Enthone-OMI, Inc., Warren, MI

Transcription

1 Electrophoretic Deposition: A New Answer to Old Questions Jonothan Holland and Robert Berger, Ph.D Enthone-OMI, Inc., Warren, MI Electrophoretic processes offer significant new opportunities for manufacturers and job shop finishers. These include higher quality finishes with greater durability and new attractive colors, including simulation of precious metals such as gold. Electrophoretic deposition produces thin uniform coatings with complete coverage of all surfaces including deep recessed areas. The coatings have excellent gloss and clarity characteristics while avoiding the drips, runs and problems associated with older topcoating technologies. Electrophoretic coatings also have low VOC emissions that meet or exceed the most difficult to reach air quality standards. The discussion will include an overview of the deposition mechanism, equipment requirements, VOC data, and deposit performance. Throughout the history of electroplating there have been applications where the last metallic finish requires a clear protective surface to enhance the product during storage or in the final functional use. Organic solvent lacquers are a major player in this area, but due to their high Volatile Organic Compounds (VOC) content. they have come under increased scmtiny by various government agencies. The plater may have further experience with other problems, i.e. drips, sags, non-uniformity, k ing. This type of coating requires the plater to have increased compentence in an area outside what is normal expertise. In addition to govertuuent and esthetic mwolls for replacing organic solvent lacquers, there is a practical reason due to the flammability of the solvents before coating, during application, and during the drying step. It was for all these reasons that the method of electrophoretic deposition was developed. The process of Electrophoretic uses the knowledge that under an applied potential, acid is generated at an anode and base (caustic) at a cathode. By incorporating a sufficiently high voltage, ionic compounds are forced to migrate to the desire electrode where acid-base neutraliation leads to the precipitation of an organic resin. After a small degree of deposition has occurred in a high current density region, this site becomes insulating and further deposition is forced onto the low current density areas. The end result is a very uniform coating throughout the target part. The insulation effect is due to a squeezing out of most of the trapped water. During the solids deposition the high voltage required by the process gives excellent packing of material and high throw power necessary to reach and coat the low current density areas. The two forms of electrophoretic deposition are anodic and cathodic. During anodic E-Coat operations, acid is generated on the part to be coated and this precipitates the quasi-solubilized resin along with cross-linking agents and flow agents on the part. Bare metal parts, Le. steel, zinc, that are succeptible to acid corrosion are not prime candidates for this process but parts protected with a chemical coating, i.e. phosphated, can be covered with an opaque E-Coat resin for primer and some topcoat applications. CIear forms of E-Coat are not available unless the metal surf' is not readily solubilized, i.e. nickel. The cathodic process was developed to ease and improve the results inherent to anodic E-Coat. In a cathodic process, the cathode generates base during processing and this precipitates the acidic quasi-solubilized resin along with its cross-linkers and flow agents. While the bath is mildly acidic, the cathode is highly basic when potential is applied and more surface active metals may be safely E-coated without extensive degrading of the metallic surface. Bare steel and brass are good examples of the types of metallic surfaces that can be successfttlly covered by clear or opaque systems that are of suitable quality for topcoat applications. 1 37

2 In both anodic and cathodic systems the coating may be cured by baking the part in an oven at some temperature sufficiently high to activate the cross-linking agent for polymerization with the resinous backbone. Prior to the moss-linking, the coating is water dried and the flow agents smooth out the surface to an acceptable level. The degree of cure is controlled by the temperature and hold time in the oven. A general rule of thumb is that greater hardness will be the results of higher temperature and longer cure time. However, a loss of gloss and color may be the result of these same conditions. Each plater must determine the best conditions for their own work. A valuable benefit of electrophoretic coating is the availability of a large number of tinting or opaque colors. By the judicious use of tinting dyes, the plater may apply a simulation surface that is visually the same as that of a more expensive metallic coating. A proper blend of yellows may Seem to duplicate brass or gold without the necessity of using a cyanide bath. This can reduce the hazwdous nature of a plating facility and greatly reduce the cost of waste treatment. Another application for E-Coat is hiding the metallic substrate through the use of opaque pigments and fillers. This type of application has primary benefit for industrial or functional coatings. Due to the high throw power of E-Coat an irregular shaped part may be covered uniformly without excessive build up in easily reached places. The inside of a tube may be covered to a degree found only in electroless metal deposition. An automobile or truck wheel may be uniformly covered throughout the edges and low current density interior regions. Due to the high throw power of the process, the variety and sizing of parts suitable for E-Coating is quite large. From fine jewelry to truck cabs, the list may be considered endless. There are several equipment requirements necessary to produce a saleable product. While the typical bath contains a solids content of 10 percent, most of this material has low conductivity and the process is heavily influenced by ionic contamination. A good grade of deionized water must be used to insure low introduction of fresh contamination. A useful specification for water is a value less than 5 micro Siemans &S). The rapid introduction of untreated water will force the dropping out of coating resin. The incorporation of iron, calcium, and-magnesium in the concentrations found in well water or other common sources of water is very detrimental to the stability of an electrophoretic bath. There are several steps in a typical system where the use of deionized water is important. The maintenance of the bath will require periodic replenishment of water to hold the level at a relatively constant value. Several pre-rinse stations must use deionized water to act as cleaning steps to reduce the risk of contaminating the bath. Also, excess dragout must be concentrated using post Mse stations. A good grade of deionized rinse water will allow retum of dragout to the bath with minimal harm while maximizing the material efficiency. The volume of the bath will be reduced through evaporation and dragout, but both will be minor effects when compared with replenishment volume of the bath. For this reason a method of volume reduction using ultrafiltration (UF) must be incorporated into the system. The technique of ultrafiltration is similar to that of reverse osmosis, except that the molecular weight cut-off and solubility effects are higher for ultrafiltration. Small molecules, such as solvents, ionic contamination, and water are removed from the bath in the permeates, while large species, Le., resin, crosslinking agent; are retained by the bath. In this manner, the bath may be cleansed or maintained to a'& working range. In cases where the ultrafiltration permeates flow in a large enough volume, they may be recycled back to the bath as the first post rinse. This allows a high degree of stability of the bath since trace amounts of solvents are retained. The retention of these solvents at the optimum level keeps the bath at a position closer to the working conditions than that found when the UF permeates are waste treated. In many cases, the recycling of permeates is practical only where there is a large working volume. The flow of permeates is not large in a single filter tube and considerable redundancy of filters may be required to support such an operation. A general rule of thumb is that ultrafiltration capacity should be maintained at a minimum flow of 2 percent permeates per hour of use. A much higher flow of permeates is necessary for use as a post rinse. A value lower than 2 percent will generally be adequate for long term maintenance of the bath, but inappropriate for emergency treatment of the bath. It is possible to incorporate a permeates cleaning cycle by the use of ion exchange resins. The major effect of such treatment is the removal of ionic contamination from the permeates. After this treatment, the retumed permeates may be considered to retain only deionized water, solubilizer, and bath solvents. 2

3 J. The return of cleansed permeates has considerable merit since trace quantities of solvent are used BS flow agents during the curing step. Their removal by ultrafiltration will lower the amount available for flow and may to an uneven appearance. It is relatively simple to replace the solvents, but due to the small quantities involved, it is possible for a bath to drift out of a useful working range. While the solvent levels in the bath may be monitored by known analytical techniques, as gas chromatography. Inappropriate maintenance of a bath may increase the number of rejects above the acceptable range while waiting for an analysis result. After the part has been electrophoretically coated, it is of importance to add a degree of permanence by curing the part in an oven. In the bath, the organic phase consists of three major parts: The resin, the cross linking agent, and flow agent@). The deposition in the bath allows all three portions to be placed on the part. However, simple low temperature drying will not give superior results to the surface. A sufficiently high temperature is used to unblock the cross linking agent and allow it to react with the resin. In many cases, the cross linking agent is derived from a very reactive compound. These materials are unstable in aqueous suspensions and must be protected before curing. By the use of a reversible blocking agent, the cross linking agent may be safely incorporated into an aqueous medium over long periods of time. Only by exceeding the unblocking temperature is the cross linking agent regenerated and made available for immediate reaction with a suitable resin. The lower VOC requirements are of growing concern to the plater. The use of lacquer may be a minor part in a shop s operation, but all non-water based volatiles must be taken into account for the location. Due to the poor efficiency of getting solvent lacquer to adhere to a surface, this type of coating can be a major source of VOC emissions. The use of electrophoretic deposition allows more of the coating material to come in contact with the surface. The use of water as the primary transport vehicle allows the VOC emissions to be greatly reduced. Even when the amount of water USBd is abstracted from the working volume, the remaining VOC materials are greatly reduced. In Table 1 a comparison between the VOC values for an organic solvent lacquer and a clear electrophoretic system are given.. TABLE 1 Volatile Organic Compounds Comparison Between A Solvent Lacquer and An Electrophoretic Lacquer Solvent Non Volatiles Water Density voc % lb/gal 2.06 lb/gal In taking into account the-non incorporation of water into the VOC calculation, there remains a 69 5% reduction in VOC emissions by switching from an organic solvent lacquer to a clear electrophoretic system..- Performance characteristics are of supreme importance in determining the suitability of a coating. Brief summaries of test data can be viewed in Tables 2, 3, and

4 TABLE 2 Typical Test Data For A Non-Pigmented Electrophoretic Lacquer 4 4

5 TABLE 3 Exposure to Household Chemicals Nom All b@ perfomed on 0.2 to 0.3 mil (5 to 8 micron) deposit cured at 320 'F (160 'C) for 20 hum

6 f? TABLE 4 Sixty Second Wipe Test I Dulling NOTE: All tests performed on 0.2 to 0.3 mil (5 to 8 micron) deposit cured at 320 'F (160 'C) for 20 minutes. 6 c