The Indiana Clean Manufacturing Technology and Safe Materials Institute,
2655 Yeager Road, Suite 103, West Lafayette, Indiana 47906-1337
NORTHERN INDIANA PLASTICS COMPANY - P021
POLLUTION PREVENTION PROJECT
The Indiana Clean Manufacturing Technology and Safe Materials Institute (CMTI) initiated a project with a northern Indiana company to investigate the application of water-based coatings to their products as an alternative to their traditionally-used, VOC-based coating.
The company produces an innovative product line of urethane foam-based products, (mantels, entrance columns, etc.,) and markets them to the wood finial industry. The innovative product is manufactured from urethane rather than the traditional wood material, and the product does not split or rot. The company employs approximately sixty people.
The company produces the urethane finial designs from a master piece (finial design) around which a room temperature, vulcanized (RTV), silicone rubber is poured. When the flexible rubber cures, it is separated from the master. The interior (or cavity of the RTV rubber mold) is a reverse image of the master’s shape. The rubber mold is then reinforced in wood and/or metal frames, which impart dimensional stability to the otherwise flexible mold. The interior cavity of the mold is then spray painted. The paint serves two functions: (1) to act as a mold release agent, which protects the mold, and (2) to act as an ultra-violet light protection layer, which protects the actual urethane finial from the degradational effects of ultra-violet light.
When the paint is dry (usually in five minutes), the mold is made ready to accept the urethane pour. The mold is exposed to thirty to sixty seconds of infra-red heat, which warms the surface of the mold’s interior cavity. The mold is then positioned under a urethane injection nozzle. The polyol and isocyanate components, which make up the urethane, are mixed internally (within the injection nozzle) just prior to their deposition onto the mold’s interior surface. The urethane mixture begins to foam and rise within the mold’s interior. At this point, a cover is placed over the mold and it is held down tightly under external pressure. The urethane continues to rise within the closed mold and fills the mold’s annular space. After approximately ten to twenty minutes (depending on mold size), the pressure exerted on the mold is released, and the mold is separated from the urethane component. The urethane is a reverse image of the mold and is an identical replica of the original master finial design. The new urethane part is trimmed of excess urethane (flash) and, usually, receives a spray paint touch-up, prior to packaging for shipment.
Avoidance of the new, federal EPA Title V Air Permit program is the key environmental issue which motivates the company to pursue VOC emission reductions and water-based coating (paint) adoption. This program is mandatory for major source companies. The program involves numerous regulations and regulatory liability if all of the permit rules and requirements are not followed.
The company and CMTI formed a team to investigate three, key manufacturing issues which the company felt might lead to reduced use of hazardous chemicals and increased yield from raw material input. Two of the manufacturing issues were related to the painting process. The first involved investigating the suitability of water-based coatings as alternatives to the traditionally-used, fast drying, VOC-based coating. The second issue entailed the investigation of more efficient coating application methods so that the mold’s interior cavity would receive a uniform coating over 100% of its surface (protecting the rubber mold from attack by the urethane). The third issue involved analyzing the urethane mold production process in order to optimize production flow and staging.
To approach the first issue, the Institute/company team, in cooperation with Purdue University’s Coating Applications and Research Laboratory (CARL), an Institute-sponsored laboratory, developed a testing program to investigate the potential of replacing the company’s VOC-based coating system with a water-based coating system. Concurrent with this project phase, the company addressed the "second issue" mentioned above (more efficient coating application methods). It investigated a high-volume, low-pressure (HVLP) spray coating system, which ultimately replaced its conventional spray coating system. The company found that the new HVLP system improved the transfer rate of paint to the mold and achieved a more uniform coverage across the part's entire surface.
The company, its paint vendor, CMTI, and CARL worked closely to analyze the coating production rates and processes so that the CARL tests would accurately replicate the company production operations. It was agreed that a worst-case scenario testing process should be designed—the samples tested would be painted with a thicker coat than usual and coated under high humidity conditions and dried with lower than normal velocity air flow.
Tests were performed in a special chamber at Purdue University’s CARL, where temperature and humidity were carefully controlled. The test’s desired temperature and relative humidity were 68° F and 75-80%, respectively. Primarily, testing was performed on two, water-based paint candidates using multiple samples at three elevated drying temperatures (115° F, 135° F, and 150° F) at a 3 mil (wet) paint application setting and using a paint application draw bar (see Figure 1). One sample for each elevated temperature and paint-type, using a 10 mil draw bar, was also tested. Finally, one sample for each paint-type (at a 3 mil draw bar setting) using ambient air for drying was also tested, as a control.
In all cases, the paint was applied to sample plaques (plates), which were precut from .0045" aluminum sheet into 6 5/8" x 4 3/4" rectangles.
Sample dryness was monitored using an encased laboratory scale with 0.001 gm. sensitivity. When the sample's weight no longer decreased, the sample was deemed dry. The heated air flow was directed through a flared duct at an average rate of 159.7 ft./min., relatively uniformly over the 22-square inch facial opening of the duct. The air from this heater unit was directed perpendicularly onto the painted surfaces of each sample plate from a distance of 1¼ inches (see Figure 1).
The laboratory-generated data clearly demonstrated that convection-heat ovens are required in order for the company to switch to water-based coatings and maintain current production as well as projected, increased production rates.
Both paint candidates appeared, in general, to have similar drying rates. Overall, the drying temperature data substantiate that which one might expect—the higher the temperature, the faster the drying time. The test showed that even the lowest elevated drying temperature of 115° F dried both water-based paint candidates approximately ten times faster than the ambient drying condition.
Two specimens of the water-based candidates were prepared by depositing the paint via an air spray system (HVLP) outside of the environmental chamber in the laboratory's spray booth. The wet film thickness of each specimen was measured at 6 mils (using a wet film thickness gauge). The actual, back calculated, wet film thickness averaged 6.71 mils. These specimens were then placed in a large convection oven which operated at a temperature of 137° F. The oven's convective air velocity is two to three times that of the air flow of the oven used in all the other tests. Both specimens achieved a 100% dryness state in less than five minutes (approximately 4.75 minutes). This result, demonstrates that the higher velocity air flow in the large oven greatly accelerates drying. This substantiates that increasing the velocity in production ovens above the test parameter of 160 ft./min will significantly reduce dry time in a production setting.
Thus, the sprayed specimen data supported the fact that the draw bar deposition of paint and the high humidity condition of the environmental chamber did, indeed, test parameters on a worst-case, most conservative basis.
The specimens that had 10 mils-wet paint deposition and were dried at the three temperature settings of 115° F, 135° F, and 155° F also displayed expected results—higher drying temperature exposure yielded faster drying times. There was a significant difference in drying time between the extreme temperature exposure of 155° F and 115° F for the 10 mil wet samples. The samples dried to nearly 100% dry in eight minutes (at 155° F), while the same product sample run (at 115° F) attained nearly 100% dry in eleven minutes. Thus, the 115° F dry time for the 10 mil wet samples dried 38% slower than the 155° F dry time. It is important to note, however, that at the higher drying temperatures of 155° F and 135° F, the 10 mil wet samples (both candidates) demonstrated dry paint film cracks, while the lower 115° F did not promote the same dry film crack formation. It is believed that the higher drying temperature creates a "skin" over the top of wet paint film. This skin later cracks as the wet paint evaporates and breaks through the skin to achieve cure. The lower drying temperature of 115° F seems to match the natural rate of the solvents' (water and co-solvents') diffusion through the paint matrix and, thus, little-to-no skin is created. However, the no-cracking advantage of the lower 115° F drying temperature is gained at the cost of increased dry time (33% longer than the 155° F drying temperature).
The 10 mil wet film thickness was run in order to observe the effects of "pooled" paint as might occur in the corners and crevices of actual molds. It is possible that some corners and recesses could receive thick applications of paint (10 mils wet). The thick wet paint test was designed to observe the manner in which thick paint applications would affect dry time and quality under the various drying temperatures.
The drying data generated at the laboratory tend to suggest that wet film thickness as high as 6 mils can be satisfactorily dried in under five minutes at temperatures approximating 135° F when exposed to convective hot air two to three times the 160 ft./min. rate. Drying rates of approximately five minutes certainly meet the production rate required by the company. The data also suggest that 10 mils of wet coating (pooling) may take eight to ten minutes of dry time and "skinning" may occur, with potential dry-crack production in the paint film. Therefore, care should be taken in selecting drying temperature and air velocity. This also indicates that special spray head wands, which allow the coating to be spray-applied to recessed areas, may be required to prevent wet film thickness pooling. There are many such spray attachments on the market which can be purchased to prevent this occurrence.
The last phase of the project, involving the analysis of production process and flow, was addressed by Purdue University's Technical Assistance Program (TAP). TAP and company personnel used the CMTI/CARL water-based coating drying test data, in conjunction with company-generated process rate data, to develop drying oven specifications and process flow plans.
POTENTIAL ENVIRONMENTAL AND COST BENEFITS
The project found that water-based coating systems could be employed by the company. The new paint system could meet the production rate requirements if drying ovens were designed and properly sized to current, and estimated, future production requirements.
It was estimated that adoption of the water-based coating in lieu of the traditional VOC-based coating could reduce VOCs by nearly ten tons per year. The water-based coating was composed of a slightly higher solids content than the VOC-based coating system. Therefore, the water-based system would require fewer gallons to coat an equivalent surface area than would the VOC-based coating system. This usage reduction, in turn, would yield an estimated material savings of $7,000, per year. A one-time saving of $8,000 was estimated as cost saving due to Title V permit avoidance. The cost of the drying oven, estimated from the drying specifications, was $35,000 to $50,000. Costs to restructure the material and production flow of the plant were estimated at approximately $7,000 to $10,000.
There was no appreciable change in hazardous waste disposal costs, and other management costs were not expected to change. Thus, the project's attractiveness would have to stand or fall on the estimated $7,000, per year, raw material cost saving.
The company has placed the project on hold. The project's capital demands result in a payback slightly over six years. The new drying technology and plant restructuring are responsible for the high costs. The payback period is beyond the company's acceptable guidelines.
However, as the company's business grows and material usage increases, the project's viability will be enhanced. The company, due to the extensive testing and work at CMTI/CARL, stands poised to make the change to water-based systems as soon as increased volume supports the investment.