Classification: thermoplastics vs. thermosets (PVC, PS, PMMA, PTFE)

Thermoplastics vs Thermosets: PVC, PS, PMMA, PTFE

1. The core distinction

• Thermoplastics: linear/branched chains held by weak van der Waals forces; soften on heating, harden on cooling — reversible, recyclable, weldable. Examples: PVC, PS, PMMA, PTFE, polyethylene.
• Thermosets: 3D cross-linked network (covalent); once cured, heating chars/degrades them — irreversible, not recyclable. Examples: bakelite, epoxy, melamine.

Property	Thermoplastic	Thermoset
Structure	linear/branched	cross-linked 3D
On heating	melts, remouldable	does not melt, burns
Recyclable	yes	no
Strength/heat resistance	moderate	high
Examples	PVC, PS, PMMA	bakelite, epoxy

2. PVC (polyvinyl chloride)

Monomer vinyl chloride $CH_2=CHCl$ → free-radical addition polymer.

• Rigid PVC (pipes, window frames, bottles) vs plasticized PVC with DOP plasticizer (cables, flooring, bags).
• Cheap, chemical-resistant, flame-retardant (Cl content) but releases HCl/dioxin if burnt — disposal concern.

3. PS (polystyrene)

Monomer styrene $CH_2=CH-C_6H_5$.

• General-purpose PS: clear, brittle (CD cases, disposable cutlery). Expanded EPS/thermocol (packaging, insulation). HIPS rubber-toughened (fridge liners, toys).
• Poor solvent/heat resistance; foamed form is bulky waste — recycling code 6.

4. PMMA (polymethyl methacrylate, Plexiglas)

Monomer methyl methacrylate $CH_2=C(CH_3)COOCH_3$.

• Glass-like transparency (>92% light transmission), shatter-resistant, UV-stable: aircraft windows, skylights, lenses, signage, bone cement.
• Scratches more easily than glass; depolymerizes cleanly to monomer on heating (chemical recycling possible).

5. PTFE (polytetrafluoroethylene, Teflon)

Monomer tetrafluoroethylene $CF_2=CF_2$.

• Strongest C–F bonds → extreme chemical inertness, heat resistance (~260 °C continuous), lowest friction coefficient of solids: non-stick coatings, gaskets, seals, PTFE tape, vascular grafts.
• Does not melt-flow (very high viscosity) — processed by sintering; pyrolysis above 400 °C gives toxic fumes.

Exam Points

• Thermoplastic vs thermoset table + one example each way (3 marks, near-certain).
• Monomer → polymer + one property + one use for PVC, PS, PMMA, PTFE (5 marks favourite — learn as 4 rows: monomer, polymerization type, property, application).