What is a composite?

Composite is the term used to describe a material that is made by combining two or more materials. Typically the individual materials have significantly different chemical or physical properties. When combined, each material’s structure is retained.

This means that the individual materials do not blend, merge or dissolve into each other, and it is quite easy to identify the original properties of each within the new material.

In a composite, the individual materials work together and have enhanced properties, resulting in a new and unique material that may be lighter in weight, have increased rigidity, flexibility or strength.

Most composites are made from just two materials, each performing a different function within the new material. One is known as the matrix, which surrounds or binds the other material, which is the fibre, also known as the reinforcement.

MODERN COMPOSITES

Within resistant materials there are three main composite groups, as follows:

• Carbon-fibre Reinforced Polymers (CFRP)

• Glass-fibre Reinforced Polymers (GRP)

• Aramid products (Kevlar®)

Why are composites used?

Composites are used because they can be engineered to meet the exact requirements of a specific application. By combining individual materials, each becomes enhanced and typically the advantages of the new material means it is more efficient, stronger and lighter.

Composites are not new, and one of the earliest known examples is combining mud (wet soil or clay) with plant material, such as straw, to make bricks from which walls and buildings could be constructed. In this example the mud is the matrix and the straw acts as the reinforcement.

Concrete, which comprises small stones and gravel with cement and sand, is another early example of a composite material. In many buildings today, concrete is further reinforced by embedding metal wires or rods into the concrete during construction.

MODERN COMPOSITES

Continuous research is helping to ensure that new innovative composite materials are developed. Within resistant materials there are three main composite groups, as follows:

• Carbon-fibre Reinforced Polymers (CFRP)

• Glass-fibre Reinforced Polymers (GRP)

• Aramid products (Kevlar®)

Glass-fibre Reinforced Polymers (GRP)

Glass-fibre Reinforced Polymers (GRP), also known as fibreglass, is a relatively modern composite material. It is made from strands of glass, which form a flexible matrix or fabric. In the finished product, variation during manufacture in the thickness of layers of glass strands produces different weights and strengths of GRP. Once assembled the strands are set into the desired shape by placing them into a mould. To set the strands in place polyester resin is added and, once the resin has cured, the process is repeated. This creates layers of GRP which is lightweight and very strong. In GRP the glass-fibre woven fabric is the reinforcement, and the polyester resin is the matrix. The fine, self-coloured finish of GRP is provided by using a female mould, which has a wax resist applied before laying up the chopped strand and encasing it with polyester resin. This method is suited to batch production and will result in the final mouldings having an identical finish.

△ Glass-fibre reinforced polymer components being manufactured.

When sufficient layers have been added, the GRP can be smoothed via sanding and, if desired, a finish can be applied. GRP is a popular composite which is widely used in the manufacture of car bodies, water tanks, swimming pool slides, canoes and boat hulls.

Carbon-fibre Reinforced Polymers (CFRP)

Carbon-Fibre Reinforced Polymers (CFRP) is one of the most expensive composite materials, but boasts the best strength-to-weight ratio of any construction material. This composite is made from high-tensile-strength carbon fibres which are woven together and then encased in a polymer resin.

△ CFRP is used in the manufacture of Formula 1 car bodies.

Carbon fibres are resistant to stretching and the result is a rigid material, light in weight yet very strong. CFRP is used extensively within the aerospace industry, it has a high tolerance to heat, and is used to replace traditional, heavier metal-based materials. Other market applications of this versatile composite include marine, automotive, defence, and in the sports and leisure industries.

Stronger than steel, when woven and manufactured into garments Kevlar® is probably best known as the material used in the manufacture of bullet-proof vests.

△ Kevlar® reinforced personal body armour.

Kevlar® is one of a family of materials developed from a synthetic fibre created by chemist Stephanie Kwolek in 1965, while she was developing a new material to make lighter tyres. Formed by combining para phenylenediamine and terephthaloyl chloride the result was a super stiff, heat resistant polymer, nine times stronger than nylon. When spun, aromatic polyamide (aramid) threads are formed which, when further refined, are woven to create this amazing high-performance, super strong, flexible, lightweight material.

On its own, Kevlar® is not a composite material, but when layers of the woven fabric are combined with resin, the result is an extremely rigid, lightweight material that has 20 times the strength of steel. A common use for this synthetically engineered material is Personal Protective Equipment (PPE), to protect people working in hazardous jobs.

Key terms:

**Composite:**a material that is made from two or more materials which have significantly different physical or chemical properties. When combined they create a single material which is often lighter and stronger than the original individual materials on their own.

**Cure:**the process of allowing a substance, for instance a polymer resin used within a composite, to harden and set.

**Matrix:**the material in a composite that performs the binding function.

**Reinforcement:**the material in a composite that provides structure and adds strength.