Polymer processing additive having improved stability

A stabilized processing aid composition that reduces melt defects in an extruded thermoplastic polymer selected from the group consisting of hydrocarbon polymers, polyamides, polyimides, polyurethanes, polyesters, polycarbonates, polyketones, polyureas, polyacrylates and polymethacrylates, the stabilized processing aid composition comprising

• (A) a fluorocarbon polymer,
• (B) a poly(oxyalkylene) polymer having the general formula:
• A((OR1)xOR2)y
• where A is an active hydrogen-free residue of a low molecular weight initiator compound having a plurality of active hydrogen atoms, (OR1)x is a poly(oxyalkylene) chain in which each R1 is a radical that can be the same or different and x is the number of oxyalkylene groups in the chain, each R2 is H or an organic radical that can contain one or more heteroatoms, and y is 2 or 3,
• (C) magnesium oxide, and
• (D) optionally a stabilizer,
• the stabilized processing aid composition comprising an amount of components (A), (B), (C) and (D) that is effective to reduce the melt defects in the extruded thermoplastic polymer.

A method of reducing melt defects during the extrusion of a thermoplastic polymer comprising the steps of:

• (A) providing a predominant amount of an extrudable thermoplastic polymer selected from the group consisting of hydrocarbon polymers, polyamides, polyimides, polyurethanes, polyesters, polycarbonates, polyketones, polyureas, polyacrylates and polymethacrylates, and an amount of an extrusion additive composition effective to reduce melt defects in the extruded thermoplastic polymer, the extrusion additive composition comprising (i) a fluorocarbon polymer, (ii) a poly(oxyalkylene) polymer having the general formula:
• A((OR1)xOR2)y
• where A is an active hydrogen-free residue of a low molecular weight initiator compound having a plurality of active hydrogen atoms, (OR1)x is a poly(oxyalkylene) chain in which each R1 is a radical that can be the same or different and x is the number of oxyalkylene groups in the chain, each R2 is H or an organic radical that can contain one or more heteroatoms, and y is 2 or 3;
• (iii) magnesium oxide, and (iv) optionally a stabilizer,
• (B) admixing the extrusion additive composition and the extrudable thermoplastic polymer to form the thermoplastic polymer composition, and
• (C) extruding the admixture.