Abstract:
The present invention relates to iodonium salts containing urethane groups of reduced crystallization tendency, to a process for their preparation, and to their use for the radiation curing of cationically curing compositions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to iodonium salts of reduced crystallization tendency containing urethane groups, to a process for their preparation, and to their use for the radiation curing of cationically curing compositions. 
     2. Description of the Related Art 
     Cationic photopolymerization is a rapid, efficient and environment-friendly means of curing cationically polymerizable monomers. Particularly efficient photoinitiators are diaryliodonium salts (I) and triarylsulfonium salts (II). 
     
       
         Ar—I + —ArMX n   −   (I) 
       
     
                               MX n   − =BF 4   − ,PF 6   − ,AsF 6   − ,SbF 6   −   
     Diaryliodonium salts (I) in particular are known from the patent literature (DE-A-25 18 639, U.S. Pat. No. 4,279,717, EP-A-0 334 056, EP-B-0 618 919) and are used as photoinitiators for polymerizing cationically polymerizable substances. The cationically polymerizable substances, however, have little or no polarity, especially if the polymerizable groups are present in organopolysiloxanes. When adding these photoinitiators, therefore, it is a very common observation that, depending on the structure of the formulation, the miscibility and solubility of the photoinitiators is limited. For this reason, the aryl radicals of such onium salts are often substituted with alkyl chains in order to increase the solubility in organopolysiloxanes (U.S. Pat. No. 4,310,469 and U.S. Pat. No. 4,374,066). 
     In the case of hydroxyl-bearing iodonium salts as described in U.S. Pat. No. 5,073,643, the poor solubility in nonpolar media is attributed to the high crystallization tendency. The particular complexation characteristics of the hydroxyl-bearing iodonium salts of the general formula (III) result in a strong crystallization tendency of the compounds (A. Kunze, U. Müller, K. Tittes, J. P. Fouassier, F. Morlet-Savary, J. Photochemistry and Photobiology A: Chemistry, 110, 115-122 (1997)):                           
     The two oxygen atoms in the molecule act as ligands for a second iodonium ion. This aggregation behavior promotes the formation of crystals. 
     In the preparation of these salts, the strong crystallization tendency is entirely desirable, since it means that the compounds can be recovered as powders in a high purity by simple recrystallization. In this way, they are easy and cost effective to prepare. Such iodonium salts are commercially available, for example, under the name CD-1012 from Sartomer. 
     The high crystallization tendency has an adverse effect, however, if the iodonium salts are to be dissolved in nonpolar media, such as organopolysiloxanes. In this case either they are insoluble or a solid precipitate forms after just a short time. 
     Because of their inhomogeneity, such coatings cure poorly on UV irradiation. However, it is also possible that, as a result of the inhomogeneity, massive surface defects (craters, wrinkles, specks, etc.) appear even during the application of a thin coat on a substrate. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to modify hydroxyl-bearing iodonium salts in a particularly cost-effective and simple manner such that the crystallization tendency is greatly reduced and good compatibility, especially with organopolysiloxanes containing epoxy groups, is established, and the resulting compounds are stable to hydrolysis, so that reformation of hydroxyl-bearing iodonium salts in the course of storage is impossible. 
     SUMMARY OF THE INVENTION 
     These objects are achieved by means of iodonium salts of the general formula (IV) 
     
       
         [R 1 —I—R 2 ] + X −   (IV) 
       
     
     where 
     I is iodine, 
     X −  is an anion of a complex metal salt and/or of a strong acid, 
     R 1  is the radical                           
      in which 
     Ar is an aromatic hydrocarbon radical, preferably, having 6 to 14 carbon atoms per radical or is an aromatic hydrocarbon radical containing at least one oxygen and/or sulfur atom and, preferably, having 5 to 15 ring atoms per radical, 
     a is 1, 2 or 3, 
     b is 0, 1 or 2, 
     c is 0, 1 or 2, 
     D, E and F are each substituents of Ar, 
     D being a radical of the formula                           
      where 
     x is 0 or 1, 
     y is 0 or 1, 
     R 3  is a linear or branched divalent hydrocarbon radical, preferably, having 1 to 40 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom and/or at least one sulfur atom and/or at least one carboxyl group, R 4  is a monovalent linear, branched and/or cyclic alkyl, aryl, haloalkyl and/or haloaryl radical, preferably, having 1 to 40 carbon atoms, which can be interrupted, if desired, by at least one oxygen atom and/or at least one sulfur atom and/or at least one urethane group and/or ester group, and which can, if desired, contain hydrophobicizing substituents and/or at least one isocyanate group, 
     E is a radical of the formula 
     
       
         —O—R 5   
       
     
     F is a radical of the formula 
     
       
         —R 6   
       
     
     R 2  is a radical of the formula                           
      where 
     R 5  is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom, 
     R 6  is a monovalent hydrocarbon radical, preferably, having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom, 
     d is 0, 1 or 2, and 
     e is 0, 1 or 2. 
     Surprisingly it has been found that the formation of a urethane from hydroxyl-containing iodonium salts makes it possible to reduce considerably the crystallization tendency, to improve substantially the compatibility with organopolysiloxanes containing epoxy groups, and to increase considerably the hydrolytic stability relative to iodonium salts containing Si—O—C bonds (see German Patent Application 19901531.7). 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred examples of aromatic hydrocarbon radicals Ar are the phenyl, naphthyl and anthryl radicals. 
     Preferred examples of aromatic hydrocarbon radicals Ar containing at least one oxygen and/or sulfur atom are the 2-furyl, 3-furyl, 2-thienyl and 3-thienyl radicals. 
     Preferred examples of the divalent hydrocarbon radical R 3  which can be interrupted by at least one oxygen atom and/or at least one sulfur atom and/or a carboxyl group are —CH 2 —, —CH 2 —CH 2 —, —CH 2 —CH 2 —O—CH 2 —CH 2 —, —CH 2 —CH(CH 3 )—, —CH 2 —CH(CH 2 CH 3 )—, —CH 2 —CH((CH 2 ) 3 CH 3 )—, —CH 2 —CH((CH 2 ) 11 CH 3 )—, —CH 2 —CH((CH 2 ) 13 CH 3 )—, —CH 2 —CH 2 —S—CH 2 —CH 2 —, —CH 2 —O—C(O)—CH 2 —. 
     Preferred examples of R 4 , the monovalent linear, branched and/or cyclic alkyl, aryl, haloalkyl and/or haloaryl radical having 1 to 40 carbon atoms, which can be interrupted, if desired, by at least one oxygen atom and/or at least one sulfur atom and/or at least one urethane group and/or ester group, and which can, if desired, contain hydrophobicizing substituents and/or at least one isocyanate group, are alkyl radicals, such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, and tert-pentyl radical; hexyl radicals, such as the n-hexyl radical, for example, the cyclohexyl radical; heptyl radicals, such as the n-heptyl radical; octyl radicals, such as the n-octyl radical and isooctyl radicals, such as the 2,2,4-trimethylpentyl radical; nonyl radicals, such as the n-nonyl radical; decyl radicals, such as the n-decyl radical; dodecyl radicals, such as the n-dodecyl radical, and octadecyl radicals, such as the n-octadecyl radical; aryl radicals, such as the phenyl, naphthyl and anthryl radical; haloalkyl radicals, such as the chloromethyl, trichloromethyl, trifluoromethyl, pentafluoroethyl and heptafluoropropyl radical; haloaryl radicals, such as the bromophenyl, chlorophenyl, fluorophenyl, difluorophenyl, fluoro(trifluoromethyl)phenyl, pentafluorophenyl, (trifluoromethyl)phenyl, (trifluoromethylthio)phenyl and trifluoromethoxyphenyl radical. 
     The abovementioned examples of the radical R 4  also apply in their entirety to the radicals R 5  and R 6 . 
     Examples of hydrocarbons R 4 , R 5  and R 6  interrupted by at least one oxygen atom and/or sulfur atom are —CH 2 —CH 2 —O—CH 3 , —CH 2 —CH 2 O—CH 2 CH 3  and —CH 2 —CH 2 —S—CH 2 CH 3 . 
     Examples of the radical R 4  interrupted by at least one urethane group and/or ester group, and being able to contain, if desired, hydrophobicizing substituents and/or at least one isocyanate group, are                           
     Preferred examples of radicals D are                           
     Preferred examples of radicals E are the methoxy, ethoxy and n-butoxy radical. 
     Preferred examples of radicals F are methyl, ethyl, propyl, 2-methylpropyl and the n-butyl radical. 
     Preferred examples of radicals R 1  are                           
     Preferred examples of radicals R 2  are the phenyl, 4-methylphenyl, 3-methoxyphenyl and 4-methoxyphenyl radical. 
     Preferred examples of anions X −  of a complex metal salt and/or of a strong acid are tosylate, SbF 6   − , PF 6   − , BF 4   − , F 3 CSO 3   − , F 3 CCO 2   − , AsF 6   − , ClO 4   − , HSO 4   − . For the purposes of the present invention, strong acids embrace, in particular, strong Brönsted acids. 
     Preferred iodonium salts of reduced crystallization tendency are those of the general formula (V)                           
     where D and X −  are as defined previously. 
     Particularly preferred iodonium salts of reduced crystallization tendency are those of the formula (VI)                           
     and of the formula (VII)                           
     The crystallization tendency of the iodonium salts modified in this way is substantially reduced in relation to the abovementioned prior art. For example, at room temperature the iodonium salt (VII) is a viscous liquid, whereas the comparable hydroxyl-containing iodonium salt having the formula (VIII) below                           
     is a powder having a melting point of 91° C., which is available under the trade name CD-1012 from Sartomer. 
     Likewise, the solubility of the iodonium salts of the invention in nonpolar media, such as n-alkanes or siloxanes, is substantially greater than that of the comparable hydroxyl-containing iodonium salts. 
     For example, the iodonium salt of the general formula (VII) has unrestricted solubility in toluol. In contrast, the comparable hydroxyl-containing iodonium salt of the general formula (VIII) is insoluble in toluol and shows no miscibility with organopolysiloxanes containing epoxy groups. 
     The invention additionally provides for a process for preparing the iodonium salts having reduced crystallization tendency. Iodonium salts of the invention are readily prepared by reaction with isocyanates.                           
     where “Photo” denotes a photoinitiator to be modified. 
     The process of the invention comprises reacting a hydroxyl-containing iodonium salt having the general formula (IX) 
     
       
         [R 1 —I—R 2 ] + X −   (IX) 
       
     
     in which 
     I is iodine, 
     X −  is an anion of a complex metal salt and/or of a strong acid, 
     R 1  is a radical                           
      which 
     Ar is an aromatic hydrocarbon radical, preferably, having 6 to 14 carbon atoms per radical or is an aromatic hydrocarbon radical containing at least one oxygen and/or sulfur atom and having 5 to 15 ring atoms per radical, 
     a is 1, 2 or 3, 
     b is 0, 1 or 2, 
     c is 0, 1 or 2, 
     D′, E and F are each substituents of Ar, 
     D′ being a radical of the formula 
     
       
         —(O)x—(R 3 )y—OH 
       
     
      where 
     x is 0 or 1, 
     y is 0 or 1, 
     R 3  is a linear or branched divalent hydrocarbon radical, preferably, having 1 to 40 carbon atoms per radical, which can be interrupted if desired by at least one oxygen atom and/or one sulfur atom and/or one carboxyl group, 
     E is a radical of the formula 
     
       
         —O—R 5 , 
       
     
     F is a radical of the formula 
     
       
         —R 6 , 
       
     
     R 2  is a radical of the formula                           
      where 
     R 5  is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted if desired by at least one oxygen atom, 
     R 6  is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted if desired by at least one oxygen atom, 
     d is 0, 1 or 2, and 
     e is 0, 1 or 2, 
     with a reagent containing isocyanate groups. 
     Preferred hydroxyl-containing iodonium salts are those of the general formula (X)                           
     where D′ and X −  are as defined previously. 
     Particularly preferred starting materials are iodonium salts of the formula (XI) below                           
     Examples of reagents containing isocyanate groups, which may be reacted with hydroxyl-bearing iodonium salts, are those of the general formula (XII) 
     
       
         OCN—R 4   (XII) 
       
     
     where 
     R 4  is a monovalent linear, branched and/or cyclic alkyl, aryl, haloalkyl and/or haloaryl radical having 1 to 40 carbon atoms, which can, if desired, be interrupted by at least one oxygen atom and/or at least one sulfur atom and/or at least one urethane group and/or ester group, and which can, if desired, contain hydrophobicizing substituents and/or at least one isocyanate group. 
     Particularly preferred reagents containing isocyanate groups which may be reacted with hydroxyl-bearing iodonium salts are the following: 
     alkyl isocyanates, such as methyl, ethyl, propyl, butyl, sec-butyl, cyclohexyl, hexyl, octyl, tert-octyl, decyl, dodecyl and/or octadecyl isocyanate; 
     aryl isocyanates, such as phenyl, tolyl, dimethylphenyl, phenylethyl, propylphenyl, methoxyphenyl, (heptyloxy)phenyl, phenoxyphenyl, acetylphenyl, nitrophenyl, benzyl, methylbenzyl, methoxybenzyl, dimethyl-m-isopropenylbenzyl, naphthyl, (trifluoromethyl)phenyl, di(trifluoromethyl)phenyl, (trifluoromethylthio)phenyl and/or (trifluoromethoxy)phenyl isocyanate; 
     haloalkyl isocyanates, such as 2-bromomethyl isocyanate; haloaryl isocyanates, such as chlorobenzyl, chlorophenyl, trichlorophenyl, 4-bromo-2,6-dimethylphenyl, fluorophenyl, difluorophenyl, fluoro(trifluoromethyl)phenyl and/or fluorobenzyl isocyanate; diisocyanates, such as cyclohexane, dicyclohexylmethane 4,4′-, diphenylmethane 4,4′-, hexamethylene 1,6-, isophorone, and/or tolylene diisocyanate; 
     polymeric isocyanates, such as Desmodur N 3300 from Bayer AG, Desmodur L from Bayer AG, Desmodur E41 from Bayer AG, Desmodur Z 4370 from Bayer AG; 
     polydimethylsiloxane isocyanates, such as                           
     The iodonium salts of the invention are suitable, for example, as photoinitiators for the polymerization of cationically polymerizable organic substances, such as epoxides, vinyl ethers, organopolysiloxanes containing epoxy groups, organopolysiloxanes containing alkenyloxy groups, such as vinyloxy groups or propenyloxy groups, and olefins. Such substances are described, for example, in U.S. Pat. No. 5,057,549, DE-A-40 02 922, and in the patent documents cited at the outset. 
    
    
     WORKING EXAMPLES 
     Example 1 
     10 g of a commercial photoinitiator of the formula (XI) were suspended in 10 ml of ethyl acetate, mixed with 1.0 g of ethyl isocyanate and two drops of COSMOS® 29* as catalyst, and heated at 60° C. in a waterbath. The mixture was stirred for 2 h. After the reaction, the solvent was removed by distillation. This left a yellow, viscous product. *Commercial product of Goldschmidt AG 
     Example 2 
     10 g of a commercial photoinitiator of the formula (XI) were suspended in 10 ml of ethyl acetate, mixed with 1.7 g of hexyl isocyanate and two drops of COSMOS® 29 as catalyst, and heated at 60° C. in a waterbath. The mixture was stirred for 2 h. After the reaction, the solvent was removed by distillation. This left a yellow, viscous product. 
     Example 3 
     10 g of a commercial photoinitiator of the formula (XI) was suspended in 10 ml of ethyl acetate, mixed with 1.6 g of phenyl isocyanate and two drops of COSMOS® 29 as catalyst, and heated at 60° C. in a waterbath. The mixture was stirred for 2 h. After the reaction, the solvent was removed by distillation. This left a yellow, viscous product. 
     Example 4 
     10 g of a commercial photoinitiator of the formula (XI) was suspended in 10 ml of ethyl acetate, mixed with 2.5 g of m-(trifluoromethyl)phenyl isocyanate and two drops of COSMOS® 29 as catalyst, and heated at 60° C. in a waterbath. The mixture was stirred for 2 h. After the reaction, the solvent was removed by distillation. This left a yellow, viscous product. 
     Comparison 1 
     The solubility of the iodonium salts of the invention was compared with the solubility of the hydroxyl-bearing iodonium salts. The results are summarized in Table 1: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Photoinitiator 
                 Solubility in toluene 
               
               
                   
                   
               
             
             
               
                   
                 Formula (XI) 
                 insoluble 
               
               
                   
                 Ex. 1 
                 soluble to a clear 
               
               
                   
                   
                 solution 
               
               
                   
                 Ex. 2 
                 soluble to a clear 
               
               
                   
                   
                 solution 
               
               
                   
                 Ex. 3 
                 soluble to a clear 
               
               
                   
                   
                 solution 
               
               
                   
                 Ex. 4 
                 soluble to a clear 
               
               
                   
                   
                 solution 
               
               
                   
                   
               
             
          
         
       
     
     The solubility difference between the hydroxyl-containing photoinitiator with the formula (XI) and the photoinitiators of the invention, containing urethane groups and intended for cationic curing, is considerable. 
     Comparison 2 
     The reduction in the crystallization tendency may be demonstrated with particular clarity on the basis of the melting point of the compounds. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Photoinitiator 
                 Melting point 
               
               
                   
                   
               
             
             
               
                   
                 Formula (XI) 
                 91° C. 
               
               
                   
                 Ex. 1 
                 viscous oil 
               
               
                   
                 Ex. 2 
                 viscous oil 
               
               
                   
                 Ex. 3 
                 viscous oil 
               
               
                   
                 Ex. 4 
                 viscous oil 
               
               
                   
                   
               
             
          
         
       
     
     It is found that by means of the modification the melting point and thus the crystallization tendency were reduced considerably. 
     Comparison 3 
     The improved hydrolytic stability of the iodonium salts of the invention containing urethane groups may best be shown in comparison with the iodonium salt with the formula (XVI), containing Si—O—C bonds (see German Patent Application 19901531.7).                           
     The results are summarized in Table 3. Indicated is the time to the formation of visible precipitates or crystals of the hydrolytic cleavage product with the formula (XI): 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Stability in 
               
               
                   
                   
                 Stability in toluene 
                 1-butanol 
               
               
                   
                 Photoinitiator 
                 (50% w/w) 
                 (50% w/w) 
               
               
                   
                   
               
             
             
               
                   
                 Formula (XVI) 
                  21 d 
                  2 d 
               
               
                   
                 Ex. 1 
                 &gt;90 d 
                 &gt;90 d 
               
               
                   
                 Ex. 2 
                 &gt;90 d 
                 &gt;90 d 
               
               
                   
                 Ex. 3 
                 &gt;90 d 
                 &gt;90 d 
               
               
                   
                 Ex. 4 
                 &gt;90 d 
                 &gt;90 d 
               
               
                   
                   
               
             
          
         
       
     
     Tests Relating to Cationic Photopolymerization 
     In order to examine the activity of the compounds shown in Examples 1-4 in cationic photopolymerization, 98 parts of a cycloaliphatic epoxysilane having an epoxy value of 3.5% and a viscosity of 125 mPas were mixed with 2 parts of each photoinitiator. 
     The mixtures were then applied to a standard OPP film (30 μm) using a pilot-scale coating machine equipped with a five-roll applicator unit. The application weight was 0.5-1 g/m 2 . The coatings were subsequently cured with a microwave-excited UV lamp (Fusion, 120 W/cm) at a web speed of 20 m/min. 
     Directly following passage under the UV lamp, it was determined whether the coatings have cured to a film which is no longer tacky. 
     The results are summarized in the table below: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Appearance of 
                   
               
               
                   
                 Photoinitiator 
                 the mixture 
                 Curing after the UV lamp 
               
               
                   
                   
               
             
             
               
                   
                 Formula (VIII) 
                 cloudy, 
                 unsatisfactory; 
               
               
                   
                   
                 crystalline 
                 cloudy, tacky film; 
               
               
                   
                   
                 precipitate 
                 crystals visible 
               
               
                   
                 Ex. 1 
                 opaque 
                 curing; 
               
               
                   
                   
                   
                 film no longer tacky 
               
               
                   
                 Ex. 2 
                 clear 
                 good curing; 
               
               
                   
                   
                   
                 clear film, no longer 
               
               
                   
                   
                   
                 tacky 
               
               
                   
                 Ex. 3 
                 clear 
                 good curing; 
               
               
                   
                   
                   
                 clear film, no longer 
               
               
                   
                   
                   
                 tacky 
               
               
                   
                 Ex. 4 
                 clear 
                 good curing; 
               
               
                   
                   
                   
                 clear film, no longer 
               
               
                   
                   
                   
                 tacky 
               
               
                   
                   
               
             
          
         
       
     
     It was found that the differences in curing behavior between the hydroxyl-containing photoinitiator with the formula (VIII) and the photoinitiators of the invention containing urethane groups are considerable. 
     The above description of the invention is intended to be illustrative and not limiting. Various changes in the embodiments may occur to those skilled in the art. These changes can be made without departing from the scope of the invention.