JP2797468B2 - Optical disc substrate molding method - Google Patents

Description

The present invention relates to a magneto-optical disk, various write-once optical disks,
Also, the present invention relates to a method for molding an optical disk substrate used for a so-called compact disk or the like, and particularly to an improvement in a method for molding an optical disk substrate made of a polycarbonate resin.

[Summary of the Invention]

The present invention is intended to reduce contamination while suppressing birefringence by plasticizing a polycarbonate resin containing no heat stabilizer in an inert gas atmosphere and injection-molding the resin onto an optical disk substrate.

[Conventional technology]

Optical discs that record and reproduce information by irradiating laser light, such as digital audio discs (so-called compact discs), optical video discs, magneto-optical discs, various write-once optical discs, and rewritable optical discs, have optical information. The recording layer is formed on a transparent optical disk substrate made of polycarbonate resin or the like, and writing or reading of information is generally performed by irradiating laser light from the substrate side. Therefore, when molding the optical disc substrate,
Various demands have been made from the viewpoint of optical characteristics.

For example, there is little birefringence (in this case, the degree of phase shift between incident light and reflected light, mainly due to internal distortion of the substrate), good transferability, and good surface smoothness. Typical examples include that there is little contamination (foreign matter or burn due to decomposition of resin), and that there is little runout (so-called skew) of the molding surface. In particular, a magneto-optical disk detects minute rotation of the plane of polarization of a laser beam to be irradiated and reads it as a signal. Therefore, the occurrence of birefringence is fatal. Techniques for minimizing refraction are essential.

Injection molding has been widely used as a method for molding an optical disk substrate. However, in order to satisfy optical characteristics (particularly birefringence), molding must be performed at a resin temperature of 330 ° C. or higher. However, in such a high temperature range, the polycarbonate resin is liable to be decomposed, which may cause contamination such as foreign matters and burns. The occurrence of contamination causes an increase in noise level, an increase in error rate, an increase in dropout, and the like, which is not preferable for an optical disc. Therefore, for example, Japanese Patent Publication No. 63-56043 and Japanese Patent Application Laid-Open No. 63-257933
As described in Japanese Patent Application Laid-Open Publication No. H10-163, attempts have been made to suppress the contamination by adding a heat stabilizer such as a phosphorus compound to a polycarbonate resin.

[Problems to be solved by the invention]

By the way, as a recording material of an information recording layer of an optical disc, a metal-based recording material is frequently used as represented by a rare earth-transition metal amorphous alloy film in a magneto-optical disc. For example, a material such as TbFeCo is used. It is reported that good characteristics are provided in terms of sensitivity, readout performance, and the like.

When the information recording layer is formed of such a metal-based recording material, corrosion is a serious problem. When corrosion occurs in the information recording layer, the number of byte errors increases and the like, resulting in poor reproduction, which greatly reduces the commercial value of an optical disc for which reliability must be guaranteed. In particular, in a magneto-optical disk using a rare earth-transition metal amorphous alloy film as an information recording layer, rare earth elements such as Tb are very easily oxidized, and this tendency is remarkable.

Therefore, in an optical disc, how to suppress corrosion of the information recording layer has become a major issue.

From this viewpoint, as described in, for example, JP-A-63-257933, addition of a heat stabilizer to the polycarbonate resin is disadvantageous, and the heat stabilizer is removed (heat stabilization). It is desirable to use polycarbonate resins (which do not contain an agent). That is, if a thermal stabilizer such as a phosphorus compound remains in the disk substrate, it will be one trigger for the occurrence of corrosion of the information recording layer, and it is preferable not to add such a thermal stabilizer if possible.

However, as described above, when injection molding is performed in a high-temperature range where birefringence does not occur, if the heat stabilizer is not added, contamination naturally increases. The fact is that heat stabilizers are unavoidably added.

In view of the above, the present invention has been proposed in view of the above-mentioned conventional circumstances, and has been proposed to provide an optical disc substrate that can be formed without adding a thermal stabilizer and without increasing birefringence or contamination. It is an object to provide a molding method.

[Means for solving the problem]

The molding method of the present invention, in order to achieve the above object,
It is characterized in that a polycarbonate resin containing no heat stabilizer is plasticized in an inert gas atmosphere and injection molded.

[Action]

When a polycarbonate resin containing no heat stabilizer is injection-molded in the atmosphere at a high temperature, contamination such as foreign matter and burns occurs in the substrate.

On the other hand, when plasticizing and injection molding a polycarbonate resin in an inert gas atmosphere, deterioration due to oxidation is suppressed even at a high resin temperature, and contamination increases even if the polycarbonate resin does not contain a heat stabilizer. Is suppressed.

Further, the heat stabilizer causes corrosion when the information recording layer is formed on the substrate to form an optical disk. In the present invention, it is not necessary to add the heat stabilizer to the polycarbonate resin. Therefore, the corrosion of the recording layer is prevented and the reliability is improved.

〔Example〕

Hereinafter, specific embodiments to which the present invention is applied will be described with reference to the drawings.

First, the configuration of the injection molding machine used in the present embodiment will be described.

As shown in FIG. 1, the injection molding machine roughly forms a resin injection section (1) for melting a resin material (here, a polycarbonate resin) and feeding it to a mold, and a cavity corresponding to the shape of the optical disk substrate. And a mold clamping mechanism (not shown) for applying pressure to the mold part (2).

The injection unit (1) is a resin pellet (4) as a raw material.
The heating cylinder (8) and the molten resin (6) are provided with a screwing hopper (5), which is provided with a heater around the charging hopper (5), and a screw (7) for sequentially feeding the molten resin (6) inside. A nozzle (9) to be ejected,
The resin pellets (4) supplied from the charging hopper (5) are sequentially melted and sent out from the tip of the nozzle (9) of the molten resin (6) to the mold part (2).

The charging hopper (5) has a sealed structure, and is provided with an inlet pipe (10) for introducing a pressurized inert gas (for example, nitrogen gas) into the charging hopper (5). Is provided with a leak prevention valve (11) which prevents leakage of inert gas and is opened only when the pellets are charged. If the pressure by the inert gas is too high, the injection of the gas into the molten resin may hinder normal injection molding. It leaks from the back of (8).

In order to injection-mold an optical disk substrate using the injection molding machine having the above configuration, first, a resin pellet (4) made of a polycarbonate resin is charged from a charging hopper (5).

The resin pellet (4) used here is mainly composed of a polycarbonate resin and does not contain a phosphorus compound such as a phosphite as a heat stabilizer. However, here, it means that a predetermined amount of a heat stabilizer is not actively added to the polycarbonate resin for the purpose of preventing decomposition by heat, for example, a trace amount of a phosphorus compound is contained due to other factors or the like. In the case where the heat stabilizer is contained or a trace amount (for example, 5 ppm or less) is contained, it is understood that the heat stabilizer is contained in the polycarbonate resin containing no heat stabilizer.

As the polycarbonate resin, any of those usually used as a substrate material can be used, and the kind thereof is not limited. For example, polycarbonate produced by a phosgene method synthesized by reacting a dihydric phenol compound (for example, bisphenol A) with phosgene in the presence of an acid binder (for example, an alkali such as sodium hydroxide) and a solvent (methylene chloride). Resins and the like are preferred. In addition, a monofunctional organic such as a branched polycarbonate resin using a trifunctional or higher polyfunctional organic compound having a phenolic hydroxyl group as a branching agent, a long-chain alkyl acid chloride or a long-chain alkyl ester-substituted phenol as a terminal stopper. It is also possible to use a terminal long-chain alkyl polycarbonate resin using a compound, a terminal long-chain alkyl branched polycarbonate resin using both the branching agent and the terminal terminator, and a mixture thereof.

Input hopper (5) into which resin pellets (4) are input
And the atmosphere in the heating cylinder (8) have been replaced by nitrogen gas introduced at a constant pressure from the introduction pipe (10) provided in the charging hopper (5). The resin (6) plasticized by heating in a nitrogen atmosphere and melted.

On the other hand, the mold part (2) is kept in a mold closed state by a mold clamping mechanism, and in this state, the polycarbonate resin (6) melted in the heating cylinder (8) is supplied to the resin injection port (12) through the nozzle (9). ) To the cavity (1
3) Inject into.

When injecting the molten resin (6), the resin temperature is preferably as low as possible if uniform kneading is possible in the heating cylinder (8), but if the resin temperature is too low, the birefringence generated on the substrate is large. In general, the temperature is preferably set to 325 ° C. or higher. However, if the resin temperature is too high, contamination will increase even if the resin is plasticized in a nitrogen atmosphere, so the upper limit is preferably set to about 350 ° C. On the other hand, the mold temperature is optional, but is preferably equal to or lower than the thermal deformation temperature of the disk from the viewpoint of improving the skew characteristics and productivity, and is usually set to about 50 to 120 ° C.

In the optical disk substrate obtained as described above, injection molding is performed at a high resin temperature, so that birefringence is suppressed, and furthermore, contamination is suppressed because the polycarbonate resin is plasticized in a nitrogen atmosphere.
Furthermore, since the polycarbonate resin does not contain a heat stabilizer, the optical disk substrate also does not contain a heat stabilizer, and for example, corrosion or the like that occurs in the recording layer when used for a substrate of a magneto-optical disk substrate. Deterioration is greatly reduced.

The present inventors have actually performed the following experiment to confirm the effect.

Experiment 1 In this experiment, the occurrence of contamination was examined when nitrogen gas was introduced during injection molding and when it was conducted in air.

That is, in this experiment, a polycarbonate resin containing no heat stabilizer was plasticized in a nitrogen atmosphere and an air atmosphere, and injection molding was performed while changing the resin temperature to a thickness of 1.2 m.
m optical disk substrates were obtained.

FIG. 2 shows the result of examining the number of contaminations on the obtained optical disk substrate. Here, the number of contaminants was obtained by counting the number of foreign substances and alterations having a diameter of 0.5 to 1 μm, and expressed the number per 1 g. In the figure, a curve A represents a case where the polycarbonate resin is plasticized in a nitrogen gas atmosphere and injection-molded, and a curve B represents a case where the polycarbonate resin is plasticized in the atmosphere.

As is apparent from FIG. 2, when the resin is plasticized in the air and injection-molded, the number of contaminations increases when the resin temperature exceeds 325 ° C., whereas the plasticization in the nitrogen atmosphere is performed. In the case of injection molding, no increase in the number of contaminations is observed up to 345 ° C. This means that a nitrogen atmosphere is more advantageous when the resin temperature is set high in consideration of birefringence.For example, when injection molding is performed at a resin temperature of 340 ° C. in order to suppress the occurrence of birefringence. When the injection molding is performed in a nitrogen atmosphere, the contamination is very small as compared with the injection molding in the air.

Experiment 2 In this experiment, the effect of the thermal stabilizer on the use of the prepared substrate as the substrate of the magneto-optical disk was examined.

A polycarbonate resin containing 15 ppm of a heat stabilizer,
Inject injection molding machine with polycarbonate resin without heat stabilizer, heater temperature 320-360 ℃, mold temperature
Injection molding was performed at 80 to 115 ° C. to obtain an optical disk substrate having a thickness of 1.2 mm. When a heat stabilizer was included, plasticization and injection molding of the resin were performed in the air, and when no heat stabilizer was included, the resin was performed in a nitrogen atmosphere.

Next, as shown in FIG. 3, on these substrates (21),
500Å transparent dielectric layer (22) [Si ₃ N ₄ film], 600 film thickness
The recording layer (23) [TbFeCo film] and the UV-curable resin layer (24) having a thickness of 5 μm were sequentially formed to form a magneto-optical disk.

With respect to the obtained magneto-optical disk, defects generated in the recording layer (23) during storage were examined. The evaluation method is 80
Each magneto-optical disk was stored under the conditions of 80 ° C. and a relative humidity of 80%, and the increase in the number of defects over time was measured. The results are shown in FIG. The curve i in the figure represents a magneto-optical disk using a substrate made of a polycarbonate resin containing no heat stabilizer, and the curve ii represents a magneto-optical disk using a substrate made of a polycarbonate resin containing a heat stabilizer. Represent.

From FIG. 4, it can be seen that in the case where no heat stabilizer was contained, the increase in defects of the recording layer due to storage was hardly observed, and good corrosion resistance was exhibited. On the other hand, in the case of containing the thermal stabilizer, the defects generated in the recording layer increased with the passage of time, and it was confirmed that the thermal stabilizer greatly affected the corrosion of the storage layer.

〔The invention's effect〕

As is clear from the above description, in the present invention, the polycarbonate resin containing no heat stabilizer is plasticized in an inert gas atmosphere and injection-molded, so that contamination is eliminated while birefringence is eliminated. Can be reduced. Further, since it is not necessary to add a heat stabilizer, it is possible to solve the problem of corrosion and deterioration of the recording layer due to the heat stabilizer, and it is possible to secure reliability during long-term storage.

Therefore, according to the present invention, it is possible to form a highly reliable optical disk substrate that has little birefringence and contamination and does not adversely affect the recording layer when the optical disk is used.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an example of an injection molding machine used for carrying out the method of the present invention. FIG. 2 is a characteristic diagram showing the relationship between resin temperature and contamination when injection molding is performed in a nitrogen atmosphere, as compared with that when injection molding is performed in the atmosphere. FIG. 3 is a schematic sectional view showing a main part of the configuration of a magneto-optical disk produced in an experiment. FIG. 4 is a characteristic diagram showing how the number of defects generated in the recording layer changes with time in a case where the thermal stabilizer is included and a case where the thermal stabilizer is not included.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) B29C 45/00-45/84 G11B 7/26 G11B 11/10 B29L 17:00

Claims (1)

(57) [Claims] 1. A method for molding an optical disk substrate, comprising plasticizing a polycarbonate resin containing no heat stabilizer in an inert gas atmosphere and injection molding.