Polymerization

The raw materials for plastics production are natural products such as cellulose,coal,oil,natural gas and salt. In every case they are compounds of carbon (C) and hydrogen (H). Oxygen (O),nitrogen (N), chlorine (Cl) and sulphur (S) may also be present. Oil,together with natural gas,is the most important raw material for plastics production.
The term plastics in the broadest sense encompasses (a) organic materials which are based on (b)polymers which are produced by (c) the conver- sion of natural products or by synthesis from primary chemicals coming from oil,natural gas or coal.
The basic building blocks of plastics are monomers. These are simple chemicals that can link together to form long chains or polymers. The type of monomer used and the way it polymerizes,or links together, give a plastic its individual characteristics. Some monomers form simple linear chains. In polyethylene,for example,a typical chain of 50000 ethylene links is only about 0.02mm long. Other monomers form chains with side branches. Under certain circumstances,the individual chains can link up with each other to form a three-dimensional or crosslinked structure with even greater strength and stability. Cross-linking can be caused either chemically or by irradiating the polymer.

To get the advantages of two different plastics, two different monomers can be combined in a copolymer. By combining the monomers in different proportions and by different methods,a vast range of different properties can be achieved (see Tables 4.13 and 4.14). The properties of plastics can also be enhanced by mixing in other materials,such as graphite or molybdenum disulphide (for lubrication),glass fibre or carbon fibre (for stiffness),plasticizers (to increase flexibility) and a range of other additives (to make them resistant to heat and light).