Introduction to polymers
Introduction to polymers

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Introduction to polymers

1.2 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.

Figure 3
Figure 3 Classification of polymers by property

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.

Figure 4
Figure 4 Classification of polymers by design function

Exercise 1

Identify the products in a typical modern house associated with the supply and disposal of water (or prevention of entry) which may be of polymeric origin, giving details of their generic type (Figure 3) and form or function (Figure 4).

Answer

A modern house might typically contain the following polymeric products for use in plumbing your water supply, and for its disposal after use. Rainwater products are also usually polymeric in origin. Your list will be determined by the age of your house and its exact design, should you choose it as the basis for your answer.

  • Water supply pipe, tank and moulded fittings, all thermoplastic

  • Bath and some fittings, composite and thermoplastic

  • Thermoplastic film covering to hot water tank insulation

  • Hot water tank insulation, if moulded around tank

  • Sewage pipe and fittings, mainly thermoplastic

  • Rainwater guttering, downpipe and fittings, all thermoplastic

  • Damp proof membranes (DPCs)

  • Flat roof coverings.

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.

Box 1 Plastics in Victorian times

The use of shellac as a moulding material was pioneered in the 1850s by Samuel Peck and Co. of the USA who added such refinements as the insertion of hinges during the moulding process. Metal inserts into thermoplastic mouldings are commonplace today. Casein made from skimmed milk was an early and reasonably successful protein plastic with a first patent in 1885 in Germany. The curds were separated from the whey, then after compounding with plasticizers and colours, they were pressed into sheets, rods, tubes or discs. Finally the casein was hardened by immersion in formaldehyde. It made a tough material capable of accepting a high polish and hence was a popular substitute for horn, ivory and amber.

The first synthetic thermoplastic was developed in the 1860s when Parkes in England and Hyatt in the USA produced a mouldable cellulose nitrate by softening it (plasticizing it) through the addition of camphor. Parkesine has not survived as a product name or material, but Hyatt's Celluloid is still used commercially. Compounding polymers with additives to give a controlled range of properties is an essential step in the production of almost all the polymers used today.

The availability of phenol from cheap coal tar and formaldehyde from the oxidation of methanol led in 1877 to the development of phenol formaldehyde resin by Baekeland and Swinburne, working in the USA and Britain. These first condensation products of the controlled reactions between phenol and formaldehyde produced hard but relatively weak and brittle materials. Swinburne commercialised his resin as a range of varnishes but Baekeland mixed the resin with significant amounts of woodflour to produce the first polymer composite – Bakelite . It was the first synthetic thermoset, a material which becomes irreversibly hard (cures) either on heating as with Bakelite or by cold curing.

Figure 5
Figure 5 Dr Leo Hendrick Baekeland
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