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

Lignin, the world's second most abundant biopolymer,hasbeen investigated as a precursor of polyurethanes due to its highavailability and large amount of hydroxyls present in its structure.Lignin-based polyurethanes (LPUs) are usually synthesized from thereaction between lignin, previously modified or not, and diisocyanates.In the present work, LPUs were prepared, for the first time, usingthe blocked isocyanate approach. For that, unmodified and hydroxypropylatedKraft lignins were reacted with 4,4 & PRIME;-methylene diphenyl diisocyanatein the presence of diisopropylamine (blocking agent). Castor oil wasemployed as a second polyol. The chemical modification was confirmedby P-31 nuclear magnetic resonance (P-31 NMR)analysis, and the structure of both lignins was elucidated by a bidimensionalNMR technique. The LPUs' prepolymerization kinetics was investigatedby temperature-modulated optical refractometry and Fourier-transforminfrared spectroscopy. The positive effect of hydroxypropylation onthe reactivity of the Kraft lignin was verified. The structure ofLPU prepolymers was accessed by bidimensional NMR. The formation ofhindered urea-terminated LPU prepolymers was confirmed. From the results,the feasibility of the blocked isocyanate approach to obtain LPUswas proven. Lastly, single-lap shear tests were performed and revealedthe potential of LPU prepolymers as monocomponent adhesives.

Automated summary

This study investigates the use of lignin as a precursor for polyurethanes through the blocked isocyanate approach. The hydroxypropylation of lignin enhances its reactivity and leads to the formation of hindered urea-terminated polyurethane prepolymers. Single-lap shear tests demonstrate the potential of these prepolymers as monocomponent adhesives.