RBSE Class 8 Science Notes Chapter 3 Synthetic Fibres and Plastics

Rajasthan Board RBSE Class 8 Science Notes Chapter 3 Synthetic Fibres and Plastics

Synthetic Fibers:
Dresses are formed from cloth, cloth is made of fibers. Some fibers obtained from nature are called natural fibers. Some fibers are made by man from different chemical reactions, these are called artificial or synthetic fibers, such as nylon, rayon, polyester etc. All these synthetic fibers are polymers.

Polymers are a long chain of small units or compounds. The polymer is formed from a Greek word where poly means more than one or many and mer means unit. In other words the long chain which is formed due to the reputation of small units is called polymer. Hence synthetic fibers are also polymers as they are formed of small units which is nothing but a chemical substance.

Types of Synthetic Fibers and their Properties:

• Rayon: They are also called artificial silk. This fiber is formed from cellulose (a natural polymer) by a chemical reaction. A thick liquid is formed by mixing sodium hydroxide (NaOH) and (CS₂) carbon-disulphide with cellulose after purifying it. This thick liquid is called viscose. Then this viscose is passed through very small pores in dilute sulphuric acid and fibers are obtained. Rayon resembles with natural silk. It is cheaper than it. It can be easily knit. By mixing cotton in rayon, bed sheets and carpets are manufactured.
• Nylon: Nylon fibres is strong durable tensile and light. It is bright and easily washable, hence it is obtained by mixing adipic acid and hexamethaline diamine, more useful in cloth manufacturing. Due to its strongness parachute and ropes to climb rocks are manufactured. Things used in our daily life like socks, tooth brushes, tents, sleeping bags, curtain are made by nylon.
• Orion: It is obtained by the polymerization of acetonitrile. It resembles with the woollen fibers.
• Dacron: It is obtained by the polymerization of ethlene glycol and terephthalic acid. It is also used to make polyester cloth.
  Special properties of decron and polyester.
  • Wrinkles are not made easily in clothes made up of polyester.
  • It is easily washable.
  • On washing thread do not shrink so iron is not needed.
• Natural fibers are costly, demand of growing population are not fulfilled, due to special properties of synthetic fibers over natural fibers they are in more demand, as Jhey absorb less water so dry up faster than cotton clothes.
• Demerits of synthetic fibers: When pieces of different clothes made up of different synthetic fibers are burnt they catch fire faster than other clothes, when they catch fire, melt and stick with body skin. They are not good absorber of sweat. Hence they stick with body in summer. To remove these drawbacks clothes by mixing natural fibers and synthetic fibers are made, called terrycot.

Plastics:
When small carbonic atoms join and form big molecules or structures are called plastics like bakelite, teflon, PVC etc. Plastics are synthesized polymer.

Classification of Plastics:
Plastics are of two types:

• Thermoplastic: Those plastics which become soft on heating and hard on cooling. They can be mould in different shapes like polythene, PVC, polystyrene etc. Things like- cans, pipes, toys, combs, buckets etc. which are used in daily life are made of thermoplastics.
• Thermosettings Plastic: Those plastics which become more hard on heating and do not melt, when once they are moulded in any shapes, they cant be remould in other shapes due to formation of more cross bonds in their structure. They become unmelting and undiluted, hence can tolerate very high or extreme heat. Bakelite is used to make switch of electricity, hands of utensils, batteries of automobiles.

Special properties of plastic:
It is a bad conductor of heat and electricity, less reactive, extensive heat tolerant and light in weight. It is cheaper than metals. Hence use of plastics is increasing day by day.

Uses of Plastics:

• Electric wires are coated with plastic to make them resistant against electricity to save from electric current.
• Handles of utensils are made of plastics, handles of pressure cooker, frypan, electric appliances like – iron, kettle, handles are made up of bakelite.
• In homes and shops plastic jar are used to keep chemicals, pickles, spices, oils, acids, etc
• Water pipes used in agriculture are made of plastics.
• In the field of medical pills, bottles for liquid drugs, syringe, gloves, thread of surgery, tools are made of plastics.
• Cooking ware used in oven and microwaves are made of special plastic.
• Non-stick pan, plate are coated with special plastic teflon.
• Dresses of firemen or fighters are coated with malamine to make them fire resistant.

Plastics and Environment:
Plastics is non-biodegradable which is harmful to our environment. It does not breakdown into small pieces easily. When it is burnt it requires a great amount of energy and during this process poisonous gases are released in such a great amount which makes the environment polluted. This problem can only be solved by minimizing the use of plastics. Biodegradable and non-biodegradable waste should be disposed separately. It should be recycled. In this way being a responsible citizen we should use the principle of 4r’s to stop making environment polluted.
These 4 principles are follows:

• Reduce
• Reuse
• Recycle
• Recover

RBSE Class 8 Science Notes