Parts cast of metals such as iron, using expandable polystyrene foams, may have an unacceptable amount of surface defects, such as lustrous carbon. The use of foams made of styrene/methyl methacrylate copolymers can improve the quality of foam molds and metal parts made using such molds. In this work, expandable copolymers were synthesized by suspension copolymerization of styrene and methyl methacrylate in the presence of blowing agents (pentane isomers). When the surrounding temperature was high enough to vaporize the blowing agent and soften the polymer shell, the beads were expanded. The effects of monomer composition and cell structure modifying aid (polyethylene wax) on the particle size, average molecular weight and molecular weight distribution of synthesized copolymer, pentane content and expansion properties (density, residual pentane of pre-expanded particles and cell structure) were investigated. The results showed that monomer composition had a significant effect on the copolymerization kinetic and the copolymer molecular weight. In addition, cell structure, cell size and cell size distribution of the pre-expanded particles were strongly influenced by cell structure modifying aid concentration. The effects of comonomer composition and copolymer particle size on pentane content of expandable particles were also studied.

Expandable core-shell beads were synthesized by dosing methyl methacrylate (and styrene) in a constant rate and polymerizing in the presence of polystyrene seeds and pentane. The morphological studies of beads indicated that there were larger poly(methyl methacrylate)-rich domains near the shell. The sizes of these domains were decreased towards the core of the particles. For better surveying the likely structures of domains and the extent of expansion agent diffusion, the beads having the shells formed with copolymerization of styrene and methyl methacrylate in the presence of polystyrene seeds were studied. From the morphological studies, the parallel regular and the onion-like structures were recognizable. In addition, another shell was formed around the dispersed phase showing a complex core/shell structure both in particle and in domain scales. The pentane content in the beads with copolymerized styrene and methyl methacrylate monomers on the polystyrene seeds was increased compared to the beads having the shells of pure poly(methyl methacrylate). This was because the onset of gel effect was longer with adding the styrene to methyl methacrylate monomer hence there was more time for expansion agent to diffuse into the beads. The different amounts of pentane distributed in polystyrene-and poly(methyl methacrylate)-rich phases were discussed in terms of the changes in glass transition temperature of two phases.