polymers Article Composite Materials from Renewable Resources as Sustainable Corrosion Protection Coatings Raluca Sanda Komartin 1,† , Brindusa Balanuca 1,2,† , Madalina Ioana Necolau 2 , Anca Cojocaru 3 and Raluca Stan 1, *   Citation: Komartin, R.S.; Balanuca, B.; Necolau, M.I.; Cojocaru, A.; Stan, R. Composite Materials from Renewable Resources as Sustainable Corrosion Protection Coatings. Polymers 2021, 13, 3792. https:// doi.org/10.3390/polym13213792 Academic Editor: Cristina Cazan Received: 12 October 2021 Accepted: 30 October 2021 Published: 2 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Department of Organic Chemistry “C. Nenitescu”, Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1-7 Polizu Street, 011061 Bucharest, Romania; ralu.komartin@gmail.com (R.S.K.); brindusa.balanuca@yahoo.com (B.B.) 2 Advanced Polymer Materials Group, University Politehnica of Bucharest, 1-7 Polizu Street, 011061 Bucharest, Romania; madalinanecolau@gmail.com 3 Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry, Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1-7 Polizu Street, 011061 Bucharest, Romania; anca.cojocaru@upb.ro * Correspondence: rl_stan2000@yahoo.com † The authors contributed equally to this work. Abstract: Epoxidized linseed oil (ELO) and kraft lignin (LnK) were used to obtain new sustainable composites as corrosion protection layers through a double-curing procedure involving UV radiation and thermal curing to ensure homogeneous distribution of the filler. The crosslinked structures were confirmed by Fourier-transform infrared spectrometry (FTIR), by comparative monitorization of the absorption band at 825 cm −1 , attributed to the stretching vibration of epoxy rings. Ther- mal degradation behavior under N2 gas indicates that the higher LnK content, the better thermal stability of the composites (over 30 ◦ C of Td10% for ELO + 15% LnK), while for the experiment under air-oxidant atmosphere, the lower LnK content (5%) conducted to the more thermo-stable material. Dynamic-mechanic behavior and water affinity of the new materials were also investigated. The increase of the Tg values with the increase of the LnK content (20 ◦ C for the composite with 15% LnK) denote the reinforcement effect of the LnK, while the surface and bulk water affinity were not dramatically affected. All the obtained composites were tested as carbon steel corrosion protection coatings, resulting in significant increase of corrosion inhibition efficiency (IE) of 140–380%, highlighting the great potential of the bio-based ELO-LnK composites as a future perspective for industrial application. Keywords: epoxidized linseed oil; lignin; composites; anti-corrosion coating 1. Introduction Polymer composites are widely used in the fields of engineering due to their perfor- mance and versatility, making them adaptable in different industrial sectors. In recent years, numerous studies have been developed for the progress of the bio-based epoxy derivatives, to face out the use of conventional epoxy resins and to achieve great industrial gains [1,2]. Due to their advantages of natural origin, structure versatility, and worldwide avail- ability, vegetable oils (VO) were considered as high-value raw materials in the synthesis of epoxy derivatives (epoxidized vegetable oil–EVO), but there are several drawbacks related to the EVO-based polymeric matrices (e.g., inadequate thermo-mechanical proper- ties or poor mechanical resistance). Many attempts were conducted to solve these issues through the development of different or sophisticated curing procedures or by using rein- forcing agents or fillers. For example, mixing different types of conventional epoxy resins with EVO, special design of the crosslinking agents or employing modified vegetable oil structures as crosslinking agents, were reported [2,3]. Polymers 2021, 13, 3792. https://doi.org/10.3390/polym13213792 https://www.mdpi.com/journal/polymers