Polyurethane adhesives from castor oil and modified lignin via reaction with propylene carbonate

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.

Lignin-Based Polyurethanes from the Blocked Isocyanate Approach: Synthesis and Characterization

“…After approximately 0.20 h, in stage II, a reduction of the slope of the N Mean vs time curves was observed, indicating a significant decrease in the reaction rate of BPUP_30KL and BPUP_30HKL_PO prepolymerization. This behavior is a consequence of two factors: (a) the decrease in reagent concentrations after their consumption and (b) the decrease in molecular mobility due to a molecular weight increase, which slows further diffusion of the reactants. , Despite showing similar trends, the system containing HKL_PO displayed higher reaction rates throughout stage II than the one containing KL. This result suggests that the modified lignin has a higher reactivity compared to the unmodified one, which is in agreement with the FTIR analysis (Figure ).…”

“…First, its use alone, as the only polyol in PU formulations, is disadvantageous because it results in highly brittle materials . For that reason, lignin is frequently blended with other polyols, such as polyethylene glycol (PEG), a conventional petroleum-based polyol, and, in a sustainable approach, vegetable oils, such as castor oil for example. , This second polyol acts as the soft segment in the lignin-based PU formulation. Even with the addition of the second polyol, lignin can only be incorporated up to 30 wt % without deterioration of the PU’s mechanical properties. , Another drawback is the low reactivity of the lignins’ hydroxyl groups against isocyanates, especially aromatic ones, due to steric hindrance .…”

Polyurethane Adhesives Based on Oxyalkylated Kraft Lignin

“…The superior performance of lignin-based adhesives compared to CPA can be attributed to the following reasons: (1) the aromatic nature of lignin, which contributes to their stiffness [ 40 , 41 ], and (2) the modification of lignin, which yields a polyether with mainly primary OH groups which react promptly with the polyisocyanate. Thus, increasing the crosslinking density of the polymer matrix and subsequently the adhesive strength [ 28 ]. Furthermore, the crude LBP contains residual catalyst and PC-oligomer, which may promote the crosslinking of the polymeric network, as well as enhance the wettability of the surface and convey some flexibility [ 25 ].…”

“…Yet, Gouveia et al [ 27 ] prepared lignin-based PU adhesives from kraft lignin oxyalkylated with PO under different conditions. In a similar manner, the same group prepared lignin-based PU adhesives by mixing castor oil with up to 30 % of LBP oxyalkylated with PC [ 28 ]. So far, to the best of our knowledge, these are the only studies that reported the use of oxyalkylated lignin with cyclic carbonate to produce PU adhesives.…”

Lignin as a Renewable Building Block for Sustainable Polyurethanes

“…Gouveia et al [ 149 ] prepared PU adhesives for wood bonding from kraft lignin oxyalkylated under mild conditions with PO. Recently, the same group prepared PU adhesives by mixing castor oil with up to 30% of LBPs oxyalkylated with PC, where the use of LBPs improved the Young’s modulus and adhesion [ 150 ]. So far, this is the only study that has reported the use of lignin oxyalkylated with cyclic carbonate to produce PU adhesives.…”