The Importance of Antifouling (Toxic) Paints in Ship Coating Applications

Paint is a material that, after being applied as a thin layer to a surface and drying, protects that surface against external effects such as wear, decay and corrosion. Its composition contains four main components: binder, pigment, solvent and additives. Marine paints are paints whose general purpose is to protect ships from the corrosive and damaging effects of the marine atmosphere. They are used to protect structures located by, on and within the sea. Marine paint applications are not carried out with a single paint. A three-stage coating system is applied.

Marine Coating Systems

Marine coating systems follow the following three stages in order: 1. Metal Primer Coat, 2. Anticorrosive Intermediate Coat, 3. Antifouling (Biocidal) Topcoat. Metal primer coat is used in preparing a suitable surface for other paints to be applied over it. Thanks to the special resins it contains, it demonstrates good adhesion to metal surfaces both physically and chemically. Anticorrosive intermediate coat creates a barrier between seawater and the metal surface. The important factor in these paints is water permeability. Low water permeability is desired. Antifouling (Biocidal) topcoat has the primary function of preventing the settlement of organisms that create fouling on the ship's hull. It will be examined in detail under the heading "Antifouling (Biocidal) Paints".

The Importance of Antifouling (Biocidal) Paint Application on Ship Hulls

The submerged portion of ships is called the hull. Some marine organisms need to attach to a hard surface to continue their life. For this reason, the ship's hull is a highly favorable environment for these organisms. The settlement of plants and animals on the hull is called "fouling". Fouling formation erodes the ship's hull, increasing the friction effect of water in this area. An increase in friction effect means a decrease in ship speed and an increase in fuel consumption. If fouling formation consists of mussels and other hard-shelled organisms, they damage the paint layer, exposing the underlying metal and eventually causing corrosion. If deposits also include vegetative contamination such as algae, a large layer called barnacles, formed by both plant and animal contamination, is observed. As mentioned above, this layer increases the ship's friction resistance, reducing the ship's speed and increasing fuel consumption. For this reason, antifouling paint application in the hull area is very important.

Antifouling (Biocidal) Paints

Biocidal paints are paints that undergo chemical reaction with seawater, dissolve, and thereby provide protection. The resin structure and biocides (agents that prevent the formation of living organisms) contained within are two important factors affecting their performance. The way they work is as follows: The paint layer reacts with seawater and softens. With the friction effect created by the ship's movement, the softened layer dissolves and the biocides inside surface to create a toxic effect. Biocide release must be controlled. If it is rapid, the paint's lifespan shortens; if it is slow, the paint does not work effectively. Factors affecting the biocide release rate can be listed as follows: • pH value of seawater, • Temperature and salinity ratio, • Type of primer and anticorrosive paint, • Thickness of biocidal paint application, • Type of resin used in biocidal paint.

Prohibition of TBT Use in Antifouling (Biocidal) Paints

In the 1950s, tributyltin (TBT) was discovered as a result of research into metal compounds with antifouling properties by van der Kerk and colleagues in the Netherlands. By the mid-1960s, it became the most widely used anti-fouling agent worldwide. Despite being an effective biocide, research has shown that this chemical has a half-life of 1-2 weeks in seawater, but when it accumulates in sediments, its half-life can extend up to 2 years; therefore, TBT levels in organisms near the sea surface were found to be higher than normal. As a result of its toxic effects on beneficial organisms and harmful effects extending to humans, legal regulations were imposed on the use of this chemical. In fact, it can cause numerous disorders, ranging from shell thickening in crustaceans to gender differentiation in some organisms. The IMO (International Maritime Organization) banned TBT, used as the main active ingredient of biocidal paints in Europe, from 1 January 2008 due to its threat to the marine ecosystem. On 17 September 2008, it was banned globally.

The Future of Antifouling (Biocidal) Paints

Technologies expected to stand out in future antifouling paint applications include natural biocides and ultra-low friction surfaces. • Ultra-Low Friction Surfaces: The most important characteristic of this technology is that paints have very low friction coefficients and create paint film layers that are so slippery that organisms cannot attach to them. This technology, which can be achieved with fluoropolymer and silicone-based paints, is currently more costly compared to the biocidal paint production methods used today. However, as it finds wider application in the future, costs will decrease. • Natural Biocides: Sponges, some mussels and crustaceans, some turtle species, in their natural environments manage living organisms that develop on their own surfaces. Through enzymes they secrete, they prevent the metabolism, reproduction and surface adhesion of algae or other marine organisms. However, adapting these natural biocides to paint technology is a time-consuming process.

References

• Fernbach C. Choisir sa peinture antifouling, quelle matrice?. (2019, 29 March). Accessed: 25.04.2020. www.bateaux.com Republic of Turkey Ministry of National Education. Shipbuilding-Surface Painting. (2016). Accessed: 25.04.2020. www.megep.meb.gov.tr Sarıışık S. General Information About Marine Paints. (2012, 28 November). Accessed: 25.04.2020. www.isgfrm.com Çelik İ. Fouling and Antifouling. (2020, 26 January). Accessed: 26.04.2020. www.ceyrekmuhendis.com Karan C. What are Anti Fouling Paints and TBT?. (2019, 11 October). Accessed: 27.04.2020. www.marineinsight.com Wikipedia. Tributyltin. Type. Accessed: 27.04.2020. en.wikipedia. org Duydu Y. Ecotoxicological Problems of Organotin Compounds. (1993, 7 December). Accessed: 27.04.2020. dspace.ankara.edu.tr The TBT Ban. (2007, 20 April). Accessed: 27.04.2020. seas-atrisk.org Masmanacı N. Biocidal Paint Systems From Past to Future. (2011, April-May). Accessed: 27.04.2020. www.teknomarin.com.tr

  Öznur Kurt Product and Service Development Specialist Aleph Trade