JPH0450662B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical information recording medium, and particularly to an optical information recording medium made of a specific thermoplastic synthetic resin.

2. Description of the Related Art Optical information recording media are used to read information by shining light onto them and using the reflected or transmitted light. This has already been used as a video disc, audio disc, information file disc, etc., and is well known.

Such an optical information recording medium is based on a transparent plate with little deviation, and information is recorded by forming grooves or irregularities on the substrate. In practice, in order to record a large amount of information on one substrate, the grooves or irregularities have a small width of several microns or less, and are attached closely to each other. In this way, light is shined on closely spaced small-width grooves or irregularities, and the reflected or transmitted light is used as information, so the base is transparent and made of a material with extremely low deviation. It is necessary that the That is, the substrate is required to have excellent shape stability, heat resistance, and low moisture absorption. Additionally, the base must be one that is easy to board.

Because of this need, the base of optical information recording bodies has been made of synthetic resin. The synthetic resins used so far are mainly polymethyl methacrylate (hereinafter referred to as PMMA) and polycarbonate (hereinafter referred to as PC). All of these have excellent transparency and, to a certain degree, excellent shape stability. However, upon closer examination, PMMA
has a drawback that it has a high hygroscopicity, and therefore the base will warp and deform in hot areas.
In addition, although PC is good in that it does not deform due to moisture absorption, its large photoelastic coefficient causes birefringence, which tends to cause disturbances in reflected or transmitted light. difficult to mold,
There was a drawback. Therefore, none of the synthetic resins used so far have been satisfactory.

Therefore, the inventor set out to find a synthetic resin suitable for forming the base of an optical information recording medium. To this end, we created various transparent hard synthetic resins and investigated their properties. Among these, the inventor found that the acetalized product of polyvinyl alcohol is hard and transparent, and that increasing the degree of acetalization reduces hygroscopicity and improves shape stability. It has been found that when appropriately selected, injection molding becomes easy, and the resulting molded product is less likely to cause birefringence, and exhibits properties suitable for making a substrate. This invention is
This was done based on this knowledge.

This invention provides an information recording medium based on a transparent synthetic resin plate, in which the synthetic resin contains 100 or more.
The present invention relates to an optical information recording medium characterized in that it is composed of an acetalized product obtained by acetalizing polyvinyl alcohol having an average degree of polymerization of 2000 to a degree of acetalization of 65% mole or more.

The synthetic resin used in this invention is polyvinyl acetal, which is made by acetalizing polyvinyl alcohol. Acetalization is carried out, for example, by adding aldehydes to an aqueous solution of polyvinyl alcohol in the presence of an acid catalyst such as hydrochloric acid. As the aldehydes, in addition to aliphatic aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, and butyraldehyde, aromatic aldehydes such as benzaldehyde, and further alicyclic aldehydes such as cyclohexanecarbaldehyde can be used.

The polyvinyl acetal thus obtained is a thermoplastic resin and has good thermal stability, and when heated, it softens and becomes easily fluid, so it can be molded into a plate by injection molding. Therefore, molding can be performed easily and efficiently. Furthermore, polyvinyl acetal has a relatively high softening point and does not deform at normal temperatures. Furthermore, polyvinyl acetal is a hard resin and has good transparency.

In addition, when the degree of acetalization is low, polyvinyl acetal has high hygroscopicity and therefore has a strong tendency to absorb moisture and warp;
When the amount is 65 mol% or more, the hygroscopicity becomes small, and accordingly, moisture absorption and deformation will not occur. Therefore, if polyvinyl acetal with an acetalization content of 65 mol % or more is used, a molded article made from this material will be less likely to deform due to moisture absorption, and will therefore have good shape stability. For this purpose, in this invention, the degree of acetalization is 65 mol%.
The above uses polyvinyl acetal.

Polyvinyl acetal, like general thermoplastic resins, increases in viscosity when melted by heating and becomes difficult to flow as the degree of polymerization increases. Therefore, polyvinyl acetal having a high degree of polymerization makes it difficult to injection mold it, and also causes a large birefringence in the molded product obtained by injection molding. Therefore, in order to reduce the birefringence of the molded article, it is preferable to have a small degree of polymerization. However, if the degree of polymerization becomes too small, the softening point will decrease, and therefore the heat resistance will decrease. Therefore, there is a certain limit to the degree of polymerization.

The degree of polymerization of thermoplastic resins is generally determined by measuring the viscosity of the resin. The same applies to polyvinyl acetal, but in the case of polyvinyl acetal, the mucous membrane differs depending on the type of aldehyde used for acetalization. Therefore, it is difficult to determine the degree of polymerization by directly measuring the viscosity of polyvinyl acetal. However, when polyvinyl alcohol is reacted with an aldehyde to form an acetal, it is thought that the degree of polymerization of polyvinyl alcohol does not change. Therefore, the degree of polymerization of polyvinyl acetal can be directly applied to the degree of polymerization of polyvinyl alcohol, which is its raw material.

The polymerization degree of polyvinyl alcohol is JIS K6726.
It is determined by measuring the average degree of polymerization specified in .

According to the inventor's experiments, when polyvinyl alcohol has an average degree of polymerization of 100 to 2000, polyvinyl acetal obtained by acetalizing it becomes easy to mold by injection molding, and The molded product has low birefringence and is suitable as a base for an optical information recording medium.

To explain this in detail with an example, the average degree of polymerization is
1400 polyvinyl alcohol is acetalized to 80% with isobutyl alcohol to produce polyvinyl acetal, which is then injection molded (at this time, a gate is provided in the center of the disk) to a thickness of 1.2 mm.
Make a disk with a diameter of 120mm, and a radius of 24mm from the center of the disk.
When we measured the birefringence at the position, the birefringence value was
It was 87nm. In addition, acetalization was performed in the same manner using polyvinyl alcohol with an average degree of polymerization of 2000,
The acetal thus obtained is formed into a similar disk,
When the birefringence at the same position was measured, the birefringence value was 710 nm. Both birefringence values are determined by the wavelength of light suitable for reading information (an example of a semiconductor laser).
780 nm) or less, and could be put to practical use. If the average degree of polymerization exceeds 2000, optical reading becomes difficult under high-density recording, so this was recognized as the limit. Thus, the upper limit of the degree of polymerization was set at 2000. On the other hand, the lower limit of the degree of polymerization was determined to be 100 based on the heat resistance of the molded product.

Thus, in this invention, polyvinyl alcohol having an average degree of polymerization of 100 to 2000 is used,
The synthetic resin obtained by acetalizing this to a concentration of 65 mol% or more is transparent, heat resistant, hard, has low hygroscopicity, has good shape stability, is easy to mold, and has birefringence. small in gender,
A foundation with the following characteristics can be obtained. Therefore, it is excellent as an optical information recording medium. In this respect,
The effects of this invention are significant.

EXAMPLES The method for producing the synthetic resin used in the present invention and its characteristics will be specifically explained below with reference to Examples.

Example 1 5000 g of polyvinyl alcohol with a degree of saponification of 98.4% and an average degree of polymerization of 800 was dissolved in 44 liters of distilled water, and while the resulting solution was kept at 5°C and stirred, 285 g of concentrated hydrochloric acid (purity 35% by weight) was added to it. was added, and 4910 g of isobutyraldehyde was added dropwise over 10 minutes.
A precipitate was deposited 32 minutes after the start of the dropwise addition. After 10 minutes had passed after precipitation, 2565 g of the above concentrated hydrochloric acid was added, and after 2 hours the temperature was raised to 55°C, and the temperature was maintained at 55°C.
Holds time. The precipitate thus obtained was washed with water, neutralized with sodium bicarbonate, further washed, and then dried to obtain polyvinyl acetal. The degree of acetalization of the obtained polyvinyl acetal was 81 mol%.

This polyvinyl acetal was put into an injection molding machine, melted at 250℃, and injected into the mold to a thickness of 1.2 mm.
mm, a disc with a diameter of 120 mm was obtained. Molding was easy. The obtained molded product was transparent and tough. The birefringence (round-trip optical path) of a portion 24 mm away from the center of this disk in the radial direction was measured and found to be 12 nm.
In addition, aluminum metal was vapor-deposited on one side of this disk, and after sufficiently drying, it was immersed in water, and the warpage was measured after 24 hours. To measure the warpage, the circumference of the disk was placed on a table with the metal-deposited surface facing downward, and the prominence at the center was determined. The warpage thus measured was 0.7 mm. Thus, it was confirmed that this disk had a small birefringence value, little warpage, and good dimensional stability. Therefore, it was confirmed that this disc is excellent as an optical information recording medium.

Example 2 5000g of polyvinyl alcohol with a degree of saponification of 99.3% and a degree of polymerization of 1400 was dissolved in 44 liters of distilled water, and the resulting solution was kept at 10°C and stirred until the purity was 35% by weight.
285 g of agricultural hydrochloric acid were added thereto, and then 4910 g of isobutyraldehyde was added dropwise over 10 minutes. A precipitate was deposited 22 minutes after the start of the dropwise addition. Thereafter, in the same manner as in Example 1, polyvinyl acetal with a degree of acetalization of 80 mol% was obtained.

This polyvinyl acetal was put into an injection molding machine, melted at 270°C, and injected into the mold.
A similar disc was obtained. Molding was easy. The discs obtained were transparent and tough. When this disc was measured in the same manner as in Example 1, the birefringence at a distance of 24 mm from the center of the disc was 87 nm.
It had good properties against light, and the warpage was small at 0.75 mm, so it was confirmed that it was sufficient to form the base of an optical information recording medium.

Example 3 In Example 2, isobutyraldehyde 4910
cyclohexanecarbaldehyde instead of g
A precipitate was precipitated in the same manner as in Example 1, except that 6360 g was used. However, in this example, a precipitate was deposited 15 minutes after the start of dropping cyclohexanecarbaldehyde. The deposited precipitate was treated in exactly the same manner as in Example 2 to obtain an acetal of cyclohexanecarbaldehyde with a degree of acetalization of 82 mol%.

This acetal was put into an injection molding machine, melted at 270°C, and injected into a mold to obtain a disk similar to that of Example 2. Molding was easy. The discs obtained were transparent and tough. When the birefringence and warpage of this disk were measured in exactly the same manner as in Example 1, they were 97 nm and 0.25 mm, respectively, and both were found to be suitable for use as a base for an optical information recording medium. .

Comparative example 1 As polyvinyl alcohol, saponification degree 99.6%,
Polyvinyl acetal with a degree of acetalization of 80 mol % was obtained in exactly the same manner as in Example 2, except that a polyvinyl acetal with a degree of polymerization of 2300 was used and the acetalization temperature was changed from 10°C to 13°C.

This polyvinyl acetal was injection molded at 270°C to obtain a disc similar to that in Example 2. Injection molding was somewhat difficult. The discs obtained were transparent and tough. The birefringence and warpage of this disc were measured in exactly the same manner as in Example 2.
Values of 930 nm and 0.6 mm were obtained. There is little warping, but
It was recognized that the birefringence value was large and the properties against light were not good.

Comparative Example 2 4910g of isobutyraldehyde in Example 2
Example 2 was carried out in exactly the same manner as in Example 2, except that 2455 g of isobutyraldehyde was used instead of . A precipitate was deposited 32 minutes after the start of dropping the aldehyde. This precipitate was treated in exactly the same manner as in Example 2 to obtain polyvinyl acetal with a degree of acetalization of 58 mol%.

This acetal was put into an injection molding machine and injection molded at 270°C to obtain a disc. Molding was easy.
The discs obtained were transparent and tough. When this disk was measured in exactly the same manner as in Example 2,
The birefringence value was 73 nm, which was good, but the warpage was 1.7 mm, indicating large deformation.

Claims (1)

[Claims] 1. In an information recording medium based on a transparent synthetic resin plate, the synthetic resin is an acetalized product obtained by acetalizing polyvinyl alcohol having an average degree of polymerization of 100 to 2000 to an acetalization degree of 65 mol% or more. An optical information recording medium comprising: