A P/Si-containing polyethylenimine curing agent towards transparent, durable fire-safe, mechanically-robust and tough epoxy resins

Epoxy resin (EP) features superior comprehensive performances, but inherent flammability seriously limits its industrial applications. Current flame-retardant strategies often bring about enhanced flame retardancy but deteriorated mechanical, thermal and optical properties and neglect flame retardant durability. Herein, a phosphorus/silicon-containing polyethylenimine (PES) is designed as a multifunctional curing agent for EP. Replacing polyethylenimine (PEI) with PES maintains high visible light transmittance of EPs and improves the UV-blocking properties. The EP cured by 30 wt% PES (EP/30PES) exhibits enhanced mechanical properties compared with the EP cured by 15 wt% PEI (EP/15PEI). EP/30PES achieves a limiting oxygen index (LOI) of 35.0% and a UL-94 V-0 rating, and its peak smoke release rate (PSPR) reduces by 63.8% relative to that of EP/15PEI. Hence, the great flame retardancy, smoke suppression and mechanical performances of EP/30PES enable it to outperform previous flame-retardant EP/aliphatic amine counterparts. Meanwhile, EP/30PES shows durable flame retardancy, which still achieves a UL-94 V-0 classification after acid/alkali/aging resistance tests. This work provides an advanced design of multifunctional curing agents to create durable fire-safe EPs with great UV-shielding and mechanical performances.