Synthetic Fibres and Plastics — AP Board Class 8 Physical Science
1. Natural vs Synthetic Fibres
Natural Fibres
Fibres obtained from plants or animals are called natural fibres.
| Source | Examples |
|---|---|
| Plants | Cotton, jute, coir (coconut fibre), flax |
| Animals | Wool (sheep), silk (silkworm) |
Synthetic Fibres
'Fibres made by humans using chemical processes are called SYNTHETIC or MAN-MADE fibres.' They are made from chemical substances called polymers — long chains of repeating chemical units called monomers.
'POLY = many, MER = unit. A polymer is a VERY long chain made of many small repeating units joined together.'
2. Types of Synthetic Fibres
2.1 Rayon — The Artificial Silk
- Made from wood pulp (cellulose) — a natural polymer.
- 'Rayon is called a SEMI-SYNTHETIC fibre because the raw material is natural but the manufacturing process is chemical.'
- It has a lustre (shine) similar to silk.
- It is CHEAPER than silk but absorbs water well (like cotton).
Properties: Soft, comfortable, breathable, lustrous.
Uses: Clothing (shirts, sarees, dresses), curtains, bedsheets, tyre cords, carpet backing.
2.2 Nylon — The First Truly Synthetic Fibre
- Made entirely from chemicals (coal, water, air).
- Discovered by Wallace Carothers in 1931 (DuPont company).
- 'Nylon is EXTREMELY strong, elastic, and lightweight. A nylon thread can support more weight than a steel wire of the same thickness!'
Properties: Very strong, elastic, abrasion-resistant, quick-drying, does not absorb water, lightweight.
Uses: Fishing nets, ropes, parachutes, seat belts, socks, stockings, tents, toothbrush bristles.
2.3 Polyester — The Wrinkle-Free Fibre
- Made from petroleum products.
- 'Polyester does NOT wrinkle easily — it is the CRUSH-RESISTANT fibre.'
- It dries very quickly.
- Examples: Terylene, PET (Polyethylene Terephthalate).
Properties: Wrinkle-resistant, strong, quick-drying, hold shape well.
Uses: Shirts, trousers, jackets, sails for boats, PET bottles, food packaging.
2.4 Acrylic — The Artificial Wool
- 'Acrylic feels like WOOL but is CHEAPER, LIGHTER, and MORE DURABLE.'
- Used as a substitute for wool.
Properties: Soft, warm, lightweight, does not absorb water easily, resistant to moths and chemicals.
Uses: Sweaters, shawls, blankets, carpets, sportswear, stuffed toys.
Comparison Table
| Fibre | Source | Characteristics | Main Uses |
|---|---|---|---|
| Cotton (natural) | Cotton plant | Absorbs water, breathable, comfortable | Clothing, bedsheets |
| Wool (natural) | Sheep | Warm, soft | Sweaters, blankets |
| Silk (natural) | Silkworm | Lustrous, smooth, strong | Sarees, ties |
| Rayon | Wood pulp (cellulose) | Silk-like, cheap, breathable | Shirts, curtains |
| Nylon | Coal/water/cair (chemicals) | Very strong, elastic, lightweight | Ropes, parachutes, nets |
| Polyester | Petroleum | Wrinkle-free, quick-drying | Shirts, PET bottles |
| Acrylic | Petroleum (chemicals) | Wool-like, lightweight | Sweaters, blankets |
3. Fibre Identification — Burning Test
| Fibre | Behaviour When Burned | Smell | Residue |
|---|---|---|---|
| Cotton | Burns readily, continues burning | Smell of burning paper | Grey ash |
| Wool | Burns slowly, self-extinguishes | Smell of burning hair | Crumbly black bead |
| Silk | Burns slowly, self-extinguishes | Smell of burning hair | Crumbly black bead |
| Nylon | Melts and burns | Chemical (celery-like) smell | Hard plastic bead |
| Polyester | Melts and burns | Chemical smell | Hard plastic bead |
| Acrylic | Burns with a sputtering flame | Acrid smell | Hard black bead |
4. Plastics — What Are They?
'Plastics are POLYMERS that can be MOULDED into any shape when soft and then SET into a permanent shape.' Like synthetic fibres, plastics are also made of long-chain polymer molecules.
Properties of Plastics
- Lightweight — Much lighter than metals.
- Durable — Do not rust or rot.
- Waterproof — Do not absorb water.
- Poor conductor of heat and electricity — Used as insulators (handles of frying pans, electrical plugs).
- Chemically resistant — Unaffected by many chemicals.
- Can be moulded — Can be made into any shape.
- Non-biodegradable — This is a SERIOUS environmental problem.
5. Thermoplastics vs Thermosetting Plastics
| Feature | Thermoplastics | Thermosetting Plastics |
|---|---|---|
| Effect of heat | Soften on heating, harden on cooling (REVERSIBLE) | Once set, cannot be softened again (IRREVERSIBLE) |
| Can be remoulded | YES — can be recycled | NO — cannot be remoulded |
| Polymer structure | Linear chains with weak bonds between chains | Cross-linked chains with strong bonds |
| Examples | Polythene, PVC, nylon, polystyrene | Bakelite, melamine, urea-formaldehyde |
| Uses | Carry bags, bottles, pipes, toys | Electrical switches, handles, dinnerware |
'Thermoplastics can be REHEATED and RESHAPED many times. Thermosetting plastics, once moulded, can NEVER be reshaped — if you heat them, they CHARR or BURN instead of melting.'
Common Plastics and Their Uses
| Plastic | Type | Uses |
|---|---|---|
| Polythene (polyethylene) | Thermoplastic | Carry bags, plastic bottles, films |
| PVC (Polyvinyl Chloride) | Thermoplastic | Pipes, raincoats, flooring, electrical insulation |
| Bakelite | Thermosetting | Electrical switches, handles of utensils |
| Melamine | Thermosetting | Dinner plates, cups, kitchenware (heat-resistant) |
| Teflon (PTFE) | Thermoplastic | Non-stick coating on cookware |
| PET | Thermoplastic | Water bottles, food containers |
6. Biodegradable vs Non-Biodegradable Materials
| Property | Biodegradable | Non-Biodegradable |
|---|---|---|
| Decomposition by microorganisms | YES — broken down | NO — cannot be broken down |
| Time taken to decompose | Days to months | Hundreds to thousands of years |
| Examples | Paper, wood, cotton, jute, food waste | Plastics, nylon, polyester, glass, metals |
'Most synthetic materials are NON-BIODEGRADABLE. A plastic carry bag takes about 450 YEARS to decompose in a landfill.'
Environmental Concerns
- Choked drainage systems — Plastic waste blocks drains, causing waterlogging and mosquito breeding.
- Soil pollution — Plastics in soil prevent water absorption and reduce soil fertility.
- Harm to animals — Animals often eat plastic bags (mistaking them for food) and die due to intestinal blockage.
- Air pollution — Burning plastic releases TOXIC gases like dioxins and furans.
- Microplastics — Plastic breaks into tiny particles that enter our food and water.
7. The 4R Principle
'The 4R principle is our STRATEGY to minimise plastic waste and protect the environment.'
| R | Meaning | Example |
|---|---|---|
| REDUCE | Use LESS plastic | Carry your own cloth bag; refuse plastic carry bags |
| REUSE | Use items AGAIN | Reuse plastic containers for storage; refill water bottles |
| RECYCLE | Convert waste into NEW products | Melt thermoplastics to make new items; PET → new bottles or fibres |
| RECOVER | RECOVER energy from waste | Burn plastic waste in controlled incinerators to generate energy |
'The most effective way is REDUCE — because the best waste is the waste that is NEVER created.'
AP Board and Government Context
'The Government of Andhra Pradesh has BANNED single-use plastics across the state under the Plastic Waste Management Rules. AP was among the first states to impose a blanket ban on plastics like carry bags, straws, and cutlery.' Students should be aware of the AP Pollution Control Board (APPCB) regulations regarding plastic waste management in their examinations.
8. Common Mistakes and Fixes
| Mistake | Why It's Wrong | Correction |
|---|---|---|
| "Rayon is a fully synthetic fibre" | Rayon is made from NATURAL cellulose (wood pulp) | Rayon is SEMI-SYNTHETIC |
| "All plastics are thermoplastics" | Some plastics (thermosetting) cannot be remoulded | Plastics are of TWO types: thermoplastics and thermosetting |
| "Nylon and rayon are the same" | Nylon is fully synthetic; rayon is semi-synthetic | They have DIFFERENT raw materials and properties |
| "Biodegradable means environmentally friendly always" | Some biodegradable items release methane in landfills | Biodegradable is BETTER but not a complete solution |
| "PET bottles can be used forever" | Repeated use can leach chemicals | PET bottles are for SINGLE use or limited reuse |
9. AP Exam Focus and Marks Distribution
| Topic | Likely Marks | Question Type |
|---|---|---|
| Natural vs synthetic fibres (definitions and examples) | 2 | Very short answer |
| Rayon, nylon, polyester, acrylic — properties and uses | 4 | Short answer / Table |
| Plastics: thermoplastics vs thermosetting | 3 | Short answer / Comparison table |
| Biodegradable vs non-biodegradable | 2 | Short answer |
| 4R principle and environmental concerns | 3 | Short answer / Long answer |
| Total | 14 marks |
10. Quick Self-Test
Q1: Why is rayon called a semi-synthetic fibre?
Q2: Which fibre is known as 'artificial wool'? Why?
Q3: Differentiate between thermoplastic and thermosetting plastic with examples.
Q4: What does the word 'polymer' mean?
Q5: Why is nylon used for making fishing nets and ropes?
Q6: What are the 4 Rs of waste management? Which one is the most important and why?
Q7: Why should we NOT burn plastic waste?
Q8: Give ONE property and ONE use each of bakelite and melamine.
Answers
A1: Rayon is called semi-synthetic because its raw material is NATURAL (cellulose from wood pulp), but the manufacturing process involves CHEMICAL treatment. It is not completely natural nor completely synthetic.
A2: Acrylic is known as artificial wool. It feels soft and warm like wool, but is CHEAPER, LIGHTER, and MORE DURABLE than natural wool.
A3: Thermoplastics soften on heating and can be remoulded (e.g., polythene, PVC). Thermosetting plastics once set cannot be softened again (e.g., bakelite, melamine). The difference lies in their molecular structure — thermoplastics have linear chains, thermosetting plastics have cross-linked chains.
A4: 'Polymer' comes from Greek: POLY = many, MER = part/unit. A polymer is a large molecule made up of many repeating smaller units called monomers joined together in a long chain.
A5: Nylon is used for fishing nets and ropes because it is VERY STRONG (stronger than a steel wire of the same thickness), ELASTIC (can stretch and return to shape), LIGHTWEIGHT, and RESISTANT to water and chemicals. It does not rot in water.
A6: The 4 Rs are: REDUCE, REUSE, RECYCLE, RECOVER. REDUCE is the most important because the best way to manage waste is to NOT create it in the first place. Reducing consumption saves resources and prevents waste generation entirely.
A7: Burning plastic releases TOXIC gases such as dioxins and furans, which are CARCINOGENIC (cancer-causing) and cause respiratory diseases. It also contributes to air pollution.
A8: Bakelite — property: poor conductor of electricity; use: electrical switches/plugs. Melamine — property: heat-resistant; use: dinner plates and kitchenware.
