Technical Issues and
Aqueous Cleaning Systems

PERFORMANCE OF AQUEOUS CLEANING SYSTEMS

KEY FACTORS LEADING TO SUCCESSFUL AQUEOUS CLEANING
Correct Cleaning Solution Composition
Obtaining Adequate Surface Impingement (Mechanical Force)
Temperature
Adequate Rinsing and Drying

LIMITATIONS/DISADVANTAGES OF AQUEOUS CLEANING

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PERFORMANCE OF AQUEOUS CLEANING SYSTEMS

Aqueous cleaners have been used for decades to meet cleaning needs in the manufacturing environment. More recently, aqueous cleaning processes have been shown in the published literature to be a technically feasible alternative to solvents that are ozone-depleting or have other undesirable characteristics. (Ref. 7-20) These studies have reported comparable or even superior cleaning results, and many have documented significant pollution prevented by using aqueous systems. Examples include:

• In a Du Pont case study, the company cited a 98 percent waste reduction while meeting all cleaning requirements. Du Pont converted from a solvent-based to a high-pressure, water-jet cleaning system for cleaning a polymer mixing tank used in a polymer production process. (Ref. 7)

• On a pilot test directly comparing automated aqueous rotary washing to perchloroethylene vapor degreasing of steel caplets at a metal finishing shop, Gavaskar reportedly found both systems met cleaning requirements on the basis of visual inspection with the aqueous system, preventing the solid hazardous waste and perchloroethylene air emissions from occurring. Instead, wastewater and oily liquid waste streams were generated, which are less hazardous and less expensive to handle. (Ref. 8)

• An automotive radiator manufacturer realized a 76.8 percent decrease in part reject rates and an 80 percent increase in production rates as a result of converting from a set of five 1,1,1 trichloroethane (TCA) vapor degreasers to a three-stage, conveyorized aqueous-cleaning system. The three stages were a tap-water wash, heated-detergent bath, and hot-water rinse. (Ref. 9) Decreases in reject rates were also seen by an automotive parts manufacturer that converted from trichloroethylene (TCE) vapor degreasers to several different aqueous-cleaning systems for the different parts made at the plant. (Ref. 15)

• The Massachusetts Toxics Use Reduction Institute's (TURI) Surface Cleaning Laboratory performed laboratory tests that showed pressure-spray aqueous cleaning worked successfully to clean aluminum and carbon steel parts used to make cooking steamers. The aqueous solution was investigated as a replacement for a petroleum distillate solvent and a naphthalene-based solvent that had been used to replace the original TCA vapor degreaser. The petroleum and naphtha-based products had been unsatisfactory to the manufacturer for two reasons: because the parts were visibly less clean than parts cleaned with TCA, and the parts produced fumes when welded subsequent to cleaning. Therefore, the aqueous spray-wash system was successfully implemented by the manufacturer. (Ref. 17)

These examples support the conclusion that aqueous systems can, in many cases, replace solvent-based systems without sacrificing cleaning performance, and often have improved performance. Successful implementation of the aqueous-cleaning systems generally required a careful design effort, and, to some degree, increased the complexity of the cleaning process. The increase in complexity can be partially attributed to the need for process steps to evaporate or otherwise remove water (e.g. blowers) after cleaning that are not required when using a highly volatile solvent. In many cases, the equipment that applies the cleaning solution to the part is also more sophisticated (e.g. ultrasonic or high-pressure spray systems), or several stages are used to accomplished the required cleaning.

KEY FACTORS LEADING TO SUCCESSFUL AQUEOUS CLEANING

Published material describing successful tests or operations using aqueous-cleaning systems identified a variety of factors that are important for aqueous systems to operate effectively. These factors, when examined as a group, show that successful aqueous-cleaning systems for manufacturing are carefully designed, multi-staged processes that exceed their solvent counterparts in complexity. Three factors — cleaning solution composition, mechanical force, and temperature — were identified as the most important to cleaning efficiency itself. Two other factors — rinsing and drying — were identified in most cases as being critical to the protection of the part to be cleaned, and to the integration of the cleaning process with downstream manufacturing steps.

Correct Cleaning Solution Composition

For the purposes of this review, cleaning solution composition is assumed to include all the constituents that make up the final cleaning solution used, including acid or alkaline chemistries, surfactants (detergents), water conditioners, corrosion inhibitors, foam stabilizers, etc. The correct choice of cleaner solution was essential in the cases reviewed, and correct composition of the solution was very case-specific. For example, strong alkaline solutions are avoided when the part is made of aluminum due to the undesired chemical reactions that take place when they come in contact with each other. (Ref. 21)

Some research has examined the affect solution composition has on cleaning. In a laboratory experiment, Monroe found that cleaning efficiency was maximized at highest chemistry concentrations tested (concentrations of 0.005, 0.05, and 0.5 percent by volume were tested). The study found cleaning efficiency increased when the solution was either acidic or alkaline, as opposed to neutral. A small improvement was also observed when using deionized water, as opposed to tap water, as the base liquid for the cleaning solution. (Ref. 22) In a separate but similarly designed experiment, Monroe, et al. found that surfactant type and concentration had a significant effect on cleaning efficiency. The surfactant type that was most efficient in the experiment had the highest hydrophilic-lipophilic balance, the longest chain length, and the highest ethoxylation level of those tested. (Ref. 23)

In a majority of the research and case studies reviewed, a tap water or deionized water wash was not adequate to meet the cleaning needs of the process. Typically, tap water was used in a multi-staged cleaning operation as a pre-and/or post-clean stage with the central cleaning step containing the surfactants and other additives mentioned above.

Aqueous cleaner composition has changed (for the most part positively) over the last decade. Quitmeyer discussed a number of the changes that have happened in aqueous cleaning solutions, including the shift to using liquid cleaners, as opposed to powders, as the basis for a cleaning solution. This is due to the "reduced safety hazards associated with their use." She also discusses a trend toward the use of deionized water instead of tap water to minimize salt buildup on the surface of parts. Water salts can cause spotting or accelerate corrosion. Finally, aqueous cleaners are becoming even more environmentally benign as constituents such as phosphates are eliminated, and surfactants that are biodegradable are made available. (Ref. 1)

Obtaining Adequate Surface Impingement (Mechanical Force)

Solvent-based cleaning applications primarily rely on the chemical properties of the solvent, as opposed to mechanical action. Aqueous cleaning installations were found in the literature to rely heavily on the mechanical properties of the system, referred to as "impingement" by Rowney. (Ref. 10) Impingement is defined by Rowney as "the surface impact caused by directing a solution, under pressure, at the part to be cleaned." The mechanical action is actually used to force the contaminants off the part in conjunction with any solvating power of the cleaning solution. Adequate impingement was found to be a key factor in successful aqueous cleaning, both in research reports and industry case studies. This was especially true for cleaning of parts with difficult-to-access surfaces or otherwise complex geometries.

Rowney presented a case study of an automotive radiator manufacturer that made a conversion from a TCA vapor degreaser to an open-air spray, aqueous-cleaning system to remove residual oil and aluminum fines. The complex geometry of this part required that the cleaning system provide aggressive impingement to ensure the all the tubes and fins were adequately cleaned. The radiators were processed in the system on a special conveyorized belt that held them at an angle to promote solution flow through and drainage. The process was found to actually improve parts cleanliness when compared to the old system.

Impingement can be obtained through methods such as open-air sprays, submerged pressure sprays, bath or part agitation, and ultrasonics.

Spray-based aqueous cleaning was used in a number of the tests and case studies found in the literature. (Ref. 7-13,15,17,18) Spray-type systems appeared to be the most popular selection for manufacturing operations. Sprayers are found in a variety of configurations, and at both low-and high-pressure, depending on the system requirements. Conveyorized, automated-sprayer systems are more typical for large volumes of small parts, while chamber-type, batch power-washers are typical for larger parts. To obtain increased impingement on the part's surface, the cleaner is forced through the spray nozzle at a higher pressure.

Ultrasonic aqueous cleaning systems were used in several cases found in the literature. (Ref. 12, 19-22) Ultrasonic cleaning systems use high-frequency sound waves to force the formation and collapse of low-pressure bubbles. The bubble formation is called "cavitation." The bubbles provide additional mechanical cleaning action, and increase the ability of cleaning agents to reach all surfaces of a work-piece. A typical system comprises four stages: an ultrasonic cleaning tank containing a water-based detergent; two rinse tanks; and a drying stage. Usually, a tap-water rinse removes any residual contamination and drag-out from the first tank. The second rinse tank, containing de-ionized water, is designed to provide a high-quality, spot-free finish. Next, the product is dried, to prevent any corrosion that might occur. (Ref. 24) Ultrasonic cleaning is not typically economically viable for very large parts due to the electrical power requirements for running the transducers that generate soundwaves in an ultrasonic system. Thompson, et al. provide a report on the laboratory and pilot testing that went into the installation of a full-scale ultrasonic cleaning facility at Corpus Cristi Army Depot. (Ref. 21)

Aqueous cleaning systems using agitation (from sources other than ultrasonics) were discussed specifically in many references in the literature reviewed. (Ref. 8, 10, 12-15, 25) Agitation is accomplished by a number of means, including tumbling and mixing with an impeller. Moving the part or mixing the cleaner both result in added impingement and, therefore, improve cleaning effectiveness.

Temperature

Temperature was cited repeatedly as a parameter that has a significant effect on the effectiveness of aqueous cleaning. Generally, increasing the temperature above ambient levels increased the cleaning efficiency, as long as the temperature was not so hot that the part being cleaned would be damaged or the properties of the cleaning solution would be altered in a negative fashion. Temperature, along with surface impingement, was cited as the most common way to improve the cleaning process that did not involve changing the make-up of the cleaning solution.

Larky reported that pilot testing by Ford Motor Company for aqueous cleaning of aluminum heat exchangers demonstrated that the effectiveness of the cleaners is influenced by temperature, concentration, and time of contact. (Ref. 11) Laboratory experiments conducted at the TURI Surface Cleaning Laboratory to assist an adhesive products manufacturing in aqueous cleaning implementation found that temperature had to be increased to 141 degrees F to obtain the desired cleanliness level in an agitated soak bath. (Ref. 25)

In the previously mentioned controlled experiment that examined the affect variations in pH, temperature, chemistry concentration, and water quality had on aqueous-based ultrasonic cleaning efficiency, Monroe found that temperature was by far the largest contributor to changes in efficiency, contributing to "over 70 percent of the variation in the experiment." (Ref. 22) Interestingly, the effect was found to be non-linear in the experiment with 77 degrees F and 194 degrees F having higher efficiency than 122 F. In related experiments, Monroe, et al. found that maintaining the cleaning solution temperature just above the cloud point resulted in the highest cleaning efficiency (the cloud point for a particular surfactant is the temperature above which the surfactant becomes insoluble in the solution). (Ref. 23) This is interesting because conventional wisdom says that optimum efficiency is at a temperature just below the solution's cloud point. The reason for this discrepancy is not understood.

Adequate Rinsing and Drying

Rinsing after the cleaning stage is almost a standard in aqueous cleaning processes using cleaning solutions other than tap water in a manufacturing environment. Rinse baths or sprays prevent chemical residues from the cleaning step from contaminating downstream processes. Some processes use multi-staged, counter-current rinsing similar to that used for other primary manufacturing processes such as metal plating and anodizing.

Most full-scale systems discussed in the literature also included a drying stage. Rowney and Monroe both discussed the importance of the drying stage in aqueous cleaning systems. (Ref. 10,15) In traditional solvent-based cleaning applications, volatile cleaners such as Freon will evaporate very rapidly after cleaning. Water, with its higher boiling point, can take hours to evaporate completely at ambient temperature and pressure. Inadequate removal of water can lead to a number of problems including corrosion or spotting of metal parts, contamination of downstream process baths, or incomplete processing of a part in downstream manufacturing steps. Therefore, aqueous cleaning solutions and/or water rinses need to be removed, but manufacturing processes cannot allow parts to sit for hours while they dry at ambient conditions. This has led to the use of a number of different drying processes, including ovens, air blowers (often used in conjunction with air knives), and centrifuges.

LIMITATIONS/DISADVANTAGES OF AQUEOUS CLEANING

The following were identified during the review as some of the limitations and/or disadvantages of aqueous cleaning:

• Aqueous cleaning generates a waste water stream that requires treatment and discharge. In addition, many currently available aqueous cleaners contain non- biodegradable components that make discharge to sewer systems or surface waters more problematic. Some cleaners that are biodegradable when new are not once they have been used, leading to mistakes by users in how they think they can treat or handle the used cleaning solutions.

• Aqueous-cleaning solutions are much more limited in their ability to remove contaminants by chemical action alone than their solvent counterparts. This leads to the requirements for mechanical action and increased temperature. In cases where the contaminants to be removed are especially thick or insoluble, large amounts of mechanical action can be required. For delicate parts contaminated with thick grease or other similar compounds, the usefulness of aqueous cleaning systems can be significantly limited.

• Spotting or corrosion of parts can be a problem with aqueous cleaning if adequate rinsing and drying are not included as part of the aqueous cleaning system. This requirement for additional process steps is a disadvantage when compared with highly volatile cleaners.

• While aqueous cleaning systems were shown repeatedly in the literature to equal or even exceed the performance of solvent systems, the initial effort required to correctly design, test, and install an aqueous-cleaning system is much greater due to the increased complexity of the aqueous-cleaning process and the increased reliance on equipment, as opposed to the cleaning solution. This limitation is very significant for manufacturers that are very short on time or capital, and can lead to a preference for "drop-in" replacements, where little time and effort is required even though the operational savings may not be as great. Manufacturers may opt for petroleum distillate or semi-aqueous cleaners in these cases.