Thermoplastics melt processing

The principles of thermoplastic melt processing can perhaps best be illustrated by reference to Figure 8.1 illustrating extrusion, injection moulding, bottle blowing and calendering operations. In order to realise the full potential of the process it is necessary to consider the following factors ... 

As with thermoplastics melt processes, the setting is achieved by cooling. It will be appreciated that such cooling is carried out while the polymer is under stress so that there is considerable frozen-in orientation. This can be maintained throughout the life of the article. It is possible with the higher molecular weight materials to heat shapes made from blanks many years previously and see them return to the original shape of the blank. 

In general, these materials produced insoluble systems after imidization and had to be processed by first casting the amic acid intermediate onto a substrate, then imidizing. More recently [8], many examples have been demonstrated where the resulting polyimide is thermoplastic, melt processable and thus can ... 

Thermoplastic Polyimides. Several types of linear high-molecular-weight polyimides have been developed, which contain enough single bonds in the polymer backbone to make them somewhat flexible and therefore usable in conventional thermoplastic melt processing (Fig. 3.40). This does, of course, sacrifice some of the inherent thermal stability of polyimides (Table 3.45). 

Copolyester elastomers can be processed by conventional thermoplastic melt processing methods such as injection molding and extrusion. They require no vulcanization. These polymers can be processed successfully with low-shear processes such as laminating, rotational molding, and casting. 

NIR spectroscopy has become an analytical tool frequently called upon in many production processes. Its use in polymer processing applications such as polymer extrusion [83] increases greatly product quality. The applications of non-destructive NIR methods to synthetic polymer studies have been reviewed [131-133]. Typical reported applications include process and pilot monitoring [134], realtime analysis of thermoplastic melt processes [129, 135], insoluble cross-linked systems [136], polymer flakes, fluffs and film. NIRA has controlled production in dyeing of textured PA6 carpet yarns with... 

BiopoF polymers are currently the only commercially available bacterially derived polyhydroxyalkanoates. The polymers can be processed on most types of conventional thermoplastic melt-processing equipment. Examples of product forms include a wide range of injection-moulded products and extrusion blow-moulded bottles. Foamed products can also be produced [60]. 

Polychlorotrifluoroethylene, (PCTFE) is less crystalline and less inert than PTFE it can be shaped by normal thermoplastic melt processing methods. Because of its relative transparency, it finds limited use as observation windows (in sheet form), or as specialist tube material. 

Polylactides are linear polymers obtained from lactic acid, CHjCHOHCOOH, commonly known as poly(lactic acid) or polylactide. Lactic acid occurs naturally in animals and microorganisms and is found in many natural foods especially in fermented foods such as yogurt, buttermilk, sourdough breads, and sauerkraut [90, 91]. Besides being biodegradable, bioresorbable, and biocompatible, PLAs can be easily conformed by conventional thermoplastic melt processing, which makes this class of biodegradable polymers very suitable for uses in bionanocomposites. 

As a tme thermoplastic, FEP copolymer can be melt-processed by extmsion and compression, injection, and blow molding. Films can be heat-bonded and sealed, vacuum-formed, and laminated to various substrates. Chemical inertness and corrosion resistance make FEP highly suitable for chemical services its dielectric and insulating properties favor it for electrical and electronic service and its low frictional properties, mechanical toughness, thermal stabiUty, and nonstick quaUty make it highly suitable for bearings and seals, high temperature components, and nonstick surfaces. 

EPDM-Derived Ionomers. Another type of ionomer containing sulfonate, as opposed to carboxyl anions, has been obtained by sulfonating ethylene—propjlene—diene (EPDM) mbbers (59,60). Due to the strength of the cross-link, these polymers are not inherently melt-processible, but the addition of other metal salts such as zinc stearate introduces thermoplastic behavior (61,62). These interesting polymers are classified as thermoplastic elastomers (see ELASTOLffiRS,SYNTHETIC-THERMOPLASTICELASTOLffiRS). 

The packaging (qv) requirements for shipping and storage of thermoplastic resins depend on the moisture that can be absorbed by the resin and its effect when the material is heated to processing temperatures. Excess moisture may result in undesirable degradation during melt processing and inferior properties. Condensation polymers such as nylons and polyesters need to be specially predried to very low moisture levels (3,4), ie, less than 0.2% for nylon-6,6 and as low as 0.005% for poly(ethylene terephthalate) which hydrolyzes faster. 

Multiblock Copolymers. Replacement of conventional vulcanized mbber is the main appHcation for the polar polyurethane, polyester, and polyamide block copolymers. Like styrenic block copolymers, they can be molded or extmded using equipment designed for processing thermoplastics. Melt temperatures during processing are between 175 and 225°C, and predrying is requited scrap is reusable. They are mostiy used as essentially pure materials, although some work on blends with various thermoplastics such as plasticized and unplasticized PVC and also ABS and polycarbonate (14,18,67—69) has been reported. Plasticizers intended for use with PVC have also been blended with polyester block copolymers (67). 

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

The inability to process PTFE by conventional thermoplastics techniques has nevertheless led to an extensive search for a melt-processable polymer but with similar chemical, electrical, non-stick and low-friction properties. This has resulted in several useful materials being marketed, including tetrafluoro-ethylene-hexafluoropropylene copolymer, poly(vinylidene fluoride) (Figure 13.1(d)), and, most promisingly, the copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. Other fluorine-containing plastics include poly(vinyl fluoride) and polymers and copolymers based on CTFE. 

These materials were first introduced by Du Pont in 1956 and are now known as Teflon FEP resins. (FEP = fluorinated ethylene-propylene.) Subsequently other commercial grades have become available (Neoflon by Daikin Kogyo and Teflex by Niitechim, USSR). These copolymers may be regarded as the first commercial attempt to provide a material with the general properties of PTFE and the melt processability of the more conventional thermoplastics. 

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