Fabrication of cellulose-based halogen-free flame retardant and its synergistic effect with expandable graphite in polypropylene

•Cellulose-based halogen-free flame retardant (HECPM) was successfully prepared.•The solid expanded and dense char residue of HECPM is higher than 43% at 600 °C.•LOI value of PP/30% HECPM reaches 29.5% and samples can pass UL-94 V-0 rating.•Synergistic flame-retarding effect of HECPM and expandable graphite (EG) was found.•Conjunct char-forming and flame-retardant effect mechanism was primarily proposed. In this paper, a mono-component, cellulose-based and intumescent halogen-free flame retardant (HECPM) was prepared by introducing phosphate groups and melamine groups onto the structure of cellulose. And the chemical structure and surface morphology of HECPM was confirmed by FTIR, EDS and SEM, respectively. The thermal degradation tests demonstrate HECPM possesses great expansion property during the inducement of heat and the char residue of HECPM is higher than 43% at 600 °C. Moreover, the synergistic effect of HECPM and EG in flame-retarding polypropylene (PP) was also verified. When 30% HECPM/EG in ratio of 1/3 were introduced into PP, the LOI value reaches 31.5% and it can obtain UL-94 V-0 rating. The conjunct action of HECPM and EG in promoting the char-forming process was also achieved in comparison with the calculated and experimental TGA curves. Along with the investigation on the residue, a potential condensed-phase flame retardant mechanism was primarily proposed.