Foam and its control in pulp and paper manufacturing: A review of chemical principles and governing factors

Abstract

This review article considers factors contributing to the development of problematic levels of foam, as well as ways to control foam, with emphasis on two critically important unit operations in pulp and paper manufacturing plants, namely the brownstock washing system and the paper machine. In general terms, hard-to-break foam bubbles can be expected when an aqueous system is subject to a means of air entrainment (such as agitation), when an air phase is present, when there are surface-active agents present, and when the solution also contains significant levels of water-soluble polymers. Brownstock washers and paper machines have all of these ingredients, and sometimes they are at problematic levels. The resulting stabilized foam bubbles can hurt production rates, interfere with displacement of pulping liquor from the fiber mat during washing, and contribute to blemishes in paper products. Considerable progress has been made over many years in understanding these phenomena and also in understanding the work of foam-control products. Engineers in modern pulp and paper mills can make use of efficient foam-control products as well as monitoring equipment. This article provides a tutorial review of such issues, based on published findings.

Full Article

Foam and its Control in Pulp and Paper Manufacturing: A Review of Chemical Principles and Governing Factors

Martin A. Hubbe 

This review article considers factors contributing to the development of problematic levels of foam, as well as ways to control foam, with emphasis on two critically important unit operations in pulp and paper manufacturing plants, namely the brownstock washing system and the paper machine. In general terms, hard-to-break foam bubbles can be expected when an aqueous system is subject to a means of air entrainment (such as agitation), when an air phase is present, when there are surface-active agents present, and when the solution also contains significant levels of water-soluble polymers. Brownstock washers and paper machines have all of these ingredients, and sometimes they are at problematic levels. The resulting stabilized foam bubbles can hurt production rates, interfere with displacement of pulping liquor from the fiber mat during washing, and contribute to blemishes in paper products. Considerable progress has been made over many years in understanding these phenomena and also in understanding the work of foam-control products. Engineers in modern pulp and paper mills can make use of efficient foam-control products as well as monitoring equipment. This article provides a tutorial review of such issues, based on published findings.

DOI: 10.15376/biores.21.3.Hubbe2

Keywords: Defoamer; Antifoam; Brownstock washing; Paper machine; Entrained air; Bubbles

Contact information: Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Campus Box 8005, Raleigh, NC, 27695-8005. Email: [email protected]

INTRODUCTION

Motivations to Control Foam in Pulp and Paper Mills

Foam-control agents, which are often called defoamers or antifoams, are among the most commonly applied additives in pulp and paper mill operations. Though they are added in relatively low amounts, they can have a big effect on operational efficiency and product quality. According to Allen et al. (1993), the paper industry is the world’s largest user of these agents. Of this amount, about 60% of the usage is for brownstock wasing, i.e. the separation of lignin-rich water from the fibers after kraft pulping. This review article mainly considers their applications in brownstock pulp washing (Pelton 1989; Santos and Hart 2014) and in the paper machine forming section (Allen et al. 1993). Foam-control agents are also used at the size press of the paper machine (Vines et al. 1997) and during preparation of coating formulations for paper (Hudson 1968; Reinhardt et al. 1998). The term “defoamer” has often been used to denote relatively fast-acting foam-control agents (Denkov 2004; Denkov et al. 2014), which are often needed in pulp and paper mill applications. The alternatively term “antifoam” has often been used to denote agents that are intended to suppress the later development of foam. It has been noted that many foam-control agents comprise both fast-acting and long-term efficacy, and their modes and speeds of action often depend on details of the industrial environment to which they are being added (Denkov et al. 2014).

A key motivation to use foam-control agents in the pulp and paper industry is to maintain a suitably high rate of production, which can depend strongly on the rate of release of water from a wet mat of fibers. Defoamers have been shown to increase rates of dewatering in brownstock washing (Hakamäki and Kovasin 1985; Pelton 1989; Lobo and Bolton 2013; Bolton et al. 2014; Santos and Hart 2014) and the forming of a paper sheet (Allen et al. 1993). In the case of brownstock washing, foam bubbles in the fiber mat not only can lower the rate of filtration, but they also interfere with the displacement of lignin-rich water by the rinse water (Pelton 1989). On the paper machine, by occupying a variable proportion of volume in a suspension of fibers, foam bubble can interfere with process control, thereby leading to variability in paper properties such as basis weight (mass per unit area) (Mudaly 2007).

Types of Foam

As illustrated in Fig. 1 (Hubbe and King 2009), three categories of air can be found within the brownstock washer systems and paper machine system, namely dissolved air, entrained air, and large bubbles. Bubbles that are small enough to move along with a fiber suspension are called entrained. Poschmann (1962) observed that the fibers in a flowing suspension can constrain such bubbles so that they are unable to collide and coalesce with each other and thereby become larger. Because the volume of a fiber-water suspension is increased by the presence of air bubbles, without appreciably contributing to the weight, there can be a loss of accuracy in the pumping and metering of flows (Mudaly 2007). Under severe cases, there can be undesired swings of basis weight due to too much compressibility of the mixture as it passes through fan pumps. Because the fiber suspension in a paper mill is under pressure, during its approach to the forming section, there will be a tendency for at least part of the entrained air to become temporarily dissolved in the process water (Matula 2000). However, the tiny bubbles form again quickly during depressurization as the jet of furnish emerges from the headbox slice, just before the dewatering process begins.

Fig. 1. Three general categories of air that are present in pulp and paper mill aqueous systems (Figure adapted from a version by Hubbe and King 2009)

Once air bubbles present in industrial processes become large, e.g. 2 mm or greater in diameter, they are expected to rise rapidly and separate themselves from the aqueous phase in response to gravity or centrifugation effects in various processes (Melzer and Poschman 1973). Visible foam becomes apparent especially at the surfaces of chests, including the trays below Fourdrinier forming sections, as well as seal pits and white water silos in a paper mill (Mudaly 2007). Not only can such foam be unsightly, but it can contribute to the concentration and subsequent deposition of pitch-like materials onto process equipment and spots in the paper product (Allen et al. 1993). When the foam is severe enough to overflow to top of a seal chest or elsewhere in a pulp and paper mill (Pelton 1989), there can be a loss of yield, since the foam can be expected to carry some otherwise valuable cellulosic fibers into sewer drains. Pinholes in paper can develop in cases where vacuum suction boxes, which are intended to remove water from the paper web, pull bubbles of air through the wet web (Allen et al. 1993).

The structure of banks of foam bubbles, often resting at the surface of water, has received detailed attention (Langevin 2008). Garrett (1993) noted that recently formed banks of foam, following agitation, often consist of polyhedral bubbles on top, with round, smaller bubbles down below, where there may still be a greater proportion of water between adjacent bubbles. Because of the interfacial tension, the smaller bubbles will have a higher pressure of gas within them. Because air is able to gradually diffuse through walls of bubbles, one can expect a gradual disappearance of the smaller bubbles in favor of growth of adjacent larger ones (Denkov et al. 2020; Brondi et al. 2022).

Intentional Foam

Though the main focus of this article is on the control and destruction of foam bubbles, it is important to point out at least two circumstances where papermakers intentionally encourage the production of foam. One is in the flotation deinking of recovered fibers (Shrinath et al. 1991; Heindel 1999). Such operations often are carried out by dissolving air into pressurized water, which is then discharged at the base of a shallow chest containing the ink-laden wash water from the deinking process. The bubbles of air attach themselves onto the hydrophobic surfaces of typical ink particles, e.g. from the offset lithographic printing process (Hutchinson 1995). By contrast, it has been found that air bubbles do not nucleate on or adhere strongly to the generally hydrophilic surfaces of papermaking fibers (Ajersch and Pelton 1994). Analogous dissolved-air-floatation (DAF) treatment is used in paper recycling operations to remove fine contaminants such as stickies from the process water (Hubbe 2026). Surfactants used in such processes, to the extent that they become recycled back to paper machine systems, may contribute to foam problems in other operations due to their tendency to stabilize bubbles.

In so-called “foam forming” processes, paper machines can be run using foam rather than water as the suspending medium (Alimadadi and Uesaka 2016; Hjelt et al. 2022; Nechita and Nastac 2022). Such processes might be considered for preparation of long-fibered paper sheets in cases where it is desirable to have fibers oriented relatively evenly in three dimensions (Alimadadi and Uesaka 2016), i.e. specialty paper grades. The control of the level of foaming in such systems is outside of the scope of this article.

Earlier Review Articles

As shown in Table 1, review articles have described various aspects of aqueous foam and its control. The present article aims not only to cover more recent findings, but also to provide needed emphasis on paper machine applications of defoamers.

Table 1. Selected Review Articles Focusing on Aspects of Foam and its Control

FACTORS CONTRIBUTING TO FOAM IN PULP & PAPER SYSTEMS

Pulp and paper mills can be regarded as favorable environments for the occurrence of unwanted entrained air and visible foam. This section will consider such foam-promoting and stabilizing factors as agitation (i.e. the mixing of aqueous solutions with air), the leaking of pump seals, variations in pressure leading to the generation of air bubbles, the role of surfactants, the role of dissolved polymers, and a possible influence of particles and emulsion droplets present in the process water or pulp and paper mills.

Agitation

Foam has been defined as gas dispersed in liquid with a sufficiently high ratio that it can have a density approaching that of a gas (Avery-Edwards et al. 1994). Though that definition does not include entrained air, it well describes the most obvious episodes of out-of-control foam that occur in pulp and paper mills during upset conditions or when there is a lack of optimized foam control technology. From a thermodynamic standpoint, every instance of foam is unstable and expected to break down over the course of time in a stationary system. A bank of foam, with no additional input of agitation or movement, will gradually collapse due to drainage of water from the bubble walls (Saint-Jalmes 2006; Langevin 2008; Wang et al. 2016). It follows that the occurrence of unwanted foam requires there to be some form of mechanical energy leading to the initial entrainment of bubbles.

In a paper machine system, water is released from the freshly formed mat of fibers as a cascade of droplets (Allen et al. 1993). A similar effect takes place during brownstock washing, especially when using rotating screen drum washing equipment (Santos and Hart 2014). Figure 2 provides a schematic illustration of a series of three conventional drum washers connected in series, such that the rinse water can be used efficiently to displace the lignin-rich water from the fiber mat and return it to the chemical recovery system of the pulp mill with a minimum of dilution. Note, however, that the rinse water is applied at the top of the drum, such that the filtrate has to fall through the air and land on a pool in the lower part of the drum interior. Both the spraying and the cascading action are expected to contribute to entrainment of air.

Fig. 2. Schematic illustration of a drum washing system set up with counter-current flow of the rinse water; figure copyrighted by the author and previously published in (Hubbe 2021)

Fig. 3. Schematic diagram of a continuous diffusion-type pulp washer in which the whole operation is completely immersed (figure redrawn from an original by Pikka et al. 1999)

An advantage of modern continuous diffusion-type pulp washing equipment is avoidance of splashing and thereby minimization of air entrainment (Allen et al. 1993). As diagramed in Fig. 3, such equipment allows the rinsing action to take place in a completely immersed situation. An observed advantage of such methods and equipment has been a reduced need for consumption of foam-control agents (Johnson 1985).

Banks of hydrocyclones, which are commonly called cleaners, are used in the approach flow to most paper machine headboxes (Bliss 1994). As shown in Fig. 4, their primary purpose is to remove dense particles, especially sand, from the fiber suspension. In addition, their design and action provide a high potential for the entrainment of air into the papermaking stock. The accepted stock from a set of hydrocyclone cleaners has only a few more seconds before it becomes formed into a paper mat, which is often a point of major concern related to entrained air.

Fig. 4. Hydrocyclones (cleaners), as commonly used in the approach flow to a paper machine’s forming section

Leaking at Pump Seals

Depending on the types of pump seals and how they are maintained, various amounts of air can leak and become entrained during the usage of modern pumping equipment (Gülich 2020). Only in the case of specialized pumps, such as diaphragm-type positive displacement pumps, can well-maintained equipment completely remove the possibility of air entrainment (Jackson et al. 2018). For the much more common centrifugal pumps, the rates of leakage during normal operations can be predicted (Gülich 2020). In addition, the wearing out or failure of seals on such pumps can lead to much larger amounts of air entrainment (Melzer and Poschman 1973; Allen et al. 1993). In fact, as a stop-gap measure to avoid adverse consequences of such a situation it has been proposed to add a defoaming agent to the fiber suspension feeding into such pumps (Melzer and Poschman 1973).

Depressurization at the Headbox Slice

In the course of industrial papermaking, the fiber suspension passes through a large pump (the fan pump), where it is combined with and diluted by filtrate that has recently been drained and pulled by vacuum from freshly formed web of paper. The elevated pressure continues to be maintained within the suspension as it travels through such devices as hydrocyclones (cleaners) and screens on its way to the headbox. During this period of pressurization, much of the air that had earlier been entrained in the slurry as small bubbles becomes dissolved in the aqueous phase. Additional air is likely to become entrained as the furnish passes through the hydrocyclones, and some of this additional air may become dissolved as well. Then, when the jet of furnish emerges from the headbox slice, there is an immediate release of pressure, which induces a re-emergence of entrained air in the form of small bubbes (Matula 2000). This situation is illustrated in Fig. 5.

Fig. 5. Schematic diagram of expected levels of entrained (while bubbles with back background) and dissolved (blue background) air in a Fourdrinier paper machine system (figure redrawn based on an original by Matula and Kukkamäki 1998)

The information in Fig. 5 came from a paper machine system in which a foam-control product was being added at the white-water silo. It also is common to inject a foam-control agent before the inlet to the hydrocylone (cleaner) system. Only a matter of seconds will pass between the time when stock is passing through the cleaners and when it is being formed into paper. A modern paper machine may run at the speed of a car on a highway or racetrack (up to 2000 to 3000 m/min for commodity grades of paper). If one assumes a 10 m length of a Fourdrinier forming section, that implies that only 0.2 to 0.3 seconds for the defoamer to act, following depressurization of the stock at the headbox slide. The time available for foam breakage is even much more constrained in the case of gap formers (paper machines in which the sheet is formed between a pair of continuous fabrics). In such systems, the water is removed over a very short distance, depending on the design of the equipment.

Surfactants

In the absence of a stabilizer, a bubble formed in water will collapse immediately. This phenomenon is well known to swimmers on a pristine beach with breaking waves. Though a foam-like appearance occurs during the breakage of each wave, most of the bubbles may be gone moments later, depending on the levels of any natural or synthetic surfactants that may be present.

Soaps from kraft pulping

Especially in mills where there is primary production of kraft pulp, the process water is likely to contain certain compounds that had been extracted from the wood under hot alkaline conditions. The most important of these are illustrated in Fig. 6. As shown, these include triglyceride fats, which are highly hydrophobic, but which can be naturally converted by enzymes to become fatty acids. Another class of surface-active compounds in kraft black liquor are the resin acids, including abietic and levopimeric acids. These two are converted to their soap forms when the pH is neutral or alkaline. In addition, there will be some compounds, such as α-pinene and β-sitosterol, which have an insufficient number of polar groups to render meaningful surface-activity. The hypothetical mycel structure shown in the middle of Fig. 6 envisions such nonpolar components as mainly occupying the center positions of pitch-like emulsion droplets or particles in the aqueous suspension, whereas the more surface-active compounds can be expected to be suitably oriented at the surface, if one were to take a snapshot in time.

Fig. 6. Types of extractives commonly found in black liquor from kraft pulping of softwood chips, as well as a hypothetical emulsion droplet that is being stabilized by the most surface-active components

While Fig. 6 shows the fatty acid and resin acid soap molecules stabilizing an emulsion of other wood extractive compounds, it is reasonable to envision them also acting as stabilizers for bubbles of foam. Figure 7 describes three relevant situations to consider. First, as diagrammed in part A, the surfactant molecules will tend of form a condensed monolayer at air-water interfaces (Hubbe et al. 2020a). The molecules within that layer are expected to be in equilibrium with surfactant molecules dissolved in the water. If the concentration is high enough, then there will be additional molecules of surfactant present as micelles. The presence of a monolayer at the air-water interface lowers the interfacial tension (Prosser and Franses 2001), as can be measured by standard tests (Lunkenheimer and Wantke 1981; Sjökvist et al. 2018). Part B of the figure describes a group of bubbles that have risen to the surface of the bulk water. In this case, each bubble, in locations above the level of the bulk water, is composed of a surfactant bilayer, with a film of water sandwiched in the middle. Part C of the figure shows the cross-sectional profile of a group of three bubbles that happen to be fully immersed in the water phase. As shown, each bubble is expected to have an approximately spherical shape, except that the shapes are modified in places where any two bubbles are in contact. Details of these shapes have been the subject of study (Koczo et al. 1994; Denkov 2004; Wang et al. 2016; Anazadehsayed et al. 2018).

Fig. 7. Diagram highlighting different expected locations of surface-active molecules in a water-air system that has been recently subjected to agitation, leading to air entrainment: A. Surfactant molecules at an air-water interface and also in equilibrium with molecules in the water’s bulk phase and with micelles; B. Three bubbles of foam (in profile) at the water surface; and C. Some bubbles (in profile) within the bulk water phase

Deinking surfactants

Surfactants also are widely used in the removal of ink from recovered printing paper. In such operations, one of the roles of the surfactants is to promote detachment of the ink from fiber surfaces (Shrinath et al. 1991; Heindel 1999; Theander and Pugh 2004). This takes place, to a large extent, during agitation of the fiber suspensions in a pulper, i.e. a chest provided with a large, robust impeller at its base. The pulping operation is often followed by a washing stage, in which the ink-laden process water is displaced from a mat of fibers (i.e. similar to what was shown in Fig. 2). The next step is to remove both the ink and much of the surfactant content of the process water in a dissolved air flotation (DAF) operation, as illustrated in Fig. 8.

Fig. 8. Schematic diagram of dissolved air floating (DAF) for removal of ink from process water after being released from the surfaces of recovered fibers from printing papers

In a DAF separation process, air is first dissolved in water at high pressure; when the same water is injected at the base of a DAF floatation unit, large numbers of tiny bubbles are released from the dissolved air. The hydrophobic nature of many kinds of ink particles, especially offset lithographic inks, causes them to cling to bubble surface and thereby become lifted to the surface of the water (Heindel 1999). The foamy froth that collects at the surface of the DAF unit becomes skimmed from the water surface, as shown schematically in Fig. 8.

Though the DAF process just described is expected to remove much of the surfactant as well as most of the hydrophobic ink particles from the process water, one can expect some of the surfactant to become recirculated into various operations within the paper mill. Two classes of surfactant are of concern, namely ionic and nonionic surfactants. Ionic surfactants that have been widely used in the past for deinking operations include fatty acid soaps and their calcium salts (Ferguson 1992; Theander and Pugh 2004; Costa and Rubio 2005). Such surfactants can cause complications due to their interactions with oppositely charged components in the system. For instance, an anionic surfactant such as sodium dodecyl sulfate (SDS) is expected to interfere with the performance of cationic additives, such as aluminum sulfate (papermaker’s alum) and with cationic polymers such as some retention aids. In recent years it has become more common for papermakers to rely on nonionic surfactants for deinking. Although such surfactants, when they are present in paper machine systems, do not interfere with charged additives, they can act as wetting agents (Schmedding and Tatum 1998). It follows that they are expected to adversely affect the performance of hydrophobic sizing agents. The term sizing agents here refers to such additives as rosin, alkylketene dimer (AKD), and alkenylsuccinic anhydride (ASA), which cure during the industrial drying of paper and develop its resistance to wetting by aqueous fluids. In addition, all forms of deinking surfactants, to various degrees, can promote the stability of foam bubbles. As noted by Kulkarni et al. (1977a), because ionic surfactants carry a charge, one can expect that the bubbles will be ionically charged. This can cause them to remain separate from each other, especially in situations where the bubbles are entrained in a fiber suspension or otherwise immersed in the water phase. Such colloidal stability can be expected to work in opposition to some potential foam control measures. However, as was already noted by Miles and Ross (1944), the behavior of ionic surfactants can be affected also by such variables as pH, water hardness levels, and interactions with other charged materials.

Surfactants in additive formulations

Nonionic surfactants also are used in the formulations of certain additives for the papermaking process. In particular, acrylamide copolymer retention aids are routinely formulated as water-in-oil emulsions to accommodate their synthesis, transportation to paper mills, and storage at the mill prior to use. The nature of the surfactant is dictated by their intended role as stabilizers of water droplets within a continuous oil phase. A base level of surfactants can be expected to be present in typical papermaking furnish, due to the continuous addition of that kind of retention aid product during the operation of a high percentage of paper machines throughout the world. In some cases, there may be significant contributions to foam in the paper mill systems. Though it is possible to avoid such usage of surfactants by dispersion of dry-bead retention aid products at the paper mill, such practices are less used in comparison to water-in-oil emulsified retention aid formulations (Lu et al. 2002).

Dissolved Polymers

If there is a goal to prepare long-lasting bubbles – as in the case of party bubbles – then it is important to include a water-soluble polymer in the formulation. For example, polyvinyl alcohol (PVOH) is a common component in bubble-making kits. Studies have shown that the presence of the PVOH contributes to aqueous foam bubble durability (Gottberg et al. 2019; Wang et al. 2019; Dehdari et al. 2020; Xu et al. 2020). Likewise, if one observes troublesome and persistent foam in a pulp and paper manufacturing facility, it is likely that there are significant amounts of a water-soluble polymer or polymers present (Gottberg et al. 2019; Wang et al. 2019; Dehdari et al. 2020; Xu et al. 2020). Some of the types of soluble polymers that can play such a role in pulp and paper mill systems are described in the subjections that follow.

Hemicellulose

The wood itself can contribute significant amounts of hemicellulose to the process water in a typical pulp and paper mill system. In principle, a portion of the hemicellulose will become dissolved from the wood material in the course of chemical pulping, e.g. the kraft process (Henriksson et al. 2024), and during bleaching (Jiang et al. 2000). In brownstock washing operations, hemicellulose present in the water is expected to render the air bubbles more persistent due to steric stabilization (Patra et al. 2021). In addition, relatively concentrated solutions of hemicellulose can be used for the preparation of solid foam packaging products (Deng et al. 2015).

Papermaking additives

Perhaps the most important source of water-soluble polymers in paper machine process water is the size press. During normal operations, soluble starch is applied to the paper surface at a size press, and the main benefits can be increased paper stiffness, increased surface strength, less dusty paper, and therefore less contamination of printing operations (Hubbe 2024). But during periods in which the size-pressed paper does not meet product specifications (e.g. due to wrinkles, the wrong basis weight, or the wrong color, etc.), the defective paper becomes repulped and sent back into the papermaking process again. Because most of the starch applied at size presses is unmodified (i.e. “pearl”) starch or oxidized starch, it does not tend to remain adsorbed onto the surfaces of cellulosic fibers in aqueous solution. Rather, it tends to stay in the water phase. For this reason, increased foam levels on paper machines that have size presses are sometimes correlated to the level of “broke” fibers that are being used currently. The dissolved starch’s contribution to solution viscosity is expected to contribute to the stability of bubbles, thus hurting the performance of foam-control agents (Jha et al. 2000). The stabilization likely can be attributed to the higher viscosity within the bulk phase decreasing the rate of stretching of air-aqueous interfaces. The slower rate of stretching provides more time in which surfactant molecules can exchange between the bulk phase and the interface to maintain stability. Such a mechanism, known as the Marangoni effect, contributes to the stability of bubbles (Chen et al. 2015).

Positively charged polymer products, including cationic starch and cationic acrylamide retention aids, are intended to adsorb onto cellulosic fiber surfaces and therefore not build up in the process water. However, there is a limit beyond which the fiber surfaces will have the capacity to adsorb the added cationic polymer. In the case of cationic starch, that limit seems to lie in a range between about 1% and 1.5% on a dry weight basis (Moeller 1966). Two limiting aspects of the fiber surfaces are space and charge density. Because cationic starch usually has a relatively low charge density (Hubbe 2014), it can be expected that limited space at fiber surfaces usually will determine how much of that product can be adsorbed onto the fibers during a papermaking process.

Often the most problematic water-soluble polymers present in paper machine process water, from the standpoint of foam, are wet-strength agents. These polymers, such as poly(amidoamine-epichlorohydrin) (PAAE), have the job of maintaining at least some of the paper strength after the paper has become completely wet, as in the case of a wet paper towel or paper banknotes unintentionally passing through a washing machine cycle. Difficulties can arise in cases where the PAAE or other cationic wet-strength product is not quite effective enough to reach the wet-strength specifications for the paper product. The natural inclination of the papermakers in such cases is to increase the dosage of the additive. But at some point, the fibers already will have a net positive charge at their surfaces, and any additional wet-strength agent will be repelled from the fiber surface and remain in the water.

Dissolved polymers vs. drainage of bubble walls and surface viscosity

In all of these cases just described, i.e. hemicellulose (if present), size-press starch recirculated in broke to the paper machine system, cationic starch used in excess, or sufficiently high levels of wet-strength agent to exceed the charge capacity of the fiber surfaces, the polymers can be expected to impede the drainage of water within the vertical segments of walls between adjacent bubbles (Wang et al. 2016). The situation is illustrated in Fig. 9. Note how, within the narrow confines of the liquid layer within a bubble wall, a relatively small amount of water-soluble polymer may effectively keep the bubble wall from draining. Gravity forces that otherwise would have tended to drain water within the bubbles that constitute banks of foam will be blocked, which is a recipe for troublesome foam.

Fig. 9. Schematic illustration of the mechanism by which dissolved polymers that happen to be present in the film of water between adjacent bubbles will tend to hinder gravity-induced drainage, which otherwise would destabilize the foam

Dissolved polymers also contribute to viscosity, which can include the viscous effects associated with bubble surfaces. Brown et al. (1953) were the first to note that increased surface viscosity led to increased stability of foams. These concepts have been supported by more recent work (Kanner 1968; Kanicky et al. 2000; Wang et al. 2019; Choudhury et al. 2020). Dehdari et al. (2023) observed very stable foams in some systems that contained both poly(vinyl alcohol) and various acidic nanoparticles, including Al₂O₃; the effects were attributed to the development of a protective layer. Such observations provide a connection to the next topic to be discussed, i.e. the likely involvement of particles in the stabilization of emulsions, as well as possible effects relative to foam.

Particulates and Droplets

The presence of various particles or emulsified droplets within process water can be expected to make foam-related phenomena more complicated. In particular, certain particles have been shown to function as stabilizers for emulsions in so-called Pickering systems (Lazaar and Hachem 2022; Deng et al. 2025; Marquez et al. 2025). It has been proposed that even emulsion droplets, if they get trapped within the walls separating pairs of bubbles, can block gravity drainage and thereby increase foam durability (Koczo et al. 1992). It follows that such particles are likely to alter the behavior of foams.

Lignin precipitation due to lower pH

A recent review article has brought attention to the fact that nano-spheres of lignin can serve as stabilizers for emulsions (Marquez et al. 2025). Such phenomena are relevant to foam in pulp and paper mills systems due to the fact that such lignin spheres may arise during the washing of unbleached kraft pulps, i.e. brownstock washing. As the highly alkaline medium is being displaced by near-neutral rinse water during brownstock washing, the pH value is lowered. This results in the protonation of phenolate groups (pK_a≅10) within the lignin, along with a substantial reduction in the amount of negative charge associated with the lignin. It has been shown that with the progressive lowering of pH, more and more lignin comes out of solution (Hubbe et al. 2019; Trovagunta et al. 2024). For instance, 90%-pure lignin can be obtained just by acidifying alkaline pulping liquor and heating it (Lynn et al. 2025). Micrographs of kraft fibers that have been collected after brownstock washing sometimes show the presence of granular particles at fiber surfaces (Koljonen et al. 2004), and these have been assumed to be lignin. Whether and how such particles might affect foaming behavior does not appear to have been studied in detail. It is possible that such particles may impede gravity drainage of films of water between pairs of bubbles (Koczo et al. 1992).

In the event that lignin particles become incorporated into foam in pulp and paper mill systems, there is reason to expect that such bubbles will have a more brittle character (Lazaar and Hachem 2022). The reason has to do with an absence of Marangoni-type effects, which explain the high stability of typical surfactant-stabilized foam system (Anazadehsayed et al. 2018). In other words, there is a smooth increase in interfacial tension when such ordinary foam bubbles are stretched, but such flexibility would not be found if particles were the main stabilizing component. Though brittle foam banks could possibly be beneficial from the standpoint of foam control, one needs to be concerned about adverse effects of deposits, as will be discussed next.

Deposit promotion

A further way that precipitation of lignin, especially in the presence of foam, can adversely affect pulp and paper operations is by contributing to deposits on the operating equipment. Foam bubbles can play a role in collecting relatively hydrophobic particles, using mechanisms related to those described earlier in the context of DAF units and removal of ink from process water (i.e. Fig. 7). Unintended foam banks can collect at multiple locations within pulp and paper mills, such as at the surface of tanks and within air-padded headboxes. If and when such processes bring about the agglomeration of particles such as lignin, there is a danger that visible spots may appear in paper products.

MONITORING OF ENTRAINED AIR AND VISIBLE FOAM

In principle, there are several ways in which air bubbles within pulp and paper mill fiber suspensions can be monitored. These can be classified as (a) ways to monitor visible foam, (b) ways to quantify the relative volume of air bubbles within water specimens, and (c) ways to detect and characterize the presence of entrained air bubbles within water.

Monitoring Visible Foam

The monitoring of visible foam is highly dependent on the layout of process equipment. In a typical brownstock washing system, foam can be expected to develop in locations where water is cascading through a mesh screen. However, that location will be hostile for placement of camera equipment. Thus, a flow channel placed after the washer itself can be recommended. In order to provide a quantitative element, a digital camera system can be set up to quantify attributes of foam (Honkanen and Eloranta 2023).

Monitoring Entrained Air by Density

To monitor the quantity entrained air, a simple approach is to accurately weigh a specified volume of flowing process water at ambient pressure (May and Buckman 1975; Allen et al. 1993). In principle, in the absence of bubbles, a dilute aqueous solution will have a density of approximately 1.000 g/mL. If fibers are known to be present, this value might be different, but still rather steady and predictable. Thus, calibration values can be obtained by occasionally placing pure water in the device. Downward deviations from the calibrated density, after correcting for the effect of fiber content, can then be attributed to the volume occupied by bubbles.

More complete information can be obtained, starting with a similar known volume of process water or dilute fiber suspension, if the pressure can be varied in a systematic way (Allen et al. 1993). Thus, Dougherty (1989) describes the use of a piston to systematically change the volume of a rigid container system, with simultaneous evaluation of the pressure. In this way, it is possible to quantify entrained air by volume. Information obtained during automatic running of the devise can be used as the basis for controlling the flow of a foam-control agent. Ajersch et al. (1992) showed that superior results could be achieved when using a vibrating U-tube containing a flowing papermaking fiber suspension, in which results were being compared at two different applied pressures. The resulting values of dispersed air content were judged to be sufficiently accurate, and the tests could be carried out continuously at high speed.

Ultrasonic Monitoring of Entrained Air

The presence of bubbles in an aqueous sample is known to affect the transmission of ultrasound (Allen et al. 1993). By using such an approach, online equipment has been developed for online monitoring of entrained air in paper machine systems (Pietikainen 1992). As shown by Bamberger (2006), the ultrasonic waves are able to effectively quantify entrained air even in the presence of cellulosic fibers. Audible sound waves also have been used in laboratory work (Al-Masry et al. 2006). The cited work showed interesting effects of foam-control agents on bubble size distributions depending on the flow rate of the water and in response to anti-foam treatments of the surfactant solutions.

Online Control of Foam

The next step after establishing an automated measurement of entrained air or visible foam in pulp and paper mills can be to close the loop and implement a controlled system of adding a foam-control agent (Sheppard et al. 2011). By this means, greater stability can be achieved with respect to such outcomes as the production rate, the steadiness of the operations, and the regularity of paper quality.

Defoamer testing

An effective basic system for laboratory evaluation of defoaming agents can be assembled with a graduated cylinder that is supplied with sparged air at the base (Allen et al. 1993). Lobo and Bolton (2013) and Bolton et al. (2014) describe a procedure in which the air is first sparged into a stirred container filled with the aqueous mixture of interest. The contents are then transferred to a graduated cylinder to measure the volume. The volumetric air content is determined by comparing the mass with a calibration amount obtained with an air-free reference specimen. The next step in the test uses vacuum to form a mat of pulp. The rate of drainage provides a measure of how much the foam bubbles are interfering with drainage. A somewhat more sophisticated arrangement can be set up with continuous pumping. By such means it is possible to measure not only the height of foam but also an indication of how long various foam-control agents can continue to be effective after their addition (Clas and Allen 1994). In addition, there is an ASTM method (D 3519, 1993) that employs a kitchen blender to generate the foam.

When running lab equipment such as that just described, it is critical to match aspects of the conditions within the pulp and paper manufacturing facility of interest as closely as practical. In particular, it is important to carry out tests at the same temperature as is prevalent in the industrial process. This is important because some of the most effective defoaming agents are likely to be formulated so that they will have greatest effectiveness within a selected range of temperature. The highest performance of such products is often just above the cloud point temperature of the surfactant contained in the foam-control product formulation (Pletnev et al. 1983; Nemeth et al. 1998; Chaisalee et al. 2003). As the temperature is raised to the cloud point, surfactants in the mixture become unstable, leading to the rapid development of emulsion droplets (Hinze and Pramauro 1993). Notably, at such conditions, a coacervate phase can essentially provide droplets of emulsion, which may play the role of foam-control agents. In view of the effects just described, there can be a strong incentive to carry out lab tests right at the industrial facility of interest. In that way, by taking quick samples from the actual process, not only can the temperature conditions be easily matched, but there may be various unknown impurities in the process that would be absent if tests were done elsewhere.

MECHANICAL DEAERATION TECHNOLOGY

Overview of Mechanical Deaeration Devices

In different paper machine systems, there are basically three kinds of approaches that are being used to mechanically remove air during continuous processing within pulp and paper mills. As described in the subsections that follow, these can be described as inherent rising of bubbles to the surface of water, vacuum deaeration, and centrifugal deaeration.

Inherent Rising of Bubbles to the Surface in a Chest

Of all methods to be considered, the “inherent rising of bubbles” approach is both the most commonly practiced and the simplest, since it merely takes advantage of the existing layout of equipment in a typical paper machine system. On a Fourdrinier paper machine, when water drains through the forming fabric, it rains down onto a shallow pool of water held by a so-called “tray”. At that location there is considerable opportunity for sufficiently large bubbles to float to the surface and perhaps pop, except that both of these events will depend on the size of the bubbles. According to the Stokes equation (Guazzelli 2011), a bubble as small as 1 µm in radius will require about four hours to rise 5 cm (perhaps the depth of water in a hypothetical pool in a tray) at a temperature of 40 °C, which is within the common range for paper machine operations. At that rate there will not be effective separation during the several seconds that it would likely take for the water to pass through the length of the tray. By contrast, if the bubbles have a radius of about 1 mm, then only about 0.015 seconds would be needed to escape to the surface of the same tray. In principle, such a growth of bubble size might be achieved by multiple coalescence events involving pairs of bubbles.

Escape of bubbles to the top of various chests, including the seal tanks and white water silo, are expected to be yet more challenging due to the depths of those units, and also the fact that there will be a downward flow in the white water silo, since the flow from the base of the silo goes back to dilute incoming thick-stock at the fan pump. Twin-wire forming sections lack the spacious tray systems, so there is even less opportunity for bubbles to be able to separate themselves by rising.

Vacuum Deaeration

Much more effective mechanical deaeration can be achieved, even removing both dissolved and entrained air from paper machine furnish, by using equipment that applies a vacuum (Allen et al. 1993; Matula and Kukkamäki 1998; Matula 2000).

Fig. 10. Schematic diagram of vacuum-type deaerator equipment acting to remove air from the accepts flow of hydrocyclone cleaning devices in a paper machine system

To take advantage of gravity, such devices are typically located well above the main floor of the paper machine forming section. In this way, a hydrostatic head of water at least partly compensates for the inherent depressurization required for the vacuum operation. Figure 10 shows a schematic diagram of a vacuum deaeration device.

Note that this device has been set up to receive the accepts from a set of hydrocyclone cleaners, such as already has been depicted in Fig. 4. This is appropriate, since, as mentioned earlier, hydrocyclones can be an important source of air entrainment. As shown in Fig. 10, vacuum is applied at the top of the deaeration unit. The negative pressure causes much of the dissolved air present in the water to emerge as bubbles. The low pressure causes such bubbles to be much larger than they would have been at ambient pressure, which favors their rapid rising and separation from the bulk phase of the water. It is also being assumed that collisions among the bubbles will result in yet larger bubbles and that those bubbles will quickly pop (presumably due to the action of foam control agents, which are the subject of the next section). Thus, water that overflows a weir and continues on its journal towards the headbox and forming section of the paper machine will contain very little air. This situation is depicted in Fig. 11, which compares a modern Fourdrinier paper machine system without and with a vacuum deaerator of the type described above. Note the large contrast in the level of dissolved air after the deaerator in the right-hand part of the figure. Also note the lack of either kind of air in the headbox, as reported in the cited work for the system fitted with the vacuum deaeration system.

Fig. 11. Schematic diagram of expected levels of entrained (while bubbles with back background) and dissolved (blue background) air in a Fourdrinier paper machine system without and with a vacuum-type deaerator (figure redrawn based on an original by Matula and Kukkamäki 1998)

Centrifugal Deaeration

When using centrifugation rather than vacuum as a means of removing air from papermaking furnish, the goal is not to remove dissolved air (Meinander and Olsson 1999; Cutts 2004). Rather, mainly the dissolved air is the target for removal. Another key difference is the placement of the device. Centrifugal deaerators, as depicted in Fig. 12, are placed on the floor below where the forming section of the machine is located. Specifically, it will be placed between the base of the white water silo and the fan pump. Note that the system needs to be actively controlled, e.g. by measuring the pressure of the air being drawn from the device, to maintain the “water wall” in a stable position for smooth operation. Again, in this case, users of centrifugal deaeration equipment don’t expect to rely on it alone; rather they will probably also be running an optimized chemical foam control product or products, as will be described next.

Fig. 12. Schematic diagram of a centrifugal-type deaerator intended to remove entrained air from papermaking process water soon after it has been released from the wet web in the forming section of the paper machine (figure redrawn based on an original from Meinander and Olsson 1999)

PRINCIPLES OF FOAM CONTROL

To shed light on current understanding of both foam stability and its control, this section starts with a historical overview of the development of foam-control products, then classical descriptions of the mechanisms of those agents, and then feeding strategies aimed at taking advantage of such mechanisms.

Foam-control Chemistry Historical Overview

The history of the development of foam control agents can provide some insight into how they work. Among the first known foam-control agents were simple oils (Allen et al. 1993). These had various bad effects, e.g. possible oily spots in paper products. One can envision such products as being able to scavenge a range of potential foam stabilizing substances, such as fatty acid soaps, from aqueous mixtures due to their generally hydrophobic nature. In addition, though it has not been much used as an industrial defoaming agent, it has long been appreciated that milk can be quite effective for control of foam in some instances (Allen et al. 1993). This fact can be regarded as a hint in the direction of modern foam-control agents for pulp and paper manufacturing applications, which mostly consist of emulsions. In the case of milk, the oil in the droplets will include triglyceride fats, and the surface-active agents include lecithin (Ho et al. 2023).

Foam control agents designed for paper machine applications have been known at least since the 1940s (Morehouse 1945; Robinson and Woods 1948). Early developers of foam control technology learned that better performance could be achieved by optimizing both the oil phase (the droplets) and the stabilizers (generally surface-active agents) (Allen et al. 1993). There was a need for further developments when it was discovered that some aromatic components in petroleum-type oils could react with chlorine bleach using in some pulp and paper mills, with the production of highly toxic chlorinated aromatic compounds (Allen et al. 1993). One of the ensuing directions of development was to use aromatic-free petroleum oil, namely low-odor paraffin solvent (Clas and Allen 1994). Another developmental path was to abandon the usage of petroleum altogether and to rely on silicone-type components (Allen et al. 1993). Such an approach had the added advantage of usually not promoting deposition of fatty and resin acids in pulp and paper mill systems (Bradt et al. 1996). The reason is that the silicone-type compounds, though being hydrophobic, generally are not mutually soluble with the hydrophobic compounds that are released during the pulping of wood (Hansen 2007; Hoekstra 2007).

Surfactants as Components in Foam-control Formulations

Types of surfactants employed

Surfactants, which generally can be defined as molecules having a hydrophilic group connected to an oleophilic group, can be regarded as required ingredients of all fast-acting foam-control agents (Pelton 1989; Garrett 1993; Denkov et al. 2014). Their effectiveness in this role can be attributed to their tendency to form monolayer films at the air-water interfaces (Jha et al. 2000). The hydrophobic groups of such surfactants are often derived from vegetable oils, e.g. C18 alkyl chains. The goal is to select surfactants having a tendency to quickly form saturated monolayers, meaning that the hydrophobic portions of the molecules comprise a densely packed monolayer at the air-water interface (Jha et al. 2000). Continual exchange of surfactants from the water and micellar phases with the interface contribute to a two-dimensional film pressure, which can lead to film spreading. A diverse range of surfactants used in various foam control products have included fatty alcohols (Joshi et al. 2005), fatty alcohol ethoxylates (Sawicki 2005), and nonylphenols with C8 to C10 length ethylene oxide hydrophilic chains (Chaisalee et al. 2003). Jha et al. (2000) demonstrated effective foam control action with twelve highly diverse surfactants, including cationic, anionic, and nonionic varieties.

Table 2 lists a variety of surfactants that have been claimed in US patents as possible components of foam-control patents. Something worth noting in this tabulation is the very wide breadth of some of the patent claims, even including cationic and amphoteric surfactants as possibly being viable components of foam-control formulations. Such options may or may not be the preferred embodiments, which sometimes are spelled out in either examples or claims of the same patents. As a general rule, nonionic surfactants have been specified most frequently in US patents of foam-control formulations. A practical reason for this preference is the fact that such surfactant systems are less likely to be sensitive to such factors as pH, salt concentrations, and differences in colloidal charge of different industrial processes.

Table 2. Examples of Surfactants Listed in US Patents Describing Possible Foam Control Formulations

Hydrophilic-lipophilic balance (HLB) of the surfactants

As a general principle, a surfactant to be employed in a foam-control formulation will need to be able to displace and spread within existing monolayers of surfactants already present in a foamy mixture that requires treatment. A rule of thumb in selecting suitable surfactants for that role is called the hydrophilic-lipophilic balance (HLB) Griffin 1954). On the HLB scale, a higher number will correspond to longer or more influential hydrophilic groups compared to lipophilic groups. When studying a series of nonionic surfactants, Rodríguez-Abreu (2019) found a strong correlation between HLB values and the packing density of the ethylene oxide groups.

Based on Fig. 13, the surfactants used in foam-control would be expected to have hydrophile-lipophile balance (HLB) values in the range of about 2 to 3, meaning that they would be strongly lipophilic (Pasquali et al. 2008). Most studies of foam-control agents have reported higher values. For instance, Zhang et al. (2007) found good foam control results when using surfactant blends with HLB in the range of 8 to 9, and Gao et al. (2023) reported excellent defoaming results with use of a surfactant having an HLB of about 10.5. Qiao et al. (2022) found increasing effectiveness of foam control formulation with decreasing HLB values in the approximate range 12 to 10. Gao et al. (2024) reported strong foam control action with a polyether modified polysiloxane surfactant having an HLB of 10.5.

A possible reason for the apparent disagreement is that there is likely to be a very wide range of prevailing surface tensions in system requiring foam control. Christiano and Fey (2003) compared 11 widely available commercial surfactants having HLB values ranging from 3 to 21 as potential anti-foam agents. All were judged to be effective, though the amount of surfactant required was found to decrease with in increasing HLB value. In other words, surfactants with higher HLB values were more effective. A second possible reason is that particulate additives to such formulations (see later) are typically highly hydrophobic, and sometimes other components of the formulation can play related roles.

Fig. 13. Summary of hydrophile-lipophile balance (HLB) values associated with various common applications of surface-active agents (redrawn based on a Wikipedia figure)

Table 3 provides HLB-related “preferred ranges” or specifications that have been provided in US Patents having the words “antifoam” or “defoamer” in the title. While patents are not always based on statistically significant findings, they can provide insight into what corporations are placing emphasis on and investing in. Note that the Table is organized according to increasing value of HLB (or mid-point in a range of preferred HLB values). It is notable that the HLB values of such surfactants spanned a huge range from 0.1 to 12. Only one of the patents listed numbers in agreement with the HLB values of 2 to 3 listed in Fig. 13 for defoamers. A possible explanation for those low values is that such components might be playing the role analogous to hydrophobic particles in some foam-control formulations.

Table 3. Hydrophilic-Lipophilic Balance (HBL) ‘Preferred Ranges’ of Specified Values for Various Surfactants in ‘Antifoam’ and “Defoamer” Agents Described in US Patents

Figure 14 describes a common observation, in the development of foam-control agents, that with increasing hydrophobic character, the surfactants contribute more and more to the stabilization of foam (Zarate-Muñoz et al. 2015). But after gradually raising the temperature and reaching a maximum in foaminess, known as the cloud point, an abrupt collapse of foam often is observed. The transition defines the point beyond which the surfactant has become slightly too hydrophobic or too insoluble for foam stabilization. Zarate-Muñoz et al. (2015) showed that the cloud point can be predicted based on the HLB value and other aspects of nonionic surfactants.

Fig. 14. Commonly observed dependency of foam characteristics of surfactants relative to their water solubility (figure adapted from TAPPI Introduction to Wet End Chemistry course)

Microemulsion-type formulating and the HLD_N concept

It can be hypothesized that optimal formulation of a foam-control agent might be governed by the same physical phenomena associated with optimization of detergent systems and microemulsions (Hubbe et al. 2022). The reason is that, just in the case of foam-control systems, microemulsions used in detergent systems tend to perform best when interfacial tensions become very low. In microemulsions systems, such as cold-water laundry detergents, it has been found that the hydrophilic-lipophilic difference or deviation (HLD_N) (Salager et al. 2000; Salager 2021) can be a key variable. This parameter represents the difference of a given surfactant system from its optimum formulation for emulsification, and it can be calculated based on physical parameters (Salager et al. 2000; Zarate-Muñoz et al. 2015; Salager 2021). Notably, some US patents contain references to the principles of microemulsions (Gammon 1980; Nguyen 1994; Furman 2002).

Spreading of oils and surfactants

Dating back at least to the time of Benjamin Franklin (1774), it has been appreciated that suitable hydrophobic oils can spread rapidly on water surfaces, forming very thin films. The extent of the spreading was such that 5 mL of the oil smoothed the waves of the pond over an area of several square meters. A requirement for such action is that the compounds need to have sufficiently large hydrophobic groups to render them insoluble in water, while at the same time having at least one hydrophilic group. Though a great deal of science has been learned since the days of Franklin, modern foam control agents generally contain at least one water-insoluble surfactant having a rapid-spreading tendency. These mixtures are often formulated by emulsification of an oil (Bergold et al. 1990; Ishida 2001). For instance, when formulating a food product, a defoaming effect can be achieved by including certain flavor oils (Qi et al. 2018).

Silicon-based products and oil-free options

Silicone-related oil products have become increasingly popular for control of foam (Sinka and Lightman 1977; Rekonen et al. 1990; Brandt et al. 1996; Campbell et al. 1997; Jha et al. 2000; Hoekstra 2007). The oil in such formulations can be poly(dimethylsiloxane) (PDMS), which is highly hydrophobic (Hill and Fey 1999; Owen 2010). However, the PDMS can optionally be omitted in formulations based on just the surface-active components. Nonionic surfactants have been combined with silicone-based formulations to fine-tune the performance (Christiano and Fey 2003). Another popular way to prepare commercial foam-control agents is to use water as a main component (Twoomey 1986, 1990); such formulations were said to be less expensive than similar products formulated with oils. The success of such products provides further support for strategies in which one is relying mainly upon just surface-active agents, along with optional hydrophobic particles, to carry out the defoaming action.

Hydrophobic Particle Additives

Another milestone was the discovery that foam-control products could be rendered more effective by their formulation with suitable particles (Allen et al. 1993; Ayeyard et al. 1994; Brandt et al. 1996; Spence 1997; Denkov 2004). These can include fumed silica that has been rendered hydrophobic. For instance, a trialkoxyalkylsilane can be reacted with the silica surfaces (Belyakova and Varvarin 1999; de Vos et al. 1999; Wang et al. 2013). Amide wax particles, such as ethylene-bis-stearamide (EBS), also have been widely used, especially in brownstock washing (Allen et al. 1993; Brandt et al. 1996). Best effects are achieved when such particles are hydrophobic (Kulkarni et al. 1977b; Aveyard et al. 1993; Garrett et al. 1994; Denkov 2004; Joshi et al. 2009) and preferentially with an angular or jagged shape (Frye and Berg 1989; Pelton 1989; Wang et al. 2013, 2024). Here, the word “angular” implies that the particles include some acute angles and sharp edges. Aveyard et al. (1994) reported effective foam breakage with cylindrical particles, whereas the spherical particles they considered actually contributed to foam stability in some cases. In addition, the performance of the hydrophobic particles often increases with increasing particle size (Kosco et al. 1994; Joshi et al. 2005). Joshi et al. found maximum foam destruction when the hydrophobic particle size was about 3 µm. Direct observations by Tamura et al. (2000) showed that bubbles broke when the thickness of their surfactant bilayer wall had been decreased to match the size of hydrophobic particles. Wang et al. (1999) showed that hydrophobic silica particles tended to situate themselves at the interface between silicone oil and air, which is consistent with their apparent role in piercing the surfactant bilayer between adjacent bubbles.

Table 4 lists some attributes of hydrophobic particles that have been specified in US patents of foam-control formulations. Though a relatively wide range of particle sizes is represented within the list, it is notable that most of the cited ranges include particles at least as large as 1 µm in diameter or somewhat larger. Such particles would be sufficiently large to be able to bridge the aqueous films between adjacent bubbles. It is a common experience that rainbow colors can be seen when viewing airborne aqueous bubbles, and such observations provide evidence that the respective bubble-wall thicknesses were one or more factors of a wavelength of visible light, i.e. at least 400 to 700 nm. Bubbles that do not show colors are likely to have bubble walls that are thinner than this range.

Table 4. Attributes of Hydrophobic Particles for ‘Antifoam’ and “Defoamer” Formulations Described in US Patents

Classic Descriptions of Foam-control Mechanisms

Cautionary note concerning mechanistic models

In the subsections that follow, it is important to bear in mind a note of caution from Wang et al. (2016). They pointed out that foam structures are complicated, and different mechanisms can be at work simultaneously. Even when there is excellent evidence to support a given mechanism, the real situation represented by a pulp and paper mill environment can be expected to pose challenges. In contrast to typical laboratory tests, the industrial system will contain such items as cellulosic fibers, changing levels of soluble polymers, a wide range of ions, different temperatures, and differences in the formulation of different foam-control agents. Thus, it will continue to be challenging to bridge the gap between the laboratory and the mill. Denkov et al. (2014) expressed an opposing, and therefore more optimistic view regarding foam-control mechanisms. In their view, although the formulations of foam-control agents are highly diverse, the effects of those agents often share a lot of similarities. Likewise, Pelton and Goddard (1994) and Garrett (1995) demonstrated that some effects of foam-control agents can be predicted based on dosage of the agent and other known factors.

Entering

There is widespread agreement in the literature that a droplet of foam-control agent must “enter” into the wall of a bubble as the first step in the mechanism of effective foam control (Robinson and Woods 1948; Ross 1950; Garrett 1993; Ayeyard et al. 1994; Ayeyard and Clint 1995; Chaisalee et al. 2003; Denkov 2004). In the discussion that follows, main attention will be directed toward a situation depicted earlier in Fig. 7, Parts (b) and (c), wherein three connected bubbles happened to be completely immersed in either air or an aqueous phase, respectively. The vulnerable locations in such assemblies of immersed bubbles appear to be (a) the relatively flat walls that separate each part of bubbles and (b) the junctions between such relatively flat areas in the bubble walls. These locations have become known as Plateau borders, in honor of one of the pioneers in learning about the structure of foam (Almgren and Taylor 1976). Figure 15 envisions a droplet of a foam control product that has been applied as a fine mist as it lands upon and enters the bubble wall. In general terms, for the entering to happen, there must be sufficient affinity between the components of the droplet and the surfactant layer that is stabilizing the foam.

Fig. 15. Schematic diagram of a droplet of foam-control formulation, applied as a mist spray in the air phase, in the process of entering a surfactant monolayer at the wall of a bubble that is resting on the water surface

The next figure envisions a similar effect, except that in this case there are three bubbles, and they happen to be submerged in an aqueous phase. The “hydrophobic particle” depicted in this figure represents a widely employed formulation option (Pelton 1989). Because Fig. 16 represents a mechanism taking places in the water phase, it is required that the foam-control agent be formulated with suitable stabilizing agents, i.e. an optimized type and amount of surfactant. The emulsion droplet also may contain an oily component, such as silicone oil, e.g. poly(dimethylsiloxane) (Jha et al. 2000; Sawicki 2005; Owen 2010; Wang et al. 2024).

Fig. 16. Schematic diagram of a droplet of foam-control formulation, applied as a mist spray in the air phase, in the process of entering a surfactant monolayer at the wall of a bubble that is resting on the water surface

In order to have its intended effect, it is widely agreed that the emulsion droplet needs to be able to insert itself into a monolayer of surfactant that is acting as the main stabilizer for the foam that needs to be controlled. An “entering coefficient” providing the criterion for entry in the situation depicted in Fig. 16 can be expressed as (Robinson and Woods 1948),

E = γ_f + γ_d’f’ – γ_d           (1)

where γ_f is the interfacial tension (water-air interfaces) in the foamy mixture, the middle term is the interfacial tension in the mutually saturated mixture of “defoamer” and “foamy system”, and the final term is the interfacial tension of the “defoamer” in pure water. Entering of the defoamer droplet into the bubble wall is expected when the value of E is greater than zero. In this equation, the word “defoamer” is being used to denote the surfactant in the foam-control agent. To meet this criterion, the value of γ_f needs to be smaller than the sum of the dominant surfactant combination that is serving as the stabilizer of bubbles in the system plus the corresponding term after the system has become fully equilibrated. As illustrated in Fig. 16, that inequality allows the defoamer droplet to enter into the first monolayer of surfactant that it encounters as it enters the bubble from the adjacent aqueous phase. The main take-away is that, to enable the “entering” step to take place, the key requirement that the interfacial tension resulting from the surfactant in the foam-control agent must be moderately low. For instance, according to Eq. 1, the value of γ_f may be higher than that of the foamy mixture itself, in some cases.

As a plausible alternative to directly entering a monolayer of surfactant layer, as was depicted in Fig. 16, another possibility is that a droplet of formulated emulsion diffuses within the Plateau border film of a group of bubbles. This possibility is illustrated in Fig. 17. In such an arrangement it is reasonable to expect that bridging between the adjacent monolayer films can take place later, for instance due to nano-scale wavelike motions within the films. Such a mechanism has been demonstrated by means of high-speed microphotography (Wang et al. 1999).

Fig. 17. Schematic diagram of a droplet of foam-control formulation, applied as a mist spray in the air phase, in the process of entering a surfactant monolayer at the wall of a bubble that is resting on the water surface

Spreading

Once a droplet of foam control agent has inserted itself into a bubble wall, the next pertinent question regards whether or not it will be able to spread (Kulkarni et al. 1977a; Garrett 1993; Avery-Edwards et al. 1994; Racz et al. 1996; Denkov 2004; Denkov et al. 2014; Pelton 1989; Yang et al. 2023). Jha et al. (2000) have presented some of the best evidence that the spreading of foam-control oils on bubble surfaces has a strong correlation with the effectiveness of foam-control products. The cited authors showed that a large spreading tendency can be attributed to a two-dimensional spreading pressure embodied within a condensed monolayer of a well-chosen surfactant (Hubbe et al. 2020a). They also reported a strong positive correlation between spreading tendency and the effectiveness of foam control. It is often assumed that the emulsion particle initially is present within just one monolayer of surfactant, rather than spanning two layers of bubble wall (Garrett 1993; Chaisalee et al. 2003). The most interesting situation is when one of the foam control droplets happens to be situated right at the Plateau border. Considering that starting point, a spreading criterion can be written as follows (Harkins 1941),

S_d/f = γ_f – (γ_d’ + γ_f’d’)         (2)

where S_d/f is the spreading coefficient, γ_f’ is the interfacial tension of the foamy system, γ_d’ is the interfacial tension the surfactant in the foam-control system in its present state of saturation with any other surfactants, and the final term is the interfacial tension of the whole system after equilibration.

Bubble wall thinning

Depending on how rapidly the foam-control agent is able to spread within a monolayer of surfactant at a bubble wall, one of the expected effects will be a thinning of the water film separating the two adjacent bubbles. This effect is illustrated in Fig. 18. Denkov et al. (2014) envision the walls between adjacent bubbles as having thinned to such an extent that droplets of foam-control agent in the intervening space cause bulges in the bubble’s double-wall. As the area between the bubbles is momentarily increased by the spreading action, there will be less water per unit area between the bubbles. This makes the structure more vulnerable to breakage. Experimental work by Tamura et al. (2000) showed that such structures can be unstable, leading to bridging by droplets of foam-control agent and subsequent coalescence of pairs of bubbles. In addition, random wavelike motions within the film may result in an opening of a window between the bubbles, and that unstable intermediate structure is expected to rapidly lead to coalescence of the two bubbles into a larger bubble (Garrett 1993). As noted by Pelton (1989), such coalescence can be regarded as the central event which, if repeated very larger numbers of times, can effectively destroy foam in aqueous systems.

Fig. 18. Schematic diagram representing the hypothesized spreading of the foam-control agents within one of the monolayers in the film between two adjacent bubbles, causing a momentary thinning of the water film between them

The role of hydrophobic particles in piercing bubble bilayer walls

Hydrophobic particles that are used in the formulation of foam-control agents have been shown to increase the effectiveness of many such products (Dippenaar 1982a,b; Frye and Berg 1989; Aveyard et al. 1993; Garrett et al. 1994; Koczo et a. 1994; Ayeyard and Clint 1995; Denkov 2004). The spreading action induced by the foam control agent provides two different opportunities for involvement of hydrophobic particles in piercing the walls between adjacent bubbles. First, the thinning of the wall between adjacent bubbles, as just described, may allow a sufficiently large hydrophobic particle to bridge across both surfactant monolayers that define the bubble wall (Owen 2010). At that point, depending on some details related to contact angles, an opening between the adjacent bubbles may occur (Owen 2010). In other cases, the spreading action may allow the hydrophobic particle to essentially tear open a window between the adjacent bubbles (Pelton 1989; Racz et al. 1996). Figure 19 illustrates the mechanical part of the particles’ apparent role in ripping apart the surfactant bilayer in the course of spreading of the foam control agent. Rapid spreading events associated with use of foam-control agents were apparently first observed experimentally by Dippenaar (1982a,b).

Fig. 19. Schematic diagram illustrating the concept of a hydrophobic particle being dragged rapidly across the bubble wall such that the thinned aqueous film becomes pierced

Another way that rapid spreading of a foam-control product potentially could lead to foam collapse is by rapid localized depletion of surface-active agent (Kulkarni et al. 1977b). The cited authors proposed that depletion could come about by rapid adsorption of surfactant onto hydrophobic particles. In view of the Marangoni effect (Pelton 1989; Garrett 1993; Anazadehsayed et al. 2018), such depletion could serve as an additional cause of spreading and thinning of the walls between pairs of bubbles.

Viscosity effects

In principle, a rapid spreading action will be favored by a low viscosity of the surfactant used in formulating the foam-control agent. Theoretical studies have identified increasing two-dimensional viscosity at water-air interfaces as an important factor in predicting instances of stable foam (Brown et al. 1953; Kanner 1968). Work by Wang et al. (2019) and Chowdhury et al. (2020) showed that higher surface viscosity (i.e. two-dimensional viscosity) tends to render foams more stable. Foam-control formulations with intermediate alkyl chain lengths of the surfactant (e.g. eight carbons) are sometimes found to be more effective than longer lengths (Garrett 1993). At the optimum length, the alkyl chain length may be sufficient to achieve a strong surface-active effects while at the same time contributing relatively little to surface viscosity. In support of these concepts, Jha et al. (2000) found that the relative antifoaming efficiency increased with decreasing surface viscosity of the surfactant monolayer.

Bridging

Bridging is widely believed to be a key milestone in the breakage of foam. The bridging agent may be either the foam control emulsion droplets themselves or in many cases the hydrophobic particles in the formulation. The bridges span across the two surfactant monolayers that compose the walls between bubbles (Frye and Berg 1989; Pelton 1989; Aveyard et al. 1993; Garrett et al. 1994; Koczo et al. 1994; Nemeth et al. 1998; Denkov 2004; Joshi et al. 2009; Yang et al. 2023).

A bridging coefficient has been defined in an effort to predict whether or not bridging will occur in a given system (Aveyard et al. 1993). The cited authors express such a coefficient as follows,

B = γ_aw² + γ_ow² – γ_oa²          (3)

where γ_aw is the air-water interfacial tension, γ_ow is the oil-water interfacial tension, and γ_oa is the oil-air interfacial tension. Bridging is expected when B > 0.

In foam-control formulations that do not contain solid particles, the role of bridging sometimes can be served by an oil component. Such an effect was demonstrated by Georgiev et al. (2023). For foamy systems stabilized by various surfactants in combination with poly(vinyl alcohol), they found that foam-control action would be described by a two-step process. First, the foam-control emulsion droplets became situated between adjacent bubbles. Second, the emulsion films at the bubble surfaces became ruptured by the foam-control material.

Pinch-off at surface of hydrophobic particles

The term “pinch-off” will be used in this article to describe an event in which the surface of a hydrophobic particle becomes dewetted due to coming together of lines of three-way contact between the particle, air, and an aqueous phase (Pelton 1989; Aveyard and Clint 1995; Denkov 2004; Owen 2010; Denkov et al. 2014; Yang et al. 2023). The process is diagramed schematically in Fig. 20. According to Kruglyakov (1994), the onset of film rupture is an indication that three-phase contact (involving air, water, and the solid surface) is not possible due to the contact angles and other factors.

Fig. 20. Schematic diagram illustrating the concept of “pinch-off” due to movement of the meniscus lines of contact at the surfaces of a hydrophobic particle that is used to enhance foam-breaking effects

Onset of bubble coalescence

When focusing on quasi-equilibrium conditions, i.e. excluding what might happen during a rapid spreading event, it has been proposed that ultimate collapse of foam, at least at one point, can be predicted based on interfacial thermodynamics (Kruglyakov 1994). Specifically, any three-way contact between water, air, and a third phase needs to conform to relationships that can be determined by experimentation. Thus, whether or not the hydrophobic particle will succeed in opening up a window between adjacent bubbles, especially as a result of relatively slow thinning of a bubble wall, will depend on angles of contact with the hydrophobic particle. The key to success is expected to lie in having a sufficiently low surface free energy of the hydrophobic particle (Belyakova and Varvarin 1999; de Vos et al. 1999; Wang et al. 2013). In addition, as discussed next, angular particles, containing acute angles and sharp edges, are likely to be more effective in both bridging between the two monolayers of surfactant defining the bubble wall and in enabling the pinch-off process.

Shape of the hydrophobic particles

Particle shape appears to be important relative to the completion of the pinch-off event (Pelton 1989; Garrett 1993; Tamura et al. 2000; Wang et al. 2024). Frye and Berg (1989) noted that the spherical shape of an emulsion droplet of foam control agent tended to be not as effective as the non-spherical shapes of hydrophobic solid particles. Likewise, though hydrophobic spheres of silica were found to be effective in such products, better effects could be achieved following grinding of such spheres, essentially making micro-shards of glass. The latter particles achieved almost 100% removal of foam. The cited authors proposed that the angular hydrophobic particles were able to achieve easier movement of the lines of three-phase content, leading to more effective bridging, and thereby leading to effective pinch-off. Related work by Joshi et al. (2005) showed that the particle must be solid to achieve superior foam-control effects; this is consistent with the necessarily round shape of completely melted particles. Pelton (1989) added a further stipulation that the particles should also be smooth on their faces, thus facilitating rapid movement of contact lines across their surfaces. Yang et al. (2023) added the stipulation of “having sharp edges” to the list of desired features in the particles.

Implications of bubble size distribution

It has been shown that treatment of an entrained air suspension with a foam-control agent resulted in heterogeneity of bubble sizes (Al-Masry et al. 2006). This situation seems consistent with the non-equilibrium processes described in previous subsections, along with a possibility that the foam-control agent might have a non-uniform distribution in the system, at least initially. Al-Masry et al. (2006) employed a sparging system to provide a relatively uniform distribution. Addition of foam-control agent at increasing amounts led to an increased average bubble size, especially at relative low flow rates, which is consistent with the pairwise coalescence of bubbles.

Specifications of surfactants for foam-control products

When developing the formulation for a new foam-control product, there are some general expectations concerning selection of the surface-active agent or agents. There are different ranges for the hydrophile-lipophile balance (HLB) values that have been found to work well for different applications of surfactants. Low values of HLB are representative of compounds in which the hydrophobic end of the molecule is somehow larger or longer than the hydrophilic end, and vice versa. Due to the diversity of categories and structures of such compounds, the HLB values can be regarded as guides rather than accurate descriptors. A research question to consider is whether or not desired foam-control effects can be fine-tuned or rendered more robust by using a calibrated ratio of two surfactants having somewhat different HLB values. There is some evidence that pure surfactants have been more successful agents of foam control in some instances (Jha et al. 2000).

HLB difference

As was noted earlier, foam-control systems share some common features with modern detergent formulations for laundering (Salager et al. 2000; Hubbe et al. 2022). In both cases, formulations are optimized in such a way as to achieve very low values of interfacial tension, either briefly or over an extended time. One of the parameters that has been utilized for the optimization of cold-water detergent system is the hydrophilic-lipophilic difference (HLD_N) (Salager et al. 2022). The value of this parameter can be calculated based on a salinity term, the alkane carbon number, and some factors that do not change in the course of numerical integration. In principle, the lowest value of interfacial tension is expected when the value of HLD_N approaches zero. However, because the goals of foam control are quite different from those of laundry detergents, it is not yet known whether the HLD_N are equally applicable to foam-control applications. On the positive side, it can be hypothesized that a microemulsion system is likely to be effective for encapsulating surface-active substances present in aqueous mixtures. However, as unfavorable aspect, the long-term persistence of very low interfacial tensions provided by such detergent systems would be expected to adversely affect the development of inter-fiber bonding in paper as it is being formed and dried (Hubbe 2006).

Another point of reference, in selection of a surfactant for a foam-control product, is the cloud point. As mentioned earlier, it is often found that foam-control agents are most effective when they are applied at a temperature near to but above the point at which their pure solution in water becomes cloudy, as the temperature is increased (Pletnev et al. 1983; Chaisalee et al. 2003). From a mechanistic standpoint, it appears that one should aim for a condition in which the surfactant has developed an unstable tendency due to its insolubility in the water phase.

Loss in effectiveness over time

Once a foam-control agent has been added to an industrial process, one can envision a strong immediate effect on foam, followed by a gradual process of equilibration over the course of time. Such an effect is shown in Fig. 21, which was redrawn in simpler form based on the work of Clas and Allen (1994). As shown, injection of air into a solution of 20% synthetic black liquor caused a rapid build-up to a foam height of about 300 mm. The plotted green triangles corresponded to an oil-based foam control agent, which had a steady effect at preventing the foam level to rise above its initial level. This behavior caused the cited authors to classify the product as an anti-foam agent, since it was effective in preventing foam from developing. In theory, the petroleum-based oil in the formulation could be expected to act as a reservoir for various surface-active compounds, thus reducing their effects as foam stabilizers. The plotted orange circles correspond to a water-based defoamer, which knocked down the foam almost completely for the first 20 minutes or so, but then became progressively less effective. The quick and strong suppression of foam means that the agent would generally be classed as a defoamer. The observed loss of effectiveness, in the latter case, can be attributed to a mixing of materials, eventually tending toward an equilibrated mixture of all the surfactants present in the system. In addition, Racz et al. (1996) suggested that the foam-control surfactant soon becomes incorporated into small, stable emulsion droplets, which have lost any defoaming activity. Since the surfactants that would be used in the foam-control product would have a lower interfacial tension than initially present in the system, one can expect that the system’s prevailing interfacial tension will end up somewhat lower than it was initially. If such a treatment were carried to extremes, the expected outcome would be such a low prevailing interfacial tension as to make the system resistant to further usage of foam-control agents. In addition, over-use of foam-control products may eventually result in stabilization of foam (Reinhardt et al. 1998).

Fig. 21. Example of loss of effectiveness of different foam-control agents as a function of the duration of time since their addition to papermaking furnish (redrawn and simplified from Clas and Allen 1994)

Based on the literature search results, there has been relatively little attention paid to the modeling of kinetic aspects of foam control. An exception was the work of Pelton (1996). The reported model calculated the likely rates of collisions between foam control emulsion particles and air bubbles in the liquid phase. The model correctly predicted effects of such variables as the dosage of foam-control agent, the initial characteristic bubble size, and changes in the height of floating foam layers in the test equipment as a function of time. The success of the modeling can likely be attributed to the focus on well-defined laboratory conditions.

Deposit problems related to foam

A further reason to avoid adding an overdose of foam-control product is to minimize contributing to unintended deposits of material onto the wetted parts of process equipment. As already has been noted, both the surfactants and any particles comprising the foam-control product can be assumed to be relatively hydrophobic. Based on thermodynamics, one can expect there to be a tendency for such components to associate themselves with other hydrophobic matter, such as wood pitch, lithographic inks, or hydrophobic sizing agents being added to the fiber suspension. Such material can precipitate from the aqueous phase and build up on various surfaces, including stainless steel or plastic. There have been reports identifying foam-control agents as being present in deposits collected at various points within paper machine systems (Dorris et al. 1985; Hoekstra 2007; Sithole and Watanabe 2013).

Foam Control Feeding Strategies

Most operations that take place within modern pulp and paper facilities can be classed as continuous processes. Some exceptions include batch kraft pulping, batch-wise pulping of recycled fibers, and preparation of certain chemical additives, can be described as continuous. Thus, most feeding strategies for foam control agents will be set up to deliver relatively steady, continuous metering of flows to the system. Turbulent flow, offering relatively rapid dispersement of the foam-control agent, is recommended as a means to achieve high effectiveness throughout the treated process stream. Positive displacement pumps, such as diaphragm-type piston pumps, can be used to meter the as-received emulsified product to the point of addition to the process (Jackson et al. 2018).

To determine a favorable rate of injection, it is recommended to carry out the kind of test that is illustrated in Fig. 22 (Hubbe and King 2009). In the example shown, the rate of addition was decreased in a stepwise, systematic way over a sufficiently long time (e.g. holding each level for 15 minutes or more) to allow the system to approach a relatively stable level of foam or entrained air, based on an online test. As the dosage of foam-control agent was being decreased, the measured level of entrained air increased. The delay effects shown in the figure are consistent with the lingering effects previously added foam-control agent at the higher dosage. Then, when the dosage was returned to its initial level, the level of entrained air soon fell back to its initial level. This example can be regarded as evidence of successful usage of a foam-control agent at a favorable level of addition. If the initial dosage of foam-control agent had been greatly in excess of the needed amount, then there would have been no increase in entrained air following a modest decrease in the addition rate.

Fig. 22. Schematic diagram of a routine dosage-response test at a paper mill to determine whether a foam-control agent is being applied at a suitable level. This figure is a redrawn version of a figure included in TAPPI’s Introduction to Wet End Chemistry course.

Multi-point addition

Given the transient nature of foam-control treatments, it is reasonable to consider strategies of multi-point addition to both brownstock washing systems and paper machines. Such procedures have been recommended as a way to achieve better overall control of the foam (Bandekar et al. 2014; Brandt et al. 1996). In some cases, there may be more than one location in a process where the operators have observed a need for control. For instance, one addition point may be intended to decrease entrained air before a filtering operation, and another addition point (and maybe even a different foam-control product) may be added such as to break down banks of foam at the top of a seal chest. In the latter case, one might consider application of a fine, low-volume spray of diluted foam-control agent at the top of the bank of foam (Garrett 1993). Bandekar et al. (2014) demonstrated the addition of foam control product to the process water (not the shower water) of a brownstock washing system at multiple points. According to Brant et al. (1996), at least two addition points for foam control additives are commonly used in brownstock washing systems.

FOAM CONTROL IN THE PULP AND PAPER MILL

This section has four areas of focus. The two primary focus areas are brownstock washing and the wet end of the paper machine. Briefer discussion is then provided related to foam control at the size press and during the preparation of aqueous coatings for paper.

Brownstock Washing

As stated in the Introduction, two priorities with respect to brownstock washing are to maintain sufficiently rapid flow of rinse water through the fiber mat and to achieve efficient displacement of lignin-rich water from that mat. As a first step in understanding such effects, the components tending to stabilize foam in such washing systems will be considered.

Composition of brownstock filtrate

The composition of the process water entering the brownstock washers can be regarded as challenging from the perspective of foam control. The term “weak black liquor” is often used to denote the liquid entering the washer system. A pH value higher than about 10 at that point will be maintaining both phenolic and carboxylic acids present in the water in their negatively charged ionic states. As shown in detail by Zhang et al. (2021), kraft lignin has abundant phenolic groups. Because the pK_a value of these groups is about 10, this means that lignin that has been released from the fibers during pulping will be present as negatively charged macromolecules. These tend to be repelled from the surfaces of fibers, which have a negative charge throughout the ranges of pH most commonly used in pulping and papermaking. Hemicellulose is expected to be the most prominent water-soluble polymer type that will be responsible for the persistent nature of foam present in brownstock system (Wallberg et al. 2006; Wallmo et al. 2009). The fatty acids, as well as any triglyceride fats originally present in the wood can be expected to have become transformed into their soap form (Niemela 1990), along with resin acid soaps when pulping involves softwood species. Typical compositions of such mixtures of carboxylic acid soaps have been reported (Käkölä et al. 2007). As is evident in the name “soap,” these substances largely account for the stabilization of foam in brownstock washing systems. According to Kanicky et al. (2000), Kanicky and Shah (2003), and Douliez and Gaillard (2014) favorable self-association among adjacent fatty acid chains in a monolayer can lead to highly stable foams at certain pH values.

High solubility of the hemicellulose in the aqueous phase can be expected due to the presence of carboxylic acid groups, which confer a negative charge throughout the pH range encountered in brownstock washers. Though much of the hemicellulose will remain part of the fibers after kraft pulping, that portion that becomes dissolved will be repelled from the fiber surfaces by like charges. Zhu et al. (2015) quantified xylans and glucomannan hemicelluloses in lignin specimens that have been precipitated at different pH values.

Operators of brownstock washing systems need to manage some tricky issues related to pH during a typical washing operation. Ultimately, the goal will be to displace almost all of the weak black liquor from the fiber mat by the time the final washing stage has been completed. The reason for using a counter-current washing system is that such a practice will minimize the extent of dilution of the weak black liquor from the fibers. The unavoidable changes in pH and composition of the filtrate during washing implies that it can be difficult to achieve an optimized foam control program that can encompass the whole washing process. As shown in Fig. 23, as the pH is lowered, more and more of the kraft lignin can be expected to precipitate out of solution (Uloth and Waring 1989).

Fig. 23. Increasing precipitation of kraft lignin (going from right to left) in response to gradual addition of strong acid (figure redrawn from original data by Uloth and Wearing 1989)

The effects shown in Fig. 23 can be understood based on expected changes in the protonation of kraft lignin as a function of pH. The initial increase in precipitation of lignin from the solution phase, as the pH is reduced in the vicinity of 10, can be attributed to protonation of the phenolic -OH groups, which have a pK_a value of about 10. The pH relationships are shown in Fig. 24 for a hypothetical simple fragment of lignin.

Fig. 24. Expected states of dissociation (with negative ionic charge) vs. protonation (with neutral charge) of two kinds of acidic species with a hypothetical fragment of lignin that is being progressively reduced in pH from kraft cooking conditions, first to pH near to 9 and then near to 3

However, in order to protonate carboxylate groups, which may be present on the aliphatic groups of lignin or remaining hemicellulose fragments remaining bound to the lignin, a much lower pH would be needed (Hubbe et al. 2019). Carboxylic acids on those entities often have pK_a values of about 3.3 (Laine 1997).

Consistent with these principles, Allen and LaPointe (2003) found that reprecipitation of extracted materials back onto the fibers in the course of brownstock washing could be reduced by raising the pH and temperature of the rinse water. The cited study was mainly concerned with such compounds as resin acids and fatty acids. However, it is reasonable to expect that NaOH addition to wash water in later stages of washing may delay or minimize reprecipitation of lignin back onto the fibers.

As a combined treatment, along with the usage of defoamers, Pelton and associates showed that brownstock washing efficiency could be further improved by taking steps to overcome channeling effects (Pelton and Grosse 1994; De et al. 1997; Lappan et al. 1997). This effect was achieved by adding a cationic polymer to the wash water. Such treatment resulted in polyelectrolyte complex formation between the cationic polymer and solubilized lignin byproducts. These complexes preferentially blocked just the larger channels of flow within the mat, thereby forcing more of the flow to take place through smaller channels. The effects were demonstrated using packed beds composed of clear solid glass beads of different sizes. Technical feasibility was also demonstrated at production scale in a pulp mill (Lappan et al. 1996).

Points of addition in brownstock washing systems

It has been proposed that a multi-point addition strategy be adopted for feeding foam-control additives (Bandekar et al. 2014). Such practices can be justified based on several grounds. First, as explained earlier, there is often a rapid decline in effectiveness with the passage of time after a foam-control agent has been added. Second, typical brownstock washing systems are comprised of separate units, and issues related to dewatering rate and displacement efficiency may need to be optimized separately in each case. And third, multiple treatment locations opens the possibility of using different foam-control formulations in different parts of the process. In addition to providing a strategy to achieve more uniform flow in a nonuniform pulp mat, the findings also confirmed the importance of pulp mat uniformity in general, relative to achieving efficient displacement of lignin during brownstock washing.

Bleaching stage washers

Foaming issues also can be encountered during the washing operations following different stages of bleaching, especially following the alkaline extraction stages (Dragan 1989). The alkaline conditions in those stages are expected to release remaining fatty acids and some of the remaining hemicellulose, both of which will contribute to stabilization of foam.

Paper Machine Foam Control

Entrained air has the potential to slow down the production of a main product from a paper mill. In addition, large bubbles in severe cases can adversely affect product quality. Subsections that follow will consider findings related to how various foam-control products may promote dewatering, affect the quality of paper products, and how they have some reported side-effects. Some particular issues related to foam can arise when a paper mill team is practicing acidic papermaking (rosin-alum systems). Likewise, some aspects of alkaline papermaking, especially when there is unintended dissolution of calcium carbonate fillers, can lead to foam problems.

Drainage issues

As has been widely reported, entrained air bubbles in a papermaking furnish tend to slow the rate of dewatering (Brecht and Kirchner 1959; Gertjejansen and Hossfeld 1967; Karras and Springer 1989; Rauch and Sangl 2000; Pelton et al. 2002; Helle and Paulapuro 2004; Martorana and Kleemann 2006). These observed effects can generally be explained as due to a tendency of the bubbles to block drainage channels within the wet web of paper (Hubbe and Heitmann 2007).

While reduced drainage due to air bubbles in papermaking fiber suspensions has been widely reported, as shown above, there also have be reports showing positive effects of surfactant addition on drainage rates. Touchette and Jenness (1960) showed that the addition of between 1 and 2% surfactant, based on dry pulp mass, increased the water removal by about 12 to 35% in laboratory tests, when considering seven commercially available surfactants representing different chemical classes. More recently, Lehmonen et al. (2020) showed that addition of sodium dodecyl sulfate at the level of 3 g/L to papermaking furnish resulted in higher dryness after forming of refined bleached softwood pulp (18 degrees Schopper-Riegler). Such results, though helping to show that the surfactant itself was not the direct cause of reduced drainage, should not be taken as a recommendation to add such high amounts of surfactant to a paper machine system, depending on the grade of paper being considered. Touchette and Jenness (1960) observed decreases of as much as 40% in tensile strength and burst strength, when individually adding the same diverse group of surfactants at the 1% and 2% levels, though some of the surfactants gave a slight positive effect on strength. Notably, the highest strength results were achieved with anionic or nonionic surfactants, i.e. those having low affinity for the cellulosic surfaces, which typically bear a negative ionic charge.

Product quality issues

As reported by Mudaly (2007), entrained air in the wet web is often given as a main cause for pinholes in paper products. The mechanism appears to involve the forceful sucking of bubbles through the wet web as it passes over suction boxes.

Campbell et al. (1997) found that foam control agents tended to render newsprint paper more water-wettable, which was undesirable. The effect was attributed to the agent’s necessary content of surface activity. In other words, the surfactants in the formulations were opposing the intended effects of rosin-alum sizing by acting as wetting agents.

Deposit issues

When foam bubbles are able to rise, form a floating layer of froth, and then the bubbles eventually break at some point in a papermaking process, such a cycle may serve as an unintentional concentrating process for relatively hydrophobic contaminants, including pitch and stickies. These may then become entrained back into the papermaking furnish, such that they contribute to spots in the paper product (Allen et al. 1993). In addition, the hydrophobic nature of components of foam control products raises a danger that efforts to control foam can sometimes contribute to deposit problems (Brandt et al.1996). In particular, ethylene-bis-stearamide, which is often used as a hydrophobic particle to enhance the effectiveness of brownstock washer defoamer products, is often implicated in deposit problems in paper mills (Brandt et al. 1996). Likewise, Dorris et al. (1985) reported finding amide-type defoamer products in pitch deposits in paper machine systems. Dunlop Jones and Allen (1989) found that such deposits tended to increase with increasing addition of the defoamer products. According to Hoekstra (2007) such problems are often dependent on the composition of the foam-control product, with some oil-based foam control products having a tendency of contributing to pitch deposition problems. Silicone products have been reported to decrease problems associated with pitch deposits (Twoomey 1990; Brandt et al.1996; Hoekstra 2007). Though silicone-based foam control products have largely displaced the oil-based products, they too are known to contribute to deposits of hydrophobic materials in some paper machine systems (Sithole and Watanabe 2013).

Acidic papermaking issues related to foam

The term “acidic papermaking” usually can be taken as an indication that the papermaking operation involves usage of a rosin product for hydrophobic sizing. The highly acidic compound aluminum sulfate (papermaker’s alum) is often used in such systems to anchor the rosin to the fibers. Background information regarding rosin-alum sizing systems has been described in more detail elsewhere (Strazdins 1981; Ekhtera et al. 2008; Xu et al. 2016). Briefly stated, most rosin purchased and used by papermakers has its origins in the kraft pulping process. However, most of it has been chemically modified by means of a Diels-Alder reaction with fumaric anhydride, giving rise to a product called fortified rosin size. The fortification reaction renders the rosin more storage-stable, easier to emulsify, and more securely anchored to fiber surfaces following interaction with alum. Some of the rosin is supplied to the paper industry as a rosin soap. The other main option is as an emulsion of the protonated form of rosin, except that a minor portion of the rosin likely will be present in its soap form, so as to serve as a stabilizer for the emulsion. The emulsion form of rosin is most often stabilized with a cationic polymer.

Episodes of excessive foam from rosin-alum papermaking operations often can be traced to an imbalance of chemical additives. In particular, foam can be expected when an insufficient amount of alum has been used to fully interact with the rosin. In the case of rosin soap, one can calculate the estimated amount of aluminum that will be needed to form the aluminum precipitate of abietic acid or levopimaric acid, which together will account for most of the material present (Nyren and Back 1958). It has been estimated that a ratio of alum to rosin (as hydrated alum solid vs. rosin solids) in the range 1:1 or 1.5:1 will be sufficient to interact with the carboxyl groups of the rosin (Strazdins 1981). The calculation can be tricker in the case of emulsified rosin, since the reaction with aluminum does not take place until the paper is passing through the dryer section of the paper machine. Another complication is the fact that some of the alum may be effectively consumed by other tasks, such as charge neutralization and reducing the pH of the process water. A further complication arises when some papermakers attempt to achieve rosin sizing above the usual acidic range. For instance, when the pH is raised above about 5, the added alum becomes progressively less effective, which favors the development of persistent foam.

Another set of problems involves the protonated form of rosin sizing products. Although only a minor portion of the rosin in such products is expected to be in its soap form, there can be a gradual deprotonation during the time that the rosin emulsion is present in the fiber suspension, especially if the pH is higher than about 5. Papermakers generally don’t start out planning to implement rosin under high pH conditions, but the pH may be forced to higher levels due to calcium carbonate, which often is present in recovered fibers from used printing paper. Thus, relatively long residence of a rosin emulsion products at relatively high temperature and relatively high pH all will be expected to increase the observed levels of foam (Marton 1989).

Alkaline papermaking issues related to foam

When calcium carbonate particles are being used as a filler, mainly to provide brightness, opacity, and reduced cost to printing papers, the pH will likely be in a range between about 8 and 8.5 (Crouse and Wimer 1991). Though rosin soap is avoided in paper mills operating under such conditions, some other kinds of surfactants may be present. For instance, the hydrophobic sizing agent alkenyl succinic anhydride (ASA) will start to undergo hydrolysis as soon as it comes into contact with the furnish. Hydrolyzed ASA, a dicarboxylic acid, will then be present in the system to an extent that will depend on such factors as the effectiveness of a retention aid system. The ASA hydrolysate can be described as a sticky, sparingly soluble surfactant that is prone to forming complexes with divalent metal ions such as calcium and magnesium (Stitt 1997). The latter can lead to deposits on the wetted surfaces paper machine equipment.

Another potential stabilizer of foam will include deinking surfactants, as mentioned earlier.

Something else that is important to know about calcium carbonate is that its dissolution, upon contact with acidic media, will cause the release of carbon dioxide, which is quite soluble in water. Thus, when the pressurized stock experiences a rapid drop in pressure, upon its emergence from the slice of a headbox, the amount of entrained air has the potential to be especially high (Duan and Sun 2003). Thus, papermakers are advised to clearly select either acidic papermaking (with an alum-buffered system) or an alkaline system (in which nothing is added that will tend to reduce the pH below about 7). Intermediate conditions can be expected to give rise to objectional amounts of foam.

Points of defoamer addition on the paper machine

Addition points for foam control agents on paper machines can be selected depending on the priorities of the operating team. Often the highest priority is placed on enhancing the dewatering rate and on avoiding pin-hole features in the paper. Both of these goals can be addressed by minimizing the amount of entrained air in the jet of stock that emerges from the headbox. As mentioned earlier, adding the foam-control agent just before the hydrocyclone clearers has the advantages of intensive mixing with the furnish, as well as the likelihood that some air will be able to be drawn from the system at the accepts from primary hydrocyclones. In cases where a paper machine has been fitted with vacuum deaeration equipment (as shown earlier in Fig. 10), there is an even stronger incentive to choose the inlet to the hydrocyclones as a preferred addition point.

Another location where problematic foam tends to collect is at the surface of seal chests, which are typically on the floor below where the forming section is located. In particular, the siphons from low-vacuum suction boxes can be expected to be discharged into such chests (Hubbe et al. 2020b), with the development of foam. In such applications there can be a preference to use mist sprays of fast-acting defoamer products that are effective in breaking existing banks of foam that has risen to the surfaces of such chests.

The Size Press and Foam Issues

The size press can be an especially challenging operation in terms of foam control. Because the size press is routinely used to apply starch solutions to the paper surface, there is an inherent polymeric water-soluble stabilizer always present in the form of dissolved starch. The relatively high viscosity can be expected to suppress rapid spreading of foam control products due to the presence of the viscous starch polymers throughout the water phase. In addition, the size press operation involves agitation and even splashing, especially if there is an overflow of starch that is directed back to a run tank.

Yet more challenging foam problems sometimes are associated with the usage of hydrophobic copolymers, which can be added at the size press together with starch as a means of developing resistance to water penetration into the paper. The chemistry and behavior of such additives has been considered in another review article (Bildik Dal and Hubbe 2021). These copolymers, which can include styrene maleic anhydride (SMA) and styrene acrylate (SA) products, are sensitive to the pH of the system; to avoid conditions leading to excessive foam, the pH may need to be monitored and adjusted, following the recommendations of the supplier. Furthermore, it may be challenging to find a foam control agent that has minimum adverse effect on the intended effect of the added copolymer in developing hydrophobicity of the paper.

Formulation of Aqueous Coatings and Foam Issues

Foam control issues can pose challenges during preparation and application of aqueous coatings for paper products (Hudson 1968; Berger and Gast 1976; Reinhardt et al. 1998). A typical coating formulation consists of pigment (e.g. clay or calcium carbonate particles of ~1 to 5 µm diameter), binder (usually latex, starch, or a combination of the two), and various additives (Lehtinen 2000). The solids content is often higher than 50%. Especially in cases where starch products play a major role as the binder, they have the potential to serve as stabilizers for foam by contributing to solution viscosity. Coating formulations can be quite viscous, though this depends on both the application equipment and the formulation. A further issue is that coating formulations are subjected to extensive hydrodynamic shear during their preparation in order to achieve a high uniformity of the mixture.

Air entrainment during coating preparation often can be minimized by adjustment of mixing conditions. In particular, the physical arrangement of an impeller, as well as its speed, can be adjusted to avoid the occurrences of vortex flow that could draw air into the mixture (Motamedvaziri and Armenante 2012).

MECHANISTIC IMPLICATIONS OF REPORTED FINDINGS

Though there appears to be sound experimental and theoretical support for the main mechanisms of foam-control action presented in articles considered in this review, some of the experimental findings suggest that at least one additional mechanistic step should be considered. As was in the foregoing discussion of paper machine applications of foam-control agents, there is a need to explain the observed enhancements in drainage rates (Hakamäki and Kovasin 1985; Twoomey 1990; Allen et al. 1993; Pelton et al. 2002; Hoekstra 2007; Mudaly 2007; Lobo and Bolton 2013; Bolton et al. 2014; Santos and Hart 2014). As was noted earlier, especially in the paper machine applications, the mechanism needs to account for the fact that only a fraction of a second may have passed between the moment of depressurization of the furnish as it emerges from the headbox and when most of the water has been removed from the wet web. Such a short time is not sufficient for very small entrained air bubbles to move by gravity, regardless of whether they have been converted into larger bubbles by the action of a foam control agent. In addition, it is worth taking seriously the early findings of Poschmann (1962), who noted that the presence of fibers tends to hold individual entrained are bubbles apart from each other, thus calling into question any drainage-promoting mechanism that requires coalescence between adjacent bubbles.

A clue to the resolution of the dilemma just described can be found in the early work of Touchette and Jenness (1960). These authors found that addition of various surface-active agents to papermaking furnish tended to increase the dewatering rates, according to laboratory tests. A further key to unraveling the mechanism can be drawn from a method that has been used for determining the characteristic size of pores in textiles (Miller and Tyomkin 1986, 1994). The test takes advantage of the following relationship between the maximum value of vacuum that can be applied before a pore of radius R will be drained,

R = 2 γ_aw cos θ / (ΔP)           (4)

In Eq. 4, the term γ_aw is the interfacial tension, θ is the contact angle, and ΔP is the applied pressure (or vacuum) that is just sufficient to draw the meniscus through the material. The physical situation is illustrated in Fig. 25, which envisions a single pore through the textile product and models that pore as a uniform cylinder. Note that according to this model, the flow is initially resisted by the maximum capillary forces at a meniscus at the top entrance to the pore, just before the value of ΔP exceeds the capillary pressure.

Fig. 25. Model corresponding to the derivation of the equation for estimating the predominant sizes of pores in textile fabrics

In order to fully apply the concept of capillary resistance to entrained air bubbles present in a wet web, there can be some modifications to governing equation and also to the diagram describing the effect. For instance, where Fig. 25 shows just a single meniscus impeding drainage within an idealized channel in a wet web, it is possible to imagine that there could be multiple bubbles, even in one channel. This could, in principle, add to the resistance to dewatering.

Ordinarily, during the test to estimate the predominant pore size, the analyst would use pure water, for which the parameter γ_aw will equal 72 mN/m. However, as was noted earlier, the most effective foam control agents are designed in such a way as to quickly and temporarily lower the values of γ_aw to fractions of that amount. It is proposed here that such effects can make it possible for any bubbles of air, which otherwise might have blocked the passage of water from the paper web (or fiber mat in a brownstock washer) to be quickly pulled through, with only minor capillary resistance. It follows from Eq. 4 that a strong decrease in interfacial tension, due to the actional of a foam-control agent, will greatly decrease the force needed to pull the air through the wet web of paper. It is proposed that such effects allow such devices as hydrofoils, forming blades, and suction boxes to more effectively pull filtrate through the fiber mat, thus overcoming the tendency of such bubbles to block drainage channels (Hubbe et al. 2020b). In addition, by reducing the interfacial tension temporarily to a very low level, the foam control treatment would allow the bubbles to easily change their shape, allowing them to be pulled through tiny openings without creating problematic pinholes. This concept is illustrated schematically in Fig. 26. Such considerations also would be relevant for drainage through mats of fibers in brownstock washing.

Fig. 26. Concept by high temporarily very low interfacial tension may facilitate high deformation of bubbles such that they fail to create significant pinholes; A: Effect of typical entrained air bubble in paper wet web as it passes over suction box; B: Proposed lack of pinhole creation when bubbles are rendered very highly deformable by foam control treatment

CONCLUDING STATEMENTS

Due to the complexities of different industrial operations, including differences in equipment, raw materials, pulping conditions, various impurities, and different kinds of paper being made, and different operating temperatures, it is inherently difficult to predict levels of foam or entrained air or to be able to forecast what will be the best-performing foam-control product. Rather, the mechanistic principles outlined in this article may serve as guideposts in combination with empirical testing. Nevertheless, based on this review of the literature related to foam control in pulp and paper mill systems, several conclusions and recommendations appear to be justified:

1. Problematic levels of foam present in pulp and paper mills systems usually can be traced to a combination of factors, which include the following:

(a) Presence of an aqueous solution

(b) Presence of an air phase

(c) Some form of agitation, sucking, raining, or splashing

(d) At least one surface-active agent

(e) At least one water-soluble polymer

2. When looking for a cost-effective long-term solution to excessive entrained air or visible foam in pulp and paper mill systems, it makes sense to place priority on minimizing any of the factors mentioned in the previous item that are at least partly under the control of the operating team. For example, known sources of surfactants to the system can be scrutinized and reduced, as in the case of excessive use of deinking surfactants. In addition, adequate washing of pulp after pulping or bleaching stages is important in order to minimize carry-over of surfactants such as fatty acid soaps. Likewise, papermakers should avoid levels of polymeric additives that exceed the adsorption capacity of the fiber surfaces.

3. Foam control agents can be formulated in ways that avoid the possibility of generation of toxic chlorinated organic compounds. Options include (a) using petroleum oils in the formulation that are free of aromatic content, (b) using silicone-based oils, such as polydimethylsilane (PDMS), or (c) formulating the foam-control agent primarily with a surfactant and optional hydrophobic particles.

4. The efficacy and cost-efficiency of foam-control treatment often can be improved by such measures as

(a) Adding fresh foam-control agent at a point of good mixing just ahead of the point where it needs to have its intended effect

(b) Using optimized smaller dosages at more than one addition point

(c) Selecting a product that is formulated for application at the temperature range prevailing in the mill application

5. Foam-control products can be enhanced by adding solid particles having the following attributes:

(a) Solid nature

(b) Hydrophobic surfaces

(c) Size large enough to span a surfactant bilayer bubble wall

(d) Angular shape

(e) Smooth surfaces

6. A generalized mechanism to describe the broad range of foam control effects, based on the published literature, can include the following:

(a) Emulsions droplets that are able to enter bubbles walls due to a sufficiently low interfacial tension

(b) Sufficiently low viscosity of the formulation, so as to allow rapid spreading on bubble walls

(c) Thinning of bi-layer type bubble walls, followed by bridging and pinch-off effects to bring about coalescence between pairs of adjacent bubbles

(d) Optional use of optimized hydrophobic particles, which can play roles in bridging of surfactant bilayers and inducing pinch-off of bubble walls.

(e) Temporary lowering of air-water interfacial tensions in the system, thereby allowing air bubbles to be easily drawn through the wet web of paper with minimal capillary resistance and enhancing the rate of dewatering

ACKNOWLEDGEMENTS

This work was supported by an endowment grant from the Buckman Foundation. The author thanks the following people who considered an earlier version and provided corrections and suggestions: Dr. Antti Koponen, VTT Technical Research Center of Finland Ltd., Jyväskylä, Finland; and Dr. Robert H. Pelton, McMaster University, Ontario, Canada.

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