Coating processes – With post-treatment of coating or coating material – Heating or drying
Patent
1998-07-16
2000-12-26
Moore, Margaret G.
Coating processes
With post-treatment of coating or coating material
Heating or drying
523446, 523427, 525523, 525526, 525934, B05D 302
Patent
active
061655589
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a storage-stable highly reactive moulding powder based on epoxy resins and aminic hardeners and to the use of said moulding powder for the preparation of moulded articles or coatings, preferably for powder coatings.
Until now, the customary method of preparing powder coatings based on epoxy resins was that using extruders. As is described, inter alia, by Becker/Braun in "Kunststoffhandbuch" (Carl Hanser Verlag Munchen Wien 1988), Vol. 10, Chapter 13.10 under powder coatings on pages 1047-1058 and, in particular, on pages 1051 and 1052, said preparation is carried out such that the individual components such as epoxy resin, hardener and the customary additives for powder coatings, typically flow control agents, pigments, fillers and catalysts, are comminuted, mixed and then extruded in the temperature range from 70 to 120.degree. C. depending on the composition of the mixture and the type of extruders used. The temperature must be such that no pre-crosslinking occurs interfering with the subsequent processing. After extruding, the extrudate is cooled and ground to a particle size of smaller than 100 .mu.m. In contradistinction to a mechanical mixture, the particles so obtained therefore contain all components combined. The application to the substrate to be coated is carried out by known methods, typically by electrostatic powder spraying or by fluidised-bed coating. The applied powder coating is then cured in the temperature range from about 130 to 240.degree. C. for 10 to 30 minutes. Disadvantages of powder coatings having low curing temperatures, e.g. 130.degree. C., are the relatively long curing times of over 15 minutes, the poor flow of the powder coating on the substrate as well as the limited storage stability of the formulated powder coatings.
EP-Patent 0 042 759 discloses a process for colouring a substrate surface by powder coating, which comprises using powder coating systems which are curable with the application of heat and pressure. The dislosed coating powders consisting of epoxy resin and polyester, or of epoxy resin and polyurethane, are cured at 180.degree. C. in 3 or 10 minutes, respectively, and are poorly suited for coating substrates susceptible to heat, such as paperboard or wood, and are not reactive enough for curing at low temperatures.
JP Application Kokai Sho 61-107980 discloses a process for coating wood, wherein the substrate consisting of wood is first coated with a liquid epoxy resin and then with an epoxy resin in powder form before the coating is cured with the application of heat and pressure. The liquid as well as the solid epoxy resin contain a catalytic hardener, e.g. 2-ethyl-4-methylimidazole, so that the storage stability of the coating systems used is low.
It has now been found that storage-stable coating powders based on epoxy resins and aminic hardeners, which are rapidly curable with the application of heat and pressure, are obtained if a powdered epoxy resin having a particle size smaller than 200 .mu.m and a softening point (according to DIN 51920) of above 60.degree. C. is mechanically mixed with a powdered aliphatic or N-heterocyclic amine or a powdered amino group-containing adduct based on an aliphatic, cycloaliphatic, araliphatic or N-heterocyclic amine having a particle size of smaller than 200 .mu.m and a softening point of above 60.degree. C. without extrusion, giving a physical mixture of the components.
Accordingly, the invention relates to a moulding powder which is curable with the application of heat and pressure, comprising a mechanical mixture consisting of group in the molecule and having a softening point of above 60.degree. C., group-containing adduct based on an aliphatic, cycloaliphatic, araliphatic or N-heterocyclic amine containing on average more than one amino group in the molecule and having a melting point or softening point of above 60.degree. C. and, optionally, being smaller than 200 um.
Suitable powdered epoxy compunds a) are all solid epoxy resins customary in epoxy resin technology having a soften
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Kovaleski Michele A.
Moore Margaret G.
Vantico Inc.
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