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In a recent blog, I touched on the fact that there is a limit to how long a cleaning solution will continue to produce clean parts as it becomes increasingly contaminated from use. Chemistry and disposing of spent chemistry are both expensive. Changing a bath usually results in a gap in production as the tank(s) are drained, refilled heated and prepared for use. It is, therefore, understandable that there is hesitation and even reluctance to dump and refill cleaning tanks until it becomes absolutely necessary. If the contaminant is non water soluble particles, it may be possible to extend the life of a cleaning solution by filtering out the particles. Oil and other soluble or emulsifyable contaminants are a little more difficult to deal with, however, because classical filtration is not effective.
Water Soluble Liquids and Particles –
The cleaning mechanism for water soluble liquid and solid contaminants is pretty straightforward. The purpose of cleaning chemistry is to enhance solubility by altering surface tension and other physical properties of, basically, water. It’s understandable that once the cleaning solution has dissolved a certain amount of contaminant, it becomes saturated and will dissolve no more. This process is described here. Indicators are contaminants remaining suspended in the cleaning solution or collecting on the bottom of the cleaning tank. More than likely, cleaning will deteriorate to the point of unacceptability as contaminants remain on the parts even before physical indications are manifest. Clearly, it is time to dump the tank and start over!
Oil –
“Oil” is a very broad category as we discussed here. Generally, oils can be divided into two groups when it comes to methods for removal. These are water soluble (or just soluble) and non-water soluble. “Oil” per-se, and almost by definition, is NOT water soluble. The addition of surfactants make “water soluble” oil appear to mix with water but, in fact, the oil is just broken into small globules even down to molecular size, which are held in suspension by the surfactants.
Note – Mixing water soluble oil and non-water soluble oil will result in a mixture that is all “water soluble” by virtue of the surfactants in the water soluble component of the mixture. If a facility is using both water soluble and non-water soluble oils, one may as well assume that everything is water soluble because of the inevitable mixing at the cleaning machine.
The life of cleaning solutions removing non-water soluble oil can often be prolonged by collecting and removing oil using a gravity oil separator (often called an oil coalescer) or other means such as a centrifuge. It should be noted, however, that oil separation in this form can ONLY work if the chemistry used for cleaning is chosen to prevent the emulsification of oil. A cleaning chemistry that emulsifies oil eliminates gravity separation as an option.
Extending the useful life of cleaning solutions containing emulsifiers to remove oil or removing water soluble oil containing emulsifiers requires the use of microfiltration or ultrafiltration. These techniques can remove very small oil globules or in some cases even molecular oil. Application of micro and ultrafiltration require careful chemistry selection and coordination to assure that the chemistry is not removed along with the oil.
In conclusion, the need for changing chemistry on a regular basis is a fact of life. Bath life is not an exact science and difficult to predict. Each application is different due to the variables in the cleaning process, production rate and contaminants being removed. It is suggested that once your cleaning machine is installed that you keep a log of the number of cycles run or number of parts processed while maintaining chemical concentration and other process parameters until the cleanliness is marginal. Once this happens, take 90% of that number and this when the bath should be changed for preventive maintenance. If longer bath life is required, consider the measures suggested above.
– FJF –