High-density polyethylene point

HDPE melts at about 135°C, is over 90% crystalline, and is quite linear, with more than 100 ethylene units per side chain. It is harder and more rigid than low density polyethylene and has a higher melting point, tensile strength, and heat-defiection temperature. The molecular weight distribution can be varied considerably with consequent changes in properties. Typically, polymers of high density polyethylene are more difficult to process than those of low density polyethylene. 

Fig. 20. Stress(er)-strain (ai) dependence for high density polyethylene samples. 1 reference sample, 2 sample obtained if molecular orientation exists, 3 super high tenacity fiber-fibril. Asterisks denote the points of fiber failure...

Voids within a sample are a major cause of internal haze. We see the effect of voiding when we stretch polymers, such as high density polyethylene and isotactic polypropylene, that have distinct yield points and clearly defined necks (as discussed earlier in this chapter). The... 

Small areas Ventilate to remove vapor. Because the boiling point of some cyanide agents is near normal room temperature (70°F), agent vapors may condense on cooler surfaces and pose a percutaneous hazard. Liquids can then revolatilize when the temperature rises. If deemed necessary, wash the area with copious amounts of soap and water. Collect and place the rinseate and place in containers lined with high-density polyethylene. 

Random ethylene copolymers with small amounts (4-10 wt-%) of 7-olefins, e.g. 1-butene, 1-hexene, 1-octene and 4-methyl- 1-pentene, are referred to as linear low-density polyethylene, which is a commercially relevant class of polyolefins. Such copolymers are prepared by essentially the same catalysts used for the synthesis of high-density polyethylene [241]. Small amounts of a-olefin units incorporated in an ethylene copolymer have the effect of producing side chains at points where the 7-olefin is inserted into the linear polyethylene backbone. Thus, the copolymerisation produces short alkyl branches, which disrupt the crystallinity of high-density polyethylene and lower the density of the polymer so that it simulates many of the properties of low-density polyethylene manufactured by high-pressure radical polymerisation of ethylene [448] (Figure 2.3). 

Although polyethylene was considered a source of useful fibers at an early date, its low melting point (110-120°C) as well as other limitations precluded active development during the period when production of other fibers based upon the petrochemical industry expanded enormously. The higher melting point of high-density polyethylene gave some promise, but it was overshadowed by the introduction of polypropylene (PP) around... 

Thus, compared to other polymers, it is easier to obtain an extended and oriented chain structure in polyethylene. High density polyethylene (HDPE) is preferred to other types of polyethylene because HDPE has fewer branch points along its backbone and a high degree of crystallinity. These characteristics of linearity and crystallinity are important to obtain a high degree of orientational order and an extended chain structure in the final fiber. 

The melting temperature for PAN is extremely high as compared, e. g., with that of isotactic polystyrene (230 °C) or high density polyethylene (137—140 °C). Generally, a high melting point, T , can be caused by a high heat of fusion, AHf, and/or by a low entropy of fusion, ASf ... 

Figure 14.39 Applied stress versus failure time (static fatigue) for a sample of high density polyethylene at various temperatures. The inflection shows the point of change from brittle failure to ductile failure. (From Ref. 49.)...

The polymers mostly used in pharmaceutical packaging are polyethylene, polypropylene, PVC, polyamide, polystyrol, nylon, cellulose acetate, polyethylene terephthtalate, and blends thereof. Copolymers and rubbers are also used. The DSC melting curve of polyethylene used for packaging purposes is characteristic. Low- and high-density polyethylene are differentiated by their melting points. " Melting point and density of polyethylene are linearily correlated. " Crystallinity may be determined as described above for amorphous state. 

Stiffer, harder than high-density polyethylene, higher melting point... 

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