Amorphous reinforcements

Athough this subsection does not really fit with the title of this book chapter, it has been inserted for the sake of completeness of the SPC topic. SPC microcomposites were produced by hot compaction (oniwconstituent approach) while nanocomposites by film stacking (two-constituents) using the same polymer, namely, poly (methyl methacrylate) (PMMA). It has to be mentioned that amorphous/amorphous (reinforcement/matrix) SPCs are studied less than those containing a reinforcing phase of semicrystalline nature. This is due to a small difference in the stiffness and strength between the polymers that overtake the role of the matrix and reinforcement, respectively. 

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). 

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. 

Cement and Concrete Concrete is an aggregate of inert reinforcing particles in an amorphous matrix of hardened cement paste. Concrete made of portland cement has limited resistance to acids and bases and will fail mechanically following absorption of crystalforming solutions such as brines and various organics. Concretes made of corrosion-resistant cements (such as calcium aluminate) can be selected for specific chemical exposures. 

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,... 

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. 

Crazing. This develops in such amorphous plastics as acrylics, PVCs, PS, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

Glass-fiber-reinforced amorphous TP RPs generally have greater creep resistance than glass-fiber-reinforced crystalline... 

In addition to the broad categories of TPs and TSs, TPs can be further classified in terms of their structure, as either crystalline, amorphous, or liquid crystalline. Other classes (terms) include elastomers, copolymers, compounds, commodity resins, engineering plastics, or neat plastics. Additives, fillers, and reinforcements are other classifications that relate directly to plastics properties and performance. 

The amorphous TPs, which have their molecules going in all different directions, are normally transparent. Compared to crystalline types, they undergo only small volumetric changes when melting or solidifying during processing. Tables 6-5 to 6-9 compare the basic performance behaviors of crystalline and amorphous plastics. Exceptions exist, particularly with respect to certain plastic compounds that include additives and reinforcements. 

Strength Fibrous minerals Ductility gaining tensile strength. Carbon fibers are more expensive fibrous minerals are least expensive but only slightly reinforcing. Reinforcement makes brittle resins tougher and embrittles tough resins. Fibrous minerals are not commonly used in amorphous resins. 

Reinforcement reduces shrinkage far more than fillers do. Fillers help balance shrinkage, however, because they replace shrinking polymer. The sharp shrinkage reduction in reinforced crystalline resins can often lead to warpage. The best mold-to-size composites are reinforced amorphous composites. 

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