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gives rise to unplasticized grades (UPVC) and plasticized grades (PPVC). The term styrene-based plastic also refers to a range of materials. These include the homopolymer polystyrene (PS), the copolymer styrene-acrylonitrile (SAN), the rubber-toughened material called high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS). The importance of commodity thermoplastics cannot be over- emphasized because of their high volume of production and usage. The advent of new catalyst systems (for example, metallocenes) will further increase the usage of bulk thermoplastics, as new types of polymer may be produced. It is also now being realized that the properties of this class of materials can be up-graded, so they can compete with many more expensive engineering ther- moplastics. Engineering Thermoplastics The term engineering thermoplastics generally refers to materials that can be used to replace metal, glass or wood in manufactured products. They include both amorphous and semi-crystalline polymers. These plastics, characterized by properties such as high strength, stiffness, impact resistance, temperature resistance and fire resistance are widely used in automotive, transportation, appliance, electronics, communications and aerospace applications. These materials are produced in smaller volumes than commodity plastics, but they command a much higher price. Materials that are considered to be engineer- ing thermoplastics include: polycarbonate (PC), polyethelene terephthalate (PET), polybutylene terephthalate (PBT), polyoxymethylene (acetal), polysmide (nylon), polymethylmethacrylate (PMMA) and acrylonitrile-butadiene-styrene (ABS). New Material Supply Raw material manufacturers or their distributors can supply a material as, for example, either prime grade material or as transitional material. Some may also supply reclaimed material. Prime grade material, also known as virgin material (or as first pass material or as first grade material) is a thermoplastic material (in the form of pellets or granules) that has not been subjected to use, or processing, other than that required for its original manufacture. Transitional material is material that does not meet the manufactur- er’s specification for prime grade material: it is off-spec or substandard mater- ial. Such material may be obtained from a continuous compounding line when a change is made from one grade to another. This means that the melt flow rate and the base color may be somewhat variable. The cost of virgin material is greater than that of these off-spec materials. Thermoplastic Grades A large number of grades may be available for any one material. These grades may differ in molecular weight, molecular weight distribution, chain branching, purity and the presence of additives or modifiers. Therefore, any properties quoted in the literature should be used only as a general guideline, as each of the many plastics available consists of a family of materials. Thus it is possible to get a wide range of properties for each material group. The properties of these thermoplastics may also be dramatically changed by the processing conditions employed. Processing may alter the material’s color, its physical properties, amount of molecular orientation and even the type, and amount, of crystallinity. 21 section 2: introduction to extrusionPDF Image | Dynisco Extrusion Processors Handbook
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