Last updated: 5 November 2023

Coatings and textures for non-toxic and non-leaching wear, aging and fouling protection

Fight against fouling is another relevant objective in the project, and two different approaches were explored: surface micro-texturing and antifouling coatings. Samples of surface micro-texture blades prepared by DCU were submerged over 8.5 months at a coastal site in Ireland. Biofilm growth was characterised through stage assessment of the protein and carbohydrate concentrations and the population dynamics of key fouling organisms. Two organisms (Amphora coffeaeformis and Nitzschia ovalis), suitable for a lab-based evaluation of the antifouling performance of candidate biomimetic textures, were selected and grown in the lab.

Four candidate biomimetic micro-textures were defined, and two alternative production methods were also tested. Samples were produced using 3D printing, and laser etching was tested to confirm feasibility. Results demonstrated that production of the desired textures would require laser technology, which was not available at DCU. An external supplier with access to a femtosecond laser was contacted and provided the sample surfaces. Also, high-fidelity turbulent flow simulations were performed to evaluate the effect of candidate micro-textures on local hydrodynamic stresses. A notable increase in mean stresses and the amplitude of fluctuations on the exposed crest planes were verified in all cases. Computational results indicated that an increase in cavity depth or ridge height does increase stresses further. Still, the enhancement was limited to low single-digit percentages. It was unlikely to justify the increased risk of a potential reduction in the blade lift due to a greater surface roughness. The final phase of the study assessed the impact of textures on how micro-organisms settled on the surface and how this in turn affected the adhesion strength. Centrifugal and hydrodynamic tests are outstanding and are required to confirm which candidate micro-textures provide the best antifouling performance.

The second antifouling strategy aimed to develop novel non-leaching antifouling coatings with permanent cavitation resistance through designing and synthesising polymers, which bear different functionalities within their chemical backbone and incorporate functionalised silica nanoparticles into such polymer formulations. Several candidate formulations based on 2K fluorinated polyurethanes were formulated with novel polyisocyanates “component A” (synthesised from a dendritic aliphatic polyester core and either fluorinated or acrylated functionalities) and applied on glass and composite substrates. As a result of their good mechanical properties, glass transition temperature and different hydrophobicity behaviour that can either prevent the biofouling or improve the release of settled biofouling by the water flow, two types of formulations were selected (coating 13 and coating 15). With the aim of increasing the thickness of the FPU topcoat, different consecutive layers were deposited. However, the overcoat time interval should be further studied and extended from 2 hours up to a minimum of 24 hours. Stable candidate polyurethane dispersions with cationic quaternary ammonium salts in the polymer backbone were synthetized. These initially selected 1K cationic PUDs would be applied and characterized on glass and composite coupons.

Silica nanoparticles were synthesised by TECNALIA and incorporated into two different types of polyurethane matrixes: a commercial polyurethane and a synthetic polyurethane prepared by FUNDITEC. The resin formulations were loaded with up to 25 % weight nanoparticles, resulting in more homogeneous coatings. The mechanical properties of the coatings and their biocide activity were measured. While the roughness of coatings increased as the concentration of nanoparticles increased, the water contact angle remained at similar values in all cases. The natural ageing with much fouling was done in the Port of Pasaia facility. Clear differences in fouling adherence between coating with and without nanoparticles were observed; however, fouling adherence did not significantly vary with the concentration of nanoparticles added.