Re: [ARSCLIST] TAPE resources online

["peter@xxxxxxxxxxxxx" <lists@xxxxxxxxxxxxx>]

During our research in the AES/ANSI technical commission on tape stability,
we got data from (unspecified) tape manufacturers that brand new tape
(immediately after manufacture) could show signs of the shorter oligomers.
This means that some tapes were partially hydrolyzed BEFORE they left the
factory.

One of the problems with pinning down what is happening is that there are a
number of issues and vectors at work.  Too often someone tries to "simplify"
things and get a "one stop" answer.  (to some extent, this applies to
"baking" tapes- a valuable process but NOT a "cure-all") With a chemically
and physically complex material such as magnetic tape, simplifying the
problem is likely to "simplify" the answer to the point where it is only
partially correct.

Take for example, the information (in the first paragraph) we received from
the manufacturers.  The polymers in the coatings of magnetic tape are more
stable (resistant to hydrolysis) the longer the polymers.  The polymers in
any one tape formulation are going to be a mix of varying lengths.  The more
long polymers, the more stable (and expensive to make) the tape.  Even brand
new tape, however, can have some shorter chain molecules.    

Now, Richardson comes along and notes that most tapes with severe hydrolysis
are back-coated- this is true.  Unfortunately, he postulates that ALL tapes
that are hydrolyzed are back-coated.  There is evidence that this is not
true.  On the other hand, since most hydrolyzed tapes are back-coated, it is
reasonable to make the connection between back-coating and hydrolysis.  Why?
Well Richard indicates that the back-coat is "binder rich".  The overall
amount of binder available may be a culprit.  In addition, there were a lot
of formula changes to the binders around the same time that back-coats were
starting to be applied- this could also be a culprit.  Finally, as the
length of the polymers makes a difference in stability- were the same
quality control procedures and the same formulas used for the binder mix in
the back-coat as for the binders in the recording surface?- if not, this
could also be a culprit.

The back-coat does have an effect on the hydrolysis problem but exactly why
and what that effect is, is not entirely clear.  Since we have other ways to
make hydrolyzed tape playable and I have handled non-back-coated tapes with
hydrolysis, I personally am not about to start removing the back-coat from
hydrolyzed tapes.

On another, slightly different tact, there is a frequently observed tape
problem that (to date) no one has been able to explain the physics/chemistry
of:

When tape is folded over on itself in a bad pack, you end up with small
sections (in the fold) where the oxide binder is pressed against the oxide
binder from the next wrap and the back-coat is pressed against the
back-coat.  In a number of instances, this results in the record-layer
binder becoming entirely detached from the base-layer where the two sections
of binder are touching: the recording layer simply falls off the tape.
Interestingly enough, the back-coat layers that are touching do not detach
from the base-layer.  Anyone want to try a stab at why this might be
happening?  Note: the rest of the tape wraps sometimes are and sometimes are
not badly hydrolyzed.

   

Peter Brothers
SPECS BROS., LLC
973-777-5055
peter@xxxxxxxxxxxxx

Tape restoration and disaster recovery since 1983

-----Original Message-----
From: Association for Recorded Sound Discussion List
[mailto:ARSCLIST@xxxxxxx] On Behalf Of Richard L. Hess
Sent: Thursday, August 16, 2007 11:13 PM
To: ARSCLIST@xxxxxxxxxxxx
Subject: Re: [ARSCLIST] TAPE resources online

At 11:00 PM 2007-08-16, Malcolm Rockwell wrote:
>Has the possibility that the winding contact between the two 
>emulsions - front and back coats - is the cause of the syndrome? 
>That it is really the interaction of the two emulsions? If this were 
>the case then removing the backcoating should halt any further 
>degradation, once the tape has been baked, that is.

Hello, Malcolm,

This theory has intrigued me, but it seems the closest it comes is 
that the long polymer chains break down with moisture (and the 
inherent instability of the polyester polyurethanes that were used) 
and then the mag coat and the back coat inter-twine the short chains 
to some extent. The back coat has more binder (is "binder rich") and 
the carbon black doesn't form any structure unlike the acicular 
(needle-like) mag particles on the mag coat. So, there is more stuff 
to degrade on the back coat.

We all agree that back-coated tapes behave worse, but Richardson 
appears to be out there alone saying that the back coat is the cause.

I've discussed this at some length with the best tape chemist I know, 
Dr. Richard Bradshaw of IBM (note all the credits in my recent paper) 
-- I owe him a lot, and I am paraphrasing what he has told me. 
Bradshaw was the one who figured out how to unwind the CHALLENGER 
tape after the Navy scooped it off the ocean floor.

>Baking and transfer still makes the best sense to me, though. Then 
>tossing the original master.
>I remember hearing - back in the early 80s -  that Ampex 456 would 
>begin absorbing moisture from the air immediately upon opening the 
>plastic bag around the reel. Then again, I have actually seen the 
>syndrome in full force upon opening the bag after boxes of tape had 
>been stored unopened for a few years.

Right--the plastic used for the bags is not an absolute water 
barrier. In fact, the PET used for the tape basefilm is also 
hydroscopic -- so it brings water around to the back of the coatings 
as well! Only metallic foil is truly a moisture barrier at the levels 
we're talking about.

>This was disappointing, to say the least. Especially with the 
>expensive 2" stock! Couldn't even sell the stuff unopened.

Disappointing yes, but better to be rid of it without some precious 
recording entrusted to it!

Cheers,

Richard

Richard L. Hess                   email: richard@xxxxxxxxxxxxxxx
Aurora, Ontario, Canada       (905) 713 6733     1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.