What are PCBs?

PCBs do not burn easily and were widely used in electrical equipment where fire was a constant threat. Beginning in the 1940s, GE used PCBs as an insulating fluid in electrical capacitors manufactured at plants along the river at Hudson Falls and Fort Edward, N.Y. GE discontinued its use of PCBs in 1977.

PCBs are a family of compounds produced commercially by directly chlorinating biphenyl. Many different combinations are possible. In chemical terminology, "phenyl" denotes a ring structure of six carbon atoms attached to something else; "biphenyl" results when two such rings are attached to each other. And polychlorinated biphenyl (PCB) is any molecule having multiple chlorine atoms attached to the carbon atoms of a biphenyl nucleus. Chlorine atoms can be placed at any or all of 10 available sites, with 209 PCB mixtures theoretically possible.

PCBs were manufactured and sold as complex mixtures differing in their average chlorination level. The 209 possible PCB compounds are referred to as "congeners." PCB congeners with the same number of chlorine atoms are known as "homologs" or "isomers" of each other. The materials now collectively referred to as PCBs are actually several dozen individual PCB congeners clustered around some average degree of chlorination. Congeners may be grouped in terms of the number of chlorine atoms attached to the biphenyl molecule. For instance, one chlorine would produce a mono-chlorobiphenyl, two a di-chlorobiphenyl, 10 a deca-chlorobiphenyl. Any biphenyl molecule with two or more chlorines is commonly referred to as a poly-chlorinated biphenyl.

Their Physical Properties

The physical properties of PCBs vary among the different homologs. Lower-chlorinated PCBs (the mono-, di-, tri- and tetra-chlorinated PCBs) tend to be light, oily fluids. Penta-chlorobiphenyls are heavy, honey-like oils. The most highly chlorinated PCBs are greases and waxy substances.

In general, PCBs are nonflammable and water-insoluble. They have high boiling points and low electrical conductivity. They are chemically and thermally stable. These physical properties made PCBs extremely desirable for a wide variety of industrial applications, including dielectric heat transfer fluids, hydraulic fluids, solvent extenders, flame retardants, organic diluents, dielectric fluids, inks, dyes, paints and adhesives. For example, PCBs were found in carbonless copy paper, newsprint and caulking compounds.