Originally, most plastics were made from resins derived from vegetable matter, such as cellulose (from cotton), furfural (from oat hulls), 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.
There are two basic types of synthetic polymers: addition polymers and condensation polymers. Addition polymers form when a radical initiator adds to a carbon-carbon double bond to yield a reactive intermediate. This intermediate reacts with another monomer molecule to yield a second intermediate. This process of monomer addition is repeated as shown in Figure l (example = polyethylene).
Condensation polymers are formed by the reaction between two molecules, producing polyesters and polyamides. Each bond in the polymer is formed independently of the others and water is also a bi-product. The monomers usually are in an alternating order and the polymer often has atoms other than carbon in the main chain. This condensation polymerisation requires a catalyst such as antimony(III) oxide at about 280oC.
The example shown in Figure 2 is the polyamide, Nylon, formed from amine and carboxylic acid monomers. Another example is a polyester known as PET (poly(ethyleneterepthalate)), commonly found in drinks bottles, which forms a reaction form 2 monomers: ethylene glycol and terephthoyl chloride. At the reaction’s end, an atom of hydrogen and an atom of chlorine are left out of each PET molecule, resulting in a bi-product of hydrogen chloride (HCl) gas.
Peptides and proteins are also formed by condensation polymerisation. A peptide link (also known as an amide link) is formed between the amino and carboxylic acid groups in amino acids, with the loss of a water molecule. Hence both polyamides and proteins and proteins contain monomer units joined by the same link.
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 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.
Because plastics are relatively inert, the final products do not normally present health hazards to the maker or user. However, some monomers used in the manufacture of plastics have been shown to cause cancer. Similarly, benzene, which is an important raw material for the synthesis of nylon, is a carcinogen. The problems involved in the manufacture of plastics parallel those of the chemical industry in general.
Most synthetic plastics are not environmentally degradable; unlike wood, paper, natural fibres, or even metal and glass, they do not rot or otherwise break down over time. (Some degradable plastics have been developed, but none has proved compatible with the conditions required for most waste landfills.) Thus, there is an environmental problem associated with the disposal of plastics. Recycling has emerged as the most practical method to deal with this problem, especially with products such as the polyethene terephlalate bottles used for carbonated drinks, where the process of recycling is fairly straightforward. More complex solutions are being developed for handling the commingled plastic scrap that constitutes a highly visible, albeit relatively small, part of the problem of solid waste disposal.
Natural polymers or synthetic polymers based on naturally occurring monomers look to be the best bet for future development. However, some work must be done before these polymers can take the place of the synthetic polymers now in use. The natural or natural-mimicking polymers now known often lack some of the properties that make current synthetic polymers so popular, such as water resistance and other physical properties in addition to the relatively low cost of synthetic polymer production.
