8

CHAPTER 2

LITERATURE REVIEW

2.1 HEALTHCARE TEXTILES

Textile products are used in medical and healthcare sector in

various forms. Healthcare textiles comprise surgical clothing (gowns, caps,

masks, uniforms etc.), surgical covers (drapes, covers etc.) and beddings

(sheets, blankets, pillow cases etc.). The complexity of applications has

increased with research and developments in the area of medical textiles. The

surgical gown, operating room garments and drapes require special anti-

bacterial properties combined with the wearer’s comfort.

A range of natural and manmade fibers with enhanced comfort

properties and anti-microbial properties have been introduced in the market.

These fibers and their blends are being utilized for developing new products

in medical textiles. It is observed that for many products, ideal set of physical

and chemical properties would not be possible to achieve from one fiber

alone. Hence, blending of two or three fibers having differing physical and

chemical properties for the desired product becomes essential. However,

some chemical characteristics of the fiber/fabric are also modified during

fiber manufacture/chemical processing. Sometimes, modified fibers are also

blended to achieve the desired end product.

Use of nanotechnology, tissue engineering, biomaterials along with

basic textile structures, viz, fibers, yarns, woven, knitted, non-woven and

braided fabrics and composite structures made it possible to widen the

9

horizon of medical textiles. Table.2.1 shows the type of fibers used for

different applications. (Rajendran and Anand 2002).

Table 2.1 Healthcare/Hygiene Products and the Fibers Used

Hospital Textile

application

Fabric Fiber Type

Surgical gowns Woven, nonwoven Cotton, polyester, polypropylene

Surgical caps Nonwoven Viscose

Surgical masks Nonwoven Viscose, polyester, glass fiber

Surgical drapes, cloths Woven, nonwoven Polyester, polyethylene

Surgical hosiery KnittedCotton, polyester, polyamide

elastomeric- yarns

Blankets Woven, knitted Cotton, polyester

Sheets, pillowcases Woven Cotton

Uniforms Woven Cotton, polyester

Protective clothing,

incontinence, diaper/sheet,

coverstock

Nonwoven Polyester, polypropylene

Absorbent layer NonwovenSuperabsorbent fibers, wood

fluff,

Outer layer Nonwoven Polyethylene fiber,

Cloths/wipes Nonwoven Viscose, lyocell

Most of the hospital textiles are made of polyester cotton and their

blends. Viscose fiber is used in the form of nonwovens for few selected

applications.

2.2 QUALITY REQUIREMENTS OF BED LINEN

Bed linen is a sheet of material used to cover the bed, which should

be soft with warm handle and easy care properties. The majority of the bed

linen is made from cotton and polyester/cotton blended yarns. Depending on

the end use, cost factor, durability of the textiles, comfort and aesthetic

10

properties, the fiber choice is made from natural fiber, regenerated cellulosic

fibers and synthetic fibers.

2.3 CHARACTERISTICS OF FIBERS USED FOR

PRODUCTION OF HOSPITAL TEXTILES

In this research work, different varieties of fibers and their blends

are used for producing hospital textiles and each fiber has its own special

characteristics which are discussed below.

2.3.1 Properties of Lyocell Fiber

Lyocell is a regenerated cellulosic fiber manufactured by a closed

loop process, involving dissolution of wood pulp in N- methyl- morpholine-

N- oxide. The manufacturing process of lyocell is different from that of other

regenerated cellulosics such as viscose rayon, in that it proceeds without the

formation of intermediate compounds and there is no curing or ripening stage.

The manufacturing process imparts to lyocell a unique combination of

properties compared to other regenerated cellulosics, such as high wet

strength, high wet modulus, high crystalline and amorphous orientation, high

crystallinity and high wet swelling, which influence its behavior in wet

treatments.

Like other cellulosic fibers, lyocell is breathable, absorbent and

comfortable to wear in conditions of high humidity because it is cellulosic

which causes moisture to be wicked away from the skin.

2.3.1.1 Moisture transport properties of Lyocell fiber

In contrast to synthetic fibers with reduced wicking properties,

lyocell offers unique moisture transport ability. This property of lyocell

guarantees optimum conditions for the skin. The figure 2.1 shows distribution

11

of moisture in the cross section of cotton, lyocell and polyester fiber (Heinrich

Firgo et al 2006).

Cotton lyocell polyester

Figure 2.1 Cross Section of Cotton, Lyocell and Polyester Fibers

The illustration shows that in contrast to cotton and polyester fibers,

lyocell controls and regularly absorbs moisture. It absorbs 50 % more

moisture than cotton. Compared to the other two fibers, lyocell features the

highest moisture absorption rate: with air humidity at 65 %, lyocell still has

unused capacity to absorb moisture from the skin. Figure 2.2 compares the

surface structure of lyocell and cotton fibers (Schurz 1994)

Lyocell Cotton

Figure 2.2 Surface Structures of Lyocell and Cotton Fibers.

Lyocell fiber has an extremely smooth surface and feels soft and

pleasant on the skin. The combination of a smooth fiber surface and excellent

moisture absorption creates a positive environment for healthy skin, making

lyocell ideal even for anyone with sensitive skin. According to recent

dermatological studies, wearing clothing made of lyocell significantly

improves comfort and promotes a feeling of well being (Thomas et al 2006).

Also, lyocell is chemical-free, ecofriendly fiber, an important factor for

sensitive skin in comparison to other fibers of natural origin.

12

Lyocell is often blended with other natural fibers such as wool,

cotton, silk, flax and various manufactured fibers. When blended with other

fibers, it gives a wonderful sheen to the yarn and adds softness to many other

fibers. Cotton blended with lyocell becomes stronger and wool/lyocell blends

are more absorbent. The properties of lyocell fiber with different deniers are

given in the Table 2.1.

Table 2.1 Properties of Lyocell Fiber

Unit Fiber DenierFiber property

Dtex 0.9 1.3 2.2 1.4 3.0

Cut length mm 34 38 50 38 75

Tenacity cN/tex 36 34 33 36 31

Elongation % 12 10 10 10 10

Tenacity (wet) cN/tex 30 28 26 25 19

Elongation(wet) % 13 12 12 10 10

MoistureRegain % 11 11 11 8 9

2.3.1.2 The structural model of Lyocell fiber

A lyocell fiber consists of countless hydrophilic, crystalline nano-

fibrils which are arranged in a very regular manner. Water absorption occurs

only in the amorphous domains and capillaries between the crystalline fibrils.

A lyocell fiber therefore is a unique hydrophilic nano-structure which is the

reason for the special water management, comfort and other positive features

of lyocell. Abu-Rous (2006) made an effort to explain the functional and

wellness properties in lyocell textile using the nano structure of lyocell. The

special functional and wellness properties of lyocell textiles is due to the

cool and smooth surface (gentle to the skin) and the wear comfort properties

due to excellent moisture transport, buffering and the temperature control.

The background for this property is the unique structure of lyocell fibers

13

which contains a compact core, a porous middle zone and a semi-permeable

fiber skin. Applying fluorescence microscopy and TEM on fiber cross-

sections, different porous zones could be discriminated, partly confirming the

model mentioned above. Figure 2.3 shows the structural model of lyocell

fibers (Christian Schuster 2006).

Figure 2.3 Structural Models of Lyocell Fibers

Presence of superfine or micro fibrils provide greater specific

surface area, greater vapour transmission, lower flexing resistance, softer

handle, greater crease resistance and greater fabric density and cover to the

lyocell fiber. These advantages provide the yarn with more pores to transport

vapour out by their superior capillary action. The higher pore density also

provides better thermo regulation.

In general, the lyocell fibers show higher porosity than natural

fibers, leading to outstanding water buffering capacity. In lyocell, the water

accessible pores are distributed very evenly in the fiber cross-section. The

cool touch of lyocell can be explained by the quick removal of water vapour

from the fiber when the textile is touched and heated up by skin contact. The

very accessible interconnected pore network seems to play a major role in the

fast conduction of moisture in the fiber (wicking effect). Additional effects of

14

the high water uptake in lyocell are minimized bacterial growth and less

odour formation.

Heinrich Firgo et al (2006) analyzed the comfort properties of

single layered and double layered fabrics made of tencel/ polyester blended

yarns in the face of the fabric and polyester as the skin contact layer. From

the experimental results the authors concluded that tencel can be used

effectively for the development of high performance sportswear provided that

the fabric is carefully designed to maximize the contribution of the tencel to

the performance of the fabric.

Friedrich et al (2002) presented a comparative analysis of thermal

insulation properties of fabrics made of cotton and tencel with different weave

structures. The fabrics made of tencel yarn showed lower values of thermal

conductivity and thermal absorption and also higher values of thermal

diffusion and resistance than fabrics made of cotton yarns. Twill woven

fabrics made of lyocell have higher air permeability and thermal resistance

compared to plain woven fabrics.

Thomas et al (2006) evaluated the skin compatibility of

commercially available tencel textiles in patients suffering from atopic

dermatitis or psoriasis in an everyday situation. The patients gave excellent

scores for tencel textiles regarding improvement of itching, skin sensitivity,

thermoregulatory properties, for its properties of cool, smooth and dry feeling,

and for its compatibility with the local topical treatment. Hence tencel textiles

significantly contribute to well-being also under dermatological conditions.

From a dermatological point of view these textiles can be recommended not

only for healthy subjects but also for people with sensitive skin or even

patients with skin disease, especially atopic dermatitis or psoriasis

(Heidelberg 2004). Avinash (2005) studied about the drying rates of resin

treated lyocell fabrics. Cheunsoon (2005) evaluated the physical and fabric

15

hand characteristics of lyocell fabrics made with different wood pulps and

stated that, the production of lyocell fibers and physicochemical traits play

interrelated roles in making lyocell with desired properties.

2.3.2 Properties of Micro fibers of Lyocell and Polyester

Synthetic fibers having fineness below 1.0 denier are called as

microfibers. Micro fibers have distinguishing properties such as softness, full

handle with excellent drapeability and comfort (Basu 2001).

Superfine or microfiber yarn enables very dense fabrics to be

produced with extended specific fabric or fiber surface area, developing more

pores to transport vapour out by their superior capillary action. The higher

pore density also provides better thermo regulation. Finer fibrils will provide

higher specific area, better vapour transmission, softer handle, higher fabric

density, better cover and lower flexing resistance (Prabhakar Bhat and

Bhonde 2006). Comfort properties of polyester microfiber fabric are more in

terms of wicking when compared with micro polyester /cotton blends and

pure polyester non- micro fiber fabrics. (Ramachandran et al 2009). Schacher

et al (2000) compared the thermal insulation and thermal properties of

classical and microfiber polyester fabrics.

Rose and Matic-Leigh et al (1993) carried out a study on the

aesthetic and comfort properties of microfiber blended knitted fabrics to

determine the effect of micro-fiber staple yarns on knit fabric performance.

Sandip et al (2007) reviewed about micro fibers, their classification,

manufacturing methods, different fiber forms, general properties and their

applications. Sampath et al (2009) studied the effect of stitch length and knit

structure on the wicking, wetting, water absorbency, moisture vapour

transmission and air permeability of moisture management finished micro

denier polyester knitted fabrics.

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2.3.3 Properties of Bamboo Fiber

Bamboo fiber is a natural, green and eco friendly textile material of

the 21st century, which is made of 100% bamboo pulp. In textile form,

bamboo retains many of the properties it has as a plant. Bamboo is highly

water absorbent, able to take up three times its weight of water.

Bamboo fiber is characterized by its good hygroscopicity, excellent

permeability, soft feel, easiness to straighten and dye, and splendid color

effect of pigmentation. It is also a new environment-friendly fiber which is

both anti-bacterial and deodorizing in nature. Bamboo fabric has an unusual

level of breathability, making it incredibly cool and comfortable to wear. This

is because the cross-section of the bamboo fiber is filled with various micro-

gaps and micro-holes; it has much better moisture absorption and ventilation

(Parameswaran and Liese 1976). This character of the fiber translates to an

excellent wicking ability that will pull moisture away from the skin so that it

can evapourate. For this reason, clothing made of bamboo fiber is often worn

next to the skin. With this unique microstructure, bamboo fiber apparel can

absorb and evapourate human sweat very quickly. In addition, bamboo fabric

has insulating properties and will keep the wearer cooler in summer and

warmer in winter. (Nazan Erdumlu and Bulent Ozipek 2008). Bamboo owns a

unique anti-bacteria and bacteriostatic bio-agent named "bamboo kun" which

gives the inherent antimicrobial property to the bamboo fiber.

The physical properties of bamboo fiber are given in Table 2.3

(China Textile Industry Testing Institute 2004)

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Table 2.3 Physical Properties of Bamboo Fiber

S.No Parameter Value

1. Dry tenacity (cN/tex) 23.3

2. Wet tenacity(cN/tex) 13.7

3. Dry elongation at break (%) 23.8

4. Whiteness 69.6

5. Moisture regain (%) 13.03

Bamboo fiber has excellent moisture regain and elongation but the

wet and dry tensile strength are comparatively less than cotton fiber. Abhijit

Majumdar et al (2010) presented the thermal properties of different knitted

fabric structures made from cotton, regenerated bamboo and cotton-bamboo

blended yarns and found that the thermal conductivity of knitted fabrics

generally reduces as the proportion of bamboo fiber increases. The interlock

fabrics have higher thermal conductivity and thermal resistance values and

lower water vapour permeability and air permeability when compared to plain

fabrics.

2.3.4 Properties of Bamboo Charcoal Fiber

Bamboo charcoal, is a non graphite form of activated carbon made

from pieces of bamboo plants which are five years or older, by carbonizing

inside an oven at temperatures over 800°C and then converted into powder

form. Fibers from bamboo charcoal can be produced in many kinds, such as

single and multi-filament; staple fiber and it can also be spun with pure

cashmere, cotton and other fibres. There are two main ways to produce

bamboo charcoal fiber: the first way is to add nano-bamboo charcoal powder

during the process of spinning in the spinning solution; the second is to add

the established bamboo charcoal composite polymer master-batch in the stage

18

of synthesizing fiber. Bamboo charcoal viscose fiber can be produced from

natural plant cellulose pulp by adding bamboo charcoal micro powder milk

dissolved by the solvent and then spinning the solution by extrusion and

solidification. Polyester based bamboo charcoal fiber is produced in the

similar way from polyester master batch with bamboo charcoal content about

50%. Similarly bamboo charcoal nylon fibers and bamboo charcoal magnetic

fiber series are also produced (Jeong-Sook Cho and Gilsoo Cho 1997; China

Textile Team 2009).

Since the Bamboo charcoal fiber contains activated carbon, which

is efficient in adsorbing odorous volatile micro-organism, thereby reducing

the odor and growth of micro-organisms, it is used in hospital textiles for

reduction of microbial growth and adsorption of wound odor (Robert Czajka

2005). The unique properties of bamboo charcoal include uniform

composition, high porosity, anti-bacterial and anti-fungal property,

breathability, thermal regulation, odor control, absorption and emission of Far

Infrared energy, preventing static electricity buildup and good wash durability

(Chin-An Lin and Ta-Chung An 2007). These fabrics can absorb and disperse

sweat fast, making them feel dry and comfortable. They also do not stick to

skin on hot summer days. Bamboo charcoal fabrics absorb and decompose

benzene, phenol, methyl alcohol, and other harmful substances. As the

bamboo-charcoal nano particles are embedded in the fiber rather than simply

coated on the surface, these fabrics are washable without diminished

effectiveness of the charcoal powder's special qualities, even after 50 washes

(Parthiban and Viju 2009). Thenmozhi et al (2010) analyzed cotton, bamboo

/ cotton and bamboo charcoal bed linens for their suitability as hospital

textiles by applying anti-microbial and blood repellant finish and it was found

that anti-microbial activity, blood repellency and odor resistance is higher for

bamboo charcoal fabrics than 100% bamboo /cotton union fabrics or 100%

cotton fabrics.

19

Jeong-Sook Cho and Gilsoo Cho (1997) have recorded that, as the

activated carbon is efficient in adsorbing odorous volatile micro-organism

thereby reducing the odor and growth of micro-organisms, it is used in

hospital textiles for reduction of microbial growth and adsorption of wound

odor.

Kuruvilla et al (2008) made an attempt to study the usefulness of

Activated Carbon Fabric mask (ACF) to prevent lead absorption. Indigenous

ACF masks were provided to eight workers involved in the manufacture of

batteries and their blood lead levels were determined before and after using

these masks. There was a substantial decrease in blood lead level after using

the mask among those who were under treatment for high blood lead levels.

Han Chien Lin et al (2008), examined on the usage of the original

bamboo vinegar collected from Moso bamboo (Phyllostachys heterocycla) at

six different temperatures to increase the fungi resistance of bamboo.

Splendore et al (2010) evaluated the thermo-physiological comfort of a

knitted polyester (PES) fabric which contains activated carbon particles in the

back-side. The activated carbon particles, added in the PES extrusion process,

give permanent attributes to the garment, such as odour resistance, UV

protection and evapourative cooling which makes the modified PES ideal for

sportswear.

2.4 PROPERTIES OF BLENDED YARNS AND THEIR

APPLICATIONS

It is apparent that the type of raw material and fabric structure

influence the properties of finished goods. Blending of different fibers is done

to enhance the performance and improve the aesthetic qualities of fabrics and

are selected and blended in certain proportions so that the fabric will retain

the best characteristics of each fiber. Fiber selection is done based on the

20

properties like moisture management, thermal conductivity, breathability,

wettability, wickability, natural stretch and dimensional stability.

Blending of different types of fibers and analysis of the comfort

characteristics of blended fabrics have been carried out by various researchers

in the past. Kothari (2006) has discussed the role of fiber properties on

comfort characteristics of fabric and studied how the blending of fibers at

yarn manufacturing stage can lead to fabrics having the desired characteristics

from comfort point of view. The results of experimental study of water

vapour permeability of polyester/viscose and polyester/cotton blended fabrics

showed that higher the polyester content in P/V and P/C fabrics, lower is the

water transmission ability. Air permeability increases with increase in

polyester content and the water vapour transmission rate also increased with

the air flow rate of the above fabric.

Yoon and Buckley (1984) determined the mechanical and surface

properties for polyester, cotton and polyester/ cotton blended knit fabrics. The

polyester fabric showed a higher resistance to tensile deformation than the

cotton fabric, while the blend fabrics showed an intermediate resistance in

accordance with the blend level.

Yoon and Buckley (1984) determined the thermal transport

properties of a series of polyester, cotton and polyester/cotton blended fabric

in an effort to understand the physical basis of clothing comfort. The results

indicated that both the fabric construction and the constituent fiber properties

affect thermal comfort. In general, thermal insulation, air permeability and

water vapour transmission rate are dependent mainly on the fabric

geometrical parameters namely thickness and porosity. Conversely, liquid

water transport is strongly dependent on the constituent fibers with cotton

generally showing advantages over polyester.

21

Brojeswari Das et al (2009) studied the moisture related properties

of plain woven polyester/viscose blended fabrics with different polyester

proportion, yarn count and twist, using a three variable factorial design

proposed by Box and Behnken and concluded that proportion of polyester in

the blended fabric affects the comfort characteristics of the fabric.

Nayak et al (2009) probed the effect of polyester content, pick

density and weave on the thermal comfort and tactile properties of polyester/

viscose blended yarn fabrics for suiting, by measuring the low stress

mechanical properties on Kawabata evaluation system and reported that

increasing polyester content increased fabric hand but decreased fabric

smoothness, softness, fullness and total hand value and increased thermal

insulation and water vapour resistance.

Behera (2007) studied the handle and comfort properties of fabrics

made of 100% linen and their blends with cotton and viscose, and reported

that total hand value (THV) of linen fabric is higher than that of cotton fabric

and blending of viscose and cotton improves the hand value of linen fabric

Tyagi (2009) sorted out the thermal comfort behavior of fabrics

made of polyester /viscose and polyester/cotton ring and MJS yarns and states

that the hydrophilic groups of man-made cellulosic component of the fiber

mix governs the liquid moisture transport through capillary interstices in

yarns and concludes that polyester/ viscose fabrics are more promising than

polyester cotton fabrics for comfort applications.

Mukhopadhyay et al (2002) used KES to determine the effects of

blend proportion on the comfort properties of polyester / viscose blended

plain and twill suiting fabrics. Increasing polyester content decreased the total

hand value and increased thermal insulation and water vapour resistance.

Sharabaty et al (2008) have explained about moisture transport through

22

polyester/cotton fabrics stating that the hospital bed sheets are commonly

produced from the mixture of cotton and PET fibers. These sheets become

uncomfortable in humid days when cotton fibers become saturated with

moisture, producing uncomfortable sensation which may cause frictional

festers on patient’s skin.

For effective moisture transports properties, several multi layered

fabrics were produced, considering the following aspects:

Hydrophobic layer: This layer must have the capability to

transfer the moisture out of contact surface while maintaining

its hydrophobicity and at the same time, to keep a dry contact

surface between patient body and the bed sheet. But the

migration of moisture stops when the other side is wet. So, to

increase the moisture transfer, its chemical potential has to be

decreased, e.g. making the external layer out of cotton.

Hydrophilic layer: This layer allows obtaining an effective

moisture management of human sweat.

Yingchun Du and Jin Li (2010), investigated the dynamic moisture

absorption behavior of polyester/cotton fabrics of different warp and weft

densities, and the results showed that the fabric moisture absorption velocity

is in reverse relation with its warp and weft densities. They also developed a

mathematical model for the fabric’s dynamic moisture absorption which is

represented in the form of a non-linear partial differential equation. Gene

Cone (2009) has analyzed on the advantages of using blended fabrics in

achieving the required comfort properties and thermoregulation of the human

body by radiation, conduction and sweating.

23

2.5 ANALYSIS ON THE PHYSIOLOGY OF HUMAN BODY

AND COMFORT

The human body continuously generates heat by its metabolic

processes. The heat is lost from the surface of the body by convection,

radiation, evapouration and perspiration. In a steady-state situation, the heat

produced by the body is balanced by the heat lost to the environment by

maintaining the body core temperature around a small range, between 36°C

and 38°C. The skin is the major organ that controls the heat and moisture

flow to and fro the surrounding environment. Gilat (1963) and Brojeswari Das

(2007) elaborated on the processes involved in human comfort such as

physical, thermo-physiological, neuro-physiological and psychological states

of comfort.

A base level of metabolism has been defined as the metabolism of a

seated person resting quietly and for a man of typical height and surface area,

the metabolic rate is about 100W. To normalize among people of different

sizes, metabolism is typically expressed in per unit skin surface area. A

specialized unit, the ‘met’ has been defined in terms of multiples of basal

metabolism: one met is equal to 58.15 w/m². A sleeping person has the rate of

0.7 met, and reclining awake is 0.8met. Office work (a mostly seated activity

but one that involves occasionally moving about) is 1.2 met: Walking slowly

(0.9 m/s or 2 mph) is 2 met, moderate walking (1.2 m/s or 2.7 mph) is 2.6

met, and fast walking is 3.8 met and jogging 8 to 12 met. In terms of energy, a

sleeping person has the rate of 40.71 w/m², and reclining awake is

46.52 w/m², Office work is 69.78 w/m² (Federico Butera 1998).

2.5.1 Mechanism of Thermal Regulation of Human Body

The body’s heat losses are through radiation, convection,

conduction, evapouration and through respiration. In a neutral environment,

24

where the body has no need to take thermo regulatory action to preserve its

balance, evapouration provides about 12% of total heat loss and sensible heat

loss provides 88%. In general, the heat transfer by conduction through the

soles of the feet or to a chair is small, around 3%. In normal indoor

environments with still air, the convective and radiation heat transfer are

about equal. In the outdoors, wind strongly affects convective heat loss or

gain and radiation can also cause large losses and gains.

Sweating is important for heat regulation, and it is also a major

source of water absolute loss. There are two types of water loss: insensible

perspiration and sweating. Insensible perspiration loss from the skin cannot be

eliminated. Daily loss is about 400 ml in an adult and the respective heat loss

is 238 kcal. The heat loss can be quite significant because there is a loss of

0.58 kcal for every ml of water evapourated. The maximum rate of sweating

is up to 5 ml/min or 2000 ml/hr in an acclimatized adult. This rate cannot be

sustained, but losses up to 25% of total body water is possible under severe

stress and could be fatal.

There is always a constant amount of trans-epidermal loss of water

vapour directly diffused through the skin resulting in heat loss by insensible

evapouration. In addition the breathing cycle involves humidifying exhaled

air producing another evapourative heat loss. The transversal moisture

diffusion is about 100 to 150 ml per day per m² of skin surface representing a

heat loss of 6% as great as the evapouration from a fully wetted surface. The

respiratory portion of the body’s total heat loss is estimated to be 12%

depending on the metabolic rate.

Clothing is used outside the skin to extend the body’s range of

thermoregulatory control and reduce the metabolic heat by thermo regulation.

It reduces sensible heat transfer, while in most cases, it permits evapourated

moisture to escape. Bed clothes are a form of clothing used for sleeping,

25

because the metabolic rate during sleep is lower than the basal rate and the

body‘s skin temperature tends to be higher during sleep, bed clothes typically

have a higher insulation value than clothing.

2.5.2 Measurement of Conductive, Convective, Radiative and

Evaporative Heat transfer through Fabrics

In order to clarify the heat transfer area involved in convective heat

exchange for the human body, which is required for calculating heat exchange

between the human body and the environment, Yoshihito Kurazumia et al

(2004) calculated the total body surface area of six healthy subjects and the

non convective heat transfer area and floor and chair contact areas for the

various body positions. The effective thermal convection area factor for nine

common body positions such as standing, sitting in a chair, sitting in the seiza

position, sitting cross-legged, sitting sideways, sitting with both knees erect,

sitting with a leg out, and the lateral and supine positions are measured. The

results showed that the effective thermal convection area factor for the naked

whole body in the standing position was 0.942, when sitting in a chair 0.860,

when sitting in a chair, excluding the chair contact area 0.918, in the seiza

sitting position 0.818, in the cross-legged sitting position 0.843, in the

sideways sitting position 0.855, in the both-knees-erect sitting position 0.887,

in the leg-out sitting position 0.906, in the lateral position 0.877 and the

supine position 0.844. For all body positions, the effective thermal convection

area factor was greater than the effective thermal radiation area factor, but

smaller than the total body surface area.

Yoshihito Kurazumia et al (2008) scrutinized the convective and

radiative heat transfer coefficients of the human body, while focusing on the

convective heat transfer area of the human body. Thermal sensors, directly

measuring the total heat flux and radiative heat flux, were employed. The

mannequin was placed in seven postures. The regression equations for the

26

convective heat transfer coefficients (hc [W/ (m2 K)]) for natural convection,

driven by the difference between the mean skin temperatures corrected using

the convective heat transfer area and the air temperature, are given below:

Standing (exposed to atmosphere) hc = 1.007 T0:406

Standing (floor contact) hc = 1.183 T0:347

Chair Sitting (exposed to atmosphere) hc = 1.175 T0:351

Chair Sitting (contact with seat, chair

back and floor) hc = 1.222 T0:299

Cross-Legged Sitting (floor contact) hc = 1.271 T0:355

Legs-out Sitting (floor contact) hc =1.002 T0:409

Supine (floor contact) hc = 0:881 T0:368

where hc is the convective heat transfer coefficient [W/(m2 K)], and

T the difference between mean skin temperature corrected using convective

heat transfer area and air temperature [K].

Richard et al (1997) analyzed the convective and radiative heat

transfer coefficients for individual human body segments and found that the

radiative heat transfer coefficient measured for the whole-body was

4.5 W/(m2 K) for both the seated and standing cases, closely matching the

generally accepted whole-body value of 4.7 W/(m2 K). Similarly, the whole-

body natural convection coefficient for the manikin fell within the mid-range

of previously published values at 3.4 and 3.3 W/(m2 K) when standing and

seated respectively. In the forced convective regime, heat transfer coefficients

were higher for hands, feet and peripheral limbs compared to the central torso

region. The ASHRAE Handbook of Fundamentals (1993) has indicated a

linearized radiative heat transfer coefficient hr=4.7 W/m2 per K which has

been widely accepted as a reasonable whole-body estimate for general

27

purposes (Fanger 1977). Byron W Jones (1998) addressed the need to include

the radiation non-uniformity commonly found in indoor environments in body

heat loss calculations.

Extensive research has been carried out to evaluate the sweating

rate. In 1998, Toshio Ohhashi et al. reviewed the methods of human

perspiration evaluation. In 1986, Kraning and his co-operator reported a new

forced-evapouration-type skin capsule for measuring local sweat gland

activity in humans. Shamsuddiny and Togawa (1998) reported a method of

continuous monitoring of sweating in which deion solution was perfused at a

constant flow rate through a chamber attached to the skin surface.

2.6 CAUSES FOR DISCOMFORT AND SKIN DAMAGE IN

MATTRESSES

One of the common manifestations of chronic disease and disability

is the abnormal loading of skin and other surface tissues unaccustomed to

bearing large mechanical forces. A result of abnormal mechanical loading of

surface tissues is breakdown. Though breakdown might appear initially as

only a slight reddening of the skin, it can develop into a significant injury that

damages tissues through the entire thickness of the body wall. If loading

continues unchanged in an area that demonstrates early breakdown,

irreversible injury and necrosis might occur. More extensive pressure ulcers

develop which extend deeper into subcutaneous tissues, sometimes into joint

or body cavities.

There is also compelling evidence that factors in addition to

pressure are contributors and must also be considered when attempting to

fully understand the pressure-sore phenomenon. Studies have implicated

factors such as shear stress (Bennett et al 1979), impact loading of tissue

(Brand 1976), elevated temperature and humidity (Hyman and Artigue 1976)

28

age, nutritional status, general health, activity level (Fisher and Patterson

1983), deformity, posture and postural change, (Hobson DA 1984; Zacharkow

1984) body stature, (Garber and Krouskop 1982) and psychological deficits.

Increased friction and shear, poor nutrition, disease, and pressure aggravate

compromised skin. Even the chemical irritation of frequent washings with

soaps can cause irritation. The adhesiveness of moist skin to bed linens is

estimated to increase the risk of ulceration fivefold. Other causes include poor

nutrition, age, very thin (emaciated), over weight (obese), and suppressed

immune system, radiation therapy, chronic diseases, incontinent, trauma,

swelling & infection.

The roll of few causative factors in pressure ulcer development

(Krouskop 1993) is listed below.

Pressure: Bedsores form where the weight of the person's body

presses the skin against the firm surface of the bed. The pressure that causes

bedsores does not have to be very intense. Pressure of less than 25% the

pressure of a normal mattress can lead to bedsores. Complete muscle necrosis

was demonstrated at 100 mmHg for 6 hours and pressures of 70 mm Hg for

2 hours resulted in pathologic changes within muscle and that lower pressures

of 35 mmHg for 4 hours resulted in no changes(Joan et al 1995).

Temperature: The metabolic heat generated by the body must be

transferred through the bed linen and failure of which leads to increase in

interface temperature between the body and mattress. Elevated body

temperature raises the metabolic activity of tissues by 10% for every one

degree Celsius of temperature increase, concurrently increasing the need for

oxygen and an energy source at the cellulose level. If the patient has impaired

circulation from local pressure and shear, tissues will starve and release

lysozymes, inducing autodigestion of cytoplasma and reducing skin integrity.

29

It has been shown in animal studies that pressure induced tissue injury

accelerates with increasing body temperature (Mahanty and roumer 1980).

Shearing and friction: shearing and friction causes skin to stretch

and blood vessels to kink, which can impair blood circulation in the skin. In a

person confined to bed, shearing and friction can occur when the person is

dragged or slid across the bed sheets. Shear stress occurs when a force is

applied in the plane of the skin surface. Friction occurs when there is

displacement between the skin and the supporting surface.

Moisture: Wetness from perspiration, urine or feces can make the

skin too soft and more likely to be injured by pressure. Moisture from

sweating or incontinence will hydrate the skin, dissolve the molecular

collagen cross links of the dermis, and soften the stratum corneum. Another

result of skin hydration is the rapid increase of the epidermal friction

coefficient, which promotes adhesion of the skin to the support surface and

increase shear, easy sloughing, and ulceration.

2.6.1 Scope and Cost of Skin Breakdown

Research studies show a prevalence of decubitus ulcers in

11 percent of the hospitalized population and in 20 percent of nursing home

residents at any given time (Sanders 1992), For patients in nursing homes, the

prevalence of pressure sores (of Grade 2 or greater) ranges from 7 to

35 percent (Allman et al 1986), resulting in a four-fold increase in mortality

(Norton et al 1975). In Spinal Chord Injured patients, pressure ulcer incidence

is as high as 42 to 85 percent in some centers (Richards 1981). Amputees

using prosthetic limbs are also at risk of breakdown, as a result of the

mechanical forces at the residual limb-socket interface. Over 43,000 new

major amputations are performed per year in the US (Kay and Newman

1975), with 58 percent of them on patients between the ages of 21 and 65

30

years (trauma, cancer, congenital). Thus, there is a significant patient

population of young people with amputation, a group likely to conduct

strenuous activities when using their prosthetic limbs. For those persons with

amputation over 50 years old, vascular causes are the etiology in 89 percent of

the cases (Sanders 1986). Their skin is typically at high risk of breakdown.

2.6.2 Measures of preventing Skin Damage

The principal approach in the past to the challenge of maintaining

healthy skin and avoiding breakdown has been prevention. For example,

patients restricted to bed rest, a subject population at high risk of pressure

ulcer formation, will be turned frequently by the nursing staff to relieve

prolonged pressure. Mattresses designed to cyclically change the distribution

of pressure have been developed. Thus, current prevention programs are

designed to reduce force levels and loading durations below those that cause

breakdown.

Prevention measures also involve the application of interface

surfaces (mattresses, cushions, liners) and frequent pressure reliefs to avoid

sustained pressures in one position. It is interesting to note that skin and body

wall tolerance for sustained pressures typically increases over time (Yarkony

1993).

Skin adaptation and breakdown prevention do exist (Griffiths 1963;

Herceg and Harding 1976; Daniel and Faibisoff 1982). A mobilization

program for an individual with spinal chord injury who has undergone

myocutaneous flap surgery for a pressure sore is an excellent example. (Joan

et al 1995) McInnes (2008) reviewed on Pressure ulcers and the preventive

measures carried out in both institutional and non-institutional settings.

Christine (2004) reviews suggested guidelines for the prevention and

management of pressure ulcers. Prentice et al (2003) published two papers on

31

An Australian model for conducting pressure ulcer prevalence surveys, and

Pressure ulcers: the case for improving prevention and management in

Australian health care settings and analyzed the prevalence of pressure ulcers

in Australia and their causes.

2.6.3 Skin Ulcers formation in Wheel Chair Users

Ulcers in the buttocks area are prevalent among wheelchair users

who have limited mobility, decreased sensation or both. Additionally, many

wheelchair users tend to sit on their cushions for extended periods of time.

Historically, high pressures at the buttocks to cushion interface and extended

sitting times have been identified as the principle contributing factors in

causing skin ulcers. (Vert Mooney et al 1971) As a result, cushion designs

have been focused on decreasing overall and peak pressures during sitting.

New data shows that skin temperature and relative humidity at the

buttocks/cushion interface may be as important as pressure in preventing the

formation of skin ulcers. (Laizzo 2004). Higher temperatures produced both

cutaneous and deep tissue damage.

The study carried out by Vert Mooney et al (1971) for comparison

of pressure distribution qualities in seat cushions confirms that presently

available cushions failed to demonstrate that any of the cushions tested were

safe for prolonged sitting by paralyzed patients. Guldemet Basal and Sevcan

Ilgaz (2009) developed a functional fabric for pressure ulcer prevention. For

this purpose, face-to-face velour weaving technique was utilized to produce a

spacer fabric from the different combinations of engineered polyester,

polypropylene, cotton and viscose fibers. Channeled polyester, cotton and

polypropylene were determined as the most promising fiber types for the final

product.

32

Xiaohua Ye et al (2008), developed a warp knitted spacer fabrics

for cushion application using PES multi filaments for surface layers and PES

mono filaments as the spacer yarn. The results showed that the warp knitted

spacer fabrics have better pressure relief properties, higher air permeability,

and lower heat resistance than PU foam, and thus could be used to substitute

PU foam, especially in the case where the comfort and recycle are highly

required.

2.7 PRESSURE RELIEVING MATTRESS OR SUPPORT

SURFACES

Many types of pressure relieving support surfaces are available

such as water beds, Sheepskins and Australian Medical sheepskin made of

high quality wool etc. An attempt to reduce pressure on a bony prominence is

based on two concepts. Either area in contact with the support surface can be

increased, or contact can be temporarily removed or shifted to other areas. In

the first case immersion and envelopment are the phenomena that produce

reduction in pressure at bony prominence. In the second, the change in areas

of contact over time is the therapeutic consideration. To be effective, support

surfaces must mould to the body to maximize contact, and then redistribute

the patient’s weight as uniformly as possible. They are designed to work on

the principle of Pascal’s law, which states that the weight of the body floating

on a fluid system is evenly distributed over the entire surface as pressure is

increasingly distributed over more body surface area the intensity of pressure

decreases over all body areas. Support surfaces also use the principle of the

deformation, meaning they must be capable of deforming enough to permit

prominent areas of the body to sink in to the support. The surfaces also must

be able to transmit pressure forces from one body area to another. (Maklebust

et al 1986; Elizabeth McInnes 2008; David et al 2000).

33

The degree of head elevation can affect the clinical effectiveness of

a support surface. When the head of the bed is elevated, pressure is shifted to

the sacral and ischial areas of the body. The patient may bottom out if the

seating area of the support surface flattens and looses volume. If the

bottoming out occurs, the support surface no longer provides therapeutic

benefit.

2.7.1 Types of Support Surfaces

The centre for Medicare services divides the support surfaces into

the following three groups

Group I: These surfaces do not require electricity and include air,

foam, gel and water mattresses and overlays (Damien J Jolley et al 2004).

These surfaces are intended for pressure ulcer prevention. Foam surfaces

come in various densities, depth and construction. To reduce pressure, foam

must be of high quality and at least four inches (10 cm) thick. Static air

overlays have multiple chambers that allow air exchange between

compartments when a person lies on the surface. The air exchange between

cells allows the surface to deform and permits the body to sink into the

surface, reducing pressure on bony prominences. Adequate air volume is

maintained with inflation or re-inflation devices. Gel mattress overlay have a

tissue life composition that reduces shear and support weight without

bottoming out. They are self sealing if punctured and can be reused. However,

gel does not deform easily and may become stiff overtime.

Group II: These surfaces are classified into dynamic powered

surfaces and advanced non powered surfaces. Dynamic air overlays are used

with mechanical pump to alternate inflation and deflation of chambers and

constantly change pressure points. Air chambers must have enough depth and

34

be close enough together to lift the body during alternate cycles (McLeod

1997; Grindley and Acres 1996).

Group III: These consists of air fluidized beds, a high air loss

system with ceramic silicon beads that become fluidized as warm pressurized

air is forced up through the beads. This gives the beads the characteristics of a

fluid, allowing the patient’s body to float on the surface and minimize the

pressure, shear and moisture (Whittemore 1998; Hargest 1969).

2.7.1.1 Cover Materials Used in Support Surfaces

Cover material is an important element of support surface. Pressure

ulcers not only result from loads normal to the body surface, but also from

loading in the plane of the skin and the micro climate at the interface between

the subject and mattress. The choice of cover material significantly influences

these factors. Normal forces, primarily, weight is taken by both deformation

of the support structure, e.g., foam top surface and the elastic stretching of the

cover material. If the cover material is relatively in elastic, then more of the

load is taken by the cover in the plane of the skin. And this tends to have

shear effect on the skin. This form of loading can significantly contribute to

the development of pressure ulcers (Shaw 1997).

Damien et al (2004), estimated the effectiveness of a new high-

performance Australian medical sheepskin (meeting Australian Standard

4480.1-1998) in preventing pressure ulcers in a general hospital population at

low to moderate risk of pressure ulcers and found that the Australian Medical

Sheepskin is effective in reducing the incidence of pressure ulcers in general

hospital inpatients at low to moderate risk. The Canadian Association of

Wound Care puts forward 12 recommendations for best practices in the

prevention and treatment of pressure ulcers that focus on an interdisciplinary

patient-centered approach.

35

Elizabeth et al (2002) interpreted on to what extent the pressure-

relieving cushions, beds, mattress overlays and mattress replacements reduce

the incidence of pressure ulcers compared with standard support surfaces and

how effective are different pressure-relieving surfaces in preventing pressure

ulcers, compared to one another and suggested that for people at high risk of

pressure ulcer, higher specification foam mattresses rather than standard

hospital foam mattresses should be used. Ken Dolynchuk et al (1992) reported

much higher pressure sore prevalence rates in the intensive care unit (ICU)

than the general hospital population.

2.7.1.2 Measurement of Interface Pressure, Temperature and Shear

Analysis of support surfaces has been carried out by various

authors (Rithalia 2005; Pring and Milman 1998, Trandel and Lewis 1975).

Steven et al (2002) studied the support surface interface pressure and

analyzed the role of implicated factors such as pressure, temperature,

moisture, shear and friction in the breakdown of the body wall tissues that

leads to pressure ulcer.

Hugo Partsch (2005) carried out sub-bandage pressure difference

measurement between active standing and lying in vivo using Kikuhime

pressure sensor. Eric Van den Kerckhove et al (2006) investigated the

reproducibility of repeated measurements with the Kikuhime pressure sensor

under two different types of pressure garments used in the treatment and

prevention of scars after burns. Narendar Reddy et al (1984) reported about

two types of semiconductors/transducers and two types of pneumatic

transducers used in clinical measurement of skin-cushion interface pressures.

Brian et al (2006) used a device which is composed of 80 transducers

arranged in an 8 × 10 array. Vert Mooney (2008), considered the effects of

seated posture and body orientation on the pressure-distribution and surface

shear (tangential) forces acting at the body seat interface.

36

2.8 HUMAN COMFORT PROPERTIES

The term comfort is defined as "the absence of unpleasantness or

discomfort". There is general agreement that the movement of heat and water

vapour through a garment is probably the most important factor in clothing

comfort. (Slater 1977). The clothing comfort can be divided into three groups,

psychological, tactile and thermal comfort. Psychological comfort is mainly

related to the aesthetic appeal. Tactile comfort has a relationship with fabric

surface and mechanical properties.

Thermal comfort is related to the ability of fabric to maintain the

temperature of skin through transfer of heat and perspiration generated with

the human body (Hatch 1983). Two aspects of wear comfort of clothing are,

(i) thermo physiological wear comfort which concerns the heat and moisture

transport properties of clothing and the way that clothing helps to maintain the

heat balance of the body during various level of activity, and (ii) skin

sensational wear comfort which is based on the mechanical contact of the

fabric with the skin, its softness and pliability in movement and its lack of

prickle, irritation and cling when damp.

2.8.1 Clothing Comfort

Today comfort is considered as fundamental property when a

textile product is valued. The comfort characteristics of fabrics mainly depend

on the structure, types of raw materials used, weight, moisture absorption,

heat transmission and skin perception. Basically, clothing comfort can be

categorized under two broad components, viz, sensorial comfort and non-

sensorial comfort.

Sensorial comfort is a perception of clothing comfort which is

sensory responses of nerve endings to external stimuli including thermal,

37

pressure, pain, etc producing neuro-physiological impulses which are sent to

the brain. Non-sensorial comfort basically deals with physical processes

which generate the stimuli like heat transfer by conduction, convection, and

radiation, moisture transfer by diffusion, sorption, wicking, and evapouration.

The heat and moisture transfer behavior of clothing has been studied

intensively by Fourt and Hollies (1970); Hollies and Goldman (1977).

2.8.2 Factors affecting Comfort

A number of properties of fibers, yarns, fabrics and garments are

significantly related to comfort and must be taken into account in producing

suitable apparel items. However, suitable fabrics, from the comfort point of

view, must be developed by textile technologists by proper selection of fiber

content, yarn and fabric construction techniques, and finishing treatments as

they influence physiological comfort level through thermal retention or

transmission etc. (Kothari 2004).

For getting thermo physiological comfort the clothing should

have suitable thermal conducting properties as well as sufficient permeability

to water vapour and / or sufficient level of ventilation (Jeffries 1990). The

textile structures can be developed to enhance the clothing comfort by

focusing principally on the thermal and mechanical properties (Prabhakar

Bhat and Bhonde 2006).There is general agreement that the movement of heat

and water vapour through clothing are probably the most important factors in

clothing comfort.

Rees (1972) concludes that the overall comfort of an apparel fabric

depends on the proper combination of values for pore size, air permeability,

water vapour permeability, thermal insulation, surface contact with skin and

several other fabric properties. Thermal wear comfort is mainly related to the

sensations involving temperature and moisture. (Prabhakar Bhat and Bhonde

38

2006). It has been recognized for a long time that it is difficult to describe

comfort positively, but discomfort can be easily described in such terms as

prickle, itch, hot and cold. Therefore, a widely accepted definition for comfort

is ‘freedom from pain and from discomfort as a neutral state’ (Hatch 1983).

A core body temperature of approximately 37° C is required by an

individual for his well being. Hence, the body temperature is the most critical

factor in deciding comfort. Heat is gained by the body from the sun or

intermediate source of energy, by internal metabolism, physical exercise or

activity, or by involuntary contractions of skeletal muscles in shivering

(Prabhakar Bhat and Bhonde 2006). Li and Holcombe (1998) developed a

mathematical model to describe the dynamic heat and moisture transport

behavior of clothing and its interaction with the human thermoregulation

system under transient wear conditions.

Sheela Raj and Sreenivasan (2009) carried out an in depth study to

understand fabric handle and wear comfort in relation to fiber, yarn and fabric

structural parameter and presented a comprehensive grading index

incorporating the air permeability, moisture and thermal transport of the

fabric. Combining different parameters responsible for thermo physiological

and tactile comforts by conferring different weighting factors, a pair of

indices of comfort have been defined viz. Thermo Physiological Comfort

Index (TPCI) and Total Wear Comfort Index (TWCI). These indices are

expected to provide a practical way for assessing overall wear comfort.

Roger L Barker (2002) discussed the evolution of objective

measurement of textile hand and comfort from Pierce through modern

methodology and approaches. Special emphasis is given to the contribution of

the Kawabata Evaluation System (KES) towards advancing the state of

objective measurement.

39

2.9 AIR PERMEABILITY OF FABRICS

The concept of ‘air permeability’ is widely used in the textile

industry to interpret the intrinsic characteristics of fabric. Air permeability is

significantly influenced by a fabric’s material and structural properties, such

as shape and value of the pores of the fabric and yarn, which in turn are

dependent on the structural parameters of the fabric, such as fabric weave, the

raw material of the yarns, the set of yarns and others. In addition, as the

results of McCullouh et al’s (2003) research show, fabrics with hydrophilic

components can change their air permeability properties under different

humidity conditions.

The application of permeability tests to textile products as well as

determining permeability and porosity has long been subjects of interests

(Berkalp 2006). The so called clean room textiles protect an atmosphere

against particles emitted from human body and vice versa, also provide

protection against particles evolved from clothing (Militky et al 1999). Air

permeability being biophysical feature of textiles determines the ability of

fabric to carry out gaseous substances, significantly influences thermal

comfort of the human body, secure the support of the proper body

temperature (Frydrych et al 2003). Air permeability (Stankovic 2005) along

with other comfort properties of hemp textiles for hospital uses was

investigated.

It was determined that nonwoven fabric weight is more important

parameter for air permeability if to compare with thickness, fiber diameter,

and density (Kothari and Newton 1974). Daukantien and Skarulskien

(2005) stated that the air permeability increased with the increase of porosity

of the fabric or decrease of its thickness. The use of spacer fabrics with air

supported layer for excellent air permeability and thermoregulation was

interesting textile-technological solution for medical applications (Heide et al

40

2005). Renata Baltakyt and Salvinija Petrulyt (2008) indicated that air

permeability was a function of knitted fabric thickness and surface porosity.

2.9.1 Air Transfer

Moisture movement and air movement through a textile fabric are

sometimes considered together under the topic of fluid flow. Air flow is

similar to diffusion of moisture vapour through a textile fabric. Air flow

through a fabric occurs when the air pressure is different on the two sides of

the fabric. It is closely related to convective heat transfer and to moisture

transport via diffusion. As fabric interstices increase in number and size, air

permeability increases.

Alibert (1972) investigates the air permeability of fabrics in relation

to structure and establishes the general laws governing the filtration of air

through his test samples. Kretsschmer (1973) introduces a ‘structure index’ to

simplify the use of his earlier permeability equation for textile materials in a

flat, single-layer configuration and gives examples to demonstrate the use of

the modified equation with his index included.

Renata Baltakyt (2008) carried out experimental investigations for

determining air permeability dependence on weave. The weft setting has a

very great influence on fabric air permeability but in the case of the same

firmness factor this influence is negligible and Q is constant for all weaves.

So, it is possible to reach constant air permeability for fabrics with different

settings by choosing a necessary weave for each setting.

2.9.2 Porosity and Cover Factor

The pores, or interstices within a fabric, are also influential factors

in moisture and air transfer. Volume porosity of a fabric is defined as the ratio

41

of the volume (fabric plus air or void) expressed as a percentage (Adanur

2006, Berkalp 2006). Apparent porosity of a fabric is the air porosity which

has the same meaning as air permeability (Booth 1968; and Benltoufa et al

2007). Skinkle (1949) defines porosity as the ratio of air space to the total

volume of the fabric expressed as a percentage. The type of finish given to a

fabric can have a considerable effect on the permeability even though the

porosity may remain the same. A roughly identical method of calculation was

presented by Hsieh (1995).

A term that is closely related to porosity is cover factor. Cover

factor combines fabric count and yarn size to give an indication of fabric

structural properties that contribute to thermal comfort. However, the term

cover factor does not take into consideration of other structural factors such as

yarn type or yarn twist (Billie and Helen 1999). Knitted structures generally

have more porosity that can retain more air and therefore provide more

warmth (Richard 2005).

2.10 Moisture Management in Fabrics

An important property associated with fabric comfort is moisture

management. Moisture management deals with the ability of a fabric to

transmit moisture away from the body. The mechanism of movement can be

either by wicking or by passage of water vapour through the fabric.

Breathability is a term often used to describe this property. The term

‘moisture management’ always refers to the transport of moisture vapours and

liquid away from the body.

The most important factors affecting moisture transfer are fiber

type, cloth construction, weight or thickness of the material, presence of

chemical treatments. In the case of cotton, the hydrophilicity of the fiber itself

wicks away the moisture, which passes through the openings in the fibers or

42

yarns, where distinctly accelerated evapouration takes place, resulting in

comfort for the wearer. On the other hand, clothing made up of synthetics

such as polyester or polyamide, etc, is unable to wick away the

moisture/perspiration due to their inherent hydrophobic nature, so the fabric

tends to stick to the skin, which impairs the comfort. To maximize comfort

and to feel cool in synthetic garments, the fabric must allow liquid to wick on

to the surface, spread away and evapourate quickly. The ability of soil release

finishes to improve water wicking also serves to improve moisture

management and is often promoted as finishes for improved comfort (Behery

2005).

The dissipation of perspiration and body heat is also influenced by

the porosity and wicking characteristics of apparel fabrics. Pore size affects

air resistance and water repellency. Usually, the larger the pore size, the lower

the resistance to air flow and the lower the water repellency. Wicking

efficiency is affected by fabric geometry of fibers in yarn and fabric. Usually,

hairiness and random fiber arrangement lead to slow rates of wicking and

surface wetting (Rees 1972).

2.10.1 Liquid Moisture Transfer: Capillarity, Wetting and Wicking

The primary cooling mechanism of the human body is evapouration

of perspiration, with water vapour carrying heat away from the body as it

evapourates out of the skin’s pores. In the garment-skin microclimate

environment, the absorption of sweat by garment and its transportation

through and across the fabric where it is evapourated are claimed by some

researchers to aid clothing comfort perception. The manner of the moisture

absorbed at the fabric inner surface, transported between the two sides and

evapourated at the outer surface significantly influences the wearer’s comfort

sensation, as the moisture is a much better heat conductor than air.

43

Capillarity can be defined as the macroscopic motion of a liquid

under the influence of its own surface and interfacial forces in narrow tubes,

cracks, and voids. The surface tension is based on the intermolecular forces of

cohesion and adhesion. When the forces of adhesion between the liquid and

the tube wall are greater than the forces of cohesion between the molecules of

the liquid, then capillary motion occurs. Flow ceases when the pressure

difference becomes zero.

Wicking in fabrics may occur in a range of conditions and

situations. Ghali et al (1994) believes that “to define the range of conditions,

researchers should attempt to distinguish between two phenomena, namely

wettability and wickability, related to liquid transport in fabrics”. According

to Harnet and Mehta (1984) “wickability is the ability to sustain capillary

flow” whereas wettability describes the initial behaviour of a fabric, yarn or

fiber when brought in contact with water.

Wetting is the displacement of solid - air interface with a solid-

liquid interface. Spontaneous wetting is the migration of a liquid over a solid

surface towards thermodynamic equilibrium. Wetting and wicking are not

different processes. Wetting is a prerequisite for wicking. A liquid that does

not wet fibers cannot wick into a fabric. When the fibers in assembly are

wetted by a liquid, the resulting capillary forces drive the liquid into the

capillaries created by the spaces between fibers in wicking process.

In general wicking takes place when a liquid travels along the

surface of the fiber but is not absorbed into the fiber. This type of flow is

governed by the properties of the liquid - solid surface interactions, and

geometric configurations of the pores structure. The pores structure of a

fibrous medium is complicated and difficult to quantify. Moreover with

hydrophilic fibers, the swelling of fibers influences the liquid flow.

44

Erik Kissa (1996) reviewed the fundamentals of wetting and

wicking. Wetting is the displacement of a fiber-air interface with a fiber-

liquid interface. Wicking is the spontaneous flow of a liquid in a porous

substrate, driven by capillary forces. You-Lo-Hsieh (1995), discussed wetting

and capillary theories and applications of these principles to the analysis of

liquid wetting and transport in capillaries and fibrous materials. Hollies et al

(1957) made a basic study of the mechanism of water transport in yarns and

found that the movement of water along fabrics is shown to depend on the

laws of capillary action and the water is carried mainly in the capillaries

formed by the fibers in the individual yarns. The speed of travel of water in

these capillaries is readily reduced by the presence of randomly arranged

fibers in the yarn, and it is this factor rather than the nature of the fiber

material which accounts for the wide range of water transport properties

found in blended fabrics.

2.10.2 Moisture Transport in Different Fibers

Cotton: Cotton garments provide a good combination of softness

and comfort. However, cotton is not recommended for use in base layer

clothing because of its tendency to absorb and retain moisture. When wet,

cotton garments cling to the skin and cause discomfort. The slow-to-dry and

cold-when-wet characteristics of cotton make this material unsuitable in

conditions in which there are high levels of moisture-either perspiration or

precipitation and where the ambient temperature is low.

Cellulosic fibers: Cellulosic fabrics absorb water into the fiber

structure and become heavy. This leads to stretching of the fabric, sticking to

the skin and when activity ceases the fabric may feel cold against the skin.

Higher levels of moisture absorbed in the fabric mean longer drying times.

However, cellulosic fabrics are generally perceived to be more comfortable

than synthetic fabrics when worn for normal day-to-day activities.

45

Viscose Rayon: The outer layer of knitted hydrophilic portion of

the twin layer sportswear can be of viscose rayon, which absorbs 2-3 times

more moisture than cotton. The wicking behavior improves by incorporation

of some hydrophobic finishes. However, if it were blended with polyester, the

absorbent capacity could be controlled to an acceptable level.

Comfort properties of polyester microfiber fabric are more in terms

of wicking when compared with polyester micro/cotton blends and pure

polyester non- micro fiber fabrics. Better wicking is found in samples having

greater proportion of polypropylene and they dry fast. Maximum water

vapour permeability and air permeability is seen on fabrics having

polypropylene on both faces of fabrics.

Toray developed a fabric made from polyester filament yarn which

has grooves which help the fabric absorb sweat quickly and disperse it along

the surface. Polyester coolmax has been claimed to increase wearer comfort

through rapid removal of perspiration by capillary. Also it has good wicking

properties and no absorbency.

Using a series of manikin and human subject experiments, Crow

and Osczevski (1998) examined the interaction between water and a range of

fabrics to investigate claims that synthetic fibers such as polypropylene do not

pick up moisture. They found that all fiber types, when made into fabrics,

pick up water, with a strong correlation between a fabric’s thickness and the

amount of water it picks up, freely expressed in absolute terms rather than as

a percentage of its mass. The amount of water wicked from one layer to

another depends on pore sizes and their corresponding volumes. While most

fabrics, both natural and synthetic have the ability to wick moisture from the

skin, not all of these are fast drying and air permeable – two factors, which

have a direct influence on cooling and perceived comfort. High tech synthetic

46

fabrics are light weight, are capable of transporting moisture efficiently and

dry relatively quickly.

It is generally agreed that fabrics with moisture wicking properties

can regulate body temperature, improve muscle performance and delay

exhaustion. While natural fibers such as cotton may be suitable for low levels

of activity, synthetic fibers made of nylon or polyester are better suited for

high levels of activity. They absorb much less water than cotton, but can still

wick moisture rapidly through the fabric.

Manas Sarkar et al (2009) developed a textile fabric simulating a

plant structure with superb liquid water transport properties. He developed

some novel weave structures, which emulate the branching structures of the

plants and create a continuous water transport passing from the bottom layer

to the top layer by interchanging the yarns from the bottom layer to the top

layer. The connecting weave has an influence on capillary rise. Fabrics

without connecting weave have the smallest wicking coefficient. When the

density of connecting crossing is increased, wicking coefficient increases.

Shinjung Yoo and Roger Barker (2005) found that Heat-resistant

fabric incorporating structural features that minimize skin contact, while also

providing liquid absorption capacity, are predicted to show enhanced comfort

performance. Blending of hydrophilic fibers and wicking finishes, however,

do not necessarily improve the comfort perceptions in the tested scenarios.

Junyan Hu et al (2005) devised a new method and instrument called

the moisture management tester to evaluate textile moisture management

properties which could be used to quantitatively measure liquid moisture

transfer of a fabric in multi directions. Hasan et al (2008) elucidated that

topographical characteristics of the fabrics strongly depend on their

47

construction parameters such as the type and fineness of filaments, yarn

fineness, yarn density, and consequently, the type of weave.

Brojeswari Das et al (2008) states that the moisture transmission

behavior of a clothing assembly plays a very important role in influencing its

efficiency with respect to thermo physiological body comfort. Part I of their

paper deals with the processes involved in moisture transmission and the

factors at play. Part II is concerned with selecting the measurement

techniques which are of great importance in determining fabric factors that

influence comfort. Brojeswari Das et al (2008) carried out an experimental

study on the effect of fiber cross sectional shape and fiber diameter on

moisture transmission properties of the fabric. With the change in shape

factor, fiber diameter and increase in fiber specific surface area, wicking rate

through fabric increases, whereas water vapour permeability of the fabric

reduces. Jakub Wiener and Petra Dejlová (2003), proposed a model of

wicking based on the simplified description of the thread structure

considering the parameters like fineness of fibers, and number of fibers at the

cross-section in the bundle and the filling.

Petrulyte and Baltakyte (2009) carried out investigations in liquid

sorption and transport for three different variants of terry woven structures.

The study carried out by Kothari and Kausik Bal (2006) established a new

approach to determine the blend proportion in polyester-viscose blends in

woven fabrics. A new rapid conditioning method by drying the samples with

infra-red and then conditioning them for a shorter duration was used to

compare the moisture content values of different blend levels. In all cases, the

moisture content values showed linear relationships with the proportion of

viscose in the fabrics.

Adler and Walsh (1984) developed a technique to study moisture

transport and made an effort to determine the mechanism by which moisture

48

is transported between fabrics under transient conditions at low moisture

contents. Wicking did not begin until the moisture content was high, more

than 30% above regain for the woven samples. Crow and Osczevski (1998),

examined the interaction between water and a range of fiber types and found

that, when made into fabrics, all fabrics pick up water, with a strong

correlation between a fabric's thickness and the amount of water it picks up

freely expressed in absolute terms rather than percent of its mass.

Scheurell et al (1985) tested the Dynamic surface wetness of fabrics

and found a correlation with skin contact comfort in wear for a variety of

fabric types and suggested that mobility of thin films of condensed moisture is

an important element of wearing comfort. Sukigara et al (1997) made an

investigation into the sensation of wetness and dampness by both subjective

and objective measurements.

2.11 DRYING TIME OF FABRICS

Drying time is the time required to evapourate water from a wet

fabric, which depends on the amount of water the fabric picks up, not the fiber

type. Raechel M. Laing et al (2007), developed a laboratory method for

wetting specimens and determining drying time and defined end-point ‘dry’,

applicable to a wide variety of apparel fabrics and made an effort to determine

the drying time of a range of apparel fabrics simulating both during and after

wear conditions using a simulated on-skin drying method.

Prahsarn et al (2005) used a test method that measures

microclimate drying time to compare the ability of different knit materials to

dissipate moisture vapour from a saturated clothing environment to the

ambient atmosphere. The thickness of fabric and the pore characteristics of

the fabric determine the drying rate.

49

Shinjung Yoo and Roger (2005) analyzed the moisture

management properties of heat resistant workwear fabrics and the effects of

hydrophilic finishes and hygroscopic fiber blends using a demand wettability

test and found that the cotton fabric shows the highest percentage of

remaining water after the evapouration test, a garment made of this fabric may

generate a clammy feeling despite its superior absorbent capacity and rate of

absorption.

2.12 WATER VAPOUR TRANSMISSION BEHAVIOR OF FABRICS

The water vapour permeability of a given material plays an

important role in evaluating the physiological wearing comfort of clothing

systems or in determining the performance characteristics of textile materials

used in technical applications. Water vapour condensation occurs when air

space size is very small, say 5mm. In such cases, water vapour permeability

has more effect on heat and mass transfer than air permeability. Vapour

permeable fabrics have a relatively lower amount of condensation heat flux

than vapour-impermeable fabrics. In other words, such a fabric can keep the

temperature and the amount of absolute water vapour concentration in the

microclimate, low. High values of these cause sensations of humidity and heat

stress in the wearer.

Wang and Yasuda (1991) developed a new experimental apparatus

for studying the transport of water vapour and liquid water through textile

fabrics during the transient period and also proposed two experimental

methods in their paper: the water vapour method and the sweat method.

Indushekar et al (2005) evaluated the water vapour transmission of a wide

range of base fabrics used in extreme cold weather protective clothing using

the conventional Dish and MDSC techniques.

50

Akshaya Jena and Krishna Gupta (2002) augmented a novel

technique for precise, accurate and fast determination of vapour transmission

rate through textiles. Vapour at a constant pressure is maintained on one side

of the textile and the increase in pressure of the vapour on the other side is

measured. The vapour permeability of the textiles is almost five orders of

magnitude lower than their air permeability.

Fourt et al (1957) measured the water vapour transmission of fibers

independent of any air space surrounding the fibers, by using sections cut

from embodiments of the fibers in polyacrylic resins. The rate of water

vapour transmission, measured as the diffusion constant, is larger for cotton

than for Rayon, Wool, Nylon and Dacron. The respective diffusion constants

are on the order of 134, 56, 39 and 8 respectively.

Scheurell et al (1985) found a correlation between dynamic surface

wetness of fabrics and skin contact comfort in wear for a variety of fabric

types, suggesting that mobility of thin films of condensed moisture is an

important element of wearing comfort. Although much study has been

dedicated to heat and water vapour transport through fabrics, there remains a

great lack of understanding on water vapour transport through fabrics under

subzero climate, particularly in terms of the effect of condensation on water

vapour transport. Xiaohong Zhou et al (2006) reported a novel apparatus,

used to measure the moisture transport through fabrics under a conventional

(20°C) and subzero (-20°C) climatic condition.

Raechel M. Laing et al (2007) developed a model of heat and water

transfer through layered fabrics by considering few special properties such as

hydrophobic, hydrophilic treatment and surface modification of textiles.

Chuang Li et al (2005) pioneered a new type of fabric micro environment

testing instrument to measure the dynamic changes of vapour pressure,

temperature, and heat flux under simulated human body sweating conditions.

51

2.13 THERMAL COMFORT PROPERTIES OF FABRICS

Thermal properties of textile materials especially thermal

conductivity have always been the major concern when the comfort properties

of clothing are concerned. Clothing has a large part to play in the maintenance

of heat balance, as it modifies the heat loss from the skin surface and at the

same time has the secondary effect of altering the moisture loss from the skin.

The properties of clothing materials critically influence the comfort and

performance of the wearer. Clothing is not just a passive cover for the skin. It

interacts with and modifies the heat regulating function of the skin and its

effects are modified by the environment. Thermal conductivity and thermal

insulation or thermal resistances and thermal absorptivity are few measures of

thermal comfort.

2.13.1 Thermal Conductivity

The ability of a fabric to conduct heat through it is of critical

importance to its thermal comfort. Thermal conductivity is a property of

materials used to describe the thermal transfer behavior of the heat flow

through a fabric due to a combination of conduction and radiation where the

convection within a fabric is negligible. The conduction loss can be

determined by the thickness of the fabric and its thermal conductivity. As

defined by ASTM, thermal conductivity is the time rate of unidirectional heat

transfer per unit distance, per unit difference of temperature of the planes.

Another relevant concept is thermal conductance (C), also defined by ASTM

as the time rate of heat flux through a unit area of a body induced by unit

temperature difference between the body surfaces. Normally thermal

conductivity can be expressed in equation 2.1

L/T

A/Qk (2.1)

52

Where Q is the amount of heat passing through a cross-section A,

causing a temperature difference T , over a distance of L . Q/A is therefore

the heat flux which is causing the thermal gradient. The measurement of

thermal conductivity, therefore, always involves the measurement of the heat

flux and temperature difference. The difficulty of the measurement is always

associated with the heat flux measurement. Guarded hot plate, as described in

ISO 8302, is a widely used and versatile method for measuring the thermal

conductivity of textiles. Another widely used simple method is directly using

a heat flow meter as described in ASTM C 518.

2.13.2 Thermal Insulation

Thermal insulation property of the fabric refers to the ability to

resist the transmission of heat by all modes. It can also be defined as

effectiveness of a fabric in maintaining the normal temperature of the body

under equilibrium conditions. The most important thermal property in most of

the apparels is the insulation against heat flow, which is measured by thermal

resistance. It is defined as the ratio between temperature difference between

the two faces and heat flux.

The thermal resistance, R and thermal conductivity, K are related as

follows,

R = d /K (2.2)

Where d is the thickness of the material. Since K is roughly

constant for different fabrics, hence thermal resistance is approximately

proportional to fabric thickness. Thermal insulation value is higher in case of

silk fabric compared to cotton fabric. It is due to openness of knit structure of

silk fabric. Silk fiber contains higher thermal insulation value as it has lower

thermal conductivity (50 mw/m/k) than cotton (71 mw/m/k).

53

In studying the thermal insulation properties of garments during

wear, it is reported that thermal resistance to transfer of heat from the body to

the surrounding air is the sum of three parameters: (i) the thermal resistance to

transfer heat from the surface of the material, (ii) the thermal resistance of the

clothing material and (iii) the thermal resistance of the air interlayer. It is

obvious that heat transfer through a fabric is a complex phenomenon affected

by many factors. The three major factors in normal fabrics appear to be

thickness, enclosed still air and external air movement. Out of which, the

entrapped air is the most significant factor in determining thermal insulation.

There are "microlayers" (those between contacting surfaces of the materials)

and "macrolayers" (between non-contacting surfaces) of air enclosed within

an assembly and an increase of either of these can increase thermal insulation.

However, the characteristics of fiber, yarns, fabrics and garment assemblies

have also a major contribution towards thermal comfort.

Most textile fibers are poor conductors of heat, but air conducts

even less heat. If air is confined in small spaces, then convection is also

minimized, and the air is ‘dead’. The higher the volume of dead air within a

textile structure, the lower the thermal transmittance, therefore, the better the

insulation value of the textile material. (Billie and Helen 1999).

2.13.3 Mechanism of Heat Transfer

To understand the thermal properties of the textile system, it is

necessary to assess the contributions of the various heat-transfer mechanisms

that may be operative. These mechanisms are conduction, convection and

thermal radiation for dry heat transfer (Gagge et al 1941; Rohsenow 1973).

2.13.3.1 Conduction

Fibers and air intermingle together in any textile yarns and fabrics

hence the fabrics are neither homogeneous nor isotropic. However, with the

54

preposition that the average heat-transfer properties of fabrics are to be

measured and calculated through the theoretical and practical work, it is

reasonable to assume that a fabric is a homogeneous and isotropic material in

heat transfer. In addition, since thickness of a fabric is substantially smaller

than the fabric width and length in normal clothing situations, it is also

feasible to consider the heat transfer through a fabric is a one-dimensional

problem. Under such assumptions, the transient heat-transfer process through

the insulating material is described as in Equation (2.3), (Yang and Tao

1999).

2

2

.x

T

cpt

T(2.3)

Where, T temperature (°K);

t time (s);

conductivity (W m-1

K-1

);

mass density (kg m-1

);

c specific heat (W S kg-1

K-1

); and

x direction of heat transfer.

2.13.3.2 Convection

As one of the basic heat-transfer mechanisms, convection involves

the transport of energy by means of the motion of the heat-transfer medium,

in this case the air surrounding the human body. When cold air moves past a

warm body, it sweeps away warm air adjacent to the body and replaces it with

cold air. It has been found that there is no convection inside clothing

insulation even with a very low density (Peirce and Rees 1946). In the FE

analysis, the convective heat transfer will be set as a boundary condition. The

heat flux due to convection can be expressed as follows (Incropera and

DeWitt 2002):

55

)( xr TThq (2.4)

Where, q heat flux (W m-2

);

h film coefficient (W m-2

K-1

);

T out surface temperature of the fabric (°K); and

T temperature of the ambient atmosphere (°K).

2.13.3.3 Radiation

The heat loss carried out by radiation from a clad human body to

the environment is a situation where the clad human body as the heat source is

enveloped by the environment. In this case, the heat flux by radiation at the

outer surface of the textile assembly is governed by the following equations

(2.5) and (2.6)( Incropera and DeWitt, 2002).

)TT(q 4

x

4

r(2.5)

)TT()TT(hn

y 4

x

4

rxr

(2.6)

Where,

Stefan-Boltzmann constant, which is 5.6703×10-8

W m-2

K-4

; and

emissivity of the surface.

2.13.4 Heat transfer through Textiles

Heat transfer through a textile assembly or a fabric system is a

complex process, involving conduction, radiation and convection. The

combined heat transfer across the fabric system, consisting of fabric and air

layers, is not simply the sum of what each mechanism would do in the

absence of the others. The three heat transfer mechanisms work together to

determine the characteristics of the overall heat transfer process.

56

Heat transfer refers to the transfer of heat energy from one

environment to another. Heat transfer occurs whenever a temperature

difference ( T) exists between the two environments; heat moves from the

warmer surface or area to the cooler surface or area. Heat transfer will

continue until the two areas attain same temperature (at equilibrium). The rate

at which heat is transferred depends on T as well as any resistance imposed

between the two environments. For people, this means that if the ambient

temperature is lower than the body temperature (37° C), heat will flow from

the body to the surrounding area. If the ambient temperature is higher than the

body, heat will flow the other way and the body will become warmer.

Clothing can provide resistance to heat transfer in either direction by serving

as insulation between the two environments (Billie and Helen 1999).

For clothing textiles, heat transfer is a complicated transient

process. Generated form the body, heat transfers through the air gap between

skin and fabric, then through the fabric system, to the outer surface of the

fabric system. During this process, conduction, convection and radiation are

all involved, may be to different extent, in determining the total heat loss.

Peirce and Rees (1946) pointed out that at the outer surface of the

clothing exposed to the air; heat is lost by means of both convection and

radiation. Farnworth (1983) presented a theoretical treatment of heat transfer

through a bed of fibers considering conduction and radiation and reported that

no detectable convective heat transfer took place inside the fiber bed. In a

more recent study, Mohammade et al (2003) presented a theoretical equation

of the combined thermal conductive, convective, and radioactive heat flow

through heterogeneous multi-layer fibrous materials.

Dul’nev and Muratova (1968) discussed heat transfer processes in

fibrous materials and derived formulae from which effective thermal

conductivity can be calculated from thermal, geometrical, and volumetric

57

parameters of the components. Mitu and Potoran (1971) used a formula

derived by earlier workers for calculating the thermal resistance of clothing

to determine a series of values of this property that represent acceptable

comfort limits for the human body when engaged in lying, sitting, walking,

running and other such activities.

As long as the air within a fabric or fabric assembly is so called

‘dead’ air, it provides good resistance to heat transfer. However, as the

volume of air space increases, the likelihood of air movement, or convection,

increases. When convection occurs, it is usually the dominant mode of heat

transfer, overpowering any effects of reduced conduction of heat (Billie and

Helen 1999). Fibers have a high surface to volume ratio; thus there are many

small spaces for dead air within a fibrous structure. In those spaces, there is

little thermal transmittance because air is a very poor conductor of heat; and

there is little radiation because although air is transparent to radiation, fibers

are not (Billie and Helan 1999).

Pile or napped constructions are often good for cold weather

because the yarns or fibers perpendicular to the surface provide numerous

spaces for dead air. This effect is maximized when such fabrics are worn with

the napped or pile surface next to the body, or when they are covered with

another layer. Otherwise, the protruding fibers in the nap structure may

conduct heat away from the body (Billie and Helen 1999).

Fabric construction also influences thermal insulation. Knitted

fabrics generally have a soft hand and higher heat-retaining properties

compared with that of woven fabrics of a specific thickness or weight. Knits

usually entrap more air than woven fabrics, although the tightness of the

weave or knit is a factor as well. In addition to the openness of the structure,

other fabric characteristics are influential in thermal insulation.

58

Heat transfer through a textile assembly consisting of fabric and air

layers can be calculated based on a theoretical model capable of dealing with

conductive, convective and radioactive heat transfer. The size of the air gaps

has a significant influence on the heat transfer. The balance heat flux drops by

40 per cent when the air gap increases from 2 to 10 mm. The influence of the

air gap tends to become smaller as the air gap is further increased. The

number of fabric layers in the textile assembly has a noted influence; more so

when the ambient temperature is lower (Yuchai Sun et al 2010).

2.13.5 Measurement of Thermal Properties Using Thermal Manikin

Since the first one segment copper thermal manikin in the world

was made for the US army in the early 1940s, more than 100 different thermal

manikins have been employed for research and product development

worldwide. Holmer (2004) reviewed thermal manikin development history

and summarized the milestones. Interest in using thermal manikins in research

and measurement standards is steadily growing and several international

testing standards have been developed in the field of the thermal comfort

evaluation.

To date, thermal sweating manikins are widely used in large scale

textiles and clothing research laboratories all over the world for analyzing the

thermal interface of the human body and its environment. Normally, the

thermal manikin is made from metal or fabric e.g. copper, plastic or water /

windproof fabric with an independent controllable heating/sweating

subsystem, data measurement and analyzing subsystems. With the

development of computer and computation technologies, visual realization

models have become more and more important and are now widely applied in

the field of thermal comfort estimation. Li et al (2004) developed a computer

based model for studies of heat moisture transfer in clothing systems. Buxton

et al in the UK are also developing a similar model that allows the use of

59

human body data from whole body scanners and motion patterns derived from

real recordings.

The thermal/sweating plate has been used for years to determine the

thermal and moisture resistance properties of fabrics. The applications and

description of sweating hot plates can be found in literature by Kawabata et al

(1977) and Holmer (2007). There are several types of skin model employed in

clothing comfort research. Kawabata reported the application of hot plate

technology for the measurement of fabric warm and cool feelings. Typical

structure and detailed description of a sweating hot plate can be found in ISO

11902-1993(E). Another newly developed apparatus is the Dynamic Sweating

Hot Plate, which was developed by NCSU and can be used to measure the

thermal capacity of fabrics under different humidity conditions.

2.13.6 Effects of Clothing Material on Thermoregulatory Responses

Today the technology applying micro PCMs (about 3 million

capsules per cm2

) to the fabrics have been achieved and the microcapsules are

added to fabrics by a conventional pad-dry-cure process. Kyeyoun Choi et al

(2004) applied Melamine formaldehyde microcapsules containing octadecane

are synthesized by the interfacial polymerization method, and the size, shape,

and thermal storage/release properties of the synthesized microcapsules are

analyzed by FTIR, SEM, and DSC and found that under the optimum

treatment concentration, temperature, and time, thermal properties after five

launderings decrease rapidly.

Nilgunozdil et al (2007) observed that yarn properties like yarn

count, yarn twist and combing process of cotton have affected different

thermal comfort properties of 1×1 rib knitted fabrics. Jintu Fan and Xiao-Yin

Cheng (2005), reported on an experimental investigation on the effect of

clothing thermal properties on the comfort sensations of wearers during sport

60

activities. A sweating manikin “Walter” was used to measure the clothing

thermal properties namely, thermal insulation, moisture vapour resistance and

moisture accumulation within clothing of five tracksuits made of 100%

polyester and 100% nylon and found that the thermal comfort sensations

during active sports wear is strongly related to the moisture vapour resistance

and moisture accumulation within clothing.

Damjana Celcar et al (2007) presented a development concept of

the mathematical model enabling an objective evaluation of the human

thermal comfort by simulation of the heat transfer from the human body to the

ambient air. Based on the simulation results, it can be concluded that the

numerical solution is able to predict heat transfer from the human body to the

ambient air.

Direct measurement of the convective and radiative heat transfer

coefficients from the clothed human body to the environment is important in

all work requiring knowledge of the human heat balance. Such measurements

can be achieved using a specially designed garment that incorporates heat flux

sensors. Sensors mounted in pockets were calibrated using a guarded hot plate

facility. Geraldes et al (2008) presented an engineering design of the principal

thermal properties in functional structures and proposed three new equations

that simulate the reality of the behaviour of the knitted structures.

Chen et al (2003) tested clothing thermal insulation on a novel

fabric thermal manikin covered with lightly breathable and highly breathable

skins to simulate low and heavy perspiration respectively. Wu (2009) found a

new method of assembling different kinds of fabrics made of cotton, wool,

lyocell, modal, soyabean, bamboo and their blends to match different sections

of the human body to improve the thermal-wet comfort of the clothing system

during exercise. Jianhua Huang (2008) designed six models for determining

air temperatures for thermal comfort of people using sleeping bags. These

61

models were based on distinctive metabolic rates and mean skin temperatures.

All model predictions of air temperatures are low when the insulation values

of the sleeping bag are high.

2.14 MULTILAYERED FABRICS

Multi layered fabrics consist of different layers of the fabrics which

has the ability to complement and maximize the essential comfort properties

for a specific end use. Various research works have been carried out to

analyze the functional properties of layered fabrics. Fohr et al (2002) have

simulated a mathematical model to determine heat and water transfer through

layered fabrics.

Guldemet Basal and Sevcan Ilgaz (2009) developed a functional

fabric for pressure ulcer prevention with a spacer fabric made of different

combinations of engineered polyester, polypropylene, cotton and viscose fibers

by face-to-face velour weaving technique and recommended the channeled

polyester, cotton and polypropylene as the most promising fiber types for

pressure ulcer prevention.

Sharabaty et al (2008) studied the wettability characteristics of

polyester, cotton and multilayered polyester/cotton fabrics to manage human

perspiration and found that wicking coefficient of multilayered fabrics is

better than cotton fabrics. Xiaohua Ye et al (2008) developed warp knitted

spacer fabrics from PES multifilament and monofilament yarns for cushion

applications and analyzed the pressure distribution, air permeability, and heat

resistance of the fabrics and concluded that these spacer fabrics could be used to

substitute PU foam, especially in the case where the comfort and recycling are

highly required.

62

Yasuda and Miyama (2002) found that, when liquid water

contacted a fabric, such as in the case of sweating, the surface wettability of

fabric played a dominant role in determining the water vapour transport rate

through layered fabrics. Wang and Yasuda (1991) designed an experimental

apparatus to permit simultaneous measurement of temperature change and

moisture flux through fabrics during the transient period after one set of

fabrics has been exposed to humidity and temperature gradients.

Markus Weder et al (2004) used Neutron radiography to study

moisture transport in textiles for the first time. Jintu Fan and Xiao-yin cheng

(2005) investigated heat and moisture transfer through clothing assemblies

consisting of porous fibrous battings sandwiched by inner and outer layers of

thin covering fabric and found that the water content accumulates with time,

and water content is higher at the outer regions than at the inner regions of

battings. Zhuang et al (2002) investigated the liquid transfer from fabric layer

to layer and liquid interaction between different fabrics in clothing systems

and found that the amount of liquid transferred largely depends on the

performance of individual fabrics as well as the way in which they contact

each other.

Another new development is a performance enhancing fabric that

cools through evapouration. Designed with three layers, when soaked with or

immersed in water, the central layer absorbs and retains moisture. As the

water evapourates from this layer, the fabric cools the wearer while its shell

and lining keep the wearer dry. Production of this fabric starts with a super

water absorbing polymer fiber that is blended into fibrous matting. This

matting is positioned between a breathable exterior shell and a conductive

waterproof lining (Prabhakar Bhat and Bhonde 2006).

Pile or napped constructions are often good for cold weather

because the yarns or fibers perpendicular to the surface provide numerous

63

spaces for dead air. This effect is maximized when such fabrics are worn with

the napped or pile surface next to the body, or when they are covered with

another layer. Otherwise, the protruding fibers in the nap structure may

conduct heat away from the body (Billie and Helen 1999).

Field sensor is a very popular high performance fabric from Toray,

which employs a multilayer structure that not only absorbs perspiration

quickly but also transports it up to the outer layer of the fabric very rapidly

using principle of capillary action. It is composed of coarser denier yarn on

the inside surface (in direct contact with the skin), and the fine denier

hydrophobic polyester yarn in a mesh construction on the outer surface to

accelerate quick evapouration of sweat (Yonenaga 1998).

Another multi layered fabrics developed using high-tech polyester

yarns with specially designed ‘W’ shape cross section speeds up fabric’s

ability to transport water away from the skin. Its increased surface area

increases the evapouration rate. So these fabrics exhibit quick drying

properties. Uncomfortable perspiration is rapidly absorbed and the skin

remains dry and more comfortable (Prabhakar Bhat and Bhonde 2006).

Another fabric developed is a light weight material consisting of Sun opaque

fiber at face side which contains a high density titanium core that blocks UV

rays and Polyester yarn with ‘W’ shape cross-section at the back side of the

fabric for moisture transfer. This is a high performance fabric that resists

sweat stains, provides comfort, sun protection and a wonderful ‘soft-hand’

feel (Prabhakar Bhat and Bhonde 2006).

The study carried out by Corinne Keiser et al (2008) showed that

the moisture content of a single layer is not only dependent on the material

properties of that particular layer, but mainly on properties of the neighboring

layers or even of the whole combination. These results suggest that the second

layer of the combination proved to be of high importance for the moisture

distribution. If the second layer is a hydrophilic layer, it absorbs moisture and

64

leads it to the outer layers, while a hydrophobic second layer only takes up

little moisture, and act as a liquid water barrier and the moisture will either be

stored in or drip off.

Behera et al (2002) produced bi-layer interlock knitted structures

using 100% polypropylene and 100% cotton spun yarn and studied for

transmission behaviours of air, water and heat in order to assess their

suitability for sportswear. Wicking coefficients in multilayered fabrics were

much better than in others of 100% cotton. Malgorzata (2006) studied the

thermal insulation properties of multi layer textiles.

2.15 FRICTIONAL PROPERTIES OF FABRICS

As a fabric is moving across the skin, displacement of skin is

increased and the perception of fabric roughness or smoothness is evoked.

The friction and mechanical interaction between fabric and skin during

contact are the key factors determining the perception of roughness,

smoothness and scratchiness. It has been identified that roughness and

scratchiness are important tactile sensations determining the comfort

performance of next-to-skin wear. The friction between skin and fabric is

smaller in fabrics with a smooth surface than fabrics with rougher surfaces.

Moisture at the skin surface can alter the intensity of perceived fabric

roughness. As moisture content increases, the friction and displacement of

skin increases, which triggers more touch receptors. Therefore, a fabric that is

perceived to be comfortable at low humidity conditions may be perceived to

be uncomfortable at higher humidity or sweating conditions.

Behmann (1990) reported a study on the perception of roughness

and textile construction parameters. The roughness was defined as

irregularities in the surface, which can be described geometrically by the size

of the rough elements, or mechanically by the friction coefficients. In 2002,

Okur found that the frictional resistance of the fabrics knitted with carded

65

yarns was higher than that of fabrics knitted with combed yarns. Protruding

fibers on the fabric surface were the most important factor affecting fabric

surface smoothness and frictional properties. Polyester fiber has higher

coefficient of friction as compared to viscose.

Apurba Das et al (2005), examined the fabric-to-metal surface and

fabric-to-fabric frictional characteristics (in both warp and weft directions) of

a series of fabrics containing 100% polyester, 100% viscose, and P/C & P/V

blends with different blend proportions. In P/C and P/V blended fabrics, the

frictional force increases as the cellulose fiber component increases, and the

blended fabrics show higher fabric-to-fabric friction than 100% polyester or

100% viscose. P/C blended fabrics show higher frictional force than that of

P/V blended fabrics for the same geometrical parameters of the fabrics.

Apurba Das et al (2007) analyzed the frictional characteristics of

woven suiting and shirting fabrics with different blends, construction

parameters and found that the fabric to metal friction is less sensitive to fabric

morphology and rub direction, whereas the fabric to fabric friction is highly

sensitive to the type of fiber, blend, yarn structure, fabric structure, crimp,

compression etc. For all fabrics kinetic friction is always lower than static

friction of different levels.

Marek Snycerski et al (2004) designed a double-layer woven fabric

with different friction coefficients on the right and left sides of the fabric, and

differential frictional coefficient along warp and weft direction. Mário Lima et

al (2009), described novel patented laboratory equipment, which was studied,

designed, and manufactured at the University of Minho, Portugal, based on a

new method of accessing frictional coefficient of fabrics. The authors

compared fabrics produced with a new generation of fibers, namely polylatic

acid (PLA) fiber and soya protein fiber (SPF) and confirmed that SPF is softer

than PLA.

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2.16 TEXTILE BASED ELECTRONIC DEVICES FOR

COMMUNICATION

The integration of microelectronic devices in textiles can be very

useful for communication and controlling aims, especially for occupational

clothing and medical applications. These smart textiles have already gained a

foothold in professional clothing (for fire, police and rescue services) where

sensors and information technologies are merged into uniforms for the benefit

of the wearer with increased ease of use as a result (Tao 2005; Tao 2002;

Friedrich 2006).

In recent years, electro textiles have emerged as a promising

material for body-worn wireless systems, which allows the smart clothes to

communicate freely through wireless network. Since electro textiles are

usually light weight, durable and flexible, they are considered as a suitable

material for wearable antennas in body-worn wireless systems (Sigurd

Wagner 2002; Ashok Kumar 2007; Salonen 1999; 2001). A lot of research

has been carried out on various materials for the construction of wearable

antennas which are designed to be an integral part of clothing. By

implementing conductive fibers, either as such as metal plated textile fibers or

thin metal wires, any textile structure, including knitted, woven or nonwoven

textiles, can be made electrically conductive (Yuehui Ouyang et al 2005;

Justin et al 2006).

Sabine Gimpel et al (2004) prepared a partially conductive textile

structure as basic substrate to integrate sensors and microelectronic devices

into textiles. The proceeding is based on two steps. The first is to create a

textile pre structure by conventional textile technologies, such as jacquard

weaving, embroidery etc. using silver-coated polyamide threads. The second

is a galvanic and/or electrochemical treatment of the textile pre structure.

Carla Hertleer et al (2008) carried out a research on the feasibility of using

textile materials for antenna design by combining nonconductive textiles for

substrate and conductive electro textiles for antenna patch and ground plane.