MONOVINYL AROMATIC POLYMER COMPOSITIONS WITH A NOVEL COMBINATION OF STIFFNESS AND STRESS CRACK RESISTANCE

1 . A rubber modified polystyrene composition comprising:

a reaction product formed by polymerizing a monomer mixture comprising:

at least 75 weight percent of one or more monovinylaromatic monomers in the presence of rubber particles to form a dispersion of rubber particles in a vinyl aromatic polymer;

wherein the dispersed rubber particles have an average particle diameter of from about 6 to about 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.

2 . The rubber modified polystyrene composition according to claim 1 , wherein the monovinylaromatic monomers are selected from the group consisting of styrene, p-methyl styrene, tertiary butyl styrene, dimethyl styrene, nuclear brominated or chlorinated derivatives thereof and combinations thereof.

3 . The rubber modified polystyrene composition according to claim 1 , wherein the rubber particles comprise one or more polymers containing at least 50 weight percent of monomer residues from monomers according to the formula:

R 1 HC═CHR 2 —CHR 3 ═CHR 4

wherein R 1 can be H or a C 1 to C 6 linear or branched alkane, R 2 can be H or a C 1 to C 3 linear or branched alkane, R 3 can be H or a C 1 to C 3 linear or branched alkane, and R 4 can be H or a C 1 to C 6 linear or branched alkane; to form a dispersion of rubber particles in a vinyl aromatic polymer.

4 . The rubber modified polystyrene composition according to claim 1 , having a Mooney viscosity (ML/4/100° C.) of from 30 to 80.

5 . The rubber modified polystyrene composition according to claim 1 , wherein the dispersed rubber particles have an average particle diameter of 7 to 9 microns.

6 . The rubber modified polystyrene composition according to claim 1 , wherein the composition has a gel content of from about 30% to about 35% by weight.

7 . The rubber modified polystyrene composition according to claim 1 , wherein the composition has a swell index of less than 12.

8 . The rubber modified polystyrene composition according to claim 1 , wherein the composition contains no more than 1 wt. % of plasticizers.

9 . A refrigerator liner comprising the rubber modified polystyrene composition according to claim 1 .

10 . The refrigerator lining according to claim 9 having an MIS ESCR value of at least 100 minutes.

11 . The refrigerator lining according to claim 9 having a tensile strength retention at 0.9% constant strain of at least 80%.

12 . The refrigerator lining according to claim 9 having ASTM flexural modulus of 250 to 300 kpsi.

13 . 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:

R 1 HC═CHR 2 —CHR 3 ═CHR 4

wherein R 1 can be H or a C 1 to C 6 linear or branched alkane, R 2 can be H or a C 1 to C 3 linear or branched alkane, R 3 can be H or a C 1 to C 3 linear or branched alkane, and R 4 can be H or a C 1 to C 6 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.

14 . The refrigerator lining according to claim 13 having an MIS ESCR value of at least 100 minutes.

15 . The refrigerator lining according to claim 13 having a tensile strength retention at 0.9% constant strain of at least 80%.

16 . The refrigerator lining according to claim 13 having ASTM flexural modulus of 250 to 300 kpsi.

17 . 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:

R 1 HC═CHR 2 —CHR 3 ═CHR 4

wherein R 1 can be H or a C 1 to C 6 linear or branched alkane, R 2 can be H or a C 1 to C 3 linear or branched alkane, R 3 can be H or a C 1 to C 3 linear or branched alkane, and R 4 can be H or a C 1 to C 6 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.

18 . The method according to claim 17 , wherein the modified HIPS resin has an MIS ESCR value of at least 100 minutes.

19 . The method according to claim 17 , wherein the modified HIPS resin has a tensile strength retention at 0.9% constant strain of at least 80%.

20 . The method according to claim 17 , wherein the modified HIPS resin has an ASTM flexural modulus of 250 to 300 kpsi.