Converting from solvent-borne coatings helps a Colorado store fixtures manufacturer lower its emissions and provide high-quality finishes on its products.
Editors Note: This article is excerpted from “Case Studies: Low-VOC/HAP Wood Furniture Coatings,” published by Julian Jones of the U.S. Environmental Protection Agency, Air Pollution and Prevention Control Division, Research Triangle Park, NC, in conjunction with Amy Marshall and Jennifer Fields of the Midwest Research Institute, Cary, NC.
Converting to high-solids coatings typically involves reformulating sealers or topcoats and generally is an easy transition for a company to make. When a coating system has a higher solids content, the amount of volatiles released as the coating cures is decreased, resulting in a direct reduction in facility emissions.
Because sealers and topcoats can account for up to 65 percent of finishing emissions, the potential emissions reduction can be significant. Traditional solvent-borne nitrocellulose sealers and topcoats have an average solids content of less than 20 percent, while high-solids sealers and topcoats can contain from 30 to 50 percent solids.
Reformulated high-solids coating systems generally are catalyzed conversion systems. In a catalyzed conversion system, the coating is cured partially through a polymerization reaction that creates a more durable and chemical-resistant coating. However, high-solids coatings are similar to traditional coatings because they are still solvent-borne, so the application method and coating behavior during application do not change significantly. This allows operators to easily adjust to the new coating system.
Other advantages of high-solids coatings include reduced solvent waste and better coverage because of the higher solids content. One application of a high-solids coating can place twice the amount of solids on an item using less solvent.
This increased coverage can lead to cost savings if the cost per gallon of coating did not increase substantially with the reformulation. However, the increase in solids content also results in an increase in viscosity, so adjustments to application equipment may be required. Some facilities heat the coating before application to reduce its viscosity.
The potential VOC/HAP emissions reductions associated with the use of high-solids coatings are not as great as the reductions that can be achieved using other low-VOC/HAP coating technologies. The coatings are still solvent-borne and a significant amount of solvent evaporates as the coating dries.
From a pollution prevention perspective, high-solids coatings are an improvement over traditional solvent-borne coatings, but other technologies can provide a finish of equivalent quality with a greater VOC/HAP emissions reduction.
Case Study: Design Fabricators
The plant operates five days a week with two shifts. The number of hours worked and number of employees vary seasonally, but averages close to 200 employees on two 8-hour shifts. Approximately 25 of those employees are in the finishing department.
In October 1994, Design Fabricators moved into its current location. Prior to the move, the local community had raised concerns over emissions and the odor of the solvent-borne coatings. The company was interested in pursuing new finishing techniques and lowering emissions, but they found that information about low-VOC/HAP coatings was not readily available.
By November 1995, alternative coatings were under serious consideration and were being tested for quality, durability, and cost effectiveness. The new coating system was fully implemented in early 1996.
Manufacturing & Coating Operations
The components are then taken directly to either the assembly area or the finishing department. Depending on the product, it may be finished before or after assembly. In the assembly area, there are specified areas for different jobs. Products are assembled and sanded by hand.
In the finishing department, the product is taken into one of the two spray booths and placed on hangers (the larger items are rolled in on trolleys). The hangers are moved manually throughout the finishing area, with the operators taking care not to touch the product until the finish is cured.
Most of the coatings are applied using air-assisted airless guns, although a small number of custom jobs require conventional spray guns. The coatings are pumped from 55-gallon drums in the paint kitchen directly to the spray guns in the booths. The only exception is the catalyzed conversion varnish, which is mixed in five-gallon batches and put into smaller pumps located in the spray booth.
The stain is applied first and is hand wiped for some products. The product is then sprayed with a sealer and sanded. Finally, a topcoat is applied and the product is allowed to air dry. After the finish has cured, the product is packaged and shipped to the customer.
Converting to a High-Solids, Low-HAP Finishing System
First, waterborne coatings were used on some of the smaller orders, but they caused several problems. The biggest problem was grain raise. When a waterborne product is applied to wood, especially the softer species, the grain of the wood absorbs the water and stands up, or raises. Grain raise results in a rougher finish that lacks the smoothness that is typically achieved using solvent-borne coatings.
In an attempt to rectify this problem and smooth the grain, additional sanding was required. However, the operators often sanded through the seal coat. This would cause the grain to raise again when the topcoat was applied because the wood was re-exposed to a waterborne coating.
Another problem encountered with the waterborne coatings was drying time. To prevent the parts from sticking together, waterborne products generally require a much longer drying time before they can be stacked or shipped. This problem can be solved by adding drying ovens to speed the curing process. However, additional equipment would not only be expensive, but would also require more space than the facility can devote to finishing.
A third problem Design Fabricators encountered with its waterborne coatings was a softer finish that was not as durable. Because of these difficulties, waterborne coatings were not chosen for the new system.
Design Fabricators next considered UV-cured coatings. Ultraviolet-cured coatings have the low-VOC/HAP advantage of waterborne products while producing a durable, high-quality finish. Grain raise is avoided because UV-cured coatings can have up to 100 percent solids and no water. The very-high solids content prevents the VOC and HAP emissions associated with traditional solvent-borne coatings.
The curing time also is very short, only a few seconds. However, UV-cured coatings are most feasible for flatline finishing, making UV finishing impractical for Design Fabricators because of the wide variety of shaped pieces that they finish.
The final coating system tested by Design Fabricators included a high-solids catalyzed conversion varnish and low-HAP sealers and stains. The VOC and HAP content of these coatings is still low due to the high solids content, and the problems experienced with the waterborne coatings tested by the facility were avoided because the coating is acetone-based.
The acetone-based coatings are applied using spray guns, allowing easy finishing of shaped pieces. The main problem with acetone-based finishes is that they tend to dry too quickly. However, drying time may be adjusted by adding other solvents. Acetone also is very flammable and fire risks are an important issue.
However, because of the high-quality finish and compatibility with the existing finishing line, the high-solids, low-VOC/HAP system was selected. The new topcoat has around 40 percent solids, where the old topcoat had about 18 percent solids.
Gradually, the old precatalyzed topcoats are being phased out and replaced with the high-solids catalyzed conversion varnishes. Catalyzed finishes have a higher solids content and result in a more durable finish. The harder finish is achieved because the coating is not only dried, but is cured by a polymerization reaction controlled by the amount of catalyst in the coating.
The transition to high-solids coatings was fairly smooth for Design Fabricators. There was a learning-curve period of six to eight months during which the operators became familiar with the new coatings and different coatings combinations were tried to achieve the best finish possible. Because the new system is compatible with the original solvent-borne system, the operators were able to make the minor adjustment rapidly.
The coating process did not undergo much change when the coatings were changed. The new coatings are applied manually using spray guns, as were the old coatings.
Looking at the Costs
According to data provided by the facility, the switch to high-solids coatings resulted in a considerable decrease in the annual VOC and HAP emissions for the plant. The coatings changes also served to address the local community’s concerns about the facility’s emissions.
The new coatings typically average around one pound of VOC per pound of solids. In 1995, before beginning the switch to the lower-emitting coatings, around 44 tons of VOCs were emitted. After the complete conversion to the new system, only about 36 tons of VOCs were emitted over a 12-month period.
Although the difference seems small, the company’s sales increased during that two year period, from $10.1 million in 1995 to $13.7 million in 1997. Design Fabricators was able to increase production and still lower their total mass emissions and their emissions per dollar of sales.
The reduction in HAP emissions was even greater. In 1995, approximately 20 tons of HAPs were emitted. By 1998, HAP emissions were almost eliminated, while production nearly doubled. The new coatings contain from 0.04 to 0.46 pound HAPs per pound of solids. In addition, the glues, cleaning solvent, and stain base contain no HAPs.
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