“Thermal Insulation of Woven Constructions”

Hafsa Jamshaid

Submitted to

Prof. Ing. Lubos Hes, PhD. DrSc

At

Faculty of Textile Engineering,Technical University of Liberec, Czech Republic

Contents

1 Introduction 4

1.1 Mechanism for Heat losses 6

1.1.1 Heat Transfer through Conduction 7

1.1.2 Heat Transfer through Convection 8

1.1.2.1 Equation Governing Convective Heat Transfer 10

1.1.2.2 Reynolds Number 11

1.1.2.3 Nusselt Number 12

1.1.3 Heat Transfer through Radiation 12

1.1.3.1 Emissivity 14

1.1.3.2 Absorptivity, Reflectivity and Transmissivity 14

1.1.3.3 Stephan-Boltzmann Equation 14

1.1.3.4 Wien’s Law of Displacement 14

1.1.4 Heat transfer through Evaporation 15

2 Heat transfer mechanism through fabric 15

3 Factors affecting Thermo-physiological comfort 19

3.1 Fibers and their properties 19

3.2 Effect of Yarn properties 22

3.3 Effect of Fabric properties 23

4 Conclusions 26

5 References 27

2

1 Introduction

Heat is an important form of energy. It is necessary for our survival. We need it to cook our food

and maintain our body temperature. Heat is also need in various industrial processes how to

protect ourselves from high as well as low temperature , needs knowledge of how heat travel .

Comfort can be defined as “a pleasant state of psychological, physiological and physical

harmony between a human being and the environment”. Comfort is affected by different fabric

components and it is very difficult to measure in a composite sense. Comport for one person may

be due to feel of textile fabric or the way a garments fits. Comfort is one the most important

requirement of human body, as human tries to achieve maximum level of comfort from their

birth till death. This effort may be conscious or subconscious. The body and mind are constantly

working in unison to provide it. [1]

Heat is termed as a energy and it is half way between internal system and its surroundings. Every

element in this world possess the thermal energy in shape of transitional rotational or vibration

energy of the element which is equal to total kinetic energy of the whole element . Thermal

energy is not considered to be the whole energy of the system it considered to be the partially.

Heat is a form of energy due to effect of gradient in temperature it transfer energy from one

element to other or vise versa. Heat is macro property of an object while temperature is a detailed

description of an object which measures the coldness and hotness of a system and it also gives

the information amount of energy that object posses.

Textile is one of the primary needs of human being. Textiles are used to cover as well as protect

the body. It comprises all the items used to protect the body from external environment. Human

body is a very complex structure which requires specific temperature and conditions to optimise

the body functions. For protection and comfort provision, external skin plays a vital role to keep

the body temperature at its optimum. This skin also holds hair in it which plays their role by

holding up still air as a natural heat flux transmission barrier to provide warmth to the body.

3

Human body requirements are changed as per climatic changes. The human body responds to

environmental factor.These environmental factors are temperature, humidity,air movement and

radiation The human body tries to maintain a constant temperature of about 37 C. This range

may be slightly vary from person to person. Normally body temperature is above the external

environment so required heat comes from internal source that is body metabolism that is

necessary burning of calories to provide power to the muscles and other internal functions. The

efficiency of human organism is such that the only 15-20% energy of food is converted into

useful work , remaining 75-80% is wasted as heat energy. Human body must always be kept in

thermal balance,i.e heat generated by body metabolism together with the heat received from the

external source must be matched by the loss from the body of an equivalent amount of heat. If

desired conditions are not provided, body will feel discomfort and in severe cases it can prove

fatal .The requirement of heat balance vary with climatic change. In summer major emphasis is

on heat dissipation whereas in winter heat conservation is main consideration. When the

external climatic conditions outmatch the body requirements, some specific fabrics are used to

provide the optimum body demands for better comfort feelings. In summer, body needs

protection against sun rays along with dissipation of heat flux out from body to keep it cool.

Owing to high external temperature and difficulty of heat flux transfer from body, body naturally

perspires in the form of a combination of water and dissolved salts. This perspiration provides

cooling effect to skin on evaporation of water and helps to maintain the body temperature at

required level. In case, fabric does not allow this water evaporation process, the body may come

under heat stress which can lead to hyperthermia. Likewise clothing used in low temperature

conditions should provide warmth by reducing the heat transfer of body to the outside

environment and also able to evaporate the moisture vapours in case of sweating to retain

comfort. During excercise in winter season, the metabolic rate of the body increases causing lots

of production of heat which increases the body temperature. This heat is surplus and naturally

can be released in form of skin sweating. During sweating, the skin surface cools itself by

evaporation and in this way maintain the required body temperature at its optimum. The clothing

which may provide warmth but does not have tendency to transfer the moisture vapours have

lower comfort level. Moreover, the thermal insulation also decreases in these clothing due to

moisture build up [2].

4

Comfort of clothing include three main aspects: sensorial, psysiological and thermo-

physiological comfort [3]. The sensorial comfort explains the feelings sensed by human senses

like touch, feel etc. The psysiological comfort deals with the actual comfort observed by the

body during wearing by individual at different activities, and what the body feels. The thermo-

physiological comfort imply both moisture management and thermoregulation by means of

transfer of moisture and heat through a fabric Thermo physiological comfort properties, include ,

thermal conductivity, thermal absorption ,thermal resistance, the water vapour and air

permeability of fabrics. [3,4]

Thermo-physiological comfort is the primary comfort which body feels on wearing any clothing.

Clothing plays a vital role in thermoregulatory process as it alters heat loss from the skin and

also changes the moisture loss from skin [2]. The clothing comfort depends upon the external

climatic conditions along with type of clothing itself. All fabric is not capable to be wear in all

climatic conditions. Every good clothing have to provide comfort level for which it is designed

which should also compatible with the required comfort of body. The Comfort level is that level

in which the temperature, moisture and air circulation are properly matched to maintain thermal

and moisture balance. Assuring thermal comfort of body is one of the main function of

clothing,and for winter season human’s heat maintenance become more crucial.[5]

The main fabric properties that are of importance for maintaining thermal comfort are:

insulation, wind proofing, moisture vapour permeability and waterproofing. These properties are

closely related to each other.[2]

Fundamental parameters which govern the thermo-physiological properties of fabrics are fibre

type, fibre conductivity, fibre moisture regain, yarn count, yarn twist per inch, yarn structure,

spinning process, fabric structure, fabric loop length, fabric thickness, fabric porosity and

finishing treatments [3,6-11]. mechanical and thermal properties of fabrics from wool and hollow

polyester fibers, 2011) (Oglakcioglu, Celik, Ute, Marmarali,

& Kadoglu, 2009) (Gunesoglu, Meric, & Gunesoglu, 2005)

(Vigneswaran, Chandrasekaran, & Senthilkumar, 2009) (

1.1 Transfer of Heat :

5

observing that when two bodies at different temperature are in thermal contact with each other .

thermal energy from a hotter body flows to a cold body in the form of heat this phenomenon is a

transfer of heat . Transfer of heat is a natural process it continues all the time as long as the

bodies in thermal contacts are at different temperature . There are are four mechanisms by

which heat can be dissipated from body to environment. Dry heat transfer takes place by

conduction, convection, and radiation. For moisture and vapour transmission, heat transfer is

occurred by evaporation [2]. The mode by which heat loss occurs depends upon the external

environmental conditions. Conduction relates to transfer of heat by contact method. The rate of

exchange of energy depends upon the thermal conductivity of contact material and also on the

temperature difference between the body and the environment Convection is a process of transfer

of heat by means of moving air. In this mechanism air, act as heat transfer carrier in clothing

systems. In radiation mechanism, the body dissipates energy in form of electromagnetic waves.

In evaporation mechanism, water is converted into vapour form by taking heat energy from skin

and thus cooling it. As warmth feeling is required in winter season, so conduction mechanism

should be lowered in these clothing by choosing that material which have less thermal

conductivity value.

1.1.1 Heat Transfer through Conduction:

The handle of metal spoon held in hot water soon gets warm but in case of wooden spoon the handle does not get warm as both material behave differently regarding the transfer of heat. Both metals and non-metals conduct heat metals are generally better conductors then non-metals .

In solid , atoms and molecules are packed at close together as shown in below figure they continue to vibrate about their mean position there is a change occur in them when one of its end is heated the atoms or molecules present at the end begins vibrate more rapidly . They also collide with their neighbouring atoms or molecules in doing so they pass some of their energy to neighbouring atoms or molecules during collisions with them with the increase in their vibration these atoms or molecules in turn pass. On a part of the energy to their neighbouring particles in this way some heat reaches the other parts of the solids .

6

Figure 1: Mechanism of heat losses

Figure 1 Heat Transfer in solid http://www.gcse.com/energy/conduction.html

same as case with heat flow in the cold parts in metal then non-metals , metals have free electron

and these free electron move with very high velocities within the metal objects they carry energy

at a very fast rate from hot to cold parts of the object as they move . Thus, heat reaches the cold

parts of the metal objects from its hot part much more quickly than non-metals .

Figure 2 Conduction of Heat in metal http://cfbt-us.com/wordpress/?p=1110

1.1.2 Conduction of heat the mode of transfer of heat by vibrating atoms and free electron in solids from hot to cold parts

of a body is called conduction of heat

all metals are good conductors of heat and the substance though which heat does not conduct

easily are called bad conductors or insulators etc wood cork wool glasses rubber.

1.1.3 Thermal conductivity

Conduction of heat occurs at different rates in different materials in metals heat flows rapidly as

compared to insulators such as wood or rubber . the amount of heat that flows in unit time is

called rate of flow of heat .

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Rate of flow of heat = Qt

Equation 1

It is observed that the rate at which heat flows through a solid object depends upon various

factors

Cross sectional area of the solid

Larger the cross sectional area A of a solid contains larger number of molecules and free

electrons on each layer parallel to its cross sectional area and hence greater will be the rate of

flow of a heat through solids thus

Rate of flow of heat =Qt

∝ A

Equation 2

Length of the Solids

Larger is the length between the hot and cold ends of the solids more time it will take to conduct

heat to the colder end and smaller will be the rate if flow of heat

Rate of flow of heat =Qt

∝1L

Equation 3

Temperature difference between ends

Greater is the temperature difference T1-T2 between hot and cold faces of the solids greater will

be the rate of flow of heat

Rate of flow of heat =Qt

(T1-T2) Equation 4

Combining the above factors we get

Qt=∝

A (T 1−T 2 )L

Equation 5

8

The rate of flow of heat across the opposite faces of a meter cube of a substance maintained at a

temperature difference of one Kelvin is called the Thermal conductivity of that substance.

K=Qt

×( L )

A (T 1−T 2)Equation 6

Heat transfer through conduction is a diffusion process. Here energy is transferred from a more energetic medium (higher temperature region) to a less energetic region (low temperature region). Conduction can take place in solids, liquids or gases. In solids conduction takes place due to collision (vibrations) and motion of free electrons. Therefore metals that are good conductors of electricity are good conductors of heat. In liquids and gases conduction takes place due to molecular collision and molecular diffusion. Amount of heat conduction depends upon the molecular arrangement, which includes space between them, their sizes and bonding, etc. Heat transfer has a direction as well as magnitude and the basic law which governs conductive heat transfer is known as the Fourier Law of conduction. Fourier's law of heat transfer through conduction in one direction describes that heat flow is directly proportional with a negative sign to the difference of heat at right angle through which heat passes. Furthermore, the amount of heat transfer per unit area is equal to the product of temperature gradient and thermal conductivity of the material. It can be interpreted in differential and integration forms. Fourier’s law is a discrete analogy of Newton’s law of cooling, whereas, Ohm’s law is an electrical analogue of Fourier’s law. Following is the simplest form of Fourier’s law: In the form of an equation the Fourier Law of heat conduction may be written as

Q=−⋋ AdTdx

Equation 7

In SI unit here Q̇ is the rate of heat flow (W or J/s), A is the area (m2), λ is the thermal

conductivity [W /m .C or W/m.K]

Heat transfer rate per unit area may be written as

q = - A {dT} over {dx} ⋋Equation 8

9

Where is local heat flux [ , λ is thermal conductivity [ and dT/dx represents

the temperature difference along the line on x-axis, which is the thickness of the material.

The negative sign indicates that heat transfer is positive when the temperature gradient is

negative. If is positive in the direction of it means that heat flows from a high temperature

region to a low temperature region in the direction . Therefore the temperature gradient is

negative.

In above equation temperature, which is a function of position and it, has been characterized in

one direction. It displays that heat flow is a vector quantity. Above equation is based on a

phenomenological law, which has been developed on the experimental values[13]. Negative sign

indicates that heat flows from a high temperature to low temperature since:

Eq:. 3

Figure 2: Heat Transfer from Higher to Lower Temperature

In above equation, is a negative value whereas; is a positive value. This justifies

the negative sign in the equation. [14]

1.1.2 Heat transfer through Convection

Convection takes place due to heat transfer between a surface and a fluid moving over the

surface. Convection to take place there should be a temperature difference between the solid and

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Q x

x

the fluid and relative movement between the solid surface and the fluid. Here the nature of the

fluid flow pays a major role in determining the amount of heat transfer. In the natural convection

process, the difference in density of fluid due to the temperature gradient in different parts of the

fluid provides force for movement of fluid. Nevertheless, forced convection is also in practice,

Here are basically two categories of convection [13]

Forced convection and

Free convection

Heat transfer from hot combustion gases to gas turbine blades is due to forced convection.

Figure 3: Heat transfer

in a gas turbine blade situation

Similarly heat transfer in heat exchangers is due to forced convection.

Textile dryers are also another example of forced convection. Free convection takes place when

heat is naturally transferred to or from a body due to naturally occurring free convective streams.

Example, cooling of a hot vertical surface or a hot cylinder as shown in figures below occurs due

to natural convection.

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Figure 4: Heat transfer in a heat exchanger

Free convection from a hot plate Free convection from a hot cylinder

Figure 5

1.1.2.1 Equation Governing Convective Heat Transfer

The basic equation for convection heat transfer is Newton's Law of Cooling. The convection heat

transfer Qc is expressed as

Figure 6: Heat Transfer through convection

In this figure, a fluid is passing over a plate with a velocity and having a temperature ,

whereas temperature of the flat plat is which (in this case) is higher than the free

flowing fluid temperature. Therefore, heat will transfer from the plate to the fluid. In such

condition we can express the relationship in the following equation [13]:

Qc = α (Ts - ) As

Eq: 4

In SI unit here Q̇ cis the rate of heat flow (W or J/s), A is the area (m2), α is convection heat

transfer coefficient of the material(W/m2K) It is assumed that α is constant, whereas in reality

there is a variation on the surface due to flow condition variation. Above equation resolves that

there are many factors, which can influence the heat flow in convection, mainly density,

viscosity, flow rate, etc.

The value of the convective heat transfer coefficient α depends on:

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type of the flow – laminar, turbulent

geometry of the situation

physical properties of the fluid

the temperature difference

position along the surface of the body

whether convection mechanism is forced or free convection

Assuming no-slip surface, there is a zero velocity near to the surface due to shear stress. There is

a continuous decrease in stress and increase in velocity with the change in position and at the end

there is no drag effect and velocity increase from . There are many dimensionless

numbers available to solve the issue of change in the density, viscosity and flow rate etc. Few are

briefly explains here.

1.1.2.2 Reynolds Number

Reynolds Number (Re) is dimensionless number and an indicator of flow either it is laminar or

turbulent. It is a ratio of inertial forces (drag) and viscose forces.

Eq: 5

Where:

L – Travel length of fluid [m]

–mean fluid velocity [

-dynamic viscosity of the fluid [

13

-is density [

- kinematic viscosity (ν = μ / ρ) [

Re numbers is indication of laminar or turbulent flow, for example, in case of flow in a pipe,

more than 2300 Re number indicate that there is a turbulent flow.

1.1.2.3 Nusselt Number

Nusselt Number (Nu) is an indicator of the ratio of convection and conduction heat flow along

the boundary. It has different numbers depends upon the shape of the material e.g. vertical pipe,

horizontal pipe etc. However, close to one, is an indicator of a slug flows or laminar flow,

whereas, values between 100 to 200 are indicator of turbulent flow.

Eq: 6

Where:

α-is convection heat coefficient and L is length and is conductive heat transfer coefficient. In

case when there is a free convection, Nu number is a function of Rayleigh and Pr numbers.

. However in case of forced convection it is function of Re and Pr number and

written as:

Eq: 7

It is also to note that mass transfer analogue of Nu number is Sherwood number.

14

There are many more numbers to describe the convection heat transfer dealing with various

shapes, variation in viscosity, having different heat transfer coefficients etc. Only few we have

discussed here.

1.1.3 Heat Transfer through Radiation :

Heat transfer radiation requires a temperature gradient in the form of electromagnetic waves or

photon emission. Thermal radiation does not require any medium. Every substance emits

radiation if its temperature is more than absolute temperature (0 K or -273.15 ). Maxwell’s

electromagnetic theory and Plank’s quantum theory are under use to define and explain the

radiation. By considering radiation a wave, then it can be said that everybody emit radiation of

all wavelengths starting from λ0 to λ∞, however, for engineering 0.1 to 100 µm wavelengths is

of more interest. Nevertheless, some text books describes that thermal energy could be

transmitted from 0.1 to 1000 µm. Solar radiation from sun is between .1 to .3 µm, whereas

visible spectrum is considered between 0.4-0.7 µm. Standard relationship between frequency and

wavelength is given as under:

Eq:8

Where, c is speed of light (3 x 108 m/s) and is frequency.

Concept of black body radiation helps in understanding the heat transfer through radiation.

Blackbody is an ideal surface which absorbs all incident radiation regardless of wavelength and

temperature, at any given temperature blackbody emits maximum energy than any other body,

and pattern of emission is constant at all temperature. [13,14]

15

Figure 7: The Electromagnetic Spectrum

1.1.3.1 Emissivity

Emissivity (ε or e) is another concept associated with radiation. It is the ratio of radiation emitted

from a surface to the radiation emitted from the surface of blackbody at the same temperature. A

pure black body has emissivity 1, whereas, highly polished materials have very low emissivity,

like, and polished silver has 0.02 emissivity. It shows that highly reflective materials have low

emissivity.

1.1.3.2 Absorptivity, Reflectivity and Transmissivity

When any radiation falls on the surface, part of it may be reflected, transmitted and absorbed. In

case of textile, there are minor chances of transmission, mainly it will be absorbed or in some

cases will be reflected. In any case the sum of three functions should be one, following the law

of energy conservation.

1.1.3.3 Stephan-Boltzmann Equation

Stephan-Boltzmann equation explains the energy emitted by any surface at a certain temperature.

16

Eq: 9

Where q is energy emitted per unit area , is Stephan-Boltzmann constant

and T is absolute temperature. Above equation is useful in case of blackbody, where there is all radiation is absorbed. However, in case of gray body, which is different from the blackbody, following equation is applied:

Eq:10

Where:

is emissivity, which is ratio of gray and black body emission power. Solar energy which earth

is receiving is called solar constant is taken between 1,353 to 1,395

1.1.3.4 Wien’s Law of Displacement

This law provide information about the relationship between the wavelength and temperature.

Eq:11

Where:

is maximum wavelength, b is Wien’s constant approximately

2890 µm K). Using this equation we can measure temperature of any body at any distance. For

example this equation tells that temperature of Sun is 5778 K,

since it has peak wavelength 0.502 µm.

1.1.4 Heat transfer through Evaporation:

Evaporation is a type of vaporization of a liquid that only occurs

on the surface of a liquid. The other type of vaporization

is boiling, which, instead, occurs within the entire mass

of the liquid. It is the opposite process of condensation.

When the ambient temperature is above body temperature,

17

Figure 8: Evaporation

then radiation, conduction and convection all transfer heat into the body rather than out. Since

there must be a net outward heat transfer, the only mechanisms left under those conditions are

the evaporation of perspiration from the skin and the evaporative cooling from exhaled moisture.

Perspiration is the conversion of liquid to vapour that removes heat from the skin. This

mechanism work well in dry hot climate but create problem in humid hot weather. [13,14]

2. Heat transfer mechanism through fabric

The transfer mechanisms of heat through fabrics are complex. When a temperature difference is

developed across such a material which is normal for its surface, heat is transferred by

conduction through the solid fibres as well as by a combination of conduction, convection, and

radiation through the air trapped. The heat transfer mechanism through textile fabric is a

complex phenomenon, since all three mechanisms (conduction, convection and radiation) coexist

in the heat transfer process. However, it is generally accepted that heat transfer by conduction is

more significant than others .Radiation and convection losses within the fabric is being

negligible. Therefore, the total heat transmitted through a fabric is the combination of the amount

of heat conducted through the air gaps and through the content of fibres [15 - 19].

Static thermal properties are characterized by thermal conductivity, thermal resistance and

thermal absorbtivity. Thermal properties are greatly affected by water vapor permeability and air

permeability, but this is out of scope of this report.

Thermal conductivity is an important property of a material. Thermal conductivity λ

is an indicator of the ability of any material to conduct heat. Thermal conductivity is the

quantity of heat transmitted through a unit thickness in a direction normal to a surface of unit area, due to a unit

temperature gradient under steady state conditions, and when the heat transfer is dependent only on the

temperature .Higher values depict that heat will pass quickly, whereas, low values tell that heat will

pass at a slow pace. [6] Following Equation 12 is used to calculate the thermal conductivity of a

fabric system.

λ = __Q_______( Wm-1K-1) Eq:12

A .Δt/h

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Where,

λ – the thermal conductivity,

Q – the heat conducted,

A – the area through which heat is conducted

Δt – the temperature gradient,

h – the sample thickness.

In clothing majority of the bulk is composed of air . For a textile material composed of fibers

and entrapped air, the thermal conductivity is a combination of the fibre polymer and air thermal

conductivity. For example in worsted clothing fabric is made of 25% of fiber and remaining of

air. Morton and Hearle have explained that thermal conductivity of fabric depends much more on

the air entrapped within it than on fibre conductivity as thermal conductivity of air (0.025 W.m-

1K-1) is lower than that of the fibre polymers [20]

Materials of low thermal conductivity are used as thermal insulation. The reciprocal of thermal

conductivity is thermal resistance. Thermal resistance is the opposition to flow of heat energy.

Thermal resistance ‘R’ is defined as the difference of the temperature across a unit area of the material of unit

thickness when a unit of heat energy flows through it in a unit of time. [8-10,15 - 21].

R= h/λ (K m2 W-1 ) Eq:13

Where,

h – the sample thickness,

λ – the thermal conductivity

The SI unit of thermal resistance is Km2/W .In textile industry different units are used including

the Tog and the Clo where 1Clo=1.55 Tog And 1 clo = 0.155 K m2 /W

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Thermal insulation of a fabric is highly related to technical performance of textiles. Winter

fabrics, fire fighter clothing and high altitude clothing are the key areas where thermal resistance

holds critical importance. It is often measured in unit “clo”, Zero (0) clo corresponds to a person

wearing nothing while one (1) clo corresponds to a person wearing a typical business suit [22]. A

thin sweater has 0.2 clo while a thick sweater has 0.35 clo of thermal insulation.

Thermal insulation of garment when it was worn not just depend on individual layer of garment,

rather it depends on whole outfit of garment. As air gaps between layers also act as insulator

assuming that air gaps are small and no heat is wasted due to conviction. The insulation value

depends mainly on thickness and can be estimated from the equation

1clo= 1.6x thickness (cm) Eq..14

Thermal resistance is directly proportional to the thickness and inversely proportional to the

thermal conductivity. ( R= h/ λ ) It displays that thermal conductivity plays a meaningful role in

thermal resistance. But due to increase of thickness of garment water vapour permeability

decreases. Presence of water in fabric decreases the thermal resistance of fabric. In a dry fabric

or containing very small amounts of water it depends essentially on fabric thickness and, to a

lesser extent, on fabric construction and fiber conductivity.

Thermal resistance provides thermal comfort to the user by keeping the human body temperature

intact. Nevertheless, at the same time, there is a need of removal of sweat from the human body.

Otherwise, accumulation of sweat will start storing heat and will create a discomfort for the

users. For wearer comfort, this sweat should be transported away from the skin surface, in the

form of liquid or vapor, so that the fabric touching the skin feels dry. The transport of both

moisture vapor and liquid away from the body is called moisture management

There is another objective measurement method for warm cool feeling called as thermal

absorptivity which determines the contact temperature of two materials.[23]Thermal

absorbtivity is a surface property which allows the fabric’s character to be assessed with regard

to its ‘cool/warm’ feeling, i.e. the feeling obtained when the human skin briefly touches any

object, such as the textile material. Hes [24] introduced the term ‘thermal absorbtivity’as a

measure of the ‘warm-cool feeling’’ of textiles. Its advantage consists in the peculiarity that

20

thermal absorption does not depend on the conditions of the experiment, and is directly related to

other thermal properties such as thermal conductivity and diffusion. Fabrics with a low value of

thermal absorbtivity give a ‘warm’ feeling, whereas those with a high value of thermal

absorbtivity give a ‘cool’ feeling [8] Thermal absorption is a surface property, and therefore the

finishing processes can change it.

It is calculated by the following Equation 15.

B=√ λρc Eq:15 Thermal

Absorptivity of a fabric system

Where: B is the thermal absorptivity (Ws1/2m-2K-1); λ is the thermal conductivity (W/m.K); ρ is

the fabric density (Kg/m3); c is the specific heat of the fabric (J/Kg.K) “(1 Joule=W.s)”

3. Factors affecting Thermo-physiological comfort

In a recent review, Li [29] explained some fundamental factors which play vital role in

explaining the comfort properties of fabrics. These factors include spinning geometry, fibre type,

yarn count, yarn twist coefficient, yarn hairiness, fabric cover factor, fabric thickness, fabric

porosity and fabric finish.

The thermal properties of textile fabrics are influenced by many factors, which can be studied at

three levels:

(1) The microscopic level (chemical composition, morphological characteristics, fineness, cross

section, porosity and water content of component fibres),

(2) The mesoscopic level (yarn structure and properties),

And

(3) The macroscopic level (the fabric’s physical and structural characteristics and finishing

treatments) [25-28]

3.1 Fibers and their properties:

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Thermal insulation is governed predominantly by the amount of air entrapped within a

structure.It is generally known that common textile fibers are low-density polymeric materials

with enclosed air in a capillary structure. Textile fibre polymers are semi-crystalline polymers

and heat flows through them depend on thermal conductivity. Many workers have explained the

effect of material parameters on thermo-physiological comfort properties of fabric including

fibre type, fibre fineness and fibre cross section [30].

3.1.1 Fiber Fineness

Wan et al. [31], Schacher et al. [32] and Ramakrishnan et al. [33] explained the effect of fibre

fineness on thermal resistance of fabrics. According to them, the micro-denier fibre gives low

thermal conductivity and higher thermal resistance. Wan explained it as optimum fibre diameter,

in comparison with the fibre fineness equivalent to penguin feather, which play their role in

blocking the thermal radiation. Das et al. in their two papers [34, 35] [studied the effect of fiber

composition, fiber fineness and twist level on various properties of plain woven fabrics from

acrylic-cotton blended bulk yarns.

3.1.2 Fiber cross-section

The effect of fiber surface and cross-sectional property was investigated by Arai [36] on multi-

porous acrylic fiber. He explained that heat retention ability of the fiber increased by entrapping

still air due to changing in fiber cross-section and fiber surface properties. By decreasing the

fiber cross-section, the number of fibers increases for the same count of yarn and hence increases

the air pockets within the yarn structure. This entrapped still air behaves as air insulation and

hence reduces the heat retention ability of the fiber.

Karaca E et al.[15] investigated the effect of polyester fiber crosssection and weave on thermal

properties .They used polyester yarns of four different fibre cross sectional shapes (round,

hollow round, trilobal and hollow trilobal). The fabrics consisting of hollow fibres had higher

thermal conductivity and thermal absorption values but lower thermal resistance, water vapour

and air permeability values than their counterparts of solid fibres.This is contaray to previous

research about hallow fibers. They concluded that as the yarn densities were the same in all the

fabrics produced, the greater diameters of the yarns produced from hollow fibres caused

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decreases in the distance between the yarns and, as a result, decreased porosity. This was an

effect which increased the thermal conductivity of the fabrics produced from hollow fibres.

3.1.3 Fiber Type

In a study, Cimilli et al. [37] investigated the effect of material type used for thermal comfort

properties of plain jersey socks by modal, micro modal, bamboo, soybean, chitosan, viscose and

cotton fibres. The results obtained suggested that there was statistical significant difference

between the fibre type and the thermal resistance of fabrics. Moreover, it was also observed that

moisture regain markedly affects the heat conduction attribute of the fabric. From the

experimental results, the thermal resistance of the socks according to fibre type was lowest for

cotton followed by modal, viscose, micro modal, bamboo, soybean and chitosan fabrics, in turn.

The lowest thermal insulation property for cotton was explained because of high moisture regain

of cotton fibres. The higher thermal insulation of chitosan fabrics was explained due to lower

moisture regain value as well as higher fibre fineness. In case of air permeability, micro modal

had the best value due to its open structure and cotton as the lowest air permeability value.

Moreover, it was also explained that water vapour transfer rate chiefly depends upon the

moisture regain and air permeability. The obtained results showed that chitosan had the highest

while cotton had the minimum water vapour transfer rate value.

Schneider et al. [38] investigated the thermal conductivity of different fibres under moist

conditions. The tests were conducted on cotton, wool, polypropylene and acrylic fabrics. The

results showed that under moist conditions, wool fabric provide better thermal insulation

property than polypropylene, cotton and porous acrylic fabrics.

Frydrych .I et.al.[8] studied the comparative analysis of thermal insulation properties of fabrics

made of cotton and Tencel The finished fabrics made of Tencel yarn showed lower values of

thermal conductivity and thermal absorption than fabrics made of cotton yarns, and higher values

of thermal diffusion and resistance.

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3.1.4 Blend Ratio

Oglakcioglu et al. [19] studied the thermal comfort properties of 1×1 rib knit fabrics with

different fibre blend ratios of cotton and angora fibre. It was found that by increasing the fibre

content ratio of angora fibre beyond 25%, it would influence the thermal comfort properties of

fabric significantly. Thermal resistance increases by increasing angora fibre ratio while water

vapour permeability value decreases.

Majumdar et al.[6] compared the thermal comfort properties of plain, rib and interlock knitted

fabrics made from regenerated bamboo cellulose and cotton fibre and their blends. From the

experimental results obtained they concluded that by increasing bamboo fibre content, the

thermal conductivity and thermal resistance values of the fabric decreases. The reduction of both

at a time was explained due to preponderance of thermal resistance over the thermal conductance

leads to reduction of both at a time. Moreover, the impact of thickness was also less which

caused such an attribute of two response variables. Thermal resistance reduces due to the fact

that by the increase of bamboo fibre content, the yarn diameter reduces and ultimately the fabric

thickness reduces which lead to reduction in thermal resistance of the fabric. They also observed

that by the increase in proportion of bamboo fibre, the air permeability and water vapour

permeability increases synergistically.

Vigneswarane,c et al [11]Studied the thermal properties of jute /cotton knitted blended fabric .

The results showed that the thermal conductivity decreased as jute blend ratio increases.

The effect of moisture content on the thermal insulation property of the fabrics was investigated

by Black and Matthew [39]. They explained that by increasing the moisture content of bone dry

weight fabric from 0 to 75%, thermal insulation of the fabric significantly reduced. This was also

in agreement with results obtained by Cimilli et al. [37]in their study.

3.2 Effect of Yarn properties

The effect of yarn parameters on the thermo-physiological properties of textiles was investigated

by many researchers. The warmth of a fabric is due to insulation provided by air trapped between

fibres and yarns. Fabrics from straight filament yarns remove heat rapidly by conduction when

placed next to the skin and in such a way produce a so-called cool feel or handle.This

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phenomenon occurs just for a moment because the skin adapts rapidly to mild stimulations.

Fabrics from hairy yarns feel warm on contact with the skin due to the insulating air held

between the fabric fibres and the skin.[40]

Pac et al. [41] studied the effect of fibre morphology, yarn and fabric structure on thermal

comfort properties of fabric. They explained that for rough surfaces, the contact interfacial area

between the fabric and skin is small allowing more free area for air trap on a hairier fabric which

ultimately provides warmer feeling. They also explained that by changing of fibre type, yarn and

fabric structure, the warm-cool feeling and roughness of the fabric changes.

Ozdil et al. [42] investigated the effect of different yarn parameters on thermal comfort of 1×1

rib knitted fabric. They explained that by decreasing count and yarn twist, the thermal resistance

increases while moisture vapour permeability decreases. This was explained due to increase in

fabric thickness. They also studied the carded and combed process for thermal comfort

properties. According to the experimental results obtained, fabrics knitted with carded yarns

provide better thermal resistance than combed yarn fabric while provide less water vapour

permeability value. This was explained due to fluffy yarn nature made by carded process, which

entrapped the air in the yarn structure. As hairiness increases, the thermal resistance of the fabric

increases. In addition, fabric tightness factor effect was also studied according to which the tight

(condensed) fabric structure provide lower water vapour permeability and thermal resistance

values were obtained.

Majumdar et al. [6] also found that by the use of finer yarn for knitted fabric formation of plain,

rib and interlock structures by blend of bamboo and cotton fibres, the thermal conductivity of

fabric reduces. In addition, they found that water vapour permeability and air permeability

decreases as the yarn becomes coarser one and vice versa. Oglakcioglu et al. [19]studied the

effect of yarn structure on thermal comfort properties of 1×1 rib knitted fabric. They observed

that ring spun yarn fabric had lower thermal conductivity and water vapour permeability values

than the open end yarn fabric. The effect is explained on the ground that ring spun yarn had hairy

structure.

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3.3 Effect of Fabric properties

Fabric works as an insulating material and can be considered a second skin. Thermal insulation

power of the fabric depends upon the type of fibre used and structure of fabric manufacturing.

Milenkovic et al. [43] investigated different fabric parameters which affect the heat transfer

through fabric. The experimental results showed that external air movement, enclosed still air

and fabric thickness are the major factors which influence the heat transfer characteristic through

fabric. Greyson [44] and Havenith [45] presented their findings that water vapour resistance and

heat resistance increases by increasing the air entrapped in the fabric as well as fabric thickness.

Hes [46] has discussed in depth the effect of moisture on thermal resistance and concludes that

porosity of the fabric plays significant role in thermal resistance. He adds that structure of fabric

is more important for higher thermal resistance than polymers. Observation of Hes has been

confirmed by having a variation in the values of thermal resistance under two different pressures.

It shows that more compressible structure has higher thermal resistance

Bogaty et al [47] explain that there is a direct link between the fibre arrangement and fabric

thickness. Direction of fibre arrangement, is very important i.e whether the fibres lie parallel or

perpendicular to the fabric surface. Finck [48] showed that thermal insulation was two to three

times greater when the fibres were arranged parallel rather than perpendicular to the fabric

surface. Parallel fibre arrangement gave a higher insulating value than a disoriented fibre mass.

There is a certain change in the thermal insulation properties when pressure is applied;

nevertheless, the effect of pressure is different on a smooth and fuzzy surface. There is an

understood change in the density of the fabric when pressure is applied. The main change occurs

in the laying direction of fibres. Bogaty et al conclude that wool and wool like fabrics are less

sensitive to pressure. This study provides enough evidence that fibre arrangement and thickness

have a significant correlation with thermal insulation .

Yoshihiroa et al. [49] also clarifies that there is a definite difference in thermal conductivity of

fabrics having different structures. Study depicts that direction of measurement (transverse or

longitudinal and in direction of thickness) also changes the value of heat flow. It reinforces the

observation that direction of material also plays a critical role in heat flow property of fabric.

26

Nayer et.al.[50] investigated the effect of polyester/viscose blended on thermal

insulation .Thermal insulation increases with the increase of polyester content and Picks/cm of

the fabric. This may be due to high thickness of fabric as polyester content increase and

thickness greatly affect thermal insulation.

The nature of weave also affects thermal insulation property. There is slow increment in the

coldness of fabric with increase in the closeness of weave. The warmer materials are produced in

twill and crepe constructions.

Karaca E et.at [15 ] investigated the thermal comfort properties of woven fabrics produced from

polyester fibres with different cross sectional shapes. In the twill fabrics, the thermal

conductivity and thermal absorption values were lower and the thermal resistance values higher

than corresponding values of the plain fabrics. The fabric with a twill pattern has lower cross

over points, higher yarn floats and, as a result, lower yarn crimps than the fabric with a plain

pattern when the warp and weft densities are the same. This results in a looser and more open

structure with lower unit weights in twill fabrics. Consequently, as also mentioned in

literature[51], the thermal conductivity and thermal absorption values of the plain fabrics were

higher and the thermal resistance values lower than corresponding values of the twill fabrics. The

twill fabrics produced from trilobal fibres showed the lowest thermal conductivity and thermal

absorption but the highest thermal resistance, water vapour and air permeability.

I. Frydrych[8] studied the effect of weaves on thermal properties when using cotton and Tencel

fiber .Thermal conductivity of plain fabrics made of both cotton and Tencel yarns have the

highest values. Thermal resistance is a very important parameter from the point of view of

thermal insulation, and is proportional to the fabric structure. Due to increase in thickness, we

can observe the increase of thermal insulation, and in the same way the decrease of heat losses

for the space insulated by the textile. Fabrics with twill and canvas weaves are characterized by a

slightly greater thickness than plain fabrics, which results from the fabric structure.

Matusiak.M et.at [52] investigated the effect of different weave and different linear density of

weft on thermal comfort properties.100% Cotton woven fabrics were used..They conclude that

the weave of woven fabrics influences the thermal insulation properties and is significant from

both a practical and statistical point of view. Plain fabrics are characterised by a lower thermal

27

resistance than twill 3/1 S, twill 2/2S, rep 2/2 (2), rep 1/1 (0,1,0) and hopsack 2/2 (0,2,0) weave

fabrics with the same linear densities of warp and weft yarns as well as the same warp and weft

nominal densities, The linear density of weft yarn influences the thermal insulation properties of

woven fabrics.

Some of the items of clothing for cold climate are made of two or more layers. Thermal

resistance of two layers of fabric is greater than the sum of the resistance of the two fabrics. This

is because of the air trapped between the layers. Fabric in garment can be used as free layer, or

joined together by stitching or by fusion. [53]

4. Conclusions

From Literature and research available we can conclude that.:

1. Fibres which have an irregular surface, a non-circular cross-section, high levels of

unevenness or a convoluted form will produce material with better insulation.

2. Yarns which are soft twisted, irregular, hairy and fancy will be better insulator

3. Fabrics loose in structure, soft, porus and thicker fabric provide better insulation properties.

4. A clothing system also helps in improving thermal insulation of garments as garment with

many layers are good insulator than garment of single layer with same thickness. Clothing for

cold weather are best designed with multiple layers, since this gives additional air trapped

between layers and also versatility in that layers can be added or subtracted to cope with different

work rates or varying environmental conditions.

28

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