Xylan and Fiber Morphology

During pulping, the xylan is dissolved and also modified to a large extent. Towards

the end of the cook [96,97], part of the xylan – which is still of oligomeric nature –

180 4 Chemical Pulping Processes

is re-precipitated at the cellulose fibers [98–102], thereby increasing pulp yield

[103], changing the mechanical characteristics (increasing strength) of kraft pulps

[104,105], and affecting the fiber quality. Hereby, the structure of the xylan affects

the adsorption characteristics to a large extent, and increasing the removal of carboxyl

groups thereby favors the retake of xylan [106].

The interaction of cellulose and hemicellulose in kraft pulps can be addressed

by a number of modern analytical approaches. Differences in hemicellulose concentrations

are observed between the surface and the inner layer of a kraft pulp

fiber. In the pulps investigated (softwood and hardwood), the amount of hemicelluloses

is generally larger at the surface as compared to the inner layer. The MWD

and sugar composition of the hemicellulose deposits can be studied with MALDI

and CE [107,108]. Analysis of the solid material involves CPMAS-13C-NMR studies

[109–112], association and localization of hemicellulose on pulps are studied by

GPC [113–116], and the use of enzymes in combination with other analytical techniques

provides insights into bonding types [117–119], as well as LCC and hemicellulose

structures [120,121].