Thermoplastic vs thermosetting
THERMOPLASTICS (LDPE, HDPE, PET, PVC, acrylic, PS, PP, ABS, nylon) are long chains NOT linked to each other; heating lets them slide, so the plastic softens, reshapes and recycles. THERMOSETTING plastics (polyester resin, melamine/urea/phenol formaldehyde, epoxy) cure into a rigid CROSS-LINKED network; once set, heat cannot soften them, so they cannot be remoulded or recycled. Cross-links make thermosets hard, heat-resistant insulators — handles, worktops, fittings.
Thermoplastics: signature property → use
Pair a named thermoplastic with a property AND a matching use. Key ones: LDPE is soft and flexible (squeezy bottles, bags); HDPE is stiffer, stronger (crates, pipes); PVC is tough and weatherproof (window frames; flexible cable insulation); acrylic is stiff, clear but brittle (signs, lenses); PP resists flexing (living hinges); nylon is self-lubricating (gears, bearings). Give property AND use, not the name alone.
Thermosets: heat-resistant insulators
Thermosets are heat-resistant, rigid insulators. Polyester resin is hard but brittle, so reinforced with glass fibre as GRP (strong, light) for boat hulls. Melamine formaldehyde (MF): hard, scratch/heat-resistant, any colour → worktops. Urea formaldehyde (UF): hard, insulating → white electrical fittings. Phenol formaldehyde (PF): hard, heat-resistant, DARK only → pan/iron handles. Epoxy: hard, chemically resistant → adhesives, PCBs.
Drawn from real examiner reports.
Thermoplastic-over-thermoset advantage
Asked the advantage of a thermoplastic over a thermoset, few give the defining answer: a thermoplastic can be SOFTENED by heat and reshaped (so remoulded and recycled), a thermoset cannot. Weak answers drift into vague cost or strength points, or wrongly say both recycle. Learn the converse — thermosets resist heat because they do not soften.
Flagged s23 P31 Q12a
Wrong or generic plastic for a product
Given a product, "plastic" alone scores nothing and a wrong choice loses the mark. Learn the standard pairings: rigid PVC for window frames and cable insulation; phenol formaldehyde for a dark, heat-resistant saucepan handle; polypropylene for crates; polystyrene for light packaging. Name a SPECIFIC plastic and tie it to a property the product needs.
Flagged s23 P31 Q5 · w23 P33 Q4
Defining by hardness, not heat
The definition trap: splitting the two families by the wrong feature. A thermoplastic softens when heated and reshapes; a thermoset sets hard on curing and cannot be re-melted. Marks are lost calling a thermoset merely "harder", or a thermoplastic "cheaper", not the heat-and-reshape behaviour that is the real divider. Anchor both to what happens ON HEATING.
Claiming a thermoset can be recycled
A thermoset undergoes an irreversible curing reaction that cross-links its chains, so reheating does not soften it — hot enough, it chars or burns. It cannot be remoulded or recycled by melting: only thermoplastics can. Do not pick a thermoforming process (vacuum forming, line bending) for a thermoset. A thermoset sets ONCE.
LDPE vs HDPE mixed up
LDPE and HDPE are both polyethylene but differ, and swapping them loses the use mark. LDPE (low-density) is soft, flexible — squeezy bottles, carrier bags, film. HDPE (high-density) is stiffer, stronger — crates, buckets, pipes. Match the grade to the job: a squeezy bottle needs LDPE to flex, a stackable crate needs HDPE's rigidity.
PF is dark only; UF/MF any colour
The formaldehyde thermosets differ on colour, which decides some uses. Phenol formaldehyde (PF) comes ONLY in dark colours (brown/black), so it suits dark saucepan and iron handles, not a white plug. Urea (UF) and melamine (MF) formaldehyde come in any colour — UF suits white electrical fittings, MF coloured worktops. Choosing PF for a white fitting misreads it.
Rigid does not mean thermoset
A common error: that any hard, rigid plastic must be a thermoset because thermoplastics are imagined "soft". Rigidity does NOT decide the family — heat behaviour does. Rigid uPVC, acrylic, polystyrene and ABS are stiff THERMOPLASTICS that soften and reshape when heated. Ask: does it soften and reshape when heated (thermoplastic) or set permanently (thermoset)?
Chain plastic → property → use
Most plastics marks are APPLICATION. Chain three links: name the plastic, its property, and why it suits the product — "acrylic is clear and weatherproof, so a sign stays bright outdoors". Plastic or property alone is not enough; the mark is the link to the use.
Describe vs explain vs evaluate
Match depth to the command word. DESCRIBE wants WHAT a property is. EXPLAIN wants WHY it suits. EVALUATE or DISCUSS wants a JUDGEMENT — weigh a plastic against an alternative, a strength and a drawback. Describing when evaluation was asked loses the top marks.
Honour the mark allocation
Let the marks tell you how much to write. A "2 reasons, 4 marks" question wants two DEVELOPED reasons — a property plus its benefit each — not four one-word properties. If it says compare, give both sides. A bare property with no link caps the mark.
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