Why Humidity Affects Paint Drying

Question:

I manage the paint line at our plant. We fabricate custom-made equipment cabinets. We started using an automotive-grade enamel. In the summer, we don’t have any problems with drying. In the winter, we are getting long drying times and are getting runs and sags. The paint supplier suggests 60% relative humidity or higher. In the winter, we are well below that. Please tell me why humidity affects our application of our solvent-based paint. Can you explain this? Thanks! D. W.

Answer:

You are probably using a moisture-cure coating which requires, in your case, an ambient relative humidity (R.H.) of at least 60%. Moisture in the air will react with the curing agent in the coating vehicle causing its resin to cross-link. From what you told me, you must be using an automotive-grade, moisture-cure polyurethane. Moisture-cure polyurethanes use isocyanate groups that react with moisture in the air, allowing them to cure the resin. As the vehicle’s resin cross links, its viscosity increases reducing its tendency to run and sag. Without moisture, the vehicle would take longer to cure causing a slower drying time for the paint. In the winter, you could increase the R.H. in the spray booth inlet air by humidification. Although this is an extra step, it will eliminate the runs and sags you now get during the lower R.H. Winter conditions