PROBLEM TO BE SOLVED: To obtain an expandable thermoplastic resin composition having improved foaming stability, uniformness and reduced foam pore size without containing boron nitride by mixing a specific binary nucleation agent system into a thermoplastic resin. ;SOLUTION: This binary nucleation system is composed of (A) a sulfonic acid, phosphonic acid or their salt (preferably a potassium salt, a barium salt or a calcium salt, etc., of perfluoroalkane sulfonic acid or perfluoroalkylethane sulfonic acid) and (B) a polyatomic anion-containing inorganic salt (preferably a tetraborate of sodium, potassium or calcium) and is mixed into a thermoplastic resin (preferably a tetrafluoroethylene/hexafluoropropylene copolymer or a polyolefin, etc.) to obtain an expandable thermoplastic resin. Preferably, 250-3,000ppm of the component A and 50-500ppm of the component B respectively based on the weight of the thermoplastic resin, are blended with the thermoplastic resin. The resultant objective composition is especially useful for producing a foamed electric wire insulator having 0.13-0.51mm wall thickness.;COPYRIGHT: (C)1998,JPO
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