Polydopamine induced natural fiber surface functionalization: a way towards flame retardancy of flax/poly(lactic acid) biocomposites

Aiming to improve flame retardancy of natural fiber reinforced poly (lactic acid) (PLA) composites, a bio-inspired fiber surface modification approach was investigated in this study. Raw flax fiber was firstly coated with a thin adhesive polydopamine (PDA) film in an aqueous solution of dopamine, followed by in situ growth of iron phosphonate on the fiber surface. The modified flax fiber was added into PLA to prepare flame retardant bio-composite. Compared with raw flax reinforced PLA with a limiting oxygen index (LOI) of 19.1% and no rating in vertical burning test (UL-94), the modified PLA composite possessed a high LOI of 26.1% and V-2 rating. Meanwhile, the fiber surface modification resulted in suppression on both peak heat release rate (decreased by 16%) and total smoke production (decreased by 21%) in cone calorimeter test (CCT). Moreover, tensile modulus of the flame retardant composite was remarkably enhanced, accompanied with a slightly decreased tensile strength compared to raw PLA. The polydopamine assisted surface modification proved to be a feasible approach to improve the flame retardancy of fiber reinforced composites.