Abstract
The current process for coating large metal substrates, such as the decks of naval ships, requires pretreatment, primer, and topcoat steps. Developing a coating that can combine these steps into one would significantly reduce the labor costs and application time, which are significantly more expensive than the coating itself. This research shows the development of a self- stratifying alkyd coating. Alkyd resins are polyesters modified with monobasic fatty acids from renewable seed oils such as tung, linseed, and soybean oil. The major drawbacks of durability, pH sensitivity, and a slow autoxidative curing mechanism will be targeted to engineer a fast curing, environmentally friendly, high performance coating. It is proposed that partial self-stratification is driven by three mechanisms; the semi-incompatibility of the two alkyds as the solvent evaporates, the low surface energy of fluorine modified alkyds, and the propensity for siloxane modified alkyds to hydrolyze with moisture on the substrate surface.
Self-stratifying process through solvent evaporation.
Fluorinated alkyds demonstrate great thermal stability, antifouling protection and resistance to dirt, oil, and water. Tung oil was modified with a fluorine group via a Diels-Alder reaction. Siloxane modified alkyds show excellent adhesion to steel and aluminum through sol-gel chemistry forming covalent bonds at the alkoxysilane group. A long oil linseed oil alkyd was modified using triethoxysilylpropyl isocyanate. Nuclear magnetic Resonance (NMR) confirmed the proposed structures. To reduce the application viscosity, reactive diluents, which crosslink with the polymer matrix upon curing, were added. One diluent, fluorine modified tung oil, showed partial stratification in the film. Finally, the use of toxic cobalt drier was eliminated using a radiation curing in the visible light spectrum to increase the crosslinking after application. It was also shown that the partial self-stratification is limited by the molecular weight of the alkyd and the concentration of fluorinated alkyds. Self-stratification can be observed by elemental analysis over the gradient of the coating through energy dispersive x-ray spectroscopy (EDAX) within a scanning electron microscope (SEM).
