Abstract:
A photohardenable composition comprising a free radical addition polymerizable or crosslinkable compound, a cationic dye-borate anion complex, and a borate salt, said complex being capable of absorbing actinic radiation and producing free radicals which initiate free radical polymerization or crosslinking of said compound.

Description:
This is a continuation of co-pending application Ser. No. 07/426/819, filed Oct. 25, 1989 now abandoned. 
    
    
     BACKGROUND 
     The present invention relates to an improved photohardenable composition of the type described in U.S. Pat. No. 4,772,541 containing a photohardenable compound, a complex of a cationic dye and a borate as a photoinitiator, and a borate salt. 
     U.S. Pat. No. 4,772,541 is incorporated herein by reference. This patent discloses photohardenable compositions which contain a dye complex of the formula (I): ##STR1## where D+ is a cationic dye; and R 1 , R 2 , R 3 , and R 4  are independently selected from the group consisting of alkyl, aryl, alkaryl, allyl, aralkyl, alkenyl, alkynyl, alicyclic and saturated or unsaturated heterocyclic groups. As noted in the patent, these groups may be substituted or unsubstituted. 
     Useful dyes form photoreducible but dark stable complexes with borate anions and can be cationic methine, polymethine, triarylmethane, indoline, thiazine, xanthene, oxazine and acridine dyes. More specifically, the dyes may be cationic cyanine, carbocyanine, hemicyanine, rhodamine and azomethine dyes. 
     Preferred dyes are cyanine dyes of the formula (II) ##STR2## where n is 0, 1, 2 or 3; R is an alkyl group and preferably an alkyl group having 4 to 10 carbon atoms; and y is --CH═CH--, &gt;NCH 3 ,&gt;C(CH 3 ) 2 , O, S, or Se 
     The borate anion is designed such that the borate radical generated upon exposure to light and after electron transfer to the dye (Eq. 1) readily dissociates with the formation of a radical as follows: 
     
         BR.sub.4 →BR.sub.3 +R.                              (Eq. 1) 
    
     For example particularly preferred anions are triphenylbutylborate, trianisylbutylborate, and triphenylsecbutyl borate anions because they readily dissociate to triphenylborane or trianisylborane and a butyl radical. As explained in U.S. Pat. No. 4,772,541, the Weller equation is useful in identifying useful complexes. 
     To enhance the film speed of photohardenable compositions described in the aforementioned patent, compounds such as N,N-dialkylanilines and more particularly diisopropyldimethylaniline may be added to the composition. Thiols such as mercaptobenzothiazole may also be added to the composition. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, an improvement in the aforementioned compositions is provided through the addition of a borate salt. The term &#34;borate salt&#34; as used herein does not mean a dye-borate complex. Rather it means a salt in which the cation is not a dye molecule. It is believed that the addition of the borate salt reduces any tendency of the complex to dissociate when it is dissolved in the monomer. As a result, the dye and the borate anion remain tightly paired, such that election transfer is very efficient. This improves short time scale reciprocity failure. There is evidence that the salt also improves the toe speed of the composition. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Useful borate salts can be represented by the formula (III): ##STR3## where Z +  is a cation which does not absorb in the region in which the dye-borate complex absorbs and which is sufficiently oleophilic to make the salt soluble in the photohardenable composition in effective concentrations. Examples of useful cations are cations such as trimethylcetylammonium, benzethonium, cetylpyridinium (CPTB), etc., R 5  -R 8  may be selected from the group consisting of an alkyl group, an aryl group, an alkaryl group, an allyl group, an aralkyl group, an alkenyl group, an alkynyl group, an alicyclic group, and a saturated or unsaturated heterocyclic group which may be substituted or unsubstituted. 
     Preferably R 5  -R 8  are the same as R 1  -R 4  in the dye complex present in the composition. This is not essential, however, because there will be ionic exchange between the borate salt and the dye complex. However, it is essential that the borate anion of the salt have the ability to dissociate and form a free radical in accordance with Eq. 1 above. If the borate anion of the salt cannot form radicals as effectively as the borate ion associated with the dye, addition of the salt may diminish rather than improve the function of the photoinitiator. Preferred R 5  -R 8  are triphenyl(n-butyl) and triphenyl(sec-butyl) as in the case of R 1  -R 4 . Triphenylbenzyl, triphenylallyl, and triphenylcyclopropyl may also be useful borates. 
     Specific examples of cationic dye-borate anion complexes useful in the present invention are shown below with their λ max. 
     
         __________________________________________________________________________ ##STR4##   n  Y.sub.1             Y.sub.2                    R&#39;  R&#34;     Max__________________________________________________________________________Compound 1   1       ##STR5##              ##STR6##                   C.sub.6 H.sub.5                        Sec-C.sub.4 H.sub.9                               550Compound 2   2       ##STR7##              ##STR8##                   C.sub.6 H.sub.5                        Sec-C.sub.4 H.sub.9                               650Compound 3   0  S              ##STR9##                   C.sub.6 H.sub.5                        Sec-C.sub.4 H.sub.7                               436Compound 4   1       ##STR10##              ##STR11##                   C.sub.6 H.sub.5                        neopentyl                               550Compound 5   2       ##STR12##              ##STR13##                   C.sub.6 H.sub.5                        neopentyl                               650Compound A (comparison)   1       ##STR14##              ##STR15##                   C.sub.6 H.sub.5                        n-C.sub.4 H.sub.9                               550Compound B (comparison)   2       ##STR16##              ##STR17##                   C.sub.6 H.sub.5                        n-C.sub.4 H.sub.9                               650Compound C   0  S              ##STR18##                   C.sub.6 H.sub.5                        n-C.sub.4 H.sub.9__________________________________________________________________________ 
    
     The most typical example of a free radical addition polymerizable or crosslinkable compound useful in the present invention is an ethylenically unsaturated compound and, more specifically, a polyethylenically unsaturated compound. These compounds include both monomers having one or more ethylenically unsaturated groups, such as vinyl or allyl groups, and polymers having terminal or pendant ethylenic unsaturation. Such compounds are well known in the art and include acrylic and methacrylic esters of polyhydric alcohols such as trimethylolpropane, pentaerythritol, and the like; and acrylate or methacrylate terminated expoxy resins, acrylate or methacrylate terminated polyesters, etc. Representative examples include ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylol-propane triacrylate (TMPTA), pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hydroxpentacrylate (DPHPA), hexanediol-1,6-dimethacrylate, and diethyleneglycol dimethacrylate. A combination of TMPTA and DPHPA is often preferred. 
     While the cationic dye-borate anion complex can be used alone as the initiator, film speeds tend to be quite low and oxygen inhibition is observed. It has been found that it is preferable to use the complex in combination with an autoxidizer. An autoxidizer is a compound which is capable of consuming oxygen in a free radical chain process. 
     Representative examples of N,N-dialkylanilines useful in the present invention are 4-cyano-N, N-dimethylaniline, 4-acetyl-N,N-dimethylaniline, 4-bromo-N, N-dimethylaniline, ethyl 4-(N,N-dimethylamino) benzoate, 3-chloro-N,N-dimethylaniline, 4-chloro-N,N-dimethylaniline, 3-ethoxy-N,N-dimethylaniline, 4-fluoro-N,N-dimethylaniline, 4-methyl-N,N-dimethylthioanicidine, 4-amino-N,N-dimethylaniline, 3-hydroxy-N,N-dimethylaniline, N,N,N&#39;,N&#39;-tetramethyl-1,4-dianiline, 4-acetamido-N,N-dimethylaniline, etc. Preferred N,N-dialkylanilines are 2,6-diisopropyl-N,N-dimethylaniline (DIDMA), 2,6-diethyl-N,N-dimethylaniline, N,N,2,4,6-pentamethylaniline (PMA) and p-t-butyl-N,N-dimethylaniline. 
     Thiols and disulfides also appear to enhance film speed although the mechanism is not clear. Particularly preferred thiols are selected from the group consisting of mercaptobenzoxazoles, mercaptotetrazines, mercaptotriazines and mercaptobenzothiazoles. Representative examples include 2-mercaptobenzothiazole, 6-ethoxy-2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 4-methyl-4H-1,2,4-triazole-3-thiol, 2-mercapto-1-methylimidazole, 2-mercapto-5-methylthio-1,3,4-thiadiazole, 5-n-butylthio-2-mercapto-1,3,4-thiadiazole, 4 methoxybenzenethiol, 1-phenyl-1H-tetrazole-5-thiol, 4 phenyl-4H 1,2,4-triazole-3-thiol, 2-mercaptobenzimidazole, pentaerythritol tetrakis (mercaptoacetate), pentaerythritol tetrakis (3-mercaptoproprionate), trimethylolpropane tris(mercaptoacetate), 4-acetamidothiophenol, mercaptosuccinic acid, dodecanethiol, 2-mercaptopyridine, 4-mercaptopyridine, 2-mercapto-3H-quinazoline, and 2-mercaptothiazoline. Disulfides as described in U.S. application Ser. No. 321,257 are also useful. 
     The cationic dye-borate anion complex is usually used in an amount up to about 1% by weight based on the weight of the photopolymerizable or crosslinkable species in the photohardenable composition. More typically, the cationic dye-borate anion complex is used in an amount of about 0.2% to 0.5% by weight. The autoxidizers are preferably used in the present invention in concentrations of about 4-5% by weight, and the borate salt is used in an amount of at least 0.2% and most typically 0.2 to 3%. 
     The invention is illustrated in more detail by the following example. 
     EXAMPLE 1 
     The following internal phase compositions were prepared and microencapsulated in melamine-formaldehyde microcapsules using the procedure described in U.S. Pat. No. 4,772,541. 
     
         ______________________________________       Composition A                   Composition B       (control)   (control)______________________________________TMPTA         100 parts     100 partsthereof        --            0.29Compound 4     0.33          0.33DIDMA          6.0           6.0Color Precursor          24.0          24.0N-100 (Mobay)  6.6           6.6______________________________________ 
    
     The microencapsulated compositions were coated on aluminized PET, dried, exposed through a step-wedge, assembled with a developer image and an image of the stepwedge was developed by passing the assembly through the nip between a pair of pressure rollers. A comparison of density versus step number showed there was approximately a four step improvement in the toe speed of the composition containing the borate salt. 
     Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.