Molecular weight distributions polyethylene

The ethylene polymerization of this catalyst was carried out in an autoclave reactor at 221°F in isopentane as the slurry solvent in the presence of triisobutylaluminum as cocatalyst and 50 psig of hydrogen and sufficient ethylene to achieve a total reactor pressure of 550 psig. The catalyst activity was 10,540 g of PE/g of catalyst/ hr, which corresponded to an activity of 146,000 g PE/g Ti/hr. The granular polyethylene product obtained was considered suitable for a particle-form slurry process such as the Phillips slurry process. The polyethylene sample displayed a Melt Index (I value of 0.70 and a High Load Melt Index ) value (HLMI) of 3 1 with a HLMI/MI ratio of 45, which indicates tfiat the polyethylene molecular weight distribution was of an intermediate value. 

Index range from 1.25-6.12, which would be suitable for polyethylene film and injection molding applications. The polyethylene molecular weight distribution was relatively narrow as indicated by Melt Flow Ratio values (HLMI/Ml) reported between 26.0 and 33.5. The narrower MWD exhibited by the polymer samples would offer improved film and injection molding properties. 

Catalysts 2 and 3 show that both TBOS and TEOS provide high-activity catalysts with a titanium-based activity of 210,000 g PE/g Ti and 130,000 g PE/g Ti, for TBOS- and TEOS-based catalysts, respectively. The important attribute of these catalysts is the relatively narrow polyethylene molecular weight distribution, as indicated by MFR values of about 26. 

It is important to note that polyethylene molecular weight distribution is a function of the sample molecular weight (MI) or the sample Flow Index (FI). Therefore, the evaluation of a new polyethylene material is carried out by producing a variety of samples over a range of Melt Index and Flow Index values. 

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. 

It may be shown that M > M. The two are equal only for a monodisperse material, in which all molecules are the same sise. The ratio MI /MI is known as the polydispersity index and is a measure of the breadth of the molecular weight distribution. Values range from about 1.02 for carefully fractionated samples or certain polymers produced by anionic polymerization, to 20 or more for some commercial polyethylenes. 

Some by-product polyethylene waxes have been recently introduced. The feedstock for these materials are mixtures of low molecular weight polyethylene fractions and solvent, generaHy hexane, produced in making polyethylene plastic resin. The solvent is stripped from the mixture, and the residual material offered as polyethylene wax. The products generaHy have a wider molecular weight distribution than the polyethylene waxes synthesised directly, and are offered to markets able to tolerate that characteristic. Some of the by-product polyethylene waxes are distHled under vacuum to obtain a narrower molecular weight distribution. 

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. 

Cocatalysts, such as diethylzinc and triethylboron, can be used to alter the molecular-weight distribution of the polymer (89). The same effect can also be had by varying the transition metal in the catalyst chromium-based catalyst systems produce polyethylenes with intermediate or broad molecular-weight distributions, but titanium catalysts tend to give rather narrow molecular-weight distributions. 

The bulk viscosity control parameter for CSM, as with other elastomers, is molecular weight M and molecular-weight distribution (MWD). Mooney viscosity for CSM is determined by selection of the polyethylene precursor. 

Studies of melt flow properties of polypropylene indicate that it is more non-Newtonian than polyethylene in that the apparent viscosity declines more rapidly with increase in shear rate. The melt viscosity is also more sensitive to temperature. Van der Wegt has shown that if the log (apparent viscosity) is plotted against log (shear stress) for a number of polypropylene grades differing in molecular weight, molecular weight distribution and measured at different temperatures the curves obtained have practically the same shape and differ only in position. 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

LDPE low density polyethylene (also MWD molecular weight distribution... 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

A prehminary study of the use of larch AGs in aqueous two-phase systems [394] revealed that this polysaccharide provides a low-cost alternative to fractionated dextrans for use in aqueous two-phase, two-polymer systems with polyethylene glycol (PEG). The narrow molecular-weight distribution (Mw/Mn of 1-2) and low viscosity at high concentration of AG can be exploited for reproducible separations of proteins under a variety of conditions. The AG/PEG systems were used with success for batch extractive bioconversions of cornstarch to cyclodextrin and glucose. 

Figure 8. Fractionation of polyethylene owing to phase splitting in ethylene solution molecular weight distributions in equuibrium phases at 260°C and 900...

The properties of a polymer depend not only on its gross chemical composition but also on its molecular weight distribution, copolymer composition distribution, branch length distribution, and so on. The same monomer(s) can be converted to widely differing polymers depending on the polymerization mechanism and reactor type. This is an example of product by process, and no single product is best for all applications. Thus, there are several commercial varieties each of polyethylene, polystyrene, and polyvinyl chloride that are made by distinctly different processes. 

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