Monovinylaromatic polymer with improved stress crack resistance

Abstract

The present invention discloses a composition of matter consisting of a high impact polystyrene exhibiting improved environmental stress crack resistance. The HIPS material is formed utilizing polybutadiene, polyisoprene, and copolymers thereof with styrene, having a Mooney viscosity exceeding about 35 and a gel content of up to about 28%. The present invention discloses the use of a combination of lubricant additives to obtain a HIPS material with higher ESCR values than either additive alone could provide, and also discloses the optimum level of each additive.

What is claimed is  
     
         1 . A high impact monovinylaromatic polymer compound consisting essentially of: (a) an impact resistant rubber modified monovinylaromatic polymer having optimum environmental stress crack resistance, said polymeric compound being polymerized in the presence of a rubber selected from the group consisting of natural rubbers, polybutadienes, isoprenes, copolymers of styrene with butadiene, copolymers of styrene with isoprene, and copolymers of styrene with isoprene, and copolymers of styrene with butadiene and isoprene; and (b) an ESCR enhancing additive consisting essentially of polyisobutylene having a viscosity range of 196-233 cst at 99° C., being added to said polymer compound in amounts of from about 1 percent to about 4 percent, by weight of said polymer compound.  
     
     
         2 . The polymer compound of  claim 1  wherein said polyisobutylene comprises 2.5 percent by weight.  
     
     
         3 . The polymer compound of  claim 1  wherein said monovinylaromatic polymer consists of polystyrene having a weight-average molecular weight of 170,000 to 285,000 and a number-average molecular weight of 10,000 to 120,000.