Supercritical Carbon Dioxide Assisted Solid-State Grafting Process of Maleic Anhydride onto Polypropylene

Abstract

The work reported in this paper aimed at exploring the advantages of using supercritical carbon dioxide (scCO 2 ) as an environmentally benign solvent and swelling agent for carrying out the grafting process of maleic anhydride onto polypropylene in the solid state. The effects of scCO 2 on the melting temperature and melting enthalpy of an isotactic polypropylene (iPP) were investigated first in order to define the upper reaction temperature and CO 2 pressure limits. The effects of various factors on the grafted anhydride content and on changes in the molecular-scale and microscale structures of the resulting iPP were then investigated. Those factors included the reaction time, monomer and initiator concentrations, reaction temperature, CO 2 pressure, and size of the iPP particles. Results showed that the scCO 2 -assisted solid-state grafting process of maleic anhydride onto iPP did have some scientifically interesting and industrially relevant advantages over the classical solid-state or melt process. Among them, it is worth pointing out that the CO 2 pressure itself constituted an additional and sensitive process parameter capable of significantly modifying the overall reaction pathway and the product quality. For example, without CO 2, the solid-state grafting process was diffusion-controlled. Under scCO 2, it became reaction-controlled. The CO 2 pressure could also regulate the anhydride content with ease. On the other hand, the degree of iPP chain scission was not reduced under scCO 2 compared to that of the classical melt process.