Impact of extended-impregnation cooking on the xylan structure in Eucalyptus urograndis kraft pulps

Compared to conventional kraft cooking, (CK), extended-impregnation kraft cooking (EIC) gives higher yield of cellulose and xylan. In the present study, the amount of xylan and its degree of substitution (DS) with 4-O-methyl glucuronic acid (MeGlcA) and hexenuronic acid groups (HexA) in unbleached and OD*(OP)D-bleached E. urograndis EIC and CK has been compared. Additionally, the molecular weight distributions of the EIC and CK pulps after oxygen delignification and of xylan isolated from these pulps were investigated. The EIC cooking resulted in pulps with xylan with higher degree of substitution with both 4-O-methyl glucuronic acid and hexenuronic acid groups. The DS of xylan with hexenuronic acid groups increased progressively when the kappa number of the pulps decreased from 34 to 16. Also after oxygen delignification and D*(OP)D bleaching, the xylan in the EIC pulps possessed up to 50% more MeGlcA groups than the xylan in the CK pulp. Almost all the HexA groups were removed from the pulps during bleaching. The weight average molecular weight of cellulose in unbleached EIC pulps was higher than for cellulose in CK pulps. Xylan in EIC pulps had a wider molecular weight distribution than xylan in CK pulps.