In this column, I will discuss the causes of foaming in polymer quenchants and metalworking fluids. We will also discuss the different types of defoamers, and antifoam agents used.
Introduction
Polymer quenchants are used extensively to quench steels and non-ferrous alloys. The quench systems can be simple immersion quench tanks with pumps or agitators, or they can be elaborate spray-type systems using machined quench rings used in induction hardening (Figure 1). These quench rings can contain hundreds of small nozzles at elevated pressure to achieve the proper heat transfer. The agitation or spraying of the quenchants can result in foaming.
Metalworking fluids, such as cleaners and water-soluble coolants are also used. Sprays and flooding of parts during machining or cleaning operations can also result in foaming.
Foaming occurs when air is entrained into the liquid. When the air comes out of solution, the air and fluid can create a stable foam at the surface. The entrained air can come from various sources, including the surrounding environment, mechanical agitation, or chemical reactions. The presence of foam can cause problems with uniform quenching, or it can cause lubricity issues with coolants.
There are several factors that affect foam stability, including surface tension of the fluid, viscosity of the fluid, temperature, pressure drops, and impurities. The lower the surface tension of the fluid, the more stable the foam will be. Higher viscosity fluids tend to entrain and hold air more efficiently, resulting in a very stable foam. Higher temperatures can reduce the surface tension of the fluid, stabilizing foam. Pressure drops, or changes in the pressure due to changes in piping, can cause the air to come out of solution, creating foam. Impurities can act as nucleation sites for bubble formation.
Primary Causes of Foaming in Quenchants and Metalworking Fluids
There are multiple potential causes of air entrainment in a metalworking fluid or quenchant.
Water Quality
The ability to create a foam, and the stability of a foam, is affected by the hardness of the water. Hard water contains dissolved salts such as Mg2+ and Ca+2 ions, that can interact with surfactants and inhibit foam from occurring. In the case of amine-type rust inhibitors present in polymer quenchants and water-soluble metalworking coolants and cleaners, the amines present can react with the calcium and magnesium ions, and create carboxylic soaps, which form on the top of the fluid or form a soap scum on the sides of the tank. Calcium and magnesium ions can cause rapid collapse of foams. Hard water can also increase the conductivity of the solution, which can cause problems with induction hardening. Classification of water hardness is shown in Table 1. Use of RO (reverse osmosis) water, while reducing conductivity, can contribute to increased amounts of foam and foaming stability.
Hard water, and the formation of metallic soaps, can clog nozzles and change nozzle flow rates and spray patterns. It is a balancing act to maintain the proper hardness to minimize foaming, while not causing other issues such as soap scum formation on parts or clogging of spray nozzles.
Air Entrainment
Most foaming problems are associated with mechanical air entrainment. This can be from a variety of sources. Hard sprays, such as induction hardening quench rings, can cause entrainment of air and foaming. Air can also be entrained into the quenchant from the vortex of suction pumps if the level of the quenchant is too low. This vortex pulls in air like a miniature tornado, and mixes with the quenchant or coolant. Use of reduced velocities or reducing the amount of flow to reduce the size of the vortex can reduce the amount of foaming.
Use of agitators or pumps in immersion systems can have the same effect. As the impeller spins, air is drawn into the vortex, mixing with the fluid. At the exit end, due to the pressure drop, air comes out of solution and creates foam. Reducing the size of the vortex, by increasing the amount of fluid above the pump inlet or fluid above the agitator, can significantly reduce the foaming tendency. Making sure that the systems are always operating at the proper level, and controlling the fluid level, helps reduce the tendency of a fluid to foam.
Contaminates
Contaminates such as dirt, dust, and tramp oil, can act as nucleation sites for bubbles to form. If there is significant drag-in from other metalworking fluids into the quench bath, then the presence of surfactants (from the previous metalworking operation), can promote foaming and foam stability. Tramp oil can be removed with rope or belt skimmers. However, the best practice is to clean the parts (and rinse them) prior to the induction hardening operation.
Bacteria and fungus can grow in metalworking fluids and quenchants. As they grow in number, they can produce secretions that promote foam stability. Fungus and bacteria can also dissolve gasses in the quenchant or metalworking fluid that cause odor problems when these gases are released. Maintaining the fluid to prevent bacteria and fungus can reduce the potential for foaming and obnoxious smells.
Additives to control foam
There are two types of additives that can be added to a quenchant to control foam. These are antifoam agents and defoamers. Antifoam agents prevent or minimize foam creation, while defoamers dissipate the foam after it forms. Often, these terms are used interchangeably.
An antifoam agent or defoamer usually consists of a carrier that is slightly insoluble in the quenchant or metalworking fluid. This is usually a mineral, vegetable, or silicone-based oil — a hydrophobe which is present as finely dissolved solids to physically break bubble walls. This could be a wax dispersion or a blend of waxes, fatty acids, or hydrophobic silica. Finally, there is the diluent, which is usually water. Antifoam agents and defoamers are usually used in very small doses (typically 0.01%) to form a stable dilution. In general, antifoam agents are formulated with the product, and defoamers are added by the user after foam occurs.
There are two common types of defoamers used – those that contain silicone and those that do not. Silicone defoamers have a silicone carrier and hydrophilic silica in silicone oil. These defoamers are chemically inert and completely soluble in water. Silicone defoamers operate by spreading evenly over a foam film, collapsing the foam film. Non-silicone containing defoamers rely on mixtures of surfactants and hydrophobic solids to be effective.
Conclusions
In this article, I briefly described some of the causes of foaming and methods of controlling foam. Mechanical initiation is probably the most common source of foaming, followed by contaminates. Antifoam and defoamers were very briefly described, with differences between antifoam agents and defoamers discussed.
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