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Effects of Relative Humidity on Moisture-Enhanced Photolysis of Acrylic Melamine Coatings: A Quantitative Study



Tinh Nguyen, Jonathan W. Martin, E Byrd, N Embree


The effects of relative humidity ranging from [approximately equal to] 0 % to 90 % on the moisture-enhanced photolysis (MEP) of a partially-methylated melamine acrylic polymer coating exposed to the ultraviolet (UV)/50 degrees C condition have been investigated. The UV source was two 1000 W xenon arc solar simulators, which provided radiation from 270 nm to 800 nm, and five different relative humidity (RH) levels were supplied by dry air/moist air mixture humidity generators. Chain scission, oxidation, and side chain cleavage of films approximately 10 {M}m on CaF^d2 substrates exposed to different UV/RH conditions were measured by FTIT transmission spectroscopy using an auto-sampling device. The total degradation under UV at a particular RH consists of four modes: hydrolysis 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 for the acrylic-melamine chemical structure increased at early exposure stage then level off, but the cleavage rate of the acrylic polymer segment was 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 photooxidation.
American Chemical Society Meeting


acrylic melamine, building technology, coating, degradation, moisture, moisture-enhanced photolysis, photodegradation relative humidity, UV


Nguyen, T. , Martin, J. , Byrd, E. and Embree, N. (2001), Effects of Relative Humidity on Moisture-Enhanced Photolysis of Acrylic Melamine Coatings: A Quantitative Study, American Chemical Society Meeting (Accessed April 23, 2024)
Created April 1, 2001, Updated February 19, 2017