Chemically Modified Walnut Shell Biochar as a Sustainable Filler for Natural Rubber: Surface Properties, Filler–Filler Interactions and Mechanical Performance

Walnut-shell biochar (BC) was investigated as a sustainable filler for natural rubber (NR), with particular emphasis on the effects of chemical purification and subsequent surface functionalization on the properties of the resulting vulcanizates. BC was treated with citric acid (CA) or sodium hydroxide (NaOH) and subsequently functionalized using 3-aminopropyltriethoxysilane (APTS) or cetyl alcohol (CAl). The applied treatments substantially modified the surface chemistry and mineral composition of BC, as confirmed by FTIR spectroscopy and elemental analysis. Incorporation of BC increased the tensile strength of NR from 14.1 to 22.1 MPa and increased the crosslink density from 2.35 × 10−5 to 2.84 × 10−5 mol cm−3. Chemical purification also markedly affected the vulcanization behavior; the NaOH-treated BC increased the torque increment to 4.55 dNm and reduced the optimum cure time to 1.50 min, compared with 2.88 dNm and 1.99 min for the unfilled NR, respectively. Surface functionalization further modified the network structure, with the highest crosslink density of 3.45 × 10−5 mol cm−3 obtained for the APTS-functionalized BC. Particularly pronounced effects were observed during thermo-oxidative aging, where the aging coefficient increased from 0.31 for NR containing untreated BC to 0.89 for the CA-treated and CAl-functionalized system. The Payne effect also depended strongly on the modification route, decreasing from 75.5% for untreated BC to 40.1% after NaOH treatment. Overall, the results demonstrate that chemical purification combined with appropriate surface functionalization provides an effective approach for tailoring walnut-shell BC–NR interactions, vulcanization behavior, mechanical performance and aging resistance.