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.