"Plastic" covers a huge range of materials, but most fall into two families: thermoplastics and thermosets. The difference comes down to how their molecules are connected — and it decides how they are processed, how they behave in use and what happens to them at the end of their life.
The short version
A thermoplastic softens when heated and hardens when cooled, and it can go through that cycle many times, a bit like wax. A thermoset is cured once — by heat, a chemical reaction or both — and after that it won't melt again; heated strongly enough, it chars or breaks down instead, more like a baked cake than wax.
What happens at the molecular level
Thermoplastics are made of long polymer chains that are tangled but not chemically linked to each other. Heat lets the chains slide past one another, so the material flows. In thermosets, curing creates permanent chemical bonds — cross-links — between chains, forming a rigid three-dimensional network.
Side by side
| Thermoplastic | Thermoset | |
|---|---|---|
| Behaviour on heating | Softens and melts | Stays solid, eventually degrades |
| Reshaping | Can be remelted and reshaped | Shape is permanent after curing |
| Typical processing | Injection moulding, extrusion, thermoforming | Compression moulding, resin casting, lay-up |
| Recycling | Often mechanically recyclable | Difficult; usually ground as filler or recovered for energy |
| Examples | Polyethylene, polypropylene, PET, PVC, ABS, nylon | Epoxy, phenolic resins, polyurethane foams, vulcanised rubber |
Where each one shines
Thermoplastics are everywhere in packaging, household goods, pipes, textiles and device housings, because they are fast to process and can be reshaped. Thermosets are chosen when a part must keep its shape under heat or load: electrical insulators, adhesives, coatings and the resin in fibre-reinforced composites.
The recycling question
Because thermoplastics can be remelted, many of them can be collected, sorted and turned into new products, although quality can drop with each cycle and mixed or contaminated plastics are harder to handle. Thermosets are much harder to recycle, which is why researchers are working on resins that can be broken down or reprocessed more easily.
For how thermoplastics are turned into parts, read our guide to injection moulding, and for the broader engineering picture see polymer and plastic technology.
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