GB2111080A - Electrodeposition bath treatment - Google Patents

Abstract

In a method of operating an electrodeposition process wherein an electrically conductive article is electrocoated from an electrodeposition bath 1 comprising synthetic resin ionically dispersed in an aqueous medium which comprises subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane 7 retains the dispersed resin and passes ultrafiltrate comprising water and solute of substantially lower molecular size than said resin including ions of the charge opposite to said resin and returning at least a portion of the ultrafiltrate 11 to the electrodeposition bath, the improvement comprises: subjecting at least a portion 19 of the ultrafiltrate to electrodialysis 21 before the ultrafiltrate is returned to the electrodeposition bath, whereby ions of the charge opposite to said resin are removed from the ultrafiltrate. <IMAGE>

Description

SPECIFICATION Electrodeposition bath treatment The present invention relates to treatment of an electrodeposition bath with the combined use of ultrafiltration and electrodialysis.

U.S. Patent 3,663,405 discloses the treatment of an electrodeposition bath with ultrafiltration. In the ultrafiltration process, the desirable components of the electrodeposition bath, mainly resin and pigment, are concentrated on the ultrafiltration membrane surface from where they can be recycled to the electrodeposition bath for further use. The objectionable materials which accumulate in an electrodeposition bath such as impurities introduced into the bath from the articles being coated, resinous decomposition products and excess solubilizing agents, pass through the membrane into the ultrafiltrate and thereby are continuously removed from the bath. A portion of the ultrafiltrate is purged to drain so as to remove the impurities from the system.

The remainder of the ultrafiltrate is used for rinsing electrodeposited articles, and the rinse water which contains loosely adhering paint commonly called "drag-out" is returned to the electrodeposition bath.

To avoid problems of stability due to coagulation of drag-out the rinse water must contain a sufficient concentration of the solubilizing agent.

The solubilizing agent is a material which reacts with groups on the resin backbone to form ionic groups so that the resin may be solubilized or dispersed in water. For anionic resinous vehicles which usually contain pendant carboxylic acid groups, the solubilizing agent is a base such as an amine. For cationic resins, which normally contain pendant amine groups, the solubilizing agent is an acid, for example, an organic acid such as acetic acid.

The solubilizing agent does not deposit with the resinous vehicle being electrodeposited.

As a consequence, it accumulates in the bath, and if not removed, will cause bath instability.

Although ultrafiltration is sufficient to control the concentration of many impurities in the bath. it is not completely satisfactory in controlling the accumulation of solubilizing agent, particularly solubilizing agents which are nonvolatile or which are used in relatively high amounts.

To control the build-up of solubilizing agents in the electrodeposition bath, it has been proposed in U.S. Patent No. 3,663,406 to conduct an electrodialysis within the bath to continuously remove a proportion of the solubilizing agent from the bath and thereby control the maximum level of solubilizing agent in the bath.

Although practicing electrodialysis within the bath provides excellent control of solubilizing agent, it is very expensive and inconvenient. To be effective, it is necessary that more than one electrodialysis unit be installed within the bath. If one unit is inactivated; there must be another unit operating, and the units are particularly susceptible to injury. For example, the article being coated often rips the membrane of the unit. Also, the membranes often become clogged with paint which inactivates the unit. Therefore, there is considerable capital expenditure and maintenance expense involved in positioning the electrodialysis units within the electrodeposition bath. In addition, it is inconvenient to replace or repair a unit when the bath is filled with paint.

As indicated above, the ultrafiltration step does remove a proportion of the solubilizing agent, but a sufficient quantity of solubilizing agent must be retained in the ultrafiltrate when, as is usual, this ultrafiltrate is to be used as rinse water to remove drag-out and return the drag-out-laden waters to the electrodeposition bath. The concentration of solubilizing agent in the ultrafiltrate is therefore considerably less than the concentration thereof in the electrodeposition bath.

It has now been surprisingly found that the electrodialysis step can be effected on the ultrafiltrate and that despite the considerably lower concentration of the solubilizing agent in said filtrate, effective control of the concentration of solubilizing agent in the electrodeposition bath can be achieved whilst still maintaining a sufficient concentration of solubilizing agent in that portion of the ultrafiltrate which is used as rinse water for drag-out removal. The use of electrodialysis in the electrodeposition bath itself and all the problems associated therewith are thereby avoided.

The present invention provides an improved method of operating an electrodeposition process wherein an electrically conductive article is electrocoated from an electrodeposition bath comprising synthetic resin ionically dispersed in an aqueous medium which comprises subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane retains the dispersed resin and passes ultrafiltrate comprising water and solute of substantially lower molecular size than said resin including ions of the charge opposite to said resin and returning at least a portion of the ultrafiltrate to the electrodeposition bath, the improvement comprising:: subjecting at least a portion of the ultrafiltrate to an electrodialysis before the ultrafiltrate is returned to the electrodeposition bath, whereby ions of the charge opposite to said resin are removed from the ultrafiltrate.

Figures 1, 2 and 3 are schematic drawings depiciting the process of the invention.

The process of electrodeposition is well known in the art and does not in itself form a part of this invention. Generally, an aqueous bath containing an electrodepositable paint is placed in contact with an electrically conductive anode and an electrically conductive cathode and upon the passage of electric current, usually direct current, between the anode and the cathode, while immersed in the electrodeposition bath containing the paint, results in an adherent film of the paint being deposited on either the anode or the cathode, depending upon what type of resin is employed. The electrodeposition process parameters may vary widely. E he voltage applied may vary from as low as. for example, one volt to as high as about 500 volts or higher. However, typically the voltage ranges from 500 to 400 volts.

The current demands are higher during the initial stages of the electrodeposition, but decrease as the deposited film insulates the particular conductive electrode. Generally, the electrode employed may be any electrically conductive surface such as steel, aluminium or copper. Also, treated metal surfaces such as galvanized steel and phosphatized steel may also be used.

A wide variety of paints may be used in the process. The paints must be anionic or cationic in character. The ionic character of the paint is usually imparted by the resinous binder used in formulating the paint. A wide variety of electrodepositable resinous binders are known in the art. Anionic resinous binders usually contain pendant carboxylic acid groups which are neutralized with a base such as an amine. Examples of such resinous binders are described in U.S. Patents Nos.

3,441,489; 3,422,044; 3,403,088; 3,369,983 and 3,366,563.

Cationic resinous binders are also well known in the art and usually involve resins with pendant amine groups which are neutralized with an acid such as an organic acid, for example, acetic acid or lactic acid, to form the cationic salt groups. Examples of cationic electrodepositable compositions which are used industrially are described in U.S. Patents Nos.

4,031,050 and 4,190,567, DE-OS 2,752,555 and European Patent Application 12463.

As described in the aforementioned patents, the electrodepositable resins are in combination with pigments, crosslinkers and adjuvant materials such as flow control agents, inhibitors and organic co-solvents to form the electrodeposition paints.

The electrodeposition bath is operated on a continuous basis, that is, articles being electrocoated are continuously entering the bath being coated and being removed. Also, it is necessary to replenish the electrodeposition with paints on a continuous basis.

After continued bath usage and aging, solubilizing agents and undesirable impurities accumulate in the bath. Examples of such impurities are oils, phosphates and chromates which are introduced into the bath from the articles being coated, carbonates and excess solubilizing agent which builds up in the bath because it does not electrocoat with the resin.

Such undesirable components deleteriously affect the coating process, for example, the voltage at which the deposited film ruptures drops significantly. conductivity of the bath increases, pH of the bath fluctuates and film thickness decreases and the chemical and physical properties of the deposited film become unsatisfactory.

To remove these impurities, and to maintain the composition of the electrodeposition bath at a relatively uniform level, a portion of the bath is withdrawn and subjected to ultrafiltration.

The basic ultrafiltration process is relatively simple. Solution to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in a cell, in contact with an appropriate filtration membrane supported to a porous support. Any membrane or filter having chemical integrity to the system being separated and having the desired separation characteristic may be employed. Preferably, the contents of the cell should be subjected to at least moderate agitation to avoid accumulation of the retained solute on the membrane surface with the attendant binding of the membrane. Ultrafiltrate is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent or solvent plus dissolved low molecular weight solute is removed.A suitable apparatus for conducting ultrafiltration is described in U.S. Patent No.

3,495,465.

In practice, a portion of the electrodeposition bath is continuously or intermittently removed and passed under pressure created by pressurized gas or by means of pressure applied to the contained fluid in contact with the ultrafilter. Obviously, if desired, the egress side of the filter may be maintained at reduced pressure to create the pressure difference.

The pressures necessary are not severe. The maximum pressure in part depends on the strength of the filter. The minimum pressure is that pressure required to force water and low molecular weight solute through the filter at a measurable rate. As practiced industrially, ultrafiltration is operated usually at about 10 to 1 50 psi, preferably about 20 to 100 psi.

Under most circumstances, the ultrafilter will have an initial flux rate of at least 8 gallons per square foot of membrane surface per minute and may even be as high as 40 gallons per square foot of membrane surface per minute.

The solutes retained on the membrane surface, i.e., resin and pigment, are usually returned to the electrodeposition bath.

Although ultrafiltration is successful in removing many impurities from the electrodeposition bath, it is not particularly successful in removing solubilizing agent from the electrodeposition bath. One of the reasons for this is that as practiced industrially, the ultrafiltrate is used as rinsing water for freshly electrocoated articles to rinse dragout from the articles and return the dragout-laden rinse waters to the electrodeposition bath. Although a portion of the ultrafiltrate is usually purged to drain, the purging will often be insufficient to remove excess solubilizing agent. Therefore, it is necessary to treat the electrodeposition bath with electrodialysis.

As described above and as shown in Figs.

1, 2 and 3, electrodialysis is employed in the practice of this invention to treat the ultrafiltrate before returning the ultrafiltrate to the electrodeposition bath. Electrodialysis is a species of dialysis which, by definition, is the separation of solutes by means of their unequal diffusion rate through a membrane. In electrodialysis, the passage of electrolyte through the membranes is accelerated by an electromotive force. The membranes employed in electrodialysis are frequently referred to as "semi-permeable membranes" and are interposed between the ultrafiltrate and the electrolyte which prevents their gross intermixture but which permits the passage of solvent and solute from the ultrafiltrate to the electrolyte. Electrodialysis is controlled by electromotive force, diffusion rate and the membrane properties.The electromotive force may be the same as that used in the electrodeposition process, however, this is not necessary. Diffusion is the force that drives the molecules and ions toward and, when possible, across the membranes. The nature of the membrane determines which molecule species can pass and which are held back.

Thus, preparation and selection of suitable membranes is of particular importance. Variety of membranes may be employed in the electrodialysis as used in the present invention. These include conventional dialysis membranes such as regenerated cellulose on fabrics or felts; films of polyvinyl compounds, as well as membrane materials which are not usually considered as dialysis membranes but which produce the desired electrodialysis when employed in the electrodeposition process. Among the useful membranes are those comprised of woven or unwoven cloth including various natural or synthetic fibres, such as disclosed in U.S. Patent No. 3,496,083.

Besides the membranes mentioned above which are non-ion selective, selectively permeable ion-exchange membranes may also be used. Using an ion-exchange membrane has advantages over using a non-ion selective dialysis membrane to separate accumulated ions from the depositable compositions in that the ion-exchange membranes normally have a lower electrical resistance than non-ion selective dialysis membranes and permit a faster and more efficient passage of ions of opposite charge through them. For a more complete description of ion-exchange membranes in electrodialysis, reference is made to U.S. Patent No. 3,419,488.

The electrode compartment used in the electrodialysis apparatus to separate the anode or cathode from the ultrafiltrate can be of any convenient shape. Perforated cylindricalshaped plastic containers having the membrane mounted on such superstructure and the electrode encased therein have been employed. However, more commonly utilized structures are rectangular-shaped boxes having the electrode centrally located and the major walls of the box parallel to the electrode and comprised of the membrane. Usually the electrode compartment is equipped with an input and outlet connection to facilitate flushing of the compartment. The electrode compartment contains an electrolyte which is usually water and, in particular, deionized water having a minimum level of electrolyte that is being removed.To prevent an accumulation of ions, periodic or continuous flushing of the electrode compartment with deionized water, or a mixture of deionized water and a minimum level of separable electrolyte, is usually provided. The construction and operation of electrodialysis cells are known in the art and are described in U.S. Patents Nos. 3,264,250 and 3,663,406; Referring now to the drawings, reference is made to Fig. 1 which schematically illustrates an embodiment of the present invention. Electrodeposition bath 1 contains the paint used in electrodeposition. The electrodeposition bath contains suitable apparatus for depositing films on the articles being coated in the bath. This apparatus is not shown in this drawing. A portion of the electrodeposition bath is continuously or intermittently withdrawn through the outlet line 3 to an ultrafiltration apparatus 5.By the process of ultrafiltration, water, low molecular weight impurities and solubilizing agent pass through the ultrafiltration membrane 7. The concentrate or retentate which comprises the high molecular weight resinous binder and pigment is retained on the ultrafiltration membrane surface and is usually returned to the bath through line 9. The ultrafiltrate is removed from the ultrafilter through outlet line 11.A portion of the ultrafiltrate may be directed either unidirectionally or proportionately in either an intermittent or continuous fashion to drain 1 3 or recycled to the electrodeposition bath through line 1 5. A portion of the ultrafiltrate can be used for rinsing electrodeposited articles at rinsing station 1 7 or simply returned to the electrodeposition bath through line 1 8.

A portion of the ultrafiltrate is passed through line 1 9 to the electrodialysis unit 21.

In the electrodialysis unit, solubilizer, undesirable contaminants and impurities which have a charge opposite the particular membrane enclosed electrode are accelerated by electromotive force through a semipermeable membrane and thereby expelled from the system with the washing liquor to drain 23. The ultrafiltrate which has been subjected to electrodialysis is returned to the electrodeposition bath through line 25. The electrodialyzed ultrafiltrate can be used for rinsing or returned to the eiectrodeposition bath without rinsing.

Reference is made to Fig. 2 which schematically illustrates aspects of the invention other than that shown in Fig. 1.

Power source 27 provides the electricity needed for the electrodeposition and the electrodialysis processes. The electrodeposition bath contains an anode 28 and cathode 30, as is well known in the art. The electrodialysis cell also contains an anode 32 and cathode 34. The anode 32 is separated from the remainder of the electrodialysis cell by a membrane 36 to form an anode compartment 38.

The membrane can be any of the typical membranes known in the art for practicing electrodialysis such as those mentioned above.

For the purpose of further describing the method of the invention as depicted by Fig. 2, it is presumed that the paint is a cationic paint and the ultrafiltrate will contain acid. However, it should be understood that the invention is also applicable to electrodeposition with anionic paints, in which case, the cathode 34 in the electrodeposition unit would be separated from the remainder of the cell and base would be removed by the electrodialysis process.

The ultrafiltrate from the electrodeposition of a cationic paint passes from the electrodeposition bath 1 through line 1 9 to the electrodialysis unit 21 where it is electrodialyzed upon passing through the electrodialysis unit 21. The anode compartment 38 is flushed in a continuous or intermittent fashion from line 40 with a flushing liquid such as ultrafiltrate or deionized water to remove the acid and other anionic materials which pass through the membrane 36. The composition in the anode compartment, commonly referred to as anolyte, is purged to drain at 23. The electrodialyzed ultrafiltrate returns to the electrodeposition bath through line 25. However, before it returns to the bath or is used for rinsing, the ultrafiltrate may pass through sensor 44.The sensor monitors the acid content of the electrodialyzed ultrafiltrate and may be, for example, a pH meter.

Removal of too much acid from the ultrafiltrate could cause a stability problem when the ultrafiltrate is used for rinsing or returned to the electrodeposition bath. If there is insufficient acid in the ultrafiltrate, the drag-out laden rinse waters may coagulate due to the absence of acid. Therefore, careful monitoring of the acid content of the ultrafiltrate may be necessary. If too much acid is being removed, the sensor 44 can open switch 47 inactivating the electrodialysis unit.

The acid removal rate of the electrodialysis unit shown in Fig. 2 is controlled by the electrical current drawn through the unit. All or part of the electrical current necessary to deposit paint in the electrodeposition paint system is passed from the power supply 27 to the paint bath 1 through the electrodialysis unit to provide acid removal. Most electrodeposition systems contain several anodes 28 supplying equal or varying amounts of electrical current for paint deposition. The electrical current drawn through the electrodialysis unit by line 46 is primarily controlled by the number of anodes it is connected in series with. The remaining electrical current, if any, is passed from the power supply directly to the electrodeposition bath via line 48.Fig. 2 also illustrates the provision for operating the electrodialysis unit during non-production periods by changing the switch position 52. The power supply 27 supplies electrical current to the electrodialysis unit and bypasses the electrodeposition bath 1 through line 50 and switch 52 which returns to the power supply 27. During this non-production mode, the electrical current is controlled primarily by the applied voltage from the power supply. The electrical current draw of the electrodialysis unit under any operating mode may be controlled or adjusted by solution conductivities in the anode compartment 38, paint bath conductivity, applied voltage to electrodialysis unit, separation of electrodes, or any voltage or current regulating unit in series or parallel with the electrodialysis unit.

The above-described electrodialysis unit may be expanded to a three-compartment unit as shown in Fig. 3 providing the capability to remove acids and bases simultaneously. This unit utilizes three compartments separted by two semi-permeable membranes. The ultrafiltrate is fed to the electrodialysis unit through line 1 9 and returned to the paint deposition system through line 25 or sent to drain. The electrical current passed from the anode 32 to the cathode 34 removes the acid and anionic materials which pass through the membrane 36, and also remove the base and cationic materials which pass through the membrane 54 into the cathode compartment 56. The solutions contained in compartments 38 and 56 are flushed through lines 40 and 23 and lines 58 and 60, respectively. The flushings can be purged to drain or, alternately, can be partially fed back into the ultrafiltrate line 25 to adjust the ultrafiltrate pH to any desired reading. Any solution could be utilized to flush compartments 38 or 56 but typically ultrafiltrate or deionized water is used.

An additional electrode may be used on the ultrafiltrate side of the membrane to provide additional control for increased or decreased acid or base removal.

Claims (9)

1. In a method of operating an electrodeposition process wherein an electrically conductive article is electrocoated from an electrodeposition bath comprising synthetic resin ionically dispersed in an aqueous medium which comprises subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane retains the dispersed resin and passes ultrafiltrate comprising water and solute of substantially lower molecular size than said resin including ions of the charge opposite to said resin and returning at least a portion of the ultrafiltrate to the electrodeposition bath, the improvement comprising: subjecting at least a portion of the ultrafiltrate to an electrodialysis before the ultrafiltrate is returned to the electrodeposition bath, whereby ions of the charge opposite to said resin are removed from the ultrafiltrate.

2. A process as claimed in claim 1, in which the counter-ion used in solubilizing the resin is removed from the ultrafiltrate.

3. A process as claimed in claim 2, in which the counter-ion is an anion derived from an acid.

4. The process of claim 3 in which the acid is an organic acid.

5. The process of claim 4 in which the organic acid is acetic acid, lactic acid, or mixtures thereof.

6. The process of claim 1, in which ions of the same charge as the resin are also removed from the ultrafiltrate.

7. The process of claim 1, in which the electrodialysis is carried out by passing the ultrafiltrate across a non-ion selective semipermeable membrane.

8. The process of claim 1, in which the electrodialysis is carried out by passing the ultrafiltrate across an ion-exchange membrane.

9. The process of claim 1, in which the electrodialyzed ultrafiltrate is used at least in part for rinsing electrocoated articles.