Coating Component Migration In Paper Coatings

In many applications such as in paints and coatings, pigments are mixed with polymeric binders to generate the final product. In the coating of paper, properties like strength, durability, and print quality are affected not only by the binder type, but the final location of the binder relative to the pigments and the fibers. During the application and drying of a paper coating, binder is known to migrate. A number of methods are available to measure the migration of latex or starch. However, all of these methods have some limitations, especially when starch, latex and kaolin are components of the coating system. Methods to tag various components with fluorescent molecules are used in this work to help understand their position within coating layers. Rhodamine B is used to tag latex using a physical adsorption process. Fluorescein isothiocyanate (FITC) is used to fluorescently tag cellulose nanofibers (CNF) and starch with water phase reactions. A confocal laser scanning microscope (CLSM) is used to image fluorescently active samples. Methods to coat, prepare samples, and analyze the results are developed. Using fluorescently tagged starch and latex, a variety of coatings were made on three different substrates to study how different coating and process parameters affect binder migration. This work is unique in that the location of both starch and latex could be determined in respect to one another, as well as the paper substrates. Two separate methods are proven successful to tag CNF with FITC: (1) covalent binding and (2) physical adsorption. Several stability tests prove that the adsorption of FITC to CNF is strong enough for coating studies. Increasing the solids content of CNF lead to a decrease in CNF penetration into the base paper. In addition, adsorption and stability of various other dyes were successful in determine the mechanism for the adsorption of FITC to CNF. A new method to tag starch with FITC in water was developed. Using this method, starch at 3.3% functionalization with FITC was produced. Mixtures of starch and latex coatings on paper showed starch deeper into the paper for all three paper substrates. However, when pigments where present, on the newsprint, latex is seen deeper in the paper than starch, while the starch is seen deeper into the substrate for both the wood free and blotter papers. These results are consistent with reverse capillary flow during drying that is able to pull the water and starch back towards the surface when pigments are present to form a fine pore structure. In addition, the blotter paper showed the least binder migration because of the reverse capillary flow. The ground calcium carbonate (GCC) pigment showed the largest binder separation with the most coating penetration into the various substrates compared to the other pigments; GCC was the bulkiest pigment that forms large pores when dried. The clay pigment which was the smallest pigment, on the other hand, showed the least binder separation and penetration into substrates. The influence of other parameters was characterized. As the amount of starch in the coating increased, migration decreased. This result likely is caused by an increase in the coating viscosity. The influence of drying rate, for the conditions studied here, were minimal with the exception of slow drying on newsprint; here rapid drying seems to trap latex and starch deep inside the paper. The amount of binder separation and penetration into the substrate increased as the coating thickness increased