DIFFUSE SPEECHIn solvent cleaning using immersion machines, one seeks the blend of solvent components that has the best compatibility with the soil materials.
In solvent cleaning using solvents dispensed from aerosol cans, the supplier selects a narrow group of soils believed to be of interest to customers, finds a blend that provides value in trial applications of cleaning those soils, and commercializes the blend as a product.
Unfortunately, the information made available to users, within the small print on the aerosol can, in the product bulletin, and sometimes in the MSDS, is very limited in specifics about where any specific aerosol product can be used to best advantage.
For example, these are some of the recommended uses found on commercial aerosol-dispensed products: “wide range of soils,” “difficult defluxing needs,” “precision and specialty cleaning jobs,” “specialty applications,” “electronics cleaning,” “medium to heavy-duty cleaning,” “cleaning electrical components,” “basic solvent,” and “cleaning of organic residues.”
My sense of industry communications about aerosol cleaners is that users: (1) already know if aerosol-dispensed cleaners are useful and so specificity about applications is not necessary, and (2) aerosol-dispensed cleaners can be used in a broad variety of applications so increased specificity would be cumbersome.PROPERTIES OF THE PROPER PRODUCTObviously, the right product is one that fulfills the needs of the application. There are three properties of an aerosol-dispensed solvent cleaner that should be considered in the context of any application: surface tension, solubility parameters, and molar volume.
The best approach for users is to consider a single physical property, choosing the aerosol-delivered solvent blend with the lowest value of:
- Surface tension for the available choices. Because aerosol cleaning is done via wetting of surfaces, it is most important to select the solvent blend that most easily wets surfaces. That’s the aerosol with the lowest value of surface tension.
- Molar volume 1 of available choices. Diffusion coefficients, and, therefore, diffusion rates, are higher for smaller molecules.
- Separation in compatibility between the solvent and the soil. Compatibility between solvent and soil is computed using Hansen solubility parameters (HSP). 2
APPLICATION TO SOIL TYPES As noted above, the reason it can be difficult to specifically characterize where specific aerosol-delivered solvent products should be used is because these products can be used in a broad variety of applications. Depending upon the application, different combinations of the three properties will be significant.
Six soil types where this is expected are:
Polymeric (undried paints and uncured coatings) soils are more likely cleaned by being softened through swelling). Here all three properties are significant, perhaps in the order of: solubility parameters, surface tension, and molar volume.
Oils (hydrocarbon or vegetable) will likely be as soluble as they are in an immersion cleaning system—though the rate of removal may be slower. Then, only two of the three properties are significant, in no particular order: surface tension and solubility parameters.
Biological materials (molds, food residue, etc.) are most likely to be soluble in some solvent displaying significant levels of polar and especially hydrogen bonding intermolecular force (such as alcohols). For these applications, the necessary properties to be considered are solubility parameters and surface tension, in that order.
Greases are less likely to be very soluble because hot immersion cleaning thins out greases, and that condition can’t be replicated with aerosol-delivered cleaning products. The order in which the three properties should be considered is: surface tension, molar volume, and solubility parameters.
Metallic residues (salts or other ionic material, which are a common application for aerosol- delivered solvent cleaners) are almost certainly soluble in a wetting solvent blend that contains an alcohol. For these applications, the necessary properties to be considered are solubility parameters and surface tension, in that order.
Particles would likely not be soluble with either aerosol-delivered cleaning products or solvent cleaning products for immersion cleaning. Yet this is an application where low-surface tension products such as CFC-113, HCFC-225ca/cb, HFC-43-10mee, HFC-365mfc, and HFE-7100 /7200 shine. Surface tension is the only property needing consideration.
One cannot help but recognize the diversity of soils that can be—and are being—cleaned with available aerosol-delivered solvent blends. Most of these blends are also sold in bulk for cleaning in immersion machines.
PREVIEW OF COMMING ATTRACTIONSIn next month’s column we’ll cover the two main—and significant—hazards in using aerosol-dispense cleaners and how they can be overcome. BIO John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0-0804-48887). He is an independent consultant specializing in metal and critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or jdurkee@precisioncleaning.com.REFERENCES
- Molar volume has the units of cc/-mole, and is calculated by dividing molecular weight by density. Both values are available on the MSDS of any commercial product.
- This topic was covered in Metal Finishing, Volume 102, Issue 4, April 2004, Pgs 42–50 and Metal Finishing, Volume 102, Issue 1, January 2004, pgs 39–42.