High density polyethylene properties

Commercial High-Density Polyethylene, Properties and Manufacture... 

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... 

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. 

The even-numbered carbon alpha olefins (a-olefins) from through C q are especially useful. For example, the C, C, and Cg olefins impart tear resistance and other desirable properties to linear low and high density polyethylene the C, Cg, and C q compounds offer special properties to plasticizers used in flexible poly(vinyl chloride). Linear C q olefins and others provide premium value synthetic lubricants linear 145 olefins are used in... 

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. 

In the mid-1950s a number of new thermoplastics with some very valuable properties beeame available. High-density polyethylenes produced by the Phillips process and the Ziegler process were marketed and these were shortly followed by the discovery and rapid exploitation of polypropylene. These polyolefins soon became large tonnage thermoplastics. Somewhat more specialised materials were the acetal resins, first introduced by Du Pont, and the polycarbonates, developed simultaneously but independently in the United States and Germany. Further developments in high-impact polystyrenes led to the development of ABS polymers. 

The electrical properties of polypropylene are very similar to those of high-density polyethylenes. In particular the power factor is critically dependent on the amount of catalyst residues in the polymer. Some typical properties are given in Table 11.3 but it should be noted that these properties are dependent on the antioxidant system employed as well as on the catalyst residues. 

In broad tonnage terms the injection moulding markets for high-density polyethylene and polypropylene are very similar. The main reasons for selecting polypropylene have been given above. In favour of HDPE is the inherently better oxidation and ultraviolet resistance. Whilst these properties may be greatly improved in polypropylene by the use of additives these may increase the cost of polypropylene compounds to beyond that which is considered economically attractive. It is for this reason that HDPE has retained a substantial part of the crate market. 

The Phillips process for the manufacture of high-density polyethylene may be adapted to produce copolymers of ethylene with small amounts of propylene or but-l-ene and copolymers of this type have been available since 1958. These soon found application in blown containers and for injection moulding. Properties of two grades of such copolymers are compared with two grades of Phillips-type homopolymer in Table 11.11. 

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. 

Akhtar has studied the morphology and physical properties of NR and high-density polyethylene blends prepared in Brabender plasticorder at 150°C at a rotor speed of 60 rpm [53]. Films were molded between two chromium plates at a pressure of 0.34 MPa. The films along with mold were... 

The main use of propylene is for polymerization to polypropylene, a process similar to the manufacture of high-density polyethylene (i.e., a low-pressure, catalytic process). Textile hhers made from polypropylene are relatively low-cost and have particularly good properties, such as high resistance to abrasion and soiling for use in furniture upholstery and indoor/outdoor carpeting. 

Kaufman, F. S., Jr., "A New Technique for Evaluating Outdoor Weathering Properties of High Density Polyethylene", in ref. 

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