Crystallisation factors affecting

If a polymer molecule has a sufficiently regular structure it may be capable of some degree of crystallisation. The factors affecting regularity will be discussed in the next chapter but it may be said that crystallisation is limited to certain linear or slightly branched polymers with a high structural regularity. Well-known examples of crystalline polymers are polyethylene, acetal resins and polytetrafluoroethylene. 

The properties of a given polymer will very much depend on the way in which crystallisation has taken place. A polymer mass with relatively few large spherulitic structures will be very different in its properties to a polymer with far more, but smaller, spherulites. It is thus useful to consider the factors affecting the formation of the initial nuclei for crystallisation (nucleation) and on those which affect growth. 

AIChESymp. Ser. (a) 65 (1969) no. 95, Crystallization from solutions and melts (b) 67 (1971) no. 110, Factors affecting size distribution (c) 68 (1972) no. 121, Crystallization from solutions Nucleation phenomena in growing crystal systems (d) 72 (1976) no. 153, Analysis and design of crystallisation processes (e) 76 (1980) no. 193, Design, control and analysis of crystallisation processes (f) 78 (1982) no. 215, Nucleation, growth and impurity effects in crystallisation process engineering (g) 80 (1984) no. 240, Advances in crystallisation from solutions. 

Suitor et al [1976] reviewed the temperature effects for inverse solubility salts. An important factor, sometimes overlooked, is that as the deposition process continues the deposit temperature rises at constant heat flux. Under these higher temperature conations, some process of additional crystallisation and reorientation are likely to occur [Taborek 1972]. The strength of the deposit, and the ease with which it can be removed, may be affected by these additional processes. 

In Chapter 8 crystallisation and scale formation are discussed and the effect of pH was demonstrated as being a factor in deposit formation. Furthermore the equilibrium of ions in solution is likely to be affected by the presence of electric currents and the current density. As a consequence, a characteristic feature of cathodically protected metallic surfaces in contact with solutions containing mineral ions, notably sea water, is the formation of deposits or cathodic protection scale. The use of controlled scale formation is mentioned as a method of corrosion control in Chapter 14. Cox [1940] proposed the deliberate formation of calcium deposits on steel by the imposition of large cathodic currents to act as anticorrosive self healing layer. 

The actual crystallisation behaviour of any given solute during freezing depends on its nucleation rate, which, in turn, is affected by the following factors ... 

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