Salicylic acid-based deep eutectic mixture as catalytic curing agent for high-performance epoxy resins: cure characteristics and mechanical properties

Epoxies have traditionally been a favourite in the coating industry. The faster and more convenient curing process, prolonged shelf life, and high performance of the end products remain the basic requirements of the epoxy industry. Deep eutectic solvents (DESs) having a hydrogen bond acceptor and a donor in their structure are being accepted very well these days. Here, we are proposing a new type of DESs formulated from non-conventional precursors, salicylic acid and N, N’-Bis(2-aminoethyl)ethane-1,2-diamine (commonly known as triethylenetetramine, TETA) as high-performance catalytic curing agent, that could replace or substitute the conventional amine-based curing agents of epoxies. The study investigated the curing efficiency of newly formulated deep eutectic solvents (DESs) in comparison to conventional triethylenetetramine (TETA) and DESs derived from choline chloride in curing DGEBA epoxy resins. Characterization via FTIR and DSC revealed distinct curing mechanisms, and DMA showed a 70% increase in storage modulus, an 85% increase in loss modulus, and a slightly reduced glass transition temperature compared to pure TETA curing agent. Additionally, DMA analysis indicated increased stiffness at elevated temperatures. Thermogravimetric analysis (TGA) confirmed comparable thermal stability and single-phase degradation. The formulated DESs exhibited near-Newtonian behavior with consistent viscosity across the shear range, underscoring its efficacy as a curing agent for epoxy resin. Notably, incorporating a 15 phr salicylic acid-TETA curing agent composition enhanced both tensile strength and fracture toughness by 107% and 8%, with greater transparency, and thermal stability. The toughening mechanism was identified as crazing, effectively maintaining stress levels. Overcoming a longstanding challenge in epoxy resin formulations, the synergistic effect of salicylic acid-based DES formulations, which involve the aromatic rings in curing reactions and the two-phase curing mechanism, significantly improves properties. This advancement paves the way for a new generation of high-performance epoxy resins.