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

The work reported in this paper aimed at exploring the advantages of using supercritical carbon dioxide (scCO2) 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 scCO2 on the melting temperature and melting enthalpy of an isotactic polypropylene (iPP) were investigated first in order to define the upper reaction temperature and CO2 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, CO2 pressure, and size of the iPP particles. Results showed that the scCO2-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 CO2 pressure itself constituted an additional and sensitive process parameter capable of significantly modifying the overall reaction pathway and the product quality. For example, without CO2, the solid-state grafting process was diffusion-controlled. Under scCO2, it became reaction-controlled. The CO2 pressure could also regulate the anhydride content with ease. On the other hand, the degree of iPP chain scission was not reduced under scCO2 compared to that of the classical melt process.