P2 Tech

Technical Issues and
Aqueous Cleaning Systems


dot PERFORMANCE OF AQUEOUS CLEANING SYSTEMS

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

dot LIMITATIONS/DISADVANTAGES OF AQUEOUS CLEANING

Return to the first page of the Aqueous Cleaning Technology Review


dot 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:

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.

dot 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.

dot 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)

dot 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.

dot 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.

dot 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.

dot LIMITATIONS/DISADVANTAGES OF AQUEOUS CLEANING

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

Continue on to the economic issues page of the Aqueous Cleaning Technology Review.
or
Return to the introduction of the Aqueous Cleaning Technology Review.

© 1999, Pacific Northwest Pollution Prevention Resource Center
phone: 206-352-2050, e-mail: office@pprc.org, web: www.pprc.org