Class 7 Science Fibre to Fabric Chapter Notes

Class 7 Science Fibre to Fabric Chapter Notese. There are many more useful educational material which the students can download in pdf format and use them for studies. Study material like concept maps, important and sure shot question banks, quick to learn flash cards, flow charts, mind maps, teacher notes, important formulas, past examinations question bank, important concepts taught by teachers. Students can download these useful educational material free and use them to get better marks in examinations.  Also refer to other worksheets for the same chapter and other subjects too. Use them for better understanding of the subjects.

This Class 7 Science chapter notes page on Fibre to Fabric covers animal fibres (wool and silk), the processing of wool and the life history of the silk moth, reeling of silk from cocoons, polymers and their types with monomers and structures, classification of polymers on the basis of molecular forces, and natural and synthetic rubber including vulcanisation.

Topics Covered

1. Animal fibre: wool and silk
2. From fibres to wool
3. Processing fibres into wool
4. Silk and life history of silk moth
5. From cocoon to silk
6. Polymers
7. Important polymers and their uses
8. Classification of polymers on the basis of molecular forces
9. Natural and synthetic rubbers and vulcanisation of rubber

Animal Fibre: Wool and Silk

Wool: Wool comes from sheep, goat, yak and some other animals. These wool-yielding animals bear hair on their body. Hair traps a lot of air. Air is a poor conductor of heat, so hair keeps these animals warm. Wool is derived from these hairy fibres.

Animals that Yield Wool

Several breeds of sheep are found in different parts of our country. However, the fleece of sheep is not the only source of wool. Sheep wool is commonly available in the market. Yak wool is common in Tibet and Ladakh. Angora wool is obtained from angora goats.

Wool is also obtained from goat hair. The under fur of the Kashmiri goat is soft. It is woven into fine shawls called Pashmina shawls.

The fur (hair) on the body of camels is also used as wool. Llama and alpaca, found in South America, also yield wool.

From Fibres to Wool

Rearing and breeding of sheep: Shepherds take their herds of sheep for grazing. Sheep are herbivores and prefer grass and leaves. Apart from grazing, rearers also feed sheep on a mixture of pulses, corn, jowar, oil cakes (material left after taking out oil from seeds) and minerals.

Certain breeds of sheep have a thick coat of hair on their body, which yields good quality wool in large quantities. As mentioned earlier, these sheep are "selectively bred" with one parent being a sheep of a good breed.

Processing Fibres into Wool

The wool which is used for knitting sweaters or for weaving shawls is the finished product of a long process, which involves the following steps:
Step I: The fleece of the sheep, along with a thin layer of skin, is removed from its body. This process is called shearing.
Step II: The sheared skin with hair is thoroughly washed in tanks to remove grease, dust and dirt. This is called scouring.
Step III: After scouring, sorting is done. The hairy skin is sent to a factory where hair of different textures is separated or sorted.
Step IV: The small fluffy fibres, called burrs, are picked out from the hair. These are the same burrs which sometimes appear on your sweaters. The fibres are scoured again and dried. This is the wool ready to be drawn into fibres.
Step V: The fibres can be dyed in various colours, as the natural fleece of sheep and goats is black, brown or white.
Step VI: The fibres are straightened, combed and rolled into yarn. The longer fibres are made into wool for sweaters, and the shorter fibres are spun and woven into woollen cloth.

Wool industry is an important means of livelihood for many people in our country. But a sorter's job is risky, as sometimes they get infected by a bacterium, anthrax, which causes a fatal blood disease called sorter's disease.

Silk

Silk fibres are also animal fibres. Silkworms spin the silk fibres. The rearing of silkworms for obtaining silk is called sericulture.

Life History of Silk Moth

The female silk moth lays eggs, which hatch into larvae called caterpillars or silkworms. They grow in size, and when the caterpillar is ready to enter the next stage of its life history, called pupa, it first weaves a net to hold itself. During these movements of the head, the caterpillar secretes a fibre made of protein, which hardens on exposure to air and becomes silk fibre. Soon the caterpillar completely covers itself by silk fibres. This covering is known as cocoon. The further development of the moth continues inside the cocoon. Silk fibres are used for weaving silk cloth. Can you imagine that the soft silk yarn is as strong as a comparable thread of steel?

[Figure: Adult silk moths, (a) male and (b) female; (c) eggs on mulberry leaves; (d) silkworm; (e) cocoon; (f) cocoon with developing moth]

The silk yarn (thread) is obtained from the cocoon of the silk moth. There is a variety of silk moths, which look very different from one another, and the silk yarn they yield is different in texture (coarse, smooth, shiny, etc.). Thus, tassar silk, mooga silk, kosa silk, etc. are obtained from cocoons spun by different types of moths. The most common silk moth is the mulberry silk moth. The silk fibre from the cocoon of this moth is soft, lustrous and elastic, and can be dyed in beautiful colours.

From Cocoon to Silk

Rearing silkworms: A female silk moth lays hundreds of eggs at a time. The eggs are stored carefully on strips of cloth or paper and sold to silkworm farmers. The farmers keep the eggs under hygienic conditions and under suitable conditions of temperature and humidity.
The eggs are warmed to a suitable temperature for the larvae to hatch from the eggs. This is done when mulberry trees bear a fresh crop of leaves. The larvae, called caterpillars or silkworms, eat day and night and increase enormously in size.

[Figure: Rearing silkworms. (a) Female silkworm moth with eggs; (b) mulberry tree with a leaf of mulberry shown separately; (c) larva (caterpillar or silkworm) feeding on mulberry leaves; (d) cocoons]

The worms are kept in clean bamboo trays along with freshly chopped mulberry leaves. After 25 to 30 days, the caterpillars stop eating and move to a tiny chamber of bamboo in the tray to spin cocoons. Small racks or twigs may be provided in the trays to which cocoons get attached. The caterpillar or silkworm spins the cocoon, inside which the silk moth develops.

Processing silk: A pile of cocoons is used for obtaining silk fibres. The cocoons are kept under the sun, or boiled, or exposed to steam. The silk fibres separate out. The process of taking out threads from the cocoon for use as silk is called reeling the silk.

Polymers

Polymers are high molecular mass substances consisting of a large number of repeating structural units derived from simple molecules called monomers.

Monomers: The simple molecules which combine to give polymers are called monomers. The process by which the simple molecules (monomers) are converted into polymers is called polymerisation.

Homopolymer: A polymer formed from one type of monomer is called a homopolymer. For example, ethylene is polymerised to polyethylene (polythene):
n CH₂=CH₂ → (-CH₂-CH₂-)_n
Ethylene (monomers) → Polyethylene or polythene (polymer)

Co-polymer: A polymer formed from two or more different monomers is called a co-polymer or mixed polymer. For example, nylon-6,6 is formed from hexamethylene diamine and adipic acid:
n H₂N-(CH₂)₆-NH₂ + n HOOC-(CH₂)₄-COOH → (-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-)_n + n H₂O
Nylon-6,6

Important Polymers and their Uses

Classification of Polymers on the Basis of Molecular Forces

A large number of applications of polymers depend upon their mechanical properties, such as tensile strength, elasticity, toughness, etc.

Depending upon the intermolecular forces, polymers have been classified into four types:
1. Elastomers
2. Fibres
3. Thermoplastics
4. Thermosetting polymers

1. Elastomers: The polymers that have elastic character like rubber are called elastomers. For example, buna-S, buna-N, etc.
2. Fibres: These are the polymers which have strong intermolecular forces between the chains. The common examples are nylon-6,6, terylene, silk, etc.
3. Thermoplastics: These are the polymers which can be easily softened repeatedly when heated, and hardened and cooled with little change in their properties. Common examples of thermoplastics are polythene, polystyrene, polyvinyl chloride, teflon, etc.
4. Thermosetting polymers: These are the polymers which undergo permanent change on heating. The common examples are bakelite, melamine, formaldehyde, etc.

Natural and Synthetic Rubbers

Natural rubber: Rubber is a naturally occurring polymer. It is obtained as latex from rubber trees. Rubber latex is a colloidal suspension of rubber in water. Rubber trees are found in tropical and semi-tropical countries such as India (southern part), Malaysia, Indonesia, Ceylon, South America, etc. Natural rubber is highly elastic. It can be elastically deformed but regains its original shape after the stress is relieved. This elasticity makes it valuable for a variety of uses.

Natural rubber is a polymer of isoprene (2-methylbuta-1,3-diene), which has the formula CH₂=C(CH₃)-CH=CH₂.

[Figure: Structure of isoprene (2-methylbuta-1,3-diene): a chain of four carbon atoms with a CH₃ group attached to the second carbon, with double bonds between the first and second carbons and between the third and fourth carbons]

In natural rubber, about 11,000 to 20,000 isoprene units are linked together in a chain-like arrangement. When n isoprene units combine, the polymer has the formula (-CH₂-C(CH₃)=CH-CH₂-)_n, which is polyisoprene, the natural rubber.

Vulcanisation of Rubber

The process of heating natural rubber with sulphur to improve its properties is called vulcanisation.

Vulcanization of Rubber