JP2889882B2 - Pickup lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is excellent in heat resistance, low moisture absorption and low birefringence,
Also, the present invention relates to a pickup lens having a small change in spherical aberration of the lens.

[Conventional technology]

Methacrylic resin is excellent in transparency and weather resistance, has properties balanced in mechanical properties, moldability, and the like, and is used in various fields as a sheet material or a molding material. Recently, utilizing such characteristics, optical information recording carriers such as video discs, audio discs, write-once discs for computers, lenses for cameras, video cameras, projection televisions, optical pickups, and other laser beam printers, projector copying Applications are expanding as various optical elements, such as machines, facsimile machines, Fresnel lenses, and optical connectors.

However, methacrylic resins have high hygroscopicity, and dimensional changes due to moisture absorption and warpage of molded products occur, and cracks occur due to long-term repeated cycles of moisture absorption and drying, so there are some fields where the use of certain products is restricted. . In particular, it is said that the influence is large on an optical information recording carrier, an optical pickup lens, a connector, and the like used for those optical systems.

As methods for imparting low hygroscopicity to methacrylic resins, copolymers of methyl methacrylate and cyclohexyl methacrylate (JP-A-58-5318) and copolymers of methyl methacrylate and cyclohexyl methacrylate and benzyl methacrylate (JP-A-58 No. 13652) has been proposed.

[Problems to be solved by the invention]

However, the copolymers shown in the above patent publications are:
Although the methacrylic resin has an improved low moisture absorption, it has a disadvantage that heat resistance is reduced. Therefore, as a method for further imparting heat resistance to a methacrylic resin, a copolymer of methyl methacrylate and a methacrylic ester having an alicyclic hydrocarbon group in an ester portion (Japanese Patent Application Laid-Open No. 60-13 / 1985)
No. 335) has been proposed. However, at present, the copolymer disclosed in this patent publication is only at the same level as ordinary methacrylic resins in terms of birefringence.

[Means for solving the problem]

In view of the above-described problems, the present inventors have conducted intensive studies with the aim of providing a pickup lens having excellent heat resistance, low moisture absorption and low birefringence, and having a small change in spherical aberration of the lens. The invention has been completed.

That is, the present invention relates to a copolymer comprising (A) 20 to 85% by weight of cyclohexyl methacrylate, (B) 70 to 10% by weight of methyl methacrylate, and (C) 5 to 30% by weight of at least one selected from cyclohexylmaleimide and isopropylmaleimide. In the pickup lens obtained from the union.

Cyclohexyl methacrylate, which is the first component of the methacrylic resin used in the pickup lens of the present invention, is a component necessary for giving useful performance as a pickup lens, and its use ratio is 20 to 85% by weight. When the use ratio of cyclohexyl methacrylate is less than 20% by weight, the obtained resin has insufficient moisture absorption,
On the other hand, if it exceeds 85% by weight, the desired low birefringence tends not to be obtained.

Next, the use ratio of methyl methacrylate, which is the second component of the methacrylic resin used in the pickup lens of the present invention, is 70 to 10% by weight, and if it is less than 10% by weight, transparency and low birefringence become insufficient. On the other hand, if it exceeds 70% by weight, the low hygroscopicity becomes insufficient.

Further, the methacrylic resin in the present invention needs to copolymerize at least one selected from cyclohexylmaleimide and isopropylmaleimide as a third component for improving heat resistance. The use ratio of this third component is 5 to 30% by weight, preferably 8 to 20% by weight. When the use ratio is less than 5% by weight, the heat resistance is not sufficiently improved, while when it exceeds 30% by weight. Although the heat resistance is improved, the mechanical strength tends to decrease. In the present invention, when another maleimide-based monomer is used, yellowing tends to occur.

Moreover, the methacrylic resin in the present invention can be copolymerized with an alkyl acrylate having an alkyl group having 1 to 4 carbon atoms, if necessary. Examples of the alkyl acrylate include methyl acrylate, ethyl acrylate, butyl acrylate, and the like, and the use ratio thereof is up to 10% by weight in consideration of heat resistance.

As a polymerization method for obtaining a methacrylic resin in the present invention, bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, and the like,
Any polymerization method can be used. Further, after the polymerization, in order to remove various polymerization aids, residual monomers and oligomers in the copolymer, a method such as degassing, reprecipitation purification, and washing with various poor solvents may be employed, which is performed by heat molding. This is effective in preventing color banding at the time.

The methacrylic resin in the present invention has a saturated water supply rate of 1.
It is designed to be 7% or less, and the deformation when absorbing moisture is greatly improved compared to 2.2% of ordinary methacrylic resin (PMMA). Therefore, it can be used as a pickup lens.

The methacrylic resin in the present invention can be applied not only by injection molding but also by any molding method such as compression molding and micromolding. When used, various additives such as oxidation stabilizers, release agents and antistatic agents are added. Agents can also be added.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited thereto. Various evaluations in the examples were performed according to the following methods.

Saturated water absorption The pellets were dried at 80 ° C for 24 hours, and the cylinder temperature was 260
Injection molding was performed at 60 ° C. and a mold temperature of 60 ° C. to obtain a 50 × 100 × 2 mm plate specimen. The plate was dried at 80 ° C., weighed, then immersed in hot water at 95 ° C. for 24 hours, weighed, and determined by the following equation.

HDT (Heat Deformation Temperature) A pressure-molded sample of 127 × 12.7 × 6.4 mm was obtained from the pellet, and measured according to ASTM D648.

HFR (fluidity) Measured at a load of 10 kg and a temperature of 230 ° C. according to ASTM D1238.

Total light transmittance The total light transmittance was measured according to ASTM D1003 using a flat plate having a thickness of 2 mm.

YI value Using a flat plate having a thickness of 2 mm, the transmitted light was measured using a spectrophotometer.

Birefringence A dumbbell specimen having a total length of 200 mm, a thickness of 3.2 mm, and a width of 12.7 mm at the center was injection molded, and the optical path difference at the center was measured with a polarizing microscope.

Experimental Example 1 1500 g of pure water was placed in a separable flask having an internal volume of 3 equipped with a stirrer, and 15 g of polyvinyl alcohol was used as a dispersant.
Was added and dissolved. Separately cyclohexyl methacrylate
N-octyl mercaptan was added to a monomer mixture of 250 g, methyl methacrylate 650 g, and cyclohexyl maleimide 100 g.
A mixture in which 1.5 g and 1.0 g of azobisisobutyronitrile were stirred and dissolved was charged into a separable flask, and stirred at 400 rpm while purging with nitrogen. Thereafter, the mixture was heated to 80 ° C. to start the polymerization, and after the peak time, the polymerization was performed at 98 ° C. for 1 hour.

The obtained beaded polymer was separated by filtration, washed with water, dried at 75 ° C. for 24 hours, fed to an extruder, extruded at a cylinder temperature of 250 ° C. and pelletized, and subjected to various evaluations. First result
It is shown in the table.

Experimental Examples 2 to 5, Comparative Experimental Example 1 Except that the monomer mixture was changed to that shown in Table 1,
An experiment was performed in the same manner as in Example 1. The results are shown in Table 1.

Reference Example The same evaluation as in Example 1 was performed using acrylet VH (product of Mitsubishi Rayon Co., Ltd.) pellets as a methacrylic resin. The results are shown in Table 1.

The flat plate of Comparative Example 1 was yellowish and transparent, had a total light transmittance of 92.6%, and a YI value of 7.2. Further, the flat plates of Examples 1 to 5 were transparent without yellowing, and had a total light transmittance of 9%.
2.6-92.8%, YI value was in the range of 1.0 ± 0.2.

Comparative Experimental Example 2 500 g of cyclohexyl methacrylate
An experiment was performed in the same manner as in Example 1 except that the amount of methyl methacrylate was changed to 500 g, and the obtained polymer was evaluated.
The saturated water absorption was 1.2%, but the HDT was 95 ° C., which was inferior in heat resistance.

Comparative Experimental Example 3 A monomer mixture was prepared by adding 100 g of cyclohexyl methacrylate,
The experiment was performed in the same manner as in Example 1 except that the methyl methacrylate was changed to 800 g and the cyclohexylmaleimide was changed to 100 g.
When the obtained polymer was evaluated, the HDT was 108 ° C., but the saturated water absorption was 2.0%.

Example 1 and Comparative Example 1 Using the polymers of Example 3 and Comparative Example 1, pickup lenses for compact discs were molded, respectively.
When the change in spherical aberration in an atmosphere of% RH was evaluated by measuring the change in interference fringes, the former (Example 7)
The change was much smaller than the latter (Comparative Example 5), and the lens performance under humid conditions was excellent.

[Effects of the Invention] As described above, the pickup lens of the present invention is excellent in heat resistance, low moisture absorption and low birefringence, and has little change in spherical aberration of the lens. It is possible to provide a pickup lens, which has an industrially superior effect.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-241011 (JP, A) JP-A-64-87649 (JP, A) JP-A-62-177009 (JP, A) JP-A 62-77009 165115 (JP, A) JP-A-62-158708 (JP, A)

Claims (1)

(57) [Claims] 1. A copolymer comprising (A) 20 to 85% by weight of cyclohexyl methacrylate (B) 70 to 10% by weight of methyl methacrylate (C) 5 to 30% by weight of at least one selected from cyclohexylmaleimide and isopropylmaleimide Pickup lens obtained from.