High-density polyethylene molecular weight distribution

High density polyethylene, shown in Fig. 18.2 a), consists primarily of linear hydrocarbon chains of the type shown in Fig. 18.1. We commonly abbreviate its name to HDPE. As with all other polymers, high density polyethylenes contain a distribution of molecular weights. The molecules have few, if any, branches. 

Low Temperature Brittleness. Brittleness temperature is the temperature at which polyethylene becomes sufficiently brittle to break when subjected to a sudden blow. Because some polyethylene end products are used under particularly cold climates, they must be made of a polymer that has good impact resistance at low temperatures namely, polymers with high viscosity, lower density, and narrow molecular weight distribution. ASTM D746 is used for this test. 

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. 

A small number of companies use metal oxide catalysts, such as the example shown in Fig. 18.6, to make high density polyethylene. The polyethylene made with this catalyst generally has a narrower molecular weight distribution than high density polyethylene made with Ziegler-Natta catalysts. 

Ethylene Polymers. Depending on the polymerization conditions, three major types of polyethylene are manufactured low-density polyethylene (LDPE) by free-radical polymerization, linear low-density polyethylene (LLDPE) by copolymerization of ethylene with terminal olefins, and high-density polyethylene (HDPE) by coordination polymerization. The processes yield polymers with different characteristics (molecular weight, molecular weight distribution, melt index, strength, crystallinity, density, processability). 

It is expected that around 500,000 tons mPE will be manufactured in 2000 in Western Europe. Resins from high-pressure- as well as from low-pressure processes suffer more difficult processability. Their narrow molecular-weight distribution results in an unfavourable viscosity behaviour [8], As with linear low-density polyethylene (LLDPE), the viscosity of mPE is independent of the shear-rate over a wide range and reduces only at high shear-rates (Fig. 9.5-11). Therefore, energy consumption is high when mPE is processed with extruders which are designed for LDPE. 

Calibration. The accepted method of calibrating a GPC system was used. Narrow molecular-weight distribution high-density polyethylene polymers were characterized by light scattering, osmometry, and sedi-... 

Much work has been done within the framework of the international programme JUPAC by 14 laboratories of different companies to reveal a characteristic of polymer material correlating with specific features of its molding into sleeve film67). The studies were conducted with two samples of low-density polyethylene and two samples of high-density polyethylene. The samples had minor differences in molecular-weight distribution and ramification characterized by the content of different side branches. 

TABLE XII Qualitative expression of effects on high density polyethylene of changes in melt flow rate, density, and distribution of molecular weight... 

Application The SCLAIRTECH1 technology (PE) process can produce linear-low-density, medium-density and high-density polyethylene (PE) with narrow to broad molecular weight distribution using either Ziegler-Natta (ZN) or proprietary single-site catalyst (SSC). 

Products SCLAIRTECH process can produce PE products with density range of 0.905-0.965 kg/m3, melt index (Ml) from 0.2 to in excess of 150, and narrow to broad molecular weight distribution (MWD). This allows producers to participate in the majority of the polyethylene market segments including among low-, medium- and high-density films, rotational, injection and blow molding applications. 

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