**4. Conclusions**

DESG was prepared by grafting SG with DOPO and EA. Flame-retardant PLA composites were obtained by melt-blending PLA with DESG and APP. Here, DESG acted as a carbon source due to its polysaccharide structure, while APP acted as an acid source and a gas source to catalyze the formation of char residue and release nonflammable gases, respectively. The flame retardancy, thermal properties, morphology and tensile properties of flame-retardant PLA composites were investigated. The results demonstrated that the synergistic effect of DESG and APP promoted the formation of an intumescent protective layer in PLA composites at a lower temperature, improved the high temperature stability of the composites and effectively enhanced the flame retardancy of PLA. When the mass ratio of DESG/APP is equal to 12/8, an LOI value of 32.2% and a UL-94 V-0 rating were achieved. A V-0 rating can also be reached at a lower addition ratio. Moreover, the addition of DESG could improve compatibility between the dispersed phase and the PLA matrix, as well as reduce the influence of the flame-retardant on the mechanical properties.

**Author Contributions:** Q.Z. and H.L. carried out the preparation work, analyzed the results and wrote the manuscript. J.G. performed the structural characterization and performance testing. X.Y. and B.L. discussed and analyzed the results. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the Program of Tangshan Normal University (2022C44).

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.
