Characterisation of kraft pulps by size-exclusion chromatography and kraft lignin samples by capillary zone electrophoresis

Sammanfattning: In the present thesis two analytical methods forcharacterisation of underivatised pulp and lignin samplesobtained from kraft pulping of hardwood and softwood areevaluated. The first method is the use of lithiumchloride/N,N-dimethylacetamide (LiCl/DMAc) for dissolution andsize-exclusion chromatography (SEC) of pulps. The applicabilityof LiCl/DMAc-SEC is demonstrated for birch wood kraft pulpswith different relations between zero-span tensile strength andviscosity. The second method concerns the applicability ofcapillary zone electrophoresis (CZE) for characterising blackliquor and isolated lignin samples with respect to mobility,i.echarge density. A method for determining the averagemobility (µav) of the mobility distributions to easecomparison between samples is also presented.The solubility in LiCl/DMAc and the elution behaviour in SECdiffer between hardwood and softwood kraft pulps. Hardwoodkraft pulps are completely dissolved in LiCl/DMAc, whereas highamounts of lignin and presence of glucomannan restrict thesolubility of softwood kraft pulps. The undissolved fraction ofsoftwood kraft pulps consists of larger amounts of mannose andlignin but has a diminished xylose content compared with theinitial pulp. Xylan and cellulose of hardwood kraft pulps arefairly well separated by LiCl/DMAc-SEC. In contrast, themolecular weight distributions (MWD) of softwood kraft pulpsare more complex. It was found that the hemicelluloses ofsoftwood pulps elute over the entire molecular weight range,indicating various degrees of association with cellulose. Theneutral monosaccharide composition implies that associationsbetween galactoglucomannan and cellulose increase withdecreasing amount of galactose. The elution behaviour ofsoftwood kraft pulp xylan suggests that this xylan consist ofsubstructures with varying propensity for associating withcellulose and/or mannan. In absence of associations betweencellulose and hemicellulose, cellulosic solutions of LiCl/DMAcconsist of cellulose aggregates, which is seen as a shoulder onthe high molecular weight end of the MWD of cellulose.According to the profiles of the MWD and light scatteringmeasurements, it is possible to break these aggregates bymechanical treatment of the solutions, without causing severecleavage of the glycosidic bonds. The relation between MWD andzero-span tensile strength was studied on hardwood kraft pulpsdegraded by gamma irradiation, oxygen/alkali or alkali. For alltreatments, the MWD of cellulose is shifted to a lowermolecular weight range as degradation proceeds. In thechemically treated pulps, a shoulder on the lowmolecular-weight end of the cellulose distribution graduallydevelops, which is not seen for the gamma treated pulps. Theobserved decrease in shape factor/fibre strength of thechemically treated-pulps is proposed to be due to a combinationof heterogeneous degradation and removal of hemicellulosewhereas the decrease in Mw of cellulose is of minorimportance.The mobility distributions obtained by CZE differ betweenblack liquor, isolated dissolved lignin and isolated residuallignin. The µav measured at pH 12 reveals that theresidual lignin isolated from pine wood kraft pulp has asignificantly lower charge density than the lignin removed fromthe pulp throughout the cook. At the end of the kraft cook ofbirch, the µav of the isolated residual lignin is aboutthe same as that of the isolated dissolved lignin, whichsuggests that the solubility is sufficient for the pulp ligninto be dissolved. Comparisons between the µav at pH 12 andpH 10 indicate that the isolated dissolved lignin samplesobtained in the middle of the cook have a lower acidity thanthe other samples. The observed difference in µav betweenblack liquor and isolated dissolved lignin may be due toassociations between lignin fragments and carbohydrate polymersin the black liquor.Keywords:Kraft pulps, Black liquor, Cellulose,Hemicellulose, Lignin, LiCl/DMAc, Dimethylacetamide,Size-exclusion chromatography, Capillary zoneelectrophoresis