A plastic is a polymer, a very long chain of repeating small molecules, and what it does when heated sorts it into one of two families: thermoplastics soften and can be remelted, while thermosets cure once and then char instead of melting. A settlement should start with thermoplastics, because a failed batch can be melted down and tried again, and keep thermosets such as Bakelite for jobs that need heat resistance, like electrical insulation.
Part of Plastics and Polymers
Polymers are materials built from long chains of linked monomer units; plastics are the polymers that can be moulded, extruded or pressed into useful shapes.
One Question About Heat Divides All Plastics Into Two Families
Think of a chain: each link is a monomer and the finished chain is the polymer. What a plastic does depends on what the monomer is, how the chains are joined, and how they sit against each other. The first and most useful question about any plastic is simpler: what happens when it is heated?
In a thermoplastic the chains lie against each other without being chemically tied together. Heat lets them slide, so the plastic softens, flows and sets again on cooling. In a thermoset the chains are tied into one network by a chemical reaction during curing. There are no loose chains to free, so heat only breaks the material down.
The Two Families Compared
| Property | Thermoplastics | Thermosets |
|---|---|---|
| Remeltable and recyclable | Yes | No |
| Heat resistance | Lower | Higher |
| Ease of production | Easier: melt and mould | Harder: mix and cure |
| Typical form | Can be flexible | Usually rigid |
| Best for | Pipes, containers, film | Electrical insulators, structural parts |
| Examples | Polyethylene, polypropylene, polystyrene, PVC, celluloid | Bakelite, epoxy, vulcanised rubber |
The table follows Plastics Manufacturing. Treat the rows as tendencies, not guarantees.
Start With Thermoplastics Because Mistakes Are Recoverable
Melt a bad thermoplastic part and you have the material back, a little weaker. Cure a bad thermoset part and you have scrap. That one fact is why the order of work in this section runs from salvage and remelting, through extrusion and moulding, to the thermosets last.
It also shapes where the material comes from. A settlement is unlikely to synthesise petroleum plastics (see Petroleum Cracking), but it can make casein, celluloid and Bakelite from natural feedstocks and can reuse the large stock of plastic left behind by the old world.
What You Need
- Knowledge of which plastic you have. Everything downstream, from melting temperature to fume hazard, depends on it. See Sorting and Identification.
- A heat source you can control. Overheating is the commonest way to ruin a plastic and to make toxic fume.
- A mould, die or press matched to the family: see Extrusion, Injection Molding and Compression Molding.
Common Mistakes
- Treating “plastic” as one material — mixing different plastics in one melt gives weak, brittle lumps.
- Assuming any plastic can be remelted — a thermoset cannot, and heating it only chars it.
- Heating before identifying — some plastics give off corrosive or poisonous gas well before they look hot.
Heated plastic gives off gas
Overheated or burning plastic releases toxic fume, and PVC releases hydrogen chloride. Identify a plastic before you heat it, work outdoors or with strong ventilation, and keep the heat low. See Fume Safety.
What’s Next
- Learn next: Polymer Chemistry — how monomers become chains and networks
- Combine with: Early Plastics — the plastics a settlement can make first