Composite materials are made from two or more different types of material. For example, MDF is made from wood fibres and glue, and fibreglass is made from a mesh of glass fibres set in a tough polymer.
The materials for a composite material are chosen because they have different properties that combine to make a more useful material. Steel-reinforced concrete is a composite material. It is made by pouring concrete around a mesh of steel cables. When the concrete sets, the material is:
Nylon has some desirable properties. It is:
Unfortunately, nylon does not let water vapour pass through it. This means that, although nylon waterproof clothing stops the rain getting in, it traps your sweat inside. After a while the clothing inside becomes wet and unpleasant to wear.
Breathable fabrics are composite materials. They have the desirable properties of nylon, but are 'breathable' – they stop raindrops getting in while letting water vapour from sweat pass to the outside. Clothing made of such material is very useful to hikers and other people who work or play outside.
Nylon or polyester is usually used for the outer layer, as they are strong. The inner layer is often made from polyurethane fibres, as these are soft and elastic. The membrane at the centre is made from PTFE, the polymer used to coat the surface of non-stick pans. It contains billions of tiny holes. Each one is too small for water droplets to pass through, but big enough to let water molecules from sweat out.