Describe two useful properties of plastics as engineering materials, differentiate thermoplastics and thermosets, and explain why thermoplastics can be recycled but thermosets cannot.

• They are lightweight with a high strength-to-weight ratio.
• They are corrosion-resistant and do not react with many chemicals.

Thermoplastics can be repeatedly softened by heating and hardened by cooling, allowing them to be reshaped. Thermosetting plastics, once molded and hardened, undergo an irreversible chemical change and cannot be softened or reshaped by heating.

Thermoplastics have polymer chains that are not cross-linked, allowing them to melt and flow when heated, making them suitable for recycling. Thermosets have extensively cross-linked polymer chains, forming a rigid, permanent structure that degrades or burns when heated rather than melting, which prevents them from being recycled.