The evolution of aqueous cleaner technology

The Evolution of Aqueous Cleaner Technology by JoAnn A. Quitmeyer, W.R. Grace and Co., Lexington, Mass.

W

ater has been used as a cleaner for centuries. The first water-soluble soaps were a blend of lye and animal fat. The chemical reaction of this mixture is a process defined as saponification. The addition of heat made the soap work better at removing the oils and greases of the day, which were also made from animal fat. As industry advanced and metal processing became more sophisticated, sugars were found to enhance the cleaning efficiency of these soap solutions. Carbonates were found to enhance the cleaning efficiency and bath life of these soaps. Phosphates were added to make the soap work better with hard or cold water. Until the last few decades, nitrites were commonly used as rust inhibitors for aqueous industrial solutions. When their potential to form cancer-causing nilrosamines (when mixed with amines) became known, chemical suppliers changed to alternate chemistries. By the mid-197Os, government regulations were starting to be felt at the job site. OSHA and material safety data sheets (MSDS) became common terms within the industrial arena. Chemicals started to be scrutinized for their effects on worker safety. Until the mid-1980s, aqueous cleaners were often dumped down the drain at the end of a cleaning cycle. Waste minimization was unheard of and filtration and recycling were reserved for only the most sophisticated applications. During the following decade, environmental issues came to the forefront. We started hearing terms like EPA, chlorofluorocarbons (CFCs), The Montreal Protocol, global warming, SARA Reportables, and air quality boards. Today, industry around the world is cognizant of the chemistry restrictions and process changes needed to meet new government regulations concerning health, safety, and the environment. Cleaners generally fall into three chemical categories: organic solvents, semiaqueous or emulsion cleaners, and aqueous-based products. With health, 34

safety, and the environment at the forefront, aqueous technology is the preferred chemical choice for many industrial cleaning applications. Water has been used to clean substrates for centuries; solvent and emulsion cleaners evolved during the 20th century. As we learn more about the health and environmental risks associated with these technologies, industrial manufacturers are again reverting to water-based systems for parts cleaning. New aqueous technology enables the user to obtain maximum cleaning efficiency with less chemistry. If less chemistry is used, less waste is generated. These new technology cleaners promote soil rejection, allowing the cleaner bath to be filtered and reused. The ingredients used to make these new technology cleaners are also selected for maximum worker safety and minimal environmental impact.

CHEMISTRY CHOICES Aqueous cleaning technology has evolved significantly over the past decade as waste minimization became an issue. Costs and liabilities associated with a particular chemistry are also factors to consider when selecting a cleaning process. Aqueous cleaners are acid, neutral, or alkaline. Acid products have a pH less than 6; they are used for removal of inorganic soils and to pickle or passivate a metallic surface. Neutral and alkaline cleaners have a pH ranging from 6 to greater than 13. These products are very effective on organic oils and greases. Additional ingredients are frequently added for increased effectiveness on inorganic soils as well. When defined as “the removal of soil or unwanted matter from a surface to which it clings,“’ cleaning can be accomplished by one or more of the following methods.2 Solubilization: The soil is dissolved in the cleaner bath. Emukificution: Mutually immiscible Q Copyright Elsevier Science Inc.

components are uniformly dispersed in the bath. Wetting: Surface and inter-facial tension are reduced with surface active agents, thus allowing the cleaner to penetrate the soil-substrate bond. Saponification: Free alkalis react with fatty acids to form water-soluble soaps. Sequestration: Also known as chelation, undesirable ions are deactivated and prevented from reacting with materials that would form insoluble products such as hard water soap scum. Displacement: Soil is displaced with mechanical force. Deflocculation: The soil is broken into fine particles and dispersed in the fluid. Organic soils tend to dissolve oily soils; semiaqueous emulsion cleaners clean by solubilization and/or emulsification. Aqueous cleaners may clean via all of the above methods, depending on the chemical composition of the cleaner and the soil. Aqueous alkaline and neutral pH cleaners are used wherever water can be tolerated. These water-based cleaners are generally divided into five major pH groups as follows: Caustic, pH > 12 High alkaline, pH l&13 Low alkaline, pH 8-10 Neutral, pH 6-8 Acid, pH < 6 Acid cleaners are generally not used for the removal of organic oily soils. Ingredients frequently contained in alkaline cleaners include alkalinity builders, water conditioners, surface active agents, inhibitors, fragrances and/or dyes, defoamers or foam stabilizers, and water. Occasionally, hydrocarbon solvents are also added to a formulation. Alkalinity Builders Alkalinity builders are selected based on the pH, detergency, inhibition, and/or cost limitations required or METAL FINISHING . SEPTEMBER 1995

TableI. Alkalinity Builders Component

Contribution

Negatives

Cost effective

Corrosive

High AIkaline Cleaners (pH 1O-13): Amines Carbonates Hydroxides Phosphates Silicates

Detergency inhibition Detergency, soil holding, low cost Cost effective Detergency, sequestration, inhibition Deteqency, inhibition

More costly Consumable Corrosive Environmental restrictions Residues, restricted use

Low Alkaline Cleaners (pH 8-10): Amines Borate Sulfates

Detergency, inhiiiion, sequestration Inhibition Filler, carrier

More costly Limited effect Restricted use

Caustic Cleaners (pp> 72): Hydroxides

desired for a specific formulation. Environmental or process restrictions must also be considered. These builders may include one or more of the items listed in Table I. Neutral pH cleaners contain little or no alkalinity builder(s) or the alkalinity reserve is neutralized with an organic or mineral acid.

Water Conditioners Sequestrants or chelators are frequently used to deactivate undesirable ions such as calcium, magnesium, or heavy metals. These ions or heavy metals are then no longer free to react with bath substances that would subsequently form undesirable compounds, such as hard water soap scum. Some of the more commonly used sequestrants include: EDTA: ethylenediamine tetraacetic acid NTA: nitrilotriacetic acid HEEDTA hydroxyethylenediamine triacetic acid STPP: sodium tripolyphosphate ATOP: amino tri(methylene) phosphoric acid HEDP: l-hydmxyethylidine-1 ,l diphosphonic acid Sodium gluconate Sodium glucoheptanate EDTA has maximum effectiveness in tying up calcium and magnesium, thereby softening the water used to dilute the cleaner bath. Sugars, such as sodium gluconate or glucoheptanate, have maximum effectiveness in tying up heavy metals. Experience shows the former has negligible effect on waste treatment processes, whereas the latter ingredients could potentially interfere 36

with chemical separation of heavy metals from an effluent.3 Surface Active Agents Surface active agents, also known as surfactants, are used to reduce the surface or interfacial tension of a water solution. Selection of the surfactant package used in cleaner formulations depends on the performance characteristics desired. Surface active ingredients frequently used in water-based cleaners include fatty acid soaps or organic surfactants. These surfactants are classified into four basic types: Anionic: negatively charged ions that migrate to the anode. Cationic: positively charged ions that migrate to the cathode. Nonionic: electronically neutral ions. Amphoteric: ions charged either negatively or positively, depending on the pH. Physical properties affected by surfactants include the cloud point, the foaming characteristics, and the detergency, emulsifying, or wetting mechanisms used to facilitate the cleaning process. Often a combination of ingredients is used to obtain the specific properties desired. Corrosion Inhibitors Corrosion inhibitors are also contained in some alkaline cleaners, depending on the application involved. If a wide variety of substrates is involved, a combination of inhibitors may be used. These inhibitors are water soluble and, therefore, are removed with a thorough rinse if desired. Inhibitors frequently added to aqueous cleaner formulations include, but

are not limited to, aldehydes, amines, benzoates, borates, carboxylates, molybdates, nitrites, thiols, triazoles, and urea. Phosphates and silicates could also be added to this list. The intended cleaner application dictates the type of inhibitor package selected. Additional Ingredients Aqueous cleaner formulations may also contain a broad spectrum of ingredients designed to affect the appearance, odor, or physical properties of the composition. These include dyes, fragrances, thickeners, defoamers, foam stabilizers, or fillers for cost reduction. Again, the intended cleaner application will dictate final composition of a formula. Hydrocarbon Solvents A variety of hydrocarbon solvents have been blended with surfactants to make emulsion or semiaqueous cleaners. Glycol ethers have been added to stabilize formulations or to increase the cleaning efficiency of a composition. Environmental regulations have identified these ingredients as volatile organic compounds (VOCs), which are regulated by air quality boards. In addition, certain glycol ethers, including 2-butoxyetbanol or “butyl cellosolve,” have been identified as health hazards. Because of their negative impact on health and the environment, many formulations of the 1990s are free of glyco1 ethers or hydrocarbon solvents. THEN VERSUS NOW Most alkaline cleaners formulated before the mid-1980s were high alkaline or caustic. Hydroxides and soaps were used to remove the more difficult soils such as carbon, grease, and heavy oils. High alkaline products contained generous amounts of phosphates, carbonates, and silicates. More often than not, these cleaners were sold in powder form; they then were mixed with water at the job site. Caking in the bottom of the drum or in the wash tank was a frequent complaint with these powdered products. Caustic powders also have dangerous exothermic reactions when mixed with water. Caustic soda and caustic potash were common ingredients years ago. Soda ash, trisodium phosphate (TSP), tetrapotassium pyrophosphate (TKPP), METAL

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TableII. pH of Commercial Chemicals PH Mateda/

1%

Causticsoda causticpotash sodaash(dense) Potassiumwbonate Trisodium phosphate (anhydrous)

12.40 12.45 11.05 11.15 11.75

Sodiumtripoiyphosphate TetrapoWium pyrophcsphate Sodium matasiliite (anhydrous) Sodium pentametasilicate Crihosiliite (anhydrous)

9.74 10.08 12.17 12.15 12.29

sodium tripolyphosphate, sodium metasilicate, sodium penta metasilicate, and orthosilicate were typically found in the high alkaline products. Table II shows the alkalinity contribution of each of these ingredients and the relative pH buffering effect each ingredient has when expressed as pH at 1 and 0.1% in deionized water. Increasing buffering, an indication of the amount of contaminant an ingredient can handle, is found with products having little difference between the pH at 1% and the pH at 0.1%. Today, many industrial manufacturers are choosing liquid cleaners based on the reduced safety hazards associated with their use. Less sophisticated formulations rely on water dilutions of the previously mentioned alkali salts. More sophisticated compositions use more water-soluble compounds to provide the alkalinity required. The use of liquid components offers increased product stability and subsequent rinsability of the bath. As the industrial cleaning application becomes more critical, more chemical restrictions are involved. Deionized water is frequently chosen over tap water in an effort to minimize the amount of salts that may be left on a part surface. Water salts, which include calcium and magnesium, can cause spotting of a polished surface and can accelerate corrosion in some instances. Silicates, especially high concentrations of metasilicates or orthosilicates, can cause insoluble residues or “white bloom” if allowed to dry on part surfaces. When using a heavily silicated product, parts should be kept wet until a thorough water rinse is incorporated. Silicates are not very stable in a water bath; therefore, a precipitate or scale is often seen in heavily silicated baths. 38

As a result, many manufacturers prefer nonsilicated products for the more critical cleaning lines. Lightly silicated products and those using the liquid forms of silicate offer increased bath stability and increased ease in the rinsing of parts. Phosphates provide excellent detergency and corrosion inhibition, while also acting as water softening agents for hard water sources. Unfortunately, these phosphates also are excellent food sources for algae. As a result, phosphates are restricted in many geographical areas due to their adverse effect on groundwaters. New technology alkaline cleaners are frequently formulated without phosphates as a result of their negative impact on the environment. Because these nonphosphated cleaners are not restricted geographically, they have more universal applicability. Due to their hard-water tolerance, excellent buffering capability, and good corrosion-inhibiting properties, organic amines are frequently used in the more recently formulated aqueous cleaning compounds. These organic amines have long been used by the personal care industry, and a significant amount of testing has been conducted on the effects these products have on worker safety and the environment. Although they are more expensive than the alkalies listed above, the safety, environmental, and performance advantages offered make them a favored ingredient in many of the newer technology alkaline cleaner formulations. One of the most significant areas of change in today’s self-cleaning aqueous cleaners is the selection of organic surfactant packages incorporated into aqueous cleaners. The surfactants used today can be selected for specific per-

formance properties desired. The newer surfactants being offered are biodegradable and can be chosen for their foaming characteristics, as well as their ability to wet out on a surface or emulsify specific soils. Frequently, a surfactant blend is used to accomplish the exact performance properties needed for a broad spectrum of applications. The newer surfactant molecules function at a greatly reduced concentration level, thereby affecting the desired cleaning task with less chemistry. As the alloying of metals and composites becomes more complex, there is a greater need for sophisticated inhibitor packages, which provide protection on a broad spectrum of substrates. The synergism of chemicals allows the formulator to obtain the inhibiting properties desired and may be limited only to the imagination of the formulator and the cost restrictions of the chemicals selected for this use.

THE CLEANING PROCESS Once the cleaning chemistry has been selected, cleaning efficiency is affected by four variables: temperature, concentration, time, and mechanical action. The choice of equipment used to facilitate the cleaning process greatly impacts the degree of cleanliness obtained with minimum chemistry. There are many equipment suppliers today. Virtually all have added features to enhance the longevity of the cleaning solution. These features include overflow troughs, skimmers, and filters used to remove contaminants. Many pieces of equipment also are designed to supply deionized or treated water and many also include some method to facilitate the drying of parts. The newer generation of aqueous cleaners is designed to reject contaminants rather than emulsify soils. This feature allows the cleaner to be filtered routinely without significant adverse effect on the cleaner chemistry. These newer formulations can be replenished with routine chemical additions of the cleaner concentrate, according to the maintenance procedures recommended by the chemical supplier. Extension of cleaner bath life obtained with regular bath maintenance results in reduced chemical consumpMETAL FINISHING

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tion, reduced waste generation, reduced waste liability, and reduced cleaning costs. Very often the newer cleaning processes also yield cleaner parts as well.

BENEFITS Many of the older alkaline cleaners have been “grandfathered” in on revised performance specifications. Specification revisions are being written to include all the formulation advantages offered by the new chemistry

cleaners. As we focus on product safety and environmental responsibility, more cleaning lines are using water. Strong alkalies are used only where needed; neutral pH products are becoming more available. Salt chemistry, including heavily phosphated and heavily silicated products, is being replaced by more efficient organic chemistry. Hydrocarbon solvents, whether CFCs or WCs, are being replaced with water wherever possible. Bath maintenance and recycling does not have to be costly and time

The Chemical Analysis of Electroplating byT.H. Irvine 182 pages $50.00

consuming. Proper selection of the chemistry and a good working relationship with the chemical supplier should result in a manageable program where parts are clean, the bath life is extended, and costs are reduced.

References Spring, S., Industrial Cleaning, Prism Press, Melbourne, Australia; 1974, p. 1 2. Cleaning Handbook, W.R. Grace Jz Co., 1.

Lexington, Mass.; 1993, p. 1 3. Foecke, T., Metal Finishing, 92(7):59;

MF

1994

Solutions

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