected that surface adsorption of the finishing agents on the individual filaments should be great and durable, but excessive hydrolysis over a 25% weight loss resulted in a decreased surface area and sharp-edged cross sections of monofilaments.
WATER ABSORPTION PROPERTY OF SPLIT MICROFIBER FABRics
Rate of Initial Water Absorption
We measured the rate of initial water absorption (mass/unit area) for 10 seconds, and the results are shown in Figure 5. Partial or complete splitting resulted in the creation of capillary pores, and capillary force affected the absorption rate. Most fabrics treated in 0.1% solution absorbed very slowly. Rapid increases in the absorption rate showed up in certain conditions where splitting seemed to be complete.
Maximum values of initial water absorption (mass/ unit area) were about 106, 124, 123, and 183 (g/cm2 x 10-3) for the fabrics treated at 100, 120, 130, and 140deg C, respectively. Represented as percentages, the values translate to 410.5, 500.3, 514.3, and 728.9% for fabrics treated at 100, 120, 130, and 140deg C, respectively. These figures are higher than for the terry cloth composed of hydrophilic cotton fibers (403.6%), which was less than for the polyester microfiber fabrics. Therefore, polyester microfiber fabrics created under these conditions should be superior in use for practical applications involving wetting.
The absorption rate of some fabrics treated at 140deg C showed a very rapid rise at the initial stages of splitting but decreased as splitting treatment time increased. The stronger the solution, the greater the rate reduction, which appeared to be related to the reduction in fabric density shown in Table I. In Figure 6a, the absorption rate seemed to be related to weight loss. It began and increased rapidly at around 2% weight loss, which was the starting point of splitting, shown in Figure 4c. The fabrics treated at 140deg C, in a range of 4.5 to 22.6% weight loss, showed the highest values in rates of initial water absorption, especially the 11.3% weight loss with splitting conditions of 0.9% for 20 minutes at 140deg C.
Rapid absorption in a short time is due to water transfer into the fabrics by high capillary forces, determined by the length of the continuous column of water.
More continuous capillary channels would have formed in the fabrics hydrolyzed at the higher temperature of 140deg C due to the complete splitting and even separation of the conjugate fibers. The microfiber fabrics split at 140deg C were superior to the cotton fiber, which absorbed water by hydrophilicity and capillary spaces rather than capillary sorption. Over 20% weight loss, as areal density decreased, capillary size grew, whereas the rate of initial water absorption (g/cm2) decreased considerably (Figure 6a).
Absorption Capacity
Maximum water absorption values of the microfiber fabrics treated under various conditions are shown in Figures 7a-d. Low absorption capacity showed fabrics treated at relatively low temperature and concentration regardless of treatment time, such as 0.1-0.3% at 100deg C and 0.1% at 120deg C. High absorption capacity showed fabrics hydrolyzed in 0.3-0.9% solutions at 130deg C. Most fibers split under any conditions, except for the 0.1% solution, showed high absorption capacity similar or superior to terry cloth of 132.4 (g/cm2 X 10-3) or 486.3%. However, the time to reach maximum water absorption was 8-193 seconds for the microfiber fabrics over 150 (g/cm2 X 10-3) or 600% and 254 seconds for the cotton fabrics.
Some of the fabrics treated at 140deg C showed the highest values of maximum water absorption in Figure 6b. The highest values of absorption capacity obtained at 140deg C were 188.7 (g/cm2 X 10-3) in a 0.9% solution for 20 minutes, and 752.4% in a 0.6% solution for 40 minutes. However, they were not distinguished from the others, which differed in absorption rate behavior, as shown in Figure 6a. There was a continuous decrease over 22.6% weight loss due to severe splitting conditions.
In the case of maximum water absorption with unlimited time, absorption behavior differed somewhat from short-term water absorption. The amount of water retained as local pore water, including continuous capillary water, was influenced by total pore volume, which would be related to both capillary sorption and pore filling. The total amount of absorbed water was the same as the total pore volumes between fibers, which varied in shape and size depending on splitting conditions. The total pore volume available to retain water in fabrics could be estimated. Fabrics in the ranges of 4.5-22.6% weight loss all seemed to have similar pore amounts. However, pore shape and size were different, particularly for fabrics split at 140deg C.
ADSORPTION PROPERTIES OF SPLIT MICROFIBER FABRICS
Adsorption properties of an antimicrobial agent on the split microfiber fabrics were determined by % add-on of the agent (Table II). Because the agent had desorbed exceedingly from the finished fabrics during only one washing cycle, the amount of remaining agent was too little to be detected by tcP-AEs analysis. Therefore, we characterized agent desorption by durability to repeated home launderings, listed in Table III, and dry cleaning, listed in Table IV. Agent retention in the cleaned fabrics was distinguished clearly by reduction of bacteria.
Compared to conventional fibers, the microfibers adsorbed a considerable amount of agent owing to their greater specific surface. The higher add-on and superior durability to repeated laundering and dry cleaning of the antimicrobial agent on the finished fabrics were caused by a much greater increase in surface area of the fibers due to splitting and irregularities by hydrolysis, shown in Figure 2. Therefore, the solution to poor durability in conventional polyester fibers, which have no sites to react with chemical bonding, could be to replace them with split microfibers.
ABSORPTION PROPERTIES OF FINISHED MICROFIBER FABRICS
Retention of excellent absorption properties after finishing microfiber fabrics with an antimicrobial agent is very important when they are being used as wetting textiles, such as damp dusters and dishcloths. We examined the absorption behavior of the finished microfiber fabrics and compared them with the conventional fibers, as shown in Figure 8. Maximum water absorption of the finished fabrics decreased as some pore spaces seemed to be filled with the adsorbed agents. On the other hand, rate of initial water absorption increased slightly due to raised capillary sorption. Because the agent had adsorbed on the capillary walls, capillary tubes were finer in size. Microfiber fabrics showed much superior absorption compared to conventional polyester fabric due to the wide-contact effect, that is, microfibers contacting over a wider area with the porous plate than conventional fibers.
Conclusions
Microfiber fabrics obtained by NaOH treatment under various conditions for splitting of NIP bicomponent conjugate filaments vary in morphology. Excellent absorption properties are obtained at 140deg C with about 10% weight loss. The optimum splitting conditions are 0.3%/40 minutes (12.6% weight loss), 0.6%/30 minutes (16.3% weight loss), and 0.9%/20 minutes (11.3% weight loss) for a faster rate and greater extent of water absorption. Complete splitting and even separation of microfibers under those conditions produce the most and best capillary channels with sharp-edged cross sections of monofilaments.
Add-on values (%) are high, and there is good durability to repeated laundering and dry cleaning of the agent on the finished tv/P microfiber fabrics, in contrast to the conventional polyester filament fabric. This is most likely due to the high surface area and surface irregularities caused by splitting and hydrolysis. Therefore, the durability problem of conventional polyester fibers could be overcome by using split microfibers. Maximum water absorption of the finished fabrics decreases slightly due to some pore spaces being filled with the adsorbed agent, but the rate of initial water absorption increases due to capillary sorption. Absorbency of the finished microfiber fabrics changes somewhat after the antimicrobial finish, but is still high. Therefore, microfiber fabrics with high water absorption and antimicrobial properties can be widely used for sanitary end-uses.
The water absorption instrument newly devised for this study is an excellent measurement system. It is possible to distinguish absorbency differences between the split tv/P microfiber knitted fabrics, which have various shapes and sizes of pore structures created and deformed during the splitting and finishing processes.
Literature Cited
1. AATCC Test Method 100-1993, Assessment of Antibacterial Finishes on Textiles on Textile Materials.
2. AATCC Test Method 86-1994, Dry Cleaning: Durability of Applied Designs and Finishes.
3. Bright, N. F. H., Carson, T., and Duff, G. M., The Heat of Wetting of Fibres, J. Textile Inst. 44, T587 (1953).
4. Burgeni, A. A., and Kapur, C., Capillary Sorption Equilibria in Fiber Masses, Textile Res. J. 37(5), 356-366 (1967).
5. Burkinshaw, S. M., "Chemical Principles of Synthetic Fiber Dyeing," Blackie Academic & Professional, London, U.K., 1994.
'6. Chartterjee, P. K., "Absorbency," Elsevier, NY, 1985.
7. Hong, C. J., and Jeong, S, H., Fluid Transfer in Knitted Pile Fabrics, J. Kor. Fiber Soc. 37(1), 44-50 (2000).
8. Hsieh, Y.-L., Liquid Transport in Fabric Structures, Textile Res. J. 65(5), 299-307 (1995).
9. Hsieh, Y.-L., Miller, A., and Thompson, J., Wetting, Pore Structure, and Liquid Retention of Hydrolyzed Polyester Fabrics, Textile Res. J. 66(1), 1-10 (1996).
10. Huang, W., and Leonas, K. K., One-Bath Application of Repellent and Antimicrobial Finishes to Nonwoven Surgical Gown Fabrics, Textile Chem. Color. 31(3), 11-16 (1999).
11. Kim, Y. H., Lee, H. M., and Kim, J. C., Alkaline Hydrolysis Behavior of Poly(trimethylene terephthalate) Fiber, J. Kor. Fiber Soc. 37(2), 118-125 (2000).
12. Leadbetter, P., and Dervan, S., The Microfiber Step Change, J. Soc. Dyers Colour. 108(9), 369-371 (1992). 13. Lee, E. J., Bok, J. S., Hong, C. J., and Joo, C. W.,
Texturing Studies on Split-type Microfine Polyester Filament Yam, J. Kor. Fiber Soc. 37(1), 25-33 (2000).
14. Lee, S., Cho, J.-S., and Cho, G., Antimicrobial and Blood Repellent Finishes for Cotton and Nonwoven Fabrics Based on Chitosan and Fluoropolymers, Textile Res. J. 69, 104-112 (1999).
15. Morton, W. E., and Hearle, J. W. S., "Physical Properties of Textile Fibers," 3rd ed., The Textile Institute, U.K., 1993.
16. Washino, Y., "Functional Fibers-Trends in Technology and Product Development in Japan," Toray Research Center, Inc., Japan, 1993.
Manuscript received October 5, 2000; accepted January 12, 2001.
MYUNG-JA PARK, SEONG HUN KIM,1 SEONG Joo KIM, SUNG NOON JEONG, AND JAE-YUN JAUNG
Department of Fiber & Polymer Engineering and Center for Advanced Functional Polymers, Hanyang University, Seoul 133-791, South Korea
1 To whom correspondence should be addressed: e-mail: kimsh@hanyang.ac.kr
ABSTRACT
Nylon/polyester (N/P) conjugate fibers are split by alkaline hydrolysis and then finished with an antimicrobial agent, and the effect of splitting and finishing on the absorption/ adsorption properties of the microfibers is studied. The split microfiber fabrics vary in weight loss and pore structure depending on the various splitting conditions. The absorption behavior of microfiber fabrics is analyzed by the degree of splitting, shrinkage, fabric density, and weight loss. Optimum splitting conditions are investigated for superior absorption rate and capacity. Even and complete splitting produces fine fibers closely packed in a parallel structure, which creates capillary channels that transport water into fabric treated at 140deg C with about 10% weight loss. Values of adsorption, add-on (%), and good durability to repeated laundering and dry cleaning of the agent on the finished rr/P microfiber fabrics are high, in contrast to a conventional fiber fabric. This is most likely due to the high surface area and surface irregularities caused by splitting and hydrolysis. The absorption capacity of the finished fabrics decreases because some pore spaces are filled with the adsorbed agent, while the absorption rate increases due to capillary sorption. The water absorption instrument newly devised for this study is an excellent measurement system. It is possible to measure the amount of water absorption with time, and to distinguish the differences in absorbency of the split rr/P microfiber knitted fabrics, which have pore structures that vary in shape and size, created by and deformed during the splitting and finishing process.
There has been a trend towards finer synthetic filament fibers over recent decades, and consequently various microfibers have been developed with novel fiber spinning techniques to reduce thickness and alter the crosssectional shape. Microfiber fabrics have enhanced drapability, luster, softness, bulkiness, and smoothness, and also high tactile aesthetics and high water absorption and chemical adsorption properties.
Split microfibers are produced by separating the bicomponent conjugate filaments through exposure to alkaline solution in combination with thermal and mechanical treatments [13, 51. This chemical splitting method-- alkaline hydrolysis-is known as a good method for complete splitting and even separation. Many studies [5, 11] of conventional polyester have shown that hydrolysis increases in proportion to treatment time and temperature and to NaOH concentration. For hydrolysis of conventional polyester fibers, the treatment temperature does not exceed 100deg C in a strong alkaline solution. On the other hand, conjugate filaments are treated in a very dilute solution at a higher temperature for greater and more even splitting as well as less hydrolysis. Optimum splitting conditions depend on the intended end-use of the microfiber fabrics, for example, whether they need to be highly absorbent.
Recently, owing to their high water absorption characteristics, microfiber fabrics, especially polyester microfiber knitted pile fabrics, have found practical application in such products as sports towels, dishcloths, and wiping cloths. However, the optimum splitting conditions for maximum water absorption are not known. Moreover, the relationship between the absorption behavior and fabric morphology of these split microfiber fabrics, depending on the extent of splitting, hydrolysis, and pore structure, have rarely been studied.
Generally, water absorption characteristics of hydrophobic polyester fibers are determined by examining the external surface of the fibers [6, 15]. Bright et al. [31 reported that the water taken up by polyester is present on the surface of the fibers, in contrast to hydrophilic fibers. Splitting creates fine, closely packed and aligned capillary columns for water transmission coupled with a large surface area [12]. The extent of splitting and changes in fabric morphology during different splitting processes affects these capillary spaces and, consequently, water absorption into the fabrics [8, 91. Currently, there is no suitable test instrument to determine the rates of water absorption and absorption by the entire capillary process, so the development of such a tester is necessary.
Microfiber fabrics with high liquid water retention also have low rates of moisture loss through evaporation, and may often be susceptible to the growth of microorganisms. Thus, they may need an antimicrobial finish for hygiene and comfort. Antimicrobial finishing of polyester fibers is rare compared with cellulose. Polyester has low reactivity, with no chemical bonding taking place between fiber and agent, so that the treatment is purely a physical coating of the fiber surface [16]. The poor durability of these treatment agents to repeated laundering has been a problem with conventional polyester fibers [10, 14]. The problem might be less serious in microfiber fabrics, since the agent ought to be more effectively adsorbed for the reasons already discussed.
Therefore, we need to determine the optimal splitting and finishing conditions for developing antimicrobial polyester microfiber fabrics with high water absorption through studies on absorption/adsorption related to fabric morphology, fiber surfaces, and pore structures between the microfibers, which were created and modified during the splitting and finishing process. The object of this study is to elucidate the effect of splitting on the water absorption and finishing agent adsorption of split microfibers related to fabric morphology, to examine the effect of finishing on the absorption of the finished split microfibers, and to develop an appropriate absorbency test instrument to measure absorption through capillary action only in fabrics.
Experimental
MATERIALS
A split N/P (nylon/polyester, 25:75 weight %) conjugate fiber (120d/72f multi filaments) pile knit (obtained from Silver Star Ltd.) was used as the material for the splitting process. Ranges of microfiber fabrics produced under various splitting conditions were used as specimens for evaluating absorption properties. One microfiber fabric treated with 0.3% NaOH solution at 135deg C for 40 minutes was used as a material for antimicrobial finishing. The antimicrobial finished fabrics were characterized to evaluate their adsorption properties, and 100% cotton (terry cloth) or 100% conventional polyester (DrY 150d/48f, FDY 250d/48f) pile fabrics were used to compare properties with the polyester microfiber fabrics.
The antimicrobial agent Ultra-Fresh, 300DDN, 1.06% bis(tri-n-butyl tin) oxide as Sn, 1.6% 5-chloro-2(2,4-- dichlorophenoxy) phenol was supplied by Thomson Research Associates. Sodium hydroxide as a splitting chemical, acetic acid as a neutralizer, and all other reagents were reagent grade. Bleach and detergent (multipurpose type) were commercially available.
WATER ABSORPTION MEASUREMENT SYSTEM
To accurately measure the rate of water absorption into the treated fabrics, we devised an instrument based on a conventional method [4, 7]; it is depicted in Figure 1. The round fabric specimen (5 cm in diameter) is positioned on a porous plate in a glass filter, below which there is a water path, a glass tube, leading to a water reservoir on an electronic balance. The electronic balance is connected to a computer that records the weight loss in the reservoir, which is equivalent to the amount of water transferred into the fabric. A reading is recorded every second. A flat round (5 cm in diameter) weight (16g) is placed on the fabric to help ensure that it absorbs water uniformly over the contact area. The levels of the porous plate and water in the reservoir are adjusted to the same height to eliminate any difference in hydrodynamic pressure. These two conditions lead to spontaneous transfer on the basis of capillary sorption and permeability by the fabric structure.
SPLITTING
Microfibers were prepared by splitting split N/P bicomponent conjugate fibers using a chemical method. Appropriate reaction conditions were determined from preliminary experiments. In these, the conjugate fibers treated in a relatively high concentration of NaOH solution below 100deg C were not split evenly, indicating that a longer reaction time was needed. While the conjugate fibers were treated in low concentration over 140deg C, most of the split polyester filaments were dissolved due to the alkaline hydrolysis reaction. Therefore, very dilute NaOH solutions under suitable temperatures were employed to obtain even and proper splitting conditions.
The pile fabric knitted from the conjugate fibers was treated in the NaOH solution in a high-pressure dyeing machine by a batch method under the following treatment conditions: 0.1-0.9% NaOH solutions at 100-- 140deg C for 20-80 minutes at a bath ratio of 50:1. Finally, the treated fabrics were neutralized in acetic acid solution.
ANTIMICROBIAL FINISHING
The antimicrobial finish was applied by the pad-dry method in various bath solutions with antimicrobial agent concentrations of 0.0133-0.1330% (owf) at a bath ratio of 1:15. Fabric padded with the finishing solution was squeezed to 100% wet pickup by two dips-two nips and dried at 50deg C for 30 minutes. Two fabrics, 100% polyester conventional fibers and the NIP microfiber fabric, were used for the finishing (see Figure 2).
CHARACTERIZATION
The weight loss (%), shrinkage (%), and fabric areal density (g/m2) were calculated from the original and final measurements before and after NaOH treatment. The degree of splitting and separation of the conjugate fiber filaments and the fabric surface were examined by scanning electron microscopy (SEM).
The rate of initial water absorption onto the microfiber fabrics was determined from the weight absorbed over 10 seconds, and the water absorption capacity of the fabrics was measured by the maximum weight absorbed. Absorption was represented as two units of percent or gram per mass of absorbed water in a specimen fabric/ mass of dry specimen or unit area of dry specimen, respectively.
Adsorption (% add-on) of antimicrobial agent on the finished fabrics was determined by measuring the Sn content, one of components in the agent, using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Desorption of the agent out of the finished fabrics was determined by the antimicrobial properties.
The antimicrobial properties of the finished and cleaned fabrics were evaluated through AATCC Test Method 100 [1] by measuring the reduction of % Staphylococcus aureus (ATCC No. 6538 gram positive organism).
Durability of the antimicrobial finish to repeated home laundering and to dry cleaning [2] was also evaluated. The finished fabrics were washed with alkaline detergent at 40deg C in an automatic washer set at the permanent press machine cycle and then allowed to line-dry; this procedure was repeated ten times. The finished specimens (15 x 15 cm) were treated in 150 ml dry cleaning solvent consisting of perchloroethylene and hexane with dry cleaning detergents in a Launder-Ometer with ten stainless steel balls at room temperature for 10 minutes. Results and Discussion
FABRIC CHANGES DURING SPLITTING
Weight Loss
The weight loss (%) resulting from the NaOH treatments (Figure 3) appeared to be approximately proportional to treatment temperature, concentration, and time, which is generally similar to the tendency observed in fabrics made from conventional rE r fibers. The ranges of weight loss were narrow at lower temperatures: 1.1 %-3.81 % at 100deg C and 1.44%-15.62% at 1200C. At higher temperatures, weight loss varied from 1.76% to 30.14% at 130deg C and from 2.3% to 47.5% at 140deg C, depending on reaction time and concentration. The rate of weight reduction from the fabric with NaOH treatment at 100-120deg C increased slowly, while at 140deg C it increased rapidly.
Shrinkage
Fabric shrinkage (%) during the splitting process was great: 27.7% in the course direction and 22.4% in the wale direction on average. Shrinkage seems to be positively correlated with NaOH concentration, but hardly affected by either treatment time or temperature. Shrinkage increases the packing density of the fibers and modifies the interfiber capillaries.
Density
Changes in the areal density of the fabric corresponded to shrinkage and weight loss and were thus also susceptible to the treatment conditions, as shown in Table I. Under conditions of 100'C for 20 minutes, fabric density increased markedly. This might be due to the great initial shrinkage and less weight loss at first. After that, the fabric showed almost constant density because shrinkage and weight loss changed little. Results were similar for conditions at 120deg C, except for the 0.9% concentration. With the 0.9% solution, fabric density decreased somewhat with treatment time, suggesting that the effect of weight loss gained predominance. In general, at relatively low treatment temperatures (100 and 120deg C), fabric density values were high and fairly constant due to much shrinkage and little weight loss. But treatments above 0.6% at 130deg C, or in any concentrations at 140deg C, made the fabrics less dense as the reaction time passed. The density of the fabrics treated in 0.6 or 0.9% solution at 140deg C was less than that of the untreated fabric. Splitting occurred in conjugate fiber filaments at the interfaces of polyester and nylon fibers, where weight loss increased with hydrolysis. Alkaline hydrolysis caused dissolution of polyester fibers as well as splitting, so levels of weight loss should be optimized.
SPLITTING CHARACTERISTICS
Pore Structure
The SEM microphotographs shown in Figure 4 present evidence of pore formation depending on the degree of hydrolysis, splitting, and separation after NaOH treatment. Capillary pores (the terms voids, micropores, holes, sites, channels, free volume, accessible space, etc. have often been used to indicate the same thing in the literature) are the spaces between the fibers where liquid molecules are either transported or become lodged. Therefore, pore structure, including the total volume of pores, average pore size, and local distribution, is the greatest structural parameter controlling transport behavior in hydrophobic fibers.
Splitting
The conjugate fibers hydrolyzed below 2% weight loss, shown in Figure 4b, were not entirely split. In Figure 4c, some conjugate filaments were split partially and irregularly. Figures of 4d and e show that all fibers were split and separated evenly and completely to become finer filaments. As a result of compact and even distribution of microfibers and alignment of the spaces, the effective capillary action between the filaments would be expected to lead to good absorbency. In Figure 4f, excessive hydrolysis of polyester fibers decreased the capillary pores and increased the pore sizes. Only nylon cores in NIP conjugate filaments remained, and some spaces between the fibers seemed too big to retain water by capillary sorption.
Surface Area
Examination of the surface morphology of the fabrics and filaments could provide an estimate of the surface area of piles in the fabrics and the shape of the monofilaments after splitting. Figure 4 shows that a large change in the surface area of the filaments occurred through splitting. Complete splitting was accompanied by an increase in the surface area per unit volume of the filament, which led to an increase in capillary walls and total surface areas. We expected that surface adsorption of the finishing agents on the individual filaments should be great and durable, but excessive hydrolysis over a 25% weight loss resulted in a decreased surface area and sharp-edged cross sections of monofilaments.
WATER ABSORPTION PROPERTY OF SPLIT MICROFIBER FABRics
Rate of Initial Water Absorption
We measured the rate of initial water absorption (mass/unit area) for 10 seconds, and the results are shown in Figure 5. Partial or complete splitting resulted in the creation of capillary pores, and capillary force affected the absorption rate. Most fabrics treated in 0.1% solution absorbed very slowly. Rapid increases in the absorption rate showed up in certain conditions where splitting seemed to be complete.
Maximum values of initial water absorption (mass/ unit area) were about 106, 124, 123, and 183 (g/cm2 x 10-3) for the fabrics treated at 100, 120, 130, and 140deg C, respectively. Represented as percentages, the values translate to 410.5, 500.3, 514.3, and 728.9% for fabrics treated at 100, 120, 130, and 140deg C, respectively. These figures are higher than for the terry cloth composed of hydrophilic cotton fibers (403.6%), which was less than for the polyester microfiber fabrics. Therefore, polyester microfiber fabrics created under these conditions should be superior in use for practical applications involving wetting.
The absorption rate of some fabrics treated at 140deg C showed a very rapid rise at the initial stages of splitting but decreased as splitting treatment time increased. The stronger the solution, the greater the rate reduction, which appeared to be related to the reduction in fabric density shown in Table I. In Figure 6a, the absorption rate seemed to be related to weight loss. It began and increased rapidly at around 2% weight loss, which was the starting point of splitting, shown in Figure 4c. The fabrics treated at 140deg C, in a range of 4.5 to 22.6% weight loss, showed the highest values in rates of initial water absorption, especially the 11.3% weight loss with splitting conditions of 0.9% for 20 minutes at 140deg C.
Rapid absorption in a short time is due to water transfer into the fabrics by high capillary forces, determined by the length of the continuous column of water.
More continuous capillary channels would have formed in the fabrics hydrolyzed at the higher temperature of 140deg C due to the complete splitting and even separation of the conjugate fibers. The microfiber fabrics split at 140deg C were superior to the cotton fiber, which absorbed water by hydrophilicity and capillary spaces rather than capillary sorption. Over 20% weight loss, as areal density decreased, capillary size grew, whereas the rate of initial water absorption (g/cm2) decreased considerably (Figure 6a).
Absorption Capacity
Maximum water absorption values of the microfiber fabrics treated under various conditions are shown in Figures 7a-d. Low absorption capacity showed fabrics treated at relatively low temperature and concentration regardless of treatment time, such as 0.1-0.3% at 100deg C and 0.1% at 120deg C. High absorption capacity showed fabrics hydrolyzed in 0.3-0.9% solutions at 130deg C. Most fibers split under any conditions, except for the 0.1% solution, showed high absorption capacity similar or superior to terry cloth of 132.4 (g/cm2 X 10-3) or 486.3%. However, the time to reach maximum water absorption was 8-193 seconds for the microfiber fabrics over 150 (g/cm2 X 10-3) or 600% and 254 seconds for the cotton fabrics.
Some of the fabrics treated at 140deg C showed the highest values of maximum water absorption in Figure 6b. The highest values of absorption capacity obtained at 140deg C were 188.7 (g/cm2 X 10-3) in a 0.9% solution for 20 minutes, and 752.4% in a 0.6% solution for 40 minutes. However, they were not distinguished from the others, which differed in absorption rate behavior, as shown in Figure 6a. There was a continuous decrease over 22.6% weight loss due to severe splitting conditions.
In the case of maximum water absorption with unlimited time, absorption behavior differed somewhat from short-term water absorption. The amount of water retained as local pore water, including continuous capillary water, was influenced by total pore volume, which would be related to both capillary sorption and pore filling. The total amount of absorbed water was the same as the total pore volumes between fibers, which varied in shape and size depending on splitting conditions. The total pore volume available to retain water in fabrics could be estimated. Fabrics in the ranges of 4.5-22.6% weight loss all seemed to have similar pore amounts. However, pore shape and size were different, particularly for fabrics split at 140deg C.
ADSORPTION PROPERTIES OF SPLIT MICROFIBER FABRICS
Adsorption properties of an antimicrobial agent on the split microfiber fabrics were determined by % add-on of the agent (Table II). Because the agent had desorbed exceedingly from the finished fabrics during only one washing cycle, the amount of remaining agent was too little to be detected by tcP-AEs analysis. Therefore, we characterized agent desorption by durability to repeated home launderings, listed in Table III, and dry cleaning, listed in Table IV. Agent retention in the cleaned fabrics was distinguished clearly by reduction of bacteria.
Compared to conventional fibers, the microfibers adsorbed a considerable amount of agent owing to their greater specific surface. The higher add-on and superior durability to repeated laundering and dry cleaning of the antimicrobial agent on the finished fabrics were caused by a much greater increase in surface area of the fibers due to splitting and irregularities by hydrolysis, shown in Figure 2. Therefore, the solution to poor durability in conventional polyester fibers, which have no sites to react with chemical bonding, could be to replace them with split microfibers.
ABSORPTION PROPERTIES OF FINISHED MICROFIBER FABRICS
Retention of excellent absorption properties after finishing microfiber fabrics with an antimicrobial agent is very important when they are being used as wetting textiles, such as damp dusters and dishcloths. We examined the absorption behavior of the finished microfiber fabrics and compared them with the conventional fibers, as shown in Figure 8. Maximum water absorption of the finished fabrics decreased as some pore spaces seemed to be filled with the adsorbed agents. On the other hand, rate of initial water absorption increased slightly due to raised capillary sorption. Because the agent had adsorbed on the capillary walls, capillary tubes were finer in size. Microfiber fabrics showed much superior absorption compared to conventional polyester fabric due to the wide-contact effect, that is, microfibers contacting over a wider area with the porous plate than conventional fibers.
Conclusions
Microfiber fabrics obtained by NaOH treatment under various conditions for splitting of NIP bicomponent conjugate filaments vary in morphology. Excellent absorption properties are obtained at 140deg C with about 10% weight loss. The optimum splitting conditions are 0.3%/40 minutes (12.6% weight loss), 0.6%/30 minutes (16.3% weight loss), and 0.9%/20 minutes (11.3% weight loss) for a faster rate and greater extent of water absorption. Complete splitting and even separation of microfibers under those conditions produce the most and best capillary channels with sharp-edged cross sections of monofilaments.
Add-on values (%) are high, and there is good durability to repeated laundering and dry cleaning of the agent on the finished tv/P microfiber fabrics, in contrast to the conventional polyester filament fabric. This is most likely due to the high surface area and surface irregularities caused by splitting and hydrolysis. Therefore, the durability problem of conventional polyester fibers could be overcome by using split microfibers. Maximum water absorption of the finished fabrics decreases slightly due to some pore spaces being filled with the adsorbed agent, but the rate of initial water absorption increases due to capillary sorption. Absorbency of the finished microfiber fabrics changes somewhat after the antimicrobial finish, but is still high. Therefore, microfiber fabrics with high water absorption and antimicrobial properties can be widely used for sanitary end-uses.
The water absorption instrument newly devised for this study is an excellent measurement system. It is possible to distinguish absorbency differences between the split tv/P microfiber knitted fabrics, which have various shapes and sizes of pore structures created and deformed during the splitting and finishing processes.
Literature Cited
1. AATCC Test Method 100-1993, Assessment of Antibacterial Finishes on Textiles on Textile Materials.
2. AATCC Test Method 86-1994, Dry Cleaning: Durability of Applied Designs and Finishes.
3. Bright, N. F. H., Carson, T., and Duff, G. M., The Heat of Wetting of Fibres, J. Textile Inst. 44, T587 (1953).
4. Burgeni, A. A., and Kapur, C., Capillary Sorption Equilibria in Fiber Masses, Textile Res. J. 37(5), 356-366 (1967).
5. Burkinshaw, S. M., "Chemical Principles of Synthetic Fiber Dyeing," Blackie Academic & Professional, London, U.K., 1994.
'6. Chartterjee, P. K., "Absorbency," Elsevier, NY, 1985.
7. Hong, C. J., and Jeong, S, H., Fluid Transfer in Knitted Pile Fabrics, J. Kor. Fiber Soc. 37(1), 44-50 (2000).
8. Hsieh, Y.-L., Liquid Transport in Fabric Structures, Textile Res. J. 65(5), 299-307 (1995).
9. Hsieh, Y.-L., Miller, A., and Thompson, J., Wetting, Pore Structure, and Liquid Retention of Hydrolyzed Polyester Fabrics, Textile Res. J. 66(1), 1-10 (1996).
10. Huang, W., and Leonas, K. K., One-Bath Application of Repellent and Antimicrobial Finishes to Nonwoven Surgical Gown Fabrics, Textile Chem. Color. 31(3), 11-16 (1999).
11. Kim, Y. H., Lee, H. M., and Kim, J. C., Alkaline Hydrolysis Behavior of Poly(trimethylene terephthalate) Fiber, J. Kor. Fiber Soc. 37(2), 118-125 (2000).
12. Leadbetter, P., and Dervan, S., The Microfiber Step Change, J. Soc. Dyers Colour. 108(9), 369-371 (1992). 13. Lee, E. J., Bok, J. S., Hong, C. J., and Joo, C. W.,
Texturing Studies on Split-type Microfine Polyester Filament Yam, J. Kor. Fiber Soc. 37(1), 25-33 (2000).
14. Lee, S., Cho, J.-S., and Cho, G., Antimicrobial and Blood Repellent Finishes for Cotton and Nonwoven Fabrics Based on Chitosan and Fluoropolymers, Textile Res. J. 69, 104-112 (1999).
15. Morton, W. E., and Hearle, J. W. S., "Physical Properties of Textile Fibers," 3rd ed., The Textile Institute, U.K., 1993.
16. Washino, Y., "Functional Fibers-Trends in Technology and Product Development in Japan," Toray Research Center, Inc., Japan, 1993.
Manuscript received October 5, 2000; accepted January 12, 2001.
MYUNG-JA PARK, SEONG HUN KIM,1 SEONG Joo KIM, SUNG NOON JEONG, AND JAE-YUN JAUNG
Department of Fiber & Polymer Engineering and Center for Advanced Functional Polymers, Hanyang University, Seoul 133-791, South Korea
1 To whom correspondence should be addressed: e-mail: kimsh@hanyang.ac.kr
Transient comfort phenomena due to sweating
Dent, Robin W
ABSTRACT
Of the many transient phenomena mentioned in the fabric comfort literature, two involve vapor diffusion (without liquid water transfer) from the surface of a perspiring "sweat-covered" body: the ". . . buffering effect of hygroscopic clothing" at the onset of sweating noted by Spencer-Smith and the "after-exercise chill" discussed by Woodcock. Both Woodcock and Spencer-Smith made laboratory studies with "sweating" hot plates to try to understand the significance of these phenomena. Their fabrics did not touch the wet plate, so there was no liquid water transfer.
Both effects involve the simultaneous transport of heat and moisture vapor through fabrics or fiber assemblies, and an analytic approximation is given here to explain aspects of these phenomena. We use the basic theoretical approach developed by Henry to study the conditioning of cotton bales. The theory can qualitatively explain the different responses or results obtained experimentally by Spencer-Smith and Woodcock in terms of fiber type and regain as well as fabric structure and ambient conditions. They discussed the connection between the physical lab tests and the field psychological comfort phenomena. We do not attempt quantitative agreement here because of space limitations, but present the next step necessary to confirm the validity of the approach and effects in this theoretical paper. Other transient phenomena such as the analogous buffering effect due to changing ambient conditions (both temperature and humidity) and the initial "cold feel" of fabrics, will be similarly analyzed in later papers. In all these cases, the question is, what is the role of fiber regain and are other effects equally significant?
In this paper, we discuss the two transient effects of "cooling" or "buffering" by an absorbent fabric at the onset of sweating in hot climates and the "chilling" due to the cessation of sweating after exercise in cool climates, using Henry's theory for coupled heat and moisture flows in an assembly of fibers (or a fabric) when the driving mechanism is gas phase diffusion alone. Cassie [1], in studying conditioning of wool fiber bales considered the corresponding case where there is forced air-- flow. With such a flow of air, the diffusion effects are normally negligible, although Daniels [3] discussed cases where both transport mechanisms are significant.
In the two effects due to sweating considered here, the subject is assumed to be resting so that there is no forced air flow through the fabric and Henry's theory is taken to be applicable. In this theory, we assume that there is sorption equilibrium locally between the fiber and air and that the diffusion into the fiber is rapid enough that the fiber moisture content M always approaches the equilibrium sorption value for the local air conditions in terms of RH or concentration C and temperature T. In order to obtain analytical answers, we restrict our considerations here generally to either short or long times.
Farnworth [6] used a numerical method to obtain graphic solutions that cover the entire time scale. He also assumed that diffusion into the fibers is extremely rapid. Wehner et al. [14] showed that correcting for diffusion into the fibers adds a small but significant improvement in comparison to experimental results for moderately sorptive fibers (with a diffusion coefficient of 2.5 X 10^sup -9^ cm^sup 2^/s and radius of 10 (mu)m).
An earlier numerical approach at C.S.I.R.O. has been increasingly refined over the years [4, 8, 9, 10]; it includes an empirical treatment of diffusion in the gas phase (with local depletion at the fiber surface) and uses a finite concentration-dependent diffusion into the fiber itself. This treatment is based on data given by Downes and MacKay [5] and Watt and McMahon [13]. However, their results equally show the classical two-stage uptake of regain predicted by Henry's theory as well as his predicted rapid transient temperature wave when humidity alone is changed. Hence, these results can also be explained in principle by Henry's theory. This means that gas phase diffusion (especially in a fabric of relatively tightly woven and twisted yarns) and the roles of surface depletion and details of diffusion into the fibers can potentially be ignored in many cases. The special situations where these effects should be included will depend on the experimental conditions. A clear distinction needs to be made between the processes of gas-- phase diffusion, internal fiber diffusion, and sorption.
Hence, analytical solutions for the two sweating phenomena are given here using Henry's theory, assuming first that sorption equilibrium has been achieved and second that there is only gas-phase resistance, so that for a body at rest, the transport is by gas-phase diffusion only. We hope these calculations will help resolve some of the continuing issues relative to the balances between sorption and diffusion and the comfort of hygroscopic and hydrophobic fiber assemblies.
Discussion
The mathematical analysis given here confirms that when the body is perspiring, fabric absorbency can become important and the buffering (or cooling) effect postulated by Spencer-Smith and the after-exercise chilling discussed by Woodcock from their lab experiments can be modeled to provide further understanding of the significance of these effects for absorbent and nonabsorbent fiber fabrics and the balance in the determining factors involved. Fabric density and thickness are particularly important if the fibers of the fabric are hygroscopic.
Note that two transient waves pass through the system, and very little heat is associated with the faster wave. The buffering and chilling effects are associated mainly with the slower, subsequent wave. Also, when buffering is large, so will be the chilling effect once perspiration ceases for the same fabric and atmospheric conditions. If the fabric has a low density, there may not be any extra cooling effect due to the transients, but rather an over-- heating, which will be larger for more absorbent fibers.
In Henry's theory used here, we assume that there is local sorption equilibrium between the fiber and air. Because most textile fibers essentially attain equilibrium with the atmosphere in a few seconds, whereas the phenomena discussed here may take minutes or hours, the effect of the time lag to attain sorption equilibrium should be small. This is not necessarily true for the fast wave, but because the heat flow associated with this first wave is small, the effect on the experimental data should not be too large. This effect has been discussed by David and Nordon [4] and more recently by Li and Holcombe [9].
The theory given here also assumes that a perspiring body essentially maintains an atmosphere at the body surface with a constant concentration (presumably, but not necessarily for the theory, equivalent to 100% RH). Farnworth [6] considered instead that the rate of sweat supply should be constant. With this boundary condition, his experiments gave somewhat different results. We have not considered his boundary condition here, although it might correspond more closely to real comfort situations. Our analysis is intended to correspond only to the laboratory experiments of Spencer-Smith and Woodcock.
Conclusions
We have shown here that Henry's theory can qualitatively explain the measurements intended to simulate the two comfort phenomena of buffering and after-exercise chill in terms of fiber type, fabric structure, and ambient conditions. Thus, our theory tends to confirm the conclusions from those measurements-that denser fabrics (say phi 0.8),-which may be the case with some staple yarn fabrics-the buffering may even be less than occurs with hydrophobic fiber fabrics, as shown by Woodcock. The effects should be larger for lighter, thicker, hygroscopic fiber fabrics and for drier conditions. Density may be as important as fiber regain in some of these conditions. Quantitative calculations are not given here, but are necessary to confirm the generality of our conclusions.
ACKNOWLEDGMENTS
I thank the late Professor J. J. Hermans for his help and guidance during this work, D. Andersen and R. Curry for their help, and the late Dr. R. Buchdahl for his encouragement.
Literature Cited
1. Cassie, A. B. D., Propagation of Temperature Changes through Textiles in Humid Atmospheres, Part II: Theory of Propagation of Temperature Change, Trans. Farad. Soc. 36, 453-456 (1940).
2. Cassie, A. B. D., Atkins, B. F., and King, G., Thermostatic Action of Textile Fibers, Nature 143, 162 (1939).
3. Daniels, H. E., Propagation of Temperature Changes through Textiles in Humid Atmospheres, Part IV: Extended Theory of Temperature Propagation through Textiles, Trans. Farad. Soc. 37, 506-525 (1941).
4. David, H. G., and Nordon, P., Case Studies of Coupled Heat and Moisture Diffusion in Wool Beds, Textile Res. J. 39, 166-172 (1969).
5. Downes, J. G., and MacKay, B. H., Sorption Kinetics of Water Vapor in Wool Fibers, J. Poly Sci. 28, 45-67 (1958).
6. Farnworth, B., A Numerical Model of the Combined Diffusion of Heat and Water Vapor through Clothing, Textile Res. J. 56, 653-665 (1986).
7. Henry, P. S. H., Diffusion in Absorbing Media, Proc. R. Soc. A 171, 215-241 (1939).
8. Li, Y., and Holcombe, B. V., A Two Stage Sorption Model of the Coupled Diffusion of Moisture and Heat in Wool Fabrics, Textile Res. J. 62, 211-217 (1992).
9. Li, Y., and Holcombe, B. V., Mathematical Simulation of Heat and Moisture Transfer in a Human-Clothing-Environment System, Textile Res. J. 68, 389-397 (1998).
10. Li, Y., and Luo, Z., An Improved Mathematical Simulation of the Coupled Diffusion of Moisture and Heat in Wool Fabric, Textile Res. J. 69, 760-768 (1999).
11. Nordon, P., MacKay, B. H., Downes, J. G., and McMahon, G. B., Sorption Kinetics of Water Vapor in Wool Fibers: Evaluation of Diffusion Coefficients and Analysis of Internal Sorption, Textile Res. J. 30, 761-771 (1960).
12. Spencer-Smith, J. L., The Buffering Effect of Hygroscopic Clothing, Textile Res. J. 36, 855-856 (1966).
13. Watt, I. C., and McMahon, G. B., The Effects of Heat of Sorption in the Wool-Water Sorption System, Textile Res. J. 36, 738-745 (1966).
14. Wehner, J. A., Miller, B., and Rebenfeld, L., Dynamics of Water Vapor Transmission Through Fabric Barriers, Textile Res. J. 58, 581-592 (1988).
15. Woodcock, A. M., Moisture Transfer in Textile Systems, Part II, Textile Res. J. 32, 719-723 (1962).
Manuscript received June 15, 2000; accepted December 15, 2000.
ROBIN W. DENT
Albany International Research Co., Mansfield, Massachusetts 02048, U.S.A.
ABSTRACT
Of the many transient phenomena mentioned in the fabric comfort literature, two involve vapor diffusion (without liquid water transfer) from the surface of a perspiring "sweat-covered" body: the ". . . buffering effect of hygroscopic clothing" at the onset of sweating noted by Spencer-Smith and the "after-exercise chill" discussed by Woodcock. Both Woodcock and Spencer-Smith made laboratory studies with "sweating" hot plates to try to understand the significance of these phenomena. Their fabrics did not touch the wet plate, so there was no liquid water transfer.
Both effects involve the simultaneous transport of heat and moisture vapor through fabrics or fiber assemblies, and an analytic approximation is given here to explain aspects of these phenomena. We use the basic theoretical approach developed by Henry to study the conditioning of cotton bales. The theory can qualitatively explain the different responses or results obtained experimentally by Spencer-Smith and Woodcock in terms of fiber type and regain as well as fabric structure and ambient conditions. They discussed the connection between the physical lab tests and the field psychological comfort phenomena. We do not attempt quantitative agreement here because of space limitations, but present the next step necessary to confirm the validity of the approach and effects in this theoretical paper. Other transient phenomena such as the analogous buffering effect due to changing ambient conditions (both temperature and humidity) and the initial "cold feel" of fabrics, will be similarly analyzed in later papers. In all these cases, the question is, what is the role of fiber regain and are other effects equally significant?
In this paper, we discuss the two transient effects of "cooling" or "buffering" by an absorbent fabric at the onset of sweating in hot climates and the "chilling" due to the cessation of sweating after exercise in cool climates, using Henry's theory for coupled heat and moisture flows in an assembly of fibers (or a fabric) when the driving mechanism is gas phase diffusion alone. Cassie [1], in studying conditioning of wool fiber bales considered the corresponding case where there is forced air-- flow. With such a flow of air, the diffusion effects are normally negligible, although Daniels [3] discussed cases where both transport mechanisms are significant.
In the two effects due to sweating considered here, the subject is assumed to be resting so that there is no forced air flow through the fabric and Henry's theory is taken to be applicable. In this theory, we assume that there is sorption equilibrium locally between the fiber and air and that the diffusion into the fiber is rapid enough that the fiber moisture content M always approaches the equilibrium sorption value for the local air conditions in terms of RH or concentration C and temperature T. In order to obtain analytical answers, we restrict our considerations here generally to either short or long times.
Farnworth [6] used a numerical method to obtain graphic solutions that cover the entire time scale. He also assumed that diffusion into the fibers is extremely rapid. Wehner et al. [14] showed that correcting for diffusion into the fibers adds a small but significant improvement in comparison to experimental results for moderately sorptive fibers (with a diffusion coefficient of 2.5 X 10^sup -9^ cm^sup 2^/s and radius of 10 (mu)m).
An earlier numerical approach at C.S.I.R.O. has been increasingly refined over the years [4, 8, 9, 10]; it includes an empirical treatment of diffusion in the gas phase (with local depletion at the fiber surface) and uses a finite concentration-dependent diffusion into the fiber itself. This treatment is based on data given by Downes and MacKay [5] and Watt and McMahon [13]. However, their results equally show the classical two-stage uptake of regain predicted by Henry's theory as well as his predicted rapid transient temperature wave when humidity alone is changed. Hence, these results can also be explained in principle by Henry's theory. This means that gas phase diffusion (especially in a fabric of relatively tightly woven and twisted yarns) and the roles of surface depletion and details of diffusion into the fibers can potentially be ignored in many cases. The special situations where these effects should be included will depend on the experimental conditions. A clear distinction needs to be made between the processes of gas-- phase diffusion, internal fiber diffusion, and sorption.
Hence, analytical solutions for the two sweating phenomena are given here using Henry's theory, assuming first that sorption equilibrium has been achieved and second that there is only gas-phase resistance, so that for a body at rest, the transport is by gas-phase diffusion only. We hope these calculations will help resolve some of the continuing issues relative to the balances between sorption and diffusion and the comfort of hygroscopic and hydrophobic fiber assemblies.
Discussion
The mathematical analysis given here confirms that when the body is perspiring, fabric absorbency can become important and the buffering (or cooling) effect postulated by Spencer-Smith and the after-exercise chilling discussed by Woodcock from their lab experiments can be modeled to provide further understanding of the significance of these effects for absorbent and nonabsorbent fiber fabrics and the balance in the determining factors involved. Fabric density and thickness are particularly important if the fibers of the fabric are hygroscopic.
Note that two transient waves pass through the system, and very little heat is associated with the faster wave. The buffering and chilling effects are associated mainly with the slower, subsequent wave. Also, when buffering is large, so will be the chilling effect once perspiration ceases for the same fabric and atmospheric conditions. If the fabric has a low density, there may not be any extra cooling effect due to the transients, but rather an over-- heating, which will be larger for more absorbent fibers.
In Henry's theory used here, we assume that there is local sorption equilibrium between the fiber and air. Because most textile fibers essentially attain equilibrium with the atmosphere in a few seconds, whereas the phenomena discussed here may take minutes or hours, the effect of the time lag to attain sorption equilibrium should be small. This is not necessarily true for the fast wave, but because the heat flow associated with this first wave is small, the effect on the experimental data should not be too large. This effect has been discussed by David and Nordon [4] and more recently by Li and Holcombe [9].
The theory given here also assumes that a perspiring body essentially maintains an atmosphere at the body surface with a constant concentration (presumably, but not necessarily for the theory, equivalent to 100% RH). Farnworth [6] considered instead that the rate of sweat supply should be constant. With this boundary condition, his experiments gave somewhat different results. We have not considered his boundary condition here, although it might correspond more closely to real comfort situations. Our analysis is intended to correspond only to the laboratory experiments of Spencer-Smith and Woodcock.
Conclusions
We have shown here that Henry's theory can qualitatively explain the measurements intended to simulate the two comfort phenomena of buffering and after-exercise chill in terms of fiber type, fabric structure, and ambient conditions. Thus, our theory tends to confirm the conclusions from those measurements-that denser fabrics (say phi 0.8),-which may be the case with some staple yarn fabrics-the buffering may even be less than occurs with hydrophobic fiber fabrics, as shown by Woodcock. The effects should be larger for lighter, thicker, hygroscopic fiber fabrics and for drier conditions. Density may be as important as fiber regain in some of these conditions. Quantitative calculations are not given here, but are necessary to confirm the generality of our conclusions.
ACKNOWLEDGMENTS
I thank the late Professor J. J. Hermans for his help and guidance during this work, D. Andersen and R. Curry for their help, and the late Dr. R. Buchdahl for his encouragement.
Literature Cited
1. Cassie, A. B. D., Propagation of Temperature Changes through Textiles in Humid Atmospheres, Part II: Theory of Propagation of Temperature Change, Trans. Farad. Soc. 36, 453-456 (1940).
2. Cassie, A. B. D., Atkins, B. F., and King, G., Thermostatic Action of Textile Fibers, Nature 143, 162 (1939).
3. Daniels, H. E., Propagation of Temperature Changes through Textiles in Humid Atmospheres, Part IV: Extended Theory of Temperature Propagation through Textiles, Trans. Farad. Soc. 37, 506-525 (1941).
4. David, H. G., and Nordon, P., Case Studies of Coupled Heat and Moisture Diffusion in Wool Beds, Textile Res. J. 39, 166-172 (1969).
5. Downes, J. G., and MacKay, B. H., Sorption Kinetics of Water Vapor in Wool Fibers, J. Poly Sci. 28, 45-67 (1958).
6. Farnworth, B., A Numerical Model of the Combined Diffusion of Heat and Water Vapor through Clothing, Textile Res. J. 56, 653-665 (1986).
7. Henry, P. S. H., Diffusion in Absorbing Media, Proc. R. Soc. A 171, 215-241 (1939).
8. Li, Y., and Holcombe, B. V., A Two Stage Sorption Model of the Coupled Diffusion of Moisture and Heat in Wool Fabrics, Textile Res. J. 62, 211-217 (1992).
9. Li, Y., and Holcombe, B. V., Mathematical Simulation of Heat and Moisture Transfer in a Human-Clothing-Environment System, Textile Res. J. 68, 389-397 (1998).
10. Li, Y., and Luo, Z., An Improved Mathematical Simulation of the Coupled Diffusion of Moisture and Heat in Wool Fabric, Textile Res. J. 69, 760-768 (1999).
11. Nordon, P., MacKay, B. H., Downes, J. G., and McMahon, G. B., Sorption Kinetics of Water Vapor in Wool Fibers: Evaluation of Diffusion Coefficients and Analysis of Internal Sorption, Textile Res. J. 30, 761-771 (1960).
12. Spencer-Smith, J. L., The Buffering Effect of Hygroscopic Clothing, Textile Res. J. 36, 855-856 (1966).
13. Watt, I. C., and McMahon, G. B., The Effects of Heat of Sorption in the Wool-Water Sorption System, Textile Res. J. 36, 738-745 (1966).
14. Wehner, J. A., Miller, B., and Rebenfeld, L., Dynamics of Water Vapor Transmission Through Fabric Barriers, Textile Res. J. 58, 581-592 (1988).
15. Woodcock, A. M., Moisture Transfer in Textile Systems, Part II, Textile Res. J. 32, 719-723 (1962).
Manuscript received June 15, 2000; accepted December 15, 2000.
ROBIN W. DENT
Albany International Research Co., Mansfield, Massachusetts 02048, U.S.A.
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