Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2001-07-19
2003-04-15
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S417000, C106S447000, C106S479000, C106S481000, C106S489000, C106S499000, C264S141000
Reexamination Certificate
active
06547869
ABSTRACT:
The invention relates to a process for preparing a powder coating material, in which binders and pigments and also, where appropriate, fillers, additives and crosslinkers, following premixing where appropriate, are processed in the melt in an extruder to give a homogeneous mixture and the extrudate, after cooling, is ground to give the finished powder coating material.
As solvent-free and environment-friendly coating systems, powder coating materials have acquired considerable importance and are preferred over solvent-borne coating materials in numerous fields of use. They normally consist of binders, pigments, fillers and, where appropriate, additives and crosslinkers. They are in finely divided form and are generally applied electrostatically to a large number of different substrates, on which they are cured by baking or by radiative energy.
In the fluidized bed sintering process, a powder is held in suspension by air in a fluidizing tank. When heated substrates are immersed in the suspended powder, the polymer powder melts on the workpiece. When the fluid-sintering powders known to date are used to produce effect coatings, the problem may arise of separation of effect pigments and polymers in the course of the fluidizing process.
In principle, powder coating materials may be pigmented using all known pigments, including the known effect pigments, in turn including pearl lustre pigments based on mica.
Pearl lustre pigments are mica platelets coated with metal oxides, and are used in a large number of technical fields. They are available, for example, from Merck under the trade name Iriodin®. These pigments are described, for example, in German Patents and Patent Applications 14 67 468, 19 59 998, 2009 566, 22 14 545, 22 15 191, 22 44 298, 23 13 331, 25 22 572, 31 37 808, 31 37 809, 31 51 343, 31 51 356, 31 51 355, 32 11 602, 32 35 017, 33 34 598 and 35 28 256.
The powder coating materials are generally prepared by mixing the aforementioned constituents and homogenizing the mixture in extruders in the melt. The solid lumps of extruded powder coating material are passed to a mill. There, the powder coating material is ground to the particle size necessary for the application.
If platelet-shaped effect pigments are used to pigment powder coating materials, however, particular problems occur. Owing to the grinding process employed during the preparation of the powder coating material, a large proportion of the pigment platelets is destroyed, leading to a reduction in the lustre effect. It has therefore not been possible to date to mix effect pigments into the starting material for preparing the powder coating material, in the same way as other pigments, and then to grind the mixture following extrusion.
In order to prevent the destruction of the effect pigment platelets, they could be admixed to the powder coating material after grinding (dry-blend). When this is done, the disadvantage of accumulation or depletion of pigments on the surface of the object to be coated is observed. The reason for this observation is the different charging behaviour of the powder coating particles and the effect pigment particles. The problem of separation can only be solved, at least in part, by means of extremely costly processes, such as the bonding process, for example, in which uniform adhesion of the admixed pigments to the surface of the powder coating particles is achieved by means of electrostatic forces.
DE 24 34 855 describes a process for preparing powder coating materials with metallic pigmentation, in which a brushing process is used to fix the pigment platelets on the surface of the powder coating particles. The union of powder coating material and effect pigment is brought about by introducing mechanical energy and possibly thermal energy as well.
The equipment which is required in order to do this is very complex, something which is reflected in the high production costs for the lustre-pigmented powder coating materials. Additionally, the lustre of the powder coating materials thus obtained is often inadequate.
It is an object of the present invention to avoid the above-described disadvantages of the prior art and to specify a process for preparing a powder coating material which can be used to give powder coating materials in which the effect pigment particles are part of the powder coating particles. The intention is that this will avoid the above-described accumulation or depletion of pigments on the surface of the object to be coated, and that coatings having outstanding lustre will nevertheless continue to be obtained.
It has surprisingly been found that the above-described destruction of the effect pigment platelets during the grinding process does not occur if surface-modified effect pigments coated with a low molecular mass polyethylene or polypropylene are used.
Surface-modified pearl lustre pigments coated with a polymer compound are known, inter alia, from German Laid-Open Specification DE-A 43 17 019. The pigments described in this patent application include pearl lustre pigments coated with polyethylene wax. Although the patent application cited states in its introduction that surface-coated and modified pigments are used both for pigmenting varnishes, powder coating materials, paints, printing inks, plastics and the like, and in cosmetic formulations, the possibility of subjecting the coated pigments to a grinding process is not addressed. Rather, the person skilled in the art has to understand the reference to the use in powder coating materials, in the light of the state of the art at that time, to mean that the pigments must be added subsequently to the powder coating material, as was described above. It could not have been expected, indeed, that the thin coating of the effect pigment platelets with a polyethylene wax or polypropylene wax would improve the mechanical stability of the pigments to such an extent that impairment of the lustre, despite the grinding process, would not occur.
The present invention accordingly provides a process for preparing the above-described powder coating materials, in which binders and pigments and also, where appropriate, fillers, additives and crosslinkers, following premixing where appropriate, are processed in the melt in an extruder to give a homogeneous mixture and the extrudate, after cooling, is ground to the finished powder coating material, wherein at least part of the pigment fraction comprises surface-modified effect pigment that has been coated with a low molecular mass polyethylene or polypropylene.
The preparation of the powder coating materials of the invention is simple in practice and easy to operate. It corresponds to the customary preparation of powder coating materials. The binder component and the pigment component, which may comprise coated effect pigment and other standard pigments, and also fillers, additives and crosslinkers, where appropriate, are mixed thoroughly, using blade mixers or tumble mixers, for example. In the course of this process, the individual components may be admixed simultaneously or in succession to the binder component. The mixture is homogenized in the melt in an extruder and upon leaving the extruder is cooled and coarsely fractionated. In a mill, it is ground down to the final fineness of the powder coating material.
The powder coating material obtained by the process of the invention offers considerable advantages over the prior art; since there is no separation, application reliability is increased. Since there is no need for a bonding process or similar complex processes, the preparation costs are reduced. It is also possible to prepare powder coating materials which can be applied tribostatically, which was hitherto—not a possibility. Furthermore, powder coating concentrates containing effect pigments may also be prepared by the method of the invention.
The present invention additionally provides a powder coating material comprising binders, pigments and also, where appropriate, fillers, additives and crosslinkers, in which at least part of the pigment fraction comprises surfac
Bell Mark L.
Hailey Patricia L.
Merck Patent GmbH
Millen White Zelano & Branigan P.C.
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