Method of forming electrical insulation by extruding polymeric compositions containing hollow microspheres

In a method of forming electrical insulation about a wire conductor by passing said wire conductor through a bath of a thermoplastic resin whereby said resin adheres to and forms a coating around said wire and pulling the resin coated wire through a circular die of an extruder while allowing the polymeric resin to solidify by cooling down in the ambient atmosphere, whereby the coated wire is pulled through the circular die at a rate equivalent to the cooling down rate of the polymeric resin, the improvement comprising using as said thermoplastic polymeric resin the composition comprising a uniform mixture of a thermoplastic polymeric material which is selected from the group consisting of polyethylene, polypropylene and polyvinyl chloride resin and, as a filler, from 5 to 50 percent by volume of inert silica-alumina, hollow, microspheres ranging from about 5 to about 300 microns in diameter and further characterized by a specific gravity of from about 0.35 to about 0.7;

• a true particle density of from about 0.4 to about 0.6 grams per cubic centimeter;
• a bulk density of from about 11 to 25 pounds per cubic foot;
• a melting point of 1200.degree. C.;
• a MOH's particles hardness of about 5;
• a shell thickness which is about 10 percent of the particle radius;
• and a composition which includes from about 55-60 percent by weight silica (SiO.sub.2);
• 30-35 percent by weight alumina (Al.sub.2 O.sub.3);
• 4-10 percent by weight iron oxide (Fe.sub.2 O.sub.3);
• and about 1-3 percent by weight of alkali and alkaline earth metals or their oxides, wherein the relationship between the operating pressure in the extruder and temperature increase (.DELTA.T) and the volume percent of the hollow microsphere filler particles is expressed in terms of the following table:
• ______________________________________ Extrusion Volume (%) Operating Pressure .DELTA.T of Microspheres (psi) (.degree.C.) ______________________________________ 5-20 1600 80 25 1550 90 30 1500 100 35 1430 110 40 1350 120 45 1210 140 50 1100 150 ______________________________________
• wherein .DELTA.T represents the difference between the temperature of the polymer resin in the extruder and the polymer temperature before it enters the extruder, the minimum diameter of said die being not less than one-sixteenth inch.