Plastics
Classification of Plastics
Plastics are primarily classified into two categories based on their response to thermal treatment:
A. Thermoplastics
These polymers soften when heated and harden when cooled. This process is reversible, allowing them to be recycled and reshaped multiple times.
- Mechanism: They consist of linear or slightly branched chains held together by weak Van der Waals forces.
- Examples: Polyethylene (PE), Polyvinyl Chloride (PVC), Polystyrene (PS).
B. Thermosetting Plastics
Once molded and cured (set), these plastics undergo a chemical change and cannot be remelted or reshaped. Overheating leads to degradation rather than softening.
- Mechanism: They form strong cross-links (covalent bonds) between polymer chains, creating a three-dimensional network.
- Examples: Bakelite, Urea-formaldehyde, Epoxy resins.
Comparison Between Thermoplastic and Thermosetting Resins
| Thermoplastic Resins | Thermosetting Resins |
|---|---|
| 1. These resins become soft on heating and rigid on cooling by regaining original properties. These can be reshaped and used. | 1. They do not soften on heating and become hard. On prolonged heating they decompose and cannot get back its structure. Hence cannot be reshaped and used. |
| 2. The heating and cooling do not alter the chemical nature of these resins but involves changes in physical nature. | 2. These resins are permanent setting resins. |
| 3. They are formed by addition polymerization. | 3. They are formed by condensation polymerization. |
| 4. Small molecular weight compounds with linear structures. | 4. Large molecular weight compounds with three dimensional networks. |
| 5. They consist of long chain linear polymer with weak secondary Van der Waals forces of attraction in between them. | 5. Highly cross-linked structure; strong covalent bonds are responsible for strength. |
| 6. They soften on heating readily because the secondary force of attraction between the individual chain can break easily by heat, pressure or both. | 6. The bonds retain their strength on heating, hence do not soften on heating. |
| 7. These plastics can be reclaimed from waste. | 7. Cannot be reclaimed from waste. |
| 8. They are soft, weak and less brittle. | 8. They are hard, strong and more brittle. |
| 9. These resins are usually soluble in organic solvents. | 9. Due to strong bonds and cross links, they are insoluble in all organic solvents. |
| 10. Curing is achieved by cooling. Examples: PE, PS, PVC, Teflon. |
10. Curing is achieved by applying heat and pressure. Examples: Bakelite, Polyester and silicones. |
General Properties of Plastics
Corrosion Resistance: Plastics are chemically inert and do not rust like metals, making them ideal for chemical storage.
Low Density: They are lightweight compared to metals and ceramics, offering a high strength-to-weight ratio.
Thermal Insulation: Most plastics are poor conductors of heat, utilized in cookware handles and insulation.
Electrical Insulation: High dielectric strength makes them essential for wire coating and electronic components.
Durability: High resistance to environmental factors and impact.
Transparency: Many plastics (like Acrylic) offer optical clarity rivaling glass but with higher shatter resistance.
Plastics as Engineering Materials
Advantages of plastics over other engineering materials.
- Low fabrication cost, low thermal & electrical conductivities, high resistance to corrosion & solvents.
- The stress – strain relationship of plastics is similar to that of the metals.
- Plastics reduce noise & vibration in machines.
- Plastics are bad conductors of heat and are used to make handles for hot objects, most plastics are inflammable.
- Plastics are electrical insulators & find large scale use in the electrical industry.
- Plastics are resistance to chemicals.
- Plastics are clear & transparent so they can be given beautiful colours.