Depolymerization of lignins and their applications for the preparation of polyols and rigid polyurethane foams: A review

Lignin, nature’s dominant aromatic polymer, is found in most terrestrial plants in the range of 15–40% dry weight and provides structural integrity. Kraft lignin (KL) is a major by-product of pulp & paper industry where, hydrolysis lignin (HL) is the solid residue left from the enzymatic hydrolysis of wood after the pretreatment processes in cellulosic ethanol plants. Currently, most of the lignin is burned to generate heat and electricity and remaining is considered as a low value material. Only 1% of the annually produced lignin is being commercialized for its application in the preparation of bio-chemicals and to limited extent for bio-materials. Although with much lower reactivity, even crude lignin (a natural polyol) can be directly incorporated into polyurethane (PU) foam formulation due to the presence of aliphatic and aromatic hydroxyl groups in its structure as the reactive sites. However, bio-replacement ratios are usually low ~20–30% and further increasing replacement ratios results in fragile and low strength foams. Lignin depolymerization with selective bond cleavage is still a major challenge for converting it into value-added precursors especially for its utilization in the preparation of rigid PU foams. Depolymerization of these macromolecules can result in the valuable products with high hydroxyl number/functionality and low molecular weights, which in turn will increase the percentage replacement of bio-based polyols in the PU foam formulations. The technical routes/technologies for the depolymerization of lignins and their effective utilization as polyols in PU foams are summarized in this review article. These include direct utilization of lignin as well as the incorporation of depolymerized lignins, with and without modification, at high replacement ratios in PU foams. The major emphasis was given on the effective utilization of low value lignin for high value applications. Some of the associated challenges for the production of materials from lignin are also discussed.