Europaisches Patentamt 19 European Patent Office Office europeen des brevets © Publication number : 0 676 428 A1 EUROPEAN PATENT APPLICATION © Application number : 95302123.5 (sT) Int. CI.6: C08G 18/38 © Date of filing : 29.03.95 © Priority : 06.04.94 JP 68479/94 © Inventor : Okazaki, Koju 1-10, Yoneomachi (43) Date of publication of application : Ohmuta-shi, Fukuoka-ken (JP) 11.10.95 Bulletin 95/41 Inventor : Kanemura, Yoshinobu 300, Hirabarucho Ohmuta-shi, Fukuoka-ken (JP) @ Designated Contracting States : Inventor : Nagata, Teruyuki DE FR GB IT 89-4, Shiraganemachi Ohmuta-shi, Fukuoka-ken (JP) © Applicant : MITSUI TOATSU CHEMICALS, Inc. 2-5 3-chome Kasumigaseki © Representative : Stuart, Ian Alexander et al Chiyoda-Ku Tokyo 100 (JP) MEWBURN ELLIS York House 23 Kingsway London WC2B 6HP (GB) © Composition for low-specific gravity urethane-base plastic lens. © The present invention is concerned with a composition for a low-specific gravity urethane-base plastic lens which comprises an iso(thio)cyanate compound and an active hydrogen compound selected so as to meet the formula (1) (A1 + kA2)D1 x D2 _ 1.136_ (1) A1 x D2 + kA2 x D1 wherein A1-M A1 A1-N A1-M : the molecular weight of the iso(thio)cyanate compound, and A1-N : the number of the functional group of the iso(thio)cyanate compound, A2-M A2 = A2-N A2-M : the molecular weight of the active hydrogen compound, and A2-N : the number of the functional group of the active hydrogen compound, D1 = the specific gravity of the iso(thio)cyanate compound, D2 = the specific gravity of the active hydrogen compound, and mole number of an active hydrogen group k — mole number of the iso(thio)cyanate group, a process for preparing a low-specific gravity urethane-base plastic lens which comprises the steps of mixing and then heating the composition to harden the same, and a plastic lens obtained by the preparation process, which lens has a low specific gravity of 1.22 or less. 00 CM CO h- LU Jouve, 18, rue Saint-Denis, 75001 PARIS EP 0 676 428 A1 BACKGROUND OF THE INVENTION The present invention relates to a low-specific gravity urethane-base plastic lens preferably having good optical properties and excellent impact resistance which are required for various kinds of optical lenses such 5 as lenses for spectacles, and it also relates to a process for preparing the lens and to a composition therefor. In recent years, plastics have rapidly spread as materials for optical elements such as lenses for spectacles and cameras, because they are light-weight and more difficult to break as compared with inorganic lenses and they can also be dyed. Nowadays, as a resin which has been widely used forthese purposes, there is a product obtained by the radical polymerization of diethylene glycol bis(allyl carbonate) (hereinafter referred to as 10 "D.A.C."). This resin has some features, for example, it is excellent in impact resistance, dyeing properties and workability of cutting, polishing and the like, and it is also light-weight. However, the refractive index of this resin is as low as nd = 1.50, as compared with a glass lens (nd = 1.52), and in order to obtain equivalent optical properties to the glass lens, it is necessary to increase the central thickness, the edge thickness and a curvature of the lens. Therefore, it is not avoidable that the lens of this 15 resin becomes thick on the whole. Accordingly, a plastic lens has been desired which has excellent character- istics corresponding to those of D.A.C. and a refractive index higher than the inorganic lens. As a lens which can meet these requirements, a polyurethane plastic lens is known. As this polyurethane lens, the present inventors have already suggested a polyurethane lens made of a polymer of a xylylene dii- socyanate compound and a polythiol compound, for example, in Japanese Patent Application Laid-open No. 20 63-46213 (46213/1988), and this kind of polyurethane lens is now spreading as optical lenses such as lenses for spectacles. However, the reduction of the specific gravity of the lenses has been presently required in addition to the above-mentioned physical properties, and in reply to this further requirement, the present inventors have al- ready suggested a urethane-base plastic lens made from isophorone diisocyanate (hereinafter abbreviated to 25 "IPDi") and 1 ,2-bis(2-mercaptoethylthio)-3-mercaptopropane (hereinafter abbreviated to "GST") in Japanese Patent Application Laid-open No. 2-270859 (270859/1990). The specific gravity of the plastic lens made from IPDi and GST is 1.23, which specific gravity is much lower than in the conventional urethane-base plastic lens. Nevertheless, this plastic lens does not sufficiently meet the requirement of the further reduction of the specific gravity. 30 Hence, the development of the urethane-base plastic lens having the further low specific gravity has strongly been desired. Preferred embodiments of the present invention may provide one or more of the following objects: a low-specific gravity urethane-base plastic lens having excellent optical properties by reducing the spe- cific gravity of the plastic lens; 35 a composition for the urethane-base plastic lens which can be used to obtain this low-specific gravity urethane-base plastic lens; a process for preparing the plastic lens by the use of this composition. The present inventors have intensively investigated and as a result, it has been found that when the following process is used, a urethane-base plastic lens having excellent optical 40 properties and a low specific gravity of 1 .22 or less can be surprisingly obtained. In consequence, the present invention has now been attained. That is to say, a first aspect of the present invention is directed to a composition for a low-specific gravity urethane-base plastic lens which comprises an iso(thio)cyanate compound and an active hydrogen compound selected so as to meet the formula (1) 45 (A1 + kA2)D1xD2 ^ <Qa m A1 xD2 + kA2xD1 w wherein A1 = A1 -N 50 A1-M: the molecular weight of the iso(thio)cyanate compound, and A1-N: the number of the functional group of the iso(thio)cyanate compound, A2-M AZ = A2-N A2-M: the molecular weight of the active hydrogen compound, and 55 A2-N: the number of the functional group of the active hydrogen compound, D1 = the specific gravity of the iso(thio)cyanate compound, D2 = the specific gravity of the active hydrogen compound, and 2 EP 0 676 428 A1 mole number of active k _ an hydrogen group mole number of the iso(thio)cyanate group. A second aspect of the present invention is directed to a process for preparing a low-specific gravity ur- ethane-base resin plastic lens which comprises the steps of mixing and then heating the composition to harden 5 the same. A third aspect of the present invention is directed to a plastic lens obtained by the preparation process. The above-mentioned iso(thio)cyanate compound and active hydrogen compound may be used in the state of a mixture of separately two or more kinds thereof, and in this case, A1, D1, A2and D2 are weighted averages of each mixture. 10 Next, the present invention will be described in detail. A composition for lenses of the present invention comprises one or more iso(thio)cyanate compounds se- lected from the group consisting of isocyanate compounds and isothiocyanate compounds which may have an isocyanate group, and one or more active hydrogen compounds preferably selected from hydroxy com- pounds and mercapto compounds which may have a hydroxy group. 15 Examples of the iso(thio)cyanate compounds which can be used in the present invention include monoi- so(thio) cyanates and polyiso(thio)cyanates. Typical examples of the monoiso(thio)cyanates include monoisocyanates such as phenyl isocyanate, butyl isocyanate and cyclohexyl isocyanate, and monoisothiocyanates such as phenyl isothiocyanate, butyl isothio- cyanate and cyclohexyl isothiocyanate. In addition, there can also be used these compounds which are sub- 20 stituted with halogens such as chlorine and bromine, alkyl groups, alkoxy groups and a nitro group. They may be used singly or in a combination of two or more thereof. Examples of the polyisiocyanate compounds include aliphatic polyisocyanates such as ethylene diiso- cyanate, trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, octamethy- lene diisocyanate, nonamethylene diisocyanate, 2,2'-dimethylpentane diisocyanate, 2,2,4-trimethylhexane dii- 25 socyanate, decamethylene diisocyanate, butene diisocyanate, 1,3-butadiene-1,4-diisocyanate, 2,4,4-trime- thylhexamethylene diisocyanate, 1,6,11-undecane triisocyanate, 1,3,6-hexamethylene triisocyanate, 1,8-dii- socyanato-4-isocyanatomethyloctane, 2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane, bis(iso- cyanatoethyl) carbonate, bis(isocyanatoethyl) ether, 1,4-butylene glycol dipropyl ether-to,to '-diisocyanate, ly- sine diisocyanatomethyl ester, lysine triisocyanate, 2-isocyanatoethyl-2,6-diisocyanatohexanoate, 2-isocya- 30 natopropyl-2,6-diisocyanatohexanoate, xylylene diisocyanate, bis(isocyanatoethyl)benzene, bis(isocyanato- propyl)benzene, a, a, a', a'-tetramethylxylylene diisocyanate, bis(isocyanatobutyl)benzene, bis(isocyanato- methyl)naphthalene, bis(isocyanatomethyl) diphenyl ether, bis(isocyanatoethyl) phthalate, mesitylene triiso- cyanate and 2,6-di(isocyanatomethyl)furan; alicyclic polyisocyanates such as isophorone diisocyanate, bis(isocyanatomethyl)cyclohexane, dicyclohexylmethane diisocyanate, cyclohexane diisocyanate, methylcy- 35 clohexane diisocyanate, dicyclohexyldimethylmethane diisocyanate,