Critical Cleaning, Wettability and Solvency

To clean effectively, the cleaning agent must access the surfaces of the product. In contrast with flat surfaces, it is very difficult to reach all the surfaces of complex, ornate, or miniature products. Electronics assemblies with spacing below 0.005” (125µ) are not as readily cleaned. The more surface area, the greater the challenge. Product produced by additive manufacturing, particularly metals can be essentially all surface. Sometimes, this complex surface is desirable. For example, some implanted medical devices are designed to be porous to promote osseointegration; over time, the bone melds with the device.

You may have heard terms like wetting, wettability, and wetting index used to extol the benefits of a particular cleaning agent, one that will supposedly solve all your cleaning problems and make you the most profitable manufacturer in the known universe.

Wetting is a simple concept that relates to physical properties of cleaning agents or ingredients of the cleaning agent. Wetting alone is not the answer to successful cleaning. Read on; we’ll show you more of the big picture.

Wetting Index

For solvents (organic solvents), the wetting index has become a widely accepted measure of wettability. It was introduced decades ago as a teaching tool geared to vapor degreasing solvents.

The wetting index is related to three physical properties of a molecule: the surface tension, the viscosity, and the density. The wetting index of a given chemical is directly proportional to the density and inversely proportional to the surface tension and the viscosity. 

Wetting Index = 1000* density/(viscosity*surface tension)

The higher the wetting index of a given molecule, the better job that molecule should do at “creeping” onto surfaces. We’ve included a table of a few cleaning agent chemicals, the pertinent physical properties, and the wetting index.

Examples: Physical properties and Wetting Index for cleaning agents (1)

It is intuitive that molecules with low surface tension and low viscosity would do a better job of accessing the surface, of flowing into tight spaces. You may catch more flies with honey; but honey does not do a good job of wetting a surface. So why is high density considered important? A chemical with a higher density would be expected to provide more kinetic energy.

A more complete picture – the wetting index in context
The wetting index is predictive for solvents that would be used in open-top liquid/vapor degreasers. The definition and implication of the wetting index include a business-related factor. As we mentioned the wetting index was introduced as a teaching tool by manufacturers of CFC-113 ((Freon 113). And (what a surprise!) CFC-113 does happen to have a high wetting index.

Isopropyl alcohol, water, hydrocarbon blends (Isopar), and modified alcohol all have a very low wetting indices. Compared with other halogenated solvents, perchloroethylene has a relatively low wetting index. However, we know that all these cleaning agents are used successfully.

Wetting is not solubility
A cleaning agent supplier may promote a cleaning agent as having a very high wetting index and assert that any product with a high wetting index is an effective cleaning agent. Nope! While a solvent with a high wetting index can readily access the surfaces of the product, if the solubility parameters of that solvent do not match those of the soil you are trying to remove, all that solvent will do is stare blankly at the soil and say “duh” (metaphorically speaking). It’s a bit like the blind date from hell; there may be proximity, but there may be no, uhm, chemistry. Without understanding solubility, you don’t know diddly squat about cleaning effectiveness.

The wetting indices of n-propyl bromide, trans-DCE, HFCO, and HFO are all high. However, they have very different solvency characteristics. Most “designer solvents” (HFCs, HFEs, HFOs) have relatively little solvency for most soils of interest to manufacturers unless they are blended with trans-DCE. HFE(Novec 7100) has an exceedingly high wetting index but very low solvency. HCFO (AGA 300) has a lower wetting index but it has moderate solvency.

Solvency (the attributes of like dissolves like) can be measured. The Hansen solubility parameters, are polar, hydrogen bonding, and dispersive (non-polar); they are a quantifiable measure of “like dissolves like.” (2) The efficacy of soil removal is related to how closely the Hansen solubility parameters of the cleaning agent match those of the soils. For non-oxygenated cleaning agents, the Kauri-Butanol (KB) number can provide an indication of likely solvency.

Wetting depends on temperature
The wetting index remains a reasonable guideline of how well a cleaning agent will access a surface under certain conditions, but not necessarily under process conditions. While the physical properties are for 25oC, many or even most cleaning processes occur at higher temperatures. where viscosities are lower, wetting indices would be higher. Water at 77C has a viscosity of .37, so the wetting index would almost triple from 25C. Modified alcohols and hydrocarbons, typically are heated, in airtight chambers; the wetting properties would improve. Perchloroethylene boils at a higher temperature, so wetting should be higher.

Wetting and Surfactants
With aqueous cleaning agents, wetting is a bit more complex. The same physical parameters of density, surface tension and viscosity apply. Surfactant also has an effect. As the concentration of a surfactant added to water increases, the surface tension decreases, but only up to a point, called the Critical Micelle Concentration (CMC). Once the surface of the liquid has been saturated, additional surfactant will create micelles, aggregates of surfactant that increase the viscosity but do not continue to reduce the surface tension. Micelles are complex aggregates of surfactants where the hydrophilic (water-loving) portion of the surfactant is in contact with water and the hydrophobic (water-hating) part of the surfactant is kept out of contact with water. Micelles serve a separate functional purpose in aqueous cleaning in that soils are trapped in the hydrophobic inner portion. This keeps soils away from the surface being cleaned. Actually, there are surfactants specifically designed for organic solutions.

As we discussed in a recent issue of Clean Source (3), cleaning is more than just chemistry. The parameters of Time, Action, Chemistry and Temperature (TACT) interact. The density parameter in the wetting index is a measure of molecular momentum. An increase in the cleaning action, such as spray or ultrasonics, might act analogous to an increase in density and allow that agent to access the surface.

References
Various sources, including Handbook for Critical Cleaning and web derived Technical Data Sheets
Handbook for Critical Cleaning, 2nd Ed., Vol. 1, Chapter 4
Clean Source—Feb 2022

Note: W.G. Kenyon was instrumental in introducing the concept of the wetting index to manufacturers. He was a practical, enthusiastic educator and a mentor. Bill was exceedingly helpful to manufacturers involved in process change.