Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2001-05-24
2003-06-17
Berman, Susan W. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S016000, C522S026000, C522S031000, C522S039000, C522S063000, C522S126000, C522S146000, C522S174000, C522S180000, C430S270100
Reexamination Certificate
active
06579913
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority of European Application No. 00201850.5, filed on May 26, 2000.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a photoactivatable coating composition comprising at least one photoinitiator and a base-catalysed polymerisable or curable organic material comprising at least one polyisocyanate and at least one compound comprising isocyanate reactive groups, and to its use for the preparation of coatings with a rapidly processable surface at ambient temperature.
2. Discussion of the Prior Art
Photoactivatable coating compositions of the type indicated above are known from, int. al., U.S. Pat. No. 4,369,206. In this document use is made of ammonium salts of &agr;-ketocarboxylic acids as photoinitiator, and the photoactivatable coating compositions comprise polyurethane or epoxide resin precursors hardening in the presence of amines. A drawback to the known compositions is that they are only cured in places which are readily accessible to UV light. In the case of three-dimensional surfaces, or where the presence of pigments does not allow the UV radiation to penetrate into lower layers, the curing speed is too low.
EP-A-0 898 202 discloses the photogeneration of amines from &agr;-amino acetophenones as latent base catalyst in a base-catalysed coating composition. The curable systems only comprise organic compounds which are capable of reacting in a base-catalysed reaction or a condensation reaction. Specific mention is made of a (poly)alcohol and a (poly)isocyanate. In order to increase the curing speed in places which are not readily accessible to UV light, mention is made of the possible use of an additional unblocked base catalyst, which, however, requires the use of an additional thermal step.
The invention now provides coating compositions which can be cured by UV radiation and have an acceptable curing speed at ambient temperature in places which are not readily accessible to UV light, which compositions comprise at least one photoinitiator and a base-catalysed polymerisable or curable organic material comprising at least one polyisocyanate and at least one compound comprising isocyanate reactive groups.
SUMMARY OF THE INVENTION
The photoactivatable coating composition according to the invention is characterised in that the isocyanate reactive groups comprise at least one thiol group and the photoinitiator is a photolatent base.
DETAILED DESCRIPTION OF THE INVENTION
It should be added that photoactivatable coating compositions comprising a photoinitiator and a polymerisable or curable organic material consisting of a (poly)isocyanate and a compound comprising thiol groups are known from, int. al., EP-A-0 188 880. The coating layers produced with the coating compositions disclosed therein can be cured first by UV radiation and then, thoroughly, in a conventional manner at ambient temperature. A major drawback to the known coating compositions is the simultaneous presence of at least two entirely different curing mechanisms. One mechanism is based on the reaction between a multifunctional alkene and a multifunctional thiol, which requires UV radiation, whereas the secondary cure comprises a great many mechanisms such as the reaction of free isocyanate with water and the reaction of free isocyanate with the thiol component. One effect of this so-called dual cure system is that unexposed places will only be cured in part, resulting in an unreacted amount of alkene in the unexposed places. Therefore, in order to still achieve a minimum degree of curing in these places, use will have to be made of compounds having a higher functionality. Using such compounds has a viscosity increasing effect, which leads to a greater quantity of solvent being required to achieve a similar spraying viscosity, which is attended with an increase in the VOC.
A further advantage of the photoactivatable coating composition according to the invention is that when a photolatent base is used, it will remain active even after the exposure has ended.
Suitable photolatent bases include N-substituted 4-(o-nitrophenyl) dihydropyridines, optionally substituted with alkyl ether and/or alkyl ester groups, and quaternary organo-boron photoinitiators. An example of an N-substituted 4-(o-nitrophenyl) dihydropyridine is N-methyl nifedipine (Macromolecules 1998, 31, 4798), N-butyl nifedipine, N-butyl 2,6-dimethyl 4-(2-nitrophenyl) 1,4-dihydropyridine 3,5-dicarboxylic acid diethyl ester and a nifedipine according to the following formula
i.e., N-methyl 2,6-dimethyl 4-(4,5-dimethoxy-2-nitrophenyl) 1,4-dihydropyridine 3,5-dicarboxylic acid diethyl ester. Examples of quaternary organo-boron photoinitiators are disclosed in GB-A-2 307 473, such as
Thus far optimum results have been obtained with a photolatent base belonging to the group of &agr;-amino acetophenones. Examples of &agr;-amino acetophenones which can be used in the photoactivatable coating compositions according to the present invention are: 4-(methylthiobenzoyl)-1-methyl-1-morpholinoethane (Irgacure® 907 ex Ciba Specialty Chemicals) and (4-morpholinobenzoyl)-1-benzyl-1-dimethylamino propane (Irgacure® 369 ex Ciba Specialty Chemicals) disclosed in EP-A-0 898 202. Preferred is an &agr;-amino acetophenone according to the following formula
The photolatent base may be used in an amount of between 0.01 to 10 wt. % on solid curable material, preferably 0.05 to 5 wt. %, more preferably 0.05 to 3 wt. %.
For most coating compositions wherein use is made of a photolatent base an acceptable balance between pot life and cure speed can be obtained by the incorporation of a certain amount of organic acid. Preference is given in said case to an organic acid that is compatible with the other parts of the composition. Thus far optimum results have been obtained with dodecylbenzene sulphonic acid. The amount used thereof varies between 0.01 and 20 wt. %, based on the total weight of the coating composition, preferably 0.05 to 10 wt. %, more preferably 0.1 to 5 wt. %.
Though irradiation of most of the &agr;-amino acetophenones known from the literature results in the generation of an active catalyst, for some of them the unexposed places detract from the performance due to a too long curing time. Said problem may be overcome by incorporating a very weak base and/or a metal compound into the composition.
Generally, good results are obtained with metal complexes and/or metal salts which are known as such as catalysts for the isocyanate hydroxyl reaction. Preference is given in said case to metal complexes or metal salts wherein the metal is selected from the group of aluminium, titanium, zirconium, and hafnium. These metals are complexed with carboxylate groups and/or diketones or alkylacetoacetates. Examples of satisfactory catalysts are disclosed in U.S. Pat. No. 5,846,897. Thus far optimum results have been obtained with the aluminium complex K-KAT® XC5218 (ex King Industries) and with organic titanates such as titanium diisopropoxide bis-2,4(pentadionate) (Tyzo® AA ex DuPont). These catalysts may be used in an amount of 0.01 to 10 wt. % on solid curable material, preferably 1 to 5 wt. %.
The coating compositions according to the invention are radiation curable after application and, optionally, evaporation of solvents. In particular, they are suitable for curing by irradiation with UV light. Combinations of IR/UV irradiation are also suitable. Radiation sources which may be used are those customary for UV, such as high- and medium-pressure mercury lamps. In order to avoid any risk involved in handling UV light of very short wave length (UV B and/or UV C light), preference is given, especially for use in automotive refinishing shops, to fluorescent lamps which produce the less injurious UV A light. However, the low intensity of the light produced by said lamps was found to have a detrimental effect on curing the known radically polymerised dual cure systems as a result of oxygen inhibition.
Surprisingly, it has now been found that when a photolatent base is used as the photoi
Klinkenberg Huig
Van Oorschot Josephus Christiaan
Akzo Nobel N.V.
Berman Susan W.
McGillycuddy Joan M.
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