Solvent Degreasing

In deciding on solvent cleaning, consider safety, regulatory and technical aspects.

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Choosing a cleaning agent and cleaning process depends on what soils need to be removed and the nature of the surface they are on. In the cleaning world, solvent cleaning (as contrasted with aqueous cleaning) means cleaning with organic solvents—cleaning agents with molecules containing carbon. The concept of “like dissolves like” is based on the inherent nature of molecules. The term “solvent degreasers” may be partly based on the fact that many soils associated with parts manufacturing are based on organic molecules, like grease. Not only do organic solvents have greater solvency for organic soils, they also frequently have low surface tension and viscosity, enabling them to enter into tight spaces.

Some solvent degreasers are single molecules, like perchloroethylene or isopropyl alcohol. If you have more polar soils, all other things being equal, isopropyl alcohol will be the better choice because it itself is polar. Mineral spirits are inherently a blend of hydrocarbons, which are non-polar molecules in chains of carbon with hydrogen attached. Hydrocarbons also tend to be very effective in removing oils and greases. Because the scope of solvency of cleaning agents that are single or closely-related molecules may be relatively limited, more complex solvent blends, containing a variety of molecules, are needed to enlarge the range of soils that can be removed.

Some manufacturers shy away from blends because of concerns about product consistency. However, even so-called single molecule cleaning agents may be a blend. Lower boiling point solvents, like chlorinated or brominated cleaning agents, contain stabilizers to inhibit acid production. Acid can damage the cleaning equipment, not to mention the product. Some higher boiling solvents use stabilizers to keep the molecules from oxidizing; this is important to keep the process constant and to avoid generating flammable products.

Some manufacturers swear by vapor degreasing, an inherently solvent cleaning process. In traditional vapor degreasers, the cleaning and rinsing can be in the liquid or vapor phase. One advantage is that because the entire process or the final rinse can take place in the vapor phase, cleaning (or rinsing) is in freshly-distilled solvent. This means less concern with contaminated process bath and reduced drying time. Degreasing with blends is also possible if you use azeotropes, which are blends where the ratio of boiling rates is the same as the ratio of the constituents so the relative constituency remains the same when the mixture boils. Using azeotropes for degreasing is a must for solvency consistency. Also, many non-azeotropes can become flammable if a non-flammable portion boils off faster than a flammable portion. It is possible to vapor degrease with low-flash-point solvents and azeotrope blends if you invest in cleaning equipment specifically designed for low-flashpoint solvents. For example, the flammable azeotrope of cyclohexane and isopropyl alcohol is effective for cleaning mixtures of polar and nonpolar soils.

Most common vapor degreasing systems are designed for lower-boiling solvents. However, many useful solvents and solvent blends have a higher boiling point than water. Mineral spirits and some bio-based solvents like those based on d-limonene (from citrus peel) and on methyl soyate (from soy beans) are higher boiling solvent cleaners. They may be used to wipe down parts or in dip tanks or with agitation or ultrasonics. While some sealed systems, often operating under a partial vacuum, are designed to use higher-boiling blends, these blends are not suited to typical open-top degreasers. With higher boiling solvents, depending on the next process steps and the end-use application, you have to be particularly concerned with drying and adequate removal of cleaning solvent residue.

There is also such a thing as too low a boiling point. Acetone, for example, has a broad spectrum solvency range—though sometimes it’s too aggressive. Some manufacturers avoid acetone on the grounds that the low boiling point causes it to flash off too rapidly to clean effectively. Since evaporation is inherently a cooling mechanism, a cooled part may condense moisture from the air. Speaking of “flash,” the flash point of acetone is -20° C; so don’t heat it or use it near ignition sources.

To decide what sort of solvent cleaning to use, consider safety/environmental regulatory as well as the technical aspects. Some important regulatory aspects include the PEL (permissible exposure level for workers), whether or not the solvent is a HAP (hazardous air pollutant) or a VOC (volatile organic compound), and whether or not it has a low flash point. Choose an effective cleaning solvent; and integrate the solvent with the cleaning process. There is probably no such thing as an inherently safe cleaning agent. If you don’t use good process design, you might harm your workers and the environment—not to mention the product. Nearly all cleaning agents can be used safely and responsibly, with the appropriate process and process controls.

Originally published in the November 2015 issue.

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