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Advanced Defoamer Technology for Controlling Foam in Metalworking Fluids Ernest C. Galgoci Münzing North America LP Introduction Foam is undesired for many industrial applications, since it can detract from the effectiveness of the associated processes. For aqueous metalworking fluids (MWF), foam minimization is required to maintain effective lubrication and heat removal and to prevent pump cavitation and overflow of sumps. Because of this, a defoamer(s) is a critical component of a fluid’s formulation. Although the criteria for choosing a defoamer will vary for a given MWF, the defoamer must generally: exhibit strong initial and persistent defoaming; be compatible (no significant separation) in the MWF concentrate; maintain defoaming despite filtration of the MWF; and not cause defects on parts that are subsequently painted. Causes and Breaking of Foam Foam can form as entrained gas bubbles in a liquid reach the surface and are stabilized by surface active agents such as surfactants, which inhibit drainage of the liquid surrounding the bubbles. The use environment of metalworking fluids is conducive to forcing large volumes of air into the fluid, and the necessary surfactants that are formulated in the MWF can stabilize that air as foam. Although formulation strategies can help reduce foam, a defoamer is inevitably needed. Defoamers exist as droplets in the foaming fluid and break foam by disrupting the surfactant stabilization by entering, spreading, and bridging of the defoamer droplets on the surfaces of the bubbles as shown in Figure 1[1]. Polysiloxane-based defoamers are the most effective, since they meet the thermodynamic requirement of having a very low surface tension. Another factor that affects the performance of a defoamer is the droplet-size distribution. If the droplets are too small, there is insufficient mass to effectively spread and bridge, while droplets that are too large will negatively affect the compatibility and the kinetics (due to a lower number of particles) of foam breaking. Figure 1. Process of Foam Rupture by a Defoamer 4 Fuels&Lubricants No. 2 JUNE 2018
FOAM BAN ® 3-Dimensional (3D) Siloxane Defoamer Technology Defoamers based on FOAM BAN 3D siloxane technology have a decades-long track record as the state-of-the-art products for metalworking fluids. Unlike silicones (e.g., PDMS = polydimethylsiloxane), which are linear polymers, the 3D technology is based on siloxane polymers that are linked together (crosslinked) to form a 3-dimensional structure as illustrated in Figure 2. The 3D structure and an optimized formulation impart stability to the defoamer droplet-size distribution, and this leads to excellent compatibility in metalworking fluid concentrates and superior initial and persistent foam control [2]. For example, Table 1 shows the enhanced stability of the FOAM BAN 3D siloxane technology (2 HP products listed) in a semi-synthetic MWF concentrate relative to 2 other products used in the industry. Figure 3 illustrates the excellent initial and superior persistent defoaming of the HP products, as they maintain relatively low foam levels over the course of the test. In addition, after the pump was turned off after 240 minutes, the foam broke in less than 20 seconds for the HP products, while some foam remained even after 5 minutes for defoamers A and B. (a) (b) Figure 2. Schematic of (a) crosslinked and (b) linear molecular structures Table 1. Compatibility ratings (1 = no issues; 5 = worst); * OMS = organo-modified siloxane Defoamer Use Level, % Rating (14 Days at 25 °C) HP753N, HP757 0.15 1.2 A (siloxane emulsion) 0.50 4.7 B (OMS*) 0.50 3.2 Figure 3. Recirculation test results. Pump turned off at 240 minutes. Filterability of 3D Technology To remove undesired metallic and other debris, filtration is important during use of a MWF. However, filtration can negatively affect the defoaming performance of the fluid. To better understand the potential impacts, different defoamer types were studied for performance in a semi-synthetic MWF during recirculation foam testing with a 10 µm filter comprising various materials [3]. The performance was assessed by integrating the total volume (liquid plus foam) of the recirculation over the course of 2 hours; the value reported is termed the integrated foam volume (IFV), for which a lower value is better and, in this case, the maximum value is 84 L•min. The results (Figure 4) show that polar filter materials (nylon = N; poly- Fuels&Lubricants No. 2 JUNE 2018 5
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