CBSE Class 8 Science Chapter 3: Synthetic Fibres and Plastics Notes

Understanding Polymers and Polymerisation

All synthetic fibres and plastics are polymers. A polymer is a giant molecule formed by joining many small chemical units called monomers in a repeating chain. The word polymer comes from two Greek words: poly meaning many and mer meaning unit or part. Think of monomers as individual paperclips and a polymer as a long chain made by linking them together. This chemical process of linking monomers to form a polymer is called polymerisation.

In nature, polymers like cellulose are found in cotton, which is made up of a large number of glucose units. Synthetic polymers, however, are made from petrochemicals using chemical processes in industries. They can have either linear or cross-linked arrangements of monomers, which deeply influences their physical characteristics and melting points.

Thermoplastics vs. Thermosetting Plastics

Types of Common Synthetic Fibres

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Important Exam Definitions

Monomer

The single, basic repeating chemical unit that combines to form a larger polymer.

Polymer

A large molecule composed of many repeating monomer units joined together chemically.

Petrochemicals

Chemical substances obtained from petroleum or natural gas, used as raw materials for synthetic fibres.

Thermoplastics

Plastics that can be melted, softened, and remoulded repeatedly upon heating.

Thermosetting Plastics

Plastics that, once set into a mould, cannot be softened or reshaped by heating.

Biodegradable

Materials that can be easily decomposed by natural biological processes such as bacterial action.

Non-biodegradable

Materials that are not easily decomposed by natural biological processes and persist in the environment.

Must Remember Points

• Synthetic fibres are human-made fibres using chemical substances, unlike natural fibres derived from plants or animals.
• Nylon is stronger than a steel wire of the same thickness, making it ideal for high-stress items like climbing ropes and parachutes.
• Rayon is classified as semi-synthetic because its raw material is natural wood pulp, though chemically processed in a laboratory.
• Plastics do not rust or corrode because they do not react with water and air, which makes them highly durable but environmentally hazardous.
• Bakelite is a poor conductor of heat and electricity, making it the perfect choice for electrical switches and cooking utensil handles.
• Melamine is highly versatile and fire-resistant; it is used to make floor tiles, break-resistant kitchenware, and uniforms for firefighters.
• Plastics take several hundred years to decompose, leading to severe soil and water pollution when disposed of in landfills.
• The 5R principle to reduce plastic pollution stands for: Reduce, Reuse, Recycle, Recover, and Refuse.

CBSE Exam Warning & Cues

CBSE Exam Target: A very common 3-mark question asks why nylon is used for making parachutes or why handles of cooking utensils are made of thermosetting plastics. Always structure your answer by citing the specific property: for nylon, state its high tensile strength and elasticity; for utensil handles, specify that Bakelite is a poor conductor of heat and does not melt when exposed to high temperatures. Also, remember that Rayon is semi-synthetic, not fully synthetic, because it uses wood pulp as its base material—this is a classic MCQ trap!

Quick Revision Check

• Why are synthetic fibres preferred over natural fibres for manufacturing activewear? Synthetic fibres like polyester are lightweight, durable, wrinkle-free, dry very quickly, and do not absorb moisture/sweat, making them ideal for high-intensity sports clothing.
• Explain why plastic containers are favored for storing industrial chemicals. Plastics are non-reactive. They do not react with water, air, acids, or chemicals. Thus, they do not corrode, keeping the stored chemicals safely contained without leakage.
• What is the difference between linear polymer and cross-linked polymer arrangements? In linear polymers, monomer units are joined end-to-end in straight chains (common in thermoplastics). In cross-linked polymers, monomer chains are interconnected in a three-dimensional network (common in thermosetting plastics).
• How does the 'Refuse' principle in the 5R system help the environment? 'Refuse' means saying 'No' to single-use plastics like disposable bags, cups, and straws. By refusing to buy or accept them, we drastically lower the demand and subsequent plastic waste generation.

Frequently Asked Questions