Pardon for the very late posting (have been massively busy with disaster
recovery projects) but the posting concerning "sticky shed" truly needs some
clarification as the way it is stated is very misleading.
Sticky shed is caused by binder hydrolysis. This is indeed a function of
chemistry- but the chemistry involved is the interaction of water (often
absorbed from humid air) with the long-chain polymers in the tape resulting
in the polymers breaking down into short-chain, low-molecular-weight
While this is a chemical reaction, the reaction is very dependent on the
moisture content of the air in the environment in which the tapes are
stored. The assertion that " they return to the sticky state eventually,
even in perfect storage" is not correct. Should this happen, they have not
been placed in "perfect storage". It has, in fact, been proven that
"sticky" tapes (without baking) will become less sticky if stored in stable,
low-RH environments. If you store polyester-base tapes in an environment of
approximately 68 degrees and an RH of 20% or less, within a year, most (some
take longer) sticky tapes are no longer sticky and further testing 2, 3 and
5 years down the road, show that the tapes continue not to exhibit "sticky
shed". As such, the "perfect storage" referred to in the earlier post is
not actually "perfect" storage for polyester-base magnetic tapes.
Another issue could be the method used for "baking" and how soon the tapes
are tested after they have been returned to storage. Many people perform
short-term baking. The issue with this is how hydrolysis affects the tape.
When hydrolysis occurs, polymers in the tape matrix as well as polymers on
the tape surface are effected. The oligomer residue created inside the tape
matrix may partially migrate to the tape surface. Heating the tape during
short-term baking primarily causes some of the oligomer residue to be
re-absorbed into the tape matrix leaving less on the surface and making the
tape, temporarily, playable. It has little effect on the oligomers other
than their absorption into the tape and away from the surface. As soon as
the tape begins to cool, these oligomers (slowly) start to migrate back to
the surface again. This is one of the reasons that individuals who do
short-term baking state that you must play back the tape as soon after
baking as possible. More sustained treatment (whether by "baking" or
exposure to very low RH environments or a vacuum) actually forces
cross-linking of the oligomer residue back into polymers. As such, there is
no great abundance of oligomer residue to migrate back to the surface and
the tapes (so long as they are not exposed to elevated humidity) remain
playable for an extended time.
This is likely more information about the subject than most people really
want to know but to state that "sticky shed" is not a function of "storage"
is extremely misleading.
Just as background, I was one of the primary authors of the National and
International Standards about magnetic tape storage and magnetic tape
handling published by ANSI, AES and the ISO.
SPECS BROS., LLC
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Audio and video restoration and re-mastering since 1983
From: Association for Recorded Sound Discussion List
[mailto:[log in to unmask]] On Behalf Of Lou Judson
Sent: Thursday, June 29, 2017 11:44 AM
To: [log in to unmask]
Subject: Re: [ARSCLIST] Cassette repair question
I believe it has been more or less proven that sticky shed is a function of
chemistry, not storage, as even after baking they return to the sticky state
eventually, even in perfect storage.
On Jun 29, 2017, at 5:11 AM, Steve Greene <[log in to unmask]> wrote:
> But bear in mind, the loss of the felt suggests that the tape probably
> wasn't stored in the best conditions, be concerned about sticky-shed,
> or other binder problems in your future.