Note: Descriptions are shown in the official language in which they were submitted.
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An optical storage medium
This invention relates to a writable optical storage medium, particularly in
the form of a
singly- or multiply-writable compact disc (CD), which comprises a specific
support layer.
Customary optical storage media, particularly CD-Rs, for which data recording
and read-out
is effected with the aid of laser light, are built up from a transparent
support, a recording
layer comprising a metal such as Te, Bi or Mn or a dye such as a cyanine,
merocyanine,
phthalocyanine or azo dye, a metallic reflection layer, and optionally a
protective layer also.
Data recording is effected by irradiation with laser light through the
transparent support. In
the course of this procedure, the recording layer is deformed, sublimed,
evaporated or
modified at the irradiated points, so that pits are formed if necessary. A
free space is also
frequently formed between the recording layer and the adjacent layer. A
reflecting metal layer
of Au, Ag, A1 or Cu is usually provided on the writable layer and a protective
layer is
provided in turn on the latter. Read-out is effected by irradiation with laser
light of lower
energy, likewise through the transparent support. The contrast between the
light reflected
from the pits and the light reflected from the remaining parts of the storage
medium is read
out as an electrical signal.
Customary non-writable CDs (ROM type: read-only-memory) do not comprise the
aforementioned writable layer, but simply comprise pits corresponding to the
data to be
reproduced, which are already formed on the support, e.g. by press moulding.
Apart from
this, their construction is relatively similar.
See A. B. Marchant, Optical Recording (1990), for a more precise explanation.
One problem with known CD-Rs is the relatively high sensitivity of the
recording layer. The
effect of heat and the effect of radiation can result in the recording layer
being altered with
the result that the CD-Rs are no longer writable and/or are no longer
readable. Thus some
manufacturers recommend that CD-Rs are not exposed to direct sunlight.
Appliance
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manufacturers advise that CD-Rs should not be played in DVD players, since it
is feared that
the shorter wavelength of the laser used in a DVD player could damage CD-Rs.
The stored
information can then only be read out subject to errors; this is measured with
the aid of
electronic test instruments and is quoted as the block error rate, amongst
other forms of
expression.
The block error rate gives, in summary form, the total number of defective
information blocks
and should be as low as possible. If the block error rate exceeds a value of
220, the data
storage medium is outside the specification. The block error rate is
determined using a CD
test player. See W. Schild "Fehler vom Player, Fehler von der Platte", from
Funkschau
23/1988, for more detailed information. It is also possible for CD-Rs to be
altered to such an
extent that they can no longer be read or written at all.
The underlying object of the present invention is to provide veritable optical
storage media,
I S particularly veritable CDs, which exhibit improved stability and which
thus have an improved
service life.
This object is achieved according to the invention by the provision of a
veritable optical
storage medium which comprises a protective layer, one or more veritable
layers, optionally a
light-reflecting layer and at least one support layer, which is characterised
in that the support
layer contains at least one dye which has a transmission of at least 70 % at
the wavelength of
the laser which is used for writing and/or reading and which has an absorption
of at least 70
at all other wavelengths of visible light which are at least 100 nm shorter
than the
wavelength of the laser light.
The support layer preferably contains at least two dyes, most preferably three
dyes, in order to
achieve the desired absorption characteristic.
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The dye or dyes are preferably selected from the group comprising
anthraquinone and
pyrazolone dyes. Makrolexgelb 3G~ (a pyrazolone dye, Colour Index, Part 1:
Solvent
Yellow 93), Makrolexrot SB~ (an anthraquinone dye, Colour Index, Part 1,
Solvent Red 52,
Part 2 No. 68210) and Hytherm Blue E~ (an anthraquinone dye, Colour Index Part
1 Solvent
Blue 101) are particularly preferred.
The material for the support layer is not critical; there are no specific
limitations. The support
layer comprises conventional material used for optical storage media,
particularly for CD-Rs,
and in particular comprises a polymer which is transparent to the laser light
used for writing
and/or reading, such as poly(co)carbonates, polymethyl methacrylates, poly-4-
methylpentene-1, amorphous cyclic polyolefines, polyesters, epoxy resins or
polyacrylates.
Poly(co)carbonate is used in particular. It may comprise information signals
in the form of
pits and/or control signals and/or guide channels, i.e. it may be a support
with or without
information signals which is optionally formatted. It may optionally be
surface-treated in the
usual manner.
The dye or dyes are incorporated in the support material in the usual manner,
e.g. by
compounding the dyes with or by depositing the dyes on granules of the support
material.
The dye may also preferably be added as a master batch.
The concentration of the dye or dyes in the support material is preferably 0.1
to 1.4 % by
weight, most preferably 0.3 - 0.8 % by weight. The support is formed from the
dye-containing
material in the usual manner, e.g. by injection moulding or stamping.
If a plurality of support layers is provided, the layers preferably comprise
the same polymer
material. In this situation, the dye or dyes can be incorporated in a single
support layer or in a
plurality of support layers.
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The writable layers) which is/are optionally provided is/are preferably the
customary
sensitive dye layers) (see EP-A 353 392, 488 231 and 410 879, for example).
These
comprise one or more organic dyes such as phthalocyanines, merocyanines,
cyanines,
indodicarbocyanines, azulenes and azo dyes. They are deposited in the usual
manner, e.g. by
spin-coating, as a solution in a solvent. Diacetone alcohol, fluorinated
alcohols such as
tetrafluoropropanol, butanol or mixtures thereof are used as solvents, for
example.
The dyes of the writable layers) are different from the dyes of the support
layer. Cyanine
dyes are particularly preferred.
The reflecting layer. which is optionally provided preferably comprises a
metal with a
reflection of more than 80% at the wavelength of laser light which is used,
such as Au, Ag,
AI, Cu or an alloy thereof for example. It can be applied, for example, by a
deposition process
from the gas phase, such as vacuum deposition, sputtering, ion-plating or
similar techniques.
The protective layer comprises a resin with an outstanding resistance to
impact, which is
usually an epoxy resin, a polyacrylate resin, or a hardened silicone resin.
The protective layer
is usually formed by deposition of the resin by spin-coating, followed by UV
irradiation for
curing. However, it may also comprise a flexible material, such as a
polyurethane resin for
example.
The layer thicknesses of the individual layers are preferably 0.5 - 1.2 mm
{support), up to 200
run (writable layer), up to 200 nm (reflecting layer) and 5 - 200 nm
(protective layer).
The wavelength of the laser for writing and/or reading is preferably either
760-800 nm, most
preferably 775-785 nm; or 620-660 nm, most preferably 635-650 nm.
The following examples further explain the invention.
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The relative viscosity of the polycarbonate was measured using a 0.5 %
solution of the
polycarbonate in methylene chloride at 25°C.
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Examples:
Example according to the invention
The following mixture was prepared as a support material:
99.6224 % by weight granular polycarbonate, Makrolon CD 2005 supplied by Bayer
AG, a
polycarbonate of bisphenol A comprising tert.-butylphenol terminal
groups, with an average solution viscosity of 1.20 (as measured in
methylene chloride at 25 °C, at a concentration of 0.5 g in 100 ml
methylene chloride),
0.06513% by weight pyrazolone dye Macrolexgelb 3G (recorded in the Colour
Index, Part 1,
as Solvent Yellow 93), Bayer AG,
0.19746% by weight anthraquinone dye Macrolexrot 5B (recorded in the Colour
Index, Part
l, as Solvent Red 52), Bayer AG
0.11501 % by weight anthraquinone dye Hythermblue E (recorded in the Colour
Index as
Solvent Blue 101).
The dyes were intensively mixed with the granular material in a closed vessel,
and thereafter
the mixture was compounded in a twin-worm Werner Pfleiderer kneader, Type ZSK
53, at a
material temperature of about 240°C.
CD-R blanks, of thickness 1.2 mm and outside diameter 120 mm, were
subsequently produced
using a CD-R stamping tool in a CD injection moulding machine. CD-R production
was
effected at a material temperature of 330°C and a mould temperature of
100°C, with a cycle
time of 10 seconds. The CD-R stamping tool had a pre-groove disposed in the
form of a spiral
with an average width of 75 nm, an average depth of 180 nm and a track spacing
of 1.6 pm.
The transmission of the CD-R blanks was measured. Figure 1 shows that the
material had a
high absorption at wavelengths less than 700 run.
(Measurement geometry: 0°/diffuse).
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Comparative example
For comparison, CD-R blanks were produced which were analogous except that no
dyes were
S added to the Makrolon CD 2005 substrate material. Figure 2 shows that the
material had a
high transmission over the entire visible light region (measurement geometry:
0°/diffuse).
A dye layer was subsequently deposited on the CD-R blanks. The cyanine dye
OM57
supplied by Fuji was used as the dye, in combination with the Kayasorb IRG-022
quencher
supplied by Nippon Kayaku Co. The dye to quencher ratio was 20:1. Dye/quencher
deposition was effected from a diacetone alcohol solution by means of spin-
coating, the
conditions being selected so that an average dye layer thickness of 150 nm was
formed.
A gold layer of thickness 60 to 80 nm was subsequently evaporated on to the
dye layer. This
gold layer was in turn provided with a protective lacquer of thickness about
10 pm.
The CD-Rs which were obtained according to the invention and from the
comparative test
were subsequently written in a Yamaha CD-R 100 burner at a write/read ratio of
4x. The
block error rate (BLER) of the CD-Rs was subsequently measured using an ISEDD
test
player supplied by the Ingenieurburos fiir Umwelttechnologie and Mel3technik,
Bielefeld, in
which a Philips and a Sony player were integrated.
The quality of the CD-Rs was of a comparable level, as shown in Table 1 below:
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T_1_1_ 1
BLER (Sony) BLER (Philips)
CD-R according to the invention 60 59
CD-R without dye in the substrate
material 62 64
The CD-Rs according to the invention and the comparison CD-Rs were
subsequently
subjected to an exposure test according to ISO 4892-2. Exposure was effected
through the
substrate material on to the CD-R dye layer, under the following test
conditions:
apparatus: Atlas Xenon-WOM
light source: 6.5 kW XENON lamp (Atlas)
intensity of irradiation: , 0.35W/(mz*nm), measured at 340 nm
filtering: pyrex / pyrex
black standard temperature: 65°C
test room temperature: 42°C
rel. atmospheric humidity: 65
spray cycle: 102/18 minutes
Thereafter, the block error rates were again measured using the ISEED test
player.
The comparison CD-R exhibited a significant increase in BLER, even after an
exposure time
of about 10 hours. After an exposure time of about 20 hours, the value of the
BLER was
about 500, i.e. way outside the specification.
The CD-R according to the invention exhibited no increase in BLER up to
practically 50
hours' exposure time.
