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

Functionalization of lignin was carried out with 4,4'-diphenyi methane diisocyante (MDI) at 90 degrees 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 similar to 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). (C) 2013 Elsevier Ltd. All rights reserved.