Molecular weight polyethylene

The 1970s saw the introduction of higher activity catalysts based on magnesium chloride-supported titanium that improved the control of the physical properties of the polyethylene—molecular weights, stereospecificity, and the degree of copolymerization. 

Figure 1.3. Degree of crystallinity <[), versus time for linear polyethylene (molecular weight = 284,000) [26].

Comparison of polyethylene molecular weights determined by light scattering in 1 chloronaphthalene and tetralin. J. Polymer Sci. 2A, 4875 (1964). 

Fig. 44. Tensile strength and elongation at break of irradiated polyethylene. Low-density polyethylene, molecular weight 21,000. Irradiated with 800 kV electrons and tested at room temperature [427].

The source and characterization of the linear polyethylene molecular weight fractions have already been described in previous publications from this laboratory (33). A Perkin-Elmer DSC-2 differential scanning calorimeter was used. The heating rates and sample sizes are indicated in the specific experiments. Special thermal history procedures that were adopted are described in the text. 

This conventional narrow standard calibration procedure yields relative molecular-weight values because the averages obtained are relative to the cahbration polymer. For example, if polyethylene molecular weights are determined with a cahbration curve produced with polystyrene narrow standards, the results would be incorrect for polyethylene. For many purposes relative molecular-weight values are adequate, since the results obtained with an unknown sample may be compared to preestablished acceptable relative values. If narrow standards of the same polymer as the unknown sample are available, then it is possible to calculate absolute values. 

Easy control of the polyethylene molecular weight using hydrogen in the polymerization process as a chain transfer reagent ... 

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. 

Figure 3.21 Chain transfer reactions that are used to control polyethylene molecular weight. The chromocene-based catalyst exhibits very high hydrogen response. Cp ligand on Cr not shown.

Figure 4.15 Correlation between catalyst activity, polyethylene molecular weight and alkyl-group substitution on Cp ring in various metallocene compounds [44].

The data in Table 4.6 show that catalyst activity increases with polymerization temperature from 1,150 g PE/g catalyst at 65 C to 5,100 g PE/g catalyst at 77.5°C. However, very importantly, the polyethylene molecular weight may be controlled over the range necessary for the manufactme of commercial grades of polyethylene for industrial appHcations, as shown by Melt Index (MI values from 4.1 to 0.6. Isopentane or isopentane containing 0.3 ppm oxygen was needed to produce the relatively higher molecular weight products. 

Figure 4.21 Polyethylene molecular weight at polymerization temperatures 110-160°C [51,52],...

The product mix of autoclave and tubular reactors are similar in terms of LDPE homopolymers (0.910-0.935 g/cc) and some specialty grades of polyethylene such as ethylene/vinyl acetate copolymers up to about 30 wt% vinyl acetate (VA). However, the autoclave process provides higher levels of vinyl acetate (40 wt%) in ethylene/VA copolymers and additional specialty grades of polyethylene such as ethylene/methyl acrylate, ethylene/acrylic acid and ethylene/n-butyl acrylate. Polyethylene molecular weight can be varied over a wide range with the high-pressure process, with Melt Index values (I ranging from 0.15 to 40. 

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