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Factors Affecting Thermal Insulation Properties of Garment
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Factors Affecting Thermal Insulation Properties of Garment

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Fabric for use in selected items of apparel must have properties that make it adaptable for various climatic conditions. Protection against cold is dependent upon thermal resistance or thermal insulation of the garment. The present article reviews various factors which determine thermal insulation of garment.


MECHANISM OF BODY HEAT LOSS


Heat can be lost from the body by conduction, convection, radiation and evaporation of perspiration. Heat loss by conduction occurs when material known to be good conductors are placed next to the body. The conductor carries the body heat to surrounding atmosphere where it is dissipated into the air cooling the body. Heat loss by convection occurs when air current move over and around the body. Heat loss by radiation means that heat is given off from the body in the form of rays. This occurs when air temperature is below body temperature. Heat loss by evaporation of perspiration results from a physiological mechanism to control body temperature.


ROLE OF AIR IN THERMAL INSULATION


Textile fibers differ in their ability to conduct heat but it is the thermal resistance of trapped pockets of air, not the conductivity of fiber which determines the thermal properties of textiles. Greater the ratio of air to the fiber in the fabric structure, the greater the insulation of fabric. Cassie opined that the air is not entrapped but clings to the fiber surface. This fact probably explains the excellent insulating property of wool fabrics as wool yarns are open yarns so expose maximum surface.


Air provides little resistance to passage of heat by radiation. Fibers do obstruct the flow of radiant heat, some more than the others and a minimum amount of fibers must be used to effectively accomplish this. As air movement causes heat loss by convection, total fabric assembly such as to reduce air flow as much as possible.


There are many fabric structures which can trap still pockets of air and so act as insulators under still air conditions. They may be pile/napped fabrics, milled or quilted fabrics and more flexible knitted and brushed fabrics. Synthetic fibers are crimped to increase the amount of air space within the fabric. The crimp prevents the fiber packing tightly in the fabric and so creates pockets of air.


FABRIC PARAMETERS AFFECTING THERMAL INSULATION


Result of studies on the thermal properties of fabric has concluded that the thickness and density of the fabrics are two chief factors which determine the insulation property. The thickness of fabric strongly affects amount of heat insulation. In general the greater the fabric thickness, greater the thermal insulation.


It has also been found that the thermal insulation is also related to the weight and compressional properties of fabric. To make an insulating material effective it should have low compression set/high resiliency to make the still air entrapped into the material.


Marsh has stated that the lesser the area of contact between the fabric and skin, the warmer the fabric. Wool fibers warmth providing property is not only on account of its crimp and curliness, but also because of its elastic nature which enables it to recover from deformation and therefore to maintain little contact with the skin.


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.


WIND CHILL


It is the stillness of the air trapped in clothing which gives it the ability to prevent heat from being conducted away from the body. The insulation value depends upon the trapped air remaining still. If wind blows through the garment, fresh cold air replaces the insulating layers and heat is rapidly lost.


Wind chill effect depends upon the temperature as well as the velocity of the wind. In the cold climate consideration of wind chill effect is very important because 80% of heat losses are due to wind chill effect.


To prevent wind from penetrating the clothing and to help trap still air, the garment should be designed to have some close fitting points.


EFFECT OF WATER ON THERMAL INSULATION


Water molecules are much closer together than air molecules so water conducts heat far more rapidly than does air. If a garment becomes wet whether from perspiration or from rain its thermal resistance drops dramatically. As little as 15% moisture can halve the insulation value of clothing.


The transmission of water through a fabric is very critical parameter from comfort point of view. In winter it is possible that water vapour produced by perspiration at the skin can be condensed to the liquid, which may subsequently freeze to ice as the water molecules move towards the cold air at the external surface. If a fabric cannot permit sufficiently rapid disposal of water to occur, perspiration related discomfort may be felt. The movement of water can be increased by either increasing water vapour permeability or enhancing the ability of the fabric to transport liquid water to the surface.


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.


 

REFERENCES


1.      Corbman B P (1985) Textile fiber to fabric 6th ed. McGraw Hill Company, New York

2.      Cassie ABD (1956) Journal of Textile Institute, vol. 37

3.      Kothari and Anderson (1991) Indian Journal of Fiber and Textile Research, vol. 29

4.      Marsh JT (1979) Textile Science B.I. Publication, New Delhi

5.      Mathur, Raj and Kasturia (1997) Indian Journal of Fiber and Textile Research, vol.22

6.      Rees (1941) Journal of Textile Institute. vol. 32


ABOUT THE AUTHOR:


Dr. Suman Pant is a Professor at the Faculty of Home Science, Banasthali University.

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