Effect of functionalized lignin on the properties of lignin–isocyanate prepolymer blends and composites

Functionalization of lignin was carried out with 4,4′-diphenyl methane diisocyante (MDI) at 90 °C for 60 min. Formation of the resulting reaction products was confirmed with the help of FTIR, FESEM–EDAX and DSC. Polymer blends were prepared using an isocyanate prepolymer (NCO/OH: 3.5) and functionalized lignin (up to 15 wt%) and evaluated their phase miscibility, wettability, rheology and mechanical properties. Under DMA run, the maximum storage and loss moduli of blends were observed at 5 wt% lignin content. Wettability results indicated that as the lignin content increased in the blends, the contact angle of probe liquids on the substrate increased while polar component of surface-free energy decreased affecting their wettability. FTIR spectra of the blends showed disappearance of –NCO peak at 2270 cm−1 supported its consumption in the network formation. Rheological test revealed that lignin-prepolymer blends needed more stress to flow and also had more viscosity recovery compared with the parent system. The single-lap joint shear strength of blends was higher with the aluminium substrate than the wood. The pull-off adhesive strength of blend at 5 wt% lignin content was ∼19% more than the parent prepolymer. The composites made with isocyanate blend and pine needles exhibited acceptable properties as specified in the standard (EN 312).