POLYETHYLENE INSULATED TELEPHONE CABLE 1247 



Fig. 1 — (a) Polyethylene disc insulated coaxial, (b) Polyethylene ribbon and 

 string insulated video pair. Note expanded polyethylene interstice fillers. 



The idea of using spongy or foamed hydrocarbons as conductor in- 

 sulation is not new. A British patent issued in 1930 contemplates such a 

 structure and numerous United States patents of more recent dates 

 cover various aspects of cellular hydrocarbon insulation. The problem 

 is one of forming the cylinder of aerated plastic in an extrusion process 

 operating at high speed and producing a closely controlled uniform 

 covering having precise physical and electrical properties. The original 

 development work was carried on by F. B. Lyons of the Western Electric 

 Company in cooperation with the author. This early work demonstrated 

 that material having the desired range of properties could be apphed in 

 a continuous extrusion process and subsequent work has shown that the 

 necessary control of properties and speed of extrusion can be achieved. 



The expansion of polyethylene is accomplished by methods similar 

 to those employed in the making of many of the numerous polymer and 

 rubber "foams". The process used to produce the cellular polyethylene 

 involves the addition at the extruder of a chemical blowing agent which 

 decomposes under heat and releases nitrogen gas. By proper mixing 

 and process control this nitrogen gas can be entrapped in the poly- 

 ethylene in the form of very small discrete bubbles, thus achieving the 

 cellular structure shown in Fig. 2. It is interesting to note that the 

 foamed plastic tends to form a desirable ''skin" of solid material on the 

 inner surface over the conductor, Fig. 2(a). 



Various degrees of expansion can be achieved as required by varying 

 the amount of blowing agent and by other means. The degree of ex- 

 pansion, or percent entrapped gas, can be determined readily by weigh- 



