Synthesis of phosphorus-containing cardanol-derived phenalkamine: a sustainable curing agent to access high-performance bio-based epoxy thermosets

Abstract

Cardanol-derived phenalkamines with different phosphorus oxidation states (Car-DCP-DETA and Car-DPC-DETA) were synthesized by the Mannich reaction and used as curing agents for diglycidyl ether of bisphenol A (DGEBA)-type epoxy prepolymers. ¹H-NMR, ³¹P-NMR and FTIR spectra confirmed that Car-DCP-DETA and Car-DPC-DETA were successfully synthesized. DGEBA/Car-DCP-DETA and DGEBA/Car-DPC-DETA thermosets were fabricated by thermally induced polymerization, while DGEBA/DDM was prepared as a control sample. The curing kinetics investigation indicated that the curing activation energy of the DGEBA/Car-DCP-DETA and DGEBA/Car-DPC-DETA systems was higher than that of DGEBA/DDM because of the larger steric hindrance of Car-DCP-DETA and Car-DPC-DETA. The cone calorimetry results showed that the peak heat release rate (PHRR) values of DGEBA/Car-DCP-DETA and DGEBA/Car-DPC-DETA were 76.8% and 44.8% lower than that of DGEBA/4,4′-diaminodiphenylmethane (DDM), respectively, and the total heat release (THR) value was reduced to 125.8 and 105.8 MJ m⁻², respectively, from 159.9 MJ m⁻² (DGEBA/DDM). These results suggested a superior flame-retardant efficiency for Car-DCP-DETA over Car-DPC-DETA. The flame-retardant mechanisms of DGEBA/Car-DCP-DETA and DGEBA/Car-DPC-DETA were studied by thermogravimetric analysis coupled with Fourier transform infrared spectroscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. It can be inferred that the outstanding flame retardancy of DGEBA/Car-DCP-DETA is mainly ascribed to the formation of thermally stable phosphorus–oxygen structures in the combustion process, which is helpful in improving the thermal oxidation resistance of char residues and thus improving the flame retardancy of epoxy resins. Tensile and flexural test results showed that Car-DCP-DETA also had good strengthening and toughening effects on DGEBA. This phosphorus-containing cardanol-derived phenalkamine (Car-DCP-DETA) imparted simultaneously improved flame retardancy, and mechanical strength and toughness to epoxy thermosets, which could be a renewable replacement for petroleum-based curing agents in fire-resistant or corrosion-resistant coating applications.