5.6 Land and water pollution

In this section we will just take a couple of examples that show how easy it is to expose ourselves to long-term damage inadvertently. Pesticides, developed to control insects and other vermin, can increase agricultural productivity. Although pesticides were originally hailed as one of the wonders of modern technology, it was quite quickly discovered that there was a downside to their widespread use. One problem was that of bioaccumulation. Pesticides tended to be stable chemicals and
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5.5 Summary of Section 5

  • The phase of a material is characterised by its physical state (e.g. solid, liquid or gas), a distinctive arrangement of the atoms, and its chemical composition.

  • Material properties can change suddenly as the temperature increases or decreases, corresponding to changes of phase and the degree of order associated with the arrangement of atoms.

  • Shape memory alloys are examples of a wide range of useful engineering materials t
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Acknowledgements

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Grateful acknowledgement is made to the following sources for permission to reproduce material in this block:

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5.6.3 Conformation and crystallinity

If there are key connections between the chain configuration and crystallisation, you might also expect some more subtle effects from rotation about chain bonds. After all, polymer chains must be able to twist into the regular conformation demanded for crystal structures (Figure 57(a)). And what influence will rotation have on
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5.5.1 Non-uniform mixtures

Moulded rubbers and plastics are compounds of a polymer matrix and a variety of additives. The mixing history of the material before and during the moulding process can have a critical influence upon the final product properties. If mixing is done badly then the microstructure of the moulding can be non-uniform. Lack of uniformity can cause variations of strength and other physical properties within the moulding. The degree of dispersion or distribution of relatively minor quantities of addit
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5.4 Dynamic mechanical properties

Viscoelasticity is not experienced just under quasi-static conditions, i.e. when the imposed stresses and strains are constant or change only slowly. Polymers, and particularly rubbers, are often deliberately selected for products which are to be subjected to dynamic mechanical loading. Tyres are an obvious example where the unique high strain elasticity and energy absorbing qualities of rubbers make them the natural choice of material. Stress analysis involves the use of the frequency-depend
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5.3.2 Effects of structure on viscoelasticity

If a single measurement of ER(t) is taken at an arbitrary but fixed interval of time, say 10 seconds, then it will vary with temperature in a way rather similar to the viscoelastic master curve. Such a curve for atactic polystyrene is shown in Figure 48, where the various zones of behaviour are identif
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4.6 Polymer grades

Polymers synthesised by a variety of routes are available in many grades from the large polymer manufacturing companies. Naturally enough, the grades of bulk tonnage polymers, such as LDPE, PVC, HDPE and PP, run into the hundreds simply because of the multiplicity of different process routes and end functions. So what are the basic differences between grades of just one polymeric material? The most important distinguishing characteristics are structure and molecular mass.

Most suppliers
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4.5.1 The copolymer equation

It can be shown that the rate of change of monomer concentration in any copolymerization is given by the equation

where [M1] and [M2] are the concentrations of monomers 1 and 2 at any instant and r1 and r2., are reac
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4.5 Copolymerization

The alloying of metals to improve their properties is widespread and although many polymers used today are relatively pure (e.g. polystyrene, nylon), an increasing number are mixtures of two or more polymers. As with metals, one reason for doing this is to increase the range of properties. The major practical problem, however, is that homopolymers blend together with difficulty and even where blends are possible, as in some thermoplastics, phase separation can occur readily.

This proble
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4.4 Step growth polymerization

Figure 41

4.3.5 Co-ordination polymerization

While most free radical and ionic polymerizations are carried out homogeneously, there is another important class of reaction which is often performed with solid catalysts. These reactions, discovered in the mid-fifties, have revolutionized polymer manufacture by permitting much less severe polymerization conditions than with other systems and by allowing a greater degree of control of polymer structure. Ziegler-Natta catalysts, as they are called, will convert vinyl and diene monomers
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4.3.4 Ionic polymerization

Free radicals are indiscriminate in the compounds they attack, and their non-selective nature in polymerization reactions leads to problems such as chain branching and transfer which affect the structure of the polymer produced. Anionic polymerization overcomes many of these problems.

A typical commercial (but also see Box 8
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4.3.3 Termination and transfer

There are basically three ways in which chains terminate.

The first is known as coupling and occurs when two free radicals join together. This can be represented by the general equation

Such a mechanism significantly increases molecular mass, if it results in
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4.3.2 Propagation

Once a small number of chains have been started, propagation involves successive addition of monomer units to achieve chain growth. At each step the free radical is regenerated as it reacts with the double bond. So in the case of styrene the propagation step is

The free radical can also add on in a
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4.1 Understanding the polymerization process

Converting monomer to long chain polymer is the final step in the polymer manufacturing sequence. Polymerization is usually highly favourable in thermodynamic terms, mainly on energetic grounds because ordering molecules into linked chains is a process where the entropy is decreased. Heat is always given out during polymerization owing to the very favourable energetics of reaction, a point you may have noticed if you have ever made GRP parts for your car, for example!

Advances in cataly
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3.4 The petrochemical industry

The four-fold increase in the price of oil in 1973–4, together with associated political events, proved a powerful stimulus in the development and exploitation of North Sea crude oil. Increasing the price of oil does not mean that the price of the final plastic moulding increases by the same amount. For example if oil prices were doubled again then naphtha prices would typically increase by about 80 per cent, although there is no simple and fixed gearing mechanism between the two prices. Th
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3.3.2 Benzene, toluene and xylene

In addition to benzene itself, the catalytic reformer also produces ethylbenzene, toluene and the isomeric xylenes directly. The demand for ethylbenzene is always great as a source of styrene monomer, but toluene does not find great use apart from a relatively small application in polyurethane. This is why most toluene is de-alkylated to increase overall benzene production. A similar problem exists with the xylenes:

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4.6 Systems engineering: the recent development of a discipline

The recent development of systems engineering can be dated from two events. First, following the lead of the US Department of Defense and its introduction of standards for contractors, systems engineering was taken up by companies such as Boeing, Lockheed Martin and, in the UK, British Aerospace, Marconi and the Lucas Group. Second, in August 1990, a group of individuals interested in systems engineering met in Seattle (Box 10 – extract from a paper presented at the International Committee
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7.4.6 Molecular beam epitaxy (MBE)

Where a suitable ALD chemistry cannot be found, or where cleanliness and high crystallinity are required, molecular beam epitaxy may be necessary. This is more akin to evaporation than to CVD, with multiple molecular beams of the separate chemical constituents each focused onto the hot wafer surface. Deposition is performed under extreme vacuum conditions (10−11 mbar) to prevent any contaminants from being incorporated, and the substrate must present a perfect cleaved crystal fac
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