Abstract:
Optionally readable information disk in which the information track is situated in a radiation-cured lacquer layer comprising at least 50% by weight of a diacrylate, triacrylate or a mixture thereof and which the cured lacquer layer has a high indentation hardness.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an optically readable information disk which comprises a substrate plate having at least on one side a radiation-cured lacquer layer in which an information track of information regions situated at a higher and a lower level is provided and is covered with a reflecting layer. 
     Such a disk is known from the published Netherlands Patent Application No. 76 11 395 in the name of Applicants. 
     Applicants have found that the known information therefrom have a considerably worse signal-to-noise ratio, abbreviation SNR, than the master plate from which the disks are derived. The master plate is a flat glass plate which is covered on one side with a photoresist layer in which the information track has been provided by exposure to laser light succeeded by photochemical development. First metal copies, the so-called father disks, are made from the master plate electrochemically and by electro-deposition, from which father plates, subsequently second metal copies, also called mother disks, are made by electro-deposition and therefrom third and subsequent metal copies are manufactured. The last copies of the &#34;family&#34; are termed dies. 
     The above-mentioned optically readable information disks are manufactured by means of the dies in which the surface of the die in which the information track is present is provided with a thin layer of a lacquer which is curable by radiation, for example ultraviolet light, the substrate plate is then provided thereon, the assembly is exposed via the substrate plate and the resulting disk comprising the substrate plate and the cured lacquer layer connected thereto in which the information track is copied, is finally removed from the die. The cured lacquer layer is provided with a reflecting layer, for example, a vapour-deposited layer of aluminum or silver, on the side of the information track. 
     The information regions present in the information track have very small dimensions. The longitudinal dimensions vary in accordance with the stored information from approximately 0.3-3 μm. The difference in height between the regions mutually corresponds to a quarter wavelength (or a multiple thereof) of the laser light with which the disk is read optically and amounts to 1-2 μm. The disk is read in reflection via the substrate plate in which the detection of the information regions is based on phase differences occurring between the forward and the reflected laser light beam. 
     In the steps leading from the master to die and the subsequent production of the optically readable disk starting with the provision of a light-curable lacquer on the die and up to the vapour deposition of a reflecting layer, all kinds of errors and deviations may occur as a result of the quality of the finished disk is diminished, in particular a deterioration in the signal-to-noise ratio with respect to that of the master. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to reduce this loss of signal-to-noise ratio considerably and hence to improve the quality, in particular the quality of the stored information in the optically readable information disk. 
     According to the invention this object is achieved with an information disk of the type mentioned in the opening paragraph which is characterized in that the lacquer layer has a minimum indentation hardness which corresponds to a maximum penetration depth of 5 μm of a needle which is placed on the lacquer layer and which comprises a ball-shaped tip manufactured from hard material and having a diameter of 18±1 μm and presses on the lacquer with a weight of 2.5±0.1 gram. 
     Favourable results are achieved with an information disk the lacquer layer of which has an indentation hardness which corresponds to a penetration depth of 1.5-3.0 μm. 
     The desired hardness of the lacquer layer can be obtained by a suitable choice of the ingredients of a radiation-curable lacquer which, after exposure, for example, to ultraviolet light, provides the cured lacquer layer. In this case it holds that those skilled in the art must make such a choice from the commercially available lacquers or lacquer components that the lacquer per unit by volume comprises a large number of cross-linkable groups so that after curing a large number of cross-links is formed. More in particular there must preferably be started from a curable lacquer which comprises components which per molecule have two or more cross-linkable groups which hereinafter also are termed functional groups. Herewith it is achieved that upon curing in addition to linear polymerization, many cross-links are realized which lead to a network structure. 
     Particularly suitable are radiation-curable lacquers comprising at least 50% by weight of a diacrylate, a triacrylate or a mixture thereof. 
     Di- and triacrylates comprise respectively two and three functional groups, in this case ethene groups, per molecule. Examples of such acrylates are alkanedioldiacrylates, such as 1,3-propanedioldiacrylate, 1,5-butanedioldiacrylate and 1,6-hexanedioldiacrylate, alkeneglycoldiacrylates, such as diethyleneglycoldiacrylate, triethyleneglycoldiacrylate and tripropyleneglycoldiacrylate, as well as trimethylolpropanetriacrylate, pentacrythritoltriacrylate and pentaerythritolpropylethertriacrylate. 
     The lacquer also comprises a usual photosensitive catalyst such as an aromatic carbonyl compound, for example, benzoinisobutylether or a ketal, for example, benzildimethylketal, in a quantity from approximately 1 to 5% by weight. 
     In a preferred embodiment the diacrylate, triacrylate or a mixture thereof used in the radiation-curable lacquer has an average molecular weight from 200 to 350. The very good results which are obtained by using said lacquer may simply be called striking. It may be expected that the comparatively low-molecular products will show a comparatively high shrinkage upon curing so that the results are influenced only negatively. The opposite is the case which, in Applicants opinion, is caused by the comparatively low viscosity of the lacquer (smaller than 30 cP) so that very thin layers can be realized in a thickness smaller than 50 μm, for example 20-35 μm. As a result of this absolute shrinkage is small. It is further believed that due to the low viscosity the mobility of the molecules not yet polymerized during the curing process is still sufficient to fill up the space between the lacquer layer and the information track present in the die arisen from shrinkage. The thickness of the cured lacquer layer then becomes slightly smaller but this has no influence on the signal-to-noise ratio. 
     The low viscosity has furthermore for its result that the radiation-curable lacquer can more easily be distributed over the die surface, while the possibility of air bubble inclusion is smaller. 
     In a further preferred embodiment the radiation-curable lacquer comprises in addition to the di- and/or trifunctional compound, also a monofunctional compound. 
     Examples of monofunctional compounds are monoacrylates which per molecule comprise one ethene group (--c═c--), for example, alkylacrylates, for example ethylacrylate, butylacrylate, hexylacrylate and ethylhexylacrylate, alkoxyalkylacrylates, for example, ethoxyethylacrylate, phenoxyalkylacrylates, for example, phonoxyethylacrylate, as well as phenylacrylate. 
     The addition of mono-functional compounds stimulates the adhesion of the cured lacquer layer to the substrate plate in particular if the plate is manufactured from a synthetic resin, for example, polyvinylchloride, polymethylmethacrylate or polycarbonate. Furthermore, the viscosity of the lacquer can be reduced by the addition of the monofunctional compound. 
     A very favourable monofunctional compound is an alkylenepyrrolidone in particular vinylpyrrolidone. This substance also improves the adhesion of the cured lacquer layer, reduces the viscosity of the lacquer and has the further advantage that the hardness of the resulting cured lacquer layer is less influenced than when the above-mentioned monoacrylates are used, while furthermore in contrast with monoacrylates the substance does not reduce the curing rate but on the contrary increases the rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail by means of the drawings and the following example, in which 
     FIG. 1 is a cross-sectional view of a micro-indentation hardness measuring instrument used in determining the hardness of the lacquer layer present in the information disk according to the invention, and 
     FIG. 2 is a cross-sectional view of an information disk according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference numeral 1 in FIG. 1 denotes an envelope for a permanent magnet 2 which may be manufactured, for example, from ferroxdur. An electric coil 3 which is connected to the needle system 5 via holder 4 is provided around magnet 2. The assembly of coil 3, holder 4 and needle system 5 has a weight of 3 g and is suspended in two flat spiral supporting springs 6 and 7, which are extremely flexible in the vertical direction and have a great rigidity in the horizontal direction. The spiral supporting strips 6, 7 are accommodated in a housing 8 which is connected to the envelope 1. Housing 8 has a reference ring 9 which is brought in contact with the surface of the information disk to be measured (not shown in FIG. 1). The needle system 5 has a magnetic body 10 of ferroxdur which cooperates with coils 11 accommodated in coil holder 12. Magnet core 10 and coils 11 together constitute a displacement pick-up 13. Needle system 5 furthermore comprises at its end remote from holder 4 a needle 15 provided in needle holder 14. Needle 15 is manufactured from sapphire and has a ball-shaped tip having a diameter of 18 μm. 
     The operation of the micro-indentation hardness measuring instrument as shown in FIG. 1 is as follows. By passing current via the coil 3 the needle system 5 is brought in a retracted position in which the needle 15 is at a certain distance above the surface determined by reference ring 9. The hardness measuring instrument is then brought perpendicular to the surface of an information disk to be measured, reference ring 9 being in contact with the surface of the disk. The current strength through coil 3 is slowly reduced in which the needle system 5 is lowered as a result of its own weight, that is to say moves in the direction of the surface of the disk. As soon as the needle 15 contacts the surface of the disk a small reduction of the current strength through coil 3 will not result in any further displacement of the needle. In this manner the needle can be brought in contact with the surface of the disk while completely balanced, that is to say without there being any downward force. The current strength through coil 3 is now adjusted so that a downward force of 2.5 g is exerted on the needle 15. Since the needle system 5 has an own weight of 3 g this means thus that by cooperating of coil 3 and magnet 2 an upward force of 0.5 g is exerted on the needle system 5 so that the resultant of the forces yields the above-mentioned downward force of 2.5 g. The depth of penetration of needle 15 caused by said force is measured in the usual manner by means of the displacement pick-up 13 known per se. It is to be noted that the indentation hardness of the optical disk can also be measured, besides by the instrument as shown in FIG. 1, also by means of the commercially available micro-indentation measuring instruments, for example, the indentation measuring instrument which is available as Wallace, type H7a, manufactured by H. W. Wallace &amp; Co., Ltd., Croydon, England. 
     Reference numeral 16 in FIG. 2 denotes a 1,2 mm thick substrate plate of synthetic resin, for example polyvinyl-chloride or polymethyl-methacrylate, which on one side has a hard light-polymerized lacquer layer 17 comprising an information track 18. Track 18 comprises information regions which are situated alternately at a lower level 19 and a higher level 20 and collectively give the track a crenellated profile. The length dimensions of the information regions vary in accordance with the stored information from approximately 0.3 to 3 μm. The difference in height between the regions is 1.15 μm. Layer 17 is covered by a vapour-deposited reflection layer not shown, in particular a layer of vapour-deposited Ag or Al. 
     The micro-indentation hardness of lacquer layer 17 was determined for various lacquer compositions. For this purpose, the depth of penetration of the needle was measured by means of the instrument shown in FIG. 1. Furthermore, the loss of signal-to-noise ratio of the resulting information disk which is covered with a reflection layer of silver or aluminium was determined in comparison with the signal-to-noise ratio of the master plate. The resulting data are recorded in the table below in which in column 1 the sequential number of the lacquer is recorded, column 2 gives the reflection layer used, column 3 the measured depth of penetration in the cured lacquer layer and column 4 gives the loss of signal-to-noise ratio with respect to the master. The composition of the lacquers is recorded after the table. It is to be noted that the information disk in which the lacquer having sequential number 5 has been used is known from the above-mentioned Netherlands Patent Application No. 76 11 395 so that the data which relate to lacquer number 5 are recorded in the table as comparison material. 
     
                       TABLE______________________________________                            loss of sig-lacquer sequential       reflection depth of  nal-to-noiseno.         layer      penetration                            ratio______________________________________1           Ag         2.1       0.81           Al         2.1       1.22           Ag         3.5       02           Al         3.5       1.33           Al         2.2       0.54           Ag         2.6       1.24           Al         2.6       1.55           Ag         9.0       8.25           Al         9.0       6.0______________________________________Composition of the lacquerslacquer No. 1:     64      wt. %   pentaerythritolpropylethertria-                     crylate     16      wt. %   N--vinylpyrrolidone     16      wt. %   hexanedioldiacrylate     4       wt. %   benzildimethylketallacquer No. 2:     78      wt. %   tripropyleneglycoldiacrylate     20      wt. %   N--vinylpyrrolidone     2       wt. %   benzildimethylketallacquer No. 3:     58      wt. %   tripropyleneglycoldiacrylate     28      wt. %   N--vinylpyrrolidone     10      wt. %   trimethylolpropanetriacrylate     4       wt. %   benzildimethylketallacquer No. 4:     78      wt. %   ethoxylated trimethylolpropane                     triacrylate                     (koskydal K-5-2445)     20      wt. %   N--vinylpyrrolidone     2       wt. %   benzoin-isobutyletherlacquer No. 5:     59      wt. %   2-ethylhexylacrylate     19.5    wt. %   butanedioldiacrylate     19.5    wt. %   trimethylolpropanetriacrylate     2       wt. %   benzoin-isobutylether______________________________________