Observation and prediction of biocide release with fluorescence techniques and mathematical modelling

Abstract

Antifouling coatings are crucial for protection of vessels’ hulls against marine biofouling. A range of technologies is available, although biocidal coatings - containing toxic or deterrent compounds – still represent a majority of the market. A long-term goal is the development of less environmentally harmful and persistent compounds; one of many potential avenues is that of synthetic analogues of natural products from marine organisms. The development of coatings using natural products has been hampered by poor performance in the field without sufficient work on their leach rates and behaviour. Furthermore, little work has been carried out on the leach rate of traditional organic biocides as used in modern coatings. Prediction of biocide diffusion is crucial to estimation of antifouling efficacy. However, diffusion in glassy polymers is a complex and oft-neglected topic; the chemically and physically changeable environment of the ocean and swelling of the polymer in such a ternary system also increase the complexity of models.

A test matrix of antifouling paint coatings was composed, including polymethylmethacrylate (pMMA), an erodible rosin-based commercial binder and a novel trityl methacrylate/butylacrylate copolymer (pTrMA/BA) as binders. Copper (I) oxide and usnic acid, a natural product biocide of interest, were incorporated into the binders and the coatings were subjected to 10 months of natural immersion and 6 months of accelerated rotor immersion tests (17 knots, 25 °C). A novel application of fluorescence microscopy was developed, allowing quantification of the usnic acid content within the test coatings from both immersion schemes. This fluorescence technique and optical microscopy techniques were applied to these coatings before and after immersion, allowing quantification of the organic biocide and pigment distribution. Existing literature models for diffusion in glassy systems were adapted with a novel method for taking into account the presence of seawater as a diluent, to obtain effective diffusion coefficients for usnic acid. These have been integrated into mathematical models of diffusion to predict biocide lifetime. These data were compared with experimental data for biocide leaching from the long term immersions.

The biocide leached completely from the p(TrMA/BA) binder during rotor testing, compared to 35% from the pMMA binder. For pontoon immersions, 61% of the additive was lost from the pMMA coating, and 53% from the rosin-based binder. An accelerated loss of usnic acid occurred in the surface of the rosin-based binder, due to rosin depletion. In all samples, release of the biocide was inhibited beyond the cuprous oxide front, which was congruent with the leached layer in samples where cuprous oxide release occurred. The erodible binder was the only one which demonstrated synchronous depletion of both additives, and it demonstrated a good resistance to fouling in immersion trials. Results of the mathematical modelling of the biocide diffusion were in good agreement with the observed data in the case of pMMA, highlighting in particular the importance of water uptake with respect to biocide diffusion. However, there was poor agreement in the case of p(TrMA/BA), for which the model under-predicted the release rate by about three orders of magnitude.

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Identifiers

ORCID for J.A. Wharton: orcid.org/0000-0002-3439-017X

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