Phase Change Materials (PCM) in Textiles In textile industry, protection from extreme environmental conditions is a very crucial requirement. Clothing that protects us from water, extreme cold, intensive heat, open fire, high voltage, propelled bullets, toxic chemicals, nuclear radiations, biological toxins, etc are some of the illustrations.

Such clothing is utilized as sportswear, defense wear, firefighting wear, bulletproof jackets and other professional wear. Textile products can be made more comfortable when the properties of the textile materials can adjust with all types of environments.

At present, for fulfilling the above requirement Phase Change Materials (PCM) is one such intelligent material. It absorbs, stores or discharges heat in accordance with the various changes in temperature and is more often applied to manufacture the smart textiles.

Phase Change Materials 'Phase Change' is the process of going from one stat to another, e.g. from solid to liquid. Any material that experiences the process of phase change is named as Phase Change Materials (PCM).

Such materials collect, discharge or absorb heat as they oscillate between solid and liquid form. They discharge heat as they transform to a solid state and absorb as they go back to a liquid state. There are three basic phases of matter solid, liquid and gas, but others like crystalline, colloid, glassy, amorphous and plasma phases are also considered to exist.

This fundamental phenomenon of science was initially developed and used for building space suits for astronauts for the US Space Program. These suits kept the astronauts warm in the black void of space and cool in the solar glare. Phase Change Materials are compounds, which melt and solidify at specific temperatures and correspondingly are able to retain or discharge large amounts of energy.

The storage of thermal energy by changing the phase of a material at a constant temperature is classified as 'latent heat', i.e., changing from a liquid state to a solid state. When a PCM experiences a phase change, a huge amount of energy is needed. The most significant characteristic of latent heat is that it involves the transfer of much larger amounts of energy than sensible heat transfer.

Quiet a few of these PCMs change phases within a temperature range just above and below human skin temperature. This characteristic of some substances is used for making protective all-season outfits, and for abruptly changing environment. Fibre, fabric and foam with built-in PCMs store the warmth of body and then release it back to the body, as the body requires it. Since the procedure of phase change is dynamic, the materials are continually shifting from solid to liquid and back according to the physical movement of the body and outside temperature. Furthermore, Phase Change Materials are used, but they never get used up.

Phase Change Materials are waxes that have the distinctive capacity to soak and emit heat energy without altering the temperature. These waxes include eicosane, octadecane, Nonadecane, heptadecane and hexadecane. They all possess different freezing and melting points and when mixed in a microcapsule it will accumulate heat energy and release heat energy and maintain their temperature range of 30-34°C, which is very comfortable for the body.

The amount of heat absorbed by a PCM in the actual phase change with the amount of heat absorbed in an ordinary heating procedure can be evaluated by taking water as a PCM. The melting of ice into water leads to the absorption of latent heat of nearly 335 J/g. If water is further boiled, a sensible heat of only 4 J/g is absorbed, while the temperature increases by one degree. Hence, the latent heat absorption in the phase change from ice into water is about 100 times greater than the sensible heat absorption.

How to assimilate PCMs in fabrics? The micro encapsulated PCM can be combined with woven, non woven or knitted fabrics.

The capsules can be added to the fabric in various ways such as:

Microcapsules: Microcapsules of various shapes - round, square and triangular within fibres at the polymer stage. The PCM microcapsules are permanently fixed within the fibre structure during the wet spinning procedure of fibre manufacture. Micro encapsulation gives a softer hand, greater stretch, more breathability and air permeability to the fabrics.

Matrix coating during the finishing process: The PCM microcapsules are embedded in a coating compound like acrylic, polyurethane, etc, and are applied to the fabric. There are many coating methods available like knife-over-roll, knife-over-air, pad-dry-cure, gravure, dip coating and transfer coating.

Foam dispersion: Microcapsules are mixed into a water-blown polyurethane foam mix and these foams are applied to a fabric in a lamination procedure, where the water is removed from the system by the drying process.

Body and clothing systems The needed thermal insulation of clothing systems mainly depends on the physical activity and on the surrounding conditions such as temperature and relative humidity. The amount of heat produced by humans depends a lot on the physical activity and can differ from 100W while resting to over 1000W during maximum physical performance.

Specially, during the cooler seasons (approx 0°C), the suggested thermal insulation is defined in order to make sure that the body is adequately warm when resting. At extreme activity, which is often a case with winter sports, the body temperature rises with enhanced heat production. To make this increase within a certain limit, the body perspires in order to withdraw energy from the body by evaporative cooling. If the thermal insulation of the clothing is decreased during physical activity, a part of the generated heat can be removed by convection, thus the body is not needed expected to perspire so much.

The quality of insulation in a garment in terms of heat and cold will be widely managed by the thickness and density of its component fabrics. High thickness and low density make insulation better. It is observed in many cases that thermal insulation is offered by air gaps between the garment layers.