Cleaning Times: What to Look For in a 'Perfect' Cleaning Solvent
by John Durkee, jdurkee@precisioncleaning.com

Table I: Physical Properties Most Wanted

Table II: Recovering Solvents

Table III: Safety, Health, and Environmental (SHE) Issues

Recently, I had an interesting consulting assignment. A manufacturer of chemicals, including solvents, had a panel to participate in a brainstorming session. The client wanted to know about which new products they should consider for development, and what approaches they should take to do that.
One of the topics was the "perfect solvent." In that context, the client wanted to know about the shortcomings of today's cleaning solvents, and what could be done to overcome them. That's the topic for this column. We'll also investigate the chances of getting just that. The art of the possible—we have to deal with that.

WISHING AND HOPING
A generation ago, our wish list would have just contained physical properties, more versatile chemical solvency, and a lower price.

Since the 1980s, however, safety, health, and environmental (SHE) aspects have become as important (or more so) than chemical solvency.
In a relatively recent development, prices have not risen in concert with gasoline prices because of the production capability of solvent manufacturers offshore.

LET'S GET PHYSICAL
The physical properties most people have always wanted in cleaning solvents are shown in the Table I.

ECONOMICS
A major drawback for users considering solvents as cleaning agents is the purchase price. It's hard to justify the working capital to fill a 15-gal. sump with solvent with a specific gravity of 1.50 that costs $15/lb. That's around $3,000.
Obviously, the capability to recycle solvent is essential (Table II).

Figure 1: CFC-113

Figure 2: HFC-4310 mee

Figure 3: HCFC 225 ca/cb

Figure 4: HFE-700

Figure 5: OS-10

Figure 6: Possible future cleaning solvent

Figure 7: Acetone

Figure 8: Trichloroethylene

Figure 9: N-propyl bromide

Figure 10: Limonene

SHE WINS
By far, the dominant item on the wish list of users is SHE issues (Table III).
This wish list is a stout order, but one which users value. This wish list was fulfilled by CFC-113 (Figure 1) prior to phaseout (in the late 1980s and 1990s) of its manufacture in the U.S. and developed countries because of justified concern about its depletion of the ozone layer.

Today, no solvent fulfills this wish list.

The "so-called" designer solvents [HFC-4310mee (Figure 2), HCFC 225 ca/cb (Figure 3), HFE-7100 (Figure 4), OS-10 (Figure 5), etc.] meet most requirements, except for being around five times more expensive.

DON'T FORGET PERFORMANCE
Please note the reason users choose solvent cleaning hasn't been mentioned: performance. This means repeatable solution of currently used soils. Per-
formance is totally soil-specific.

Here also, the "designer" solvents don't generally match up well with common soils. That's why they're seldom used in metal cleaning work.

BAD NEWS
There is no "perfect" solvent. That molecule hasn't been yet synthesized or identified.

IMAGES
The purpose of the molecular drawings illustrating this column is to show the differences among long-standing and recently commercialized cleaning solvents. Atomic composition and structure, which determine SHE and cleaning performance, are not similar.

IMAGE OF SUCCESS
One molecular composition and structure identified during this consulting assignment is shown in Figure 6. It is a common cleaning solvent used in metal cleaning (n-Methyl Pyrrolidone, nMP), modified to eliminate some of the drawbacks that keep it from being on the "perfect" cleaning solvent list.

LIMITATIONS AFFECT NEW SOLVENT
The main obstacle to obtaining this or any solvent considered "perfect" is how to produce multiple copies of the desired structures at a price that competes with other alternatives.

That's basically a function of three factors, which are:

• The volume expected to be sold in all markets at an assumed price structure. Acetone, Figure 7, is a commonly used feedstock for synthesis of other chemicals. Use of acetone as a cleaning solvent is minuscule compared to other uses. That means those doing cleaning work with acetone will be assured of a competitive price. That is what's wanted with any new solvent;
• difficulty expected of obtaining regulatory compliance with the issues listed as SHE. Chiefly, this is the cost and time to develop a credible body of toxicology information;
• if trichloroethylene, Figure 8, was a new metal cleaning solvent, expected cost and time to commercialization might inhibit suppliers from bringing it to market;
• if today's suppliers of n-propyl bromide, Figure 9, knew in 1995 that in 2005 the American Congress of Governmental Hygienists (ACGIH) might propose an exposure limit of 10 ppm, they might not have supported the toxicological tests upon which that decision might be based;
• and technical fit with their existing manufacturing processes. Limonene, Figure 10, is a good example of a common cleaning solvent that is readily available because manufacturing capacity for it exists.

So the hope for a "perfect" cleaning solvent rests on issues outside its performance as a cleaning solvent.

CHANCE OF HAVING A PERFECT SOLVENT

It's about zero.

Even if the three factors listed above and the physical property/economic/SHE items on our wish list could all be provided in some new solvent, there would probably have to be at least a dozen new "perfect" solvents.
There are at least that number of different soils found in metal cleaning applications.

OTHER SOURCES OF A PERFECT SOLVENT
Should those doing solvent cleaning technology abandon it because it is a "dead end?"

No—at least for those doing vapor degreasing.

There are at least 650 binary azeotropes that are not commercial products—because there hasn't been demand for them. That was my advice to one client—commercialize binary azeotropes as that path avoids the three factors above.

We'll cover that topic in a subsequent column.

John B. Durkee, II, PhD, PE, is a consultant in metal and critical cleaning. You can contact him at 830-238-7610; (e-mail) jdurkee@precisioncleaning.com; or (fax) 612-677-3170