Additive blend for enhancing water vapor permeability and increasing cell size in thermoplastic foams

A composition for forming a closed cell, rigid thermoplastic polymer foam having a water vapor permeability ranging from 1 to 1.5 perm inch, the composition comprising:

• a styrene maleic anhydride copolymer having the formula
• where m=100-2500 and n=100-2500;
• a cell size and water permeable enhancing agent consisting of polyethylene oxide having the formula
• where n=5-50 and R and R′ are independently H, CH3, C2H5, C3H7, or other homologs;
• and at least one blowing agent;
• wherein the styrene maleic anhydride copolymer and the cell size and water permeable enhancing agent is present in an amount from 0.5 to 3% by weight of the composition.

A rigid, closed cell thermoplastic polymer foam product comprising:

• an extruded foamable composition having a water vapor permeability ranging from 1 to 1.5 perm inch, said foamable composition including:
• a foamable polymer material;
• at least one blowing agent;
• and an additive blend including polyethylene oxide having the formula (I)
• where n=5-50 and R and R′ are independently H, CH3, C2H5, C3H7, or other homologs and a styrene maleic anhydride copolymer having the formula (II)
• where m=100-2500 and n=100-2500;
• wherein the additive blend is present in an amount from 0.5 to 3% by weight of the extruded foamable composition.

A method of forming a rigid, closed cell foam product comprising:

• heating at least one alkenyl aromatic polymer material and an additive blend including polyethylene oxide having the formula
• where n=5-50 and R and R′ are independently H, CH3, C2H5, C3H7, or other homologs and a styrene maleic anhydride copolymer having the formula
• where m=100-2500 and n=100-2500 to a first temperature sufficient to melt said at least one polymer material and foam a polymer melt;
• incorporating one or more blowing agents into said polymer melt at a first pressure to faun a foamable gel;
• cooling said foamable gel to a second temperature, said second temperature being less than said first temperature;
• and
• extruding said cooled polymer melt at a pressure sufficient to form a rigid, closed cell extruded foam product having a water vapor permeability ranging from 1 to 1.5 perm inch;
• wherein the additive blend is present in an amount from 0.5 to 3% by weight of the rigid, closed cell extruded foam product.