The paint industry has long been aware of the problems of microbial attack on painted surfaces. Microbial growth can lead to both aesthetic and physical degradation of the coating or painted surface. In addition to the obvious aesthetic effects of mold, mildew and algae growth, physical deterioration by their enzymes can lead to physical degradation. This degradation can include an increase in porosity of the surface coating or a loss of adhesion to the substrate. Moisture penetration can lead to fungal decay of the underlying wood. Biodegradation is not limited to the surface coating or dry paint films; it can also occur during production and storage of the paint.
Coatings formulators face a number of challenges in the selection of dry film biocides. One of the formulator’s main goals is to select dry film biocides so as to achieve broad-spectrum and long-lasting protection of the paint film. One of the challenges is that relatively few biocide active agents are available, and those actives must simultaneously meet multiple requirements. Evaluation of fungicides and algaecides in the context of a paint formulation biocide program invariably involves outdoor exposure testing. Testing of this type involves a complex mix of variables. This is particularly true when it is desired that the results have relevance for more than one geographical region. In addition to consideration of the properties, microbial spectrum and regulatory status of the biocide active agents, there are procedural variables that must be considered. These include the type of substrate to be coated, the coating application method, the panel orientation and direction, the number of replicates, and the inclusion of appropriate controls. Given the number of variables involved, choices must be made to narrow the scope of the panel study experimental design. As constraints for each variable are selected, the resulting matrix forms the study design. One such selection matrix approach is described in this article.