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59.5 MODIFICATION OF PROPERTIES OF POLYMERS

Plasticizers, fillers and dyes are often added in various quantities to modify the physical properties of plastics. For example, it is a common feature among many plastics that they do not soften to workable extent on heating.

a)      Synthetic resins and cellulose derivatives are horny tough materials. Such plastics can be easily softened by the addition of some organic compounds which are known as plasticizers. For example, polyvinyl chloride (PVC) is extremely- hard and brittle. It was
found that it often decomposed before it could be moulded. However, it was discovered in 1926 that the addition of certain substances like, di-n-butylphthalate, makes it soft and workable. The plasticizing effect is due to the solubilization action and an accompanying reduction in intermolecular forces which permits free movement of molecules relative to each other. In simple word plasticizers are molecules which get in between the chains, allowing them to slide over one another more easily.

Some important plasticizers are:

(i)                 Triceryl phosphate

(ii)               Dimethyl pthalate

(iii)             Triphenyl phosphate

(iv)              Camphor.

 b)      The moulding of a polymer can also be achieved by using the thermoplastic or thermosetting polymers. The method of moulding is to use an expansion method. In this method either a chemical reaction is done or a gas is bubbled into the polymer. For example,

                    i.            Polyurethane foam is made by mixing isocyanates and diols. The isocyanate reacts to give carbon dioxide gas, which produces foaming. The polyurethane foams are used as filling for furniture.

                  ii.            Similarly expanded polystyrene is made by bubbling gas into the polymer. Pentane has been used for this purpose. However, CFCs (Chlorofluoro hydrocarbons) have been increasily used for making polymer foams.

                iii.            Vulcanisation of rubber. The natural as well as synthetic rubber are very soft and sticky. These are hardened by a process known as vulcanisation. Charles Goodyear found that when rubber is heated with sulphur, it becomes hard and more flexible. He called the heating of rubber with sulphur as vulcanisation, (The name is coined after vulcan, the Roman god of fire.)

Heating rubber with sulphur causes cross-linking of polymer chains through disulphide bonds. Thus, the individual chains which were entangled together in the rubber now get together into a giant molecule through vulcanisation. As the polymer has double bonds, the chains have bends and kinks that prevent them from forming a tightly packed crystalline polymer. When rubber is stretched, the chains straighten out along the direction of the pull. Cross-linking prevents the polymer from being torn when it is stretched. When the stress is removed it springs back to its original shape and size. This is shown in Fig. 59.6.

Fig.59.6