۱۳۸۷ مرداد ۱۴, دوشنبه

The Chemistry of Polychlorinated Biphenyls

By Roberta C. Barbalace
Polychlorinated biphenyls commonly known as PCBs are man made chemicals that never existed in nature until the 1900's when they started to be released into the environment by manufacturing companies and consumers. First manufactured by Monsanto (the only American company to manufacture PCBs) in 1929, PCB's were quickly acclaimed as an industrial breakthrough. These chlorinated oils have a low degree of reactivity. They are not flammable, have high electrical resistance, good insulating properties and are very stable even when exposed to heat and pressure. All in all, they seemed to be the perfect oil for use in dielectric fluids, and as insulators for transformers and capacitors. Not only were PCBs hailed for their role in preventing fires and explosions, they were actually required by fire code. Uses for PCBs quickly expanded to include hydraulic fluids, casting wax, carbonless carbon paper, compressors, heat transfer systems, plasticizers, pigments, adhesives, liquid cooled electric motors, fluorescent light ballasts, and the list goes on.
Polychlorinated Biphenyls make up a group of 209 individual chlorinated biphenyl rings known as congeners. They were typically manufactured as mixtures of 60 to 90 different congeners. In the concentrated form, PCBs are either oily liquids or solids with no discernable taste or odor. As the number of chlorines in a PCB mixture increases the flash point rises and the substance becomes less combustible. Also, PCBs with large numbers of chlorines are more stable and thus resistant to biodegradation. The most highly favored PCBs tended to be the ones with large numbers of chlorines. These congeners are also proving to be the ones that present the greatest environmental and health risks.
PCBs were used for many different industrial purposes, including the afore mentioned electrical industry and:
Hydraulic fluids; casting wax; carbonless carbon paper; compressors; heat transfer systems; plasticizers; pigments; adhesives; liquid cooled electric motors; fluorescent light ballasts.
More: http://electricalsafety.blogfa.com/post-80.aspx

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