CBSE Notes Class 8 Science - Synthetic Fibres And Plastics

Synthesis involves creating something, and the term synthetic refers to something artificial. Thus, fibres produced by humans are termed synthetic fibres. These fibres are chains of small chemical units joined together. When numerous such units come together, they form a large, cohesive structure called a polymer.

1.0Introduction  

Fibres are thin, long, and flexible thread-like structures used to make various materials, especially fabrics. These are classified into two main types based on their origin:

• Natural Fibres: Sourced from plants or animals, e.g., cotton (from cotton plants), wool (from sheep), and silk (from silkworms).
• Synthetic Fibres: Man-made fibres from chemical substances, often petrochemicals. Examples include rayon (semi-synthetic), nylon, and polyester.

2.0Synthetic fibres 

Synthetic fibres are created through synthesis, where chemical substances form long chains called polymers. Polymers are made of repeating units called monomers. Examples: Nylon, Polyester, Acrylic, etc                          

These fibres offer specific properties like durability, flexibility, and strength, making them suitable for clothing, ropes, and other products.

1. Rayon: A Regenerated Fibre

Rayon is a semi-synthetic fibre made from natural cellulose (wood pulp) through chemical processing. It combines the characteristics of natural and synthetic fibres.

Characteristics:

• Versatility: Mimics silk, wool, cotton, and linen in texture and drape.
• Appearance: Known as "Artificial Silk" due to its resemblance to silk.
• Cost-Effective: Rayon is a more affordable alternative to silk, derived from silkworms but costly.

Rayon bridges the gap between natural and synthetic fibres, offering a luxurious feel at a more affordable price.

2. Nylon

Nylon, a fully synthetic fibre, was first created in 1931 and is notable for being the first synthetic fibre made without using any natural raw materials. It was synthesised using coal, water, and air. Nylon is highly valued for its strength, elasticity, and lightweight nature, and is also lustrous and easy to wash. Due to these properties, nylon is widely used in various products.

3. Polyester

Polyester is another synthetic fibre made from repeating units of a chemical compound known as ester. Polyester is highly durable, doesn’t wrinkle easily, and remains crisp, making it an ideal fabric for clothing. It is easy to wash and care for, enhancing its popularity for dress materials. A well-known example of polyester is terylene, which can be drawn into fine fibres and woven like any natural yarn.

PET (Polyethylene Terephthalate) is one of the most common forms of polyester. It is widely used in fabrics, bottles, utensils, films, and wires.

4. Acrylic 

Acrylic is a synthetic fibre composed of repeating units of the chemical compound acrylonitrile. It is commonly used as an alternative to natural wool. Known for its durability, softness, and wide range of available colours, acrylic is a popular material for making sweaters, blankets, and other garments designed for cold weather. Acrylic clothes are typically cheaper than wool and provide the same warmth, making them an affordable and convenient alternative.

3.0Characteristics and Advantages of Synthetic Fibres

The following features make synthetic fibres ideal for clothing, textiles, and various industrial uses.

• Cost-Effective: Cheaper to produce, making them affordable.
• Strength and Durability: Stronger and more durable than natural fibres.
• Abrasion Resistance: Highly resistant to friction and wear.
• Quick Drying: Absorb less water, drying faster than natural fibres.
• Low Maintenance: Easy to wash, resistant to moths and mildew.
• Wrinkle Resistance: Retain shape without wrinkling after washing.
• Lightweight: Comfortable and easy to wear.
• Smooth Texture: Provide a polished, sleek appearance.
• Affordable: Mass production makes them more affordable.

Disadvantages of Synthetic Fibres

• Low Moisture Absorption: Can feel uncomfortable in hot or sweaty conditions.
• Flammability: Easily catches fire when exposed to open flames.
• Heat Sensitivity: Melt quickly, making them difficult to iron or expose to high temperatures.

4.0Plastics 

Plastic is a polymer similar to synthetic fibres but with varying molecular structures. Plastics can have linear or cross-linked arrangements of units. They are highly versatile and can be moulded into various shapes and sizes.

Characteristics of plastics

Mouldable: Easily shaped into various forms.

Recyclable: Can be processed and reused through recycling.

Customisable: Can be coloured, melted, rolled into sheets, or formed into wires.

Poor Conductors: Do not conduct heat or electricity effectively.

Chemical and Water Resistant: Highly durable against chemicals and water.

Lightweight and Strong: Combines lightness with strength, making it versatile.

Non-Reactive: Suitable for a wide range of applications due to low reactivity.

Environmental Impact: Non-biodegradable, takes years to decompose, leading to ecological challenges.

Plastics are categorised into two main types based on their response to heat:

Thermoplastics

Properties: When heated, these plastics can be easily deformed and bent. They can also be re-melted and reshaped multiple times. Examples: Polythene and PVC.

1. Polythene

Characteristics:

• Low strength, hardness, and rigidity.
• Highly ductile, with good impact strength and low friction.
• Waxy feel, good electrical insulation, and resistance.

Types:

• Low-density polythene is made at 350-570 K with dioxygen or peroxide and is used for bags and film wraps.
• High-Density Polythene: Made at 333-343 K with metallocene catalysts; used for bottles and tubs.

Uses:

• Moulded into various shapes like bottles and pipes.
• Used for plastic bags and stretch films.
• High chemical resistance but degrades under UV radiation.

2. Polyvinyl Chloride (PVC)

PVC is a strong, rugged thermoplastic for insulation, pipes, hoses, raincoats, and seat covers. It is made from vinyl chloride monomer through condensation polymerisation.

Types:

• Rigid PVC: Used in construction (pipes, doors, windows), bottles, non-food packaging, and cards.
• Flexible PVC: Made softer with plasticisers, it is used in plumbing, electrical insulation, imitation leather, and flooring.

Characteristics:

• Hardness: High, increasing with molecular weight but decreasing with temperature.
• Heat Stability: Poor; requires a heat stabiliser.
• Insulation: Good electrical insulation properties.
• Chemical Resistance: Resists acids, salts, bases, fats, and alcohols, making it suitable for sewer piping.

Thermosetting Plastics

Properties: Once moulded and set, these plastics cannot be softened or re-shaped by heating. They maintain their shape and strength even at high temperatures. Examples: Bakelite, Melamine: 

a. Bakelite

• Type: Thermosetting polymer (phenol and formaldehyde)
• Characteristics: Hard, tough, heat and scratch-resistant, solvent-resistant, and good electrical insulator.
• Uses: Electrical switches, plugs, sockets, machine parts.

b. Melamine

• Type: Heat-resistant plastic with fire-retardant properties
• Characteristics: High heat tolerance, releases nitrogen gas when burned (fire retardant)
• Uses: Microwave-safe cookware, high-pressure laminates (dinnerware, flooring)

4. Teflon

• Type: Slippery fluorocarbon plastic
• Characteristics: Solid at room temperature, repels water and oil
• Uses: Non-stick cookware, waterproof fabrics, anti-friction devices, medical and lab coatings