CLASS VIII | SCIENCE | CHAPTER 3
notes prepared by subhankar Karmakar
Definition
FIBRES: A very thin, thread-like strand from which cloth is made, is called a fibre. Fabric means cloth. Fabric is made by weaving or knitting long, twisted threads called 'yarn' made from fibres. The clothes which we wear are made of fabrics. Fabrics are made from fibres obtained from 'natural' or artificial' sources (synthetic sources). Thus, all the fibres can be divided into two groups:
(1) Natural fibres, and (ii) Synthetic fibres.
NATURAL FIBRES:
The fibres obtained from plants and animals are called natural fibres. Cotton, flax, jute, wool and silk are natural fibres. Cotton, flax and jute fibres come from plants whereas wool and silk come from animals.
SYNTHETIC FIBRES:
The synthetic fibres are made by human beings. Rayon, nylon, polyester and acrylic are synthetic fibres.
FIBRES ARE MADE OF POLYMERS:
POLYMER: A polymer is a 'very big molecule' formed by the combination of a large number of small molecules.
The small molecules (of chemical compounds) which join together to form a polymer are called 'monomers'.
The monomers which make a polymer may all be of the same compound' or of 'two different compounds'.
So, a polymer is made of many small 'repeating units' (of chemical compounds) called monomers.
Polymers are of two types :
Natural polymers and Synthetic polymers.
NATURAL POLYMERS:
Cotton, wool and silk are natural polymers. For example, cotton fibre is made of a natural polymer called cellulose. Cellulose is a polymer which is made up of a large number of small glucose molecules (or glucose units) joined one after the other. The walls of all the plant cells are made up of cellulose. So, wood contains a large amount of cellulose polymer. Thus, polymers occur in nature too.
SYNTHETIC POLYMERS:
Nylon, polyester, acrylic, polythene, polyvinyl chloride (PVC), bakelite, and melamine are synthetic polymers (or man-made polymers). For example, nylon fibre is made up of nylon polymer in which two different types of molecules (or monomer units) are combined alternately to form long chains.
SYNTHETIC FIBRES:
a. PRODUCTION OF SYNTHETIC FIBRES:
The man-made fibres produced from chemical substances are called synthetic fibres, Synthetic fibres are made in industry by the chemical process called 'polymerisation'. A synthetic fibre is a long chain of small units joined together. Each small unit is a chemical compound (called organic compound). Many, many such small units join together one after the other to form a very large single unit called polymer. It is this man made polymer which forms synthetic fibres. Thus, a synthetic fibre is a polymer made from the molecules of a monomer (or sometimes two monomers) joined together to form very long chains. Synthetic fibres are also known as man-made fibres or artificial fibres.
b. TYPES OF SYNTHETIC FIBRES:
Depending upon the type of chemicals used for manufacturing synthetic fibres, there are four major types of synthetic fibres (or man-made fibres). These are :
1. Rayon,
2. Nylon,
3. Polyester, and
4. Acrylic.
c. RAYON IS NOT FULLY SYNTHETIC:
Rayon is a man-made fibre made from a natural material called cellulose (obtained from wood pulp).
WOOD PULP:
Wood pulp is a soft, wet mass of fibres obtained from wood. Wood pulp contains a large amount of natural polymer called 'cellulose'.
d. FULLY SYNTHETIC FIBRES:
Nylon, polyester and acrylic are fully synthetic fibres which do not require a natural material (like cellulose) for their manufacture. These fully synthetic fibres are prepared by a number of processes by using raw materials (or chemical compounds) of petroleum origin, called petrochemicals.
RAYON & ITS CHARACTERISTICS:
Rayon is often regarded as artificial silk. It is a man-made fibre prepared from a natural raw material (called cellulose) by chemical treatment. The cellulose required for making rayon is obtained from 'wood pulp'. So, we can also say that is obtained by the chemical treatment of wood pulp (which contains cellulose).
• PRODUCTION OF RAYON:
Rayon is produced as follows:
(i) Wood pulp is dissolved in an alkaline solution (sodium hydroxide solution) to form a sticky liquid called 'viscose'.
(ii) Viscose is forced to pass through the tiny holes of a metal cylinder (called spinneret) into a solution of sulphuric acid when a silk like thread of rayon is formed.
• RAYON IS NOT FULLY SYNTHETIC FIBRE:
Since rayon is made from naturally occurring polymer (cellulose) present in wood pulp, therefore, rayon is neither a fully synthetic fibre nor a fully natural fibre. It is a semi-synthetic fibre. Rayon is different from truly synthetic fibres because it is obtained from a natural material (wood pulp).
• RAYON, THE ARTIFICIAL SILK:
Although rayon is obtained from a natural resource called wood pulp, yet it is said to be a man-made fibre. This is because it is obtained by the chemical treatment of wood pulp in factories. Rayon fibre is chemically identical to cotton but it has shine like silk. Since rayon resembles silk in appearance, therefore, rayon is also called artificial silk.
• ADVANTAGES OF RAYON:
Rayon is cheaper than natural silk and can be woven like silk fibres. Rayon can also be dyed in a variety of colours.
• USES OF RAYON:
1. Rayon is used in textile industry for making clothing like sarees, blouses, dresses, socks, etc.
2. Rayon (mixed with cotton) is used to make furnishings such as bed-sheets, curtains, blankets, etc.
3. Rayon (mixed with wool) is used to make carpets.
4. Rayon is used in medical field for making bandages and surgical dressings.
5. Rayon is used in tyre industry for the manufacture of tyre cord.
NYLON & ITS CHARACTERISTICS:
Nylon is a synthetic fibre. In fact, nylon is the first fully synthetic fibre made by man without using any natural raw materials (from plants or animals). It was made in the year 1931.
• SOURCE OF NYLON:
The chemical compounds (or monomers) used in making nylon are now obtained from petroleum products called petrochemicals. It is made up of the repeating units of a chemical called an 'amide'. So, nylon is a polyamide (which is a polymer).
The name NYLON comes from the fact that it was developed in New York (NY) and London (LON).
Nylon is a thermoplastic polymer (which can be melted by heating). Molten nylon is forced through the tiny holes in a spinneret to make nylon fibres (or nylon threads), or cast into desired shapes.
• PROPERTIES OF NYLON:
Some of the important properties of nylon fibres are as follows:
(i) Nylon fibres are very strong fairly elastic, lightweight and lustrous.
(ii) Nylon fibres absorb very little water, so clothes made of nylon are easy to wash and dry.
(iii) Nylon is wrinkle resistant.
(iv) Nylon fibres have high abrasion resistance (high wear and tear resistance), so they are very durable (long lasting).
(v) Nylon is not attacked by moths and ordinary chemicals.
Due to all these properties, nylon fibres have become very popular for making clothes.
• USES OF NYLON:
1. Nylon is used for making textiles (fabrics) like sarees, shirts, neck-ties, tights, socks and other garments.
2. Nylon is used in making curtains, sleeping bags and tents.
3. Nylon is used in making ropes, car seat belts, fishing nets, tyre cord, strings for sports rackets and musical instruments, bristles for toothbrushes and paint brushes.
4. Nylon is used for making parachutes and ropes for rock climbing.
5. Nylon is used as a plastic for making machine parts.
"All these uses of nylon are due to the high strength of nylon it is actually stronger than a steel wire of similar thickness."
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