Anti-fouling coating for marine vessels
Dr Michael Ford MIMMM at Haseltine Lake Kempner explores a patent for an anti-fouling coating on marine vessels.
Marine vessels and engineering components that are regularly submerged in seawater can suffer from biofouling. This typically involves the accumulation of microorganisms, plants, algae and animals, such as barnacles and mussels, on ship hulls and other exposed surfaces.
Biofouling can lead to component degradation and accelerated corrosion. For example, corrosion pitting in metals often starts at barnacle attachment sites. In addition to material damage, biofouling poses a significant problem for vessels because the build-up of organic matter can increase frictional resistance or drag in the water, reducing hydrodynamic performance and increasing fuel consumption.
Techniques used to prevent or reduce marine accumulations include anti-fouling coatings or paints, ultrasonic systems, pulsed laser irradiation and regularly scheduled cleaning operations.
Anti-fouling paints are one of the most commonly used methods for preventing build-up on marine vessels and can be divided into two broad classes – self-polishing paints and fouling-release coatings (FRCs).
Self-polishing paints incorporate a water-soluble or hydrolysable binder material (such as an acrylate binder combined with hydrolysable silyl esters). When exposed to seawater, the paint is gradually eroded by dissolution or hydrolysis of the binder, constantly revealing a fresh surface. Marine organisms attached to the eroded layers of paint are swept away by the mechanical action of the water. Gradual breakdown of the binder material can also provide controlled release of biocidal agents included in the paint.
Unfortunately, however, self-polishing paints are typically only suitable for use on lower-speed vessels (i.e. with speeds up to 30 knots). At high speeds, excessive paint erosion may occur.
Fouling-release coatings, in contrast, are formulated to produce smooth, hydrophobic surfaces that hinder the initial attachment of sea organisms. Suitable materials therefore have low surface tensions and low moduli of elasticity and are commonly based on polysiloxanes.
Such coatings are effective at preventing attachment of macroscopic organisms and are suitable for high-speed vessels. However, micro-organisms such as bacteria and algae are still able to attach themselves to some FRC-coated surfaces. Eventually, these organisms can form slime-like biofilms, which then enable larger marine creatures to attach.
Attempts have therefore been made to reduce biofilm formation on FRCs. Polyether-modified silicone oils have been incorporated to reduce slime and algae attachment. Biocides such as zinc or copper pyrithione can also be effective.
Nevertheless, there is still room for improvement. For example, adding solid biocidal anti-fouling agents requires increased use of paint-thinning solvents, which can release volatile organic compounds (VOCs) to the environment.
Norwegian paint and coating manufacturer Jotun A/S appears to have developed a solution to this problem. In January 2024, Jotun was granted European patent EP4065649B1 - Fouling release coating.
The patent claims protection for a coating comprising: a) a curable polysiloxane-based binder comprising at least 50wt.% polysiloxane parts; b) an anti-fouling agent; and c) 10-30% by dry weight of a non-ionic hydrophilic-modified polysiloxane.
The coating has i) a hydrophilic-lipophilic balance (HLB) of 1-12, or a relative weight of hydrophilic moieties in the range 5-60wt.%, and ii) a number average molecular weight (Mn) of 500-18,000g/mol, or a weight average molecular weight (Mw) of 1,000-50,000g/mol.
Protection is also claimed for a marine structure comprising the FRC composition on at least a part of its outer surface; a process for preparing the FRC; and a kit including a first container of the curable polysiloxane-based binder, the anti-fouling agent and/or the non-ionic hydrophilic-modified polysiloxane and a second container containing a crosslinking agent and/or a curing agent.
According to the patent, the non-ionic hydrophilic-modified polysiloxane is different from the curable polysiloxane-based binder and does not contain chemical groups that react with the binder or crosslinker during curing at relevant temperatures.
Such polysiloxanes are already known as surfactants and emulsifiers. However, the inventors have found that these non-ionic hydrophilic-modified polysiloxanes enhance the dissolution of solid biocides in curable polysiloxane coatings. They also appear to facilitate transport of the biocides to the surface of a coating film, while forming a hydrated layer at a coating-water interphase, which is important for fouling protection performance.
While the compositions described in the patent preferably include at least some solvent, the inventors have been able to control the composition’s viscosity primarily through using non-ionic hydrophilic-modified polysiloxane. This requires less solvent than found in existing products. The quantity of potentially dangerous VOCs released can therefore also be reduced.
The patent shows how the amount, molecular weight and hydrophilicity of the non-ionic hydrophilic modified polysiloxanes can be selected to control both the paint’s viscosity and the biocide leach rate.
