Hello all:
Coming very late to this thread. I don't intend to talk here about Mr.
Richardson's process but, in answer to Tom's post, I'll try to address some
of the issues with "sticky shed". So, here goes:
First off, the primary cause (not necessarily the only cause) of sticky shed
has been shown in laboratory testing to be binder hydrolysis. Each time a
different observation is made, it seems like someone wants to say "aha" it
doesn't fit with the hydrolysis model. In actuality, most of the
"exceptions" are well within the hydrolysis model if one looks closely at
the science.
Hydrolysis of polyester is a well-documented and well known chemical
reaction in polymer science. When binder hydrolysis affects magnetic tape,
water, often absorbed into the binder matrix from humid air, interacts with
the polyester molecules in the binder and breaks the polymers down into low
molecular weight oligomers and carbolic acid. The oligomer residue produced
has a different frictional coefficient than the original polymers and the
residue has less internal cohesion than the polymer matrix. Thus, the
oligomers create a "sticky" effect and "shed" from the matrix- "sticky
shed"..
So, why do tapes with backcoating display SS more often than non-backcoated
tapes? The answer may be fairly simple if one considers the manufacturing
of the different parts of the tape from the viewpoint of polymer science.
The longer the polymer chain, the more stable. Short or mid-length polymers
are more subject to hydrolysis than long-chain polymers. Due to the fairly
exacting physical requirements of the recording surface on magnetic tape,
the polymers used in the recording layer are longer, more stable polymers.
Unfortunately, it takes significant effort, QC and money to create
relatively long, stable, uniform polymers. The backcoat of a tape requires
different (and less exacting) characteristics so the polymers used in the
backcoat are shorter and less uniform in length than the polymers in the
recording surface. They are much more subject to hydrolysis.
That doesn't mean that the polymers in the recording layer can't hydrolyze;
the do. It is just not as likely and, the shorter polymer chains in the
backcoat will hydrolyze before the long chains (in either the recording
surface or backcoat) will. We have tested residue from non-backcaoted tapes
and detected oligomer residue. It happens, but there are also many
circumstances where short-chain molecules will hydrolyze and the
longer-chain molecules will not.
Another complication is that the oligomer residue from the hydrolysis
reaction is more hydrophilic then the polymers. The amount of hydrolysis
that can occur is dependent on the amount of water available in the tape.
This is usually a factor of both RH and absolute humidity. RH determines
how "willing" the environment is to allow the tape to absorb water for the
reaction and absolute humidity determines how much water is actually
available. If a tape is already partially hydrolyzed, the presence of the
oligomer residue in the polymer matrix allows the tape to absorb more of the
available water from the atmosphere than a tape with no oligomer residue and
will "catalyze" the hydrolysis reaction. This partially explains why
treatment for hydrolysis is becoming more intense over time- in some tapes
the reaction has progressed further and needs more remediation.
This could also partially explain why a small amount of tape known to
hydrolyze, wrapped in a larger reel of tape that is not greatly subject to
hydrolysis, might not show the same level of hydrolysis as a full reel of
the hydrolysis-prone tape; the lack of oligomer residue in the majority of
the reel reduces the water available in close proximity to the short section
of tape.
As for "baking" to temporarily counteract SS, it is not the only procedure
that works- just the most convenient. It is possible to get similar (but
not identical) results using a vacuum chamber or a "desiccating" chamber.
Both methods have been tested and shown to be effective ( the treated tapes
play back) but they require significantly more time. The "not identical"
results are part of the key to the easy success of baking that is generally
overlooked. Baking, vacuum and extreme desiccation can all cause oligomer
residue to cross-link back into polymers. Yes, binder hydrolysis is a
bi-directional chemical reaction. Please let us avoid the argument about
hydrolysis not being reversible. The reaction is not reversible; it is
bi-directional. The two are not the same but do have the same practical
result so far as returning a tape to potentially playable condition. If you
remove sufficient water/moisture from a hydrolyzed tape, the oligomers will
cross-link "back" into polymers but not the same polymers as were there
originally and the new polymers will tend to be shorter than the originals.
We have removed oligomer residue from a tape, treated it and shown through
chemical extraction in a laboratory that the treated material had
significantly more polymer content than the material had before treatment.
The effect that is frequently overlooked during the baking process is the
re-absorption of residue into the binder matrix due to the elevated
temperature. When tape hydrolyzes, the polymers don't just break down on
the tape surface. They also break down inside the depth of the matrix.
When the polymers interact with water, and oligomer residue is produced, the
volume of the residue produced for any given volume of polymer is somewhat
greater than the volume of the original polymers. Thus, not only does the
tape surface become sticky due to surface breakdown, this is aggravated by
migration of additional oligomer residue from inside the binder matrix to
the tape surface. Initial, short-term baking does not cause sufficient
cross-linking of oligomers into polymers to make a tape playable. What it
does, initially, is raise the absorption capacity of the binder matrix and
some of the oligomer residue on the tape surface is re-absorbed back into
the binder matrix. Depending on the condition of the tape, it is possible
for sufficient oligomer residue to be absorbed into the tape ( and off of
the surface) to make the tape playable without significant cross-linking to
occur at all. If the tape is badly enough hydrolyzed, however, the matrix
cannot absorb the volume of oligomer necessary to make the surface usable
and significantly longer treatment is required to actually cause
cross-linking.
As for whether cold/dry storage has any effect on sticky-shed, we have taken
tapes with sticky-shed that would not play, placed them in cool/dry storage
for extended times and were later able to play the tapes without any other
treatment. We have done this numerous time so the test is repeatable. The
key is more the issue of dry than of cool. Once again, following the
chemistry of hydrolysis, water is removed from the tapes due to the long
exposure to dry conditions and the volume of free oligomers is reduced.
That said, the environment we put the tapes in was very dry (20% RH and 60
degrees Fahrenheit).
All that said, hydrolysis of the polyester binder is not the only decay
vector identified in/on magnetic tape. Laboratory testing has identified a
number of other decay residues. Among these are cyclic tri-mers from the
polyester base, sodium or calcium impurities from the magnetic powder,
various fatty acids or stearates from the lubricant and surfactants left
over from the manufacturing process. Further testing has shown that "baking"
does not seem to have a positive or "corrective" effect on these other
residues. In some testing, "baking" has made the surface effects of a few
of these other residues worse.
As for Tom indicating that getting the "recipes" for the different tape
binders likely being a "dead end", I would have to agree. I had a somewhat
unique relationship with some manufacturers' R&D departments- we shared data
that was not, otherwise, available. From correspondence and personal
contact, I know that, in at least some instances, the information on the
"recipes" no longer exist.
Peter Brothers
SPECS BROS., LLC
973-777-5055
[log in to unmask]
Audio and video restoration and re-mastering since 1983
-----Original Message-----
From: Association for Recorded Sound Discussion List
[mailto:[log in to unmask]] On Behalf Of Tom Fine
Sent: Tuesday, December 22, 2015 7:12 PM
To: [log in to unmask]
Subject: Re: [ARSCLIST] One more sticky-shed data point - Richardson treated
tape
I was hoping for more input from the tape-degradation experts on the list. I
know Richardson's treatment is very controversial with some people. I was
_very_ skeptical, which is why I asked him to treat that test tape for me.
To be honest, I'm pleasantly surprised that it's still not sticky.
I'm not ready to say, I believe all his claims, but I am ready to say that
more independent scientific testing should be done, especially given that
it's been tested and shown (by Goran
Finberg) and heard (by me and others) that baking, at some point, causes
audio degradation. That, too, should be tested in a more scientific manner.
I'd like to know WHY the AM distortion increases, heard by me and others as
a "fuzzing out" of the sound quality. My theory -- totally a theory since I
don't have a powerful microscope or other tools to study it -- is the
surface of the tape gets less smooth from baking, and there's a tape-to-head
contact issue or a sort of scrape-flutter is being caused by less-smooth
tape rubbing against the face of a stationary head. We also need more
scientific testing on whether cold/dry storage has any preventative effect
on sticky-shed. Perhaps the ARSC board could consider research grants, maybe
working with AES and NARAS and perhaps the government? One avenue I think is
a dead end, which has had plenty of talk and speculation over the years, is
ever getting any reliable documentation on "the recipes" used to make the
sticky-shed tapes -- or even knowing for sure if "the recipes" varied over
time during the sticky-shed era.
-- Tom Fine
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