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

Area of Science:

• Polymer Chemistry
• Biomaterials Science

Background:

• Lignin, a renewable biopolymer, is abundant and rich in hydroxyl groups, making it a promising precursor for polyurethanes.
• Traditional synthesis of lignin-based polyurethanes (LPUs) involves reacting lignin with diisocyanates.

Purpose of the Study:

• To explore the feasibility of using a blocked isocyanate approach for LPU synthesis.
• To investigate the prepolymerization kinetics and structure of LPUs derived from unmodified and hydroxypropylated Kraft lignins.
• To evaluate the potential of these LPUs as adhesives.

Main Methods:

• LPUs were synthesized using unmodified and hydroxypropylated Kraft lignins with 4,4'-methylene diphenyl diisocyanate and diisopropylamine (blocking agent).
• Castor oil was used as a co-polyol.
• Chemical modification and structure were analyzed using 31P nuclear magnetic resonance (31P NMR) and bidimensional NMR.
• Prepolymerization kinetics were studied using temperature-modulated optical refractometry and Fourier-transform infrared spectroscopy.
• Lap shear tests were conducted to assess adhesive properties.

Main Results:

• Hydroxypropylation enhanced the reactivity of Kraft lignin.
• The formation of hindered urea-terminated LPU prepolymers was confirmed.
• The blocked isocyanate approach proved feasible for LPU production.
• LPU prepolymers showed potential as monocomponent adhesives.

Conclusions:

• The blocked isocyanate method is a viable route for synthesizing LPUs from Kraft lignin.
• Modified lignin (hydroxypropylated) exhibits improved reactivity.
• The resulting LPU prepolymers demonstrate promise for adhesive applications.