Chemical and structural characterization of hardwood and softwood LignoForce™ lignins

Knowledge about lignin structure after isolation is paramount for demonstrating its potential for industrial applications. This paper presents for the first time the detailed characterization of LignoForce (TM) hardwood (LFHL) and softwood (LFSL) kraft lignins that were obtained from industrially realistic lignin production facilities. The moisture content was circa 8 % for both LFHL and LFSL, and their total carbon contents were 62.8 and 64.4 %, respectively. Both samples had low ash and carbohydrate contents and total lignin contents higher than 97 %. The macromolecular properties and functional groups of lignin were characterized by nuclear magnetic resonance, analytical pyrolysis (Py-GC-MS), thermal analysis, mass spectrometry, and size exclusion chromatography. LFHL presented lower aliphatic and phenolic OH groups, higher methoxyl content (S/G ratio of 1.97), lower weight-average apparent molecular mass (Mw) and degree of condensation, higher solubility in organic solvents, and lower thermal stability compared to LFSL. C1-Guaiacol and C1-syringol predominated in the PyGC-MS analyses of LFSL and LFHL, respectively, whose beta-O-4' contents were 5.8 and 6.2 per 100 aromatic units. The high total OH groups of both lignins suggests their suitability for replacing petroleum-based polyols in the preparation of polyurethane foams. LFHL would be preferable in this application due to its higher solubility in the reaction medium. Also, the lower Mw and glass transition temperature of LFHL would facilitate its blending with polymers in thermoplastic applications. LFSL would be a suitable replacement of phenol in phenolformaldehyde resins, and its higher Mw and improved thermal stability are expected to be useful for making carbon fibers.