Thermal Stability and Fire Retardant Properties of Polyamide 11 Microcomposites Containing Different Lignins

This study investigates the influence of various lignins and their content on the thermal stability and fire retardancy of biobased polyamide 11 (PA). Microcomposites based on PA and containing 5, 10, 15, and 20 wt % different lignins were prepared with a twin-screw extruder. Morphological analysis showed good interfacial interaction and uniform distribution of lignin particles within the resulting microcomposites. Further, thermogravimetric analyses carried out in inert atmosphere indicated that, unlike kraft lignin, which is able to give rise to the formation of lower char residue (41–48 wt % at 600 °C), the sulfonated counterpart provides a higher thermal stability as well as a higher char residue (55–58 wt %). Furthermore, vertical flame spread tests clearly showed that 15 wt % is the optimum of kraft or sulfonated lignin loading to achieve improved flame retardant properties and a V1 rating. In addition, a cone calorimeter was exploited to study forced combustion behavior; in particular the microcomposites containing sulfonated lignin revealed significant reductions of the peak of the heat release rate (−51%) and of the total heat release (−23%) and a lower average mass loss rate together with a noticeable increase of the final residual mass (about 9 wt %). Conversely, the microcomposites containing kraft lignins showed opposite effect, since heat release rate (HRR) and total heat release (THR) values increased in the presence of kraft lignin.