Agglomeration of particles in the coating mixture is an undesirable effect and has a negative impact on the final coated paper product. Each base paper coming out of the paper machine has a natural embossed structure which is uneven and rough. If we want to improve its surface characteristics, it is necessary to add a coating to the surface of the paper. Paper or cardboard is coated with different coating mixtures. These consist pigments, binders, metal binders, auxiliaries and water. In coating mixtures, the particles are connected together to form larger aggregates or agglomerates. Agglomeration of calcium carbonate as a pigment is an unavoidable negative process of all dispersions and is a direct result of the interaction between the dispersed particles and the liquid, which affects the microstructure of the paper. The uneven formation of agglomerates on the surface still results in a slightly rough surface on micro level, and roughness is an effect whose impact we want to reduce by coating the paper and try to make the surface smoother. The purpose of coating the paper and making the surface as even as possible is to improve its printability or its printability characteristics. This mainly affects on the improvement of surface and optical properties such as smoothness, gloss, whiteness, opacity, surface uniformity, print quality, uniform ink uptake and the abičity to varnish. The aim of the MSc thesis was to study how different coating formulations affect on the level of agglomeration of calcium carbonate, what is the role of binders, which component contributes the most to reducing or increasing the level of agglomeration, and to determine how suitable is the physico-mechanical method of reducing the level of agglomeration by ultrasonication. Trough research we studied the properties of the coated papers (basic, surface, structural and optical properties) and trough image analysis using a scanning electron microscope (SEM), obtained a more detailed insight into the micro-evolution of the components of the coating mixtures used by Papirnica Vevče in its production process. Six different samples with different contents of calcium carbonate and kaolin were studied. The samples were further divided into 3 sets according to the purpose of the treatment. These were samples of coated mixtures, samples of coated mixtures treated by ultrasonication for 5 min at 35 °C and samples treated by ultrasonication for 5 min at 45 °C. The results showed that agglomeration was higher in samples containing a higher proportion of CaCO3 compared to the set of samples with a higher proportion of kaolin. Calcium carbonate has the highest values of roughness, porosity, stiffness and gloss. The higher agglomeration is partly due to the binder and the change in temperature. Physico-mechanical treatment with ultrasound cannot sufficiently overcome the chemical attraction between the pigment particles, resulting in partial agglomeration. Although the larger agglomerates were broken down into smaller ones and the size of the pigment particles CaCO3 and kaolin was reduced, ultrasound did not cancel the agglomeration process.
|
