NCERT Class 8 Science Chapter 3 - Synthetic Fibres and Plastics Notes

Chapter 3 - Synthetic Fibres and Plastics Notes

1. Natural Fibres
• A fibre is a thin thread of a natural or artificial substance, especially one that is used to make cloth or rope.
• Fibres can be classified into 2 main categories: natural and synthetic.
• Natural fibres are obtained from natural sources such as animals and plants

2. Natural fibres
• Cotton from plant
• Silk from Silk worms
• Wool from sheep
• Linen from flax plant
• Hair fibres from rabbits and goats

3. Plant Fibres
• Cotton: Cotton fibre is obtained from the cotton plant. It is one of the traditional fibres used in the textile industry.
• Linen: Linen fabric is obtained from the flax plant. The properties of linen fabric are very much similar to cotton fabric.
• Jute: It is obtained from the jute plant. On account of its high strength, it is perfect for use in packaging material.

4. Animal Fibres
• Silk: Silk is obtained from silk worms. The most popular kind of silk is obtained from the mulberry silk worm.
• Wool: Wool is commonly obtained from sheep. Wool fabric is soft to the touch and provides warmth to the weather, due to which it is the preferred choice for winter apparel.
• Yak fibre: The yak is an animal that is largely found in the Himalayas in India and Tibet. The hair of the yak is very useful in the production of warm clothes, mats and sacks.

5. Synthetic Fibres
• Synthetic fibres are man-made fibres.
• Synthetic fibres are made from different chemicals, hence each kind of synthetic fibres have their own properties.
• Synthetic fibres are more in length and are long lasting.
• The only limitation in synthetic fibres is that they are poor absorbents of moisture and they catch fire easily.
• Many items of clothing contain materials such as polyester, polyamide, terylene, rayon etc.
• The items previously mentioned are all man-made fibres, and are called synthetic fibres.

6. Synthetic Fibres
• A synthetic fibre is a chain of small chemical units joined together.
• Each small unit is called a monomer.
• Large chain formed by many monomers is called a Polymer.
• The process of combining small monomers to form a large polymer is called Polymerisation.

7. Rayon
• Rayon is prepared from cellulose.
• Though cellulose is a natural polymer it needs extensive chemical treatment to form rayon.
• It is also called artificial silk.
• It absorbs moisture and is comfortable to wear.
• It can be easily dyed in vivid colors.

8. Properties of rayon
• Easily dyed and wooven into cloth
• Drapes well
• Lustrous in appearance
• Good absorbent of sweat

9. Uses of rayon
• Mixed with fibre glass for making helmets
• Used to make jackets and tracksuits
• Surgical Dressings
• Reinforcing nylon tyres
• Upholstery for luxury cars and office and home furnishings
• Curtains because it drapes well
• Dress material because it is soft, silky and moisture absorbent

10. Nylon
• Chemically it is a ‘polyamide’, a polymer. It is the strongest synthetic plastic material which can be moulded to any shape.
• It is strong and easy to dye.
• Nylon fibres do not absorb water easily.
• It is elastic in nature and hence, highly durable.

11. Properties of nylon
• Toughest elastic material
• High tensile strength
• Slightly water absorbent, dries quickly
• It can be moulded into fibres, bristles, sheets, rods, tubes and coatings
• Resists oil, grease, moths, fungus and other pests
• Can be made into palates or powdered
• Is lustrous and easy to wash

12. Uses of nylon
• Stretchable wears
• Seatbelts
• Upholstery
• Ropes
• Reel thread
• Thread
• Sleeping bags
• Swimming wears
• Raincoats
• Tracksuits
• Fishing nets and fishing lines
• Making sarees

13. Polyester
• Alcohol and organic acid react together to make compounds called esters which are polymers.
• Polyesters come under the brand names of ‘Terylene’, ‘Dacron’, ‘Terene’ and ‘Polyester’.

14. Properties of polyester
• Resistant to pests and chemicals
• Dries quickly
• Easy to wash, does not shrink or stretch
• Forms a permanent crease if heated and pressed
• Tough, light weight and elastic

15. Uses of polyester
• Making magnetic audio and video cassettes and floppy disks
• Highly suitable for shirting, suiting, sarees and draperies mixed with fibre glass for moulding it into helmets, protective sheets and hulls in boats

16. Acrylic
• Acrylic is often used as a substitute for wool.
• Acrylic fibres are strong and wrinkle-resistant.
• It is easy to wash and dries quickly.
• It is resistant to moths, oils, chemicals and deterioration on exposure to sunlight.

17. Properties of acrylic
• Soft with crimps resembling wool
• Is durable light and washable
• Strong and stain resistant
• Also available in sheets
• Can be dyed in different colors

18. Uses of acrylic
• Acrylic knitting wool is good for baby wears because they are soft, warm and washable.
• Used for making artificial fur, blankets, carpets etc.
• A transparent type of acrylic is used to cover automobile lights, lenses surgical tools etc

19. Blending Fibres
• Terylene and cotton blend, known as terrycot is used for making dress materials.
• Terylene and wool blend is known as terrywool is used as a substitute for costly woollen clothes.
• Polyester and cotton blend is more crease-resistant.
• Acrylic and wool blend trousers are less expensive.

20. Advantages of Synthetic fibres
• Synthetic fibres are strong so they can take up heavy things easily.
• Synthetic fibres retain their original shape so it's easy to wash and wear.
• Can easily be stretched out.
• Synthetic fibres are generally soft so they are used in clothing materials.
• Varieties of colours are available as they are manufactured.
• Clothes made by synthetic fibres are generally cheaper than those made by natural fibres.

21. Disadvantages of Synthetic Fibres
• Synthetic fibres do not absorb sweat, trapping heat in our body.
• Synthetic fibres may give rough feel, making it unsuitable for pyjamas, underwear, etc.
• It is dangerous to wear near fire, as they catch fire easily.
• They cannot be easily ironed as they melt very easily.

22. Types of plastics
• Thermosetting Plastics
• Thermoplastics

23. Thermosetting plastics
• Thermoset plastics are hard and rigid. Example is Bakelite and melamine.
• Thermoset can be moulded to set it in any shape but it cannot be remoulded.
• It is dark in color, hard and resistant to heat and electricity.
• It is being widely used for the handle of kettles and pans.
• Examples of thermoset plastics are thermoset handles, a fireman’s uniform coated with thermoset plastic to make it fire resistant etc.

24. Thermoplastics
• Thermoplastics are soft and flexible.
• They are not elastic like rubber and steel springs.
• They melt on warming and regain their shape on cooling.
• Thermoplastics can be drawn into fine fibres, moulded to any desired shape or stretched or spread as sheets.
• Some of the better known thermoplastics are nylon (polyamide), polyesters, polyethylene, polyvinyl chloride (PVC), acrylic, polyurethane, polypropylene (PP), poly- tetra-fluoro-ethylene (PTEE) etc.
• Examples of Thermoplastics are plastic table and chairs, plastic food containers etc.

25. Properties of Plastics
• Resist corrosion
• Resist high temperatures
• Insulate heat and electricity
• Elastic
• Thermoplastics can be recycled
• Not reactive to Chemicals and water
• Water resistant
• Soft when hot

26. Plastics and the environment
• Recycling waste plastic is very important. It may kill many animals as well as polluting Earth.
• Plastics are not biodegradable.
• Few years ago, scientists have discovered photodegradable plastics that can be broken down by sunlight, but it takes very long time to rot.
• Recently, scientists developed biopolymers, biodegradable polymers. They rot very easily, for example plastics made from corn starch, dissolves rapidly in water.

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