Several efforts have been dedicated to the development of lignin-based polyurethanes (PU) in recent years. The low and heterogeneous reactivity of lignin hydroxyl groups towards diisocyanates, arising from their highly complex chemical structure, limits the application of this biopolymer in PU synthesis. Besides the well-known differences in the reactivity of aliphatic and aromatic hydroxyl groups, experimental work in which the reactivity of both types of hydroxyl, especially the aromatic ones present in syringyl (S-unit), guaiacyl (G-unit), and p-hydroxyphenyl (H-unit) building units are considered and compared, is still lacking in the literature. In this work, the hydroxyl reactivity of two kraft lignin grades towards 4,4′-diphenylmethane diisocyanate (MDI) was investigated. 31P NMR allowed the monitoring of the reactivity of each hydroxyl group in the lignin structure. FTIR spectra revealed the evolution of peaks related to hydroxyl consumption and urethane formation. These results might support new PU developments, including the use of unmodified lignin and the synthesis of MDI-functionalized biopolymers or prepolymers.

Owing to its chemical structure, wide availability and renewable nature, lignin is a promising candidate for the partial replacement of fossil-based raw material in the synthesis of epoxy resins. Its poor compatibility has been reported to be one of the main drawbacks in this domain. On the other hand, a well-established modification method for lignin epoxidation has been used for many years for the improvement of lignin compatibility. However, the extent of the effect of lignin epoxidation on the improvement of bio-based epoxy mechanical properties, applied as adhesives, is still an open question in the literature. In this context, a pristine and industrial grade kraft lignin (AKL) was reacted with epichlorohydrin to yield epoxidized lignin (E-AKL) in this work. Afterwards, AKL or E-AKL were separately blended with petroleum-based epoxy resin at 15 and 30 wt% and cured with a commercial amine. The adhesive curing kinetic was evaluated using a novel technique for thermal transition characterization, Temperature Modulated Optical Refractometry (TMOR); the results showed that the incorporation of AKL reduces the crosslinking rate, and that this effect is overcome by lignin modification. Mechanical tests revealed an improvement of impact and practical adhesion strength for samples containing 15 wt% of E-AKL. These results elucidate the effect of lignin epoxidation on the application of lignin-based epoxy adhesives, and might support the further development and application of these bio-based materials.

No abstract

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.

No abstract

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.

No abstract

No abstract