Q. How do I keep my e-coat bath bacteria-free? N.B.
A. Since the introduction of solvent-free e-coat systems around the world, microbiological contamination of those systems now occurs with more frequency, as many of the solvents eliminated also provided antimicrobial properties to the formulations. With the elimination of certain solvents, such as hexyl cellosolve, the probability of biological contamination of your system has gone up.
Other system factors also contribute to this increase in biological activity, such as the tempered operating temperature encountered in new e-coat systems, the water-based environment of the e-coat and the high percentage of closed-loop operations accomplished.
Bacterial contamination may appear in your e-coat in many different ways and forms. Sometimes this contamination can be easily detected visually. You can see the presence of biofilm on your system’s walls and reservoirs or by the discoloration of permeate or anolyte fluids. In other cases, you may smell foul odors around the system or observe your UF output go down. In other e-coat systems, the presence of bacteria can be detected by change in the system chemical parameters or patterns.
Many e-coat systems require bag filters changed too frequently, or maintain poor material efficiency and drop too many paint solids into the bottom of the tank. Other systems have problems maintaining pH because it shifts toward neutral and there’s a need to add too much external solubilizer to maintain it within range (acid for cathodic or amine for anodic systems). Other systems have the need to purge excessive amounts of permeate to maintain system conductivity. All of those anomalies may be the cause of bacterial contamination.
Bacteria are the most common type of biological microorganisms found in an e-coat system. When active for long periods of time, the bacteria create colonies that form a protective layer. This accumulation is known as biofilm and is the result of long and extreme bacterial activity and contamination.
The presence and activity of bacteria in an e-coat system can severely interfere or alter chemical system control parameters such as pH, conductivity and solubilizer level in paint bath and permeates, as well as DI and RO systems. Other critical elements of system operation affected by the bacterial activity are UF membranes and anolyte box membranes. DI columns and RO membranes can also be victims of bacterial contamination.
Complete elimination of bacterial activity in an e-coat system involves implementing a minimum of three procedures for identification, treatment and prevention, eliminating the need for costly, time-consuming treatments.
Identification of bacteria involves a method for determining the types, counts and specific locations of the entire e-coat system and ancillary equipment where they exist. Bacteria are typically detected using dip slides exposed to the sample to be tested and incubated for a determined amount of time, typically 24–48 hours. The dip slides are fed to speed up the creation and multiplication of colonies so that they can be easily counted and identified.
The result of this testing and counting serves as the barometer to decide if contamination has occurred and when further treatment is required. The measurement used in the industry is Colony Forming Units (CFUs) of bacteria detected per milliliter or gram of sample. Bacterial contamination is considered present in the system when the CFUs are greater than 10,000 or 100,000 CFU/mL. A treatment procedure and level must be determined for each specific system depending on unique system characteristics and operation.
There are two kinds of bacterial treatment procedures used by industry for treatment in e-coat systems: biocide-based and enzymatic-based. Several different treatment procedures are used for DI/RO systems.
The most common e-coat biocide treatment procedure involves use of a biocide such as Kathon. Kathon is fairly toxic and requires use of protective equipment and only by authorized personnel. This biocide requires concentrations of 0.05–0.15% by volume of the biocide to total volume treated. All piping and systems volumes must be taken into account for proper concentration and effectiveness of dosage and time. The procedure applies to paint bath, post rinses, UF and anolyte subsystems.
The biocide treatment must be carried out with mechanical agitation and recirculation of the entire volume for at least 24 hours. Heavy filtering with bag filters of 5–10 µ during the 24 hours should be used. Solubilizer should also be added to the system for pH adjustment during this period if necessary. The treatment should be performed during non-production hours and always under supervision of your e-coat paint supplier. The system should then be left to stabilize for 24 hours after treatment and prior to any significant production time.
Enzymatic procedures are only used for the treatment of biofilms and should never be used for the treatment of bacteria alone. Biofilms are nests of bacteria that grow and multiply rapidly and biocide treatments alone are not sufficient for complete treatment.
Enzymatic treatment involves draining the tank or system to be cleaned, and treating it with a solution of an enzymatic material while rubbing and removing it from the surfaces, hand brushing and/or pressure spraying affected areas. The enzymatic material is used during this removal treatment procedure to kill all colony-forming biofilm. Like Kathon, these materials are dangerous and require use of protective equipment by authorized personnel under supervision of your e-coat supplier.
Enzymatic removal must be followed by an e-coat biocide system treatment using Kathon and should never be added directly to an e-coat bath or post-rinse tanks. It should only be used for removal of biofilms.
A different treatment is used for DI/RO systems, which can only be treated with strong oxidants such as hypochlorite, hydrogen peroxide or chlorine. Treatment involves adding the material to the holding tank and circulation pump system for up to four hours or until sanitized, maintaining a positive-free residual chlorine. This kind of treatment is not intended to be used for DI columns or RO membranes, just the holding tank and circulation system. The intent is to sanitize the system that feeds contaminated DI/RO water to the e-coat system.
Treatment and cleaning of e-coat baths, as well as removal of biofilm, can be costly and time consuming. Because of this, it’s best to establish a proactive approach to preventing microbial contamination in the pre-rinses, and DI/RO post-rinses. This strategy requires maintaining an acceptable level of free residual chlorine in the DI/RO rinse prior to the e-coat tank, and the last DI/RO post-rinse tank, if the system has one. Acceptable chlorine levels should be in the order of 0.5–2 ppm.
Prevention also involves regular (about once a week) detection testing using dip slides or another method of bacterial detection and count. With this testing, the level of free residual chlorine can be maintained to keep the e-coat system bacteria free.
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This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 13, 2012.