The prior art is well aware of defoaming compositions which are used in many types of manufacturing processes to break bubbles and defoam aqueous systems. Major industries which require the use of defoamers are in the manufacture of paper, the manufacture of paints and coatings, and the manufacture of textiles. A defoamer is necessary in such industries to reduce the amount of foam and permit the manufacturing process to operate at full efficiency. Water based defoamers are oil-in mater emulsions, usually containing about 75% to 90% water, and are dispersible in water. See Wamsley et al, Pulp & Paper in Canada, 95:5 (1994), p. 47-50.
Foam in paper mills and pulp mills may interfere with many processes and can affect the efficiency of refiners, pumps, screens, centrifugal cleaners, deinking plants and water removal equipment. Foam will adversely affect final sheet formation, porosity, opacity, printability, smoothness, and plybond strength. Excessive foaming leads to loss of fibers, raw materials, Further, the drainage rate of a sheet on the wire can be affected by the presence of trapped air in a wet web. When a sheet or mat has a large number of trapped bubbles, the drainage rate is slowed because the water passing through the sheet or mat must also pass between the bubbles.
Precipitated silicas are known to be used in formulation of anti-foaming agents, particularly in pulp and paper applications. Such precipitated silica formulations generally comprise an oil dispersion of silica particles which have been surface treated to render them hydrophobic. As pointed out in the publication by Patterson, Colloids and Surfaces A: Physico, Chemical and Engineering Aspects 74 (1993) pps. 115-126, the hydrophobic silica particles are the active bubble breakers and the oil serves as the means of carrying these active particles to the bubbles in the form of fine droplets which are insoluble in the aqueous foam. These oil dispersions may also be emulsified with water and surfactants and often contain other proprietary ingredients designed to enhance stability and activity. It is pointed out in this publication that silica may be made hydrophobic by various methods including dispersion in a silicone oil and heating the resultant mixture. The heating step causes the silicone to react with the silica surface and render it hydrophobic. The publication notes that if the silica is merely dispersed gently in the silicone without heating, the silica remains hydrophilic and does not contribute to anti-foaming activity.
Various proposals are found in the literature to modify precipitated silicas to improve properties for various uses including defoaming agents. For example, in U.S. Pat. No. 4,224,295, finely divided silicic acid is prepared by spray drying in the presence of a base or basic reacting substance to destroy unwanted acidity and produce a composition useful as an additive in defoaming agents.
U.S. Pat. No. 3,993,497 discloses the production of precipitated silicas by addition of sulfuric acid and aluminum sulfate to sodium silicate, this product being a reinforcing agent or flatting agent in paints and varnishes, a carrier for flow condition, or liquid drying agent. U.S. Pat. No. 4,036,663 discloses a finely divided pigment produced by dry blending mixtures of sodium aluminum silicate and aluminum sulfate, such pigments being useful in paper, rubber, paint and adhesives. U.S. Pat. No. 4,038,098 discloses a method for producing precipitated silica abrasive compositions for dentifrice use wherein an aluminum sulfate adduct is added to the composition to control refractive index. U.S. Pat. No. 5,168,082 discloses a ceramic composition comprising an additive of a functionalized colloidal silica sol prepared by mixing alumina and colloidal silica as a dispersion and then spray drying.
The present invention provides a new and improved defoaming agents based on precipitated silica and silicate compositions.