RE: [AV Media Matters] Print-through

[Peter Brothers <lists@specsbros.com>]

>Sent: Friday, July 07, 2000 10:17 AM
>To: AV-Media-Matters@topica.com
>Subject: RE: [AV Media Matters] Print-through
near the hub.

>According to conversations I have had with engineers at Ampex
>(now Quantegy)
>the "bake first" idea may not be a good practice.
>
>The current thinking is that baking is finite and has a limit to
>the number
>of
>times you can recover a tape by the baking process.  It seems
>that each time
>you bake, you drive out some of the components that aid the process, thus
>making it progressively more and more difficult to achieve a stable and
>usable
>result.
>
>The end point is a condition where baking will NOT reverse the sticky
>condition
>and your tape is trashed for good, unless some other process is worked out
>that
>will compensate accordingly.
>
>It is already well known that a baked tape will revert to the sticky
>condition
>within a matter of time depending on humidity of storage
>conditions and thus
>will require baking every time it is to be re-used, presuming enough time
>has
>elapsed between the baking and the next usage requirement.

This is the first I have heard about a limit to the "repeatability" of
baking to deal with excess oligomer residue.  I know of numerous
circumstances where tapes have been rebaked with success but none where a
tape was successfully baked once and a later attempt was unsuccessful.
There are, of course, a number of other problems that result in tapes
sticking or clogging heads that are not directly due to sticky oligomer
residue.  Since these other problems do not respond to baking in the first
place, the problem will not be fixed by the baking process no matter how
often it is applied.  If there is information on limitations to the
repeatability of "successful baking", I would very much be interested in the
data.  Additional caution may need to be used in applying the process if
there are some terminal effects or limitations associated with the process
that have previously not been published or disseminated.  Of course, to make
that determination, we need the data.

As to the second point concerning tapes reverting to the "sticky shed"
condition after baking, this is an inherent problem with the chemistry of
the tape and not specifically related to the baking process.  If a tape has
polymers in the binder that hydrolyze, exposure to high moisture levels will
result in oligomer residue (sticky shed).  Baking does not create new
"hydrolysis-resistant" polymers.  Polymers subject to hydrolytic breakdown
will hydrolyze over time in moist conditions whether they have been baked or
not.  We have treated tapes with sticky shed that were later stored in a
low-humidity environment and were still playable after three years without
retreatment.

Peter Brothers
President
SPECS BROS., LLC
peter@specsbros.com
web-site: http://www.specsbros.com
TAPE RESTORATION AND DISASTER RECOVERY
SINCE 1983