Partially Crystalline Plastics

The densities of the crystalline region and amorphous regions are very different. The density of the polymer, P, will therefore depend on its percent crystallinity x  

The percent crystallinity of commodity plastics generally increases as follows  

Extending a plastic material in one direction (anisotropically) results in forcing the chains into aligning themselves in that direction. As the chains get closer to each other, further crystallization is encouraged. At very high extensions, the plastic becomes a highly crystalline, anisotropic fiber such as those used in textiles. 

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The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

A number of organic pigments cause distortion in certain types of polyolefins, especially in HDPE. Pigments act as nucleating agents in such partially crystalline plastics i.e., they promote crystallization, which creates stress within the plastic product (Sec. 1.6.4.3). These pigments also enhance the shrinkage of polyolefins, particularly in the direction of the flow. 

Transparent and reduced 1/3 SD polyolefin systems are stable up to 300 to 320°C. 0.25% pigment is required in order to formulate 1/3 SD HDPE samples containing 1% TiOz. P.O.43, incorporated in injection-molded partially crystalline plastics such as HDPE, affects the shrinkage of the medium, which precludes its use in nonrotation symmetrical injection-molded articles (Sec. 1.6.4.3). 

The influence of temperature on the stress-strain behavior of polymers is generally opposite to that of straining rates. This is not surprising in view of the correspondence of time and temperature in the linear viscoelastic region (Section I l.5.2.iii). The curves in Fig. 11-23 are representative of the behavior of a partially crystalline plastic. 

Furthermore, monomers from which crystalline homopolymer can be produced, such as high-density polyethylene and polypropylene, can be copolymerized to produce resins with controllably reduced crystallinity and thus greater transparency. The ethylene/propylene copolymers may range from partially crystalline plastics to amorphous elastomers. 

Linear or branched, that is, not crosslinked, thermoplastic materials, usually first begin to soften on heating and then on further heating (amorphous polymers) begin to flow over a rather ill-defined temperature range (see Figure 3.1). Partially crystalline plastics in general have narrow melt-... 

A new area of development is to incorporate the filler permanently into the polymer matrix, by use of coupling reactions. This can increase impact strength and thermal properties of polyamides and modify the anisotropy of partially crystalline plastics, such as polyamides and polyesters. In polypropylene, bonding with kaolin can also improve scratch resistance, which is a useful benefit for automobile interior applications. Surface modification of fillers such as silica, mica, and wollastonite allows these to penetrate markets that were formerly the province of reinforcements such as carbon black and glass fibre. 

Eliminating the fall in melt temperature which occurs in a cold runner, has enabled the use of a longer flow path in the cavity. This is of significance in the case of partially-crystalline plastics, which feature a narrow temperature window that prevents compensation of the temperature drop in the CR by raising the injection temperature. 

P.O. 72 is a yellowish orange of high heat stability and excellent light fastness. It does not induce warping in partially crystalline plastics and can be used for spin... 

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