Indian Journal of Fibre & Texti le Research Vo l. 3 L March 2006. pp. 177-186

Thermo-physiological comfort characteristics and blended yarn woven fabrics

V K Kothari"

Department of Tex tile Technology. Indian Institute of Tech nology. New Del hi I I()() 16. India

Clothing protec ts from cold or heat to ma in ta in body thermal comron throughout the full range of human activit ies. Various types of tac tile. moisture and thermal interactions between the clothing material and the human skin determine the comfort leve l of a person at a given environmental condi ti on whil e engaged in a spec ific level of activity. The fabric type and its blend compositi on, the tactile and therma l insulation behaviour or the fab ric assembly and the moisture management capabilities of the clothing can affect the comfort. This pnper discusses the ro le of fib re properties on comfort characte ri stics or fabric s and why the blending of fibres at yarn mamtfacturing stage can lead to fabri cs hav ing the desi red characteri stics from comfort poi nt of view. The detail s about the properties of diffcrem fibres and their rel ationship to different comfort auributes have also been provided. The resu lts o f experimentnl study of wnter vapour permeability conduct of po lyester/viscose (PlY) and polyes ter/cotton (PIC) blended yarn fabric s show that the higher polyester content in P!V and PIC fabri cs is detrimenta l to water vapour transmission. The water vapour u·ansmi ssion rate also increases with the air now rate above the fabric.

Ke)'WOrds: Rlencled yarn. Comfort. Therma l insulation behav iour, Water vapour permeability. Woven fabric IPC Code: Int. C l. 8 D03D 15/00

I Introduction Comfort may be defined as a pleasant state of

psychological, phys iological and physical harmony between a human being and the env ironment. All three aspects are eq uall y important, since people feel uncomfortable if any one of them is absent. Comfort is not a property bu t a condit ion of mind. The human mind responds with various degrees of satisfaction to the ever chang ing environment. This perception incl udes the effect of clothing between body and environment. A number of properties of fibres, yarns, fabrics and garmen ts are significantly related to comfort and must be taken in to account in producing sui tab le apparel items. Designers of clothing can take care of psychological and physical aspects of comfort by suitable selection of colour, des ign, texture, style, garment fit, etc. However, suitable fabrics from the comfort point of view must be developed by textile technologists by proper selection of fibre content, yarn and fab ri c construction techniques and fi nishing treatments as they influence physiological comfort level through thermal retention or transmission, moisture vapour permeability, water resis tance, static charge build-up, etc .

Fabric properties depend on fibre properties, ,ya rn structure, fabric structure and the mechanical and chemical fin ishing treatments given to the fabric . Of

"E-mai l: kotharivk @gmail.com

the vanous fib re properties, fibre type, fine ness, cross-sec ti onal shape, crimp, length and surface properties are extremely important. Yarn structu re governs the yarn properties for yarns produced from a fibre with a given set of fi bre propert ies. Type of yarn li ke filamen t yarn , textured yarn, spun yarn produced on different spinning systems, twist level , uneven ness and hairiness of yarns have significant infl uence on comfort and other properties of fabrics. Fabric structure includes yarn linear densities, sett, weave. crimp levels and can in fluence such critical fabr ic properties (cover, thickness, bulk density, mechanical and surface behaviour) which have direct relation with fab ric comfort . Finishes whi ch affect the properties of the fab rics and appearance can also significantly change the performance of a fabr ic in clothi ng.

Human body can be regarded as an open system, which is always in a state of interaction with its surrounding environment. In thi s system, there are fo llowing three types of processes involved:

(i) Physical processes: the processes in clothing and sLmound ing environments, such as heat and moisture transport in clothing and mechanical behav iour of the fabric during wear.

(ii) Physiological processes: • Neurophysiological processes-the neuro­

physiological mechanisms of the sensory

178 INDIAN 1. FIBRE TEXT. RES .. MARCH 2006

recepti on system of the body and their in teractions w ith clothing during wear.

• Thermophysio logical processes-the the rmal balance and comfort of the body, its thermoregulato ry responses and dynamic inte racti ons with c lothing and env iron ment.

(iii ) Psycho logical processes: the processes forming subjective perception of comfo rt sensations and preferences from various senso ry s ignals.

These three types of processes are relatively independen t. However, they interact with each o ther to determine the comfo rt status of a wearer at any specific moment.

2 Human-Clothing-Environment System Cloth ing is an in tegral pan of hu man li fe. Th e

primary role of clothing is to prov ide a layer of barriers . that protect the body against unsui table environments as show n in Fig. 1.

The environ men tal factors , such as temperature and air movement, can have drastic effec t o n the perceived temperature of the ex posed sk in , thereby strong ly affectin g the cooli ng o r heating sensation.

C lothing provides a mi c roclimate between the nude body and the ex tern al environment. The body responds to thi s microcl imate, and the thermoregul atory responses of the body and the heat transfe r and vapour permeati on properties of the clo thing detcnnine the microcl imate. For a continuously heated body (by metabolic heat production), a dy namic cq ui libriu·m is mainta ined where. ge nera ll y the

Body temperature> Skin temperature> C lothing surface temperature> Ambien t temperature

That temperature of c lothing is higher than the ambient temperature emphas izes that the environment a lso prov ides insulat io n by the boundary or air layer. The propert ies of thi s layer are very important to heat exchange and can be affected by the external environment.

3 Thermal Comfod In the ISO 7730 standard, thermal comfort is

defi ned as 'That condition of mind wh ich expresses sat isfaction with th e thermal enviro nment ' . A definition most people can agree on, but a lso a defi nition whi ch is not easily converted into physical parameters.

Environmental

Body 1-. Clothing f---- Stress Elements:

Metabolism Transmission Temperature

And I.,__ Properties Humidity

Activity Level ~ Radiation Air Movement

Fig. t- Role of clothing in protecting the body :llld maintaining the therma l ba lance between body and t.! nvironment

The sensati on o f comfort is an unco nscious reaction to the efforts of the body to cope with the prevailing cond iti ons, and warmth is undou btedly o ne of the more important contri butory factors. T he perception of warmth is determined largely by the imbalance or the rate of loss of heat from the skin surface and the heat flow produced at the skin by physical activity (metabolic heat). The range of condi tions over which the body can adjust its rate of heat loss , either by altering the bl ood fl ow or moisture release rates at the sk in surface, is quite large . lt is not unt il cond iti ons outside thi s range are experienced that o ne becomes aware o f any di scomfort. .

Man has a very effec tive temperature regulatory system, which ensures th at the body's core temperature is kept at approximately 3rC. When the body becomes too warm, two processes arc ini tiated . In firs t process, the bloo~l vesse ls vasod ilate, increas ing the blood flo w through the sk in and subsequently one begins to sweat. Sweating is an effect ive coo ling tool, because the energy requ ired for the sweat to evaporate is taken from the skin. Only a few tenths of a degree inc rease in the core body tem pera ture can s timul ate a sweat production which quadrupl es the body's heat loss. If the body is getting too cold, the firs t reacti on is for the blood vessels to vasoconstrict, reducing the bl ood fl ow through the ski n. The second process is to increase the internal heat production by stimul ati ng the musc les, which causes shive ri ng . This sys tem is a lso very effec ti ve. and it can increase the body 's heat product ion dramatica lly .

The control sys tem · which regu lates the body temperature is complex and is not yet fu ll y understood. The two most important set of sensors for the control sys tem are however known. They are located in the sk in and in the hypothalamus. The hypothalamus-sensor is a heat sensor which starts the body's cooling func tion when the body's core temperature exceeds 37°C. The sk in-sensors are cold sensors which start the body's defence against cooling down when the skin temperature fa ll s be low 33-34°C. Jf the hot and cold sensors o utpu t s ignals at the same

KOTH ARI: THERMO-PH YS IOLOGICAL COMFORT CHARACTERISTICS 179

time, ou r bra in will inhibit o ne or both of the body's defence reactions. Figure 2 shows the effect o f dev iati on in body core temperature o n hu man beings.

T wo conditions must be ful f illed to mainta in thermal comfort. One is that the actu al combination of sk in tempe rature and the body's core temperature provide a sensati on of thermal neutra li ty . The second is the fulfil lment of the body's energy balance, i.e. the heat produced by the metaboli sm sho uld be equal to the amount of heat los t from the body.

3.1 Thermal Insulating 1\llaterials

Category of f ib res whi ch provides thermal insul at ion entrap as much a ir as poss ible and have low compressibi lity, high res ili ence and hi gh bulk. Natural fibres have lower thermal conductivity than sy nthetic fibres in the dry s ta te but they absorb aro und 8- 13% mo isture and such a h igh moisture conten t inc reases their thermal conducti v ity significantly. Among sy nthet ic fibres. polyeste r and polyp ropy lene have thermal conduct ivity on the lower s ide as shown in Fig. 3.

Main fac to rs that govern thermal insul ati on value of a fab ric a re the thickness vihich dete rm ines the degree of insul atio n prov ided by the c lo thing; the cons truction-open stmcture has lower thermal insu lation cl ue to high convective losses; and the bulk density which inf luences the amount of a ir entrapped in the structure.

3.2 Heat Transfer l\·techanisms

Heat can be transfe rred within appare l in the fo rms of conduction, convectio n, radiation and late nt heat transfer by moisture transport. Conduction, convect ion and radi ati on are dominated by the temperature diffe rence between the ski n surface and the environment, and are therefore grou ped as dry heat transfer. On the o ther hand, latent heat transfe r is achieved by mo isture transmission related to water vapour pressure between the ski n surface and th e environment.

A person wearing li ght c lo th ing engaged in li ght acti vity in a tem perate environment loses up to 75 % of hi s metabolic heat by transfer of ' d ry' heat. The rest is lost when water evaporates from the ski n and through lungs . As activity levels rise, the pro porti on of evaporative heat loss increases, a lthough the abso lute rate of dry heat loss can be very hi gh. Dry heat is lost from the o uter surface of the body, i.e . both cloth ing and bare skin , by a co mbinati on of radi ation to the surroundin gs, natural or forced

Hypothermia -(3-4"C)

Skin Temperature 33"C Tolerance! (20-30''C)

Fig. 2-Effect of body core temperature on humans

Fig. 3- Therma l conduct ivi ty or some common tex tile ri bres

convection in the adjacen t air and the air which 'adheres' to th e oute r sur face and by co nduct io n through fibres and ai r.

The fit of the garment, the proporti on of the body surface area covered by the garment, geo metry o f the human body and body movements are few of the important facto rs that affect the transfe r of heat from skin to the environment. For example, in colder cli mates, according to di ffe rent des ign and f it of the clothi ng, the air gaps between laye rs of fabrics and openings aro und the body wi ll result in different effects o n thermal insul ation efTiciency. It was reported that as much as 75% of the total heat loss can be att ri buted to the loss of heat th rough open ings at the pl aces li ke the neck, the waist, the w ris ts and the ank les by be llows action when the body is mov in g in wi ndy conditions.

W here the heat mu st pass out thro ugh c lothing, the insulation of that c lo th ing partly de termines the rate at which metabo lic heat can be lost. The res istance of the clothing to the transfer of dry heat, known as the thermal res istance, has considerable infl uence on the co mfo rt o f the wearer, and is fundamental to the des ig n o f c lo thing systems.

3.3 Factors Influencing Thermal Resistance of Fabric Heat is transferred through tex tile mate ri als by a

combinatio n of conducti on through the air and fi bres. convection and infrared rad iation. Radi ati o n may pass fro m one fib re to the nex t, in w hich case the rate of

180 INDIAN J. FIBRE TEXT. RES .. MARCH 2006

heat transfer is very low, or in an open construction it may pass directly through the interstices of the materia'] without absorption and contribute significantly to the heat transfer. The interchange of heat between layers of clothing occurs by conduction through the a ir gap between them and unrestricted infrared radiation. Natural convection in the air trapped between the layers of clothing also contributes to the heat transfer from layer to layer, whilst forced convection influences the heat now in both the air gaps <:md fabrjc layers.

Thickness is the most important fac tor determining the thermal resistance .of textile materials. A number of vvorkers have demonstrated that a I i near relationship exists be{ween these two parameters.

It has been shown that the thermal resistance of a mixture of fibres and air is approximately equivalent to the resistance of a layer of still air of the same thickness. less an amount due to conduction in the fibres. This component is related to the amount of the fibres present, which is measured by the packing factor of the material and their conductivity. Packing factors vary from about 0.2 for a densely woven , cropped fabric to 0.05 for loosely knitted underwear, and down to about 0.0 I for fibre batting. The conductivities of all the fibres are of the same order of magnitude, whilst the conductivity of air is much lower.

As a consequence, the thermal resistivities of most conventional textile materials are very similar. The thermal resistivity of still air without infrared rad iation is 38 K.m/W, whilst that of textile material s is in the range 23-28 K.m/W . Thickness therefore is the factor which determines the greater part of the thermal resistance of any given product.

Textile materials manufactured from yarn are not a homogeneous blend of polymer and air. The packing density varies from a maximum in the region of the yarn core to a minimum at the fabric surface. The surface fibres contribute to the insulation in still air conditions by immobili zing the air which surrounds them. These surface t\bres are very eas ily deflected by the appl ication of pressure such as used in the measurement of fabric thickness. Although these surface fibres contribute to the overall thermal res istance of the fabric , they muy not be accounted for in the measured thickness.

The technique of rai sing or brushing the fabric surface to increase the thickness is one means of producing a material of low density and high thermal

insulation. Thi s is particularly advantageous in underwear, as it also increases the apparent warmth to the touch. The feeling of warmth in a object is determined by the rate at which the object conducts heat away from the skin . If a fabric s urface contains fibres which keep the skin away from the more conductive core of the yarns, then the rate of heat loss will be low and the fabric will be subjectively as-sessed as warm, irrespect ive of its actual thermal resistance.

Resistance of fabrics to air fl ow is useful as a measure of the pro tection agrunst penetration by wind. This can be an important factor in determining the loss of body heat through clothing in cold conditions. Alternatively, in warm conditions, it is one factor controlling the degree of ventilation provided by a garment.

4 Water Vapour Permeability The resis tance to the flow of air and heat. both

affect comfort provided by a fabric used as clothing. A third such quantity is the resistance to the diffusion of water vapour across the fabric. For comfort the permeability to water vapour of a clothing fabric should be as high as possible to allow the escape of water vapour which is constantly being released from the skin, even when the person is inactive. High water vapour permeability is, to some extent, incompatible with high resistance to air tlow and to protection from rain, and the art of des igning clothing for extreme conditions (tropics or Arctic) lies in finding a satisfactory compromise. A line of development that has a degree of success is the di scovery of so-call ed ' breathab le ' coatings. These provide some permeability to water vapour but restrict air permeability and the penetration of liquid water.

4.1 Moistm·e and Clothing Material

Under the normal conditions of atmospheric temperature and humidity, and body activity , the human body is continuously producing perspirations which evaporate within tlie skin layers and escape in the form of water vapour. This is referred to by physiologists as ' insensible perspirations' in contrast to sensible perspiration. i.e. liquid sweat which normally appears only when the ambient temperature is abnormally high or the individual indulges in s trenuous exercise. As long as the perspirations remain insensible, i.e. in vapour form, it is reasonably comfortable. Studies have shown that when a man at

KOTHARI: THERMO-PHYSIOLOGICAL COMFORT CHARACTERISTICS 181

rest or engaged in light act1v1ty is physiologically comfortable, the relative humidity at skin level is about 35%, being appreciably lower (because of the hi gher temperature) than that of the ambient air (say 50-60 %). This comfortable state can only be maintained if the clothing is adequately permeable to water vapour. If water vapour cannot escape sufficiently quickly through the clothing the rela tive humidity at skin level will increase. Thi s yields an uncomfortable sensation of clamminess, and if the conditions and the type of clothing are such that the relative humidity within th e clothing is increased to 100%, the liquid moisture is formed by the condensation of water vapour and the sensation of discomfort is accentuated.

All textile fibres irrespective of their chem ical co mposition are imper meable to air, and the passage of air through a fJ.bric can only take place through the pores in the fabric, i.e. through the spaces between fibres. The air permeability of a fabric is , therefore, dependent almost entirely on the structure of the fabri c and the yarn or fibre assembly , and is practically independent of the type of fibre used.

With regard to water vapour permeability the position is quite different. Most textile fibres are able to absorb a certain amount of moisture from the adjacent air, the damper th e air, the more water vapour will be absorbed by the fibre. With a garment during the wear, the concentration of water vapour in the air between the garment and the body exceeds that of the ambient air because of the insens ible perspiration emitted by the body, and the fibres will emit water vapour to the ambient air in addition to that which passes through the air spaces between them. The rate at which water vapour passes through a fibre depends on the nature of the fibre. With hydrophobic fibres the rate is very slow, whereas with hydrophi lic fibres it is relatively fast. Thus, although fibre type is relatively unimportant in connection with the water vapour permeability of fabrics of low bulk density because of the very large proportion of air present in the fabric , it is important for fabrics of close construction. Very closely woven windproof cotton fabrics, for example, have very low . air permeability, but because of the hydrophilic nature of the cotron fibres they have high water vapour permeability, and this means that they are comfortable Lo wear.

5 Comfort Characteristics of Blended Yarn Fabrics Modern clay living conditions require clothing that

is light weight , comfortable, safe, elegant, easy care and hard wearing. No single textile fibre has all the desirable attributes. Synthetic fibres have better wear and easy care properties but they lack many comfort related properties. Natural and regenerated cellulosic fibres have better feel and higher moisture absorbency leading to good comfort in vvear and low static charges but have poor strength and abrasion resistance. The blended yarns composed of two or more fibre components of different types, such as wool/polyester, wool/acrylic, polyes ter/cot ton and polyester/viscose in intimate blend, can produce yarns with desirable propert ies . For instance, blending of polyester fibre with cotton/viscose has become popular because of the complementary nature of the properties as indicated in Table I .

The blend of polyester with cotton/viscose reduces most of . the negative features of polyester, while negative features of cotton/viscose are overcome by the presence of polyester in the blend. In general. main motives behind blending are:

• Combination of merits of constituent fibres, • Opportunity to produce coloured effects, and • Reduction ol" cost by use of cheaper fibre.

Mehta and Narrasimham 1 summarized some comfort related properties and positive as well as

Table !- Positi ve and negative auributes of polyester and cellu losic fib res

Polyester

Good crease retention Good wrinkle recovery Good tenacity Better abrasion resist ance Lower stain ing tendency Ease of washing Higher co lour fas tness

Poor moi sture absmption Poor static di ssi pation Poor moi sture vapour transmission Poor fee l Warm. crisp hand Lower comfort Non bio-degradable

Con on/vi scose

Poor crease retention Poor wrinkk recovery Poor tenacity Lower abrasion resistance Higher stain ing tendency Difficulty of washing Lower colour fastness

Good moisture absorpt ion Good static dissipation Good 111oi sturc vapout· trans111iss ion Good feel Cool. silky hand lligher comfort Bio-degradable

BLEND WITH POSITI VE ATTRIB UTES OF BOTH FIBRES

182 INDIAN J. FIBRE TEXT. RES .. MARCH 2006

negative attributes of these properties for natural and man-made fibres (Table 2). Comfort attributes of fibres like cotton. wool and viscose can be combined with hard wearing and heat settab le attributes of sy nthet ic fibres to produce fabrics with right balance of propert ies. Desi rable or undesirable attributes of fibres can be affected by the actual textile construction. Yarn and fabric parameters, together with finishing treatments and garment design, can considerably change the comfort level produced by a garment.

While textile mill s are vitally concerned with yarn tensile strength, ex tensibility and their variations , the consumer IS very little concerned wi th these properties. A rev iew of these and othc1' mechanical propert ies has been given by Paj grt and RcichsUidtcr. ~

Table 3 shows the properties for typical fibres belonging to each of the fibre generic groups, namely natural cel lulose (cotton), natural protein (woo l),

regenerated cel lulose, polyamide, polyester and acrylic.

5.1 Optimum lllend of Yarns An optimum blending ratio can be determined from

statis ti cally sign ificant laboratory and practical \.vear trials of yarns and products made of them. as suggested in fi g. 4 . The optimum blend h<L to meet both technical and economical requirements.

It is apparent th at an optimum blend represents the sum of al l techni cal factors of the blend components wh ich approach closest to the complex of techni cal factors required from the given product and of all economical factors of the bl end whi ch are most attract ive fo r the product. It should be admitted, however, that ac tually every additiona l fibre component in th e yarn makes the technological process more comp licated. It makes not only work organization and stock records more complex . but it also requ ires higher organizational, technical and

Table 2-Propert ics and comfort amihutcs·' of SlllllC na tural and man-made fibres

Propert y Co lion Wool Silk Rayon Nylon Polyester Acrylic Pol ypropylcnc

Stretch and e lasticity

Elongation-at- 3-7 35 15 15 30 20-30 20-55 40 break . ci~ ·

Recovery at 2'Yr. 70 100 90 Varies wi th 100 97-1 00 95 J()(J

elongation. lft1 production method

Resiliency Low Exn:pti onall y Moderate Low Good: good Exccl kntto Good; resists Excellent to gootl wrink le res istan t good wrinkle wrin kles Good

propert ies recovery wel l

Abrasion fa ir to good Good Good Low Excc lknt, good Exceptionally Good as Excel lent resistance resistance to good compared to

flex ing wool

Dimensional Fabric may Poor, Good Fabri c wi ll Can be heal se t If properly Excelle nt. if Unaffected by stability shrink du ring untlcrgocs rcs i stance to relax Uuring to maintain heat set, wi ll not \Valer, wi ll not

lau ndering fe ll ing when stretch and laundering, shape not stretch or overheated shrink unless wet shri nkage. stretch eas il y shrink heated to

Fab rics m<ty du ring yarn temperatu re be stretched or and fab ric 150°C ironed back in manufacture shape

M oistu re regain . % 8.5 13.6- 16.0 I I 12-13 4.2-5 at 0 .4-0 at 1.5-3.0 0 .1 standard standard cond itions conditions

Comfun all ributcs" High- MA High-P High-MA Good-MA Smooth-I-I Low-MA Proper-W Exce llem- \V Gooci-W Good-W Good-W Good-Or Low-MR Less but Very low-

Smooth- If Smooth- H Good-S&SR Improper-Or present - MR or waxy Soft dry-11 Low-M A Poor-W sc

nature Low-A P SC-Prcscnt

·' Predominant property responsi ble for comfort is given in each col um n and promine nt feature which mars comfort is al so spec ified. "MA- Moistu re absorption. MR- Moi sture regain. W- Wi ck ing, AP- Air permeabi lity. II- Hand, Dr- Drape, SC- Static charge. and S&SR-Stretch & stretch recover

KOTHARI: THERMO-PHYSIOLOGICAL COMFORT CHARACTERISTICS 183

Table 3-Typica l fibre propert ies

Fibre Tenacity Breaking Modulus Work of Mois ture Water Electri ca l Flam rna Light Heat Alkali Acid N/tcx extension N/tex rupture regain reten ti on conductivity bili ty resistance resistance res istance resistance

lft, mN/tex % (fr! log Rs- 1 0 1% months oc h h

Colt on <UO 5 5 10 7 so 0.14 18 3.5 105 10 0.5

Wool 0.14 40 2.5 40 17 44 0.10 25 3.5 90 0.5 4

Vi scose 0.20 15 6 20 12.5 100 0.10 18 2.5 105 10 0.5

Poly:unidc 0.45 20 2.5 70 4.3 15 0. 13 2 1 3" 100 4

Polyester 0.45 20 10 60 0.4 20 ().(}8 22 3.5 140 2 50

Acrylic 0 27 30 5 50 u 10 0.07 I <J 19" 120 5 100

Polypropylene 0.65 15 7 70 0 5 0.06 17 4" 105 10 10

Average fibre (J.3() 15 5 40 5 50 0 .10 21 3 120 2 2

"Semi-dull fibres; and ''UV swbili zcd t'ihre . Electrica l conductivity - Reciproca l log o f' r~s i stu ncc. Rs of fib re at 65 RH; ]]eat resi stance - Max. tempe rature for continued usc. 0 C: Light res istance­Time to lose 50'/f· strength in Florida sunl ight: Alkali resistance- T ime to lose 50'K strength in I O'if· Nu0!-1 at 100°C: and Acid resistance- Time to lose 50'!f· strength in I 0'/f HC I at I 00°C.

I OPTIMUM BLEND I

TECHN ICAL FACTORS (performance characteri stics required for the

ci\'en end usc) l111X of all performance characteris tics of blend

-components which approaches closest to the

required performance characteristics in the gi\'en

product.

ECONOI'v11 C FACTORS

'---Sum total of all economic factors of the blend

which is attractive for the given product tor the p:n1icular economic conditions.

rig. 4-Factors affecting opt imum blend in fa brics

commercial knowledge and skill from people at all levels of production.

5.2 Static Charges Textile fibres normally used in clothing have very

lovv electrical conductivity. Static charges which are generated in die lectric or non-conducting material s either due to repeated contact or rubbing between dissimilar surfaces can create nui sance and cause discomfort. Fabric can cling to other fabrics or to the sk in , Charge generation increases dry soi lin g and also causes spark discharge which is extremely unpl easant for the wearer. Numerous problems can arise during manufacturing of yarns and fabrics and it is always desirable to prevent high static build-up in textiles. The problems ari sing from the development of electrostatic charges are more serious when fibres of re latively low moi sture regain are used and at low relative humidi ty 3

. Viscose blends cause a sharp reduction in static charge generation. Pajgrt and

Table 4-Stat ic c harges o n ble nded yarn woven fabrics

Fabric

Wool/polyester ( I 00:0)

Wool/polyeste r (70:30)

Wool/polyester (50:50)

Wool/po lyester (30:70)

Wool/po lyester (0: 1 00)

Rayon/polyester ( 1 00:0)

Rayon/polyes ter (70:30)

Rayon/polyester (50:50)

Rnyon/polyeste r (30:70)

Rayon/polyes ter (0: 1 00)

S tat ic charge. V

420

770

980

1100

1700

110

320

42 0

720

1700

Reichs tUdter2 suggested that the rayon content of 25 % is enough to su ppress static charges to limi ts acceptable for normal wear. Fabrics with 30% rayon content generally give adequate protection in outerwear. Static charge generation of a few fibres and fibre blends is g iven in Table 4 .

It is evident that the polyester fabrics blended with wool or rayon reduce the static charge generation considerably which agrees with the practical experience where blended fabrics can be worn normally without an antistatic finish, while 100% polyester fabrics have a tendency to stick to the body and are uncomfortable to wear.4

5.3 Moisture Absorbency The property of absorbing moisture is a valuable

feature of clothing materials. Apart from its direct utility in keeping the skin dry, the absorption of water causes the fabric to act as a heat reservoir, protecting

184 INDIAN J . FIBRE TEXT. RES .. MARCH 2006

Table 5- Moisture absorption of fibres

Mat.:ri al

Cotton

Mercerized col!on

Hemp

Flax

Jute

Viscose rayon

Secondary acetate

Tri ace tate

Silk

Wool

Casein

Nylon 6.6. ny lon 6

Polyester llbre

Acryl ic fibre

Modacry lic fibre

Poly( vinyl chloride)

Poly(vi nyl alcohol)

Glass. polyethylene

Absorpt ion regain

(65% RH . 20°C) %

7-8

up to 12

8

7

12

12- 14

6. (i<}

4.5

10

14, 16-!8

!4

4.1

0.4

1-2

0.5- 1

0

4.5-5.0 ()

Difference in desorption and

absorption regains (65% RH, 20°C)

0.9

1.5

1.5

1.8

2.6

1.2

2.0

1.0

0.25

the body from sudden changes o f ex ternal conditions. However, it may be a disadvantage in drying the hygroscopic fibres .

Moisture absorpti on changes the properties o f fibres. It causes swelling which alters the dimensio ns of the fibre, and this , in turn , causes changes in the size, shape, stiffness and permeability of yarns and fabrics. The mechanical and the frictional properties are altered, •vhich affect the behaviour o f the fibres during process in g and in subsequent use. Wetting and drying may lead to permanent set or creas ing. The moi sture conditi o n of the material is one of the most important fac tors in determining its e lectrical properti es; static charge generation is less likely to occur in damp conditi ons. The above examples show the technological importance of moisture absorptio n in fibres. Table 5 shows the absorption regain va lues and width of the hyste rsis curve fo r some of the textile fibres.

Moisture regain of blended yarn fabr ics can be obtained on the basis of wei ghted average of two compo nents in the blend as shown in Table 6.

5.4 Moisture Vapour Transmission Characteristics

C lothing serves to protect the body agai nst excessive hea t loss , to keep ou t detrimental

Table 6--Moisture rega in values o f blended yarn fabr ics

131end rati o Moisture reoai n. % Polyt:s ter/viscosc Polyester/cotton

100:0 0.40 0.40

90:10 1.66 1.16

80:20 2.92 1.95

70:30 4.1 8 2.68

65:35 4.!\ 1 3.06

60:40 5.44 3.44

50:50 6.70 4.20

40:60 7.96 4.96

30:70 9.22 5.72

0:100 13.00 8.00

Fig.S - Water troug h appa ratus

environmental and weather effects, and to enhance the appearance. Clothing is des ig ned to maintain a hygienic and comfortable zone about the human body in which o ne feels good , even if outer environment changes rapidly. The zone in which the temperature. moi sture and air circulation are properly matched so as to maintain th e thermal and the moi sture balance is called the 'comfort zone'. The ab ility of a fabric to transm it the water vapour emitted from the body is an important factor in assess ing the comfort characteristics of fabrics and clothit;g assemb lies . 5-

7

Polyester/vi scose (PlY) and polyester/cotton (P/C) are popular fibres blends for a wide variety of app li cations and the water vapour permeability in bottom weight PIV and P/C fabtics is of interest from the point of v iew of comfort properties of these fab rics. Fabrics were tested on three instruments. In water trough apparatus (Fig. 5), two sampl es of ~ 50mm x I SOmm each were placed o n two water troughs with indi vi dual water dosi ng sys tem to maintain a constant air gap of 10 mm between water surface and fabric in each trough throughout the

KOTHARI: THERMO-PHYSIOLOGICAL COMFORT CHARACTERISTICS 185

ex periment for 10 h. Weight of water lost through the fabrics was used to calculate water vapour permeabili ty of the fabric (WVPI ). In the perforated

plate apparatus (Fig. 6) , a sample of 150mm x 150mm was placed over a sinte red brass plate which is in contact with wate r in a water trough. The si ntered plate si mulates human porous skin. A wate r closing system was used to maintain a constant water level in the trough throughout the experiment for I 0 h . Weight o r water lost through the fabri cs was used to calculate water vapour permeability of the fabric (WV P2). In Permetes t (Fig. 7), a copper plate with water o n top was supplied with heat to keep its temperature equal to the environment temperature (iso thermal conditions). The heat supplied to maintain a constant temperature with and w ithout the fabric o n top of the

Specimen

Water Dispencing System

Fig. 6- l'errora ted plnt e apparatus

Airflow

C.h cbJ ~ Temp Heat flow sensor sensor Heater

Measuring head of Permetest

Fig.7- Pennetest appara1us

:trl plate I

Water film

Stand

plate was measured and relative water vapour permeability (RWVP) was calculated.

5.5 Effect of Blend Percentage on Water Vapour Permeability The water vapour permeability decreases with the

increase in the polyester fibre content in the blend (Fig . 8). Same trend can be observed in all the three experimental setups. This is because abso rption or water molecules decreases with the increase in po lyester content. Hence. the evaporation is less from th ese fabrics.

Permetest gives the highest water vapour pe rmeabili ty followed by water trough apparatu s and perforated plate respectively. This is because in Permetest air is blown over the fabric at a speed of 3-5 m/s, hence the removal of water vapour f rom the surface of the fabri c per un it time is much higher. In case of perforated plate apparatus, the sintered brass

3000

2500

2000

1500

1000

-"' "" ~

500 N" "" "" -~ ] 0 u 5 2500 0.. M

" 0 E-> .!1 ~

1500

1000

500

0

0

PIV blended fabric

•'MI~er Trougn M e:hod

• ParforatadP~IaMethOt:l

4 Perm&!est Apparet.:s

PIC blended fabric

•

+ Walr.rlrough ,_,athod

•

• •

• Partof•tad P lata Methcd

A P l)fTTIOtc6t Ap011nrtus

20 40 60

Polyester, %

I'"' ·5.3V.7x .. 264~3 R1 •00059

v• ·3.63B3x • '.:1ti 1 R1 =0SBD4

v= -3.655>: •1367 a R1=09756

y = -3 . 002x•21~ .9

R1 .. 0 '0'12

v= -3 .~61lc + 156:) 1

R1 o:0.9431

y = -2.6~1C + 099 3

R1 "' 0.9535

80 100

Fig. S-Effect nf polyester co ntent in different blends on the water vapour permeability measured for different experimental se tup

186 IND IAN J. FIBRE TEXT. RES ., MARCH 2006

18lKl

1700

1600

1500

1400

1300

"" '<t 1200 ~

""' ~ g 1100

:0 3 1000

6 0.. 1800 !3 ~ 1700 > g

1600 3:

1500

1400

1300

1200

1100

1000

0

PIV blended fabric

_;.- ,.. -- -- - ..... --

PIC blended fabric

....... x .... ~ ..... ..,.- ... .. .- · :x:

J . ..,.- .. . .. . .. - .. -~ •_;. . .. .. --..... .;~--.," .. - .,­. . -. ..-

.. .. _;.,--! _,. ....... ......

0.2

PolyuhtConhnt,% + • ao • 75 ._70 Xl36

0. 4 0.6 0.8 1.2 1. 4 1.6 Wind Velocity , m/s

Fig. 9-EITect of wind velocity on the water vapour permeabi lity of different fabr ics measured by water trough apparatus

plate blocks the water vapour from reaching the fabric surface and hence evaporation takes place from a relatively lesser surface area.

5.6 Effect of Wind Velocity on Water Vapour Permeability With the increase in wi nd velocity, the water

vapour permeability obtai ned on water trough apparatus (WV PI ) increases for both PIV and P/C fabrics (Fig. 9). The effect of the air flow rate in the range of 0- 1.6 m/s over the fabrics is si milar irrespective o f the polyester fibre content. The fab rics w ith the lower value of the polyester content show higher water vapour permeabili ty as compared to the fabrics with the higher polyester content at all wind velocities.

It is observed from the above study that the (i) water vapour permeab il ity reduces with the increase in polyester content in both PIV and P/C fabrics; (i i) perforated plate method gives the lowest and Pennetest apparatus gives the highest water vapour permeabi lity; and (iii) water vapour permeability increases with the increase in wind velocity for both P/V and P/C fab rics

References I Meh ta R & Narrasimham K V, Clothing comfort: A review of

related properties. M{111 -111ade Te.rf India, .July ( 1987) 327-335. 2 Pajgrt 0 & RcichsWdter B. Processing of Polye.\'ler Fibres

(Elsevier Scientific Publish ing Co., Amsterdam), 1979, 149-155.

3 Morton W E & Hcarle J W S, Physical Properties of Texlile Fibres (The Texti le Institute, Manchester). 1993.

4 Shah R S & Dweltz N E. Static electricity in text il es: Problems and remedies, Indian Texl J, August ( 1994) 50-60.

5 Holcombe B. Thermal insulation performance of textile fabrics, Wool SciRe\'. 60 ( 1984) 12-22.

(j Mehta P, Requirements of moist ure transport in underwear. Wool Sci Nev. 00 ( 1984) 23-46.

7 Yi Li, Measure111en1 and researc/1 on 1her111o-wer COII(/im

properties r!/ textiles, paper presented at the Third Japan­Austra li a Joint Symposium on Object ive Measurement Applications to Product Design and Process Control, Kyoto. 1985.