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Relating Laboratory and Outdoor Exposure of Coatings III. Effect of Relative Humidity on Moisture-Enhanced Photolysis of Acrylic-Melamine Coatings

Published

Author(s)

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

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 ultraviolet (UV) radiation at 50 oC 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 mm on CaF2 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 photo-oxidation.
Citation
Polymer Degradation and Stability
Volume
77
Issue
No. 1

Keywords

Acrylic Melamine, building technology, coating, degradation, mechanism, moisture, moisture-enhanced photolysis, photodegradation relative humidity, UV

Citation

Nguyen, T. , Martin, J. , Byrd, E. and Embree, E. (2002), Relating Laboratory and Outdoor Exposure of Coatings III. Effect of Relative Humidity on Moisture-Enhanced Photolysis of Acrylic-Melamine Coatings, Polymer Degradation and Stability (Accessed May 20, 2024)

Issues

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Created January 1, 2002, Updated February 19, 2017