Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-03-12
2003-09-23
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
active
06624238
ABSTRACT:
The present invention relates to a novel powder clearcoat slurry, free from organic solvents and external emulsifiers, which possesses pseudoplasticity. The present invention also relates to a novel process for preparing this powder clearcoat slurry. The invention relates not least to the use of the novel powder clearcoat slurry for producing clearcoats for the automotive sector and for the industrial sector, and also for coating moldings.
For the coating of automobile bodies, preference is given today to the use of liquid coating materials, i.e., spray paints. These give rise to numerous environmental problems owing to their solvent content. Aqueous basecoat materials for use in a basecoat/clearcoat system are described in DE 196 52 842. As the film-forming medium they comprise a dispersion of acrylic polymers and a nonassociative thickener. EP 0 038 127 discloses aqueous basecoat materials which are based on a dispersion of crosslinked polymer microparticles having a particle diameter of 0.01-10 &mgr;m. In both cases, after the continuous polymer film has formed, coating with a clearcoat material takes place. The use of waterborne clearcoat materials likewise gives rise to numerous environmental problems, since these still always contain certain amounts of organic solvents.
Waterborne clearcoat materials of this kind are known from the German patent DE-A-196 23 371. Directly after application, the conventional waterborne clearcoat materials do not dry to a powder but instead flow out to form a continuous film. They comprise aqueous secondary dispersions and are used in the automotive sector for aqueous multicoat systems or aqueous one-component or two-component clearcoats. The aim here is for sedimentation-stable dispersions having an average particle size of from about 10 to about 200 nm. The reason for this is the experience, familiar to the skilled worker, whereby, the better the stabilization and the smaller the size of dispersion particles, the less their tendency to settle. For reliable application behavior and in order to reduce the popping tendency, however, it is necessary to use up to 20% by weight of solvents as well.
For this reason, increased efforts have been made in recent years to use powder coating materials for the coating. The results to date, however, have not been satisfactory; in particular, powder clearcoat materials still show weaknesses with regard to chemical resistance and yellowing.
In the meantime, many developments have aimed to provide powder clearcoat materials in the form of aqueous dispersions that can be processed using liquid coating technologies. The patent U.S. Pat. No. 4,268,542, for example, discloses a process using a powder coating dispersion based on acrylic resins which are suitable for coating automobiles. In this case, first of all a conventional powder coat is applied to the body, after which the powder coating dispersion is applied as clearcoat. This powder clearcoat dispersion, also referred to those in the art as a powder slurry or powder clearcoat slurry, uses ionic thickeners, which lead to a relatively high sensitivity of the applied clearcoat film to moisture, especially to condensation.
The European patent EP-A-0 652 264 discloses a powder clearcoat slurry in which the solid binder and crosslinker components and, if desired, additives are, as is usual for the production of powder coating materials, first of all coextruded and then subjected to dry grinding, after which, in a further step of wet grinding, they are converted into a powder clearcoat slurry with the aid of emulsifiers and wetting agents. The aqueous powder coating dispersions described in the German patent DE 195 40 977 are also prepared by wet grinding. They comprise addition polymers containing epoxy groups, and also at least one nonionic thickener, and have a viscosity of 10-1 000 mPas at a shear rate of 500 s
−1
. However, pseudoplasticity is not attained.
These customary and known powder clearcoat slurries, in contradistinction to the powder clearcoat materials, may be processed in conventional wet coating plants and may be applied at substantially lower coat thicknesses of about 40 &mgr;m, relative to about 80 &mgr;m in the case of powder coating materials, with good leveling and with a chemical resistance which is comparable with that of the powder coating materials.
However, the conventional grinding processes do not always ensure the degree of homogenization of the constituents which would in fact be desirable, or else such a degree must be achieved by means of a complicated multiple extrusion.
Normally, in the conventional powder clearcoat slurries, relatively large particles are unwanted since they tend toward sedimentation. Moreover, the powder clearcoat slurries, on application and crosslinking, exhibit an increased tendency to form popping marks (blisterlike cavities enclosed in the coating film).
The situation is similar with the cracking, known as “mudcracking” in the pulverulent, dry films which have been predried at room temperature or a slightly elevated temperature but not yet baked. On baking, drying cracks of this kind no longer flow out fully, and in the baked film they form visible flow defects in the form of furrows with a leathery texture, these drying cracks increasing in pronouncedness and frequency as the dry film thickness increases. Where automobile bodies are coated electrostatically, relatively high coat thicknesses may occur locally if there is a higher field line density at positions which, owing to their geometry, are particularly exposed. Such areas of excessive coating are particularly susceptible to mudcracking.
The German patent DE-A-196 17 086 discloses a powder clearcoat slurry in which the average size of the solid particles is from 0.1 to 10 &mgr;m. It is preferred in this case to employ average particle sizes of from 0.23 to 0.43 &mgr;m. For stabilization, it is necessary, in addition to the ionic stabilization, to employ external emulsifiers as well—generally polyethylene oxide adducts, which reduce the resistance of the coating to water and moisture. Moreover, these known powder clearcoat slurries still always include certain amounts of organic cosolvents or leathering agents, which cannot be removed since they are essential to the leveling properties of the partly dried film. Moreover, special equipment such as pressure release homogenizing nozzles are necessary for their preparation. Prior to their application, they are adjusted to the application viscosity using thickeners; however, a complex viscosity behavior is not described. Furthermore, the patent does not give any teaching as to how the problem of mudcracking in connection with powder clearcoat slurries might be solved.
It is an object of the present invention to provide a new powder clearcoat slurry which no longer has the disadvantages of the prior art. In particular, the novel powder clearcoat slurry should be preparable with a smaller number of processing steps than the conventional powder clearcoat slurries. However, on the basis of its typical powder slurry properties, with residual solvent contents of <1%, and its comparable particle sizes, it should have a similarly advantageous application behavior to said conventional powder clearcoat slurries. In contrast to the known waterborne clearcoat materials, the novel powder clearcoat slurries should ensure reliable application behavior with regard to popping marks at the required film thicknesses of approximately 40-50 &mgr;m, even without the assistance of organic solvents.
A further object of the present invention was to find a novel process for preparing powder clearcoat slurries which continues to ensure the essential advantage of the mixing of the components in solution: the very good homogeneity of the resulting particles.
The invention accordingly provides the novel, pseudoplastic powder clearcoat slurry which is free from organic solvents and external emulsifiers and comprises solid spherical particles with an average size of from 0.8 to 20 &mgr;m and a maximum size of 30 &mgr;m, at
Bendix Maximilian
Ott Günther
Poth Ulrich
Raka Fatmir
Röckrath Ulrike
BASF Coatings AG
Cain Edward J.
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