Chemistry only
A composite material is a material made from two or more individual materials with significantly different properties that, when combined, produce a material which is different from the separate individual materials. The idea is to mix two or more materials together and hopefully end up with a new material which has only the useful or "good" properties and hopefully none of the bad unwanted properties. Perhaps one of the first composite materials was made by the ancient Egyptians who mixed mud and straw together to make bricks for building their homes. As a simple example of a composite material consider the example shown in the image below.
There are two keywords you words you need to know which are often used in describing composite materials; these keywords are: reinforcement and matrix (or binder). The matrix is the background material; in the example above the matrix is material B. The reinforcement material is usually a fibre or particle which is added to the background material; the matrix. In the example above the reinforcement is material C. The reinforcement is there to absorb the forces placed on the material in the matrix and the job of the matrix is to hold the reinforcement in place and to protect it from heat, friction or the weather. The image below gives more details on the use of reinforcement materials and the matrix in a composite material.
Concrete is used mainly in the construction industry; it is used to build up tall building such as car parks and
sky scrapers. It is also used for roads and foundations for buildings. As a building material it is very strong
in compression, hard wearing and durable but it is brittle and it has a low tensile strength (it will crack if it is
forced to stretch). This will obviously limit it uses.
Metals on the other hand are weak in compression but very strong in
tension (can support heavy loads when stretched). The solution to concrete problem is to embed
reinforcing rods
(often called rebar) made of metal into the wet concrete. When the concrete sets hard you will have a
composite
material which is both strong in compression and strong in tension. The reinforcement here is the steel rods or
rebar which is embedded in the concrete matrix or binder.
Perhaps one of the most common composite materials is
fibreglass or glass reinforced plastic (GRP) as it is sometimes
called. Here glass fibres; which are normally woven into a sheet; are embedded in a polymer material; often polyester
or epoxy. The glass fibre is the reinforcement, it is strong and stiff but brittle, whereas the polymer, which is
the matrix or binder is usually flexible. Thus the resulting fibreglass is stiff, strong and flexible. The image opposite
shows some of the many uses of fibreglass, the most common uses include:
Laminates are also a type of composite material; they are made by joining together 2 or
more layers of material
together. Plywood is a laminate; it is made by bonding many layers of wood together using an adhesive. Alternate
layers of wood are rotated by 90o and the glued to each other; this increases the strength of the plywood and increases its resistance to bending.
Other examples of
common laminate composite materials are car windscreens. Here 2 layers of glass have a thin polymer layer sandwiched between them. This
laminated glass has the advantage over "normal" glass in that it will not shatter into large sharp pieces and so will
reduce the risk of serious cuts to anyone unfortunately enough to be involved in a car accident. It will also stop most
objects from passing through it, so will offer some protection to passengers if the windscreen is hit by a small
object.
Many outdoor jackets and sportswear are also laminates. The ski jacket shown in the image below has three different layers of materials. The outside layer is waterproof but breathable and stops most of the rain getting through to the inside of the jacket but allows sweat to pass out. The middle layer is made of a polymer which is also breathable, but this layer stops all water but allows sweat to pass through, this helps keep the wearer comfortable and warm. The inner layer is soft and warm to the skin. In many fabrics the layers maybe bonded to each other. Shoes, in particular trainers are also made of laminated materials. Different laminates and materials can be used for different parts of the shoe or boot.
