Sustainable polymer foaming using high pressure carbon dioxide: A review on fundamentals, processes and applications

In recent years, carbon dioxide (CO2) has proven to be an environmentally friendly foaming agent for the production of polymeric foams. Until now, extrusion is used to scale-up the CO2-based foaming process. Once the production of large foamed blocks is also possible using the CO2-based foaming process, it has the potential to completely replace the currently used foam production process, thus making the world-wide foam production more sustainable. This review focuses on the polymer–CO2-foaming process, by first addressing the principles of the process, followed by an overview of papers on nucleation and cell growth of CO2 in polymers. The last part will focus on application of the process for various purposes, including bulk polymer foaming, the production of bioscaffolds and polymer blends

Ultrasound-induced polymerization of methyl methacrylate in liquid carbon dioxide : a clean and safe route to produce polymers with controlled molecular weight

Ultrasound-induced cavitation is known to enhance chemical reactions as well as mass transfer at ambient pressures. Ultrasound is rarely studied at higher pressures, since a high static pressure hampers the growth of cavities. Recently, we have shown that pressurized carbon dioxide can be used as a medium for ultrasound-induced reactions, because the static pressure is counteracted by the higher vapor pressure, which enables cavitation. With the use of a dynamic bubble model, the possibility of cavitation and the resulting hot-spot formation upon bubble collapse have been predicted. These simulations show that the implosions of cavities in high-pressure fluids generate temperatures at which radicals can be formed. To validate this, radical formation and polymerization experiments have been performed in CO2-expanded methyl methacrylate. The radical formation rate is approximately 1.5*1014 s-1 in this system. Moreover, cavitation-induced polymerizations result in high-molecular weight polymers. This work emphasizes the application potential of sonochemistry for polymerization processes, as cavitation in CO2-expanded monomers has shown to be a clean and safe route to produce polymers with a controlled molecular weight