Ammonium polyphosphate surface-modified with l-lysine as an intumescent flame retardant for epoxy resin

To improve the flame retardance and compatibility of ammonium polyphosphate (APP) via an environmentally friendly method, l-lysine (Lys), a bio-based amino acid, was selected as the surface-modifier for APP in this work. Lys-modified APP (L-APP) was prepared through the cation exchange reaction, and its morphology and chemical composition were characterized comprehensively. The characterization results demonstrated the fabrication of Lys shell structure on the surface of APP. Additionally, the thermogravimetric analysis illustrated that the char-forming ability of APP was significantly improved by the Lys shell structure. The flame retardance of APP and L-APP on epoxy resin (EP) was evaluated by the limiting oxygen index (LOI), vertical burning (UL-94), and cone calorimeter tests (CCT). The results showed that EP blending with 15 wt% L-APP (EP/L-APP15) passed V-0 rating, and got an LOI value of 31.7%. In the CCT, compared to the pristine EP, the peak heat release rate and total smoke production of EP/L-APP15 were decreased by 77.6% and 47.1%, respectively. It was confirmed that the good flame retardance and smoke suppression performance of L-APP can be attributed to the phosphorus-containing intumescent char residuals formed during combustion process. The tensile strain tests were conducted to assess the mechanical properties of EP composites. The results demonstrated that compared to pristine APP, L-APP possessed weakened impact on the tensile strain property of EP, which was beneficial to the practical application.