composite heading

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.

composite materials

There are two keywords you words you need to know which are often used in describing composite material, these 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 material in the matrix and the job of the matrix is to hold the reinforcement in place and to protect it from heat, or friction or the weather. The image below gives more detail on reinforcement and the matrix in a composite material.

matrix and binder composite material

Examples of composite materials

1. Steel reinforced concrete

steel reinforced concreteConcrete 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 very strong in compression, hard wearing and durable but it is brittle and it has low tensile strength (it will crack if it is forced to stretch). This will 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.

2. Concrete

Concrete itself is a composite material. It consists of loose stones and gravel mixed with sand, often called aggregate. This aggregate is then mixed with water and cement. Cement acts as a glue or binder and once the water evaporates, cement crystals grow to form a matrix/binder which embeds the sand and gravel to form a solid mass. The sand/stones would be the reinforcement here.

Fibreglass

fibreglass surfboards 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, flexible.

Laminates

plywood 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 are rotated by 90o, this increase the strength of the plywood and increases its resistance to bending. Other examples of common laminate are car windscreens. Here 2 layers of glass have thin polymer layer sandwiched between them. This laminated glass has the advantage over "normal" glass in that it will not shatter in 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.

Clothing

Many outdoor jackets and sportswear are 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 breathable also, 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.

laminates in clothing

Key points

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