Browsing by Author "Maloney, Thaddeus C., Prof., Aalto University, Department of Bioproducts and Biosystems, Finland"

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Starch-Calcium Carbonate Hybrid Pigments for High Filler Content Papers
(2018) Kuusisto, Jonna
School of Chemical Technology | Doctoral dissertation (article-based)
The replacement of fibers with novel materials in paper is a potential way for the forest industry to gain market competitiveness. This thesis demonstrates a new method to produce hybrid pigments with one potential application in high filler content papers. The target was to integrate precipitated calcium carbonate (PCC) and polysaccharide into a single bondable pigment with a cost-efficient and straightforward technology that could then be used to produce paper with increased filler content while maintaining good strength properties. Two methods were investigated for the hybrid pigment manufacture, both of which utilized co-precipitation i.e. crystallization of calcium carbonate (CaCO3) in the presence of polysaccharide. The first technique examined the addition of dissolved polysaccharide (native corn starch and carboxymethylcellulose, CMC) into the crystallization process. These dissolved polysaccharides were found to interfere with the crystal morphology control and cause aggregation already at low polymer contents (1.5% of PCC). Only minor paper strength benefits were achieved with the hybrid pigments produced by this methodology. In the polysaccharide core-PCC shell procedure, up to 50% of the starch was incorporated into the hybrid pigment structure. This process consisted of starch activation (in order to swell the granules), precipitation of a permeable CaCO3 shell around the swollen starch granules and hybrid pigment cooking to exploit the bonding potential of starch through the porous PCC shell. Native corn and rice starches were subjected to various activation temperatures in order to determine their optimum swelling degree. The retention of starch in the pigment structure after cooking varied from 69% to 94% depending on the level of starch swelling during activation. High retention of starch in the pigment structure resulted in good dewatering during sheet preparation. The cooking stage was found to be essential in order to develop starch's bonding potential and increase paper strength. The optical performance of paper delivered by the hybrid pigments was affected by the starch content, process parameters and starch origin. Various reactor technologies and reaction conditions required for hybrid pigment synthesis were also examined. Batch and fed-batch reactors were examined for the dissolved polysaccharide-PCC method, whereas a circulation reactor with an adjustable high shear mixer was utilized in the core-shell procedure. The high shear reactor demonstrated a more efficient precipitation process when compared to the conventional batch reactor and control of shear in the carbon dioxide feed zone enabled modification of crystal properties, such as particle size and specific surface area. Overall, this thesis provides a simple, novel technique for the encapsulation of starch with PCC. It clearly highlights how hybrid pigment could be used to increase the filler content and the amount of low-cost native starch in paper. The hybrid pigment provides considerable improvements in paper strength while simultaneously maintaining paper optics and sheet drainability.