The process and high-temperature performance of bisphenol-A based benzoxazine were tailored by copolymerizing with bi-functional cycloaliphatic epoxy resin in order to be used as a matrix for the advanced composite fabrication. The melt viscosity, cure kinetics, thermo-mechanical, and tensile properties of the systems with various cycloaliphatic epoxy resin and benzoxazine concentrations were studied using rheometer, differential scanning calorimetry, dynamic mechanical analyzer, and micro-tensile tester, respectively. Mixtures with cycloaliphatic epoxy resin concentration greater than 25 wt.% provided a suitable viscosity for composite processing by resin infusion. The isothermal and nonisothermal cure kinetics of the benzoxazine and cycloaliphatic epoxy resin reaction was studied. The predicted kinetic curves based on Kamal and Sourour phenomenological reaction model expanded with a diffusion factor resulted in an agreement with the experimental kinetic curves. The investigation of copolymers with various mixture compositions showed an increase of 56% in glass-transition temperature ([Formula: see text] and 79% in cross-linking density by increasing epoxy concentration up to 40 wt.% in comparison with homopolymerized benzoxazine.

In this study, graphite nanoplatelets (GNPs) were dispersed in a copolymer matrix consisting of bisphenol‐A based benzoxazine (BZ) and bi‐functional cycloaliphatic epoxy (CER), using two solvent‐free techniques: ultrasonication and three‐roll mill (3RM). The effects of GNP addition on the tensile performance, storage modulus, glass‐transition temperature (Tg), and electrical conductivity were evaluated. A maximum increase of nearly 46% and 20% in tensile modulus and strength, respectively, was found at 1.8 wt% of GNP content dispersed using the ultrasonication technique. In comparison, a superior enhancement with 55% and 37% increase in the tensile modulus and strength could be obtained at a lower GNP content, 0.9 wt%, dispersed via 3RM calendering, respectively. In the electrical conductivity measurement, a percolation threshold was achieved in the range between 0.6 wt% and 0.9 wt% of GNP content using the 3RM technique, which was in agreement with the predicted values. The theoretical stiffness obtained from the simplified Halpin‐Tsai model corresponded with the experimental data at low fractions. The incorporation of GNPs into the BZ/CER copolymer resulted in the full recovery of all the performance losses from the addition of CER to BZ. Choosing a proper dispersing technique, the 3RM calendering in this case, could lead to a minimum required GNP content for achieving superior nanocomposite performances.

Two toughening approaches were investigated in this study to improve the interlaminar fracture toughness (ILFTs) of carbon fiber/benzoxazine composites. A matrix modification as well as an incorporation of thermoplastic polyamide (PA) veils were utilized to investigate the change in the ILFT properties. Two PA veils with different melting temperatures were analyzed and the ILFT properties of the resultant laminates were determined by the Mode-I and Mode-II critical strain energy release rates (GIC and GIIC). The incorporation of both veils was found to increase both GIC and GIIC performances. The veil with a lower melting temperature than the curing temperature of the resin resisted a Mode-I crack propagation better than the veil with a high-melting temperature. A significant increase in the GIC measurements could therefore be achieved with the use of the toughened matrix system. In the Mode-II ILFT testing, the high-melting temperature veil outperformed the low-melting temperature veil in resisting both crack initiation and propagation. No significant toughening improvement was observed with the use of a toughened matrix system in the Mode-II experiment. A combination between matrix toughening and interleaving with the high-melting temperature veil led to an enhancement in Mode-II properties. The fracture surfaces of the tested specimens were then examined for a determination of toughening effects of the interleaved veils on the composites.