High-density polyethylene temperature

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... 

Formic acid is commonly shipped in road or raH tankers or dmms. For storage of the 85% acid at lower temperatures, containers of stainless steel (ASTM grades 304, 316, or 321), high density polyethylene, polypropylene, or mbber-lined carbon steels can be used (34). For higher concentrations. Austenitic stainless steels (ASTM 316) are recommended. 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

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. 

The Phillips-type catalyst can be used in solution polymerization, slurry polymerization, and gas-phase polymerization to produce both high density polyethylene homopolymers and copolymers with olefins such as 1-butene and 1-hexene. The less crystalline copolymers satisfy needs for materials with more suitable properties for certain uses that require greater toughness and flexibiUty, especially at low temperatures. 

Low pressure (0.1 to 20 MPa) and temperatures of 50 to 300°C using heterogeneous catalysts such as molybdenum oxide or chromium oxide supported on inorganic carriers to produce high density polyethylene (HDPE), which is more linear in nature, with densities of 0.94 to 0.97 g/cm. ... 

Figure 10.9. Specific heat-temperature relationships for low-density polyethylene, high-density polyethylene and polystyrene." (The Distillers Company Ltd.)...

High-density polyethylene (p = 0.94-0.96 g/cm ) has up to five times the stiffness of low-density polyethylene at ambient temperatures and can be used at much higher temperatures. Its chemical resistance is similar to that of the low-density grades, but the resistance to swelling by solvents is higher. 

High-density polyethylene is very susceptible to environmental stress corrosion cracking, especially if used at the high end of the temperature range. 

High-density polyethylene is characterized by a higher crystallinity and higher melting temperature than LDPE due to the absence of branching. 

High density polyethylene produced by a low-pressure low-temperature process involving Ziegler-Natta catalysts. This creates low levels of branching and hence a high degree of crystallinity. 

The most common backbone structure found in commercial polymers is the saturated carbon-carbon structure. Polymers with saturated carbon-carbon backbones, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyacrylates, are produced using chain-growth polymerizations. The saturated carbon-carbon backbone of polyethylene with no side groups is a relatively flexible polymer chain. The glass transition temperature is low at -20°C for high-density polyethylene. Side groups on the carbon-carbon backbone influence thermal transitions, solubility, and other polymer properties. 

On the other hand, the polymer prepared by the embedded catalyst shows T around 130 °C, which is a typical melting temperature of high density polyethylene. There was little activity difference between the polyethylene produced by embedded particles and those by homogeneous catalysts. The results of ethylene polymerization using embedded catalyst and homogeneous catalyst are summarized in Table 1 and Fig. 2,... 

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. 

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. 

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