Plastics and polymers

Polymers and Plastics 

Polymers is an extremely large molecule consisting of a chain or network of small molecules called monomers. The process of forming polymer by large number of monomers is known as polymerization. For example, polyethene is a polymer obtained by joining large number of ethene (CH2 = CH2 ). Similarly, Vinyl chlorides are monomer units of polymer PVC (Polyvinyl chloride) 

i)              

                   Ethene                                 Polyethene                                                                                    

ii)        n(CH2 = CHCl) polymerization • CH2 - CHCI. .. ... ... CHCl- CH2

              Vinyl chloride                                 PolyVinyl chloride                                                                    

Polymers may be natural such as protein, starch, cellulose, silk, wool, rubber or synthetic such as plastics, nylon and terylene. 

Plastics 

The word plastics comes from the greek word 'plastikos' which means capable of being moulded. So, plastics means a manmade polymer which can be moulded to any shape when they are warm. The starting monomers for plastics are obtained from crude oil and small quantities other substances often used to improve performances or to reduce cost. 

Types of plastics 

Plastics are of two types 

1. Thermoplastics

2. Thermosetting plastics

1. Thermoplastics'

Plastics which becomes soft on heating and hard on cooling are known as thermoplastics. The monomers in this type of plastics are connected via straight chain. They can be moulded again and again into different shapes. For example, polyethene, polyvinyl chloride (PVC), polystyrene or polyster, etc.

2. Thermosetting plastics

Plastics which are hard and strong and are resistant to heat is known as thermosetting plastics. The monomers in thermoplastic setting plastics are arranged in cross linkage, which cannot be broken down by heating. So, they cannot be moulded again and again. For example, bakellite, melamine, etc.

Advantages:

1. They can be easily shaped and moulded.

2. They do not get rot, rust. They are unaffected by chemicals. So they are very durable.

3. They are cheap and light.

4. They can be used as insulators for heat and electricity.

5. They can be recycled (except thermosetting plastics).

Disadvantages:

1. They are non biodegradable and cause pollution.

2. They produce poisonous gases and smoke when burnt.

3. They easily catch fire.

4. They do not look as good as wood, stone or metals.