Indian Journal of Fibre & Textile Research Vol. 26, September 2001 , pp. 287-295

Influence of wool-synthetic fibre blends on thermal insulation

R Indu Shekar", Nishkam Kasturiya, Hans Raj & Susheel Nigam

Textiles Divi sion, Defence Materials and Stores Research & Development Establishment, Kanpur 208 0 13, Indi a

Received 25 November 1999; revised received 7 April 2000; accepted 15 May 2000

Five different types of wool fibre blended blankets and four different types of knitted undergarments have been prepared and their thermal insulation evaluated by Thermolobo technique using dry and wet methods. It is observed that the optimum thermal insulation can be achieved in wool-acrylic blends, both in woven and knitted constructions. A comparison of woven and knitted constructions shows that the knitted clothings offer better advantages because of the more entrapped air. Data have been stati stically analyzed to relate the observed performance to the operative mechanisms of thermal transport.

Keywords: Acrylic fibre, Polypropylene fibre, Thermal conductivity, Thermal insulation, Thermal resistance, Wool fibre

1 Introduction The use of wool fibre in various forms, such as

felts , nonwovens and woven fabrics, for protection against varying levels of cold is well known. Woollen fibres are semi-crystalline proteinaceous polymer consisting of crystalline microfibrils embedded in an amorphous matrix , which is crosslinked by disulphide bonds. In addition. due to the presence of amine, amide and carboxyl groups there are a large number of hydrogen and ionic bonds which have a significant influence on the physical properties of the fibres l

. The excellent warmth provided by the wool fibre is primarily because of its structural features and low thermal conductivity (54 mwm-I k· I

), although the amount of air entrapped plays a vital role in providing warmth. In recent years, the major manufacturers of textiles have introduced innovative insulating materials designed for use in cold climatic conditions. The use of acrylic, polypropylene, hollow thermo­plastic fibres, fibre battings, thinsulate, textile reinforced aluminum sheets, etc have been tried all around the world to achieve protection against varying levels of cold. Of late, polyester battings (Polarguard and Hollowfil) have been widely used as a filling material in extreme cold weather clothings because of its high insulation-to-weight ratio and good compressional recovery. The polyester battings Polarguard and Hollowfil have very similar fibre

"To whom all the correspondence should be addressed. Phone: 450404; Fax: 0091-0512-404774; E-mail : indushekars@ rediffmail.com

diameters and per cent fibre volumes and hence they exhibit similar absorption constants, thermal resis­tance to thickness and resistance to mass ratio. Recently, hollow fibres filled with functional gels have also been produced by Teijin Ltd2

.

Wool-synthetic blends are of special interest in protective clothing against cold, especially to get enhanced strength, abrasion, low creep behaviour, aesthetic appeal, etc. Wool in various woven and knitted forms gives varying levels of warmth, depending upon the structural features like crimp level , maturity, diameter of the medulla and quality of wool.

The designing of the protective clothing for defence forces to meet both functional and comfort properties is of paramount importance, as the soldiers have to be combat-effective besides their protection against hostile weather and climate. The selection of fibres and the system design of clothing, therefore, require a careful consideration to reduce the level of discomfort and physical stress of the soldiers to the minimum possible extent. Heat transfer from the body through a fabric is a complex phenomenon affected by numerous factors, such as fabric thickness, quantity of the entrapped air around the body, moisture content and its transport, and the motion of the external air3

.4. The extent to which the textile structures can provide warmth is indicated by the th'ermal resistance values. The laboratory methodo­logy for characterizing the insulative properties of protective clothing has been studied extensively in recent years5

-9

. The Thermolobo equipment, used for

288 INDIAN J. FIBRE TEXT. RES ., SEPTEMBER 2001

measuring the thermal insulation, offers special advantages under varying simulating conditions of air velocity, wet condition, and entrapped air between skin and fabric.

The present study was aimed at evaluating the thermal performance of selected woven blankets and knitted undergarment and to study the influence of blend proportion, thickness and other critical properties of fabrics that influence the thermal resistance of wool blended fabric (blankets) and knitted structures. The performance was compared with respect to four different methods of measuring thermal insulation , viz. dry contact (DC), dry spacer (OS), wet contact (WC) and wet spacer (WS).

2 Materials and Methods 2.1 Materials

Five different types of woven blankets (twill weave) were prepared using the following different blends: wool:polypropylene (50:50), wool:polypro­pylene (70:30), wool:acrylic (50:50) , wool:acrylic (70:30), and wool:jute (85: 15).

Four different types of knitted undergarments were also prepared: wool:polypropylene (plain knit), wool:acrylic (rib knit), wool:acrylic (plain knit), and 100% polypropylene (terry) .

Fabric construction details of blankets and under­garments are given in Tables 1 and 2 respectively.

2.2 Methods The thermal insulation was measured by placing

the fabric on one side of an isothermal hot body and the energy required to maintain the hot body at constant temperature was measured.

All the samples tested were conditioned for 2 h at 65% RH and 20± 5°C before evaluation. The thermal insulation test was carried out using the following four different methods : dry contact method (specimen in contact with the hot plate), dry spacer method (5 mm space between the hot plate and the specimen), wet contact method (wet filter is kept between the hot plate and the specimen), and wet spacer method (5 mm space between the wet filter and the specimen).

The measurements were carried out at room temperature (27°-28°C), maintaining the hot plate at 3rC (to simulate the human body temperature) and air velocity at 130 cm/s.The sample size used was 20 cm x 20 cm.

The Thermolobo-II provided thermal insulation from which thermal resistance in togs was calculated using the following formula:

Table 1- Woven fabrics (blankets) construction details

Parameter Wool: PP Wool : PP Wool: Acrylic Wool : Acrylic Wool: Jute

Composition, % 70:30 50:50 70:30 50:50 85: 15 Mass per unit area, g/m2 430 420 420 380 630

Wool fineness , ).l 20.6 (64' ) 20.6 (64' ) 20.6 (64' ) 20.6 (64' ) 3].0 (48')

Thickness at 7g/m2, mm 2.0 1.7 2. 1 1.6 4.3 Air permeability, cc/cm2/s 34 37 25 38 60 Cloth cover, Kc 22.8 21.3 21.7 21.1 22.1 Breaking strength, kg

Warp 137 178 125 144 117 Weft 142 149 100 118 108

Relaxation shrinkage, % Warp 3.I(s) 1.6(s) 2.0(s) 1.5(s) 0.7(s)

Weft 0.2(e) 0.7(e) 0.5(e) 0.6(e) 0.9(e)

(s)-Shrinkage; (e)-Elongation; and PP- Polypropylene

Table 2- Knitted fabrics (undergarments) construction details

Sample Thickness Composition Wool quality mm %

Wool: PP undervest (plain) 1.1 50:50 62'

Wool: Acrylic undervest (rib knit) 2.0 50:50 64'

Wool : Acrylic undervcst (rib knit) 2.0 50:50 58'

Wool : PP undervest (terry) 2.3 0:100

INDU SHEKAR et al.: INFLUENCE OF WOOL-SYNTHETIC FIBRE BLENDS ON THERMAL INSULATION 289

. Ax(T - T )xlO Thermal resIstance (Togs) = I 2

C x V where A is the area of the specimen; T" the temperature of the hot plate; T2, the room temperature; C, the current required to maintain the temperature; and V, the voltage.

Air permeability, breaking strength, thickness and thermal resistance tests were performed according to IS: 11056, IS : 1969, IS: 7702 and IS : 2702 respec­tively .

3 Results and Discussion 3.1 Woven Blankets 3.1.1 Dry Contact (DC) and Dry Spacer (OS) Methods

The thermal insulation and thermal resistance (calculated) of blankets measured by different techni­ques are shown in Table 3. In all the blankets (except wool-jute blended blanket), the wool fibres of 64s

were intimately blended with acrylic and polypro­pylene fibres. A comparison of the wool-synthetic blankets shows that the wool-acrylic blanket (70:30) has high insulation and thermal resistance values in both dry contact and dry spacer methods. This may be attributed to the combination of following properties: (i) the proportion of wool fibre in the blend is more; (ii) the thickness of the blanket (2.1 mm) is in favour of the wool-acrylic blanket. The higher proportion of the wool fibre ensures more scattering of woollen fibres on the surface which helps in entrapping the air; (iii) the influence of thermal conductivity and the natural crimp of wool fibre contribute to a great extent as the finer wool is characterized by more crimp; the crimpiness varies from 15 crimps to 30 crimps per inch in finer wool and this helps in entrapping the air. According to Fonesca and Hoge IO

, the differences in thermal conductivity between the fibres of similar

organic compounds are not large. The thermal conductivity of air being lower than the fibre, the more the air entrapped, the better would be the insulation. In the present work, 64$ quality wool was used, which is a finer variety. Finer wool generally has no medulla, whereas in coarse wool, the medullation is around 36-70% (ref. 11); and (iv) from the structural point of view, the proportionately more number of scales in a finer wool also contribute to higher insulation.

The wool-polypropylene blankets (50:50 and 70:30) are comparable with wool: acrylic blankets (50:50 and 70:30) in terms of thickness, mass per unit area and cloth cover but show less thermal resistance

. than the wool-acrylic blankets in both dry contact and dry spacer methods. However, the differences in dry spacer method are marginal and cannot be considered significant. As compared to wool-polypropylene (70:30) blanket, the wool-acrylic blanket (70:30) shows higher insulation in dry contact method, whereas a marginal difference is observed in dry spacer method. The higher insulation observed in wool-acrylic (70:30) sample may be ascribed to the structure of acrylic fibre in addition to the influence of other parameters discussed previously. It is also interesting to note that in dry spacer method, the insulation does not vary significantly in all the four blended blankets. It varies from 70% to 73% in the wool-synthetic blankets. It appears that the dry spacer technique, wherein 5 mm spacer is used, is influenced more by the amount of air entrapped by spacer rather than by the actual structural parameters of the sample. The thermal insulation and tog values of wool­polypropylene (50:50) and wool-acrylic (50:50) blankets are comparable and do not show significant difference except that the wool-polypropylene sample

Table 3--Thermal insulation and tog values of blankets by four different methods

Blanket Dry contact method Dry spacer method Wet contact method Wet spacer method

Insulation, % Tog Insulation, % Tog Insulation, % Tog Insul ation, % Tog

Wool :Acrylic 55.26 1.1 8 71.11 1.96 52.11 1.48 64 .96 2.49

(50:50)

Wool:Acrylic 60.00 1.33 73.33 2.10 55.63 1.59 69.7 1.47

(70:30)

Wool : PP 52.63 1.13 70.56 1.93 50.70 1.44 63.25 2.31

(50:50)

Wool : PP 56.32 1.21 72.78 2.08 50.70 1.45 65.3 1.73

(70:30)

Wool : Jute 66.32 1.57 73 .89 2.15 54.73 1.59 63.25 2.38

(85: 15)

290 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2001

shows marginal difference in insulation in dry contact method. The influence of thickness and blend composition on insulation is shown in Table 4 and Fig. 1 respectively. A comparison of the wool-jute (85: 15) blanket with wool-acrylic (70:30) blanket shows that the correlation between thickness and insulation is less clearly defined. The wool-jute blanket having 50% more mass per unit area and double the thickness compared to the other samples shows an insulation of 66% in dry contact method. The increase in insulation value for a 2-fold increase in thickness and 50% increase in mass per unit area is only 10%. This explains that the thickness does not necessarily show a definite linear relationship. The insulation value observed in dry spacer method for wool-jute blanket does not show significant change compared to wool-acrylic (70:30) blanket and this explains that the technique is less affected by the structural parameters of the blanket. The results show that the insulation is primarily decided by a host of properties and not by the thickness alone. The influence of amount of air entrapped can be seen when the insulation values of wool-polypropylene blends (50:50 and 70:30) are compared with wool­acrylic (50:50 and 70:30) blends individually. It was expected that at the comparable thickness, the blanket comprising low density fibre such as polypropylene gives better thermal insulation as it involves greater amount of trapped air in the material. However, contrary to our expectation, wool-acrylic blanket showed better insulation at comparable thickness. The comparative performance of wool-synthetic blankets indicates that the high thermal resistance can be

achieved in wool-acrylic blends. Hence, further extensive studies on the influence of fibre conduc­tivity alone and of amount of air entrapped on insulation are needed to get better understanding of various properties towards insulation as the factors affecting the insulation are wide and varied. Further, it is observed that the dry contact and dry spacer methods demonstrate a linear relationship between insulation and tog values for the samples having higher insulation and higher thermal resistance. It may be concluded that the thermal protection achievable in a clothing assembly is primarily a function of thickness, air space between the skin and

20

18 -

16

14 &

12 -d' .!2 10 :;:; ::l 8 '" .s

6

4

2

0

Fig. I-Effect of blend composition on insu lation C- Dry contact method and 0 Dry spacer method)

Table 4-Effect of thickness on thermal insulation for blankets

Thickness

mm

Calculated

1.6 55.26

I.7 52.63

2.0 56.32

2.1 60.00

4.3 66.32

Total

aValues are not significant. For Dry Contact:

COD=85%

Y=4.3972X+47 .817

r=O.92

Dry contact method

Expected

54.852

55.292

56.611

57.051

66.724

Thermal insulation, %

Chi square· Calculated

0.003 71.11

0.128 70.56

0.001 72.78

0.1 i.2 73.33

0.002 73.89

0.286

For Dry Spacer:

COD=53%

Y=0.9453X+ 70.122

r=0.72

Dry spacer method

Expected Chi square"

71.634 0.003

71.729 0.019

72.012 0.008

72.107 0.020

74.186 0.001

0.051

INDU SHEKAR el al .: INFLUENCE OF WOOL-SYNTHETIC FIBRE BLENDS ON THERMAL INSULATION 291

the material, density of fibres, thermal conductivity of fibre, heat transfer co-efficient of air, and structural properties of fibres. The influence of cloth cover, weight and air permeability are shown in Tables 5-7. The bar diagram showing comparative performance of blankets in terms of weight-to-insulation ratio are shown in Fig. 2. To test the statistical significance of test results of insulation, the chi square test was applied and the values are shown in Tables 4-7 .

3.1.2 Wet Contact (WC) and Wet Spacer (WS) Methods

The insulation and thermal resistance of blankets measured by wet contact and wet spacer methods are shown in Table 3. The wet contact method simulates the condition when the human body develops sweat

due to metabolic heat generated in the body or when the fabric is wet due to exposure to rain, snow, etc. In wet contact method, where the wet filter is used between the hot plate and the specimen, the insulation decreases significantly as compared to that in dry contact and dry spacer methods. The insulation varies between 50% and 55% for wool-acrylic (70:30) blankets, showing the highest insulation value in both wet contact and wet spacer methods. It is observed that the wool-polypropylene blankets (50:50 and 70:30) do not show any difference in their wet insulation values. It could be due to very low moisture regain (0.01 %) of polypropy lene fibre and indicates that the insulation of wool-polypropylene blankets is not affected by the wet conditions. The wool-acrylic

Table 5- Effect of cloth cover on thermal insul ation for blankets

Cloth cover Thermal insulation , %

Kc Dry contact method Dry spacer method

Calculated Expected Chi square" Calculated Expected Chi square"

21.14 55.26 56.208 0.0 16 71.11

21 .31 52.63 56.680 0.289 70.56

21.74 60.01 57.874 0.078 73.33

22.10 66.32 58.873 0.942 73.89

22.83 56.32 60.899 0.344 72.78

Total 1.669

"Values are not significant.

For Dry Contact:

COD=12%

Y=2 .776X-2.4767

/'=0.35

For Dry Spacer:

COD=42%

Y=1.3919X +41.956

/'=0.65

Table 6-Effect of weight on thermal insu lation for blankets

Thermal insul ation, %

71.38 1 0.001

71.617 0.015

72.2 16 0.017

72.717 0.018

73.733 0.012

0.063

Weight

g/m2 Dry contact method Dry specer method

380

420

420

430

630

Total

Calculated

55.26

52.63

60.00

56.32

66.32

"Values are not significant. For Dry Contact:

COD=76%

Y=0.0463X+36 .989

r=0.87

Expected Chi square"

54.583 0.008

56.435 0.256

56.435 0.225

56.898 0.005

66.158 0.001

0.495

Calculated Expected

71.1 1 68.614

70.56 68.609 73.33 68.636

72.78 68.631

73.89 68.64 1

For Dry Spacer:

COD=44%

Y=0.0097X+67.925

r=0.67

Chi square"

0.087

0.053

0.300

0.236

0.372

1.048

292 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 2001

Table 7-Effect of air permeability on thermal insulation for blankets

Air Thermal insulation, %

permeability Dry contact method Dry spacer method

cc/cm2/s Calculated Expected Chi square" Calculated Expected Chi square"

20

16

4 f-

o

25 60.00

34 56.32

37 52.63

38 55 .26

60 66.32

Total

"Values arc not significant. For Dry Contact :

COD=38%

Y=0.253X +48 .289

r=0.61

Itl Dry contact method o Dry spacer met hod r-

~ ~ ~ ~ ~ ~

/: ~ ~

% ~ % ~ . % ~ :%

~ ~ ~

~ ~ ~ ~ ~

54.614

56.891

57.650

57.903

63.469

~

~ ~ ~ ~ ~ ~ ~ ~ k:

WloI:PP WloI:PP WloI:Acryl ic v.,bOt:Acrytic WlokJute (50:50) (70:30) (50:50) (70:30) (85:15)

Fig. 2-Weight-to-insulation ratios of blankets

blanket again exhibits high insulation in wet contact method as in dry contact and dry spacer methods and the insulation values are comparable with wool-jute sample, even though the thickness and weight of the wool-jute sample are comparatively high .

A comparison of dry contact and wet contact methods shows that the insulation values of wet contact method decrease to the extent of 4-21 %. Wool-jute (85: 15) blanket seems to be more affected by the wet contact and wet spacer methods. The sample shows 17-21 % reduction in insulation when compared with dry spacer and dry contact methods respectively. The wool-jute blankets, in which both the constituents are natural fibres, have high moisture content as compared to other blends. The low

0.531

0.005

0.437

0.120

0.128

1.221

73.33 71.898

72.78 72.185

70.56 72.280

71.11 72.312

73 .89 73.014

For Dry Spacer:

COD=8%

Y=0.0319X +71. I

r=0.28

0.028

0.004

0.040

0.020

0.0 10

0.101

insulation observed may be due to the fact that the air of normal water content" is replaced by water vapour through wet filter, which reduces the insulation and also the effect of moisture on the constituents of the fabric. The change in insulation is affected by the water sorbed by substrate as the water molecules replace the entrapped air in the interstices and reduce the insulative effect of the material because water conducts heat more readily than air. It is expected that heat loss through moisture is 25 times higher than that through air at the same temperature l2. Most of the insulating materials, including down feathers, exhibit poor insulation under compression and moist condition. Recently, the Primaloft, a synthetic insula­ting material made from a blend of polyester micro­fibres l3 is claimed to maintain its performance properties even under wet conditions.

Hollies and Bogatyl4 measured the thermal resistance of clothing fabrics at varying moisture content and found that the loss in insulation is proportional to the moisture content. The heat of the sorption of fibre increases with an increase in moisture content, and is generally high for fibres having high moisture content. This is in agreement with the observations made by Hollies and Bogatyl4. It is found that when the fibres absorb moisture, they become warmer and release the sum of heat of condensation of the water and heat of chemisor-

. II ptIOn .

It is interesting to note that the tog values in wet method do not exhibit a strong linear relationship with

INDU SHEKAR et al.: INFLUENCE OF WOOL-SYNTHETIC FIBRE BLENDS ON THERMAL INSULATION 293

insulation, which is observed in the dry contact method.

Woodcock and Dee'5 observed that the thermal losses are greater if the layer next to the heat source is wet rather than separated from a spacer. In the present work, compared to wet contact method , the wet spacer method shows -25% increase in insulation. The findings in the present work confirm the observations of Woodcock and Dee '5.

3.2 Knitted Undergarments The protective clothings made from knitted

construction are of special interest in extreme cold weather as they provide excellent insulation pro­perties and snug fitting. The construction particulars of undergarments are given in Table 2 and the insulation and thermal resistance are given in Table 8.

3.2.1 Dry Contact Method Table 8 shows that in case of dry contact method,

wool-acrylic (rib knit) undergarment with a thickness of 2 mm has an insulation of 55 %. On the other hand, wool-acrylic (plain knit) undergarment with compa­rable thickness exhibits insulation of 50%. This indicates that the structure of the fabric plays a vital role in determining insulation and thus the structure ' has to be carefully designed as it influences the amount of entrapped air. The insulation values of 100% polypropylene (teITY) are comparable with wool-acrylic plain knit. However, the wool-polypro­pylene undergarment with plain knit shows low insulation at 1.1 mm thickness compared to other undergarments wherein the thickness varies from 2 mm to 2.3 mm. It appears that the insulation-to­weight is a critical parameter which is to be considered while designing the fabrics for extreme cold region 3S the relationship between insulation and thickness is not simply linear as observed in wool­acrylic undergarments and blankets.

3.2.2 Dry Spacer Method Table 8 shows that the technique is not much

influenced by the fabric paJticulars. Similar trend was also observed in woven blanket samples. All the four undergarment samples show insulation in the range of 69-71 %. It may be concluded that the insulation of knitted fabrics primarily depends on a combination of various parameters. The high insulation values observed in knitted fabrics as compared to that in woven fabrics are due to the higher proportion of air to fibre volume. This should be taken into

consideration while designing the fabrics for extreme cold region. Theoretically, the influence of above factors may not show any significant difference in insulation values, but it is expected to influence the performance of a soldier during practical usage as the various parameters influencing functional and comfort properties come into picture. Thi s is especially true in case of wool fibre based fabrics .

3.3 Correlation Properties Different blend properties have been correlated to

provide a wider understanding of the relationship between these properties. The correlation between fabric thickness and insulation is shown in Table 4, cloth cover and insulation in Table 5, weight and insulation in Table 6, and air permeability and insulation in Table 7. The relationships between various properties were measured on the basis of strength of the linear association between each pair. The cOlTelation coefficient (r) and the coefficient of determination (COD) were determined with the help of regression line and are shown in Tables 4-9. Since

Table 8--Thermal insulation and tog values of undergarments

Undergarment Dry contact method Dry spacer method

Insul ation, Tog Insulation, Tog

% %

Wool : PP 44.74 0.97 69.44 1.84

(plain knit)

Wool:Acrylic 55.79 1.20 71.11 1.97

(rib knit)

Wool:Acrylic 50.00 1.06 69.44 1.84

(plain knit)

100% PP 49.47 1.07 71. II 1.96

(terry)

Table 9--Correlation coefficient (r) and coefficient of determination (COD) of some important properties with

insulation for blanket samples

Dry contact method Dry spacer method r COD r COD

%

Thickness vs

insulation 0.92 85 0.72 53

Air permeability vs

insulation 0.61 38 0.28 8

Cloth cover vs

insulation 0.:3 12 0.65 42

Weight vs

insulation 0.87 76 0.67 44

294 INDIAN 1. FIBRE TEXT. RES. , SEPTEMBER 2001

Table IO-Null hypothesis to analyse correlation combination between various properties

Relationship Dry contact method Dry spacer method

between to when to> t Remark to when to>t Remark

Thickness 6.72 2.36 Null hypothesis Linear 2.97 2.36 Null hypothesis Linear vs insulat ion rejected rejected

Air permeability 2.19 2.36 Null hypothesis Non-linear 10.83 2.36 Null hypothesis Linear vs insulation accepted rejected

Cloth cover 1.05 2.36 Null hypothesis Non-linear 2.41 2.36 Null hypothesis Linear vs insulation accepted

Weight vs 2.46 2.36 Null hypothesis Linear insulation rejected

r in this work has a wide range of values, null hypothesis technique was used to test its significance. When the null hypothesis 16 is true, it can be shown as:

The to has a t-distribution with /1-2 degree of freedom and may be tested for significance by comparing with the values in a t-distribution table. Table lO shows the results of null hypothesis technique for dry contact and dry spacer methods.

3.3.1 Dry Contact Method

The relationships between thickness & insulation and weight & insulation show strong correlations in dry contact method and are linearly correlated when tested at 95% confidence level. This is confirmed by the null hypothesis results also. However, a moderate correlation is observed in the case of air permeability & insulation, suggesting that the other variables are influencing the thermal resistance. Contrary to our expectation, a very poor correlation is observed in the case of cloth cover & insulation. Is was expected that the cloth cover influences insulation significantly since the degree of closeness influences the amount of entrapped air. The linear association of thickness and weight with insulation is reasonable and expected as discussed earlier. The coefficient of determination (COD) of thickness and weight again show a high percentage. The COD indicates that 85% (thickness vs insulation) of the total varIatIOn in thickness is explained by the linear influence of thickness on insulation.

rejected

2.55 2.36 Null hypothesis Linear rejected

3.3.2 Dry Spacer Method In dry spacer method, a moderate correlation was

observed in the case of thickness & insualtion and cloth cover & insulation and poor correlation when air permeability & insulation were associated. The statistical analysis (Table 10) again confirms that the thickness and weight have a significant influence on insulation. The results were also analysed by applying chi square test to see whether the results are stati stically significant or not and are shown in Tables 4-7.

4 Conclusions The potential advantages of wool-acrylic blends

which, when viewed in the long-term, can be effectively used as an alternative for lOO% wool blankets and undergarments as the production of wool fibre cannot be increased arbitrarily in the future. The study also encompases varying simulating conditions of measuring insulation so as to characterize the thermal resistance values that directly help in the design of fabrics taking into account the comfort factor.

References I Feughelman M, i Macromol Sci fB} Phys, 7 ( 1973) 569. 2 Chem Abstr, 127(2) (1997) 19405y. 3 Morris G 1, i Text but, 44(1953) 449. 4 Rees W H, i Text inst, 32(1941) 149. 5 Lyman Fourt & Norman R S Hollies, in Clothing Comfort

and Function, Chap 2 (Marcel Dekker inc., New York), 1970.

6 Behnke W P, Fire Technol, 13 (1977) 6. 7 Benisek L & Philips W A, Text ResJ, 51 ( 198 1) 191. 8 Perkins R M, Text Res i , 49 (1979) 202. 9 Shalev I & Barker R L, Text Res i . 53 (1983) 474. 10 Fonseca G F & Hoge H J, The thermal conductivity of

multilayer sample of underwear material under a variety of experimental conditions. Tech Report PR-8 , Pioneering

INDU SHEKAR et al .: INFLUENCE OF WOOL-SYNTHETIC FIBRE BLENDS ON THERMAL INSULATION 295

Research Division, (R& Eng. Center Natick, Massachusetts), October 1962, AD 290744.

11 Gupta N P, Shakyawar D B & Sinha R D, Indian J Fibre Text Res. 23( 1998) 32.

12 Nishkam Kasturiya, Subbu Lakshmi M S, Gupta S C & Hans Raj, System design of cold weather protective clothing, paper presented at the National Snow Science Workshop 99, SASE, 29-30 October 1999.

13 Goldman R F, Military operations and impact of load or combat clothing on operational tolerance limits for military

operations in hot alld/or cold environments, paper presented at the 12th Commonwealth Defence Conference, Ghana, 1978.

14 Hollies N R S & Bogaty H, Text Res J, 35 (1965) 187. 15 Woodcock A H & Dee T E (Jr), Wet cold: Effect of moisture

on heat through insulating materials, Report No. 170, Environ. Prot. Sec & D. Br (Quartermaster Clim. Res. Lab., Lawrence, Masachusetts), December 1950, PB iii 639.

16 Leaf G A V, Practical Statistics for the Textile Industry . Pt II (The Textile Institute, UK), 1987, 78.