Biobased Carboxylic Acids as Components of Sustainable and High-Performance Coating Systems

Carboxylic acids are widely used as building blocks in several types of coating systems, including as hardeners for epoxy resins. The chemical structure of the acid significantly impacts the coating properties. While petrochemical-derived acids have traditionally dominated the field, the structural diversities of biologically occurring molecules, as well as potential for improved sustainability, are leading to an increased focus on biobased raw materials. Recent advances in microbial engineering are paving the road toward commercial availability of additional suites of poly(carboxylic acid) structures which have not been evaluated in epoxy coating systems. Herein, we select a series of 12 biological di- and tricarboxylic acids whose structures could prove advantageous in coating applications to be evaluated as components for epoxy-acid coatings. We tested several of the selected acids as hardeners for two polyfunctional epoxy resins: biobased epoxidized sucrose soyate (ESS) and petrochemical Araldite MY 721. This was done using a catalyst-free epoxy-acid-solvent system with an equimolar epoxy:COOH ratio. If the biobased acid is water soluble, the use of an ESS-acid system with water as a solvent could give rise to 100% biobased coatings. Kinetic studies carried out using ESS-acid systems demonstrated that under selected thermal curing conditions a complete conversion of carboxylic groups of biobased acids is achieved. Changing the solvent from water to 1,4-dioxane had little effect on the properties of the epoxy-acid formulations. Films made from the developed formulations demonstrated excellent solvent resistance and adhesion to metal substrates. Using isomeric acids (both cis-trans and structural isomers) for cross-linking epoxy resins resulted in highly varying properties in cured coatings. The hardness and flexibility of the cross-linked films can be tuned by changing the chemical structure of the acid and epoxy resin. The results highlight the potential for unexplored biobased compounds to expand the currently accessible property portfolio of acid-derived coatings while also improving sustainability.