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
