Composites

Composites consist of two main parts:

• reinforcement material
• binding material (the matrix)
  

Examples of composites include, 

• concrete, which consists of small stones bound together by a cement matrix and 
• glass reinforced plastic (GRP) which consists of strand glass fibres bound together by a polyester resin matrix.

Composites are used because they have better physical properties than their constituent parts and they can easily be moulded into complex shapes.

Composites can be pre-formed into sheets, tubes, beams, channels etc. or they can be made during the construction of a product.  

For example, the concrete frame structure opposite was pre-formed in moulds and brought to the building site. The concrete floors of the building will be cast at the building site.

The fishing rod opposite is made from a number of carbon fibre reinforced plastic (CFRP) tubes that fit one inside the other. The structure is immensely strong, yet very light. This composite material consists of carbon fibres fixed in a polyester resin matrix.

Moulding GRP

During the moulding of a glass reinforced plastic (GRP), the GRP material is made as the glass fibres are laid into the mould and resin is stippled or sprayed into it.

Reinforcing Materials for Plastic Composites

The reinforcing material of most plastic composites is in the form of strands, fibres, woven rovings or cloths. Materials with special qualities can be made by choosing the reinforcing material and matching a binding, or matrix material that will give the desired results.

Wood Composites

Wood particles, fibres and chips are used as the reinforcing material in wooden composite sheet materials such as particle boards, Medium Density Fibre boards (MDF), and chipboard.  

Other Manufactured Boards

Plywood and Blockboard are manufactured boards that are made from thin layers of wood (called veneers) and battens. They may be classified as composites if we call the veneers and battens the reinforcing material and the adhesive as the binder or matrix.

Plywood is made by bonding layers of wood veneers. There are always an odd number of layers and the direction of the grain on a veneer is always at right angles to the direction of the grain of the layer next to it.  

Blockboard and laminboard are made by sandwiching battens between veneers.

The quality of the materials used in a manufactured board determines the quality of the board. For example, plywood that will be used in boat building has to withstand being totally immersed in water for long periods of time without the plywood de-laminating. Marine grade plywoods also have to have a hardwearing surface veneer that is attractive. On the other hand, plywood used for making moulds for concrete (shuttering) is of a more inferior quality as appearance is not important.