Study on heat and moisture vapour transmission characteristics through multilayered fabric ensembles

Indian Journal of Fibre & Textile Research Vol. 36, December 2011, pp. 410-414 Study on heat and moisture vapour transmission characteristics through multilayered fabric ensembles A Das a, Shabaridharan & B Biswas Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016, India The present paper reports a detailed study on heat and moisture vapour transmission characteristics of different types of multilayered fabric ensembles. Two sets of multilayered fabrics have been prepared. In first set, two types of carded web produced from acrylic and polyester fibres with five different areal densities in each fibre type have been used as middle layer. In second set, different types of fabrics, namely woven, knitted, felt, single-sided fleeced and double-sided fleeced fabrics made from wool fibre, have been used as middle layer. In both the sets, two types of inner and outer layer fabrics have been used. Silk and polyester woven fabrics have been used as inner layer and plain woven nylon coated and polytetrafluoroethylene (PTFE) coated fabrics have been used as outer layer. In both the sets, silk inner layer fabric ensembles show higher thermal resistance and lesser water vapour permeability than polyester fabric ensembles. Among the first set of multilayered fabrics, the fabric ensembles with acrylic fibre web show slightly higher thermal resistance and lesser moisture vapour permeability than fabric ensembles with polyester fibre web. Of all the fabrics, the type of outer layer used has no significant effect on the thermal transmission properties of multilayered fabrics. In the first set of multilayered fabrics, irrespective of the type of fibres, the increase in areal density of the fabric increases the thermal resistance and reduces moisture vapour transmission of the fabrics. In the second set of multilayered fabrics, woven, felt and knitted fabric ensembles show lesser thermal resistance and moisture vapour transmission than the fleeced fabric ensembles. Keywords: Moisture vapour transmission, Multilayer fabrics, Porosity, Thermal transmission, Thickness 1 Introduction Multilayered fabrics are preferred in unusual conditions such as cold or extreme cold weather. Mainly, the thermal and moisture vapour transmission characteristics of such fabrics govern the comfortness of human being 1. The heat and moisture vapour transmission from human being to the environment through textile materials can be given by the following heat balance equation: M W = C k + C + R + E sk + (C res + E res ) (1) where C k, C, R and E sk are the heat transfer by conduction, convection, radiation and evaporation respectively; M, the metabolic heat generation in W/m 2 ; W, the external work done in W/m 2 ; and C res and E res, the heat loss by respiration such as sensible heat loss and evaporative heat loss respectively 2,3. Many researchers have studied the thermal comfort properties of different types of fabrics. Studies on the properties of yarns and fabrics produced from cotton acrylic bulked yarns have been made by Das et al. 4,5. Shrinkable acrylic fibre has been used to spun a To whom all the correspondence should be addressed. E-mail: apurba65@gmail.com / apurba@textile.iitd.ernet.in the yarn. It has been reported that the increase in proportion of shrinkable acrylic fibre improves the thermal resistance and moisture vapour transmission appreciably, due to the increase in bulkiness of the respective fabric. Similar study has been conducted by Das et al. 6 on the transmission behaviour of needle-punched nonwoven made from blends of shrinkable and non-shrinkable acrylic fibres. It has been reported that the thermal insulation and moisture vapour transmission improve with the increase in proportion of shrinkable acrylic fibre due to open structure and bulkiness offered by the fabric. Absorption characteristics of layered napkins have been studied in detail 7. The transmission behaviour of fabrics can be controlled by proper alignment of fibres in the yarn, which are related to the openness of fibres 8, 9. Thermal insulation behaviour of single and multiple layers of textile materials has been studied by Haq et al. 10. It reveals that the increase in number of layers increases the thermal insulation property and that the increase in velocity of air reduces the thermal insulation. They have also suggested to study the thermal properties of fabric ensembles consisting of wool, silk and fur. Effects of fibre diameter and crosssectional shape on moisture transmission through

DAS et al.: HEAT AND MOISTURE VAPOUR TRANSMISSION CHARACTERISTICS 411 woven fabrics have been reported by Das et al 11. Frydrych et al. 12 has studied the thermal and moisture vapour transmission properties of different types of laminated fabrics along with other fabric layers. It has been reported that PTFE coated fabrics show highest water vapour permeability. It has also been mentioned that an additional insulation layer will be used during the manufacturing of clothing for the protection from cold. Use of such additional layers may change the transmission properties of the materials. In order to thoroughly investigate the thermal and moisture vapour transmission of such multilayered fabric ensembles, a study has been conducted using three layered structure, namely woven inner layer, insulating middle layer and coated fabric outer layer. 2 Materials and Methods 2.1 Materials Two sets of multilayered fabrics consisting of inner layer, middle layer and outer layer were prepared. In both the sets, two different fabrics, namely silk woven fabric and polyester woven fabric were used as inner layer. Similarly, two types of coated fabrics, namely plain woven polytetrafluoroethylene (PTFE) coated fabric and plain woven nylon coated fabric were used as outer layer. The details of the fabrics used in inner and outer layers are given in Table 1. In first set, carded web was used as middle layer which is produced from acrylic and polyester fibres using five different areal densities, namely 200, 250, 300, 350 and 400 g / m 2 in each fibre. The polyester fibre of the staple length 58mm & fineness 3den and acrylic fibre of the staple length 60mm & fineness 3den were used to produce carded web. In second set, different types of fabrics, namely woven, knitted, felt, single-sided fleeced and double-sided fleeced fabrics made from wool fibre were used as middle layer. The particulars of fabrics are given in Table 2. The coding of the two sets of multilayered fabrics is given in Tables 3 and 4. 2.2 Methods 2.2.1 Measurement of Fabric Particulars The mass per unit area and thickness were measured for different fabrics using the standards Inner layer Outer layer ASTM D1059 and ASTM D1777-96 (2002) respectively. An average of 10 readings was taken for each test. Fabric bulkiness depends on physical characteristics of fabric, namely mass per unit area and thickness. The bulk density of the fabric (g/cm 3 ) was calculated using the following equation: Bulk density = M / T (2) where M is the mass per unit area of the fabric (g / cm 2 ); and T, the thickness of the fabric (cm). Porosity of the fabric depends upon the volume of air entrapped in the material which can be given by the following equation: Porosity (%) = 1 [d F / d f ] 100 Table 4 Coding of fabrics in which wool fabric is used in the middle layer Wool knitted (WK) Wool woven (WW) Middle layer Wool felt (WF) Two- sided fleece (TF) (3) Table 1 Particulars of fabrics used for inner and outer layers Position of fabric Inner layer Outer layer Material used Material used Mass per unit area, g/m 2 One-sided fleece (OF) Silk (S) Nylon coated (N) SN-WK SN-WW SN-WF SN-TF SN-OF Silk (S) PTFE coated (P) SP-WK SP-WW SP-WF SP-TF SP-OF Polyester (P) Nylon coated (N) PN-WK PN-WW PN-WF PN-TF PN-OF Polyester (P) PTFE coated (P) PP-WK PP-WW PP-WF PP-TF PP-OF Thickness mm Polyester 60 0.15 Silk 60 0.15 Nylon with 40 0.15 teflon coating Nylon polyester 45 0.20 Table 2 Particulars of fabrics used for middle layer Mass per unit area g/m 2 Thickness mm Bulk density g/cm 3 Porosity % Wool knitted 396 2.1 0.0396 85.61 Wool woven 410 2.3 0.0410 86.39 Wool felt 405 2.1 0.0405 85.28 Fleece white 200 2.2 0.0200 93.06 Fleece black 220 2 0.0220 91.60 Table 3 Coding of fabrics in which carded web is used in the middle layer Fabric code Inner layer Middle layer SA Silk Acrylic web SP Silk Polyester web PA Polyester Acrylic web PP Polyester Polyester web

412 INDIAN J. FIBRE TEXT. RES., DECEMBER 2011 where d F is the bulk density of fabric (g/cm 3 ); and d f, the density of fibre (g/cm 3 ). The density 8 of the wool fibre is assumed as 1.31 g/cm 3. 2.2.2 Thermal Resistance and Relative Water Vapour Permeability The thermal resistance and relative water vapour permeability were measured in Permetest instrument. The instrument consists of measuring head which is heated up to the core temperature of the human body (35 C). The ambient conditions were maintained at 25 C ± 2 C temperature, 65 % relative humidity and 1.6 m/s air velocity. The thermal resistance of the fabric is given by the following equation: R ct ( Ts Ta ) = (4) Q where R ct is the thermal resistance of the fabric along with air layer (mk.m 2 /W); T s, the temperature of test plate ( C); T a, the temperature of the ambient air layer ( C); and Q, the heat flux (W / m 2 ). The heat flux (Q) can be calculated using the following formula: Q = u.s (5) where u is the output voltage (mv); and S, the sensitivity of the sensor (W/m 2 mv). The relative water vapour permeability was measured by placing the moisture vapour permeable membrane over the porous measuring head. Deionized water is injected in to the measuring head for each test, so that a layer of water is formed between the test plate and the permeable membrane, and it can be evaporated when the test plate is heated. The temperature of the measuring head is maintained at room temperature for isothermal conditions. When the fabric is placed the water is evaporated into vapour form which simulates the latent heat of the human body and the rate of evaporation depends on the type of fabric, which can be indirectly measured by a heat flow sensor mounted on the measuring head. The output of the sensor is given in terms of output voltage (mv). The relative water vapour permeability is given by the following relationship: 3 Results and Discussion 3.1 Thermal Resistance Figures 1 and 2 show the thermal resistance of different types of multilayered fabrics. In both the sets of multilayered fabrics, the fabric ensembles having silk fabric as inner layer show higher thermal resistance than the polyester fabric as inner layer, which may be due to the lesser transverse thermal conductivity of the silk fibre than polyester fibre. In general, the fibres tend to align towards the axis of the yarn owing to the drawing and parallelization at various stages of spinning. Since the inner layer is woven fabric, the fibres in the fabric may be aligned parallel to the axis of the fabric, i.e. fibres in the warp and weft yarns tend to be aligned parallel to axis of the warp and weft yarns respectively. So, the transverse conductivity of fibres plays a significant role than the longitudinal conductivity of the fibres. The transverse thermal conductivity of the silk and polyester fibres is 0.118 and 0.127 W/m K respectively 13. Since the transverse conductivity of silk fibre is lesser than that of the polyester fibre, the thermal resistance of the multilayered fabrics consisting of inner layer of silk fibre shows slightly higher thermal resistance than that of the fabrics consisting of inner layer of polyester fibre (Figs 1 and 2). Among the first set of multilayered Fig. 1 Thermal resistance of fabric ensembles consisting of carded web in middle layer with nylon coated polyester fabric in outer layer Relative water vapor permeability (p wv ) = Q 1 / Q 0 (6) where Q 1 is the heat flux when fabric is placed on the measuring head; and Q 0, the heat flux from bare measuring head, i.e without fabric sample. The average of 10 readings was taken for the each sample. Fig. 2 Thermal resistance of fabric ensembles consisting of carded web in middle layer with teflon coated nylon fabric in outer layer

DAS et al.: HEAT AND MOISTURE VAPOUR TRANSMISSION CHARACTERISTICS 413 fabrics, the fabrics having middle layer as carded web produced from acrylic fibre show slightly higher thermal resistance than the carded web produced from polyester fibre. It may be due to the lesser density of acrylic fibre than the polyester fibre. The density of acrylic and polyester fibres is 1.19 and 1.39 g/cm 3 respectively 8. Lesser the density, the higher is the number of fibres accommodated for the given mass per unit area, which may reduce the radiative heat transfer by back scattering effect. In the first set of multilayered fabrics, irrespective of the type of fibres, the increase in areal density of the fabric increases the thermal resistance of the fabric, which may be due to lesser radiative heat transfer offered by increased back scattering effect with higher number of fibres (Figs 1 and 2). The increase in number of fibres offers more surfaces for absorbing radiative heat transmitted from the heat source 14,15. In the second set of multilayered fabrics, knitted, woven and felt fabric ensembles show lesser thermal resistance than fleeced fabric ensembles, which is due to the higher porosity of the fleeced fabrics (Table 2). Knitted, woven and felt fabrics are higher in mass per unit area than the fleeced fabrics for almost same thickness of fabric. So, the fleeced fabric offers higher porosity and hence results in better entrapment of air. The thermal conductivity of air is much lesser than the thermal conductivity of textile fibres. So, the fleeced fabrics offer higher thermal resistance than other fabrics (Fig. 3). Of all the fabrics, the type of outer layer fabric has no significant effect on the thermal transmission properties of multilayered fabrics. 3.2 Relative Water Vapour Permeability Figures 4-6 show the relative moisture vapour permeability (%) of different types of multilayered fabrics. In both the sets of multilayered fabrics, the fabrics having polyester fabric as inner layer show slightly higher water vapour permeability than the fabrics having silk fabric as inner layer. The diffusion coefficient 16 of water vapour through hydrophobic fibres like polyester is 10 9 cm 2 /s, whereas through hydrophilic fibres like silk it is 10 7 cm 2 /s. Even though the diffusion of moisture vapour through silk fibre is higher than the polyester fibre, the silk fibre has the tendency of retaining the moisture within the fibre, since moisture regain of the silk fibre is much higher (11%) than the moisture regain of polyester fibre (1.5%) 13. As the silk fabric is used in the inner most layer, the absorbed moisture cannot be transported to the environment readily. So, the water vapour permeability of polyester fabric ensembles is slightly higher than the silk fabric ensembles. At the Fig. 4 Relative water vapour permeability of fabric ensembles consisting of carded web in middle layer with nylon coated polyester fabric in outer layer Fig. 5 Relative water vapour permeability of fabric ensembles consisting of carded web in middle layer with teflon coated nylon fabric in outer layer Fig. 3 Thermal resistance of fabric ensembles consisting of wool fabrics in middle layer Fig. 6 Relative water vapour permeability of fabric ensembles consisting of wool fabrics in middle layer

414 INDIAN J. FIBRE TEXT. RES., DECEMBER 2011 same time, if the successive fabrics are also hydrophilic in nature then moisture vapour transmission to the environment may be higher, due to the fact that the absorbing fabric can act as a moisture source to the environment, which is comparatively drier (Figs 4 and 5). In both the sets of multilayered fabrics, nylon coated fabric ensembles show higher water vapor permeability than PTFE coated fabric ensembles, which may be due to better sorption desorption phenomena of the former one. The moisture regain of nylon is around 4% at 65% relative humidity which is much higher than the moisture regain of PTFE polymer. So, the nylon coating can act as a moisture source to the environment which is just above the fabric and can transport higher amount of moisture vapour. Among the first set of multilayered fabrics, the fabrics which have middle layer as carded web produced from polyester fibre show slightly higher water vapour permeability than web produced from acrylic fibre, which may be due to the higher number of fibres offered by acrylic web, that reduces the diffusivity through the fibre component. The diffusion coefficient 17 of moisture vapour through air is 0.239 cm 2 /s, whereas through the textile fibre component it is 10-7 -10-9 cm 2 /s (Figs 4 and 5). Increase in mass per unit area of the fabrics decreases the relative water vapour permeability, which may be due to the fact that the higher number of fibres reduces the diffusivity of the moisture vapour through fibre component of the fabrics than the air with higher areal densities. There is no significant difference between relative water vapour permeability of second set of multilayered fabrics, except the slightly higher permeability for fleeced fabric ensembles, which may be due to the higher porosity of the fabrics. The fleecing operation increases the thickness of the fabric, thereby increasing the entrapment of air in the fabric. So, the moisture vapour transmission is slightly higher in fleeced fabric ensembles than in others (Fig. 6). 4 Conclusion 4.1 The thermal resistance of multilayered fabrics increases with the increase in mass per unit area. 4.2 The fleeced fabric offers higher thermal resistance than the other structures, such as knitted, woven and felt fabrics, even though the mass per unit area is lesser. 4.3 Relative water vapour permeability of the multilayered fabrics decreases with the increase in mass per unit area of the fabric. 4.4 Type of fibre used to produce the fabric and type of finish play vital role in transmission of thermal and moisture vapour transmission properties. 4.5 Among the coated fabrics, nylon coated fabric shows slightly higher moisture vapour permeability than PTFE coated fabric. 4.6 Of all the fabrics, the type of outer layer used, has no significant effect on the thermal transmission properties of multilayered fabrics. References 1 Milenkovic L, Skundric P, Sokolovic R & Nikolic T, Working and Living Environmental Protection, 1 (1999) 101. 2 Apurba Das & Alagirusamy R, Sciences of Clothing Comfort (Woodhead Publishing, New Delhi, India), 2010, 87. 3 Tuğrul Oğulata R, Fibres Text Eastern Eur, 15 (2007) 67. 4 Das A, Kothari V K & Balaji M, J Text Inst, 98 (2007) 261. 5 Das A, Kothari V K & Balaji M, J Text Inst, 98 (2007) 363. 6 Das A, Alagirusamy R & Banerjee B, J Text Inst, 100 (2009) 350. 7 Das A, Kothari V K, Makhija S & Avyaya, J Appl Polym Sci, 107(2007) 1466. 8 Ishtiaque S M, Das A & Chaudhari S, Indian J Fibre Text Res, 28(2003) 399. 9 Ishtiaque S M, Das A & Chaudhari S, Indian J Fibre Text Res, 28(2003) 405. 10 Babus Haq R F, Hiasat M A A & Probert S D, Appl Energy, 54 (1996) 375. 11 Das B, Das A, Kothari V K, Fanguiero R & De Araújo M, Fibers Polym, 9(2008) 225. 12 Frydrych I, Sybilska W & Wajzsczyk M, Fibres Text Eastern Eur, 17 (2009) 50. 13 Morton W E & Hearle J W S, Physical Properties of Textile Fibres (The Textile Institute, CRC and Woodhead Publishing Limited, Cambridge, England), 2008, 163-167, 175. 14 Farnworth B, Text Res J, 53 (1983) 717 15 Woo S S, Shalev I & Barker R L, Text Res J, 64 (1994) 149 16 Yoon H N & Buckley A, Text Res J, 54 (1984) 289. 17 Das B, Das A, Kothari V K, Fangueiro R & De Araújo M, AUTEX Res J, 7 (2007) 100.