Analytical protocols for characterisation of sulphur-free lignin

Abstract

Interlaboratory tests for chemical characterisation of sulphur-free lignins were performed by five laboratories to develop useful analytical protocols, which are lacking, and identify quality-related properties. Protocols have been established for reproducible determination of the chemical composition and functional groups, such as phenolic hydroxyl and carboxyl groups of alkaline lignins from annual fibre sources. The lignins studied, were precipitated from black liquors of alkaline pulping of annual fibre crops, flax, hemp and straw. The total (Klason and acid soluble) lignin contents of these lignins range between 83 and 97%. Contaminants may be present, such as sugar residues (<3.2%), ash (<5.2%), including Si (<0.9%), and relatively high amounts of nitrogen (<1.6%). Furthermore, the (oxidised) acid precipitated soda lignins were found to contain significantly less phenolic hydroxyl and more carboxyl groups than organosolv lignin (Alcell™). A comparison of FTIR and wet chemical methods indicated that acetylation for determination of hydroxyl groups under the conditions used is incomplete, resulting in an unreliable total hydroxyl determination. Molecular weight analysis of the round robin lignins using HPSEC with different eluents gave incomparable results. Number average molecular weights of the lignins, as found by VPO, are relatively low (2–3 phenylpropane units). Compared with Alcell™ lignin, soda lignins from flax, hemp and straw have different compositions, functional groups and molecular weights, which confirms that different functional properties are related to the chemical structure.