Using KB Ratings for Solvent Evaluation

When discussing the cleaning strength or aggressiveness of cleaning solvents, we often refer to the solvent’s KB (Kauri-Butanol) number. This  test  measures the ability of a hydrocarbon solvent to overcome molecular  forces in a standard solute. The KB test is one of a number that can be used to rank solvents based upon relative solvent power.  Although some KB values can give misleading results the value still proves useful when used to determine the relative solubility of different solvents.

Kauri resin is a  resin sourced from the sap of the Kauri  tree, which grows  in New Zealand. This resin dissolves easily in normal butyl alcohol (butanol) but will not dissolve very well in hydrocarbon solvents. To run the KB test dissolve the Kauri resin in a fixed amount of n-butanol. This solution is then titrated with the particular solvent to a “cloud point” or until the clear solution first turns turbid or hazy. The volume of  solvent used to reach the cloud-point is the KB value of the solvent. The greater the volume of solvent needed to reach the cloud-point, the “stronger” the  solvent. A solvent with a KB value of 1000  such as Purasolve MS it is a much stronger solvent than one with a KB value of 50 to 100 such as Perchloroethylene.

The KB value definition according to the ASTM (method D 1133), is the volume given in milliliters of any solvent which is at 25 °C (77 °F). A standard KB solution is titrated and a certain amount of turbidity is identified. Each solvent has its own capacity to dissolve a certain type of substance more or less effectively than others depending on its own physico-chemical properties as well as those of the substance that has to be dissolved.

With regard to chemicals and solubility, we are usually referring to dissolving a small amount of a liquid or solid, the solute, in a larger volume of liquid, called the solvent. When a liquid solvent dissolves a solute the solvent breaks the forces that bind the molecules of the solute together. Solvent molecules force themselves between and around the solute molecules, until the molecules that made up the solute are  dispersed within the larger volume of solvent molecules.

In spite of the lack of strong and consistent theoretical test methods in our cleaning industry, the KB value has been typically accepted as a guide to determine the cleaning power of the solvents.

The range of safety solvents with one of the highest KB rankings globally can be found at https://envirofluid.com/worksafe-environmental-chemistries/products-by-brand/purasolve