Foam plays an important role in numerous products and technical processes – either intentionally or as an undesired side effect. In metalworking processes, foam interferes with efficient operation and must be avoided or at least controlled as much as possible.
According to two presentations at the 19th Colloquium Tribology at the Technische Akademie Esslingen in Germany in January, foam control is becoming even more critical with the higher cutting speeds and higher pressures being used today. Michael Stapels of Kao Chemicals GmbH and Ernest Galgoci of Munzing NA LLP described emerging technologies that not only help control foam but also improve fluid life.
Choosing an Antifoam
Galgoci, based in Bloomfield, New Jersey, United States, explained, Although the criteria for choosing an antifoam will vary depending on the specific fluid and requirements, generally the antifoam must exhibit strong initial defoaming, persistence (longevity) and compatibility (no separation) with the fluid.
Unfortunately, the optimal antifoam is often best determined empirically because variations among fluid formulations and specific performance requirements produce a complexity that makes the best antifoam difficult to predict. However, he said, experience can be used as a guide to organize the search for an antifoam more efficiently.
Stapels, based in Emmerich, Germany, said, The standard approach in the industry is to test foaming behavior with a shaking cylinder. However, the discriminative power for unstable foams is limited because the observation time for foam breakdown typically is too short.
Galgoci said, For aqueous systems, antifoams are typically comprised of an active liquid, mulsifiers/wetting agents and other ingredients that help to improve efficiency, stability and compatibility. The primary liquids are generally mineral oils and silicones that serve as defoaming agents and may contain hydrophobic particles such as modified silicas or waxes.
The emulsifier facilitates the dispersal of the droplets/particles in the fluid and enables the formation of the optimal droplet/particle size distribution for the most effective and long-lasting defoaming. Another important function of the emulsifier is to increase the efficiency of the antifoam in entering the foam bubble and breaking it, Galgoci said.
The other components may include a carrier liquid that [reduces] the viscosity of the formulation and sometimes [enhances] defoaming, compatibility and dispersion, he added. For water-based defoamer formulations, auxiliary additives may include rheology additives and biocides.
Kaos Stapels explained that ether carboxylic acid based on saturated C16 fatty alcohol was developed in 2011 during an oleyl alcohol shortage. This technology closely matches the performance of ether carboxylates based on oleyl alcohol in terms of formulation ability, emulsification and stabilization.
The key [alteration] was the insertion of an additional block of propylene oxide between the hydrophobic tail and the hydrophilic head group of the molecule, Stapels said. This technology made possible a wide range of new products by varying the length of the carbon chain and the degree of propoxylation and ethoxylation.
Experience shows that propoxylated ether carboxylic acids can be a viable alternative to standard oleyl based ether carboxylic acids because they have significantly lower foaming potential as well as more efficient emulsification, Stapels said. This combination of properties makes them highly attractive for foam critical applications like high pressure coolants.
Emulsifiers are a critical component of water-miscible metalworking fluid formulations. They stabilize the resulting emulsion and should facilitate tramp oil removal and filtration during their lifetime, Stapels said. In addition, emulsifiers need to fulfill a whole range of other requirements such as good toxicological and dermatological profile, low foaming tendency and good heat stability.
In modern fluids, ether carboxylic acids are an essential component of the emulsifier package because they increase the robustness of the fluids overall performance, Stapels continued. In particular, using ether carboxylic acids in the emulsifier package significantly decreases the fluids sensitivity to water quality. This helps increase cutting fluid stability and longevity, regardless of water hardness.
An ether carboxylic can be regarded as interrupted soap with the addition of a number ethylene oxide groups between the alkyl chain and the carboxylate group, Stapels said. Generally, the shorter the alkyl chain, the better the foam control. On the other hand, a short carbon chain will result in comparatively lower dispersing and emulsification power.
Another performance factor is the degree of alkoxylation. He said, Generally, higher ethoxylation boosts dispersing power; however, it also reduces both foam control and corrosion protection.
Testing Ether Carboxylates
Researchers at Kao investigated the effectiveness of ether carboxylic acids consisting of a saturated C16 carbon chain with six methyl branches produced by the propoxylation of fatty alcohols. Particular emphasis was placed on determining adsorption characteristics, emulsification and foaming behavior.
Stapels explained, Surfactants are substances that alter the surface properties of liquids, even when present in small quantities…. A surfactant should be able to adsorb on the surface when added to a liquid at low concentration and reduce the excess free energy at the surface.
Surface activity typically is produced when the number of carbon atoms in the hydrophobic tail of the surfactant is higher than eight. Typically, he said, surface activity is minimal below eight (very soluble) and above 18 (insoluble) carbon atoms in the hydrophobic tail. Surfactant activities are at a maximum if the number of carbon atoms is between 10 and 18, Stapels said, at which level a surfactant has good but limited solubility in water.
He added that all ether carboxylic acids based on a C16 alkyl chain length and containing a propylene oxide block show significantly higher adsorption speeds compared to standard oleyl based ether carboxylic acids. Also, the higher the degree of ethoxylation, the less mobile the surfactant molecule becomes; that is, the more the adsorption speed declines.
Surfactants that are added to assist in forming an emulsion are called emulsifiers, and emulsifying power measures the ability of a surfactant to assist the formation of an emulsion…. [E]mulsifying power is assessed by stability studies of standard emulsions through visual observation, Stapels said. He pointed out that a further extension of the propoxylation concept is the development of low foaming microemulsions that combine both high emulsification and low foaming.
A usual testing practice in the metalworking industry is to formulate an unstable preblend by removing a part of the emulsifier. Then, small volumes of emulsifier are added, and the emulsion is stirred thoroughly. The emulsifier that converts the unstable preblend to a stable one at the lowest concentration is regarded as the most efficient emulsifier.
In the Kao study, a conventional soluble oil formulation was used to compare the emulsifier efficacy of propoxylated ethoxylated ether carboxylic acids to that of standard ether carboxylic acids. The results showed that only one-half as much ether carboxylic acid with a propylene oxide block in the carbon chain was required to form a stable emulsion, compared to the conventional ether carboxylic acid, Stapels said.
Ether Carboxylate Foam Control
Kao used a Dynamic Foam Analyzer DFA 100 to characterize the foaming behavior of metalworking fluids containing different ether carboxylic acids. Measurements were made in a five percent emulsion of the base metalworking fluid formulation containing 3.2 percent ether carboxylic acid. All emulsions had a pH of 9, temperature was 20 degrees C and water hardness was adjusted to 179 to 267 ppm CaCO3.
In each measurement, 40 milliliters of emulsion was placed in the analyzer. Air was passed through the porous vessel base for 15 seconds to generate foam. During and after foam generation, the heights of the liquid column and the foam column were recorded by measuring the light transmission as a function of time.
The results showed that the propoxylated ether carboxylic acids reduced maximum foam height by 90 percent compared to standard oleyl based ether carboxylic acids.
Another measure of foamability is foam capacity, which is the ratio of generated foam volume to the volume of introduced air. When the foam is unstable and part of the gas is released due to bursting bubbles, foam capacity will be less than one. Foam capacity will be greater than one if the foam is stable during formation. The analysis of foam capacity showed that propoxylated ether carboxylic acids generate less volume and more unstable foam than standard ether carboxylic acids based on oleyl alcohol, Stapels said.
Foam stability can also be evaluated from foam collapse measurements. Again, testing showed [that] propoxylated ether carboxylic acids [generate] unstable foams….Foam collapsed completely in less than 10 seconds, whereas the foam generated by standard metalworking fluid formulations [did not] show a 50 percent collapse within 30 seconds. He concluded that propoxylated emulsifiers provide formulators with a powerful new toolbox for tailor-made fluids performance. Inside this toolbox, you can identify synergetic combinations to optimize either fluid life or foam control.
Compared to conventional silicone-based chemistries, Munzings Galgoci said, antifoams based on crosslinked (3D) siloxane technology deliver the most effective defoaming and persistence for metalworking fluids. Unlike the linear or branched structures of conventional silicones, [in] 3D siloxane technology … the siloxane molecules are linked to form gel-like, microscopic particles. When formulated with a suitable emulsifier package and carrier, 3D siloxanes produce relatively stable, minute droplets in aqueous media. The small droplet size coupled with the low surface tension provides superior and persistent defoaming in dilutions as well as excellent compatibility in concentrates.
Munzing evaluated the defoaming effectiveness of 3D siloxanes in the three main metalworking fluid types: soluble oil, semisynthetic and synthetic. Antifoams were tested for compatibility in the fluid concentrate and for defoaming effectiveness in dilution.
Galgoci explained, Compatibility ratings were based on visual observations such as haze and separation…. A recirculation test rig was [used because our] experience has shown this to correlate more closely with application performance.
The recirculation test consisted of placing 200 ml of the test dilution (usually 5 percent of the concentrate in water) into a standard 1,000-ml graduated cylinder fitted with a nozzle on the bottom. Each sample was recirculated from the bottom of the cylinder to the top, and the total volume in the cylinder was recorded at specific time intervals.
At the end of the test, the foam collapse was recorded for up to five minutes, Galgoci said. Lower foam volumes during the test and a faster rate of foam collapse after the recirculation pump is turned off indicate better defoaming efficiency.
Soluble Oil Metalworking Fluid: 3D siloxane antifoams were evaluated in a commercial soluble oil metalworking fluid that required a defoamer with improved compatibility and defoaming compared to modified silicone. At one-half of the use level, the 3D siloxane antifoam showed better compatibility and significantly improved defoaming performance compared to the control, said Galgoci.
Semisynthetic Metalworking Fluid: A commercial semisynthetic metalworking fluid was tested to determine if 3D siloxane technology could provide better compatibility and persistence than a modified silicone. For compatibility, both control samples showed sediment and haze, whereas the samples containing the 3D siloxane antifoams exhibited less haze and no sediment after aging for 28 days, Galgoci said.
Even at double the concentration, the 3D sample showed good compatibility after 28 days of aging compared to the other samples, he added. Even after aging the concentrates for one month at room temperature, both of the 3D siloxane-containing samples showed little foam during the recirculation test and complete foam knockdown in less than one minute after recirculation was stopped.
Synthetic Metalworking Fluid: The performance of two 3D siloxane antifoams was compared to that of a modified silicone antifoam in a commercial synthetic metalworking fluid. Galgoci said, Both 3D samples provided comparable compatibility compared to the control. However, they had outstanding defoaming performance in the recirculation test with regard to total foam and foam break at the end of the test. One 3D antifoam showed no foam in less than one minute after recirculation stopped.