Properties of flexible polyurethane foams containing isocyanate functionalized kraft lignin

A commercial kraft lignin (k-lignin) was successfully functionalized by means of the reaction of an isocyanate group from isophorone diisocyanate (IPDI) with hydroxyl groups present in lignin (k-IPDI), for their further incorporation in a flexible polyurethane foam (PUF) formulation. The functionalization was confirmed by FTIR and 13 C NMR, as well as by the OH number determination. It was observed that approximately half of the hydroxyl groups present in k-lignin were functionalized. Afterwards, two series of PUF containing 3, 5 and 10% of k-lignin and k-IPDI were prepared by one-shot free rise polymerization. It was observed that the presence of unmodified lignin decreased the reactivity of the system, yielding to longer foaming stages that required the addition of more catalyst to the system. Apart from that, more rigid foams were obtained with k-lignin than with k-IPDI, as it was observed by DSC and compression force deflection (CFD), which values reflected that the presence of k-IPDI contributed to the formation of more flexible foams. It was also found that the functionalization of lignin contributed to a higher chemical attachment of the lignin to the polyurethane matrix as it was observed by lignin extraction experiments. This chemical attachment was reflected on the stress-strain curves which showed that k-lignin containing foams had higher elastic modulus and higher ability to absorb energy than k-IPDI containing ones, which resulted to be more soft and flexible.