Quantification and Comparison of Natural and Accelerated Photodegradation of Alkyd Paint

Abstract

Photodegradation is a major contributor to materials’ weathering resulting in deterioration over time due to changes on the molecular level of the surface of materials exposed to sunlight. In order to test the photostability of a material or its resistance to photodegradation, exposure tests conducted in the expected service environment of the material are required. Furthermore, assessment criteria and measures of degradation need to be determined and quantified. Natural outdoor exposure testing of materials could last over long periods and the time element could represent an obstacle to commercialization and to the rapid advancement of research and development of new materials. As a result, accelerated testing became a common alternative to natural testing without taking into consideration the potential discrepancy in the photodegradation mechanism occurring in accelerated tests compared to natural conditions. Using alkyd paints as a reference material, this work attempts to answer the question of whether accelerated photodegradation tests induce degradation mechanisms identical to natural outdoor exposure tests on the chemical level. In order to answer this question, a photodegradation quantification method is developed using Attenuated Total Reflectance-Fourier Transform Spectroscopy and applied to outdoor aging experiments then extended to accelerated aging experiments using xenon-arc lamps as an artificial light source. The developed quantification method shows consistent results for the natural outdoor testing with a maximum standard deviation value of 0.22%. The comparison of accelerated testing and natural testing results using the developed quantification method indicates that differences in the photodegradation mechanisms of natural and accelerated tests are mainly observed in the rate of degradation of certain characteristic chemical bonds in the alkyd paint’s binder polymer relatively to each other.