also serves an important function in that it prevents flow of the isobutylene rubber material at temperatures up to 200 u00b0 c thus the filler material can withstand temperatures encountered during heavy loads on the power transmission lines without softening and having its viscosity become so low that it will flow out of the cable at cable ends or flow lengthwise of the corrugated metal shield at locations where the cable is on a substantial slope some additional material can be added if necessary as a processing aid for example a hydrocarbon oil such as used in rubber compounding or a chlorinated paraffin or isobutylene liquid plasticizer can be used to bring the isobutylene rubber compound to a pumping consistency without utilizing excessive heat it is preferable however to use as little processing air as possible or none at all when it is not necessary for obtaining a pumping consistency the disadvantages of the processing aids are that they may migrate into the insulation shield and cause swelling and a consequent reduction in the conductivity of the shield the amount of electrical conductive carbon black or graphite material which is mixed with the isobutylene rubber material is from 40 to 150 parts by weight of the.