[AV Media Matters] Detailed look at history of sticky tape syndrome

[<rohre@ARLUT.UTEXAS.EDU>]

I am soliciting input of anyone who has worked with old analog magnetic
tapes that have developed stiction.  First some background:

Stiction in Mylar based magnetic tapes first came to industry attention
in
the scientific community in the 1980's.  It was found that exposure to
modest ambient humidity, in even office environments, could cause a
hydrolysis of the binder holding the back coating to tapes.  Some tapes
eventually developed failures of the binder holding the magnetic oxide,
and
the oxide would depart the base film, leaving clear film.

Ampex Tape Division developed a technique that could be safely done to
recover the tapes to playability, at least long enough for new copies to
be
made from Masters.  This was a low heat, baking process.  In the science
community, environmental chambers were used which controlled the
temperature
for the 24 to 48 hour process very exactly.  (24 baking, 24 cooling).

However, Ampex personnel also found that a consumer grade convection
oven
was adequate for this process for smaller size reels used in the audio
industry and very large archives were processed for that customer base.

In my scientific instrumentation recording use; I have also experienced
the
occasional sticky tape.  Some seem prone to it from certain batches of
tape.

In one famous incident, a batch of tape used for the Galileo space probe
became sticky during the mission past Jupiter, but JPL successfully
applied
tape command techniques remotely, to free the tape and continue to use
it.
This overcame a failure of their high speed data link, and allowed use
of
the lower speed link, by the recorder serving as an analog buffer
memory.

This command was, once tape stuck to guides or heads, to reverse the
tape
motion, and it would free itself.  As long as the tape movement was
continued, you often could play back data just fine.

Examination of my own sticky tape incidents has now lead me to believe
this
problem always starts with the back coating on tapes.  Back coating was
applied widely to instrumentation and top of the line audio tapes to
reduce
print through artifacts, reduce cinching in storage, and reduce friction
between tape layers.  It is thicker than oxide coatings, and thus has in
my
opinion, larger area to absorb moisture.  (It is possible its binder
differs
from the binder that is used with oxide particles, but I have no
confirmation of that.)

In any case, the sticky syndrome has always seemed to start with the
back
coating causing sticking to tape guides, and this migrates around and
sometimes coats the head to tape interface as well.  I have several
times
managed to clean sticky residue from guides and heads and completed
playback
of such tapes in mild cases of "sticky shed" as it is called.

Once the baking formula was perfected, I have recovered tapes that were
even
shedding the oxide by the baking process.  Then in 30 days we reproduced
the
tapes and had time to transfer wanted data or audio.

Now my question to the tape users is; has anyone else noticed that this
failure starts first with the back coating?   I have three 27 year old
tapes
that seem to have excellent oxide and base film condition, but the back
coating is sticky when played over tape guides.  My impression is in any
tape over 25 years, one might well want to bake it first, to save the
debris
buildup on the tape playback equipment.

Incidentally, the manual tape stiction check was done before mounting
the
tape, ie it, and its fellow tapes, unwind easily by hand rotation of the
reel allowing gravity to pull the tape from the reel.  No tendency to
adhere
to the adjacent layer is seen with that easy test.  Of course, stiction
can
be a depth- in- the- tape reel dependent failure as well, but as a first
check, the outer layer test is a good beginning.

Has anyone experienced sticky tapes without back coatings?  And has
anyone
noticed back coating failure before oxide failure or delamination?

Thank you,
Stuart M. Rohre
Applied Research Labs, Tape Archives
Univ. of Tx., Austin