TECHNOLOGY FOCUS

KNOW FOAM - PART 1

by Dom Ruggeri

January 2004:

Foam is a dispersion of a gas in a liquid.  The globules may be of any size from colloidal to macroscopic as in soap bubbles.  Bubble bath and beer foam are examples of desirable foams, however, in machining and grinding applications, any foam is undesirable.

Metalworking fluids generally contain alkanolamines and surface-active agents.  These are added to emulsifiable oils to provide emulsion stability.  In the case of synthetic fluids, the alkanolamines are used to neutralize the organic acids to produce a pH that will minimize corrosion.  In both systems, soap is formed, and it is a well-known phenomenon that soaps reduced with water will produce large quantities of stable foam when agitated.

Foam will cause many problems such as loss of cooling and lubricity, resulting in striations on ground surfaces, distortion of machined parts, and shorter tool life.  The above problems are directly related to the bottom line - production.  However, persistent foam will cause other problems such as pump cavitation, wasted volume in equipment, and housekeeping problems in general.  Left unchecked, the housekeeping problem may become a safety hazard and an OSHA violation.

Foam in metalworking fluids is generally caused by the presence of alkanolamine soaps and certain anionic and non-ionic surfactants.  These compounds, when added to water and subjected to the high shear and agitation encountered in high-speed machining operations, will generate copious quantities of foam.  When a metalworking fluid is mixed with water, a certain amount of foam will develop.  This foam will either collapse or remain stable.

Stable foams consist of mixtures in which at least one component is surface active.  Pure liquids cannot sustain a stable foam.  An explanation for this observation follows from thermodynamics.  For a one component system with significant surface area, the Gibbs Free Energy is given by:

dG = 6dA + VdP - SdT        (Eq.  1)

Where "6" sigma is the surface tension of the liquid, "A" is the surface area, "V" is the volume, "P" is pressure, "S" is entropy and "T" is temperature.  At constant temperature and pressure this equation becomes:

6= (dG/dA) _{p, t}                    (Eq2)
integrating Eq.  2 yields

AG = 6/\A                           (Eq.  3)

where "/\G" is the change in the Gibbs Free Energy at constant temperature and pressure.  Surface tension is always positive and bubble coalescence leads to a loss of surface area ("/\A" is negative).  Therefore, bubble coalescence in a pure liquid should be a spontaneous process because it must result in a reduction in the Gibbs Free Energy.

The situation is more complex for mixtures containing surface-active agents concentrated at the air - liquid interface.  This surface-active agent reduces the surface tension by being positively absorbed at the gas - liquid interphase, causing the gas - liquid phase to concentrate in the surface layer.  This absorption of the surface-active agent requires the expenditure of work.  This work requires an additional term in equation 3 of opposite sign to the "6/\A" term.  If this term is large, enough spontaneous coalescence need not occur.

The above is directed to a theory of foam stability and a theory of why a pure liquid cannot sustain stable foam.  Let us now turn our attention to foam control.  One way to control foam in metalworking fluids is to raise the temperature of the emulsion above the cloud point.  This method works well for cleaner baths, but it is impractical for coolants.

Drainage of liquid from the bubble walls under the influence of gravity acts to reduce film thickness and makes the film more fragile.  Hence, the foam should break.  This is not the case owing to the Maragoni effect.  The Maragoni effect applies to liquids moving under the influence of a surface tension gradient.  As films drain, surfactant molecules become concentrated in the lower regions of the film, leading to a lower surface tension, relative to the upper regions.  The higher surface tension in the upper regions thus tends to counteract downward flow.  Of course, the foam can be broken by the action of chemical antifoams.

The incompatibility of certain chemical agents will not only suppress foam, but it will inhibit its formation.  Such chemicals are, in most instances, completely insoluble in the system.  Droplets of these insoluble liquids spontaneously enter the bubble and spread across it, and the mechanical disruption thus imparted to the bubble causes it to rupture.

Choosing the proper antifoam is as important as matching the product to the application.  However, should a foam problem develop, before the introduction of a chemical antifoam, the recirculation systems should be checked thoroughly. 

When choosing a chemical antifoam, the hardness of the customer's water should be checked.  Machining and grinding emulsions have a tendency to foam in soft water.  Calcium acetate slurry will add hardness to the fluid in the customer's system.  The extra calcium will react with the excess fatty acids yielding calcium salts.  These salts are very active antifoams.

Droplets of a silicone oil antifoam spontaneously enter the foam bubbles, spread across them, and the mechanical disruption thus imparted to the bubble causes them to rupture.

Bis-stearamide wax dispersions in oil function by dewetting of solid antifoam particles.  Dewetting is the spontaneous withdrawal of the foamy liquid from the solid antifoam particle resulting in the rupture of the bubble.

Many chemical antifoams consist of liquids that are insoluble in the foamy medium.  We have all noticed that over a period of time most chemical antifoams will lose effectiveness.  Therefore, a program of antifoam treatments must be developed and followed.

Use rates vary greatly depending on the antifoam employed.  Typical concentrations range from 100 to 5000 ppm in the foamy system.  Your coolant supplier can recommend starting points but optimum concentrations must be determined empirically.

Until next month, good luck.

Dom