Solid state NMR studies on inhomogeneous structure of fibre wall in kraft pulp

Solid state NMR spectroscopy was used to analyse the inhomogeneous structure of kraft fibre. Primary fines, isolated from the original kraft pulp, and secondary fines, isolated after refining, were compared with the corresponding long fibres. To fractionate fibre wall surface material, the kraft pulp was refined in stages. After each stage, the secondary fines and long fibres were separated and the long fibres refined again to peel off the surface material. It was found that the crystallinity of cellulose is lower in fines compared with the corresponding long fibres. When the fines fractions were compared with each other, the cellulose crystallinity was observed to increase towards the inner parts of the fibre surface. A very clear adient was also seen in the amount of extractives, which was highest in the primary fines rich in ray gr cells. The contents of lignin and some hemicelluloses, mainly xylan and glucomannan, were also higher in fines. Residual lignin isolated from the fines was found to be slightly more condensed than residual lignin from long fibres.