Modelling the fluid phase behaviour of polydisperse polyethylene blends in hydrocarbons using the modified Sanchez–Lacombe equation of state

One or more polyethylene (PE) samples with different molecular configurations can be mixed together to produce a blend with different physical characteristics. The blending of multiple polyethylenes from different reactors, each with their own molar mass distribution and degree of branching, can be carried out in a hydrocarbon solvent. Modelling the fluid phase behaviour of polydisperse polyethylene blends would be an important tool for the design, optimization and operation of the production process.The modified Sanchez–Lacombe equation of state (MSL-EOS) was used to calculate the cloud points of a blend of two polydisperse linear low-density polyethylenes (LLDPEs) in a hydrocarbon solvent. The type of linear low-density polyethylenes considered were hydrogenated polybutadienes (hPBDs). The calculated cloud points were compared to experimental data that were available for the systems: hPBD-1 + hPBD-2 + n-hexane and hPBD-3 + hPBD-4 + n-pentane. The four hPBD samples have considerably different molar mass distributions (MMDs) though the other properties of the samples are unknown (degree, type and frequency of branching in the polyethylene molecule).The polyethylene molar mass distributions were approximated using probability distribution functions that match the reported number- and weight-average molar masses. The molar mass distributions were represented in the model using 100 pseudocomponents. The temperature-dependent LLDPE + hydrocarbon binary interaction parameters had been previously adjusted to correlate the cloud points of the binary mixtures. Previous investigations have demonstrated that for the same LLDPE parameters, the magnitude of the binary interaction parameter is in part related to the branch content of the polyethylene sample.The cloud point isotherms calculated exhibit a distinctive bend when the solvent mass fraction is held constant and the ratio of the two polymers is allowed to vary. The model predicts a three-liquid phase region inside the two-liquid phase region for the LLDPE-1 + LLDPE-2 + n-pentane system although none was observed experimentally. The molar mass distributions of both of the polyethylenes in each of the three liquid phases will be presented.