3. Principles and process of polymerisation in plastics production

Condensation polymerisation and addition polymeri­sation are the two main processes in plastics production. The manufacture of plastics depends upon the building of chains and networks during polymerisation.

A condensation polymer is formed by a synthesis that involves the gradual reaction of reactive molecules with one another, with the elimination of small molecules such as water. The reaction gradually slows down as polymers are built up.

An addition polymer forms chains by the linking of small identical units without elimination of small mol­ecules.

The most important concept in condensation polymers is that of «functionality», i.e., the number of reactive groups in each molecule participating in the chain build­up. Each molecule must have at least two reactive groups, of which hydroxyl (-OH), acidic endings (-COOH), and amine endings (-NH) are the simplest.

Hydroxyl is characteristic of alcohol endings, combin­ing with an acid ending to give an ester, the polymer be­ing known as a polyester. Examples are polyethylene terephthalate obtained by reaction of ethylene glycol con­taining hydroxyl groups at each end and terephthalic acid containing two acidic groups and polycarbonate resins.

Alcohols are a particular class of oxygen-containing chemical compounds with a structure analogous to ethyl alcohol (C-HOH). Amines are various compounds derived from ammonia by replacement of hydrogen by one or more hydrocarbon radicals (molecular groups that act as a unit). Esters are compounds formed by the reaction between an acid and an alcohol or phenol with the elimi­nation of water.

Bulk addition polymerization of pure monomers is mainly confined to styrene and methyl methacrylate The process is highly exothermic, or heat producing. The dis­sipation of heat (necessary to maintain chain length) is achieved in the case of styrene by intensive stirring of the viscous, partially polymerized mixture, which is then passed down a tower through zones of increasing tem­perature. Alternatively, polymerization may be com­pleted in containers that are small enough to avoid an excessive temperature rise as a result of the heat released during polymerization.

Methyl methacrylate is also partially polymerized be­fore being poured into molds consisting of between sheets of plate glass, to produce clear acrylic sheet.

Ethylene is polymerized in tubular reactors about 30 metres long and less than 25 millimetres in diameter at pressures of 600-3,000 to give 10-20 percent conver­sion to low-density polyethylene. Residual gas is recy­cled.

Polymerization of monomers in solution allows easy temperature control, but the molecular weight of poly­mers formed is reduced because of chain transfer reac­tions

Solvent removal from such a solution may also be very difficult. The process can be applied advantageously to vinyl acetate and acrylic esters.

Suspension polymerization producing beads of plas­tic is extensively applied to styrene, methyl methacr­ylate, vinyl chloride, and vinyl acetate. The monomer, in which the initiator or catalyst must be soluble, is main­tained in droplet form suspended in water by agitation in the presence of a stabilizer such as gelatin, each drop­let of monomer undergoing bulk polymerization.

In emulsion polymerization the monomer is dispersed in water by means of a surface-active agent (a substance slightly soluble in water that reduces the surface tension of a liquid), its bulk aggregating into tiny particles held in suspension. The monomer enters the hydrocarbon part of the surface-active micelles and is polymerized there by a water-soluble catalyst.

This process is particularly useful for the preparation of very high molecular weight polymers.

Exposure of certain substances to X-ray or ultravio­let radiation initiates chain reactions that can be used for manufacture of such thermoplastics as polyethylene and polyvinyl chloride.