Preparing substrates for waterborne coatings...
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Since more stringent regulations limiting emissions of VOCs from paints and coatings are in effect and enforced in most areas, paint manufacturers have developed a range of highly effective, compliant waterborne coatings. Some waterborne resins commonly in use today include acrylics, epoxies, alkyds and polyurethanes. There are several advantages to using coatings in which water is the major solvent. In addition to providing a means for achieving compliance with VOC regulations, these coatings often meet or exceed the finish properties found in their solvent-borne counterparts. Because they are less flammable and less toxic, they also lower insurance costs for shop owners.
However, there are a number of factors that must be considered when a finishing system is set up or reconfigured for waterborne coatings. Often, some modification to the finishing process is needed to accommodate this type of coating system, including changes to the cleaning and pretreatment of parts to be coated, the surface temperature of parts and the temperature and humidity levels in the shop.
Waterborne paints commonly have a pH of 8.0 to 9.5. Therefore, dissimilar metals submerged in the paint can corrode. These metals may be found in pumps, agitators or tanks. This effect must be considered when choosing finishing equipment for use with waterborne coating systems. Recirculation systems, for example, may have to be replaced with stainless-steel pumps and piping to avoid corrosion.
Also, special attention must be given to electrostatic applications. Because waterborne coatings are highly conductive, electrical isolation by means of an isolation stand or voltage blocking system is required, which may limit application to air spray only. Another area requiring special attention is the surface preparation of the parts to be coated.
Many waterborne coating failures result from inadequate surface preparation. These include both cosmetic defects and performance defects. Cratering and fisheyes, typical cosmetic defects, occur because surface contaminants prevent the waterborne paint from sufficiently wetting the substrate. Cosmetic defects also may include streaks or stains that occur when surface contaminants bleed through the coating.
Performance defects can include poor adhesion, which contributes to early corrosion or blistering in extreme humidity or soft films due to an incomplete cure. These defects occur when surface contaminants interfere with proper film formation. One example is a paint shop that power washed hot-rolled steel in preparation for a waterborne coating. Although no visible residue was present, the coating peeled off in sheets after only two weeks. The underside of the paint chips was covered with a dark mill-scale residue, a sign that contamination was not removed during cleaning.
Waterborne coatings can offer excellent adhesion, corrosion resistance and impact resistance. Effective cleaning and pretreatment is necessary to maximize these characteristics. Cleaning of all surfaces must be sufficient so that coating appearance and performance will not be compromised.
The degree of cleanliness required for waterborne coatings is higher than that required for most solvent-borne coatings due to the low solvent content available for "secondary" cleaning of residual oils and contaminants after the wash process. As the saying goes, oil and water do not mix. Solvent-borne coatings are more forgiving of residue because the solvents contained in the coatings may dissolve cutting oils and assist with substrate wetting. Even though waterborne coatings typically contain "co-solvents," which can comprise as much as 30% of the product, they still contain much less organic solvent than solvent-borne coatings. Therefore, there is little chance of dissolving or cutting oily residues.
Consequently, complete removal of soil, surface oils and rust is essential when using waterborne coatings. Cutting fluids, drawing compounds, rust inhibitors, grease and oil as well as everyday dirt must be removed prior to coating any product with waterborne coatings. Molded parts, in particular, can carry residual mold release agents; stamped parts (commonly steel) can retain oils; and plastic parts can attract dust from the air through static buildup. Thus, manual cleaning usually does not provide sufficient, consistent cleanliness. For metal parts, solvent cleaning with hand-held rags (manual cleaning) is not an acceptable process. Streaks and smears usually occur as the rags become contaminated with oils. While solvent washing or vapor degreasing processes may work adequately for many solvent-borne paint operations, waterborne operations typically require some type of alkaline cleaning process and sufficient rinsing to eliminate soapy residues.
On metal substrates, a prep system using an alkaline cleaner, rinse, phosphate conversion coating and final rinse with a corrosion passivator is the optimum choice for waterborne coatings. Each step in the process requires careful adherence to supplier specifications, and equipment maintenance is crucial to achieving a good result. The pretreatment chemical supplier is best able to assess the condition of the substrate and to recommend the proper concentrations and maintenance of cleaners and conversion coatings. Sufficient rinsing is the key to any surface preparation system. The more fresh rinsing, the better.
Plastic parts may be manually sanded prior to finishing to remove molding defects and to improve the surface profile. After sanding, all parts should be hand wiped to remove any dust, oil or mold release agents remaining on the surface. Prior to entering the primer application booth, it is recommended that plastic parts pass through an ionized air blowoff chamber to remove any remaining dust particles from the surface and neutralize residual static charges that can attract airborne particles.
For plastic parts, a five-stage power spray wash is recommended after the second sanding stage to ensure removal of all surface contaminants prior to finishing. The first three stages of the system may use tap water and a cleaner solution to ensure complete removal of all surface contaminants. These stages should be followed by a rinse-aid package and a deionized water rinse to ensure proper sheeting action for complete water removal. After washing, a second ionized air blow off system is recommended to remove the remaining water and dissipate surface static electrical charges prior to entering a dry-off oven. Rapid water droplet removal is vital on plastic parts to eliminate water spot defects in the final finish.
Once cleaned, parts must be thoroughly dried prior to coating. Like solvent-borne paints, waterborne coatings can be air-dried, force-dried or baked, depending on the system used, the composition of the parts coated and space and production restrictions. Adequate flash-off time between the spray booth and oven is necessary when force drying or baking the coating, otherwise solvent popping may result.
It is also necessary to ensure that the surface temperature of the part is greater than the dew point to prevent condensation from forming. The dew point is the temperature at which water condenses from air. While this is generally not a problem during warmer months, it can be a significant concern during colder months.
Of more concern during warmer weather is high humidity, which can extend flash-off and dry times. If the humidity is high, the water vapor released during drying has no place to go, and the film will not cure. By providing moderate air flow and increased temperature, a continuous supply of fresh air can be provided to the paint to give the air more capacity to hold moisture.
Drying ovens should be equipped with two types of ventilation: exhaust to remove solvent and water vapor as well as cure reaction by-products; and circulation to improve heat transfer and provide temperature uniformity. The most important ventilation is the exhaust requirement to keep the oven at a safe condition. Although the water vapor coming from waterborne finishes is not flammable, a portion of the vapor-laden air must be exhausted. Water vapor not removed will build up and retard further evaporation of additional water from the coating.
Circulation ventilation in the oven is important in improving heat transfer and providing temperature uniformity. It also increases the rate of evaporation of liquids from the coatings. Ventilation can be provided by a main circulation fan that removes air from the oven and then returns it to the shop through a special duct designed for recirculation.
Another method of circulating air within the oven uses large propeller-type fans with the fan motors located outside the oven chamber. When propeller fans are used, an external heat source supply fan and distribution system are required to warm the oven. Either method is acceptable to provide heat transfer, evaporation of liquid and drying of the coating.
In addition, all water must be removed from the coating before parts are exposed to freezing temperatures. Failure to do so may result in a loss of adhesion, as the remaining water will expand upon freezing.
Changing from solvent-borne to waterborne coatings usually requires some modifications to existing equipment and operating procedures, particularly in the area of surface preparation. But, for many finishers seeking to reduce VOC emissions and decrease finishing costs, waterborne coatings are the best choice.
Waterborne coatings are just one of several coating options available today. Other options include high-solids, 100% solids, UV-curable and powder coatings. Each coating system has its advantages and disadvantages. A thorough comparison of each coating system is necessary to make a sound decision. Certainly, given the greatly improved performance characteristics of waterborne coatings available today, making the switch to this type of coating system is well worth considering.