CHANGES IN THE STRUCTURE OF SISAL AND FLAX LIGNINS DURING SODA-AQ PULPING AND TCF/ECF BLEACHING

The structural modifications of lignins from sisal and flax during their soda-anthraquinone (AQ) pulping and subsequent totally chlorine-free (TCF) and elementary chlorine-free (ECF) bleaching have been studied. For this purpose, residual lignins were isolated from pulps, analyzed by Py-GC/MS and 2D-NMR, and their characteristics compared to the “milled-wood” lignin of the raw materials. Soda-AQ pulping caused a preferential removal of S-lignin and cleavage of β–O–4 ′ linkages but the structure of the residual lignin remained relatively similar to native lignin. TCF bleaching barely affected the lignin structure, and noticeable amounts of β– O–4 ′ linkages still occurs in these pulps. In contrast, ECF bleaching caused strong modifications in the lignin structure with the complete removal of lignin markers in ECF-bleached sisal pulp. However, residual lignin was still present in ECF-bleached flax pulp, with a predominance of G-and H-lignin units and the presence of β–O–4 ′ linkages