Structure of hardwood glucuronoxylans: modifications and impact on pulp retention during wood kraft pulping

The behaviour of different hardwood glucuronoxylans during the kraft pulping process was investigated. Woods and pulps xylans were isolated and characterized by size exclusion chromatography, methylation (linkage) analysis and 1H NMR. Eucalyptus globulus and Eucalyptus urograndis showed xylan retention significantly higher than that of Betula pendula. The higher retention of Eucalyptus xylans was assigned to (i) their higher average molecular weight (31 against 24 KDa in B. pendula) and to (ii) the presence of O-2 substituted 4-O-methyl-α-d-glucuronic acid groups ([→2)-GlcpA-(1→]) with galactopyranosyl/glucopyranosyl residues belonging to fragments of galactan/glucan chains that were absent in B. pendula xylans. A significant part of uronic acids, particularly [→2)-GlcpA-(1→] units, remain in fibres until the end of pulping. The acetylation degree and distribution of acetyl groups between Xylp units, in general terms, was similar in the three types of xylans. Unexpectedly, about 20% of the acetyl groups persisted in pulps xylans till the end of pulping.