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20 of tonnage, thermoplastics are by far the most important, as they are approxi- mately 80% of all plastics used throughout the world. Advantages of Plastics Plastic materials are widely used because they can be formed into complex components, or shapes, relatively easily. They also have a useful combination of properties that can be modified for use in a wide range of applications. Some of these desirable properties (which are not possessed by any one plastic) include physical strength, resilience, corrosion resistance, elasticity, electrical insulation, wide color range, thermal insulation, lightness in weight, chemical resistance and moldability. These materials are therefore versatile in their applications. Their use frequently simplifies production, due to the fact that they often allow several discrete parts to be incorporated into one component. The ease with which complex parts may be produced from plastic compositions at high speed, in many cases outweighs the disadvan- tages of plastic materials. Attractive decoration of such products adds to their sales appeal. However, thermoplastic materials sometimes lack strength, stiff- ness, temperature resistance, and fire resistance. Not only do some plastics burn relatively easily, but also they often evolve large quantities of smoke and fumes when they do burn. Many plastic components will creep or change their dimensions if subjected to relatively small loads for prolonged periods of time. Environmental stress cracking (ESC), changes of dimensions with tem- perature and humidity, and a sudden change from tough to brittle behavior are other factors that should be investigated before thermoplastics are used. Major Thermoplastic Materials There are two main categories of plastic materials, thermoplastics and ther- mosetting plastics (thermosets). Because of the commercial importance of thermoplastic materials this book will concentrate on their extrusion process- ing. This important group of materials has been divided into four groups: 1. Commodity or Bulk Thermoplastics 2. Engineering Thermoplastics 3. Thermoplastic Elastomers or Rubbers 4. Blends and/or Alloys Commodity thermoplastics, such as the polyolefins (polypropylene and polyethylene), polyvinyl chloride plastics and styrene-based plastics com- prise, the major tonnage of material produced. Commodity or Bulk Thermoplastics Because of their relative ease of manufacture from readily available mono- mers, their low cost and versatility, usage of polyolefins has grown dramati- cally in recent years. Both homopolymers and copolymers in this family are available, as are rubbers and thermoplastics materials. The members of the polyethylene family include; linear low-density polyethylene (LLDPE), low den- sity polyethylene (LDPE), high-density polyethylene (HDPE) and very low den- sity polyethylene (VLDPE). The term polyethylene (PE) therefore covers a range of materials. A new class of these resins has recently been produced “using “metallocene” or “single site constrained geometry” catalysts. These catalysts allow the production of homopolymers and copolymers with mole- cules of more uniform size and structure. The term polyvinyl chloride (PVC) plastic refers to homopolymers and copolymers, both of which may be used with or without plasticizers. ThisPDF Image | Dynisco Extrusion Processors Handbook
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