The Influencing Mechanism Of Magnesium Alkali On The Strength Properties Of Peroxide Bleached CTMP

Chemical thermo-mechanical pulp (CTMP), as one of the main high-yield pulps, has a wide range of application in specialty paper, newspaper and core layers of coated paperboard. Sodium hydroxide (NaOH) is usually used as the alkali source when CTMP is bleached with hydrogen peroxide (H2O2), which is advantageous to gain the bleached pulp with higher strength and brightness. However, some disadvantages appear at the same time. As a strong base, sodium hydroxide can cause the dissolution of carbohydrates into the bleaching effluent, which leads to the decrease of bleaching yield and increase of chemical oxygen demand (COD) load.In order to decrease dissolution of wood components, increase pulp yield and decrease cost of production and effluent treatment, magnesium oxide (MgO) or magnesium hydroxide (Mg(OH)2) have been used as one alkali source to replace NaOH in peroxide bleaching of poplar CTMP.In this paper, effect of magnesium oxide on the strength properties of peroxide bleached poplar CTMP was studied. First of all, MgO substituting NaOH at different substitution percentages was used as the alkali source in H2O2 bleaching and the strength properties of the corresponding bleached pulp were measured. After that, the effects of chemical components (holocellulose, lignin and benzyl-alcohol extraction), charge characteristics (fiber total charge, fiber surface charge and fiber carboxyl), beating properties, fiber morphology, surface lignin content, the relative bonding area, and hydrogen bond of bleached pulp on the strength properties of the bleached pulp were discussed, respectively. Finally, the main reasons that caused the changes of the strength properties of the bleached pulp in H2O2 bleaching when MgO partially substituting for NaOH was used as the alkali source were obtained.Results showed that the tensile index, tear index and burst index decreased as substitution percentage of MgO for NaOH increased. As for the bleached pulp, when the substitution percentage increased from 0% to 75%, tensile index decreased from 18.8 to 12.4 N-m/g, tear index decreased from 2.65 to 1.93 mN-m2/g and burst index decreased from 0.790 to 0.438 kPa-m2/g. In addition, as the substitution percentage increasing, the contents of holocellulose, ligin and benzyl-alcohol extraction increased gradually. When the substitution percentage of MgO for NaOH increased from 0% to 75%, the content of surface lignin in the bleached pulp increased by 12.0%, the content of fiber charge decreased by more than 30%, the average length of fiber decreased by 10.1%; the relative bonding area decreased by 37.4%, and hydrogen bond in the cellulose decreased.The fitting results of the tensile, tear and burst indices of the bleached pulp with the above factors showed that these strength properties had a good negative linear correlation with the fiber surface lignin content, a positive linear correlation with the fiber carboxyl content and the relative bonding area, and a good quadratic correlation with the fiber total charge content. At the same time, the fiber surface charge content had a good quadratic correlation with the tear and burst indices of the bleached pulp, and the fiber length had a positive linear correlation with the tensile and burst indices of the bleached pulp.In general, it could be concluded that the weaker alkalinity of the bleaching system is the main reason that caused the decrease of the strength properties of the bleached pulp in H2O2 bleaching when MgO partially substituting for NaOH is used as the alkali source. The weaker alkalinity of the bleaching system leads to the less chemical components dissolving, which results in the decrease of the fiber charge content and the surface lignin content. And then a negative influence on the fiber swelling occurs. Less fiber swelling causes less fiber fibrillating in the beating process, which leads to the pulp bonding property weakening. Then the tensile and burst indices of the bleached pulp reduce. Meanwhile, less fiber swelling can also cause more fiber cutting in the beating process. The shortened fiber length results in the decrease of the tear index of the bleached pulp.