Biobased Flame-Retardant Polylactic Acid Foams through Lignin-Based Nanocarriers Encapsulating Deoxyribonucleic Acid

Lignin-based nanocarriers loaded with deoxyribonucleic acid (DNA) are investigated as a potential biobased flame-retardant additive in polylactic acid (PLA). The progressive ban of halogenated-based flame retardant has pushed toward developing more benign alternatives like phosphorus-containing biomaterials. DNA is thus of interest, since it is a biobased and abundant source of phosphates containing synergistic nitrogen and aromatic units in its structure. Here, we report the use of DNA to prepare biobased and flame-retardant PLA foams, which might be used for lightweight packaging or building insulation. For the first time, DNA was successfully encapsulated in lignin-based nanocarriers in situ via cross-linking in an inverse miniemulsion. Surface-grafting of the lignin nanocarriers (LNCs) with OH-terminated PLA is conducted to enhance the compatibility in the PLA matrix during the following composite preparation and adjustment of the LNC amounts from 1 to 20 wt %. Pressurized CO2 allows foaming the PLA composites, indicating that LNCs are promoting nucleation, leading to smaller cells and lighter foams even at low loading (1 wt %) compared to neat PLA, making them efficient biobased nucleating agents. Thermal stability of the PLA nanocomposites was reduced systematically when LNCs were added up to ca. 50 °C compared to neat PLA, and the flame retardant properties show an expected trend, i.e., the more DNA is introduced, the lower total heat release is observed. The LOI values of the foams, ranging from 25 to 30, are higher compared to those of their respective unfoamed composites. Overall, this strategy shows the potential of biopolymers to enhance flame retardancy of bioplastics and avoid toxic polymer additives.