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Synthetic Fibres and Plastics

CBSE Class 8 Science • Chapter 13 • Detailed Master Notes

Chapter Overview

We use many items made of plastic and synthetic materials. In this chapter, we will learn how synthetic fibres are made, their characteristics, and how plastic affects the environment.

13.1 What are Synthetic Fibres?

Fabrics are made from fibres. Fibres are obtained from natural or artificial sources.

Natural Fibres: Fibres obtained from plants and animals (e.g., Cotton, wool, silk).
Synthetic Fibres: Fibres made by human beings in factories using petrochemicals.

Polymer: A synthetic fibre is a chain of small chemical units joined together. Many small units combine to form a large single unit called a Polymer.

(Poly = many, mer = part/unit).

13.2 Types of Synthetic Fibres

A. Rayon (Artificial Silk)

Source: Rayon is obtained by the chemical treatment of wood pulp. It is a man-made fibre synthesized from a natural source.
Uses: It is similar to silk but cheaper. Rayon mixed with cotton forms bed sheets, and mixed with wool makes carpets.

B. Nylon

Formation: Prepared in 1931 from coal, water, and air. It is the first fully synthetic fibre.
Properties: Nylon is strong, elastic, and light. It is lustrous and easy to wash.
Uses: A nylon thread is stronger than a steel wire. It is used for parachutes, ropes, and tents.

C. Polyester and Acrylic

Polyester: A synthetic fibre that does not get wrinkled easily. Terylene is a popular polyester. PET is used for making water bottles.
Acrylic: Synthetic wool used to make sweaters and blankets for winter. It mimics real wool but is cheaper.

Topic Practice: Fibres

Q1. Why is it dangerous to wear synthetic clothes near fire?

Ans: Synthetic fibres melt upon heating. If a synthetic fabric catches fire, it melts and sticks to the body of the person wearing it, causing severe burns.

13.3 Plastics

Plastic is a polymer. In some plastics, the arrangement of units is linear, whereas in others it is cross-linked.

Type of Plastic	Description	Examples & Uses
Thermoplastics	Plastics that get deformed easily on heating and can be bent.	Polythene and PVC. Used for toys and containers.
Thermosetting Plastics	Plastics that cannot be softened by reheating once moulded into a shape.	Bakelite: Used for electrical switches. Melamine: Resists fire; used for floor tiles.

13.4 Plastics and the Environment

Plastics are non-reactive, strong, light, and poor conductors of heat and electricity. However, disposal is a major problem.

Biodegradable: Material that decomposes through natural processes. (e.g., paper, vegetable peels).
Non-biodegradable: Material that is not easily decomposed by natural processes. Plastics take hundreds of years to decompose.

The 4R Principle

To save our environment, we must follow the 4R principle:

Reduce → Reuse → Recycle → Recover