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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.

A series of novel phosphaphenanthrene/phenylsiloxane bigroup macromolecules (DDSi-n) were synthesized and applied to obtain high-performance epoxy thermosets. DDSi-n macromolecules simultaneously enhanced the anti-impact and flame-retardant performance of epoxy thermosets. The impact strength of the DDSi-n-containing thermoset (DDSi-n/EP) was maximally increased by nearly 140% in 8% DDSi-1/EP because of the flexible phenylsiloxane block and the polar phosphaphenanthrene group interacting with matrix in DDSi-n macromolecules. Meanwhile, the evidently elevated anti-ignition and self-extinguishing performance, the suppressed combustion heat, and the enhanced charring capability of DDSi-n/EP were all caused by the flame-retardant group synergistic effect of phosphaphenanthrene and phenylsiloxane groups of DDSi-n macromolecules in thermosets. Compared with the individual phosphaphenanthrene or phenylsiloxane group in monogroup contrasts, the phosphaphenanthrene and phenylsiloxane groups in bigroup DDSi-n macro...