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This study aims to evaluate the influence of cyclohexane/ethanol, alkali, and silane treatments on the waste hemp fibers and their polybenzoxazine composites. The Fourier transform infrared (FTIR), scanning electron microscope (SEM), and thermogravimetric analysis (TGA) tests were carried out to understand the influence of treatments on the fibers. FTIR results revealed that hemicellulose and lignin of the fibers were removed after washing with cyclohexane/ethanol, and thermal stability of the fibers was improved after the alkali and silane treatments. The consequences of the treatments on the hemp fiber composites were studied by evaluating the mechanical, thermomechanical, thermal, and water uptake properties. The flexural, tensile and impact tests results confirmed that the silane treated fiber composites displayed highest properties than the other treated fiber composites. Furthermore, the stiffness and glass transition temperature of the silane treated fiber composites were also best among the studied composites. Moreover, the thermal stabilities values showed the lowest decline for the silane treated composites; due to the better thermal stability of the silane treated fibers, and lowest water absorption was also recorded for the silane treated fiber composites. The morphological studies confirmed that the silane treated fiber composite had stronger fiber/matrix adhesion interface.

In this current study, alkali treated eco-friendly hemp fibers were chosen to reinforce thermosetting resin blend composed of cyanate ester and benzoxazine resin. For various volume fractions ranging from 5 to 20 vol % with a regular increment of 5 vol%, their effects on the morphological, mechanical behaviour and water uptake properties were experimentally evaluated using the scanning electron microscopy, flexural, tensile and water uptake tests. The produced composites possessed much ameliorated flexural properties as compared to those of the unfilled resin. Data from the tensile test revealed a continuous increase in the composites' tensile strength and modulus as the proportion of the treated fibers augmented. Furthermore, the tensile experimental results were compared to the most commonly used empirical models like the rule of mixture (ROM), revised rule of mixture (RROM), inverse rule of mixture (IROM), Halpin Tsai (HT), Lewis and Nielson (LN), and Halpin Kardos (HK) models. It was found that the alkali treated fibers were arranged in a 3D random fiber arrangement according to the RROM model and the HK model was identified to fit well the tensile experimental results. The experimental results and theoretical modeling confirmed that the water absorption in the produced composites was increased and governed by the Fickian diffusion. These effects took place due to the chemical and morphological changes in the treated fiber external surfaces, which promoted better interfacial adhesion between the plant fibers and the polymer blend matrix.

A novel self-exothermic curing agent, effectively able to cure the diglycidyl ether of bisphenol-A (E51 epoxy resin) at room temperature, has been synthesized by blending 4, 4’-diaminodiphenylmethane (DDM) and acrylic acid (AA). The chemical structure of prepared curing agent was characterized by 1 H nuclear magnetic resonance ( 1 H NMR) and Fourier transform infrared (FTIR). The curing behavior of epoxy with synthesized room temperature hardener was studied by differential scanning calorimetry (DSC) and FTIR. Results confirmed that the curing reaction was completed in only 3 h, which can be attributed to the high reactivity of the acryl amide groups with epoxy and high heat release from DDM and AA reaction. The 5 wt% NaOH treated short hemp fibers (TF) were sandwiched in room temperature curing epoxy resin to obtain rapid prototyping high performance composites. The effects of fiber content on the mechanical properties of composites were studied in terms of tensile, flexural, and impact load. The tensile strength of the composites was increased with the increase of the TF content, and the elongation at break was decreased. Compared with the cured neat epoxy resin, an increase of about 233% in impact strength, 52% in flexural modulus, and 213% in Young's modulus were recorded for the cured composite having 7.5 wt% TF content. We used dynamic mechanical analyzer (DMA) and thermogravimetric analysis (TGA) to study the effect of fiber content on the thermal properties of composites. The scanning electron microscope (SEM) revealed excellent adhesion between TF and epoxy resin cured at room temperature.

Typically, hybrid fibers reinforced composites have the potential to provide materials with excellent mechanical properties. However, cost-effective, lightweight, easily manufactured and high-performance materials are even more attractive for the market and the industry. In this work, newly designed hybrid poly-pyridobisimidazole (PIPD) and basalt fibers (BFs) reinforced polybenzoxazine composites were investigated. Initially, the fibers were chemically modified with silane GX-560 coupling agent. Afterwards, the hybrid combinations were included in the resin and cured. The Fourier transform infrared spectra showed the chemical bond that results when the silane is grafted. The X-ray photoelectron spectroscopy scan provided the fibers atomic content and showed that the silane content was effectively raised after treatment. The microhardness and impact properties were markedly improved, reaching the maximum value of 570 MPa and 18.44 kJ/m2, respectively, compared to the reference resin with 347 MPa and 1.6 kJ/m2. This was due to the high-performance PIPD and BFs inclusive of their good interfacial adhesion and scattering around the surface confirmed by the fibers’ pull-out void and the random dispersion seen in the scanning electron microscope images.