  Citation: Zuccarello, B.; Bongiorno, F.; Militello, C. Basalt Fiber Hybridization Effects on High-Performance Sisal-Reinforced Biocomposites. Polymers 2022, 14, 1457. https://doi.org/10.3390/ polym14071457 Academic Editor: Alexey Iordanskii Received: 17 March 2022 Accepted: 1 April 2022 Published: 3 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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 Basalt Fiber Hybridization Effects on High-Performance Sisal-Reinforced Biocomposites Bernardo Zuccarello * , Francesco Bongiorno and Carmelo Militello Dipartimento di Ingegneria, Università degli Studi di Palermo, 90128 Palermo, Italy; francesco.bongiorno01@unipa.it (F.B.); carmelo.militello01@unipa.it (C.M.) * Correspondence: bernardo.zuccarello@unipa.it Abstract: The increasing attention given to environmental protection, largely through specific regula- tions on environmental impact and the recycling of materials, has led to a considerable interest of researchers in biocomposites, materials consisting of bio-based or green polymer matrixes reinforced by natural fibers. Among the various reinforcing natural fibers, sisal fibers are particularly promising for their good mechanical properties, low specific weight and wide availability on the current market. As proven in literature by various authors, the hybridization of biocomposites by synthetical fibers or different natural fibers can lead to an interesting improvement of the mechanical properties or, in turn, of the strength against environmental agents. Consequently, this can lead to a significant enlargement of their practical applications, in particular from quite common non-structural applica- tions (dashboards, fillings, soundproofing, etc.) towards semi-structural (panels, etc.) and structural applications (structural elements of civil construction and/or machine components). Hybridizations with natural fibers or with ecofriendly basalt fibers are the most interesting ones, since they permit the improvement of the biocomposite’s performance without an appreciable increment on environ- mental impact, as occurs instead for synthetic fiber hybridizations that are also widely proposed in the literature. In order to further increase the mechanical performance and, above all, to reduce the aging effects on high-performance sisal-reinforced biocomposites due to environmental agents, the hybridization of such biocomposites with basalt fibers are studied with tensile, compression and delamination tests performed by varying the exposition to environmental agents. In brief, the experimental analysis has shown that hybridization can lead to further enhancements of mechani- cal performance (strength and stiffness) that increase with basalt volume fraction and can lead to appreciable reductions in the aging effects on mechanical performance by simple hybridization of the surface laminae. Therefore, such a hybridization can be advantageously used in all practical outdoor applications in which high-performance sisal biocomposites can be exposed to significant environmental agents (temperature, humidity, UV). Keywords: biocomposites; natural fibers; sisal; basalt; aging; mechanical performance 1. Introduction The use of biocomposites consisting of eco-friendly matrices reinforced by natural fibers has increased rapidly in many industrial sectors such as the automotive, shipbuilding, and civil construction industries. In particular, the use of natural fibers characterized by good mechanical properties, low specific weight, and wide availability in the current mar- ket, as is the case of sisal fiber, not only allows compliance with the recent and increasingly stringent regulations of environmental protection, but also has the benefit of an appreciable reduction in production and operating costs, especially for applications in the transport sector (automotive, nautical, etc.) [1,2]. Several studies reported in the literature have shown that, generally, the hybridization of these biocomposites with synthetic fibers or different natural fibers, can allow the improvement of their performance, and therefore the expansion of this field of application, Polymers 2022, 14, 1457. https://doi.org/10.3390/polym14071457 https://www.mdpi.com/journal/polymers