Photocationic initiator combinations

Details Photocationic initiator combinations Photocationic initiator combinations

2002-05-28

2004-08-03

Berman, Susan (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C016S024000, C016S027000, C016S197000, C016S083000

Reexamination Certificate

active

06770686

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photocationic initiator combination, a UV-crosslinkable epoxide mixture containing this initiator combination, as well as its use as a lacquer, printing ink or adhesive.
2. Description of Related Art
The photocationic crosslinking of difunctional and polyfunctional epoxides and vinyl ethers is employed for light-hardening lacquers and printing inks since the crosslinking takes place without any problem in the presence of atmospheric oxygen, in contrast to the photo-radical crosslinking of acrylates, and lacquers, above all those based on cycloaliphatic epoxides, exhibit on account of their low degree of shrinkage a very good adhesion to various substrates such as tinplate, aluminium, and to many types of plastics materials. The low level of odour development and the low vapour pressure of the cycloaliphatic epoxides that are principally used is also an advantage.
Suitable initiators for the photocationic crosslinking are onium salts with complex, non-nucleophilic anions, in which connection triarylsulfonium salts and diaryliodonium salts in particular with hexafluorophosphate, hexafluoroarsenate or hexafluoroantimonate anions, as are described by J. Crivello and various co-authors in, inter alia, J. of Polym. Sc., Polym. Chem. Ed., 18, 2677-2695 (1980); in J. Radiation Curing 10 (1), 6-13 (1983) and in U.S. Pat. Nos. 4,058,400, 4,102,687, 4,161,478 and 4,423,136, have been introduced in industry.
These initiators decompose under irradiation with light of a wavelength range <350 nm first of all into radicals that then release protons in secondary reactions with the solvent or the monomer, as has been shown by S. P. Pappas in J. of Imaging Technology 11 (4), 146-157 (1985) and by J. L. Dektar and N. P. Hacker in J. Am. Chem. Soc. 112, 6004-6015 (1990). The strong acids HPF
6
, HSbF
6
or HAsF
6
that are thereby formed then constitute the actual initiators for the ring-opening polymerisation of epoxides, preferably epoxycyclohexyl compounds, and the chain polymerisation of high electron density olefins, such as for example vinyl ethers.
The light-induced crosslinking initiated by the protons is adversely affected only by acidic and, above all, basic impurities and by high atmospheric humidity, but not by atmospheric oxygen. Epoxycyclohexyl compounds, in particular 3,4-epoxycyclohexyl-3,4-epoxycyclohexylmethyl ester, are on account of their substantially higher crosslinking rate with respect to aliphatic and aromatic glycidyl ethers and their lower toxicity, the industrially preferred materials for photocationically crosslinkable lacquers. In addition they may also contain in minor amounts of in each case up to 20 wt. %, polyfunctional aliphatic and aromatic glycidyl ethers, olefin diepoxides and OH-containing compounds as co-reactants in order to influence the lacquer properties. Higher weight proportions of such additives reduce the crosslinking rate however.
The triarylsulphonium salts preferably used in industry are mixtures of various aromatic sulfonium salts and are employed as fifty percent solutions in propylene carbonate or &ggr;-butyrolactone. Their hexafluorophosphates are particularly preferred on account of their lower toxicity, although they give rise to lower crosslinking rates than the corresponding hexafluoroantimonates and hexafluoroarsenates. For this reason a mixture of hexafluorophosphates and hexafluoroantimonates is often employed in the case of lacquers that have to harden rapidly but do not come into contact with foodstuffs. The concentration of the photocationic initiator solutions in the case of lacquers and printing inks with wet film thicknesses of 5 and 25 &mgr;m is between 2 and 10 wt. %.
Major disadvantages of the photocationic lacquers based on cycloaliphatic epoxides and triarylsulfonium-hexafluorophosphates are their significantly lower crosslinking rate with respect to photoradically crosslinkable acrylate lacquers, and the relatively weak UV absorption of the triarylsulfonium salts and diaryliodonium salts above 320 nm, with the result that they overlap only slightly between 350 and 420 nm with the main emission maxima of the industrially preferred high-pressure mercury vapour lamps. Although this can be compensated by adding photosensitisers such as anthracene or thioxanthones, in particular isopropylthioxanthone, which absorb in the aforementioned wavelength region, the resulting lacquers however then either contain toxicologically harmful polycyclic aromatic hydrocarbons or exhibit a tendency to yellowing.
A further significant disadvantage of the industrially used triarylsulfonium salt photoinitiators is the emission of unpleasantly odiferous and toxic photolysis products such as aromatic sulfides and benzene.
On account of the short wavelength UV maxima of the aforementioned initiators it has therefore hitherto also not been possible to crosslink pigmented epoxide lacquers photocationically since titanium dioxide in its preferred rutile modification completely absorbs light below 400 nm, and also the anatase form with its higher transmission between 350 and 400 nm largely overlaps the absorption of the initiators. At best, only thin pigmented layers a few microns thick, such as are present in printing inks, can be photocationically crosslinked.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is accordingly to provide a photocationic initiator system whose UV absorption significantly better overlaps the emission range of the metal-doped medium-pressure and high-pressure mercury vapour lamps that are mainly used industrially, between 350 and 420 nm, and that is also suitable for crosslinking pigmented epoxide lacquers.
The present invention accordingly provides a photocationic initiator combination that includes the following components:
(a) at least one 2,4,6-triarylpyrylium salt of the general formula (I)
wherein X
−
denotes a non-nucleophilic anion and Ar
1
, Ar
2
and Ar
3
independently of one another in each case denote a phenyl group of the general formula (II) or a biphenyl group of the general formula (III),
wherein R
1
to R
8
independently of one another in each case denote hydrogen or an alkyl or alkoxy group with 1 to 12 carbon atoms, and
(b) at least one electron donor that forms cations under the action of light in the presence of the aforementioned 2,4,6-triarylpyrylium salt,
wherein the electron donor is
(i) a combination of an aromatic olefin and an aromatic carbonyl compound with in each case at least two unsubstituted phenyl groups or C
1
-C
12
-alkyl or C
1
-C
12
-alkoxy substituted phenyl groups and/or
(ii) a radical-forming peroxy or azo compound.
The invention is based on the surprising finding that the photocationic initiator combination according to the invention can very rapidly crosslink unpigmented as well as pigmented epoxide lacquers accompanied by complete decolouration, on irradiation with UV light in the wavelength range from 320 to 420 nm.
This finding is all the more surprising since 2,4,6-triarylpyrylium salts exhibit very strong UV absorptions with molar decadic extinction coefficients of 27-30,000 l/mole·cm between 350 and 450 nm and their solutions in organic solvents and also in cycloaliphatic epoxides are intensely coloured and strongly fluoresce.
DETAILED DESCRIPTION OF THE INVENTION
It is particularly preferred to use such 2,4,6-triarylpyrylium salts in the photocationic photoinitiator combination according to the invention whose long wavelength UV absorption maxima lie between 350 and 420 nm.
Compounds that can generate radicals, such as organic dialkyl, diaryl and diacyl peroxides, peresters and hydroperoxides and radical-forming azo compounds are suitable as components of the initiator combinations according to the invention containing 2,4,6-triarylpyrylium salts.
X
−
in the aforementioned formula (I) preferably denotes PF
6
−
, SbF
6
−
, AsF
6
−
or [B(C
6
F
5
)
4
]
−
.
The initiator combinations according to the invention, in contra

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