A review on the free radical grafting of vinyl monomers onto polyethylene and polypropylene by reactive extrusion

Polyethylene (PE) and polypropylene (PP) have been widely used in films, pipes, plates, fibers, and electronic devices, etc. due to a good balance between mechanical properties, processability and cost. They are apolar and chemically relatively inert. Sometimes these characteristics may restrain their application. A solution to making them polar and/or chemically reactive is to graft polar and/or functional groups onto their backbones. This can be done by free radical chemistry via a reactive extrusion process which is continuous, solventless and of short residence time (<a few minutes). This review describes problems related to free radical grafting: low grafting yield, monomer residual, and side reactions (especially chain scission for PP and gel formation for PE), and strategies to solve them: addition of comonomers, use of nano-reactors, optimization of screw profile and mixing, sequential feeding of monomers, assistance of supercritical carbon dioxide (scCO2), and devolatilization.