Thermomechanical analysis of isolated lignins

The thermal behavior of kraft lignin (KRL), periodate lignin (PIL), steam-exploded lignin (SEL) and acetic acid lignin (AAL), with emphasis on changes in volume upon heating, was investigated by thermomechanical analysis (TMA) in an attempt to evaluate the fusibility of lignin. All lignins underwent a glass transition but, with the exception of AAL, they all had infusible characteristics. The TMA curve for birch AAL (B-AAL) revealed two clear inflection points, assigned to the glass transition point (Tg) and the softening point (Ts) for transformation into a fluid liquid. Thus, only B-AAL among the lignins examined in this study had a fusion state. A fraction of B-AAL with almost the same weight-average relative molecular mass (Mw) as original B-AAL but with less polydispersity was found not to be transformed into a fused state. By contrast, fractions with lower relative molecular mass, namely, with Mw of less than 1,000, which accounted for 30 % of AAL, had good fusibility. Therefore, the low-Mw fractions were responsible for the fusibility of B-AAL. Thermostable fusion states of acetylated KRL could not be confirmed by results of TMA and visual inspection. Thus, lignins could not be converted to fusible materials solely by the introduction of acetyl groups. Furthermore, from the results of TMA of fir AAL (F-AAL), which did not have a clear fusion state, it appeared that the fusibility of lignins was related to their molecular structures, for example, the extent of condensation of aromatic nuclei.