Citation: Periasamy, K.; Kandare, E.; Das, R.; Darouie, M.; Khatibi, A.A. Interfacial Engineering Methods in Thermoplastic Composites: An Overview. Polymers 2023, 15, 415. https://doi.org/10.3390/ polym15020415 Academic Editor: Victor Tcherdyntsev Received: 9 December 2022 Revised: 28 December 2022 Accepted: 5 January 2023 Published: 12 January 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 Review Interfacial Engineering Methods in Thermoplastic Composites: An Overview Kailashbalan Periasamy , Everson Kandare, Raj Das, Maryam Darouie and Akbar A. Khatibi * School of Engineering, RMIT University, Melbourne, VIC 3001, Australia * Correspondence: akbar.khatibi@rmit.edu.au; Tel.: +61-3-9925-6105 Abstract: The paper critically analyzed different interfacial enhancing methods used in thermoplastic composites. Although the absence of cross-linked polymer chains and chemical bonds on solidifi- cation enables the thermoplastics to be remelted, it creates weak interfacial adhesion between fibre reinforcements and the thermoplastic matrix. The weak fibre-matrix interface bonding reduces the efficiency with which the applied load can be transferred between these composite constituents, caus- ing the composite to fail prematurely. Their need for high-temperature processing, poor compatibility with other polymer matrices, and relatively high viscosity render thermoplastics challenging when used to manufacture composite laminates. Therefore, various methods, including nanoparticles, changing the polarity of the fibre surface by plasma etching, chemical treatment with ozone, or an oxidative attack at the fibre surface, have been applied to improve the fibre/matrix bonding in thermoplastic composites. The fabrication steps followed in these techniques, their progress in research, and the associated toughening mechanisms are comprehensively discussed in this paper. The effect of different fibre-matrix interfacial enhancement methods on the mechanical properties of thermoplastic composites is also deliberated. Keywords: interfacial bonding; thermoplastic composites; nanoparticle inclusion; fibre surface treatment 1. Introduction Thermoplastic polymers offer a more sustainable solution for fabricating high-toughness components since they can be thermoformed, manufactured at high production rates, and recycled without affecting their physical properties [1]. High-performance thermoplastics, such as polyamide6 (PA6), polyphenylene sulfide (PPS), and poly (ether-ether ketone) (PEEK), are widely used in aircraft components [2] and consumer products [3] to meet elevated temperature service requirements. These polymers, after the addition of fibre reinforcement, exhibit excellent thermo-mechanical properties [4]. Although the fabrica- tion of thermoplastic composites is much more complex and expensive than traditional thermoset composite manufacturing, the introduction of additive manufacturing for ther- moplastic composites has created a new image of ease of processing and sustainability. There is a growing trend toward the additive manufacturing of thermoplastic composites for primary structures. Therefore, enhancing the mechanical performance of thermoplastic composites and overcoming manufacturing deficiencies is a primary need for the full utilization of these composites. Because of long and inert macromolecular chains in thermoplastics, further enhance- ment of the mechanical properties using copolymerization or blending with other high- performance polymers is minimal [5]. One possible solution for improving the mechanical properties of thermoplastic composites can be achieved by cultivating fibre-matrix inter- facial bonding. Fibre-matrix interfacial properties play a vital role in structural integrity and stress transfer in fibre-reinforced composites. Three main interactions govern the adhe- sive strength at the interface: (i) physical adhesion related to surface energies of the fibre and the matrix, (ii) chemical bonding, and (iii) mechanical interlocking created by rough Polymers 2023, 15, 415. https://doi.org/10.3390/polym15020415 https://www.mdpi.com/journal/polymers