Phosphorylated cardanol-formaldehyde oligomers as flame-retardant and toughening agents for epoxy thermosets

Presently, flammability and brittleness are two major problems for EP which has restricted its development and application in industrial. Under the global advocacy of green and sustainable development, developing bio-based flame-retardant and toughening agents which can endow epoxy resins with high fire resistance and satisfactory mechanical properties is of significant demand for its industrial applications. Herein, three kinds of phosphorylated cardanol-formaldehyde oligomers (CF-PO(OPh)2, CF-POPh2, and CF-PPh2) were synthesized and introduced into epoxy thermosets. The incorporation of CF-PO(OPh)2, CF-POPh2, and CF-PPh2 can remarkably improve the mechanical performance of EP composites, especially the tensile strength and elongation at break. With 10 wt% of CF-PO(OPh)2, CF-POPh2, and CF-PPh2, the tensile strength of epoxy thermosets reached 67.1, 74.5, and 70.3 MPa, respectively; the elongation at break of epoxy thermosets was raised by 133.3%, 162.5%, and 150.0%, respectively, as compared to neat EP. EP composites containing CF-PO(OPh)2, CF-POPh2, and CF-PPh2 also exhibited remarkably enhanced flame retardancy simultaneously. Besides, CF-PO(OPh)2 exhibited the best flame-retardant efficiency by comparison with CF-POPh2 and CF-PPh2, and the results manifested that epoxy composite with only 2.5 wt% CF-PO(OPh)2 could pass the UL-94 V-0 rating as well as a high LOI value of 28%, and the LOI value of EP/CF-PO(OPh)2–10 reached up to 32%. Moreover, CF-PO(OPh)2 also displayed an excellent catalytic charring effect and great suppression effect on heat and smoke release. The cone calorimeter test results showed that EP composite containing 10 wt% CF-PO(OPh)2 exhibited the most significant PHRR reduction which was 53.8% lower than pure EP, and the char yield of EP/CF-PO(OPh)2–10 after the cone calorimeter test was 14.0% which was much higher than that of pure EP (5.7%). This work opens a new vision for the synthesis of bio-based flame-retardant and toughening agents with a special oligomeric structure combining rigidity with flexibility, and contributes to the production of high-performance epoxy thermosets.