Synergistic advancements in sustainable coating technologies : polyurethane microencapsulation of soybean oil for autonomous self-healing and anticorrosive paints

Abstract

The objective of this thesis was to develop and characterize a sustainable, self- healing polyurethane (PU) coating containing microencapsulated soybean oil for improved anti-corrosion performance applicable particularly in automotive topcoats. Soyabean oil microcapsules were synthesized with phases of 100 µm to 7000 µm based on the interfacial polymerization and particle size was confirmed via scanning electron microscopy (SEM). Microencapsulation technique was successfully able to generate spherical microcapsules however the particle size was difficult to control. The largest of the microcapsules exceeded the required final dry film thickness of >250 µm, resulting in an uneven coating appearance. This made it visually unappealing and compromised the need for a smooth finish, which is critical in automotive applications. The self-healing capacity was tested by introducing microcapsules into PU-based paint at the content of 3, 5 and 7% wt of base pigmented PU composite on steel substrate. Optical microscopy assessments of X-shaped scratches at 1 and 7 days revealed that healing efficacy enhanced with higher concentration of microcapsules while excessive dosages caused surface roughness. The self-healing function and the surface properties of the coating were at odds with each other, showing that these properties need to be optimally dosed for high-finish applications. Corrosion resistance was assessed in a Neutral Salt Spray (NSS) test (ISO 9227) for 72, 168 and 240 hours, showing that increasing the dosage of microcapsules resulted in a significant decrease in the formation of rust and improved the corrosion resistance. Cross-hatch cross cut tape adhesion testing demonstrated consistent adhesion performance between all samples regardless of content, showing that incorporation of microcapsules does not appear to negatively impact adhesion. The results from this investigation highlight the utility of microencapsulated soybean oil for sustainable mitigation in PU coatings, which could lead to future self-healing automotive topcoats with potential corrosion protection properties. The results pave way for future efforts to focus on controlling the particle size and optimizing loading, which shall target simultaneous improvements in functional properties as well as aesthetic quality of protective coatings