Structure–property relationships of flexible, non-isocyanate polyurethane foams from lignin and castor oil-based reagents

Polyurethane foam is a valuable material with applications across the automotive, packaging, construction, appliance, and furniture industries. However, traditional polyurethane foams are made from petroleum-based precursors including harmful isocyanates. Isocyanates are the leading cause of workplace asthma and are made from toxic phosgene gas, representing a challenge to green chemistry principles. To address these concerns, non-isocyanate polyurethane (NIPU) foams replace the traditional polyurethane reaction with a safer, non-isocyanate process most often employing cyclic carbonates and diamines. Current approaches to NIPU foams struggle to meet the low-density and flexibility of commercial foams and also employ many petroleum-derived and toxic agents. This work demonstrates the use of kraft lignin, a crosslinked aromatic polymer produced by the dominant kraft pulping process, as an effective raw material for flexible NIPU foams, reaching densities below 100 kg m⁻³ while displaying flexible compression properties. A biobased, aliphatic agent was added in various amounts to introduce flexibility to the polymer structure while modifying the viscosity of the reaction mixture to enable increased rise height of foams containing over 30% lignin. The thermal, mechanical, and burn testing results of a series of NIPU foams are compared to a conventional PU reference foam and the structure–property relationships of the novel materials are explored based on the lignin-to-aliphatic carbon content. The results show a fully biobased, flexible, NIPU foam from lignin that can approach commercial properties, helping to demonstrate the relevance of NIPU foams for many applications.