The rheological and structural properties of blends of polyethylene with paraffin wax.

摘要

Motivations: For dealing with polyethylene (PE) mixtures, two sets of theories have been available: those applicable to solutions and those applicable to blends. Both theory sets are limited in application. Solution theories generally break down as concentrations of high molecular weight polymer solutes increase beyond certain levels in low molecular weight polymer solvents. Blend theories suffer similar failure as ratios of component molecular weights (or viscosities) increase. Research has thus far treated such systems only as solutions in which very high concentrations were neglected or as blends in which very low concentrations were neglected. This study provides a much-needed bridge between the two theories by identifying crossover points between them, and the behaviors exhibited therein. It accomplishes this by production and analysis of cases in which the solute and solvent, although chemically identical, comprise a wide range of molecular weight ratios and blend compositions. Results demonstrate that this “bridge” realm offers significant manufacturing applications and advantages.;Means and Outcomes: Several polyethylene blends were employed in this study. The low molecular weight component is fixed as 3134 paraffin wax, which possesses a molecular weight well below that of entanglement, but is solid at room temperature. The wax was blended with several grades of high-density polyethylene with widely varying molecular weights from 41 kg/mol to 2,000 kg/mol. The blends were prepared by melt-mixing the components across a wide range of compositions, and quenching them in ice water. The zero-shear viscosities were then determined at a fixed temperature.;Within the purview of polyethylene/wax blends, of particular interest is that of ultra-high molecular weight polyethylene (UHM_wPE) and paraffin wax. UHM_wPE is of great value as it provides the best mechanical properties of the PE grades. However, it is notoriously difficult to process due to its incredibly high viscosity in melt. Because of this high viscosity, it is usually processed only in solution. Solution processing brings with it a significant problem in that the solvents employed are usually both expensive and environmentally unfriendly. This study indicates paraffin wax to be a viable, inexpensive, safe and “green” alternative to these problem solvents.;For blends of PE and paraffin, the paraffin typically remains to some degree not fully intercalated into the PE, though the PE is partially miscible with the paraffin. This behavior was demonstrated by melting and observing endotherms of the blends. In doing so, the wax heat peak remained at a fixed temperature but changed in intensity as a function of composition. The PE rich heat peak changed in both temperature and intensity as a function of composition. From these peaks, it is possible to determine the fraction of wax in the sample that was intercalated into the PE. This was further evidenced by examination of the blends under microscopy whereby after dissolving away the wax component of the blend, a web-like co-continuous structure was observed.;It was demonstrated that a substantial portion of the wax can be literally squeezed out of a sample by rapidly applying mechanical stress to an UHM _wPE/wax blend (either tensile or compressive) at a temperature above the melting point of the wax but below the melting point of the polyethylene. Viability of this method was verified by compressing a cylindrical sample at such a temperature, then determining the composition by TGA of the resulting film at different radial distances. The interior wax content proved lower than the neat blend; and the exterior wax content, higher. The sample remains mechanically intact. (Abstract shortened by UMI.).