Sometimes, a processor cannot solve production problems on his own. One source of troubleshooting assistance is the coating or equipment supplier's research and development center. Using design of experiments software, one R&D center was able to optimize a processor's curing cycle and eliminate finish quality concerns.

Table 1. These are the factors and levels chosen by Akzo Nobel researchers to simulate the client's paint finishing production line.

The harsh environment that specialty outdoor equipment is exposed to presents challenges for the product finisher. Subject to wind, rain, sun, snow and salt exposure, the coating must remain durable and attractive. Any small defect in the paint finish when applied will quickly degrade the product's appearance (and cause customer dissatisfaction), so the coating must be free of defects.

When one manufacturer of heavy duty specialty equipment began experiencing finish problems, the company was determined to quickly identify and eliminate any and all sources. It turned to the Research and Development Center at Akzo Nobel Coatings Inc., Louisville, KY. There, Akzo researchers were able to construct a series of experiments to determine the cause of the defects -- mainly, solvent popping -- and the best solution.

The first step was to determine the optimum factors for increasing paint film thickness without having solvent popping occur.

"Our client asked us to identify a production line process in which the painted panels exited the ovens hard and cured, without any film blistering or softness," Steve Hazelwood, a researcher at Akzo Nobel, explained.



Figure 1. At high convection oven temperatures, i.e., 200oF (93oC), infrared curing temperature was not significant because the majority of popping already had occurred in the convection oven.

Using design of experiments (DOE) methodology, Hazelwood and other researchers performed a five-factor, two-level, 16-experiment design. The five key factors that affect paint finish quality first were identified (table 1); then, high and low levels for each of these factors were determined.

"We evaluated the five key factors and the ten, two-factor interactions by taking an average of the eight high and low responses for each of the 15 factors and their interactions," Hazelwood said.

Once the factors and levels were identified, a series of experiments were performed.

"The experimental design and analysis work was simplified using Design-Ease DOE software from Stat-Ease Inc. in Minneapolis," Hazelwood added. "The software allowed us to consider all factors simultaneously by revealing how interconnected factors responded over our range of values."

The designed experiment simulated the customer's paint curing ovens to determine the best settings for reducing solvent pop. The solvent pop response was measured as the film thickness at which popping began. The desired outcome was a high, tack-free film build without any popping.

"In particular, we needed to pay careful attention to the grueling factors encountered during the hottest part of summer, when solvent pop is most common -- especially in facilities that are not climate controlled, like our client's," Hazelwood said.

As film thickness increases, solvent popping is more likely to occur in hot environments, so it is important to carefully control the ovens.

"This coating was being applied to heavy-duty specialty outdoor equipment, so the paint finish must endure extremely harsh conditions, year after year, in all types of soil, weather and temperatures," Hazelwood noted. "A high quality, defect-free finish was imperative."



Figure 2. Contour lines show film thickness at varying oven temperatures. Akzo Nobel recommended a reduced convection oven temperature that produces properly cured film while simultaneously reducing solvent pop.

Manipulating Factors To Attain an Ideal Response

The research objective was to vary and test important factors for solvent pop. Hazelwood and other Akzo researchers began by preparing eight, 1 x 4" test panels, which were run through the laboratory's simulation paint line. The simulated coating and curing involved a multi-step process:

  • With airless sprayer, apply 1 to 1.5 micron dry

    primer on each of the eight panels.

  • Flash all panels for 6 min. Flashing is an air drying

    technique that dissipates some, but not all, of the solvent.

  • Apply two coats of paint to each of the eight panels.

  • Remove one panel from the simulation.

  • Apply one coat of paint to each of the remaining panels.

  • Continue removing one panel and applying a coat of paint to each of the remaining panels until the panels contain between two (low film thickness) and nine coats (high film thickness) of top coat.

  • Flash all panels for 6 min.

  • Cure all panels in a convection oven for 12 min.

  • Cure all panels in an infrared oven for 12 min.


Figure 3. This cube plot generated by DOE software shows a winnging combination of factors for Akzo's customer.

The DOE software's analysis showed that the most significant factors causing solvent pop were convection and infrared oven temperatures (figure 1). Although there was significant interaction between the oven types, the convection oven was a bigger factor than the infrared (figure 2).

"The software allowed us to identity the factors leading to solvent pop," Hazelwood said. "By combining the correct oven temperatures with proper accelerator levels and activator types, we were able to consistently minimize solvent pop with acceptable film hardness and cure."

The results of the DOE software analysis recommended the following settings to minimize solvent pop (figure 3):

  • Low convection heat (Factor A-).

  • Low infrared heat (Factor B-).

  • High catalyst or accelerator.

  • Activator B.

"By simulating our client's curing oven factors in a laboratory DOE, we optimized the settings that reduce solvent pop," Hazelwood concluded. "Our client can now expect fewer application problems and higher production efficiencies."



For more information from Akzo Nobel Coatings Inc., Louisville, Ky., call (502) 254-0470 or visit www.akzonobel.com. For more information from Stat-Ease Inc., Minneapolis, Minn., call (612) 378-9449 or visit www.statease.com.

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