In response to increasing concerns about climate change, the demand for innovative materials derived from renewable biomass is rapidly growing. Rubber elastomers are commonly employed in a wide variety of industries including tyre manufacturing, packaging, engineering and construction. The mechanical performances of rubber itself are unsatisfactory for the desired applications so their improvement is necessary and are commonly enhanced by adding reinforcing fillers.

The most popular reinforcing filler is carbon black whose use, however, is associated with health and environmental concerns. For this reason, many tyre manufacturers are concentrating their efforts in replacing carbon black with more sustainable alternatives.

Within the framework of a circular economy, one promising strategy is the valorisation of industrial waste and side streams such as lignin, the most abundant aromatic biopolymer on Earth. Lignin is mainly obtained as a by-product of the kraft pulping process in the paper industry. However, due to its intrinsic recalcitrance, complex structure, and limited solubility, lignin is commonly incinerated for energy recovery rather than converted into high-value products. The development of lignin nanoparticles (LNPs) has recently attracted significant attention because their high surface area and reduced particle size can enhance lignin valorization in advanced materials. Current research focuses on the functionalisation of LNPs to improve their stability and compatibility with various matrices [1,2].

The current work presents the design and production of sustainable functional reinforcing fillers based on selectively surface-functionalised kraft lignin nanoparticles and a bio-derived crosslinker for the partial replacement of carbon black in technical rubber compound.

Initially, a fractionation process was implemented to reduce the structural complexity and polydispersity of lignin by isolating fractions with more homogeneous properties. Solvent fractionation using solvents of different polarity enabled the separation of lignin fractions according to their solubility, which is primarily governed by molecular weight and the presence of polar functional groups such as phenolic, aliphatic hydroxyl, and carboxylic acid moieties [3]. Subsequently, selective chemical modifications were performed on the lignin fractions, particularly on the low molecular weight fraction, with a bio-based molecule which could covalently bind to the polymeric matrix during vulcanisation, ensuring reinforcement.

Building on previously reported self-assembly mechanisms of LNPs [4], surface-modified nanoparticles were prepared by coating pre-formed LNPs (derived from the insoluble fraction) with the chemically modified lignin fraction, in order to obtain a complete valorisation of the initial kraft lignin. Core-shell LNPs were obtained through a two-step anti-solvent precipitation process. The morphology and stability of the resulting LNPs were characterised. Selectively-functionalised LNPs were included in technical rubber compounds in partial substitution to carbon black and their reinforcement performances were systematically evaluated.

References

1- F. Ferruti, M. Carnevale, L. Giannini, S. Guerra, L. Tadiello, M. Orlandi, L. Zoia ACS Sustain. Chem. Eng. 2024, 12, 14028.

2- M. Carnevale, F. Ferruti, L. Tadiello, S. Guerra, L. Giannini, M. Orlandi, L. Zoia ACS Sustain. Chem. Eng. 2026, 14, 5460.

3- M. H. Sipponen, H. Lange, M. Ago, C. Crestini ACS Sustainable Chem. Eng. 2018, 6, 9342.

4- F. Ferruti, I. Pylypchuk, L. Zoia, H. Lange, M. Orlandi, A. Moreno, M. H. Sipponen Green Chem. 2023, 25, 639.

Acknowledgments

All the authors acknowledge financial support from CORIMAV (Consortium for the Research of Advanced Materials).