Solvent-free and electron transfer-induced phosphorus and nitrogen-containing heterostructures for multifunctional epoxy resin

The preparation of effective phosphorus-nitrogen flame retardants (PNFRs) is still limited using organic solvent. In this work, a solvent-free mechanochemical method was introduced to prepare a phosphorus-containing hypercrosslinked aromatic polymer (HCAP) from triphenylphosphine. Subsequently, nitrogen-rich graphitized carbon nitride was introduced to prepare a series of phosphorus and nitrogen-containing heterojunctions named HCN. The formation of the HCAP and HCN was verified by a combination of density functional theory (DFT) calculations and experiments. Afterwards, upon addition of 5 wt% 20HCN to epoxy resin (EP), the limiting oxygen index and vertical combustion test level reached 30.3% and V-0, respectively. The peak heat release rate, total heat release, peak smoke production rate and total smoke production were reduced by 41.2%, 38.4%, 34.9% and 36.0%, respectively, relative to those of the pure EP. The combined scores of multiple flame retardant properties were evaluated through machine learning. The mechanical properties and thermal conductivity remained at the same level as those of EP. These results confirmed the role of HCN in reducing the fire hazard of EP, stemming from its lamellar char and ability to blow out flames. This work provides a new method of preparing PNFRs.