Parts Cleaning: Does the Cleaning Agent Work?

 

Cleaning is a process. For this discussion, let’s focus on the cleaning agent itself. Choosing the right cleaning agent (aqueous or solvent) involves a number of factors. The cleaning agent must meet safety and environmental requirements. It must not destroy the surface, or you have rather defeated the purpose of surface prep. It has to be cost-effective; and most of all, it has to remove soils effectively. Cleaning efficacy involves solvency, wetting properties
and temperature.

Solubility: The Hansen solubility parameters work on the principle of “like dissolves like.” They are measures of three intermolecular forces: dispersive (oil-like), polar (water-like), and hydrogen bonding (another polar property). Chemists look at the Hansen parameters in somewhat the same way we may look at paint chips; choosing the right color depends on the balance of red/yellow/blue as well as the intensity (vibrant purple versus lilac). Surface finishers have used classic chlorinated or brominated solvents to remove oils and metalworking fluids, because they are effective. This effectiveness is due in part to the similarity of the Hansen parameters to the oils and greases of interest. 

Wettability: Would you use a really viscous liquid like honey for surface prep? Probably not, even if honey turned out to be a really good solvent for the soil you were trying to remove. The viscosity limits the ability of the fluid to wet, to flow into and out of small spaces. The wetting index was developed as a teaching tool by W. Kenyon to describe how well the cleaning agent could get into tight spaces like helicoils or into the recesses of a highly-carved decorative metal product. Wettability is probably important even for relatively flat surfaces in that agents with better wettability might be better able to penetrate layers of soil and to lift particles off surfaces of the substrate. The wetting index is directly proportional to the density and inversely proportional to the surface tension and the viscosity. The table shows the physical parameters and wetting index for a few cleaning agents; in general, a higher wetting index is desirable. Compared with some organic solvents, the relatively low wetting index of water makes it behave at bit like honey–water alone may not be able to reach soils trapped in tight spaces. Some organic solvents have excellent wetting properties, but poor solubility. Others have both favorable solvency and great wetting properties; but they may have safety and environmental issues. Increasing the wettability is one reason why surfactants and other additives are included in aqueous cleaning agents.

Temperature: Arrhenius’s rule says that, as a very rough rule of thumb, for every 10°C increase in temperature, the reaction rate doubles. Barbara’s old organic chemistry teacher used to recount that Arrhenius based his rule on his observation of the chirping rate of crickets. Crickets are complex creatures with many variables of construction. In terms of cleaning, we can focus in on fewer variables. One is the melting point of soils, such as waxes or solid lubricants. A cleaning agent with relatively unfavorable solubility parameters and a relatively low wetting index may still be an acceptable choice if melting the soil is sufficient for good cleaning efficacy. 

Consider also that the wetting index is generally calculated at room temperature. Many cleaning processes operate at elevated temperatures. The physical parameters, especially viscosity, change with increasing temperature. If you want honey to flow more effectively, heat it up a bit. Some cleaning agents are more effective at higher temperatures because the wetting properties are more favorable. From the table, you can see that the wetting index of water is significantly greater at 50°C compared with 25°C. Of course, if you were to clean with honey, and perhaps with higher boiling cleaning agents, rinsing may be required.

The cleaning process: A cleaning process has to be developed; many factors have to be considered. This means looking not only at solvency but also at wettability. This means looking at the cleaning solvency and wettability not only at ambient temperature but also at likely process temperatures. This means considering materials compatibility at the operating temperature; higher temperatures may be more effective for removing soils, 

but the part you are cleaning may be damaged.

Testing cleaning agents takes a bit of experimentation. Please also consider worker safety, neighborhood safety and environmental issues. Look at things like flashpoint and lower and upper flammability limits. Look at the worker inhalation exposure level; “unknown” or “not established” does not mean “safe.” It means “use with caution.” Don’t dunk your fingers in the cleaning agent; use the right gloves. What’s the right safety gear? It depends on the cleaning agent, so ask your safety professional. Oh, and by the way, you are experimenting so use scrap, not valuable hardware.

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 info@bfksolutions.com.  

 

 

 

 

 

Physical Parameters and Wetting Index
Cleaning Agent	Temperature C	Density g/ cm3	Surface Tension Dynes/cm3	Viscosity Centi-poises	Wetting Index
n-Propyl Bromide	25	1.35	25.5	.49	106
d-limonene	20	0.84	25	1.28	26
H2O	25	1.0	73	.89	16
H20	50	0.99	68	.55	27