Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1996-06-21
1998-04-14
Sellers, Robert E.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
525 65, 525110, 525118, 525454, 525471, 525522, 427519, 428412, C08L 6302, C08L 6304, C08L 6706, C08L 6102
Patent
active
057392145
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a radiation-curable coating composition comprising at least one cationically curing epoxy resin, at least one free-radically curing, ethylenically unsaturated compound, and photoinitiators.
The invention also relates to a method of coating metallized surfaces and to the use of the radiation-curable coating compositions for the coating of metallized surfaces such as, for example, of data carriers, and in particular to their use for the coating of the metallized side of compact disks.
BACKGROUND AND SUMMARY OF THE INVENTION
Compact disks are pressed from high-purity polycarbonate under the action of pressure and heat. The side carrying the data is then vapor-coated with a reflecting layer of aluminum. This aluminum layer is only about 7 to 8.times.10.sup.-8 m thick and requires protection against damage by external influences and against corrosion, in order to ensure error-free sampling of the stored data. The metallized layer is therefore usually provided with a clear protective coat which is then in turn provided with a label, printed in one or more colors with a conventional printing ink.
In many cases conventional, i.e., solvent-containing, coating materials are still used to produce the protective layer. High-volume production and the increasingly stringent statutory regulations on industrial coating plants with the aim of limiting the emission of pollutants are leading, however, to the increasing use of more environmentally friendly, solvent-free coating materials.
One alternative is to coat the metallized side of compact disks with radiation-curable coating materials. However, a number of specific problems of radiation-curable systems are associated with this technology. For instance, a shrinkage of the film is observed on curing radiation-curable materials because the transition from the liquid to the solid state is extremely rapid, and the polymerization is accompanied by a reduction in volume. On metal surfaces which have not been chemically pretreated this results in serious problems of adhesion. Because of the danger of early corrosion of the aluminum layer, conventional adhesion promoters cannot be used to solve this problem.
There are also problems with regard to the printability of the protective coat with solvent-containing printing inks. This is because radiation-curable coating materials generally lead to coatings of extremely high solvent resistance. This causes the adhesion of the printing ink to protective layers of radiation-curable coating materials to be inadequate. Finally, the inadequate scratch resistance of the cured coatings also causes problems.
U.S. Pat. No. 4,657,779 discloses radiation-curable coating compositions whose shrinkage is so low that they are also suitable for coating deformable substrates such as, for example, polycarbonate compact disks. These radiation-curable coating materials comprise a cationically curable polyepoxide, an acrylate-functional component containing at least one reactive group, an acrylate-functional component to establish the viscosity, and a photoinitiator component. These radiation-curable coating materials known from U.S. Pat. No. 4,657,779 have the disadvantage, however, of inadequate scratch resistance in the cured coatings. In addition, there is no attempt made to address the problem of the printability of the cured protective layers with conventional printing inks.
From EP-B-138 381 it is also known to coat compact disks with radiation-curable coating materials. The examples given in EP-B-138 381 of suitable coating materials are mixtures which comprise a free-radically polymerizable monomer and/or oligomer, a cationically polymerizable compound such as, for example, an epoxy resin, and a photoinitiator. The use of these radiation-curable coating materials also leads to coatings having an inadequate scratch resistance.
The combination of free-radically and cationically curing systems to improve the technical properties of the resulting coatings is also described in other document
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patent: 4514439 (1985-04-01), Rounds
patent: 4657779 (1987-04-01), Gaske
patent: 5093386 (1992-03-01), Bishop et al.
BASF Lacke & Farben AG
Sellers Robert E.
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