Polyethylene high-density form

United States The Ziegler route to polyethylene is even more important because it occurs at modest temperatures and pressures and gives high density polyethylene which has properties superior to the low density material formed by the free radical polymerization described m Section 6 21... 

In 1971 a metal-backed polyethylene acetabular cup was introduced. This cup provided an eccentric socket which was replaceable, leaving the metal and replacing only the polyethylene. Because of the success of this component, metal-backed high density polyethylene (HDPE) liner is standard for prosthetic acetabular components. Research confirms that metal-backing reduces the peak stresses in the bone cement, and that HDPE forms a successfiil articulating surface for the prosthetic joint. 

High density polyethylene is widely used for pipes and drains, especially in large-diameter cormgated forms. The cormgations provide stronger walls at less thickness, which reduces the materials cost of the pipe. 

Chlorination may be carried out with both high-density and low-density polyethylene. When carried out in solution the chlorination is random but when carried out with the polymer in the form of a slurry the chlorination is uneven and due to residual crystalline zones of unchlorinated polyethylene the material remains a thermoplastic. 

High-density polyethylene has more CH2 groups per unit volume than low-density polyethylene. Explain why this is so in terms of the structures of the two forms of the polymer. 

Storage stability Store DF in lead and wax-lined carboys, high-density polyethylene bottles, or nickel-lined containers in well-ventilated areas. Never store DF with alcohols DF will react with alcohols to form lethal chemicals, such as crude GB. Incompatible with water, glass, concrete, most metals, natural rubber, leather, and organic materials like glycols. The acidic corrosive hydrolysis products may react with metals, such as Al, Pb, and Fe, to give off hydrogen gas, a potential fire and explosive hazard. 

Chapter 661 of the USP provides criteria for the interchangeability of low- and high-density polyethylene for dry, oral dosage forms. In addition, there are standards for polyethylene terephthalate bottles and polyethylene terephthalate G bottles. USP criteria for interchangeability are listed in Table 17. These criteria allow usage of alternate materials in the same plastic class to be used prior to obtaining prior stability data. 

The two polymer substrates investigated as part of the study of DBDPO mixtures were polypropylene (PP) and linear high density polyethylene (HDPE). while both PP and HDPE decompose by similar random chain scission, radical mechanisms, chain transfer occurs much more teadily during the pyrolysis of PP because of the presence of the tertiary hydrogens. In addition, only primary chain end radicals are formed when the HDPE chain cleaves homolytically. Therefore, a comparison of the PP/DBDPO and the HDPE/DBDPO mixtures volatile product distributions was undertaken. 

A massive explosion in Pasadena, Texas, on October 23,1989, resulted in 23 fatalities, 314 injuries, and capital losses of over 715 million. This explosion occurred in a high-density polyethylene plant after the accidental release of 85,000 pounds of a flammable mixture containing ethylene, isobutane, hexane, and hydrogen. The release formed a large gas cloud instantaneously because the system was under high pressure and temperature. The cloud was ignited about 2 minutes after the release by an unidentified ignition source. 

The development of medium and high density polyethylene pressure pipes over the last thirty years has led to an extensive database on their stress-rupture behaviour, with associated standards for their testing and the interpretation of results [1, 2], Various forms of PVC are also used. The accelerated testing of gas and water pipes is introduced as an example of a field where there is a large database of pressure testing on which current regulations are based, the number of polymers is limited and accelerated tests are widely used (unusually, with two accelerating parameters). 

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