Organic Chemistry Of Plastics (Thermoplastics vs. Thermosetting plastics)
Definition of Terms
Plastics: Synthetic or semi-synthetic organic polymers with high molecular mass, which can be molded into various shapes when soft and then hardened for use.
Thermoplastics: Polymers that soften and melt when heated, and harden when cooled. This process is completely reversible, allowing them to be remolded and recycled multiple times because their polymer chains are held together by weak intermolecular forces.
Thermosetting Plastics: Polymers that change irreversibly into an infusible, insoluble mass upon heating. Once molded and set, they cannot be softened, remolded, or recycled due to the formation of strong covalent cross-links between the chains.
WAEC Theory Questions & Solutions
Question 1
(a) State two major differences between thermoplastics and thermosetting plastics.
(b) Give two examples of each type of plastic.
(a) State two major differences between thermoplastics and thermosetting plastics.
(b) Give two examples of each type of plastic.
SOLUTION
(a) Differences:
| Feature | Thermoplastics | Thermosetting Plastics |
|---|---|---|
| Effect of Heat | Soften and melt on heating. | Do not soften; decompose on excessive heating. |
| Structure | Linear or branched long chains with no cross-links. | Heavily cross-linked three-dimensional structures. |
| Recyclability | Can be remolded and recycled. | Cannot be remolded or recycled. |
- Thermoplastics: Polyethene (Polythene), Polyvinyl Chloride (PVC), Polystyrene.
- Thermosetting Plastics: Bakelite, Urea-formaldehyde, Melamine.
Question 2
Explain why a plastic bucket made of polyethene can be melted and reshaped, whereas a plastic electrical switch made of Bakelite chars and ruins when exposed to high heat.
Explain why a plastic bucket made of polyethene can be melted and reshaped, whereas a plastic electrical switch made of Bakelite chars and ruins when exposed to high heat.
SOLUTION
Polyethene is a thermoplastic consisting of linear polymer chains held together by weak Van der Waals forces. When heated, these weak forces are easily broken, allowing the chains to slide over one another, melt, and be reshaped.
On the other hand, Bakelite is a thermosetting plastic. During its manufacturing, strong covalent cross-links form between its polymer chains, creating a rigid 3D network. Heating does not break these cross-links to allow melting; instead, excessive heat breaks the covalent bonds within the structure itself, causing the material to char, decompose, and ruin irreversibly.
WAEC Objective Questions & Solutions
1. Which of the following polymers is a thermoplastic?
Answer: B
Solution: Polyethene is a classic example of a thermoplastic as it consists of linear chains without cross-linking. Bakelite and urea-formaldehyde are thermosetting materials.
Solution: Polyethene is a classic example of a thermoplastic as it consists of linear chains without cross-linking. Bakelite and urea-formaldehyde are thermosetting materials.
2. The primary reason why thermosetting plastics cannot be remolded after setting is the presence of:
Answer: C
Solution: Thermosetting plastics undergo a chemical change during setting, forming strong covalent cross-links between polymer chains that permanently lock the structure into place.
Solution: Thermosetting plastics undergo a chemical change during setting, forming strong covalent cross-links between polymer chains that permanently lock the structure into place.
3. Which of the following items is most likely manufactured from a thermosetting plastic?
Answer: B
Solution: Electrical switches must handle heat without losing shape or melting for safety reasons. They are made from Bakelite, which is a thermosetting plastic.
Solution: Electrical switches must handle heat without losing shape or melting for safety reasons. They are made from Bakelite, which is a thermosetting plastic.
4. A plastic that softens on heating and hardens on cooling repeatedly without changing its chemical properties is classified as:
Answer: B
Solution: This physical, reversible behavior under temperature changes defines thermoplastic materials.
Solution: This physical, reversible behavior under temperature changes defines thermoplastic materials.
5. Which of the following pairs contains only thermosetting plastics?
Answer: B
Solution: Both Bakelite and Urea-formaldehyde develop heavily cross-linked structural networks and cannot be remolded or recycled.
Solution: Both Bakelite and Urea-formaldehyde develop heavily cross-linked structural networks and cannot be remolded or recycled.
6. What type of forces exist between the linear chains of thermoplastics?
Answer: C
Solution: Thermoplastics are held together by weak secondary intermolecular forces (Van der Waals forces), which easily yield when thermal energy is applied.
Solution: Thermoplastics are held together by weak secondary intermolecular forces (Van der Waals forces), which easily yield when thermal energy is applied.
7. Which of the following statements is NOT true of thermoplastics?
Answer: B
Solution: Option B describes thermosetting plastics. Thermoplastics soften when heated rather than hardening permanently.
Solution: Option B describes thermosetting plastics. Thermoplastics soften when heated rather than hardening permanently.
8. PVC stands for Polyvinyl Chloride. It is widely used for making sewage pipes because it is a thermoplastic. This means PVC:
Answer: C
Solution: Because PVC is a thermoplastic, technicians can safely apply mild heat on-site to alter its shape, bend it, or widen the joints for plumbing networks.
Solution: Because PVC is a thermoplastic, technicians can safely apply mild heat on-site to alter its shape, bend it, or widen the joints for plumbing networks.
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