Sustainable Adhesives—Exploring Boronic Ester Vitrimer Composites Containing Lignin Microparticles

The production of epoxy resin-based adhesives has been continuously growing in recent years since they are key components in several industries, such as automotive, aerospace, construction, and electronics. The attention is focused on formulations that combine lightweight materials with outstanding properties in terms of high bonding strength, durability, and insulation. Despite these great advantages, many commercial formulations have drawbacks related to their utilization. Firstly, the presence of permanent and non-processable crosslinking bonds prevents chemical and mechanical recyclability, leading to accumulation of recalcitrant materials in landfills. Secondly, the petroleum-based origin of the monomers involved in the crosslinking process, such as bisphenol A diglycidyl ether, raise concerns about their potential harm to the environment and human health. In this regard, the United Nations asked for a decisive change towards the production of bio-based materials that could be evenly reprocessed, recycled, and biodegraded.
To overcome the recyclability problem of a general thermoset, dynamic covalent bonds, which can be thermally activated, has been introduced in the 3D covalent network. The resulting materials, called vitrimers, combine the resistance and structural features of thermosets with the reprocessability of thermoplastics. In literature, examples of reversible exchange reactions that could be involved in vitrimer structures include transesterification, disulfide exchange, imine metathesis, and boronic ester exchange.¹
In this work,² a series of bio-based vitrimer composites, made from two renewable feedstocks, namely epoxidized linseed oil and commercial Kraft lignin, were prepared and used as alternative sustainable adhesives on several substrates, such as aluminium, stainless steel and ash tree wood. Specifically, lignin microparticles were added in 20 to 50 wt% to the vitrimer matrix formed by epoxidized linseed oil and a dithiol boronic acid without any previous chemical modification or the assistance of additional solvents. The lignin/vitrimer composites showed lap shear strengths in the range of 9-17 MPa and good to excellent rewelding abilities for at least four cycles depending on the filler amount in the matrix. The presence of a vitrimer matrix allowed the processability and recyclability of the materials at temperatures above 160 °C, while the inclusion of a pulp industry waste product positively influences the thermal and mechanical properties, improving the rewelding abilities of the adhesive and enhancing the biodegradability in seawater and soil. The reported materials want to pave the way for a new class of bio-based, biodegradable, and highly processable and recyclable adhesives.

¹ V. Schenk, K. Labastie, M. Destarac, P. Oliver, M. Guerre, Vitrimer composites: current status and future challenges, Mater. Adv. 3 (22) (2022) 8012–8029.
² M. Nardi, L. Ceseracciu, V. Scribano, M. Contardi, A. Athanassiou, A. Zych, Sustainable adhesives: Exploring boronic ester vitrimers containing lignin microparticles, Chem. Eng. J. 495 (2024) 153400.