Tuesday, September 9, 2014

Strength of a Cleaning Solvent – Kauri-Butanol Values & Solvent Solubility – Guest Blog

by Steve Allen, VP Marketing and Innovation, ACL Staticide

Our guest blogger, Steve Allen from ACL, returns with an examination of cleaning solvent strengths and Kauri-Butanol numbers

When talking about the cleaning strength or aggressiveness of electronics cleaning solvents, we often refer to the solvent’s Kauri-Butanol number, or KB. The KB value is a test that measures the ability of a hydrocarbon solvent to overcome molecular binding forces in a standard solute. The KB test is one of a number of “cloud-point” determinations that can be used to measure and order solvents in a ranking based on relative solvent power. ACL Staticide uses this relative measure to determine and publish the general cleaning strengths of its broad range of electronics rework and repair products.

Other cloud-point determinations are aniline cloud point, solubility grade, wax number, and heptane number. All have their specific uses, but the KB value is the most widely used test for gauging the relative solvent power of most hydrocarbon solvents.

Kauri resin from the Kauri Tree
Though KB values can give misleading results for some solvents, the KB still proves useful when used to determine the relative solubility of different solvents. Kauri-Butanol value is only a small part of the subject of solubility, and there are many different solubility scales.

How does KB actually work? Kauri resin is a fossilized resin derived from the sap of the Kauri pine tree, which grows primarily 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, one dissolves 20 grams of the Kauri resin in a fixed amount of n-butanol. This solution is then titrated with the hydrocarbon solvent to a “cloud point” or until the clear solution first turns slightly turbid or hazy. To make the cloud point easier to see, the test is usually performed over a page of ten-point type. When the ten-point type becomes slightly blurred or not quite crystal clear, you have reached the cloud point. The volume or number of milliliters of hydrocarbon solvent used to reach the cloud-point is reported as the Kauri-Butanol or KB value of the hydrocarbon solvent. By this methodology, the greater the volume of solvent needed to reach the cloud-point, the “stronger” the hydrocarbon solvent. A solvent with a KB value of 100 (ml) is a much stronger solvent than one with a KB value of 50.

The Kauri–Butanol value definition is, according to the ASTM (method D 1133), the volume given in milliliters of any solvent with which at 25 °C (77 °F), a standard Kauri–Butanol solution is titrated and a certain amount of turbidity is detected. The accepted KB value range goes from 20 or 25, to 120 or 130. The KB values for ACL precision solvent cleaners, which are mixtures of various hydrocarbon solvents, are determined by running the Kauri-Butanol titration with the cleaner product. They can also be estimated using the KB value of the blended product from the KB values of its individual solvent components and their proportions in the solvent mixture.

KB values for most of ACL’s solvent cleaners are found on the product’s technical data sheet. Most of the ACL solvent cleaner blends have KB values of 50 or less, but the stronger and more aggressive products like Precision Wash NF, or Precision Wash NS have KB values above 100. The higher the KB value of an ACL cleaner, the more likely it is that this product will attack soft plastics like polystyrene and polycarbonate. You would normally select an ACL cleaner with a high KB value only if you need heavy-duty cleaning of thick or encrusted residues and have no sensitive plastics in the application. If only light-duty cleaning or plastic safety is required, then the user should select an ACL cleaner that has a KB value of 50 or less.

Even today, many people still misinterpret the KB value’s meaning. They relate the KB value with the titrating solvent “power.” As a matter of fact, there is no such thing as “solvent power.” Each solvent has its own capacity to dissolve a certain type of substance more or less effectively than others. This depends on its own physico-chemical properties, as well as those of the substance that has to be dissolved. In the beginning, the KB value was only used to evaluate the behavior of petroleum fractions. They would yield KB values of up to 30 or 35. Since this method was arbitrarily taken as a measure of the “solvent’s power,” very soon it was adopted to measure other type of solvents. For instance, chlorinated hydrocarbons, such as perchloroethylene, may be the most industry representative, legacy solvent used in our industry. It yields a KB value of about 90.

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 molecules of the solvent break the electrostatic or binding 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 finely dispersed within the larger volume of solvent molecules. There are some solvents such as alcohols, ketones, and glycol ethers that cannot be evaluated using the KB method, because they can readily dissolve the Kauri gum--so they can hardly form any turbidity when titrated into a Kauri–Butanol standard solution.

These ACL precision cleaners have KB values above 100
In spite of the lack of strong and consistent theoretical test methods in our cleaning industry, the KB value has been accepted as a guide to determine the cleaning power of the solvents that are used in dry-cleaning machines (yes, dry cleaning). Indeed, there is a certain relation between the KB value of a solvent and its capacity to clean garments. Solvents with low KB values, lesser than 35, tend to dissolve greases somewhat easily. That is because the low KB value solvents can be relatively light fractions of petroleum which are similar in chemical nature to grease. These kinds of solvents tend to have some difficulties removing lacquer and varnish stains from garments. On the other hand, this family of solvents normally doesn’t damage the fabrics that are treated with them.

As a case in point, Perchloroethylene with its aforementioned KB value of about 90. Not that this solvent is “stronger” than the hydrocarbons. It has simply different properties that allow an easier dissolution of substances such as varnishes and lacquers (and conformal coatings). This solvent can still dissolve greases and oils, but it begins damaging some synthetic garments over time. It may also provoke a certain degree of color bleeding. Perchloroethylene can dissolve most of the stains found in the garments while the potential damages that it can cause are still controllable. That is why “Perc” continues to be the leading solvent in the dry-cleaning industry. There are some other types of solvents, such as 1,1,1, trichloroethane, with a KB value of 124. This solvent has been found to be too aggressive for most of the natural and synthetic fabrics found in most garments today. That is why it hasn’t been successful in the electronics industry.

However, with all this said, Kauri-Butanol values are not the final word in cleaning strength or solubility tests. In fact KB values tend to assign a higher solubility to a solvent than it really has. Also certain kinds of solvents cannot be tested with the Kauri-Butanol protocol as they prove to be infinitely soluble in this test procedure. For this reason ketones like acetone and MEK and glycol ethers cannot be tested by the Kauri-Butanol procedure to determine solvent solubility for these. As we can see, the Kauri–Butanol value can be useful to evaluate the proper solvent to be used for a given cleaning task. Those who have a variety of cleaning applications or multisolvent cleaning equipment (vapor degreasing, parts washers, and spray cabinets), can choose the kind of solvent that best serves the particular cleaning needs of their plant and application. The KB value is one of the parameters that have to be taken in account for that purpose. You can find all the necessary KB values for our solvents on ACL’s tech data sheets.

Q Source thanks Steve for another informative Guest Blog contribution to The Q Source Resource.

For information about ACL Staticide products, please visit our ACL Staticide Department at QSource.com. You may also contact us via email or phone at 800-966-6020 and one of our associates will be happy to assist you.

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