Surface Chemistry and Molecular Dynamics of Epoxy Resin: Insights from Analysis During Curing and Post-Curing Processes

The surface chemistry of epoxy resin and its composites is critical for their long-term performance across various applications. In this study, we investigate the main reactions occurring on the surface of DEGBA/DEGBF epoxy resin following curing, post-curing, and thermal post-curing processes using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). ToF-SIMS analysis elucidated molecular details, including curing and cross-linking progression, cross-link characteristics, cured resin structure, residual unreacted hardener, cross-linking density, and reaction pathways. Principal Components Regression analysis (PCR) was applied to distinguish between cured and post-cured samples, focusing on specific ions indicative of the curing process. The completion of curing was associated with ions such as C₁₄H₇O⁺, CHO⁺, CH₃O⁺, and C₂₁H₂₄O₄⁺, while unreacted hardener was indicated by C₂₁H₂₄O₄⁺ ions. Cross-linking density and the intensities of aliphatic hydrocarbons were crucial in differentiating curing stages. Calibration ensured that all ion intensities totaled to one, and specific ions were tracked to monitor the states from uncured to post-cured. Negative spectra provided insights into the consumption of hardener molecules during curing and post-curing. The results demonstrated that post-curing enhances the properties of epoxy resin by promoting further cross-linking, reducing residual unreacted groups, and forming a more extensive covalent network. This results in improved mechanical and thermal stability. The molecular changes observed through ToF-SIMS data effectively distinguish between curing and post-curing reactions, contributing to a better understanding and optimization of epoxy resin properties for various applications.