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RE: [AV Media Matters] The Functions and Science of 'CD Polishing '.


Graeme Jaye wrote:

But the unintended penetration of water through the porous polycarbonate and
protective lacquer layer. *Is* polycarbonate 'porous'?  It is my
understanding this is not the

Response:The Permeability or permeation coefficient (P) is the amount of
substance passing through a polymer film of unit thickness, per unit area,
per second and at a unit pressure difference. If you wish to pursue this
subject I recommend D.W. van Krevelen, "Properties of Polymers"-their
correlation with chemical structure; their numerical estimation and
prediction from additive group contributions.It is published by Elsevier,
1997. ISBN: 0-444-82877-x. The section on permeability of polymers is
specifically relevant to the question. It shows the relatively high level of
permeability of polycarbonate as compared with other polymers (see Fig. 18.6
on p. 554).

Another reference for the porousness of polycarbonate disc media is from the
book entitled: "Permeability And Other Film Properties of Plastics and
Elastomers, published by the Plastics Design Library, 1995, ISBN:
1-884207-14-6. I quote: "Polycarbonate is not generally considered a good
barrier material". Various grades of Polycarbonate were subjected to the
ASTM D2752 Test Method for gases using three penetrants: Nitrogen had a
permeability of between 27-57 cm3.mil/100in2.day; Oxygen had a permeability
of between 230-314 units; Carbon dioxide  ranged between 1720-2100 units.
Gaseous penetrants, if they reach the reflective layer of an optical disc,
will make it unplayable.

Another set of tests done with water vapor, carbon dioxide and oxygen
demonstrate the phenomenon as well. Water vapor as a penetrant at approx. 40
degrees C and 90% RH produced a vapor transmission rate of 170.5 g.25
micron/m2.day. Carbon dioxide permeability was 780 cm3.mil/100 in2.day. And
oxygen rendered a permeability coefficient of 102 cm3.mm/m2.day.atm.

Just to give a sense of relative permeability to water vapor, I will give
you the results of Test Method ADTM F372 applied to Polyethylene Naphthalate
(PEN) commonly used as a digital magnetic tape substrate. Using a sample
thickness of 0.25mm, the water vapor transmission rate was 0.73 g.mm/m2.day.
And the oxygen gas permeability was 1.5 cm3.mm/m2.day.atm. Or 3.8
cm3.mil/100 in2.day.atm.

Another material used for magnetic tape is Polyethylene Terephthalate (PET).
Using test Method ASTM D1434, its permeability to water vapor was at 40
degrees C and 90% RH 18.6 g.25 micron/m2.day.

That is one of several reasons why warnings are printed on many optical
discs not to clean them with water! Test measurements of the warp caused to
optical discs just by UV drying of various types and thicknesses of printing
inks, show that considerable risks are involved in wetting and subsequent
drying of polycarbonate.

Graeme states: The question of destroying the reflective coating through
damage to
the lacquer integrity is a more serious issue, I grant - but I have
seen no evidence of this so far.  Obviously more work needs to be done
in this area - certainly from the point of view of archival use.

Response: Well such work has been going on in Paris, France for a number of
years and at the Joint Technical Symposium there last January I have seen
what damage is done when the lacquer is penetrated in a relatively short
time. A number of discs could not be played back as the reflective layer had
disappeared. The manufacturers have also reported the premature oxidation of
reflective layers. In fact, I have initiated a dialogue with the
manufacturers as to the possibility of molecule hopping mechanisms that
might cause this.

Graeme states:
e> The standards of successful treatment to recover music from a CD is
e> not the same as that of a disc used to preserve valuable heritage
e> content.True - but, as far I am aware, no manufacturer of such devices
made a pitch at the archival market.  Possibly, this is because the
requirements which would be placed on the machine in question would
raise its cost to a point where no one could afford it anyway :-).

Response: The cost of such devices (if they were able to provide reasonable
success rates) may not be justifiable for an individual archive. But
commercial facilities that serve archives are willing to buy technology that
helps solve customer problems. Motion picture film and magnetic tape
rejuvenation and content restoration is a thriving business. Obviously such
services could evolve for disc content.

Graeme states: Perhaps someone out there is already working on a 'wet gate'
CD player
- we can but hope :-)

Response: That is not an unreasonable assumption, it obviously has solved
the problem for film with base scratches. One of the challenges would be to
develop a tool that would replicate disc signals while the disc surface is
in a wetted state, so-called 'wet application playback', as is used in
projection and in contact printing. The challenge is to find the right
solution that would not dry until after the signal is played back. Given the
hot environment inside the drive, I doubt whther that would be a workable
approach. It would get more sophisticated and more successful when immersing
the read-out mechanism in the solvent as well. Whether it can do so is to be
explored, ultimately resulting in an aquarium.

Graeme wrote:

As a direct result of the discussion which has been taking place here,
I am seriously considering doing this for my own interest. Obviously,
I can not do this on behalf of the manufacturers I represent (although
I expect they would be interested in the results, particularly if

However, although I am sure there *is* someone here with the necessary
equipment and a bit of spare time (not to mention an acceptable rate
for the job) I have yet to find them.

Response: I appreciate your approach, after all it will help you to qualify
your systems and services for a wider range of users!

Best regards,

Ed H. Zwaneveld,
Technological Research and Development,
National Film Board of Canada, and
Chair AMIA Preservation Committee
August 16, 2000

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