Living Science 2020 2021 for Class 8 Science Chapter 3 - Synthetic Fibres And Plastics

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Rayon is a regenerating natural fiber (cellulose fiber), which is made from the wood pulp. Whereas nylon or polyester, fibers are purely synthetic polymers, made entirely from chemicals. 
 

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A polymer is made of two words, 'poly' means many and 'mer' means unit or part. These units are called monomers which are bonded together by covalent bonds and form larger molecule known as a polymer. For example, polythene is made of many ethene monomer units.

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Even with so many advantages, we do not wear clothes made of pure synthetic fibres because:

1. They do not absorb sweat or water and they stick to the body, which makes them uncomfortable to wear in hot humid conditions.
2. They catch fire very readily, they melt on heating and stick to the body. Therefore, they can cause severe burn injuries.

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The common feature between the molecular structures of synthetic fibre and plastics is that they both are polymers and are made up of a long chain of monomers. Plastics are basically synthetic fibres but all synthetic fibres are not plastics.

 

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Bakelite is a thermosetting polymer i.e. they can be softened by heating only once i.e. once they are moulded and harden on cooling, they cannot be softened again. Whereas, polythene is a thermoplastic polymer i.e. softening and hardening can be repeated again and again.
 

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Plastics are non-biodegradable i.e. they cannot be decomposed by the microorganisms such as bacteria.

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(b) cotton
Natural fibres are greatly elongated hair-like substances obtained from plants or animals. They can be spun into filaments, threads or ropes. Cotton fibre is almost pure cellulose that grows in a protective capsule around the seeds of cotton plants.

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(c) nylon
Fibres derived from chemicals are polymers. They do not occur naturally but are produced entirely in chemical plants or laboratories. Nylons are man-made polymer fibres.

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(a) rayon
Rayon is a manufactured regenerated cellulose fibre that is made from purified cellulose, primarily from wood pulp.

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(b) bakelite
Thermosetting plastics are rigid and heavily cross-linked polymers. They cannot be remoulded once they set. Bakelite is a thermosetting phenol formaldehyde resin.

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(d) all of these
A polymer is a macromolecule composed of many repeated subunits, known as monomers. Bakelite is a phenol formaldehyde resin. Polystyrene is a synthetic aromatic polymer made from the styrene monomer. Polythene is a thermoplastic polymer consisting of long hydrocarbon chains.

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(b) polythene
Polythene is a thermoplastic polymer consisting of long hydrocarbon chains. Thermoplastic polymers can be repeatedly heated and moulded into any desired shape.

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(c) perspex
Perspex is a thermoplastic. It is transparent like glass but it is much stronger. It is used as a substitute for glass in windows.

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(d) soluble in water
Plastics are insoluble in water. That is why plastic buckets are used to store water and plastic glasses are used to drink water.

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Natural materials that are used to manufacture human-made materials are known as raw materials.

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The most common source for the synthetic fibres is petroleum.

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Rayon is an artificial fibre that is made from wood pulp.

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True.
Natural polymers are the products of raw materials found in nature that can be extracted.
Wool is obtained from sheep and certain other animals. Cotton is almost pure cellulose that grows in a ball, or a protective capsule, around the seeds of cotton plants.

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Nylon is a synthetic fibre which is very strong, even stronger than steel wire. Therefore, it is commonly used to make strong ropes.

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Rayon, a synthetic fibre, is also called artificial silk.

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All plastics are made of long chain molecules called polymers.

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Polythene sheets are used for packing liquids.

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Thermocol is made from polystyrene plastic.

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Teflon is used to make non-stick pans. This is because teflon is a very slippery material. In addition to being slippery, the material also has a number of other features. It has high temperature resistance, little reaction to most chemicals and reduced stress-cracking and corrosion.

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Bakelite, a thermosetting plastic, is used to make plugs and switches. Thermosetting plastics are bad conductors of heat and electricity. They do not allow electricity to pass through, which is why they are used to make plugs and switches.

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Examples of natural fibres:
1) Cotton
2) Silk
Examples of synthetic fibres:
1) Nylon
2) Polyester

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The word ‘polymer’ comes from two Greek words ‘poly’ meaning many and ‘mer’ meaning part or unit. When smaller units of a chemical substance are combined to form a large single unit, it is called a polymer. Examples are cellulose, polythene etc.

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Rayon is called a regenerated fibre because its preparation involves the chemical breakdown of cellulose using an alkaline solution. This is followed by the regeneration of solidified synthetic fibres by passing them through spinnerets.

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Two disadvantages of synthetic fibres are:
1) Synthetic fibres can’t absorb moisture, which makes them unsuitable during summer. They stick to the body and cause skin irritation when the body sweats during summers.
2) They are dangerous to be worn near fire or heat as they can catch fire easily.

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Plastic is a synthetic material made from a wide range of organic polymers such as polyethylene, PVC, nylon etc., that can be moulded into shape while soft, and then set into a rigid or slightly elastic form.
The common property of all plastics is that they are all made up of polymers.

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Plastic bottles are commonly used to store chemicals in a chemical laboratory because plastics are non-reactive. They are inert to air and water. They are non-corrosive, therefore we can store chemicals in them safely.

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Following are the properties of nylon which make it a useful synthetic fibre:
1) Nylon fibre is very strong and elastic.
2) It is light and wrinkle resistant.
3) It is easy to wash and it absorbs less water, and so it can be dried quickly.

The uses of nylon are:
1) It is used to make ropes.
2) It is useful in making garments, socks and stockings.

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The uses of polyester are as follows:
1) Being very strong and crease resistant, polyester is used to make pants, shirts, suits and bed sheets.
2) Because of its water-resistant properties, it is used to make jackets and garments that are used in wet and damp environments.
3) Polyesters are used to make bottles, utensils and wires as they are light weight and elastic.

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Clothes purely made of synthetic fibres are not comfortable in hot and humid weather as they are water resistant and do not absorb sweat from the body. They catch fire easily and stick to the body and cause severe burns and injuries.Therefore, they are not suitable in kitchen and laboratories. On the other hand, clothes spun out of fibres made by a mixture of synthetic and natural fibres do not cause such problems. Moreover, mixing synthetic fibres with natural fibres enhances their properties, such as strength, elasticity and ease of washing and ironing.

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Plastics are useful but today they have become a serious global environment and health concern due to their excessive use. This is because plastics are non-biodegradable. They cannot be decomposed by the microorganisms in the soil. Inappropriate disposal of plastics is a serious problem. It chokes drains and blocks the pores of soil, which hinders the absorption of water by soil. Plastic bags contaminate food because of the poisonous dyes associated with them. Moreover, burning of plastics releases poisonous fumes, which causes air pollution. These fumes also cause cancer.

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The following three steps can be taken to reduce the danger that plastics pose to the environment:
1) We can support recycling schemes.
2) We can buy products with less plastic packaging.
3) We can practise or promote proper disposal of plastics at home and its surroundings and can encourage others to do the same.

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Rayon is regenerated cellulose, made from wood pulp by dissolving wood pulp in an alkaline solution. The thick liquid thus produced is passed through tiny holes to make fibres, which are hardened by passing them through a bath of sulphuric acid. The fibres are then spun into yarns and woven into clothes.
Rayon is called artificial silk because it is produced from naturally occurring polymers and looks like silk. It is woven like silk fibres but is man-made, unlike silk.

The important properties of  rayon fibres are:
1) They are easier to dye in various colours.
2) Clothes made from rayon are very comfortable in summer because they can absorb sweat from the body.

The uses of rayon are:
1) It is used to make curtains, table clothes, blankets, bed sheets, carpets etc.
2) It is used to make apparels such as jackets, suits, etc.

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The four properties of plastics are:
1) Plastics are poor conductors of heat and electricity. That is why they are used to make handles of cooking utensils and electrical wires. Cords of electrical appliances and cables also have plastic coating.

2) Plastics are insoluble in water and that is why we use plastic buckets to store water and use plastic glasses to drink water.

3) Plastics are inert to air and water. They are non-corrosive and therefore plastic bottles are used in chemical laboratories to store chemicals.

4) Plastics easily melt on heating and therefore they can be moulded into different shapes. Hence, they are used to make things such as toys, buckets etc.

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(a) The properties and uses of polythene are as follows:
Properties:
1) It is water resistant.
2) It is strong but flexible and can be rolled into sheets and moulded into different shapes.

Uses:
Polythene sheets are used for the packaging of liquids such as milk. Polythene pipes are used to transport liquids such as oil and water. Polythene containers are used to store liquids.

(b) The properties and uses of polystyrene are as follows:
Properties:
1) It can be easily moulded.
2) It has poor chemical resistance.

Uses:
It is used for the packaging of delicate materials like electronic items. It is used to make thermocol. It is also used to insulate the hollow walls of the refrigerator.

(c) The properties and uses of teflon are as follows:
Properties:
1) It is slippery and is not affected by heat.
2) It does not react chemically with other substances.

Uses:
It is used as a non-stick coating on pans and other cooking utensils. It is used for making gaskets, pump packings, valves, seals etc.

(d) The properties and uses of bakelite are as follows:
Properties:
1) It is a good electrical insulator.
2) It is harder than other plastics.

Uses:
It is used to make electrical goods, combs, fountain pens, buttons, plugs and switches.

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Synthetic fibres catch fire very easily. They melt on heating and stick to the body of the person wearing them. However, rayon is regenerated from cellulose and it doesn't melt on burning, rather it flares up like paper and smells like it.

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Cotton smells like burning paper because both cotton and paper are extracted from plants and constitute mainly cellulose, whereas burning wool smells like burning hair because both wool and hair are extracted from animals and both have sulphur in them.

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Plastics are non-reactive. They last much longer than any other material. They cannot be decomposed by bacteria and so are non-biodegradable in nature. These properties make plastics useful but also make them as environmental hazard.

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Clay is a 'plastic' material because we can mould it into different shapes, which is a property of plastic. But clay is not a polymer because polymers are made up of monomer units while clay is not made up of monomer units. Clay is a mixture of compounds.

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Nylon is a synthetic fibre. Synthetic fibres catch fire easily. They are inflammable. They melt on heating and stick to the body of the person wearing them which causes severe burn injuries. Therefore, these type of clothes are not suitable in a kitchen.

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Lata always prefers to buy things from the market that use fewer plastics in packaging because of the following reasons:
1. They are non-biodegradable in nature.
2. They produce toxic fumes when burnt and gases produced due to the burning of plastics can cause cancer.
3. They harm the environment by choking the drains.
4. Recycling of plastic is more costly than its manufacturing.