Olive stone as a filler for recycled high-density polyethylene: A promising valorization of solid wastes from olive oil industry

The valorization of solid residues generated from olive oil production is a challenging issue for olive oil-producing countries such as Greece, Italy, Spain, etc. Olive stone waste is a type of lignocellulosic biomass with potential use as a natural additive in polymeric composites. In the present study, olive stone waste was compounded with recycled high-density polyethylene (rHDPE) via melt mixing in 5%, 10%, and 20% wt. Additionally, the effect of Joncryl (JC) and polyethylene-grafted maleic anhydride (PE-g-MA) as compatibilizers on the properties of the green composites was also investigated. It is worth mentioning that JC was employed for the first time as a compatibilizer for polyolefin composites with olive stone. The materials underwent structural, thermal, and mechanical characterization using multiple complementary techniques. Scanning electron microscopy showed good dispersion of the filler for all composites, while in the compatibilized samples a markedly better adhesion was observed. Tensile tests revealed an increase in the elastic modulus for the composite with 10% wt. olive stone. Furthermore, all materials demonstrated a strong antioxidant activity, rendering them interesting for active packaging applications. Overall, the solid residue from olive processing can enhance the properties of rHDPE, paving the way to promote circular economy.