Composite materials

Composite materials (composites) are made when two or more materials with different properties are combined to produce a new material. The physical and chemical properties of each of the constituent materials remain distinct in the new material. These constituent materials work synergistically to produce a composite material that has improved properties when compared with the individual constituent materials.

Modern composites

In modern society, composites are commonplace. Fibreglass, developed in the late 1930s, is a well known modern composite. It consists of fine glass fibres woven into a cloth then bonded together with a plastic or resin. The glass makes the material strong, and the plastic makes it more or less flexible. Fibreglass is used for making surfboards, swimming pools and spas, boat hulls and a wide variety of exterior automobile parts.

The different materials that make up a composite function as either the matrix or the reinforcement. The matrix surrounds and binds together a cluster of fibres or fragments of a material with different properties – the reinforcement. At least one of each type of material is needed to make a composite. In fibreglass, the fine glass fibres are the reinforcement and the matrix is the plastic or a resin. Other common composites include plywood (wood veneer sheets and glue), concrete (loose stones and cement) and carbon fibre composites (carbon fibre and polymer resin).

Increasingly, researchers are exploring biodegradable materials to use in making composites. Some are looking into how plant residues (such as kiwifruit or harakeke) can be used. These new biocomposites have less impact on the environment than materials such as plastic and rubber, as the plant materials are from a sustainable resource and the end product is designed to degrade. Research developments like this mean that there may be a future where we don’t have to rely on petroleum-based plastic products.

Composites in nature

Composites also exist in nature. Wood is a composite material made of cellulose and lignin. The long fibres of cellulose (the reinforcement) are held together by the matrix (lignin). The binding strength of the lignin is what makes a piece of wood much stronger than cotton, which is also made of cellulose.

A seashell is a composite material made of calcium carbonate crystals (the reinforcement) embedded in a scaffold of protein and carbohydrate fibres (the matrix).

The reinforcement

The reinforcement is the material that provides strength, rigidity and the ability to carry a load. Glass fibres are the most common reinforcement material. However, many advanced composites use fibres of pure carbon. Carbon fibres are much stronger than glass fibres, but they are more expensive to produce.

Carbon fibre composites (often simply called ‘carbon fibre’) are light and strong. They are used in a variety of products including aircraft, high-end bike frames, golf clubs and even in bone and joint replacements.

The matrix

The matrix is usually a viscous material that hardens to give shape to the composite product and to protect the fibres from damage. Modern composites often use plastic to hold the reinforcement together. Plastics can be made malleable at high temperatures and moulded into different shapes. The plastic then hardens as it cools.

For some highly specialised purposes, advanced ceramics, carbon or metals are used as the matrix. Carbon is used for products that are exposed to friction and wear, for example, disc brake pads. The disc brake pads have a framework of carbon fibres (the matrix) in-filled with silica (the reinforcement). The resulting material has high wear characteristics as well as being thermally resistant

Why use a composite?

Composite materials are often matched to a particular application. By carefully choosing the reinforcement, the matrix and the process that brings them together, the properties of composites can be tailored to meet specific requirements. Composites have the advantage that they can be moulded into complex shapes. This is a great advantage when producing products such as surfboards, boat hulls or bike frames.