Combustion Carbonization of Cellulose Nanocrystal-based Flame Retardant to Enhance Fire-safety Behaviors of Poly(butylene succinate)

Based on the strategy of normal combustion carbonizaiton, highly crystalline structure of cellulose nanocrystals (CNCs) is expected to favor a conversion to char. Thus, graphene with stable carbon skeleton, phosphorous compound and solid acid metal ion are specifically introduced to hybridize with CNC as one integration system through covalent bonding and complexation. In this case, phosphorous compound and solid acid metal ion might play the roles of free radicals capturing and dehydrogenation/char-forming catalysis, respectively. Moreover, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide firstly reacted with graphene oxide (GO) to obtain phosphorus-containing GO (P-GO), while the CNCs were esterificated by maleic anhydride to give the nanoscaled products with carboxyl groups-coverd surface (CNC-COOH); subsequently, a novel flame retardant CNC/P-GO hybrid (CNC@P-GO) was prepared via the complexation of Fe3+ with carboxyl groups on the surface of P-GO and CNC-COOH. Compared with no char residue of individual CNC-COOH or P-GO in air atmosphere under high-temperature alone, the char of the CNC@P-GO hybrid with the equivalent ratio of CNC-COOH to P-GO as 2:1 could reach up to 37.6%, showing a graphitized continuous structure. When this kind of the CNC@P-GO hybrids was applied to promote flame retardant of bio-based polyester, only very low loading-level of 5 wt% could result in 17% char residue rate of CNC@P-GO-filled poly(butylene succinate) (PBS) in contrast to almost no char of neat PBS. At the same time, the peak heat release rate and total heat release dramatically dropped down by 71% and 66%, respectively. The predominant reduction of the combustion heat indicated an enhancement of fire safety, which was mainly attributed to the fact that the CNC@P-GO hybrid as flame retardant resulted in the formation of a dense and continuous carbon layer with high graphitization degree. Overall, this study enriches the ideas on the design of high-efficiency flame retardants derived from biomass resources and the fire-safety enhancement of bio-based polyester.