Indian Journal of Fibre & Textile Research Vol. 38, December 2013, pp. 387-394
Thermal properties of single and double layer fabric assemblies Deepti Gupta1,a, Ashish Srivastava1 & Sunil Kale2 1
Department of Textile Technology, 2Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India
Received 23 November 2012; revised received and accepted 14 February 2013 This study has been undertaken to generate objective data regarding the thermo physiological comfort characteristics of fabrics used in the manufacturing of inner and outer layer fabrics worn by uniformed personnel in North India. Effect of layering fabrics with and without air gaps between them has been assessed to simulate the effect of a multilayered garment assembly. Results show that thermal insulation increases markedly when an inner layer is paired with an outer layer of fabric, although breathability is not affected much. Proposed equations can be used to accurately predict the comfort properties of layers, if the comfort properties of individual fabrics are known. Thermal resistance of a garment assembly can be controlled by regulating the air gap between fabric layers. This data can be used to engineer uniform fabrics, so as to maximize the thermal comfort of wearers during hot weather conditions. Keywords: Comfort, Fabric layers, Moisture control, Thermal resistance, Uniform fabrics
1 Introduction Men’s clothing in North India typically comprises two layers of clothing worn for most part of the year when the weather conditions vary between hot and dry to hot and humid. These layers comprise an inner and an outer garment covering the torso and upper limbs and a similar combination for the lower body and limbs. The outer layer for the torso comprises shirting material which may be made from woven or knit fabric. For the bottom wear, trousers made from suiting materials are usually used in India. The inner layer on the top as well as bottom is generally a knit cotton undergarment. Sometimes these men are exposed to weather conditions for extended periods of time ranging between 8 h and 12 h. Given that temperatures in most parts of Northern plains in India touch a maximum of 46 - 47°C during the summer months with RH varying from 35% in May-June to 75-80% in August-September, selection of correct fabrics and fabric assemblies for uniforms becomes a critical consideration in reducing thermal stress and ensuring the comfort of wearer. Uniforms currently worn or available in the country have been designed paying scant attention to —————— a Corresponding author. E-mail:
[email protected]
the hot weather conditions in which most of the uniformed personnel have to operate for at least six months in a year. A survey of traffic policemen in Delhi, carried out by the authors in the month of June, showed that they had rashes over the body and chaffing around the neck area due to excessive exposure to heat and profuse sweating. All of them reported that the uniforms were extremely uncomfortable to wear. Though uniforms are used everywhere in India, no data is available in literature on the comfort characteristics of fabrics commonly used in their manufacturing. The designs and styles, originated during the colonial times, are based on western styles and sensibilities rather than on tropical weather or local work conditions. With this motivation, the current study was undertaken to assess and systematically study the thermal comfort characteristics of fabrics commonly used in the production of uniforms worn by policemen, bus and auto drivers and security guards in North India. Testing has also been done to evaluate the effect of layering the fabrics with and without an air gap, on the thermo physiological comfort properties. Clothing, by its very presence and nature impedes the transfer of heat from the body1. Thermal properties of the fabrics covering the body further affect the rate of heat transfer. However,
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when ambient temperature is greater than that of the body, no heat can be lost from the body to the environment. The body breaks into a sweat and the only means of cooling the body then is by the evaporation of sweat, rate of which is affected by the moisture vapour transport through the fabric. If the RH is also high then the moist air next to skin leads to building up of vapour pressure within the clothing till the air becomes saturated and liquid sweat condenses on the skin. Subsequently, the temperature of the body rises causing heat strain to the user 2. The only way to improve the situation in this case is to remove the body heat by ventilation through the clothing which is affected by garment design and the air gaps between the body and garment and the garment layers. Therefore, selection of fabric as well as garment design assumes importance in ensuring the comfort of the wearer 3. Wang et al.4, reported an interesting study wherein they used a sweating thermal manikin to propose empirical equations which can be used to estimate the total thermal and evaporative resistance of multilayer clothing assemblies by summing the resistance values obtained for individual garments. Researchers have studied the effect of fabric layers on thermal and moisture transport properties of clothing assemblies5. Benisek and Philips6 reported that the addition of a layer of under wear beneath the outer layer increased the thermal resistance. This was further enhanced when layers of still air were introduced between the fabric layers7. Increase in the number of layers as well as the thickness of the air gap between the two layers of fabric, increased the protection from environmental heat. At the same time, an increase was observed in the thermal stress experienced by the body due to reduced flow of heat from the body to the environment. The problem is compounded manifold when the environmental temperature is much higher (>400C) than the body temperature. In this study, 11 suiting fabrics (typically used in production of uniforms), two types of vest fabrics and 2 woven shirting fabrics were tested and characterised in terms of their physical parameters as well as comfort characteristics. The aim of the study was to test the thermal parameters of fabrics as single layers and then as a combination of an inner layer and outer layer so as to be able to design fabrics with optimal thermal and moisture transport properties for the defined weather conditions.
2 Materials and Methods 2.1 Materials Inner Layer (IL)
Two 100% cotton knit fabrics were procured from the local market. One was a single jersey plain knit (IL1) and second a rib construction (IL2). Both fabrics are representative of the fabrics used in locally available vests. Outer Layer (OL)
Eleven polyester-viscose and polyester-cotton blend suiting fabrics (OL 1-11) were obtained from a uniform fabric manufacturer in Mumbai, India. Fabric codes, weave and blend ratio for the test samples are given in Table 1. Two 100% cotton woven shirting fabrics (OL12 and OL13) were procured from the local market. Fabrics obtained from the producers were used as such without any processing. 2.2 Methods
All tests were carried out under standard atmosphere conditions (270C and 65% humidity). 2.2.1 Physical Characterisation of Fabrics
Thickness, weight (g/m2), wale and course density and linear density of yarns were measured. Essdiel thickness gauge was used for thickness measurement at a pressure of 20gf/cm2. Average of 10 readings was taken for each sample. Weight was measured with a sample size of 10cm × 10cm on Table 1—Characteristics of fabrics used in the study Sample code
Weave/knit
Application
IL 1 IL 2 OL 1 OL 2 OL 3 OL 4 OL 5 OL 6 OL 7 OL 8 OL 9 OL 10 OL 11 OL 12 OL 13
Plain knit Rib knit Plain weave Plain weave Plain weave Plain weave Plain weave Plain weave Twill weave Twill weave Twill weave Twill weave Twill weave Plain weave Twill weave
Vest Vest Suiting Suiting Suiting Suiting Suiting Suiting Suiting Suiting Suiting Suiting Suiting Shirting Shirting
Fibre blend Blend ratio %
P— Polyester, V— Viscose, C— Cotton.
Cotton Cotton P/V P/V P/V P/V P/C P/C P/V P/V P/V P/C P/C Cotton Cotton
100 100 67/33 67/33 70/30 67/33 67/33 70/30 67/33 75/25 67/33 67/33 67/33 100 100
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Shimadzu LIBROR EB-280 electronic weighing balance. Thread density was measured with the help of a pick glass. 2.2.2 Testing of Comfort Parameters Thermal Resistance
Thermal resistance (Rct, Km2W-1) of a fabric represents a quantitative evaluation of how good a thermal barrier the fabric is8. Thermal resistance of test samples was measured on Alambeta instrument by Sensora of Czech Republic9. The Alambeta can be used to measure several thermal parameters of fabric including thermal conductivity, thermal absorbtivity, thermal resistance and sample thickness. It simulates the dry human skin and is based on the principle of measurement of heat power passing through the test fabric due to the difference in temperature between the bottom measuring plate (22°C) and the top measuring head (32°C). The hot plate comes in contact with the fabric sample at a pressure of 200 Pa. As soon as the plate touches the fabric, the amount of heat power transferred from the hot surface to the cold surface through the fabric is detected and processed to calculate the thermal parameters of fabric10,11. Average of 10 readings was taken for each sample. Air Permeability
Air permeability (AP, cm3/ cm2 / s) is the rate of air passing perpendicularly through a known area under a prescribed air pressure differential between the two surfaces of a material12. It was measured on a Textest FX 3300 model air permeability tester by Textest AG of Switzerland. Tests were done as per the BS 5636. Average of 10 readings was taken. Relative Water Vapour Permeability
Relative water vapour permeability (RWVP, %) was measured on Permetest instrument by Sensora of Czech Republic, as per modified (0.5 m2, 23°C, RH 50%) ISO 11092 standard13,14, if the digital version of the device is used. The instrument works on the principle of heat power sensing. The relative water vapour permeability of the fabric sample is calculated by the ratio of heat loss from the measuring head with fabric (u1) and without fabric (u0), as given by the following equation15: RWVP (%) =
u1 ×100 u2
...(1)
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2.2.3 Testing of Fabric Layers
Each fabric sample was first tested individually and then each outer layer fabric (OL1-11) was tested in combination with each of the inner layer fabrics (IL12) to assess the effect of 2 layers on the comfort properties. In testing of double layer fabrics, the outer layer fabric was kept directly over the inner layer fabric and two were tested together. 2.2.4 Testing Thermal Resistance with Air Gap
Three individual wooden rings of thickness 5mm, 7mm and 10mm were used for creating air gap between the inner and the outer layer. The diameter of rings was kept exactly equal to the diameter of Alambeta bottom plate. Each outer layer suiting fabric (OL 3-9) was tested in combination with each inner layer (IL1-2) with a fixed air gap in between them. Mathematical modeling was done to find out correlations between various parameters using MS Excel software. 3 Results and Discussion 3.1 Physical Properties of Fabrics
Weight and thickness of fabric are important parameters as far as the thermal properties of fabrics are concerned. The physical characteristics of test fabrics are given in Table 2. Single jersey knit fabric (IL1) has a thickness of 0.57 mm while rib knit (IL2) is much thicker (0.83mm). The thickness of woven uniform fabrics (OL1- OL11) varies in a wide range between 0.36 mm and 0.63 mm. Shirting fabrics (OL12 and OL13) have the least thickness at 0.27mm and 0.3 mm respectively. Weight of uniform fabrics ranges from 190 GSM to 280 GSM. Shirting fabrics (OL12 and OL13) are of much lower weight at 115 GSM and 135 GSM respectively. Knit fabrics lie on the lower end at 155 GSM (ILI) and 145 GSM (1L2). The count of yarns used in knit fabrics is higher than that used in suiting fabrics but lesser than that in shirting fabrics. So knit fabrics used as innerwear (IL1 and IL2) are in general thicker and lighter than woven fabrics and as a consequence density is minimum for knit fabrics. Shirting fabrics are the thinnest and lightest of the entire lot. 3.2 Comfort Properties
The air permeability, thermal resistance and relative water vapour permeability values of the 11 outer fabrics and 2 inner layer fabrics are reported in Table 3. It can be seen that knit fabrics have higher
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Table 2—Physical properties of test fabrics Sample code
Thickness mm
Weight g/m2
Warp density ends/inch
Weft density picks/inch
Warp count Ne
Weft count (Ne)
Cover factor
IL 1 IL 2 OL 1 OL 2 OL 3 OL 4 OL5 OL 6 OL 7 OL 8 OL 9 OL 10 OL 11 OL 12 OL 13
0.57 0.83 0.39 0.63 0.57 0.44 0.36 0.44 0.49 0.42 0.48 0.5 0.51 0.27 0.3
155 145 200 280 255 225 190 205 250 235 260 245 252 115 135
46 40 75 40 80 58 70 64 100 110 70 84 26 190 145
46 60 70 36 64 54 50 50 64 72 56 54 13 90 75
26 30 18 8 30,9 13 16 13 18 18 13 12 80 64 40
26 30 18 8 30,9 11 16 13 18 36 13 14 58 64 40
16.2 15.98 23.7 20.44 17.15 23.04 22.19 22.85 25.96 26.82 24.2 26.2 22.79 25.25 25.19
Table 3—Comfort characteristics of test fabrics Sample code
Air permeability cm3/cm2/s
Thermal resistance ×10-3 Km2W-1
Relative water vapour permeability, %
IL 1 IL 2 OL 1 OL 2 OL 3 OL 4 OL 5 OL 6 OL 7 OL 8 OL 9 OL 10 OL 11 OL 12 OL 13
36.033 92.842 6.697 12.037 12.03 9.857 19.49 16.34 2.304 4.973 3.486 8.073 7.381 24.22 14.466
20.12 27.025 14.22 19.65 19.35 15.21 12.76 15.12 15.4 14.875 15.98 16.78 17.62 17.15 17.7
62.4 63.475 56.5 46.075 51.45 51.96 63.15 56.3 49.2 56.15 48.66 53.96 48.33 63.1 61.85
thermal resistance than woven constructions. Their air permeability is much higher and relative water vapour permeability (RWVP) is slightly higher than that of the woven fabrics. This can be attributed to the higher porosity of knit constructions which makes them more breathable than woven fabrics. At the same time, their thermal resistivity is higher which means that they prevent the loss of heat from the body. While this makes the knits an excellent choice for cold weather conditions, they may make the wearer uncomfortable during hot weather conditions by preventing heat loss
from the body. Within the woven fabrics, shirting fabrics (OL12 and OL13) have properties different from suiting fabrics. They are lighter in weight and have greater air and water vapour permeability, while the thermal resistance is similar to suiting fabrics. So they can be expected to be more comfortable than suiting fabrics. Not much differentiation is observed among the comfort properties of woven suiting fabrics. 3.3 Relation between Physical Characteristics and Comfort Properties Thermal Resistance
Thermal resistance (Rct) is used to express the heat insulation properties of a fabric. Rct of textiles is affected by fibre conductivity, fabric porosity and fabric structure. It is also a function of fabric thickness, as shown by the following expression: Rct = h/λ
... (2)
where Rct is the thermal resistance (m2K/W); h, the fabric thickness (m); and λ, the thermal conductivity of fabric (W/m/K). Plot of thermal resistance (Rct) vs thickness for all fabrics is shown in Fig. 1(a). While most woven suiting fabrics are clustered together (marked by rectangles), the knit fabrics show a different trend because of their much higher thickness and corresponding thermal properties. Because of these distinct differences in the properties of knit and woven fabrics, in all subsequent analyses these
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two categories have been analyzed and discussed separately. The plot of thickness vs Rct was thus replotted only for the woven fabrics [Fig.1(b)]. A high correlation (R2 = 0.938) is obtained, indicating that Rct is directly related to fabric thickness for a given class of fabrics. Similar findings have been reported by Hes et.al.16 in their studies with fabric assemblies. No clear effect of fibre blend is observed on Rct, probably because the volume of air entrapped in a fabric structure affects Rct more than the fibre composition. Within knit fabrics, the rib structure (OL2) shows higher thermal resistance and it also has the maximum volume. Ramachandran et. al.17 also reported similar findings in their study with single jersey, rib and interlock knitted fabrics. Air Permeability
Knit fabrics have significantly higher air permeability as compared to woven fabrics. But the air permeability varies from one construction to another – with the rib construction showing 3 times higher permeability than the single jersey construction. It can thus be seen that air permeability can vary within a large range for knit constructions. In woven fabrics, the value for air permeability is generally low. Shirting materials show somewhat higher values, as they are made from finer yarns.
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Finer yarns have lower diameter so they cover less area and give more space for air and water vapour to travel11. Relative Water Vapour Permeability (RWVP)
RWVP is a more complex phenomenon in comparison to the two characteristics discussed above. It is not just a function of inter yarn pores, but it also depends on intra yarn and intra fibre pores present in a fabric. Knitted and shirting fabrics show marginally higher water vapour permeability than suiting fabrics while polyester-cotton blend fabrics have slightly higher (avg. 55.43%) RWVP in comparison to polyester-viscose blends (avg. 51.42%). It is also observed that RWVP is inversely related to fabric thickness. In knit fabrics, IL1 has lower AP, lower Rct but higher RWVP although both are made from 100% cotton and have comparable weight. The only difference is in the thickness of the two fabrics. Therefore, it may be inferred that lower thickness of IL1 results in higher RWVP. This hypothesis was tested out by plotting RWVP of woven fabrics against fabric thickness (Fig.2). An inverse correlation (R2= 0.796) is observed, indicating that thickness of fabric is an important parameter affecting the RWVP of a fabric. This can be explained by the fact that thinner fabrics are made up of finer yarns and therefore, they allow more space for water vapour to pass through. Similar results have been reported by Özdil et al.18. 3.4 Effect of Physical Properties on Thermal Comfort
Correlation coefficients were plotted for the three comfort parameters with fabric physical properties to see if any relationship could be established between them. Results are shown in Table 4. As can be seen from the results and also established by other authors, Rct is a function of fabric thickness19,20.
Fig.1—Thickness vs thermal resistance for (a) all test fabrics and (b) only woven fabrics
Fig. 2—Relation between fabric thickness and RWVP (%) for woven fabrics
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Table 4—Correlation between physical and comfort characteristics of fabrics Property
Thickness
Weight
Cover factor
0.938
0.708
0.628
Air permeability
0.249
0.496
0.755
RWVP
0.796
0.81
0.16
Thermal resistance
RWVP is almost equally affected by thickness and the weight of fabric. Air permeability is related inversely to fabric thickness and shows good correlation with the cover factor in case of some fabrics. No correlation is observed between AP and RWVP. Knit fabrics have higher water and air permeability as well as higher thermal resistance as compared to woven fabrics. Thus, they allow more air and water vapour to flow through them but at the same time resist the flow of heat. For hot weather conditions, it is desirable to have as low a value of Rct as possible for the inner layer. One way to offset the thermal load of these fabrics can be through garment design. If the knits are designed for a loose fit, the porosity of structure will allow air movement between the body and environment, thereby reducing the heat build up in the body microclimate. But knitted structures would be uncomfortable in case of tight fitting clothes, where a thin layer of still air will be entrapped between the body and clothing and a very low amount of heat will be transferred from the body, thus heating up the body. 3.5 Effect of Layering on Comfort Properties
In the second part of this study, tests were conducted to study the effect of combining two layers of fabrics on comfort characteristics. Relationship between the properties of single and double layers has also been established. Each outer layer fabric (OL1 - OL13) has been combined with each inner layer fabric (IL1 and IL2) and tested. On comparing the values of layered fabrics with those of single layers (Table 3), thermal resistance of double layer is found to be higher than that of a single layer in each case, though it is slightly less than the sum of values for component layers. A typical graph for OL4 fabric layered with single jersey and rib knit fabric is shown in Fig. 3. Similar trend is observed for all OL fabrics. It has already been established that thermal resistance
Fig. 3—Comfort properties of OL2 fabric layered with plain and rib knit inner fabric
of a fabric is directly related to its thickness. When layered, the net thickness of combined layers goes up and so does the thermal property. In this experiment, the layers are kept one over the other with no air gap between them. Air permeability of knit fabrics is reduced drastically when a layer of woven suiting material is placed over it; in most cases being even less than that of the single suiting fabric alone. This indicates that woven fabrics block out all the pores or openings of the knit fabric. The water vapour permeability is slightly reduced when fabrics are layered. It continues to be around 40-45% in most cases. Comparing the combinations of two inner layer knit constructions, it is observed that values obtained for air permeability and water vapour permeability for rib fabric are slightly more in each case when compared with the corresponding combination with single jersey fabric. From these results, it can be inferred that combining two layers of fabrics reduces their comfort properties as compared to single layers. High air permeability of inner layer knits is reduced when it is combined with an outer layer. 3.6 Thermal Resistance of Layered Fabrics with Air Gap
This set of measurements was carried out with the help of wooden rings of different thickness which were used to separate the two fabric layers. Rings of 5, 7 and 10mm thickness were used one at a time to create the corresponding air gap between fabric layers. Thermal measurements were recorded on the Alambeta instrument.
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Table 5—Thermal resistance of IL1 layered with OL fabrics at different air gaps Sample code OL 1 Ol 2 OL 3 OL 4 OL 5 OL 6 OL 7 OL 8 OL 9 OL 10 OL 11 OL 14
0 mm 29.00 34.82 33.64 32.11 28.82 31.90 29.75 32.00 33.74 33.85 35.55 36.24
Rct ×10-3, m2 KW-1 5 mm 7 mm 141.49 162.37 147.25 170.11 148.12 173.28 146.88 168.39 143.51 166.87 145.23 169.46 145.13 165.35 149.86 168.52 148.76 171.27 149.22 173.96 151.38 172.86 153.25 173.28
10 mm 178.12 184.23 185.02 184.15 179.19 182.34 182.45 181.72 188.33 191.79 192.11 193.03
Table 6—Thermal resistance of rib knit fabric layered with OL fabrics Sample code OL 1 Ol 2 OL 3 OL 4 OL 5 OL 6 OL 7 OL 8 OL 9 OL 10 OL 11 OL 14
Rct ×10-3, m2 KW-1 0 mm
5 mm
7 mm
10 mm
29.74 33.42 32.30 33.23 28.40 36.11 32.16 30.34 39.66 37.51 35.12 35.18
152.75 158.22 157.81 159.49 155.21 163.85 161.98 158.36 166.37 165.97 164.93 164.25
178.66 180.19 182.36 183.98 180.91 187.14 181.27 182.33 189.43 188.86 186.67 189.05
196.23 198.14 199.56 201.96 198.35 203.18 197.56 199.45 205.37 204.08 200.77 203.84
No difference is observed in the results obtained for plain and rib knit as the inner layer. Results for plain knit fabric and rib knit fabric combined with OL fabrics are shown in Tables 5 and 6 respectively. It can be seen from these data that Rct of double layer fabric increases with increasing air gap between them. When the gap is increased from 0 mm to 5 mm the Rct increases by 4 times, indicating a dramatic increase in the insulating effect on introduction of a small air gap between layers. With subsequent increase in the air gap to 7 mm and 10mm, the increase observed is not as much. This trend can be explained by the fact that there is a change in mode of heat transfer between samples with change in the thickness of air cushion. When the gap is less (5mm), a thin layer of air is trapped between the layers. This thin layer is perfectly still as there is no scope
Fig. 4—Effect of air gap on thermal resistance (with rib knit)
for its movement and therefore has the maximum insulating effect which is reflected in the high increase in Rct (Fig. 4). As the gap between layers increases, convection currents start dominating as the medium of heat transfer and thus the increment observed in Rct values is not as much as that observed in the first case. Thus, it can be inferred that two layers of fabric, when lying directly on top of each other, have low Rct which can be increased almost four times by introducing a 5-10 mm gap between them. Though a decrease in Rct is not observed in this study, it can be expected that it may start decreasing when the gap between layers is increased beyond 10mm or whatever is the critical thickness of air gap. This is an important finding as it can be used to control the Rct of a garment by regulating the fit of clothing layers in terms of looseness and tightness around the body. 4 Conclusion Innerwear knit fabrics of same or similar thickness are more breathable as well as more insulating as compared to woven suiting and shirting fabrics. This makes them suitable for use in cold weather conditions. A high correlation exists between thermal resistance and thickness of fabric. But this relation holds true for a specific class of fabrics only such as shirting, suiting or knit fabrics. If all classes are taken together, the relation no longer holds true. In woven fabrics, the relative water vapour permeability is related to fabric thickness. Thermal comfort goes down drastically when an inner layer is paired with an outer layer of fabric although breathability is not affected much. Rct of a garment assembly can be controlled by regulating the air gap between fabric layers. Results show that the conventional practice, in North India, of wearing a close fitted knitted vest and a uniform garment over
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it is unsuitable for hot months as it would add to thermal stress. A single layer of clothing of a suitably designed fabric will be more comfortable.
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