Solubility and Diffusivity of Propylene, Ethylene, and Propylene–Ethylene Mixtures in Polypropylene

Abstract

The solubility of propylene, ethylene, and propylene–ethylene mixtures in isotactic polypropylene (iPP) is measured with a high‐pressure sorption balance at different temperatures and pressures. The obtained sorption isotherms of both binary systems (propylene–iPP and ethylene–iPP) are fitted with the Sanchez–Lacombe equation of state. It is found that both binary interaction parameters for ethylene and propylene in iPP decrease with temperature. At temperatures above 70 °C, the overall solubility of ethylene plus propylene mixtures can be modeled using purely binary systems. However, at temperatures under 70 °C it is necessary to use a ternary solubility model that accounts for interactions between the penetrants and the polymer. Furthermore, at lower temperatures, the total solubility of the gas mixture is higher than the sum of gas solubilities of individual components because the anti‐solvent effect of ethylene (on propylene) is weaker than the co‐solvent effect of propylene on ethylene. Moreover, the molar ratio of ethylene to propylene dissolved in iPP increases with temperature and pressure. Desorption studies show that the effective diffusion coefficient increases with pressure and temperature and that the effective diffusivity of gaseous ethylene–propylene mixtures is lower than that of either of the individual gases.

The Effect of Reactor Conditions on High‐Impact Polypropylene Properties and Gas Phase Polymerization Kinetics

“…On the other hand, the high aEPC content in the center of A70‐0.6‐40‐8 particles can be explained by the higher partial pressure of ethylene in the bulk during its copolymerization. As it has been recently shown by our group, a higher partial pressure improves solubility and diffusivity, which results in an enhanced penetration of ethylene in the particle.…”

Estimation of diffusion coefficients for multiple penetrant/polyolefin systems based on sorption data

“…Additionally, it should be pointed out that the D 0 parameter used in these calculations was obtained by another author [48], and not fitted to this particular set of data. The binary model was also validated with a single set of parameters using two independent data sets of the diffusivity of propylene and ethylene in iPP obtained by Cancelas et al [29] and Gonzalez et al [49]. The table below shows the results obtained Cancelas et al [29] and how they compare to the model results.…”

“…The binary model was also validated with a single set of parameters using two independent data sets of the diffusivity of propylene and ethylene in iPP obtained by Cancelas et al [29] and Gonzalez et al [49]. The table below shows the results obtained Cancelas et al [29] and how they compare to the model results. diffusivity would be a stronger function of pressure.…”

“…To check the validity of the model, we used the characteristic length scale provided by Cancelas et al [29] for their sorption experiments, and our value of the predicted mutual diffusivity value in Crank's [52] analytical expression for diffusion in a sphere: (22) where M(t) is the mass sorbed at a given time t, M eq is the total mass sorbed at equilibrium, r…”

“…However, this approach falls short when polymer swelling is significant. Cancelas et al [29] studied the diffusivity of ethylene, propylene and ethylene-propylene mixtures in isotactic polypropylene (iPP) and found that the effective diffusivity of ethylenepropylene mixtures is lower than that of either of the individual gases.…”