Back to basics: A guide to alkaline parts cleaners.
This guide provides metal substrate manufacturers an overview of metal cleaners, the differences between them, and discusses what to look for in choosing an effective cleaner.
Cleaning is an essential phase in metalworking, fabricating, and finishing processes. The cleaning method can vary depending on the contaminant to be removed, the amount of material involved, the possible need for an automated process, and the overall effect on subsequent operations.
Alkaline cleaners are specifically formulated chemical blends consisting of alkaline salts, wetting agents, and sequestrant (chelating) agents. They owe their detergency, or cleaning ability, to the displacement of contaminants by surface-active materials and alkaline builders. These constituents remove the contaminants and allow them to be easily rinsed away.
The term "alkaline builders" covers a broad group of chemicals: caustic soda (NaOH), caustic potash (KOH), phosphates, silicates, and carbonates. This group supplies the alkalinity for the cleaner. High alkalinity products saponify fats and vegetable oils into soluble soaps. These alkaline salts also neutralize acidic contaminants and aid in dispersing oils.
Caustics (KOH, NaOH), the most common type of alkalinity builders, are highly alkaline (pH 12-14). They saponify fats and work with surfactants to disperse contaminants. This type of builder is not safe for use on soft metals like aluminum and zinc.
Silicates provide medium alkalinity (pH 11-12.5) and contribute to detergency. They inhibit attack on soft metals, but become insoluble at pH values less than 10.
Phosphates have slightly lower alkalinity values (pH 9.5-11.5) and provide more detergency than the other builders listed. They provide some protection for soft metals and tie up hard water ions, preventing them from interfering in the cleaning process.
Carbonates, mildly alkaline (pH 9.0-9.5), are mainly used to neutralize acidic contaminants. They also buffer solutions to maintain a specific pH range.
Wetting agents (surfactants and synthetic detergents) aid in removing contaminants by lowering the surface tension of the solution, allowing the cleaner to get under the contaminant and displace it from the metal surface. Once the contaminant is in the solution, the wetting agent creates an emulsion, preventing redeposition onto the part being cleaned. Surfactants have one end that is soluble in water (hydrophilic), and one end soluble in oil (hydrophobic). This di-functionality allows the surfactant molecule to create an oil-water emulsion that is easily rinsed away.
Soil is defined as matter out of place. Regardless of the type or category, all cleaners remove contaminants from a substrate by one or more of the following principles:
* solvent action enables the cleaner to dissolve oils present on the metal surface;
* saponification, a highly alkaline process, chemically converts drawing compounds (organic oils and fatty acids) into water-soluble soaps that can add to cleaning efficiency;
* detergency allows surfactants to reduce the interfacial tension between solution and contaminant, enabling cleaning solutions to better penetrate and displace contaminants from the metal surface;
* emulsification occurs when surfactants in the cleaning solution suspend contaminants in the aqueous phase for easy rinsing; and,
* deflocculation disperses contaminants into very fine particles which are suspended in the cleaning solution.
Materials to be removed are classified two general categories--oil and particulate matter. Oil, by definition, is a petroleum-based product. However, for discussion purposes, simple waxes, vegetable oils, or animal fats, which may have been applied to facilitate metal processing operations or for rust prevention, may be included. Particulate matter is finely divided contaminants present on the surface of the substrate to be cleaned.
Solvent wiping uses a terpene, petroleum, or chlorinated solvent applied to an absorbent material. Contaminant removal is achieved by a combination of solvent action and mechanical rubbing action. Since the rags and solvent quickly become dirty, they must be replaced frequently, making solvent wiping costly. Many of the solvents are flammable and may pose a health risk to the workers.
Vapor degreasing involves a chlorinated solvent or freon to remove oily contaminants. The part is placed in a chamber containing a di-phase system of vaporized and liquid solvent. The solvent condenses on the part, removing the oily contaminant. To remove inorganic and other contaminants, accessory equipment like spray or ultrasonic tanks are needed. A chief advantage of vapor degreasing is that an oil-free, dry part is achieved in one step. This method can be automated, but disposing of spent solvent can be expensive, and the process poses possible health hazards.
Emulsion cleaning employs an organic solvent suspended in water. The solvent-water combination is applied to the part. The solvency promotes very effective cleaning of the oils and, along with the action of the surfactants, other contaminants are removed. Since the major constituent is water, this is an economical cleaning method. Emulsion cleaning can be automated and, by adding alkalinity, contaminant removal can be improved. Since this method involves a solvent, it can present a fire hazard, and disposal can also be a problem.
Alkaline cleaning blends alkalinity sources, usually NaOH, KOH, silicates or phosphates, together with a balanced amount of surfactants to constitute a highly effective metal cleaner. The concentrate is dissolved in water and, in combination with mechanical action and temperature, generally removes most contaminants. This method is very cost effective to run, and disposal of the spent material is easy.
Acid cleaning involves the use of acids, both mineral and organic (sulfuric, phosphoric, or citric), in combination with surfactants. The acid water solution can remove contaminants from the metal surface as well as remove rust and scale. Acid cleaners used in pickling applications are examples of this type of cleaning.
Mechanical cleaning uses abrasive brushes, sandpaper, or some other media to dislodge the contaminant off the part's surface.
To select a cleaner it is important to consider which metal(s) will be processed to prevent attack of the substrate. This is particularly important with aluminum, zinc, and certain exotic metals. The cleaner must be formulated to remove the contaminants it will encounter. Simple light rust preventative oils and water-soluble coolants are easily removed with mildly alkaline cleaners at moderate temperatures. Waxes, heavy oil rust inhibitors, and other durable corrosion prevention compounds require a more aggressive product. Typically, a high alkaline product with a good oil solubilizing surfactant package is needed in conjunction with high temperatures.
The cleaner must also be suitable for the mechanics of the operating system. Immersion cleaners normally require different surfactant systems from spray cleaners. Also, the use of chelates is desirable to counteract the undesirable effects of hard water salts.
Concentrate form also needs to be considered. Liquids are easy to use and can be automated. Powders are usually added manually, but they are more cost effective for most operations.
An important part of the cleaning process is the rinse stage. If not immediately rinsed once the substrate leave the cleaner stage, contaminants such as emulsified soils can redeposit on the part and become difficult to remove. The rinse must remove the materials without interfering with subsequent operations.
Caustic and silicate, two major ingredients of cleaners, are poor rinsers. Phosphate and phosphate blends, on the other hand, are relatively easy to rinse, along with blends of caustic and silicate. Hot water can assist in rinsing, however care must be exercised so that drying does not occur just before the rinse tank. Double rinsing is common, using either deionized water for spot free parts, or adding corrosion inhibitor in the final rinse if rusting is a problem.
1. Single stage, clean only, no rinse
This method should contain inhibition if parts are ferrous steel. It should be skimmed frequently to remove floating oils, extending cleaner life and preventing redeposition.
An immersion tank should be used for low volume or batch work. An automated cabinet spray washer is more efficient for low volume batch work, while a conveyorized spray is more efficient for high volume, continuous production work.
2. Two stage, Clean/Rinse
With this method, frequent skimming to remove oils is needed in stage one. This extends cleaner life and prevents redeposition. The rinse should also be skimmed and changed frequently, and a rust inhibitor may be added as well.
An immersion tank may be used, an automated cabinet spray equipment with rinse cycle, a conveyorized spray, auger washer, or ultrasonic.
3. Three Stage, Clean, Rinse, Rinse
This system can produce the cleanest part. Stage one should be skimmed, while the stage two rinse should be kept clean by overflowing. Stage three can utilize deionized water for spot free parts and/or incorporate a water-soluble rust inhibitor if needed.
Equipment options include immersion tanks, automated cabinet spray equipment with three indexed cycles (wash, rinse, conditioned rinse), conveyorized spray, auger wash, and ultrasonic.
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|Comment:||Back to basics: A guide to alkaline parts cleaners.|
|Publication:||Tooling & Production|
|Date:||Dec 1, 1999|
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