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A Water Barrier Film to Stabilize the Dimensional Properties of Single-Sided Plastic Optical Disks

IP.com Disclosure Number: IPCOM000123649D
Original Publication Date: 1999-Feb-01
Included in the Prior Art Database: 2005-Apr-05
Document File: 2 page(s) / 104K

Publishing Venue

IBM

Related People

Yen, Y-S: AUTHOR [+3]

Abstract

The requirements of the characteristics of a protective overcoat on a single-sided plastic optical disk that will adequately suppress disk deformation due to water absorption are quantified. The requirements are expressed in terms of controlling gradients in the absorption of water into the plastic substrate, whereas prior art is concerned with the absolute amount of water admitted at an equilibrium state. The requirements to suppress the transient water gradient are orders of magnitude more stringent than for suppressing the equilibrium steady state absorption.

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A Water Barrier Film to Stabilize the Dimensional Properties of Single-Sided
Plastic Optical Disks

   The requirements of the characteristics of a protective
overcoat on a single-sided plastic optical disk that will adequately
suppress disk deformation due to water absorption are quantified.
The requirements are expressed in terms of controlling gradients in
the absorption of water into the plastic substrate, whereas prior art
is concerned with the absolute amount of water admitted at an
equilibrium state.  The requirements to suppress the transient water
gradient are orders of magnitude more stringent than for suppressing
the equilibrium steady state absorption.

   Usually one side of a single-sided optical disk is coated
with a metal layer, which is impermeable to water.  Such optical
disks are asymmetrical with respect to water absorption, and the
resulting asymmetrical water concentration gradient within the
thickness of the disk causes the disk to deflect.  For typical 130 mm
diameter optical disks subjected to a change in temperature or
humidity, the gradient maximizes in roughly six hours (with a
corresponding disk tilt of 10 milliradians) and disappears in a
couple of days, at which point the disk reaches an equilibrium state
at the new temperature or humidity.  Even changes within normal usage
conditions (20-50 degrees Celsius, 20-80% RH) can give rise to
unacceptably high disk tilt.  High tilt degrades the quality of the
focused beam on the disk and leads to lower signal-to-noise ratio,
increased crosstalk, and reduced recording sensitivity.

   This disclosure claims the addition of a protective
overcoat with specific characteristics described below on the side
opposite the recording metal layer to limit the disk deformation to a
specific value when exposed to large humidity or temperature
gradients.

   To identify the requirements of the overcoat, the
diffusion of water through a protected single-sided disk structure is
modeled.  The volumetric expansion coefficient is combined with the
water absorption capacity of the substrate as a function of
temperature and humidity to derive the disk strain as a function of
water concentration.  From geometry, the disk strain is related to
disk tilt and axial deflection.  Given the diffusion constants and
thicknesses of the overcoat and substrate, the diffusion is modeled
to find the water concentration as a function of time and position in
the substrate.  In net, the water concentration gradient within the
d...