Simply Water for Parts Cleaning
When it comes to critical cleaning, plan the way you use water.
Water, the most common compound on the earth’s surface, has a simple chemical structure. This unique structure results in complex properties that affect how water can be used for cleaning processes. The benefits of understanding the properties of water include achieving a more effective, profitable aqueous cleaning process and a realization of where there might be limitations.
Many of the chemical and physical properties stem from the shape and size of the water molecule. The V shape causes water to be polar, with the hydrogen side positively charged and the oxygen side negatively charged.
Water molecules are small. Methanol is almost twice the molecular weight of water, but only about 80 percent as dense. The combination of small size and polarity impart unusual properties in comparison with any other chemical.
Universal solvent. The forces between water molecules are enhanced by hydrogen bonding, between hydrogen and an adjoining oxygen. The polar and hydrogen bonding forces enable so many materials to dissolve in water that it is sometimes referred to as a universal solvent. Water alone is useful as a cleaning compound, especially for polar materials, such as salts.
Too much of a good thing, even solvency, can lead to undesirable consequences. Because of the affinity of water for many materials, it can be difficult to obtain or maintain high purity water. Very pure water leaches ions from the pipes and containers holding it, so by the time it actually contacts the product to be cleaned, it is no longer as pure. The affinity for leaching ions can make pure water very corrosive, disrupting the structure of glasses and alloys by leaching desirable materials from the surface.
There is no true universal solvent. Many organic compounds, including oils and greases, have little or no polarity and are not readily water soluble. Therefore, chemicals are combined with water to enhance cleaning and avoid corrosion. The large number of these additives results in a wide diversity of aqueous-based cleaning agents.
Physical properties. Beyond solvency, the physical and physical/chemical properties of water have many desirable consequences. The high density is a basis for oil splitting cleaning chemistries, where oils that have been separated from a substrate float to the water surface and can be skimmed off, prolonging bath life.
The high density means that water droplets in a directed spray have high momentum. Soil particles and film can be blasted off the surface. In addition, the kinetic energy of warm or hot water improves the scrubbing action, which also holds true in soaking and agitation. However, there are limitations to spray cleaning and high temperature. Spray cleaning is line-of-sight, so it may not reach all surfaces, and high temperature and pressure can damage parts. After all, spray cleaning uses the same mechanism as water-jet machining.
Surface effects. Have you ever seen a water slider, an insect that walks on water? They can do that because of high water surface tension. Surface tension is a result of water molecules at the surface experiencing strong attractive forces from molecules below that are not countered by the air above. The surface acts like a “skin” that can support objects that are denser than water, like insects or particles.
High surface tension has positives and negatives. Water beading on nonpolar surfaces is the basis of contact angle or water break tests for surface cleanliness. However, high surface tension impedes water from penetrating small holes and crevices, so cleaning may not be effective. This is why typical aqueous cleaning agents contain surfactants that lower the surface tension. Since surfactants are typically removed with water alone, the rinse process has to be thoughtfully engineered.
Mighty molecule. We take water for granted, probably because there is so much of it. However, when it comes to critical cleaning, plan the way you use water. Water has physical and chemical properties that make it behave differently than other molecules. Many of these properties make it a valuable cleaning chemical, but effective and profitable cleaning is a process not a chemical. Once you understand both the positive and negative properties of water, you can apply them to your product and application to design a great cleaning process.
Barbara and Ed Kanegsberg are industrial cleaning consultants with BFK Solutions, and industry leaders in critical/precision and industrial product cleaning. For questions or to receive their newsletter, call 310-349-3614 or firstname.lastname@example.org.
Originally published in the March 2016 issue.
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