Influence of Depolymerization Conditions on Birch Bark Suberinic Acids Evaluated by Fractional Factorial Design

Birch (Betula spp.) outer bark is a renewable side stream rich in suberin, a natural polyester that can be depolymerized into suberinic acids (SA) containing hydroxyl, carboxyl, epoxy, and other functionalities relevant for bio-polyol synthesis. A two-level fractional factorial design (25−1), combined with regression modelling and response-surface visualization, was applied to evaluate the effects of alkali type, alkali concentration, temperature, reaction time, and acidification pH on SA composition and properties. Sixteen depolymerization experiments were performed, and the resulting SAs were characterized by epoxy content, acid and hydroxyl numbers, saponification number, total phenolic content, molecular weight (MW) distribution, extractives fraction, ash content, and GC–MS-detectable low-MW compounds. Most statistical models showed satisfactory explanatory performance within the investigated experimental domain (R2 = 0.77–0.93), while the model describing hydroxy acids and corresponding esters exhibited lower explanatory power (R2 = 0.49) and was excluded from the response-surface optimization. Among the investigated factors, acidification pH exerted the strongest influence on acid number, MW distribution, and recovery of depolymerization products. Alkali concentration generally increased depolymerization severity, resulting in higher proportions of low-MW compounds and increased functionality. Differences between KOH and NaOH were observed in several compositional parameters, suggesting differences in depolymerization and recovery behavior. The results identify experimentally relevant regions within the investigated factor space that may be suitable for producing SAs with different compositional profiles depending on subsequent purification requirements. These findings provide an initial quantitative description of the relationships between depolymerization conditions and SA characteristics and support further development of birch-bark-derived SAs as renewable feedstocks for bio-based polyol production.