Question

Which of the following is a thermoplastic?

• A. Bakelite
• B. Polycarbonate
• C. Ebonite
• D. Vulcanized

Hint:

Thermoplastics can be recycled easily because they soften on heating.

Answer 1

The Correct Option is B

Step 1: Understanding thermoplastics.
Thermoplastics are polymers that soften when heated and harden upon cooling. This process is reversible, allowing thermoplastics to be reshaped multiple times without chemical change.
Step 2: Classification of given materials.
Bakelite, ebonite, and vulcanized rubber are thermosetting polymers. Once set, they cannot be reshaped on heating due to extensive cross-linking. Polycarbonate, however, is a linear polymer with little cross-linking.
Step 3: Analysis of options.
(A) Bakelite: A thermosetting plastic, not thermoplastic.
(B) Polycarbonate: Correct — It softens on heating and can be reshaped.
(C) Ebonite: A hard form of vulcanized rubber, thermosetting in nature.
(D) Vulcanized: Vulcanized rubber is thermosetting.
Step 4: Conclusion.
Polycarbonate is a thermoplastic polymer, hence option (B) is correct.

Answer 2

The clearest practical test for whether a polymer is thermoplastic is to ask what happens if you reheat a finished part made from it: does it soften and become reshapeable, or does it stay rigid until it chars? Applying that test to each option settles the question.

1. Bakelite: Bakelite is heavily cross-linked during its initial curing (a phenol-formaldehyde thermoset). Reheating a finished bakelite part does not soften it back into a mouldable state, it simply stays rigid until it eventually degrades, confirming it is a thermoset, not a thermoplastic.
2. Polycarbonate: Polycarbonate consists of long linear chains with only weak secondary bonds between them, no permanent cross-links formed during moulding. Reheating a polycarbonate part softens it, allowing it to be reshaped or remoulded, and this reversible softening on heating is exactly the definition of a thermoplastic.
3. Ebonite: Ebonite is rubber that has been heavily vulcanized with a large amount of sulfur, creating an extensive, permanent cross-linked network. Like bakelite, reheating it does not make it mouldable again, confirming it as a thermoset.
4. Vulcanized (rubber): Vulcanization itself is the cross-linking reaction (sulfur bridges between rubber chains) that converts soft raw rubber into a fixed, permanently set network, again making it a thermoset once cured.

Only polycarbonate softens reversibly on reheating rather than staying permanently set, which is the defining behaviour of a thermoplastic.

Therefore, the correct answer is Polycarbonate.