Ultrasonic cleaning replaces chlorinated solvents.

The virtual elimination of the use of CFCs in the rubber molding industry is now possible. Aqueous ultrasonic cleaning is a mature, proven and readily available technology. Successfully used by many industrial sectors for almost a half century, water based ultrasonic cleaning has a reputation for being reliable, safe, efficient and inexpensive. Concern over the environment and the medically harmful consequences of solvent use has focused worldwide attention toward the development of viable non-polluting alternatives. In addition, rising costs to obtain, use and dispose of solvents has made alternative cleaning techniques a competitive necessity. The cost of using chlorinated solvents has increased dramatically over the past five years, and will get worse in the future. This article will address the use of ultrasonic cleaning within the rubber molding industry, and how this use can reduce or eliminate the use of chlorinated solvents.

Ultrasonic energy as a cleaning system

Ultrasonic cleaning occurs when sound waves are focused through a cleaning solution, creating an effect called cavitation. Cavitation is the rapid formation and collapse of countless tiny cavities (or bubbles), produced by alternating high and low pressure waves within the detergent solution. During the low pressure phase, these bubbles grow from microscopic size until, during the high pressure phase, they are compressed and implode. This powerful energy is localized at the microscopic level, making the cleaning process safe for parts.

Historically, ultrasonic cleaning has been recognized as being particularly suitable in difficult, complex and critical cleaning applications. The 8th ASM Metals Handbook identified six typical application areas: removal of tightly adhering or embedded particles from solid surfaces; removal of fine particles from powder metallurgy parts; cleaning of small precision parts; cleaning parts made of precious metals; precision cleaning of complexly configured parts; and parts cleaning for hermetically sealed units. This identification predated the financial, legal and environmental concerns incumbent upon solvent use.

Ultrasonic cleaning is inherently a reliable process. In the past five years there has not been a single industrial transducer (which produces the ultrasonic energy) failure. Electronic controls in the newest models make the ultrasonic cleaner extremely easy to set up and operate. Reliability in the cleaning process, therefore, can be easily maintained. Controlling the ultrasonic cleaning process

There are many considerations important to ultrasonic cleaning, each of which must be taken into account to properly control the process. If any one of them is incorrect, the process could be jeopardized by improper cleaning, damage to the parts being cleaned or damage to the cleaner itself. These considerations include the size and type of ultrasonic cleaner, length of cleaning cycle, solution temperature and solution chemistry.

The choice of cleaning unit is important. It must be physically large enough and generate power sufficient enough to clean the parts properly. The cleaning method, direct or indirect, must be addressed when choosing the size of the cleaner.

* Direct cleaning occurs when the parts are cleaned in a cleaning solution which fills the cleaner, usually inside a perforated tray or mesh basket. The limitation of direct cleaning is that a solution must be chosen that will not damage the ultrasonic cleaner.

* Indirect cleaning involves placing the parts to be cleaned in an inner nonperforated tray or beaker that often contains a solution that the user may not want directly filling the ultrasonic tank. Indirect cleaning requires that the cleaner's inside dimensions be sufficient for the parts tray(s) or beaker(s).

Solution reactivity with the materials being cleaned and the contaminating soils is directly related to solution temperature. A cleaning temperature below the optimal will either insufficiently (or not at all) clean, or increase the cleaning time with an associated decreased process throughput. A temperature higher than required may react adversely with the material being cleaned, create material handling difficulties and shorten solution life.

Cleaning time adjustment is the easiest (and most often misapplied) factor used to compensate for process variables. Although new application cycle duration can be approximated by an experienced operator, it usually must be validated by actual use with the chosen solution and the actual soiled parts.

Solution selection and concentration are the most critical process parameters. Modern ultrasonic cleaning solutions are compounded from a variety of detergents, wetting agents and other reactive components. Proper selection is crucial for acceptable cleaning activity and to preclude undesirable reactivity with the part being cleaned. Improper solution selection can cause extended or indeterminate cleaning cycles. The correct solution choice will prevent damage to both the parts being cleaned and the ultrasonic cleaner itself.

Testing for the rubber molding industry - first phase

To investigate the effectiveness of ultrasonics cleaning techniques for use in the rubber molding industry, discussions were initiated with Airex Rubber Products. During those discussions, Airex indicated that their existing methods (using chlorinated solvents) were costly, slow, and presented significant disposal problems after use. A variety of test samples, chosen to represent a worst case set of conditions, were selected by Airex. In the parts selected, contaminants included cutting oils, dirt, grease, particulates, release compounds and other unidentified soils. Some of the parts were further covered by a fine soot residue remaining from burning off rubber previously bonded to metallic parts.

Also included in the first set of samples was tooling which had not been cleaned for several years. This sample carried dirt, oil, baked-on rubber, a variety of mold release compounds and other soils which could not be identified or categorized. This item was included in the test samples to evaluate the effectiveness of the ultrasonic cleaning process on a part considered "uncleanable" by normal means.

The contaminants on all samples could not be effectively wiped off of the parts nor washed off with water. Particles of rubber adhered to areas where rubber had been burned off. Additionally, some of the test samples contained difficult to access areas, such as annular grooves, cavities and recesses.

The purpose of the test was to develop a controlled cleaning system that would clean metallic and rubber parts without causing damage, injury or dimensional changes to the materials being cleaned. It also had to be aggressive enough to consistently remove the most difficult soils and contaminants. This would require that reactivity of various metals and compounds be taken into consideration. Another concern was that the medical and food processing industries toxicity and non-contamination requirements, as defined by FDA GMP, would not be compromised.

Test 1

The first test was conducted to determine if ultrasonic cleaning could acceptably clean salvage parts. The test samples to be cleaned - stainless steel in a variety of configurations - were inner and outer rings between which rubber had been previously bonded. This rubber had been burned off in an oven, leaving the parts covered with a black soot and adherent rubber particulates.

The solvent cleaning method

The usual method to prepare these parts for remolding was to first vapor degrease the parts. After initial rubber removal, the parts would be placed in an oven in order to burn off the remaining rubber. This would be followed by an overnight immersion in 1,1,1,trichloroethane (to remove the soot and rubber residual), followed by an extended drying cycle.

The parts would then be glass bead peened to roughen the bonding surface and vapor degreased to remove the glass residue. This process had certain disadvantages:

* Time - Successful process completion took up to 12 hours. If not properly performed, the process would leave residual solvents on the parts necessitating additional processing, because any such residue would cause bond failure.

* Hazards - Chlorinated hydrocarbon vapors could be released into the atmosphere if cleaning procedures were not closely followed. This could cause hazards to personnel who might be overcome by fumes while cleaning the tanks, or exposed to vapor from the parts. Absorption of the solvent through skin contact or accidental ingestion must also be guarded against.

Costs - Airex estimates that a 55 gallon drum of 1,1,1trichloroethane will cost approximately $540. The direct disposal cost of this material, after use, is an additional $270. These figures do not include the cost of administration, internal controls, records, penalties, fines or other costs which accrue due to the use of hazardous and/or toxic materials in the workplace.

The aqueous ultrasonic cleaning method

It was decided that the time period for ultrasonic cleaning would be empirically determined by cleaning for a series of short time cycles, with evaluation between the cycles. Separate time cycles had no effect on the quality of the cleaning, as they were cumulative in effect on the samples being cleaned. The temperature of a mildly acidic solution was set at 50[degrees]C. After the first two-minute cycle, it was determined that additional cleaning was necessary. The parts were returned to the solution and the parts were cleaned for an additional minute. At the end of the aggregate three minutes, there was no residue observed on the units. After rinsing, the parts were free of wetting action and, with only a few residual drops of water left, they were hand wiped. During this wiping process, no staining of the cloth was observed, indicating that the soils and residues had been completely removed by the cleaning process. The parts were immediately ready for remolding. Soils removed from the parts were captured in the cleaning unit, and could be easily scooped or filtered from the solution to allow the reuse of the solution. While there is some depletion of the solution with use, it can be refreshed by careful addition of solution stock. The cleaned parts were returned to Airex for rubber bonding and test.

According to Airex, the parts are tested for bond integrity after bonding rubber to the inner and outer rings. The test, which is qualitative in scope, is performed by securely restraining the outer ring. The test arbor is then fastened to the inner ring, such that the ring is securely held to the arbor, and is forced to move with the arbor motion. The arbor is then cycled in a reciprocal motion, flexing the rubber bonded between the rings. This motion exceeds the real life deflection expected in the field. The test is conducted for a period of 24 hours, within a hot water environment. The criteria for success of the test is that the rubber bond between the rings does not show separation after completion of the test period. This is determined by magnified visual inspection. Observed debonding is cause for rejection of the unit.

The test pieces showed no evidence of bond failure in any area. No manual touchup was required after bonding, and the process output, with zero observable failures, was superior to the historical bond failure rate of up to 5% with new parts.

Test 2

The second test involved the cleaning of a group of flat stainless steel plates, from which the molded rubber had been burned off. These parts were flat mounting plates for shock mounts, and had been salvaged from worn out units. The parts had been vapor degreased upon receipt at Airex, and the bulk of the rubber cut off. They were then placed in an oven for eight hours at 800[degrees]C. The parts were covered with heavy soot and particulates from the process when received at Master Sonics for test cleaning. These soils were not removable by hand methods.

The solvent cleaning method

The typical procedure employed to clean these parts after burning off the rubber would have been to soak them in solvent to remove the residual rubber and soot. The parts would then have been glass bead peened, followed by vapor degreasing to remove the glass residue. Finally, the parts would have been painted with adhesive, bonded to rubber, and again vapor degreased.

The aqueous ultrasonic cleaning method

Cleaning was performed at 55[degrees]C with a mild acidic cleaning solution for 15 minutes. Cleaning action was observed immediately upon immersion, and continued vigorously for approximately 12 minutes, whereafter the actions slowed substantially.

After completion of the cycle, the parts were examined and found free of residues. In this case, the cleaning had also restored the original coloration of the metal, having removed the heat stains as well as soils. The parts, after water rinse and wiping, were ready for surface bead peening and molding. These parts, due to their non-critical application, only require visual testing for debonding and obvious separation. The cleaned parts, returned to Airex for evaluation and process completion, were successfully bonded with rubber and experienced no evidence of bond failure. As in the previously tested and cleaned parts, no manual touchup was required after bonding. Airex again reported that the process output, with zero observable failures, was superior to those normally experienced with virgin parts. Normal reject rates are up to 5% on these items.

Test 3 (figure 1)

The final test conducted at Master Sonics was an attempt to clean the excessively soiled tooling component described earlier. This part was a mold cover which had not been cleaned for several years. It was a flat plate approximately .500 inch thick and 12 inches in diameter. It had a series of three grooves in one surface, which formed a sealing surface on the part being molded. Rubber had adhered within the contour of the grooves which were deep "V" shaped, and narrow at the bottom. The plate had areas of staining and was totally covered on the molding surface with a dark viscous coating of mold release compound, dirt and rubber residue.

[ILLUSTRATION OMITTED]

The solvent cleaning method

Because of the extreme level of soiling, the more typical method of cleaning (soaking the part in solvent to soften the residue, and then hand brushing the surfaces to remove the soils) promised doubtful success.

The aqueous ultrasonic cleaning method

The varied and profound level of contamination exhibited by this unit was such that a highly aggressive cleaner was required. Based upon the known contaminating substances and the tooling material, it was decided to use a cleaning solution which had been developed principally as an oxide remover. This solution is mildly acidic, and specifically formulated to not attack mild steels. It was developed to remove light oxides, welding and brazing residues, fluxes and other high tenacity soils. It is especially effective where the item being cleaned has been subjected to high operating temperatures over extended periods.

The soils and contaminants present on this item, having aged for several years, were adherent to such an extent that an attempt to remove them using a wire brush or scraper failed, even employing an extremely vigorous effort. In addition, the unit had areas where excess rubber had bonded to surface imperfections becoming strongly adherent to the base metal.

The tooling sample was cleaned with a Master Sonics model 8210-DTH ultrasonic cleaner, using a cleaning solution temperature of 60[degrees]C. Cleaning action became apparent within 30 seconds of immersion in the cleaner, and rapid removal of the surface contaminants began before 60 seconds elapsed. This was indicated by the moving of soil flakes away from the part and into the solution. Oils and release compounds on the part could be observed being emulsified, and transported from the locus of the part. At the end of 20 minutes, all contaminants had been removed from the unit. Post cleaning processing was a simple tap water rinse and manual wipe of the few remaining water drops from the tooling surface.

Airex had presented this mold cover to Master Sonics to see if the ultrasonic process could prevail in a cleaning difficulty of such magnitude. They "never expected it would be possible to remove" the accumulated soils and contaminants, and they were "amazed that the process was able to return the unit to virtually new condition."

Second phase

During this phase of testing, a Master Sonics model 1210-MTH ultrasonic cleaner was installed at the Airex facility. The initial application involved the removal of residual soils and mold release compounds from production parts. A requirement of this application was that all contaminants be removed from the parts prior to customer delivery.

The solvent cleaning method

Prior to using ultrasonic cleaning technology, cleaning had been accomplished by an activated soak in solvent. While this would remove the soils and release compound, it required constant, diligent and expensive monitoring and supervision. The solvent usually used is flammable, toxic and dangerous, requiring meticulous process controls. Breathing the fumes can cause serious illness. If overheated, or subjected to flame, it can ignite explosively. It is toxic if ingested, and can cause skin problems.

The process had been to soak the units in solvent for an extended period of time under a fume hood, then remove the units from the bath and let them air dry. This had a negative side effect of softening the surface of the rubber and partially dissolving it, at times resulting in dimensional changes and process losses.

The aqueous ultrasonic cleaning method

A solution was chosen which had a neutral pH of 7.0 and met FDA standards. Experimentation with the time cycle in two-minute increments indicated that the optimum cycle time would be five minutes. The temperature was set at 60[degrees]C.

At the end of the cleaning cycle, it was once again established that a cold water rinse was sufficient to remove any residual solution from the parts. Since the parts had not been exposed to a solvent, there was no material dissolution of the surface. There was no need for extensive drying, and no dimensional changes as a result of the cleaning process.

Prior to using ultrasonic cleaning technology, shipments had been restricted by delays generated by the use of the solvent. Many days of process time have been eliminated through the use of ultrasonic cleaning in this application.

Conclusions

Aqueous ultrasonic cleaning is a low cost, safe, effective and efficient way to limit (and possibly eliminate) the use of chlorinated solvents within the rubber molding industry. In each of the test situations described in this discussion solvent use was eliminated.

Each test situation produced unexpected beneficial results. In the first test, not only was cleaning improved and salvage parts successfully reclaimed, but the inner and outer rings were cleaned in only three minutes, a significant reduction from a more typical twelve hour process. In the second test, not only were the mounting plates cleaned, but their original color was restored. In the third test, all contaminants were removed from an "uncleanable" mold cover, inside of 20 minutes, cleaning a profound level of contamination that would surpass virtually any cleaning problem likely to be seen in the normal course of business.

Achieving desired results is dependent upon meeting the requirements of a variety of application specific parameters. These parameters include the quality of the ultrasonic cleaning unit, solution temperature, use of the proper solution, length of cleaning time and the addition of a wetting agent or surfactant to the bath. If these parameters are not addressed satisfactorily, results will usually be mixed or minimal.

Creating a partnering relationship between the rubber molder and the ultrasonic cleaning expert is helpful to establish proper parameter adjustments and achieve positive application results. We were able to work hand in hand with Airex, achieving improvements for their cleaning applications, and reducing their need for the use of chlorinated solvents.

The most obvious applications for ultrasonic cleaning in the rubber molding industry include degreasing parts, cleaning parts after sandblasting, final cleaning after rubber molding and cleaning pure rubber parts before shipping.

Airex Rubber's current goal is to reduce their use of chlorinated solvents by half. With the use of ultrasonic cleaning, they are well on their way to completely eliminating the use of chlorinated solvents in their processes.

References

(1.)Handbook of Physics, Condon & Odishaw; McGraw-Hill Book Company, 1958; p. 3-118; 13. Technical Aspects of Sound Generation.

(2.)Metals Handbook 8th Edition Volume 2, ASM Handbook Committee, American Society for Metals, 1964.

(3.)Optimizing the Bransonic Ultrasonic Cleaning Unit; Branson Ultrasonics Corporation, 1990.

(4.)Understanding Physics, Vol. l; Isaac Asimov; Dorsett Press, 1988.

(5.)Wave Physics, Newton, R.E.I.; Hodder & Stoughton, 1990.

Acknowledgements

"Rubber degradation and stabilization" is based on a paper given at the April, 1994 Akron Rubber Group meeting.

"Thiophosphates - helpful in the design of nitrosamine-free accelerator systems" is based on a paper given at the October, 1993 Rubber Division meeting.

Table 1 - benefits of ultrasonic cleaning

Task

Remove burned-off rubber residue

Solvents

8 hr. solvent soak; 12 hr. dry

Ultrasonics

Ultrasonically clean for 10 min.; water rinse and wipe

Benefit

93% reduction in process time; no solvent use

Task

Remove mold release compound (parts and tools)

Solvents

2 hr. solvent soak; air dry and wipe; reinspect for growth

Ultrasonics

Ultrasonically clean for 5 min.; water rinse and wipe

Benefit

98.8% reduction in process time; no solvent use

Task

Clean molds

Solvents

Soak in solvent; hand brush to remove solvent

Ultrasonics

Ultrasonically clean for 20 min.; water rinse and wipe

Benefit

No solvent use

Task

Clean machine parts (maintainence)

Solvents

Soak in solvent; vapor degrease; wipe dry

Ultrasonics

Ultrasonically clean; water rinse; wipe dry

Benefit

No solvent use

Task

Removal of trim dust

Solvents

Clean out with Q-tip

Ultrasonics

Ultrasonically clean; water rinse; wipe dry

Benefit

97% reduction in process time

RELATED ARTICLE: Ultrasonic cleaning

Research conducted during World War II demonstrated that sound energy introduced into water at a controlled frequency and amplitude caused a reduction of total system pressure, resulting in a condition called cavitation - the formation of bubbles in a liquid. These bubbles in the solution form when the instantaneous pressure falls below the vapor pressure.

Cavitation occurs in one of two general modes. The first mode occurs in non-degassed solution (pseudo-cavitation), when dissolved gases are forced out of solution and form gas filled cavities.

The second mode occurs after gases have been removed from the solution ("degassed"). As ultrasonic energy is added, a process is triggered which causes ruptures in the solution, breaking the bonds between the solution molecules. A series of microscopic cavities are generated which are energy wells enclosing a virtual vacuum. This is accomplished by the use of a coupled transducer that is designed specifically for the frequency determined to be most effective in producing the desired energy transfer.

These cavities are not concentrated at any one level or position in the solution, but are statistically present throughout. Their concentration is a function of the strength, orientation and frequency of the energy field.

The sonic energy generates a series of high and low pressure waves in the solution. As the high pressure wave encounters the cavity boundary the cavity collapses, giving over its excess energy to the solution. This is then converted to heat and kinetic energy. Thus, the collapse of the cavity will generate high temperature gradients in microscopic portions of the solution, saturating the space of the solution with high energy regions. This energy is then available to assist the selected detergents in breaking the bonds between soils and the adherent parts.

Local motion of the solution is away from the surface of the parts being cleaned. Thus, loosened soils will tend to move along with the general motion of the solution. These soils are then emulsified and trapped so that redeposition is minimized. Highly visible removal of soils starts almost immediately after the ultrasonic cleaning action begins. If activity is not observed, the operator has rapid feedback that a problem exists. Simple adjustment of the cleaning solution and/or temperature will usually resolve the problem.