Processing of polymers

As already mentioned, the results and rules found in dependencies of properties with the morphology of polymeric materials are in most cases limited to special classes of polymers or even one polymer species only. Therefore, the intention of this subsection can only be to demonstrate the potential and the possibilities which exist to influence the properties of polymers by modification of their morphology. This will be done with relevant examples that are described in detail in Chaps. 3-6. 

Just as was the case for a metal, a polymer must have formability in order to be shaped. Although an exact definition of formability is as difficult to give for polymers as it was for metals, it involves such polymer physical properties as viscosity, crystallinity, and 

Source H. Yanagida, K. Kouraoto, and M. Miyayama, The Chemistry of Ceramics. Copyright i John Wiley Sons, Inc. 

Permission to reproduce this image online was not granted by the copyright holder. Readers are kindly requested to refer to the printed version of this article. 

For square pitched screws, Ls = Ds, and the flights are all perpendicular to the screw axis. Extruder screws range from 25 to 150 mm in diameter, with screw lengths typically of 25 to 30 screw diameters. They rotate at speeds from 50 to 150 rpm, and deliver melt at rates from 10 to 1000 kg/h with pressures up to 40 MN/m.  

To obtain the velocity profile of the polymer in down channel direction, v, we start with the appropriately simplified equation of motion in rectangular coordinates 

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Other chemical apphcations being studied include the use of microwaves in the petroleum (qv) industry (175), chemical synthesis (176,177), preparation of semiconductor materials (178), and the processing of polymers (179). 

Whilst the development of flame retarders has in the past been largely based on a systematic trial-and-error basis, future developments will depend more and more on a fuller understanding of the processes of polymer combustion. This is a complex process but a number of stages are now generally recognised and were discussed in Chapter 5. 

As previously stated, molecular orientation occurs during melt processing of polymers. On removal of the deforming stresses the molecules start to coil up again but the process may not go to equilibrium before the polymer cools to below its Tg. This leads to residual orientation (frozen-in strain) and corresponding frozen-in stresses. 

Bastiaansen, C.W.M. (1997) High-modulus and high-strength fibres based on flexible macromolecules, in Processing of Polymers, ed. Meijer, H.E.H. Materials Science and Technology, A Comprehensive Treatment, vol. 18, eds. Cahn, R.W., Haasen, P. and Kramer, E.J. (VCH, Weinheim) p. 551. 

Charlesby, A. (1988) Radiation processing of polymers, in Supplementary Volume I of Encyclopedia of Materials Science and Engineering, ed. Cahn, R.W. (Pergamon Press, Oxford) p. 454. 

It is necessary to consider the micro-mechanical processes of polymer glasses and elastomers separately as their mechanical properties are so different. In addition, cross-linking profoundly affects the deformation processes in glasses but very little is known about the micro-mechanical processe.s that occur in single phase cross-linked glasses so the latter materials will not be discussed further. 

A polymer such as polyethylene is a long-chain molecule with repetitions of the same monomer. Due to topological constraints, the crystallization process of polymer chains is expected to be different from that of simple molecules as discussed so far [160]. 

Tomer RV (1972) Basic processes of polymer processing (Theory and calculations methods), M., Chemistry (in Russian)... 

Improving the chemical synthesis and processing of polymers and ceramics ... 

Complex liquids are ubiquitous in materials manufacture. In some cases, they are formed and must be handled at intermediate steps in the manufacture of materials (e.g., sols and gels in the making of ceranucs, mixtures of monomer and polymer in reactive processing of polymers). In other cases (e.g., composite liquids), they are the actual products. Understanding the properties of complex fluids and the imphcations of fluid properties for the design of materials processes or end uses presents a formidable intellectual challenge. 

Radiation processing of polymers was introduced after World War II with the development of the nuclear reactor. In the current years, various radiation sources, e.g.. X-rays (soft and hard), gamma (7) and ultraviolet (UV) rays and electron beam (EB) are being widely used. 

Material containing conventional vulcanization system, once formed (e.g., profile) is normally cured immediately. If the product is off-size, or undercured, it is not possible to run again. In the case of EB-cross-Iinked equivalent, the product, in the case of being undercured, can be treated with additional dose to make up to the required level. Hence, EB-processing of polymers is expected to generate less scrap [47]. 

Among the various radiation-induced modifications, the EB-processing of polymers has gained special importance as it requires less energy, is simple, fast, and versatile in application. The overall properties of EB-irradiated polymeric materials are also improved compared to those induced by other ionizing radiation. 

The Flory principle, whose validity has been verified for a wide range of macromolecular reactions, is a good approximation for the description of the kinetics of many processes of polymer preparation. However, strong experimental evidence for the violation of this principle is currently available from the study of a number of macromolecular reactions. Possible reasons for such a violation may be connected with either short-range or long-range effects. 

When there is a need to calculate only the statistical moments of the distribution of molecules for size and composition, rather than to find the very distribution, the task becomes essentially easier. The fact is that for the processes of polymer synthesis which may be described by the ideal kinetic model the set of equations for the statistical moments is always closed. 

Figure 1. Scheme depicting erosion process of polymer disc containing acid labile linkages. (Reproduced with permission from ref. 7. Copyright 1991 Elsevier Science Publishers.)... 

Zabotin, K. P., Malysheva, L. V. Proceedings Chemistry and Chemical Technology. Physico-chemical Foundations of Synthesis and Processing of Polymers Vol. 1, p. 474, Gorky, GGU 1973, (Russ.)... 

During commercial processing of polymers, primary crystallization is normally complete within a matter of a few seconds or, at most, a minute or two. 

The term ageing of polymers is usually reserved for long-term changes in properties of polymers exposed to weathering conditions. It may involve any of the above processes and include physical processes of polymer recrystallization and denaturation of, for example, protein structure in biopolymer chemistry. The term corrosion, used essentially for the deterioration (ageing) of metal... 

Recently computer simulations have come to be recognized as powerful and promising tools to investigate the molecular process of polymer crystallization, to attack problems that are hard to access by experiments [15-35]. The potential power to directly reproduce the crystallization thereby enabling... 

Fig-1 Schematic illustration of the crystallization and melting processes of polymers. The crystallization process corresponds to processes of disentanglement and chain sliding diffusion. The melting process is the reverse of the crystallization process. Between equilibrium melt and ideal crystal, there exists metastable melt and crystal. Cross marks indicate entanglement... 

This means that the Mn dependence of I is controlled by the diffusion process of polymer chains and not by the formation process of a critical nucleus. 

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