Many plastics are manufactured as composites. This involves a system where reinforcing material (usually fibres made of glass or carbon) is added to a plastic resin matrix. Composites have strength and stability comparable to that of metals but generally with less weight. Plastic foams, which are composites of plastic and gas, offer bulk with low weight.

Shaping and Finishing

The techniques used for shaping and finishing plastics depend on three factors: time, temperature, and flow (also known as deformation). Many of the processes are cyclic in nature, although some fall into the categories of continuous or semicontinuous operation.

One of the most widely used operations is that of extrusion. An extruder is a device that pumps a plastic through a desired die or shape. Extrusion products, such as pipes, have a regularly shaped cross section. The extruder itself also serves as the means to carry out other operations, such as blow moulding and injection moulding. In extrusion blow moulding, the extruder fills the mould with a tube, which is then cut off and clamped to form a hollow shape called a parison. The hot, molten parison is then blown like a balloon and forced against the walls of the mould to form the desired shape. In injection moulding, one or more extruders are used with reciprocating screws that move forwards to inject the melt and then retract to take on new molten material to continue the process. In injection blow moulding, which is used in making bottles for carbonated drinks, the parison is first injection moulded and then reheated and blown.

In compression moulding, pressure forces the plastic into a given shape. Another process, transfer moulding, is a hybrid of injection and compression moulding: the molten plastic is forced by a ram into a mould. Other finishing processes include calendering, in which plastic sheets are formed, and sheet forming, in which the plastic sheets are formed into a desired shape. Some plastics, particularly those with very high temperature resistance, require special fabrication procedures. For example, polytetrafluoroethene (Teflon) has such a high melt viscosity that it is first pressed into shape and then sintered--exposed to extremely high temperatures that bond it into a cohesive mass without melting it. Some polyamides are produced by a similar process.

Uses

Plastics have an ever-widening range of uses in both the industrial and consumer sectors.

Packaging

The packaging industry is a leading user of plastics. Much LDPE (low-density polyethene) is marketed in rolls of clear-plastic wrap. High-density polyethene (HPDE) is used for some thicker plastic films, such as those used for plastic waste bags and containers. Other packaging plastics include polypropene, polystyrene, polyvinyl chloride (PVC), and polyvinylidene chloride. Polyvinylidene chloride is used primarily for its barrier properties, which can keep gases such as oxygen from passing into or out of a package. Similarly, polypropene is an effective barrier against water vapour. Polypropene is also often used in housewares and as a fibre for carpeting and rope.

Construction

The building industry is a major consumer of plastics, including many of the packaging plastics mentioned above. HDPE is used for pipes, as is PVC. PVC is also used in sheets for building materials and similar items. Many plastics are used to insulate cables and wires, and polystyrene in the form of foam serves as insulation for walls, roofs, and other areas. Other plastic products are roofing, door and window frames, mouldings, and hardware.

Other Uses

Many other industries, especially motor manufacturing, also depend on plastics. Tough engineering plastics are found in vehicle components like fuel lines, fuel pumps, and electronic devices. Plastics are also used for interior panelling, seats, and trim. Many car bodies are made of fibreglass-reinforced plastic.

Among the other uses of plastic are housings for business machines, electronic devices, small appliances, and tools. Consumer goods range from sports equipment to luggage and toys

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.

4. RESINS

Resins that cannot be softened by heating include the phenolics, furan resins, aminoplastics, alkyds, allyls, epoxy resins, polyurethanes, some polyesters, and silicones.

Phenolics or phenol-aldehydes

The important commercial phenolic resin Bakelite is based on phenol and formaldehyde. The two processes in general use are the one-step process producing resol resins (the first stage in the formation of a phenolic resin) that are either liquid or brittle, soluble, fusible solids, from more than one molecule of formaldehyde per phenol molecule; and the two-step process, using an excess of phenol to produce novolacs, resins that have no reactive methylol groups and must be mixed with an aldehyde to undergo further reaction.

Resol resins thermoset on heating and are used for adhesives. Novolacs require a further source of formaldehyde in the form of hexamethylenetetramine to produce molding powders. Both resins are run out from the reaction vessel, after removal of water by distillation, and ground up, then compounded on heated rolls with fillers that vary from wood flour to mica; for strength and heat resistance fibrous asbestos is used as a filler (hexamethylenetetramine is also added at this stage in the case of the two-step resin). Final grinding produces the molding powders, which on further heat treatment will yield the typical thermoset resin.

Phenolic moldings are resistant to heat, chemicals, and moisture and are preferred for wet-dry applications as in washing machines. Their stability to heat and low heat conductivity suit them for use in appliance parts, and their electrical insulation qualities qualify them for electric fittings such as switches, plugs, and distributor caps; resistance to hydraulic fluids has led to their use in automotive parts. All these applications have been made more economical by the development of injection molding and extrusion methods. Complex phenols are used in manufacture of brake linings.

Furan resins

Furfural is a five-membered ring compound (i.e., the basic molecule has a ring shape and contains five atoms) of four carbon atoms and one oxygen atom, carrying the aldehyde group, -- CHO; it reacts like formaldehyde with phenols in the presence of an acid catalyst to give a rigid polymer with high chemical resistance, used for coatings in industry. It can be prepared in semiliquid form with a low viscosity and remarkable penetrating power when applied to porous forms such as foundry sand cores or graphite blocks, being in this respect superior to other liquid resins.

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