Is It Clean? – Oil and Hydrophobic Films

In most manufacturing processes where metal is formed or cut, oil is used in one form or another as a lubricant.  Oil and other lubricants, in order to do their job, are formulated to have very low surface tension, and, because of their low surface tension, are difficult and often impossible to control or confine.  Oil spreads on surfaces, easily becomes airborne and, in general, is an unavoidable contaminant in facilities where it is used in any quantity.

In some applications, a residual oil film is beneficial on the finished part as it continues to provide lubrication and protection against oxidation.  In other cases, painting and plating for example, oil is a troublesome contaminant requiring thorough removal.  In the next few blogs, we will address techniques to detect residual oil on the surface of cleaned parts.

You may have noticed in the title that we will not limit our discussion to “oil.”  Oil is just one example of a hydrophobic substance.  Liquids that are not miscible with or will not go into solution with water are “hydrophobic.”  Many hydrocarbons are hydrophobic but there are many other hydrophobic liquids as well.  There are also hydrophobic solids but, for the purposes of our discussions over the next several blogs, we will be dealing exclusively with liquids.  Many of the techniques for detecting oil and other hydrophobic films described in the following are not applicable for similar contaminants on hydrophobic substrates.

For purposes of clarification it is important to understand that miscible liquids and emulsifiable liquids are two different things.  “Water-soluble” oil, for example, includes an ingredient, a “surfactant,” that allows an otherwise hydrophobic liquid to mix with water.  Surfactants are comprised of molecules called “micels” that are water-loving (hydrophilic) on one end and oil-loving the other end.  The water-loving end bonds to the water and the oil loving end latches on to the oil or otherwise hydrophobic liquid.  It is the surfactant that promotes and allows the “mixing” of oil and water in an emulsion.  On a micro scale, the oil and water are still discreet entities.  They are not really “mixed” and will separate on standing or if the surfactant allowing them to co-exist is removed or becomes ineffective.

Surfactants are important ingredients in most cleaning chemistries.  The surfactant allows the chemistry to emulsify oil and other hydrophobic films.  BUT, surfactants have different properties and “strengths” (for lack of a better term).  A surfactant, to be effective, must be powerful and plentiful enough to prevent hydrophobic liquids from re-attaching themselves to the substrate thereby remaining as contaminants.

Above, it was stated that residual hydrophobic films are valuable in some cases but not in others.  The “others” are when another material must adhere to the substrate.  Hydrophobic films will prevent the adhesion of paint and other coatings, hamper plating process and, in general, wreak all kinds of havoc in metal finishing operations.  Residual films may also act as contaminants in the application of a finished part.  It would be best, for example, if a hip or knee replacement was not contaminated with a foreign material that might cause the body to reject it.

OK, with that out of the way, we have set the stage to discuss methods of detecting oil and other hydrophobic films on surfaces.  The next several blogs will these methods in some detail.

–  FJF  –