High-Temperature Thermoplastics

The heat release rates for thermoplastics with glass fibers and charring-type thermoplastics, high-temperature polymers, and halogenated polymers from the Cone Calorimeter (Table 11.10) are in the range predicted for the burning of solid polymers and similar to those from the Fire Propagation Apparatus (Table 11.9). 

Classification with respect to the methods of reaction is discussed in Sect. 12.3 to include thermosets, thermoplastics, high-temperature adhesives, elastomers, sealants, anaerobics, moisture curing adhesives, and primers. 

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. 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

Plastic packagiag materials are thermoplastic, ie, reversibly fluid at high temperatures and soHd at ambient temperatures. These materials may be modified by copolymerization, additives ia the blead, aHoyiag, and surface treatment and coating. Properties of principal plastic packagiag materials are givea ia Table 1. 

Polyester. Poly(ethylene terephthalate) [25038-59-9] (PET) polyester film has intermediate gas- and water- vapor barrier properties, very high tensile and impact strengths, and high temperature resistance (see Polyesters, thermoplastic). AppHcations include use as an outer web in laminations to protect aluminum foil. It is coated with PVDC to function as the flat or sealing web for vacuum/gas flush packaged processed meat, cheese, or fresh pasta. 

Ja.cketingMa.teria.ls. Besides the metallic protective coverings (based on aluminum, copper and copper alloys, lead, steel, and zinc), the most popular jacketing materials are based on polymeric materials that can be either thermoplastic (with limited high temperature use) or thermosetting. 

Thermosetting-encapsulation compounds, based on epoxy resins (qv) or, in some niche appHcations, organosiHcon polymers, are widely used to encase electronic devices. Polyurethanes, polyimides, and polyesters are used to encase modules and hybrids intended for use under low temperature, low humidity conditions. Modified polyimides have the advantages of thermal and moisture stabiHty, low coefficients of thermal expansion, and high material purity. Thermoplastics are rarely used for PEMs, because they are low in purity, requHe unacceptably high temperature and pressure processing conditions. 

Polymers are used as inserts for pins and contacts. Important properties of the commonly used insert materials have been compiled (31). Polysulfones are high temperature thermoplastics that have high rigidity, low creep, excellent thermal stabiHty, flame resistance, low loss tangents, and low dielectric constants. The principal weakness of polysulfones is their low chemical resistance. 

These thermoplastic natural mbber elastomers have a place in the modem world, where recycling has become so important, and when excessive heat is not found in service. Thus, footwear, gla2ing seals, sports goods, hose, domestic products, and a whole range of automotive products have already been identified for such use. It must be noted, however, that tines are not a potential market for these materials, because of the high temperatures which result from emergency braking. 

Pseudothermoplastic resin systems, which are formed as conventional thermoplastic materials and then cured or postcured in a manner similar to that used for thermosetting resins to enhance high temperature properties. 

LARC-CPI has a T of 222°C and melts at 350°C (37). The high melting temperature iUustrates one disadvantage of the high temperature thermoplastics ia order to process or melt coasoUdate the prepreg, it must be processed at temperatures greater than 360°C. 

Because of increased production and the lower cost of raw material, thermoplastic elastomeric materials are a significant and growing part of the total polymers market. World consumption in 1995 is estimated to approach 1,000,000 metric tons (3). However, because the melt to soHd transition is reversible, some properties of thermoplastic elastomers, eg, compression set, solvent resistance, and resistance to deformation at high temperatures, are usually not as good as those of the conventional vulcanized mbbers. AppHcations of thermoplastic elastomers are, therefore, in areas where these properties are less important, eg, footwear, wine insulation, adhesives, polymer blending, and not in areas such as automobile tires. 

Newer resins include polysulfone, polyethersulfone, polyetherimide, and polyetherketone. Some of these newer materials are high temperature thermoplastic, not thermoset, resins. They are being promoted for the design of injection-molded printed circuit boards in three-dimensional shapes for functional appHcations as an alternative to standard flat printed circuit boards. Only semiadditive or fully additive processing can be used with these devices. 

These LCT materials have very high tensile and flexural strength, and excellent mechanical and chemical resistance properties. Some commercial LCT are Vectra (Hoechst-Celanese) and Xydar (Amoco). Du Pont, ICI, GE, and Dow Chemical are also suppHers. Their appHcation in electronic embedding is stiU. in its infancy because of the high temperature processing requirement. Nevertheless, this class of thermoplastic polymers will play an important role in electronic embedding. 

COLLYER, A. A., A Practical Guide to the Selection of High Temperature Engineering Thermoplastics, Elsevier Advanced Technology, London (1990)... 

The presence of the either linkages is sufficient to allow the material to be melt processed, whilst the polymer retains many of the desirable characteristics of polyimides. As a consequence the material has gained rapid acceptance as a high-temperature engineering thermoplastics material competitive with the poly-sulphones, poly(phenylene sulphides) and polyketones. They exhibit the following key characteristics ... 

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