Here's a datapoint I know from the beverage business. PET is actually quite permeable. That's why
there are "good-by" dates on carbonated soft drinks. Back in the 90s, when the whole industry moved
big-time to PET bottles, soft drink bottlers noticed that drinks left on the retail shelves too long
went "flat" (lost carbonation), and consumer complaints would follow. Furthermore, aspartame breaks
down faster due to airflow through the PET and also light exposure in clear-PET bottles. So,
expiration dates were implemented and the problem pretty much went away. Notice how much longer a
drink lasts in an aluminum can -- no leaching of CO2 through the sidewalls and no light/air exposure
to break down the sweetness of the aspartame. Heating makes PET more permeable, as I understand it.
So I'm not so sure that the base film (a VERY thin layer of older PET) doesn't first of all pass
"contamination" from the back-coat to the oxide coat and also break down as a barrier during heating
In the beverage business, it's well known that, the larger the percent of total drink contents
touching the surface area of a PET bottle (ie smaller-sized bottle has more of the drink touching
the sidewalls vs. a 2- or 3-liter bottle), the shorter the shelf life because more of the
carbonation leaches out throught the PET faster. In the case of diet drinks, you usually see the
smaller-sized PET bottles in fridge-retail settings, because colder temps staves off some of the
aspartame deterioration. In fact, cold-storage staves off some of the CO2 leaching, now that I
remember discussions with bottle makers. So heat is not good for PET's boundary properties.
Now that I think about this more, I wonder if part of the sonic degrading from repeated bakings
might have to do with the PET base becoming either un-flat or un-straight or actually shrinking a
-- Tom Fine
----- Original Message -----
From: "Richard L. Hess" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, January 26, 2016 8:51 AM
Subject: Re: [ARSCLIST] One more sticky-shed data point - Richardson treated tape
> 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?
>> 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  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.
> Richard L. Hess email: [log in to unmask]
> Aurora, Ontario, Canada 647 479 2800
> Quality tape transfers -- even from hard-to-play tapes.