Engineering plastics also

The repeating units of the engineering plastics also identify these as being predominantly condensation polymers, the class discussed in Chaps. 20 and 21. The commodity plastics are prime examples of the utility of the vinyl-type or chain reaction, polymers to be discussed here. 

The combination of properties that makes fiuorocarbons highly desirable engineering plastics also makes them nearly impossible to heat or solvent weld and very difficult to bond with adhesives withont proper snrface treatment. Fluorocarbons such as polytetraflu-oroethylene (TFE), polyfluoroethylene propylene (FEP), polychlorotrifluoroethylene (CTEE), and polymonochlorotriflnoroethylene (Kel-F) are notoriously difficult to bond because of their low surface tension and chemical resistance. However, epoxy and poly-nrethane adhesives offer moderate strength if the fluorocarbon is treated prior to bonding. 

The combination of properties that makes fluorocarbons highly desirable engineering plastics also makes them nearly impossible to heat or solvent weld and very difficult to bond with adhesives without proper surface treatment. The most common surface preparation for... 

An example of the latter is Dow s development of syndiotactic polystyrene, which has the properties of an engineering plastic. Also important is the incorporation of comonomers in a highly uniform manner. Again, Dow has produced an 80% ethylene/20% octane elastomer (89). Metallocene polymers have mainly been used in higher priced specialty applications (). 

Cyclic ether and acetal polymerizations are also important commercially. Polymerization of tetrahydrofuran is used to produce polyether diol, and polyoxymethylene, an excellent engineering plastic, is obtained by the ring-opening polymerization of trioxane with a small amount of cycHc ether or acetal comonomer to prevent depolymerization (see Acetal resins Polyethers, tetrahydrofuran). 

Copolymers are typically manufactured using weU-mixed continuous-stirred tank reactor (cstr) processes, where the lack of composition drift does not cause loss of transparency. SAN copolymers prepared in batch or continuous plug-flow processes, on the other hand, are typically hazy on account of composition drift. SAN copolymers with as Httle as 4% by wt difference in acrylonitrile composition are immiscible (44). SAN is extremely incompatible with PS as Httle as 50 ppm of PS contamination in SAN causes haze. Copolymers with over 30 wt % acrylonitrile are available and have good barrier properties. If the acrylonitrile content of the copolymer is increased to >40 wt %, the copolymer becomes ductile. These copolymers also constitute the rigid matrix phase of the ABS engineering plastics. 

Resins for advanced composites can be classified according to their chemistry typical resins are polyaryletherketones, polysulfides, polysulfones, and a very broad class of polyimides containing one or more additional functional groups (Table 2) (see also Engineering plastics). 

Derivatives of CPD have also been incorporated into these resins. CPD and 2-butene-l,4-diol have been condensed in ethanol and catalyticaHy hydrogenated in situ to give 2,3-bis(hydroxymethyl)bicyclo[2.2.1]heptane (51). This latter compound is used as a chain extender in polyesters for engineering plastics (52). 

The forecasts made in 1985 (77) of 8—8.5% worldwide aimual growth have not materialized. The 2 x lOg + /yr engineering plastic production reported for 1985—1986 has remained fairly constant. Whereas some resins such as PET, nylon-6, and nylon-6,6 have continued to experience growth, other resins such as poly(phenylene oxide) have experienced downturns. This is due to successhil inroads from traditional materials (wood, glass, ceramics, and metals) which are experiencing a rebound in appHcations driven by new technology and antiplastics environmental concerns. Also, recycling is likely to impact production of all plastics. 

With the expiry of the basic ICI patents on poly(ethylene terephthalate) there was considerable development in terephthalate polymers in the early 1970s. More than a dozen companies introduced poly(butylene terephthalate) as an engineering plastics material whilst a polyether-ester thermoplastic rubber was introduced by Du Pont as Hytrel. Polyfethylene terephthalate) was also the basis of the glass-filled engineering polymer (Rynite) introduced by Du Pont in the late 1970s. Towards the end of the 1970s poly(ethylene terephthalate) was used for the manufacture of biaxially oriented bottles for beer, colas and other carbonated drinks, and this application has since become of major importance. Similar processes are now used for making wide-neck Jars. 

As regards the general behaviour of polymers, it is widely recognised that crystalline plastics offer better environmental resistance than amorphous plastics. This is as a direct result of the different structural morphology of these two classes of material (see Appendix A). Therefore engineering plastics which are also crystalline e.g. Nylon 66 are at an immediate advantage because they can offer an attractive combination of load-bearing capability and an inherent chemical resistance. In this respect the arrival of crystalline plastics such as PEEK and polyphenylene sulfide (PPS) has set new standards in environmental resistance, albeit at a price. At room temperature there is no known solvent for PPS, and PEEK is only attacked by 98% sulphuric acid. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

Plastic also refers to a material that has a physical characteristic such as plasticity and toughness. The general term commodity plastic, engineering plastic, advanced plastic, advanced reinforced plastic, or advanced plastic composite is used to indicate different performance materials. These terms and others will be reviewed latter in this chapter. Plastics are made into specialty products that have developed into major markets. An example is plastic foams that can provide flexibility to rigidity as well as other desired properties (heat and electrical insulation, toughness, filtration, etc.). 

Nylon (Polyamide) PA is a crystalline plastic and the first and largest consumption of the engineering thermoplastic. This family of TPs are tough, slippery, with good electrical properties, but hygroscopic and with dimensional stability lower than most other engineering types. Also offered in reinforced and filled grades as a moderately priced metal replacement. 

Another method of reducing the quantity of plastics that has been used in certain products is to use engineered plastics with higher performance than the lower-cost commodity plastics. When applicable, this approach permits using less material to compensate for its higher cost. With a thinner-walled construction there could also be additional cost savings, since less processing heat, pressure, and time cycle is required. 

Poly(2,6-dimethyl-l,4-oxyphenylene) (poly(phenylene oxide), PPG) is a material widely used as high-performance engineering plastics, thanks to its excellent chemical and physical properties, e.g., a high 7 (ca. 210°C) and mechanically tough property. PPO was first prepared from 2,6-dimethylphenol monomer using a copper/amine catalyst system. 2,6-Dimethylphenol was also polymerized via HRP catalysis to give a polymer exclusively consisting of 1,4-oxyphenylene unit, while small amounts of Mannich-base and 3,5,3, 5 -tetramethyl-4,4 -diphenoquinone units are always contained in the chemically prepared PPO. 

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