1246 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



The various electrical and mechanical characteristics^ • ^ of polyethyl- 

 ene are shown in Table I, along with some of the other materials con- 

 sidered. Among these other materials only polytetrafluoroethylene (Tef- 

 lon) and polystyrene have power factors and dielectric constants in 

 the same low range as polyethylene. There are basic objections to both 

 of these materials. Polytetrafluoroethylene is so expensive as to be un- 

 economical for this application and polystyrene in thick sections is too 

 stiff and brittle to handle in a satisfactory manner. 



The use of polyethylene in telephone cables is not altogether new but 

 it has heretofore been confined to special types of high-frequency cable. 

 For example, the coaxial cable and video pair have polyethylene disc 

 insulation and strip-and-string insulation respectively (see Fig. 1). These 

 cables were designed for low attenuation in the megacycle range and 

 the use of polyethylene or a similar low power factor material was a 

 necessity. A low power factor is of lesser importance in the carrier sys- 

 tems for which the multipair cables are used and the polyethylene in- 

 sulation on these cables must show other advantages in order to prove in. 



Polyethylene insulation is applied to the wire by an extrusion proc- 

 ess; the insulation may be either solid or expanded depending on the 

 application. Generally the polyethylene is supplied as granules pre- 

 viously compounded with an antioxidant. In the case of solid poly- 

 ethylene insulation the granules, in which the pigment has been in- 

 corporated, are fed into the extruder and formed on the conductor as a 

 uniform close fitting tube of insulation. 



Table I — Characteristics of Insulating Materials 



Density — gms/cc . 

 Tensile strength- 



psi 



Elongation % 



Water absorption 



% in 24 hrs 



Diel. strength, 



RMS volts/mil 



^* thickness*. 

 Power factor, 1 



300 kc 

 Diel. constant, 1- 



300 kc 



Polyethylene 



0.92 



1400-2000 

 600 



<0.01 



4(X>500 



0.0002 



2.3 



Plasticized 



Polyvinyl- 



Chloride 



1.2-1.4 



1500-3000 

 200-450 



0.4-0.65 



300-700 



0.09-0.16 



3.5-5.0 



Polystyrene 



1.06 



500-9000 

 2-5 



<0.05 



500-700 



0.0002 



2.5-2.6 



Polytetra- 

 fluoroethylene 

 (Teflon) 



2.2 



1500-2500 

 100-200 



Nil 



400-500 



0.0002 



2.0 



Poly amide 

 (Nylon) 



1.09 



7000 

 100-200 



0.4-2.0 



400 



0.04-0.2 



3.5-8 



xu-*!^*®*®*^*"®.'**"®."^*" ^^® greater for thinner sections— for example, in 14 mil 

 thicknesses polyethylene has a dielectric strength of approximately 2500 RMS 



