Interactions between Polymers in Dispersion Coating and Surface Sizing of Paper and Board
Sammanfattning: Barrier dispersion coatings are applied to paper or board in order to lower the permeability of the material to liquids and gases. In the present study, the film formation and barrier properties of latices used for coating applications were investigated. The film topography and surface chemical composition of styrene/butyl acrylate latex films were studied by Atomic Force Microscopy, AFM, and Electron Spectroscopy for Chemical Analysis, ESCA, to see the effects of carboxylation and neutralizing agent on polymer interdiffusion. Barrier properties were determined for materials coated on different substrates, particularly the water vapor permeability. Latex dispersions were dialyzed in order to evaluate the effect of purity on film properties.
Different three-ply base papers were coated with a pre-metering roll coater on a pilot scale, and the effects of drying conditions and soft calendering on the coating properties were studied. Heat sealability, barrier properties and blocking tendency of the coated material were studied.
Surfactant migration in annealed films was confirmed with both AFM and ESCA and was taken to be a measure of polymer interdiffusion. The results showed that interdiffusion was retarded in carboxylated latex films and that this process was further slowed down when sodium hydroxide was used as neutralizing agent. Drying of the films at temperatures above Tg of the rigid outermost shell was required to achieve interdiffusion in carboxylated films. Non-carboxylated dispersions formed smooth films at the temperatures expected from their bulk Tg values. Dialysis removed excess amounts of electrolytes and surfactants and gave more densely packed films with a smoother surface topography.
The lowest water vapor permeability was exhibited by carboxylated latex films, presumably because of the formation of a rigid polymer network with a low free volume available for water diffusion, combined with lower surfactant migration. Surfactant enrichment on the coated surface had a negative effect on the heat sealability and gave rise to an increased blocking tendency.
The distribution of hydrophobic sizing agent and starch in surface-sized porous materials was studied by microscopy and ESCA. The degree of penetration of starch could be controlled by achieving a weak flocculation between the components through regulation of the pH and salt concentration. The results indicated that the sizing agent remained on the substrate surface, i.e. simultaneous penetration of the two components did not occur.
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