The answers to questions like “What’s the best solvent?” or “What’s the strongest cleaner?” are the same: it depends! 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 solvent 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. Solvents 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 Kb value refers to a standardized ASTM test (Method D-1133) that calibrates the relative strength of a solvent by dissolving a gummy resin. The working ranges of Kb values go from 10 (very mild) to 200 or even higher (very strong cleaning). For years, this has been used to judge the relative cleaning “power” of solvents used on electronics, metals, optics and elsewhere.
Even Kb Values have their Limits
While it is convenient to state that higher Kb values indicate a “stronger” solvent, 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 solvent and its capacity to clean fluxes, oils and grease. At MicroCare, our product specifications for cleaning solvents always include a Kb value to help you decide on the best solvents for the job you’re facing.
Solvents with lower Kb values will dissolve greases but may not handle ionics and fluxes. Solvents 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.
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.