Improvement of High Yield Pulps

Abstract

A variety of chemical treatments were employed in the production of high yield pulps from softwood (spruce) and hardwoods (birch, oak) by thermo -mechanical pulping. The effects of these chemical treatments on the physical, chemical and morphological properties of pulps were investigated.
Of the pretreatment chemicals investigated, neutral and bisulfite appeared to offer the greater potential for strength development of high yield pulps than that of alkaline and acid sulfite. Motphological studies on the fibers and fines show that the fiber surface of the pulps from neutral and bisulfite pretreated chips were smooth and the fines fractions of them after defibration consisted of compound middle lamella fragments and undamageed ray cells. On other hand, pretreatments with alkaline and acid sulfite yielded damaged fibers and fines fraction of them contained the fragments generated from secondary wall as well as from middle lamella and ray cells. These results show that the fiber separation of the chips pretreated with neutral and bisulfite occurred mostly in the middle lamella, while the fiber separation of chips pretreated with alkaline and acid sulfite occurred between middle lamella and secondary wall. These behavior are supported by the results of determination of lignin contents in the cell wall by the method of UV microscopy. The results showed that topochemical preference for the removal of lignin from the secondary wall was found in the case of acid sulfite pretreatment, while bisulfite pretreatment not showed the topochemical effect on the lignin removal.
Significant improvements in physical properties of pulps were obtained by chemical pretreatments of hardwood chips. However, the qualities of the pulps from pretreated hardwood chips were inferior to that of the pulps from pretreated softwood chips. Therefor, ozone treatment was used after defibration to obtain higher quality pulps from hardwood. The effects of ozone on the beatability and physical properties of the pulps were remarkable with only a little reduction of pulp yield. Treatment with ozone for 5 min resulted in 50-70% decrease in the beating energy. In breaking length, 90-100% increase were obtained after 30 min ozone treatment. Lignin contents of the pulps were decreased with the progress of the reaction with ozone. The mode of dissolution of lignin from cell walls of the pulps by the ozone treatment was also followed by means of UV microscope spectrometry. In the ozonization process, lignin was dissolved stepwise from the outermost layer of the cell walls, and lengthening of the reaction time led to an increase in the area of the lignin-free region in the cell walls of fibers. UV spectra in limited areas in the cell wall showed that small amount of aliphatic conjugate structure were remained in the areas where no absorption at 280nm was observed under the UV microscope. Residual lignin in the area where distinct absorption was observed under the UV microscope was subjected to chemical change during ozone treatment.