Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2002-02-25
2004-09-07
Tsoy, Elena (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S407100, C427S409000, C427S514000, C427S517000, C427S518000, C522S096000
Reexamination Certificate
active
06787197
ABSTRACT:
The present invention relates to a process for producing scratch-resistant weathering-stable coatings on the basis of UV-curable coating compositions.
Coating compositions which cure by UV radiation are used in industry to produce high-quality coatings. Radiation-curable coating compositions are generally flowable formulations based on polymers or oligomers containing crosslinking-active groups which on exposure to UV radiation undergo a crosslinking reaction with one another. This results in the formation of a high molecular mass network and thus in the development of a solid polymeric film. Unlike the heat-curable coating compositions often used to date, radiation-curable coating compositions may be used free from solvents or dispersants. They are further notable for very short curing times, which is particularly advantageous in the case of continuous processing on coating lines.
Coating compositions curable by UV radiation generally give high surface hardness and good chemical resistance. For some time there has been a desire for coatings which possess high scratch resistance, so that when it is cleaned, for example, the coating is not damaged and does not lose its gloss. At the same time, the coatings should retain the properties normally achieved with radiation-cured coatings.
In the literature there have been various descriptions of the physical processes involved in the appearance of scratches and the relationships between scratch resistance and other physical parameters of the coating (on scratch-resistant coatings cf., e.g., J. L. Courter, 23
rd
Annual International Waterborne, High-Solids and Powder Coatings Symposium, New Orleans 1996).
A variety of test methods have been described to quantify the scratch resistance of a coating. Examples include testing by means of the BASF brush test (P. Betz and A. Bartelt, Progress in Organic Coatings 22 (1993) 27-37), by means of the AMTEC wash brush installation, or various test methods analogous to scratch hardness measurements, as described for example by G. Jüttner, F. Meyer, G. Menning, Kunststoffe 88 (1988) 2038-42. A further test to determine scratch resistance is described in European Coatings Journal 4/99, 100 to 106.
In accordance with the present state of development, three routes to scratch-resistant surfaces are being discussed, which in principle may also be transferred to UV-curing systems.
The first route is based on increasing the hardness of the coating material. Since a harder material cannot be scratched by a softer one, a high level of hardness is a sufficient prerequisite for scratch resistance. However, the high level of hardness is at the expense of other properties, such as the penetration depth or the adhesion, which are vital to coating materials.
The second route is based on selecting the coating material such that on scratching it is stressed in the reversible deformation range. The materials involved are those which permit high reversible deformation. However, there are limits on the use of elastomers as coating materials. Coatings of this kind usually exhibit poor chemical stability. This approach has to date played no part in practical application.
A third approach attempts to produce coatings having a ductile, i.e., plastic deformation behavior and at the same time to minimize the shear stress within the coating material that occurs on scratching. This is done by reducing the friction coefficient, using waxes or slip additives, for example. Coatings additives for UV-curing systems are described, for example, in B. Hackl, J. Dauth, M. Dreyer; Farbe & Lack 103 (1997) 32-36.
U.S. Pat. No. 5,700,576 describes a UV-curing, scratch-resistant coating which comprises 1-30% by weight of a prepolymeric thickener containing thiol groups and 20-80% by weight of one or more polyfunctional acrylates or methacrylates, and also diluents, especially reactive diluents containing a free-radically polymerizable group, free-radical initiators, and further customary additives for producing coatings. The polymerization and thus curing of the coating is initiated by irradiation with UV light.
EP 0 544 465 B1 describes a UV-curable, scratch-resistant coating whose hardness is increased by incorporating colloidal silica into the coating matrix, the intention being at the same time to retain the flexibility of the organic matrix. A scratch-resistant coating of this kind contains 1-60% by weight of colloidal silica, 1-50% by weight of the hydrolysis product of an alkoxysilyl acrylate, especially 3-methacryloyloxypropyltrimethoxysilane, and 25-90% by weight of acrylate monomers, comprising a mixture of 20-90% by weight of hexanediol diacrylate and 10-80% by weight of a monofunctional branched or carbocyclic acrylate, and also a sufficient amount of free-radical initiator.
It is an object of the present invention to provide a process for producing scratch-resistant and weathering-stable coatings on the basis of UW-curable coating compositions.
We have found that this object is achieved, surprisingly, by a process which involves applying a liquid, UV-curable coating composition based on aliphatic urethane (meth)acrylate prepolymers to the target substrate and curing the still-wet coating subsequently by UV radiation in the substantial absence of oxygen.
The present invention accordingly provides a process for producing scratch-resistant coatings, which involves applying at least one UV-curable coating composition comprising as its photochemically crosslinkable constituent
at least one aliphatic urethane (meth)acrylate prepolymer PU having at least two double bonds per molecule, or
a mixture of at least one urethane (meth)acrylate prepolymer PU and at least one reactive diluent,
to the target substrate and curing the resulting wet coating by exposure to ultraviolet radiation under an inert gas atmosphere.
The term inert gas atmosphere refers to an essentially oxygen-free atmosphere of chemically inert gases, such as nitrogen, carbon-monoxide, carbon dioxide and noble gases, e.g., argon, or mixtures of said gases. Inert gases generally contain not more than 2000 ppm of impurities and normally not more than 500 ppm of oxygen. These slight traces of oxygen do not impair the effect of the invention. Even amounts of oxygen of up to 2% by volume do not detract from the effect of the invention. Higher grades of nitrogen contain less than 10 ppm of oxygen. Typical argon grades contain less than 6 ppm of oxygen. The preferred inert gas is nitrogen.
The UV-curable coating compositions used in the process of the invention comprise as their photochemically crosslinkable constituents and, accordingly, their film formers at least one aliphatic urethane (meth)acrylate prepolymer PU having at least two double bonds per molecule, or a mixture of such prepolymers PU with at least one reactive diluent, selected preferably from difunctional and polyfunctional esters of acrylic acid and/or of methacrylic acid with aliphatic diols or polyols (reactive diluent R).
Aliphatic urethane (meth)acrylate prepolymers are polymeric or oligomeric compounds which have urethane groups and acryloxyalkyl and/or methacryloxyalkyl groups or (meth)acrylamidoalkyl groups. Normally, the (meth)acryloxyalkyl and/or (meth)acrylamidoalkyl groups are attached via the oxygen atom of the urethane group. The term acryloxyalkyl groups refers to C
1
-C
10
alkyl radicals, preferably C
2
-C
5
alkyl radicals, substituted by one, two or three, preferably one, acryloxy group. Similar comments apply to methacryloxyalkyl groups. Accordingly, (meth)acrylamidoalkyl groups are C
1
-C
10
alkyl radicals, preferably C
2
-C
5
alkyl radicals, substituted by one, two or three (meth)acrylamido groups, preferably by one (meth)acrylamido group. In accordance with the invention, the aliphatic urethane (meth)acrylate prepolymers PU have at least two double bonds per molecule, preferably from three to six double bonds per molecule. The aliphatic urethane (meth)acrylate prepolymers PU of the invention are essentially free from aromatic structural elements, such as phenylene or naphthylene or su
Jaworek Thomas
Schwalm Reinhold
BASF - Aktiengesellschaft
Tsoy Elena
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