OPTICAL RECORDING MEDIUM SUBSTRATE, OPTICAL RECORDING
MEDIUM, AND METHOD OF MANUFACTURING OPTICAL RECORDING
MEDIUM SUBSTRATE
5 CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Priority Patent Application
JP 2012-251413 filed November 15, 2012, the entire contents of which are
incorporated herein by reference.
10 BACKGROUND
The present disclosure relates to an optical recording medium substrate that
reduces warpage, an optical recording medium using the optical recording medium
substrate, and a method of manufacturing the optical recording medium substrate.
An optical recording medium is very excellent as a medium for supplying
15 software (including music, videos, etc.), and has become widespread in a
comprehensive range of fields due to the convenience of handling, the ease of mass
production, the reduction of manufacturing cost, etc. In addition, improvement in
high density of an optical recording medium is in progress, and a Blu-ray disc
(registered trademark) or the like implementing a significantly high capacity
20 compared to, for example, a compact disc (CD) and a digital versatile disc (DVD) of
the related art, has become widespread.
When these optical recording media are warped, it is difficult to exhibit
good reproduction characteristics. This is because aberration occurs in a light spot.
In Japanese Unexamined Patent Application Publication No. 2009-271970,
25 technology for laminating an optical transmission layer (transparent cover layer)
formed by an active energy line curable resin having predetermined characteristics in
order to suppress the warpage of an optical recording medium (particularly, a Blu-ray
disc) has been disclosed.
30 SUMMARY
Incidentally, when the optical recording medium of the related art does not
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have sufficient rigidity and, for example, is stored for a long period of time in a
horizontal state in which both ends of the optical recording medium between which
the center is interposed are inserted into holding grooves of a storage cartridge, there
is a possibility of warpage occurring and it is difficult to exhibit good reproduction
5 characteristics. In storage in a high-temperature environment, this may be
remarkably shown. This is one factor shortening a life of the optical recording
medium. If the warpage of the optical recording medium stored in such aliorizontal
state can be reduced, this will greatly contribute to long-term storage of the optical
recording medium.
10 It is desirable to provide a long-life optical recording medium substrate
which reduces warpage and has good reproduction characteristics even when an
optical recording medium is stored for a long period of time in a normal- or hightemperature
enviroimient in a horizontal state, and an optical recording medium
using the same.
15 According to an embodiment of the present disclosure, there is provided an
optical recording medium substrate including a polycarbonate in which an average
molecular weight is in a range from 15000 to 16000, and mass per unit volume at
25°C is in a range from 1.1930 g/cm^ to 1.2000 g/cml
According to another embodiment of the present disclosure, there is
20 provided an optical recording medium including an optical recording medium
substrate which is a polycarbonate in which an average molecular weight is in a
range from 15000 to 16000, and mass per unit volume at 25°C is in a range from
1.1930 g/cm^ to 1.2000 g/cml
Further, according to still another embodiment of the present disclosure,
25 there is provided a method of manufacturing an optical recording medium substrate,
including filling and injection-molding a polycarbonate in which an average
molecular weight is in a range from 15000 to 16000 in a mold, and performing
cooling in the mold for 6 seconds or more.
Thereby, high rigidity of the optical recording medium is secured.
30 According to one or more embodiments of the present disclosure, even
when an optical recording medium is stored for a long period of time, warpage due to
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its own weight can be suppressed and music, videos, etc. recorded on the optical
recording medium can be reproduced in a good state.
In addition, even when the optical recording medium is stored in a hightemperature
environment, it is possible to suppress warpage due to its own weight
5 and similarly obtain good reproduction characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory diagram of a layer structure of an optical recording
medium of an embodiment;
10 FIG. 2 is a diagram illustrating a method of measuring a density of an
optical recording medium substrate;
FIG. 3 is a diagram illustrating a change in a shape when the optical
recording medium is held in a horizontal state;
FIG 4 is a diagram illustrating a measurement result of warpage of the
15 optical recording medium when stored in the horizontal state;
FIG. 5 is a diagram illustrating a relationship between a density and warpage
of the optical recording medium substrate; and
FIG. 6 is a diagram illustrating a relationship between a cooling time in
injection molding of the optical recording medium substrate and a density.
20
DETAILED DESCRIPTION OF THE EMBODIMENT(S)
Hereinafter, preferred embodiments of the present disclosure will be described in
detail with reference to the appended drawings. Note that, in this specification and
the appended drawings, structural elements that have substantially the same function
25 and structure are denoted with the same reference numerals, and repeated
explanation of these structural elements is omitted.
Hereinafter, contents of the present disclosure will be described in the
following order.
[1. Structure of optical recording medium]
30 [2. Manufacturing method]
[3. Density measuring method]
4^
f
[4. Experiment results]
[5. Conclusion]
Hereinafter, an optical recording medium, an optical recording medium
substrate, and a method of manufacturing the optical recording medium substrate
5 according to an embodiment will be described with reference to FIG. 1.
Technology of the present disclosure is applied to the optical recording
medium substrate described here and applied to an optical recording medium using
the optical recording medium substrate.
The present disclosure includes an example of a Blu-ray disc as the optical
10 recording medium, an example of a Blu-ray disc substrate as the optical recording
medium substrate, and an example of a method of manufacturing the Blu-ray disc
substrate as the method of manufacturing the optical recording medium substrate as
will be described hereinafter.
A range of application of the present disclosure is not limited to an optical
15 recording medium serving as the Blu-ray disc, a Blu-ray disc substrate, and a method
of manufacturing the Blu-ray disc substrate, and, for example, can be widely applied
'to various optical recording media such as a CD and a DVD, a substrate for various
optical recording media, and a method of manufacturing the substrate for various
optical recording media.
20 [1. Structure of optical recording medium]
In FIG. 1, an example of a layer structure of the Blu-ray disc (optical
recording medium 1) is included.
FIG. lA illustrates a layer structure of an optical recording medium 1 in
which a so-called recording layer 5 is a single layer.
25 The optical recording medium 1 has an optical recording medium substrate
4 having one side on which the recording layer 5 is formed. As the optical
recording medium substrate 4, a polycarbonate is used. In the polycarbonate, an
average molecular weight is in a range from 15000 to 16000 and mass per unit
-J -2
volume at 25°C is in a range from 1.1930 g/cm to 1.2000 g/cm .
30 The recording layer 5 is formed in a concave/convex shape as a pit array
pattern including pits and spaces. The optical recording medium substrate 4, for
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example, is formed with a thickness of about 1.1 mm.
A reflective film 6 for reflecting laser light 9 is formed for reproduction on
the side having the recording layer 5 of the optical recording medium substrate 4
having the pit array pattern. A side opposite the recording layer 5 in the reflective
5 film 6 serves as an incidence surface of the laser light 9 on which light is collected by
an objective lens 14.
Although silver, a silver alloy, aluminum, or an aluminum alloy is usually
used as the reflective film 6, the present disclosure is not limited thereto because the
fimction is satisfied as long as light of 405 nm which is a wavelength of reproduction
10 laser light can be efficiently reflected.
A transparent cover layer 7 having uniform and excellent surface
smoothness is formed on an incidence surface side of the laser light 9 in the
reflective film 6, that is, an information readout surface side. Further, a hard coat
layer 8 serving as a protective layer is formed on a surface side of the transparent
15 cover layer 7, that is, a side on which the laser light 9 is incident.
In this case, although the protective layer is formed by the transparent cover
layer 7 and the hard coat layer 8, a total thickness of the transparent cover layer 7 and
the hard coat layer 8, for example, becomes 95 to 105 |^m. Although a ratio
between the transparent cover layer 7 and the hard coat layer 8 is not particularly
20 regulated, for example, the thickness of the hard coat layer 8 is usually 1.5 to 5 j^m.
In addition, when viewed fi-om an incidence side of the laser light 9 of the
optical recording medium substrate 4, a surface (so-called label surface) of an
opposite side is formed as a printing side 3 on which a label printing operation for
displaying contents of the optical recording medium 1 has been performed.
25 The hard coat layer 8 may not necessarily be formed. For example, when a
surface protection fimction by the transparent cover layer 7 is obtained, an example
in which no hard coat layer 8 is formed may be considered. A disc manufacturing
process can be simplified by omitting the formation of the hard coat layer 8.
In the Blu-ray disc, a two-layer disc structure having two recording layers
30 10 and 12 is also standardized. In FIG. IB, a layer structure of a two-layer optical
recording medium 2 is illustrated. As illustrated in FIG. IB, the optical recording
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medium substrate 4 is provided as in the optical recording medium 1. As described
above, a polycarbonate is used as the optical recording medium substrate 4. In the
polycarbonate, an average molecular weight is in a range from 15000 to 16000 and
mass per umt volume at 25°C is in a range from 1.1930 g/cm to 1.2000 g/cm .
5 On an incidence surface side of the laser light 9 of the optical recording
medium substrate 4, a first recording layer 10 and a second recording layer 12 are
formed via an intermediate layer 11.
The first recording layer 10 and the second recording layer 12 are formed in
a concave/convex shape as a pit array pattern including pits and spaces. The
10 intermediate layer 11, for example, is formed with a thickness of 20 to 30 |am.
The reflective film 6 is formed between the first recording layer 10 and the
intermediate layer 11. A semi-transparent reflective film 13 is formed on a surface
of the second recording layer 12. On the surface, the transparent cover layer 7 is
formed. Further, the hard coat layer 8 serving as the protective layer is formed on a
15 surface side of the transparent cover layer 7, that is, on a side on which the laser light
9 is incident. A total thickness of the transparent cover layer 7 and the hard coat
layer 8, for example, is 75 jam.
As in the case of the optical recording medium 1, Ae hard coat layer 8 may
not necessarily be formed.
20 The second recording layer 12 is formed by pressing a stamper on which a
pit array pattern is pre-formed against the intermediate layer 11.
Because the Blu-ray disc (optical recording media 1 and 2) has a one-side
reading method, that is, the transparent cover layer 7 only on one side of the optical
recording medium substrate 4, as illustrated in FIG. 1, the Blu-ray disc is asymmetric
25 in a thickness direction and is easily deformed due to the effect of a residual stress of
each layer.
Therefore, the Blu-ray disc is constituted of a polycarbonate in which an
average molecular weight is in a range from 15000 to 16000 and mass per unit
volume at 25°C is in a range from 1.1930 g/cm^ to 1.2000 g/cm^, so that the
30 occurrence of warpage is particularly efficiently suppressed and good reproduction
characteristics can be obtained.
1
[2. Manufacturing method]
Hereinafter, a method of manufacturing the optical recording medium 1 will
be briefly described.
(a) First, injection molding is performed to create the optical recording
5 medium substrate 4. The injection molding is performed by melting polycarbonate
pellets at a high temperature and filling the melted polycarbonate pellets in the mold.
(b) Next, the optical recording medium substrate 4 is cooled for a fixed time
within a mold and extracted from the mold. Thereby, the optical recording medium
substrate 4 with a thickness of about 1.1 mm is formed.
10 (c) Next, the recording layer 5 and the reflective film 6 are laminated on one
side of the optical recording medium substrate 4 extracted from the mold using
techniques such as sputtering, vapor deposition, and coating.
(d) Subsequently, the transparent cover layer 7 is formed by a spin-coating
method or laminating a film of about 100 |am on the reflective film 6.
15 (e) Next, the transparent cover layer 7 is hardened by radiating ultraviolet
light to the transparent cover layer 7 formed by spin coating or film lamination,
(f) Ultimately, the hard coat layer 8 is formed.
A manufacturing method associated with injection molding for the mold is
also the same in manufacturing of a CD and a DVD.
20 Next, a method of manufacturing the two-layer optical recording medium 2
will be briefly described.
The method of manufacturing the two-layer optical recording medivim 2 is
substantially the same as in the case of the above-described single-layer optical
recording medium 1 vmtil the first recording layer 10 is formed.
25 After the first recording layer 10 is formed, the intermediate layer 11 is
formed. The intermediate layer 11 is hardened by radiating ultraviolet light thereto.
On the intermediate layer 11, the second recording layer 12 and the semi-transparent
reflective film 13 are formed by a sputtering method, etc. Thereafter, the
transparent cover layer 7 and the hard coat layer 8 are formed as in a single-layer
30 disc.
In the technology of the present disclosure, injection molding is performed
%
by filling a polycarbonate of a range in which an average molecular weight is 15000
to 16000 in the mold. A cooling time within the mold is set to 6 seconds or more.
By setting the cooling time to 6 seconds or more, it is possible to have a
characteristic that a density of the polycarbonate is 1.1930 [g/cm ] to 1.2000 [g/cm ].
5 Ultimately, the optical recording medium substrate 4 is formed by a polycarbonate in
which an average molecular weight is in a range from 15000 to 16000 and a density
at 25°C is in arange fi-om 1.1930 [g/cm^] to 1.2000 [g/cm^].
Thereby, it is possible to suppress the deformation of the optical recording
media 1 and 2, prevent warpage due to long-term storage, and obtain good
10 reproduction characteristics as a result.
[3. Density measuring method]
Hereinafter, an experiment in a warpage state when the optical recording
medium 1 was stored for a fixed period of time in an upper limit of a use temperatvire
environment v^U be described. First, a method of measuring the density of the
15 optical recording medium substrate 4 used in this experiment will be described vdth
reference to FIG. 2.
As the density measuring method, an underwater substitution method was
used to accurately obtain the density even in a complex shape of the optical
recording medium substrate 4.
20 As a gravimeter for measuring the density, an electronic force balance 30
capable of measuring up to a precision of 0.1 [mg] was used.
A classical balance or simply a balance is a measuring device which obtains
the mass of a sample from the mass of a balance weight when it is balanced by
applying leverage, placing the sample on one side of a force point or an action
25 position, and placing the balance weight serving as a reference on the other side.
On the other hand, in an electronic force balance 30, a numerical value of weight is
immediately obtained by merely placing a physical object to be measured without the
need of miscellaneous operations.
In addition, in the optical recording medium substrate 4 to be measured, an
30 inorganic film through which water of 10 [nm] did not pass was formed on both sides
of the optical recording medium substrate 4 in order to suppress the degradation of
4
precision of a density due to absorption.
First, the weight of the optical recording medium substrate 4 was measured.
The weight was measured by placing the optical recording medium substrate 4 on the
electronic force balance 30 as illustrated in FIG 2A. At this time, the weight was
5 assumed to be m [g].
Next, the weight of a container 31 containing water 32 (pure water) was
measured as illustrated in FIG. 2B. At this time, the weight was assumed to be Ml
[g]-
Further, the weight was measured when the optical recording medium
10 substrate 4 was hung and floated by a support rod 33 under the water 32 as illustrated
in FIG. 2C. At this time, the weight was assumed to be M2 [g].
From the above-described measurement results, when a density of water
corresponding to a water temperature at the time of weight measurement was p
[g/cm"], (M2 - Ml)/p was the volume of the optical recording medium substrate 4.
15 A density A of the optical recording medium substrate 4 could be obtained by
dividing m, which is the weight of the optical recording medium substrate 4, by (M2
-Ml)/p.
Ultimately, an expression for obtaining the density A [g/cm^] of the optical
recording medium substrate 4 is as follows.
20 A=pm/(M2-Ml).
According to the above method, the density of the optical recording medium
substrate 4 was obtained.
[4. Experiment results]
Next, the experiment results of a warpage state when a general optical
25 recording medium was stored for a fixed period of time in an upper limit of a usage
environment temperature and a warpage state of the optical recording medium 1
having the optical recording medium substrate 4 of this embodiment wdll be
described with reference to FIGS. 3 to 6.
FIG 3 visually illustrates a storage state and a deformation state of the
30 general optical recording medium (optical disc) and illustrates a deformation state
when the optical recording medium was supported at two points and stored in the
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horizontal state. FIG 3A corresponds to the case in which there is no deformation
and no warpage occurs.
FIG. 3B illustrates a deformation state when storage was performed within a
constant-temperature layer at a temperature of 55°C for 96 hours. For ease of
5 understanding, FIG. 3B is illustrated in an exaggerated manner. Although portioiis
(arrows) supported as illustrated in FIG. 3B are not deformed, the other portions are
bent and drooped due to their own weight or deformed in an overall potato chip
shape. In this case, a recording or reproduction operation using a laser is not
normal. This deformation is a cause of malftmction.
10 FIG. 4 is a graph obtained by specifically measuring a deformation amount
(warpage) of the general optical recording medium illustrated in FIG. 3 as a tilt
(angle) from the center of the optical recording medium. The horizontal axis
represents a radius of the optical recording medium and the vertical axis represents
the angle from the center of the optical recording medium. This tilt (angle)
15 corresponds to the warpage of the optical recording medium. In FIG. 4, •
represents a minimum value, A represents an average value, and • represent a
maximum value. When the tilt exceeds ±0.3 degrees, recording or reproduction
using a laser is impeded. This is because aberration occurs in a light spot (focus).
FIG. 4A corresponds to the case of FIG. 3 A, and illustrates the tilt of the
20 optical recording medium before storage (immediately after manufacturing). In this
case, a difference between a maximum value and a minimum value of the tilt is in a
range of ±0.2 degrees, which is substantially flat. This is obviously within ±0.3
degrees. Accordingly, neither recording nor reproduction using laser is impeded.
FIG 4B corresponds to the case of FIG. 3B, and is a graph illustrating a tilt
25 of the optical recording medium after storage at a temperature of 55°C for 96 hours
in a two-point support state. A difference between the maximum value and the
minimum value reaches ±0.6 degrees (a radius of about 58 mm) as illustrated in FIG.
4B. In this case, a recording or reproduction operation is impeded, resulting in
inconvenience.
30 FIG. 5 illustrates a relationship between a density and warpage (tilt) of the
optical recording medium substrate 4 having a polycarbonate in which an average
11
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molecular weight is 15000 or 16000 as the optical recording medium substrate 4.
Here, the density refers to mass per unit volume.
While the warpage of the optical recording medium substrate 4 is abruptly
increased in a region in which a density at a temperature of 25°C is 1.1926 [g/cm^] or
5 less as illustrated in FIG. 5, a warpage amount is gently decreased in a region in
which the density is above 1.1926 [g/cm ]. When the density is 1.1920 [g/cm ], the
warpage amount is about 0.56 degrees.
On the other hand, when the density at the temperature of 25°C exceeds
1.1926 [g/cm ], the warpage amount is decreased to 0.28 degrees or less. Therefore,
10 the use of a polycarbonate in which the density is 1.1930 [g/cm ] or more for the
optical recording medium substrate 4 may sufficiently suppress the warpage and may
be practically preferred.
From the above-described measurement results, if a polycarbonate in which
an average molecular weight is in a range from 15000 to 16000 and mass per unit
15 volume at 25°C is 1.1930 [g/cm ] or morels used as the optical recording medium
substrate 4, the warpage can be reduced and good recording or reproduction
characteristics of the optical recording media 1 and 2 can be obtained.
The density of the polycarbonate can be controlled under appropriate
conditions such as molding conditions and an annealing process. Here, the density
20 can be set to a predetermined value by setting a cooling time within the mold to, for
example, 6 seconds, as will be described later.
In addition, an upper limit of the density of the polycarbonate is generally
1.2000 [g/cm^]. Therefore, the polycarbonate of the present disclosure in which an
average molecular weight is in a range from 15000 to 16000 and mass per unit
25 volume at 25°C is in a range from 1.1930 [g/cm ] to 1.2000 [g/cm ] is preferred.
FIG. 6 is a diagram illustrating a relationship between a cooling time within
a mold of the optical recording medium substrate 4 and a density. The horizontal
axis represents the cooling time and the vertical axis represents the density. When
the cooling time is increased as illustrated in FIG. 6, the density of the optical
30 recording medium substrate 4 is increased. If the cooling time is 6 seconds or more,
the density of the optical recording medium substrate 4 can be set to 1.1930 g/cm or
11-
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more.
Therefore, it is possible to suppress the occurrence of warpage of the optical
recording medium substrate 4 and the optical recording medium 1 having the same
by setting the cooling time within the mold to 6 seconds or more upon molding the
5 optical recording medium substrate 4.
[5. Conclusion]
According to the above, when an optical recording medium substrate w4iich
is a polycarbonate in which an average molecular weight is in a range from 15000 to
16000 and mass per unit volume at 25°C is in a range from 1.1930 g/cm^ to 1.2000
10 g/cm is used for the optical recording medium, the occurrence of warpage is
particularly eflFiciently suppressed and good reproduction characteristics can be
obtained.
In addition, it is possible to prevent the deformation of a disc in a CD and
DVD as well as a Blu-ray disc, prevent the warpage of the disc due to long-term
15 storage, and obtain good reproduction characteristics.
It should be understood by those skilled in the art that various modifications,
combinations, sub-combinations and alterations may occur depending on design
requirements and other factors insofar as they are within the scope of the appended
claims or the equivalents thereof.
20
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20
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^ % ^ ^ "-^^ t ^ C^ What is claimed is: V-^
1. An optical recording medium substrate comprising:
a polycarbonate in which an average molecular weight is in a range from
15000 to 16000, and mass per unit volume at 25°C is in a range from 1.1930 g/cm^
5 to 1.2000 g/cm^
2. An optical recording medium comprising:
an optical recording medium substrate which is a polycarbonate in which an
average molecular weight is in a range from 15000 to 16000, and mass per unit
10 volume at 25°C is in a range from 1.1930 g/cm^ to 1.2000 g/cml
3. The optical recording medium according to claim 2, fiirther comprising:
a transparent cover layer on an information readout surface side of the
optical recording medium substrate.
4. A method of manufacturing an optical recording medium substrate, comprising:
filling and injection-molding a polycarbonate in which an average molecular
weight is in a range from 15000 to 16000 in a mold; and
performing cooling in the mold for 6 seconds or more.
( - j>
5. The method of manufacturing an optical recording medium substrate according to
claim 4, comprising:
forming a transparent cover layer on an information readout surface side of
the optical recording medium substrate after extracting the optical recording medium
25 substrate from the mold.