Class 8 Notes

Class 8 Science Notes Chapter 3 (Synthetic Fibres and plastics) – Science Book

Ajay Mehra

— 5 min read

Science
Okay, let's begin with the detailed notes for Chapter 3: Synthetic Fibres and Plastics, focusing on aspects relevant for government exam preparation.

NCERT Class 8 Science - Chapter 3: Synthetic Fibres and Plastics

1. Introduction to Fibres

  • Fibre: A thread-like strand from which textiles are made.
  • Types of Fibres:
    • Natural Fibres: Obtained from plants (e.g., Cotton, Jute, Flax) or animals (e.g., Wool, Silk).
    • Synthetic Fibres: Man-made fibres produced using chemical processes, primarily involving petrochemicals. Also known as artificial fibres.

2. What are Synthetic Fibres?

  • Polymers: Synthetic fibres (and plastics) are polymers.
    • A polymer is a very large molecule formed by the combination of many small repeating units.
    • The word 'Polymer' comes from Greek: 'poly' meaning 'many' and 'mer' meaning 'part' or 'unit'.
  • Monomers: The small, repeating chemical units that join together to form a polymer.
  • Polymerization: The process of joining together a large number of small monomer molecules to form a large polymer molecule.
    • Example: Cellulose (found in cotton) is a natural polymer made of repeating glucose units.

3. Types of Synthetic Fibres

  • a) Rayon (Artificial Silk)

    • Source: Obtained by chemical treatment of wood pulp (cellulose, a natural polymer).
    • Key Point: Although derived from a natural source, it requires extensive chemical processing, hence classified as synthetic or sometimes semi-synthetic.
    • Properties:
      • Similar appearance and texture to silk.
      • Cheaper than natural silk.
      • Can be dyed in a wide variety of colours.
      • Absorbs moisture (more than other synthetics, less than cotton).
    • Uses:
      • Clothing (sarees, dresses, shirts).
      • Home furnishings (bedsheets, curtains).
      • Mixed with cotton to make bedsheets.
      • Mixed with wool to make carpets.
      • Medical applications (surgical dressings).

  • b) Nylon

    • Source: First fully synthetic fibre, made without using any natural raw material (plant or animal). Prepared from coal, water, and air (in 1931).
    • Properties:
      • Very strong (high tensile strength).
      • Elastic (stretches and regains shape).
      • Lightweight.
      • Lustrous (shiny appearance).
      • Easy to wash and dries quickly.
      • Abrasion-resistant (durable).
      • Does not absorb much water.
    • Uses:
      • Ropes (for climbing, fishing nets).
      • Parachutes and tents (due to strength and lightness).
      • Clothing (socks, stockings, swimwear).
      • Toothbrush bristles.
      • Car seat belts, sleeping bags.

  • c) Polyester

    • Source: Made from repeating units of a chemical called an ester. Esters are chemicals which give fruits their smell.
    • Key Examples: Terylene is a popular polyester. PET (Polyethylene terephthalate) is a very familiar form of polyester.
    • Properties:
      • Does not get wrinkled easily; remains crisp.
      • Easy to wash and dries quickly.
      • Durable and strong.
      • Resistant to chemicals and abrasion.
    • Blended Fabrics: Often blended with natural fibres:
      • Poly-cot: Polyester + Cotton
      • Poly-wool: Polyester + Wool
      • Terry-cot: Terylene + Cotton
    • Uses:
      • Dress materials, sarees.
      • PET: Bottles, jars, utensils, films, wires.
      • Water hoses for fire-fighting.
      • Conveyor belts.
      • Sails for boats.

  • d) Acrylic (Artificial Wool)

    • Source: Synthetic fibre made from polymerisation of acrylonitrile monomer.
    • Properties:
      • Resembles wool (lightweight, soft, warm).
      • Cheaper than natural wool.
      • Available in a variety of colours.
      • Resistant to moths, oils, chemicals.
      • Durable and wrinkle-resistant.
    • Uses:
      • Sweaters, shawls, blankets.
      • Carpets and upholstery.

4. Characteristics (Advantages) of Synthetic Fibres

  • Durability: Last longer than natural fibres.
  • Cost: Generally less expensive than natural fibres.
  • Availability: Readily available.
  • Maintenance: Easy to wash, dry quickly, wrinkle-resistant, require less ironing.
  • Strength: Generally high tensile strength.
  • Elasticity: Many are elastic.
  • Weight: Often lightweight.
  • Resistance: Resistant to moths, chemicals, and abrasion.

5. Disadvantages of Synthetic Fibres

  • Melting: Melt on heating. Can stick to the body if they catch fire, making them dangerous to wear near fire (e.g., kitchen, labs).
  • Comfort: Do not absorb sweat well, making them uncomfortable to wear in hot and humid weather.
  • Static Electricity: Can build up static charge, especially in dry weather.
  • Non-Biodegradable: Most synthetic fibres are non-biodegradable, contributing to pollution.

6. Plastics

  • Definition: Plastic is also a polymer, like synthetic fibres.
  • Arrangement of Units: Monomers in plastics can be arranged in:
    • Linear: Monomers form long straight chains.
    • Cross-linked: Monomers form chains that are linked sideways.
  • Mouldability: Plastics can be easily moulded into various shapes and sizes. They can be recycled, reused, coloured, melted, rolled into sheets, or made into wires.

7. Types of Plastics

  • a) Thermoplastics

    • Definition: Plastics which get deformed easily on heating and can be bent easily. They soften on heating and harden on cooling, and this process can be repeated.
    • Structure: Usually linear or slightly branched polymers.
    • Examples:
      • Polythene (Poly + ethene): Used for making carry bags, packaging films, containers.
      • PVC (Polyvinyl Chloride): Used for making pipes, footwear, insulation for electric wires, window frames, flooring, toys.
    • Properties: Can be remoulded.

  • b) Thermosetting Plastics (Thermosets)

    • Definition: Plastics which, when moulded once, cannot be softened by heating. They retain their shape even at high temperatures.
    • Structure: Heavily cross-linked polymers.
    • Examples:
      • Bakelite: Poor conductor of heat and electricity. Used for making electrical switches, handles of various utensils (cookers, pans), telephone casings.
      • Melamine: Resists fire and can tolerate heat better than other plastics. Used for making floor tiles, kitchenware (unbreakable dinner sets), fire-resistant fabrics (uniforms for firefighters).
    • Properties: Cannot be remoulded once set. Hard and rigid.

8. Plastics as Materials of Choice

  • Properties leading to widespread use:

    • Lightweight: Easy to handle and transport.
    • Lower Price: Generally cheaper than metals.
    • Good Strength & Durability: Long-lasting.
    • Non-reactive: Do not react with water, air, or most chemicals. Do not rust or corrode like iron. Used to store various materials, including chemicals.
    • Poor Conductors: Poor conductors of heat and electricity. Used as insulators (electrical wire coverings, handles of tools and utensils).
    • Mouldability: Can be moulded into complex shapes.
    • Versatility: Can be made transparent, opaque, coloured.

  • Specific Uses based on Properties:

    • Healthcare: Packaging tablets, threads for stitching wounds, syringes, gloves, medical instruments.
    • Cookware: Special plastic cookware used in microwave ovens (heat cooks food but doesn't affect plastic vessel). Teflon (a special plastic) used for non-stick coating on cookwares (oil and water do not stick).
    • Fire-proof Plastics: Melamine used in uniforms of firefighters has fire-resistant coating.

9. Plastics and the Environment

  • Biodegradable: Material which gets decomposed through natural processes, such as action by bacteria. (e.g., paper, cotton cloth, wood, vegetable peels).
  • Non-biodegradable: Material which is not easily decomposed by natural processes. Takes many years to decompose. (e.g., plastics, tin, aluminium, metal cans).
  • Environmental Problems caused by Plastics:
    • Pollution: Accumulate in the environment (landfills, water bodies), causing land and water pollution.
    • Harm to Animals: Animals may ingest plastic bags, leading to choking or internal blockages.
    • Clogging Drains: Plastic bags can clog drainage systems.
    • Soil Pollution: Affects soil fertility.
    • Air Pollution: Burning plastics releases poisonous fumes into the atmosphere.

10. Solutions: The 5R Principle

  • To minimize environmental damage from plastics, follow the 5R principle:

    • Reduce: Minimise the use of plastic items (e.g., use cloth or jute bags instead of plastic bags).
    • Reuse: Reuse plastic containers and bottles whenever possible.
    • Recycle: Collect and send plastic waste for recycling. Most thermoplastics can be recycled. (Note: Thermosetting plastics are difficult to recycle). Look for recycling symbols.
    • Recover: Generate energy from plastic waste (e.g., through incineration under controlled conditions).
    • Refuse: Avoid buying products with excessive plastic packaging or single-use plastics.

  • Other Measures:

    • Do not throw plastic bags in water bodies or on the road.
    • Segregate biodegradable and non-biodegradable waste separately.
    • Develop environmentally friendly alternatives to plastic.

Key Terms for Revision:

  • Synthetic Fibres, Polymer, Monomer, Polymerization, Rayon, Nylon, Polyester, PET, Acrylic, Thermoplastics, Thermosetting Plastics, Polythene, PVC, Bakelite, Melamine, Biodegradable, Non-biodegradable, 5R Principle.

Exam Focus Points:

  • Distinction between Natural and Synthetic Fibres.
  • Definition of Polymer, Monomer, Polymerization.
  • Specific properties and uses of Rayon, Nylon, Polyester, Acrylic (Match the Following, Identify the Fibre based on Use/Property).
  • Why Rayon is called 'artificial silk' and Acrylic 'artificial wool'.
  • Why Nylon is strong (uses like ropes, parachutes).
  • PET as a type of Polyester and its uses.
  • General advantages and disadvantages of synthetic fibres (especially melting property and non-absorbency).
  • Difference between Thermoplastics and Thermosetting plastics (definition, remouldability, structure, examples).
  • Specific examples: Bakelite and Melamine (properties and uses - especially heat/electrical resistance).
  • Reasons for widespread use of plastics (properties like non-reactive, lightweight, poor conductor).
  • Environmental impact of plastics (non-biodegradability, pollution).
  • Meaning of Biodegradable and Non-biodegradable.
  • The 5R Principle and its components.

These notes cover the core concepts of the chapter with details relevant for competitive exams. Remember to correlate properties with uses, understand the classifications, and be aware of the environmental aspects.

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