High Reliability Solvent Cleaning in Today’s Environment
Cleaning systems must be safe, efficient, effective and environmentally compliant.
With the ever tightening rules and regulations imposed on solvent cleaning by federal, state, and other governing bodies, today’s cleaning systems must be safe, efficient, effective and environmentally compliant while providing the required throughput to meet the end user’s needs. As existing solvents are more heavily regulated or phased out and new materials emerge, it is important to understand the guidelines for using these approved solvents in conjunction with cleaning systems available on the market today.
To understand today’s use of solvent cleaning systems, one must first take a retrospective look at how our current environmental rules and regulations were developed, including the criteria that ushered in the current regulations for solvent systems. Common regulatory objectives include achieving lower limits of solvent usage to better our environment, preserving processes currently required for specific applications and providing a cost effective means and safe working environments.
For decades, solvent degreasing was the preferred cleaning method for most manufacturers. Compared with aqueous cleaning, most factories found degreasing to be a simple, one-step process, requiring less floor space, maintenance and cost, while providing better compatibility for numerous soils and substrates. Degreasing is especially effective for parts with complex geometries because of solvents’ ability to penetrate the smallest tolerances. If any solvent remained on the part, it evaporated into the atmosphere leaving minimal residue without affecting the substrate.
As scientists studied the earth’s ozone layer, the scientific community theorized that the use of these solvents or the associated family of compounds for degreasing, refrigerants, carrier fluids and other applications was harmful to the ozone layer around the earth. Public awareness increased, governments formed investigative committees, and environmental organizations analyzed these theories. The result was a worldwide pact to decrease or eliminate the use of certain ozone-depleting chemicals/solvents. This joint agreement became known as the Montreal Protocol, first going into effect in 1989.
Solvent degreasing processes were suddenly facing challenges both from applied chemicals and operational perspectives. Ozone-depleting solvents had phase-out dates. Companies faced new limits on air emissions and operator exposure. The National Emissions Standards for Hazardous Air Pollutants (NESHAP) was created/regulated by Federal mandate. Degreasers nationwide were modified to meet these new rules or users switched to alternative processes based on corporate directives eliminating ozone depleting solvents.
For some users, the switch from solvent degreasing to aqueous cleaning occurred rapidly. Often, the transition went well, but sometimes not, resulting in less than ideal equipment, chemistry or process selections. Some users were able to successfully transition to aqueous cleaning, while others returned to limited degreasing for those challenging applications where the process truly excelled.
Fast Forward to 2015
Since the Montreal Protocol was first established, new solvent molecules and blends that react with certain lubricants to remove them quickly and effectively have been discovered. These materials are designed for environmental compliance regarding VOC content, lower air emissions, global warming, stability, non-flammability, and other chemical concerns. While these materials are more friendly for both the environment and operators, containment and minimized exposure is still required.
Modern degreasers meet and typically exceed all federal guidelines concerning design criteria, operator exposure and atmospheric emissions.
Just as today’s automobiles are more reliable with better fuel economy, degreaser manufacturers have also improved their new designs by incorporating new technology that capitalizes on advanced solvent properties. The full benefit of manufacturing higher quality products while having the lowest operator exposure and solvent emissions by pairing the right solvent technology and equipment for the application is here now.
Basic degreasers can be generally classified in four categories:
- Traditional open-tops
- Enclosed low-emission maximum efficiency
- Airless or vacuum
The current solvent landscape is a mixture of old and new. Chlorinated or brominated degreasing fluids that have been used for years are now facing higher scrutiny, but are still available, while others have been regulated out of existence. Some companies have also turned to other more benign fluids that are not generally thought of as degreasing fluids for that purpose; as such, they typically have a limited scope of effectiveness. Environmental impact and regulatory compliance remain the deciding factor for most solvents today.
For example, the ban on production of AK-225—effective January 1, 2015—impacts numerous users and their precision cleaning processes. While certain fluids are more effective on specific contaminants, any solvent in consideration should be evaluated for personnel safety, regulatory compliance, actual performance, ease of operation and overall cost.
A novel solvent is now available within the marketplace. From a performance perspective, this solvent has excellent solvency for a range of soils. The solvent has extremely low surface tension to penetrate and remove soils from blind holes and crevices, and it meets and exceeds regulatory and safety requirements. It is non-flammable and carries an OEL of 800 ppm. The solvent is a suitable replacement for nPB, AK 225, HFEs, HFCs, TCE, Trans and Stoddard solvents.
The biggest challenge for solvent cleaning is that a universal “green” solvent that removes a range of contaminants (lubricants and oils, flux residues, waxes, particulates and more) simply doesn’t exist. In general terms, the more user-safe and environmentally friendly the solvent is, the more narrow its efficacy on a range of soils. Therefore, users are often faced with the dilemma of selecting materials that can be effectively removed using solvents allowed by their corporate EH&S policies or regionally dictated mandates, rather than using lubricants or materials that they would prefer from a production standpoint. If it has to be clean, then it has to be cleanable using available technology.
Years of research and development have evolved into a two-solvent or DuoSolvent process. This method opens the process window for soils removed and compatibility with materials processed by first immersing the part in a primary cleaning solvent focused on contaminant removal, and second, processing through an environmentally preferred degreasing solvent for complete, water-free rinsing.
Figure 1 (on page 52) shows that bulk cleaning occurs in the first solvent, which is designed to provide maximum effectiveness to the specific soils being removed. This is a high boiling point, liquid immersion non-vapor cleaning process. After the initial cleaning is completed, the work is transferred to the first process tank in the rinse degreaser and immersed, where any residual cleaning solvent carry-over is solubilized and captured by the rinsing agent. Those familiar with aqueous cleaning processes often refer to this as a chemical-isolation rinse. Two stages of immersion in the rinse tanks and vapor zone of the degreaser then provide a high-purity rinse, producing a dry and completely residue-free part.
Pairing the solvents takes care to ensure the solvating agent is easily removed by the rinsing agent, yet can be efficiently separated to maintain purity of the rinse and provide long process life. Studies performed using gas chromatography analysis demonstrate that at normal carryover levels, the first rinse step can be effectively maintained at greater than 98 percent purity. Figure 2 summarizes the results of three controlled tests where specific volumes of solvating agent were added to the first rinse tank on an hourly basis for a full shift.
By shifting the main cleaning step out of a typical degreaser, the process eliminates the biggest challenges of co-solvent applications—the need to maintain specific solvent ratios and eventual rinse contamination because of partial pressures and contamination buildup in the degreaser.
Properly matched DuoSolvent pairings can fit a range of process needs, from drying to lubricant and coating removal to flux cleaning.
Equipment suppliers are experiencing a limited increase in solvent degreasers based on the need for modern environmentally compliant equipment. While this market may never reach the level it was 30-plus years ago, solvent cleaning is necessary in certain specific applications. From the sheer size and complexity of some products to be cleaned and the cleanliness criteria that today’s aerospace, military and medical industries demand, solvent degreasing offers a simple yet effective solution.
Environmental impact is driving equipment design toward enclosed systems when possible. Whether an atmospheric shielded tank or hermetically sealed chamber, it’s the preferred choice of many companies. The advantages of these systems are readily apparent: lower solvent usage costs and emissive losses, combined with a controlled process that yields repeatable results with minimum operator interface.
What is the future of solvent cleaning from today’s view? Equipment and solvent manufacturers readily admit that the market has shrunk over the last decade, but it is far from dead. The industry as a whole is seeing new machines being ordered. Small, precision cleaning operations are desiring totally enclosed systems featuring automation that will minimize operator interface while conserving as much solvent as possible.
The old adage that solvent cleans better, faster, and in more restrictive places than water can ever reach is still true. The fact that today’s cleanliness specifications for precision and critical cleaning cannot have any contaminant residue or rinse water residue on the end product still drives some users toward solvent.
The cost to clean will always be a part of this equation, as will cost for environmental compliance and end user confidence. Some applications can go either solvent or aqueous, and there will remain alternate choices for all.
Solvent and equipment manufacturers believe the existing process will remain viable for many years into the future. We know the equipment will evolve as well as the solvent that goes into it. The basic process will remain similar. The improvements will come in new solvent blends and processes, material handling concepts, solvent recovery systems, and even tighter environmental compliance.
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