The effect of pH on fiber and paper properties, during beating and sheet formation, was investigated for three different pulps. The pulps were pH adjusted to four different pH levels between 3 and 9. Isotropic laboratory sheet were made of both unbeaten and beaten pulps. The beaten neutral sulfite semi-chemical pulp and bleached softwood kraft pulp were affected by changes in pH; bleached softwood kraft pulp in a minor extent due to less fiber surface charges. Compared to the other pH levels, pH 3 showed a lower fiber surface charge, water retention value, tensile index, tensile stiffness index, compressive index and edge crush resistance index. SEM pictures showed a denser network at pH 9 than for pH 3. This was seen for both neutral sulphite semi-chemical and softwood kraft pulp. The unbeaten pulps and beaten bleached hardwood kraft pulp were not affected by changes in pH. SEM pictures showed no difference in the fiber network for bleached hardwood kraft pulp. A mill trial, with neutral sulphite semi-chemical pulp, at pH levels between pH 4.8 and pH 5.6 was completed. No significant difference was seen for any mechanical property.

This study first examined the kinetic changes of fiber carboxyl group content in bulk fiber, polysaccharide, and residual lignin of oxygen delignified pulps during one-stage oxygen delignification of a low kappa (32.5) kraft pulp. The carboxyl group contents determined in different chemical components of oxygen delignified pulps was used to establish the distribution of carboxyl groups in lignin and pulp polysaccharide and decouple the responses from residual lignin and polysaccharide. Following this study, two high kappa (~ 49.0) SW kraft pulps prepared were delignified through two-stage oxygen delignification. Fiber carboxyl group profiles of these pulps were elucidated to investigate the effect of lignin content of incoming unbleached kraft pulps on fiber carboxyl group formation. Due to a limitation to enhance fiber carboxyl groups only by parameter optimization during one- and two- stage oxygen delignification, a catalytic oxidation program was developed to enhance fiber carboxyl groups by 52.2 116.0 % employing 0.10 - 0.18% of a bismuth ruthenium pyrochlore oxide catalyst during oxygen delignification. The mechanism of fiber carboxyl group formation through the catalytic oxidation was proposed. The main factor on carboxyl group formation in pulp carbohydrate was identified to follow the order: NaOH > oxygen pressure> reaction temperature through a 3-factor at 3-level (L933) orthogonal experimental design and the optimal conditions were found at 2.5% NaOH, 85-100 oC, and 800-960 kPa O2 during the catalytic oxidation. ECF bleaching study was also conducted on these pulps with higher amount of fiber carboxyl group enhanced at early pulping and oxygen delignification processes. The bleaching results demonstrated that the early-stage enhanced fiber carboxyl groups were partially retained through ECF bleaching. Additionally, fiber carboxyl groups of fully bleached kraft pulps were ~ 20% different from typical bleaching protocols, depending on bleaching chemicals used and the bleaching sequences such as DEDED, (D+C)EDED, ODEDD, and OQPZP. This study finally demonstrated that an increase of fiber carboxyl groups by 17.4-62.1% through chemical oxidation resulted in reduced fiber curl, increased fiber WRVs, 4.3-25.5 % increase in paper tensile index at comparable pulp viscosity; and 4.4 -30.1% increase in paper dry tensile stiffness.

Esteves et.al. 2020 visade att syrgasdelignifiering ökar den totala fiberladdningen men leder inte alltid till ökad pappersstyrka. Denna studie syftar till att undersöka hur fiberladdning påverkar massans egenskaper.  För att testa hypotesen att ökningen av dragstyrkan beror på den större mängd sekundärt finmaterial som orsakas av malningen, testades vissa egenskaper hos massan, såsom vattenabsorption, med och utan malning avmassan. Två kraftmassor med olika initialt kappatal, 91 respektive 52, syrgasdelignifierades med olika alkalilsats under varierad tid. De massor som hade den lägsta och högsta mängden totala fiberladdningar maldes med PFI-kvarn. Water Retention Value (WRV), Schopper-Riegler (SR), Fiber Saturation Point (FSP) och finmaterial analyserades. WRV testades också efter att finmaterialet avlägsnats från massan.  Resultaten tyder på att massan med en större ökning av den totala fiberladdningen jämfört med den kraftkokta massan vid ett givet kappantal kan resultera i en högre ökning av FSP och WRV. Massor med större ökning av den totala fiberladdningen bildade mer finmaterial vid malning, vilket resulterar i en högre ökning av SR. / According to a previous study from Esteves et. al. 2020, it was found that oxygen delignification can increase the total fiber charge but does not always increase the paper strength. This study aims to investigate how fiber charge has an impact on pulp properties.  To test the hypothesis that the increase in mechanical strength for the fiber is due to the bigger increase of secondary fines caused by the refining process, some properties of the pulp such as water holding capacity were tested with different treatments: refining and no refining. Two kraft cooked pulp with different initial kappa numbers of 91 and 52 were subjected to oxygen delignification with different alkali charges and reaction times. When the characterization tests such as, kappa number and total fiber charge measurement have been determined, the samples which had the lowest and the highest total fiber charge were PFI refined and other analysis such as, water retention value (WRV), Schopper-Riegler degree (SR), fiber saturation point (FSP), and fine content, were done. WRV was also tested without fines.  The results suggest that the pulp with a higher increase in total fiber charge when compared to the kraft cooked pulp at a given kappa number can result in a higher increase in FSP and WRV. A higher increase in total fiber charge can also have a high tendency to create fines, due to the higher swelling ability, resulting in a higher increase in SR.

Over the past 10 years, growing concerns over the modification of fibers have led researchers to focus on enriching the carboxyl group content of fibers by chemical oxidation and topochemical grafting. The current series of experiments continues this line of research by investigating the carboxyl group content of fibers during kraft pulping, alkaline peroxide bleaching, and 2,2,6,6-tetrametyl-1-piperidinyloxy radical (TEMPO)-KBr-NaClO oxidation system. The first experiment characterizes changes in the carboxyl group content of fibers for two sets of kraft pulps: 1) conventional laboratory cooked loblolly pine kraft pulps, and 2) conventional pulping (CK) versus Lo-Solids pulping (LS) pulps. The results indicate that effective alkali (EA), temperature, and H-factor are the primary factors controlling fiber charge during kraft pulping. Another set of kraft pulps distinguished by conventional pulping and Lo-Solids pulping were investigated to determine the effect of H-factor and pulping protocol on fiber charge. The second experiment examines the influence of alkaline peroxide treatment on elementally chlorine-free (ECF) bleached softwood kraft pulp. The effect of increased fiber charge on refining, cationic starch adsorption, and hornification was examined. The final experiment investigates the effect of TEMPO-mediated oxidation of an ECF bleached softwood kraft pulp on carboxyl group content, carbonyl group contents, degree of polymerization, and water retention value of fibers. The results show that TEMPO-mediated oxidation is useful in enriching the carboxyl and carbonyl groups to fibers, as well as enhancing the property of water adsorption of fibers. These findings suggest that: (1) kraft pulping process can be modified to obtain the target carboxyl group content, (2) terminal peroxide bleaching provides higher fiber charge which can save energy and chemical charge of subsequent refining and wet-end processes, respectively, as well as reduce hornification during drying, (3) TEMPO-mediated oxidation of fibers is capable of improving the properties of fibers, including fiber charge and water adsorption, and enhancing final paper strength.