A synthesized P/N/S oligomer flame retardant to construct superior flame-retardant, transparent, and mechanical properties epoxy resin

The construction of epoxy resin (EP) composites with remarkable flame retardancy, transparency, and mechanical properties was a challenging subject in the industry. A novel P/N/S oligomer flame retardant (PALO) was synthesized of terephthalaldehyde (TPAL), 4-aminothiophenol (PATP), and 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO), and employed in EP to fabricate a series of EP/PALO composites with different PALO amount. Thermogravimetric analysis (TGA) revealed that PALO exhibited remarkable thermal stability, retaining a substantial char residue of 32.2 wt% at 800 °C. Thanks to the synergy of P, N, and S, EP/PALO composites exhibited excellent flame retardancy properties. The LOI value of the EP/PALO composites saw a rise from 31.0 % to 34.2 % as the PALO content increased from 2.5 to 10 wt%, while UL-94 remained at the V-0 rating. EP/PALO also showed significant heat suppression and smoke suppression effects. The combined effects of the physical barrier from phosphate-containing char in the condensed phase, along with the flame-retardant and dilution effects of phosphorus radicals, nitrogen, and sulfur inert gases in the gas phase, contributed to the significant enhancement in flame retardancy observed in the EP/PALO composites. A notable increase in the tensile strength, flexural strength, and elongation at break of EP was triggered by the incorporation of 2.5 wt% PALO, with enhancements observed by 18.2 %, 16.6 %, and 53.1 %, respectively. Simultaneously, PALO effectively maintained the favorable transparency of EP composites under different addition amounts. Therefore, this study outlined a holistic approach for the development of oligomer flame retardants containing P/N/S and had good application potential for constructing EP composites with superior flame retardancy, transparency, and mechanical properties under different addition amounts.