Humidity Effect on Painting

Q 1. I run a paint finishing system in the Northeast. We run high solids paint through two electrostatic shrouds, and then through a cure oven. Recently, we have been having problems with the curing of some of our paints. Some seem to cure, while some of them do not. I tend to think that the issue is color specific, but also related to humidity levels in the painting environment. Have you heard of this before? Please let me know your thoughts on my situation. I can be a lot more specific if need be. W.Z.

A 1. Humidity certainly does affect paint drying rates. Simply stated, in periods of high Relative Humidity (RH), the great amount of moisture inhibits solvent evaporation, thereby increasing curing time. I’m not so sure about the effect of differing RH on different colored paints’ curing times. Let me think about that. Do the different colored paints contain the same resins and otherwise different formulations? Let me know and we’ll get to the bottom of this problem.

Q 2. Thank you for your response. As far as I know, the resins remain the same in all of my colors. I am almost positive that humidity plays a role in how these colors come off of my line. I have three or four colors that slowly lose gloss as the humidity rises. It is these same colors that I have noticed a curing issue with. I have not been able to pinpoint exactly when this begins to happen, but I am positive that it does. I also have some colors that do not seem to be affected by the humidity as drastically as others. To make things even more complicated, the colors that seem to be affected are from one vendor. I only seem to have issues with product from one vendor. This is telling me the problem is related to the humidity, but also directly related to one vendor. W.Z.

A 2. The differences in curing times from one vendor to another are probably due to the paint formulation. Despite the fact that both vendors use the same resin types, their basic resin compositions could be different. Furthermore, even if they use the same resin from the same supplier, the other components in the formulation, e.g. pigment type, solvent type, solvent balance, etc. could also affect curing times with respect to ambient RH. An indication that your two vendors’ formulations differ is the loss of gloss you mentioned. Paint films having fast evaporating solvents will blush (lose gloss) in the flash-off zone of a curing line. As you know, evaporation is cooling. As the faster solvents evaporate, they condense moisture in the high RH air which falls as fine droplets onto the paint film affecting a gloss reduction.