Microplastics from fouling control coatings

Fouling control coatings are an essential tool for combatting biofouling growth on ship hulls. Recent environmental concerns have been raised regarding the presence of microplastics from fouling control coatings in the marine environment. Several pathways have been suggested for the release of these microplastics, yet the magnitude of each pathway is unclear. Additionally, the release of microplastics from these coatings are
not inherent to their design and not addressed in the working mechanisms of these coatings. This thesis investigates two release pathways, during sailing and during underwater cleaning. These scenarios are simulated under controlled laboratory conditions and the microplastic release is analysed and characterised for each.

First, an exposure method and a microplastic analysis procedure were designed and presented. Exposure to sailing conditions was achieved through a small-scale lab rotor with artificial seawater which enabled practical exposure with small seawater volumes and minimal interference from contaminating particles. Microplastic analysis was performed using a semi-automated quantitative SEM/EDX procedure. This method was able to analyse particles down to 1 μm and evaluate the elemental composition of each measured particle. With these two methods, an brief investigation of a self-polishing antifouling coating showed that the initial state of the coating and its leached layer had a great influence on the microplastic release rates.

Using these methods, a broader investigation was performed where six model coatings and six commercial coatings were exposed for 5 weeks. The influence of formulation was determined through the model coatings and their known compositions. The commercial coatings reflected this influence and gave an overview of the relative release from different coating technologies. In essence, a positive correlation was determined for rosin content and microplastic release rates. This is due to the increased hydrophilicity of rosin, compared to acrylic-based binder components, which promoted swelling from water uptake, thereby lowering the local mechanical strength of the binder and promoting fragmentation at the surface. Overall, the two coatings with the lowest microplastic release rates were a high end self polishing antifouling coating and a fouling release coating.

The microplastic analysis method was further exploited to study microplastic release during in-water cleaning of a commercial antifouling coating. Two cleaning methods were investigated; brush cleaning which serves as the current standard method, and ultrasonic cleaning which is an example of an upcoming contactless method. Through controlled lab-scale simulation of these cleaning procedures, the influence of brush properties and cleaning parameters was shown. Brush cleaning resulted in partial removal of the leached layer from some samples, observed as visible scratches on the coating surface. Ultrasonic cleaning resulted in near-complete leached layer removal if performed with a small probe-coating distance. The volume of microplastics released was directly related to the size of the damage spot from this leached layer removal. Regardless of observable damage, all samples showed varying degrees of microplastic release with gentle ultrasonic cleaning releasing the lowest volume of particles followed by brush cleaning with soft outward-oriented bristles. This highlights the importance of appropriate choice of cleaning methods and parameters to reduce the impact on the coating and the environment.

Furthermore, copper content of the microplastic particles was compared to the overall copper release and it was found that most copper was in the particles. As such, the capture of material released during cleaning could also prove an effective measure of mitigating microplastic release during cleaning. This thesis lays the foundation for measuring and understanding microplastic release from fouling control coatings. Studying microplastic release during long-term high-speed expo-sure is still required to fully assess the magnitude of environmental pollution from fouling control microplastics. Practical improvements to the exposure methods and simplified microplastic analysis methods are required to facilitate this .