PSI - Issue 64

Angelo Savio Calabrese et al. / Procedia Structural Integrity 64 (2024) 1832–1839 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Toughening methods for epoxy polymers can be broadly classified into homogeneous and heterogeneous systems, Mi et al. (2022). Homogeneous toughening systems disperse modifiers uniformly within the epoxy matrix, forming interpenetrating networks without distinct phases. This can be achieved by introducing bio-based materials, flexible chain segments, or hyperbranched polymers into the cross-linked network of epoxy polymers. On the other hand, heterogeneous toughening involves phase separation between the modifier and epoxy matrix during curing, resulting in distinct phases. This method includes incorporating rubber and nanomaterials. Epoxy toughening not only enhances fracture and impact toughness but, in some cases, also strengthens epoxy polymers. However, this can lead to high viscosity, opaqueness, and limited flowability, constraining its application in advanced settings. This section describes two heterogeneous toughening methods suitable for EB-FRP strengthening/repair applications of existing structures, namely rubber- and nanomaterial-toughening. 3.1. Rubber-toughening Toughening epoxy resins with rubber is a traditional method within heterogeneous toughening systems, dating back to the 1950s when Merz et al. (1956) first proposed its mechanism. The addition of rubber materials introduces a multifaceted toughening effect, enhancing energy absorption, flexibility, ductility, and promoting crack deflection and pinning within the material. Consequently, the toughened epoxy composite exhibits improved toughness, impact resistance, and higher durability, rendering it suitable for a wide range of industrial applications, Sultan and Mcgarry (1973). Rubbers employed for toughening epoxy can be broadly categorized as reactive liquid rubbers or rubber particles. Reactive liquid rubbers, characterized by low molecular weight, can dissolve in the epoxy matrix, leading to improved toughness efficiency with increased content. On the other hand, spherical rubber particles dispersed within the matrix serve as centers for mechanical energy dissipation during the fracture process, contributing to enhanced impact strength. The incorporation of rubber particles can be effective in toughening of epoxy polymers, due to crack deflection and pinning effect, often combined with particles debonding from the matrix, followed by plastic voids growth. Those effects lead to an increase in fracture energy, albeit at the expense of reduced strength and modulus. Furthermore, other open challenges associated with rubber-toughened epoxy is the increase in viscosity of the epoxy monomer mixture, and the potential decrease of the modified-polymer glass transition temperature, depending on particle size and type, Johnsen et al. (2007). 3.2. Nanomaterial-toughening The integration of inorganic nanomaterials into epoxy resin has emerged as a highly effective method for enhancing polymer toughness while maintaining mechanical and chemical properties, Johnsen et al. (2007). Nanomaterials suitable for this purpose encompass nanofibers, silica, graphene, clay, and carbon nanotubes. Nanomaterial-modified epoxies exhibit diverse toughening mechanisms. Due to the high surface area-to-volume ratio and to the presence of active groups on their surface, nanomaterials interact effectively with the surrounding matrix, determining optimal adhesion with the epoxy polymer. This enhances the formation of an immobilised layer of polymer around the nanoparticles, which act as reinforcement agents for pinning and deflection of cracks, Zhang et al. (2006). Additionally, nanomaterials can effectively bridge microcracks within the epoxy matrix. Nanomaterials also influence the curing process, improving cross-linking density and network formation, consequently enhancing mechanical properties such as tensile strength and elastic modulus. Due to their small size, nanoparticles penetrate deeply into the matrix, resulting in enhanced reinforcement density throughout the material. Furthermore, the incorporation of nanomaterials enhances the rheological properties of epoxy resin, leading to improved processability by reducing viscosity pre-cure. Nanoparticle agglomeration, arising from strong molecular interactions, poses a limitation to their utilization in epoxy composites, necessitating strategies for achieving good dispersion. Techniques such as the sol-gel method, involving nanoparticle dispersion in a solvent followed by gelation, proved to be effective in ensuring uniform dispersion within the cured matrix, Zhang et al. (2006). Consideration must be given to health and environmental concerns associated with nanomaterial use. Nanomaterial toxicity, which varies based on type and physicochemical characteristics, encompasses dermal, carcinogenic, and