Influence of Kraft Pulping on Carboxylate Content of Softwood Kraft Pulps

This study characterizes changes in fiber charge, which is the carboxylate content of fibers, for two sets of kraft pulps: (1) conventional laboratory cooked loblolly pine kraft pulps and (2) conventional pulping (CK) versus low solids pulping (LS) pulps. Laboratory kraft pulping of loblolly pine was carried out to study the influence of pulping conditions, including effective alkali (EA), sulfidity, and pulping temperature, on fiber charge. The results indicated that, when pulping to the same H-factor, low EA charge and low pulping temperature are favorable for increasing bulk carboxylic acid group content of fibers. Sulfidity did not have an obvious effect on bulk or surface carboxylic acid group content of fibers. Surface charge was not significantly affected by pulping temperature. Bulk fiber charge has a linear relationship with water retention value. Roughnesses of holocellulose fiber handsheets were determined by atomic force microscopy (AFM). No apparent relationship between roughness and surface charge of holocellulose fibers was found, but unrefined holocellulose fibers exhibited at least 26% lower root-mean square (RMS) roughness values compared to refined holocellulose fibers. Another set of kraft pulps distinguished by conventional pulping and low solids pulping were investigated to determine the effect of H-factor and pulping protocol on fiber charge. When bulk fiber charge was plotted against kappa number, pulps from low solids pulping had a higher slope value than the conventional pulping pulps. The charge on holocellulose fibers approaches a constant value as pulping advances for both types of pulping processes.