Solvent Versus Aqueous: Busting the Myths

As the regulatory pressure on several halogenated solvents continues to increase, many manufacturers are considering a conversion to aqueous cleaning because of its presumably “greener” profile compared to solvents. But is aqueous cleaning truly better, safer and more sustainable?

How ‘green’ is aqueous cleaning?

Often, aqueous cleaning is regarded as the environmentally friendly choice of cleaning methods because often it is not associated with chemicals. However, water alone doesn’t clean, which is why soap must be added when cleaning dirty dishes in the kitchen, for example. Water-based cleaning does rely on chemical additives such as detergents, surfactants, emulsifiers or buffers to facilitate and enhance its cleaning performance. The process can be further aided by heat, agitation and time.

Some manufacturers also believe that aqueous cleaners are biodegradable. Some are, but many are not. Because most aqueous agents contain surfactants, biocides, complexing agents, dyes, fragrances and so on, many are not biologically degradable and, depending on their concentration, may be harmful to fauna and flora in surface waters. Therefore, even if the aqueous cleaner is biodegradable, it still needs to be treated and disposed of properly if its contaminations are not biodegradable.

Fixating on the cleaning agent alone can easily lead to wrong assumptions and conclusions. When the technology and processes within which a cleaning medium is deployed are overlooked, a negative perception of solvents can be construed.

Water alone is not an effective cleaning agent. Chemicals must be added as well as proper processes. 

Every cleaning method has its own environmental impact that manifests in different ways. A holistic parts cleaning approach must consider different factors such as energy usage, water consumption, recyclability, waste treatment and disposal, as well as chemical impact on the environment.

Which method consumes more resources?

In the U.S., solvent cleaning is usually associated with traditional open-top degreasers. A less familiar but increasingly popular technology is the sealed, reduced pressure cleaning system often referred to as an airless/airtight degreaser.

The sealed construction enables the safe and sustainable use of solvents whereby solvent emissions are virtually eliminated. Operators have no direct contact with the solvent during operation. The working chamber only opens following vacuum drying, so there are no solvent residues left on parts, and no drag out losses. The use of a closed-loop solvent transport and delivery system can further facilitate a practically emission-free and spill-free handling of solvents.

The built-in distillation unit within such an enclosed system is instrumental in optimizing cleaning media consumption. It can effectively and consistently recover and purify used solvent, thereby significantly reducing solvent consumption and waste while lowering overall cleaning costs. Some applications do not necessitate a solvent bath exchange until after 10 years or longer.  

In contrast, many aqueous machines are not equipped with distillation units. Because dirt and soils are emulsified and rinsed off in aqueous cleaning, these baths might have to be replaced frequently if not being properly treated.

Also, aqueous cleaning systems are often open systems, so the evaporation losses must be monitored, measured and compensated on a regular basis to maintain a constant cleaner concentration.

Not only does this constant replacement of the cleaning agent add to ongoing operating costs but effluent treatment prior to disposal at a sewage plant also represents another cost factor.

In terms of energy requirements, solvent cleaning does not require heaters or blowers because parts readily come out dry due to the vacuum condition. While energy is required to keep the machine operation under vacuum, this also lowers the boiling points of the solvent in a vacuum, hence accelerating its evaporation and enabling quick drying of metal parts within a shorter cycle time.

In comparison, the energy consumption in a water-based process can be significant, because of the energy requirement for operating high-pressure pumps: heating water, drying metal parts, as well as treating and purifying used water for reuse or disposal. The typical horizontal setup of an aqueous system — with multiple cleaning and rinsing baths — also demands more space, hence more electricity to run.

As a relatively slow-drying cleaner, water requires 10 times more of the latent heat of vaporization (2,259 joules per gram) than that of solvents (200-300 joules per gram).

Fixating on the cleaning agent alone can easily lead to wrong assumptions and conclusions.

Finally, unlike aqueous cleaning, there is no water consumption in a solvent cleaning process (and water itself is a precious and finite resource.) It is not unusual for industrial companies to have such difficult water requirements in place for their aqueous cleaning applications that they end up producing thousands of gallons of wastewater per week. These results present significant financial and environmental considerations. 

So ... which media is more effective?

Solvents, when used in combination with an airtight/airless degreaser, can have an even better environmental profile than many other cleaning methods. However, solvent is not always better. Both solvent and aqueous cleaning can be effective cleaning methods, depending on the application. And they both have their limitations.

Aqueous cleaning is particularly suited for cleaning polar contaminations. It can be more efficient when working wet in wet (for example, galvanization) or when combined with surface finishes (such as phosphating, chromating and so on). 

Solvent cleaning, on the other hand, can be a great choice for cleaning nonpolar contaminants such as oils and greases. It excels in cleaning parts with complex geometry and has a high compatibility with many different metal types. 

Modified alcohol solvents — which are chlorine-free, bromine-free and fluorine-free and are not under evaluation of the Toxic Substances Control Act (TSCA) — can represent a viable alternative to the traditional halogenated solvents.

The investment in a proper cleaning setup is a considerable financial commitment. When done correctly, it can add tremendous value to the operation in terms of cost, efficiency and quality. Nowadays, great technological advances in closed-loop solvent vapor degreasers are leading to environmentally sound and responsible cleaning options that are not only compliant with but are even a step ahead of the tightening regulatory requirements. Companies would be doing themselves a disservice if they hastily rule out the use of solvent without first understanding how modern technology can contribute to the highest safety and sustainability standards.