On revealing the effect of alkaline lignin and ammonium polyphosphate additives on fire retardant properties of sustainable zein-based composites

The paper deals with the evaluation of the efficacy of both ammonium polyphosphate, APP, and alkaline lignin, AL, as suitable additives for the development of innovative and sustainable composites based on thermo-plasticized zein, TPZ. The chemical and physical interactions which occur between AL and APP and the secondary structure of thermo-plasticized zein were investigated by torque analysis and FTIR spectroscopy and were exploited to rationalize macroscopic properties such as the thermal stability, fire behavior and the mechanical properties. In details the fire behavior of the composites was evaluated by both cone calorimetry and self-sustained vertical combustion tests in order to simulate different fire scenario. The results highlighted that AL intercalates both the α-helix and β-sheets structures and thus improves the fire retardant behavior of composites in the vertical burning scenario by enabling the withdrawal effect of the film from flame due to the removal of PEG plasticizer during the burning test. On the other side, APP enhances the fire resistance of composites exposed to pool forced combustion due to an enhanced charring effect which protects the substrate during burning. Finally the composite containing simultaneously 10 wt% APP and 3 wt% AL exhibited excellent fire resistance behavior in different fire scenario and satisfying mechanical properties with a toughness which is around 30% higher than that of pristine TPZ. Due to this peculiar combination of functional and structural properties the composite materials may be considered as potential substitutes of traditional polymeric materials in several application fields (i.e packaging, electrical components, …).