Thermoplastic resins: choose the statement that best characterises thermoplastics compared with thermosetting polymers.

Introduction / Context:
Thermoplastics and thermosets are the two broad classes of polymeric materials encountered in engineering. Distinguishing their behaviours, especially under heat and mechanical stress, is foundational for process selection (injection, extrusion, thermoforming) and end-use performance (toughness, impact, recyclability).

Given Data / Assumptions:

• We compare general properties, not edge-case formulations.
• Thermoplastics soften on heating and harden on cooling reversibly.
• Thermosets form crosslinked networks that do not melt.

Concept / Approach:
Thermoplastics possess linear or branched chains with limited crosslinking, enabling chain mobility when heated. This makes them reprocessable and, in many cases, tougher and less brittle than highly crosslinked thermosets. Thermosets, in contrast, cure irreversibly to rigid networks that resist creep at elevated temperatures but tend to be more brittle and cannot be remelted or readily reclaimed by melt processing.

Step-by-Step Solution:
Assess option (a): incorrect; thermoplastics soften when hot and harden when cooled—statement is internally inconsistent.Assess option (b): incorrect; many thermoplastics can be reprocessed/recycled via melt routes.Assess option (c): describes thermosets, not thermoplastics.Option (d) aligns with typical behaviour—thermoplastics are usually less brittle than thermosets.

Verification / Alternative check:
Impact tests (e.g., Izod/Charpy) and DMA measurements generally show higher toughness and ductility in many thermoplastics versus thermosets.

Why Other Options Are Wrong:
(a) Misstates temperature behaviour.(b) Ignores recyclability of thermoplastics.(c) Confuses with thermoset definition.

Common Pitfalls:
Assuming all plastics behave identically upon heating. Crosslink density dictates melt/flow capability.

Final Answer:
They are generally less brittle than thermosetting resins.