Paint It Green: Alternatives to Solvent-Based Painting Michael Heaney I. INTRODUCTION A myriad of technologies are available today that are significantly greener, safer, and less costly than conventional solvent-based spray painting. There is no single greenest painting technology. Because of the high performance demands for surface coatings in terms of appearance, protection and other properties, the best technology depends on the specific application. Organic paints for original equipment manufacturing (OEM) products, the focus of this paper, account for 30% of the paints and coatings market. OEM paints are distinguished from architectural and maintenance paints by the fact that they are applied at high production volumes and are often thermally cured. Painting is a multi-step process. To gain optimal corrosion protection and adhesion, paints are usually applied as primer and one or more topcoat layers. This paper focuses primarily on paint application and curing. The cleaning and/or pretreatment steps that precede painting are also critical part of overall surface coating application with their own potentially significant potential environmental impacts. However, this paper does not address these surface preparation steps. II. EMISSIONS FROM CONVENTIONAL SOLVENT-BASED PAINTING Solvent-based painting generates several hazardous wastes including: l volatile organic compound (VOC) air emissions, l paint sludges, l waste solvents, l spent air filters, l off-spec or left-over paint, particularly with multi-component paints, and l paint stripping wastes from hangers and other equipment. Solvent-based painting is one of the largest sources of hazardous wastes. Although the amount of hazardous painting waste is difficult to quantify because it is generated across 38 many industrial sectors, painting results in more hazardous waste than any other process in several major industrial categories including automobile manufacturing. VOC emissions from spray painting and are a precursor for 6% of photochemical smog.’ Typical coating solvents include methyl ethyl ketone, methyl isobutyl ketone, toluene, and xylene. These solvents are regulated as Hazardous Air Pollutants (HAPS) under Title III of the Clean Air Act Amendments of 1990. Facilities emitting more 10 tons per year of any one of these solvents will be subject to much anticipated industry- specific Maximum Achievable Control Technology (MACT) standards. Many painting MACT standards, which may include many of the painting technologies discussed in this paper, are scheduled to be issued in draft form in 1999 with start-up deadlines occurring around 2004. Existing non-compliance with ozone standards and the new more stringent ozone standards recently announced for 2010 provide additional regulatory pressure to reduce VOC emissions from painting. In the past, only 30 to 40% of all paint used was deposited on the product. The remainder, called overspray, was removed by ventilation equipment. The mass fraction of paint solids deposited on the product divided by the total paint solids applied is called transfer efficiency. Transfer efficiencies as low as 15% are not uncommon even among high-tech robotic systems.* Improving transfer efficiency is one of the simplest and most effective strategies to minimize pollution from painting. Benefits associated with improving transfer efficiency include:3 l Reduced VOC emissions, l Reduced hazardous waste, l Less frequent cleaning, l Reduced control equipment residuals (filters, etc.), and l Reduced costs. The effect of transfer efficiency on VOC emissions is shown in Figure 1 .4 Operator technique can have a significant impact on transfer efficiency. Spray technique training is almost always a worthwhile investment even for experienced operators. Transfer efficiency 39 Figure 1 can also be improved by relatively minor equipment changes. The transfer efficiencies of various spray-application alternatives are:5 l Air-atomized spray 30 to 40% l Air-electrostatic spray 60 to 70% l Airless and air-assisted electrostatic spray 70 to 75% l Airless Electrostatic Spraying 70 to 80% l Airless Spraying 50 to 60% l High Volume Low Pressure (HVLP) 70 to 90% Although tests have shown that HVLP guns to be generally more efficient, this may not be true in every case. Only application testing can determine conclusively which gun works best for a given paint rheology and substrate. The basis for HVLP’s higher transfer efficiency is less reflection of paint particles at the low pressure (0.5 - 10 psig). What has made HVLP sprayers especially popular since their introduction in the mid- 1980’s is that they require no additional capital purchases other than the guns themselves which cost less than $1000. 40 The solid wastes generated by solvent-based painting are often classified as hazardous, based on either their heavy metal or solvent content. Solvent-based paints also pose worker health and flammability hazards. Because of these risks, spray-booths and curing areas require large volumes of make-up air to bring the atmosphere below the OSHA Permissible Exposure Limit (PEL) and the lower flammability limit. These ventilation requirements carry a high energy cost. Overspray in most spray paint booths is controlled with simple air filters. Often these filters must be disposed of as hazardous waste, based on their levels of leachable heavy metals. All paints contain heavy metals either as additives or through process contamination. Lead is typically the heavy metal of greatest concern. Paints may contain as much as 600 parts per million (ppm) of lead and still be called “lead-free” 6 The hazardous waste threshold for leachable lead is only 5 milligrams per liter (5 ppm) as measured by the Toxic Characteristic Leaching Procedure (TCLP). Approximately 20% of existing paint booths control overspray with a water-wall instead of air filters. Water-wall booths can handle higher application rates. The water, which functions as a wet-scrubber, often must be disposed of a hazardous waste. Water-walls and air filters control only particulate emissions. Only 20% of all painting operations have VOC emissions controls.7 The regulatory limits for “compliant coatings” in most industrial sectors is often the Reasonably Available Control Technology (RACT) limit of 3.5 pounds VOC per gallon. Whereas older paint formulations may have contained 39 to 40% solids, high solids coatings, a loosely-used term, typically contain more than 52% solids. Higher solids coatings require special sprayers because of their high viscosity. Because less solvent is used in the paint, less is available to wet metallic surfaces that are contaminated with oil; therefore, surface cleaning is more critical. The average solids content of commercial high solids paint dropped roughly 1% per year during the late 1980’s. This illustrates the rapid technical development that is ongoing in paint chemistry and in surface cleaning. As MACT standards replace IUCT, many facilities above the 10 ton per year threshold will be driven from high solids coatings to non-solvent based paints. 41 III. WATER-BASED PAINTING Water-based paints have water as the primary solvent, but most also contain on the order of 1.0 pounds per gallon of VOC, often glycol ether compounds, as a co- solvent. The ratio of water to VOC is typically 4:1. Paints are always regulated on the basis of VOC content less water. On this basis, the VOC contents are typically below 2.0 pounds per gallonand some are as low as 1.5 pounds per gallon. Very recently a few zero VOC paints have become commercially available, but their performance has not been established and is probably not suitable for industrial applications. Almost all types of resins can be formulated as water-based including vinyls, two- component acrylics, epoxies, polyesters, carboxyl-terminated alkyls, and urethanes. Water-based paints generally can use the same spraying equipment as the solvent-based paints they replace, although generally, low-pressure guns produce fewer air-bubbles in the finished coating. Drying times are often longer for water-based paints; however this is not always the case, especially for architectural coatings.* To maintain production capacity with water-based paints it may be necessary to install drying ovens. Humidity control is more critical when using water-based chemistry. The absence of dehumidifiers. can eliminate water-based paints from consideration in some situations. Water-based coatings pose much less flammability risk and do not require special flammable storage rooms. It is important to note, despite popular misconceptions, that lead levels in water- based paints are typically as high as in modem solvent-based paints. Frequently paint booth filters for water-based coatings can contain enough leachable lead to be classified as hazardous waste. The likelihood of a filter containing high lead levels depends on the length of time in service, the transfer efficiency, and, of course, the contents of the paint. If water-based paint clean-up water is drained to a septic tank where sediments accumulate, the tank contents may also be classified as hazardous waste. 42 IV. POWDER COATING Powder coating technology uses dry, finely-ground thermoplastic or thermoset resins and pigments. The powder can be applied by one of two methods: an electrostatic spray gun or a fluidized bed tank. The coating is applied at a temperature between 325 and 400°F depending on the type of powder coating selected. Powder coatings are usually applied in a single coat. The thickness