Thermoplastics processing options

More process options are available for thermosetting composites than thermoplastic composites at present, but the latter range is gradually being extended (Chapter 11). 

There are now a variety of biopolymers, having different chemical structures and different properties. They offer a large field of applications and after use, composting is a sustainable option. They can be processed by the traditional methods of thermoplastic processing. Table 1.3 summarises the possibilities offered by some of the commercial biopolymers. 

The GMP s process eliminates the use of sheet metal for the skin of the refrigerator door. In this application, the thermoplastic film forms a durable, protective outer skin with a wide choice of color options that are applied directly to the film. In addition more innovations exist apart from the film and thermoplastic interior liner, the doors consist entirely of polyurethane. GMP backs the thermoplastic film with an approximately 4 mm thick layer of the Baydur 110 structural foam polyurethane RIM system from Bayer AG that creates a rigid, dimensionally stable outer shell with no need for sheet metal. Then, GMP fills the space between this shell and the inner liner with insulating polyurethane foam, a rigid, low-density foam. The result is a self-supporting door that satisfies all stability, thermal insulation, and surface finish requirements. 

Generally, processing temperatures for THV are comparable to those used for most thermoplastics. In extrusion, melt temperatures at the die are in the 230°C to 250°C (446°F to 482°F) range. These relatively low processing temperatures open new options for combinations of different melts (coextrusion, cross-head extrusion. 

In the early 1980s, workers at Shell could demonstrate melt processability of polyketone produeed by palladium cyanide catalysts, after extensive extraction of catalyst residues from the polymers and blending these with other polymers such as styrene/acrylonitrile copolymer. From these studies, it was suggested that thermoplastic properties were possible in principle, and that the polyketone backbone was not inherently unstable in the melt as previously concluded. However, catalyst extraction did not offer a viable production option from a technical and economic viewpoint. 

It should be noted that starch is more readily broken down into small molecules than cellulose, making it an economic option for the production of monomers or as a raw material in the synthesis of other biopolymers. Another advantage over cellulose is the possibility of converting starch to thermoplastic material by physical processing alone, which will be the main topic discussed in this chapter. 

As natural rubber is a product of nature, its properties are determined by the biochemical pathway by which the polymer is synthesized in the plant. In the case of natural rubber the polymerization process cannot be tailored like that of synthetic rubbers. The only option to modify natural rubber is after it has been harvested from the tree. The important modified forms of natural rubber include hydrogenated natural rubber, chlorinated natural rubber, hydro-halogenated natural rubber, cyclized natural rubber, depolymerised liquid natural rubber, resin modified natural rubber, poly(methyl methacrylate) grafted natural rubber, poly(styrene) grafted natural rubber, and epoxidized natural rubber [33,34]. Thermoplastic natural rubber prepared by blending natural rubber and PP is considered as a physically modified form of natural rubber. 

Olefinic Thermoplastic Elastomers Blends of EPDM or EP rubbers with polypropylene or polyethylene, optionally eross-linked. Has low density, good dielectric and mechanical properties, and processibility but low oil resistance and high flammability. Processed by extrusion, injection and blow-molding, thermoforming, and calendering. Used in auto parts, construction, wire jackets, and sporting goods. Also called TPO. 

On the other hand, Crosslinking also, at least partially, transforms the thermoplastic melt into a duroplastic, which is often undesirable from the point of view of further processing and may limit recycling options. 

Competitive penetration of polypropylene into other applications has primarily taken place in polyethylene, polystyrene, polyvinyl chloride (PVC), thermoplastic polyester, nylon-6 or -6/6, and sometimes directly from metals or thermoset polymers like phenolic or reinforced reaction injection molded (RIM) urethane. The reasons for market penetration by PP replacement vary widely with an assortment of material design options chemical resistance, heat resistance, recycleability, processability, economics, and aesthetics. 

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