Peroxide crosslinked carbon black polyethylene compositions

Polyethylene containing large quantities of carbon black can be crosslinked by a chemical free radical mechanism utilizing the thermal decomposition of dicumyl peroxide as the source of free radicals. Crosslinking in the presence of different types of blacks was studied in a liquid paraffin model system. The information obtained from this study provides a logical basis for speculation on the mechanism of crosslinking of carbon‐black‐loaded polyethylene compositions. Evidence is presented that the grade of carbon black is an important factor and that the course of the peroxide decomposition is largely determined by the chemical nature of the carbon surface. Carbon blacks having a highly oxygenated surface cause an ionic decomposition which is not effective for crosslinking, while those of low oxygen content allow the decomposition to follow a free radical mechanism which results in the crosslinking of polyethylene. Marked changes in the physical characteristics of the polymer are brought about as a result of pigmentation and subsequent crosslinking. Such compositions display a surprising degree of flexibility and strength over a wide range of temperatures. Moreover, significant strength is observed at temperatures above the crystalline melting point of the polymer. These improved properties are attributed to the reinforcing ability of the carbon black and the crosslinked network of polymer chains.