Introduction to polymers

1 Polymer materials

1.1 Polymer types

Traditionally, the industry has produced two main types of synthetic polymer – plastics and rubbers (Figure 3). The distinction is that plastics are, by and large, rigid materials at service temperatures while rubbers are flexible, low modulus materials which exhibit long-range elasticity. Plastics are further subdivided into thermoplastics and thermosets, the latter type being materials where the long chains are linked together by crosslinks, a feature they share with conventional vulcanized rubbers. As Figure 3 shows, however, the distinction in terms of stiffness has become blurred by the development of thermoplastic elastomers (TPEs). Moreover, all polymers, irrespective of their nature, can be reinforced by a very wide range of fillers to produce composite materials.

Another way of classifying polymers is in terms of their form or function, varying from additives to other bulk materials (e.g. viscosity modifiers in plaster), coatings to products (e.g. paints), film and membranes to fibres (e.g. textiles) and bulk products such as pipe, containers and mouldings (Figure 4). Some of these materials are of course used as products in their own right, or manipulated further into finished products. This does not always happen, however, some polymers being a disposable intermediary in certain industrial processes. Thus photoresists are used to create the circuit patterns on semiconductor chips through controlled degradation, and are entirely absent in the final product.

The first polymeric materials to be used were entirely natural in origin and required relatively little modification to be adapted for useful purposes. Such materials included wood from various species of tree, fibres for rope and textile fabrics, and amber adhesive for attaching stone and metal tools to wooden handles. Rubber was used by the early Americans for containers, shoes and balls. Ways of processing them to shape and improving their properties were developed during the Victorian era (see Box 1), but it was not until the growth of the organic chemical industry that the first synthetic polymers were made. The true molecular nature of materials like natural rubber and synthetics like Bakelite was not understood until about the 1920s when Hermann Staudinger recognised their chain-like structure. That period saw the growth of polymer chemistry, by which monomers could be synthesised and polymerized in a controlled way to give macromolecular materials. Some of today's major polymers were discovered in this period and were commercialised in the 1930s and 1940s. They included materials like polychloroprene (Neoprene rubber), nylon, polyester (Terylene or Dacron) and polyethylene (Polythene – note that trademarks for polymers are shown as proper names).

The raw materials for making the monomers had at first been based on coal tar derivatives but, with the rise of the petrochemical industry based on oil and natural gas, a much wider range of basic chemical building blocks became available. Fundamental advances in the understanding of catalysis led to the discovery of many new polymers in the post-1945 period – variations on simple polymers like polyethylene as well as entirely new stereoregular polymers like polypropylene. That progress has continued at an increasing rate up to the present. Novel polymers, like aromatic polyamides and polyimides which were discovered only in the 1960s, have been developed, while speciality, high temperature materials like polysulphones have penetrated new markets hitherto inaccessible to the traditional range of commercial polymers. The achievement has been a direct result of pioneering scientific research closely linked to development by industry.

1.1.1 Natural and synthetic rubbers

Natural rubber was the first major polymer to be imported and used for commercial purposes. Long ago the natives of South America learned to tap the indigenous Hevea Brasiliensis trees to collect, dry and coagulate the latex. Today the main rubber plantations are in Malaysia and Indonesia. Natural rubber is well established as an important and versatile engineering material with an excellent balance of properties. However, almost two-thirds of the rubber now consumed world-wide is synthetic. The development of synthetic rubbers in Western Europe and the USA was accelerated by the demands of the Second World War and the associated loss of access to the natural rubber plantations in the Far East. Today's engineers have a complete spectrum of synthetic rubbers available to them, with properties ranging from the general-purpose to the highly specific. Hence the term ‘rubber’ or ‘elastomer’ is more properly the generic name for a class of polymeric materials of widely varying properties. The properties and common uses of a selection of both general-purpose and speciality rubbers are shown in Table 1.

1.1.2 Thermoplastics and thermosets

As already stated, polymers including rigid plastics were first developed in the last century from natural precursors. The sealing wax employed by the Victorians, for example, was usually based on the natural polymer shellac, an exudate of the Indian lac insect. Shellac is an early natural thermoplastic – defined as a material which softens and hardens reversibly on heating and cooling. In theory these reversible physical changes will take place without a corresponding change in the chemical structure of the material. This is why scrap thermoplastic can be re-used. In practice, some thermal and oxidative degradation occurs and recycling must be done only with an understanding of the effect that it has upon the properties of the final moulding.

Thermosets can be defined as those polymers which become irreversibly hard on heating or by addition of special chemicals. This hardening involves a chemical change (curing) and hence scrap thermoset cannot be recycled except as a filler material. The curing process invariably involves a chemical reaction which connects the linear molecules together to form a single macromolecule. These connections are known as crosslinks.

Scrap rubbers cannot be recycled easily, because of vulcanization which crosslinks the chains during moulding. It will be seen later that rubbers at the early stages of their processing can be considered to fit the definition of thermoplastics, but in their final moulded state they are properly defined as thermosets.

As with rubbers, the impetus for the development of new and better synthetic plastics followed supply and demand. Initially the demand was for cheaper substitutes for traditional materials, but today no plastic is cheap and some are extremely expensive with unique properties designed to satisfy the stringent requirements of sophisticated products. Table 2 lists the names and acronyms of the bulk-use commodity plastics and some of the more specialised and expensive materials, and comments on their important properties and uses.

1.1.3 Consumption of plastics

The consumption of plastics in the UK today is shown in Figure 6, with usage being dominated by PVC, closely followed by a range of polyolefins (polypropylene, various polyethylenes) and materials based on styrene monomer (PS and HIPS). These are the ‘big five’ bulk commodity polymers which dominate the market, and which have found application in almost every sphere of human activity (Figure 2). PVC is produced either in a flexible, elastomeric form (plasticized PVC) or as the rigid material familiar in pipes and profiles (unplasticized or uPVC). The ubiquitous plastic bag is usually made from any of the grades of polyethylene, and polypropylene is widely used for wrapping of consumer products. Polystyrene by itself is quite brittle, so is often used in its reinforced form (HIPS), or alternatively as a lightweight foam for insulation or protecting sensitive goods in transit.

These five are followed by thermosetting melamine and urea formaldehyde, widely used for electrical insulation products as well as for reinforcing wood products. Polyurethanes are a very versatile group of polymers – they may be thermoplastic or thermosetting, and range in stiffness from flexible elastomers to stiff plastics. Stiff grades are used for car body panels, and in foam form for insulation, flexible foam being used in furniture. Another grade is widely used in paint varnishes. Polyester, PET, originally mainly used as a strong fibre, is often nowadays used in blends with cotton in textile fabrics. But its greatest application today is in tough lightweight bottles, displacing glass for its safer properties. ABS is a derivative of PS, being much tougher by blending with rubber particles, and so is widely used for enclosures. The oldest synthetic thermoset, PF, is used for reinforcing wood products, while unsaturated polyester forms the main matrix material for glass-fibre reinforced composites. Acrylics, which comprise PMMA and related polymers, first found use in transparent aircraft windows during the Second World War, especially since they could be easily formed into complex shapes. They should not be confused with acrylic fibre, actually based on a quite different chain structure, that of polyacrylonitrile or PAN.

Although consumption of the remaining named polymers in Figure 6 is relatively low, they represent a growing class of so-called engineering polymers, whose properties are often so unusual or interesting as to find unique application in addition to displacing traditional materials. The family of nylons centres around nylon 6,6 and 6, both invented in the 1930s, the former still popular as a fibre, although both forms are used for mouldings. A more spectacular example is aramid fibre, developed in the late 1970s in the USA, for its very high stiffness and resistance to high temperatures. These properties have resulted in aramid fibre being widely used in textiles, ropes and composite structures such as aircraft tailplanes and rotor blades (often in combination with epoxy resins). Acetal is a tough, crystalline polymer which until recently was widely used for domestic kettles. Polycarbonate also finds use as a tough material for consumer products, and displaces PMMA owing to its greater toughness. PBT is another kind of thermoplastic polyester mainly used as a moulding material. The fluoropolymers are a unique class of polymer exemplified by PTFE, the parent material used most frequently for its exceptionally low coefficient of friction and temperature resistance as in non-stick frying pans, plumbing tape and support pads for moving heavy equipment. A related elastomer, Viton rubber, is used for engine seals and aircraft hose.