Re: [AV Media Matters] Detailed look at history of sticky tapesyndrome

["Jim Wheeler" <Jimwheeler@aol.com>]

Stuart

This is in response to your Nov 15 posting.

For videotape, the backcoat is usually about 50 micro-inches thick
and the oxide/binder coating is usually around 200-300 micro-inches
thick.  I have only seen sticky-shed on tapes with backcoat but most
tapes made since 1968 have backcoat.  The main exception is
VHS-because of the expense of running the basefilm through the
coating process twice.

Below is a little bit of sticky-shed history.

Jim Wheeler

                 Brief Background about Sticky-Shed

I first ran into shedding mag tape around 1983.  The Yale audio
archivist, Richard Warren, called Ampex about a problem with a
powder on some Ampex audio tapes.  I visited Richard at his facility
at Yale and he demonstrated several tapes with the problem.
Fortunately, he had a case of the same batch of Ampex tape and the
carton was still sealed.  I took the carton of tapes back to
California.  The unused tapes exhibited the same powder shed
problem. It was analyzed and determined to be oligimers of the
binder.

Dr. Neal Bertram (an Ampex Physicist at that time) developed a
chemical equation for the binder breakdown.  The equation indicated
that the process could be countered by placing the tape in a cold
and/or dry environment for several days.  It would not be truly
chemically reversible but close enough to make the tape usable.

Several experiments were done over a three year period and placing a
shedding tape in a hot environment was the favorite because it was
the fastest. The high heat is a way of getting a dry environment.
There are several variables so there is no set time/temperature that
covers all cases.  115-120 F overnight is usually adequate.  If not,
repeat it.  The oven should have a fan.  Real Goods has a fruit
dryer that has a fan and keeps a steady temperature.  I am leaving
out some details so don't just pop your tapes in an oven without
knowing the entire process!

Several tapes were baked several times over a three year period and
they all played back ok.  Some tapes were playable for only a few
days and some for a few weeks but rebaking cured the problem each
time.

Tests made by Kevin Bradley at the National Library of Australia
indicated that about one db loss in high frequency response would
occur because of baking.  I consider this acceptable but some people
may not.

If you don't like the idea of baking your tapes then use the Dry
procedure. I place each tape in a zip-lok freezer bag with a
desiccant that is in its own little bag.  Do not let the desiccant
spill into the zip-lok bag. You can obtain desiccant at RV stores or
get the color-coded (reusable) type at Gaylord.

The sticky-shed problem is due to long-length binder molecules being
split by moisture.  Drying the tape makes the oligimers that oozed
to the surface, go back into the binder.  The oligmers can migrate
to the oxide surface or to the backcoat surface.

Around 1968, most tape manufacturers started using polyurethane as
the binder.  When the sticky-shed problem came to light in the
1980s, a reasonably stable polyurethane was developed.  The problem
with the "stable" binders were that they increased the mix volume
and that made the tape thicker and not interchangeable in some
special cases.

No tape manufacturer will publish anything that mentions the words
"long-term" or "archival" because that would infer that other tapes
are not and that would create a potential liability problem.  If the
Specifications for the tape mention something like "more durable",
that gives you a hint that it probably has an improved binder.  Ask
the Representatives of the tape manufacturers which tape they
recommend for long-term storage.  They will usually give you a
verbal answer but not one in writing.

Tapes with a heavy amount of sticky-shed debris should not be
cleaned first because that may remove oxide.  Use the Drying process
before cleaning.