Low-molecular-weight thermoplastic modifiers as effective healing agents in mendable epoxy networks

Solid-state healing of epoxy networks is shown to be an effective and robust mechanism for highly cross-linked epoxy networks. Using diglycidyl ether of bisphenol A epoxy resin and diethyltoluenediamine, the cured epoxy network is transformed into a mendable system using phenoxy resin and low-molecular-weight polybisphenol A-co-epichlorohydrin) thermoplastic modifiers. Using functionally terminated low-molecular-weight poly(bisphenol A-co-epichlorohydrina) thermoplastic modifiers as healing agents reveals that salicylic acid or neutralized sodium salicylate groups produce healing similar to high-molecular-weight non-functional phenoxy resin as measured using single-end notched beam testing. The miscibility of both thermoplastics in the diglycidyl ether of bisphenol A/diethyltoluenediamine system was evaluated using differential scanning calorimetry and dynamic mechanical thermal analysis and identified as being important to promoting healing. Near-infrared spectroscopy showed that the network structure was unaffected by the thermoplastic modification, suggesting that healing occurred primarily through physical or non-covalent mechanisms rather than covalent bonding. The potential for self-assembly of the salicylic acid and neutralized sodium salicylate groups to form a high-molecular-weight thermoplastic in situ was also discussed as being a possible reason for the improved level of healing with the low-molecular-weight thermoplastic.