Oil and water do not mix (at least not readily); so how can manufacturers successfully remove oil from product using water? The key is in the formulation, in the secret blend of “herbs and spices;” or, for fully-formulated aqueous cleaning agents, in the blend of chemical additives, most of which are proprietary.
In Parts 6 and 7 of this series (1, 2), we discussed how the high surface tension of water impedes the ability of water to reach and remove oily contaminants from surfaces and how the addition of surfactants reduces that surface tension, and increases the wetting. Surfactants are not the only additives that allow water based cleaning agents to effectively remove oils and grease.
These chemical additives are often referred to as ‘builders.’ Builders is a general term to include chemicals that are added to cleaning agents to make them work better. Some builders have multiple functions. Here are a few of the myriad additions to cleaning formulations.
pH Adjusters
The acidity or alkalinity of the cleaning bath strongly affects the ability to clean. Alkaline (high pH) baths can dissolve most oils, essentially reacting with them, in a process called saponification, to produce a water soluble soap. This is why most aqueous cleaning agents are alkaline. Additional ingredients, including surfactants, help to carry the organic soils away from the surface.
While acids (low pH) are effective in pickling and passivation, these processes do not work if there is oil on the surface. Oils or organic materials must be removed before an acid process.
pH neutral (near 7) solutions are sometimes used because they are presumed to be safer for workers. However, in order to compensate for the reduced cleaning efficiency of a non-alkaline bath, other ingredients may be added. Higher temperatures may be required, and higher cleaning forces may be employed, for example, from spray nozzles. All these have the potential to be hazardous to both workers and product, so one cannot tacitly assume that neutral is necessarily safer.
Solvents
Organic solvents can found in aqueous formulations. Solvents that are water miscible, such as alcohols and glycol ethers, can also lower the surface tension and act in a similar manner as surfactants (3). In some applications, surfactant-free formulations are available (4). Organic solvents, also enhance the solvency of the agent for organic soils that are present.
Sequestrants
Sequestrants or chelating agents act to trap and separate metal ions from the solution in order to reduce the likelihood of creation of insoluble scum that can redeposit soils onto cleaned parts (5).
Oil splitting, solubilization, emulsification additives
Once the oil has been physically removed from the surface, how do you keep it from settling back onto the surface?
Solubilizers and emulsifiers keep the soil away from the part and in the bath. However, at high soil loading, there will be redeposition. If you plan to discard the bath fairly often, solubilization and emulsification may be great choices. However, bath life is limited; and disposal of spent baths can be costly.
Other formulations are oil splitting; you have a suspension during the cleaning process, but then the oil rises to the surface. Oil splitting chemistries are more of a manufacturing process than a “dishpan” type of operation. You can’t just lift the parts through the oil, or you will undo the cleaning process, so something to separate the oil (like a weir) is a must.
Defoamers
Many soaps and detergents trap air inside bubbles, creating foam, especially in processes using agitation or spray. Foams combine attributes of gasses (they are mostly air), liquids (they can flow) and solids (they can retain a shape—think of a meringue or whipped cream). Foam can spill over the wash tank and fill the room (we’ve seen it!), Foam can be hard to rinse; foam can increase carry-over of cleaning agent into rinse tanks. We don’t want too much foam.
Therefore, aqueous cleaning agents designed for agitation or spray applications contain additives, such as non-ionic surfactants, to act as a foam suppressant or defoamer.
Corrosion inhibitors
Sometimes an additive is not present to improve cleaning but to protect the surface. Corrosion inhibitors leave a thin coating or beneficial contaminant on metal surfaces in order to keep oxygen in the air from reaching the surface and causing oxidation or rusting. They are frequently added to the final rinse; and they may also be added to the cleaning agent.
Corrosion inhibitors deter oxidation; they do not prevent oxidation forever. Therefore, understand the role of the corrosion inhibitor in the product you use. Know, for instance, how long such protection lasts and how to further process the surface such as drying and packaging to protect against long term oxidation.
Builders you may NOT want or need
Sometimes chemicals are added to cleaning agents that may actually detract from cleaning effectiveness. Examples include dyes and fragrances. Color may be added to distinguish one aqueous cleaning agent from a competing product or to give the allusion of clean (blue) or ecologically superior (green). A fragrance may be added to mask an unpleasant chemical aroma and make the agent more acceptable to workers.
However, any additive might make the product harder to rinse and therefore compromise product quality. Odor maskants, might compromise worker safety, especially if the hazardous chemical being disguised has an objectionable odor.
Do you know what you have?
An aqueous cleaning formulation may contain dozens if not hundreds of builders. The Safety Data Sheet may or may not disclose all of them or their concentration. Chemicals that are present in small enough concentration that they do not pose a hazard risk to employees need not be listed. An additive that is safe for workers could still impair product performance. In addition, many builders are organic materials and are considered to be VOCs; this can be a problem in areas of poor air quality.
References
- Clean Source, June 2008
- Clean Source, April 2010
- B. Kanegsberg, “Cleaning Agents: Overview,” Handbook for Critical Cleaning: Cleaning Agents and Systems, Chapter 1, CRC Press, 2011.
- H. Wack, Handbook for Critical Cleaning: Applications, Processes and Controls, Chapter 20, CRC Press, 2011
- Clean Source, May 2013
