UV-CURED OPTICAL ELEMENTS

Abstract: The invention relates to UV-cured optical elements, a process for the manufacture of such optical elements, and the use of UV-curable compositions in the preparation of UV-cured optical elements. The UV-cured optical element comprises the reaction product of a polyalkylene glycol di(meth)acrylate and a benzyl (meth)acrylate or derivatives thereof. Preferably, the optical element is an ophthalmic lens.

Description: UV-CURED OPTICAL ELEMENTS The invention relates to UV-cured optical elements, a process for the manufacture of such optical elements, and the use of UV-curable compositions in the preparation of UV-cured optical elements. Recently, the use of ultraviolet light (UV-curing) in manufacturing optical elements, such as audio and video discs, prisms, camera and telescope lenses, television screens, and ophthalmic lenses, has begun to replace thermal curing to some extent. Compared to thermal curing, UV-curing reduces the manufacturing time considerably (15-25 hours and several minutes, respectively), resulting in a more efficient use of the usually expensive moulds. This would be an advantage especially in the preparation of ophthalmic lenses based on organic glass. However, conventional materials used for organic lens manufacturing include, for example, diethylene glycol bisallyl carbonate and other allyl-based compositions, and UV-curing such materials does not lead to complete polymerisation, due to the relatively low reactivity of the allyl group. Therefore, manufacturing ophthalmic lenses still requires the lengthy thermal curing process. Attempts to shorten the long thermal curing time by increasing the temperature have failed, because in that case the polymerisation will generate more heat per time unit than can be dissipated, causing the temperature inside the curing composition to rise. This will result in a more rapid decomposition of the initiator and, consequently, an even faster polymerisation rate. Eventually, this self accelerating process will lead to an uncontrolled and exothermic polymerisation reaction, causing a very inefficient use of the initiator and resulting in inhomogeneous and soft lenses. Over the last few years, optical research has moved in the direction of (meth)acrylate based compositions for use in UV-cured ophthalmic lenses. Such compositions are reactive enough in UV-curing. In the case of UV- curing, the polymerisation heat will not lower the efficiency of the initiator, as photo-initiators are only activated by UV-radiation. Therefore, a temperature rise inside the mould of approximately 100°C can be readily accepted. Furthermore, any excessive polymerisation heat can be prevented by interruptions of the irradiation process. However, in the preparation of ophthalmic lenses it is required that the properties of the resulting UV-cured lens do not differ from those of lenses prepared by thermal curing. Especially important are colour, homogeneity, impact strength (brittleness), and hardness. European patent application 0 441 383 discloses compositions comprising polybutylene glycol di(meth)acrylate having at least 5, but preferably more than 7 butylene glycol units (having at least 26 and at least 36 atoms in the main chain, respectively) and a monomethacrylate, e.g., benzyl methacrylate. These compositions may be used to obtain UV-cured lenses. However, a lens prepared from such a composition shows a hardness which is too low to be acceptable. EP-A-0 441 383 therefore discloses the use of a third component, e.g., a urethane- or epoxy poly(meth)acrylate which imparts hardness to the ophthalmic lens. It has now been found that the use of a polyalkylene glycol di(meth)acrylate wherein the main chain has an average chain length of fewer than 26 atoms will impart the necessary hardness to an ophthalmic lens without the addition of a third rigid polymer such as mentioned by EP-A-0 441 383. The present invention relates to a UV-cured optical element, substantially halogen free, comprising the reaction product of a composition comprising: a) 20-80 wt% of (a) polyalkylene glycol di(meth)acrylate(s) of formula I, having a polyalkylene glycol main chain -0-(R-0) n - with an average chain length of 4.5 to 25 atoms, wherein n represents an integer of 1 or higher; Ri and R 2 are independently chosen from hydrogen or methyl groups; R is a linear or branched C 2 -C 23 alkylene group optionally substituted with: - epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups; and/or - C 2 -C 20 alkenyl, C 2 -C 20 alkynyl, C 3 -C 20 cycloalkyl, C 6 -C 20 aryl, C 7 -C 20 aralkyl, and C 7 -C 20 alkaryl groups, which groups may be linear or branched, may optionally contain one or more oxygen atoms, nitrogen atoms, sulphur atoms, sulphoxy groups, sulphone groups, -O-CO groups, or -CO-O- groups, and may optionally be substituted with epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups; b) 80-20 wt % of at least one benzyl (meth)acrylate or derivative thereof of formula II, wherein R 3 is hydrogen or a methyl group; R 4 , R 5 , R 6 , R 7 , and R 8 are independently selected from hydrogen, epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups, C r C 20 alkyl groups, C 2 -C 20 alkenyl groups, C 2 -C 20 alkynyl groups, C 3 -C 20 cycloalkyl groups, C 6 -C 20 aryi groups, C 7 -C 20 aralkyl groups, and C 7 - C 20 alkaryl groups, which groups may be linear or branched, may optionally contain one or more oxygen, nitrogen, sulphur, sulphoxy, sulphone, -O-CO, or -CO-O- atoms or groups, and may optionally be substituted with epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups; and c) 0-20 wt% of comonomers. As a result of their synthesis, compounds presented by the formula I are mostly part of a mixture containing compounds that differ in their n value. Therefore, the length of the polyalkylene glycol main chain may be defined by an average chain length. All polyalkylene glycol di(meth)acrylates present in the composition have to be taken into account to calculate the average chain length. The average chain length of the main chain -0-(R-0-) n - in a mixture of compounds is the sum of the multiplications of the mole fraction of a certain compound by the particular main chain length of that compound. For example, a known polyethylene glycol dimethacrylate satisfying formula I wherein R=CH 2 CH 2 and R 1 =R 2 =CH 3 , and in which the average molecular weight of the polyethyleneoxide backbone equals 200 (PEG200DiMA) has the following mole distribution in mole fractions: The average chain length of PEG200DiMA is calculated as follows: 0.081 *7 + 0.304*10 + 0.317*13 + 0.185*16 + 0.079 * 19 + 0.026*22 + 0.007*25 + 0.001*28 = 13. When mixtures of products of polyalkylene glycol di(meth)acrylates defined by the formula I are used in the composition of the present invention, the average chain length is again calculated by taking all polyalkylene glycol di(meth)acrylate compounds present into account. For example, a mixture of 50 wt% of ethylene glycol dimethacrylate (average chain length = 4 atoms) and of 50 wt% of tetrapentylene glycol dimethacrylate (average chain length = 25 atoms) has a mole distribution of 72 mole% ethylene glycol dimethacrylate and 28 mole% tetrapentylene glycol dimethacrylate. Accordingly, the average chain length is: 0.72 * 4 + 0.28 * 25 = 9.9. It is possible to have different alkylene glycol units present in one compound. Each R group may, then, be selected independently from the other(s) from the definition provided above for R, e.g., as presented in the following structure: CH CH 3 CH 2 = C-C-0-CH 2 CH 2 CH 2 CH 2 -0-CH 2 CH 2 -0-CH 2 CH 2 CH 2 -0-C-C = CH 2 II II o O Preferably, R is a C2 and/or C3 linear or branched alkylene group, i.e. 1 ,2- ethylene, 1 ,3-propylene, and/or 1 ,2-propylene group. Preferably, in formula I as mentioned above R., and R 2 are methyl groups. Also preferred is n ranging from 1 to 20, more preferably from 1 to 15, most preferably 1 to 10. The average chain length is preferably 5 to 22 atoms, more preferably 5.5 to 19 atoms, and most preferably 6 to 16 atoms. With regard to the compound represented by formula II, R 4 , R 5 , R 6 , R 7 , and R 8 preferably are hydrogen atoms. More preferably, the compound of formula II is benzyl methacrylate. The comonomers may be acrylic, vinylic, allylic or mixtures thereof. Examples include methyl acrylate, methyl methacrylate, phenyl methacrylate, vinyl acetate, vinyl benzoate, ethoxylated Bisphenol A dimethacrylates, diallyl isophthalate, and diallyl phthalate. Preferably, the composition comprises less than 5 wt% of a comonomer. More preferably, the composition consists of 30-70 wt% of at least one polyalkylene glycol di(meth)acrylate of formula I and 70-30 wt% benzyl (meth)acrylate or a derivative thereof of formula II. Most preferred are compositions comprising 30-70 wt% of a polyalkylene glycol di(meth)acrylate, or a mixture of polyalkylene glycol di(meth)acrylates, with an average chain length from 5 to 22 atoms, and 70-30 wt% of benzylmethacrylate. The composition of the present invention is preferably applied in an ophthalmic lens. Japanese patent application 59 136310 discloses liquid compositions comprising benzyl methacrylate and polyethylene glycol dimethacrylate for optical elements. However, the compositions are stated to be cured thermally using a peroxide. Moreover, the compositions also comprise 40- 90 wt% of tribromophenyl methacrylate and 3-40 wt% of a brominated phenyl acrylate. Accordingly, at least 43 wt% of bromine-containing compounds are present in the composition. It is known to the skilled man that bromine-containing lenses will turn yellow with time (due to UV- radiation of, e.g., sunlight). Accordingly, UV-curing of compositions containing bromine compounds will result in lenses with an unacceptably high yellowness. Therefore, the present invention relates to a substantially halogen free lens, i.e., less than 10 wt% of halogen-containing compounds, preferably less than 5 wt%, while most preferably the composition does not contain any halogen-containing compounds. GB-2176794-A discloses a copolymer characterised by, inter alia, a high refractive index which is suitable for processing into optical lenses. To this end a monomer mixture comprising at least a) a (meth)acrylate which optionally is chlorinated or brominated, b) a di(meth)acrylate which optionally is chlorinated or brominated, and c) a third monomer which typically is chlorinated or brominated and gives the desired high refractive index, is polymerized. In all examples where a benzylmethacrylate is included in the mixture, a brominated monomer is also used. Furthermore, the mixtures are polymerized by means of an organic peroxide using heat. The use of brominated and/or chlorinated monomers in UV-cured compositions, as argued above, is unacceptable. There is no suggestion in GB-2176794-A that lenses according to the invention can be made. Japanese patent application 61 1141716 discloses optical elements prepared from 10-80 wt% methyl methacrylate, 20-80 wt% of a monomethacrylic ester, and 0.3-10 wt% of a crosslinking monomer, e.g., n- C 4 - 12 alkylene glycol di(meth)acrylate. At least 90 wt% of such a composition consists of monofunctional, relatively small monomers. Therefore, the high shrinkage upon polymerisation of these compositions will prohibit their use in most applications in the lens casting area. For, such a high shrinkage rate will inevitably result in very low yields due to a phenomenon called premature release, i.e., the casting separates from the mould during polymerisation, with the intended shape not being maintained. Furthermore, the corresponding polymers will have a highly thermoplastic character resulting in reduced chemical resistance. The composition may also contain one or more conventional additives to act as release agents, dyes, pigments, ultraviolet light absorbers, etc., each preferably in quantities not higher than 1 wt%. Compositions according to the present invention are UV-curable. When, after addition of a photo initiator, the compositions are poured into glass moulds and irradiated, they result in hard and homogeneous castings that are not yellowed and are not brittle. The hardness of the lens is usually expressed as Barcol hardness. Preferably the lenses have a Barcol hardness greater than 15. More preferably, the Barcol hardness is greater than 25, while a Barcol hardness greater than 30 is most preferred. The UV-polymerisation process is initiated by photoinitiators in a conventional way. The cast composition comprising 0.001 to 1 wt% of photoinitator is subjected at least once to UV-light to polymerise the composition. The time necessary to complete polymerisation ranges from 20 seconds to 5 hours. It may include one or more dark time periods, in which the sample is taken out of the UV-irradiated area, in order to prevent the cast composition temperature from getting too high. Some non-limiting examples of photoinitiators are methylphenyl glyoxylate, n-butylbenzoin ether, 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-benzoylcyclohexan-1-ol, benzophenone, 2,4,6-trimethylbenzoyldiphenyl phosphine oxide, and mixtures thereof. After the curing process, the lens can be subjected to further physical or chemical treatment such as surface polishing, antistatic treatment, hard coating, non-reflective coating, dyeing or photochromic treatment as needed in order to prevent reflection, improve abrasion resistance, improve chemical resistance, impart hardness, impart anti-mist properties or impart fashionability. The invention will be further illustrated by the following examples, which are not to be construed as limiting the invention in any way. The scope of the invention is to be determined from the claims appended hereto. Experimental The properties of the materials indicated in the examples were determined as follows: the index of refraction (η D 20 ) and the Abbe value with the use of a Zeiss refractometer; the Barcol hardness with a Barber-Coleman impressor (model GYZJ 934-1). Abbreviations: BMA: benzyl methacrylate EGDiMA: ethylene glycol dimethacrylate DEGDiMA: diethylene glycol dimethacrylate TEGDiMA: triethylene glycol dimethacrylate TTEGDiMA: tetraethylene glycol dimethacrylate PEG200DΪMA: polyethylene glycol dimethacrylate in which the average molecular weight of the polyethylene backbone equals 200 PEG400DiMA: polyethylene glycol dimethacrylate in which the average molecular weight of the polyethylene backbone equals 400. The ethylene glycol distribution of the polyethylene glycol dimethacrylates used in Examples 1-8 and A-E is listed in Table 1. TABLE 1 1 Av. chain length = average chain length of the main chain in number of atoms Examples 1-8 Prepared were 30 g of a monomer composition, consisting of polyethylene glycol dimethacrylate and benzyl methacrylate in the weight ratios listed in Table 2, and 0.015 g (0.05 wt%) of 2,4,6-trimethylbenzoyldiphenyl phosphine oxide. Next, the mixtures were poured into glass moulds having a 5 mm thick moulding cavity, and subsequently irradiated by a Philips HOK lamp (2000 W; 100 W/cm). The temperature inside the moulds was monitored by means of a thermocouple. Irradiation periods were alternated with periods of dark time (in which the mould was taken out of the irradiated area) to prevent the samples from having a temperature above 150°C. Moreover, the lenses were turned after every irradiation period. After completion of the curing cycle, the lenses were removed from the mould. TABLE 2 As can be seen from the results listed in Table 2, the Barcol hardness of the lenses of the present invention is satisfactory. Also, none of these lenses were yellow or cracked. Cracking is indicative of the lens being too brittle. Comparative examples A-E The procedure of Examples 1-8 was repeated. However, in comparative Examples A and B use was made of EGDiMA, and in comparative Examples C, D, and E PEG400DiMA was employed. The results are given in Table 3. As can be seen from the results in Table 3, the lenses made from the compositions of Comparative Examples A-B were far too hard and brittle (and therefore cracked) to be employable. On the other hand, the compositions of Comparative Examples C-E resulted in lenses which were soft and flexible, due to the fact that the average chain length is too long. TABLE 3 n.m. = not measured

Claims: Claims 1. A UV-cured optical element, substantially halogen free, consisting of the reaction product of a composition comprising: a) 20-80 wt% of (a) polyalkylene glycol di(meth)acrylate(s) of formula I, having a polyalkylene glycol main chain -O-(R-0) n - with an average chain length of 4.5 to 25 atoms, wherein n represents an integer of 1 or higher; Ri and R 2 are independently chosen from hydrogen or methyl groups; R is a linear or branched C 2 -C 23 alkylene group optionally substituted with: - epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups; and/or - C 2 -C 20 alkenyl, C 2 -C 20 alkynyl, C 3 -C 20 cycloalkyl, C 6 -C 20 aryl, C 7 - C 20 aralkyi, and C 7 -C 20 alkaryl groups, which groups may be linear or branched, may optionally contain one or more oxygen atoms, nitrogen atoms, sulphur atoms, sulphoxy groups, sulphone groups, -O-CO groups, or -CO-O- groups, and may optionally be substituted with epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups; b) 80-20 wt % of at least one benzyl (meth)acrylate or derivative thereof of formula II, wherein R 3 is chosen from hydrogen or methyl groups; R 4 , R 5 , R 6 , R 7 , and R 8 are independently selected from hydrogen, epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups, C C 20 alkyl groups, C 2 -C 20 alkenyl groups, C 2 -C 20 alkynyl groups, C 3 - C 20 cycloalkyl groups, C 6 -C 20 aryl groups, C 7 -C 20 aralkyi groups, and C 7 -C 20 alkaryl groups, which groups may be linear or branched, may optionally contain one or more oxygen, nitrogen, sulphur, sulphoxy, sulphone, -O-CO, or -CO-O- atoms or groups, and may optionally be substituted with epoxy, cyano, amino, thiol, hydroxy, nitro, and amido groups; and c) 0-20 wt% of comonomers. 2. An optical element according to claim 1 , characterised in that the groups R< and R 2 are methyl groups. 3. An optical element according to claim 2, characterised in that R is a C2 and/or C3 alkylene group, i.e. 1 ,2-ethylene, 1 ,3-propylene, and/or 1 ,2-propylene group. 4. An optical element according to claim 3, characterised in that the average chain length ranges from 5 to 22. 5. An optical element according to any one of the preceding claims, characterised in that R 4 , R 5 , R 6 , R 7 and R 8 are hydrogen atoms. 6. An optical element according to claim 5, characterised in that the compound of formula II is benzyl methacrylate. 7. An optical element according to any one of the preceding claims, characterised in that the composition comprises 30-70 wt% polyalkylene glycol di(meth)acrylate(s) of formula I and 70-30 wt% benzyl (meth)acrylate or a derivative thereof of formula II. 8. An optical element according to any one of the preceding claims, characterised in that the optical element is an ophthalmic lens. 9. A process for the production of an optical element according to any one of the preceding claims, comprising the steps of mixing a compound of formula I as defined in claim 1 , a compound of formula II as defined in claim 1 , optionally, a comonomer, and a photoinitiator, casting the resulting mixture into a mould, and subjecting the mixture at least once to UV-light. 10. Use of a composition comprising a compound of formula I as defined in claim 1 , a compound of formula II as defined in claim 1 , and, optionally, a comonomer in the preparation of UV-cured optical elements.

Inventors: PAAY JANET DIEUWERTJE FREDERIK, VAN DE GRAAF ROBERT WINSTON, PAAY, JANET, DIEUWERTJE, FREDERIKE, VAN DE GRAAF, ROBERT, WINSTON
Applicants: AKZO NOBEL NV, GRAAF ROBERT WINSTON VAN DE, PAAY JANET DIEUWERTJE FREDERIK, AKZO NOBEL N.V., PAAY, JANET, DIEUWERTJE, FREDERIKE, VAN DE GRAAF, ROBERT, WINSTON
Application year: 1997
Main classification: G02B1
IPC codes: C08F220, C08F222, C08F290, G02B1, G02C7