Coating processes – Solid particles or fibers applied – Uniting particles to form continuous coating with...
Reexamination Certificate
2000-01-13
2001-10-02
Cameron, Erma (Department: 1762)
Coating processes
Solid particles or fibers applied
Uniting particles to form continuous coating with...
C427S202000, C427S407100, C427S409000
Reexamination Certificate
active
06296903
ABSTRACT:
The present invention relates to an aqueous powder coating dispersion which is particularly suitable for preparing pigment pastes, solid-color topcoats, and clearcoats.
The present invention additionally relates to a process for preparing the aqueous powder coating dispersion arid to the use of the powder coating dispersion, especially for the coating of automobile bodies.
For the coating of automobile bodies, preference is currently given to the use of liquid coating materials. These cause numerous environmental problems owing to their solvent content. The same applies to cases where water-based coating materials are employed.
For this reason, increased efforts have been made in recent years to use powder coating materials for coating. The results so far, however, are not satisfactory; in particular, increased coat thicknesses are necessary in order to achieve a uniform appearance. On the other hand, the use of pulverulent coating materials entails a different application technology. The plants set up for liquid coating materials cannot, therefore, be used for powder coatings. Therefore, the attempt is being made to develop powder coating materials in the form of aqueous dispersions which can be processed using liquid coating technologies.
U.S. Pat. No. 4,268,542, for example, discloses a process in which a powder coating slurry is used which is suitable for the coating of automobiles. In this case, a conventional powder coat is first applied to the bodywork, and the clearcoat slurry is applied as second coat. In the case of this clearcoat slurry, acrylate resin-based binders and ionic thickeners are used. With these clearcoat slurries it is necessary to operate at high baking temperatures (more than 160° C.).
In the text below, the term transparent powder coating dispersion is used synonymously with powder clearcoat slurry.
DE-A-196 18 657 discloses an aqueous powder coating dispersion which can be prepared by first producing a powder coating material from a binder and crosslinker and, if desired, further auxiliaries and additives by means of extrusion and milling of the extrudate, dispersing this powder coating material in water, and then subjecting the aqueous powder coating dispersion to a grinding process at a shear rate of 500 s
−1
while maintaining a temperature of from 0 to 60° C.
In addition, WO 94/09913 discloses a process for preparing powder coating materials, in which the binder and the crosslinker are dissolved using supercritical gas, especially supercritical carbon dioxide, and the powder coating particles are generated by subsequent spraying. The preparation of aqueous powder coating dispersions is not described in WO 94/09913.
The object of the present invention is to provide an aqueous powder coating dispersion which can be applied to automobile bodies using the existing liquid coating technology and in particular can be baked at temperatures as low as 130° C. Furthermore, the powder coating dispersion ought to be very easy and inexpensive to prepare, very fine, exhibit good leveling, and lead to coatings having good technological properties, examples being good appearance and high gloss. Moreover, it ought also to be possible to use powder coating materials comprising components which are reactive at ambient temperature, such as, for example, powder coating materials based on free isocyanates, for these powder slurries without the occurrence of coagulum and the like.
This object is surprisingly achieved by means of an aqueous powder coating dispersion which is preparable by processing the binder of the powder coating material and/or the crosslinker of the powder coating material or the entire powder coating material to a powder by spray drying and then preparing an aqueous powder coating dispersion using the spray-dried powder.
The present invention additionally provides a process for preparing the aqueous powder coating dispersion and also provides for its use, in particular for coating automobile bodies, and for its use in processes for producing a multicoat paint system.
It is surprising and was not foreseeable that the powder coating dispersion of the invention is significantly easier and less expensive to prepare than conventional powder coating dispersions and yet exhibits good leveling and leads to coatings having the desired good technological properties, examples being good appearance and high gloss. Further advantages are that the powder coating dispersion of the invention can be applied to automobile bodies using the existing liquid coating technology and in particular can be baked at temperatures as low as 130° C. Finally, the powder coating dispersions of the invention are very fine.
It is essential to the invention that the powder coating dispersion is prepared using a spray-dried powder. In this context, for preparing the powder coating dispersion of the invention, it is preferred to subject the entire powder coating material to spray drying. One further embodiment of the invention comprises processing only the main binder of the powder coating material to a powder, by means of spray drying, and incorporating the crosslinker and any catalysts, auxiliaries and additives of the powder coating dispersion of the invention by subsequent admixing. Finally, it is also possible to subject only the crosslinker to spray drying and to incorporate the binder and any catalysts, additives and auxiliaries into the powder coating dispersion of the invention by subsequent admixing.
The spray drying that is essential to the invention can be conducted with the aid of conventional techniques. For example, it is possible to operate with the aid of a rotary atomizer, a pressure atomizer, or by means of pneumatic atomization. It is preferred, however, to operate using a rotary atomizer.
For the spray drying, a heated gas, in particular a heated inert gas, preferably heated air or heated nitrogen, is employed for drying. The gas is preferably being heated to a temperature such that at the respective exit speed through the nozzle the temperature of the sprayed particles is on the one hand high enough to ensure effective evaporation of the water and/or solvent and on the other hand not so high that instances of sticking/crosslinking or the like occur. Preferably, spray drying takes place at a product temperature which is below the glass transition temperature of the binder of the powder coating material. With particular preference, the temperature of the sprayed particles should be not more than 80° C., with very particular preference from 40 to 60° C. Particular preference is given to the use of gas which has been heated to a temperature of from 100 to 200° C. The amount of heat required to evaporate the water and/or the solvent can of course, however, also be supplied in whole or in part by means of other methods, an example being radiative heating.
The solution or dispersion that is used for the spray drying and is to be sprayed preferably has a solids content of from 30 to 60% by weight.
The particles can be sprayed in particular by means of a rotary atomizer, by means of a pressure atomizer, or by means of a pneumatic atomizer, preferably by means of a rotary atomizer. The use of a rotary atomizer has the advantage in this case that very finely divided powders having a narrow particle size distribution can be obtained. Moreover, especially in the case of spray drying using a rotary atomizer, the relatively large powder particles which are obtained are predominantly aggregates of smaller particles, so that the subsequent preparation of the powder slurry is easy and requires relatively little energy input.
If desired, spray drying may be followed by a classifying operation (in particular by means of air classifiers).
In the case where a rotary atomizer is used, the liquid pressure is generally from 2 to 3 bar. In the case where a pneumatic atomizer or a pressure atomizer is used, the liquid pressure is preferably from 20 to 50 bar.
The spray drying can take place with the aid of conventional spray dryers, which are available commercially, for example, from
Sapper Ekkehard
Woltering Joachim
BASF Coatings AG
Cameron Erma
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