Lignin is a complex, abundant natural polymer that has attracted significant attention as an alternative to petroleum-based polyols in rigid polyurethane foams, owing to its aromatic structure, high functionality, and high hydroxyl (OH) value. However, the limited reactivity of its sterically hindered phenolic hydroxyl groups towards isocyanates has limited the utilization of solid lignin to approximately 50% polyol substitution, with a noticeable decline in properties beyond that point. Oxyalkylation with cyclic carbonates has emerged as a green and effective method for improving lignin reactivity by converting the phenolic hydroxyl groups into more reactive aliphatic OH groups. This thesis focuses on the development of rigid polyurethane and polyisocyanurate foams using oxypropylated lignin polyols. First, a simple yet novel method was employed to increase the hydroxyl value of lignin polyols by the in-situ generation of propylene glycol. Then, rigid polyurethane (PU) foams were produced by fully replacing petroleum-based polyols with synthesized lignin polyols. The developed lignin polyols had very high reactivity, necessitating the addition of a delayed-action catalyst to control foaming. The lignin-based foams also met most of the standard specifications for type II spray-applied rigid cellular polyurethane thermal insulation and had superior mechanical properties compared to the foam made with conventional fossil fuel-based polyol. The biobased foams, however, had slightly lower thermal stability due to the presence of propylene glycol and other oligomers of propylene carbonate. Finally, urethane-modified polyisocyanurate (PUR/PIR) foams were prepared with oxypropylated lignin polyols. These foams exhibited improved flame retardancy, with significantly shorter burn times than the control, even without the addition of flame retardant, and showed less shrinkage compared to the petrochemical-based foams.
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