When carbon black back coated magnetic tapes were taken out of storage and playback attempted, one often found a gooey substance, called sticky shed, on the oxide surface of the tapes. This sticky material caused squealing, binding, blocking, poor playback quality and other problems. The sticky shed debris also collected on the tape machines heads, guides and posts. In some severe cases these contaminated tapes could not be unwound because the gooey substance acted as a blocking adhesive, sticking together the wound tape layers.



Many believed that the sticky shed phenomenon was the hydrolysis by-product of the irreversible deterioration of only the oxide binder chemical. Most efforts to restore sticky shed contaminated tapes focused solely on the oxide surface to enable playback and migration of content to digital media, often causing the loss of the tape. None of these efforts have been successful in eliminating sticky shed contamination.

Independent forensic laboratory analyses of the chemistry of magnetic tapes have revealed that the major cause of sticky shed contamination is the deterioration of the tapes carbon black back coating. The carbon black back coating binder has been found to make up most, if not all, of the gooey substance found on the oxide surface of contaminated tapes and on the tape heads, guides, etc.

Carbon black back coating is made of carbon black particles combined with polyurethane, an organic binder chemical. Tape manufacturers began adding carbon black back coating to magnetic tapes in the early 1970s. It was intended to reduce static and friction. Before the introduction of carbon black back coating, some tapes used a

plastic back coating that did not contain carbon black and few, if any, developed sticky shed syndrome.

Carbon black coating has proven itself to be a ticking time bomb causing tape deterioration. Carbon is the most variable, flexible, versatile, and often unstable of all the chemical elements. Carbon will change from one substance compound into another any time it gets the chemical and environmental chance to do so.

Carbon black plus its binder chemical provides the chemical structure, breeding ground, and living organic micro universe within which the carbon black coating changes into the unwanted gooey sticky shed material. The carbon black back coating is very hygroscopic, acting like a moisture sponge, with an addictive appetite for moisture.

In the presence of moisture, carbon black, along with its organic binder chemical, is also a seed-bed for the growth of mold and fungus. When the tapes carbon black binder coating is exposed to moisture, the moisture, combined with its environmental heat energy, starts up, sustains, and accelerates hydrolysis. Hydrolysis produces the by-products of carboxyl acid, alcohol, polymer fragments, carbon particles, mold, and fungus. These chemicals, aided by the direct mechanical contact of the carbon black back coating with the oxide surface, attacks the oxide coating causing major hydrolysis reactions. Some of this hydrolysis chemical activity also injects itself into the base film upon which it is coated.

By correctly removing the deteriorated carbon black coating from both sides of the tape and proper storage, sticky shed syndrome is eliminated and the tape will resume a long archival functional life span and produce playback sonic qualities essentially as good as originally recorded.

Copyright 2008 Charles A. Richardson