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Re: [AV Media Matters] The Permeability of Polycarbonate (PC) used in optical c


Sorry I'm a bit late getting back on this - but Spain is on holiday
this month and I'm trying to take it a bit easy as well.

e> One of our CD polishing discussion partners last week, dismissed the
e> permeability issue of polycarbonate substrates that are used for optical
e> compact disc media, implying that a 10 minute inundation with water will do
e> no harm because he has never seen it to do harm.

Guess that would have been me :-).

Actually, just to set the record straight, the disc is not immersed in
anything. While it is spinning, water, at the rate of a few drops per
second is dripped onto the play side of the disc. This serves to keep
the disc cool, carry away material removed by the grinding surface and
stops 'loading' of the grit with polycarbonate (which would rapidly
reduce its efficacy, of course).

As the disc spins very fast, much of the water is thrown clear (taking
the removed material with it) and a disc removed from the machine
after treatment is nearly dry anyway.

e> While respecting his point of view, I would like to quote Dr.
e> Martin Dubs, Optical Storage R&D Manager of Balzers Process Systems
e> (BPS)

Interesting - but, without wishing to imply that I don't care what the
man says, my original comments were made with reference to the real
world that most of us live in.

e> The question might be asked as to why this newest CD product has
e> such a humidity barrier layer in it, while this is not the case
e> with any other CD products.

e> The answer lies in the purpose for which this disc has been designed, which
e> includes high on the list, the archiving of audio.

Well, the archivists will probably take a great interest in such a
disc if the claims being made for it can be substantiated.

e> today's archival copy must capture "all of the original recording, down to
e> the merest hint of harmonics, buried in noise. The technology behind the
e> Suepr Audio Compact Disc must support such ultra-high quality archiving."

Without wishing to open another 'can of worms' - one which has
surfaced here before - it seems to me that *no* digital copy of
analogue information can meet the basic requirement.  In which case,
does it matter?

e> The original Compact Disc Design engineers calculated that it might take as
e> long as 30 years for humidity and other pollutants to penetrate through the
e> polycarbonate substrate in temperate climates. This implies that in tropical
e> climates optical CD life expectancy (LE) will be a fraction of that, and
e> when the disc is exposed to water or solvents that also will reduce the time
e> needed to permeate and harm the information/reflective layers.

For this I can only refer to Joseph Wrobel's far more technical a
lucid explanation;

"The latter point is relevant to the discussion of whether a brief (10
minute) immersion of a CD in water will do any harm.  The answer is
no.   The diffusion constant, D, of water in polycarbonate is
7.5*10^-8 cm^2/s  at 25C.  The depth of water diffusion over time t is
roughly equal to  the square root of the product of D and t.  In ten
minutes, the depth of  moisture diffusion in polycarbonate at room
temperature is calculated to  be 0.07 mm.  In that depth, the moisture
content will rise from its  equilibrium value (0.15% at 25C, 50%RH) to
its saturated level (0.3% at  25C).  And, if the disc is dried,
approximately ten minutes later the  moisture equilibrium in the
polycarbonate will be reestablished. "

Which, pretty much says what I did - I haven't had a problem.

e> In addition, as stated before, warping will occur 'every time'.

Apparently not, according to Mr Wrobel.

e> The day when SACD writers will become available could be the time
e> that audio archivists have been waiting for, to gain protection for
e> their audio collections beyond the 30 year Life Expectancy barrier
e> of regular CD media.

Maybe so - then again, maybe not.  What is more important is that the
rest of the world will *not* be purchasing these things (as they will,
no doubt, be more expensive than their equivalent 'domestic' machine)
and the rest of us will have to struggle along with whatever we can

e> Meanwhile we should not assume that we can expose optical disc
e> media that are vulnerable when exposed to water, high humidity and
e> solvents.

While I would draw the line at *solvents* which, by definition, are
going to be harmful in one way or another, I am quite happy to assume
that a few drops of water or a short-term high humidity environment is
unlikely to be any more detrimental to the disc than the damage to
which has already been subjected by the time it gets into my hands.

The point which I keep making - and to which no one has, as yet,
provided an alternative - is this.  If the disc is damaged, what other
courses of action are open to you (other than throwing it in the bin)?

Maybe the methods we are discussing are harmful to a disc - the jury
is still out on that issue - in the longer term. However, in the
shorter term it can at least be played and (more importantly) copied
to a fresh disc.

e> I would also like to commend Jim Lindner of VidiPax in New York,
e> NY., for his offer to test 100 discs free of charge. I look forward
e> to reading the resultrs of the evaluations and commit to consider
e> them as we develop recommended practices for extended-term life
e> expectancy of optical disc media.

I second this.  I have written off-list to Jim about this and I am
anxious to try and get my principals involved in such a study, as I
believe it would be of great help to everyone.

Graeme Jaye


Audio Restoration and CD Repair

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