Monovinyl aromatic polymer compositions with a novel combination of stiffness and stress crack resistance

A refrigerator lining comprising a rubber modified polystyrene composition comprising:

• the reaction product formed by polymerizing a monomer mixture comprising at least 75 wt. % of one or more monovinylaromatic monomers in the presence of rubber particles containing one or more polymers containing at least 50 weight percent of monomer residues from monomers according to the formula:
• R1HC═CHR2—CHR3═CHR4
• wherein R1 can be H or a C1 to C6 linear or branched alkane, R2 can be H or a C1 to C3 linear or branched alkane, R3 can be H or a C1 to C3 linear or branched alkane, and R4 can be H or a C1 to C6 linear or branched alkane;
• to form a dispersion of rubber particles in a vinyl aromatic polymer;
• wherein the dispersed rubber particles have an average particle diameter of 6 to 10 microns;
• wherein the composition has a gel content of from about 28% to about 36% by weight;
• wherein the composition has a swell index of less than 13;
• and
• wherein the composition contains no more than 2 wt. % of plasticizers.

A method of down gauging a refrigerator lining while maintaining performance characteristics comprising:

• modifying a rubber modified polystyrene resin by replacing at least 50 weight percent of the rubber with a second rubber containing one or more polymers containing at least 50 weight percent of monomer residues from monomers according to the formula:
• R1HC═CHR2—CHR3═CHR4
• wherein R1 can be H or a C1 to C6 linear or branched alkane, R2 can be H or a C1 to C3 linear or branched alkane, R3 can be H or a C1 to C3 linear or branched alkane, and R4 can be H or a C1 to C6 linear or branched alkane, to form a modified HIPS resin containing rubber particles dispersed in the polystyrene;
• and
• thermoforming the modified HIPS resin to form a refrigerator lining;
• wherein the rubber particles have an average particle diameter of 6 to 10 microns;
• wherein the modified HIPS resin has a gel content of from about 28% to about 36% by weight;
• wherein modified HIPS resin has a swell index of less than 13;
• wherein the modified HIPS resin contains no more than 2 wt. % of plasticizers;
• wherein the gauge of the thermoformed modified HIPS resin is at least 5% less than the gauge of a thermoformed rubber modified polystyrene;
• and
• wherein the performance properties of the thermoformed modified HIPS resin are at least equivalent to the performance properties of the thermoformed rubber modified polystyrene.