Solvent Strength (by Dr. Julian Lakritz)

If there is one thing that cleaners know about solvents it is solvent strength. Specifically, solvent strength as defined by its Kb. It is generally understood that the greater the Kb, the more aggressive and better cleaner the solvent is. We also believe that if the Kb of a solvent is too high, it can pull color and possibly cause other damage to a garment. As an example hydrocarbon solvents with Kb values around 30 are considered gentle. They do not dissolve sequins and beads, and do not cause appreciable bleeding of dyes. Perc on the other hand with a Kb of 90, dissolves beads and is more prone to pull dyes. Trichloroethylene with a Kb of 129 is even more aggressive than Perc. The more aggressive or “stronger” solvents are better cleaners. Tars and dirty motor oil stains have a better chance of coming out of a garment with Perc or trichloroethylene than with a hydrocarbon solvent, however, sugar, salt or other water soluble stains will not dissolve in any of the previously mentioned solvents. When we speak of “stronger”, we should stipulate “stronger” to what. We do not consider water as a strong solvent and yet it dissolves sugar and salt that other solvents do not.

Understanding Strength

To have a better understanding to strength, we should examine the origin of “Kb” so that we can appreciate its significance. The concept of Kb was developed by the paint industry. Oil based paints contain solvents, resins, and pigments. A properly formulated paint must contain solvents that can dissolve the resins and give it a suitable viscosity to permit brushing or spraying. Once applied, the solvent must remain long enough to level the paint but volatile enough to avoid dripping or excessively long drying. In 1927 S.R. Kiel together with Philadelphia Paint and Varnish club developed a procedure to minimize the amount of trial and error needed to obtain proper solvent blends. It assigned a numeric value to the solvents that permitted a more rapid formulation of such blends. Specifically, if a particular resin is found to be soluble in a solvent that has a Kb value of 30, an equal part mixture of a solvent with Kb of 20 with another solvent with a Kb of 40 will probably also dissolve the resin. The Kb value of a solvent is obtained by determining how much of the solvent in questions needed to cause Kauri Gum, to drop out of a Butanol solution. Kauri gum is a natural tree resin and was chosen because it resembles the solubility characteristics of many of the resins that were in use by the paint industry. Butanol is an alcohol that is chemically similar to drinking alcohol (ethanol) and rubbing alcohol (isopropyl alcohol). Kauri gum is very soluble in Butanol but not in hydrocarbon or the chlorinated solvents. Adding such a solvent to a Butanol solution of the Kauri resin will cause the resin to come out of solution. Perc is a slightly better solvent than a hydrocarbon solvent such as Stoddard solvent or DF-2000 for this resin and therefore more Perc than hydrocarbon can be added before the resin drops out and the solution becomes cloudy. It takes 3 times as much Perc (Kb=90) as Stoddard (Kb= ~30) to cloud the solution. The principle behind this si that only a very small amount of a very poor solvent is enough to precipitate the resin out of solution and cause it to become cloudy. A larger quantity of a better or “stronger” solvent will be needed to achieve the same level of cloudiness.

Like Dissolves Like

There is a general chemical rule that “like tends to dissolve like”. Alcohols are structurally similar to water and are called polar solvents. Polar solvents such as water will dissolve polar substance like salt, while non-polar solvents such as hydrocarbon solvents will dissolve non-polar materials like oils. Of the substances mentioned, the most polar is water, followed by alcohol, perc, and hydrocarbon. On a relative scale, perc is close to hydrocarbon and alcohol is close to water. Since perc is closer in polarity to Butanol than hydrocarbon, you would expect it to be a better solvent for the Kauri gum. From a cleaner’s point of view, neither solvent will effectively remove this resin.

The term “stronger solvent” can be misleading. From the foregoing, Butanol would be categorized as an infinitely strong solvent since it will not precipitate Kauri gum under the Kb test procedure. Drinking alcohol or rubbing alcohol, which are chemically similar to Butanol will also have very high Kb values. Yet we certainly would not consider any of these solvents as particularly aggressive as it relates to dry cleaning. These solvents are excellent for dissolving kauri gum but not effective at dissolving oils and greases. They are also much better at dissolving sugar than perc or hydrocarbon.

Kb and Dry Cleaning

How is Kb then reflected in practice to dry cleaning? Since perc is similar to hydrocarbon it will dissolve oils. In addition however, it will also remove some slightly more polar substance that hydrocarbons will not such as some dyes and plastics. Perc will not appreciably remove lipstick or shoe polish. These stains require solvents whose chemical structure is closer to their own. These types of solvents are found in spotters. Spotters contain blends of solvents. Because stains are composed of substances varying chemical structures, any given spotter may not completely remove a stain. Dry cleaners therefore often use more than one brand of POG to accomplish this.

Rynex is More Effective

As one can see, the Kb concept has a limited significance and primarily to substances that closely resemble hydrocarbon solvents. Even then, it only relates to a specific kauri resin or materials that closely resembles it chemically. There are a number of other solvent rating systems that are used by different industries they include “aniline cloud point”, “wax number”, “heptanes number”, and “aromatic character”. The most comprehensive is the “Hildebrand solubility parameter”. This system assigns values to solvents and the substances, solutes, that one wishes to dissolve. The parameter incorporates chemical and physical properties of these substances. By matching the value of the solvent with the solute one can easily find an appropriate solvent for a given material. Some of the newer solvents such as Rynex and carbon dioxide cannot be properly classified under the Kb system. They are more meaningfully categorized using the Hildebrand solubility parameter.

Rynex, a glycol ether, for example is more polar than hydrocarbon and less polar than water. It is more effective in removing water-soluble soil than perc or hydrocarbon solvent but still effective in removing oils. It not only safely cleans the majority of garments including sequins and beads, but also removes many stains such as shoe polish and lipstick that are not removed by the other solvents. Since Rynex also is non-toxic, biodegradable, and high flash, it is a good, safe and effective dry cleaning solvents.

In summary, dry cleaning is a very complicated operation. It deals with many fabrics and dyes and an endless variety of soils that have to be removed without damaging the garment. At times it is helpful to use devices such as Kb and solubility parameters to pick suitable dry cleaning solvents however, ultimately the only way to evaluate a solvent is to try it.

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