Exploitation of Arundo donax L. Hydrolysis Residue for the Green Synthesis of Flexible Polyurethane Foams

Flexible polyurethane foams were prepared from solid waste residue derived from the hydrothermal acid treatment of the Arundo donax L. herbaceous biomass, which produced a very high yield of levulinic acid. An innovative, sustainable, and green liquefaction route was adopted to produce lignin-based flexible polyurethane foams by partially replacing fossil-fuel source polyols with an abundant and renewable hydroxyl source, the Arundo donax L. "lignin-like" residue. Lignin liquefaction was performed in polyolic solvents using microwave irradiation, saving time and energy while ensuring a more sustainable and green approach. Foam production was performed with controlled expansion using the "one-shot" technique. Water was adopted as the only blowing agent, and the isocyanate index (NCO/OH) was kept to less than 100, which reduced the cross-linking degree of the desired foam and increased its flexibility. About 7 wt.% of the conventional petrochemical polyether polyol was replaced with the Arundo donax L. hydrolysis residue. The chemical and mechanical properties of the synthesized foams were compared with those obtained by using a pure technical soda lignin, ProtoBind 1000. The results were characterized by satisfactory mechanical properties, thus closing the biorefinery cycle of Arundo donax L. exploitation.