Expanded polystyrene via stabilized water droplet by in-situ modified starch nanocrystals

Abstract

Synthesis of polystyrene beads containing well-dispersed water microdroplets viasurfactant free Pickering emulsion polymerization in water-in-oil-in-water system. The water microdroplets with a diameter of 8–15 μm were stabilized using in situ modified starch nanocrystals (SNCs). The morphological investigation revealed that the water droplets were surrounded by a densely layer of the modified SNCs. Since the acid hydrolyzed SNCs were sensitive to pH, the basic feature of polystyrene beads was changed with pH, so that the incorporated water content, encapsulation efficiency and droplet number density were increased with pH, but the volume-weighted mean diameter (d4,3) was decreased with pH. The thermogravimetric analysis confirmed two type of water inside the beads; free water and bound water with release temperatures of 103 and 120 °C, respectively. The bound water was responsible for higher water preservation ability and therefore long shelf-life of the beads, which over 40% of the initial entrapped water was kept 7 months after synthesis. The beads can be expanded at an optimum temperature of 135 °C due to water vapor pressure developed inside the beads. The best max expansion ratio of around 16 was found for the beads included 1.8 wt.% of water. During the expansion process, the SNCs were located in the cell wall to reinforce it from rupturing. Eventually, we report a new type of water expanded polystyrene beads synthesized by the use of SNCs, named SNCWEPS, with good expandability and extended shelf-life.