Optical disc and method and apparatus for making same

An optical disc comprises two facing disc units, at least one of which has a recording layer formed on its surface facing to the other disc unit. The disc units are bonded together by a layer of adhesive which may be a room-temperature curing thermosetting adhesive having a glass transition temperature higher than the upper limit of the range of temperatures of environments in which the disc may be used. Alternatively, the adhesive may be a room-temperature curing two-pack epoxy adhesive which comprises a bisphenol epoxy resin base agent and a modified aliphatic polyamine curing agent, and which has a viscosity of 100-1000 cps, a pot life of one hour or more, a cure shrinkage of 1.0% or less, a water absorption, after curing, of 0.2% or less, and a Shore hardness, after curing, of 80-90.

This invention relates to an optical disc, and, more particularly to an 
erasable optical disc, which may be used as, for example, an external 
device of computers This invention relates also to a method and apparatus 
for manufacturing such optical discs. 
BACKGROUND OF THE INVENTION 
FIG. 1 shows a cross-section of an ordinary optical disc, designated as 10. 
A recording layer 2 is formed on a transparent substrate 1 of resin, such 
as, for example, polycarbonate, and a protective film 3 is formed over the 
recording layer 2. The substrate 1, the layer 2 and the film 3 form a disc 
unit 4. Two such disc units 4 are bonded to each other at the exposed 
surfaces of the respective protective films 3 with an adhesive layer 5. 
Thus, the optical disc 10 is formed. Conventionally, in order to bond the 
disc units 4 together, a thermosetting adhesive, an ultraviolet-curable 
adhesive, or a thermoplastic adhesive are used. A corrosive component 
contained in such adhesives, however, tends to degrade the recording 
layers 2, and, in particular, when the water absorption of the adhesive 
layer 5 is high, the protective films 3 ana the recording layers 2 are 
oxidized, which lowers the reliability of the disc 10. A moisture-curing 
adhesive, such as one-pack epoxy resin adhesive, may be used. However, 
when disc units having large areas are bonded with a thin layer of a 
moisture-curing adhesive, air or moisture hardly penetrates to reach 
center portions of the bonded disc units, resulting in incomplete curing 
of the adhesive. Furthermore bonding disc units with such an adhesive 
requires a long time and is difficult to do. When two-pack non-mixing type 
resins and microcapsule type resins are used, some components may remain 
uncured, which will erode the discs. Furthermore, such adhesives can 
provide insufficient adhesion. For such reasons, two-pack non-mixing type 
and microcapsule type adhesives are considered unsuitable for bonding disc 
units. 
Another problem which may be encountered when curing type adhesives are 
used is that contraction of the curing adhesive may cause distortion of 
the disc units. In particular, when a thermosetting adhesive is used, not 
only does contraction of the adhesive occurs when it cures, but also heat 
applied for curing the adhesive distorts the adhesive layer 5 itself. 
Distortion of the adhesive layer 5 causes deformation or warpage of the 
disc units, and, therefore, resulting optical discs may have to be 
rejected. 
The use of a thermoplastic resin adhesive rather than a thermosetting resin 
as the adhesive layer 5 is disclosed in, for example, Japanese Patent 
Publication No. SHO 63-67258. In the invention disclosed in this patent 
publication, a hot-melt type adhesive, which is thermoplastic, is used for 
the adhesive layer 5. A hot-melt adhesive which has been heated and melted 
is applied over the protective film 3 of a first disc unit 4. Then, the 
other disc unit 4 is placed on the first disc unit 4 in such a manner that 
the protective film 3 of the other disc unit 4 comes into contact with the 
adhesive which has been applied over the protective film 3 of the said 
first disc unit 4. Then, the assembly is cooled to room temperature so 
that the hot-melt adhesive cures to bond the two disc units 4 to each 
other. 
Since such a thermoplastic resin adhesive need not be heated for its 
curing, distortion of the adhesive layer 5 is small, which, in turn, can 
advantageously reduce warpage of the disc units 4. However, if the 
adhesive is applied non-uniformly, portions of the adhesive layer 5 may 
swell from moisture which penetrates through the substrate 1, and the disc 
units may crack or may separate from each other. 
Usually, optical discs are used in different environments, from cold 
districts to hot districts, and, accordingly, the temperature at which 
discs are designed to perform desired functions (hereinafter this 
temperature is referred to as usable environment temperature) ranges for 
example, from -20.degree. C. to 60.degree. C. The inventors have conducted 
humidity-resistance tests and temperature-humidity cycle tests on optical 
discs comprising two disc units 4 bonded together with a hot-melt adhesive 
at the above-stated temperature range of from -20.degree. C. to 60.degree. 
C. at which discs may be used. They found that the recording layers 2 were 
readily peeled off, pin holes were produced and, when the number of test 
cycles increased, bit error rates increased abruptly. Therefore they 
concluded that such optical discs were not sufficiently reliable. 
One conventional technique for bonding two disc units 4 together is as 
follows. A first disc unit 4 is positioned with the recording layer 2 
facing upward. An adhesive is applied over the protective film 3 on the 
recording layer 2 in generally concentric circles. A second disc unit 4 is 
aligned with the first disc by means of a center shaft of a disc 
manufacturing apparatus, and the protective film 3 of the second disc unit 
is brought into contact with the adhesive. The resultant assembly is left 
as it is so that the adhesive is spread over the entire surfaces of the 
protective films due to the weight of the disc unit, and the adhesive is 
caused to cure to bond the two disc units together. In this bonding 
technique, bubbles may be disadvantageously formed in the adhesive when it 
is applied over the protective film or when the second disc unit is 
brought into contact with the adhesive on the protective film of the first 
disc unit, and the bubbles may remain after the adhesive cures. 
one method for preventing such bubbles from being formed in the adhesive 
layer is shown in Japanese Unexamined Patent Publication No. SHO 61-50231. 
According to the method shown in this patent publication, an adhesive is 
applied over the entire surface of the protective film 3 by spin-coating, 
and the two disc units are bonded together with their center axes aligned 
with each other. According to another method which is shown in Japanese 
Unexamined Patent Publication No. SHO 61-292244, an adhesive is applied 
over portions of the protective film 3 of one disc unit, the other disc 
unit is placed over the first disc unit with the center axes of the two 
disc units aligned, and pressure is applied to bond the two disc units 
together with the adhesive spread over the entire surfaces of the 
protective films. 
When an adhesive is applied over the entire surface of the protective film 
3 of one disc unit 4 and the other disc unit is bonded to the first disc 
unit, the adhesive may forced out into the center holes of the disc units 
and also around the bonded disc units, and, when the adhesive cures, burrs 
7 may be formed on the periphery of the center hole 6 of the optical disc 
10 and on the periphery of the disc 10, as shown in FIGS. 2(a) and 2(b). 
Burrs 7 formed within the center hole 6 could make the disc 10 eccentric, 
and, accordingly, they should be completely removed. In order to deburr, a 
deburring device, such as one shown in Japanese Unexamined Patent 
Publication No. SHO 61-00534, may be used. However, the use of a deburring 
device will undesirably increase the number of manufacturing steps, which, 
in turn, increases the manufacturing costs. On the other hand, even when 
the adhesive is applied over portions of the protective film, it may be 
forced out or ooze out as in the case when the adhesive is applied over 
the entire surface as stated above, or, sometimes, the distribution of the 
adhesive may be non-uniform, so that the disc 10 may flutter. 
In order to prevent the adhesive from oozing out into the center hole or 
out of the periphery of the disc, a precisely adjusted pressure must be 
applied, which requires high-precision, expensive equipment. One of the 
simplest techniques for bonding two disc units is the use of the weight of 
a disc unit itself with an adhesive placed between the two disc units. In 
this technique, however, if an adhesive having viscosity of less than 100 
cps is used, it may ooze out and form burrs 7 like the ones shown in FIGS. 
2(a) and 2(b). In contrast, if the viscosity of the adhesive is above 1000 
cps, the adhesive may not spread over the entire space between the two 
disc units 4, as shown in FIGS. 3(a) and 3(b). Even if the amount of the 
adhesive to be applied is precisely measured, oozing out of the adhesive 
as shown in FIGS. 2(a) and 2(b) or absence of the adhesive at some 
portions as shown in FIGS. 3(a) and 3(b) may occur if the adhesive is 
applied to disc units at inappropriate positions. 
A reduced-pressure bonding apparatus as shown in FIG. 4 has been 
conventionally used for bonding, with an adhesive, two disc units without 
leaving bubbles in the adhesive layer. In FIG. 4, a vacuum chamber 11 
houses mounts 14 and 15 coupled respectively to shafts 12 and 13 which can 
move up and down. Disc units 4 with adhesive layers 16 and 17 applied over 
the surfaces of protective films 3 of the respective disc units 4 are 
mounted on the mounts 14 and 15, respectively. A vacuum pump (not shown) 
is operated to reduce the pressure in the vacuum chamber 11 through an 
exhaust pipe 18 to a pressure of about 20 Torr or less. Then, the shafts 
12 and 13 move the mounts 12 and 13 toward each other for bonding the disc 
units 4 together. The adhesive may be cured under a reduced pressure or 
under normal pressure, but the pressure under which the bonding of disc 
units is carried out must be about 20 Torr or below. When the pressure is 
higher than that, bubbles may form in the adhesive. 
When the reduced-pressure bonding apparatus of FIG. 4 is used, the adhesive 
is applied to at least portions of the protective films 3 of the disc 
units 4, and the disc units 4 are bonded with the adhesive which is spread 
over the entire surfaces of the films 3 due to application of pressure. It 
is, therefore, necessary to control precisely the movement of the shafts 
12 and 13 in order to prevent the adhesive from oozing out into the center 
hole or to the outer periphery of the disc or from being non-uniformly 
distributed. Furthermore, it is necessary to determine precisely the 
amount of adhesive to be applied and also the position where the adhesive 
is to be applied. In addition, it is also necessary to maintain the 
pressure in the reduced-pressure bonding apparatus at about 20 Torr or 
below. If one wants to use this type of apparatus for mass-production of 
optical discs, the size of the apparatus must be large. 
Therefore, a first object of the present invention is to produce optical 
discs which are free of warpage of the discs and peeling off of a 
recording layer of the discs when they are used at a usable environment 
temperature within a range of, for example, from -20.degree. C. to 
60.degree. C. 
A second object of the present invention is to produce, at low costs, 
highly reliable optical discs in which degradation of recording layers and 
deformation of disc units are eliminated by the use of such an adhesive 
that can reduce the number of discs to be rejected in the step of bonding 
two disc units. 
A third object of this invention is to provide an improved method of 
manufacturing optical discs, according to which, when two disc units are 
bonded together with an adhesive, the adhesive does not ooze out into the 
center hole or to the outer periphery of a resulting disc, and, 
accordingly, a step for removing burrs can be eliminated. The resulting 
optical discs are free of eccentricity and free of surface fluttering. 
SUMMARY OF THE INVENTION 
In a first example of an optical disc according to the present invention, 
the optical disc comprises two disc units facing each other, at least one 
of which has a recording layer formed on the surface facing the other disc 
unit. The two disc units are bonded together by means of an adhesive layer 
which is disposed between the facing surfaces and which comprises a 
thermosetting resin that has a glass transition temperature higher than 
the highest usable environment temperature of the optical disc and that 
cures at room temperature. 
In a second example of an optical disc according to the present invention, 
two disc units, each comprising a transparent substrate and a recording 
layer formed on one surface of the substrate, are disposed with the 
recording layers facing each other, and the disc units are bonded together 
by means of an adhesive layer interposed between the recording layers. The 
adhesive layer is formed by curing a room-temperature curable two-pack 
epoxy adhesive which comprises a bisphenol epoxy resin as a base agent and 
a modified aliphatic polyamine as a curing agent, and which has a 
viscosity of 100-1000 cps, a pot life of one hour or more, and a cure 
shrinkage of the adhesive of 1.0% or less. The adhesive, when cured, has a 
water absorption of 0.2% or less, and has a Shore hardness of 80-90 (D 
scale). 
For example, the bisphenol epoxy resin of A-type or F-type may be 
advantageously used. The modified aliphatic polyamine having a viscosity 
of, for example, 100 cps or less may be advantageously used. 
According to one aspect of the method of manufacturing optical discs of the 
present invention, an adhesive, which forms the adhesive layer of the 
above-described first and second examples, is applied to a surface of one 
of two disc units along the circumference of one or more circles about the 
center of the disc unit having a radius of from 0.5a to 0.85a, where a is 
the radius of the disc unit, and then the two disc units are bonded 
together. 
A second example of the method of manufacturing optical discs of the 
present invention comprises the steps of: applying an adhesive to the 
surface of the first of two disc units to be bonded together which faces 
the second disc unit; placing the second disc unit slantwise with one end 
of a diameter of the second disc unit abutting one end of a diameter of 
the first disc unit; supporting the first and second disc units by means 
of a center shaft of a base; holding the second disc unit at the other end 
of the said diameter, while maintaining the second disc slanting with 
respect to the first disc unit, by holding means extending from driving 
means mounted on an axis which is substantially in parallel with the line 
connecting the respective other ends of the diameters of the first and 
second disc units; moving the driving means downwardly to bring the two 
disc units into contact with each other in such a manner that portions of 
the disc units near to the one ends of the diameters first contact with 
each other and portions of the disc units near to the other ends contact 
with each other last; and curing the adhesive to bond the disc units 
together. 
In each of the examples of the method of manufacturing optical discs 
described above, the adhesive used in the previously described first and 
second examples of optical discs may be used. 
An optical disc manufacturing apparatus of the present invention includes a 
center shaft mounted on a base for supporting two disc units, holding 
means for holding one end of a diameter of one of the two disc units in 
such a manner that the one disc unit is slanting with respect to the other 
disc unit, driving means supporting the holding means and movable up and 
down at an adjustable speed, and angle adjusting means for adjusting the 
slanting angle of the driving means in accordance with the angle of the 
one disc unit relative to the other disc unit.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is described in detail with reference to the 
accompanying drawings. 
As shown in FIG. 5, an optical disc according to the present invention may 
be manufactured in the following manner. A layer 22 of dielectric 
material, such as silicon nitride (SiN.sub.x), is formed over a surface, 
in which grooves may be formed, of a transparent substrate 21 of synthetic 
resin, such as polycarbonate, having a glass transition temperature of, 
for example, 130.degree. C. Then, over this dielectric layer 22, a 
recording layer 23 is formed. The recording layer 23 may comprise an 
amorphous magnetic material having perpendicular magnetic anisotropy, such 
as terbium-iron-cobalt (Tb-Fe-Co). A protective film 24 of, for example, 
silicon nitride (SiN.sub.x) is formed over the recording layer 23 to 
complete a disc unit 20. Two such disc units 20 are bonded together, with 
their respective protective films 24 facing each other, with an adhesive 
layer 25 interposed between them. The basic structure of this optical disc 
is substantially the same as that of conventional ones. The optical disc 
of the present invention is characterized by the material of the adhesive 
layer 25. 
According to the present invention, the adhesive layer 25 comprises a 
thermosetting adhesive which has a glass transition temperature higher 
than the upper limit of the usable environment temperature range of, for 
example, from -20.degree. C. to 60.degree. C., of the disc, and which can 
cure at room temperature. An example of adhesive usable in the present 
invention is a two-pack room-temperature curable epoxy adhesive which 
comprises a bisphenol epoxy resin, as a base, and a modified aliphatic 
polyamine, as a curing agent, has a viscosity of from 100 to 1000 cps, and 
has a pot life of one hour or longer. This adhesive can cure at room 
temperature and has a glass transition temperature of about 70.degree. C., 
and, therefore, it can satisfy the above-described conditions. The 
above-described two-pack epoxy adhesive has to have a cure shrinkage of 
1.0% or less, a water absorption of 0.2% or less, and a Shore hardness of 
from 80 to 90 (D scale), after it cures. 
Another example of usable adhesive is a two-pack room-temperature curable 
epoxy adhesive which comprises a bisphenol epoxy resin, as its base, and a 
modified aliphatic polyamine, as the curing agent, having a viscosity of 
100 cps or less at room temperature. This adhesive has a viscosity of 
100-1000 cps and a pot life of one hour or longer, and can be cured at 
room temperature. Like the first described example, this adhesive should 
have a cure shrinkage of 1.0% or less, a water absorption of 0.2% or less, 
and a Shore hardness of 80-90 (D scale). 
Bisphenol epoxy resins usable as the base agent of the adhesives of the 
present invention are a bisphenol A epoxy resin (commercially available as 
TB2022 (trade name) from Three Bond Co., Ltd. Hachioji-shi, Tokyo, Japan), 
and a bisphenol F epoxy resin (commercially available as TB2023 from Three 
Bond Co., Ltd.). Experiments have revealed that two-pack epoxy adhesives 
other than the above-described ones and one-pack epoxy adhesives cause the 
recording layers to be oxidized and degraded due to corrosive components 
contained in such adhesives. 
In some applications, adhesives to be used for manufacturing optical discs 
of the present invention may be selected from the viewpoint of glass 
transition temperature and warpage of substrates or disc units. 
EXAMPLE 1 
The adhesive layer 25 comprises a two-pack epoxy adhesive comprising a 
mixture of a bisphenol A epoxy base agent (available as TB2022 from Three 
Bond Co., Ltd.) and a modified aliphatic polyamine curing agent (available 
as TB2131D from Three Bond Co. Ltd.) which are mixed in a ratio of 3:1. 
The mixture is then deaerated. This adhesive can cure at room temperature, 
and it has a glass transition temperature of about 70.degree. C. As 
indicated by a solid line in FIG. 6, the adhesive has a thermal expansion 
coefficient substantially the same as that of the polycarbonate substrate 
21 indicated by a dash-and-dot line, in the temperature range below the 
glass transition temperature (T.sub.g2) of the adhesive. The polycarbonate 
substrate 21 has a glass transition temperature T.sub.g1. 
Since the adhesive of Example 1 could cure and form the adhesive layer 25 
at room temperature, distortion of the layer produced when the adhesive 
cures was small, and the amount of warpage of the substrates 21 measured 
in terms of tilt angle after the two disc units were bonded together was 
1.5 mrad, which was small relative to the values for Comparisons 1, 2 and 
3 described later. Incidentally, a tilt angle is the angle of tilt of the 
normal to a surface of a disc with respect to the axis of rotation of the 
disc and can be a measure of the amount of warpage of the disc. 
Optical discs having a similar structure to the disc of Comparison 1 and 
comprising two disc units 20 bonded together by the above-described 
adhesive were placed in a thermo-hygrostat whose temperature and relative 
humidity (RH) were held at 60.degree. C. and 90%, respectively, for 100 
hours to test their humidity resistance, and the tilt angle was measured. 
The measured tilt angle was 1.6 mrad, which was only slightly different 
from the tilt angle measured before the test. Furthermore, this value was 
smaller than the tilt angles of Comparisons 1, 2 and 3, described later, 
measured immediately after the bonding. The optical discs of Example 1 
were also subjected to composite temperature/humidity cyclic tests 
according to JIS-C5024 Procedure I (in which the temperature was between 
-10.degree. C. and 65.degree. C. and thirty test cycles were performed). 
(As for the JIS-C5024, Procedure I composite temperature/humidity cyclic 
test, see IEC 68-2-38 Basic Environmental Testing Procedures, Part 2.) 
After the tests, the recording characteristics of the optical discs were 
measured. As indicated by a solid line a in FIG. 7, substantially no 
increase in bit error rate from the initial value was observed, which 
means that the optical discs of Example 1 are highly reliable. 
EXAMPLE 2 
The adhesive layer 25 of Example 2 comprises a mixture of the same 
bisphenol A epoxy base agent as in Example 1 (available as TB2022 from 
Three Bond Co., Ltd.) and a modified alicyclic polyamine curing agent in a 
ratio of 2:1. This adhesive has a glass transition temperature of 
90.degree. C. and cures at room temperature. 
The tilt angles of optical discs measured immediately after two disc units 
20 were bonded together with the adhesive of Example 2 was 1.4 mrad, which 
was small relative to that of the later-mentioned Comparisons 1-3. The 
tilt angles indicative of the amounts of warpage measured after the discs 
were subjected to humidity resistance tests was 1.8 mrad which was also 
smaller than the tilt angles of the discs of Comparisons 1-3 measured 
immediately after the bonding. Furthermore, the recording characteristics 
of the optical discs employing the adhesive of Example 2 measured after 
temperature/humidity cycle tests were also good. 
EXAMPLE 3 
The adhesive layer 25 used in this example is an acrylic ultraviolet-curing 
resin (commercially available, as TB3033, from Three Bond Co., Ltd.) which 
has a glass transition temperature of 80.degree. C. and cures at room 
temperature. 
The tilt angle of the disc measured immediately after two disc units were 
bonded together with the adhesive of Example 3 was 1.7 mrad, which was 
small relative to those of Comparisons 1-3. The tilt angle of the discs 
measured after humidity resistance tests was 1.9 mrad which was also 
smaller than those of Comparisons 1-3 measured immediately after the 
bonding. Further, the recording characteristics of the optical discs 
employing the adhesive of Example 3 measured after temperature/humidity 
cyclic tests were also good. 
EXAMPLE 4 
The adhesive forming the adhesive layer 25 of this example is a two-pack 
epoxy adhesive which comprises a base agent (TB2022C available from Three 
Bond Co., Ltd.) comprising a bisphenol A epoxy resin to which a reactive 
diluent having an epoxy group is added, and a modified aliphatic polyamine 
curing agent (TB2131D available also from Three Bond Co., Ltd.), mixed in 
a ratio of 3:1. The adhesive of Example 4 has a glass transition 
temperature of 60.degree. C. and cures at room temperature. 
The tilt angle of optical discs employing the adhesive of Example 4 
measured after two disc units were bonded was 2.0 mrad; This is as small 
as that of Comparison 1 which was the smallest in the three Comparisons. 
The amount of warpage of the optical discs of this Example slightly 
increased after they were subjected to humidity resistance tests, but such 
an increase was practically negligible. A-dash-and-dot line c in FIG. 7 
indicates the bit error rates measured for the optical discs of Example 4 
after respective cycles of temperature/humidity cyclic tests. As is seen, 
the bit error rate remained substantially the same as the initial value 
when the discs were subjected to twenty or less test cycles. This means 
that the optical discs of Example 4 have sufficient durability and 
reliability. The bit error rate begins to increase more or less when the 
discs are subjected to thirty cycles of the test, but is still practically 
negligible. 
In order to demonstrate the advantageous features of the adhesives used in 
accordance with the present invention, various characteristics of optical 
discs using conventionally used adhesives are described hereinafter. 
COMISON 1 
In Comparison 1, the adhesive used for forming an optical disc adhesive 
layer 25 is a hot-melt adhesive which has a glass transition temperature 
of 5.degree. C. and cures at room temperature. 
The tilt angle of discs of Comparison 1 measured after two disc units were 
bonded together was 2.0 mrad, which was small enough. However, the tilt 
angle of substrates 21 increased considerably to 21.5 mrad, after the 
discs were subjected to humidity resistance tests in which the discs were 
placed within a thermo-hygrostat of 60.degree. C. and 90% RH for 100 
hours. It is understood that the discs were not usable. The discs of 
Comparison 1 were also subjected to composite temperature/humidity cyclic 
tests according to JIS-C5024 Procedure I (temperature ranging from 
-10.degree. C. to 65.degree. C., the number of test cycles being 30 
cycles). As is shown in FIG. 7, by a broken line b, the bit error rates of 
the discs after the cycle tests were relatively high, and, in particular, 
when the discs were subjected to 20 or more cycles of tests, the bit error 
rate increased sharply, which means that the discs of Comparison 1 are not 
sufficiently reliable. 
COMISON 2 
The adhesive of the adhesive layer 25 of optical discs of Comparison 2 
comprises a mixture of a bisphenol A epoxy resin (Epicoat 828 commercially 
available from Shell Chemical, Co.), as a base agent, and a curing agent 
comprising polythiol. They are mixed in a ratio of 1:1. This adhesive has 
a glass transition temperature of 50.degree. C. and cures at room 
temperature. 
The tilt angle of the optical discs of Comparison 2 measured after two disc 
units were bonded together was 2.4 mrad, which was small and desirable. 
However, the tilt angle of substrates 21 increased to 17.2 mrad after the 
discs were subjected to humidity resistance tests similar to those 
conducted for Comparison 1, and the bit error rate rapidly increased after 
the discs were subjected to temperature/humidity cyclic tests. That is, 
the optical discs of Comparison 2 were neither durable nor reliable. 
COMISON 3 
The adhesive of the adhesive layer 25 of optical discs comprises a mixture 
of a base agent which is a bisphenol A epoxy resin (Epicoat 828) and an 
imidazole compound. They are mixed in a ratio of 100:2. This adhesive is 
heated to 80.degree. C. for curing. The glass transition temperature is 
130.degree. C. 
The tilt angle of the discs of Comparison 3 measured after two disc units 
were bonded together with the adhesive was 10.3 mrad, which is too large 
for practically usable optical discs. 
Data concerning Examples 1-4 of the present invention and Comparisons 1-3 
are shown in the following Table. 
TABLE 
__________________________________________________________________________ 
Tilt Angle After 
Glass Transition 
Tilt Angle After 
Subjecting To 60.degree. C., 
Recording Characteristics 
Curing 
Temperature (.degree.C.) 
Bonding (mrad) 
90% RH for 100 Hours 
After Temp./Hum. Cyclic 
Temperature 
__________________________________________________________________________ 
Example 1 
70 1.5 (good) 
1.6 (good) (good) room temperature 
Example 2 
90 1.4 (good) 
1.8 (good) (good) room temperature 
Example 3 
80 1.7 (good) 
1.9 (good) (good) room temperature 
Example 4 
60 2.0 (good) 
2.6 (rather good) 
(rather good) room temperature 
Comparison 1 
5 2.0 (good) 
21.5 
(bad) (bad) room temperature 
Comparison 2 
50 2.4 (good) 
17.2 
(bad) (bad) room temperature 
Comparison 3 
130 10.3 
(bad) 
-- -- 80.degree. C. 
__________________________________________________________________________ 
As is seen from this Table, all of the adhesives of the present invention 
forming the adhesive layers 25 disposed between two disc units of optical 
discs have a glass transition temperature (T.sub.g2) higher than the upper 
limit (60.degree. C.) of the usable environment temperature range T.sub.d 
and are curable at room temperature. Further, when they are used at a 
temperature within the usable environment temperature range T.sub.d, the 
warpage of the substrates 21 is small and has sufficient durability and 
reliability. 
The reasons why such good results can be obtained are considered to be as 
follows. As is shown by the, temperature-linear expansion coefficient 
characteristic in FIG. 6, the glass transition higher than the upper limit 
(60.degree. C.) of the usable environment temperature range (T.sub.d), 
and, therefore, the linear expansion coefficients of the substrates 21 and 
the adhesive layers 25 are substantially equal to each other in 
environments where the discs are normally used. Accordingly, even if large 
temperature changes occur, the adhesive layers are not distorted and, 
accordingly, neither warping of the discs nor peeling off of the recording 
layers 23 occurs. When an adhesive such as the one of Comparison 1 which 
has a glass transition temperature T.sub.g3 that is lower than the upper 
limit (60.degree. C.) of the usable environment temperature range T.sub.d 
is used, the linear expansion coefficient of the adhesive become greatly 
different from that of the substrates even in the temperature range 
T.sub.d, which may cause discs to warp or may cause the recording layers 
to be peeled off. 
Because water absorption of adhesives of resins is very high at 
temperatures above the glass transition temperature, the adhesives of 
Comparisons 1 and 2, whose glass transition temperatures are low, absorb 
moisture even within the temperature range T.sub.d. Water absorbed in the 
adhesives accelerates warping and corrosion of optical discs, which causes 
bit error rates to greatly increase. These are the reasons why the optical 
discs with the adhesives having a glass transition temperature higher than 
the upper limit (60.degree. C.) of the usable environment temperature 
range can produce good results. 
The adhesive of Comparison 3 has a high glass transition temperature of 
130.degree. C. However, since this adhesive needs to be heated to cure, 
the adhesive layer 25 is distorted while it is curing and, therefore, 
optical discs using this adhesive greatly warp. Thus, adhesives like the 
one of Comparison 3 are not suitable for use in optical discs. 
In some applications, adhesives may be chosen from the viewpoint of 
corrosion, warpage and the number of swells of discs caused by adhesives 
used, distribution of applied adhesives, etc. 
FIG. 8 shows corrosion of optical discs according to some embodiments of 
the present invention in comparison with corrosion of conventional optical 
discs. The percentage of corroded area to the total disc area is the 
ordinate. The values were measured after the respective discs were stored 
in an atmosphere the temperature and relative humidity of which were 
60.degree. C. and 90%, respectively. The discs were placed in the 
atmosphere for 1,000 hours. Discs A.sub.1 -A.sub.5 use Adhesives each 
comprising a mixture of a bisphenol A epoxy resin (TB2022 available from 
Three Bond Co., Ltd.) and modified aliphatic polyamine (TB2131D available 
from Three Bond Co., Ltd.), but the viscosities of the curing agents in 
the respective adhesives are different. (For instance, the viscosity of 
the curing agent of A.sub.4 is 110 CPS, and that of A.sub.5 is 2800 cps.) 
Discs B.sub.1 -B.sub.3 use adhesives each comprising a mixture of the 
bisphenol A epoxy resin and a modified polyamideamine (polyamide 
accelerated by polyamine) curing agent, but the viscosities of the curing 
agents are different. Discs C.sub.1 -C.sub.3 use adhesives each comprising 
a mixture of the bisphenol A epoxy resin and a polythiol curing agent, but 
the viscosities of the curing agents are different. Further, discs D.sub.1 
-D.sub.3 use different thermoplastic adhesives for optical disc use. 
FIG. 9 shows the percentage of corroded areas of respective optical discs 
in which adhesives comprising a bisphenol A epoxy resin (TB2022 available 
from Three Bond Co., Ltd.) and modified aliphatic polyamine (TB2131D 
available from Three Bond Co., Ltd.) are used. The viscosities of the 
modified aliphatic polyamine in the respective adhesives are different. 
The ordinate is the percentage of corroded area, and the abscissa is the 
viscosity of the modified aliphatic polyamine (in cps at 25.degree. C.). 
Except for the adhesives used, the optical discs shown in FIG. 9 have been 
made under similar conditions. As shown in FIGS. 8 and 9, the combinations 
of bisphenol A epoxy and modified aliphatic polyamine provide less 
corrosion, and those in which the viscosity of the curing agent is 100 cps 
(at 25.degree. C.) or less provide much less corrosion. The disc D.sub.1 
(FIG. 8) has a relatively low percentage of corroded area, but it tends to 
have a large number of swells, as will be described later, and, therefore, 
is not suitable for use as an optical disc. 
FIG. 10 shows the number of corroded regions in optical discs which use 
adhesives comprising a bisphenol A epoxy resin (TB2022 available from 
Three Bond Co., Ltd.) and modified aliphatic polyamine (TB2131D available 
from Three Bond Co., Ltd,.), in relation to water absorptions (%) of the 
adhesives after they cure. The adhesives used have different water 
absorptions after they have cured, which are determined after boiling them 
for one hour in water. The number of corroded regions were measured after 
storing the discs at 60.degree. C. and at 90% RH for 1000 hours. The 
number of corroded regions is the ordinate, while the water absorption in 
percent (%) is the abscissa. As shown in FIG. 10, discs with layers of 
adhesives of which the water absorption after curing is 0.2% or less have 
no corrosion. Thus, adhesives to be used for optical discs should have a 
water absorption after curing of 0.2% or less. 
FIG. 11 shows maximum tilt angles (solid line) and maximum increases in 
birefringence caused by bonding (broken line), of optical discs in which 
adhesives of different curing types are used. In FIG. 11, (A) is a disc 
which uses a room-temperature curing adhesive, such as one comprising a 
combination of a bisphenol A epoxy resin (TB2022 available from Three Bond 
Co., Ltd.) and modified aliphatic polyamine (TB2131D available from Three 
Bond Co., Ltd.), (X) is a disc using a conventional adhesive which cures 
when heated to 30.degree. C., (Y) is a disc using a conventional adhesive 
which cures when heated to 80.degree. C., and (Z) is a disc using a 
conventional ultraviolet curing adhesive. 
FIG. 12 shows maximum tilt angles (solid line) and maximum increases in 
birefringence caused by bonding (broken line), on the ordinate, in 
relation to cure shrinkages, on the abscissa, of the optical discs using 
adhesives of the above-stated room-temperature curing type. 
The data show that, whether the adhesive used is of heat curing type which 
cures at a temperature above 30.degree. C., of the ultraviolet curing 
type, or of the room temperature curing type, if the cure shrinkage of the 
adhesive is above 1.0%, distortion after curing is too large. This 
produces disc deformation and/or increase of warpage and birefringence. 
This means that the adhesives for bonding disc units together must be 
curable at room temperature and have a cure shrinkage of 1.0% or less. 
FIG. 13 shows the number of swells produced in optical discs with adhesives 
of various curing types used for bonding disc units when the discs are 
subjected to 30 cycles of composite temperature/humidity cyclic tests 
conducted according to JIS-C5024 Procedure I. In FIG. 13, the ordinate 
indicates the number of swells and the abscissa indicates optical discs 
which employ adhesives of various curing types for their adhesive layers. 
A.sub.1 -A.sub.4 represent discs using room-temperature curing epoxy 
resins which comprise, for example, a bisphenol A epoxy resin (TB2022 
available from Three Bond Co., Ltd.) and modified aliphatic polyamine 
(TB2131D available from Three Bond Co., Ltd.) and have a Shore hardness of 
85 (D scale). D.sub.1 -D.sub.4 represent discs using thermoplastic 
adhesives applied by means of a roller. It is seen from FIG. 13 that 
non-uniform distribution of roller-applied thermoplastic adhesives may 
permit moisture to penetrate through the substrates and stay at portions 
between the substrates and the adhesive layer, which causes swellings to 
be formed in the protective films and recording layers. Such swells may 
cause the protective films and/or recording layers to be peeled off or to 
be cracked. This shows thai the adhesive layers must be uniform and also 
have hardness of a given value or more. 
Next, adhesives of different hardness values were tested. FIG. 14 shows the 
number of swells (solid line) formed in optical discs and also drop 
strength (broken line) of the discs after they are subjected to 30 cycles 
of temperature-humidity cycle tests, in relation to the Shore hardness of 
the adhesives. The tested discs include layers of different hardness 
adhesives which essentially comprises a room-temperature curing expoxy 
resin comprising, for example, a bisphenol A epoxy resin (TB2022 available 
from Three Bond Co., Ltd.) and modified aliphatic polyamine (TB2131D 
available from Three Bond Co., Ltd.). The drop strength is expressed in 
comparison with the value, which is 100, of a disc using an adhesive 
having a Shore hardness of 85 (D scale). As shown in FIG. 14, the number 
of swells in optical discs steeply decreases substantially to 0 when the 
Shore hardness (D scale) is above 80. However, drop tests for determining 
the breaking strength of discs revealed that discs using adhesives having 
a Shore hardness of 90 or below have substantially the same drop strength, 
but when a Shore hardness of the adhesive used in discs is above 90, the 
discs are easily broken when dropped. Accordingly, it is necessary that 
the adhesives have a Shore hardness (D scale) of from 80 to 90. 
Curing-type adhesives essentially comprising a room-temperature curing 
epoxy resin which comprises, for example, a bisphenol A epoxy resin 
(TB2022 available from Three Bond Co., Ltd.) and modified aliphatic 
polyamine (TB2131D available from Three Bond Co., Ltd.), and having 
different viscosities, were applied over disc units with no pressure 
applied to the adhesives. FIG. 15 shows a ratio (%) of area of portions 
left uncoated to the total area of discs (solid line) or a ratio (t) of 
the length of protrusion of adhesives out of the peripheries of the discs 
(broken line), in relation to the viscosity of the adhesives. Along the 
abscissa, the viscosity of the adhesives when they are applied over disc 
units is indicated in cps at 25.degree. C. Along the ordinate, the 
percentage of the area of uncoated portions arid the percentage of the 
length of adhesive protrusions are indicated. From FIG. 15, it is seen 
that the viscosity of adhesives to be used for bonding disc units must be 
from 100 cps to 1000 cps in order to form uniform adhesive layers. In the 
bonding step, when the viscosity of adhesives upon application to disc 
units is above 1000 cps, the percentage of portions left uncoated 
increases rapidly. On the other hand, if the viscosity is less than 100 
cps, or if pressure is applied to adhesives so as to leave no uncoated 
portions, the adhesives will protrude from the inner periphery of the disc 
center hole or the outer periphery of the disc, and, therefore, the 
mechanical characteristics of discs are adversely affected, or an extra 
step for removing such protruding adhesive becomes necessary. 
Further, in order to form a uniform adhesive layer free of bubbles, the pot 
life of adhesives should be more than one hour, since mixing, deaeration 
or debubble, coating and bonding steps require at least one hour in total. 
Now, bearing in mind the above results, the present invention is described 
in greater detail by means of further examples. 
EXAMPLE 5 
A two-pack epoxy adhesive was prepared by mixing, at room temperature, a 
bisphenol A epoxy resin base agent (TB2022 available from Three Bond Co., 
Ltd.) having a viscosity of 13,000 cps (at 25.degree. C.) and a modified 
aliphatic polyamine curing agent (TB2131D available from Three Bond Co., 
Ltd.) having a viscosity of 10 cps, in a ratio of 3:1. The mixture was 
deaerated. The resultant adhesive had a vicosity of 400 cps (at 25.degree. 
C.), a pot life of 5 hours, a water absorption after curing of 0.1% (after 
boiling in water for one hour), a cure shrinkage of 0.05%, and a Shore 
hardness of 85 (D scale). 
As shown in FIG. 16, in a normal pressure environment, one of highly 
corrosion-resistant, Tb-Fe-Co disc units 201 having a diameter of, for 
example, 130 mm was placed horizontally on a support 30 secured to a 
center shaft 31 which extended from a base 29, with the center hole in the 
disc unit 201 being fitted over the shaft 31. 0.5 g of the adhesive of 
Example 5 was applied in a circle having a radius of 40 mm on the disc 
unit 201. This quantity of the adhesive was to provide a thickness of from 
20 to 70 microns of the adhesive layer when it cured. Then, the second 
disc unit 202 to be bonded to the first disc unit 201 was fitted over the 
center shaft 31, and only one point on the peripheral edge of the second 
disc unit 202 was brought into contact with one point on the periphery of 
the first disc unit 201. Thus, the second disc unit 202 was held slanting 
relative to the first unit 201. Preferably, an adhesive 32 is applied 
along the circumference of a circle having a radius of from 0.5a to 0.85a 
from the center of the disc unit 201, where a is the radius of the disc 
unit 201. In the example, the adhesive was applied along the circumference 
of a circle having a radius of about 0.6a from the center. 
A holding mechanism for the second disc unit 202 comprises a base 61 of 
which the slanting angle is adjusted by angle adjusting means 33, a shaft 
60 attached to the base 61, drive means 62 movable up and down along the 
shaft 60, holding means 63 held by the drive means 62, and an extension 64 
extending from the holding means 63. The slanting angle of the base 61 is 
adjusted so that the shaft 60 becomes substantially parallel with the line 
which connects points on the peripheral edges of the first and second disc 
units 201, 202 diametrically opposite to the aforementioned points which 
are in contact with each other. The tip end of the extension 64 is in 
engagement with the aforementioned diametrically opposite point on the 
outer periphery of the second disc 202 to hold the unit 202 slanted as 
shown. The drive mean& 62 is then lowered along the shaft 60 at a rate of, 
for example, about 1 mm/sec. so as to slowly place the second disc unit 
202 on the first disc unit 201. When the second disc unit 202 has been 
lowered, the tip end of the extension 64 is still in engagement with the 
second disc unit 202, although the engagement is slight. Then, the holding 
means 63 is actuated to retract the extension 64 to disengage from the 
second disc unit 202. Then, the weight of the disc unit 202 causes the 
adhesive to spread over the entire space between the two disc units 201 
and 202, while causing no oozing of the adhesive from the inner periphery 
of the center hole or from the outer periphery of the disc and also 
leaving no portions uncoated with the adhesive. 
The disc was left at room temperature for 24 hours to cure the adhesive. 
The maximum tilt angle of the disc measured was 0.9 mrad, and the maximum 
amount of increase of the birefringence was 2.1 nm. These values show that 
the disc is satisfactory. This disc was left in a 60.degree. C., 90% RH 
atmosphere for 3,000 hours to see how much the disc was corroded. FIG. 17 
shows the results. Because no corrosion was produced, the C/N 
(carrier/noise) ratio did not decrease, or the B.E.R. (bit error rate) did 
not increase. This means that the optical disc is highly reliable. 
Further, the disc was subjected to severe temperature/humidity cyclic tests 
to determine how the protective films and the recording layers stood up. 
Even after the disc was subjected to 30 cycles of the tests, no swell 
was-formed, neither the protective films nor the recording layers peeled 
off, and neither the C/N nor the B.E.R. changed. 
EXAMPLE 6 
The same process and the same apparatus as used in Example 5 were employed 
for making an optical disc, but, in place of the bisphenol A epoxy base 
agent (TB2022 of Three Bond Co., Ltd.) used in Example 5, a bisphenol F 
epoxy (TB2023 available from Three Bond Co., Ltd.) was used. Tests similar 
to the ones conducted for the optical disc of Examples 1-5 were conducted, 
and similar good results were obtained. 
As described above, according to the present invention, in order to bond 
together two disc units, each including a transparent substrate of 
synthetic resin and a recording layer, with the recording layers 
facing-each other, a layer of adhesive is disposed between the facing 
recording layers, which adhesive is a room-temperature curing two-pack 
epoxy adhesive comprising a base agent of bisphenol epoxy resin and a 
curing agent of modified aliphatic polyamine. The adhesive has a viscosity 
of 100-1000 cps, and a pot life of more than one hour. After curing, the 
adhesive layer has a cure shrinkage of 1.0% of less, a water absorption of 
0.2% or less, and a Shore hardness of 80-90 (D scale). The use of such an 
adhesive can avoid degradation of the recording layers, deformation of the 
resultant optical discs, and formation of swells. Thus, products to be 
rejected due to undesirable bonding are reduced, and highly reliable, high 
quality optical discs can be manufactured. 
According to an aspect of the present invention, as the curing agent of the 
above-described adhesive, modified aliphatic polyamine which has a 
viscosity of 100 cps or less is used. The use of such a curing agent can 
provide optical discs having, in addition to the above-described 
advantages, higher reliability. 
For manufacturing the optical disc of the present invention, an apparatus 
as shown in FIG. 18 may be used, as well. In FIG. 18, a base 29, a center 
shaft 31 for supporting disc units 201, 202, and a support 30 are similar 
to the corresponding components shown in FIG. 16, and the position on the 
disc unit 201 where the adhesive is to be applied is the same as in FIG. 
16. In the apparatus of FIG. 18, rotation means 42 is mounted on vertical 
drive means 41. An arm 44 pivots about a pivot 43 on the rotation means 
42. Holding means, such as a suction means 45 is attached to the tip end 
of the arm 44 to hold the disc unit 202 slanting as shown. 
Under a normal pressure condition, the first disc unit 201 having a 
diameter of, for example, 30 mm is placed horizontal on the support 30 
with the center shaft 31 extending through the center hole in the disc 
unit 201. Then, 0.5 g of the adhesive 32 is applied onto the disc unit 201 
along the circumference of a circle having a diameter of about 40 mm. The 
second disc unit 202 is then place in the apparatus, with the center shaft 
31 extending through the center hole of the disc unit 202. In this case, 
one point on the outer periphery of the disc unit 202 is brought into 
contact with the corresponding point of the first disc unit 201. The 
second disc unit 202 is held by the suction means 45 at the portion 
diametrically opposite to that one point. Thus, the second disc unit 202 
is held slanting as shown. In this case, the vertical position of the 
drive means 41 and the angle of the arm 44 are so adjusted that the arm 44 
is in parallel with the surface of the second disc unit 202. 
Then, the rotation means 42 is activated so as to cause the arm 44 to pivot 
in such a manner that the tip end of the arm 44 is lowered at a rate of, 
for example, 1 mm/sec. Thus, the second disc unit 202 is placed on the 
first disc unit 201. Seeing that the adhesive is spread over the entire 
space between the two disc units by the weight of the second disc unit 
202, the adhesive 32 is cured at room temperature. In place of the 
illustrated jack-type device, any other types of drive means 41 can be 
used.