Biobased rigid polyurethane foam using gradient acid precipitated lignin from the black liquor: Revealing the relationship between lignin structural features and polyurethane performances

Rigid polyurethane foams (RPFs) are a type of versatile polymers with a variety of applications, which can be produced by condensation polymerization of isocyanates with renewable polyols that are derived from biomass materials. This study investigates the characteristics of bio-sourced synthesized RPF using lignin fractionated from the alkaline black liquor of corn stover. GPC, FTIR, TGA, 2D-HSQC, and 31P NMR were used to characterize the isolated lignin specimens. The fractionated lignin specimens were then pre-polymerized followed by using them as the substitutes of RPFs. The characterization and properties of the RPFs substituted with the isolated lignins were analyzed. Compared with other lignin specimens, the specimen isolated in pH range of 5.5–2.0 afforded to higher compressive strength (0.39 ± 0.01 MPa), lower apparent density (41.35 ± 0.21 kg m-3), lower thermal conductivity (0.043 ± 0.0012 W m-1 K-1), and better thermal stability of the RPF product. In addition, the relationships between structural features of lignin specimens and the properties of the RPFs were investigated. Results show the content of β-O-4 bond inner lignin structure was the main factor that affected on the thermal conductivity of RPF, while a higher hydroxyl groups content in lignin would largely improve the compressive strength and leading to the reduction of the apparent density. This research helps to comprehensively understand the features of lignin on RPF’s performance, providing a new method for its functionalization which is significant in their commercialization.