What Is a 'Kb Value' and What Does It Mean?

Kb value refers to a standardized ASTM test that measures the relative strength of a non-aqueous cleaning fluid. The test involves measuring the solubility of a very specific type of contamination, called “kauri gum.” Kb values range from 10 (very mild) to 200 or even higher (very strong). Mild solvents with lower Kb values will may not be strong enough to dissolve ionics and fluxes. Cleaning fluids with higher Kb values may be speedy and effective cleaners but may attack plastic components. The Kb values for the MicroCare products generally range from 20 to 119.

It is difficult to predict the cleaning effectiveness of a fluid without tests. The results of a cleaning process can vary by the contamination, the selected cleaning fluid, the application, the cleaning system and the physical shape of the parts being cleaned. The MicroCare Critical Cleaning Lab is capable of conducting cleaning tests for clients to determine the optimal cleaning fluid for the contamination being removed.

Kb Values

Not-In-Kind Benchmarks

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pH Versus Kb

People often ask about the pH of a cleaning fluid, because everybody knows the pH of a fluid can be a good indicator of cleaning strength. It is — but the pH test only works for fluids containing water. For non-aqueous fluids a better measure of cleaning effectiveness is the Kauri–Butanol (Kb) value. Let’s compare these two measurements.

What is pH? First introduced by S.P.L. Sørensen in 1909, pH measures the acidity or alkalinity of a fluid in comparison to water. The range goes from 0 – 14, with 7 being neutral (water). A pH lower than 7 indicates an acidic fluid (which means it has more hydrogen ions than plain water) while a pH greater than 7 indicates a basic, or alkaline, fluid (more hydroxyl ions than standard water). Most aqueous cleaning systems use alkaline cleaning agents, often with pH ratings around 12-13. So for contamination that is water-soluble, looking at the pH makes sense.

But many types of contamination are not water-soluble so non-aqueous cleaning fluids are a better choice for those cleaning jobs. Non-aqueous cleaning fluids are neither acidic nor alkaline; the pH test simply isn’t relevant because non-aqueous cleaning fluids don’t contain any hydroxyl ions or hydrogen ions. So Kb value is the alternative measurement.

Kb value refers to a standardized ASTM test (Method D-1133) that attempts to calibrate the relative strength of a cleaning fluid by dissolving a gummy resin called Kauri gum (see note, below). The benchmark for the test is butanol solvent. The working ranges of Kb values go from 10 (very mild) to 200 or even higher (very strong). For years, this has been used to judge the relative cleaning “power” of cleaning fluids used on fluxes found on electronics, metals, optics and elsewhere.

Limitations of Kb Values

While it is convenient to state that higher Kb values indicate a “stronger” cleaning fluid, it is not that precise. There are many limitations to the Kb test. For example, the test must be performed at 25°C (77°F) and one atmosphere pressure. Conducting the test under other conditions yields different results. Some cleaning fluids cannot be evaluated using the Kb method since they dissolve the kauri gum almost instantly. Certain types of fluxes, such as OA fluxes and some synthetics, simply don’t react predictably to the Kb test. The test can produce some exotic results especially with alcohol formulas. Real-world cleaning results also can be affected by other factors such as surface tension, density and viscosity. Also, the original Kb test calibrated petroleum products (butanol, hence the name Kauri-butanol test) but now it has been stretched to measure brominated, fluorinated, chlorinated, and other liquids. In other words, people are asking the Kb test for to do more than it can consistently provide.

So, there is a general relation between the Kb value of a cleaning fluid and its capacity to clean fluxes, oils and grease. Most product specifications for cleaning fluids  include a Kb value in one form or another. Cleaning fluids with lower Kb values may displace particulates but  but may not be strong enough to handle grease, ionics and fluxes. Cleaning fluids with higher Kb values may be speedy and effective flux removers but may attack plastic components or remove inks.

Many other factors affect cleaning performance as well: surface tension and density affect wetting; toxicity and flammability affect the handling of a material; costs certainly drive the economic analysis. Therefore, while the Kb value is a handy benchmark and a good starting point for an analysis, a Kb number — like a pH value — is only a the first of many predictors of cleaning performance. Selecting a cleaner with the optimal Kb value is a balancing act with numerous trade-offs.

In short, we use the term Kb Value and we know the test method, but it should only be considered an approximation that needs to be validated with real-world cleaning tests. MicroCare offers a large family of cleaning fluids with a variety of Kb values, which enables engineers to fine-tune their cleaning process.

Note: What Is Kauri Gum?

The benchmark resin used in the Kb test actually is the sap from the kauri tree (Agathis Australis) that only grows in New Zealand. In the 1890s, chemists searched for a solvent to liquefy kauri resin to make it useful industrially, particularly in the furniture finishing industry. They found that kauri gum was easily dissolved in butyl alcohol (butanol) so that became the benchmark solvent and is the source of the name of the test, the Kauri-Butanol test.