(180p) Accelerated Aging of Hull Coatings for Ocean Energy Devices

Biological fouling on hulls has hindered the performance of seagoing vessels for ages. Fouling organisms reduce vessel speed, increase fuel consumption and damage ship hulls through secretion of harmful chemicals. Throughout the last century, inorganic biocides such as Cu₂O and tributyl-tin (TBT) were incorporated into paint to prevent biofouling. Incorporation of these inorganic biocides came at a significant cost to aquatic environments.  Now, TBT is banned in many places around the world, and Cu₂O may soon face similar regulations. Discussion with marine energy development firms has indicated a strong desire for coatings with operational lifetimes longer than 10 years, which exceeds the time obtainable even with the best current coatings technologies. Many new biocides and surface treatments have been proposed to provide less environmentally harmful and longer life solutions to the problem. The current method of testing new coatings requires applying the new surface treatment to a raft or small section of a ship and then waiting to observe the effectiveness. Since it would be impractical to test the effectiveness of novel coatings over such long periods in real time, one of the goals of the current project is to provide an accelerated validation of surface treatment efficacy.

Experimental studies are carried out in the Yaquina Bay, Newport, OR to determine if biofouling will grow on painted test coupons. Fouling is observed within two weeks on biocide-free polyurethane painted, neat PVC, and neat fiberglass substrates. Two month residence time of samples in the bay with no biological growth is considered validation of biofouling resistance. The limiting factor in useful life of biocide paints is anticipated to be biocide flux, which is a function of the biocide concentration profile. Accelerated aging is necessary to match the concentration profile expected in the future, as opposed to simply matching the surface concentration with a diluted concentration of biocide in a film. Accelerated aging is achieved by exposing painted test coupons to high temperature flow conditions in the lab. The temperature dependence of solubility and mass diffusivity are accounted for to identify the exposure time in hot seawater needed to produce a sample which would match one exposed to ocean conditions for a predetermined length of time. Leaching experiments were conducted with 3,4-dichloroaniline dissolved in polyurethane to validate the accelerated leaching temperature dependence assumptions.