Polyethylene partially degraded

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

Interpretation of these curves show that Poly (vinyl chloride) (PVC) first loses HC1 later the mixture of unsaturated carbon-carbon backbone and unchanged poly (vinyl chloride) partly degrades to small fragments. Poly (methyl methacrylate) (PMMA), branched polyethylene (HPPE), and polytetrafluorethylene (PTFE) degrade completely to volatile fragments, while a polyimide (PI) partially decomposes, forming a char above 800°C. 

The fluid mechanical aspect of luminous gas is also evident with non-polymer-forming plasma. It has been observed that the degradation of polymer exposed to N2 plasma is very severe in the constricted portion a polyethylene film inserted in the constricted portion suffered permanent deformation due to partial melting, whereas polyethylene films placed in wider portions of the tube (before and after the constriction) did not show any visible difference after they were exposed to N2 glow discharge simultaneously as depicted in Figure 20.27. 

Transparency, in general drops with crystallinity (e.g. polyethylene), and with an increase of crystallite size which causes light scattering. Most fillers, colorants and auxiliary additives lead to opacity. Transmittance depends on the refractive index, so that some fillers may preserve full or partial transparency (translucent). There are also dyes that dissolve in the polymer, so that a colored transparent polymer is produced. It is also possible to find stabilizers (including antioxidants or UV absorbers) that do not affect the polymer transparency. Any chemical change in the polymer like degradation or oxidation, or diffusion of some components, may reduce light transmission. 

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