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 important from the point of view of polymer recycling, because the most suitable method for the degradation of a given polymer is closely related to both its thermal properties and its polymerization mechanism.

Classification According to Thermal Behaviour

Plastics are divided into two major groups depending on their behaviour when they are heated:

• Thermoplastics are plastics which undergo a softening when heated to a particular temperature. This thermoplastic behaviour is a consequence of the absence of covalent bonds between the polymeric chains, which remain as practically independent units linked only by weak electrostatic forces (Figure 1.4(a)). Therefore, waste thermoplastics can be easily reprocessed by heating and forming into a new shape. From a commercial point of view, the most important thermoplastics are high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyamide (PA), polymethyl methacrylate (PMMA), acrylonitrile–butadiene–styrene copolymer (ABS), and styrene–acrylo-nitrile copolymer (SAN).

• Thermosets are plastics whose polymeric chains are chemically linked by strong covalent bonds, which lead to three-dimensional network structures (see Figure 1.4(b)). Once formed into a given shape, thermosets cannot be reprocessed or remoulded by heating. Examples of thermosets with significant commercial applications are polyurethanes (PU), epoxy resins, unsaturated polyesters and phenol–formaldehyde resins. Thermosets are produced in smaller amounts than thermoplastics, as can be seen in Table 1.2.

Table 1.2 summarizes the production of different plastics in Western Europe over the period 1994–1996. Thermosets account for just 16% of total plastic production. A similar ratio of thermoset to thermoplastic production is found in the USA.

Elastomers constitute a third class of polymers. Similarly to thermosets, elastomers have a network structure formed by crosslinking between the polymer chains. However, the number of links is less than in the case of thermosets which gives these materials elastic properties. Elastomers can be deformed by the application of external forces. When these forces are suppressed, the polymer recovers its original form. From a commercial point of view, rubbers are the main class of elastomers, being mainly used in the manufacture of tyres.

The repeating units corresponding to a variety of organic polymers are shown in Figure 1.5.

Classification According to Polymerization Mechanism

Depending on the mechanism of polymerization, two groups of plastic materials can be identified:

• Addition polymers. The polymerization proceeds by a sequential incorporation of monomeric molecules into the growing polymer chain, without the release of any molecules or fragments to the reaction medium. As a consequence, the repeating units of addition polymers have the same chemical composition as the monomers. Examples of addition polymers include PE, PS, PVC, PMMA, etc.

• Condensation polymers. In this case the polymerization reactions take place with the liberation of small molecules, such as water, hydrochloric acid, etc. Nylon-6,6, obtained by polycondensation of adipic acid and hexamethylenediamine is a classic example of a condensation polymer. As shown in Figure 1.6, this polymerization reaction proceeds with the release of two water molecules by each repeating unit.

Thermoplastics

Thermoplastics account for the majority of plastics consumption. They are used in a wide variety of products and applications. It can be seen from Table 1.2 that about 90% of the total thermoplastics consumption in Western Europe corresponds to just five thermoplastics: PE, PP, PVC, PS and PET. The main properties of these resins are briefly described below.

Polyethylene (PE)

Polyethylene is synthesized by polyaddition of ethylene molecules, which leads to different types of PE depending on the reaction conditions:

• High density polyethylene (HDPE) is produced at relatively low temperature (60–200 °C) and pressure (1–100 atm) and is a highly linear polymer having a specific gravity in the range 0.94–0.97 and a high degree of crystallinity (80–95%). The main applications of HDPE are for the manufacture of films, food and domestic containers, crates, toys, gas tanks, pipes, etc. by blow moulding and injection moulding. The production of blown films for bags accounts for about 7% of the HDPE market.

• Ultrahigh molecular weight polyethylene (UHMWPE) is really a variety of HDPE with a molecular weight greater than 3 x 106. UHMWPE is a strong and lightweight plastic used in the fibre industry and for specialized applications such as its use in medicine for the manufacture of artificial hips.

• Low density polyethylene (LDPE). Unlike HDPE, this type of polyethylene is synthesized at very high pressures (1200–1500 atm) and at temperatures of about 250 °C. LDPE is a highly branched polymer characterized by its lower crystallinity and specific gravity than HDPE but with greater flexibility. Both the flexibility and crystallinity of LDPE can be controlled by adding low concentrations of acryl or vinyl monomers during the polymerization. LDPE has widespread use in films for bags and food packaging, greenhouses, bottles, cable insulation and injection moulded products.

• Linear low density polyethylene (LLDPE) is synthesized by copolymerization of ethylene and α-olefins, mainly 1-butene and 1-hexene. The role of the α-olefinic comonomers is to control both the number and the length of the side branches. As a consequence, LLDPE is a polymer with intermediate properties with respect to LDPE and HDPE. Main applications for LLDPE Dare films, injection moulded parts and wire insulation.

Polypropylene (PP)

Polypropylene is synthesized by polymerization of propylene, which may result in two main types of PP with commercial applications:

• Isotactic polypropylene (1-PP) is the most widely produced type. In this polymer, all the pendant methyl groups are located on the same side of the backbone, which results in a high crystallinity (80–5%). Isotactic polypropylene is synthesized at temperatures in the range 50–80 °C and at pressures of 5–25 atm. The main commercial applications of 1-PP are the manufacture of injection moulded containers, pipes, sheets and textile fibres for carpets. 1-PP is more rigid and crack resistant than HDPE, having good electrical insulation properties. Moreover, i-PP has a higher crystalline melting temperature (Tm) which enables its use in products that must be steam sterilized. These facts explain the continuous increase in the use of i-PP in various sectors.

• Syndiotactic polypropylene (s-PP) is produced at lower temperatures than 1-PP in the presence of Ziegler–Natta catalysts. The side methyl groups in this case are in alternating positions along the chain, which results in a non-crystalline polymer with lower density, mechanical strength and Tm than i-PP. Accordingly, s-PP is consumed in significantly lower amounts, being used as a coating material and in hot melt adhesives.

Polystyrene (PS)

Polystyrene is produced by styrene monomer polymerization, which leads to an amorphous, non-flexible polymer having good electrical insulation properties and a density of about 1.04 g/cm3. However, its high brittleness and low softening temperature (< 100 °C) are important limitations on its industrial application. PS is used in the manufacture of radio and TV parts, toys, electronic components, etc.

Expanded polystyrene (EPS) is prepared by impregnation of commercial PS beads with a blowing agent, such as isopentane. Steam heating of the impregnated beads leads to a cellular structure with a very low density. EPS is commercialized as beads or foams having widespread use in the packaging and building insulation sectors.

High impact polystyrene (HIPS) is synthesized by emulsion polymerization of styrene in styrene–butadiene latex. The higher impact characteristics of HIPS make it suitable for use in the manufacture of sheets, food containers, window frames, household goods, etc.

Polyvinyl Chloride (PVC)

PVC is a plastic of low crystallinity, prepared by polymerization of vinyl chloride at temperatures of about 50°C. There are two main grades of PVC, rigid and flexible. Rigid PVC is the product directly obtained from the polymerization and, as its name indicates, it is a stiff, hard and often brittle polymer. Flexible PVC is obtained by blending with a variety of plasticizers, which leads to a soft and pliable material. Rigid PVC is used in the manufacture of sheets, pipes, window profiles, etc., whereas the applications of flexible PVC include wire coating, toys, floor coverings, films and tubing.

In addition to plasticizers, PVC usually incorporates other components such as impact modifiers, fillers and extenders.

Polyethylene Terephthalate (PET)

Several routes are available for synthesis of PET, starting from different monomers: terephthalic acid (TPA), dimethylterephthalate (DMT) and bishydroxyethylterephthalate (BHET). The most common method of PET synthesis is based on the copolymerization of TPA and ethylene glycol. PET is a thermoplastic which can exist in amorphous, partially crystalline and highly crystalline states. For most PET applications, crystallinity is desired because it leads to enhanced strength and increases the maximum working temperature.

PET is widely used in the manufacture of fibres, bottles and films. In recent years, rapid growth in the use of the moulding grades of PET has occurred.

Thermosets

Thermosets are used in a similar proportion for both plastic and non-plastic applications. Plastic uses of thermosets include vehicle seats, sports equipment, electrical and electronic components, etc., while typical non-plastic applications include coatings and adhesives.

The main commercial thermosets are urea–formaldehyde resins (UF), melamine-formaldehyde resins (MF), phenol–formaldehyde resins (PF), epoxy resins, unsaturated polyesters, alkyd resins and polyurethanes. Changes in thermoset consumption in Western Europe during the period 1994–1996 are shown in Table 1.2. UF/MF resins and polyurethanes are produced in the greatest quantities, making up about 70% of the total thermosets market.