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
2002-10-26
2004-06-08
Wu, David W. (Department: 1713)
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...
C523S223000, C523S300000, C522S074000, C522S086000, C522S098000, C522S103000, C522S104000, C427S508000, C428S544000
Reexamination Certificate
active
06747091
ABSTRACT:
The present invention relates to a novel powder slurry curable with actinic radiation and, if desired, thermally. The present invention also relates to a novel process for preparing this powder slurry. The invention relates not least to the use of the novel powder slurry to prepare clearcoat materials and also single-coat and multicoat clearcoat systems for the automotive sector and the industrial sector.
Automobile bodies are nowadays coated preferably using liquid coating materials, i.e., spray paints. These coating materials give rise to numerous environmental problems owing to their solvent content. The same applies to the use of waterborne clearcoat materials, since they always still contain certain amounts of organic solvents.
Thermally curable 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 clearcoat materials. The aim here is for sedimentation-stable dispersions having an average particle size of from about 10 to about 200 nm. The reason, familiar to the skilled worker from experience, is that the better the stabilization and the lower the size of dispersion particles, the less their tendency toward sedimentation. Reliable application behavior and a reduction in popping tendency, however, necessitate the use of 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, owing in particular to the need for increased film thicknesses in order to obtain a uniform appearance.
Further problems of powder coating materials for thermal curing arise from the requirement for blocking resistance on storage, and storability even in summer temperatures. In order to ensure that this requirement is met, the softening point of the coating powders must be high. Because of the high softening point of the coating materials, however, the thermally activated crosslinking reaction begins as early as during the melting of the powders on the substrate, before the film surface has achieved optimum leveling.
To solve this problem, the German patents DE-A-24 36 186 and DE-A-26 47 700, the European patents EP-A-0 098 655, EP-A-0 286 594, EP-A-0 410 242, EP-A-0 522 648, EP-A-0 585 742, EP-A-0 636 669 and EP-A-0 650 979, the international patent application WO 93/25596, and the U.S. Pat. Nos. 4,064,161, 4,129,488, 4,163,810, 4,208,3130 and 5,639,560 propose UV-curable powder coating materials in which it is possible to separate the melting operation from the crosslinking. The UV powder coating materials disclosed to date are all based on substances containing acrylic or vinylic unsaturation, which owing to the high melting temperature required for effective blocking resistance may also undergo thermal polymerization prior to UV irradiation. In order to guarantee blocking resistance, the binders employed for the UV powder coating materials must be absolutely solvent-free polymers, which, however, are highly problematic to obtain owing to their tendency to undergo thermal polymerization.
A problem of UV powder coating materials, however, is that they are only of limited suitability for coating three-dimensional objects, since, with such objects, shadow regions occur in which UV powder coating materials undergo little or no curing. The same applies to UV powder coating materials comprising hiding pigments.
Attempts have been made to solve this problem by means of powder coating materials which are curable thermally and with actinic radiation. A so-called dual-cure powder coating material of this kind is known from the European patent EP-A-0 844 286. It comprises an unsaturated binder and a second resin copolymerizable therewith, and also a photoinitiator and a thermal initiator, and is therefore curable thermally and with actinic radiation. However, this dual-cure powder coating material is used as a pigmented topcoat material, which is cured superficially with UV light and thermally in the regions close to the substrate. The aforementioned patent does not reveal whether this known powder coating material is also suitable for producing clearcoats in multicoat systems.
The general problem with the use of powder coating materials, namely that, owing to the different application technology, they cannot be used on existing installations designed for liquid coating materials, is not solved by the dual-cure powder coating material either, however.
This was the reason for the development of thermally curable powder coating materials in the form of aqueous dispersions which can be processed using liquid coating technologies. These powder clearcoat dispersions, known to those skilled in the art also as powder slurries or powder clearcoat slurries, and their preparation and application are described in the German patents DE 196 13 547, DE 196 17 086, DE 196 18 657, DE 195 40 977 and DE 195 18 392, the European patent EP-A-0 652 264, the international patent application WO 80/00447, and the U.S. Pat. No. 4,268,542.
Thus, in the process known from the U.S. Pat. No. 4,268,542, a powder clearcoat slurry based on acrylate resins is used which is suitable for coating automobiles. In this case, a conventional powder coat is applied first of all to the body, after which the powder coating dispersion is applied as a clearcoat material. In this powder clearcoat slurry ionic thickeners are used, which lead to relatively high sensitivity of the applied clearcoat film with respect to moisture, especially with respect to condensation. Moreover, it is necessary to operate at high baking temperatures of more than 160° C.
The powder clearcoat slurry known from the European patent EP-A-0 652 264 is prepared by first coextruding the solid binder and crosslinker components and any additives, as normal with the production of powder coating materials, and then subjecting the coextrudate to dry milling, after which it is converted into a powder clearcoat slurry in a further step of wet milling, using emulsifiers and wetting agents.
Unlike the powder clearcoat materials, these customary and known powder clearcoat slurries may be processed in conventional wet coating installations and may be applied at substantially lower film thickness of approximately 40 &mgr;m as against approximately 80 &mgr;m in the case of powder coating materials, with good leveling and with a chemical resistance comparable with that of the powder coating materials.
However, the conventional milling processes do not always ensure such a high degree of homogenization of the constituents as would actually be desirable, or else said degree of homogenization must be achieved by means of multiple extrusion, which is laborious.
In the conventional powder clearcoat slurries, relatively large particles are normally unwanted, since they tend toward sedimentation. Moreover, on application and crosslinking of the powder clearcoat slurries there is an increased tendency to form popping marks (blister-shaped cavities enclosed in the coating film).
The situation with cracking, known as mud cracking, in the pulverulent dry films which have been dried initially at room temperature or slightly elevated temperature but not yet baked, is similar. Such drying cracks no longer flow out fully on baking, and in the baked film they form visible flow defects in the form of leather-textured furrows, these drying cracks being more pronounced and more frequent the higher the dry film thickness. In the case of the electrostatic coating of automobile bodies, relatively high film thicknesses may occur locally when there is a higher field line density at sites which are particularly exposed owing to the geometry. Such sites of overcoating are particularly susceptible to mud cracking.
The powder clearc
Baumgart Hubert
Meisenburg Uwe
Ott Günther
Poth Astrid
Röckrath Ulrike
BASF Coatings AG
Sastri Satya
Wu David W.
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