Solvent-kraft pulping and papermaking properties of aspen (Populus tremuloides) and spruce (Picea exelsa)

Solvent effect on the chemical phenomena is known to have a strong influence on the reaction rate and chemical equilibria. If the kraft delignification is carried out in an aqueous organic solvent, this strategy could provide new insight into the interaction and reactivity between ions and wood polymers. Experiments were conducted to investigate the properties of the solvent-kraft cooking liquor. Aspen (Populus tremuloides) and Norway spruce (Picea exelsa) wood chips were subjected to solvent-kraft pulping in a laboratory-scale digester. The effect of the ethanol concentration on the lignin removal, pulp yield, reaction kinetics, and papermaking properties were examined.;With an addition of ethanol to the kraft cooking liquor, solution viscosity, ionic transfer free energy, and solvent basicity were increased, whereas the ionization was decreased. Higher ionic mobilities were observed at low ethanol compositions. Ethanol addition to kraft pulping had a significant impact on delignification and the resultant pulp yields. Substantial gains in pulp yield was obtained as a result of the increased delignification selectivity. Significantly higher hemicellulose and alpha-cellulose contents were observed in solvent-kraft pulps compared to control pulps. A kinetic analysis indicated that the delignification rate in the solvent-kraft process was remarkably faster resulting in an increased productivity than in the kraft process. The delignification reactions were controlled by chemical reaction, whereas the carbohydrate dissolution reactions were controlled by both diffusion and chemical reaction process. Significantly lower activation energy of delignification was observed in solvent-kraft process compared to control. The activation Gibbs free energy of delignification monotonously and slowly increased, but the activation enthalpy and entropy of delignification exhibited an extrimum behaviour as a function of ethanol composition. Faster beating response was observed in spruce solvent-kraft pulps compared to kraft pulps. Water retention value of solvent-kraft pulps did not significantly differ from that of control pulps, but higher bulk properties was observed in solvent-kraft pulps. Solvent-kraft pulp handsheets exhibited lower tensile and comparable burst properties to kraft, but marked reductions in tear were observed.