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formed components may be greatly affected by the degree of crystallization and crystalline morphology (structure) that occurs during solidification. In addition, they exhibit greater shrinkage variations than amorphous polymers. Therefore a greater emphasis on the uniformity of cooling is required when processing a semi-crystalline material. Orientation The most commercially important polymer conversion processes involve melt- processing methods in which the polymer is heated and made to flow. In the melted form all polymers are amorphous; that is they have no structure. When the polymer flows, the polymer chains slip, or slide, over each other while rotation around the carbon-to-carbon bonds also occurs. Layers slip one over the other in what is called laminar or shear flow. As the individual polymer molecules move relative to each other, they may change their direction or ori- entation as a result of chain entanglements and friction between layers. The chains thus become “drawn out” in the direction of flow. Because of the rapid cooling often employed in polymer processing, this orientation may be “frozen-in” and the product will contain, what are referred to as, frozen-in stresses or, frozen-in strains. These products thus contain molecules that are orientated in the flow direction, which means that there is a “grain effect” pre- sent – just like that which exists in wood. As the result of the grain, the wood is stronger in one direction that it is in another. Because of orientation, thermoplastic products may be much stronger in one direction than in another; that is, the product is anisotropic. Commercial use is made of this “uni-axial” effect in extrusion in the produc- tion of the tape used for strapping packages. Biaxial orientation (orientation in two perpendicular directions) allows the production of strong, stiff, film from brittle polymers, such as polystyrene (PS). Table 1. Standard (Based on ISO and ASTM) Abbreviations For Selected Thermoplastics Abbreviation Name ABAK acrylonitrile-butadiene-acrylate (was A/B/A in ISO) ABS acrylonitrile-butadiene-styrene A/B/A See ABAK A/CPE/S See ACS ACS acrylonitrile-chlorinated polyethylene-styrene (was A/CPE/S in ISO) AES acrylonitrile-ethylene-propylene-diene-styrene (was A/EPDM/S in ISO) A/EPDM/S See AES A/MMA See AMMA AMMA acrylonitrile-methyl methacrylate (was A/MMA in ISO) AS acrylonitrile-styrene (SAN is more usual in Europe) ASA acrylonitrile-styrene-acrylate CA cellulose acetate CAB cellulose acetate butyrate CAP cellulose acetate propionate CN cellulose nitrate CP cellulose propionate E/EA See EEAK EEAK ethylene-ethylene acrylate (was E/AK in ISO) E/MA See EMA 17 section 2: introduction to extrusionPDF Image | Dynisco Extrusion Processors Handbook
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