What is a thermoplastic?
A thermoplastic is a plastic that becomes soft and mouldable when heated and hard again when it cools, and can go through that cycle many times. Its long molecules are not chemically bonded to each other but lie tangled side by side. When warm, they slide past one another; when cool, they lock in place again. That is why thermoplastics can be welded, bent and recycled by remelting.
Examples and uses in construction
- PVC: soil and rainwater pipes, window frames, electrical conduit and floor coverings.
- PE (polyethylene): water and gas pipes, sheeting, vapour and root barrier membranes.
- PP (polypropylene): waste pipes, infiltration crates and fibres in concrete.
- PS (polystyrene): EPS and XPS insulation.
- PMMA and PC: acrylic and polycarbonate sheets for rooflights, canopies and greenhouses.
- PEX and PE-RT: plastic pipes for underfloor heating and water supply (PEX is lightly cross-linked and behaves partly like a thermoset).
Properties
- Tough and often impact-resistant, but more sensitive to heat than a thermoset.
- Can creep under sustained load and heat.
- Expands considerably with temperature changes. A long PVC gutter or downpipe therefore needs expansion joints or sliding sockets.
- Joined by solvent welding (PVC), heat welding (PE, PP) or compression fittings.
Tips
- On long outdoor pipes and plastic roof edges, always allow for movement in length; fit clips so that the pipe can slide.
- Use the right adhesive for each plastic: PVC cement does not work on PE or PP.
- Many thermoplastics degrade under UV light. Outdoors, only use material that is suitable for it.
Related terms
- Thermoset
- Plastic
- PVC
- Polyethylene
- Polystyrene
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