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Whether it’s a five-year-old washing their dad’s car with a wire brush and turpentine, or a supposedly ‘eco-friendly’ citrus cleaner transforming a high-gloss piece of furniture into a dull antique, it makes us realise that there is far more science to the job of cleaning than many of us might think. The story starts with the kauri tree, which is indigenous to the northern territories of New Zealand. It can grow to a height of more than 50 metres, and reach circumferences of 18 metres or more. Because of the quality of its wood and its long, straight trunk, the kauri tree was heavily logged (predominantly for ship building) in the late 1800s. At approximately the same time, chemists were looking for a solvent that could effectively dissolve kauri resin for possible use in furniture-finishing and a variety of other industrial applications.
They turned to butyl alcohol, also known as butanol, which proved to be excellent at dissolving the resin. In fact, the results were so conclusive that this test, known as the Kauri-butanol test, became the benchmark for all solvent strengths. Today, we still use these Kb values to determine the strength of household and industrial solvents. In short, the speed (or ease) in which a solvent can dissolve kauri gum is what determines the strength (or Kb value) of a solvent. Typically speaking, Kb values range from very mild (10 Kb) to very strong (200 Kb or more) – you can refer to the graph below to better understand solvency strengths. It should be noted, though, that Kb relates only to non-aqueous solvents, as the strength of most water-based cleaners is measured according to their pH. This can range from 0 to 14, with 7 indicating the midway neutral mark.
Anything below 7 indicates an acidic fluid, and anything above 7 indicates an alkaline fluid. Generally speaking, most water-based cleaners use an alkaline solution, and in cases where the dirt (or contamination) is water-soluble (such as food grease), it makes perfect sense to use a water-based cleaner with the correct (or preferred) alkaline reading. However, some contaminants are not water-soluble, and in these cases, a solvent with a specific Kb value is preferred. But there’s a catch: If you get the solvent strength (or Kb value) wrong in relation to the component you’re cleaning, there’s a good chance you’ll damage the component. In most cases, the higher the Kb value, the greater the chance of damaging plastic, rubber or electrical parts. As mentioned in last month’s issue, solvents that are petroleum-based and have higher Kb values are very often cheaper than their safer, naturally derived alternatives. And, depending on the complexity of the contamination, it may also be necessary to make use of more than one type (or formula) of cleaner. This, in turn, raises the question of whether to choose a multi-use formula, or focus on a purpose-specific solution.