Abstract:
A process for polymerizing/curing a photopolymerizable monomer composition comprising at least one diepisulfide monomer which comprises the steps of: 
     a) mixing to the monomer composition an effective amount of a cationic photopolymerization initiator, and 
     b) irradiating the mixture of a) with an ultraviolet radiation to at least partially cationically photopolymerize the monomer composition.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns in general a process for polymerizing/curing diepisulfides which comprises cationic photopolymerization of diepisulfides and the use of such a process for making optical lens materials, in particular optical lens materials having refractive indices of 1.7 or more. 
     2. Description of Prior Art 
     Document U.S. Pat. No. 5,807,975 discloses thermal polymerization and curing of diepisulfides in particular for the manufacture of optical materials such as a lens material for spectacles. This thermal polymerization/curing of diepisulfides lasts for several hours, generally about 20 hours. 
     Photo-initiated cationic polymerization of monoepisulfides is disclosed in the article “Photocationic crosslinking of poly(2,3-epithiopropyl methacrylate) and Photoinitiated cationic polymerization of its model compounds” M. Tsunooka et al, Journal of Polymer Science: polymer chemistry edition, vol. 22, 2217-2225 (1984). However, the photoinitiators used in the photopolymerization are colored material that are not suitable for making optical lens materials. 
     Cationic polymerization of diepisulfides to form polymer networks has not been reported. The diepisulfides show different reactivities than monoepisulfides due to the neighboring episulfide functional groups. 
     It, thus, exists a need for fast processes for polymerizing dicpisulfides and diepisulfide base compositions for the manufacture of lens materials having ultra high refractive indices of 1.7 or more with good optical properties. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a process for cationic photopolymerization of diepisulfides. 
     It is another object of the invention to provide a process for making in a relatively short time an optical lens material by cationic photopolymerization of a diepisulfide base composition. 
     It is still another object of the invention to provide a process for making an optical lens having a refractive index equal to or greater than 1.7. 
     These and other objects are provided by a process for polymerizing/curing a polymerizable monomer composition including at least one diepisulfide monomer which comprises mixing to the composition an effective amount of at least one cationic photopolymerization initiator (photoinitiator) to obtain a photopolymerizable composition and irradiating the photopolymerizable composition with an UV radiation to at least partially cationically photopolymerize the photopolymerizable composition. 
     In a preferred embodiment, the process for polymerizing/curing the photopolymerizable composition comprises first preheating the photopolymerizable composition to a predetermined temperature, irradiating with a UV radiation to cationically partially polymerize the photopolymerizable composition and then heating the partially polymerized composition to a predetermined temperature for a predetermined time to complete polymerization and cure. 
     It was found that the above process is particularly useful for making plastic lenses of high optical quality, both finished or semi-finished, in a relatively short time. 
     In particular the above process can be used for making high optical quality lenses which are clear, hard, processable and free of striations. 
     Thus, the present invention also provides a process for making plastic lenses in particular having a refractive index of 1.7 or more, which comprises the steps of: 
     a) pouring in a mold a photopolymerizable monomer composition including at least one diepisulfide monomer and at least one cationic photopolymerization initiator; 
     b) preheating the photopolymerizable monomer composition to a temperature ranging from 50 to 100° C.; 
     c) irradiating the photopolymerizable monomer composition with a UV radiation to partially polymerize the composition; 
     d) heating the partially polymerized composition at a temperature ranging from 30 to 100° C. for a predetermined time to complete polymerization; and 
     e) recovering the plastic lens from the mold. 
     During the UV irradiation step, the polymerization temperature is preferably monitored so that this temperature is kept within a predetermined range. For controlling the exothermic reaction during photopolymerization, the exposure to UV radiation is repeatedly turned on and off as needed to maintain constant the polymerization temperature which in turn results in a uniform polymerization or curing. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the invention, the photopolymerizable monomer composition includes at least one diepisulfide monomer. 
     Suitable diepisulfide monomers are diepisulfides of formula:                           
     in which R 1  and R 2  are, independently from each other, H, alkyl, aryl, alkoxy, aryloxy, alkylthio or arylthio; R 3  and R 4  are, independently from each other,                           
     R a  designates H, alkyl, aryl, alkoxy, aryloxy, alkylthio or arylthio and, n is an integer from 0 to 4 and m is an integer from 0 to 6. 
     A preferred class of diepisulfides is comprised of diepisulfides of formula:                           
     in which R 1 , R 2 , R 3  and R 4  are defined as above. 
     Another preferred class of diepisulfides is comprised of diepisulfides of formula:                           
     in which R 1 , R 2 , R 3  and R 4  are defined as above. 
     In R 1 , R 2 , R 3  and R 4  the alkyl and alkoxy groups are preferably C 1 -C 6 , more preferably C 1 -C 4  alkyl and alkoxy groups such as methyl, ethyl, propyl, butyl, methoxy, ethoxy, propoxy and butoxy. 
     preferred diepisulfides are those of formula:                           
     The photopolymerizable monomer compositions also include at least one cationic photoinitiator for cationic photopolymerization of the monomers of the composition. 
     Any cationic photoinitiator can be used in the photopolymerizable composition of the invention, but preferably the photoinitiator shall not color the resulting photopolymerized material. 
     Among the preferred photoinitiators, there may be cited the compounds of formula:                           
     where X is Sb or P,                           
     and mixtures thereof. 
     Cationic photoiniators are commercialy available under the tradenames of RHODORSIL® 2074 (compound of formula V), Irgacure® 261 (compound of formula IV), CYRACURE® (mixed triarylsulfoniumhexafluoroantimonate salts and mixed triarylsulfoniumhexafluorophosphate salts of formulac I and III), SATCAT® (mixed triarylsulfoniun and iodonium salts) and TTAS (compound of formula VI). 
     In general, the photoinitiator is used in an amount of 0.005 to 5% by weight based on the total weight of the polymerizable monomers of the composition and preferably in an amount of 0.25 to 1% by weight. 
     The photopolymerizable compositions of the invention may include a photosensitizer. Any known photosensitizer may be used in the composition of the invention. 
     Among usual photosensitizers, there may be cited alkoxyacetophenones, benzoinethers, phosphine-oxides, benzoyloximes, benzophenones, benzyls, xanthones, anthrones, thioxantones, fluorenones, suberones, acridones and anthracene. 
     Such kind of photosensitizers are known in the art and described in particular in US patents U.S. Pat. No. 4,026,705; U.S. Pat. No. 4,069,054 and U.S. Pat. No. 4,250,053. 
     Preferred photosensitizers are selected from the group consisting of: 
     a) aromatic tertiary amines having the formula:                           
     where Ar 1 , Ar 2  and Ar 3  are aromatic groups having 6 to 20 carbon atoms; Z is selected from oxygen, sulfur,                           
      a carbon to carbon bond; or                           
     where R′ 1  and R′ 2  are selected from hydrogen, alkyl radicals of 1 to 4 carbon atoms and alkenyl radicals of 2 to 4 carbon atoms and wherein n 1  is zero or 1; 
     b) aromatic teriary diamines having the formula:                           
     where Y is a divalent radical selected from arylene and Ar 8 —Z—Ar 9 , where Z is as described above; Ar 4 , Ar 5 , Ar 6 , Ar 7 , Ar 8  and Ar 9  are aromatic groups having 6 to 20 carbon atoms; and wherein n 1  and m 1  are zero o r 1; and; 
     c) aromatic polycyclic compounds having at least three fused benzene rings and further having an ionization energy less than about 7.5 cV. 
     d) fluorescent polyaryl compounds selected from the group consisting of polyarylenes, polyarylpolyens 2,5-diphenylisobenzofurans, 2,5-diarylfurans, 2,5-diarylthiofurans, 2,5-diarylpyroles, 2,5-diarylcyclopentadienes, polyarylphenylenes, cournarins, and polyaryl-2-pyrazolines. 
     Most preferred photosensitizers are anthracene, 9-methyl-anthracene, 7-dimethylamino-4-trifluoromethylcoumarin, acetone, perylene, 1,5-diphenyl-3-styryl-2-pyrazoline, 1-phenyl-3-(p-methoxy-styryl)-5-(p-methoxyphenyl-1,2-pyrazoline), isopropylthioxanthone. 
     The amount of photosensitizer in the photopolymerizable composition is classical and ranges generally from 0.005 to 1% by weight of total weight of photopolymerizable monomers. 
     The photopolymerizable monomer compositions of the invention may comprise solely as polymerizable monomers one or more of the above defined diepisulfides. However, as will be indicated later, the photopolymerizable monomer compositions may also include other polymerizable monomers different from the diepisulfides. 
     These monomers are compounds having two or more functional groups capable of reacting with an episulfide group or compounds having one or more of these functional groups and one or more of other homopolymerizable groups. 
     These additional monomers include epoxy compounds, polythiols, unsaturated ethylenic compounds such as vinyl ether or (methy)acrylate compounds, polyvalent carboxylic acids and anhydrides, thiocarboxylic acids, thio alcohols, thiophenols, polyphenols, amines and amides. 
     Preferred additional monomers are epoxy compounds polythiol, acrylate and (meth)acrylate compounds. 
     Among the preferred epoxy compounds there may be cited the compounds of formula:                           
     where X is O or S, R b  is H, alkyl or aryl and preferably alkyl or aryl, and R 1 , R 2 , R 3 , R 4 , m and n are defined as above. 
     Typical examples of the unsaturated ethylenic compounds include ethylene glycol dimethacrylate, propylene glycol dimethacrylate, trimethylene glycol dimethacrylate, tetramethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, trimethylolpropane triacrylate, pentaerythritolpropane trimethacrylate, ethoxylated bisphenol A diacrylate, 1,6-hexanediol dimethacrylate, urethane diacrylate, epoxy diacrylate, dially phthalate, and divinyl benzene. 
     Examples of polythiol compounds include 1,2,3-trimethylolpropane tri(thioglycolate), pentaerythritol tetra(thio-glycolate), pentaerythritol tetra(3-mercaptopropionate), 1,2,3-trimethylolpropane tri(3-mercaptopropionate), thioglycerol, dithioglycerol, trithioglycerol, dipentaerythritol hexa (2-mercapto acetate), and 3,4,5,6-tetrachloro-1,2-dimercapto benzene. 
     Among the (meth)acrylate compounds, there may be cited alkyl (C 1 -C 6 ) (meth)acrylate such as methyl(meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate and butyl(meth)acrylate 
     The additional monomers usually represents 0 to 80% by weight, preferably 0 to 50% by weight, of the total weight of polymerizable monomers present in the composition. 
     The photopolymerizable composition may also include other optional ingredients classically used in the formulation of polymerizable compositions such as mold release agents, antioxidants, dyes and UV light absorbers. These and other ingredients may be employed in their customary amounts for their customary purposes. 
     Diluents or solvents may also be added to adjust the viscosity of the polymerizable composition. 
     The preferred solvents are dimethylsulfoxide (DMSO), dimethylfornamide (DMF), acetonitrile and tributylphosphate (TBP). 
     The process of the invention includes a step of partially polymerizing the photopolymerizable composition by irradiating the composition with an UV radiation. 
     The ultraviolet radiation used in the photopolymenization step can be selected from any suitable source, including low, medium or high pressure mercury lamps, laser, xenon, luminescence such as fluorescence or phosphorescence, and the like. The irradiation intensity may vary from 5 mW/cm 2  to 350 mW/cm 2  depending upon the photopolymerizable composition and the desired degree of photopolymenization. 
     As previously indicated, the temperature of the exothermic photopolymerization reaction is preferably monitored by turning on and off the irradiation in order to keep the reaction temperature within a predetermined range. This temperature range usually varies from 30° C. to 100° C. typically 60-80° C. 
     Monitoring of photopolymerization of a composition is disclosed in U.S. Pat. No. 5,422,046. 
     Preferably, the process of the invention includes a step of preheating the polymerizable composition. Preheating can be carried out by using a water bath, vat, IR-radiation or by an air-oven. 
     The most preferred process of the invention includes preheating the composition generally at a temperature of 50° C. to 100° C., typically 60° C. to 80° C., before the partial photopolymerization step and completion of polymerization by heating to a temperature of 30° C. to 100° C., preferably of 60° C. to 80° C. 
     The partial photopolymerization step may comprise a single pre-polymerization step resulting in the obtention of a gel or a first pre-polymerization step for forming a gel and a second polymerization step for increasing the polymerization rate beyond the mere obtention of a gel. 
     Subsequent to partial polymerization by radiation, the final step of the process involves a thermal heating to complete the polymerization. This is done either in an oven or using an IR source. 
     This heating step generally lasts for 1 to 5 hours in order to achieve complete polymerization, and typically is of 3 hours. 
     The process of the invention is advantageous for making plastic lenses having refractive indices as high as 1.7 or greater. 
     It can be implemented using classical two part glass molds. 
     When using the preferred process including preheating step and final heating step, the resulting lenses do not exhibit striations. 
     The following examples illustrate the present invention. 
    
    
     In the examples unless otherwise indicated all parts and percentages are by weight. 
     EXAMPLES 1 TO 8 
     Diepisulfide of formula IA was mixed at 15° C. with Cyracure 6974 in the proportions indicated in Table I. The mixture was degrassed and injected into a 1mm thick finished single vision lens mold with a rubber gasket. The mold assembly was heated to 60° C. and then subjected to an on/off irradiation with UV light as indicated in table I. The mold assembly was then heated as indicated in Table I for completion of polymerization. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 EX 1 
                 EX 2 
                 EX 3 
                 EX 4 
                 EX 5 
                 EX 6 
                 EX 7 
                 EX 8 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Composition 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Diepisulfide IA (g) 
                 9.8 
                 9.9 
                 9.9 
                 9.8 
                 9.8 
                 9.9 
                 9.925 
                 9.95 
               
               
                 Cyracure ® 6974 (g) 
                 0.2 
                 0.1 
                 0.1 
                 0.2 
                 0.2 
                 0.1 
                 0.075 
                 0.05 
               
               
                 Preheating 
               
               
                 Temperature ° C. 
                 60 
                 60 
                 60 
                 60 
                 60 
                 80 
                 80 
                 80 
               
               
                 Irradiation with 
               
               
                 UV light 
               
               
                 Intensity mW/cm 2   
                 200 
                 330 
                 320 
                 150 
                 200 
                 100 
                 100 
                 100 
               
               
                 Duration of 
                 30/30/30/15 
                 45/15/15 
                 45/30/30 
                 30/30/30/30 
                 45/30/30/15 
                 27/10/10/17/17 
                 27 
                 48/13/17/45 
               
               
                 irradiation (shots) 
               
               
                 Time between 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 shots (minutes) 
               
               
                 Final heating 
               
               
                 Temperature ° C. 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 Time (hours) 
                 3 
                 3 
                 2.5 
                 3 
                 3 
                 3 
                 3 
                 3 
               
               
                   
               
             
          
         
       
     
     EXAMPLES 9 TO 12 
     Photopolymerizable compositions were prepared as in the previous examples using diepisulfide of formula IB and Cyracure® 6974 in the amounts indicated in Table II. The mold assembly was preheated to 60° C. prior to irradiation. Irradiation conditions with UV light are given in Table II. After irradiation with UV lights, the mold assembly was heated at 100° C. for 3 hours. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                   
                 EX9 
                 EX10 
                 EX11 
                 EX12 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Composition 
                   
                   
                   
                   
               
               
                   
                 Diepisulfide IB (g) 
                 9.9 
                 9.9 
                 9.9 
                 9.9 
               
               
                   
                 Cyracure ®6974 (g) 
                 0.1 
                 0.075 
                 0.5 
                 0.25 
               
               
                   
                 Preheating 
               
               
                   
                 Temperature ° C. 
                 60 
                 60 
                 60 
                 60 
               
               
                   
                 Irradiation with UV light 
               
               
                   
                 Intensity mW/cm 2   
                 125 
                 125 
                 125 
                 125 
               
               
                   
                 Duration of 
                 30 
                 60 
                 60 
                 60 
               
               
                   
                 exposure (s) 
               
               
                   
                 Final heating 
               
               
                   
                 Temperature ° C. 
                 100 
                 100 
                 100 
                 100 
               
               
                   
                 Time (hours) 
                 3 
                 3 
                 3 
                 3 
               
               
                   
                   
               
               
                   
                 CYRACURE ®6974: mixture of compounds of formulae II and III  
               
               
                   
                 X = Sb  
               
             
          
         
       
     
     EXAMPLES 13 AND 14 
     Photopolymerizable compositions having the fomulations given in Table III were degassed for I minute at 60° C. and poured in a mould as in the previous example. 
     The internal walls of the mould were coated with a layer of a a standard external mould release agent. 
     The compositions were then polymerized according to the present process and as specified in Table III. 
     Release property (demoulding), cracking, light transmission and Barcol values of the resulting lenses are also given in Table III. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                   
                 EX. 13 
                 EX. 14 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Composition 
                   
                   
               
               
                 *Diepisulfide IA(g) 
                 — 
                 19.8 
               
               
                 *Diepisulfide IB (g) 
                 14.835 
                 — 
               
               
                 *Cyracure ® 6974(g) 
                 0.075 
                 0.2 
               
               
                 *Blue dye solution 426 (g) 
                 0.09 
                 — 
               
               
                 Preheating 
               
               
                 *Infrared heater 
               
               
                 *Temperature (° C.)/duration (mn) 
                 60/5 
                 80/5 
               
               
                 Irradiation with UV light 
               
               
                 *UV intensity (mW/cm 2 ) 
                 40 
                 40 
               
               
                 Prepolymerization to gel 
               
               
                 *Number of shots 
                 1 
                 3 
               
               
                 *Duration of shots (s) 
                 45 
                 60/60/15 
               
               
                 *Temperature reached (° C.) 
                 60-70 
                 80-90 
               
               
                 Further polymerization 
                 For 5 minutes 
                 — 
               
               
                   
                 with 15-30 s shots and 
               
               
                   
                 20-30 time intervals to 
               
               
                   
                 maintain 60-70° C. 
               
               
                   
                 temperature 
               
               
                 Final heating 
               
               
                 *Temperature (° C.) 
                 75 
                 80 
               
               
                 *Duration (hours) 
                 1 
                 3 
               
               
                 Properties of moulded lens 
               
               
                 *Mold release 
                 good 
                 good 
               
               
                 *Cracking 
                 no 
                 no 
               
               
                 *Light Transmission 
                 83.4 
                 83.3 
               
               
                 *Barcol Hardness value 
                 79.61 
                 — 
               
               
                 *Refractive index 
                 1.7 
                 1.73 
               
               
                   
               
             
          
         
       
     
     Acceptable Barcol hardness value range is 72-90. 
     The Barcol hardness of the lens is measured by using a Barcol impressor according to ASTM D 2583-93, being precised that the instrument is first calibrated with a standard test metal disc. 
     An uncoated lens specimen is placed at 23±5° C. temperature and 50±5° RH for 8 hours and placed under the indentor. 
     The average of three readings are taken as the final Barcol value.