New generation of dispersants by grafting lignin or xylan
Doctor of Philosophy
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Synthetic dispersants are commonly used in the stabilization of various colloidal suspensions. However, their non-biodegradable and toxic natures hamper their industrial use. The use of natural polymers as dispersants for various colloidal suspensions has been reported in the past, but the incentive for producing highly efficient natural polymeric dispersants is high. Lignin and xylan are the most abundant and sustainable natural polymers on earth, which are produced as by-products of pulping and cellulosic ethanol industries. These chemicals can be considered as the raw materials for producing value added products such as dispersants. In this dissertation, the anionic modification of kraft lignin and xylan via carboxymethylation, sulfomethylation and oxidation were comprehensively investigated and the applications of the products as dispersants in kaolin and coal suspensions were systematically assessed. The influence of reaction conditions on charge density and solubility of kraft lignin or xylan were systematically investigated. The anionic products obtained were characterized using NMR, FTIR, TGA, molecular weight and elemental analyses. The adsorption of anionic lignin or xylan on kaolin and coal particles and their effect on zeta potential of the suspensions was comprehensively assessed. The relative turbidity and viscosity analyses of colloidal suspensions confirmed the better dispersion performance of the products compared with commercial ones. The impact of dispersant dosage, pH of the suspension and time of mixing on dispersant performance of anionic lignin or xylan in kaolin and coal suspensions was also studied. The dispersion efficiency of suspensions found to increase with anionic lignin or xylan dosage and time of mixing. The dispersion efficiency of anionic lignin or xylan was at maximum under neutral conditions. The influences of molecular weight and charge density on the dispersant performance of anionic lignin in kaolin suspensions were also studied. The results showed that a highly charged anionic lignin adsorbed more compared with the low charged ones, and an increase in the molecular weight of anionic lignin showed no significant effect on its adsorption performance. Both charge density and molecular weight had an influence on dispersant activity of anionic lignin in kaolin suspensions. The dispersion studies of anionic kraft lignin or xylan in kaolin and coal suspensions showed that the increase in the dispersibility of these colloidal suspensions depends on adsorbed amount of anionic lignin or xylan on kaolin and coal particles, but not on the unadsorbed amount present in the suspensions. In the presence of anionic lignin or xylan dispersants, the zeta potential of kaolin or coal particles cannot determine directly the stability of colloidal suspensions. The adsorbed anionic lignin or xylan on kaolin and coal particles induced electrostatic repulsion between the particles and thereby improved the dispersibility of suspensions. The results obtained in this work confirms the possibilities for use of kraft lignin or xylan as dispersants in kaolin and coal suspensions and provides more insights on how the dispersability of these suspensions will be impacted by the properties of the produced dispersants. The results of this thesis contributed to knowledge on the chemical modification of lignin and xylan, characterization of modified products and to the fundamentals associated with the performance analysis of dispersion systems.