Highly pure curing agent from tomato waste for bio-based anti-corrosion epoxy coatings

Tomato peels have been proposed as a promising biowaste for producing bio-derived anti-corrosion polymer coatings potentially more sustainable than traditional petrol-based ones. Nonetheless, there is no proof that the cutin monomer, i.e. 10,16-dihydroxy hexadecanoic acid (10,16-diHHDA) largely found in tomato peel, can be used as a curing agent, due to a lack of sufficiently pure samples of this fatty acid. In this work, the cutin monomer was firstly successfully isolated by up-cycling tomato-dried peels with a purity averaging 92 % and a 67 % recovery rate. 10,16-diHHDA was then used as a trifunctional reactive component to develop fully bio-based epoxy coatings formulated with three different epoxidized precursors. Using a phloroglucinol epoxidized precursor, an increase in the ratio between epoxy groups and hydrogen-active nucleophiles increased the glass transition temperature, up to an average of 104 °C, along with enhanced hardness and adhesion to steel and aluminum alloy surfaces. Additionally, all the cutin-derived coatings exhibited higher hydrophobicity than a reference bisphenol-A-containing coating. The most promising bio-based resin formulations were finally used to coat carbon steel samples and the protection ability against corrosion in a 3.5 wt% NaCl solution was assessed through a multi-step electrochemical testing protocol to investigate continuum phenomena at both medium|coating and coating|metal interfaces. As a result, two resin compositions (namely PC_4:1_5 and PC_5:1_3, differing for the amounts of the cutin monomer and catalyst) were identified as the most protective coatings in terms of both instant and long-term performance. Pore resistance, initially ≥ 100 GΩ cm2 on average, double that of the reference petrol-based coating, decreased to a still well-satisfactory 1 GΩ cm2 value even after one week of accelerated aging test. Within the same time frame (i.e., 7 times longer than the recommended period by the ASTM industrial standard test) the water uptake was finally set at <5 % values without any delamination.