Fungal Growth on Coated Wood Exposed Outdoors: Influence of Coating Pigmentation, Cardinal Direction, and Inclination of Wood Surfaces

Abstract

Four coating systems were exposed for one year outdoors at 45° south. They consisted of solventborne (alkyd based) and waterborne (acrylic based) systems in both clear and pigmented versions. Fungal growth visually assessed was compared to fungal enumeration, and the influence of exposure time on the main fungal species was studied. Results clearly showed that fungal growth was lower on the pigmented coating systems compared with their pigment-free versions. Although the clear solventborne coating included a higher amount of biocide, it was more susceptible to blue stain than the pigmented version. A new multifaceted exposure rig (MFER) also contributed to the study of fungal growth. It allowed samples to be exposed with nine different exposure directions and angles. Exposure using this MFER has shown that the worst cases (highest area and intensity of blue stain fungi) were for samples with the clear coating system exposed to north 45° and at the top of the MFER (horizontal surfaces). For any cardinal direction, all surfaces inclined at 45° displayed more blue stain fungi than vertical surfaces, due to a higher moisture content of the panels. Depending on the cardinal direction and the orientation, some surfaces were free of visible cracking, but colonized by fungi. It was concluded that the growth of blue stain fungi was not linked with cracking development.

The Effect of Polyacrylate Emulsion Coating with Unmodified and Modified Nano-TiO2 on Weathering Resistance of Untreated and Heat-Treated Wood

“…The results of surface hardness in Figure 4a show that accelerated weathering increased the surface hardness of coated untreated and heat-treated ash wood samples. A number of authors also found an increase in the surface hardness of wood coated with an acrylic water-based coating after weathering [30][31][32]. The obtained increase in the hardness of the coated wood surface under the influence of accelerated weathering can be related to structural changes and crosslinking in coatings caused by UV radiation, which was determined for free films of emulsion coatings [26].…”

Assessing the Bioreceptivity of Biobased Cladding Materials

“…It has previously been reported that the cardinal direction of exposure affects the colour and chemical changes in the wood due to weathering [19,[34][35][36]. Few reports are available on the effects of cardinal direction on fungal growth and/or associated surface discolouration [14,36]. To investigate this, samples exposed in all directions were swabbed and colony forming units (CFUs) were counted (Figure 2).…”

“…In long-term experiments, Aureobasidium sp., a polyextremotolerant fungus [52], has consistently been shown to be the prevalent species on wood-based materials [14,17,36]. Podgorski et al, who performed sampling every 3 months, reported that the diversity of fungal species decreases over time and that it is after 9 months that Aureobasidium sp.…”

“…In most studies where fungal growth on wooden materials was investigated, the earliest time point for fungal infestation evaluation is 3 months after sample exposure [14][15][16] or more [17,18]. We would like to argue that earlier time points deserve more attention as they can provide information about the initial steps of fungal attachment and growth on surfaces [19], which is relevant both for their prevention and promotion.…”

“…In the literature, samples are often exposed in a single cardinal direction [15][16][17][18], yet Podgorski et al, who exposed samples in all four cardinal directions, found they did differ [14]. In real life, cladding materials are exposed in all cardinal directions (on buildings) and the information about their respective performance is of relevance.…”