Over the last several years, lignin hydroxypropylation with propylene oxide (PO) has been used as the main strategy to overcome lignin limitations, such as its low reactivity, poor dispersion, and high stiffness, in the context of polyurethane (PU) synthesis. However, PO is a flammable, toxic, and carcinogenic compound. Propylene carbonate (PC), a compound with low toxicity and biodegradability, has emerged as a feasible alternative to PO. Although this compound has potential for use in syntheses, lignin hydroxypropylation with PC has not yet been explored in PU synthesis. In this work, PU adhesives are synthesized using castor oil (CO) and hydroxypropylated lignin via the reaction with PC (HKL_PC) as a polyol. HKL_PC is incorporated in CO at different concentrations (10 wt%, 20 wt%, and 30 wt%). Curing kinetics is investigated with temperature‐modulated optical refractometry (TMOR). Dynamic mechanical analysis (DMA) demonstrates an increase in Tg after lignin addition. Furthermore, Young's modulus and practical adhesion are improved with increasing HKL_PC concentration. Results prove that hydroxypropylation with PC is a feasible method for lignin‐based PU synthesis and reveals the potential of this approach as an alternative for the PO‐based method.

Polypropylene (PP) is a multifunctional and widely applied polymer. Nevertheless, its low energy surface and poor adhesion are well-known and might impair some prospective applications. Aiming to overcome these limitations, PP composites can be applied as a tool to enhance PP surface energy and then increase its practical adhesion. In this work, Kraft lignin (KL) was chemically modified and blended with PP. In short, KL was hydroxypropylated and further reacted with acetic anhydride (A-oxi-KL) or maleic anhydride (M-oxi-KL). Lignin modifications were confirmed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). PP-composites with different lignin contents, as well as pristine PP, were characterized in terms of their thermal behavior, morphology, surface energy, and practical adhesion by DSC, scanning electron microscopy (SEM), contact angle measurement, and peeling tests, respectively. Lignin incorporation did not affect the PP degree of crystallization. The lignin modifications led to a better compatibility with the PP matrix and surface energies up to 86% higher than neat PP. Increases of up to 66% in the peel strength were verified. Composites with M-oxi-KL showed the best adhesion performance, confirming the lignin functionalization is an efficient approach to improve the practical adhesion of PP films.