Choosing materials for the matrix

Making a composite.

Part II

Composite Materials

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Putting it Together – the Science and Technology of

Most composites are made up of just two materials. One material (the matrix or binder) surrounds and binds together a cluster of fibres or fragments of a much stronger material (the reinforcement). In the case of mud bricks, the two roles are taken by the mud and the straw; in concrete by the cement and the aggregate; in a piece of wood by the cellulose and the lignin. In fibreglass the reinforcement is provided by fine threads or fibres of glass, often woven into a sort of cloth, and the matrix is a plastic.

The threads of glass in fibreglass are very strong under tension but they are also brittle and will snap if bent sharply. The matrix not only holds the fibres together, it also protects them from damage by sharing any stress among them. The matrix is soft enough to be shaped with tools, and can be softened by suitable solvents to allow repairs to be made. Any deformation of a sheet of fibreglass necessarily stretches some of the glass fibres and they are able to resist this, so even a thin sheet is very strong. It is also quite light which is an advantage in many applications.

Over recent decades many new composites have been developed, some with very valuable properties. By carefully choosing the reinforcement, the matrix and the manufacturing process that brings them together, engineers can tailorthe properties to meet specific requirements. They can, for example, make the composite sheet very strong in one direction by aligning the fibres that way, but weaker in another direction where strength is not so important. They can also select properties such as resistance to heat, chemicals and weathering by choosing an appropriate matrix material.

For the matrix, many modern composites use thermosetting or thermosoftening plastics (also called resins). The use of plastics in the matrix explains the name 'reinforced plastics' commonly given to composites. The plastics are polymers that hold the reinforcement together and help to determine the physical properties of the end product.

Thermosetting plastics are liquid when being preparedbut hardenand become rigid (i.e., they cure) when they are heated. The setting process is irreversible, so that these materials do not become soft under high temperatures. These plastics also resist wear and attack by chemicals making them very durable even when exposed to extreme environments.

Thermosoftening plastics as the name implies are hard at low temperatures but soften when they are heated. Although they are less commonly used than thermosetting plastics they do have some advantages, such as greater fracture toughness, long shelf life of the raw material, capacity for recycling and a cleaner, safer workplace because organic solvents are not needed for the hardening process.

Ceramics, carbon and metals are used as the matrix for some highly specialized purposes. For example, ceramics is used when the material is going to be exposed to high temperatures (e.g., heat exchangers) and carbon is used for products exposed to friction and wear (e.g., bearings and gears).

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