Thermoplastics are polymers whose chains are not chemically linked, so they soften when heated, can be shaped, and harden again on cooling, and the same material can be melted and remoulded many times. Sort salvaged plastic by type before melting, because mixed plastics give a weak, lumpy product and PVC gives off corrosive hydrogen chloride when heated. Heat only until the plastic flows, because overheating breaks the chains and releases harmful fumes.
Part of Polymer Engineering
Thermoplastics are melt-processable polymers such as polyethylene, polypropylene, nylon, acrylic and shellac that can be softened and reshaped repeatedly by heat.
Separate Chains Slide When Heated
In a thermoplastic the long chains are held to each other only by weak attractions. Heat gives them enough motion to slide, so the plastic softens and flows. On cooling, the attractions take hold again and the plastic sets. Because no chemical bond is made or broken, the process is reversible. The Advanced Materials article tabulates the practical result: thermoplastics soften and melt and can be reshaped, are fully recyclable by remelting, have moderate strength, and are processed by melting and moulding or extruding. Its examples are polyethylene, nylon and acrylic.
Natural thermoplastics exist too. Shellac softens at about 65 °C and melts at 75-80 °C, which is low enough to mould by hand but means it cannot stay in a warm place.
Sorting Decides Whether Recycling Works
Plastics Manufacturing notes that salvaged plastic can be sorted by float and burn tests and reprocessed, and the Advanced Materials article says to sort by the resin identification code, the triangle with a number stamped on most items. Standard code meanings are 1 for PET, 2 for high-density polyethylene, 3 for PVC, 4 for low-density polyethylene, 5 for polypropylene, 6 for polystyrene and 7 for other.
- Read the code first. Keep each number separate.
- Float test. Polyethylene and polypropylene are lighter than water and float. PET, PVC and polystyrene sink.
- Use the easy ones. Polyethylene and polypropylene (bottles, containers, automotive parts) are the Advanced Materials article’s recommended salvage.
Do Not Over-Cook It
Plastic degrades when melted at too high a temperature or held too long. Chains shorten each time (a remelted batch is a little weaker than the last) and the material discolours and fumes. Heat gently, stir and mould promptly. Keep sizes uniform so that melting is even.
Why mixing fails: two different thermoplastics rarely mix at the molecular level, so they separate into patches with weak boundaries, in the same way that a bad grain boundary weakens a casting (see Grain Boundaries). The result is brittle.
Shaping
- Pressing or compression moulding. Place the softened plastic in a mould and press.
- Extrusion. Push molten plastic through a die, the route to pipe for water and drainage.
- Sheet forming. Warm a sheet until it sags, then drape it over a former.
Worked Example: Mass For A Mould
A mould cavity of 250 cm³ filled with polyethylene of density about 0.95 g/cm³ (typical value, not a corpus number) takes 250 × 0.95 = 238 g. Add a few percent for shrinkage on cooling, since thermoplastics contract and leave gaps, so weigh out about 250 g.
Fumes from heated plastic
Overheated or burning plastic gives off toxic smoke. PVC releases hydrogen chloride, which corrodes equipment and the lungs, so keep PVC out of every batch and never burn it. Fluoroplastics such as PTFE (non-stick coatings) give off fumes that cause polymer fume fever. Work outdoors or under strong ventilation, keep the heat low, and have no open flame near molten plastic. Hot molten plastic sticks to skin and causes deep burns.
Common Mistakes
- Melting mixed plastics together — weak lumps.
- Including PVC — fumes and contamination.
- Overheating — discolouration and brittleness mean it has degraded.
What’s Next
- Learn next: Thermosets — the plastics that cannot be remelted
- Combine with: Plastics Manufacturing — turning reprocessed plastic into pipe