Influence of network structure determined by Time-domain [formula omitted] Double Quantum NMR on the creep properties of non-stoichiometric epoxy-amine resins aimed for chemical anchoring applications

Abstract

Chemical anchors, used to fasten structural components, are prone to creep under sustained loading, which significantly affects their long-term performance and service life. In this study, the molar ratio, that determines the ultimate network structure of an epoxy-amine resin system, is systematically modified to investigate the creep properties. The percentage of physical entanglement and defects (free and dangling chains) for each formulation is obtained by Time-Domain 1H Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) measurements. The crosslink density values from the dynamic mechanical analysis (DMA) and the network parameter from the NMR measurements show a strong linear correlation. Tensile tests were performed to determine the influence on the mechanical properties and set the load levels for tensile creep measurements of the selected formulations. Epoxy-rich systems (Epoxy-Amine adduct) show the highest modulus and tensile strength but inferior creep properties compared to stoichiometric and amine-rich (Amine-Epoxy adduct) compositions. This can be attributed to the build-up of several network structures with higher content of free and dangling chains leading to lower deformation resistance. At high loads, the entanglement density has an increasing impact on the creep behavior compared to defect-rich systems. Modeling using the Findley approach was carried out to link the obtained creep parameter with the material properties. Therefore, this work aims to deepen the understanding of the molecular-scale structures of epoxy resins required for material and design optimization, by investigating the correlation between network structure and creep properties.