AbstractThe development of new sustainable material solutions in the processing of thermoplastic polymers concerns both the application of biopolymers and the use of valorized plant derivatives as fillers and modifiers of petrochemical polymers. Herein, the possibility of using unprocessed raw parts of two commonly used in the food industry leaves, i.e., lemon (LL) and stevia (ST), as active and functional fillers for high-density polyethylene (HDPE) has been verified. The series of composites containing 1, 2, and 5 wt% of ground leaves produced in the melt-mixing process were analyzed for thermal properties (DSC and TGA), and the antioxidant potential of the fillers was evaluated. Verifying the active effect of the ground leaves on the resistance to oxidation in the molten state was carried out by oxygen induction time (OIT by DSC) analysis and oscillatory rheology under steady-state shear conditions combined with spectroscopic (FTIR) carbonyl index (CI) analysis. Studies have shown that the introduction of 5 wt% of both types of leaves allows for a significant increase in the melt oxidation resistance (above 2 times longer OIT concerning HDPE, ~ 35 min) of composites without substantial changes in their crystalline structure and thermal stability. Determined after the long-term rheological measurements in an oxidative atmosphere CI showed 70 and 82% lower values for 5 wt% LL and ST composites compared to unmodified polyethylene. Graphical abstract