The Kauri-Butanol Number – Unconventional Wisdom

It would be terrific to have a numerical comparison of how effective various cleaning agents are at removing soils. Can you pick a single number to choose the best cleaning agent? Manufacturers have come to rely on the Kauri-Butanol (Kb) number as an indication of cleaning agent strength. 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 that is based on actual testing. It is assumed that, all other things being equal, a high number means effective cleaning. When we teach, we ourselves often show a table indicating Kb number and boiling point. The reason we include the boiling point is that cleaning agents tend to work more work aggressively at higher temperature. The conventional wisdom would be to choose a cleaning agent with a high Kb number and a high boiling point. As a manufacturer who cleans critically, you may need unconventional wisdom. Let’s take a deeper dive into Kb testing and where this single number may or may not be sufficient to achieve effective critical cleaning. 

Cloud point test
The Kb number is determined using a cloud-point test. In cloud-point testing, 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 solution, so the solution becomes cloudy. The more solvent that can 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 clearly specified. The temperature is controlled. Visual inspection is related to visibility of printed material of defined font and size on a card that is placed under a defined flask containing specified amounts of the standard Kauri solution. The end point is when the outlines of the print become blurred but are still legible. 

Kauri resin – the solute
The ASTM method specifies reagent grade of the Kauri resin-butanol solution. The solution is available from several sources. It is derived from the New Zealand Kauri tree; and it 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 it finds 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.” (we have highlighted some key sections) 

“Significance and Use

4.1 The kauri-butanol value is used as a measure of solvent power of hydrocarbon solvents. High kauri-butanol values indicate relatively strong solvency.”

“Scope

1. 1. This test method covers the determination of the relative solvent power of hydrocarbon solvents used in paint and lacquer formulations.”

The Kb number is designed to be predictive of 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, oxygen. The molecular arrangement of the elements may include ketones, alcohols, esters. If the molecule to be tested can readily dissolve the Kauri resin, a very high number will be obtained. In addition, Kb numbers may be indicated for blended cleaning agents. How far can or should 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

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.” In fact 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 as 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. While “Blended Solvent” does a good job of dissolving Kauri resin. this is not necessarily an indication of 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. Actually, given the range of cleaning chemicals, it’s more like comparing apples with peaches or rutabagas or 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. One reason people may rely on the Kb is 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 the same way. A universal solvent is also a fantasy. A solvent that dissolves all soils would dissolve the product to be cleaned as well as the storage container.

To achieve a more realistic measure of cleaning effectiveness, get to know the Hansen Solubility Parameters (HSP) (8). HSP are a measure of 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 a soil and will not damage a polymeric substrate. We’ll explain more about HSP real soon!

References:

1. 1. ASTM D1133-13 (2021), “Standard Test Method for Kauri-Butanol Value of Hydrocarbon Solvents.” https://www.astm.org/d1133-13r21.html 
   2. https://www.unistarchemical.com/news.htm 
   3. https://producerschemical.com/media/PC%20Solvent%20Chart.pdf
   4. https://www.sciencedirect.com/book/9780080523781/handbook-of-organic-solvent-properties 
   5. https://www.envirofluid.com/wp-content/uploads/2018/09/Purasolve-MS-TDS.pdf
   6. https://www.techspray.com/kauri-butanol-Kb-values-and-solubility-parameters
   7. https://www.linkedin.com/pulse/using-Kb-ratings-solvent-evaluation-ben-ohlmeyer/
   8. https://www.hansen-solubility.com