Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2002-02-25
2004-08-17
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S006000, C522S090000, C522S096000, C522S100000, C522S104000, C522S107000, C522S150000, C522S120000, C522S121000, C522S152000, C522S153000, C522S157000, C522S169000, C522S182000, C522S902000, C427S487000, C427S491000, C427S508000, C427S510000, C427S495000, C264S494000, C204S157150, C204S157440, C204S471000, C204S478000
Reexamination Certificate
active
06777458
ABSTRACT:
The present invention relates to a process for producing scratch-resistant coatings on the basis of radiation-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. For example, EP-A 544 465 describes a coating composition for scratch-resistant coatings which comprises colloidal silica and alkolysis products of alkoxysilyl acrylates. The increase in hardness is based here on the incorporation of the silica into the polymer matrix of the coating. However, the high level of hardness is at the expense of other properties, such as the penetration hardness 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.
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 in 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, under inert gas, for example.
However, the solutions proposed for producing scratch-resistant coatings are unsatisfactory because they are comparatively expensive and because the other coating properties are not satisfactory.
In another invention, which is the subject of a parallel application, it has been found that scratch-resistant coatings having a balanced profile of properties can be produced if a radiation-curable coating based on urethane acrylates is cured under inert gas conditions. Inert gases generally contain not more than 500 ppm of oxygen, which under standard conditions corresponds to an oxygen partial pressure of less than 0.05 kPa. The substantial exclusion of oxygen requires an expensive technology. In order to exclude oxygen, the curing of the coating on structures, i.e., nonplanar articles having a three-dimensional form, has to be carried out in chambers closed off to the outside and maintained strictly under an inert gas atmosphere. Especially in the case of continuous coating lines, this would necessitate an expensive airlock technology and would therefore be uneconomic.
It is an object of the present invention to provide a simple process for producing scratch-resistant coatings which overcomes the disadvantages of the prior art.
We have found that this object is achieved if a conventional radiation-curable coating composition is cured by exposure to ultraviolet radiation in an oxygen-containing, protective-gas atmosphere having a oxygen partial pressure of not more than 18 kPa, without the need to observe strict inert gas conditions.
The present invention accordingly provides a process for producing scratch-resistant coatings, encompassing the following steps:
applying at least one UV-curable coating composition to at least one surface of an article to be coated, said coating composition comprising at least one polymer and/or oligomer P1 containing on average at least one ethylenically unsaturated double bond per molecule, and
curing the coating composition by exposure to UV radiation,
which comprises conducting the curing of the coating composition under an oxygen-containing protective gas which has an oxygen partial pressure in the range from 0.2 to 18 kPa.
In the case of a protective gas under atmospheric pressure, an oxygen partial pressure of 18 kPa corresponds to an oxygen fraction of about 20% by volume. Under the same conditions, an oxygen partial pressure of 0.2 kPa corresponds to a volume fraction of 2200 ppm of oxygen in the protective gas (cf. also E. W. Bader [Ed.], Handbuch der gesamten Arbeitsmedizin [Handbook of complete occupational hygiene], Vol. 1, Urban und Schwarzenberg, Berlin, Munich, Vienna, 1961, p. 665). An oxygen partial pressure of 9 kPa corresponds to 10% by volume of oxygen in the protective gas.
For the process of the invention all that is necessary is for the coating compositions to be subject to an oxygen concentration of less than 18 kPa in the regions where curing takes place at the time of their exposure to UV radiation. The relevant regions are the surface regions of the article to be costed which have been provided with the radiation-curable coating compositions, at the time of their exposure to UV radiation. In order to attain optimum scratch resistance, the oxygen partial pressure is preferably not more than 17 kPa (≈19% by volume), in particular not more than 15.3 kPa (≈17% by volume), and with particular preference not more than 13.5 kPa (≈15% by volume). Optimum curing results are generally obtained at oxygen partial pressures in the range from 0.5 kPa to 10 kPa (≈5500 ppm−11% by volume), in particular in the range from 0.5 to 6.3 kPa (≈5500 ppm−7% by volume). Typically, the oxygen partial pressure will not be below a level of 0.5 kPa, especially 0.9 kPa (≈1% by volume), 1.8 kPa (≈2% by volume), or 2.5 kPa (≈3% by volume).
Suitable prote
Jaworek Thomas
Königer Rainer
Kranwetvogel Reiner
Schwalm Reinhold
BASF - Aktiengesellschaft
McClendon Sanza L.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Seidleck James J.
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