Azeotropes – Critical cleaning with “Best Friend” Molecules

Some children hang out together constantly – they are inseparable. Many solvent blends for critical cleaning applications are marketed as “azeotropes” or “azeotrope-like”. In some ways azeotropes behave like childhood best friends – under certain conditions, the molecules seem inseparable. As a product manufacturer, it makes great business sense to understand azeotropes. There are at least three reasons. Well-designed azeotropic blends that are used correctly, provide very effective, consistent product cleaning. Well-designed azeotropes have a relatively favorable worker safety profile. You may also be aware that halogenated solvents are being reevaluated by the EPA and that some azeotropes, commonly marketed to manufacturers, contain a chemical that is being reviewed by the U.S. EPA.

Here’s what’s going on

Azeotropes are blends of two or occasionally more chemicals where the blend components boil at the same rate as their constituency in the blend. Therefore, the blend stays the same constituency. Azeotropes are beneficial for manufacturers doing vapor degreasing. In a vapor degreaser, boiling solvent vapors distill and condense on a part suspended in the vapor zone above the solvent bath. If the mixture is azeotropic, the liquid that condenses on the part, and then drips back into the bath, has the same consistency as the solvent in the bath. The process is consistent for many cleaning cycles. Examples of azeotropes used as vapor degreasing cleaning agents include mixtures of Isopropyl alcohol (IPA) with cyclohexane and so-called “designer solvent” blends of hydroflurocarbons (HFCs), hydroflueroethers (HFEs) or hydrofluoro-olefins (HFOs) with trans-1,2,-dichloroethylene (trans-DCE). Note that not all of these “designer solvent” blends are azeotropes; we discuss this further later on.

One reason parts and components manufacturers are increasingly moving to “designer solvent” azeotropes is because classic chlorinated and brominated solvents (perchloroethylene, trichloroethylene, methylene chloride, and n-propyl bromide) will be regulated by the U.S. EPA under TSCA reform (1). Some may no longer become available or may require costly equipment to use. Since the mid-1990s, there have been regulations for chlorinated solvents under the Federal NESHAP (National Emissions Standards for Hazardous Air Pollutants). TSCA Reform is almost certain to increase restrictions.

Manufacturers are looking for other options; but many azeotropic blends are relatively costly. Why are they so expensive? Molecules containing fluorine tend to be costly – hence the popularized term “designer solvent.” While IPA and cyclohexane are relatively inexpensive, both are flammable. To use flammable solvents, appropriately designed cleaning systems must be used. Well-designed systems for low flashpoint solvents can work reliably (we have set up such processes from time to time). However, there can also be extra costs for facilities modification, fire department concerns, and insurance issues.

On balance, azeotropes containing designer solvents may be a more accessible option. HFCs, HFEs, and HFOs have good wettability, evaporate efficiently, and leave low residue. They don’t have a flashpoint. Unfortunately, they are relatively ineffective cleaning agents – they don’t do much of a job of dissolving and removing most soils of concern to components manufacturers. Trans-DCE has good wettability, evaporates efficiently, leaves low residue. And, like many of the chlorinated and brominated solvents it has the solvency to actually get the part clean. Ok, trans-DCE does have a low flashpoint – you can’t have it all! Enter the HFC/HFE/HFO/trans-DCE azeotrope. You have a mixture that cleans consistently and effectively, that can often be used in the current degreaser, and that does not produce an annoying conflagration. It would seem to be a win/win situation.

Trans-DCE
Please be aware that trans-DCE is also under regulatory review as part of the U.S. EPA TSCA reform (2); the scope of the risk review was announced last year (3), and just this month asked suppliers of to provide more test data (4). Trans-DCE may remain a great option for blending, or, there could be restrictions and real problems for manufacturers. At this point, we simply don’t know what will happen.

Do not assume that the EPA will move slowly. Comparatively, the agency appears to be moving at lightning speed.

Do not assume that the talented folks who synthesize and formulate chemicals will come up with a magic answer. There are relatively few solvents that do not have a flashpoint. We have mainly just a few of the designer solvents. We know that trans-DCE blends relatively well with the designer solvents and can form azeotropes. We also know – from experience – that it is really difficult to make azeotropes without a flashpoint and with good solvency, if trans-DCE is removed from consideration. We know this from studies conducted with Dr. Darren Williams and the Cleaning Research Group at Sam Houston State University (5). We’re getting into the issue of thinking outside the box versus thinking outside the periodic table of the elements.

“Near azeotrope, azeotrope-like”
In a search for effective cleaning options, many products, currently marketed for use in vapor degreasers, are marketed as “azeotrope-like” or “near-azeotrope.” Many of these products are “designer solvent” blends with trans-DCE. The liquid and vapor constituencies may be close but not identical. Sometimes, extra trans-DCE, alcohol, or some other material is added to enhance solvency. For example, a blend with enhance levels of an alcohol, some providers recommend using the blend in a degreaser with two coils to avoid preferential loss of the alcohol.

We strongly recommend using true azeotropes for vapor degreasing applications. A blend either is azeotropic – or it isn’t. John Durkee used to refer to near-azeotropes as “close but no cigar.(6)” The “Cleaning Lady” considers saying a mixture is a “near azeotrope” as like saying someone is “a little pregnant.”

Limits
An azeotropic blend is not some sort of “super molecule.” The components may be “friends” and “hold hands” but they are not inseparable. There are limits to the ratios in any given azeotropic blend. That is, there is a finite range of temperatures over which the components of the blend boil at a rate proportional to their constituency. Outside of that range, one will boil off faster than others and the overall constituency will change over time. Also, in a cleaning application such as a vapor degreasing, soils that are removed from the parts accumulate in the solvent bath. Eventually, the build-up of soils in the bath potentially could alter the vapor pressures and destroy the azeotropic ratio.

A note about IPA
Why is the recommended COVID antiseptic 91% isopropyl alcohol (IPA) much more expensive than the commonly found purity of 72% IPA? One reason has to do with the fact that IPA forms an azeotrope with water, alcohol (IPA)/water (72%/28%); that ratio can be achieved by multiple distillations. Using typical distillation, you will not get a higher proportion of alcohol; so additional steps are needed to obtain higher purity. Achieving 91% IPA requires more effort, more specialized processes.

References

    1. US EPA, “Assessing and Managing Chemicals under TSCA,” https://www.epa.gov/assessing-and-managing-chemicals-under-tsca
    2. US EPA, “Risk Evaluation for Trans-1,2- Dichloroethylene,” https://www.epa.gov/assessing-and-managing-chemicals-under-tsca/risk-evaluation-trans-12-dichloroethylene
    3. US EPA, “Final Scope of the Risk Evaluation for trans-1,2-Dichloroethylene,” https://www.epa.gov/sites/production/files/2020-09/documents/casrn_156-60-5_trans-12-dichloroethylene_final_scope.pdf
    4. US EPA, “Order Under Section 4(a)(2) of the Toxic Substances Control Act,” https://www.epa.gov/sites/production/files/2021-01/documents/tsca_section_4a2_order_for_12-dichloroethane_on_ecotoxicity_and_occupational_exposure_0.pdf
    5. D. Williams, “Development of Azeotropic Blends to Replace TCE and nPB in Vapor Degreasing Operations”, December. 2016, https://www.shsu.edu/~chm_dlw/WP-2522-DRAFT-Final-Report.pdf
    6. J. Durkee, “Yesterday’s Solvents Tomorrow, Part II,” Materials Today, November 2004 https://www.materialstoday.com/metal-finishing/features/yesterdays-solvents-tomorrow-part-ii/
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