Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Fungal infestation in fire protection coatings in outdoor areas

Discolouration of a façade surface as a result of a microbiological infestation can indicate a severe defect. In the case of wooden façades, microbiological attacks on the substrate itself can lead to its destruction and are therefore far more serious than optical defects. Wooden façade elements therefore require a protective coating, in order to ensure the functionality of the component. However, coating systems on exterior wooden components are exposed to increased infestation through mould and blue-stain fungi. Selective use of biocides enables the avoidance of damage. 

Biocides on the basis of 3-iodo-2-propynylbutylcarbamate (IPBC) are one of the most important classes of biocides for the film protection of water-based systems in Europe. A further wood-specific problem is fire protection. Within the framework of a bachelor thesis, the WKI, in co-operation with the Institute for Microbiology at the TU Braunschweig, examined the influence of a biocide on the basis of IPBC on the fungal infestation in a multi-layered, water-based, intumescent fire protection coating system for wooden façade elements.

The coating system comprises a base coat (dry layer thickness 25µm), an acrylate-based fire-protection coating (dry layer thickness 150µm) and a topcoat. The layer directly in contact with the wood, the base coat, contains the biocide. The biocide concentration was varied. For the topcoat, a solvent-based commercial product with a layer thickness of 30µm and 50µm was applied.

Biocide tests were carried out to examine the fungal growth using various biocide concentrations. Five concentrations were examined. These were, respectively, the maximum (3.5%) and minimum (0.1%) concentrations as suggested by the manufacturer, as well as a medium (1.5%) concentration and two lower concentrations (0.05% and 0.01%). The concentration refers to the supplied product and the entire coating quantity applied. In addition, a biocide-free sample was also tested. For the execution of the tests, spore suspensions from the following four test strains were used: Cladosporium cucumerinum, Altenaria alternata (mould fungus), Aureobasidium pullulans and Sydowia polyspora (blue-stain fungi). A spore suspension with varying biocide proportions was added to the 2% malt extract medium. Following inoculation and incubation of the samples, the fungal growth was evaluated.

The fungal infestation was carried out on the basis of the EN 152 standard. For the substrate, pine sapwood (Pinus sylvestris) was used, and glass was used for the negative control. The dimensions of the wood and glass samples were 90mm x 40mm x 10mm. The coated upper surface of the wood samples and the glass plates were sprayed with a spore suspension mixture. The first mixture consisted of Aureobasidium pullulans and Sydowia polyspora, whilst the second consisted of Altenaria alternata and Cladosporium cucumerinum. Four samples respectively were then subsequently sealed in a sterilised jar. For each variant, eight samples were coated. The jars with the sprayed wood samples were subsequently incubated at 20°C (± 2°C) and after six weeks, the fungal infestation was evaluated under a stereomicroscope.

The water permeability of the coating system in accordance with the standard EN 927-5:2006 was determined by measuring the water absorption (g/m²) of a coated spruce test specimen. For this, the test specimen was subjected to distilled water for 72 hours. The result is the average of six samples.

© Fraunhofer WKI
Fig. 1: Fungal infestation for fungus mixture of Aureobasidium pullulans and Sydowia polyspora.
© Fraunhofer WKI
Fig. 2: Water absorption of the coating system in relation to time.

The results of the biocide tests show that very low biocide concentrations are sufficient for impeding the fungal growth of the test strains Cladosporium cucumerinum, Altenaria alternata, Aureobasidium pullulans and Sydowia polyspora. Irrespective of the concentration of the biocide used, no fungal growth was determined for the samples which contained biocides. Solely the biocide-free malt extract showed uninhibited fungal growth. With the help of these results, it was possible to determine that a low concentration of biocides (0.05% and 0.1%) should be used for the base coat.

The fungal infestation was determined microscopically by counting the colonies per sample. The statistical evaluation of the results of the fungal infestation test was carried out by calculating the average of eight samples with standard deviation and a T-test. Figure 1 shows, as an example, the results of the fungal infestation test for the biocide-containing variants compared to the biocide-free variants for the fungus mixture comprised of Aureobasidium pullulans and Sydowia polyspora.

By applying the T-test to compare the quantities of colonies which grew on the biocide-free samples (with topcoat thicknesses of 30µm and 50µm) with the colony quantities on the samples with 0.05% and 0.1% IPBC and the same topcoat thicknesses, it could be determined that the various concentrations of the biocides have a significant influence on the results. The fungal infestation on the biocide-free samples is higher for both topcoat thicknesses. The biocide IPBC inhibited the fungal growth in both the lower (0.05%) and the higher (0.1%) concentration. For the variant with 0.1% IPBC, the influence of the topcoat thickness is not significant. For the coating system with 0.05% IPBC, the influence of the topcoat thickness is discernible; the thicker the topcoat layer, the lesser the degree of fungal infestation. Interpretation of the results should take into account the fact that the evaluation of the growth was based on the quantity of colonies. As, however, the size of the individual colonies varied, a certain degree of inaccuracy is present.

The water absorption in relation to time is shown in Figure 2. Uncoated spruce samples absorbed the most water. The topcoat thickness had an influence on the water permeability of the coating system. The variants with a topcoat thickness of 50µm show the lowest values. In contrast, the variants with a layer thickness of 30µm absorb more water. The lowest degree of water absorption is shown by the variants with 0.05% IPBC and a topcoat thickness of 50µm. The proportion of biocide had no significant influence on the water permeability of the coating system.

The results of this project prove that the biocides have an influence on fungal growth. When the spores are brought into direct contact with a biocide, e.g. in a liquid culture medium, extremely low concentrations are sufficient in order to impede the fungal growth. Adding the biocide to the base coat resulted in a reduction of the fungal growth, but not in complete prevention. The effectiveness of the biocide was obviously reduced by the coating. In order to achieve better protection from fungal infestation, higher concentrations of the biocides should be applied. In addition, HPLC can be used to define the quantity of biocide which diffuses from the base coat into the topcoat, in order to adjust the concentration accordingly.

A systematic correlation between water absorption and the composition of the base coat was not determined, as they absorb similar quantities of water, depending on the topcoat thickness. There is a correlation between the absorbed quantities of water and the fungal infestation; the higher the water absorption of the coating system, the greater the degree of fungal infestation. Solely the variants with a 50µm topcoat from the coating system are suitable for applications with restricted dimensional stability, as the water absorption here is under the recommended limit stipulated by the standards DIN EN 927-1 and DIN EN 927-2.

Förderung

Internationaler Verein für Technische Holzfragen (iVTH) e.V.

(International Association for Technical Issues Related to Wood))