EP0296167B1

EP0296167B1 - Method of cationic electrodeposition using dissolution resistant anodes

Info

Publication number: EP0296167B1
Application number: EP87901876A
Authority: EP
Inventors: Henry T. Austin
Original assignee: PPG Industries Inc
Current assignee: PPG Industries Inc
Priority date: 1986-03-03
Filing date: 1987-02-18
Publication date: 1993-06-02
Anticipated expiration: 2007-02-18
Also published as: DE3786079T2; KR880700870A; AU7085987A; JPH01501488A; AU580475B2; EP0296167A1; JPH0572480B2; WO1987005340A1; MX164828B; KR900006661B1; CA1308058C; ATE90117T1; DE3786079D1

Abstract

Cationic electrodeposition of an aqueous cationic resinous composition with an anode comprising a self-supporting substrate to which is adhered a coating of a conductive material selected from the group consisting of platium, palladium, rhodium, ruthenium, osmium, iridium, gold, oxides thereof, and mixtures thereof. The anode is more resistant to dissolution than stainless steel anodes which are conventionally used in cationic electrodeposition.

Description

The present invention relates to a method of cationic electrodeposition of aqueous dispersions of cationic resinous compositions using an anode which does not dissolve nor deteriorate during the process.
Cationic electrodeposition has been used industrially since 1972. The early cationic electrodeposition compositions comprised quaternary ammonium salt group-containing resins in combination with aminoplast curing agents. In 1976, cationic compositions comprising amine salt group-containing resins in combination with blocked isocyanate curing agents were introduced for priming automobile bodies. Today, over 90 percent of the automobile bodies are primed by cationic electrodeposition and practically all of the cationic compositions use the amine salt-blocked isocyanate resins.
In cationic electrodeposition, the part being coated is of course the cathode. The counter-electrode or anode is usually made of a corrosion-resistant material such as stainless steel since most cationic electrodeposition baths are acidic in nature. Because of the electrochemical reactions which occur at the anode, the stainless steel electrode slowly dissolves during the cationic electrodeposition process. The rate of dissolution depends principally on the current density, temperature of the electrodeposition bath to which the anode is exposed; the greater the current density and the higher the temperature, the faster the rate of ion dissolution. Also, the composition to which the electrode is exposed can affect the rate of dissolution. The presence of chloride ion greatly accelerates dissolution, and other unknown constituents of the electrodeposition bath can also affect dissolution. It has been found, for example, that electrodeposition baths in one location may be relatively passive to the stainless steel anodes, whereas electrodepositon baths in another location employing the same cationic paint may be very aggressive towards the stainless steel anode. The dissolution of the anode results in low film builds and poor appearance. Eventually, if the dissolution is great enough, the anode must be replaced resulting in a time-consuming and expensive shut down of the electrodeposition process.
From Ullmanns Encyclopedia of Technological Chemistry, 4th Edition, Volume 4 (1975), page 337, it is known to use metal anodes in electrolytic cells for the electrolysis of brine. The anodes comprise a metal support of titanium with a ruthenium oxide coating thereon. The anodes are commercially available as DSA-electrodes.
Chlorine generation at the anode requires a high concentration of chloride salts and the solution is subjected low voltages.
From US-A-3,682,814 a cathodic electrocoating process is known which is operated at pH-values of the bath between 1.5 and 5.5 and in the presence of an oxidation promotor to control the decomposition of the acid.
There is mentioned that the steel anodes may be coated with an oxidation catalyst to assist controlling of the oxidation conditions. As catalysts are disclosed platinum, other precious metals, chromates, manganates, vandates, molybdates, cobalt, nickel, chromium and various oxides of the metals or other heavy metals which do not materially suppress the passage of electric current.
It is the object of the present invention to provide a method of cationic electrodeposition in the event that the bath is very aggressive towards stainless steel anodes.
The object is attained by a method of electrocoating an electrically conductive surface serving as a cathode in an electrical circuit comprising said cathode and an anode having a conductive coating adhered to a substrate immersed in an aqueous dispersion of a cationic resinous composition which will dissolve stainless steel anodes by passing electric current between said cathode and anode at a constant voltage of from 50 to 500 volts to cause a coating to deposit on the cathode characterized by using an anode which does not dissolve nor deteriorate during the electrocoating process and comprises a titanium including alloys of titanium substrate and a coating of a material selected from the group consisting of ruthenium oxide, iridium oxide and mixtures thereof and operating at a current density of from 0.5 to 10 amperes per 929.034 cm² (square foot).
The aqueous dispersion can contain chloride ion in amounts of at least 10 parts, preferably 10 to 200 parts per million based on the weight of the aqueous dispersion.
Practicing cationic electrodeposition in this manner would insure consistent results in terms of coating quality and would also result in considerable savings from not having to replace the anodes because of dissolution.
The electrode does not dissolve nor deteriorate in the cationic electrodeposition environment, provides for consistent quality coatings, and provides for considerable maintenance savings associated with not having to replace the dissolved stainless steel electrodes because of dissolution.
In the process of cationic electrodeposition, an aqueous electrodeposition bath containing an electrodepositable paint is placed in contact with an electrically conductive anode and an electrically conductive cathode and upon passage of an electric current, usually direct current, between the anode and cathode while immersed in the electrodepositon bath, an adherent film of paint is deposited on the cathode. The electrodeposition of the paint occurs at a constant voltage between 50 and 500 volts, and at a current density of 0.5 to 10 amperes per 929.03 cm² (square foot) with higher current densities being used during the initial stages of the electrodeposition and the current density gradually decreasing as the deposited coating insulates the cathode.
Usually the cathode, such as a series of automobile bodies, are introduced into the electrodeposition bath or tank sequentially and continuously. The cathode passes through the bath where it passes a series of anodes arranged from the beginning to the end. The anodes first in line or towards the entrance end of the tank are subjected to the greatest current flows, and in the case of the stainless steel electrodes, dissolve the fastest. It is these anodes which are preferably replaced with the anodes to be used according to the present invention. Although all the stainless steel anodes may be replaced with the specific electrodes, it may not be necessary to replace the stainless steel anodes which are positioned more towards the exit end of the tank since these electrodes may not have that great a current flow (due to the insulating effect of the deposited coating) and may not significantly dissolve in the bath. Therefore, the electrodes in the bath towards the entrance end of the tank should be those of the invention, whereas the other electrodes more towards the exit end of the tank may be of the conventional stainless steel type.
The anodes may be exposed directly to the electrodeposition paint or as is more usually the case, they may be part of an electrodialysis cell positioned within the electrodeposition bath, in which instance, the anodes are separated from the electrodeposition paint by semi-permeable membranes which are permeable to ionic materials such as acid anion and water-soluble anionic impurities such as chloride ion but impermeable to resin and pigment of the paint. The ionic materials which are attracted to the anode and pass through the membrane can then be removed from the bath by periodically flushing the anode area with water. In an electrodialysis cell, the anode area is commonly referred to as the anolyte cell and the liquid in which the anode is in contact the anolyte solution. Using the anodes in this manner is particularly desirable when the buildup of excess acid from the cationic electrodeposition resin is a particular problem.
The electrodeposition paints which are used in the process of electrodeposition comprise cationic resins, pigments, crosslinkers and adjuvant materials such as flow control agents, inhibitors, organic co-solvents and of course the dispersing medium, water. Specific examples of cationic electrodeposition compositions are those based on cationic resins which contain active hydrogens and include amine salt groups, for example, the acid-solubilized reaction products of epoxy resins and primary or secondary amines in combination with capped isocyanate curing agents. Cationic electrodeposition paints employing these resinous ingredients are described in U S -A-4,031,050 to Jerabek. Specially modified cationic resins such as those containing primary amine groups formed from reacting polyepoxides with diketimines containing at least one secondary amine group, for example, the methyl isobutyl diketimine of diethylene triamine, are also well known electrodeposition resins and cationic paints employing these resinous ingredients are described in U S -A-4,017,438 to Jerabek et al. Modified cationic resins such as those obtained by chain extending the polyepoxide to increase its molecular weight can also be used in the method of the invention. Such resins are described in U S Patent No. 4,148,772 to Jerabek et al in which the polyepoxide is chain extended with a polyester polyol and in U S - A- 4,468,307 to Wismer et al in which the polyepoxide is chain extended with a particular polyether polyol. Also, chain extension such as described in CA -A- 1,179,443 can be used.
The cationic electrodeposition paints preferably contain capped isocyanate curing agents because these curing agents provide for low temperature cure and the development of optimum cured coating properties. However, cationic electrodeposition paints based on epoxy resins and capped polyisocyanates are often contaminated with chloride ion which is a by-product of the method of preparation of the epoxy resins and capped polyisocyanates. Many epoxy resins are made from epichlorohydrin and certain polyisocyanates are made from phosgene. Chloride has a very adverse effect on the dissolution of the conventional stainless steel electrodes. It is therefore with cationic paints containing chloride ion that the invention is particularly useful. Such paints typically have a chloride ion concentration of at least 10, usually 10 to 200 parts per million (ppm) based on total weight of the aqueous dispersion.
The anodes which are useful in the process of the invention comprise a substrate of titanium including titanium alloys as self-supporting material which is chemically resistant and to which the coating of the specific metal oxides described below will adhere. By chemically resistant is meant the substrate is resistant to the surrounding electrolyte, that is, the electrodeposition paint or the anolyte solution, and is not subject to an appreciable extent to erosion, deterioration or to electrolyte attack.
Examples of suitable titanium alloys include titanium, alloys with tantalum, niobium and titanium with 1 to 15 percent by weight molybdenum.
It is not essential that the entire substrate be of the titanium or alloy. Rather, a core of metal such as copper or aluminum may be cladded or coated with the titanium or alloy.
To the self-supporting substrate is adhered a coating or a layer of a material which is electrically conductive and which functions as an anode in an electrical circuit. Also, the material is be chemically resistant under anionic conditions to the surrounding electrolyte. The suitable materials are selected from, oxides of ruthenium oxide and iridium oxide and mixtures of two or more oxides. Because of cost and performance in an electrodeposition environment, ruthenium oxide and iridium oxide are the selected oxides with ruthenium oxide being the most preferred.
The thickness of the substrate and the outer layer of the metal oxide is not critical. It only is necessary that the thickness of the substrate furnish a self-supporting structure and the metal oxide layer be present in an amount sufficient to function as an anode, that is, to be able to combine current density requirements with corrosion resistance.
Typically, the substrate is from 1270 µm to 12 700 µm (50 to 500 mils) in thickness and the metal oxide layer is from 0.254 µm to 254 µm (0.01 to 10 mils) in thickness. The coating of the metal oxide layer can be on both sides of the substrate or on one side, that is, the side facing the cathode. Preferably, the substrate is entirely covered with a metal oxide layer.
The configurations of the anodes are not particularly critical but for use in electrodeposition tanks, they are usually square or rectangular. Typically, for use in industrial electrodeposition tanks, electrodes having an area of from 9290 cm² to 46 450 cm² (10 to 50 square feet) are used, and as mentioned above, usually a series of electrodes are positioned in the tank extending from the entrance to the exit end of the tank.
The procedure for making the electrodes is generally a proprietary process with the manufacturers. In general, the metal oxide can be applied by evaporative techniques, thermal decomposition of suitable metal oxides in organic medium, and by electroplating. is In the instance the oxide is desired, the oxide is precipitated by chemical, thermal or electrical means. Oxides of the group of metals can also be applied directly to the titanium support in a molten bath of the oxide.

Examples

In the following examples, the corrosive effects of typical cationic electrodeposition paints towards a stainless steel anode and ruthenium oxide-coated titanium and iridium oxide-coated titanium anodes were evaluated. One cationic electrodeposition paint was based on an acid-solubilized epichlorohydrin-bisphenol A type epoxy resin-amine reaction product and a capped isocyanate curing agent. The epoxy resin was an epichlorohydrin-bisphenol A type. The paint was available from PPG Industries, Inc. under the trademark UNI-PRIME®. The second paint was a cationic acrylic prepared from glycidyl methacrylate and contained a capped polyisocyanate curing agent. The paint was available from PPG as ED-4000. Samples of anolyte solutions from the paints were collected and used for testing. The anodes being tested were 1524 mm by 25.4 mm (6 inches by 1 inch) and were made part of an electrical circuit inserted between two 1524 mm by 25.4 mm (6 inch by 1 inch) steel cathodes. The electrode spacing was about 50.8 mm (2 inches) and the electrodes were immersed to a 50.8 mm (2-inch) depth in the anolyte solutions. The effects of temperature, amperage and time on the loss of weight of the electrodes was measured and is reported in Table I below.

Claims

A method of electrocoating an electrically conductive surface serving as a cathode in an electrical circuit comprising said cathode and an anode having a conductive coating adhered to a substrate immersed in an aqueous dispersion of a cationic resinous composition which will dissolve stainless steel anodes by passing electric current between said cathode and anode at a constant voltage of from 50 to 500 volts to cause a coating to deposit on the cathode
characterized by
using an anode which does not dissolve nor deteriorate during the electrocoating process and comprises a titanium including alloys of titanium substrate and a coating of a material selected from the group consisting of ruthenium oxide, iridium oxide and mixtures thereof and operating at a current density of from 0.5 to 10 amperes per 929.034 cm² (square foot).
The method of claim 1,
characterized in that
the conductive coating material is ruthenium oxide.
The method of claim 1,
characterized in that
the aqueous dispersion contains chloride ion in amounts of at least 10 parts per million based on weight of the aqueous dispersion.
The method of claim 1,
characterized in that
the aqueous dispersion contains chloride ion in amounts of from 10 to 200 parts per million based on weight of the aqueous dispersion.