Relating laboratory and outdoor exposure of coatingsIII. Effect of relative humidity on moisture-enhanced photolysis of acrylic-melamine coatings

Abstract

The effects of relative humidity ranging from ≈0 to 90% on the moisture-enhanced photolysis (MEP) of a partially-methylated melamine acrylic polymer coating exposed to the ultraviolet (UV)/50 °C condition were investigated. The UV source was two 1000 W xenon arc solar simulators, which provided radiation with wavelengths from 270 to 800 nm. Five different relative humidity (RH) levels were supplied by dry air/moist air mixture humidity generators. Degradation of coating films approximately 10 μm thick applied to CaF 2 substrates exposed to different UV/RH conditions were measured by FTIR transmission spectroscopy using an auto-sampling device. The total degradation under UV at a particular RH consists of four modes: reactions during post curing, dark hydrolysis at a particular RH, photolysis, and MEP. Experiments were designed so that changes in MEP with RH were measured. Both the rates and magnitudes of the MEP increased with increasing RH. The MEP rates of the acrylic-melamine crosslink loss and coating oxidation increased during the early exposure stage then leveled off, but the MEP cleavage rates of the acrylic polymer segment were nearly constant with time. The two-stage degradation of melamine-acrylic structure is attributed to the heterogeneous microstructure of the coating, and the enhanced degradation is explained by a mechanism based on hydrolysis-generated formaldehyde molecules, which act as chromophores to absorb UV light and accelerate photo-oxidation.