Weathering of wood coated with semi-clear coating: Study of interactions between photo and biodegradation

In order to clarify the relationship between photodegradation and biological degradation, wood specimens finished with a semi-clear coating were degraded under a xenon lamp for different periods of time. The specimens were then inoculated one of two black stain fungi (Aureobasidium pullulans and Epicoccum nigrum). Colonization was monitored visually and by colorimetric analyses. The extent and the nature of the degradation were evaluated using different techniques. The chemical composition was studied by Fourier Transform Infrared Spectroscopy (FTIR) and the physical changes by both microscopic analyses and adhesion tests. The results obtained from FTIR did not provide relevant information. Coating thickness and adhesion were found to decrease as photodegradation increased. Microscope observations detected numerous bubbles trapped in the coating films. These bubbles were found to become holes following photodegradation and the decrease in coating thickness. It was established that more severe photodegradation led to more extensive colonization of the specimens. The fungi seemed to use transpressorium to go through the protective layer and take advantage of organic matter present at the wood/coating interface. Funguscolonization was also found to decrease the coating adhesion in the early stages of the exposure process.