Fully Biobased Surface-Functionalized Microcrystalline Cellulose via Green Self-Assembly toward Fire-Retardant, Strong, and Tough Epoxy Biocomposites

The design of renewable and fully biobased flame retardants (FRs) with high efficiencies and mechanical reinforcement functions for epoxy resin (EP) can greatly advance their potentials to satisfy sustainability. Although some biobased fire retardants have been successfully developed so far, most of existing biobased FRs are often not fully biobased and their syntheses normally involve the use of a large volume of organic solvents in addition to complicated synthesis processes. Herein, we report a facile and green strategy to synthesize fully biobased FR (P-MCC@CS@PA-Na) by surface-functionalizing microcrystalline cellulose (MCC) with chitosan (CS) and sodium phytate (PA-Na) via layer-by-layer assembly in water. The results show that incorporating 15 wt % P-MCC@CS@PA-Na enables EP composite to pass a UL-94 V-1 rating with a limiting oxygen index of 26.2%. Meanwhile, the peak heat release rate, total heat release, peak smoke production release, total smoke production, the fire growth rate, and the fire retardancy index of the EP/15 wt % P-MCC@CS@PA-Na are greatly reduced, indicating a good fire retardance. Notably, the well-designed P-MCC@CS@PA-Na simultaneously strengthens and toughens the EP because of uniform dispersion and a favorable interfacial compatibility between P-MCC@CS@PA-Na and the EP matrix. This work provides a green strategy for the fabrication of highly efficient multifunctional fully biobased FRs for polymers.