Defoamers and Deaerators For Paper Coating
In the coating process, entrapped air or gas can give rise to production problems and quality loss. The major part of the air seems to get mixed into the coating color during the coating application and circulation. Such air is mostly dispersed as small bubbles that, due to the high viscosity of some coating colors, may remain in the color until drying. Higher coating speeds (2000–3000 m min–1) and total solid content of coating formulations possibly contribute to more stable micro-foam. Formation of foam is a topic in many industrial processes including the coating of paper and board.
Foam is formed through an interaction of mechanical forces and physical chemical properties of the chemicals. From a physical chemical point of view, foam can be defined as a dispersion of air or a certain gas in a liquid or fluid medium. The formation of foam always involves a substantial increase in surface area between the dispersed air or gas and the liquid. Foams in general are in¬herently unstable and are susceptible to decay with time. On the other hand, it is well known that under certain circumstances foams can be quite stable and may persist over long periods of time. Pure liquids and liquid mixtures, which do not contain a surface-active ingredient, are not able to produce stable foams.
Surface-active ingredients or surfactants reduce the surface tension of the liquid and, in consequence, the surface energy renders the system more stable than without surfactants. Besides surface activity, other parameters like foam structure, lamel¬lae thickness, foam drainage, surface rheology, and elasticity contribute to the stabilization or destabilization of foams. These parameters are dependent on time, pH, the presence of surfactants, polymers, proteins, and/or salts as well as the chemical composition and the physical properties of the liquid. Foam generation can occur at different levels of mechanical energy input to the liquid system. Dif¬ferent foam volumes can be generated using the same liquid composition but with different mechanics involved.
Foam reduction, defoaming or deaeration can be achieved by various means. Apart from pressure reduction or mechanical influences (e. g., skimmer or separa¬tion grids), special chemical additives are mainly applied to control the foam. In general, it can be stated that there is a need for formulations of specific defoamers for each specific foaming liquid system.
In those cases where surfactants are already a part of the chemical composition of the liquid medium, high-foaming surfactants may be replaced by low-foaming surfactants, e. g., methyl- or butyl-capped alkyl- or fatty alcohol ethoxylates or pro¬poxylate or polymeric materials like polyoxyethylene or polyoxypropylene or both as block copolymers. In contrast to these water-soluble defoamers, usually insolu¬ble hydrophobic additives are widely used as very efficient defoamers. Among these should be mentioned hydrocarbon/fatty acid/ester or wax blends, (poly) siloxanes, fluorocarbons, dispersions of solid particles (hydrophobic silica, organic microwaxes, etc.) in hydrocarbons or (poly)siloxanes.
The chemical composition of the defoamer and the particle size distribution of the dispersed additive in the liquid medium determine the efficiency of these defoamers. In these defoamers, the fluid component serves as a dispersal aid for the solid, crystal-like particles. When added to the foam, the hydrophobic liquid component spreads out on one side of the foam lamella surface, producing a nonzero contact angle to the sur¬rounding water phase. After further drainage of the foam lamella, the hydrophobic drop finally bridges the lamella and causes lamella breakdown due to the high contact angles and ongoing drainage.
In this respect, the dispersed solid particles enhance these processes and improve the efficiency of the defoamer system to a great extent. Foam control agents are usually applied during coating preparation. In systems where the pigments, binder(s) and the other additives are made up simultaneously, the antifoam agent is added before the pigment and adhesive. Normal dosages lie in the range 0.05 to 0.2 % on dry coating solids. Defoamers are sometimes added pre-diluted and should be properly mixed into the system to ensure an even distribution throughout the volume. Spots in the finished coating layer, commonly referred to as fisheyes and birdeyes, can have a variety of causes. Such deficiencies can be related to the type of antifoam or to too high a dosage of antifoam, where the foam control agent counteracts the complete spreading or wetting of the paper surface at these positions.