Tuning the molecular diffusion in microcapsules for a sustained release of antifoulants

Abstract

Marine biofouling is a serious economic and environmental concern as fouling organisms on ship hulls cause pollution by increasing the friction drag and forcing a higher fuel consumption. Fouling can be avoided temporarily by dispersing biocides in marine paints. A long-term protection, however, is difficult to achieve as the small molecular size of the biocides renders a fast diffusional leakage through the coating matrix. A promising approach to sustain the release, and thus prolonging its functional lifetime, is microencapsulation of the biocides. In this project the biocidal compound tralopyril has been encapsulated into polymeric microcapsules through a solvent evaporation method. Several microcapsule systems have been assessed by their loading capacities and release characteristics, through a screening based on morphology and capsule constituents. The material selection was limited to biodegradable capsule constituents to be applicable in a real marine environment. Further, the most promising capsules have been incorporated in a marine paint system, where their performance has been evaluated relative to commercial paint formulations with freely dispersed biocide. It was found that a core-shell morphology was not suited for encapsulation of tralopyril, due to a disadvantageous solubility preference. Encapsulation in monolithic microspheres was therefore superior and could be optimized by selecting a polymer with a suitable chemical composition and molecular weight. Lactic-based polymers performed significantly better than polyhydroxybutyrate, with pure polylactic acid (PLA) being superior in terms of encapsulation efficiency and lower diffusivity. Increasing the molecular weight led to a more sustained release, but potentially also a lower loading capacity. Incorporation of PLA spheres in a marine paint was successful from a formulation and release perspective. Coatings containing encapsulated biocide had a significantly more sustained release compared to coatings with freely dispersed biocide. Thus, by tuning the molecular diffusion in microcapsules an overall sustained release from antifouling coatings can be achieved.