Dr. Sushil Pawar, Tech. Officer 'C', DRDO
Thesis title: Hydrophilic and Hydrophobic Antifouling Coatings for Marine Application
Protecting an environment without compromising with development is the need of an hour. This can be done by making the materials and processing technology greener and greener. The limitation of conventional systems can be overcome by means of a self-stratification concept which provides formation of different functional coatings layers in single coat application. The concept of self stratification was introduced long ago in which phenomena of phase separation process was explained during film formation by a mixture of polymeric incompatible powders. Number of driving forces is responsible such as gravitational forces, interfacial surface tension between different phases and substrate, the layer thickness and solvent evaporation rate on self stratification behavior in paint for Self-stratifying coating formulation in single coat application provides desired functional coating layers after application. Thus stratified coating will offer two principal advantages. First, there will be an economic advantage of applying effectively two coats in one operation. This type of coating will be extremely useful in reducing solvents present in paint formulation and regulation on environmental pollution. Secondly, the bonding between the two layers of the stratified coating shall be stronger than that of separately applied coating system. Consequently, there will be a low risk of intercoat adhesion failure and the need for application of a tie coat will be eliminated. An extensive R & D work on nontoxic approaches, particularly hydrophilic and hydrophobic low-surface-energy Foul Release Coatings (FRCs) have been carried out worldwide by the coating researchers. Foul-release coatings do not necessarily deter the attachment of marine organisms, but allow only a weak bond to form between marine organisms and the surface, so that organisms are released by the movement of ships in water by hydrodynamic forces. Epoxy resin based anticorrosive coatings are well known for imparting excellent adhesion and corrosion resistance property. Owing to its low surface energy, siloxane is a preferred candidate for foul-release coatings. Silicone resin based antifouling coatings are known to impart excellent weathering resistance and low surface energy results in the formation of hydrophobic surfaces. Silicone and epoxy resins are different in chemical nature as well as in their inherent physical properties.
The present thesis describes on hydrophilic and hydrophobic concept on antifouling coating formulations based on photocatalytic nano TiO2 based silicone soya alkyd compositions are prepared and evaluated. Formation of hydrophilic coating’s surface is confirmed by measuring water contact angle under the influence of UV light.
Non-biocidal hydrophobic antifouling coatings based on polydimethyl siloxane-epoxy fouling release coating compositions which impart anticorrosive & fouling release properties in a single coat application. This coating is formulated based on self-stratification concept. During curing the low surface energy polydimethylesiloxane based coating enriches the air interface while the epoxy coating with a higher surface energy preferentially migrates towards the substrate interface. The low surface energy hydrophobic top surface ensures foul release property, while the underneath epoxy coating layer provides anticorrosive properties and also provides excellent adhesion to metallic substrates.
The principal advantages of such coatings over conventional sequentially applied multi layer coatings are higher intercoat adhesion between two layers of resin owing to inter diffusion, reduced VOC emission because of the single coat application and reduced coating duration. Fouling-release coatings (FRC) were developed as an environmentally friendly alternative after the ban of highly toxic
antifouling coatings based on tributyltin. Polydimethylsiloxane (PDMS) has been the most widely used polymer for FRC, and its fouling-inhibition properties have been enhanced by addition of bimodal network of –OH terminated silicone resin system. Examples of this bimodal network include high Mol.wt (22000) and low Mol. wt (1800) –OH terminated silanols. These bimodal blend systems enhance and modify the physicochemical properties of the surface. FRC provide superior fuel savings to the shipping industry on the first stages of immersion compared to other current technologies.