Types of plastics

Question 1 What are thermoplastics?

Question 2 Give example of thermoplastics?

Question 3 What are thermosetting plastics?

Question 4 Give example of thermosetting plastics?

Question 5 What are the various types of plastics?

Question 6 Why are thermoplastics not used for making frying pan handles?

Question 7 Why frying pan handles are made of thermosetting plastics?

Question 8 Why are electric switches, plugs and sockets made of thermosetting plastics?

Question 9 Give few differences between thermosetting and thermoplastic?

Question 10 Why the handle and bristles of toothbrush are made of different plastic?

Question 11 Why thermoplastics become soft on heating ?

Question 12 Why thermosetting plastic does not become soft on heating?

Question 13 What is the structure of polythene?

• NCERT Solutions Class 8 Science

Types of Plastic

1)Thermoplastic

A plastic which can be softened repeatedly by heating and can be moulded into different shapes again and again is called thermoplastic.

They are flexible so can be bent easily.

They are known as thermo softening plastics.

For Example : Polythene, Polyvinylchloride

They are used for making those articles which do not get too hot, and are flexible.

They are used for making insulations of electric wires and cables, various types of plastic containers, combs, toys, raincoats, seat cover, bristles of brushes, chains.

2)Thermosetting Plastic

A plastic which once set, does not become soft on heating and cannot be moulded a second time, is called thermosetting plastic.

Once set in a given shape and solidified, a thermosetting plastic cannot be re-softened or re-moulded.

They are known as thermosets.

For Example : Bakelite and melamine

They are hard and rigid.

They are not flexible. Due to this, they cannot be bend. When the article made of thermosetting plastic is forced to bend, it breaks.

Activity

If we take an electric switch and put it in hot water for some time, we will find that the electric switch does not become soft. This is because electric switch is made up of thermosetting plastic .This shows that thermosetting plastics do not become soft on heating.

Thermosetting plastics are used for making those articles which may get too hot during use and are hard and rigid.

They are used for making handles of cooking utensils, plates, cups, floor tiles, electrical fitting, ballpoint pens, telephones.

Thermosetting plastics are used for making the handles of cooking utensils (fry pans and pressure cooker) because:

1)they do not become soft on getting heated

2)they are poor conductors of heat.

Thermosetting plastics are used for making electrical fitting(electric switches, plugs, sockets) because 

1)they do not become soft on getting heated

2)they are poor conductors of electricity.

The handle and bristles of a toothbrush are made of different plastics. Because the handle of a toothbrush has to be hard and rigid(made of thermosetting plastic) whereas bristles have to be soft and flexible(made of thermoplastic)

Structure of Thermoplastic and Thermosetting plastic

Thermoplastic are made up long chain molecules called polymers. The long polymers chains are not cross-linked with one another. Due to this, on heating the individual polymer chains can slide over one another and thermoplastic material becomes soft and ultimately melts.

For Example : Polythene is a thermoplastic having linear polymer chains with no cross-linkage, so it becomes soft on heating.

Thermosetting plastics are made up long chain molecules called polymers. The long polymer chains are cross linked with one another. These cross links prevent the displacement of individual polymer chains on being heated. Due to this, thermosetting plastic do not become soft on heating once they have been set into a particular shape.

For Example : Bakelite is a thermosetting polymer having long polymer chains connected through cross-links due to which it does not become soft on heating.

Other Notes from Chapter 3 Synthetic Fibres and Plastics

• Synthetic Fibres
• Types of synthetic fibres
• Rayon
• Nylon
• Polyester
• Poly Ethylene Terephthalate and Acrylic
• Characteristics of Synthetic Fibres
• Plastics
• Types of plastics
• Properties of plastics
• Biodegradable and Non-biodegradable material
• Harmful effects of Plastics
• Steps to save environment from plastic waste