Suberinic Acids from Birch Outer Bark as a Renewable Feedstock for Polyurethane Polyols: Influence of Depolymerization Conditions on Structure and Reactivity

Suberinic acids obtained from birch outer bark represent a promising renewable feedstock for bio-based polyols suitable for polyurethane applications. Their chemical structure and reactivity depend strongly on depolymerization conditions, reagent type, and moisture content. In this study, extracted birch outer bark was depolymerized using NaOH, KOH, and KOH followed by FeCl3 precipitation to selectively remove phenolic components. The resulting suberinic acid mixtures (SAM) were characterized by GC-FID, GC-MS and GPC, and subsequently converted into polyols via oxyalkylation with propylene carbonate. FeCl3-treated SAM produced polyols with the highest hydroxyl values (435 mg KOH/g), while NaOH-derived SAM yielded the lowest viscosity polyols (5361 mPa·s). KOH-derived SAM generated the most viscous polyols due to the presence of long-chain oxidized structures and oligomeric fragments. Moisture was identified as a critical parameter: undried SAM caused uncontrolled reaction acceleration and catalyst deactivation during oxyalkylation. The results provide mechanistic insight into how depolymerization chemistry shapes SAM composition and polyol performance, supporting the development of optimized bio-based polyurethane systems.