Polymer classification — thermosets: Thermosetting plastics are materials that, when formed under heat and pressure, result in a product that is __________.
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Aare formed into shape under heat and pressure and results in a permanently hard product
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Bdo not become hard with the application of heat and pressure and no chemical change occurs
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Care flexible and can withstand considerable wear under suitable conditions
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Dare used as a friction lining for clutches and brakes
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Ecan be remelted repeatedly without degradation
Answer
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:
- Thermosets undergo crosslinking during cure.
- Processing typically involves heat and pressure (or catalysts/UV).
- After cure, reheating does not soften the material.
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