Biofouling on ships causes deleterious effects such as increased drag leading to reduced speed and increased fuel consumption. Controlling biofouling on ships is generally accomplished with biocide-based antifouling (AF) coating systems. As more restrictive environmental regulations are introduced (reduce or eliminate need for cuprous oxide) and as more rigorous service life demands emerge (extend drydocking intervals), the need for a next-generation long-life environmentally friendly coating system increases. Sustained and long-term biocide release is critical to effective AF coating performance. Microencapsulation of biocides results in increased biocide loading capacity in coatings as well as reduced and controlled biocide release rates.

The objective of this project was to develop a better understanding of how microencapsulation of biocides could be used in AF coatings. Specific objectives were to:

1) Identify and redefine key performance parameters of microencapsulated 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) and coatings containing those capsules to ensure focus on capsule/coating combinations suitable for effective performance as well as commercial viability.

2) Understand biocide loss over time under field exposure conditions and use these data to validate effective field performance.

3) Understand how modifications to capsule properties affect biocide release rate from capsules, and then from capsule-containing coatings.

4) Demonstrate sustained, low, and constant controlled release over longer periods of time with laboratory release rate analysis of capsules in AF coatings.

Typical microcapsules. Diameters from 5-45 microns. (L) Capsule in solvent slurry. (R) Optimized dried powder of microcapsules. No drying agent required; few agglomerates.

This project demonstrated that microcapsules with DCOIT cores, that meet all of the identified key performance parameters, can be produced with at least two wall chemistries. Coatings containing microencapsulated biocide retain more DCOIT over long periods of time than coatings that are formulated with free DCOIT. This is true in both the laboratory and in the field. Microencapsulated DCOIT, when incorporated into commercially relevant AF coating systems, enhances overall coating performance. DCOIT release rates into seawater and xylene can be controlled through microencapsulation. Both laboratory release rate studies and field biocide loss studies produce predictable results based on capsule properties and coating formulation. Microencapsulation allows high levels of DCOIT to be loaded into coatings without negatively impacting liquid or cured coating properties.

Controlled release technology in the form of microencapsulation has the potential to fill the performance gap that currently exists between the current and next generation of AF coating systems for the Department of Defense. Benefits of microencapsulation of biocides include (1) control of the rate of biocide release, resulting in highly predictable, steady-state rates of release; (2) assurance that more biocide remains in the coating over a longer period of time, resulting in extended utility and more effective performance; and (3) reduction in the rate of release of cuprous oxide. This project improved understanding of how capsule formulation modifications impact diffusion and the mechanics of diffusion/release of biocide from capsules and from capsules in coatings.