In polymer science, thermoplastic materials are those polymer materials which show what characteristic behaviour when heated and cooled under normal processing conditions?

Introduction / Context:
Polymers are broadly divided into thermoplastics and thermosetting plastics based on how they respond to heat. Understanding the difference between these two classes is essential in materials science, mechanical engineering and manufacturing. This question asks you to choose the definition that correctly describes thermoplastic materials, focusing on how they behave when repeatedly heated and cooled.

Given Data / Assumptions:

• Thermoplastics soften when heated and harden again on cooling.
• Thermosetting plastics undergo chemical cross-linking and become permanently hard.
• The question is asking specifically about thermoplastic behaviour.
• No unusual processing conditions are assumed; we consider normal industrial practice.

Concept / Approach:
Thermoplastic polymers consist of long linear or branched chains that are not heavily cross-linked. When heated, the intermolecular forces between chains weaken, allowing the material to soften and become moldable. Upon cooling, the intermolecular forces strengthen, and the material hardens again, returning to a solid state without undergoing a permanent chemical change. This process can usually be repeated many times, which is why thermoplastics can be recycled by remelting. In contrast, thermosetting plastics form extensive cross-links when first molded; once set, they cannot be remelted without decomposition and are permanently hard.

Step-by-Step Solution:
Step 1: Recall that thermoplastics are defined by their ability to soften on heating and harden on cooling repeatedly.Step 2: Recognise that no significant chemical change occurs during this cycle; changes are mainly physical.Step 3: Examine option A, which states that the material does not become permanently hard and that no chemical change occurs when heat and pressure are applied.Step 4: Compare this with the definition of thermosetting plastics, which become permanently hard due to chemical cross-linking (described in option C).Step 5: Note that option B is a general statement about flexibility and wear resistance, not a defining feature of thermoplastics.Step 6: Conclude that option A correctly describes thermoplastic behaviour.

Verification / Alternative check:
Common thermoplastic materials include polyethylene, polypropylene, PVC and polystyrene. These are routinely melted, molded and sometimes reprocessed by remelting. Their processing relies on the ability to soften upon heating without decomposing. On the other hand, thermosetting materials such as Bakelite, epoxy resins and some polyesters are cured once and then cannot be melted again; they will char or burn if heated excessively. This clear practical difference aligns with the definitions in the options and confirms that option A is the proper description for thermoplastics.

Why Other Options Are Wrong:
Option B describes properties like flexibility and wear resistance, which may apply to some thermoplastics or thermosets but are not the defining characteristic. Option C describes thermosetting materials, which become permanently hard after being formed and cannot be re-melted without degradation. Option D refers to a specific application (friction linings) that can involve various materials and does not define thermoplastics as a class.

Common Pitfalls:
Students often confuse thermoplastics with thermosetting plastics because both are polymers and both may initially be processed using heat and pressure. The key difference is whether the material undergoes a reversible physical change (thermoplastic) or an irreversible chemical cross-linking (thermoset). To avoid confusion, remember that the word plastic here refers to plasticity, and thermoplastic materials can be reshaped with heat many times.

Final Answer:
Thermoplastic materials soften on heating and harden on cooling without permanent chemical change; they do not become permanently hard after one heating cycle.