KR900006661B1 - Method of cationic electro deposition using dissolution resistant anodes - Google Patents

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

[Name of invention]

Cationic electrodeposition method using solvent resistant anode

Detailed description of the invention

(Background of invention)

FIELD OF THE INVENTION The present invention relates to electrodeposition, and more particularly to cationic electrodeposition of aqueous dispersions of cationic resin compositions.

Brief description of the prior art: Cationic electrodeposition has been used industrially since 1972. The initial cationic electrodeposition composition included an aminoplast curing agent and a resin containing a group of quaternary ammonium salts. In 1976, a cationic composition comprising a base-containing resin of an amine salt and a blocked isocyanate curing agent was introduced during the priming of the vehicle body. Today, over 90% of automotive bodies are undercoated by cationic electrodeposition, and virtually all cationic compositions utilize amine salt-blocked isocyanate resins.

In cationic electrodeposition, the part to be covered is, of course, the negative electrode. The counter-electrode or anode is usually made of a corrosion resistant material such as stainless steel, since most cationic electrodeposition baths are acidic. Because of the electrochemical reactions occurring at the anode, the stainless steel electrode dissolves slowly during the cationic electrodeposition process. The dissolution rate mainly depends on the current density and the temperature of the electrodeposition bath to which the anode is exposed. The higher the current density and the higher the temperature, the faster the ion dissolution rate. In addition, the composition to which the electrode is exposed may affect the dissolution rate. The presence of chloride ions greatly accelerates dissolution, and other unknown components in the electrodeposition bath can also affect dissolution. For example, it may be relatively unresponsive to an electrodeposition stainless steel anode in one place. Electrodeposition baths in another location using the same cationic paint can be very aggressive against stainless steel anodes. Due to the decomposition of the anode, only a small film is made and the appearance is poor. As a result, if the degree of decomposition is very large, the anode must be changed, which results in a large amount of time in the electrodeposition process and a high process price.

It is an object of the present invention to overcome the above problems and to provide a cationic electrodeposition method using a cathode resistant to deterioration and dissolution in all cationic electrodeposition environments. In this way, the quality of the coating is improved, and the cost is not significantly reduced because the anode does not need to be changed.

(Summary of invention)

According to the present invention, there is provided a method of electrocoating an electrically conductive surface acting as a cathode in an electrical circuit comprising a cathode and an anode immersed in an aqueous dispersion of a cationic resin composition. This method involves passing a current between the cathode and the anode so that the coating is deposited on the cathode. The anode is a self-supporting film having a conductive ruling film, which is platinum, galvanization, rhythm, luteum, ossop, rhythm, gold, an acidic product of the metal or a mixture of the metals or a mixture thereof. self-supporting).

These anodes are costly in that they are neither soluble nor thermal in an amphide electrodeposition environment, provide an improved mass of coating, and do not require the replacement of the source stainless steel electrode in a soluble medium.

(details)

In the cationic electrodeposition process, an aqueous electrodeposition bath containing an electrodepositable paint is placed in contact with an electrically conductive anode end electrically conductive cathode, and when a current (usually direct current) passes between the anode and the cathode immersed in the electrodeposition bath, An adhesive film of paint is electrodeposited on the cathode. Electrodeposition of paint occurs at a constant voltage, typically 50-500 volts, and at a current density of about 0.5-10 amps per 0.093 n square (1 square foot), during which the higher current density is used, As the electrodeposited film insulates the cathode, the current density gradually decreases.

Usually, an object to be plated, such as one of a series of automobile bodies, is introduced into an electrodeposition bath or tank. The body is passed through the tank in a continuous and continuous operation. The article passes through the bath, which is electrically connected for use as a cathode, through a series of anodes arranged from beginning to end. The anodes in the front row or towards the inlet of the tank receive the greatest current flow and dissolve the fastest in the case of stainless steel electrodes. It is these anodes that are preferable to change to the anode of the invention. All stainless steel anodes can be changed to the electrode of the invention, but the stainless steel anodes facing the outlet of the tank do not need to be grounded, because these electrodes cannot have such a large current flow ( This is due to the insulating effect of the electrodeposited vesicles). It is therefore more important that the electrode in the electrodeposition bath facing the inlet side of the tank has the coating according to the invention. On the other hand, other electrodes that are more towards the outlet of the tank may be conventional stainless steel type electrodes.

The anode may be directly exposed to electrodeposition paint, or even more commonly, the anode may be part of an electrodialysis cell located in an electrodeposition bath, in which case ionic substances such as acid anions and water-soluble anionic impurities such as chloride ions Although it is permeable to the pigments and resins of the paint, it is separated from the cathodic electrodeposition paint by a semipermeable membrane which is impermeable to paint. Ionic substances that are sucked into the anode and pass through the membrane can be removed from the bath by periodically flushing the anode zone with water. In an electrodialysis cell, the anodic zone is commonly called an anolyte cell, where the anode is in contact with the anolyte. The use of a positive electrode in this manner is particularly desirable when the accumulation of excess acid from cationic electrodeposition resins poses a particular problem.

Electrodeposition paints used in the electrodeposition process include cationic resins, pigments, crosslinker meet adjuvant materials such as flow control agents, inhibitors, organic cosolvents as well as water as a dispersion medium. Specific examples of cationic electrodeposition compositions include those based on acid-soluble and reaction products of sulfuric hydrogen and cationic resins containing amine salt groups such as epoxy resins and primary or secondary amines and capped isocyanate curing agents. Cationic electrodeposition paints using such resin components are described in Gerabec, US Pat. No. 4,031,050. Specially modified cationic resins such as primary amine group-containing resins obtained by reacting polyepoxides with one or more secondary phosphorous groups with anti-flowing diketamines such as ethyl isobutyldiketamine of diethylenetriamine, are well known electrodeposition resins, Cationic paints using such resin components are described in Gerabeck, US Pat. No. 4,017,438. Modified cation resins, such as resins obtained by extending the chain of polyepoxides to increase molecular weight, can also be used in the process of the present invention. Such resins are described in US Pat. No. 4,148,772 to Gerazin for extending the chain of polyepoxide with polyester polyols and US Pat. No. 4,468,307 for Whismer for extending the chain of polyepoxide to specific polyetherpolyols. . It is also possible to use chain extension techniques as described in Canadian Patent No. 1,179,443.

It is desirable to include capped isocyanate curing agents in the cationic electrodeposition paint, because these curing agents provide low temperature curing and allow the cured coatings to achieve optimal properties. However, cationic electrodeposition paints based on epoxy resins and capped polyisocyanates are often mistaken for chloride ion, a byproduct of the epoxy resin and capped polyisocyanate process. Many epoxy resins are made from epichlorohydrin and certain polyisocyanates are made from phosgene. Salts have a very detrimental effect on the dissolution of conventional stainless steel electrodes. Therefore, the present invention is particularly useful in the case of using a cationic paint containing chlorinated chloride ions. Such paints typically have a chloride ion concentration of at least 10, usually l0-200 ppm, based on the weight of the aqueous dispersion.

Anodes useful in the process of the present invention are chemically resistant and consist of a substrate left of a self-supporting ruling to which a coating of metal-met metal oxides described below will be attached. The deception may be a metal, but is preferably a valve metal. "Valve metals" are defined as metals that are oxidized under anionic conditions to form chemically resistant acid-academic rules on the surface and that are resistant to transmembrane. That is, it means that it is resistant to electrodeposition paint or catholyte and is not corroded or contaminated enough to be strongly peeled and is not attacked by electrolyte.

Examples of suitable valve metals include titanium, tantalum, niobium mit alloys of these metals, such as alloys of titanium with 1-15 weight 96 molybdenum. Titanium is regarded as a preferred valve metal because of its excellent corrosion resistance, low cost, availability in use and adhesion to a metal or metallographic coating.

The entire period does not necessarily have to be a valve metal. It can pour or coat the valve metal on a metal core such as copper or aluminum.

A self-supporting substrate is attached to a layer or film of electrically conductive material that acts as an anode in the electrical circuit. The material must also be chemically resistant to the surrounding electrolyte under anionic conditions. Examples of suitable rulings include metal platinum, palladium, rhythm, lutetum, ossop, rhythm, gold, and alloys of two or more of these metals. Also mixed oxides of these metals, such as ruthenum oxide oxidization rhythms, and two or more products may be used. Mixtures of metals and metal oxides may also be used. Performance in Electrodeposition Environments Luteum oxide and iridium oxide are preferred for rudimentary cost, and the most preferred is ruthenium oxide.

Period thickness The thickness of the metal or metal oxide outer layer is not critical, although the thickness of the base layer is sufficient to provide a self-supporting structure and the metal or metal oxide layer to act as an anode, ie to combine current density requirements and corrosion resistance. It is only necessary that it exist in a quantity.

Typically the thickness of the substrate is about 50-500 days and the thickness of the metal or metal oxide layer is 0.0l-10 mils. The metal or metal oxide layer mass may be on both sides of the substrate, or on one side, ie the side facing the cathode. Preferably the organ is completely covered with a metal or metal oxide layer.

The shape of the anode is not particularly important, but for use in electrodeposited tanks it is usually square or rectangular. Typically for use in industrial electrodeposition tanks electrodes having an area of about 0.93-4.65 I (about 10-50 zebophytes) are used, as described above, usually a series of electrodes from the inlet to the outlet of the tank. It is located in the tank lined up.

The electrode manufacturing process is generally a patent process according to the manufacturers. In general, metals or metal oxides may be applied by evaporation techniques, pyrolysis of suitable metals or metal oxides in organic media or by electroplating. In most applications, the valve metal is etched and then coated with liquid metal. If an oxide is desired, the oxide is precipitated by chemical, thermal or electrical means. Such groups of metal products may also be applied directly to the valve metal support in a bath of oxide.

EXAMPLE

In the following examples, the corrosion effects of a typical cationic electrodeposition paint on a titanium steel anode and an anode coated with iridium oxide were coated with a stainless steel anode and an acidic ruthetum. The first cationic electrodeposition paint was based on acid-solubilized epichlorohydrin-bisphenol A type epoxy resin-amine reaction products and capped isocyanate hardeners. Epoxy resins were epicoolohydrin-bisperal A type. This paint was available from PPG Industries under the trademark “Uni-Prime.” The second paint was a cationic acrylic resin made from glycidyl acrylate, and contained a capping polyisocyanate curing agent.

This paint was available from PPG Industries under the trademark "ED-4000". Anolyte solution samples were collected from the paint and used for testing. The test anode had a dimension of 6 ″ × 1 (l5.2 cm × 2.54 cm) and achieved a boolean with Jin Ki-hee inserted between the steel cathodes having dimensions of 15.2 cm × 2.54 cm. The spacing was about 5.1 cm (2 inches) and immersed in the anolyte solution at a depth of 5.1 cm. The effects of on, ampere number and time on the weight loss of the electrode were measured and shown in the following PoI.

TABLE 1

Anolyte solution for 1-uni-prime cationic paint has a pH of 3.8 and 0.03982 milliliter equivalents (MEQ) of acid per gram of anolyte, 0.0007 MEQ of chloride (24 ppm of chloride ions) Hang oil. The anolyte solution for EP-4000 has a pH of 2.8 and contains 0.0583 MEQ acid per gram and 0.0006 MEQ cineul (21 ppm of chloride) per gram of basel of 0.00l8 MEQ per gram.

2Measured according to ASTM D-A262.

3 Available from the Alteg System 1s as EC-200.

Available as TIR-2000 from 4Legt Systems.

Claims (6)

In a method of electrically coating an electrically conductive surface acting as a cathode in an electric circuit comprising a cathode and an anode immersed in an aqueous dispersion of a cationic resin composition by passing an electric current between the cathode and the anode so as to deposit a film on the cathode. Characterized by a self-supporting substrate having a coating of platinum, palladium, rhythm, ruthenum, ossop, rhythm, gold, an oxide of the metal or a mixture of the metal or a conductive material which is an oxide of the mixture. How to. The method of claim 1, wherein the chloride ion is present in the aqueous dispersion in an amount of at least 10 ppm based on the weight of the aqueous dispersion. The method of claim 1 wherein the self-supporting device is a valve metal. The method of claim 3 wherein the valve metal is titanium. The method of claim 1 wherein the coating finish is selected from the group consisting of rudennium oxide, rhythm oxide mixtures thereof. In an electric circuit containing a cathode and an anode immersed in an aqueous dispersion of a cation resin composition rule containing at least 10 ppm of chloride ions based on the weight of the aqueous dispersion by passing an electric current between the cathode and the anode so that the coating is deposited on the cathode. A method of electrically collapsing an electrically conductive surface that acts as a cathode, wherein the anode is acidic luthenium. Wherein the coating of the oxidative rhythm and mixtures thereof is of a source titanium engine.