Polypropylene grafting in supercritical carbon dioxide

Polypropylene (PP), one of the most widely low-cost commodity polymers, has a hydrophobic nature which prevents its use from a large variety of applications. Functionalisation, through free radical grafting of polar vinyl monomers offers an effective route towards higher added value applications especially for environmental remediation, wastewater treatment, and hydrometallurgy. Classic processes for the modification of polypropylene by polar monomers includes reactive extrusion in the molten state and chemical reaction in the solid-state as well as in liquid solvents. Recently, the use of supercritical carbon dioxide (Sc-CO2) has appeared as an innovative and interesting medium because of its environmentally friendly characteristics and its properties of sorption, swelling and foaming of polymers. In this study, PP was put in an autoclave in different forms: pellets, fibres or powders. Vinylbenzyl chloride (VBC) was added along with benzoyl peroxide as initiator of the polymerisation. The autoclave was filled with CO2, pressurised and let for a certain amount of time under controlled pressure and temperature. After opening, the swollen polymer was washed with acetone, analysed by infra-red spectroscopy and the chlorine content was determined. When there is sufficient chlorine the samples were used for further modification with a phosphate ligand in order to perform metal ions complexation from water solutions. Microphotographs as well as infra-red spectra show clearly that effective grafting with covalent binding is obtained under sc-CO2 conditions not only at the surface but also into the pellets. It also appears that the reaction is more complete when polypropylene has previously been pulverised. This may come from a larger surface area on one hand and a different crystalline ratio on the other hand. A two-step process, one aimed at the polymer swelling and the second aimed at the grafting has been successfully compared to the single step process. The influence of several operating parameters was tested: duration of the experiments, pressure and temperature, peroxide content and VBC/PP ratio. After reflux treatment with triethyl phosphite, the best result achieved was a phosphorus elemental analysis of 1.25 mmol/g. This value shows that a high degree of polymerization has been reached and that the polyVBC is grafted on the PP chains, rather than coated on the surface. The next study will deal with the capacity of such grafted PP samples to remove heavy metal ions from wastewater.