Synergetic enhancement on flame retardancy by melamine phosphate modified lignin in rice husk ash filled P34HB biocomposites

Lignin can be employed as a sustainable functional additive and reinforcement filler for polymers. In this work, melamine phosphate modified lignin (MAP-lignin) was synthesized as a bio-based halogen-free flame-retardant agent. The synergetic effects between MAP-lignin and rice husk ash (RHA) on the thermal stability, dynamic mechanical performance and flame-retardant properties of the poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) biocomposites were investigated. The results revealed that MAP-lignin and RHA could significantly increase the char residue and MAP-lignin had a good miscibility with P34HB. The incorporation of MAP-lignin and RHA as stiff fillers could enhance the storage modulus of the P34HB matrix. Moreover, the tensile strength and elongation at break of the P34HB/RHA composite could be improved due to the MAP-lignin could serve as a compatibilizer to improve the interaction between RHA and P34HB. Cone calorimetry data demonstrated that the addition of 30 wt% MAP-lignin and 5 wt% RHA reduced the peak heat release rate (PHRR) and total heat release (THR) of P34HB noticeably by 42.8 and 24.3%, respectively. In addition, the CO2 production rate and CO concentration could be significantly suppressed by the addition of MAP-lignin. The enhanced flame retardancy of P34HB composite should be ascribed to the barrier effect of the increased char residue with compact and intact structure, as well as the released gases from the decomposition of melamine.