NEW EXTRUSION TECHNIQUE FOR FOAMED HIGH DENSITY POLYETHYLENE INSULATION,

The paper presents a new technique for producing thin-walled foamed high density polyethylene insulation over fine gauge telephone cable conductors using a conventional extruder now commonly associated with solid low density polyethylene insulation. In this technique, high density polyethylene formulated with a chemical blowing agent is swelled with an organic solvent prior to extrusion. This reduces the materials melt viscosity, enabling it to be extruded at a temperature low enough to prevent premature decomposition of the blowing agent and resultant poor cell structure in the insulation. In this study, a foamable compound of high density polyethylene 0.2 melt index, 0.95 density was swelled with xylene and experimentally extruded over No. 26 AWG 0.4 mm and No. 24 AWG 0.5 mm conductors at line speeds of 3,000 ftmin or more. Smooth thin-walled insulation of uniform cell structure was obtained. The results indicate that xylene swelling is a very effective technique for improving the extrudability of foamable high density polyethlene. Abrasion resistance of thin foamed high density polyethylene insulation was found to be much superior to that of thicker solid low density polyethylene insulation. From heat aging test data, it can be predicted that the service life of foamed high density polyethylene insulation should be equal to or longer than that of solid low density polyethylene insulation. Trial cables with foamed high density polyethylene insulated conductors were manufactured, and their properties were compared with those of the same type commercial cables comprised of solid low density polyethlene insulated conductors. Author