While the principal strength and stiffness of the composite is provided by the fibres, the matrix material also has a large part to play in the overall mechanical properties. The matrix is not designed to bear much of the load. Instead, the matrix binds the fibres together and distributes the load. It also provides ductility and protects the fibres from surface damage. It separates the fibres and prevents propagation of cracks from one fibre to the next. Also, unless the matrix chosen is a particularly flexible one then it will assist in prevention of the fibres buckling under compression.

The requirements of a good matrix material are that it can infiltrate between the fibres and form a strong interfacial bond. It is also essential that there is no chance of chemical reaction between the matrix material and fibres and that the matrix material does not cause damage to the fibres.

There are three main types of composites based on the type of matrix they employ – polymer matrix composites (PMC), ceramic matrix composites (CMC), and metal matrix composites (MMC).

The most common type are polymer matrix composites. They are produced in the largest quantities, due to their good room temperature properties, ease of manufacture and low cost.

There are two main types of polymer composite matrix materials, thermoplastics and thermosetting resins.