"Precision Cleaning - The Magazine of Critical Cleaning Technology"
Parts Cleaning

Hybrid Chemistries: Mixed Solutions for Solvent Substitution - View in pdf

by: Joseph Lucas and Zeljko Halar
Pages: 25 - 28 ; May, 1994

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.

Application Emphasis

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

  • Typically does not lend itself to manual surface cleaning.
  • Parts with blind holes and small crevices may be difficult to clean and require expensive process optimization.
  • Less effective on non-polar soils than polar.
  • Potential for galvanic corrosion to occur during the process.
  • Some materials and processes are incompatible with water.
  • Higher energy consumption.
  • Rinsing difficulties--some surfactants and other components can be difficult to rinse.
  • High concentrations of organic coupling compounds sometimes contribute to organic emissions.
  • Process equipment tends to be large, expensive, and requires considerable space.

Disadvantages of Semi-Aqueous Cleaning

  • Typically does not lend itself to manual surface cleaning.
  • Flammability -- especially if sprayed.
  • Odors.
  • Some components, such as terpenes, can auto-oxidize in the presence of heat, water, and air to increase non-volatile residue problems.
  • Surfactants are sometimes difficult to rinse.
  • The chemistry of maintaining proper emulsion characteristics during the process can be difficult.
  • Higher organic concentration can lead to higher organic emissions.

Disadvantages of Straight Hydrocarbon Cleaning

  • Flammability problems.
  • High vapor pressure, leading to high VOC emissions.
  • The need for low non-volatile residue tends to require the use of lower flash point, lighter fractions that evaporate rapidly, and contribute massively to organic emissions.
  • Low-volatility hydrocarbons typically leave objectionable residue.
  • Typically not effective on all soils.
  • Contaminate build-up in the cleaning tank can quickly cause a failure of the follow-on process.

Hybrid Chemistries

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.

Plotting Formulas

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.

Collective Forces

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.

Effectiveness Study

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.

{short description of image} 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.

Table 2
Trimsol Contamination Residues on Glass slide Surfaces after Cleaning
Cleaner Result (%)
EP-92 10.5
1,1,1-TCA 0.5
Freon TE 1.0
Freon 113 2.0
MEK 3.0

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.

Strategic Assessment

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

  • Can be designed to be nearly or essentially non-volatile.
  • Even though they may be non-volatile, they can be designed to be extremely free-rinsing.
  • Can have extremely high flash points.
  • Can contain design components that exhibit widely varied polarity, widely varied dispersion forces, and widely varied hydrogen bonding characteristics, enabling one cleaning agent to be effective on a wide variety of contaminants.
  • Exhibit a high capacity for contaminant loading while maintaining specified cleaning requirements.
  • Capable of delivering the exquisitely clean surfaces mandated by the most demanding of precision cleaning requirements.

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.

  • What is the solvent being substituted? Why is it being used for this process? This defines the chemical and physical characteristics of the material presently in use. The follow-up question helps in determining solvent characteristics important to this particular cleaning process.
  • What is the motivation for this substitution? A likely answer would cite regulations discouraging use of this chemical. We want to avoid "jumping out of the frying pan and into the fire." The answer also helps in determining the goals of the substitute chemical.
  • What process is the solvent being used in? A chemical's ability to clean varies, depending on the method of application; manual wipe, vapor degreasing, ultrasonics, etc. In comparative testing of potential alternatives, the method of application helps to further define the characteristics necessary to do the job.
  • What is the substrate? It is important to know what materials are in contact with the chemical to verify that they will not be adversely affected by the alternative solvent.
  • What are the contaminants being removed? What is their origin? The characteristics of the contaminate influence what materials might be used to remove it. The follow-up question addresses how the contaminate got on the substrate, and whether the contamination can be avoided.
  • What are the follow-up processes? Do they require this cleaning step? This help define the cleanliness requirements of the job. Does the follow-on process require the current level of cleaning?
  • How clean do you need the surface to be for the follow-on process? For comparative testing purposes, "clean" must be defined. "No residue" is often the answer, but even TCA and Freons leave residue on surfaces. Is there an analytical or an application test procedure to verify the cleaning efficiency of the alternative in question?
  • Are there any contra-indications? This helps to determine any known materials to avoid--sulfur with aluminum at high temperature, electrically conductive materials, etc.
  • Has anything else been tried as a substitute? What were the reasons for its non-acceptance? This information helps avoid repeated effort, saving time and money. The reasons behind the failure or non-acceptance of an alternative may be the most revealing part of this process.

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