Degreasing Solvents, Old and New

Article From: Products Finishing, , , from Creative EnterpriZesM

Posted on: 7/1/1996

Cleaning chemistry 101: why solvents do what they do for us and to us...

There are two reasons for wanting to understand the chemistry of halogenated cleaning compounds. The first is that you and other users may be reconsidering the use of hazardous halogenated cleaning solvents because of unsatisfactory performance with aqueous and other cleaning systems. The second is that there are some new halogenated cleaning compounds that are safe to use. We're going to take a look at the chemistry and performance of both the old and new solvents. You'll be more comfortable with the old, and more open-minded toward the new after you read this!

CFC's Are Out?

Well, it's finally over! No, not the wrangling between President Clinton and the Congress, nor the acrimonious remarks of Princess Di and Prince Charles, nor even the last day of O.J. in court. The phase-out of CFCs and methyl chloroform (1,1,1 trichlo-roethane) is finally over.

Well...not exactly. Actually, there are two forces that will keep CFCs (and methyl chloroform) in use until around when O.J. finishes his court appearances. The forces are: (1) the EPA has exempted HCFC 141b from phase-out until the end of 1996 for those who are already using it; and (2) there will be legally imported methyl chloroform (and maybe CFC-113) available for some time, as predicted in Products Finishing.

By the time the banned solvents are truly no longer available, HFC 43-10 mee and HCFC-225 ca/cb will have been evaluated by industry, and HFE-A should be commercially available.

OK. Most people thought what they needed to know about cleaning chemistry was that MEK (methyl ethyl ketone) was different from MiBK (methyl isobutyl ketone), and that there was no such chemical as MRK (methyl Ruth ketone)!

But now we have all these numbers describing solvents we should consider. Just what do they mean? Are these descriptive numbers made up by vendors, like WD-40, 501, or KZ-7? Just what are these chemicals, and how can we tell them apart? When should we use any of them, and just what should we expect from their use?

Cleaning Chemistry

HCFCs are chemicals whose structures contain hydrogen, chlorine, fluorine and carbon. That's why they're referred to as HCFCs (for hydro-chlorofluorocarbons). Why is the one called HCFC-141b in disfavor?

Let's start with the numbering system. We know what an HCFC is. The numbers 141b refer to the composition of the chemical in terms of carbon, hydrogen, fluorine, and chlorine atoms. You need the following code to translate between those numbers and something you might want to know (its structure):

  • The number [1] on the right refers to number of fluorine atoms. So a value of 1 means 1 fluorine atom.
  • The second number [4] from the right is one more than the number of hydrogen atoms. So a value of 4 means 3 hydrogen atoms.
  • The third number [1] from the right refers to one less than the number of carbon atoms. So a value of 1 means 2 carbon atoms linked together, which is ethane.
  • Ethane has connections for 6 atoms, so we have 1 fluorine, 3 hydrogens; and by subtraction, 2 chlorines. So HCFC-141b means C2H3Cl2F. Its chemical name is dichlorofluorethane.

    What does the "b" mean? Well the "b" is necessary to distinguish between three different chemical structures that have the same formula, called isomers. There are different positions of the chlorine atoms:

    The naming code requires the most symmetrical isomer to be indicated by the number alone, without any lower-case letter following it. Here, symmetry means the sum of the atom weights attached to each of the two carbon atoms. Lower case letters higher in the alphabet are assigned to isomers being increasingly more asymmetrical. For example, HCFC-141b has three halogen atoms on one carbon and three hydrogen atoms on the other; while HCFC 141 a has the halogens split among the carbon atoms.

    • If the compound were unsaturated (ethylene vs. ethane), there would be a fourth number from the right which would indicate the number of double bonds. Ethylene has one double bond.
    • If bromine is present in place of part or all of the chlorine, the capital letter B follows the naming of the parent, on the right. And the number of bromine atoms is given by a number immediately to the right of the capital B.

    The reason you have heard of HCFC-141b is that it is a replacement for a very common cleaning agent: CFC-113. Incidentally, CFC is an abbreviation for chlorofluorocarbon. For CFC-113, we know that its formula must be C2H0F3Cl3. More conventionally, the formula would be given as C2F3Cl3. The chemical name of CFC-113 is trichloro-trifluoroethane.

    The reason chemical structure is important is that it tells us about how the chemical will perform. For example, chlorine atoms in molecules provide solvency, but increase concern about toxicity; fluorine atoms provide inertness and volatility, and add cost; and hydrogen atoms provide solvency and reduce inertness. Increased molecular weight increases boiling and flash points. Branched or isomeric molecules have lower boiling and flash points than their paraffinic relatives at the same molecular weight.

    Vaporized chemicals that are reactive with sunlight to form ozone in the earth's lower atmosphere are called VOCs (Volatile Organic Compounds). We see the by-products of reactions with VOCs as smog. Examples include MEK, IPA, toluene/ xylene, aliphatic hydrocarbons, and D-Limonene. The photochemical reactivity of other compounds can be low enough that these compounds are defined by the EPA as not being VOCs. Examples are ethane, acetone, and parachlorobenzotrifluoride.

    But some compounds are so inert that they survive and populate the earth's upper atmosphere. Recent scientific data have shown that chlorine atoms in these compounds can be liberated by reaction with high-intensity UV light from the sun. The chlorine atoms react with ozone. These chemicals, containing chlorine atoms, are called ozone-depleting chemicals (ODCs).

    Cleaning solvents perform because of their solvency and the temperature of use. Both can be determined by structure. In vapor-degreasing applications, temperature is the boiling point. Generally, cleaning action is better at higher temperatures.

    The reason 1,1,1 TCA is so hard to replace in vapor-degreasing applications is that it is a wonderful solvent (high Kb) and it is used at a higher boiling temperature. HCFC 141b is a better solvent than CFC-113, but in vapor degreasing applications, some of that advantage is lost because it is used at a lower temperature. Finally, the addition of one chlorine atom to HCFC 141b should improve its solvency, but that gain is more than offset in CFC-113 by the loss of three hydrogens atoms and the gain of two fluorine atoms.

    HCFC-141b, 1,1,1 TCA, and CFC-113 are all ODCs. From the structures above, HCFC-141b and 1,1,1 TCA are very similar. You might speculate that they have similar reactivity with ozone; and the EPA rates them as being so. U.S. manufacture of all three is banned by Section 604 of the 1990 Clean Air Act.

    In an important announcement in June, 1995, however, EPA modified the restriction for HCFC-141b. The current situation is as follows:

    • Manufacture of 1,1,1 TCA and CFC-113 are banned after January 1, 1996. This is unchanged.
    • Manufacture of HCFC-141b is banned after January 1, 2003. This is unchanged.
    • Applications, such as solvent cleaning, are banned at an earlier date. To be legally using HCFC-141b now, you had to be using it on April 18, 1994. All solvent cleaning applications were to have been banned on January 1, 1996.

    Many users and vendors of HCFC-141b, had petitioned the EPA to postpone the effective date of the ban. In June, EPA did!

    Existing users doing high-performance electronics cleaning and precision cleaning can continue to use HCFC-141b for an additional year: until January 1, 1997.

Existing users doing metal cleaning with HCFC-141b were required to stop using it on the original ban date: January 1, 1996.

These users are likely to be readers of Products Finishing. You can get information on replacement chemicals and processes from a variety of sources--including articles in a special series appearing in PF over the last two years.

OK, that's what an HCFC is. Apparently, the chemicals we just reviewed will be around only in the short-term. With what chemicals will users replace them? Let's use our knowledge to learn how other cleaning solvents differ from one another, and we'll be able to draw some conclusions about potential replacements. Another HCFC, 225 ca/cb, is being considered by some users:

There is a switch of chlorine and fluorine atoms between the first and third carbon atoms of the two isomers, "ca" and "cb." The product sold is an equal blend of both isomers. Its boiling point is 129F and its Kb value is 31.

HCFC-225ca/cb, relative to HCFC 141b, has significantly higher molecular weight, but significantly more fluorine atoms per carbon atom. The boiling point of HCFC-225ca/cb is quite low for a compound with a molecular weight of 201 because the high fluorine content raises evaporation rate. Solvency is somewhat poorer because of the reduced chlorine and hydrogen content relative to HCFC-141b. But this is more than offset, as cleaning in vapor degreasers is done at the boiling point, and that of HCFC-225ca/cb is 30F higher.

HCFC-225ca/cb is an ODC because the inertness conferred by the five fluorine atoms allows survival into the upper atmosphere, where the two chlorine atoms can react to decompose ozone. Because of its inertness and lesser chlorine content, however, HCFC-225ca/cb is not as harmful to the ozone layer as is HCFC-141b. Manufacture of HCFC-225ca/cb is not banned until 2020.

HFC 43-10 mee is used both as a cleaning and a drying agent. It is called an HFC (NOT an HCFC, because it contains no chlorine). From the designation 43-10, you should know that its formula is C5H2F10, and there are no unsaturated double bonds, because there is no fourth digit in the naming code. The dash is used to show that there are only three digits in the naming code. The reason for the isomer designation "mee" is beyond the scope of this work.

HFC 43-10 mee has no chlorine atoms which is why it is not an excellent cleaning solvent. But there are low concerns about toxicity, and it is not an ODC. HFC 43-10 mee can be used with light soils and particulates. Broad cleaning capability is obtained with azeotropes, and in cosolvent machines. This product is commercially available now.

Both HCFC-225ca/cb and HFC 43-10 mee are significantly more expensive than HCFC-141b because they are new products and have high fluorine content. Thus they may not be of interest to those doing general metal cleaning.

Certain compounds were developed recently to be both solvents and drying agents.

They are called hydrofluoroethers (HFE). The "A" and "B" designations by the manufacturer are purely arbitrary, and are not part of the chemical naming system. Since they have no chlorine atoms, they won't be great solvents and won't be ODCs. Solvency of HFEs will come from the ether (oxygen) linkage and the hydrogen atoms. Also, the oxygen linkage will probably allow these compounds to form azeotropes with other compounds, such as alcohols. In this way, the solvency can be increased to allow metal-cleaning in vapor degreasers. Because HFEs don't have chlorine atoms, concerns about toxicity should be reduced. HFEs are forecast to be available by mid-1996.

Another product is CH2BrCl (called BCM for bromochloromethane). Bromine is less stable than chlorine. It also raises more concerns about toxicity, and is more reactive with ozone. You are now experienced enough as cleaning chemists to predict that this molecule will have some solvency, but there will be serious concerns about toxicity and ozone-depletion. As of December 1995 EPA was evaluating the suitability of CH2BrCl for solvent-cleaning applications.

There is a product called parachloro-benzotrifluoride (PCBTF). For metal cleaning it has the best solvency of the products we have examined. This is because of the presence of the chlorine atom and the benzene structure (the six-carbon ring). But both of those features also enhance concerns about toxicity.

The three fluorine atoms enhance rate of evaporation, and PCBTF dries much faster than one would expect for its boiling point (282F). PCBTF is being used in cold-cleaning applications. The virtue of PCBTF is that since its photochemical reactivity is low, the EPA has exempted it as a VOC.

Structures: How They Affect Cleaning and the Environment

The four compounds: Trichloroethane, Trichloroethylene, Perchloroethylene, and Methylene Chloride; are the chlorinated solvents most commonly used for metal cleaning. We have learned that the high chlorine content per carbon atom guarantees excellent solvency. 1,1,1 trichloroethane (TCA or methyl chloroform) is stable enough that it survives to reach the earth's upper atmosphere and decomposes while consuming ozone. That's why its manufacture is banned after January 1, 1996. The other three compounds are somewhat less stable, and are not ODCs. They also are commonly used for metal cleaning. But because of the presence of the chlorine atoms, there is concern about toxicity.

We have examined the structure of various chemicals that are or could be used for metalcleaning. We can understand why their use or manufacture is banned; why they are or are not good solvents; why there is or is not concern about toxicity; and why they evaporate rapidly.

As to questions about how to use them, these are subjects for another article2,3 or book; or you may want to contact the authors.

 


2Durkee, J.B., "Changing from Chemicals to Processes," presented at the ACS National Meeting, Chicago, Illinois, August 24, 1995.

3Durkee, J.B., "New Process Developments in Replacement Cleaning Systems," presented at the 1995 CFC/Halons Conference, Washington, DC, October 25, 1995.


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