High molecular weight polymers: Supercritical solvents as blend demixing and grafting reaction medium

Abstract

Near and supercritical fluids offer very interesting alternatives for separation processes, reaction medium and in the synthesis of new materials. Particularly, the applications of supercritical fluids (SCF) in the polymer industry are growing up in the last decades. These applications include polymerizations [1–3], properties modifications [4–6], processing [7–9], recycling [10], etc. In this sense, the use of SCF extraction is an interesting method to improve the separation of thermoplastic polymer blends. Some authors have studied the phase equilibria for polypropylene-hydrocarbon systems in terms of the effects of polymer solubility, solvent quality and polymer molecular weight [11]. Also, the study of phase equilibria of a combination of solvents (system type: polymer-solvent A-solvent B) was carried out [12]. The adding of a new solvent can improve or diminish the polymer solubility acting as co-solvent or antisolvent, respectively depending on the affinity. In this chapter, the use of high pressure-high temperature n-alkanes for high molecular weight Polypropylene/Polystyrene (PP/PS) blends separation is presented. The selectivity of two solvents (n-pentane and n-heptane) on pure polymers, at high pressure and over a wide range of temperatures, is evaluated. Based on this study, a method for polymer blend demixing, physical and reactive ones, is proposed and the influence of the blend morphology and composition on the separation efficiency for the physical blend (PP/PS) it is discussed. Additionally, a diffusion model is proposed to describe the selective dissolution of a high molecular weight polymer, from an immiscible polymer blend. Also, a discussion of polymer mechanical degradation by the use of high pressure process is performed, analyzing the effects of temperature and concentration on chain scission. Finally, the knowledge of polyolefin solubilization in high pressure – high temperature n-alkanes, allows to design a new method to graft styrene onto commercial polyethylene (PE) by using near critical n-heptane as reaction medium and AlCl3 as catalyst, with very good results.