Citation: Samyn, P.; Lembrechts, W.; Cosemans, P.; De Vriese, L. Mechanical Performance of Protective Epoxy Coatings with Bio-Based Ingredients for Flax–Fiber Composites. Eng. Proc. 2023, 56, 39. https://doi.org/10.3390/ ASEC2023-15327 Academic Editor: Ana Paula Betencourt Martins Amaro Published: 26 October 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Proceeding Paper Mechanical Performance of Protective Epoxy Coatings with Bio-Based Ingredients for Flax–Fiber Composites † Pieter Samyn * , Wannes Lembrechts, Patrick Cosemans and Linde De Vriese SIRRIS, Department Innovations in Circular Economy, Gaston Geenslaan 8, B-3001 Leuven, Belgium; wannes.lembrechts@sirris.be (W.L.); patrick.cosemans@sirris.be (P.C.); linde.devriese@sirris.be (L.D.V.) * Correspondence: pieter.samyn@sirris.be † Presented at the 4th International Electronic Conference on Applied Sciences, 27 October–10 November 2023; Available online: https://asec2023.sciforum.net/. Abstract: Due to its long and continuous cellulose fibers, flax offers excellent specific tensile strength and stiffness relative to other natural fibers such sisal or jute, and it is widely used as fiber re- inforcement in composites with relevance in industries such as automotive, sports and maritime environments. However, the use of natural fibers poses additional challenges relative to synthetic fibers in ensuring the functional lifetime of composites; in particular, water resistance and resistance against UV conditions should be improved for outdoor use. Therefore, a protective coating that offers high resistance against environmental conditions and mechanical damage can be applied to avoid direct surface exposure of natural fibers. The linseed oil or wax coatings increase the hydrophobic surface properties and limit water ingress, but they have drawbacks such as extended curing periods via oxidative crosslinking and weak mechanical performance. In seeking alternatives for natural fiber composites, the potential of bio-based crosslinked coatings to enhance mechanical robustness, surface protection and durability was explored by screening various coating grades, including bio-based epoxy resin, diluents and crosslinkers. The epoxy coatings with a bio-based phenalkamine crosslinker offer higher hardness and scratch resistance, and the water resistance was improved in the presence of an amine crosslinker with long alkyl chains. In parallel, the mechanical abrasion resistance of the crosslinked coatings significantly increased in relation to the intrinsic mechanical properties and crosslinking density of the coatings. The processing of the epoxy coatings was further enhanced by adding a bio-based trifunctional diluent with low viscosity while providing limited shrinkage and good compatibility with the composite substrate. Moreover, the UV resistance was better for epoxy coatings with a bio-based diluent, likely via migration effects and the formation of a protective layer at the outer surface. Keywords: composites; lifetime; protection; coating; water resistance; mechanical resistance 1. Introduction Natural fiber composites can offer a sustainable replacement for common composite materials, where naturally sourced fibers from annual plants or agricultural residues may replace traditional reinforcement fibers such as glass or carbon fibers. Typical composite applications are products that require high structural efficiency and long-time reliability, e.g., those used in the aerospace, energy, construction and sports industries. The fibers determine the material strength and stiffness, and the polymer matrix dominates the surface properties such as roughness and outdoor resistance. The production of glass and carbon fiber is very energy-intensive; in addition, it is a challenge for traditional composites to preserve the material value during the end-of-life [1]. Alternatively, the long flax fibers that are specifically grown and harvested from the linseed plant are preferred as they present good mechanical properties combined with high availability compared to other natural fibers such as hemp or jute. Furthermore, flax fibers show similar or even higher specific Eng. Proc. 2023, 56, 39. https://doi.org/10.3390/ASEC2023-15327 https://www.mdpi.com/journal/engproc