Information recording substrate

Abstract: A methacrylic resin having reduced water absorption properties is produced by polymerizing a mixture consisting of (A) 50 to 85% by weight of a methyl methacrylate, (B) 15 to 50 by weight of a cyclohexyl methacrylate and (C) 0 to 30% by weight of the other copolymerizable with said monomer (A) and (B), or partially polymerized product thereof. The methacrylic resin can suitably be used to various optical equipments, information recording substrates and so on.

Description: This invention relates to an information recording substrate comprising an optical resin composition consisting essentially of a copolymer of cyclohexyl methacrylate with methyl methacrylate. Methacrylic resin comprising methyl methacrylate as the main constitutional unit are not only very much excellent in transparency and weather resistance but also well-balanced in mechanical and thermal properties and fabricability, and they are widely used in various fields of application such as sign-boards, parts of illumination equipments, electric equipments and automobile as well as miscellaneous goods while utilizing their advantageous features. While on the other hand, with the development of their application uses, demands have more and more increased for the performances of the raw material resins and the improvement therefore has been desired depending on the fields of the application. One of such demands is directed to the problem of hygroscopic property. Since methacrylic resins are relatively more hygroscopic than polyolefin resins or polystyrene resins and, in view of the dimensional changes and warpage due to the moisture absorption, or generation of cracks due to the repeating cycle of moisture absorption and drying over a long time, their uses have been restricted depending on the commercial goods in certain application fields. EP-A-83 301 506.8 which was published after the filing date of the present patent under Publication No EP-A-0090588 (Art. 54(3) EPC), relates to an optical element prepared by an injection molding, compression molding or the like, which comprises an optical resin composition consisting essentially of a polymer selected from the group consisting of a copolymer of a cyclohexyl methacrylate with at least one other methyl methacrylate and a copolymer of a methylcyclohexyl methacrylate with at least one other ethyl methacrylate. US-A-2 129 666 covers a polymeric methacrylic acid ester of an alcohol selected from the group consisting of an alicyclic alcohol and tetrahydrofurfuryl alcohol in which there is no branch-chain substitution on the carbon atom adjacent to the carbinol group prepared by heating the monomeric ester to a temperature of from 60 to 100°C in the presence of benzoyl peroxide. DE-A-2 730 370.4 discloses the use of chemically uniform copolymer for the manufacture of optical elements having a low water absorption rate. The low water absorption rate is achieved by means of a copolymer containing 95 to 10% by weight of a cyclohexyl methacrylate, 5 to 20% by weight of maleic anhydride and 0 to 70% by weight of further comonomers. DE-A-2 635 691 relates to an optical resin consisting essentially of copolymers of cyclohexyl methacrylate and styrene exhibiting low water absorption. The hygroscopicity in the methacrylic resin is essentially attributable, to some extent, to chemical structure of the polymer thereof. Then, there is scarcely a proposal to find for improving the nature of methacrylic resins, and, naturally, no attempts have been made for developing them into commercial goods up to now. While on the other hand, it has already been recognized long since that a homopolymer of cyclohexyl methacrylate has a higher refractive index as compared with other methacrylate type polymers, as well as has an excellent light dispersing property thus having excellent basic performances as the material for plastic lenses. However, since polymers of cyclohexyl methacrylate are much brittle, it is difficult to mold under usual conditions and presents a very significant problem for putting them into practical use. In order to overcome the foregoing defects and for developing methacrylic resins with excellent optical properties, the present inventors have made detailed studies on various types of copolymers of cyclohexyl methacrylate and, as the result, have accomplished the present invention based on the discovery of a quite novel effect that they can reduce the hygroscopicity of methyl methacrylate polymers by the preparation of copolymers containing cyclohexyl methacrylate and methyl methacrylate. The copolymers have optical properties quite different to those of the known polymers. Moreover, the copolymers having a specific range of compositions have excellent properties such as transparency, weather resistance, mechanical and thermal properties, as well as fabricability. The object of the present invention is an information recording substrate comprising an optical low hygroscopic methacrylic resin having a water adsorption rate of no greater than 1.4 % consisting essentially of a copolymerized mixture of at least 50% by weight of methyl methacrylate, at least 15% by weight of cyclohexyl methacrylate and 1-15% by weight of a third monomer selected from methyl acrylate or butyl acrylate. Having excellent optical properties and in view of the significant improvement in the dimensional changes and warpage in products due to moisture absorption as compared with conventional methacrylic resins, the low hygroscopic methacrylic resin used according to the present invention provides information recording substrates such as audio discs, video discs and computer information discs. Description of the Preferred Embodiments The methyl methacrylate in the present invention is a component necessary to provide the optical properties, weather resistance, mechanical and thermal properties or fabricability inherent to the obtained resin and the component is used in the monomeric mixture at a ratio of at least 50 % by weight and preferably 55 to 80 % by weight. Most suitable amount of methyl methacrylate Is 55 to 75 % by weight. If it is less than 50 % by weight, the above-mentioned properties are impaired and, on the other hand, if it is exceeds 85 % by weight, the improving effect for the hygroscopicity is decreased. The cyclohexyl methacrylate in the present invention is a component essential to the improvement of the hygroscopicity in the obtained resin and it is used in the monomeric mixture at a ratio of at least 15 % by weight and preferably, 20 to 45 % by weight. If it is less than 15 % by weight, no sufficient improvement can be obtained for the hygroscopicity and, while on the other hand, if it exceeds 50 % by weight, the mechanical properties of the resin is significantly reduced, as well as chips or cracks are resulted upon molding to provide problems in view of the fabricability. The third monomer is selected from butyl acrylate or methyl acrylate 1 to 15 % by weight of methyl acrylate or butyl acrylate is used. In preparing the resin, although the monomeric mixture may be poured together with the radical polymerization initiator into a mold, it is generally preferred to pour it as a partially polymerized product thereof into a mold. In the use of the partially polymerized product, a preferred embodiment for the process can include generally of carrying out a preliminary bulk polymerization to obtain a partially polymerized product with 5 to 40 % by weight of polymer content. Upon casting into the mold, the radical polymerization initiator is added. The radical polymerization initiator can specifically include, for example, azo compounds such as 2',2'-azobis(isobutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile) or organic peroxides such as benzoyl peroxide, lauroyl peroxide and bis-4-tertiary-butyl-cyclohexyl peroxydicarbonate. The radical polymerization initiator is added by 0.0001 to 5 parts by weight and, more preferably, 0.01 to 1 parts by weight based on 100 parts by weight of the monomeric mixture or the partially polymerized product thereof. The monomeric mixture or the partially polymerized product thereof incorporated with the radical polymerization initiator is polymerized in a ordinary manner by pouring the same to the cell between two opposed tempered glass plates sealed with a gasket at the circumference of the glass plate, and then is heated, or by continuously pouring from the upper stream into a space sealed with two endless belts mirror-polished on one surface and gaskets advancing in the same direction at the same speed and is heated to be continuously polymerized. Although depending on the various types of the radical polymerization initiators employed, the temperature for the polymerization is generally between 40 to 140°C and it is generally preferred to carry out the polymerization at two steps of polymerization temperature, that is, 40 to 90°C for the first step and 90 to 140°C for the second step. The thickness for the resin plate obtained through casting polymerization is preferably within a range of 0.5 to 20 mm although it is not particularly restricted thereto. In carrying out the present invention, various additives employed for the production of ordinary methacrylic resin may be added. Specific examples of the additives can include antioxidants or dyes employed for coloration, stabilizers such as UV absorbers, or release agents which facilitate the release of the resin plate from the mold. This invention will now be explained referring to specific examples, wherein "parts" means parts by weight and "%" means % by weight. In the examples, the water absorption rate means the equilibrium water adsorption rate measured according to ASTM D 570 (long-term immersion at 23°C), the total luminous transmittance, parallel light transmittance, and haze represent the values measured according to ASTM D 1003, the bending strength and the heat distortion temperature represent the values measured according to ASTM D 790 and ASTM D 648 respectively. Example To a polymerizing receptor equipped with a stirrer and a condenser, was charged a monomeric mixture comprising 60 parts of methyl methacrylate, 30 parts of cyclohexyl methacrylate and 10 parts of methyl acrylate and circulating hot water was adjusted to 100°C to increase the temperature. When the inside temperature reached 80°C, 0.03 parts of 2,2'-azobis(2,4-dimethyl valeronitrile) were added and when the inside temperature reached 95°C, polymerization was proceeded for 20 minutes. Thereafter, the contents were cooled to obtain a partially polymerized product with 23.1 % of polymer content and having a viscosity of 4,5 d Pa.s. (450 cps) at 20°C. Then, a composition comprising: partially polymerized product obtained as above 100 parts 2,2'-azobis(2,4-dimethyl valerontrile) 0.05 parts Tinuvin-P (Registered Trade mark of UV absorbers manufactured by Ciba-Geigy AG) 0.03 parts sodium dioctylsulfo succinate 0.001 parts was continuously poured from upper stream into a space sealed with two endless stainless steel belts mirror-polished on one surface and gaskets advancing at the same speed in an same direction in an ordinary manner, was passed for 28 minutes through a hot water shower zone at 82°C and was then passed for 14 minutes through a far infrared ray heater heated zone to be heated up to a maximum temperature of 135°C. and an annealing zone to continuously obtain at the downstream a resin plate of 1.5 mm in thickness. The resin plate had the following physical properties. Water absorption rate 1.2 % Total luminous transmittance 93 % Parallel light transmittance 92 % Haze 1 % Bending strength 900 kg/cm² Heat distortion temperature 81°C Comparative Example 1 The same procedures as in Example 1 were repeated except that the charging composition for the monomeric mixture in Example 1 was changed to 100 parts of methyl methacrylate to obtain a resin plate of 1.5 mm in thickness. The water absorption rate and other physical properties were measured for the resin plate to obtain the result as shown in Table 1. Example Comparative Example 1 Water absorption rate (%) 1.2 2.1 Total luminous transmittance (%) 93 93 Parallel light transmittance (%) 92 92 Haze (%) 1 1 Bending strength (kg/cm²) 900 1200 Heat distortion temperature (°C) 81 100 Comparative Examples 2-3 A composition to be poured in was made according to the following composition in which the methyl methacrylate and cyclohexyl methacrylate were made the amounts mentioned in Table 2: methyl methacrylate varied amount cyclohexyl methacrylate varied amount 2,2'-azobis(isobutyronitrile) 0.08 parts Tinuvin-P (Registered Trade Mark of UV absorber manufactured by Ciba-Geigy AG) 0.05 parts Toshiba silicone oil YF 3063 (release agent) 0.0005 parts The composition was poured in between two tempered glass plates with a gasket at the circumference of glass plates in an ordinary manner, dipped in a water bath at 60°C for 16 hours and then heated in an air bath at 120°C for 2 hours to complete polymerization and obtained a resin plate of 2 mm in thickness. The water absorption rate and other physical properties were measured for the resin plate to obtain the result as shown in Table 2.

Claims: An information recording substrate comprising an optical low hygroscopic methacrylic resin having a water absorption rate of no greater than 1.4 % consisting essentially of a copolymerized mixture of at least 50% by weight methyl methacrylate, at least 15% by weight of cyclohexyl methacrylate and 1 - 15% by weight of a third monomer selected from methyl acrylate or butyl acrylate. The information recording substrate of claim 1, wherein the water absorption rate is not more than 1.2%.

Inventors: ABE KAZUNORI, KAMADA KAZUMASA, SIMOMURA YASUNORI, TAMAI KATSUMI, ABE, KAZUNORI, KAMADA, KAZUMASA, SIMOMURA, YASUNORI, TAMAI, KATSUMI
Applicants: MITSUBISHI RAYON CO, MITSUBISHI RAYON CO., LTD.
Application year: 1983
Main classification: C08F220
IPC codes: G02B1, C08F20, C08F220, C08F2, G11B23