Citation: Stajcic, I.; Veljkovic, F.; Petrovic, M.; Veliˇ ckovic, S.; Radojevic, V.; Vlahovi´ c, B.; Stajcic, A. Impact- and Thermal-Resistant Epoxy Resin Toughened with Acacia Honey. Polymers 2023, 15, 2261. https:// doi.org/10.3390/polym15102261 Academic Editors: Yun Lu, He Liu and Jingpeng Li Received: 7 April 2023 Revised: 26 April 2023 Accepted: 29 April 2023 Published: 10 May 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/). polymers Article Impact- and Thermal-Resistant Epoxy Resin Toughened with Acacia Honey Ivana Stajcic 1, * , Filip Veljkovic 1 , Milos Petrovic 2 , Suzana Veliˇ ckovic 1 , Vesna Radojevic 2 , Branislav Vlahovi´ c 3 and Aleksandar Stajcic 4 1 Department of Physical Chemistry, “Vinˇ ca” Institute of Nuclear Sciences—National Institute of the Republic of Serbia, University of Belgrade, Mike Petrovi´ ca Alasa 12-14, P.O. Box 522, 11001 Belgrade, Serbia; filipveljkovic@vin.bg.ac.rs (F.V.);vsuzana@vin.bg.ac.rs (S.V.) 2 Faculty of Technology and Metallurgy, University of Belgrade, 11000 Belgrade, Serbia; mpetrovic@tmf.bg.ac.rs (M.P.);vesnar@tmf.bg.ac.rs (V.R.) 3 Mathematics and Physics Department, North Carolina Central University, Durham, NC 27707, USA; vlahovic@nccu.edu 4 Center for Microelectronic Technologies, Institute of Chemistry, Technology and Metallurgy—National Institute of the Republic of Serbia, University of Belgrade, 11000 Belgrade, Serbia; stajcic@nanosys.ihtm.bg.ac.rs * Correspondence: ivana_r@vinca.rs Abstract: High performance polymers with bio-based modifiers are promising materials in terms of applications and environmental impact. In this work, raw acacia honey was used as a bio-modifier for epoxy resin, as a rich source of functional groups. The addition of honey resulted in the formation of highly stable structures that were observed in scanning electron microscopy images as separate phases at the fracture surface, which were involved in the toughening of the resin. Structural changes were investigated, revealing the formation of a new aldehyde carbonyl group. Thermal analysis confirmed the formation of products that were stable up to 600 ◦ C, with a glass transition temperature of 228 ◦ C. An energy-controlled impact test was performed to compare the absorbed impact energy of bio-modified epoxy containing different amounts of honey with unmodified epoxy resin. The results showed that bio-modified epoxy resin with 3 wt% of acacia honey could withstand several impacts with full recovery, while unmodified epoxy resin broke at first impact. The absorbed energy at first impact was 2.5 times higher for bio-modified epoxy resin than it was for unmodified epoxy resin. In this manner, by using simple preparation and a raw material that is abundant in nature, a novel epoxy with high thermal and impact resistance was obtained, opening a path for further research in this field. Keywords: bio-modifier; acacia honey; epoxy resin; thermal stability; impact-resistant 1. Introduction Epoxy resins are widely used materials, due to their suitable properties such as a high Young’s modulus, chemical and thermal resistance, good corrosion resistance, and excellent insulation [1–6]. They were extensively studied for various industrial applications, including construction, automobile manufacture, and military and aerospace industry applications, which require high-performance materials [7–9]. In order to satisfy the application demands, numerous epoxy matrix composite materials with a versatility of ceramic and organic nanoparticles and fiber reinforcements were developed over the years [10,11]. However, cross-linked epoxy shows low toughness, which raises the concern of the composites’ performance under impact. The formation of micro-cracks that are not visible to the naked eye leads to a sudden breakage, which poses a great problem for manufacturers of parts that are placed in critical mechanical points. The heterogeneous nature of microscale epoxy-based composites could also be regarded as a weakness. Fiber- reinforced composites show high impact resistance in the direction of fiber orientation, Polymers 2023, 15, 2261. https://doi.org/10.3390/polym15102261 https://www.mdpi.com/journal/polymers