Optical disk

An optical disk is provided with a protective layer which is an adhesive which is non-tacky at room temperature in the case of a one sided disk; and which is useful as a tacky adhesive at room temperature in the case of a two sided disk made from two one-sided disks bonded to one another.

BACKGROUND OF THE INVENTION 
This invention relates to an improvement in optical video disks or digital 
audio disks. 
An optical video disk is fabricated according to the following method: As 
shown in FIG. 1, signal pits 1a are formed in one side of a layer 1 of 
synthetic resin such as acrylic resin, and a reflecting film 2 of aluminum 
or the like is formed on the side of the layer 1 thus treated. A 
protective film 3 is formed on the reflecting film 2, to form one disk. 
Another disk is prepared according to the same method. Then, the 
protective films 3 and 3' are bonded to each other with an adhesive. In 
FIG. 1, like parts are designated by like reference numerals, but with 
primes added. 
Heretofore, a solvent type adhesive has been employed to bond the 
protective films to one another. Therefore, the solvent type adhesive must 
be such as will not affect the synthetic resin and the reflecting 
surfaces. Thus, the selection of a suitable adhesive is considerably 
difficult. Furthermore, as the adhesive is liable to flow to the surfaces 
of the synthetic resin layers, the resultant product is often low in 
practical worth. On the other hand, as the adhesive is dried after being 
applied to bond the two disks, dust may stick to the adhesive during the 
drying operation, thus spoiling the product; that is, with such a product, 
noise is produced during reproduction. 
SUMMARY OF THE INVENTION 
In view of the foregoing, an object of this invention is to provide an 
optical disk in which the disks are bonded together with a hot-melt type 
adhesive, whereby the disk material and the reflecting films are not 
adversely affected. The resultant product is improved in quality, and not 
only the manufacturing procedure can be simplified, but the cost of the 
required equipment can be reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A method of manufacturing an optical disk according to this invention will 
be described with reference to FIG. 2. 
In FIG. 2, reference numeral 4 designates a roller conveyor for conveying a 
disk 5 in the direction of the arrow, which disk has been prepared by 
forming signal pits on a blank of synthetic resin such as acrylic resin 
and by forming a reflecting film of aluminum or the like on the disk thus 
treated. Reference numeral 6 designates a hot melt type filming agent. For 
the disk 5, only one side of which is reproduceable, the filming agent is 
such that, the tacky adhesion thereof is smaller than 4 kg 1 inch at a 
temperature of, 95.degree. C., and the surface tack is less than that of 
EVA resin at room temperature (that is, not tacky at room temperature). 
For instance, "062" adhesive made by Sela Tilney Japan Ltd. is suitable 
for the disk 5 mentioned above. The composition of "062" adhesive is as 
follows: 
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weight percent 
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Ethylene/vinyl acetate copolymer 
approximately 40% 
Microcrystalline wax approximately 30% 
Petroleum resin approximately 30% 
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Where two disks 5 are to be bonded together so that signals can be 
reproduced from both sides thereof, a suitable filming agent is such that 
the tacky adhesion thereof is smaller than 4 kg at a temperature of 
95.degree. C., and it is useful as a tacky adhesive at room temperature. 
(For instance, "HM406" adhesive made by Cemedine Co., Ltd. is employable). 
The composition of "HM 406" is as follows: 
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approximate weight 
proportion 
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Styrene/isoprene block polymer 
(0 to 100) 
Styrene/butadiene block polymer 
(0 to 100) 100 as a 
Styrene/ethylene/butadiene 
(0 to 100) mixture 
block polymer 
Mineral oil 0 to 100 
Hydrogenated petroleum resin 
100 to 600 
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The mineral oil operates as a plasticizer and the petroleum resin operates 
as a tackifier. 
Further in FIG. 2, reference numeral 7 designates a hopper into which the 
aforementioned hot-melt type filming agent 6 is poured, the hopper 
operating to heat and melt the filming agent 6 and apply the filming agent 
thus heated to the surface of a roll coater 8. More specifically, the roll 
coater 8 operates to apply the filming agent 6 onto the reflecting surface 
side of the disk 5 which is being conveyed on the roller conveyor 4. The 
roll coater 8 is turned in the direction of the advancement of the disk 5 
and is coated with the filming agent 6 by the hopper 7. 
As the disk 5 is conveyed by the roller conveyor 4, the hot-melt type 
filming agent 6 is thus applied to the reflecting surface side of the disk 
5 when the latter passes through the roll coater 8. 
In the case of a single-sided disk, the filming agent 6 thus coated is 
solidified by being cooled to room temperature while the disk is further 
conveyed. Thus, the filming agent thus solidified serves as a protective 
film for the reflecting layer of the disk. 
On the other hand, with the two sided disk, two disks, to at least one of 
which the filming agent 6 has been applied, are prepared. The two disks 
are then bonded to one another using the filming agent 6 as the adhesive, 
to thus form one unit, namely, a two-sided disk. It goes without saying 
that disks formed with the aforementioned adhesive "062" used as 
protective films thereon may be bonded together using the above-described 
adhesive "HM406". 
As is apparent from the above description, according to the invention, two 
disks are bonded together with a hot-melt type adhesive. Accordingly, in 
the optical disk of the invention, the influence of dust motes or the like 
on reproduction is eliminated; that is, the quality of the disk is 
remarkably improved. Further, in the optical disk of the invention, the 
flow of the adhesive to the disk surface during manufacture can be 
minimized, i.e., the effect of the flow of the adhesive on the resultant 
quality is minimized; and the manufacturing equipment and process can be 
simplified and costs correspondingly reduced.