Estimation of organic biocide leaching rate using a modified cavity jump diffusion model

Estimation of biocide lifetime in marine antifouling coatings is of great use to improve and develop technologies. An existing model simulating the diffusion of molecules in polymer networks below glass transition temperature was employed to estimate leaching. This model was modified to allow for swelling due to water uptake and to permit evaluation of copolymer binders as well as homopolymers. This enabled prediction of biocide diffusion coefficients in polymeric coatings of various binder types, including pMMA, a pMMA/butylacrylate binder containing rosin, and a trityl copolymer, using usnic acid as a 'model' biocide. For comparison with modelling results, coatings fomulated using each binder type were also submitted to static and dynamic seawater immersion. Fluorescence microscopy techniques were used to quantify biocide leaching from these coatings relative to unimmersed coatings. Agreement of the modified diffusion model with experimental data was good for pMMA, reasonable for the rosin-based binder, and poor for the trityl binder. Comparison of predicted and experimental biocide profiles in the binder demonstrated deviation from the expected Fickian mechanism for the pMMA binder, despite the accurate rate prediction. This work demonstrates a first approach to predicting organic biocide diffusion, and highlights the areas for future attention.