Fifth in a series » Among the many specifications applied to finished heat-treated goods, the cleanliness of those goods is increasingly important in appearance, performance, and customer satisfaction. In this series of articles, I’ll break down the why and how of cleaning for heat treatment.
In this column, I will discuss solvent cleaning of parts for heat treatment. This process is particularly appropriate for vacuum heat treating.
Introduction
In the previous articles in this series, we have described the various sorts of soils that can show up on parts prior to heat treatment. These soils can be divided into two classes: polar and non-polar (Figure 1).
Polar solvents have a positive and negative charge at different locations in their structure. For instance, water, a strong polar solvent, has H+ and OH– as it is broken into the basic ions. Other polar solvents include acetone, acetonitrile, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), isopropanol, and methanol.
Non-polar solvents include alkanes (pentane, hexane, and heptane) and aromatics (benzene, toluene, and xylene). These are often used as spot removers, paint thinners, or glue solvents. They are also used industrially as solvents. However, non-polar solvents have their issues. The U.S. Environmental Protection Agency (EPA) has banned or restricted several solvents, including trichloroethylene (TCE), perchloroethylene (PCE), and methylene chloride. These bans are intended to protect human health and the environment from cancer-causing and toxic chemicals. Examples of chemicals banned as solvents include trichloroethylene (TCE), perchloroethylene (PCE), and methylene chloride. These chemicals were often used in vapor degreasing applications [1].
Many of the aromatics and alkanes (and alcohols) have very low flash points (less than 100°C), so special precautions need to be taken to avoid fires and the emission of volatile organic compound (VOC) emissions. Volatile organic compounds readily evaporate, and have many environmental, health, and safety restrictions. Regulations regarding VOCs can range from national (EPA) to regional (Southern California Air Quality Review Board). There, there is a strong interest in solvents that contain either low or no VOC solvents to meet increasingly stringent cleaning demands, while decreasing environmental and health and safety concerns.
With pigmented drawing compounds, the presence of these pigments makes cleaning difficult, especially after the heat generated by a drawing operation and the impacting of pigment into the metal. With unpigmented drawing compounds, oils, and greases, hot alkaline cleaning normally removes such soils. However, this process is very corrosive and tends to destroy equipment. Further, special precautions are needed to protect equipment and personnel from the elevated temperatures and very high pH. In addition, very thorough rinsing is required to prevent residues on the part.
Insoluble cutting oils, greases, and wax-based corrosion inhibitors are very difficult to remove from a part using water-based alkaline cleaners. The presence of sulfur and chlorine components may make the soil more difficult to remove with alkaline cleaners.
Methods
There are many different methods of organic solvent cleaning.
Vapor Degreasing
Vapor degreasing is a method that uses solvent vapors to remove contamination from parts. Essentially, a non-flammable solvent is boiled, and the vapors coat the parts to be cleaned. Refrigeration coils are placed at the top of the tank to contain vapors and help condense the vapors onto the part. As the vapor condenses into a liquid, it cleans the parts, and the contamination and condensed solvent drips back into a reservoir. Because the solvent is boiling, it is always pure and acts like a distillation process.
The solvent in the reservoir is then distilled, and the solvent recovered, and the remaining soils are recovered and disposed. Because the solvent is a gas, it can get into small spaces, making very clean parts.
Solvents used in vapor degreasing must be non-flammable. The solvent must also maintain a constant boiling temperature and composition during the cleaning operation. Solvents such as acetone cannot be used because of the low flash point. However, many of the chlorinated vapor degreasing solvents that were commonly used, such as trichloroethylene, perchloroethylene, and methylene chloride, have been banned.
Brominated solvents such as n-propyl bromide have been used to replace the chlorinated solvents. However, there are presently many restrictions on its use, and will likely be banned in the future.
Fluorinated solvents are the newest vapor degreasing solvents to come to market. These have much lower vapor points but have much better health and safety profiles. There are also lower regulations regarding its use, emissions, and disposal.
Immersion
In immersion cleaning, the parts are lowered into a bath containing the solvent. Agitation is usually included to help lift contaminants from parts. These washers can be either manually operated or automated. Due to the typical solvents used, strong fire suppression is needed because of the flammable nature of the solvents. Ventilation and vapor capture is also needed because the solvents typically used emit VOCs. Typical solvents used are dearomatized white mineral spirits and alkoxypropanols (modified alcohols).
Dearomatized white mineral spirits are used for cleaning very greasy or oily parts, such as gear boxes. They are commonly used in the remanufacturing industry for engines and transmissions. They have lower EHS restrictions than aromatic mineral spirits, but still have a low flash point temperature and higher VOC.
Alkoxypropanols, or modified alcohols, are synthetic solvents which contain alcohol and ether groups. Because they are a mix of polar and non-polar chemistries, they have a broad application in cleaning polar and non-polar residues from parts. Alkoxypropanols generally do not have a flash point and have some miscibility in water. These chemicals are rapidly replacing traditional chemicals for vapor degreasing and immersion cleaning.
Vacuum Solvent Cleaning
Vacuum solvent cleaning, or vacuum degreasing, is a closed-circuit specialized type of vapor degreasing. In this type of operation, the parts are placed in a mild vacuum chamber, where the solvent (modified alcohol, chlorinated solvent, or hydrocarbon) is introduced into the chamber. Because of the high vapor pressure of the solvents, the solvent immediately turns to gas and fills the vacuum chamber. The operator never encounters the solvent, so environmental, health, and safety concerns are minimized. The drying of the solvent is done completely under a vacuum. This process can be done either manually or with complete automation.
The solvent is continuously vacuum distilled, so that the solvent remains stable and clean. This ensures consistent cleaning and reducing costs.
Conclusion
In this article, we briefly described different types of solvent cleaning. We also discussed the typical types of non-polar solvents used, and some of their replacements due to EHS concerns.
Should there be any questions regarding this article, or suggestions for further articles, please contact the editor or myself.
References
- Kanegsberg, B., Kanesgsberg, E.; Handbook for Critical Cleaning: Applications, Processes, and Controls, Boca Raton, FL, CRC Press, 2011.
