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

Lignin, the world's second most abundant biopolymer, has been investigated as a precursor of polyurethanes due to its high availability and large amount of hydroxyls present in its structure. Lignin-based polyurethanes (LPUs) are usually synthesized from the reaction between lignin, previously modified or not, and diisocyanates. In the present work, LPUs were prepared, for the first time, using the blocked isocyanate approach. For that, unmodified and hydroxypropylated Kraft lignins were reacted with 4,4'-methylene diphenyl diisocyanate in the presence of diisopropylamine (blocking agent). Castor oil was employed as a second polyol. The chemical modification was confirmed by 31P nuclear magnetic resonance (31P NMR) analysis, and the structure of both lignins was elucidated by a bidimensional NMR technique. The LPUs' prepolymerization kinetics was investigated by temperature-modulated optical refractometry and Fourier-transform infrared spectroscopy. The positive effect of hydroxypropylation on the reactivity of the Kraft lignin was verified. The structure of LPU prepolymers was accessed by bidimensional NMR. The formation of hindered urea-terminated LPU prepolymers was confirmed. From the results, the feasibility of the blocked isocyanate approach to obtain LPUs was proven. Lastly, single-lap shear tests were performed and revealed the potential of LPU prepolymers as monocomponent adhesives.