Crystalline reinforced

Figure 9.24 Crystalline-reinforced engineering thermoplastics, Stiffness-toughness positioning chart. (Courtesy of DSM Engineering Plastics,)...

With respect to the research in this field, all-polypropylene (PP) composites have been extensively studied. Hine et al. investigated the influence of fabrication parameters that control the hot-compaction behavior of woven-oriented PP [23,24], The results indicated that molecular weight, crystallinity, reinforcement shape and the weave style are important for controlling the composites performance. Alcock et al. created unidirectional all-PP composite laminates from co-extruded three layer tape of PP (with a copolymer homopolymer copolymer structure) via filament winding [12]. The specific mechanical properties were comparable to those reported for a commercial unidirectional glass fiber reinforced PP. 

Baird DG, Ramanathan R (1990) The in-situ generation of liquid crystalline reinforcements in engineering thermoplastics. In Culbertson BM (ed) Contemporary topics in polymer science, vol 6. Plenum, New York, pp 73-93... 

Stretching a polymer sample tends to orient chain segments and thereby facilitate crystallization. The incorporation of different polymer chains into small patches of crystallinity is equivalent to additional crosslinking and changes the modulus accordingly. Likewise, the presence of finely subdivided solid particles, such as carbon black in rubber, reinforces the polymer in a way that imitates the effect of crystallites. Spontaneous crystal formation and reinforcement... 

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. 

Crystallinity. Generally, spider dragline and silkworm cocoon silks are considered semicrystalline materials having amorphous flexible chains reinforced by strong stiff crystals (3). The orb web fibers are composite materials (qv) in the sense that they are composed of crystalline regions immersed in less crystalline regions, which have estimates of 30—50% crystallinity (3,16). Eadier studies by x-ray diffraction analysis indicated 62—65% crystallinity in cocoon silk fibroin from the silkworm, 50—63% in wild-type silkworm cocoons, and lesser amounts in spider silk (17). 

Shipment ndStora.ge. The crystalline material is shipped as a nonha2ardous material, in polyethylene-lined fiber dmms. The solution can be shipped in dmms or bulk. Suitable materials of constmction for handling ammonium thiocyanate are aluminum, 316 stainless steel, mbber, poly(vinyl chloride), and glass-reinforced epoxy. Steel, 304 stainless steel, and copper alloys should be avoided (375,376). 

Polymers with differing morphologies respond differentiy to fillers (qv) and reinforcements. In crystalline resins, heat distortion temperature (HDT) increases as the aspect ratio and amount of filler and reinforcement are increased. In fact, glass reinforcement can result in the HDT approaching the melting point. Amorphous polymers are much less affected. Addition of fillers, however, intermpts amorphous polymer molecules physical interactions, and certain properties, such as impact strength, are reduced. 

Some interesting differences are noted between amorphous and crystalline polymers when glass fibre reinforcement is incorporated into the polymer. In Figure 9.2 (ref. 10) it will be seen that incorporation of glass fibre has a minimal effect on the heat deflection temperature of amorphous polymers (polystyrene,... 

Polymers below the glass transition temperature are usually rather brittle unless modified by fibre reinforcement or by addition of rubbery additives. In some polymers where there is a small degree of crystallisation it appears that the crystallines act as knots and toughen up the mass of material, as in the case of the polycarbonates. Where, however, there are large spherulite structures this effect is more or less offset by high strains set up at the spherulite boundaries and as in the case of P4MP1 the product is rather brittle. 

The reinforcing filler usually takes the form of fibres but particles (for example glass spheres) are also used. A wide range of amorphous and crystalline materials can be used as reinforcing fibres. These include glass, carbon, boron, and silica. In recent years, fibres have been produced from synthetic polymers-for example, Kevlar fibres (from aromatic polyamides) and PET fibres. The stress-strain behaviour of some typical fibres is shown in Fig. 3.2. 

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