Utilization of phenylglyoxalic acid esters as photoinitiators

Details Utilization of phenylglyoxalic acid esters as photoinitiators Utilization of phenylglyoxalic acid esters as photoinitiators

2001-09-21

2003-05-13

Pianalto, Bernard (Department: 1762)

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C427S372200, C427S375000, C427S384000, C427S487000, C427S508000, C427S517000, C427S519000, C427S521000, C427S553000, C427S558000, C430S056000, C430S270100, C430S581000, C522S001000, C522S006000, C522S008000, C522S009000, C522S037000, C549S465000

Reexamination Certificate

active

06562464

ABSTRACT:

The present invention relates to the use of certain phenylglyoxalic esters as photoinitiators in powder coating materials for exterior applications.
Conventional radiation-curable coating materials generally include reactive monomers for adjusting the application viscosity (reactive diluents) and are cured at room temperature. These coating materials have the disadvantage that the monomers used as reactive diluents are able to penetrate into porous substrates, such as wood or paper, thereby escaping the UV cure. Furthermore, the exposure of three-dimensional objects may give rise to the incidence of shadow regions, which are insufficiently exposed, meaning that liquid coating materials may remain tacky in these regions. Aqueous UV coating materials have therefore been developed which are heat treated for drying and are cured at relatively high temperatures, see W. Reich et al., RadTech 98, Conference Proceedings, Chicago 1998, 258-265.
In order to avoid the abovementioned problems, furthermore, UV-curable powder coating materials have been proposed which are applied to the substrate by conventional methods and then melted and cured at relatively high temperatures; see, for example, RadTech, Conference Proceedings, Chicago 1998, 170-176; JOT 1998, 2, 44-47; PPCJ 1997, (9), 18,20; EP 636 669 A; 650 985 A; 650 978 A and U.S. Pat. No. 5,639,560. Also known are Dual Cure Systems where curing takes place by means of radiation and atmospheric humidity (U.S. Pat. No. 4,138,299; 4,173,682; 4,415,604 and EP 549 116 A) or which comprise a heat-activatable initiator as well as a UV free-radical initiator (EP 844 286 A).
It has been found that, under the thermal stress of the melting of powder coating materials, many of the common photoinitiators are too volatile to permit curing by UV irradiation. Furthermore, the depletion of photoinitiators at the surface of the coating material is particularly undesirable, since it is there that there is the greatest oxygen inhibition, so resulting in inadequate curing and thus poor film properties, such as low chemical resistance and inadequate blocking resistance. Consideration has therefore been given to the less volatile arylglyoxalic esters as photoinitiators; see, for example, DE 26 39 742 A, U.S. Pat. No. 4,024,297 and DE 198 26 712 A, which latter application has a priority date earlier but a publication date later than the present specification. Polymer-bound arylglyoxalates are known from DE 28 25 955 A and Macromolecules 1998, 31, 322-327.
WO 98/33761 describes diesters of arylglyoxalic acids with diols. These esters are low-volatility photoinitiators which are used in liquid systems. The possibility of their use in powder coating materials is only mentioned in general terms.
It is an object of the present invention to provide improved photoinitiators for use in powder coating materials for exterior applications.
We have found that this object is achieved by certain esters of arylglyoxalic acids with diols, which are particularly suitable as photoinitiators in powder coating materials for exterior applications. In particular it has been found that the esters used in accordance with the invention can be exposed to heat without disadvantage prior to irradiation. We have also found that the photoinitiators exhibit particularly low yellowing levels.
The present invention therefore provides for the use of phenylglyoxalic esters of the formula I
in which the two radicals R
1
independently of one another are a radical of the formula
R
2
, R
3
and R
4
independently of one another are H, C
1
-C
6
-alkyl, which is unsubstituted or substituted by OH, OC
1
-C
6
-alkyl or OCOC
1
-C
6
-alkyl, or are OH or OC
1
-C
6
-alkyl;
A is C
2
-C
6
-alkylene or a radical of the formulae
 the radicals R
5
independently of one another are H or COCOR
1
, and
A
1
is C
2
-C
6
-alkylene or
 as photoinitiators in powder coating materials for exterior applications.
C
1
-C
6
-alkyl is a straight-chain or branched alkyl group having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms. Examples of alkyl groups are methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl and hexyl.
C
2
-C
6
-alkylene is a straight-chain or branched alkylene group having 2 to 6 carbon atoms. Examples of alkylene groups are methylene, ethylene, propylene, isopropylene, n-butylene, sec-butylene, isobutylene, tert-butylene, pentylene and hexylene.
Preferably, one of the radicals R
2
, R
3
and R
4
is C
1
-C
6
-alkyl and the other two are H, and with particular preference R
2
, R
3
and R
4
are all H.
A is preferably
The compounds of the formula I in which A has the preferred definitions indicated are likewise provided by the invention.
The compounds of the formula I are either known or can be prepared by the methods described in WO 98/33761. Furthermore, it is possible to react the desired arylglyoxalic acid with the alcohol in the presence of an acidic catalyst, such as sulfuric acid, p-toluenesulfonic acid, in the manner of a conventional esterification to give the desired ester. The esterification is judiciously conducted in a water-immiscible solvent, for example a hydrocarbon, such as methylcyclohexane, toluene, xylene, etc. The water formed is removed from the reaction mixture in a conventional manner.
The compounds of the formula I in which A is
can be prepared by reacting the corresponding arylglyoxalic acid with a diglycidyl ether of the formula
in which A
1
is as defined above. The reaction is generally conducted without solvents and in the presence of an appropriate catalyst, such as a tetraalkylammonium halide. The reaction temperature is generally in the range from 80 to 120° C.
The compounds of the formula I can be used as photoinitiators for the photopolymerization of ethylenically unsaturated compounds. In particular, they are suitable as photoinitiators in powder coating materials for interior and, preferably, exterior applications. They can be used alone or together with other photoinitiators. Examples of photoinitiators with which they can be used in a mixture are mono- or bisacylphosphine oxides, benzophenones and hydroxyacetophenones. Further cophotoinitiators are described, for example, in WO 98/33761.
The compounds of the formula I are generally used in radiation-curable compositions. These compositions include at least one ethylenically unsaturated, radiation-curable substance and at least one photoinitiator compound of the formula I, together if desired with further photoinitiators and/or additives. The amount of compounds of the formula I in the compositions is generally in the range from 0.1 to 10% by weight, preferably from 0.5 to 8% by weight, based on the weight of the radiation-curable substance. Where mixtures with other photoinitiators are used, said mixtures generally contain from 0.1 to 50% by weight of said other photoinitiators, based on the overall weight of the mixture.
The amount of radiation-curable substance in the compositions is generally in the range from 10 to 90% by weight, preferably from to 90% by weight, based on the overall weight of the composition. In the case of powder coating materials, the amount of radiation-curable substances is generally in the range from 50 to 90% by weight, preferably from 60 to 90% by weight, based on the overall weight of the composition.
The radiation-curable substance can comprise one or more olefinic, free-radically polymerizable double bonds and be monomeric or polymeric, especially oligomeric, in nature. Examples of monomeric substances (reactive diluents) having a double bond are alkyl or hydroxyalkyl acrylates or methacrylates, such as methyl, ethyl, n-butyl and 2-ethylhexyl acrylate or methacrylate, and 2-hydroxyethyl acrylate or methacrylate. Further examples are (meth)acrylonitrile, (meth)acrylamide, N-C
1
-C
4
-alkyl substituted (meth)acrylamides, vinyl esters, such as vinyl acetate, vinyl ethers, such as isobutyl vinyl ether, vinylaromatic compounds, such as styrene, N-vinylpyrrolidone, vinyl chloride and vinylidene chloride.
Examples of monomers having two

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