Technical Paper Characteristics of Artificial Leather for Footwear － Heat and Moisture Transport Properties － Won Young Jeong !1,Jung Woo Park !2,Masayoshi Kamijo !1,YoshioShimizu !1,andSeung Kook An !2 ＊1 Dept. of Kansei Engineering, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan ＊2 Dept. of Textile Engineering, Pusan National University, Busan 609-735, Korea Abstract :Artificial leather used in this study was prepared by wet coagulation method with polyurethane resin in a laboratory instrument. Various needle-punching nonwoven fabrics were used as substrate material, which are widely used in the shoe industry as the substrate ofinsole or artificial leather. To estimatecomfort properties with the finishing for artificial leather, we measured air, water vapor, and thermal transport properties ; and then we evaluated the changes of each property of finished nonwoven fabrics which were dipped and coated with polyurethane resin. The changes due to the dipping and coating processes for artificial leather may give rise to a feeling of discomfort. Water vapor permeability and heat keeping rate decreased, but thermal conductivity and a cool feeling increased with the finishing process. After dipping and coating processes, there was little significant difference in the transport properties of base materials. Therefore, the transport characteristics were more closely related to the finishing condition than the constructive characteristics of the base materials. (Received 14 December, 2006 ; Accepted 4 July, 2007) 1. Introduction some situations, the evaporation rate of perspiration from wet skin is less than that of perspiration secretion. So Artificial leathers have the advantage over natural accumulated water vapor may condense inside the inner products in uniformity of quality and availability in roll clothing of a wearer undertaking arduous exercise in a form, which facilitates production planning and cold and wet climate. This condensation problem is minimizes waste. The principal difficulty in substituting particularly acute in the popular garments manufactured artificial leather for natural leather is to develop special from coated and laminated fabrics because the standard fiber and polyurethane resin that is able to imitate the polymers have generally low water vapor permeability delicate inner structure and surface of natural leather [1,2]. [3,6] The perception of discomfort in the active case The artificial leather manufactured in this study has depends on the long wetness due to perspiration, and on particular function. It is generally known that water vapor how long the wetness persists after activity stops. easily transports through the fabric structure while it has The transport properties of water and moisture resistance to the passage of water. The principle of this through fabrics have been discussed from various points particular function is the enormous difference in size of view [7,8]. Recently, introduction of the aspect of between water vapor molecules and water droplets [3,4]. comfort is important in many fields of clothing and Most of the artificial leathers for footwear are textiles, so the relationship between these transport manufactured by needle-punching nonwoven fabric properties and comfort has been considered [9-11]. In this substrates [5]. Needle-punching nonwoven fabric is a 3- study, various transport characteristics of needle- dimensionally entangled structure by mechanical bonding punching nonwoven fabrics were measured to evaluate process, and has physical strength and high transport the changes of transport properties with dipping and characteristics. In particular, since there is a lot of coating processes of polyurethane resin. perspiration that comes from the feet, transport properties are important factors in determining comfort. 2. Experimental Human beings rely on the evaporation of perspiration to remain comfortable and prevent 2.1 Materials and preparation of artificial leather overheating in hot environments and during exercise. In In this study, six needle-punching nonwoven fabrics （57） SEN’IGAKKAISHI（報文）Vol.63, No.11(2007) 271 W !WD Table 1 Characteristics of nonwoven fabrics. Heart Keeping rate(%) = !100 (3) W 2 (W :heat loss of bared BT box (W/m ), WD :heat loss when covering the sample using the dry method (W/m2)). 3. Results and discussion 3.1 Air permeability of artificial leather for footwear that are commercially used for footwear were chosen, and Anew microclimate is created around the foot in the the specifications of the specimens are given in Table 1. shoes. It is relatively higher in temperature, humidity, and The nonwoven fabrics were used for base material of the pressure than the outer environment. Therefore, air and artificial leather. Artificial leathers were prepared with the moisture transport properties are very important factors wet coagulation method using laboratory coating for footwear. instruments. The specifications of the dipping and coating Airflow through a fabric occurs when the air process of polyurethane resin to nonwoven fabrics were pressure is different on the two sides of the fabric. Air described in the preceding paper [12]. permeability is expressed as the rate of airflow through 2.2 Measurement of physical properties the fabric where a different air pressure is on either The mass per unit area was measured before and surface of the fabric. Figure 1 shows the results of air after finishing, and the thickness was measured with a permeability measured by the Frazier analysis. The rate of KES-FB3 compression tester at 0.5gf/cm2 load. Using the airflow is adjusted so that a prescribed pressure difference characteristics, apparent specific gravity and free air is achieved between the two sides of the fabric. The volume were calculated by the following equations. difference was 12.7 mm of water, which is equivalent to 125 Pa. In case of the artificial leather finished from ρ"ρ # a! = ρ 100, (1) nonwoven substrates, airflow was not detected, because the pressure difference was relatively low compared to where ρ is the apparent specific gravity of fiber, and ρa is polymer membrane-fabric composites. the specific gravity of fiber. The results of A group were higher than those of B group. It was caused by high-density structure of B group, Wf !Wa Wf ρ = = (2) which was more compact in construction compared to the Vf +V a A+T other group. Air permeability is correlated with the mass (Wf :fiberweight, Wa :airweight, Vf :fibervolume, per unit area, and the correlation coefficient was very V a :airvolume, A : area, T :thickness). high. In particular, the coefficient for A group is higher, 2.3 Measurement of transport properties which consists of relatively low-density nonwoven We measured various transport properties, which substrates. influenced comfort perception of the foot. Water vapor permeability was measured by the permeation cup method (JIS 1099 A-2) to determine the transport property of water vapor emitted from the foot. Air permeability was measured with a Frazier Precision Instrument in accordance with Frazier analysis, and the air pressure at which air passed through the specimen was set to be 125 Pa. Thermal transport properties were measured with a KES-F7 (Thermoabo II, Kato Tech. Co., Ltd., Japan). The measuring condition was 20±2oC, 65± 5% RH, and 10cm/sec air movement. Thermal conductivity and warm/hot feeling (Qmax)were measured, and the heat keeping rate was calculated with Equation (3). The results are compared with intrinsic properties of Fig. 1 Air permeability of needle-punching nonwoven nonwoven fabrics for foot comfort. fabrics for footwear. 272 SEN’IGAKKAISHI（報文）Vol.63, No.11(2007) （58） Table 2 Apparent specific gravity and free air volume of nonwoven fabrics. Apparent specific gravity and free air volume of the nonwoven substrates were calculated and shown in Table 2. Generally, it is known that nonwoven fabric has a high air-volume compared to woven fabric. It was found that all specimens used in this study have about a 70-90% level of air volume. In each group, free air volume increased, and apparent specific gravity decreased with increasing mass per unit area. We analyzed the relation between the characteristics, and air permeability decreased with increasing free air volume. This means that the passage of air flow in nonwoven structure was higher because the volume and thickness of the Fig. 2 Water vapor permeability of nonwoven fabrics for nonwoven increased. footwear. 3.2 Water transport properties of artificial processes, water vapor permeability remarkably leather for footwear decreased regardless of base materials. It means that Water transport in textile material is measured in water vapor characteristics mainly depend on finishing humidity state and liquid state of water. Transport of conditions. water in the humidity state occurs as it diffuses through 3.3 Thermal transport property of artificial the free space of material, and water liquid state is leather for footwear transported by a capillary phenomenon. The water Figure 3 shows the results of thermal transport transport in the artificial leather for footwear, is closely properties after finishing on needle-punching nonwoven related to the transport of sweat, which is in humid state, fabrics. In case of heat keeping rate, mean values of the and water vapor permeability was mainly considered in rate for A group were higher than those for B group and this study. The water vapor permeability of nonwoven they remarkably decreased by dipping and coating fabric before and after coating is shown in Figure 2. In a processes. Nonwoven fabric is the composition of air and state of the base material, the water vapor permeability of fiber, and the air volume in the fabric affects the heat group A was higher than that of group B, which was keeping rate of the material. Polyurethane resin was high-density material. The permeability of group A decreased while that of group B slightly increased, with increasing fabric mass. As referred to above, transport of humidity through textile material depends mainly on the free volume inside the material, while a small quantity of humidity is transported through fiber itself.