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The increasing efforts aimed to design structures with reduced weight and better mechanical performances has led in recent years to a growing use of fiber reinforced polymer materials in several fields such as marine. However, these materials can be composed of chemically very different elements and, hence, may be difficult to joint. This research aims to improve the adhesion between a thermoplastic matrix of polyamide reinforced with short carbon fibers (PA12-CR) and a carbon fiber reinforced epoxy matrix (CFRP). Two different silane coupling agents, (3-Aminopropyl)trimethoxysilane (AM) and (3-Glycidyloxypropyl)trimethoxysilane (EP), were applied, through the spray deposition method, on the PA12-CR substrate to create a reactive layer between the adherents. Different deposition methods and coupling agents curing conditions were also investigated. The wettability of the PA12-CR surface as well as the chemical modifications induced by silane treatments were investigated through contact angle and Fourier Transform Infrared spectroscopy (FTIR) analyses. Furthermore, the interfacial adhesion between PA12-CR and CFRP substrates was evaluated through Mode I delamination tests (DCB). The effectiveness of the most promising treatment was finally verified on sandwich structures, having PA12-CR printed as internal core and CFRP laminates as external skins, through quasi-static three-point bending mechanical tests. Overall, the epoxy-based silane (EP) allowed significantly better resistance to the delamination up until the tensile failure of the CFRP substrate.

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The goal of this paper is to evaluate the effectiveness of a cost-effective and eco-friendly treatment based on the use of sodium citrate (Na3C6H5O7) on the mechanical properties of flax fiber reinforced composites. To this scope, flax fibers were soaked in mildly alkaline solutions of the sodium salt at different weight concentration (i.e., 5%, 10% and 20%) for 120 h at 25 °C. The modifications on fibers surface induced by the proposed treatment were evaluated through Fourier transform infrared analysis (FTIR), whereas scanning electron microscope (SEM) and helium pycnometer were used to obtain useful information about composites morphology. The effect of the concentration of the treating solution on the mechanical response of composites was determined through quasi-static tensile and flexural tests, Charpy impact tests and dynamical mechanical thermal (DMTA) tests. The results revealed that composites reinforced with flax fibers treated in 10% solution exhibit the best mechanical performances as well as the lowest void contents. SEM analysis supported these findings showing that, by treating fibers in solutions with concentration up to 10%, composites having better morphology can be manufactured, in comparison to untreated ones. Conversely, higher Na3C6H5O7 concentrations negatively affect both the morphology and the mechanical properties of composites.

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Nowadays, the world requires more sustainable and eco-friendly materials to replace or limit the usage of synthetic materials. Moreover, several researchers focused their attention on the use of agricultural sources as reinforcement for biocomposites since they are abundant, cost-effective and environmentally favorable sources. In such a context, purpose of the present paper is the evaluation of lemongrass plant (Cymbopogon flexuosus) as possible source of natural reinforcement for biocomposites. To this aim, natural fibers were obtained from the leaf and the stem of lemongrass and their main properties were compared for the first time. To this scope, mechanical and thermal characterizations, chemical investigation, Fourier-transform infrared spectroscopy, X-Ray diffraction and scanning electron microscope analysis were carried out. The experimental campaign showed that, despite having similar chemical composition (i.e., cellulose, hemicellulose and lignin contents equal to 44–45%, 28–29% and 17%, respectively), leaf fibers possess higher mechanical properties (i.e., + 55% and + 76% in the tensile strength and modulus, respectively) than stem ones. This result can be ascribed to different factors such as larger amount of absorbed water (i.e., + 4%) and ash content (+ 2%) shown by stem fibers in addition to a more compact structure evidenced by leaf fibers which also present higher density (i.e., 1.139 g/cm3 versus 1.019 g/cm3).

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