Introduction

1 Significance of Plastic Materials in Today's Society

Plastics are not, as many people believe, new materials. Their origin can be traced to 1847 when Shönbein produced the first thermoplastic resin, celluloid, by reaction of cellulose with nitric acid. However, the general acceptance and commercialization of plastics began during the Second World War when natural polymers, such as natural rubber, were in short supply. Thus, polystyrene was developed in 1937, low density polyethylene in 1941, whereas other commodity plastics such as high density polyethylene and polypropylene were introduced in 1957.

Today, plastics are very important materials having widespread use in the manufacture of a variety of products including packaging, textiles, floor coverings, pipes, foams, and car and furniture components. Plastics are synthesized mainly from petroleum-derived chemicals, although only about 4% of total petroleum production is used in the manufacture of plastics.

The main reasons for the continuous increase in the demand for commodity plastics are as follows:

• Plastics are low density solids, which makes it possible to produce lightweight objects.

• Plastics have low thermal and electric conductivities, hence they are widely used for insulation purposes.

• Plastics are easily moulded into desired shapes.

• Plastics usually exhibit high corrosion resistance and low degradation rates and are highly durable materials.

• Plastics are low-cost materials.

Engineering plastics, particularly thermosets, are also used in composite materials. Their excellent technological properties make them suitable for applications in cars, ships, aircraft, telecommunications equipment, etc. In recent years, important new areas of application for plastics have emerged in medicine (fabrication of artificial organs, orthopaedic implants, and devices for the controlled release of drugs), electronics (development of conductive polymers for semiconductor circuits, conductive paints, and electronic shielding), and computer technology (use of polymers with non-linear optical properties for optical data storage).

The above paragraphs show that today plastic materials are used in almost all areas of daily life. Accordingly, the production and transformation of plastics are major worldwide industries. Consumption of plastics in Western Europe is forecast to grow from 24.9 million tonnes in 1995 up to about 37 million tonnes in 2006, an annual growth rate of 4%. This prediction places plastics among the most important materials in the next century also.

Table 1.1 summarizes the changes in total plastic consumption in Western Europe from 1992 to 1996. These data refer to the final market for plastic products consumed by end-users but they do not include sectors such as textile fibres, elastomers, coatings, or products in which plastics are present in small quantities, because these are not considered as plastic products. If non-plastic applications are also taken into account, the total plastic consumption in Western Europe in 1996 increases up to 33.4 million tonnes. By comparison, the consumption of plastics in the USA and Japan in 1995 were 33.9 and 11.3 million tonnes, respectively.

The main sectors of plastic consumption in Western Europe are shown in Figure 1.1. The major field of plastic consumption is packaging, accounting for more than 40% of the total volume, followed by the building and automotive sectors. The most important uses of plastics in packaging are the production of films and sheets, sacks, bags, bottles and foams. In the building sector, plastics are used in a variety of applications: insulation, floor and wall coverings, window and door profiles, pipes, etc. The automotive sector is a good example of the continuous increase in the use of plastic materials. A car's weight can be reduced by 100–200 kg through the replacement of conventional metallic materials by plastics. Fuel tanks, bumpers, bonnets, insulation, seats, dashboards, textiles, batteries, etc. are examples of car components commonly manufactured with plastic materials. Plastics are used for a variety of applications in the agricultural sector such as greenhouses, tunnel and silage films, pipes for both drainage and irrigation, drums and tanks, etc.

Figure 1.2 illustrates plastic consumption in Western Europe by product for 1995, confirming that plastics are versatile materials which can be found in a wide range of products. The production and consumption of plastics have continuously increased over recent decades. The plastic consumption per capita in Western Europe has increased from ~1 kg per inhabitant in 1960 to about 65 kg per inhabitant in 1995.

2 Classes of Organic Polymers and their Main Applications

Polymers are long-chain molecules composed of a large number of identical units called repeating units. A polymer can be expressed as follows:

— (RU)n —

where RU is the repeating unit and n the number of units present in the polymer molecule. The number of repeating units must be large enough that no variations in the polymer macroscopic properties occur by small changes in the number of repeating units. This concept enables a distinction to be made between polymers and oligomers. Oligomers are molecules with a small number of repeating units, hence their properties vary significantly by just adding or removing a repeating unit.

Most of the polymers with commercial applications are synthetic materials. They are prepared by polymerization reactions involving the chemical linkage of small individual molecules (monomers) to give long-chain polymeric molecules. In some cases, polymers are synthesized by reaction between several monomers. The product so obtained is called a copolymer while the starting molecules are known as comonomers. The structure of copolymers depends on both the relative proportion and the sequence of the different comonomers along the macromolecular chain. Depending on the polymerization conditions, it is possible to obtain random, alternating, block or graft copolymers, as illustrated in Figure 1.3.

It is not easy to define the term 'plastic', which is usually considered as equivalent to the term polymer. Plastics are polymeric materials, but not all polymers are plastics. In general, the term 'plastic' is used to refer to any commercial polymeric material other than fibres and elastomers. Moreover, commercial plastics include other components such as additives, fillers, and a variety of compounds incorporated into the polymers to improve their properties. The term 'resin' is usually used to describe the virgin polymeric material without any of these components.

Classification of Polymers

Polymers are commonly classified according to two main criteria: thermal behaviour and polymerization mechanism. As explained further below, these classifications are...