THE WARMTH OF TEXTILES

For comfort of human body, the temperature of textile materials for clothing must be near 98.6^oF. Our clothing should be capable of adjustment to changes in climate, as light as possible in weight, permeable to air and to water vapour, but capable of repelling liquid water when necessary.

Textile fibres do not possess great insulating powers, and that the thermal insulation value of textile fibre is largely due the air entrapped within the mass. Importance of the entrapped air is not only responsible for warmth of the material, but also the layer of air between the fabric and the skin. It may be slated broadly that the greater the area of contact between the fabric and the skin, the ‘cooler’ the fabric; similarly, the less the area of contact, the ‘warmer’ the fabric. For instance, Linen- cool feeling for smoothness and maximum contact with skin; Wool- highly crimped fibre whose yarn construction comprises a random arrangement, the composition of these two factors producing minimum contact with skin and resulting in the sensation of warmth.

The nature of weave is another factor. There is a slow increase in the coldness of fabrics with increase in the closeness of the weave; this is due to the fact that in a closely woven structure there will be more fibres per unit area to make contact with the skin. [Raising increases the warmth of the fabrics.]

Elastic nature of wool is very high even when damp, so minimum contact with skin; but cotton is plastic when damp and so establishes greater contact with skin and feels cooler.

Another factor is conductivity of fibres. Heat conductivity of cotton is about 5 times that of wool, so for cotton feels cooler.

Thermal insulation is important which is due to air entrapped in the fibrous mass and in a windily environment the thin cotton fabric acts as a wind breaker and helps to immobilize the air between it and the skin. [Thermal insulation value is determined by 2 methods: (a) to measure the rate of cooling of some warm object which is surrounded by textile material and (b) to measure the energy required to keep the wrapped object at a constant temperature. (b) is more accurate.] The chief objects of heat retention of fabrics are thickness and density; wool is therefore, is one of the best fibres because it permits the construction of fabrics of great thickness and low density.

Finally, the most important factor in the warmth of fabrics is the entrapped air; this necessitates thick fabrics of low density wool. Thermal properties are of small moment in assessing the warmth of textiles.

Swelling: All textile fibres swell, when immerged in water diameter is increased and increase in length is insignificant. There are two factors which determine the amount of water which a fibre will relative when in equilibrium with the surrounding atmosphere. 1^st is the extent of amorphous regions which determines the accessibility to water molecules. 2^nd is the number of polar groups is the molecular chain which can exercise secondary affinities for water.

Moister content is important. The absorption of water vapor from air is a reversible process. The fibre will take up more moisture if the atmosphere becomes damp and will give it up if the air becomes dry. Relative humidity is the term used to express the state of air with regard to its moisture content.

Relative humidity =(p/p1)x100

p is the actual pressure of water vapor

p₁ is the maximum pressure at the same temperature.