Low molecular weight HDPE

Low-molecular weight HDPE—spray coatings, emulsions, prinking inks, wax polishes, and crayons... 

Figure 12.32 Comparison of storage and loss tensile relaxation moduli of a paraffin [C94H190, ( ) and (O) respectively] and two low molecular weight HDPE fractions [(A) and (A) I860 MW ( ) and ( ) 3800 MW], (From Ref. 42.)...

A wide variety of chromium oxide and Ziegler catalysts was developed for this process (61,62). Chromium-based catalysts produce HDPE with a relatively broad MWD other catalysts provide HDPE resins with low molecular weights (high melt indexes) and resins with a narrower MWD (63,64). 

Figure 9- Log-normal distribution of HDPE molecular weight after 1.5 h at 165 °C and 10 h at 135 °C 1 (upper line) - HDPE with antioxidant circles - HDPE without antioxidant 2 (lower line) upper broken line has been deplaned vertically by a factor of 2.2 to coincide with the low molecular weight data of unstabilized sample.

The polymer has a low cohesive energy density (the solubility parameter 8 is about 16.1 MPaU2) and would be expected to be resistant to solvents of solubility parameter greater than 18.5 MPa1/2 (Chapter 4). Since polyethylene is a crystalline hydrocarbon polymer incapable of specific interaction, there are no solvents at room temperature. Materials of similar solubility parameters and low molecular weight will however cause swelling, the more so in low-density polymers. LDPE has a gas permeability in the range normally expected with rubbery materials. HDPE has a permeability of about one-fifth that of LDPE. 

Table 2 Diffusion data for low molecular weight organic substances in Polyethylenes (PE). Medium and High Density Polyethylenes (MDPE HDPE) 498...

The data in table 9.2 show that HDPE cannot be efficiently fractionated when the HDPE is a solid at extraction conditions. Less than 5wt% of the HDPE charged to the column could be dissolved at pressure levels as high as 663 bar (9,615 psia) at a temperature below the second critical end point of the polymer. The material that was extracted in this fractionation was likely amorphous low molecular weight polymer present at or near the surface of the... 

As has been indicated, the catalyst particle sites control the structural arrangement of the polymer. Unfortunately, not all of the sites on a ZN catalyst behave the same way. There are always some sites that produce a low molecular weight, highly branched material. In HDPE, this waxy material must be removed to produce a polymer with the desired characteristics. 

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