It would be terrific to have a numerical comparison of how effective various cleaning agents are at removing soils.

Barbara and Ed KanegsbergBarbara and Ed KanegsbergCan you pick a single number to choose the best cleaning agent? Manufacturers have come to rely on the Kauri-Butanol (Kb) number to indicate the strength of the cleaning agent. Suppliers of cleaning agents sometimes proudly indicate a high Kb number as a feature of the product, along with a notation that it has been tested using an ASTM method. 

People use the Kb number because, in contrast to the Hansen solubility parameters, it is a single value based on actual testing. It is assumed that, all other things being equal, a high number means effective cleaning. When we teach, we often show a table indicating the Kb number and boiling point. The reason we include the boiling point is that cleaning agents tend to work more aggressively at higher temperatures.

Conventional wisdom would be to choose a cleaning agent with a high Kb number and a high boiling point. However, as a manufacturer who cleans critically, you may need unconventional wisdom. Let’s dive deeper into Kb testing and where this single number may or may not be sufficient to achieve effective critical cleaning. 

Image courtesy of www.emerson.com

Cloud Point Test

The Kb number is determined using a cloud-point test. In cloud-point testing, the solvent under consideration is gradually added to a solution. At the cloud point, the solute (the material that is dissolved) begins to come out of the solution, so the solution becomes cloudy. The more solvent that can be added before one observes the cloud point, the higher the number. 

The official ASTM method for the Kb number, ASTM D1133-13 (2021) “Standard Test Method for Kauri-Butanol Value of Hydrocarbon Solvents” (1), outlines a well-defined visual inspection. While there are limitations to any visual inspection, the test conditions are well-specified. The hydrocarbon to be tested is added gradually to a solution of Kauri resin dissolved in butyl alcohol. The attributes of the reagents are specified. The temperature is controlled. Visual inspection is related to the visibility of printed material of defined font and size on a card placed under a defined flask containing specified amounts of the standard Kauri solution. The endpoint is when the outlines of the print become blurred but are still legible. 

The ASTM method specifies the reagent grade of the Kauri resin-butanol solution, which is available from several sources. It is derived from the New Zealand Kauri tree and is sometimes referred to as Kauri Gum or Kauri Copol. When it is fully fossilized and polymerized, it is referred to as New Zealand amber. The material can be aesthetically pleasing and has found its way into the world of art. Do a few web searches to find beautiful examples of Kauri resin. 

What is the Test Designed For?

To go beyond conventional wisdom, let’s be persnickety. The first clue to the limitations is in the title of the ASTM method – “hydrocarbon solvents,” and we have highlighted some key sections.

  • Significance and Use
    • 1 The kauri-butanol value is used to measure the solvent power of hydrocarbon solvents. High kauri-butanol values indicate relatively strong solvency.”
  • Scope
    • This test method determines the relative solvent power of hydrocarbon solvents used in paint and lacquer formulations.”

The Kb number is designed to predict the solvent power of hydrocarbon solvents. Hydrocarbons contain two elements: hydrogen and carbon. Among hydrocarbon solvents, the Kb number depends on the molecular structure. While we might infer from the scope that the major use of this test is for coatings formulation, the number is also used to describe the effectiveness of cleaning. Further, in critical cleaning and surface prep, we see the Kb number used for cleaning agents that are more diverse than hydrocarbons.

In addition to hydrogen and carbon, they may contain chlorine, fluorine, bromine, nitrogen, and oxygen. The molecular arrangement of the elements may include ketones, alcohols, and esters. A very high number will be obtained if the molecule to be tested can readily dissolve the Kauri resin. In addition, Kb numbers may be indicated for blended cleaning agents. How far can we extrapolate from the original scope of the ASTM method?

The table below lists the Kb numbers for some cleaning chemicals with boiling points. 

Table: Kb numbers and Boiling Points

Chemical Kb Boiling Point (DEG C)
Trichloroethylene (2) 129 87
Perchloroethylene (2) 90 121
n-Propyl Bromide (2) 125 71
Hydrofluoroether (Novec 7100) (2) 10 55
Trans-Dichloroethylene (2) 117 48
Benzene (3) 112 80
Cyclohexane (3) 58 81
Hexane (4) 29 67
Xylene (4) 98 140
Blended solvent (5) 1000 154

 

What the Numbers Mean

Are these numbers useful to compare cleaning? Sometimes, the Kb number is indicated to be “not applicable.” (6) or “misleading” (7) without indicating why. ASTM D1133-13 is designed to compare hydrocarbons. PCE, nPB, HFE, and trans-DCE are not hydrocarbons; they are halogenated – they contain chlorine, fluorine, or bromine. They differ from hydrocarbons by substituting a hydrogen (charge +1) with a halogen (charge -1). Because the Kb numbers for the halogenated solvents fall within the range of hydrocarbons, it may not be an unreasonable reach to group them with hydrocarbons. 

Some people may consider halogenated solvents “honorary hydrocarbons.” The actual solvency properties are far more nuanced; the chlorinated and brominated solvents have a wider solvency range. There is a point at which solvents cannot considered honorary hydrocarbons. For example, “Blended Solvent,” indicated to have a Kb number of over 1,000, is approximately 80% “soybean oil, methyl esters,” and the remaining 20% are unspecified on the SDS. Meanwhile, “Blended Solvent” does a good job dissolving Kauri resin. This does not necessarily indicate how well it will dissolve the soils and residues you encounter. 

The Kb number is probably best thought of as indicating solvency style for similar solvents. To do otherwise becomes analogous to comparing apples and oranges. Given the range of cleaning chemicals, it’s more like comparing apples with peaches, rutabagas, avocados, or Brussels sprouts. You could substitute peaches for apples in a pie. But a rutabaga pie? Not really. We think the Kb number is best restricted to what the test was designed for – to compare hydrocarbons. 

A Universal Solute?

Selecting a cleaning agent based on the Kb number alone is not a path to successful manufacturing. People may rely on the Kb because of the unspoken assumption that the Kauri resin is a universal solute. A universal solute is a fantasy. A universal solute would represent all soils, and not all soils behave similarly. A universal solvent is also a fantasy. A solvent that dissolves all soils would dissolve the product and storage container to be cleaned.

To achieve a more realistic measure of cleaning effectiveness, get to know the Hansen Solubility Parameters (HSP) (8). HSP measures three intermolecular forces: polar, non-polar (dispersive), and hydrogen bonding. HSP are available for thousands of compounds, including cleaning solvents, soils, and polymeric substrates. HSP provides a way to determine if a solvent is efficient at dissolving soil and will not damage a polymeric substrate. We’ll explain more about HSP real soon!

References

Barbara and Ed Kanegsberg founded BFK Solutions in 1994 as a critical cleaning consulting service, and the go-to resource to make cleaning, surface quality, and contamination problems go away or — even better — to avoid problems in the first place. Barbara, widely known as “The Cleaning Lady,” is an expert and trusted adviser in critical cleaning. Ed is known as “The Rocket Scientist,” they write Clean Source, an approximately monthly e-newsletter that provides practical ideas to improve cleaning, contamination control, and product quality. They are co-editors and contributors to the acclaimed two-volume “Handbook for Critical Cleaning,” CRC/Taylor & Francis, 2011. Visit https://bfksolutions.com