Principles and process of polymerisation in plastics production

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2. MANUFACTURING OF PLASTICS

The manufacture of plastic and plastic products in­volves procuring the raw materials, synthesizing the basic polymer, compounding the polymer into a material useful for fabrication, and moulding or shaping the plas­tic into its final form.

Raw Materials

Originally, most plastics were made from resins de­rived from vegetable matter, such as cellulose (from cot­ton), oils (from seeds), starch derivatives, or coal. Ca­sein (from milk) was among the nonvegetable materials used. Although the production of nylon was originally based on coal, air, and water, and nylon 11 is still based on oil from castor beans, most plastics today are derived from petrochemicals. These oil-based raw materials are relatively widely available and inexpensive. However, because the world supply of oil is limited, other sources of raw materials, such as coal gasification, are being ex­plored.

Synthesizing the Polymer

The first stage in manufacturing plastic is polymeri­zation. As noted, the two basic polymerization methods are condensation and addition reactions. These methods may be carried out in various ways. In bulk polymeriza­tion, the pure monomer alone is polymerized, generally either in the gaseous or liquid phase, although a few solid-state polymerizations are also used. In solution polymerization, an emulsion is formed and then coagulated. In interfacial polymerization, the monomers are dissolved in two immiscible liquids, and the polymerization occurs at the interface of the two liquids.

Additives

Chemical additives are often used in plastics to pro­duce some desired characteristic. For instance, antioxidants protect a polymer from chemical degradation by oxygen or ozone; similarly, ultraviolet stabilizers pro­tect against weathering. Plasticizers make a polymer more flexible, lubricants reduce problems with friction, and pigments add colour. Among other additives are flame retardants and antistatics.

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

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.

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