Reinforced lignin-based polyurethane foams with improved heat-insulating performance by using cellulose fibers

The incorporation of natural polymers into the synthesis of functional materials such as polyurethanes provides an effective approach for value-added biomass utilization. In this study, Lignin-containing polyol (LP) was prepared by simply mixing Kraft lignin, polyethylene glycol, and glycerol at mass ratio of 3 : 3.5 : 0.5. Lignin-based polyurethane foam (LPUF) was then prepared by mixing LP and polymethylene diphenyl diisocyanate (PMDI) using water as a foaming agent. Cellulose fibers from wood at various loadings (0%, 0.2%, 0.5%, 0.7%, and 1.0%) were incorporated as reinforcing skeletons to investigate the effects on the mechanical properties of LPUF. The results showed that the compressive strength, the flexural strength, and the tensile strength of LPUF were significantly increased by 72.3%, 154.5%, and 244.1% by 0.5% cellulose fiber addition, and reached 647.6 kPa, 1.28 MPa, and 1.17 MPa, respectively. Furthermore, improvement of thermal insulation performance of LPUF was also observed by the decrease of thermal conductivity from 0.0413 W m^-1 K^-1 to 0.0378 W m^-1 K^-1 by 0.5% cellulose fiber addition. However, cellulose fiber addition over 0.5% resulted in irregular pore morphology, ultimately impairing the mechanical performance of the LPUF. Cellulose and lignin are the major constructional constituents of woody biomass. The application of these natural polymers in polyurethane materials contributes to sustainable development and carbon neutrality.