Other amorphous thermoplastics

Polycarbonates are an unusual and extremely useful class of polymers. The vast majority of polycarbonates are based on bisphenol A [80-05-7] (BPA) and sold under the trade names Lexan (GE), Makrolon (Bayer), CaUbre (Dow), and Panlite (Idemitsu). BPA polycarbonates [25037-45-0] having glass-transition temperatures in the range of 145—155°C, are widely regarded for optical clarity and exceptional impact resistance and ductiUty at room temperature and below. Other properties, such as modulus, dielectric strength, or tensile strength are comparable to other amorphous thermoplastics at similar temperatures below their respective glass-transition temperatures, T. Whereas below their Ts most amorphous polymers are stiff and britde, polycarbonates retain their ductiUty. 

Mechanical Properties. The room temperature modulus and tensile strength are similar to those of other amorphous thermoplastics, but the impact strength and ductility are unusually high. Whereas most amorphous polymers arc glass-like and brittle below their glass-transition temperatures, polycarbonate remains ductile to about — 10°C. The stress-strain curve for polycarbonate typical of ductile materials, places it in an ideal position for use as a metal replacement. Weight savings as a metal replacement are substantial, because polycarbonate is only 44% as dense as aluminum and one-sixth as dense as steel. 

Table 3.28 Comparision of bisphenol-TMC polycarbonate, bisphenol-TMC copolycarbonates and other amorphous thermoplastics [94]...

Amorphous thermoplastics were some of the earliest plastics, excepting polycarbonate, to find general acceptance indeed, cellulose plastics were the first thermoplastics available commercially, and poly(methyl methacrylate), (PMMA), was a commercial product in the 1930s. Their continued application some 50 years later is a comment on their usefulness, and on their properties compared with those of polystyrene, since PMMA is almost twice the price of PS, and the cellulose plastics are significantly more expensive than PMMA. Polycarbonate, a development of the late 1950s, is some three times more expensive than PS, and so finds use in critical applications where performance rather than cost is the criterion of acceptability. There are other amorphous thermoplastics with yet more advantageous properties which have not reached the status of commodity materials a selection of these with elevated service temperatures is reviewed in a later chapter. 

Crystalline polymers undergo a discontinuous decrease in volume when cooled through (Fig. 4). This can lead to nonuniform shrinkage and warping in molded objects. On the other hand, it also causes the polymer to "lock on" to reinforcing fibers, eg, glass (qv), so that crystalline thermoplastics benefit much more than amorphous thermoplastics from fiber reinforcement. 

The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks each chain is cross-linked in many places to other chains, so that, if stretched out, the array would look like a piece of Belgian lace, somehow woven in three dimensions. These are the thermosets if heated, the structure softens but it does not melt the cross-links prevent viscous flow. Thermosets are usually a bit stiffer than amorphous thermoplastics because of the cross-links, but they cannot easily be crystallised or oriented, so there is less scope for changing their properties by processing. 

As with other rigid amorphous thermoplastic polymers such as PVC and polystyrene (see the next chapter) poly(methyl methacrylate) is somewhat brittle and, as with PVC and polystrene, efforts have been made to improve the toughness by molecular modification. Two main approaches have been used, both of which have achieved a measure of success. They are copolymerisation of methyl methacrylate with a second monomer and the blending of poly(methyl methacrylate) with a rubber. The latter approach may also involve some graft copolymerisation. 

Plasticised amorphous thermoplastics Certain plastics may be mixed with high-boiling low-volatility liquids to give products of lower T. The most important example occurs with p.v.c. which is often mixed with liquids such as di-iso-octyl phthalate, tritolyl phosphate or other diesters to bring the below room temperature. The resultant plasticised p.v.c. is flexible and to some degree quite rubbery. Other commonly plasticised materials are cellulose acetate and cellulose nitrate. 

Notes. Within amorphous thermoplastics some are, due to asymmetry, non crystallisable while others are usually amorphous because they are too slow to crystallise during normal processing. 

PEEK is miscible with poly(ether imide) (PEI). PEI is less expensive than PEEK it is used as an amorphous thermoplastic. The kinetics of crystallization and other properties of such blends have been presented in the literature. 

Fig. 2 Temperature-dependence of the modulus of elasticity (Young s modulus) of plastics (diagram). As an alternative to this modulus, tension a can also be plotted against constant elongation e or viscosity i), or other properties [2]. MSRe x,d- main softening range of elastomers, thermoplastics, duroplastics, Tgt associated glass transition temperature, Tfi flow point of the amorphous thermoplastic, //////// application range, application range...

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