Difficulty: Easy
Correct Answer: are formed into shape under heat and pressure and results in a permanently hard product
Explanation:
Introduction / Context:
Plastics are broadly divided into thermoplastics and thermosets. Understanding the difference is essential for selecting materials for electrical insulators, adhesives, coatings, and structural components where heat resistance and dimensional stability are required.
Given Data / Assumptions:
Concept / Approach:
Thermosetting polymers cure via chemical reactions forming three-dimensional crosslinked networks. Once set, they become rigid and infusible; reheating does not cause flow, distinguishing them from thermoplastics, which soften upon reheating. Examples include phenolics, epoxies, and melamine-formaldehyde resins. Therefore, the correct description emphasises permanent hardening after forming under heat and pressure.
Step-by-Step Solution:
Identify thermoset behaviour: crosslinking → permanent set.Relate to processing: heat/pressure cure.Select the option that states “permanently hard product”.Reject options implying remeltability or generic usage statements.
Verification / Alternative check:
DSC/DMTA data show absence of melting transition after cure in thermosets; degradation occurs instead of melting upon reheating.
Why Other Options Are Wrong:
No chemical change/soften on heat describes thermoplastics.
Flexibility/wear resistance may describe some thermoplastics; it is not defining for thermosets.
Friction lining is an application, not a definition (and many linings use specific composites).
Remeltability contradicts the essence of thermosets.
Common Pitfalls:
Assuming all plastics can be reshaped; ignoring cure schedules that determine final properties of thermosets.
Final Answer:
are formed into shape under heat and pressure and results in a permanently hard product
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