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

Claim 1

The process for forming styrene polymer beads having a styrene monomer-divinylbenzene copolymer relatively uniformly dispersed throughout the bead 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 styrene monomer and divinylbenzene; (c) forming an emulsion comprising suitable emulsifier and a portion of said comonomer solution; (d) adding the emulsion to the styrene polymer bead suspension at 25°-75° C. to allow the comonomers to be absorbed into the beads; (e) forming a second emulsion comprising a suitable emulsifier, the remaining portion of said comonomer solution, and free-radical-producing catalysts; (f) adding the second emulsion to the suspension of said beads absorbed with comonomers at 25°-75° C., at a rate such that the rate of absorption of comonomer/catalyst by said beads is at least equivalent to the rate of addition of emulsified comonomer/catalyst solution, then allowing said suspension to stand for 30 minutes to 4 hours, and then raising the temperature to 80°-95° C., and maintaining the suspension at said temperature to polymerize said comonomers; and (g) heating said suspension at a temperature of 115° C. to about 135° C. to substantially complete the polymerization of said styrene monomer and divinylbenzene to form beads having copolymer relatively uniformly distributed throughout the beads; (h) cooling the suspension to 105° C. or less; (i) 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; (j) heating the suspension at a temperature of from 80°-150° C. to impregnate the polymer beads with blowing agent; and (k) cooling the suspension to room temperature, acidifying the suspension, and separating the impregnated beads from the aqueous system.