PROCESS FOR MAKING A HIGH-REFRACTIVITY PLASTIC LENS RESIN

Abstract: Disclosed herein is a colorless transparent plastic lens having high refractivity and low dispersion. The lens is light, has heat resistance sufficient for post processing, and is excellent in grinding machinability and impact resistance. It is made of a sulfur-containing urethane resin, which is a polymer of a specific polythiol and a particular poly­ isocyanate of at least bifunctionality.

Description: This invention relates to a Process for making a plastic lens resin having a high refractive index (hereinafter may also be called "refractivity"), low dispersion, and excellent heat resistance. Plastic lenses have been finding utility as eye lenses and camera lenses and in optical devices in recent years since they are lighter, less fragile and dyeable compared with inorganic lenses. As a resin employed widely for these purposes, there is a resin obtained by subjecting diethylene glycol bisallylcarbonate to radical polymerization. Its refractive index is however small (n D =1.50) compared with inorganic lenses (n D =1.52). In order to achieve optical characteristics equivalent to the inorganic lenses, it is necessary to increase the curvature of a lens so that the central thickness and peripheral thickness of the lens increase and the overall thickness of the lens hence becomes greater unavoidably. There is thus an outstanding demand for the development of a lens resin having a higher refractive index in order to overcome this inconvenience. It has therefore been proposed to introduce one or more aromatic rings, chlorine atoms or bromine atoms in the chemical structures of various monomers. The introduction of one or more aromatic rings alone can however hardly achieve a high refractive index, whereas the sole introduction of one or more halogen atoms results in a greater specific gravity and impairs the advantageous light weight feature of plastic lenses. Since high refractivity, low dispersion and low specific gravity can be expected from the introduction of one or more sulfur atoms into the chemical structure of a monomer, sulfur-containing urethane resins have been proposed (Japanese Patent Laid-Open Nos. 194401/1985 and 217229/1985). These sulfur-containing urethane resins are however inferior in heat resistance to conventional radical-polymerization olefin resins, they are susceptible to deformations upon post processing such as lens dyeing and surface coating so that the processing temperature must be controlled low during such post processing. A limitation is hence imposed on their wide-spread use. With the foregoing in view, the present inventors have carried out an extensive investigation on various polyols and polythiols having at least trifunctionality in order to find out monomers capable of providing sulfur-containing urethane resins which have heat resistance sufficient to withstand heating conditions in post processing such as dyeing and surface coating while maintaining good physical properties as lenses such as high refractivity, low dispersion and low specific gravity. As a result, it has been found a process for making a Plastic lens resin by reacting at least one polyisocyanate being at least bifunctional, with at least one polythiol represented by the following general formula: wherein K is a substituent in which R means a hydrogen atom or methyl group, m stands for an integer of 0 - 3, n for an integer of 0 - 1, p for an integer of 0 - 1 and q for an integer of 1 - 3, wherein the ratio of polyisocyanate to the polythiol of the formula (I) is in the range of 0.5 to 3 in terms of NCO/SH molar ratio. The term "at least bifunctionality" as used herein means either bifunctionality or any higher functionality". The present invention therefore provides a high-refractivity plastic lens resin having high refractivity, low dispersion and low specific gravity and excellent heat resistance and is colorless and transparent, and does not undergo deformations even when heated upon post processing such as dyeing and surface coating. It is therefore suitable as eye lenses and camera lenses and as a resin useful in other optical devices. As illustrative examples of the polythiol of the general formula (I) useful in the practice of this invention, may be mentioned tris(2-mercaptoethyl) cyanurate, tris(3-mercapto-n-propyl) isocyanurate, tris(2-methyl-3-mercapto-n-propyl) isocyanurate, tris(mercaptomethyl) isocyanurate, the mercaptopropionate of tris(2-hydroxyethyl)isocyanuric acid, the mercaptoacetate of tris(2-hydroxyethyl)isocyanuric acid, the mercaptopropionate of tris(hydroxymethyl)isocyanuric acid, the mercaptoacetate of tris(hydroxymethyl)isocyanuric acid, etc. On the other hand, as exemplary polyisocyanates which have at least bifunctionality and are useful in the practice of this invention, may be mentioned xylylene diisocyanate, nucleus-chlorinated xylylene diisocyanates, α,α,α',α'-tetramethylxylylene diisocyanate (hereinafter abbreviated as "tetramethylxylylene diisocyanate"), hexamethylene diisocyanate, isophorone diisocyanate, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, the biuret reaction product of hexamethylene diisocyanate, the adduct reaction product of hexamethylene diisocyanate and trimethylolpropane, the trimer (isocyanurate) of hexamethylene diisocyanate, methylcyclohexane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, ethyl lysine isocyanate-β-isocyanate, etc. In view of the weatherability of the resulting resins, preferred are aromatic isocyanates obtained by substituting one or more isocyanate groups for the corresponding number of alkyl groups as side chains of an aromatic ring, such as xylene diisocyanate and tetramethylxylylene diisocyanate; and aliphatic isocyanates such as isophorone diisocyanate, methylcyclohexane diisocyanate and hexamethylene diisocyanate. In the present invention, said at least one polyisocyanate having at least bifunctionality and said at least one polythiol represented by the general formula (I) are used at a ratio in a range of 0.5 - 3.0, preferably, 0.5 - 1.5 in terms of NCO/SH molar ratio (which means the ratio of -NCO groups of the polyisocyanate to -SH groups of the polythiol; this will apply to all subsequent molar ratios). In order to increase the crosslinking degree of the polymer (resin), the present invention also allows to incorporate one or more polyols and/or polythiols such as pentaerythritol, trimethylolpropane, pentaerythritol tetrakis(thioglycolate), pentaerythritol tetrakis(mercaptopropionate), trimethylolpropane tris(mercaptopropionate), trimethylolpropane tris(thioglycolate) and pyrogallol. In this case, the amount of the polyisocynate is increased to maintain the NCO/(SH + OH) molar ratio within a range of 0.5 - 3.0, preferably, 0.5 - 1.5 as calculated on the basis of the whole monomers, further the ratio of polyisocyanate to the polythiol of formula (I) is 0.5 to 3.0 in terms of NCO/SH molar ratio. It may also be feasible in the present invention to mix one or more polythiols having an aromatic ring, such as xylylene dithiol, benzene dithiol and toluene dithiol, their nucleus-halogenated derivatives, bismuthiol. In this case, the amount of the polyisocyanate is increased to maintain the NCO/SH molar ratio within a range of 0.5 - 3.0, preferably, 0.5 - 1.5 as calculated on the basis of the whole monomers, further the ratio of polyisocyanate to the polythiol of formula (I) is 0.5 - 3.0 in terms of NCO/SH molar ratio. In the present invention, a catalyst such as dibutyltin laurate, dimethyltin chloride or lead 2-ethylhexanoate may be added in an amount of 0.01 - 1.0 wt.%. It may also be feasible in the present invention to add suitably one or more additives such as ultraviolet ray absorbent, oxidation inhibitor, coloring inhibitor and/or fluorescent dye as needed in order to maintain physical properties of the resultant resin. For the production of the lens resin of this invention, at least one polythiol represented by the general formula (I), at least one polyisocyanate having at least bifunctionality, and if necessary, a cross-linking agent such as the above-mentioned polyols or polythiols or a polythiol having an aromatic ring such as the above-mentioned aromatic polythiols are mixed. The resultant mixture is then processed by a known cast-molding technique, namely, is poured into a mold formed in combination of a glass or metal mold, which has beforehand been treated with a parting agent, and a TEFLON (trade mark), polyethylene or polyvinyl chloride gasket, followed by its heating and curing. The reaction temperature and reaction time vary depending on the kinds of monomers to be used but may generally be -20°C - 150°C and 0.5 hour - 72 hours respectively. (Examples) The present invention will hereinafter be described in more detail by the following Examples, in which all designations of "part" or "parts" mean part or parts by weight. The refractive index, Abbe number, lens polishing machinability, impact resistance and dyeability of a lens resin obtained in each of the following Examples were measured or determined respectively in accordance with the following testing methods. Refractive index and Abbe number: Measured at 20°C by means of a Pulfrich refractometer. Lens polishing machinability: The resin was ground and polished by a eyeglass lens polisher. The machinability is rated good (O) where the thus-polished surface was good, whereas the machinability is rated poor (X) where the polisher was loaded and the grinding and polishing were no longer feasible. Impact resistance: Using a flat plate having a central thickness of 2 mm, a falling ball impact test was conducted in accordance with the FDA standards. The impact resistance is rated good (O) where the flat plate did not shatter. Dyeability: In 300 parts of water, were dispersed 0.9 part of "Miketon Polyester Red FB" (trade name; product of Mitsui-Toatsu Dyes, Inc.), 0.1 part of dialkyl sulfosuccinate and 0.1 part of the sodium salt of a condensation product of β-naphthalenesulfonic acid and formaldehyde, followed by an addition of 1.5 parts of "Tetromin OEN" (trade name for an orthophenylphenol type carrier; product of Yamakawa Chemical Ind. Co., Ltd.) as a carrier. The resulting mixture was heated to 85°C under thorough stirring. Lenses were immersed in the thus-prepared dye bath. After maintaining the lenses at the same temperature for 10 minutes, they were pulled out of the dye bath and then washed thoroughly with water. Those colored in a red color were rated good (O) while those not colored were rated poor (X). Heat resistance: Test pieces were separately heated at a rate of 2.5°C/min under a load of 5 g by means of a "Thermomechanical Analyzer" (trade name; manufactured by Parkin-Elmer Company, U.S.A.) to measure their heat-distortion starting temperatures. Example 1: Mixed into a uniform liquid were 188 parts of m-xylylene diisocyanate, 234 parts of tris(3-mercapto-n-propyl) isocyanurate and 0.1 part of dibutyltin dilaurate. The homogeneous liquid was poured into a mold composed of a glass mold, which had been treated with a silicone parting agent of the baking type, and a polyethylene gasket. After heating the homogeneous liquid at 55°C for 3 hours, at 60°C for 2 hours, at 80°C for 1 hour and then at 90°C for 1 hour, the mold was cooled to release the resultant lens. Its refractive index (η²⁰), Abbe number and specific gravity were 1.615, 34 and 1.33 respectively. The lens had good polishing machinability, impact resistance and dyeability. Its heat-distortion starting temperature was 120°C. Examples 2 - 15: Following the procedure of Example 1, lenses were produced separately from the compositions given in Table 1. Test results are also shown in the same table. Comparative Example 1: Without using any isocyanurate compound of the general formula (I), 222 parts of isophorone diisocyanate, 244 parts of pentaerythritol tetrakis(mercaptopropionate) and 0.3 part of dibutyltin dilaurate were mixed into a uniform liquid. The homogeneous liquid was poured into a mold composed of a glass mold, which had been treated with a silicone parting agent of the baking type, and a polyester gasket. After heating the homogeneous liquid at 55°C for 3 hours, at 60°C for 2 hours, at 80°C for 1 hour and then at 90°C for 1 hour, the mold was cooled to release the resultant lens. Its refractive index (η²⁰), Abbe number and specific gravity were 1.55, 44 and 1.32 respectively. The lens had good polishing machinability, impact resistance and dyeability but its heat-distortion starting temperature was as low as 90°C. Comparative Examples 2 - 4: Without using any isocyanurate compound of the general formula (I), lenses were produced separately by using m-xylylene diisocyanate,namely, from the compositions given in Table 1 in the same manner as in Comparative Example 1. Test results are also shown in the same table.

Claims: A process for making a resin by reacting at least one polyisocyanate being at least bifunctional with at least one polythiol of formula (I) wherein K is a substituent in which R means a hydrogen atom or methyl group, m stands for an integer of 0 - 3, n for an integer of 0 - 1, p for an integer of 0 - 1 and q for an integer of 1 - 3, wherein the ratio of polyisocyanate to the polythiol of formula (I) is in the range of 0.5 to 3 in terms of NCO/SH molar ratio. The process as claimed in Claim 1, wherein the polythiol is tris(3-mercapto-n-propyl) isocyanurate. The process as claimed in Claim 1, wherein the polyisocyanate is one member of the group consisting of xylylene diisocyanate, α,α,α',α'-tetramethylxylylene diisocyanate, isophorone diisocyanate and methylcyclohexane diisocyanate. A process for making a resin by reacting at least one polyisocyanate being at least bifunctional with at least one polythiol of formula (I) wherein K is a substituent in which R means a hydrogen atom or methyl group, m stands for an integer of 0 - 3, n for an integer of 0 - 1, p for an integer of 0 - 1 and q for an integer of 1 - 3, and additionally with one or more polythiols having an aromatic ring selected from: xylylene dithiol, benzene dithiol and toluene dithiol, their nucleus-halogenated derivatives, bismuthiol, or a crosslinking agent selected from: pentaerythritol, trimethylolpropane, pentaerythritol tetrakis (thioglycolate),pentaerythritol tetrakis (mercaptopropionate), trimethylolpropane tris (thioglycolate), trimethylolpropane tris (mercaptopropionate) and pyrogallol, wherein the ratios of polyisocyanate to the total of the polythiol of formula (I) and the crosslinking agent when used or the aromatic polythiol when used in terms of NCO/OH+SH molar ratio is in the range of 0.5 to 3, and polyisocyanate to the polythiol of formula (I) is in the range of 0.5 to 3 in terms of NCO/SH molar ratio. The process of Claim 4 wherein the NCO/OH+SH molar ratio is in the range of 0.5 to 1.5. The process of any one of Claim 4 or Claim 5, wherein the polythiol is tris(3-mercapto-n-propyl) isocyanurate. The process of any one of Claim 4 or Claim 5, wherein the polyisocyanate is one member of the group consisting of xylylene diisocyanate, α,α,α',α'-tetramethyl-xylylene diisocyanate, isophorone diisocyanate and methylcyclohexane diisocyanate. The lens comprising the resin obtained by the process of any one of Claims 1 to 7.

Inventors: IMAI, MASAO, KAJIMOTO, NOBUYUKI, KANEMURA, YOSHINOBU, NAGATA, TERUYUKI, SASAGAWA, KATSUYOSHI
Applicants: MITSUI TOATSU CHEMICALS, INC.
Application year: 1987
Main classification: C08G18
IPC codes: G02B1, C08G18