Influence of pigments on phototransformation of biocides in paints

 
Biocides are commonly applied to construction materials such as facade renders and paints in order to protect them from microbial spoilage. These renders and paints are exposed to weathering conditions, e.g., sunlight and rain. Thus, biocides might be affected by sunlight already on the material´s surface. Often, biocides do not interact directly with the sunlight, but other components of the materials do and then transfer the energy to the biocides, which then might transform (so-called indirect phototransformation). Pigments are interacting intensively with the spectrum of the incoming light; thus, an effect of paint pigments on phototransformation rates and reaction pathways of the biocides was hypothesized. Accordingly, we investigated the phototransformation of four commonly used biocides (carbendazim, diuron, octylisothiazolinone (OIT) and terbutryn) in four different paint formulations differing solely in pigments (red and black iron oxides, white titanium dioxide, and one pigment-free formulation). Paints surfaces were irradiated under controlled conditions. The results show that biocides degrade most rapidly in the pigment-free formulation. The degradation was considerably slower in the pigment-containing paints. The determination of several phototransformation products of terbutryn and octylisothiazolinone showed that different transformation products where formed dependent on the pigment. In conclusion, pigments reflect the incoming light and thus protect biocides from phototransformation, but additionally, they interact with the phototransformation of biocides.