Over several years, this column has covered parts cleaning as if there were only two major types—aqueous and solvent.
Well, there is a third—one that is becoming more common in the finishing industry. And I want to cover it in detail in this piece and the succeeding two columns.
This third type of cleaning entails applying rapidly evaporating cleaning fluids to a surface through an aerosol delivery system, and then wiping the soil and cleaning fluid from the surface.
As I researched materials for this article, I was surprised to find how versatile aerosol-dispensed cleaning can be in terms of the diversity of soils that can be removed. Those findings will be covered in the second part of this series.
In this column, I describe what aerosol-dispensed cleaners are, and how they are used.
Prior to the 1990s, aerosol-dispensed cleaners were noted for their low cost and high flammability. The reason for both aspects was their composition—volatile low-cost hydrocarbons, alcohols, ethers, and the like. The propellant was usually also a volatile and flammable hydrocarbon, such as propane.
The value provided by these aerosol-dispensed products was threefold, in that:
- They applied cleaning fluid by spray only to locations on a surface where it was wanted
- The cleaning agent had been selected to wet the soiled surface, penetrate/dissolve the soil structure, and to fulfill the needed cleaning requirement, and
- The cleaning agent was immediately evaporated without operator action, leaving a dry surface
Almost none of these products was/is based on water, because of its extraordinarily high surface tension (even combined with detergents), and its slow evaporation rate.
The Perfect Aerosol
The second defect in these products was that the surface tension of the coalesced solvent droplets (~25 dynes/cm) was high so surfaces weren’t well-wetted—outside of the obvious defect with solvent-based aerosols of concern about fires. (This will be covered in the third part of this series).
In the 1980s and 1990s the global search for non-ozone-depleting replacements for CFC-113 and like chemicals led to the development of three new and different generations of solvents: HCFCs, HFCs, and HFEs. These solvents: weren’t flammable as they didn’t have measured flash points; displayed little polar and hydrogen-bonding intermolecular forces; were compatible with other more useful solvents that displayed significant polar and hydrogen-bonding intermolecular forces; and left dry surfaces because they evaporated rapidly.
The new defect was that they were very expensive relative to previous offerings.
So, the perfect aerosol single-component solvent possessed the virtues of these HCFCs, HFCs, and HFEs while displaying substantial polar and hydrogen-bonding intermolecular force, dried by evaporation “in a heartbeat,” and being priced more cheaply.
In the absence of the perfect, the presently accepted “good aerosol” solvent product is a blend of these HCFCs or HFCs or HFEs with another solvent that possesses significant polar and hydrogen-bonding intermolecular forces, and whose packaged selling price is less than a twenty-dollar bill.
How Aerosol Delivery Works
The condition that produces an aerosol is a high velocity of propellant relative to liquid. Here, tiny droplets of liquid are produced via shearing of larger droplets using the force released when the pressurized propellant is allowed to expand when the control nozzle in the aerosol can is opened (See Figure 1)1.
Lower propellant velocities, relative to liquid, produce larger liquid droplets; higher relative propellant velocities shear the liquid more effectively (and often) and produce smaller droplets.
Only to a modest extent is propellant velocity controllable by adjusting the position of the top plunger of the aerosol can. This means that the quality of surface coverage might not be at the level of consistency as desired by the user.
How Aerosols Clean
Effluent from an aerosol can is not a high-velocity moving stream of liquid solvent (see Figure 3.), which could flush a surface. Rather, an aerosol is a suspension of fine liquid droplets in a slowly moving gas stream. These droplets can be sub-microscopic in size (< 1 micron in major dimension) to just a few or a few dozen microns in size at ambient pressure and temperature. Other aerosols can support droplets significantly larger in size. (The droplets visible in Figure 1 must be above ~50 to 60 microns in size, as that is the limit of visual resolution.)
So the aerosol spray does not apply force to a surface to liberate soils. Sprays from aerosol cans do not apply significant impact to surfaces, which could displace or dislodge soil materials. Sprays from aerosol cans only wet surfaces.
Don’t confuse a high-velocity spray from a nozzle connected to a pump in a spray cleaning machine with a low-velocity spray connected to a small can.
In solvent cleaning with aerosols, the purpose of these droplets is to wet a surface so soils on it may be swollen (or possibly dissolved) and liberated from surfaces by later mechanical action, usually with a hand-applied wiper.
- Wetting is done when the submicron-to-micron-size drop lets coalesce on the soiled surface (see Figure 2).
- Swelling is done by diffusion of solvent molecules into the large volume of voids within a polymeric structure, and absorption of the solvent within those voids. This weakens the bonds between the polymeric structure and a surface (see Figure 3)3.
So the prime mechanism by which blends (and single solvents) dispensed from aerosol cans clean is by wiping of wetted surfaces where the bond between the soil and the surface has been weakened via wetting, and the soil may have been dissolved/softened via solutioning.
Contact time is certainly a variable in cleaning with aerosol sprays. In an immersion cleaning machine, it might be seconds to a few minutes; with aerosol sprays, it could be many minutes.
In the contact within the immersion machine, diffusion of the solvent(s) through the soil matrix is aided by the elevated temperature—which raises diffusion coefficients—and continued agitation, which avoids static concentration gradients.
Application by hand of mechanical force with a wetted wiper, and extended contact time of the solvent(s) on the soiled surface, are the only variables that can enhance soil removal.
Often after spray wetting of a soiled surface, and short-time application of mechanical force with a wiper to spread the droplets, operators allow 5 or 15 minutes of time for the solvent to diffuse into the soil matrix, before starting the substantial wiping operation.
The Aerosol Can
An aerosol can may be any sealed container that contains a liquid under gas pressure, and has an internal fixture that is externally activated to allow a controlled release of pressure so as to allow emission of the liquid suspended in the gas being expanded through the fixture (See Figure 4)4. The fixture is the heart of the aerosol delivery system—it’s called the actuator valve, or crimp valve.
Contents of The Aerosol Can
The aerosol can contains two chemical components: the cleaning solvent(s) and a propellant. Ideally, the propellant should be a liquid whose boiling point is slightly lower than the temperature of use. This allows the can to contain the liquid solvent and have the propellant be in the vapor phase under a modest (not high) pressure, sufficient to expel the can’s contents on demand. In use, as the propellant gas expands, its volume is replaced by evaporation (and cooling) of the cleaning solvent.
To avoid management of a two-phase blend, the propellant is chosen to be miscible with the cleaning solvent (or blend).
After World War II, chlorofluorocarbons (CFCs) were commonly used as propellants because of their compatibility with other solvents, and (then) low price. In the 1970s, it was discovered that emitted CFCs collected in the Earth’s stratosphere and acted as catalysts to destroy the ozone in that region. To that end, their manufacture was banned by a global fiat (the Montreal Protocol) in 1988.
Replacements for CFCs have chiefly been volatile and flammable low-cost hydrocarbons, typically propane, n-butane and isobutane; compressed gasses, such as carbon dioxide or occasionally nitrogen; and substantially more expensive HFCs, such as HFC-134a, HFC-365mfc, and HFC-227. Hydrocarbons, carbon dioxide and HFC-365mfc are the most commonly employed today in aerosol cans used to dispense cleaning solvents.
Under certain circumstances, both carbon dioxide and HFC-365mfc are useful solvents for some soils. In use as propellants, one assumes they do not act only as carriers and do contribute solubility performance to the dispensed cleaning solvent.
The solvent contents of an aerosol can of cleaning solvents is only rarely a single component—such as isopropanol, aliphatic hydrocarbons, or HFC-43-10mee.
This enables suppliers to differentiate their respective offerings from competitors.
That’s the main reason solvents dispensed from aerosol cans are blends or azeotropes containing at least two chemical components—to make offerings unique.
There is a second and commonly accepted reason, which is—unfortunately—not technically sound.
The reason is a belief that when one component of binary azeotropes or blends has no measured flash point (such as HFC-43-10mee, HFE-7100/7200, HCFC-225ca/cb, HFC-365mfc, etc.), and if it is present in sufficient amount, the azeotrope or blend can’t be ignited. That can be a most valuable outcome. It’s just not certain—that’s all!
(The next column—which will appear in the June issue of Metal Finishing—will provide further information about the difference between two measures of ignitability flash point and flammability limits.)
There is a third reason: solvent blends dispensed from aerosol cans are often used in a less-than-scientific fashion. For example, the label on the aerosol can almost never reveals the ingredient list. Instead, the print is usually consists of just a name chosen by marketers—and some rudimentary safety warnings. The formulation of aerosol products is highly proprietary, which is why their ingredients are concealed to the extent consistent with federal and local regulations.
There are many types of aerosol products. Each is formulated with a base solvent (such as the four pictured above) whose unique properties often engender formation of an azeotrope with an additional one or more components, or a base solvent whose properties can be made more versatile by addition of another one or more components to form a blend.
The base solvent is undoubtedly one in which the manufacturer has a strong financial interest. It is likely they are the only global supplier. It is likely they hold patents on its method of manufacture, molecular structure, and azeotropic compositions.
Preview of Coming Attractions
In next month’s column we’ll cover the many applications for aerosol-dispensed cleaners, and how they are specified for success.
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 firstname.lastname@example.org.
- The image of Figure 1 is courtesy of featurepics.com.
- The image of Figure 2 is courtesy of the Koc University Nano-Optics Research Laboratory (nano-optics.ku.edu.tr). The magnified image depicts droplets that have yet to coalesce.
- The image of Figure 3 is courtesy of the Martin Luther Universitat of Halle Wittenberg (physik.uni-halle.de/fachgruppen/nmr/research/).
- The image of Figure 4 is courtesy of boxvox.net.