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Appl. Sci. 2020, 10, 3023; doi:10.3390/app10093023 www.mdpi.com/journal/applsci Review A Review on Thermoplastic or Thermosetting Polymeric Matrices Used in Polymeric Composites Manufactured with Banana Fibers from the Pseudostem Deesy G. Pinto 1, *, João Rodrigues 1 and Luís Bernardo 2 1 CQM-Centro de Química da Madeira, MMRG, Universidade da Madeira, Campus da Penteada, 9020-105 Funchal, Portugal; joaor@uma.pt 2 C-MADE-Centre of Materials and Building Technologies, University of Beira Interior, Calçada Fonte do Lameiro, 6201-001 Covilhã, Portugal; lfb@ubi.pt * Correspondence: deesy.pinto@staff.uma.pt; Tel.: +351-291705150 (Secretariat) or +351-291705121 (Office) Received: 22 March 2020; Accepted: 20 April 2020; Published: 26 April 2020 Abstract: Recent manufacturing advancements have led to the fabrication of polymeric composites (PC) reinforced with fibers. However, to reduce the impact on the environment, efforts have been made to replace synthetic fibers (SF) by natural fibers (NF) in many applications. NF, e.g., as banana fibers (BF) possess higher cellulose content, a higher degree of polymerization of cellulose, and a lower microfibrillar angle (MFA), which are crucial factors for the mechanical properties (MP), namely tensile modulus (TM) and tensile strength (TS), and many other properties that make them suitable for the reinforcement of PC. This review paper presents an attempt to highlight some recent findings on the MP of PC reinforced with unmodified or modified BF (UBF, MBF), which were incorporated into unmodified or modified (synthetic (SPM) or a bio (BPM)) polymeric matrices (UPM, MPM). The experimental results from previous studies are presented in terms of the variation in the percentage of the MP and show that BF can improve the MP of PC. The results of such studies suggest the possibility to extend the application of PC reinforced with BF (PCBF) in a wide range, namely from automotive to biomedical fields. The meanings of all the acronyms are listed in the abbreviations section. Keywords: banana fibers; thermoplastic; thermosetting; polymers; reinforcement; mechanical properties 1. Introduction The use of natural fibers (NF) as a renewable material, which can be extracted from renewable resources (e.g., leaves, peels, and trunks of plants), were discovered in the early 1900s [1]. Natural fibers act as biodegradable reinforcing materials and as an alternative for the man-made synthetic fibers (SF) due to their inherent properties. For instance, they are eco-friendly, unlike SF, because they are biodegradable and non-abrasive. On the other hand, NF enhances the mechanical properties of the polymers, i.e., tensile strength, flexural, and impact strengths properties of natural fiber-reinforced polymer composites with different fiber loadings, as presented in the following sections [2,3]. The most commonly used extract methods to obtain NF have been the mechanical method, chemical method, enzymatic (EM), biological, and combined procedures. The mechanical method has been the most common fiber–matrix procedure because it is fast and can be performed by a machine (e.g., mechanical decorticator), which acts by disintegration [4–7].