Polyethylene shrinkage

Polyethylene Typ Melt Index (hl6 g) Density Range (giccy Molecular Weight Distribution 

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Polycarbonate melts adhere strongly to metals and if allowed to cool in an injection cylinder or extrusion barrel may, on shrinkage, pull pieces of metal away from the wall. It is therefore necessary to purge all equipment free of the resin, with a polymer such as polyethylene, after processing. 

FIGURE 31.15 Variation of percent heat shrinkage and amnesia rating with radiation doses of low-density polyethylene/ethylene-vinyl acetate (LDPE/EVA) film from 50 50 blend without ditrimethylol propane tetra-crylate (DTMPTA). (From Chattopadhyay, S., Chaki, T.K., and Bhowmick, A.K., Radiat. Phys. Chem., 59, 501, 2000. With permission.)... 

More than 0.3% pigment is required to formulate 1/3 SD HDPE samples. P.Gr.36 is thus much weaker than P.Gr.7 types, although it withstands more than 300°C and is thus equally heat stable. Considerable influence on the shrinkage of injection-molded polyethylene parts necessitates some caution in using P.Gr.36 for this purpose. Many types are difficult to disperse in plastics, especially in polyolefins. 

Low-profile additives are generally materials such as poly (vinyl acetate), polystyrene, polyethylene or polycarbonate. During the unsaturated polyester cure cycle, the low-profile additives separate into a second phase, which expand to counteract the shrinkage of the curing unsaturated polyester resin. Material development and the science of low-profile additives have helped create substantial markets for unsaturated polyesters. Their use in automotive markets, where Class A show room quality surfaces is a requirement, is an example of this. 

ISO 14236 2000 Plastics pipes and fittings - Mechanical-joint compression fittings for use with polyethylene pressure pipes in water supply systems ISO 14616 1997 Plastics - Heatshrinkable films of polyethylene, ethylene copolymers and their mixtures - Determination of shrinkage stress and contraction stress ISO 14632 1998 Extruded sheets of polyethylene (PE-HD) - Requirements and test methods... 

General drawbacks are the innate relative sensitivity to heat and creep, low rigidity, significant shrinkage and poorer low-temperature behaviour than polyethylene. 

The specimens are then immersed in a molten wax or natural resin and the Cellosolve slowly distilled off. This step invariably involves some shrinkage. Antishrink efficiencies of 80% are, however, obtainable in this way with mixtures of beeswax and rosin. This treatment appears suitable for the preservation of wood artifacts. Christensen (43) has treated wood artifacts by replacing the water with tertiary butanol and this with polyethylene glycol-4000. 

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