Method and apparatus for forming substrate sheet for optical recording medium

A method for forming a substrate sheet for optical recording media having a preformat on the surface is disclosed. This method has the steps of extruding a melted resin to form a melted resin sheet, pressing the melted resin sheet between a mirror roll and a resin roll prior to the curing of the melted resin sheet, thereby forming a resin sheet, the resin roll being disposed in the face of the mirror roll and being covered on the peripheral surface thereof with a resin, and forming a photocurable resin composition layer on the surface of the resin sheet with which the resin roll has come in contact, and forming the preformat on the photocurable resin composition layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for forming a substrate for an 
optical recording medium on which optical recording and reproducing or 
playback are carried out by a laser beam or the like. 
2. Related Background Art 
Heretofore, an optical recording medium has been formed by providing a 
recording layer on a substrate transparent to light such as a 
semiconductor layer by which recording and playback of information are 
carried out, and then laminating a protective layer on the recording 
layer. On the surface of this substrate for the optical recording medium, 
there are engraved fine preformats of micron order or submicron order such 
as grooves for tracking and address information pits. As conventional 
methods for forming these preformats, for example, an injection molding 
method and a compression molding method are known. However, these methods 
are unsatisfactory in points of mass productivity and cost. 
In order to solve this problem, the present applicant has suggested, in 
EP-A-0 369 780 and EP-A-0 369 781, methods for preparing substrate sheets 
for optical recording media which comprises the steps of melting and 
extruding a resin to mold resin sheets, and then pressing the melted resin 
sheets 104 between a molding roll 102, having a preformat pattern on the 
surface thereof, and an oppositely disposed mirror roll 101 to transfer 
preformats thereto as shown in FIG. 3. 
According to the above-mentioned methods, the substrate sheets for optical 
recording media can be continuously formed which are excellent in 
transferability of the preformat and which controls double refraction 
which will be the cause of the decline in a C/N value, even when a resin 
such as polycarbonate which easily causes the double refraction is used. 
However, as a result of the investigations on the above-mentioned methods, 
it has been found that the double refraction and the transfer accuracy of 
the substrate sheet for optical recording media change owing to the 
increase or decrease of the amount of the extruded melted resin 
attributable to the slight alteration of conditions at the time of the 
molding, for example, the heating temperature of an extruder or a T-die. 
This increase or decrease of the amount of the extruded melted resin 
causes the change of the thickness of the melted resin sheet pressed 
between the molding roll 102 and the mirror roll 101. On the other hand, 
the molding roll 102 and the mirror roll 101 which can be used are each 
made usually from a steel roll plated with chromium, and therefore, it can 
be considered that when the thick melted resin sheet 104 is extruded from 
a T-die 105 and then pressed by the rolls, excessive pressure is applied 
to the resin sheet, with the result that the value of the double 
refraction and the transfer accuracy of the substrate sheet for optical 
recording media are uneven. 
In order to solve the above-mentioned problem, there is a technique for 
covering the surface of the mirror roll 101 with a resin. According to 
this technique, when the polycarbonate sheet coming from the T-die is 
molded by a resin roll and the molding roll, the resin is not forcedly 
pressed, so that the resultant substrate sheet for optical recording media 
is less strained in the process of the sheet's preparation. 
However, in the case that the resin roll is used, it is necessary to 
subject the surface of the resin roll to a mirror processing, but the 
resin roll is poorer in durability as compared with a metal roll. Thus, as 
molding is repeatedly carried out, the surface of the resin is damaged, 
and the surface of the roll must often be polished or the roll is required 
to be exchanged. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished to solve the above-mentioned 
problems, and an object of the present invention is to provide a method 
and an apparatus for forming a substrate sheet for optical recording media 
having small and uniform double refraction and the good transfer accuracy 
of a preformat pattern. 
The method for forming the substrate sheet for optical recording media of 
the present invention is a method for forming a substrate sheet for 
optical recording media having a preformat on the surface thereof which is 
characterized by comprising: 
a step of extruding a melted resin to form a melted resin sheet, 
a step of pressing the melted resin sheet between a mirror roll and a resin 
roll prior to the curing of the melted resin sheet, thereby forming a 
resin sheet, the resin roll being disposed in the face of the mirror roll 
and being covered on the peripheral surface thereof with a resin, and 
a step of forming a photocurable resin composition layer on the surface of 
the resin sheet with which the resin roll has come in contact, and forming 
the preformat on the photocurable resin composition layer. 
Furthermore, an apparatus for forming a substrate sheet for optical 
recording media of the present invention is an apparatus for forming a 
substrate sheet for optical recording media having a preformat on the 
surface which comprises: 
a means for extruding a melted resin to form a melted resin sheet, 
a mirror roll and a resin roll for pressing the melted resin sheet 
therebetween to form a resin sheet, the resin roll being disposed in the 
face of the mirror roll and being covered on the peripheral surface 
thereof with a resin, 
a molding roll disposed in the face of the first surface of the resin sheet 
with which the resin roll has come in contact, the molding roll being 
equipped with a pattern corresponding to the preformat and being rotated 
in a predetermined direction, 
a means for forming a photocurable resin composition layer on the first 
surface of the resin sheet or the peripheral surface of the molding roll, 
said means being disposed on the first surface of the resin sheet, 
a means for pressing the resin sheet against the molding roll with the 
interposition of the photocurable resin composition layer, 
a means for irradiating the photocurable resin composition layer with 
light, this means being disposed at a position apart from the press 
position of the resin sheet against the molding roll in a rotational 
direction of the molding roll, and 
a means for separating, from the molding roll, a photocured resin 
composition layer which has been integrated with the resin sheet by the 
light irradiation of the irradiation means and to which the preformat 
pattern has been transferred. 
That is, in the present invention, the step for extruding the melted resin 
sheet to mold the resin sheet is separated from the step for forming the 
preformat on the resin sheet, and in the molding step of the resin sheet, 
the resin roll having elasticity and heat insulating properties is used, 
whereby excessive pressure is not applied to the melted resin sheet and 
the resin sheet having less strain and less double refraction can be 
obtained. 
In the transfer step of the preformat, the photocurable resin is used, and 
therefore the transfer of the preformat can be achieved with a high 
accuracy. In addition, the photocurable resin composition layer is formed 
on the resin roll contact side of the resin sheet, and hence it is not 
necessary to process the surface of the resin roll into a mirror surface, 
which makes the roll processing easy. Moreover, since the resin roll is 
not required to be often exchanged, and productivity is improved.

DETAILED DESCRIPTION OF THE INVENTION 
Next, the present invention will be described in detail in reference to 
drawings. 
FIG. 1 is an explanatory view showing one embodiment of a method for 
forming a substrate for optical recording media of the present invention. 
In the same drawing, reference numeral 1 is an extruder, numeral 2 is a 
T-die, numeral 3 is a resin roll whose peripheral surface is covered with 
a resin, and reference numerals 4 and 5 are a first mirror roll and a 
second mirror roll, respectively. Reference numeral 6 is a molding roll 
equipped on the peripheral surface thereof with a preformat pattern 
corresponding to prepits and/or pregrooves, numeral 7 is a feed roll for 
forming a photocurable resin composition layer 12 on the molding roll 6, 
and numeral 8 is a photocurable resin composition. Furthermore, reference 
numeral 9 is a light irradiation means for curing the photocurable resin 
composition layer 12, numeral 10 is a press roll for pressing a resin 
sheet 15 against the molding roll 6 with the interposition of the 
photocurable resin composition layer 12, and numeral 11 is a separator 
roll for separating, from the molding roll, a substrate sheet 16 for 
optical recording media which has been obtained by curing the photocurable 
resin composition layer with the light irradiation means 9 to integrate 
the photocurable resin composition layer with the resin sheet 15 and to 
which the preformat pattern has been transferred. 
Next, a method for forming a polycarbonate substrate sheet for optical 
recording media by the use of the above-mentioned apparatus will be 
described in reference to FIG. 1. 
In FIG. 1, pellets of the polycarbonate are first thrown into the hopper 11 
of the extruder 1. The thrown pellets are heated and melted in the 
extruder 1, and the melted resin is then extruded into the shape of sheet 
by the T-die 2 connected to the extruder 1. The melted resin sheet 14 
which has been extruded by the T-die 2 is introduced into a gap between 
the resin roll 3 and the second mirror roll 4 and then pressed. At this 
time, the resin roll is used to relieve the pressure to some extent which 
has been applied to the melted polycarbonate resin sheet 14, and the 
strain of the polycarbonate can be reduced. That is, the polycarbonate 
sheet extruded through the T-die has uneven thickness, and the pressure 
which is applied to the thick portion is high and the strain which is 
applied thereto is also high. However, when the above-mentioned resin roll 
is used, such an unevenness of the thickness can be absorbed to some 
extent by the elasticity of the resin, whereby the pressure which is 
applied to the polycarbonate can be made constant. As a result, the 
polycarbonate resin sheet 15 having less double refraction and the uniform 
thickness can be molded. Then, the thus obtained resin sheet 15 is 
delivered in the direction of an arrow A. 
On the other hand, the molding roll 6 is rotated in the direction of an 
arrow B, and on the peripheral surface of this molding roll 6, the 
photocurable resin composition layer 12 is continuously formed by the feed 
roll 7. 
Next, the resin sheet 15 is pressed against the molding roll 6 by the 
pressing action of the press roll 10, with the interposition of the 
photocurable resin composition layer 12 therebetween and the photocurable 
resin composition layer 12 is then cured by light from the light 
irradiation means 9 disposed at a position apart from a press position 17 
in the rotational direction B of the molding roll so as not to irradiate 
the press position 17 of the resin sheet 15 against the molding roll 6, so 
that the resin composition layer 12 is integrated with the resin sheet 15 
and a preformat pattern is transferred to the resin composition layer 12, 
thereby obtaining a substrate sheet 16 for optical recording media which 
has the preformat 13 on the surface thereof. 
In the present invention, when the resin sheet 15 is pressed against the 
molding roll 6, it is preferred that the surface (hereinafter referred to 
as "first surface") of the resin sheet 15 which had been in contact with 
the resin roll 3 is brought into contact with the molding roll 6. That is, 
the surface of the resin roll 3 generally tends to be damaged and it is 
difficult to maintain the first surface of the resin sheet 15 in a mirror 
state, but since the first surface is pressed by the molding roll 6, the 
photocured resin composition layer 12 is brought into contact with the 
first surface of the resin sheet 15. Simultaneously, the preformat pattern 
on the peripheral surface of the molding roll 6 is transferred to the 
surface of the photocured resin composition layer 12. Thus, the surface 
state on the first surface of the resin sheet 15 is not influential any 
more, and as a result, it is unnecessary to accurately control the surface 
state of the resin roll 3. 
Next, some examples of the photocurable resin composition which can be used 
in the present invention are ultraviolet rays curable resin compositions 
and electron beam curable resin compositions. Examples of the ultraviolet 
rays curable resin compositions include usual ultraviolet rays curable 
resin compositions each comprising a photopolymerizable oligomer, a 
photopolymerizable monomer, a photopolymerization initiator and the like. 
Examples of the photopolymerizable oligomer include acrylates such as 
unsaturated polyesters, epoxyacrylates, urethane acrylates and polyether 
acrylates. Examples of the photopolymerizable monomer include 
mono-functional monomers such as lauryl acrylate, 2-ethylhexyl acrylate, 
1,6-hexanediol monoacrylate and dicyclopentadiene acrylate, and 
polyfunctional monomers such as dicyclopentenyl acrylate, 1,3-butanediol 
diacrylate, polyethylene glycol diacrylate, pentaerythritol triacrylate, 
dicyclopentenyloxyethyl acrylate, 1,4-butanediol diacrylate, 
1,6-hexanediol diacrylate, diethylene glycol diacrylate, neopentyl glycol 
diacrylate, hydroxypivalic acid ester neopentyl glycol diacrylate, 
tripropylene glycol diacrylate, 
1,3-bis(3'-acryloxyethoxy-2'-hydroxypropyl)-5,5-dimethylhydantoin, 
diacrylates of hydroxypivalic acid ester neopentyl glycol derivatives and 
trimethylolpropane triacrylate. Furthermore, usable examples of the 
photopolymerization initiator include benzophenone, benzoin and their 
derivatives, benzoin ether and benzyldimethylketal. 
The photocurable resin composition is preferably regulated so that the 
refractive index of the resin composition layer cured by the light 
irradiation may be substantially equal to that of the resin sheet. That 
is, when the cured film of the photocurable resin composition layer 12 
having about the same refractive index as in the resin sheet 15 is formed 
on the first surface of the resin sheet 15, refraction and/or irregular 
reflection of the interface between both the layers can be controlled 
sufficiently, even if the first surface of the resin sheet 15 is rough. 
When this kind of substrate sheet is used, the excellent optical recording 
media which can reproduce signals having a high S/N ratio can be prepared. 
Incidentally, the above-mentioned "substantially equal" in the present 
invention means that the refractive index of the resin sheet material is 
in the range of an error of .+-.0.02, particularly in the range of an 
error of .+-.0.01. 
Furthermore, the refractive index of the photocurable resin composition 
depends upon the total values of the refractive indexes of the respective 
materials of the composition, for example, the photopolymerizable oligomer 
and the photopolymerizable monomer, and therefore the photocurable resin 
composition having the desired refractive index can be obtained by 
suitably combining the materials of the resin sheet. 
Next, in the case that a polycarbonate (refractive index 1.59) is used as 
the resin sheet, a bifunctional or a trifunctional oligomer is preferable 
as the ultraviolet rays curable resin composition suitably usable in the 
present invention which has a substantially equal refractive index and 
which does not warp the substrate sheet 16 for optical recording media. 
Moreover, a preferable example of the photopolymerizable monomer is a 
bifunctional or a trifunctional monomer of the above-mentioned monomers. 
From these oligomers and monomers, what have a refractive index of about 
1.59 to 1.70 and a refractive index of about 1.45 to 1.59 are suitably 
selected and mixed. 
In the present invention, when the oligomer of n.sub.D.sup..degree. =1.605 
represented by the undermentioned formula (I) is mixed with the monomer 
(1,6-hexanediol diacrylate) of n.sub.D.sup.20 =1.458 represented by the 
undermentioned formula (II) in a ratio of 9:1, the resultant composition 
can have about the same refractive index as in the polycarbonate. In 
addition, this composition prevents an organic dye from having a bad 
influence on the recording layer, if the organic dye is contained in the 
recording layer, and it also prevents the substrate sheet 16 for optical 
recording media from warping. Moreover, the above-mentioned composition 
improves the adhesive properties between the photocured resin composition 
layer and the resin sheet, and therefore it is a composition which can be 
particularly suitably used in the present invention. 
##STR1## 
wherein m+n=2 to 10 
EQU CH.sub.2 .dbd.CHCOO(CH.sub.2).sub.6 OCOCH.dbd.CH.sub.2. Formula (II) 
Next, a resin for the resin layer 31 of the resin roll 3 in the present 
invention is required to be heat-resistant. The roll itself will be heated 
up to a temperature of 120.degree. to 150.degree. C. and will be then 
brought into contact with the polycarbonate at about 280.degree. C., and 
therefore the resin preferably has heat resistance which can withstand at 
least 200.degree. C., particularly 220.degree. C. or more. 
Here, the heat resistance means that the resin layer does not melt, deform, 
decompose or bring about other mechanical physical changes at 200.degree. 
C. for at least 2 hours, preferably 50 hours, between a roll base and a 
stamper at the time of the molding of the substrate sheet. 
When the resin layer 31 is formed on the peripheral surface of the roll 
base 32 constituting the resin roll 3, it is preferable to use a resin 
having a tensile elongation of 200% or less, preferably 180% or less, more 
preferably 150% or less, and it is more preferable to use a resin having a 
tensile elasticity modulus of 200 to 1000 kg/mm.sup.2 in accordance with 
ASTM D-882. 
As such a resin, any kind of resin can be used, so long as it meets the 
above-mentioned requirements. When there is used a polyimide, a 
fluororesin, a polyether-ether ketone (PEEK), a polyether sulfone (PES), a 
polyparabanic acid resin, a polyphenylene oxide, a polyphenylene sulfide 
(PPS), a polyarylate resin, an epoxy resin or a silicone resin, the double 
refraction of the resin sheet can be decreased In particular, the 
polyimide, PEEK, PES, the polyparabanic acid resin, the polyphenylene 
oxide, PPS, the polyarylate resin or the fluororesin can control the 
influence of the fluctuation of the molding conditions on the substrate 
sheet. 
The above-mentioned resins will be described in more detail. An example of 
the polyimide resin is a resin represented by the formula 
##STR2## 
which can be prepared from an aromatic dicarboxylic anhydride such as 
pyromellitic anhydride and an aromatic diamine such as diaminodiphenyl 
ether. Examples of the fluororesin include homopolymers such as 
polytetrafluoroethylene (PTFE), polymonochlorotrifluoroethylene (PCTFE), 
polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF) as well as 
copolymers such as tetrafluoroethylene perfluoroalkylvinyl ether copolymer 
(PFA) and fluorinated ethylenepropylene copolymer (FEP). 
PEEK is a polymer represented by the formula 
##STR3## 
PES is a polymer represented by the formula 
##STR4## 
Examples of the epoxy resin are products having a molecular weight of about 
5000 or more, preferably 8000 or more which can be obtained from bisphenol 
A and epichlorohydrin and from an alicyclic compound and derived from 
polybutadiene. 
The polyparabanic acid resin (PPA) is a polymer represented by the formula 
##STR5## 
(wherein R is 
##STR6## 
The polyphenylene sulfide (PPS) is a polymer represented by the formula 
##STR7## 
The polyarylate (PA) is an aromatic polyester resin which can be made by 
the polycondensation reaction of an aromatic dicarboxylic acid with a 
bisphenol, and when the bisphenol is, for example, bisphenol A, the 
polyarylate has a structure represented by the formula 
##STR8## 
Furthermore, the thickness of the resin layer 31 is preferably from 0.1 to 
15 mm, particularly 0.3 to 5 mm, since the double refraction of the resin 
sheet can be uniformly decreased and its thickness can also be 
uniformized. 
The molding roll 6 is formed on the peripheral surface thereof with a 
preformat pattern corresponding to the preformat 13 on the surface of the 
substrate sheet 16 for optical recording media, and this kind of molding 
roll 6 can be prepared by attaching a plate stamper made by a usual method 
onto the surface of the roll with an adhesive, by mechanically securing 
the stamper on the roll surface by screws, by directly engraving on the 
mirror roll, or by another technique. 
The preformat pattern which can be used in the present invention is a 
spiral pattern, a concentric circle-like pattern and a stripe pattern 
corresponding to tracking grooves for optical discs and optical cards, 
each pattern having a width of 1 to 4 .mu.m, a pitch of 1 to 20 .mu.m and 
a depth of 200 to 5000.degree. as well as a pattern corresponding to 
address pits, this pattern having a width of 0.6 to 10 .mu.m, a length of 
0.6 to 20 .mu.m and a depth of 200 to 5000.ANG.. 
Next, the melted resin sheet 14 extruded from a T-die 2 in the molding step 
of the resin sheet 15 of the present invention is preferably pressed 
between the resin roll 3 and the mirror roll 4, while the sheet 14 is 
extremely close to a melting state. Accordingly, the temperature of the 
T-die preferably is as high as possible, so long as the resin does not 
decompose, and it is preferably in the range of the glass transition 
temperature (hereinafter abbreviated to "Tg") of the molded 
resin+110.degree. C. to Tg+200.degree. C., particularly Tg+130.degree. C. 
to Tg+190.degree. C. For example, in the case of the polycarbonate resin, 
the temperature of the T-die is 260.degree. to 340.degree. C., preferably 
280.degree. to 330.degree. C., more preferably 290.degree. to 320.degree. 
C. 
If the resin sheet is cooled between the T-die 2 and the molding roll, the 
preformat pattern is not sufficiently transferred and the double 
refraction tends to occur. Therefore, the distance between the T-die 2 and 
the pressing point of the molding roll 3 and the first mirror roll 4 is 20 
cm or less, preferably 15 cm or less, more preferably 10 cm or less, and 
the temperature of an ambient atmosphere therebetween preferably is 
60.degree. C. or more. 
In order to precisely extrude the resin sheet toward the pressing point, 
the constitution of vertical extrusion is preferably taken as shown in 
FIGS. 1 and 2 in which the pressing point is disposed vertically under the 
T-die. This reason is that the resin is nearly in the melting state and so 
the vertical extrusion permits extruding the resin toward the pressing 
point more accurately than horizontal extrusion. 
Surface temperatures of the resin roll 3 and the mirror rolls 4, 5 depend 
upon the kind of resin to be used, but for example, in the case that the 
polycarbonate is used, the temperature of the mirror roll 4 is usually set 
to 110.degree.-145.degree. C. considering the heat deformation temperature 
of the polycarbonate, that of the resin roll is set to 
90.degree.-135.degree. C., and that of the mirror roll 5 is set to 
120.degree.-150.degree. C. In the case that amorphous polyolefin is used, 
the temperature of the mirror roll 4 is set to 120.degree.-145.degree. C., 
that of the resin roll 3 is set to 100.degree.-135.degree. C., and that of 
the mirror roll 5 is set to 120.degree.-150.degree. C. The temperatures of 
these rolls can be controlled, for example, by heating them with a heater 
incorporated in the rolls or by circulating a hot medium to the central 
portions of these rolls. 
A resin which can be used as the material for the resin sheet 15 in the 
present invention is preferably a thermoplastic resin having high 
transmission to light for record and reproduction Examples of such a resin 
include acrylic resins, polyester resins, polycarbonate resins, vinyl 
resins, polysulfone resins, polyolefin resins and cellulose derivatives. 
In the present invention, a base 32 of the resin roll and bases of the 
mirror rolls 4, 5 and the molding roll preferably have high hardness, good 
thermal conductivity and easy mirror surface workability on peripheral 
surfaces. For example, steel, chromium steel, aluminum or steel (maraging 
steel) for molds can be used as the base materials. 
FIG. 2 shows another embodiment of the preparation method of the substrate 
sheet for optical recording media regarding the present invention, and the 
steps until the molding of the resin sheet 15 are identical with those of 
the embodiment in FIG. 1. Afterward, the photocurable resin composition 
layer 12 is formed on the first surface of this resin sheet 15 by the use 
of a coating roll 7 for the photocurable resin composition 8, and the 
resin sheet 15 is pressed against the molding roll 6 with the 
interposition of the photocurable resin composition layer 12 by means of 
the press roll 10. Then, the photocurable resin composition layer 12 is 
then cured by light from the light irradiation means 9 disposed at a 
position apart from a press position of the resin sheet 15 against the 
molding roll 6 in the rotational direction B of the molding roll 6, so 
that the resin composition layer is integrated with the resin sheet and a 
preformat pattern is transferred to the resin composition layer, thereby 
obtaining the substrate sheet 16 for optical recording media which has the 
preformat 13 on the surface thereof. 
The thus obtained substrate sheet 16 for optical recording media is wound 
up and then delivered to a recording layer and/or refractive layer forming 
step, a protective layer forming step or a step of forming a hard coat 
layer on the incident surface of recording and/or reproduction light on 
the substrate sheet 16 for optical recording media. Alternatively, after 
the molding of the substrate sheet 16 for optical recording media, the 
above-mentioned respective steps may be continuously carried out. 
As materials of the recording layer which is formed on the surface of the 
substrate sheet for optical recording media on which the preformat has 
been transferred, there can be used oxides of Te, Sb, Mo, Ge, V and Sn; 
inorganic compounds such as Sn and TeOx--Ge; composites of metals and 
organic compounds or inorganic sulfides such as Te--CH.sub.4, 
Te--CS.sub.2, Te-styrene, Sn--SO.sub.2, GeS--Sn and SnS--S; thermoplastic 
resins such as nitrocellulose, polystyrene and polyethylene containing the 
dispersed particles of a metal such as silver; chalcogen elements; 
magnetic films of Tb--Fe--Co, Gd--Fe--Co, Tb--Fe--Co--Cr, Gd--Fe--Co--Cr 
and the like; and organic dyestuffs. 
In the case that the organic dyestuff is used as the recording layer and 
the wave length of the energy beam of light such reproduction light is 650 
nm or more, particularly 700 to 900 nm, it is preferable that a difference 
between the reflectance of pits which are recorded portions and that of 
unrecorded portion is large. In addition, in order to achieve the 
recording, it is necessary that absorption is present in the 
above-mentioned wave length section. Moreover, energy necessary to cause 
the change of the reflectance by the irradiation of the energy beam is 
preferably small. It is also preferable that the reflectances of the 
recorded portions (pits and the like) and the unrecorded portions scarcely 
change under the influence of the energy beam of the reproduction light. 
Examples of such an organic dyestuff include anthraquinone derivatives (in 
particular, compounds having an indanthrene skeleton), dioxazine compounds 
and their derivatives, triphenodithiazine compounds, phenanthrene 
derivatives, cyanin compounds, merocyanin compounds, pyrylium series 
compounds, xanthene series compounds, triphenylmethane series compounds, 
croconium series compounds, azo dyestuffs, crocons, azines, indigoids, 
polymethine dyes, azulenes, squalium derivatives, sulfur dyes and 
dithiolate complexes of metals. 
The above-mentioned dye may be mixed with a stabilizer. Examples of the 
stabilizer include metal chelate compounds, multidentate ligands 
containing Zn, Cu, Ni, Cr, Co, Mn, Pd and Zr as the central metals, for 
example, tetradentate ligands such as N.sub.4, N.sub.2 O.sub.2, N.sub.2 
S.sub.2 S.sub.4, O.sub.2 S.sub.2 and O.sub.4 and combinations thereof, 
aromatic amines and diamines, nitrogen-containing aromatic compounds and 
onium salts thereof, for example, aminium salts, diimonium salts, 
pyridinium salts, imidazolinium salts and quinolinium salts, pyrylium 
salts which are the salts of oxygen-containing aromatic compounds, and 
mixture thereof. 
The desirable stabilizer should be selected from the above-mentioned group 
considering the compatibility with a solvent which is used together with 
the organic dye The amount of the stabilizer is preferably 1 to 50% by 
weight based on the organic dye, and in particular, when the amount of the 
stabilizer is from 10 to 30% by weight, the deterioration of sensitivity 
is inhibited and the large effect of the stabilizer can be obtained. 
The solvent which is used to dissolve the above-mentioned organic dye and 
stabilizer therein must not affect the resin sheet, and examples of the 
solvent include diacetone alcohol, cellosolve, 1-methoxy-2-propanol and 
mixed solvents of the same and halogen series solvents. 
Furthermore, the coating of the recording layer can be achieved by a 
technique such as gravure coating, curtain coating, spray coating, dip 
coating, bar coating and blade coating. 
In the present invention, the thickness of the recording layer is 
preferably in the range of 500 to 5000.ANG.. 
As described above, according to the method for forming the substrate for 
optical recording media of the present invention, the molding step of the 
resin sheet is separated from the transfer step of the preformat pattern. 
First, in the molding step for the resin sheet, the elastic resin roll is 
used, and therefore the resin sheet is not forcedly pressed, so that the 
substrate sheet having less strain and less double refraction can be 
obtained. 
In addition, in the transfer step of the preformat pattern, the 
photocurable resin composition layer 12 is formed on the resin roll 
contact surface of the resin sheet 15, and so it is not necessary to 
process the surface of the resin roll into a mirror surface, which makes 
the roll processing easy. Moreover, the resin roll is not required to be 
often exchanged, and productivity is improved. 
In the case that either surface (the surface on the side of the photocured 
resin layer) of the resin sheet substrate is rough, the adhesive 
properties between the resin sheet 15 and the photocured resin composition 
layer 12 are further improved 
The refractive index of the photocured resin composition layer can be 
adjusted to be substantially equal to that of the resin sheet 15, whereby 
the substrate for optical recording media which can reproduce signals 
having less noise and a high S/N ratio can be prepared. 
EXAMPLE 1 
Chromium plating having a thickness of 0.3 mm was deposited on the 
peripheral surface of an iron roll having a diameter of 300 mm and a width 
of 200 mm, and the surface of the chromium plating was then 
mirror-finished to a surface roughness of 0.1 .mu.m. Afterward, a silicone 
resin layer (trade name: SE 1188; Toray Silicon Co., Ltd.) having a 
thickness of 2 mm was formed on the peripheral surface of the chromium 
plating to obtain a resin roll. On the other hand, chromium plating having 
a thickness of 0.3 mm was deposited on the peripheral surface of an iron 
roll having a diameter of 300 mm and a width of 200 mm, and the surface of 
the chromium plating was then mirror-finished to a surface roughness of 
0.1 .mu.m, thereby obtaining mirror rolls 4 and 5. 
An Ni stamper having a pattern corresponding to a preformat for optical 
cards having a groove width of 2.5 .mu.m, a pitch of 12 .mu.m and a groove 
depth of 3000.ANG. was stuck on the peripheral surface of the mirror roll 
having a diameter of 400 mm and a width of 150 mm by the use of an epoxy 
adhesive (trade name EP-170, made by Cemedine Co., Ltd.), and an apparatus 
for forming a substrate sheet for optical cards shown in FIG. 1 was then 
prepared. As a resin sheet material, polycarbonate (trade name Panlite 
L-1250; made by Teijin Chemicals Co., Ltd.) was used, and a resin sheet 
was extruded under conditions of an extrusion width of 200 mm, a roll gap 
of 0.4 mm and an extrusion speed of 3 m/minute. With regard to the molding 
conditions, the surface temperature of the resin roll 3 was 120.degree. 
C., the surface temperature of the mirror roll 4 was 130.degree. C., and 
the temperature of a T-die was 300.degree. C. 
On the other hand, an ultraviolet curable resin composition layer having 
the following composition and a thickness of 15 .mu.m was formed on the 
peripheral surface of the molding roll 6 by the use of a feed roll 7. 
Resin Composition 
(a) 87.3 parts by weight of a photopolymerizable oligomer represented by 
the formula 
##STR9## 
wherein m+n=2 to 10. (trade name NK Ester ABPE-4, made by Shin-Nakamura 
Chemicals, Inc.) 
(b) 9.7 parts by weight of a photopolymerizable monomer represented by the 
formula 
EQU CH.sub.2 =CHCOO(CH.sub.2).sub.6 OCOCH.dbd.CH.sub.2 
(trade name NK Ester A-HD, made by Shin-Nakamura Chemicals, Inc.) 
(c) 3 parts by weight of a photopolymerization initiator represented by the 
formula 
##STR10## 
(trade name Irgacure 184, made by Ciba-Geigy). 
The above-mentioned ultraviolet curable resin composition was regulated so 
that the refractive index of the cured film of this resin composition 
might be the that of the polycarbonate, i.e., 1.59. 
Next, the extruded polycarbonate resin sheet 15 was mounted on the molding 
roll 6 with the interposition of the ultraviolet cured resin composition 
layer 12. At this time, the surface of the polycarbonate resin sheet which 
had came in contact with the resin roll at the time of the previous 
extrusion step was brought into contact with the molding roll with the 
interposition of the ultraviolet curable resin composition layer 12. 
Next, after the polycarbonate resin sheet 15 was mounted on the molding 
roll, the ultraviolet curable resin composition layer 12 was irradiated 
through the resin sheet with ultraviolet rays from a position 15 cm above 
the resin sheet by the use of an ultraviolet lamp (a 4 KW high-pressure 
mercury vapor lamp) to cure the ultraviolet curable resin composition 
layer. Then, the polycarbonate resin sheet which had been integrated with 
the ultraviolet cured resin composition layer was released from the 
molding roll, thereby preparing a substrate sheet for optical cards in 
which a relief pattern was transferred to the resin sheet. 
For the thus obtained substrate sheet for the optical cards, double 
refraction was measured at optional 9 points in each preformat-transferred 
portion. As a result, average values of the double refraction in all the 
portions were 20 nm or less. The double refraction was measured with a 
laser beam having a wave length of 830 nm and a spot diameter of 1 .mu.m 
by the use of a polarimeter (SP-224 type; made by Sinko Seiki Co., Ltd.), 
and the double refraction was the value of single pass. The obtained 
substrate was optically homogeneous, and it was excellent in the 
transferability of the relief pattern. 
Next, the surface of the substrate sheet for optical cards on which the 
preformat was formed was coated with a 2% diacetone alcohol solution of 
1,1,5,5-tetrakis(p-diethylaminophenyl)-1,3-pentadienyl perchlorate which 
was a polymethine series dye, thereby forming a recording layer having a 
thickness of 1000.ANG.. Afterward, the polycarbonate sheet having a 
thickness of 0.3 mm was laminated on the recording layer with the 
interposition of an ethylene-vinyl acetate copolymer hot-melt adhesive 
sheet, and the resultant laminate was then passed between a pair of 
heating rolls having a surface temperature of 110.degree. C. to thermally 
press the same. Then, the sheet was cut into a size (length 85 
mm.times.width 54 mm) of the optical cards to obtain the optical cards. 
The thus obtained optical cards were moved at a speed of 400 mm/second by 
the use of an optical card recording/reproducing device (made by Canon 
Inc.), and a signal at a frequency of 100 KHz under a recording power of 
10 mW was recorded by the use of a semiconductor laser having a wave 
length of 830 nm. Next, the signal was reproduced under a semiconductor 
laser power of 0.2 mW. The noise level of the reproduced signal and the 
level of a carrier signal are shown in Table 1. 
The incidence of the recording light and the reproducing light to the 
optical cards was carried out through the substrates. 
COMATIVE EXAMPLE 1 
The same procedure as in Example 1 was effected except that an ultraviolet 
curable resin composition layer was formed on the surface of a resin sheet 
15 with which a mirror roll 4 had been come in contact, thereby obtaining 
a substrate sheet for optical cards The double refraction of this 
substrate sheet was measured in the same manner as in Example 1, and as a 
result, it was 20 nm or less. 
Next, optical cards were prepared from this substrate sheet for optical 
cards in the same manner as in Example 1, and recording and reproduction 
were then carried out. 
The results are set forth in Table 1. 
TABLE 1 
______________________________________ 
Comp. 
Example 1 Ex. 1 
______________________________________ 
C/N Value 51.0 dB 43.2 dB 
Carrier Level 
-25.9 -19.2 
Noise Level -76.9 -62.4 
______________________________________ 
EXAMPLE 2 
In the resin sheet molding step of Example 1, a heat shrinkage tube (made 
by Gunze Limited) of perfluoroalkylvinyl ether copolymer having a 
thickness of 0.5 mm was used in place of a silicone resin as a resin roll 
3. A resin sheet was molded under conditions of the temperature of the 
resin roll=140.degree. C., the temperature of a mirror roll 4=150.degree. 
C. (a temperature of a T-die =300.degree. C.), a roll gap=0.4 mm and an 
extrusion speed=4 m/minute. 
A molding roll was prepared in the same manner as in Example 1 which had a 
diameter of 400 mm and a width of 200 mm. 
The peripheral surface of this molding roll 6 was coated with an 
ultraviolet curable resin composition layer comprising the following 
composition and having a thickness of 10 .mu.m. 
Resin Composition 
(a) 74.3 parts by weight of a photopolymerizable oligomer represented by 
the formula 
##STR11## 
(wherein m+n=2 to 10). (trade name NK Ester ABPE-4, made by Shin-Nakamura 
Chemicals, Inc.). 
(b) 22.7 parts by weight of a photopolymerizable monomer represented by the 
formula 
##STR12## 
(wherein m+n=4). (trade name Kayarad R-712, made by Nippon Kayaku Co., 
Ltd.). 
(c) 3 parts by weight of a photopolymerization initiator represented by the 
formula 
##STR13## 
(trade name Irgacure 184, made by Ciba-Geigy). 
The above-mentioned ultraviolet curable resin composition was also 
regulated so that the refractive index of the cured film of this resin 
composition might be about 1.59. 
The subsequent steps were carried out by the same procedure as in Example 1 
to obtain a substrate sheet for optical cards. 
The double refraction of this substrate sheet was measured in the same 
manner as in Example 1, and as a result, it was 10 nm or less. The 
obtained substrate optically homogeneous, and it was excellent in the 
transferability of the relief pattern. 
Next, the surface of the substrate for optical cards which had been formed 
with a preformat pattern was coated with a 1.6% by weight dichloroethane 
solution of an azulene series dye having the structure 
##STR14## 
to form a recording layer having a thickness of 1000.ANG., and an optical 
card was prepared in the same manner as in Example 1. This card was 
evaluated in the same manner as in Example 1. The results are set forth in 
Table 2. 
TABLE 2 
______________________________________ 
Example 2 
______________________________________ 
C/N Value 51.9 dB 
Carrier Level -27.7 
Noise Level -79.6 
______________________________________ 
EXAMPLE 3 
The same procedure as in Example 1 was effected except that a resin layer 
31 on the peripheral surface of a resin roll was replaced with a polyimide 
resin layer (trade name LARC-TPI; made by Mitsui Toatsu Chemicals, Inc.) 
having a thickness of 3 mm, thereby preparing a substrate sheet for 
optical cards. The thus prepared substrate sheet was evaluated in the same 
manner as in Example 1, and as a result, the double refraction of the 
substrate sheet was 10 nm or less. 
Furthermore, a recording layer of the same azulene series dye as in Example 
2 was formed on this substrate sheet for optical cards by the same 
procedure as in Example 2 to prepare optical cards. Evaluation was then 
made, and at this time, a signal having less noise and an excellent C/N 
value could be reproduced as in Example 2.