High pressure phase equilibria in the systems linear low density polyethylene + n-hexane and linear low density polyethylene + n-hexane + ethylene: Experimental results and modelling with the Sanchez-Lacombe equation of state

Abstract

Cloud point isopleths, bubble-point isopleths and liquid–liquid–vapour bubble point isopleths were measured for a binary system of linear low density polyethylene (LLDPE) and n-hexane and for the ternary system LLDPE + n-hexane + ethylene. The experiments were performed according to the synthetic method in the temperature range 400–500 K and at pressures up to 14 MPa. The LLDPE used was a hydrogenated polybutadiene and was almost monodisperse ( M w/ M n = 1.19). Measured experimental data for the system LLDPE + n-hexane and experimental data for the system LLDPE + ethylene taken from literature [H. Trumpi, Th.W. de Loos, R.A. Krenz, R.A. Heidemann, High pressure phase equilibria in the system linear low density polyethylene + ethylene: experimental results and modeling, J. Supercrit. Fluids 27 (2003) 205–214.] were modelled with the modified Sanchez-Lacombe equation of state. The same LLDPE sample was used in both experiments. The parameters for LLDPE were found by performing a sequence of non-linear regressions on pressure–volume–temperature reference data for molten polyethylene and the experimental cloud point data for the systems LLDPE + n-hexane and LLDPE + ethylene. From this information and a Sanchez-Lacombe fit to n-hexane + ethylene data the phase behaviour of the ternary system LLDPE + n-hexane + ethylene can be predicted. Using this procedure the influence of the ethylene concentration on the cloud point pressure is slightly under predicted. Therefore, the LLDPE–ethylene binary interaction parameter was adjusted to ternary LLDPE + hexane + ethylene cloud point data. In this way the modified Sanchez-Lacombe equation gives a very good description of the ternary cloud point curves and an almost quantitative prediction of the ternary bubble point and liquid–liquid–vapour boundary curves.