Effects of Curing Schedules on Carbon-Fiber-Reinforced Laminates with a Bio-Based Epoxy Matrix

Abstract

Carbon-fiber-reinforced polymer (CFRP) composites fabricated with bio-based epoxy resins represent a promising approach for sustainable lightweight structures produced by out-of-autoclave (OoA) technologies. The curing schedule influences the state of the epoxy matrix and, consequently, can affect the fiber–matrix interaction and laminate performance. This study investigates the effect of practical curing conditions on 2 × 2 twill woven carbon fiber laminates fabricated by vacuum infusion using a commercially available bio-based epoxy resin IB2. The manufacturer’s recommended room-temperature conditions (25 °C for 24 h) were compared with accelerated mold heating schedules at 40, 50, 60, and 70 °C for 12 h. The laminates were characterized by three-point tensile and flexural tests, heat deflection temperature (HDT) measurements, differential scanning calorimetry (DSC), and SEM fractography. The tensile response showed limited sensitivity to the investigated curing conditions, with mean tensile strengths ranging from 634.01 to 672.95 MPa; T60 exhibited the highest mean numerical tensile strength (672.95 ± 53.60 MPa) and tensile modulus (53.39 ± 11.53 GPa), although the differences were small relative to the experimental spread. In contrast, the flexural response was more sensitive to the processing conditions. T70 exhibited the highest average flexural strength (980.60 ± 129.03 MPa), strain at maximum flexural stress, and strain energy density to maximum stress (8.75 ± 1.89 MJ/m3). The heat deflection temperature (HDT) systematically increased from 65.20 °C for T25 to 85.83 °C for T70. DSC revealed clear differences in the calorimetric response after curing during the first heating cycle, while the glass transition temperatures at the middle of the second heating occupied a relatively narrow range of 80.9–85.6 °C. SEM fractography revealed mixed tensile failure mechanisms related to fibers, matrix, and interface under all curing conditions. Overall, the results demonstrate that accelerated 12 h heated mold programs can reduce cure time while maintaining tensile performance generally comparable to the 24 h room-temperature IB2 reference condition and providing higher average flexural performance and thermal deformation resistance under load. These findings establish processing–property relationships relevant to the development of biomass-based CFRP OoA laminates for lightweight aerospace applications.