THE PROCESSING OF PLASTICS

The various processes discussed in this chapter are used to fabricate all types and shapes of plastic products, ranging from household convenience packages to electronic devices and many others—including the strongest products in the world, used in space vehicles, aircraft, building structures, and so on. Proper process selection depends upon the nature and requirements of the plastic, the properties desired in the final product, the cost of the process, its speed, and the product volume. (Remember that a plastic may also be called a polymer or a resin.) Some materials can be used with many kinds of processes, but others require a specific or specialized machine (see Fig. 7-1). Plastics consumption by process is given in Table 7-1. Numerous fabrication process variables play an important role and can markedly influence a product s aesthetics, performance, and cost. 

Combine two or more parts with ultrasonics, adhesives, etc. 

Many product designs are inherently limited by the economics of the process that must be used to make them. For example, TSs cannot be blow molded, and to date they have limited extrusion possibilities. Many hollow parts, particularly very large ones, may be produced more economically by the rotational process than by blow molding. The need for a low quantity of parts may eliminate certain molding processes and indicate the use of casting or others. The extrusion process has fewer problems with TPs than does injection 

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Differences among the processes have a major impact on the use of the products. Products from a particular process or manufacturer may dominate one market, while products from a different process may be preferred in a different appHcation. Major uses include hot-melt adhesives for appHcations requiring high temperature performance, additives to improve the processing of plastics, sHp and mb additives for inks and paints, and cosmetic appHcations. 

Principles of the Processing of Plastics 8.2.6 Thermal Properties Affecting Cooling... 

In this book, the process of plastic deformation and the related crystal defects liave been discussed repeatedly. In Section 2.1.6, the distinction between continuum... 

These examples show the kinds of alterations that the processing of plastics can have on the performance of the product. As discussed throughout this book, the many different plastics tend all to behave in different patterns, so where a particular problem could develop during processing with one mate-... 

In dust-free and dispersed form, they are supplied as concentrated plastic granules (masterbatch pellets), as concentrated pastes, and as liquid colors. These products are added at different stages in the processing of plastics. 

In the processing of plastic masses the destruction is traditionally considered as a negative factor deteriorating physical and mechanical properties of products and manufacturers try to avoid it in every possible way. Mechanical destruction of molten polymers takes place, primarily, under the action of shear strains effectuating the tension of macromolecules 65-661 in this case, molecules with a high molecular mass... 

Polymeric macromoleculcs of types V-VII are needed for applications requiring enhanced molecular siite (see also Chap. III). Thus, linear polymers of type V are prepared by Mannich polymerization or from bis-Mannich bases by exchange reaction, for example, with bis-thiols, as in the case of poly(ketosulfide) 501, which is useful as a high-molecular-weight antioxidant in the processing of plastics.Macromolecules of type VI are generated by the functionalization of polymers, as shown by 502, which is obtained by aminomethylation of polyacrylamide and used as flocculant in water treatment." - ... 

Temperature is one of the most important factors that affect the process of plastics pyrolysis. The required temperature varies with different types of plastics and the desired composition of products. At a temperature above 600°C, the products are mainly composed of mixed fuel gases such as H2, CH4 and light hydrocarbons At 400-600°C, wax and liquid fuel are produced. The liquid fuel products consist mainly of naphtha, heavy... 

Catalysts tend to be deactivated in the process of plastics pyrolysis because of coke deposition on their surface. The deactivation of HZSM-5, HY, H-zeolite and silica-alumina was compared by Uemichi et al. [86]. In the case of PE pyrolysis and HZSM-5 added as catalyst, no deactivation occurred due to the low coke deposit, and high yields of light hydrocarbons (mainly branched hydrocarbons and aromatics) were achieved. In the case of PS, however, coke production increased dramatically, so HZSM-5 was deactivated very quickly. Silica-alumina catalyst was deactivated gradually and slowly with the increase of cracking gas, while HY- and H-zeolite molecule sieve catalysts were deactivated very quickly. Walendziewski et al. [87] studied the catalytic cracking of waste... 

The predicted value is derived from the combined processes of plastic-hole growth and shear-band growth, assuming that the particle has cavitated. The processes of plastic-hole growth and shear-band growth cannot be separated because hole growth arises from plastic deformation of the matrix. 

In a further work,2 the authors studied the processing of plastic wastes in mixtures of tetralin and used automotive oil. Figure 6.2 shows the results obtained with different waste oil/tetralin ratios. ZSM-5 was again more active than the ferrihydrite-based catalyst, while the latter led to oil yields very close to those of the thermal degradation. In both thermal and catalytic experiments, the activity was strongly enhanced by an increase in the waste oil/tetralin ratio, which shows the latter is not the best solvent for the liquefaction of aliphatic plastics. 

Briefly, antioxidants quench free radicals that are formed in the process of plastic degradation by oxygen and initiated by temperature and UV light, and assisted by moisture, stress, presence of metals, and other catalysts of plastic oxidation. If not intercepted by antioxidants, the polymeric plastic is degraded (depolymerized) so much that it loses its integrity and ceases to be a plastic anymore. It is converted to a loose powderous material, mainly a filler. 

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