Polymers: thermoplastics and thermosets

Plastics are polymers: long chains of molecules. How a polymer reacts to heat decides how we shape it and whether we can recycle it.

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What are polymers?

Most polymers are made from crude oil, which is a finite resource: it will run out.
Some polymers are made from plants, such as PLA, which is made from corn starch or sugar cane and is often used for 3D printing.
Polymers are usually light, waterproof, good electrical insulators and easy to mould into complex shapes.

Thermoplastics

Thermoplastics soften when heated and can be reshaped. They harden again when cool. This can happen many times, so they can be recycled.
ThermoplasticUses
AcrylicSigns, display stands, laser-cut products
HIPS (high impact polystyrene)Vacuum-formed trays and packaging
PETDrinks bottles
PolypropyleneChair seats, lunch boxes with flexible hinges
PVCPipes, window frames, cable insulation

Thermosets

Thermosetting polymers (thermosets) go through a chemical change when they are first heated and set. After that, heating will not soften them: they char or burn instead. They cannot be remoulded, so they are hard to recycle.
They are good where heat resistance is needed.
ThermosetUses
Melamine formaldehydeKitchen worktop laminates, tableware
Urea formaldehydeElectrical plugs and sockets
Epoxy resinStrong adhesives (glues)
Polyester resinGlass-reinforced boat hulls
Worked example

Explain why an electric plug is made from urea formaldehyde rather than acrylic.

  1. A plug can get warm and must stay rigid and safe.
  2. Urea formaldehyde is a thermoset: it does not soften when heated, and it is a good electrical insulator.
  3. Acrylic is a thermoplastic, so it would soften if it got hot.

Answer: Urea formaldehyde is a thermoset that stays rigid when warm and insulates electricity; acrylic would soften with heat.

Key idea

Thermoplastics soften when heated and can be reshaped and recycled (acrylic, HIPS, PET, polypropylene, PVC). Thermosets set permanently after one chemical change and resist heat (melamine formaldehyde, urea formaldehyde, epoxy resin).

Key words
Finite resource
A resource that will run out because it cannot be replaced, such as crude oil.
Mould
A hollow shape that material is formed in to give it a shape.

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Extra practice

2 more questions

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  1. Which type of polymer can be reheated and reshaped many times?

    1. Thermoset
    2. Ceramic
    3. Hardwood
    4. Thermoplastic
    Show the answer

    Answer: Thermoplastic

    Thermoplastics soften when heated.

  2. What happens to a thermoset if it is heated strongly after setting?

    1. It chars rather than softening
    2. It softens and can be reshaped
    3. It melts into a liquid
    4. It becomes metal
    Show the answer

    Answer: It chars rather than softening

    Thermoset bonds are permanent.

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Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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