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

[Joseph Wrobel <mediaguy@kodak.com>]

For those of you who would like to learn more about the diffusion of
water into polycarbonate, I suggest you obtain a copy of the paper I
wrote that discusses the subject in depth:  "Ramp profiles for optical
disc incubation", SPIE Vol. 2338 Optical Data Storage, 191-202, 1994.
Ed, you should be aware of this paper as it is referenced in the ANSI
standard for estimating life expectancy of CD-ROM (ANSI/NAPM
IT9.21-1996) and that for CD-R (ANSI/PIMA IT9.27-1999).

The purpose of the paper was to address the changes in polycarbonate
water content that occur as conditions are cycled during the high
temperature, high humidity incubations that are performed in accelerated
keeping studies of optical media using polycarbonate substrates.  As Ed
points out, the rate of water diffusion in and out of the polycarbonate
substrate is very different at its two surfaces.  Water diffusion
through the metal coated side is essentially nil; water diffusion
through the free polycarbonate surface, though slow, is appreciable over
the duration of an incubation experiment, especially those run at high
temperature.

The problem is not the moisture intake.  The saturated moisture content
of polycarbonate at 20C is 0.3%; at 80C it rises only to 0.5%.  The
problem is that unless due precautions are taken, when a disc held at
high humidity and high temperature for a long time is introduced into a
room environment, the moisture preferentially diffuses out the free
surface and, as it does, leaves behind a non-uniform distribution of
water in the substrate.  Since polycarbonate expands when it picks up
water, the gradient in water content causes the polycarbonate substrate
to symmetrically deform ("bowl") to a spherical surface to accommodate
the gradient in expansion.  If the gradient is extreme, the disc
deflection could exceed the CD deflection specification until the disc
equilibrates with the environment.  And at room temperature, this would
take a rather long time, up to several days.

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.

Now consider the Super Audio CD system to which Ed refers.  This disc,
without the "functional humidity barrier layer", is asymmetric to
moisture absorption, just like a CD.  With this layer, moisture
diffusion at both surfaces becomes essentially zero.  This means that it
is very unlikely that this disc will ever experience a significant
moisture gradient and hence it will avoid the deflection that could
result.  Why is this important for SACD?  Because the tilt tolerances
for DVD type media are much tighter than they are for CD, and the inner
information layer of an SACD disc is meant to play back in a DVD type
system.  The answer to your question, Ed, is that purely CD products do
not require this protection, however SACD, a CD/DVD hybrid, does.
Interestingly enough, since pure DVD discs are symmetric, they require
no humidity barrier layer and have none.  Moisture comes and goes
depending on the environment, but because any resultant gradients are
symmetric about the center of the disc, no deflection results.

With regards to your non-acceptance of "ridiculous and misleading claims
of life expectancy", who could argue.  We all reject ridiculous and
misleading claims.  But there are also serious and data-based claims.
Kodak spent a lot of time and effort to establish the lifetime of their
recordable CD media.  The report summarizing the work can be viewed at
http://www.cd-info.com/CDIC/Technology/CD-R/Media/Kodak.html .  Yes,
it's an Arrhenius study, and I've heard all the objections.  And yes, it
only deals with temperature and humidity.  But I have yet to hear anyone
suggest a better way to estimate lifetime, and I've been listening.

With regards to SACD, could you please direct me to the data that
suggests it will have a long life?  In fact, could you direct me to any
study on the estimated lifetime of any DVD format?

Joe Wrobel
CD-R Media Development & Commercialization
Eastman Kodak Company