Thermoplastics and thermosetting polymers, their properties and uses, and how composites combine materials to give a better performance than either part alone.

What this dot point is asking

AQA wants you to tell thermoplastics from thermosetting polymers, give examples and uses, and explain how a composite combines materials for a better result. The most examined idea is the cross-link structure that explains why thermosets cannot be re-melted, and why a composite outperforms its separate parts.

Thermoplastics

Examples include acrylic (PMMA) for signs and machine guards, polypropylene (PP) for crates and living hinges, HDPE for bottles and pipes, and PET for drinks bottles. They are light, good electrical insulators and easy to mould, which is why most everyday plastic products are thermoplastics.

Thermosetting polymers

Examples include epoxy resin (adhesives and the matrix in composites), melamine formaldehyde (worktops and tableware) and urea formaldehyde (electrical fittings). The cross-links are the key idea: they are permanent chemical bonds, so heating a cured thermoset only chars or burns it rather than softening it.

Composites

The matrix holds the fibres in place, protects them and shares load between them, while the fibres carry the main forces. The combination beats either part alone: the fibres are strong but cannot hold a shape, and the resin holds a shape but is weak, so together they give a high strength-to-weight ratio that neither has by itself.

Try this

Q1. State one difference between a thermoplastic and a thermosetting polymer. [2 marks]

• Cue. Thermoplastics soften and reshape when reheated; thermosets set permanently and cannot be re-melted.

Q2. Name the two parts of a composite material. [2 marks]

• Cue. A reinforcement (fibres) and a matrix (resin).