Water vapour resistance of knitted fabrics in the function of thermophysiological comfort.

Salopek, Ivana ; Skenderi, Zenun

Abstract: In the recent decades, the achievement of human-comfortable clothing has become an important aim for the textile experts. The comfort of textile depends on various factors such as the type of material, constructional parameters, parameters of finishing, environmental conditions, the feeling of the wearer, etc. The purpose of this study is to evaluate the role of cotton yarn in knitted fabrics in the thermophysiological comfort achievement. For the investigation have been used single jersey fabrics knitted on circular machine from single cotton yarns in counts of 12, 14 and 20 tex and elastane. The water vapour (sweat) resistance of fabrics has been measured using the sweating guarded hotplate. The results have indicated that the important factors that increase the water vapour resistance are the structural density and addition of elastane to the structure of fabric.

Key words: water vapour resistance, permeability index, sweating guarded hotplate, thermophysiological comfort, knitted fabric

1. INTRODUCTION

Nowadays, comfort is considered one of fundamental properties when a textile product is valuated. According to the literature, it could be divided into physiological, psychological and sensorial comfort. The interest and need for the physiological research of clothing started with the approaching World War II. At the beginning, the intention of military approach was to quantify clothing impacts under various conditions. As the accent of scientific investigations has been added to human-related studies and the achievement of human-comfortable clothing has become important aim for the textile experts, many investigations have been carried out in order to improve the thermophysiological comfort or textile materials, either single- or multi- layered. Therefore, textile experts, physiologists, physicists and climatologists have gathered to cover the full range of the problem.

The investigations related to the topic, reported during the last decades, have mainly been focused to the following directions:

1. The investigation of fabrics in static state (McCullough et. al., 2004; Bartels et. al., 2003, etc.)

2. The investigation of clothing in dynamic state (Nelson et. al., 2005, etc.)

3. Wear trials on humans (Fanger, 1970, etc.)

The importance of the comfort of apparels has been recognized, but not enough objectively and systematic expressed. The main reason is because it depends on various factors that include the type of material, method of construction, different finishing treatments, impact due to environmental conditions, the feeling of the wearer, etc. (Salopek et.al. 2007).

The ability of clothing ensembles to transport water vapour is an important determinant of physiological comfort. In transient conditions it is often not possible to avoid sweat accumulation in the clothing. Therefore it is preferable to remove the sweat from the skin surface to the surface of the underwear or to fabric layers further out in the clothing ensemble. That is why the underwear has an important function of the sweat regulator. To obtain the characteristic of comfortable, the fabric should transmit the vapour when the body sweats. After the body has stopped sweating, the textile fabric should release the hold vapour to the atmosphere in order to reduce the humidity at the skin.

In earlier studies (Salopek et. al., 2006), the influence of the cotton yarn characteristics and the process of finishing on the knitted fabric hand (as a parameter of sensorial comfort) have been observed.

The main purpose of this study is to evaluate the influence of yarn and finishing process to the water vapour resistance of knitted fabrics. The measurements have been carried out on fabrics that are mostly used for the production next-to-skin clothes. All the fabrics, used for this investigation, were commercially produced. The intention of the investigation was to determine the fabric with optimal water vapour resistance that should result in better thermophysiological comfort for the wearer.

2. EXPERIMENT

For the investigation have been used single jersey knitted fabrics. The fabrics were knitted on circular knitting machine Relanit E 28, produced by the Mayer & Cie Company. In the process of knitting, the sinking depth was setup to 8-9 mm and the input tension was kept 5-6 cN. For knitting have been used single cotton yarns in three different counts--12, 14 and 20 tex. In the process of knitting with yarns in the counts of 12 and 14 tex, elastane yarn in the count of 33 dtex had been added. After the knitting, the fabrics were finished. They were optically bleached at 98[degrees]C, softened and finally dyed. The specimen description is shown in Table 1.

For all the fabrics, the structural parameters--horizontal density and vertical density, as well as the mass per area have been measured. The sweating guarded hotplate (SGHP) has been used to the determinate the water vapour resistance. SGHP is often referred as the "skin model" because of its ability to simulate the heat and moisture transfer from the body surface through the clothing layer to the environment. The measurement of the process has been carried out, according to the ISO standard (ISO 11092: 1993). During the tests, the air temperature and relative humidity have been set to 35[degrees]C and 40% R.H. The air velocity has been kept constant at 1 m/s.

The water vapour ([R.sub.et]) resistance has been determined according to:

[R.sub.et] = ([p.sub.s] - [p.sub.a])/H/A - [R.sub.et0] (2),

where:

[R.sub.et] = evaporative resistance of sample only ([m.sup.2]Pa/W)

[p.sub.s] = saturation vapour pressure at hotplate surface (Pa)

[p.sub.a] = ambient partial vapour pressure (Pa)

H/A = zone heat flux (W/[m.sup.2])

[R.sub.et0] = bare plate evaporation resistance ([m.sup.2]Pa/W)

The thermal resistance ([R.sub.ct]) has been measured in order to calculate the water-vapour permeability (Wd) and water-vapour permeability index ([i.sub.mt]). The calculations have been carried out according to the following formulas:

[W.sub.d] = 1/[R.sub.et] x [phi][T.sub.m] (3) and

[i.sub.mt] = S x [R.sub.ct]/[R.sub.et] (4),

where:

[W.sub.d] = water vapour permeability (g/[m.sup.2]hPa)

[R.sub.et] = evaporative resistance of the sample ([m.sup.2]Pa/W)

[i.sub.mt] = water vapour permeability index

S = constant equals 60 Pa/K

[R.sub.ct] = dry Resistance of sample ([m.sup.2][degrees]C/W)

[PHI][T.sub.m] = the latent heat of vaporization of water at the

temperature [T.sub.m] of the measuring unit (Wh/g).

The calculated [i.sub.m] value indicates moisture-heat permeability through the material, where 0 means totally impermeable, while 1 means totally permeable.

3. RESULTS

The results of knitted fabric structural parameters are shown in the Table 2, while the results of the water vapour resistance are shown on Figure 1.

The results of the water vapour permeability and water vapour permeability index for measured fabrics are shown in the Table 2.

[FIGURE 1 OMITTED]

4. DISCUSSION

It is seen from the Table 2 that the fabric mass per area has decreased after the process of finishing. That should be the main reason for the decrease of water vapour resistance for all the finished fabrics. The cotton fabrics (samples C1R and C1F) have rather low resistance to water vapour transfer due to the absence of elastane component and lower structural densities. According to the [i.sub.mt] results (shown on the Table 3), the raw fabrics with elastane have significantly lower values on the permeability scale. The permeability increases after the finishing for all the fabrics.

5. CONCLUSION AND FURTHER RESEARCH

It could be concluded from the presented discussion that important factors that increase the water vapour resistance are the structural density and the presence of elastane component in the structure of fabric. The commercial process of finishing decreases the resistance values. As the fabrics for summer wear are worn in hot and humid environment, it is to expect for 100% cotton fabrics to provide better water vapour (sweat) transfer than fabrics with elastane component. In further research, our intention is to continue the investigation of thermophysiological comfort on fabrics knitted from different raw materials, both in normal and transient wear conditions, in order to obtain more precise conclusion about the influence of different parameters.

6. REFERENCES

Bartels, V.T. (2003). Thermal comfort of aeroplane seats: influence of different seat materials and the use of laboratory test methods. Applied Ergonomics, 34, 4, 393-399, ISSN: 0003-6870

Fanger, P.O. (1970). Thermal comfort--Analysis and applications in environmental engineering, McGraw-Hill Book Company, ISBN 0-07-019915-9, USA

ISO 11092: 1993 (E); Textiles--Physiological effects--Measurement of thermal and water-vapour resistance under steady-state conditions (sweating guarded hotplate)

McCullough, E.; Huang, J. & Kim, C.S. (2004). An Explanation and Comparison of Sweating Hot Plate Standards. Journal of ASTM International, 1, 7, 1-13, ISSN: 1546-962X

Nelson, D.A. et.all. (2005). Determining localized garment insulation values from manikin studies: computational method and results. European Journal of Applied Physiology, 95, 464-473, ISSN: 1439-6319

Salopek, I.; Skenderi, Z.; Srdjak, M. (2006). The knitted fabric hand in the function of yarn characteristics, Proceedings of the XLIII Congress of the IFKT--Knitting today and tomorrow, Angelova, Y. (Ed.), pp. 19-22, ISBN-10 954-91951-1-2, 13 978-954-91951-1-8, Plovdiv, Bulgaria, 01-05 October 2006., Scientific and technical union of textiles, ready-made clothing and leathers, Sofia

Salopek, I., Skenderi, Z., Srdjak, M. (2007). Melliand Textilberrichte. The knitted fabric comfort--aspect of fabric hand, 6, 2007., ISSN: 0947-9163

Table 1. The specimen description

Description Finishing Designation

Cotton 20 tex - C1R
Cotton 14 tex + Lycra 33 dtex - E1R
Cotton 12 tex + Lycra 33 dtex - E2R
Cotton 20 tex + C1F
Cotton 14 tex + Lycra 33 dtex + E1F
Cotton 12 tex + Lycra 33 dtex + E2F

Table 2. The structural parameters

Fabric Structural parameters

 Dh Dv C Mass per area
 (l/cm) (l/cm) (Dh/Dv) (g/[m.sup.2])

C1R 13,5 18,5 0,73 150
E1R 16,2 26,2 0,62 184
E2R 18,8 32,8 0,57 262
C1F 13,8 18,8 0,73 140
E1F 16,0 26,4 0,61 157
E2F 19,2 32,9 0,58 220

Table 3. Water vapour permeability and permeability index

Designation [W.sub.d] (g/[m.sup.2]hPa) [i.sub.mt]

C1R 0.36 0.31
E1R 0.25 0.22
E2R 0.25 0.22
C1F 0.48 0.30
E1F 0.46 0.27
E2F 0.44 0.27