Polymers and Composites
Polymers: Two Fundamental Families
Polymers are large molecules built from repeating chemical units called monomers. In engineering they divide cleanly into two families based on how they behave when heated. Thermoplastics soften on heating and harden again on cooling — the long molecular chains can slide past one another when warm. This means they can be remelted and reshaped, which makes them recyclable and suitable for injection moulding, vacuum forming and extrusion. Thermosetting polymers (thermosets) form permanent cross-links between their molecular chains during curing. Once set, reheating does not soften them — it chars or burns them instead. This gives thermosets superior heat and chemical resistance, making them useful for electrical fittings, cookware handles and adhesives.
Thermoplastics: ABS, acrylic, nylon, polycarbonate, polystyrene. Thermosets: epoxy resin, polyester resin, melamine, polyurethane, vulcanised rubber. A critical exam trap: polypropylene is NOT on this specification — only polystyrene is listed as the named foam/rigid thermoplastic.
Named Thermoplastics and Their Uses
ABS (acrylonitrile butadiene styrene) is tough, rigid and easily moulded into complex shapes. It is used for computer keyboard keys, printer housings, crash helmets and LEGO bricks. Acrylic (PMMA) is transparent, UV-stable and weather-resistant, with a glass-like clarity at half the density of glass. It is used for shop display units, rear light lenses and aircraft windows. Nylon is self-lubricating, tough and wear-resistant: ideal for gear wheels, bearings and cable-tie heads that must operate without added grease. Polycarbonate combines high impact resistance with transparency — it is used for safety visors, CD/DVD discs and bullet-resistant glazing. Polystyrene is rigid and a good thermal insulator in its expanded (foam) form; in its high-impact form it is used for food packaging trays, model kits and disposable cutlery.
Epoxy resin is a two-part adhesive and matrix material: once mixed and cured, it is strong, rigid and resistant to chemicals and moisture. It bonds metals, ceramics and composites. Polyester resin is the most common matrix for glass-reinforced plastic (GRP) boat hulls and automotive body kits — cheaper than epoxy but slightly less strong. Melamine is hard, heat-resistant and hygienic: used for kitchen worktops and children’s tableware. Polyurethane provides thermal insulation in rigid foam panels and is used as a coating resin. Vulcanised rubber is natural rubber with sulfur cross-links added: it is tough, elastic and resistant to wear — essential for vehicle tyres, seals and conveyor belts.
Composites: Matrix Plus Reinforcement
A composite consists of a reinforcing material embedded in a matrix. The matrix (usually a polymer resin) holds the fibres in position and transfers load; the reinforcement (fibres or particles) provides strength and stiffness. Neither component alone has the combined performance of the composite. The properties of a composite depend on: the type of reinforcement, the direction of the fibres, the volume fraction of fibres, and the type of matrix.
Glass-reinforced plastic (GRP) uses glass fibres in a polyester or epoxy matrix. It is lightweight, corrosion-resistant and relatively cheap: used for boat hulls, roof panels and sports car bodies. Carbon-fibre reinforced polymer (CFRP) uses carbon fibres in an epoxy matrix. It is stiffer and stronger than GRP for the same weight — the Boeing 787 uses a CFRP fuselage and F1 cars use CFRP monocoque chassis because the material combines very high stiffness with very low density. Fibre direction matters critically: fibres aligned along the load direction give maximum strength in that direction but are weak across it. Woven fabrics or cross-ply laminates are used when loads come from multiple directions.
In a composite, strength and stiffness are controlled by the reinforcement, but the fibres must be oriented along the direction of the applied load. Fibres running across the load direction contribute almost nothing to that load. This is why CFRP components are carefully designed as multi-directional laminates.
Structural Timber, Board Products and Ceramics
Structural-grade timber is graded by strength and used where a lightweight, renewable structural material is needed. C16 and C24 are the most common grades (the number indicates characteristic bending strength in N/mm2). Timber is strong along the grain but weaker across it — this is why structural beams run lengthwise. Plywood is made from thin veneers glued with alternating grain directions. Cross-graining overcomes timber’s natural weakness across the grain: the result is a panel that is strong and stiff in both directions, resists splitting and is dimensionally stable. It is used for structural flooring, concrete formwork and furniture carcasses. MDF (medium density fibreboard) is made from fine wood fibres bonded with resin under heat and pressure. It has no grain direction, machines cleanly and takes paint well, but it is not moisture-resistant and swells if wetted. OSB (oriented strand board) uses strands of wood bonded at specific angles for high strength and is used for structural wall sheathing. Ceramics (including engineering ceramics such as aluminium oxide and silicon carbide) are extremely hard and wear-resistant but brittle, so they are used for cutting tool inserts and heat-resistant linings — not structural applications subject to impact.
Structural concrete is a composite of cement, aggregate (sand and gravel) and water. Concrete is very strong in compression but weak in tension. Reinforced concrete adds steel rebar in the tension zones: the concrete resists compression, the steel resists tension, and together they can carry bending loads that neither could handle alone. Prestressed concrete uses tensioned steel tendons to put the concrete into permanent pre-compression, so it can resist higher tensile loads before cracking. HS2 tunnels and multi-storey car parks are typical applications.