polymers Article Basalt Fibre Composite with Carbon Nanomodified Epoxy Matrix under Hydrothermal Ageing Tatjana Glaskova-Kuzmina 1, *, Aldobenedetto Zotti 2 , Anna Borriello 2 , Mauro Zarrelli 2 and Andrey Aniskevich 1   Citation: Glaskova-Kuzmina, T.; Zotti, A.; Borriello, A.; Zarrelli, M.; Aniskevich, A. Basalt Fibre Composite with Carbon Nanomodified Epoxy Matrix under Hydrothermal Ageing. Polymers 2021, 13, 532. https://doi.org/10.3390/ polym13040532 Academic Editor: Angels Serra Received: 1 February 2021 Accepted: 8 February 2021 Published: 11 February 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 Institute for Mechanics of Materials, University of Latvia, 3-635, Jelgavas Str., LV-1004 Riga, Latvia; Andrey.Aniskevich@pmi.lu.lv 2 Institute of Polymers, Composites and Biomaterials, National Research Council of Italy, 80055 Portici, Italy; aldobenedetto.zotti@unina.it (A.Z.); anna.borriello@cnr.it (A.B.); mauro.zarrelli@cnr.it (M.Z.) * Correspondence: Tatjana.Glaskova-Kuzmina@lu.lv Abstract: This work aimed to investigate the effect of hybrid carbon nanofillers (e.g., carbon nan- otubes/carbon nanofibers in the ratio 1:1 by mass) over the electrical and flexural properties for an epoxy matrix and corresponding basalt fibre reinforcing composite (BFRC) subjected to full-year seasonal water absorption. Hydrothermal ageing was performed by full immersion of the tested materials into distilled water according to the following model conditions (seasons). The mechanical properties were measured in three-point bending mode before environmental ageing and after each season. Upon environmental ageing, the relative change of flexural strength and elastic modulus of the epoxy and NC was within 10–15%. For nanomodified BFRCs, the slightly higher effect (approx. by 10%) of absorbed moisture on flexural characteristics was found and likely attributed to higher defectiveness (e.g., porosity, the formation of agglomerates etc.). During flexural tests, electrical resistance of the nanocomposites (NC) and BFRC/NC samples was evaluated. The electrical conduc- tivity for UD BFRC/NC, before and after hydrothermal ageing, was by 2 and 3 times higher than for the NC, accordingly, revealing the orientation of electrically conductive nanoparticles and/or their agglomerates during lay-up manufacturing which was evaluated by the rules of the mixture. Based on all results obtained it can be concluded that the most potentially applicable for damage indication was UD BFRC/NC along fibres since full-year hydrothermal ageing improved its electrical conductivity by approx. 98% and, consequently, the ability to monitor damages was also enhanced. Keywords: polymer composite; carbon nanofiller; epoxy; hydrothermal ageing; mechanical proper- ties; water absorption; electrical resistance 1. Introduction Environmental ageing is one of the most important reasons for the preliminary failure of fibre reinforced plastics (FRP) applied in engineering structures for outdoor applica- tions [1,2]. Basalt fibre reinforced composites (BFRCs) are sustainable materials that can be positioned between glass fibre reinforced composites (GFRCs) and carbon fibre reinforced composites (CFRCs) in terms of performance and cost-effectiveness. Besides, BFRCs also have high stability to the action of water, acids, and chemically active media which results in prolonged durability of FRPs during operation in the outdoor environments in compari- son with CFRCs and GFRCs [1,3,4]. Based on these advantages, the applicability of BFRCs as structural materials for outdoor applications is highly expected [5,6]. Nevertheless, the durability of FRP composites is a complex phenomenon and it depends on the degradation of both polymer matrix and fibres, as well as on their interface bond behaviour [7–9]. The epoxy resins which are usually applied for the impregnation of FRPs may have changes in their physical and chemical properties due to environmental effects (e.g., moisture, temperature, and UV radiation) [10–12] leading to plasticization of the epoxy resins, chemical reactions, and post-curing. Polymers 2021, 13, 532. https://doi.org/10.3390/polym13040532 https://www.mdpi.com/journal/polymers