US3476668A - Electrophoretic coating process in a medium containing a resin,plus powdered plastic material - Google Patents

Description

United States Patent Int. Cl. C23b 13 00; B01k 5 02 U.S. Cl. 204181 7 Claims ABSTRACT OF DISCLOSURE Carrying out electrophoretic coating by the use of an electrolyte liquid phase which is aqueous and has contained therein a liquid resinous coating composition either dissolved or dispersed in water and a suspended powdered plastic material having a particle size of about 0.1 to 100 microns, preferably 0.5 to 20 microns.

This invention relates to the art of applying a surface coating to an object. It more particularly refers to a process for applying a surface coating by electrolytic means.

It is known that surface coatings can be applied to shaped objects by electrolytic means provided that the shaped objects are themselves conductive and can serve as electrodes. The material to be used as the surface coating is suitably dissolved or dispersed in an aqueous medium and an electric field is maintained in the medium, such as will cause the coating material to migrate toward the electrode and becomes coated thereon in a coherent layer. This process has generally been referred to as electrophoretic coating or enameling.

Electrophoretic coating or enameling has considerable advantages over conventional varnishing or enameling methods. For example, it is possible by this method to produce a solid, water-insoluble film with a solids content of about 95 to 97% in a thickness of about 20 to 30 microns within two minutes. The object is provided on all sides with an adherent and close-pored, practically waterfree coating, even corners and escutcheons, as well as places of diflicult access being well covered. One special advantage of eletrophoretic coating is the safety in handling water-base coatings, which to a great extent eliminates the danger of fire or explosion and the nuisance of bad odors. The coating material is utilized efficiently, and the danger of so-called runs or curtains such as occur in conventional methods of application does not exist.

In spite of the technical advance which has been achieved by the introduction of the electrophoretic meth- 0d of varnishing and enameling, this process still has a number of disadvantages. If a certain maximum voltage is exceeded or if excessively high current densities are used, electrolytic phenomena and the production of gases occur which can result in porosity in the deposited film. Also, care must be taken to maintain a predetermined and constant temperature and a predetermined pH. Another disadvantage is that the liquid bath must be stirred constantly to prevent the settling of coating varnish and pigment particles of greater specific weight, but it must not be excessively agitated because otherwise the thickness of coating achieved within a given time decreases.

M a 1C6 While the bath is steadily being depleted of the coating material, it is nevertheless particularly difficult to replenish it and maintain its viscosity by the timely addition of the coating substance in such quantity that a solid concentration of about 10 to 15% by weight is maintained.

A problem that is just as diificult is the use of coating materials made up of different resin components, for in this case there is the danger that the components may not be uniformly deposited out of the bath, resulting in uncontrollable changes in the composition of the coating material in the bath, the development of poor reproducibility conditions, and the formation of coatings which differ considerably from the original coating composition.

The main disadvantage of the processes that have hitherto become known for electrophoretic coating with plastics, however, consists in the fact that only relatively thin coatings could be obtained and these only in dark color tones, so that electrophoretic varnishing has hitherto been limited to the production of prime coatings and these with thicknesses of only up to about 30 microns.

It is an object of this invention to provide an improved electrophoretic coating process.

It is another object of this invention to provide a process for electropheritic coating of objects to provide relatively thick surface coatings thereon.

It is another object of this invention to provide an electrophoretic coating process adaptable to the deposition of light colored coatings.

Other and additional objects of this invention will become apparent from a consideration of this entire specification including the claims appended hereto.

In accord with and fulfilling these objects, one aspect of this invention resides in an electrophoretic coating process using as the coating medium one or more suitable resins dissolved or dispersed in water and having at least one insoluble, powdered plastic material suspended therein.

It has surprisingly been found that, if plastic powders with a grain size under 0.1 micron are used, film thickness of no more than the prior art magnitude of about 20 to 30 microns can be achieved electrophoretically. It has also been found that, if plastic powder in a grain size of over about microns is used as a suspension component, the coated films become irregular and porous, so that, in spite of the coating thicknesses of approximately 100 microns that can be achieved, these films are technically unusable. Thus, it is an important aspect of this invention that the suspended plastic material should have a particle size of about 0.1 to 100 microns. The optimum grain size of the suspended plastic powder runs between about 0.5 and 20 microns. It has been found that, when such a powder is used, films can be achieved with thicknesses of 100 microns and more, which are substantially non-porous and substantially uniform in structure.

Within the scope of the invention, thermosetting plastics and/or thermoplastics are used, preferably thermoplastics, on the basis of polyolefins, cellulose esters, polyurethanes, polyamides, polystyrenes, polyepoxies and polyethers, and also halogenated plastics and/or copolymers or mixtures of these plastics.

The dissolved or dispersed resin may suitably be pigmented or not as the case may be and as desired. It is within the scope of this invention to utilize as such vehicle resin thermosetting or thermoplastic materials as exemplified by phenolic resins, which may be modified acrylic, melamine or alkyd resins, epoxy resins, polyamides, resins on a trimellitic acid basis, or mixtures of these resins.

According to another aspect of this invention, the plastic powder may contain 0.5 to 5% by weight, of an organic, inorganic or metallic pigment. This pigment should be homogeneously incorporated into the plastic powder, i.e., it can have been added to the monomer of the plastic prior to its polymerization, or it can have been mixed uniformly into a fused thermoplastic before the latter is processed into a powder.

According to the invention, the mixture of vehicle emulsion and insoluble plastic powder is diluted with desalted water to such an extent that the suspension has a content of solids of 8 to 20% by weight, preferably 10 to by weight.

The ratio at which the plastic powder is mixed with the resin solution or dispersion is on the order of 1:3 to 1:6.

According to an improvement of the invention, up-to by weight of an organic solvent, such as alcohol, is added to the solution or dispersion.

Another feature of the invention is that wetting agents or emulsifiers can be added to the dispersion.

The process of the invention makes it possible within a certain voltage range to achieve a substantially perfect enamel coating, there being no need to fear any settling out of the pigment particles, even if naught but moderate agitation is used. The consumed coating material can be replaced by the addition of fresh material, without any complications, because the concentrate can be very easily distributed into the bath liquid. Since the pigment particles and the fine plastic powder particles are well encased by the resins dissolved or dispersed in the water, a uniform migration of the components to the anode and a uniform depositing without separation of the components is achieved, thereby assuring substantial identity between the composition of the solid content of the bath liquid and the film that is produced.

The process of the invention produces coating thicknesses such as have not been able to be obtained hitherto. The production of single coats has been achieved, which have uniform thicknesses of 50 to 100 microns, even at the corners and arrises of the workpiece. The coatings are characterized by high adhesion, substantial freedom from porosity, and maximum resistance to solvents and chemicals. Since coatings produced by the process of the invention are finishes, they are particularly well suited for protection against corrosion and require no additional coating. The production of light-color finishes has also been successful by the process of the invention.

This electrophoretic coating process can generally be carried out within the prior art operating parameters; for example, the electrode voltage may be about 50 to 250 volts at a current density of about 15 to 65 amps. per square meter. The operating temperature may be about 15 to 35 C.

The following examples will serve to illustrate this invention without in any way being limiting thereon. In these examples, parts and percentages are by weight unless specified to the contrary.

EXAMPLE 1 30 parts by weight of finely powdered polyundecanamide containing 1 weight percent rutile type titanium dioxide as pigment, and having a grain size of 0.5 to 5 microns and a density of 1.04 g./cm. were carefully stirred in 100 parts by weight of a dispersion of 60 parts of a vinyl-modified water-soluble phenolic resin in 100 parts by weight of pure water, with the addition of 10 parts of methanol, and then another 600 parts of desalted water and 0.5 part of an emulsifier of the ethoxypolyglycol type are added. In an electrophoresis apparatus, at a DC voltage of 80 volts and a current of 20 amp./m. at a temperature of 21 C., and at an adjusted pH of 8.0 in the bath liquid, a coherent coating was deposited on a 1.5 mm. thick mild steel plate serving as the anode, which had previously been degreased and slightly roughened. After removal from the bath, the coated specimen was rinsed repeatedly with water and given a final rinse with fully desalted water and dried with warm air to remove the adhering water droplets. The specimen was then heated for 10 minutes at 250 C. in a circulated-air drying oven.

The coating obtained had an everywhere uniform thickness of microns, even on the raised portions thereof, and was characterized by great adhesion and freedom from porosity, as well as good mechanical properties and outstanding resistance to chemicals.

EXAMPLE 2 40 parts of very finely powdered polyethylene that had been intimately mixed with 2 parts of red iron oxide as pigment and had a grain size of 0.5 to 1.0 micron and a density of 0.94 g./cm. were carefully stirred together with 120 parts of a 60 weight percent aqueous solution of vinyl-modified acrylic resin, and mixed with a mixture of 650 parts of fully desalted water and 50 parts of isopropanol. At volts DC, a bath temperature of 22 C. and a pH of 7.9 in the solution, a coating of plastic microns thick was deposited within 2 minutes on a phosphatized mild steel specimen plate whose surface was covered with a zinc phosphate coating approximately 2 microns thick. After rinsing and removal of the water by heated compressed air, the coating was heated for 15 minutes at C. in an oven.

The plastic coating obtained had an extraordinarily good adhesion and was characterized by outstanding corrosion resistance and low gas permeability.

EXAMPLE 3 50 parts of finely powdered polystyrene with a grain size of 1 to 3 microns, a softening point of 92 C., a density of 1.05 g./cm. and a k-value of about 80, were carefully suspended in 300 parts of a dispersion consisting of 40 parts of alkyd melamine resin in 100 parts of pure water, and another 900 parts of desalted water were added. Under the same circumstances as given in Example 2, a plastic coating was deposited on a cleaned copper sheet. After repeated rinsing, finally with fully desalted water, the specimen was heated for 10 minutes at 160 C.

The coating obtained was 90 microns thick at all points. The coating was pore-free, showed very good water resistance, and had good electrical properties.

What is claimed is:

1. In the process of coating electrophoretically with an electrolytic liquid phase coating composition, which coating is accomplished by passing an electric current through the electrolytic phase whereby coating said coating composition onto a substrate; the improvement which comprises said electrolytic liquid phase being an aqueous phase containing liquid coating composition and a suspended, powdered, solid plastic material having a particle size of about 0.1 to 100 microns, wherein said liquid coating composition is a thermosetting resin which is at least one member selected from the group consisting of phenolic, alkyd resins, melamine resins, epoxy resins, trimellitic acid resins and thermoset acrylic resins and wherein said plastic powder is at least one member selected from the group consisting of polyolefins, cellulose esters, polyurethane, polyamides, polystyrenes, polyepoxide polyethers, halogenated polyolefins, copolymers of such materials and mixtures of such polymers, and wherein said plastic powder is present in a weight ratio of said liquid coating composition of about 1:3 to 1:6.

2. The improved process as claimed in claim 1, wherein said particle size is about 0.5 to 20.

3. The improved process as claimed in claim 1, wherein said plastic powder contains a pigment therein.

4. The improved process as claimed in claim 1, wherein said coating composition is in an aqeuous solution.

5. The improved process as claimed in claim 1, wherein said coating composition is in an aqueous suspension.

6. The improved process as claimed in claim 1, Wherein said electrolyte contains up to about 20* weight percent of an organic solvent.

7. The improved process as claimed in claim 6, Wherein said organic solvent is an alcohol.

References Cited UNITED STATES PATENTS 2,851,408 9/1958 Cerulli 204-481 2,858,256 10/1958 Fahnoe et a1, 204-181 6 Shyne et a1 204-181 Fahnoe 204-181 Satriana et a1. 204181 McNeill et al. 204181 McNeill et a1. 204-181 McCoy 204181 Gilchrist 204-181 JOHN H. MACK, Primary Examiner 10 E. ZAGARELLA, JR., Assistant Examiner.