Carbon Fiber-Reinforced Prepregs And Composites Of One-Component Epoxy Resins Containing Thermal Latent Curing Agents

Carbon fiber-reinforced composites are increasingly replacing metals in sectors such as aerospace and defense due to their key advantages, including low density, high specific strength, and superior corrosion resistance. High-performance, state-of-the-art composites are typically fabricated using thermoset prepregs. However, the susceptibility of these prepregs to premature curing at ambient temperatures limits their shelf life and necessitates cold storage, leading to increased energy consumption, reduced storage flexibility, and higher processing costs. To overcome these limitations, prepregs formulated with thermal latent curing agents (TLCs) are designed to remain stable at ambient temperatures and initiate curing only upon exposure to elevated temperatures, thereby addressing issues related to premature reactivity, storage stability, energy consumption, and processing reliability.This thesis focuses on the design, synthesis, and application of thermal latent curing agents (TLCs) in one-component epoxy resins (OCERs) to enhance the processing of carbon fiber-reinforced prepregs and the mechanical performance of the resulting composites. This research encompasses molecular-level advancements in TLCs and prepregs and their translation into composite manufacturing through four interconnected studies.The first study investigated the formulation and cure behavior of an OCER incorporating the newly developed small-molecule TLC, phenylurea propyl imidazole (PUPI), in combination with diglycidyl ether of bisphenol A (DGEBA). This OCER demonstrated high conversions at moderate temperatures while maintaining ambient temperature stability for up to three days. Moreover, PUPI synergistically accelerated cure kinetics in the presence of dicyandiamide (DICY), significantly lowering the onset and peak curing temperatures as confirmed by DSC analysis.The second study involved the synthesis of a novel sulfonyl urea-based TLC, PTSU-EDA, from p-toluenesulfonyl isocyanate and ethylenediamine, and its incorporation into DGEBA to formulate an OCER. This OCER demonstrated superior thermal latency compared to imidazole-based formulations and reduced cure temperatures relative to OCERs with conventional urea accelerators. However, the cured network’s low cross-link density highlighted the need for further structural optimization or stoichiometric adjustment.To overcome limitations of small-molecule TLCs, the third study focused on the synthesis of poly(2-phenyl-2-oxazoline)-Im (PPhOZ-Im), a polymeric TLC featuring a relatively hydrophobic polyoxazoline backbone and a terminal imidazole group. PPhOZ-Im was incorporated into DGEBA either alone as a curing agent or combined with dicyandiamide (DICY) as an accelerator to formulate OCERs, which were subsequently used to prepare prepregs and composites. Incorporation of PPhOZ-Im into OCERs significantly enhanced composite mechanical properties, increasing tensile strength by 31.3% and compressive strength by 4.5% relative to controls.Building on these material developments, the fourth study investigated the relationship between prepreg processability-specifically drapeability and tackiness-and the interlaminar shear strength (ILSS) of cured composites. Prepregs with varied degrees of cure (DoC) were produced using off-stoichiometric curing ratios and cured under identical conditions. Results demonstrated a strong relationship between prepreg drapeability and composite ILSS, highlighting the importance of carefully controlling processing conditions when optimizing prepreg formulations with TLCs and dual-cure systems.These studies collectively establish a robust framework for designing ambient temperature-stable, more efficient OCERs tailored for high-performance composite applications. Further optimization-including predictive modeling of prepreg behavior and the development of tailored TLC architectures-can broaden the applicability of these resins. Moreover, integrating in-line characterization tools may enable precise fine-tuning of prepreg and composite properties, ultimately improving manufacturing efficiency and reducing costs.