Effect Of Alkaline Peroxide Treatment On The Fibre Charge And Lignin Structure Of P-RC APMP And CTMP

Wood fibres and fines in an aqueous system are negatively charged due to ionization of functional groups associated with hemicellulose, oxidized cellulose and oxidized lignin.These functional groups consist of carboxylic acid groups, sulfonic acid groups, phenolic hydroxyl groups, alcoholic hydroxyl groups and so on. Fibre charge is an important parameter during pulp and papermaking process. It not only determines the consumption of cationic additives used in papermaking, such as retention aids, sizes, wet and dry strength resin etc., but also is very important for the swelling ability of wet fibres, fibre flexibility, fibre-fibre bonding, and refinability. The number of fibre charge depends on the origin of fibre, and on the chemical treatments such as pulping and bleaching applied to the fibres, including oxygen delignification, ozone bleaching, and peroxide bleaching.In this paper, the effect of alkaline peroxide treatment on the charge characteristics of aspen P-RC APMP and CTMP was firstly discussed. The conductometric and polyelectrolyte titration were used to determine the carboxyl content and surface charge content, dissolved charge of fibres respectively. Secondly, the technology of surface analysis, such as ESCA, was used to study fibre surface properties of treated and control pulps. Finally, the structural changes of residual lignin after alkaline peroxide treatment were analyzed.The results indicated that alkaline peroxide treatment could increase carboxyl content and surface charge of aspen P-RC APMP fibres. Moreover, fibre charge was increased with increasing sodium hydroxide, peroxide, elevating the reaction temperature and extending the reaction time. The increased charge groups in the pulp were beneficial to the fibre swelling ability and pulp refinability. ESCA analysis showed that O/C was higher than that of the control pulp, but surface lignin and surface extractive content decreased after alkaline peroxide treatment.Milled wood lignin was extracted from control and alkaline treated pulps of aspen P-RC APMP, and the structure of MWL was characterized by GPC, ³¹P-NMR and 2D-NMR techniques. The functional groups in MWL could be quantitatively analyzed by ³¹P-NMR technique. After alkaline peroxide treatment, the contents of both aliphatic OH and COOH groups in residual lignin were increased while total phenolic OHs were decreased to very low contents. In addition, Mn , Mw and Mw / Mn of MWL extracted from treated pulp were slightly higher than those of control pulp from the results of GPC. 2D-NMR results showed that guaiacyl unit was easier to be oxidized by peroxide than syringyl unit. Carboxylic acids were formed by oxidative attack of peroxide toward the structures ofα-carbonyl and double bond groups in lignin side chain during alkaline peroxide treatment.β-βandβ-5 type sub-structure in lignin were stable to peroxide treatment, whileβ-O-4 type sub-structure withα-carbonyl andα- ether linkage in side chain were degraded.Fibre charge characteristics and paper strength properties of aspen CTMP during alkaline peroxide treatment were also studied in this paper, and the influence of this treatment on the behavior of fibre morphology was further evaluated. The results showed that those parameters, such as sodium hydroxide, peroxide, reaction temperature and time, could affect the carboxyl content, surface charge, and dissolved charge of the pulp. But to the zeta potential, the dosage of sodium hydroxide was more important than the other parameters. The alkaline peroxide treatment improved strength properties but reduced light scattering coefficient of papers. Compared with control pulp, the length and width of treated pulp fibres changed little, but the curl and kink index declined sharply. Furthermore, water retention value was increased, which indicated that the swelling ability of treated fibres was improved. ESCA observation revealed that surface lignin content decreased sharply, while surface extractive content decreased from 7.16% to 5.04% after alkaline peroxide treatment.The structural analysis of MWL obtained from both aspen CTMP control pulp and alkaline peroxide treated pulp was also used GPC, ³¹P-NMR and 2D-NMR techniques. According to the results of ³¹P-NMR, the disciplinarian of CTMP lignin structural changes during alkaline peroxide treatment was similar to P-RC APMP. GPC analysis showed that Mn , Mw and Mw / Mn of MWL obtained from treated pulp were lower than those of the control pulp, indicating that lignin was partly oxidized and degraded. 2D-NMR results showed that guaiacyl unit was easier to be oxidized by peroxide than syringyl unit. Carboxylic acids were formed by oxidative attack of peroxide toward the structures ofα-carbonyl and double bond groups in lignin side chain during alkaline peroxide treatment.β-βandβ-5 type sub-structure in lignin were stable to peroxide bleaching, whileβ-O-4 type sub-structure withα-carbonyl andα- ether linkage in side chain were degraded.