Foaming Resin Composition, Foam Using the Same and Coaxial Insulated Cable

[0024]A composition comprising 99.5% by weight of a tetrafluoroethylene-fluoroalkylvinyl copolymer (FEP5100J manufactured by Mitsui du Pont Fluorochemical) and 0.5% by weight of an aluminum borate whisker (Alborex Y manufactured by Shikoku Chemicals) was extrusion-molded with an extruder at a die temperature of 380° C., and 0.52 MPa of nitrogen was charged to form a foam layer with an outer diameter of 1.44 mm on a silver-plated soft copper wire with a diameter of 0.511 mm running on a head mounted on the extruder. Thus, a cover made of the foam layer was formed around a central conductor for a coaxial insulated cable. An electrostatic capacity of the resulting foam layer was measured. A dielectric constant ε was calculated according to the following formula 1, and a porosity V (%) was calculated on the basis of the resulting dielectric constant.

ε=(C×log (D / d)) / 24,16  Formula 1

[0025]wherein

[0026]C: electrostatic capacity per meter (pF)

[0027]D: outer diameter (mm)

[0028]d: diameter o