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
1999-07-27
2001-09-18
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
06291579
ABSTRACT:
The present invention relates to an aqueous powder coating dispersion which is particularly suitable as a coating for car bodies which have been coated with water-based paint.
For the coating of car 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.
Entirely solvent-free emulsion paints have been used to date only in areas of application (for example, as wall paints in architectural preservation) which impose relatively low requirements on the resulting surface properties. Where the coating is required to meet very stringent optical and mechanical requirements, as in the case of coil coating or automotive topcoats, for example, the complete renunciation of organic solvents is at present impossible. Therefore, many water-based coating materials have a residual organic solvent content in the range from 10 to 20% and should therefore really be referred to as solvent-reduced coating systems.
For this reason, increased efforts hare been made in recent years to use powder coating materials for the coating operation. The results so far, however, are not satisfactory; in particular, increased coat thicknesses are necessary in order to achieve a uniform appearance. Furthermore, powder-based coating films are still not at an optimum in terms of their optical properties, such as leveling and gloss, and the technomechanical properties such as chemical resistance, weather resistance and water drop resistance. In addition, the crosslinking reactions which are suitable for the line conditions of powder coatings in the automotive segment in some cases exhibit weaknesses in chemical resistance and scratch resistance.
On the other hand, the use of pulverulent coating materials entails a different application technology, so that special coating equipment is required for these materials. Consequently, integrating powder coating technology into existing coating lines is often economically unprofitable and so delays the change to solvent-free coating systems until the entire coating plant is made the subject of new installation. At present, owing to the relatively large particle size of about 25 &mgr;m, it is almost impossible using powder coating technology to produce coating films having a coat thickness of less than 50 &mgr;m.
The plants set up for liquid coating materials cannot be used for powder coating materials. On the other hand, liquid coating materials have the disadvantages set out above. In particular, the more environment-friendly, aqueous coating systems constitute merely a compromise between solvent content and coating quality. An improvement in the film properties can in may cases be obtained only at the expense of a higher solvent content. Therefore, the attempt is being made to develop powder coatings in the form of aqueous dispersions which can be processed using liquid coating technologies (U.S. Pat. No. 3,737,401, U.S. Pat. No. 3,787,230, DE-B 2 601 618, DE-A 2 140 843, DE 2 716 118, U.S. Pat. No. 4,477,530, U.S. Pat. No. 4,686,249, U.S. Pat. No. 4,510,275, U.S. Pat. No. 4,122,055, U.S. Pat. No. 4,385,138, WO 96/37561).
These so-called powder slurries are stable aqueous dispersions of powder coating materials. The first powder slurries were prepared by suspending powder coating materials in water. In their case the particle size of the powder coating resin particles was 0.5-80 &mgr;m. Coating formulations having a solids content in the range of 20-70% could be realized. In contrast to water-based coating materials, the addition of organic solvents as leveling agents is unnecessary with powder slurries since following application of the coating material the coating particles are readily able to stretch out through the aqueous phase. This is possible since at this point in time the resin particles are in a relatively low-viscosity medium and so possess sufficient mobility. In the course of the subsequent stoving process, the binders and crosslinker particles react with one another, so that this mobility is lost as the development of the network increases. Following the initial drying of a powder slurry, it can be regarded in simplified terms like a powder coating material. In terms of the particle size of the resins, however, the systems are markedly different from one another, which in connection with the application of powder slurries is manifested in a lower coat thickness (20-45 &mgr;m) of the stoved coating. The advantage of powder coating materials, namely the temporal separation between the melting of the resin particles to form a smooth film and the subsequent reaction to form a coherent network, is therefore also found in powder slurry systems.
Powder slurries and their preparation are the subject of numerous publications and patent applications. For instance, DE-A 2 140 843 describes the preparation of a powder coating material by spraying or squirting a paint into water and separating and drying the precipitated powder. Alternatively to this preparation process, attempts have been made to convert powder coating materials into powder slurries by mechanical precomminution followed by fine wet grinding in water. U.S. Pat. No. 4,268,542, moreover, discloses a process in which a powder coating slur is used which is very suitable for the coating of automobiles. In this case, first a conventional powder coat is applied to the body and the clearcoat slurry is applied as a second coat. In the case of this clearcoat slurry it is necessary to operate at high stoving temperatures (more than 160° C.).
U.S. Pat. No. 5,379,947 describes a powder slurry system for the coating of automobiles which is based on a hydroxy-functional binder and on an epsilon-caprolactam-blocked isophorone diisocyanate crosslinker. Following pneumatic-electrostatic application of the powder slurry, the slurry is first dried initially at 49° C. for 10 minutes and then stoved at 177° C. for 30 minutes.
For OEM automotive finishes, it has not hitherto been common practice to employ stoving temperatures higher than 150° C., on economic grounds and owing to the color stability of the base coats. Furthermore, the fineness of the powder particles is unsatisfactory.
The preparation processes described in the above-mentioned patents relating to the powder slurry start from the micronization of solid resins. These resins are first of all mixed in a solids mixer, then homogenized in an extruder, pulverized using a mill, and finally wet-ground in water in stirred mills with the addition of various additives such as wetting agents and dispersants to the final particle size in the range of 3-20 &mgr;m.
This process not only is very complex and susceptible to faults but also leads to restrictions which must be taken into account even at the stage of binder synthesis and crosslinker synthesis. For instance, the glass transition temperature of the solid resins must not be below a certain value in order to ensure sufficient blocking resistance, which is one of the preconditions for the micronization and unproblematic storage of the pulverized resins. The upper limit on the glass transition temperature is defined by the requirement for very good leveling at low stoving temperatures, i.e., high reactivity of the binder/crosslinker combination.
The present invention has now set itself the object of providing a process for preparing an aqueous powder coating dispersion which can be applied to car bodies using the current liquid coating technology and which in particular can be stoved even at temperatures of less than 150° C.
This object is achieved by mixing a liquid melt comprising binders and crosslinking agents and, if desired, catalysts, auxiliaries and further additives such as devolatilizing agents, UV absorbers, free-radical scavengers and/or antioxidants, placing said mixture in an emulsifying apparatus preferably with the addition of water and stabilizers, and cooling and filtering the resultant emulsion.
In order to be able to obtain high-qu
Antonietti Markus
Bendix Maximilian
Borho Klaus
Kalck Ulrich
Rück Sven
BASF Coatings AG
Cain Edward J.
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