What is the key difference between a thermoplastic and a thermoset polymer?

Thermoplastics are always transparent; thermosets are always opaque Thermosets conduct electricity; thermoplastics do not Thermoplastics can be re-melted and reshaped repeatedly; thermosets cure permanently and cannot be re-melted Thermosets are biodegradable; thermoplastics are not

PET (polyethylene terephthalate) is used to make drinks bottles because it is:

Extremely rigid and hard to dent Transparent, lightweight, tough, and capable of forming a good gas barrier Biodegradable within a few weeks The cheapest polymer available

PTFE (Teflon) is used as a non-stick coating and in low-friction bearings mainly because it:

Is extremely hard and scratch-resistant Conducts heat very efficiently, preventing hot spots Has an extremely low coefficient of friction and is chemically inert Is transparent and allows chefs to see the pan surface

Polycarbonate is used for bullet-resistant glazing because it combines:

High hardness with zero water absorption High optical clarity with exceptional impact toughness Very low density with high electrical conductivity Excellent UV stability with self-healing surface properties

Ceramics are widely used as cutting tool inserts and spark plugs because they are:

Ductile and easy to re-sharpen without specialist equipment Very hard, maintain hardness at high temperatures, and are electrically insulating Corrosion-resistant and electrically conductive Lightweight and able to be welded to metal frames

The Space Shuttle's thermal protection tiles were made from a silica ceramic because ceramics:

Are magnetic and can be controlled by ground-based electromagnets Have very low thermal conductivity and can withstand temperatures exceeding 1,600°C without melting Reflect all infrared radiation back into space Melt and reform as a protective liquid film during re-entry

In a composite material, what roles do the matrix and reinforcement play?

The matrix provides strength; the reinforcement merely adds colour Both matrix and reinforcement must be metals for the composite to be structurally useful The matrix binds and protects the reinforcement; the reinforcement provides the primary strength and stiffness The reinforcement increases density; the matrix reduces it

Carbon fibre reinforced polymer (CFRP) is preferred over aluminium for Formula 1 car chassis because it:

Is transparent so cameras can see inside the chassis Has a higher specific strength and can be designed to fail in controlled ways that protect the driver Is cheaper to produce than aluminium sheet metal Conducts electricity efficiently for the car's electronic systems

Fibreglass (glass-fibre reinforced polymer, GFRP) is commonly used for boat hulls and wind turbine blades because it:

Is transparent, allowing visual inspection of internal components Has the highest tensile strength of any composite Is strong, corrosion-resistant, relatively inexpensive, and easy to mould into complex curved shapes Can be recycled as easily as steel scrap

Reinforced concrete combines concrete and steel rebar because concrete is strong in compression but weak in tension. What does the steel rebar do?

It adds weight to prevent the structure from floating It carries the tensile stresses in regions where concrete would crack under bending or stretching It prevents the concrete from setting too quickly in hot weather It acts as a conductor to ground lightning strikes

Nylon is used for gears and mechanical components in preference to metal because it:

Has higher strength than steel at elevated temperatures Is self-lubricating, lightweight, quieter in operation, and resistant to corrosion Can be welded easily using conventional arc welding Is transparent so engineers can see wear occurring in real time

Epoxy resin is a thermoset used as the matrix in high-performance composites. After it cures, it cannot be melted again. This is because:

The curing process removes all hydrogen atoms, making the polymer insoluble Curing creates covalent cross-links between polymer chains, forming a rigid 3D network The resin reacts with atmospheric nitrogen to form a ceramic shell Heating causes the polymer chains to align perfectly in one direction