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
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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
