Comparison of calorimetry and NMR relaxometry measurements of epoxy cure kinetics

Abstract

Epoxy resins are widely used as coatings and adhesives in various industries. Epoxy cures when mixed with a hardener, causing molecular crosslinking. Analysis techniques such as differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) are limited because they cannot be used in situ for large sample volumes. We demonstrate that single-sided nuclear magnetic resonance (NMR) can be used to non-destructively measure characteristics of epoxy cure through changes in NMR spin-spin relaxation times (T2). A time-dependent extent of conversion, α(t), was measured using both DSC (via the heat of reaction) and single-sided NMR (using T2), then phenomenologically fitted to a modified Kamal-Malkin model to extract kinetic information. Results show that DSC and NMR data are well correlated, indicating that NMR measurements that reflect molecular mobility can be used to characterize chemical reactions in a non-destructive manner. Further directions involve connecting NMR-derived kinetic values with molecular geometry and other kinetic models.