Polychlorinated Biphenyls (PCB’s) are a range of substances consisting of a biphenyl molecule with or without alkyl or aryl substituents, in which more than one chlorine atom is substituted in the biphenyl nucleus. It is believed that PCB’s do not occur naturally. They are resistant to chemical and biochemical process. PCB’s are excellent dielectrics, stable to thermal, chemical and biological degradation and are fire resistant. These have been in use commercially, since 1930 as dielectric and heat exchange fluids and in a variety of other applications. Their thermal stability, chemical stability e.g. general inertness to oxidation and hydrolysis and dielectric properties have made these very useful in a variety of industrial applications. PCB’s were developed as high boiling point stable, heat-transfer fluids for use in transformers and heat exchangers. Other major uses of PCB’s included hydraulic fluids, plasticizers, carbonless carbon paper, inks, lubricants, waxes, cutting oils, and adhesives. They have been used as coolant – insulation fluids in capacitors and transformers, as agents for impregnating cotton and asbestos, in braided insulation of electrical wiring; as high – pressure hydraulic fluids; as heat transfer agents (Gustafson, 1970). In United States of America approximately 50% of total production was used in capacitors and transformers and 20% as plasticizers between 1930 to 1970. The degree of chlorination of PCB’s being adjusted according to the intended use.

The PCB’s had been discovered before the turn of 19^th century and were first produced in 1929 as Aroclor. The bulk of commercial Aroclors consists of about 50 PCB’s isomers. The commercial mixtures of PCB’s in term of Aroclor is designated by four digit numbers of which the first pair (e.g. 12 in Aroclor 1242) designate the category of PCB’s and the second pair of digits designate the approximate percentage of chlorine, e.g. Aroclor 1242 has 42% chlorine, equivalent to the composition C₁₂H₇Cl₃ and is a liquid with a boiling point of 38⁰C, while Aroclor 1260 is 60% Chlorine, equivalent to C₁₂H₄Cl₆ and is a solid. PCB’s are very resistant to chemical and bio-chemical degradation, their stability increases with the degree of chlorination and therefore they persist in the environment. Individual PCB isomers can differ sharply in their persistence, with the monochlorinated compounds generally being quite biodegradable. PCB’s with three or more chlorines on each phenyl rings are generally quite persistent, highly lipophilic (soluble to 10% in fat), water soluble, and extremely stable in the environment.

Back to Content