In one line
Thermoplastics soften on heating and harden on cooling repeatedly because their linear chains are held by weak forces, while thermosets set permanently on heating into a cross-linked network and cannot be remelted.
Explained simply
A thermoplastic is like chocolate: melt it, reshape it, cool it, and melt it again. A thermoset is like a boiled egg or a baked cake: once heated and set, heating again will not make it liquid; too much heat just burns it. This classification depends on molecular structure and bonding.
How it works
- Thermoplastics: linear or slightly branched chains held by secondary (van der Waals, dipole, H-bond) forces. Heating overcomes these forces, chains slide, polymer softens and flows. Cooling restores the forces. The process is reversible (physical change).
- Thermosets: prepolymers with reactive groups are shaped and then heated (cured). Covalent cross-links form in three dimensions, giving a giant network. Re-heating cannot break these bonds without decomposing the polymer (chemical change, irreversible).
Key formula
No formula. Comparison:
| Thermoplastic | Thermosetting |
|---|---|
| Linear or branched chains | Three-dimensional cross-linked network |
| Weak secondary forces between chains | Strong covalent cross-links |
| Soften on heating, harden on cooling, repeatedly | Set permanently; do not soften on reheating |
| Usually formed by addition polymerisation (also some condensation, e.g., nylon) | Usually formed by condensation polymerisation |
| Can be reshaped and recycled | Cannot be reshaped; difficult to recycle |
| Softer, weaker, less brittle | Harder, stronger, more brittle |
| Soluble in suitable organic solvents | Insoluble (may only swell) |
| e.g., PE, PP, PVC, polystyrene, nylon, PET | e.g., Bakelite, epoxy resin, melamine-formaldehyde, urea-formaldehyde |
Quick example
PET water bottles are recycled by melting and re-moulding (thermoplastic). Electrical switches, plugs and pan handles are made of Bakelite (thermoset) because it does not soften when hot and is a good insulator.
Common mistakes
- Saying thermoplastics are always addition polymers. Nylon 6,6 and PET are condensation thermoplastics.
- Writing that thermosets "melt at high temperature"; they char/decompose instead.
- Giving only property differences without the structural reason (cross-linking).
How MU asks it
"Distinguish between thermoplastic and thermosetting polymers" (3–5 marks; one mark per 1–2 points). For full marks write at least 5–6 points in a table, with structure and bonding first, and two examples each.