Process for producing modified styrenic polymer beads for high strength foamed articles

Claim 1

A process for forming styrenic polymer beads modified with styrenic monomer/divinylbenzene copolymer for use in producing foamed cups with improved strength and thermal properties comprising: (a) forming an aqueous suspension of initial polystyrene beads having a weight average molecular weight of 230,000 to 350,000 with a polydispersity (Mw/Mn) of 2 to 3.1, with the aid of a suitable suspending agent system; (b) forming a comonomer solution of a styrene monomer and divinylbenzene; (c) forming an emulsion comprising a suitable emulsifier and said comonomer solution with free-radical-producing catalysts dissolved therein; (d) adding the emulsion to the styrene polymer bead suspension at 25°-75° C. to allow the comonomers and catalyst to be absorbed therein/thereon during the addition and shortly after the addition is complete, heating the resulting bead suspension to a polymerization temperature of about 80°-95° C., and maintaining said temperature for 0.10 to 6 hours to copolymerize said comonomers; and (e) heating said suspension to a temperature of 115° to about 135° C. to substantially complete the polymerization of said styrene monomer and divinylbenzene to form beads having copolymer diffused throughout but distributed primarily at or near the surface of the bead; (f) cooling the suspension to 105° C. or less; (g) adding a non-ionic surfactant and 3-12% by weight based on polymer of a blowing agent selected from the group consisting of aliphatic hydrocarbons containing 4 to 6 carbon atoms, halogenated hydrocarbons which boil at a temperature below the softening point of the polymer, and mixtures of these; (h) heating the suspension at a temperature of from 80°-150° C. to impregnate the polymer beads with blowing agent; and