Incorporating Phenolated Lignin into Polyurethane Materials: Impact on Mechanical, Thermal, and Adhesion Performance

The complex and resistant nature of lignin's structure impedes its direct use in synthesizing polymeric materials. Consequently, modifying its backbone chemically is necessary to incorporate compatible functionalities into the polymer matrix. In this paper, we present phenolation as a simple and straightforward chemical modification method to improve lignin's reactivity and compatibility toward the synthesis of polyurethane (PU) materials. A series of phenolated lignin-based polyurethane (PhLPU) films were synthesized using phenolated lignin (PhL) and polytetrahydrofuran as a polyol mixture and either an aromatic or an aliphatic isocyanate, namely, poly[(phenyl isocyanate)-co-formaldehyde] (pMDI) and hexamethylene diisocyanate trimer (HDI trimer), respectively. The influence of PhL concentration as well as isocyanate's chemical structure on the thermostability and mechanical performance of PhLPU films was systematically studied. The findings revealed that PhLPU films formulated with a small fraction of PhL exhibit notably improved thermomechanical properties compared to those of lignin-free networks. Furthermore, PhLPU films produced with aromatic pMDI displayed superior strength characteristics when compared to those prepared with the aliphatic HDI trimer. Finally, PhLPU blends were tested as wood adhesives for PU, measuring adhesion via single lap shear tests. The obtained shear strength values matched or surpassed those of a commercial PU glue. This research underscores the efficacy of phenolation as a means to boost the chemical reactivity of lignin, thereby contributing to the development of high-performance PU films and adhesives with potential applications in various industries.