Synthesis and characterization of novel foams by pyrolysis of lignin

We report the synthesis and characterization of lignin-based foams by pyrolyzing cold pressed lignin compacts at 300°C, 500°C, 700°C, and 900°C in an argon (Ar) atmosphere. Detailed thermogravimetric analysis (TGA), thermomechanical analysis (TMA), Fourier transform infrared spectroscopy (FTIR), and microstucture analysis was performed on these samples. Scanning electron microscopy (SEM) and X-ray tomography analysis showed that all the foams had cellular structure, and had a total porosity of 93.3%, 94.5%, 93.9%, and 93.4% after pyrolysis at 300°C, 500°C, 700°C, and 900°C, respectively. Comparatively, the fraction of open porosity of the total porosity increased from 89.7% at 300°C to 97.1% and 98.1% at 500°C and 700°C, respectively, and then decreased to 93.0 % at 900°C. The averaged ultimate compressive strength (UCS) of the foams fabricated at 300°C was ~0.77 MPa. It decreased to ~0.49 MPa at 500°C, after which it increased to ~0.95 MPa at 700°C and then retained similar strength at 900°C. Detailed FTIR analysis showed a decreased in intensity of peaks for all the samples fabricated at higher temperatures, which was further correlated with changes in lignin molecular structure. By analyzing TGA, TMA, FTIR, and microstructure, a novel mechanism for designing lignin-based cellular solids has been proposed in this work. The wettability study by sessile drop test showed that the foams are hydrophobic after pyrolyzing at 300°C and 500°C, after which the compacts became hydrophilic after pyrolyzing at 700°C and 900°C, respectively.