Electrodeposition has been the dominant primer finish through-out the world. Significant advantages of electrocoat over other application methods include film uniformity, improved appearance and quality, complete coverage of complex geometries, high transfer efficiencies, high throughputs due to automation, reduced air and wastewater emissions and reduced fire hazards.
A summary of the evolution in the development of cathodic electrodeposition technology is as follows:
First Generation, 1976, Corrosion Protection
Fourth Generation, 1986, Reduced Film Shrinkage and VOCs
Fifth Generation, 1991, Improved throwing power
Seventh Generation, 1995, HAPS-free, reduced VOCs, heavy-metal free, low film build.
The first generation material has been the workhorse of the automotive parts and accessories (APA) market because it has been considered the "best performance" primer. Although the best performer, it has relatively high VOCs and lead and chrome content compared to the others. The development of future generations was targeted to improve specific undesirables of the original (high VOCs and heavy metals, film shrinkage, better ultrafiltration). In comparison, fourth and fifth generations have been the most used in the automotive OEM market.
Metokote Corporation currently (as of this writing, in 1996) has 37 electrodeposition systems, of which 28 are black cathodic epoxies. Started in 1969 in Lima, Ohio, MetoKote presently operates 17 plants in eight states. We specialize in applying protective coatings on parts for the automotive and other industries. Our services include electrodeposition, powder, wet paint and plating. We electrocoat more than 250,000,000 sq ft/year for our more than 500 customers.
We have been using first generation electrocoating since 1982. We use or have used the fourth and fifth generation cathodic epoxies and other electrodeposition systems. One of our biggest challenges was to get approval from OEMs and other customers to change the first generation cathodic epoxy tanks to the newer, lead- and chrome-free generations. Our customers were almost always hesitant to change because corrosion performance was never comparable to the first generation coating at extended salt spray hours. Increased film builds and appearance changes (gloss level) have also been a problem for many customers.
In 1993, MetoKote was facing 1990 Clean Air Act Amendments and consequently Title V regulations with permitting thresholds of 10 tons for any particular HAP and 25 tons for total combined HAPS. We were in the process of developing a strategy to minimize the business impact of Title V permitting and realized that existing electrocoat technologies would not get us below the thresholds in plants with multiple lines.
Our primary strategy was to ask the coating manufacturer to develop an electrocoating that had no VOCs, HAPS or heavy metals. And to make it even tougher, we requested a coating that exhibited performance comparable to that of the original cathodic epoxy at low film builds. As a good partner and supplier, the manufacturer accepted the challenge and in 18 months seventh generation cathodic epoxy was available and approved for a production trial.
MetoKote filled the first and only R&D/trial/production line in May 1995. Our goal now is to convert all 28 tanks before the summer of 1996. Our "mission" is to quickly change as many tanks to the seventh generation material as possible.
Solvent/VOCs. The seventh generation system has been formulated with low solvent/VOCs. The resin has significantly less solvent than the first generation resin, but virtually the same as the fourth and fifth generation resins. The paste for the latest material has about half the amount of solvent as paste for the earlier coatings.
The real environmental benefit hinges not on the coating's solvent/VOC content but on actual solvent/VOC emissions. Emission reduction is more striking when film weight loss in the oven, film build and solvent replenishment are taken into consideration.
HAPS. The seventh generation system has zero HAPS. A major improvement of the resin as compared to earlier resins is the zero HAPS released during the curing process. Although some solvents are released during the crosslinking process they are not listed HAPS. In addition, the paste corresponding to the seventh generation resin is HAPS free.
Heavy Metals. The seventh generation system is both chrome and lead-free. It is the first cathodic epoxy electrocoat to exhibit excellent performance at low film builds without using heavy metals. Currently fourth and fifth generation systems can be used without heavy metals but the lack of extreme performance at low film builds has forced many users to apply films greater than one mil. Even at high film builds extreme performance is not equivalent to the same materials when pastes containing lead and chrome are used.
Although electrocoat is known for high transfer efficiencies (95 pct plus) and closed-loop arrangements with the use of ultrafiltration units, some paint is always depleted from the system due to replacement of bag filters, periodic maintenance of the bath to eliminate sludge accumulated in the bottom of tank (especially if agitation is not adequate), carryover to final post rinse tank and accidental spills. These wastes are regulated by the Resource Conservation and Recovery Act (RCRA) or the Clean Water Act (CWA). In an electrocoat system that uses a no chrome or lead paint, any waste that needs to be disposed of off-site most likely will be classified as non-hazardous. This results in less regulatory burden on the user and lower disposal and liability costs. Additionally, the discharge of the final post rinse overflow (if deionized water) could be made to the local sewer system after pH adjustment, instead of going to wastewater treatment.
Application Considerations. The seventh generation system has introduced several operating or application differences as compared to previous generations of electrocoats. Percent solids and P/B (pigment/binder) ratios are virtually the same as fourth and fifth generation systems but much lower than the first generation system. Conductivity and pH are slightly lower than all prior generations.
Bath temperature is maintained slightly higher than prior generations. Operating range is 90-100F with optimum at 92-93F. This higher temperature actually reduces the cooling load on the chilling equipment, which translates into lower operating costs. Cure temperature is slightly higher than prior generations. While earlier systems can develop good properties at metal temperatures of 350F for 20 min, the seventh generation system requires at least 375F metal temperature for 20 min to develop comparable properties.
The amount of permeate to drain in a seventh generation system has been found to be lower than a first generation system, but virtually equal to fourth and fifth generation systems. This translates into lower operating costs by minimizing the amount of wastewater going into in-house wastewater treatment systems.
Oven fouling in a seventh generation system has also greatly improved over prior generations. Oven fouling is the amount of chemical released from the curing film that accumulates on internal oven surfaces. These deposits, if not cleaned periodically, can dislodge from the oven walls and end up on the work pieces. Oven fouling has been reduced primarily by formulating the latest system with less solvent and by minimizing the amount of film shrinkage.
The material's increased coulombic efficiencies of the material over prior generations are significant. Earlier systems needed a minimum of 150 volts to properly coat, whereas the latest system coats at voltages of 100 volts or less. This is unusual considering the paint bath conductivity is slightly lower than prior generations. Electrical efficiency of the latest system is approximately 1.2-1.7 amps/sq ft. In the earlier systems it is 2.0-2.5 amps/sq ft. The higher electrical efficiency of the seventh generation system translates into lower electrical consumption, smaller rectification units and lower waste heat going into the paint bath.
The throwing power for the seventh generation system has also improved. Throwing power is the ability to coat in hard-to-reach areas. The latest system has greater throwing power than originally thought. It surpassed the fifth generation system with a throwing power of 16-18 inches. This could be explained by the higher electrical efficiency of the latest system and its ability to deposit film quickly.
Edge coverage of the seventh generation system has also improved. Edge coverage is the ability of a curing film to not flow away from peaks in the substrate and leave a film of lower thickness than the average that could lead to premature corrosion failures. This is critical on castings or forgings with high profile surfaces. Testing has shown older generations with 75-100 rust spots (GM 9632P, 38 deg), whereas the new system shows about 45 rust spots at a 0.15 P/B ratio.
Film weight loss in the oven, also known as shrinkage, has greatly improved. Film weight loss occurs in the oven during crosslinking. Less shrinkage translates into lower material consumption and therefore less material cost. Film weight losses as low as one pct at 400F have been reported.
Seventh generation cathodic epoxy represents the latest generation in an evolutionary chain of technology that extends back more than 30 years. It is an electrocoating system built around the requirements of a custom coater. It is fully approved under small parts specifications for Chrysler, Ford and General Motors. It meets requirements at low film thickness (12-20 microns) over properly zinc phosphated substrates. It is a heavy-metal free, HAPS-free, low-VOC and solvent-free operation. It has no solvent-replenishment costs, reduced wastewater treatment costs, zero hazardous waste generation and higher transfer efficiencies. Seventh generation is the only available electrocoat technology that meets the performance, environmental and operating costs of the present and future automotive parts market.