Other crystalline thermoplastics

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. 

A related class of polymers is the crystalline, thermoplastic materials. These also are fabricated by heating to a high temperature so diat they flow but when they are cooled ordered regions develop within them, which makes them translucent. They have much tendency to flow because of these mechanical crosslinks" and have good dimensional stability. Polyediylene and polypropylene belong to this class. Here the chain is simple and regular so that different polymer molecules, or different parts of the same molecule, can pack next to each other. The same situation exists with Teflon" (polytetrafl uoroethylene). 

Figure 3.13 shows the shift factors aT determined from time-temperature superposition as a function of temperature for melts of two semi-crystalline thermoplastics as well as the Arrhenius plot. For the two polyethylenes (HDPE, LDPE), the progression of log ax can be described with the Arrhenius equation. The activation energies can be determined from the slope as Ea(LDPE) 60 kj/mol and Ea(HDPE) 28 kj/mol. Along with polyethylenes (HDPE, LDPE, LLDPE), other significant semi-crystalline polymers are polypropylene (PP), polytetrafluoroethylene (PTFE) and polyamide (PA). 

Polyoxymethylene (POM) plastics are highly crystalline thermoplastics that are obtained by polymerization of formaldehyde and can also be in the form of trioxy-methylene oligomers (trioxane). The world-wide consumption in 1997 was 0.5 x 106 t for car parts and other articles processed by injection moulding. Polyacetals are primarily engineering materials being used to replace metals. 

SPS has high a melting point almost comparable to those of other engineering plastics (Table 18.2). In comparison with other crystalline engineering thermoplastics, of particular interest is that SPS has the lowest specific gravity and dielectric constant. 

Owing to its partial crystallinity, sPS is strongly resistant to concentrated acid and bases, oils and greases and most organic solvents, except chlorinated and aromatic compounds that cause swelling. Its density (about 1 g/cm3) is significantly low compared with other engineering thermoplastics such as polyamide-6, poly(butylene terephthalate) and poly(phenylene sulfide), and this is... 

Due to its high crystallinity, polyacetal is more rigid and stronger than other thermoplastics especially at elevated T (50-120°C). Because of its semi-crystalline character, POM requires a greater heat input for melting than amorphous resins. It has a fairly high heat of fusion (160-200 kJ/kg) but because of lower melt temperature (210°C), it requires considerably lower total heat energy than other crystalline polymers (viz., POM = 435-475, HDPE = 720, PA-66 = 756-786 kJ/kg). 

Rubber consumption is dominated by tyre production. In these, conveyor belts, and pressure hoses, thin layers of either steel wire or polymeric fibre reinforcement take the main mechanical loads. These layers, with rubber interlayers, allow flexibility in bending, whereas the reinforcement limits the in-plane stretching of the product. The applications are dominated by natural rubber and styrene butadiene copolymer rubber (SBR). Other rubbers have specialised properties butyl rubbers have low air permeability, nitrile rubbers have good oil resistance, while silicone rubbers have high and low temperature resistance. Rubbers play a relatively small role in this book, but the rubbery behaviour of the amorphous phase in semi-crystalline thermoplastics is important. 

Polyallomer Crystalline thermoplastic block copolymers of ethylene, propylene, and other olefins. Has good impact strength, flex life, and low density. 

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