Characterisation and Release Mechanisms of Aerogel-encapsulated Biocides for Antifouling Coatings

Due to stricter regulation of biocide loading in antifouling coatings, and related environmental concerns, there is an urgent need for solutions that can reduce the high loading of biocides in antifouling coatings. In this thesis, an aerogel-based encapsulating technology, developed by the company EnCoat, to improve the utilisation of biocides in antifouling coatings, is investigated. The PhD project focuses on silica aerogel encapsulated copper pyrithione (CuPT) crystals, one of the presently most widely used antifouling biocides. To analyse the inuence on the particle size distribution, morphology and physical properties of the biocide loading, aerogels with dierent CuPT contents were characterised and the  maximum loading was quantied. The results of the characterisation study were subsequently used to describe the CuPT release mechanism with respect to improved biocidal utilisation.

A microscopic examination revealed that a highly porous silica layer, a few hundred nanometres in thickness, encapsulated the elongated CuPT crystals. The CuPT loading of the silica aerogels was inuenced by the particle size, pore-volume, and specic surface area, which are all key parameters that can aect the CuPT release rate. Furthermore, the study proved that, in contrast to hydrophobic CuPT crystals, the aerogelencapsulated CuPT crystals are hydrophilic with a high anity for water in a coating lm. Relative to the case of pure CuPT crystals, hydrophilicity and the physical connement of solid CuPT crystals contributes to a more ecient and controlled release of active CuPT.

To investigate the inuence of aerogel-encapsulated CuPT on coating properties, including rheology, water absorption and hardness, model coatings containing aerogels with varying CuPT-loadings were investigated. When using 2 wt% of rosin saturated aerogels, the CuPT loading level had an insignicant inuence on both the water absorption and the rheological properties of the coating. However, an increase in the CuPT loading of the aerogel from 0 to 80 wt% reduced the coating's pendulum hardness from 61 to 37 s.

To demonstrate the antifouling performance of aerogel-encapsulated CuPT in rosin/acrylic coating systems with low biocidal content, coated panels were statically exposed under real-life environmental conditions at the CoaST Maritime Test Centre. During the growth season, the panels were exposed for seven months and assessed bi-weekly with a fouling resistance rating according to guidance from the European Chemicals Agency. When compared to a reference coating with non-encapsulated CuPT, results show that a coatingcomprised of silica aerogel-encapsulated CuPT provides a 17% higher antifouling resistance.The eects of types and concentrations of cuprous oxide (Cu₂O) on the performance of aerogel-containing coatings were also studied. When using a surface-treated Cu₂O in combination with 2 wt% CuPT-loaded aerogels in a coating, a high performance with a Cu₂O content of only 5 wt% was obtained. Also, the panel exposure testing demonstrated that the level of CuPT loading did not inuence the overall antifouling performance. This suggests that a coating with highly CuPT-loaded aerogels will have an extended service life.

Lastly, accelerated dynamic testing on a laboratory rotor setup was performed. For examination of the inuence of CuPT-loaded aerogels in combination with dierent pigments, including red iron oxide (Fe₂O₃) and surface-treated Cu₂O, self-polishing coating systems were formulated. In comparison with a reference coating, the coating containing aerogels provided a slight reduction in the polishing rate. In summary, the investigations have claried the visual appearance and physical properties of aerogel encapsulated CuPT with dierent loadings, thereby establishing the basis for understanding the CuPT release mechanisms. Investigation of coating properties provided insights into the formulation of coatings with silica aerogel-encapsulated CuPT, and static panel experiments proved the benet of encapsulation, as well as the ability to reduce the total biocidal content of an antifouling coating. Moreover, the dynamic study showed that aerogels have a limited inuence on the polishing rate of a self-polishing antifouling coating system.