Foam (Entrained air, Visible foam)
Air that is present in papermaking stock or white water can cause various kinds of problems, including pin-holes, circular marks on the paper, reduced dewatering rates, unsightly foam, deposit problems related to foam buildup, and loss of solid material due to overflow of foam from tanks and chests.
The three main classes of air that are of concern to papermakers are dissolved air, entrained air, and large bubbles. Dissolved air behaves as part of the water phase, except that it can come out of solution as tiny bubbles (entrained air) if the pressure is reduced. Entrained air consists of bubbles that are small enough (say less than 1 mm) to move along with the fibers. Large bubbles have sufficient buoyancy to rise to the surface; however they still can be a problem if the bubbles are persistent and build up as visible foam or froth.
Though our main concern in this website is in regards to chemical factors, it is important to consider mechanical factors that can be contributing to air problems. Leaky seals on pumps are a leading cause of unnecessary air entrainment. Also it is important to realize that the paper machine environment contains all of the key elements needed to create foam - water, air, lots of agitation and splashing, and various substances that can act as stabilizers. On a basic, traditional paper machine the main mechanism of air release is the coalescence, creaming, and breakage of bubbles in the white water tray and white water silo areas, just after the paper has been formed. Often the breakage of surface foam is promoted by spray showers. It is important also to make sure that the showers within air-padded headboxes are working effectively. Many modern paper machines, especially those producing fine and specialty paper grades, contain deaerating equipment. These involve application of vacuum and some type of fluid motion to separate dissolved and entrained air from the water phase.
The most common short-term solution to air problems is to add a chemical defoamer product. A wide variety of chemical formulations has been found to be effective to promote coalescence of air bubbles within papermaking stock and white water or to break bubbles at the surface of water. Some common ingredients of these formulations include water-insoluble surfactants, oils, water, and hydrophobic particles. An essential feature is that the liquid phase has to have a low viscosity and a tendency to spread rapidly on bubble surfaces. Different defoamer products may be needed in different paper machine environments. In particular, many defoamers have an optimum temperature range. Over-use of defoamers should be avoided due to cost and in order to minimize deposit problems. Defoamers often are added in dilute solution either at a fan pump, just before hydrocyclone cleaners, and as surface sprays.
A long-term answer to air problems in a paper mill also should include consideration of chemical factors that tend to stabilize foams. Sometimes these factors can be minimized. Surface active materials tend to stabilize foams by lowering the interfacial tension between water and air. Likely sources of surface active materials include deinking agents, black-liquor carryover (in unbleached kraft operations), excessive rosin soap size or saponified rosin acid emulsion size, and in the formulations of various wet-end additives, including some biocides. Water-soluble, high-molecular-mass polymers such as cationic starch and wet-strength resins can be expected to stabilize foam bubbles if the additives are used at levels beyond what can be efficiently retained on the fiber surfaces. Alternative solutions include either reducing the amounts of such additives or carefully managing the balance of positively and negatively charged additives to improve the retention of all of the polymeric materials on fiber surfaces.
Some other measures that can be considered to reduce various possible root causes of foam include repair of pump seals, repair and adjustment of pulp-washing equipment, and use of foam-control agents and channel-blocking polymers to improve displacement efficiency during pulp washing operations. Bubbles coming out of the headbox sometimes can be reduced by decreasing the level of turbulence in the headbox, adjusting the impingement of the jet to minimize splashing, and adjusting hydrofoils to prevent excessive action on a Fourdrinier table.
Increased problems with dissolved and entrained air (in addition to some deposit problems) usually can be expected if furnish that contains calcium carbonate filler is exposed to acidic conditions below a pH value of about 6.5. Just as in the case of ant-acid tablets, the reaction between calcium carbonate and acid leads to the release of carbon dioxide gas. The high solubility of CO2 means that a high proportion will tend to remain dissolved in the water. Some of this elevated level of dissolved air will tend to come out of solution as entrained air when the jet of furnish emerges from the headbox, greatly lowering the pressure. The tiny bubbles can act just like fiber fines in impeding dewatering from the stock.
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PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (email@example.com). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.