iv. Emulsion Polymerization : This is the most widely used method of polymerization. This process overcomes the difficulty of heat control and low degree of polymerization. The organic reaction mass is emulsified with soap in a continuous aqueous phase. The dispersed particles are smaller in size than in Suspension Polymerization ( ≤ 0.1 μm) . In addition, due to stabilizing action of soap, the emulsion obtained is stable and agitation may not be necessary. In some methods, a water – soluble initiator may be used.

The main product of Emulsion Polymerization is latex, which forms the basis of the popular latex paints. By coagulating the latex with ionic salts and acids, solid rubber may also be obtained.

v. Homogeneous Polymerization : In case of homogeneous bulk polymerization, the feed is a gas , liquid or solid monomer. No initiators or additives are used. For homogeneous Solution polymerization, the monomer is completely dissolved in a solvent.

vi. Heterogeneous Polymerization : In heterogeneous Emulsion polymerization, the monomer molecules are emulsified in aqueous media in the form of micelles. For heterogeneous Suspension polymerization, the monomer is suspended in a n aqueous or other type of media as large droplets.

 

37.2. CLASSIFICATION ACCORDING TO PHYSICAL PROPERTIES :

Polymers can also be classified according to physical properties as –

• i.  Thermoplastic

ii.   Thermosetting

iii.   Elastomers

iv.   Fibers

 

i. Thermoplastic : The polymers in this category are composed of monomers which are linear or have moderate branching. They can be melted repeatedly and casted into various shapes and structures. They are soluble in solvents, but do not have appreciable thermal resistance properties. Vinyls, cellulose derivatives, polythene and polypropylene fall into the category of thermoplastic polymers.

ii. Thermosetting : There are some polymers which, when heated, decompose, and hence, cannot be reshaped. Such polymers have a complex 3-D network (cross-linked or branched) and are called Thermosetting Polymers. They are generally insoluble in solvents and have good heat resistance quality. Thermosetting polymers include phenol-formaldehyde, urea-aldehyde, silicones and allyls.

iii. Elastomers : These are resistant solids which have considerable flexibility. They are composed of polymers with glass transition temperature below room temperature.

One major difference between elastomers and plastics is that the elastomer is in a liquid state, while plastics are in the glassy state. Examples of elastomers are Butadiene, Butadiene co – polymers and their derivatives, polycondensation products , silicones and thiokols.

iv. Fibers : These are solids which can form thread – like structures and have high tensile strength.

Examples of fibers are Polyamides, Polyesters, Polyurethanes, Protein derivatives.