Polyurethane (PU) foam consumers and producers are actively looking for ways to improve the sustainability of their products by replacing petroleum-based raw materials with biobased, sustainable raw materials. Lignin contains both aliphatic and aromatic hydroxyl groups, making it a suitable natural polymer to replace petroleum-based polyol in the formulation of polyurethanes. Previous studies have shown that the incorporation of lignin in PU foams increased flame retardancy, antimicrobial, and mechanical properties of the foams such as compressive strength and Young's modulus. This study is focused on evaluating the suitability of a wide range of lignins isolated from different plant sources and chemical processes in replacing 20 wt% of petroleum-based polyol in the formulation of PU flexible foams. The main emphasis was to study the effect of lignin incorporation on the foam's structural, mechanical, and thermal properties. The results showed that incorporating 20 wt% lignin increased tensile, compression, tear propagation strengths, and support factor of the developed PU flexible foams. Additionally, statistical analysis showed that foam properties such as density and compression force deflection were positively correlated to lignin's total hydroxyl content. In contrast, ultimate elongation was negatively correlated with the lignin's total hydroxyl content. Among tested lignins, organosolv lignins were the most suitable lignins for partial replacement of polyol in PU flexible foams designed for automotive applications, because they had higher solubility in co-polyol, lower glass transition temperature (Tg) and lower hydroxyl content compared to kraft lignins.
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