Chemically-resistant epoxy novolac coatings: Effects of size-fractionated technical Kraft lignin particles as a structure-reinforcing component

Abstract

To provide protection against corrosion in harsh environments, high performance anticorrosive coatings are applied on steel structures at all scales. However, to also limit the use of fossil-based ingredients, there is a growing demand to incorporate renewable raw materials in the coating formulations.In this study, to replace pigments and fillers of an epoxy novolac coating, technical Kraft lignin particles were ground and size fractionated (i.e., sieved), and used for formulation work. The effects of sieved and unsieved Kraft lignin, as structure-reinforcing components, on the anticorrosive and mechanical performance of epoxy coatings were subsequently investigated using the following methods: size exclusion chromatography (SEC), phosphorous nuclear magnetic resonance spectroscopy (31P NMR), scanning electron microscopy (SEM), differential scanning calorimeter (DSC), salt spray exposure, pull-off, König pendulum hardness, and chemical resistance tests.Compared to the unsieved-lignin reference (U-L EN), the coating based on lignin fines (S-L EN) showed about 31 % lower rust creep after 70 days of salt spray exposure. However, no surface defects or chemical degradation were observed for any of the coatings.For the S-L EN coating, excellent adhesion strength (23 MPa) and impact resistance (0.49 N), relative to reference values of 17 and 13 MPa and 0.41 and 0.07 N for commercial and lignin-based diglycidyl ether bisphenol F (L-DGEBF) coatings, respectively, were measured.The addition of lignin particles did not influence the chemical resistance, the hardness, and the glass transition temperature of the epoxy novolac coatings.In summary, chemically unmodified Kraft lignin particles, after grinding and sieving, can be incorporated in epoxy novolac coatings (up to 25 vol%), thereby providing a bio-based alternative to pigments and fillers in heavy duty coatings (primers in particular).