Polyurethane Coatings on Wood

Q. I would like to add to your response in “Curing Polyurethanes on Wood” in your March column.

W.J. and his specialty woodworking company need to control the moisture content of their wood before coating. High moisture content will interfere with the reactions between resins and catalyst. I have seen cases where the paint never cured because too much of the isocyanate is taken up by moisture in the wood that was caused by high humidity in storage. They need to control storage temperatures and humidity.

Unless they are going to cure for extended periods of time, which is a waste of money, they only need to cure to the point that the material is tack-free and will no longer pick up dust. The remaining curing (as much as 90 percent of the total cure) will take place after heating in the oven. The coating forms the best film this way. The exception to this rule is if the business has no storage space and then has to force dry for a longer period in order to accommodate rapid shipping of product.

Most acrylic-based systems dry more quickly but can become brittle over time. Polyesters have higher impact resistance but typically need a longer time to become tack-free. W.J. will want to use a system that is a blend of resins, perhaps including levels of epoxy less than 5 percent to avoid chalking when it is exposed to prolonged direct sunlight, for better abrasion resistance. This is especially true for the railings.

Depending on the system selected and the amount of heat used, only 20-40 percent of the cure will take place in the first day. The remainder will take place over the next one to three weeks, depending on storage temperature, accelerators selected and isocyanate selected by the supplier. 

A good rule of thumb to pass along is for every 10°C you maintain the temperature (above 20°C), you basically double the reaction rate. That means one hour at 80°C equals two hours at 70°C, four hours at 60°C, eight hours at 50°C, etc. But at 10°C or lower, the reaction is stopped. If they really want to control their processes, the most important thing to control is the temperature. That includes nights, weekends, etc. It is okay to turn the temperature down, as long as it is always turned down. I had issues with a major U.S. manufacturer who turned the heat off over the weekends. Units were damaged in shipping about 1/4 of the time, but okay the rest of the time. It turned out that almost all the damaged units were painted on Friday and shipped Monday or Tuesday. Parts painted any other day were fine. B.R.

A. Thank you for your comments, B.R., and for mentioning the “10° rule,” the well-known time–temperature relationship for curing coatings. Your explanation should help W.J. and others solve their coating problems on wood and other substrates.