Kb Value Measures Solvent Strength

How well a fluid will clean depends on several variables including:

the contamination
the selected solvent
the application
the cleaning system
the physical shape of the parts being cleaned

When is pH a Good Enough Measure?

While the pH of a cleaning fluid can be a good indicator of a fluid’s cleaning strength, 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.

pH measures the acidity or alkalinity of a fluid in comparison to water. The range goes from 0 – 14, with 7 being neutral. 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.

Why Use the Kb Value?

Many types of contamination are not water-soluble. Non-aqueous solvents are a good choice for those cleaning jobs. Cleaing fluids are neither acidic nor alkaline; the pH test simply isn’t relevant because solvents don’t contain any water. That’s when the Kb value is your best guide to how strong a cleaner is.

The cleaning strength or solvency of non-aqueous cleaners usually is measured by an industry benchmark called the Kari-Butanol test. It is an industry standardized ASTM test (Method D-1133) that calibrates the relative strength of a cleaning fluid by immersing a test material called butanol — a thick, rubbery, eraser-like gum — in the solvent for a certain period of time under controlled conditions (e.g., temperatures, pressures, etc.). The lab technician then measures the remaining gum to determine the quantity of material that was dissolved by the solvent. It’s simple, fast and easy. The result of the test is an index called the Kari-Butanol, or Kb value.

The working ranges of Kb values go from 10 (very mild) to 200 or even higher (very strong cleaning). For years, the Kb value has been used to judge the relative cleaning “power” of cleaning fluids used on electronics, metals, optics and elsewhere. The Kb value is usually reported on MicroCare product spec sheets as “Kb value.” A higher Kb value is better and means the cleaner is more aggressive or active. Mild cleaners have low scores in the tens and twenties; powerful cleaners like the old chlorinated solvents have ratings in the low hundreds. Soft, mild MicroCare cleaners like the Alcohol-Enhanced Flux Remover – ProClean™ have Kb values in the mid-30s. Stronger cleaners, like the Heavy Duty Flux Remover – SuprClean™, will have values of 75-130.

Even Kb Values Have Their Limits

While it is convenient to state that higher Kb values indicate a “stronger” cleaner, 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.

There are some solvents that cannot be evaluated using the Kb method since they dissolve the kauri gum almost instantly. Also, the original Kb test calibrated petroleum products but now it has been stretched to measure brominated, fluorinated, chlorinated, and other liquids. In other words, people are asking for the Kb test to deliver more than it can consistently provide.

Using Kb Values to Guide Cleaning of Fluxes, Oils and Grease

There is a general relation between the Kb value of a cleaning fluid and its capacity to clean fluxes, oils and grease. At MicroCare, our product specifications for cleaning fluids always include a Kb value to help you decide on the best solvents for the job you’re facing. Cleaning fluids with lower Kb values will dissolve greases but may not handle ionics and fluxes. Cleaning fluids with higher Kb values may be speedy and effective flux removers but attack plastic components or remove inks. Selecting a cleaner with the optimal Kb value is a balancing act. There are trade-offs. Many 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.

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. In addition to the Kari-Butanol test, many engineers are also using systematic procedures to rigorously test cleaning fluids on their components, their contamination and their cleaning processes. This makes sense because it’s a test of real-world conditions. Often, the results are unexpected: the relatively mild fluids clean as well as or even better than cleaners with much higher Kb values. Informally, we call these surprising results “operational Kb values.” In these real-world cleaning of oils and fluxes, many MicroCare fluids clean much more effectively than the Kari-Butanol test would suggest while maintaining excellent materials compatibility. Special Note: You cannot compute a Kb value for a water-based cleaner, and you cannot compute a pH value for a solvent cleaner. Aqueous cleaners are profiled using the pH test for acidity or alkalinity. Different chemistries require different tools.

Special Note: The benchmark resin actually is the sap from the kauri tree (Agathis Australis) that only grows in New Zealand. Chemists searched for a solvent to liquefy that resin to make it useful industrially. They found that the gum was easily dissolved in butyl alcohol, so that became the benchmark solvent and is the source of the name of the test, the Kauri-Butanol test.