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THE RETURN OF WATERBORNE BASECOATS
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Figure 1–Paint makers are focusing on developing waterborne basecoats to meet stricter VOC limits.
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April 22, 2002 -The interest in waterborne materials is increasing again in the refinishing industry due mainly to the spread of regulations that limit the amount of volatile organic compounds (VOCs) allowed in refinish coatings. Regulatory agencies at all levels continue to review and adjust allowable VOC levels. States and provinces that were not regulated before are now being regulated, or will be regulated soon. Paint makers continue to improve their products to comply with these regulations, and waterborne coatings are a key for meeting low levels of allowable VOCs. The two coatings that get most of the attention for formulating into waterbornes are primer-surfacers and basecoats. This article will focus on the resurgence in developing waterborne basecoats (see Figure 1). There's not much interest so far in developing waterborne clearcoats, since the durability of nonsolvent-borne coatings against environmental fallout and the ultraviolet rays of the sun is questionable. So the solution has been that if the VOC levels in the basecoat can be reduced substantially, the clearcoat can remain solvent-borne and the combined topcoats will still be within the required VOC limits.
Waterborne basecoats are not new. OEMs have been applying waterborne basecoats for over 20 years. Though some paint makers have been selling waterborne basecoats successfully for over 5 years, the experience of most paint makers has been less than successful. That's been mostly due to low demand, but also because of application concerns. Some early forms of waterborne basecoats sprayed out with a pebbly look, and unless the material was sprayed wet, the basecoat would not lay out, creating severe orange peel with successive coats. Newer forms of this material still have an orange peel appearance when first applied, but the effect goes away.
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Figure 2–Spot repairs require portable venturi air blowers for drying.
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Some concerns with waterborne basecoats still exist with the newer materials. Spraying equipment that doesn't corrode is required. Waterborne coatings require a very large volume of air for drying. Spraybooths must have an airflow velocity at least 20 meters (70 feet) per minute. Portable blow dryers are required in the spot repair process (see Figure 2). There's also the hazardous waste issue. Waterborne does not mean excess material can be poured down the drain. Waste from this product is considered hazardous, and must be properly disposed. And part of the proper disposal method is not mixing waterborne waste with solvent-borne waste (see Figure 3). Depending on local waste regulations, the collision repair facility may also need to have lined containers. Gross waste may need to be separated from rinse water waste.
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Figure 3–Waterborne waste must be separate from solvent-borne waste.
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In addition to these concerns, there's the matter of applying waterborne basecoats. There's a learning curve required, in both setup and actual application.
Not Pure Waterborne
Waterborne does not mean that water is the only solvent. That's the case with most latex coatings, but waterborne automotive finishes are not latex. Waterborne basecoat does use certain solvents, including some hydrocarbon types, to reduce the resins and pigments. Co-solvent is the blending of these various synthetic and modified liquids with purified water.
Learning Waterborne
For those refinishers not familiar with applying waterborne basecoats, some knowledge and retraining is needed. Shop and environmental conditions may be a determinant in whether a facility can use this type of material. Temperature may also become important. Humidity plays a major role in the drying process. If the atmosphere is full of water, these materials won't be able to expel the water from within, severely slowing the drying process. Heat, either infrared lighting or hot air, greatly assists the dry time. Heated spraybooths provide good drying when the temperature is maintained at 30°C (80°F) or above, both during the spraying and drying operations.
Application
Preparation for applying waterborne basecoat follows the standard process of cleaning the surface with soap and water, wax and grease remover, and sanding with P600-P1000 sandpaper. The surface is re-cleaned after sanding, then the area is completely dried, final wiping with a tack rag.
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Figure 4–The inner passages of the spray gun must be corrosion-resistant, like this stainless steel design.
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Spray gun fluid tips range from 1.0–1.3 mm. The spray gun must have stainless steel or plastic fluid passages (see Figure 4). Spray tests have shown that the gravity-feed style works best and is easy to clean. Spraying waterborne basecoat requires the best from the technician. Gun settings must be correct to provide an even pattern, top to bottom, and equal and even atomization throughout the pattern. Spraying technique must apply even coats, sprayed at 90° angles, constant 50% overlap, and even constant speed.
For spot repairs, apply 3–4 light coats until hiding is achieved. Use portable air blowers between coats. Blending can be done following the paint maker’s process for reducing air pressure or further reducing the material. Larger panels, sections, or complete vehicle refinishing can be sprayed with medium coats. Accelerated drying may not be required due to the size of the area being refinished. Metallic colors will normally require a final mist coat to even out the metallic effect. Be careful to not overwet the surface while mist coating. This step is done only to control metallic position and orientation.
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Figure 5–This orange peel appearance soon lays down as the basecoat begins to dry.
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When first applied, waterborne basecoats tend to have a severe orange peel appearance (see Figure 5). This is normal and will disappear in a very short time. The flash time between coats can be as short as 1–2 minutes, based on drying conditions. Flash time before applying the clearcoat under normal conditions will be at least 15 minutes. These recoating times will also be indicated by the surface having a matte finish, and being dry to the touch. There usually isn't a specific type of clearcoat required for use over waterborne basecoat.
Equipment Cleaning
Spray equipment must be thoroughly cleaned immediately after each use. Following the paint maker’s recommendation, only use water to clean, and follow with isopropyl alcohol if recommended. Do NOT use solvent, as it will set the waterborne material and make it difficult to remove. After cleaning completely, dry the spray gun, attach it to an air supply, and spray until all residue is gone.
Other Tips
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Figure 6–Mix waterborne basecoat only by stirring, not by agitating in a paint shaker.
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Never mechanically agitate waterborne materials. Paint shakers will cause the material to capture micro-size air bubbles in the material. Hand stir only (see Figure 6). Use nylon strainers. Paper strainers use water-based glue that separates when contacted with waterborne products. Paper mixing containers will do the same. Use plastic mixing containers. Protect waterborne materials from freezing. Most paint makers recommend temperatures no lower than 4°C (40°F).
Airflow, temperature, and humidity all play a major role in the process of drying these products. Selection and use of these materials may increase the speed of repair due to the fast dry time.
Conclusion
Waterborne basecoats are making a comeback in the refinishing industry, but just how much of a comeback will depend on how strict VOC regulations become, and how much the VOC regulations spread into states and provinces that are currently only required to meet national rule requirements. Some areas are already being forced to get used to the process, and are being targeted as testing beds for the new wave of waterborne basecoats.
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