A previous blog Cleaning – Soluble Contaminants – “Solvents” described soluble contaminants as those contaminants that can be dissolved using a solvent or engineered liquid such as a surfactant. Contaminants that are not soluble are the other distinct classification of contaminants. Sometimes the line between soluble and insoluble is not well defined since there are both liquid and solid contaminants that are not soluble using one cleaning agent but which will dissolve using a different cleaning agent or solvent. For the purposes of this blog, we will consider solid contaminants which do not dissolve short of extraordinary means such as the use of an acid or other means that would also attack the underlying substrate.
Insoluble solid contaminants range from well-bonded or integral surface coatings such as an oxide layer to those which occur as discreet particles which are loosely adhered to each other and/or the underlying substrate. Well-bonded surface coatings usually require treatment that goes beyond what we call “cleaning” since they are an integral part of the surface of the substrate. Removal requires actually changing the substrate surface either chemically or through the use of mechanical means such as abrasives and buffing, processes which are commonly called “surface finishing,” not “cleaning.” Surface finishing varies from “cleaning” in that cleaning only removes contaminants and does not involve any change in the substrate surface itself. Admittedly, the lines of definition between surface finishing and cleaning are not clear with a lot of “grey area.” What is surface finishing to one person may be cleaning to another but, in the blog, I will maintain the distinction as described above.
Insoluble particles are attracted to and each other and to substrates by a number of means which include ionic bonding, adhesion and electrostatic forces. In some cases, a secondary contaminant is actually the “glue” that holds particles on substrates. In that case, the challenge is to first remove or dissolve the secondary contaminant thereby leaving only the loosely attracted particles to be removed.
Removing particles, once all other bonding means have been removed, simply requires physically displacing the particle far enough from the substrate or neighboring particles to break whatever forces of attraction are present. In most cases, this is relatively easy leaving the only remaining challenge to prevent subsequent re-attraction which would re-contaminant the cleaned substrate. Later blogs will define means to break the attractive forces between particles and substrates as well as means (both chemical and mechanical) to prevent re-contamination of cleaned surfaces by the particles which have been removed previously.
Magnetic particles and/or substrates provide a special challenge in cleaning. Magnetism is a very strong attractive force which, if present either in the substrate or in the contaminating particles, is extremely difficult to overcome. In most cases, substrates and particles are “de-magnetized” to eliminate the force of magnetic attraction prior to cleaning. For a more thorough discussion of magnetism and its effect on cleaning see the blog Is It Clean? – Particles – Exceptional Particles – Magnetism.
Successful removal of both soluble and insoluble contaminants often requires the use of mechanical force as in spraying, brushing or agitation. Upcoming blogs will explain the benefits of various means of supplying mechanical force including ultrasonics to enhance removal of both soluble and insoluble contaminants.
– FJF –