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On 1/25/2016 8:47 PM, Tom Fine wrote:
> Am I correct in thinking that tape is made of a base material
> (polyester, in the case of the sticky tapes we are discussing here),
> with the recording surface being a mix of metallic flakes and a binding
> "glue" material?

I would say mostly correct. I would modify the "recording surface" to 
"mag coat" and "metallic flakes" to "oxide" or "oxide particles." 
"Metallic" implies what was later used in "metal particle" tape or 
"metal evaporated" tape, MP and ME, respectively, so they are to be 
avoided when talking about an oxide-based mag coat. I would also add to 
the mix an important element that stays in the matrix, lubricant.

The mag coat is loaded with enough lubricant to allow for squeal-free 
playback (squeal being caused by stick-slip) under normal conditions. 
The term Loss of Lubricant (LoL) has been applied to tapes that do not 
have enough lubricant to play back in their current state and exhibit 
stick-slip, manifested as squeal. This appears to be a faulty concept as 
Benoit Thiebaut in Paris a decade plus ago identified that there was 
still what looked like a normal load of lubricant in such tapes using a 
mass spectrometer. My conjecture is while the normal load is still 
there, it is insufficient to achieve stick-slip free playback with the 
current condition of the mag coat (as demonstrated by the fallen Tg, 
among other things).

> And on the other side of the base, a back-coat of
> mostly carbon black with a binder material?

Correct

> So this begs a question --
> because polyester is a plastic and inherently changes over time, is
> there any interaction through the polyester between the chemistry of the
> magnetic surface and the back-coat? Might this have to do with this
> gooey-ness that doesn't respond to baking, might it be some sort of
> breakdown of the polyester cause by some sort of chemistry on and
> through the polyester? Or, in the case of non-back-coated tapes that
> have this behavior (Sony 150 and maybe the 3M [175] type Richard has
> mentioned), might there be some sort of chemistry that is turning the
> polyester itself gooey, and that gooey-ness is "bubbling up" into the
> magnetic layer?

That is a very good question, but my answer is, at this point, I believe 
the PET to be stable enough on the time scales we're discussing and in 
the environments we are discussing so that this is not the case. As you 
may recall, my tape from the economizer cycle storage location was 
showing "clear leader" sections where the mag coat had fallen off at the 
head of the tape, but the base film was clear and fine looking.

I think PET film is slightly permeable, so it is possible that the back 
coat can diffuse through and affect the mag coat, so I won't rule that 
out, but I don't think the PET itself is entering into the 
equation...yet under normal storage conditions.

As we discussed a day or two ago, we're not certain if the stiffening in 
Maxell UD and 3M 206/207 is base film or coating related (two coats in 
the 3M 206/207, single coat in the Maxell UD).

I am so pleased how this discussion is turning out this time around. 
Thanks Peter, Tom, and others for some great information and ideas.

Cheers,

Richard
-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada                             647 479 2800
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.