Modelling biocide release based on coating properties

Growth of micro-organisms on coated substrates is a common problem, since it reduces the performance of materials, in terms of durability as well as aesthetics. In order to prevent microbial growth biocides are frequently added to coatings. Unfortunately, early release of these biocides reduces the biocidal protection of these coatings. Furthermore, since biocides leach to the environment, their release rate is a key parameter determining the environmental impact of biocides. As a result of legislation, the use of biocides and the corresponding concentrations is restricted. Understanding how coating and biocide properties determine biocide release rate is crucial to understand durability, and enable new product development and evaluation of environmental impact. The present study aims to establish this connection via modelling of the release process. The proposed model is presented with an initial validation. The release is viewed as a combination of water diffusion, dissolution of the embedded biocide and release from the material. The key parameters necessary for the model are bulk parameters determined from experiments. The resulting model shows that release can be proportional to water exposure time or to its square root, depending on the dominating processes in a certain regime. In application conditions high variabilities exist in terms of materials and conditions, therefore the effect of environmental conditions is investigated by a statistical approach. The results show that the variability in the time of protection (the time during which biocides are still present in the coating) is high, due to the variabilities in the weather conditions. A variation of 1 year (95% interval) was seen on an average time of protection of 1.5 years. The model can be used to assess new approaches towards material development and helps underpinning product performance claims.