Surfactant free Pickering emulsion polymerization of styrene in w/o/w system using cellulose nanofibrils

Here, we report a method to synthesize Volatile Organic Compounds (VOCs)-free expandable polystyrene beads containing well-dispersed water microdroplets as blowing agent. The water expandable beads were prepared through surfactant free Pickering emulsion polymerization of styrene in water/oil/water (w/o/w) system using cellulose nanofibrils (CNF). The water microdroplets were shown to be surrounded by a dense layer of CNF/polystyrene composite (CNF/PS) nanospheres which formed in the initial stages of the process and were in situ surface-modified by styrene-maleic anhydride (SMA) to act as stabilizer. The formation mechanism of the CNF/PS nanospheres was confirmed by infra-red spectroscopy and contact angle analysis. Morphological characterization of unexpanded polystyrene beads showed that the number droplet density as well as total entrapped water in the beads increased with the CNF content. Furthermore, a bimodal distribution of water droplet size, centered at 3 and 6 mu m and a linear correlation between the average water droplet size and the average size of the CNF/PS nanospheres was found. Finally, it was shown that the under appropriate condition, the beads can be excellently expanded to form a low density polystyrene foam. (C) 2015 Elsevier Ltd. All rights reserved.