Thermoplastic materials: identify the correct definition and behavior under heat and pressure Consider the behavior of thermoplastic polymers during heating, forming, cooling, and any associated chemical change. Choose the most accurate description of thermoplastics.

Given

• Definition-centric question about thermoplastic behavior.

Concept
Thermoplastics soften and become formable when heated and solidify when cooled, without a chemical curing reaction. They can be reheated and reshaped multiple times. In contrast, thermosets cure irreversibly into a permanently hard product upon heat and pressure.

Reasoning
Thermoplastic: softens ⇄ hardens with temperature cycling; no cross-linking reaction.Thermoset: permanent set due to chemical cross-linking under heat/pressure.

Eliminate options
Option (a) describes thermosets. Options (c) and (d) are application-specific and not defining features.

Final Answer
Do not become permanently hard by heat and pressure; they soften on heating, harden on cooling, and undergo no chemical change