Grafting of ammonium polyphosphate onto poly(lactic acid) and its effect on flame retardancy and mechanical properties

Poly(lactic acid) (PLA) can be functionalized with maleic anhydride (MA) to obtain MA-grafted PLA (PLA- g -MA), which in turn, can be functionalized with ammonium polyphosphate (APP) to obtain PLA- g -APP. This functionalization should facilitate the obtaining of compounds with flame-retardant properties through intumescence and also could function as a compatibilizer for the addition of bio-fillers. To achieve this, the PLA was first functionalized with MA using dicumyl peroxide (DCP) as free radical former, at varying peroxide and maleic anhydride concentrations. FTIR and NMR confirmed the functionalization. In addition, it was found that at certain DCP and MA concentrations, the attained grafting values were close to 1% MA into PLA. Thereafter, APP was grafted onto PLA- g -MA, in order to obtain PLA- g -MA/APP. XPS analyzes showed the effective functionalization of PLA with MA and subsequently, the grafting of APP. The SEM images showed that the “new” material (PLA- g -APP) does not show a brittle fracture as that of pure PLA, although a tough fracture and an interfacial adhesion between PLA and APP is improved revealing its compatibilization effect. This compatibilization allowed an improvement in tensile strength, impact resistance and a slight increase in HDT of the PLA. Finally, it was observed that the use of PLA- g -APP in a pure PLA matrix has a positive effect on its mechanical properties. The flame retardancy was tested by cone calorimeter which showed that pHRR and THR are reduced at 30% and 35%, respectively. In addition, the better flame retardancy was obtained when using PLA- g -APP with 15% (by weight) of grafted APP. This functionalized PLA (PLA- g -APP) is a new and good option to prepare bio-fire retardant composites with enhanced flame-retardant properties, in sustainable and environmentally friendly applications. Graphical abstract