In Saul’s Gloss Loss Problem, Saul Vance had changed the coupling solvent in the water-reducible alkyd formulation from ethylene glycol butyl ether (EB) to ethylene glycol propyl ether (EP) when he discovered the EB solvent came out of aqueous solution at the elevated cure temperature. Lindsay Doyle explained that EB solvent exhibited a Lower Critical Solution Temperature (LCST) that caused the coating to lose gloss upon cure. Now the EP solvent was demonstrating the same problem. Even though EP solvent didn’t have a LCST, what was the other critical property that caused the loss of gloss?
Saul Vance knew that when applying waterborne coatings, like his water-reducible alkyd, controlling the humidity of the air is critical. The evaporation of water is directly related to the relative humidity of the air. Unlike the evaporation rate of most solvents, which is not affected by the relative humidity, the relative evaporation rate (RER) of water decreases linearly at 25º C from 0.31 RER at 0 relative humidity to 0 RER at 100% relative humidity. The higher the moisture in the air above the wet applied coating, the slower the water comes out of the coating film.