Lubricants and Mold-Release Agents
Lubricants are added to most polymers at low levels to increase the overall rate of processing or to improve surface properties [1–4]. They have been used in the past to facilitate extrusion, injection, compression, etc., of many polymers, mainly PVC, acr
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Lubricants and Mold-Release Agents
2.1 Lubricants 2.1.1
Principles of Their Action
Lubricants are added to most polymers at low levels to increase the overall rate of processing or to improve surface properties [1-4]. They have been used in the past to facilitate extrusion, injection, compression, etc ., of many polymers, mainly PVC, acrylonitrile-butadiene-styrene terpolymer (ABS), PS, PMMA, cellulose acetate (CA), polytetrafluoroethylene (PTFE), and so on. Figure 2.1 shows the relative importance of these polymers as far as the use of lubricants is concerned [5]. The most used lubricants and their relative importance in the United States in 1978 were [6]: Ca and Zn stearates, 45% by weight Waxes, 23.5% by weight Fatty acid amides, 19% by weight Fatty acid esters, 12% by weight When used with a good knowledge of the problem to be solved, lubricants often improve the properties of products (brightness, heat stability in the course of a processing operation, light stability, resistance to degradation by corrosive agents and to water absorption, better dispersion of additives, improved electrical, optical, and mechanical properties, etc. [7]). Some examples of lubricants are stearic acid and its Ca, Li, Ba, AI, Pb, etc ., salts, natural waxes, microcrystalline paraffins with a high melting point (to avoid a cloudy appearance and greasy touch of finished products), mineral and vegetable oils, and whale fat (used to transform Teflon powder into some products). J. Štěpek et al., Additives for Plastics © Springer Science+Business Media New York 1983
2.1: Lubricants
35 Fig. 2.1. The use of lubricants for different plastics . A, PVC, 58.6% by weight. B, Polyolefins , 14.5% by weight. C, PS and its copolymers, 20.9% by weight. D, Other polymers, 6% by weight [5]. A
A main function of lubricants is to prevent polymers that have a tendency toward tackiness from sticking on metallic parts of processing machinery; this is external lubrication. Another important function is to provide internal lubrication to the polymer under processing to improve the free flow ofthe melt bulk polymer by reducing friction between chain segments. This allows, for example, the operation of the mixing process at a higher temperature for a better homogenization of the mixture of the polymer with additives. These two combined functions give a better output from the processing equipment, and they also facilitate mold release. There are many different ways of thinking about the properties, functions , and classification of lubricants [5,8-20]. For example, in the case of internal lubricants for PVC, it is usually believed that, contrary to plasticizers, lubricants solvate only the surface of PVC spherical particles or "nodules" produced by agglomeration of PVC chains in the course of the polymerization reaction and that behave as independent units during the melt flow of the bulk polymer [21]. Thus the lubricant acts exactly as an oil in the case of bearing lubrication and the spherical agglomerates slide directly on a lubricant layer [22]. With
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