Ultrasonic Cleaning with Aqueous Based Detergents

Chlorinated solvents have been used throughout industry as well as United States Army depots for a number of years for cleaning applications. Recently, the following solvents have been implicated for health and environmental problems.

Also, all of these solvents are considered Resource Conservation and Recovery Act (RCRA) wastes, which means stringent handling and disposal requirements. Because of their many regulations, alternatives are being sought for these solvents that are more environmentally and worker friendly.

Personnel at the Oak Ridge Y-12 Plant (operated by Lockheed Martin Energy Systems, Inc., for the U.S. Department of Energy) began evaluation of ultrasonic aqueous cleaning in the early 1980s as a replacement for vapor degreasing with chlorinated solvents. This evaluation included:

Also, many studies regarding alternative solvents for chlorinated solvents were conducted at Y-12. These efforts led to a 92% decrease in the purchase of chlorinated solvents at the Y-12 Plant from 1987 to 1992.

Because of the success of the Y-12 program, the U.S. Army Environmental Center (USAEC) sponsored a project to evaluate and demonstrate alternatives for chlorinated solvents used at the Army depots. Initially, assessments were conducted at several depots, which led to a feasibility study evaluating ultrasonic aqueous cleaning. The results of the initial assessment can be found in a report entitled "Feasibility Testing of Ultrasonic Cleaning for Army Depot Operations."

The success of the feasibility testing resulted in an ultrasonic unit being purchased and installed at the Corpus Christi Army Depot (CCAD). Evaluations were conducted at CCAD to determine optimum temperatures and operating times for four different detergents:

Each of these detergents had passed hydrogen embrittlement tests a requirement for use at CCAD. Many detergents do not pass this test that is used to determine the amount of hydrogen that may be absorbed in a part during cleaning and plating and to determine if the level is so high that embrittlement may occur.

Comparative tests also were conducted on site between the ultrasonic cleaner and a smaller ultrasonic unit, a methyl chloroform spray, and a mechanically agitated cleaner.

Ultrasonic cleaning utilizes high-frequency sound to cavitate a liquid medium. This cavitation forms small microbubbles that burst on the surface of the metal being cleaned providing mechanical action as well as the chemical cleaning action of the liquid medium. Ultrasonic cleaning has several factors that can influence the cavitational intensity of the ultrasonics and thus the cleaning effectiveness: (1) the frequency of the sound waves used to generate the cavitation, (2) the electrical energy driving the transducers, (3) the packing of the transducers that generate the sound, and (4) the liquid medium itself.

In order to cavitate a liquid medium, a frequency of 18 kHz is required. Ultrasonic cleaners are available in frequencies ranging from 18 to 100 kHz. Higher frequencies commonly are used for cleaning of jewelry and electronics. Equipment operating in the 20-40 kHz range often are found in the metal cleaning industry. The electrical energy is an important factor because the higher the energy, the greater the cavitational intensity. However, a maximum amount of energy can be transmitted to the liquid medium. Energy supplied above this amount is unnecessary and can have detrimental effects such as surface cavitation. Surface cavitation is the cavitation that takes place just at the surface of the transducer. This prevents the ultrasonic wave from radiating throughout the liquid. The packing (or areal density) of the transducers in the tank also affects the cleaning efficiency. A dense packing reduces nodes or dead zones in the bath (that occur from the sound waves canceling out each other) and increases cleaning efficiency. Studies conducted at Y-12 have shown that ultrasonic equipment that operates at 10 watts/in² and 20 kHz is very effective for cleaning large metal parts.

Coupling (or the transfer of energy) between the transducers and the liquid medium is critical. A quick test that can be conducted to determine if the intensity of the cavitation is adequate to provide aggressive cleaning action is the aluminum foil erosion test. To conduct this test, simply submerge a 0.001-in. thick piece of aluminum foil in the tank while it is operating for 30 sec. A good aggressive ultrasonic tank will cause holes to form in the foil. This test also is useful in determining where any dead zones may occur in the tank. To ensure that parts are not left in dead zones during cleaning, it is advisable to rotate the parts in the tank.

The liquid medium used in the ultrasonic cleaner also is very important. Fluids cavitate at different intensities depending on properties such as surface tension and density. Water has been found to offer excellent cavitational properties. Thus, aqueous-based systems are an excellent choice for use in ultrasonics. In choosing an aqueous-based detergent, factors such as compatibility with the materials being cleaned must be considered. For instance, high pH solutions are generally not compatible with aluminum. Aqueous-based detergents usually can be disposed of in the sanitary sewer. Selection of the detergent also should consider local sanitary sewer regulations if the detergent will be disposed of in this manner (e.g., the amount of oil, phosphates, or silicates may be regulated).

The temperature of the bath is another important variable that must be considered with ultrasonic cleaning. Higher temperatures help to melt waxes or oils for easier removal and also increase the solubilization of the oil by the detergent. However, the cloud point of the detergent also should be considered. At the cloud point, phase separation of the detergent occurs, and the detergent is not as effective. Temperature also has an effect upon the intensity of the cavitation of the liquid.

When using aqueous cleaning, several mechanisms such as dissolution, wetting, emulsification, deflocculation, saponification, and sequestration generally are employed.

Dissolution—Dissolution is a process whereby water-soluble salts are dissolved in the alkaline solution and then flushed away in the rinse.

Wetting—Surfactants are used to enable wetting to occur. They act by lowering the surface tension of the solution. When this low interfacial surface tension is achieved, oils or soils can be undercut and displaced.

Emulsification—Emulsification is the primary means of removing oils when using alkaline or aqueous cleaning. The oils are broken up into tiny droplets that are suspended in solution and are flushed away in the rinse. Detergents vary in their ability to emulsify oils and keep them is suspension. Detergents that are poor emulsifiers will allow the oil to float to the surface, where it then must be skimmed off.

Deflocculation—Deflocculation is used in removing solid soils that have aggregated. It works by breaking the attractive forces holding the particles together, thus breaking up the solids into small, fine particles that can be easily dispersed.

Saponification—Saponification is the breaking up of fats into water-soluble fatty acids and glycerine. An example of this process is the old-fashioned method of making lye soap. It called for mixing lard with lye and cooling the mixture to form soap. Saponification also is used in describing the neutralization of fatty acids and generally is the method employed when removing fluxes or other acidic compounds.

Sequestration—Sequestrants "capture" hard water ions such as calcium and magnesium or heavy metals and prevent them from forming insoluble soaps with fatty acids.

Prior to conducting the demonstration testing at CCAD, preliminary tests were conducted at the Y-12 Plant to determine if any obvious relationships existed between the operating parameters and effectiveness of the detergents. These tests consisted of (1) determining cloud points for each of the four detergents being tested, (2) determining cavitation properties as a function of temperature, and (3) conducting cleaning analyses of the detergents as a function of concentration and temperature.

Cloud Point—The cloud point should be considered when using a detergent since this is a sign of the detergent losing its effectiveness. Some detergents are low-temperature detergents (which means they will cloud at higher temperatures), while other detergents are formulated to be higher temperature detergents and may appear cloudy at room temperature but become clear as heated. The cloud points of the detergents in this study were determined by slowly heating a 10% solution of the detergent in demineralized water and noting the temperature when the solution became cloudy or hazy.

Cloud Points of Various Detergents
Detergents	Cloud Point (C°) Determined Experimentally
Brulin 815GD	Did not cloud up to boiling (100)
Daraclean 212	63-64
Daraclean 282	34-35
HurriSafe Hot Immersion Degreaser	79-80

Cavitation Properties—The cavitational intensity of each of the detergents as a function of temperature also was determined. Past studies have indicated that cavitation created by ultrasonic cleaning varies not only with detergent composition but also with the temperature of that composition. Cleaning is directly proportional to the cavitational intensity.

The Brulin 815GD gave the best overall cavitational results of these four detergents. The Brulin 815GD cavitated very well over the entire temperature range tested, showing little if any decrease in intensity. The remaining detergents performed similarly in cavitational intensity, exhibiting decent capability. beakers These detergents also showed very little loss in cavitational intensity as a function of temperature.

Cleaning Analysis—In order to test the effectiveness of the four detergents as a function of concentration and temperature, preliminary coupon studies were conducted at the Y-12 Plant. Stainless steel coupons initially were cleaned in order to establish a baseline level of cleanliness. The coupons then were coated with lubricant sampled from parts from CCAD that were used in the feasibility testing.

Generally, operating at ~55°C is recommended for ultrasonic aqueous cleaning. At this temperature, detergents generally cavitate well, and the temperature helps to solubilize oils more readily. None of the detergents appeared to have a relationship between cleanliness and concentration. The four detergents selected for testing appeared to operate well within the 5 to 15% concentration range.

Testing of Ultrasonic Cleaner at CCAD

tank An ultrasonic cleaning system consisting of an ultrasonic cleaning tank and an air-sparged rinse tank was purchased and installed for demonstration purposes at the main plating shop of CCAD.

The cleaning tank has a working volume of 4 ft. x 4 ft. x 4 ft. with 10 transducers that operate at a frequency of 20 kHz. This frequency was selected based upon test results obtained in testing conducted at the Y-12 Plant. Lower frequencies also afford greater cavitational intensity, which in turn yields better cleaning results. The rinse tank has the same working volume as the cleaning tank but is equipped with air spargers for agitation instead of ultrasonic transducers.

5% Brulin 815GD: Results of Cleaning Experiments in Ultrasonic Cleaner
Cleaning Temperature(°C)	5 Minutes	Time Cleaned 15 Minutes	30 Minutes
40	Spots of oil remaining	Some oil in threads on a few shafts	Appeared clean
60	Appeared clean	---	---
80	Appeared clean	Slight flash rusting; Appeared clean	Appeared clean

5% Daraclean 212: Results of Cleaning Experiments in Ultrasonic Cleaner
Cleaning Temperature(°C)	5 Minutes	Time Cleaned 15 Minutes	30 Minutes
30	Oil remaining in threaded areas of shafts	Shafts appeared clean	---
41	Shafts appeared clean	No difference detected from 5 minute clean	---
60	Oil present on sides of housing and deep in holes	Sides appeared clean but oil and sludge still present in holes	Oil and sludge still present in holes

5% Hurrisafe: Results of Cleaning Experiments in Ultrasonic Cleaner
Cleaning Temperature(°C)	5 Minutes	Time Cleaned 15 Minutes	30 Minutes
40	Slight ammount of oil on sides of part	Parts appeared clean on sides; still some grease in holes	Still some sludge left in holes
60	Sides appeared clean; sludge in holes	Sludge in holes	Sludge in holes
80	Oil remaining on sides of parts, sludge in holes, Oil floating on bath	Sides appeared clean; sludge in holes, flash rusting more prevalent	Oil and sludge still present in holes

5% Daraclean 282: Results of Cleaning Experiments in Ultrasonic Cleaner
Concentration (%)	Temperature(°C)	5 Minutes	Time Cleaned 15 Minutes	30 Minutes
5	30	Oil still present, oil floating on bath	Oil still present	Oil still present
5	50	Oil on contours and in threads. Oil floating on bath. Ultrasonic cleaner squeals.	Slight oil left in threads. Cleaner still squealing.	Oil remaining in some holes. Cleaner still squeals.
5	70	Oil remaining on sides. Oil floating on surface. Slight squealing.	Oil remaining on threads. Oil sheen on surface.	Parts so rusted that could not tell if oil or rust was coming off. Cleaner squealing.

As shown, of the four detergents tested, the best overall results were obtained using Brulin 815GD. The Brulin offered the best cavitational properties over a wide range of temperatures and cleaned parts in a shorter period of time.

Analytical Results

graph Several sample pieces were prepared at the Y-12 Plant and taken to CCAD, where they were cleaned and then returned for analysis using X-ray photoelectron spectroscopy or auger electron spectroscopy. Generally, three different surface areas were evaluated for each sample, and an average of the elemental makeup of those areas was taken. The results of these tests are shown to the left, with cleanliness measured as a function of atomic percent carbon present on the surface.

Comparative Testing

graph At CCAD, comparative testing was conducted on various types of cleaning equipment available. The equipment tested included the ultrasonic cleaning unit being demonstrated, a Ramco mechanically agitated cleaner, a small Blackstone ultrasonic cleaning unit (approximately 1 ft. x 1 ft x 1 ft), and a methyl chloroform spray (TCA).

As shown here, the demonstration unit procured for the Army yielded much lower amounts of atomic percent carbon, indicating a much cleaner surface than the other methods tested. Nonstatistical difference is evident between the Ramco cleaner, the Blackstone ultrasonic unit, and the TCA.

Additional Testing

In the past, methylene chloride was the choice for paint stripping of both epoxy and urethane paints. Even though methylene chloride is not regulated as an ozone-depleting substance, many concerns exist regarding its use—primarily that it is a suspect carcinogen—and stringent national emission standards were issued for it in December 1994. The U.S. Army depots formerly used methylene chloride for stripping chemical-agent-resistant coating (CARC) from parts and another epoxy-polyamide coating used in various applications. CARC consists of a waterborne epoxy primer and polyurethane topcoat. The depots have switched to a commercial product but are not satisfied with its overall performance.

Work is being conducted to determine alternative formulations or enhancements to the current formulation that may improve stripping ability. Ultrasonics as a means to enhance paint removal also is being investigated.

Conclusions

Both visual and analytical results indicated that the ultrasonic unit when used with the Brulin 815GD cleaned faster and better than the Ramco mechanically agitated tank, the Blackstone ultrasonic unit, and the spray. However, all the cleaning methods did yield "clean" parts. No statistical difference was noted in cleanliness levels obtained with the Ramco cleaner, the Blackstone cleaner, and the spray.

Several of the parts that were cleaned with ultrasonics were then plated. No problems were noted in plating these parts. The ultrasonics did appear to be an effective means of cleaning for the plating shop. Cleaning of heavily greased engine parts was not accomplished completely by using ultrasonics. Areas where the grease was extremely heavy did not clean well. The thick grease probably attenuates the sound, causing it to deaden and decreasing its effectiveness. However, if high-pressure spray or wiping could be used to remove thick areas of grease, the ultrasonics would work well in cleaning the remaining grease. Ultrasonic cleaning proved itself to be a viable cleaning method for depot-type operations.