High density polyethylene, HDPE general

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

Although some polymers may be satisfactory when used under the stress of static loads, they may fail when subjected to impact. The impact resistance, or resistance to brittle fracture, is a function of the molecular weight of a polymer. Thus uhmwpe is much more resistant to impact failure than general purpose high-density polyethylene (hdpe). The impact resistance of brittle polymers is also increased by the addition of plasticizers. Thus polyvinyl chloride (PVC), plasticized by relatively large amounts of dioctyl phthalate, is much less brittle than unplasticized rigid PVC. 

A major example of the second branched polymer type is the polyethylene that is made by free radical polymerization at temperatures of 100-300°C and pressures of 1,000-3,000 atm. The extent of branching varies considerably depending on reaction conditions and may reach as high as 30 branches per 500 monomer units. Branches in polyethylene are mainly short branches (ethyl and butyl) and are believed to result from intramolecular chain transfer during polymerization (described later in Chapter 5). This branched polyethylene, also called low-density polyethylene (LDPE), differs from linear polyethylene (high-density polyethylene, HDPE) of a low-pressure process so much so that the two materials are generally not used for the same application. 

Materials. High density polyethylene (HDPE) having different molecular weights, and specific gravity of 0.951 (Marlex 5202, HXM 50100, made by Phillips 66 Co), were used for extrusion applications. Polyamides used were a semicrystalline copolyamide of adipic acid, hexamethylene diamine and caprolactam, and a copolyamide containing isophthalic acid as well. An anhydride modified polyethylene (3-5) as an interlaminar adhesive/compatibilizer was also used. The combinations are generally included in "Selar" barrier materials supplied by E. I. du Pont de Nemours Co. 

Polyethylene now tends to be marketed in three general grades high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE or 1-LDPE), and low-density polyethylene (LDPE). The essential structural differences for each are shown in Table 15.7. 

From a mechanical point of view, high density polyethylene (HDPE)/isotactic polypropylene (iPP) blend have generally been considered as very unsatisfactory materials In particular, they show very poor ultimate mechanical properties at room temperature in comparison with those of the blend constituents. This fact precludes their use for most commercial purposes. In previous papers we found that it is possible to improve the mechanical tensile performance of these blends by appropriately varying the testing conditions such as the temperature and the drawing rate. 

Plastics are subject to attack by some aggressive fluids and chemicals. However, not all plastics are attacked by the same materials. It is generally possible, therefore, to select a plastic matrix to meet a particular condition. Some plastics, such as high-density polyethylene (HDPE), are immune to almost any commonly found solvents. A few such as polytetrafluoroethylene (PTFE) are immune to almost any corrosive conditions. 

Polyethylene (PE) is inherently less sensitive to oxidative attack than PP, but stabilization of PE is also mandatory for outdoor use. The stability varies with the type of polyethylene and manufacturing process. Linear low-density polyethylene (LLDPE) (1-octene comonomer) is significantly less sensitive to photooxidation than low-density polyethylene (LDPE) with comparable density and molecular weight [20, 21]. Generally, LDPE is less susceptible to photooxidation than high-density polyethylene (HDPE). The most fundamental difference between polyethylene homopolymers and polypropylene is the behavior of hydroperoxides toward photolysis. On photooxidation, hydroperoxides accumulate in PP, but decrease rapidly on UV exposure of PE. In copolymers of polyethylene with vinyl acetate, the stabihty depends on the content of vinyl acetate. The higher the content, the more the copolymers act like polyvinyl acetate, which is more susceptible to photooxidative degradation than polyethylene. 

The Moody diagram is a general graph for the Darcy factor versus the Reynolds number, It is applicable to a very large number of different pipe materials. There are four principal pipe materials associated with slurry flows plastic pipes [high-density polyethylene (HDPE)], plain steel pipes, rubber-lined steel pipes, and concrete pipes. The absolute... 

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