Most composites are made of two materials, a matrix or binder surrounding…

The idea

A composite combines two or more materials so that the product has properties better than either material alone. In most composites, one material forms a matrix or binder that surrounds and holds together the other material, called the reinforcement, which may be fibres or fragments. The matrix keeps the reinforcement in position and transfers stress between the parts, while the reinforcement provides strength and stiffness. For example, in fibreglass, a polymer resin matrix binds together glass fibres; the resin alone is flexible and the glass fibres alone are brittle, but together they form a strong, lightweight material used for boat hulls and sports equipment. Reinforced concrete, with steel bars in a concrete matrix, is another common example.

What earns the marks

• A composite is made from two or more materials combined so that the product has improved properties compared with the separate materials.
• The matrix, also called the binder, surrounds and binds together the other material.
• The reinforcement is the material being bound, and it may be in the form of fibres or fragments.
• The matrix holds the reinforcement in place and helps transfer forces through the composite.
• The reinforcement usually provides strength and stiffness to the composite.
• Examples include fibreglass, where a polymer resin matrix binds glass fibres, and reinforced concrete, where a concrete matrix binds steel bars.

Common mistakes and how to put them right

• Confusing the matrix with the reinforcement.

  Put it right: The matrix surrounds and binds, while the reinforcement is the fibres or fragments being held.

• Thinking a composite is simply a mixture with no bonding.

  Put it right: The matrix binds the reinforcement together into a single material.

• Assuming a composite must be stronger than both starting materials in every way.

  Put it right: It combines useful properties, such as strength with low density, rather than being superior in all respects.

Exam tips

• Name the two roles clearly in every answer: matrix or binder surrounds and binds, reinforcement is the fibres or fragments being bound.
• Support your explanation with a named example such as fibreglass or reinforced concrete and state the benefit of combining the materials.
• Use the phrase 'properties of the composite are better than those of the separate materials' to show why composites are useful.