Question
What is the difference between thermoplastic and thermosetting plastic? Give two examples of each.
Solution — Step by Step
Thermoplastics are plastics that become soft and pliable when heated and harden again when cooled. This cycle of heating-softening and cooling-hardening can be repeated many times without chemical degradation.
The polymer chains in thermoplastics are linear or branched and held together by weak van der Waals forces. Heat provides enough energy to break these weak forces, allowing chains to slide past each other.
Thermosetting plastics (also called thermosets) are plastics that, once set (cured) by heat or chemical reaction, cannot be re-melted or reshaped. They undergo a permanent chemical change when heated.
Thermosetting plastics have cross-linked polymer chains — covalent bonds form between chains during curing, creating a rigid three-dimensional network. These strong covalent bonds don’t break on reheating; instead, the plastic chars or decomposes.
| Feature | Thermoplastic | Thermosetting Plastic |
|---|---|---|
| Behaviour on heating | Softens, can be reshaped | Does not soften; chars or decomposes |
| Polymer structure | Linear or branched chains | Cross-linked 3D network |
| Recyclability | Yes — can be remoulded | No — permanent set |
| Strength | Lower (generally) | Higher (generally) |
| Examples | PVC, Polythene, Nylon | Bakelite, Melamine, Urea-formaldehyde |
| Common uses | Bags, pipes, buckets | Electrical switches, cookware handles |
Thermoplastic examples:
- Polythene (Polyethylene) — used for plastic bags, bottles, wrapping film
- PVC (Polyvinyl Chloride) — used for water pipes, window frames, electrical insulation
Thermosetting plastic examples:
- Bakelite — used for electrical switches, plug casings, pan handles (doesn’t conduct heat or electricity)
- Melamine — used for utensils (plates, cups), fire-resistant laminates for furniture surfaces
The reason Bakelite is used for electrical equipment is directly because it doesn’t soften on reheating — if the switch got hot, a thermoplastic switch could deform and cause a short circuit.
Why This Works
The behaviour difference comes from molecular architecture:
Thermoplastics: Long chains with only intermolecular (weak) forces between them. Heating → chains gain energy → forces overcome → material flows. Cooling → forces re-establish → material hardens. The chains themselves are unchanged.
Thermosets: Chains are covalently bonded to each other through cross-links. These bonds are as strong as any covalent bond — heating cannot break them without destroying the molecules themselves. The structure is essentially one giant covalently bonded molecule.
Alternative Method — Memory Device
Think of it this way:
- Thermo-PLASTIC = plastic in behaviour = can be shaped repeatedly (like modelling clay, but with heat)
- Thermo-SETTING = sets permanently = like cement — once it sets, it’s set for good
CBSE Class 8 board exam tip: If the question asks “Why are thermosetting plastics preferred for making electrical switches?”, the key points are: (1) they don’t soften on heating, so they maintain their shape even when hot, and (2) they are non-conductors of electricity and heat. Give both reasons for full marks.
Common Mistake
Students sometimes say thermoplastics are “weaker” and thermosets are “stronger” as an absolute rule. This is often true (cross-linking increases rigidity), but it’s not always the case. High-density polyethylene (HDPE) is a thermoplastic that is very strong and tough. The key distinguishing feature is not strength — it’s whether the plastic can be remelted and reshaped, which depends on chain structure (linear vs cross-linked).