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This lesson covers thermosetting polymers — plastics that undergo an irreversible chemical change when heated and shaped, forming permanent cross-links between their molecular chains. Thermosets are a key topic in AQA GCSE Design and Technology (8552), Section 3.1.6.
Thermosetting polymers (thermosets) have molecular chains that form permanent cross-links during the curing (setting) process. Once set, they cannot be reheated and reshaped — they will char and burn rather than soften.
| Feature | Thermoplastics | Thermosetting Polymers |
|---|---|---|
| Molecular structure | Long chains with weak intermolecular forces | Cross-linked chains with strong covalent bonds |
| Effect of heating | Soften and can be reshaped | Do not soften — char and decompose |
| Recyclability | Can be melted and reformed | Cannot be recycled by melting |
| Strength | Generally flexible or semi-rigid | Generally rigid and hard |
| Heat resistance | Soften at relatively low temperatures | Resist heat well; do not deform |
| Examples | Acrylic, PP, HDPE, PET, ABS, nylon | Epoxy, melamine, polyester resin, UF |
AQA Exam Tip: The key distinction is: thermoplastics can be reheated and reshaped (weak intermolecular bonds); thermosets cannot (strong covalent cross-links). This is the single most important fact about polymer classification and appears in almost every D&T exam.
| Property | Detail |
|---|---|
| Form | Two-part system — resin + hardener mixed before use |
| Curing | Chemical reaction (exothermic) when resin and hardener are mixed |
| Adhesion | Excellent adhesive strength on most materials (metals, ceramics, plastics, wood) |
| Strength | Very high tensile and compressive strength |
| Chemical resistance | Excellent — resists acids, alkalis and solvents |
| Electrical insulation | Good |
| Shrinkage | Very low during curing |
| Application | Why Epoxy Is Used |
|---|---|
| Adhesive (Araldite) | Bonds almost any material; extremely strong joint |
| Carbon fibre matrix | Epoxy is the matrix (binder) in CFRP composites |
| Fibreglass (GRP) matrix | Alternative to polyester resin — stronger but more expensive |
| PCB manufacture | Epoxy-glass laminate (FR4) is the standard circuit board material |
| Flooring | Epoxy floor coatings in factories and hospitals — seamless, chemical-resistant |
| Encapsulation | Protects electronic components from moisture and vibration |
| Property | Detail |
|---|---|
| Hardness | Very hard surface |
| Heat resistance | Excellent — withstands hot dishes and pans |
| Stain resistance | Non-porous surface resists staining |
| Food safety | Food-safe — widely used in kitchen and dining products |
| Scratch resistance | Good — maintains appearance with regular use |
| Electrical insulation | Good |
| Application | Why Melamine Is Used |
|---|---|
| Kitchen worktop surfaces (laminate) | Heat-resistant, stain-resistant, hard-wearing |
| Children's dinnerware | Lightweight, unbreakable (unlike ceramic), food-safe |
| Electrical sockets and plugs | Good insulator; heat-resistant (won't melt near hot wires) |
| Whiteboard surfaces | Smooth, hard, wipes clean |
| Decorative laminates (Formica) | Hard-wearing, available in many colours and patterns |
AQA Exam Tip: Melamine formaldehyde is the standard answer for "a thermosetting polymer suitable for kitchen use." Its heat resistance and stain resistance make it ideal for worktops and dinnerware. This is a very commonly tested material.
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