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Hybrid Chemistries: Mixed Solutions for Solvent Substitution - View in pdf
Solvents such as methyl ethyl ketone (MEK), 1,1,1-trichloroethane (TCA), CFC-113, methylene chloride, xylene, toluene, mineral spirits, trichloroethylene, or perchloroethylene have worked well for a wide variety of industrial uses for the last 50 years.
One could literally specify "vapor degrease with 1,1,1" without knowing how clean it made a surface, how effective it was at removing specific contaminants, or what the follow-up requirements might be, and yet still be fairly certain that welds would perform, paints would adhere, and adhesives would bond.
The pressure to eliminate these solvents has created a major push to discover and develop alternative chemistries and processes to accomplish the same surface preparation tasks without incurring the same regulatory pressure.
Unlike the traditional surface preparation formulas, alternate chemistries tend to be application specific. For example, MEK has been used as a prep prior to painting, sealing, bonding, and welding. It can also be used to remove adhesives, remove cured sealant, remove machinist's layout dye, and for cleaning painting equipment.
One low-VOC alternative may perform some of these functions, such as prepping prior to painting, bonding, sealing, or welding; however, it may not remove machinist's layout dye, part marks, cured sealant, or protective wing coatings.
It may be necessary to develop a different substitute that removes machinist's dye, cleans paint equipment, and removes some adhesives. A third substitute would be needed to remove cured sealant and adhesives, and yet a fourth to remove protective wing coatings.
Adding to this complexity, very little information exists regarding the cleanliness requirements for any of the typical industrial follow-on processes. This means there is not way to effectively compare cleanliness results achieved via a potential substitute against a known requirement.
This lack of good test data -- probably the primary impediment to the efficient design and adoption of low-VOC chemistries and processes -- continually raises the specter of the unknown. Will the weld hold as well, will the paint bond as well, will the circuit board last 20 years? Or will some unobserved or unknown difference in the process put the company's product and reputation in jeopardy?
No Drop-In Answer
Currently, when presenting an alternative, many chemical companies will market a replacement for a hazardous chemical, but not to a particular cleaning process. Rarely is there help in the adjustment of the process to succeed with the substitution. All that might be said is, "This is our new non-ozone-depleting substitute for TCA."
Often the chemical may present a whole range of other problems. We have seen re-labeled MEK and toluene blends as substitutes for ozone-depleting solvents. Those are highly volatile VOCs with their own health and environmental concerns. Perchloroethylene and methylene chloride have also been labeled as non-ozone-depleting. If TCA is a health concern, these two are definitely "jumping from the frying pan into the fire."
A drop-in replacement might be the dream, but the reality is usually something quite different: Almost all substitutions require some modification in procedure.
To date, most of the chemical design work in the low-VOC substitute arena has focused on aqueous, semi-aqueous, and hydrocarbon cleaning chemistries. Each of these approaches have several inherent disadvantages that have impeded their universal adoption.
Disadvantages of Aqueous Cleaning
Disadvantages of Semi-Aqueous Cleaning
Disadvantages of Straight Hydrocarbon Cleaning
As a result of the difficulty experienced in attempting to replace the ozone-depleting substances and highly volatile solvents with more benign chemistries, a new approach has been developed that, from preliminary test data, promises to solve many of the inherent problems associated with the more traditional approaches. This new approach, for lack of a better term, is being referred to as "hybrid chemistry."
This hybrid chemistry incorporates many of the advantages of aqueous, semi-aqueous, and hydrocarbon cleaning systems while eliminating or markedly reducing many of their inherent disadvantages.
Aggressive removal of a wide range of contaminants is possible with a hybrid cleaning system. The components considered for these formulations must first be of very low potential hazard. These materials might customarily be used in food, cosmetics, perfumes, personal hygiene, or other such products. They must also exhibit very low vapor pressure in order to minimize VOC loading. Many are made from natural renewable sources, such as essential oils.
The possible components possess a large variety of different physical and chemical characteristics. Depending on how they are processed, modified, and/or blended, they can offer and infinite variety of resulting physical and chemical properties.
Components with potentially large differences in inter-molecular forces can be combined to optimize contaminant removal. For non-particulate contaminants, their solubility in the cleaner can play a key role in the cleaner's effectiveness. Once solubilized, the contaminants can be removed.
A good starting point for this work stems from liquid phase thermodynamic theory. The solubility of materials can be interpreted by various solution theories to help in the formulating process. With the Hanson Solubility Parameters, the inter-molecular forces of a chemical can be divided into three components: dispersion (non-polar), polar, and hydrogen bonding.
A chemical can be represented as a point in a three-dimensional graph. If the points of the two materials are close enough to each other within this graph, they should be soluble. The rules for computing the resulting solubility parameters are well established.
One important function of hybrid systems is to expand the sphere of solubility so that completely different contaminants can be removed by one blend of materials.
On the molecular level, this means that components with the greatest affinity to the contaminant tend to dominate the surface interface between the liquid and the contaminant; and are also most likely to solubilize the contaminant. As a result, one blend might provide the characteristics necessary to remove several very different contaminants.
Hydrophobic contaminants are attracted by the high dispersion, low polarity, and low hydrogen bonding components of the system. Various "modified" hydrocarbons -- oxygenated, nitrogenated, but not halogenated -- are used to attack polar and hydrogen bonding contaminants. Special agents may be added to facilitate the rinsing and solubilizing of all materials, including the contaminant.
Another effect of these hybrid solutions is that the contaminants seem to become emulsified within the mixture. The resulting colloidal mixture appears to inhibit redepositing of contaminants. The mixture seems to accommodate larger amounts of contaminate loading and still continue to clean. This provides for less material usage while the chemistry's low volatility provides for less VOC loading.
An example of the precise cleanliness that some of the new hybrid chemistries are capable of delivering was developed in a series of test comparing film thickness after cleaning.
One hundred and forty-four samples of Oxygen Free High Conductivity Copper and 6061 Aluminum were machined using mineral oil as the cutting lubricant, then immediately set into a mineral oil bath for storage until they could be cleaned and tested.
Ten cleaning materials were tested: TCA, eight hydrocarbon solvents, and one hybrid solution called X-Caliber, consisting of hydrophobic and hydrophilic chemistries. The coupons were immersed, ultrasonically cleaned for five minutes, rinse cycled, then dried with clean air.
Ellipsometry measurements were taken of the cleaned surfaces, and thickness of the residual film after cleaning was then calculated. The measurements were taken at multiple locations on the coupons, averaged, and the film thickness of each test sample was compared.
The hybrid cleaning chemistry of X-Caliber clearly outperformed the TCA in this test for surface cleanliness with film thicknesses ranging from 1.5 to 2.4 nanometers versus film thicknesses of 6.5 to 7.8 nanometers for TCA's baseline performance.
Hybrid EP-921 was tested against TCA and CFC-113 for hydrocarbon and Trimsol (coolant fluid) removal. Table 1 lists the hydrocarbon contamination residues on aluminum after cleaning; Table 2 shows Trimsol contamination residues on glass slide surfaces.
Value in Versatility
Hybrid systems seem to compete favorably with traditional solvents, especially when faced with a variety of different contaminants.
Another example of the versatility of hybrid systems for replacing TCA takes a much broader approach. The cleaning effectiveness of 14 different contaminants was evaluated. These included not only non-polar contaminates like grease, mineral oil, WD-40, and fingerprints, but also polar materials such as three types of inks, Trimsol, and machinist's layout dye.
In addition to cleaning effectiveness, other parameters considered were corrosion, residual chlorides and acids, and cryogenic compatibility. Prior to any testing, the client's Safety & Health and Environmental Resource Management Departments screened all material to be used.
Bare aluminum and stainless steel coupons were soiled by 14 different contaminates. The cleaning process involved a wipe with a deionized (DI) water-dampened cloth followed by a dry wipe with a clean, dry cloth. The coupons were examined with fluorescent light under magnification.
For the purposes of this test, if any visible contamination remained, the coupon was rated "fail." The TCA failed to remove the machinist's dye, three inks, and the indelible pencil on steel. The hybrid EP-921 succeeded with the removal of all contaminants, and was ultimately recommended for TCA replacement.
When replacing a targeted chemical, particularly with hybrid chemistries, look beyond it. Step back and examine the entire process. Analyze the objectives of the substitution by knowing several facts: What is being removed? What material is it being removed from? How did it get there? How is the cleaner applied? What is clean enough? Plus, evaluate any other important process-specific information.
Hybrid chemicals have proven to be safe, effective, acceptable alternatives to hazardous varieties.
About the Author
Joseph Lucas, president of Inland Technology (Tacoma, Wash.), is a recognized authority on the source reduction of hazardous waste as well as solvent substitution. In 1990 he was one of 300 experts invited to present papers at the International Workshop on Solvent Substitution, sponsored by the Department of Defense and the U.S. Environmental Protection Agency.
Advantages of Hybrid Chemistries
Key Questions for Substitute Search
The level of frustration, risk, and confusion common with the adoption of a substitute cleaning chemistry can be minimized by developing a database of essential information for use as an outline guide.
An extensive diagnostic protocol aids in deriving the necessary data from the solvent usage process. Detailed answers to the following questions can help define the objectives of the surface cleaning replacement project.
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