Which of the following is not an example of thermoplastic?
🔬 Understanding Thermoplastics vs. Thermosetting Plastics
Plastics are broadly divided into two main categories based on how they behave when heated. This fundamental difference is due to their molecular structure.
Thermoplastics: These plastics soften and become liquid when heated and harden when cooled. This process is reversible, like melting and freezing wax. Their molecular structure consists of long polymer chains that are not chemically linked to each other.
Thermosetting Plastics (Thermosets): These plastics are cured or "set" into a permanent shape using heat. During this initial heating, the polymer chains form strong, irreversible chemical cross-links. Once set, they do not soften upon reheating; instead, they will char and decompose at very high temperatures.
⚖️ Detailed Analysis of the Options
Let's classify each of the given plastics.
(a) Polyethylene, (b) Nylon, and (c) PVC
Classification: All three of these are classic examples of thermoplastics.
• Polyethylene (PE) is used for plastic bags, bottles, and films.
• Nylon is a strong engineering thermoplastic used for gears, bearings, and fibers.
• Polyvinyl Chloride (PVC) is used for pipes, window frames, and flooring.
All of them can be melted down and remolded, which is why they are widely recycled.
(d) Bakelite
Classification: Bakelite (a phenol-formaldehyde resin) is the original industrial thermosetting plastic.
Why it's the answer: When Bakelite is first produced, it is heated in a mold. This heat causes a chemical reaction that creates a rigid, three-dimensional cross-linked network. Once this reaction is complete, the Bakelite is permanently set. It cannot be melted again. This is why it was used for heat-resistant applications like electrical insulators, pot handles, and old telephone casings. It is not a thermoplastic.
💡 Study Tips for Plastic Types
- Thermo-Plastic = Plastic when Heated: These plastics become soft and moldable (plastic) every time you heat them. They are generally recyclable.
- Thermo-Set = Set by Heat: These plastics are "set" into a permanent shape by the initial heating process. This change is irreversible. Think of it like baking a cake—you can't turn the cake back into batter by heating it again.
- Bakelite is the Classic Thermoset: Remember Bakelite as the original, old-fashioned, hard, black plastic that doesn't melt.