Influencing the properties of LigninPU films by changing copolyol chain length, lignin content and NCO/OH mol ratio

Abstract

The polyurethane films with chemical incorporation of lignin (ligninPU) films were synthesized using kraft lignin (KL), 4,4′-Methylene diphenyl diisocyanate (MDI) and polypropylene glycol (PPG) copolyols and characterized. The effect of PPG chain lengths, lignin content, and NCO/OH mol ratio on crosslink density, thermal and mechanical properties of solution casted LigninPU films were studied. In general, homogenous and good quality films were obtained, irrespective of poor miscibility between PPG and KL. FTIR established the formation of urethane as well as urea linkages and covalent incorporation of lignin macromers within the PU network. The crosslink density (CD) values were between 0.2 and 1.2 × 10−3 mol cm−3. They were mainly influenced by lignin W% and increased with lignin content. A complex effect of change in NCO/OH ratio at constant lignin content and PPG copolyol on the CD values was observed. The thermal stability was higher for the films prepared with longer PPG copolyols at constant NCO/OH mol ratio and lignin content. The char yield for the ligninPU films increased as the NCO/OH ratio was increased from 1.1 to 1.5. The glass transition temperatures of the films were mainly influenced by the PPG chain length. PPG 425 based films were brittle at room temperature and exhibited Tg above 40 °C. Whereas the longer PPG derived ligninPU displayed more flexibility and featured Tg near or below room temperature. The mechanical properties of the films were found to deteriorate with increasing PPG chain length. The ultimate tensile strength and Young’s modulus increased with increasing lignin content, however % elongation at break decreased.