These types of materials are those that can revert from the current shape to a previously held shape, usually due to the action of heat. This technology has been extensively pioneered by the UK Defence Clothing and Textiles Agency. When these shape memory materials are activated in garments, the air gaps between adjacent layers of clothing are increased, in order to give better insulation. The incorporation of shape memory materials into garments thus confers greater versatility in the protection the garment provides against extremes of heat or cold.
Shape memory alloys, such as nickel-titanium, have been developed to provide increased protection against sources of heat, even extreme heat. A shape memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the alloy is easily deformed. At the activation temperature, the alloy exerts a force to return to a previously adopted shape and becomes much stiffer. The temperature of activation can be chosen by altering the ratio of nickel to titanium in the alloy.
Cuprous-zinc alloys are capable of a two-way activation and therefore can produce the reversible variation needed for protection from changeable weather conditions. They will also react to temperature changes brought about by variations in physical activity levels.
In practice, a shape memory alloy is usually in the shape of a spring. The spring is flat below the activation temperature but becomes extended above it. By incorporating these alloys between the layers of a garment, the gap between the layers can be substantially increased above the activation temperature. In consequence, considerably improved protection against external heat is provided.
Shape Memory Polymers have the same effect as the Ni Ti alloy but, being polymers, they will potentially be more compatible with textiles. They could also be employed as flame retardant but as they are thermoplastic polyurethanes they melt at a much lower temperature than the alloys and this limits their use to threats such as hot liquids. The shape memory effect is observed when a plastic conforming to one shape returns, at a particular temperature, to a previously adopted shape.
For clothing applications, the desirable temperatures for the shape memory effect to be triggered will be near body temperature.
Polyurethane films have been made which can be incorporated between adjacent layers of clothing. When the temperature of the outer layer of clothing has fallen sufficiently, the polyurethane film responds so that the air gap between the layers of clothing becomes broader. This broadening is achieved if, on cooling, the film develops an out-of-plane deformation, which must be strong enough to resist the weight of the clothing and the forces induced by the movements of the wearer. The deformation must be capable of reversal if the outer layer of clothing subsequently becomes warmer.
Bi-Material Film Laminates rely on differing coefficients of thermal expansion to produce a reversible bending effect in the same manner as a bi-metallic strip. This temperature dependant bending movement can be harnessed to provide the reversible fabric separation required for variable insulation. A dimensional change of at least 3% is required in the film laminate materials and special joining techniques are needed to avoid delamination problems.
Encapsulated Bi-Gels absorb liquid at differing rates according to temperature which causes them to bend. This offers possibilities for their use as actuators in a variable insulation system. Exploratory work using polyacrylamide and poly-N-isopropylacrylamide did not produce the envisaged robust actuator behaviour.
Established applications for shape memory alloys also include domestic appliances: shower mixer valves, coffee makers, rice cookers; utility applications: safety shut off valves for fuel lines in the event of fire and air conditioning systems. The shape memory alloys can also contribute to the miniaturization of equipment and systems, decrease the number of parts required and extend the life expectancy too due to the favourable fatigue properties of the alloy.
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