James A. Mertens
Technical Service and Development
The Dow Chemical Co.
Up until 1995, cleaning metal machinery was a nobrainer. Companies almost universally used vapor degreasing and solvents, mainly 1, 1, 1-trichloroethane and CFC 113. Then the U.S. government outlawed those chemicals for being ozone-depleting substances (ODS) and phased out their production for use in all developed countries.
There are still some chlorinated solvents on the market, most notably trichloroethylene (TCE), perchloroethylene (PERC), and methylene chloride (MEC), and they are the closest to drop-in replacements for the outlawed chemical cleaners. But there are other alternatives often promoted on the basis of health, safety, and environmental and economical advantages. Comparing these "green" alternatives to chlorinated chemicals lets designers and manufacturers see how they actually stack up against one another.
Aqueous cleaning uses detergent and water or an alkaline solution, and is extremely popular. It's not as effective at removing greases and oils as vapor degreasing, but in cases where precision cleaning isn't needed, aqueous cleaning can usually do the job. If more effective cleaning is needed, chemical, thermal, mechanical, or ultrasonic agents can be added. Aqueous systems pose little threat to the environment, although used solutions and rinse water usually must undergo standard water treatment.
The disadvantages of aqueous cleaning, as compared to chlorinated solvents, include:
- It's difficult to clean crevices and small holes with aqueous solutions. Chlorinated solvent vapors, on the other hand, can reach all regions of the most convoluted parts.
- Aqueous cleaning processes must be carefully designed and controlled. Vapor degreasing is a forgiving process, which continually generates a clean solvent vapor.
- In addition to the washing stage, aqueous cleaning also calls for rinse and drying stages, which use more floor space than degreasing. And supplying energy to heat the water and power the drying stage makes aqueous cleaning more costly and energy-intensive than degreasing.
In this method, parts to be cleaned are exposed to a nonchlorinated solvent which usually includes a surfactant.
Parts are next immersed in an emulsion bath, where the solvent mixes with water. The bath is continually cleaned, letting the solvent return to the solvent tank and the water to the emulsion tank. Parts are then rinsed and dried. This method provides good cleaning and low levels of vapors. Its disadvantages include:
- Recycling the solvent is difficult. So when oil contamination of the solvent reaches 30%, the solvent has to be replaced. (Hydrocarbon-based materials cannot be distilled and purified without special equipment.) Chlorinated solvents, however, can be continuously distilled in the vapor degreaser, and can be easily concentrated to mixtures of more than 60% oil in conventional equipment.
- Most solvents used in emulsion cleaning are either flammable or combustible. Their use requires special equipment to protect against fire. Chlorinated systems, on the other hand, are considered nonflammable because they have no flash point, according to standard tests.
- Most emulsion solvents are volatile organic compounds (VOCs) and contribute to smog formation. They must be carefully controlled. PERC and MEC are exempt from VOC regulations.
- Like aqueous cleaning, emulsion systems require more floor space and energy than vapor degreasing.
- Not all emulsion cleaners have been fully studied for toxicity. Chlorinated solvents are about the most fully studied and best understood chemicals on the market.
- Some emulsion cleaners have problems of their own, including gelling in low-water-content solutions and auto-oxidation.
Cleaning can also be done with nonchlorinated solvents like acetone, mineral spirits, nmethylpyrrolidone, and terpenes. While these provide good cleaning, they are flammable or combustible and therefore unsuitable for vapor degreasing. Instead, nonchlorinated solvents are restricted to cold-cleaning applications such as simple wiping or immersion. Their evaporation rates (drying times) also present a problem. Some are too slow, requiring extra time to dry, while others are too fast, requiring more solvent to clean the parts.
Several new "designer" solvents recently put on the market include methyl siloxanes, oxygenated solvents (glycol ethers and dibasic esters), the hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), hydrofluoroethers (HFEs), and n-propyl bromide.
Siloxanes are good at attacking silicone-based oils and lubricants, but they're flammable. Oxygenated solvents have good solvency properties, but they, too, are flammable and may be limited due to VOC concerns as well.
HCFCs, HFCs, HFEs and bromide were developed as attempts to mimic chlorinated solvents. Unfortunately, their high cost (12 to 15 times that of chlorinated versions) and poor performance offset any advantages they have.
N-propyl bromide has been rediscovered as a solvent. It had been overlooked because of flammability. Recent tests, however, show that its flash point is outside the temperature range of current tests. Because it is relatively new, there have been few toxicity tests and reliable data won't be available for several years. There are also unresolved questions regarding its ozone depletion potential, environ-mental impact, and health effects.
Supercritical fluids are another alternative to chlorinated solvents. The fluids may be gaseous (like CO2) or liquid (like water) at ambient pressures and temperatures, but they reach a supercritical state when compressed and heated past their critical pressure and temperature. In the case of CO2, for example, the critical temperature is 31.1°C and the critical pressure is 73.9 atm. Such pressures require costly equipment and this method needs more development before it is practical. Other methods include plasma and laser cleaning, which are much too expensive. Thermal vacuum cleaning removes oils but leaves other forms of dirt and soil behind. And mechanical cleaning blasts parts with a medium such as dry-ice pellets, starches, or pressurized gas. Blasting is good for some applications, but it risks harm to the part and calls for special part handling to clean all the surfaces of a part. And if blasting is used to clean oily parts, the oil clogs the media and makes recycling it difficult or impossible.
Each of these cleaning systems has advantages and disadvantages, and each is appropriate for a limited type of cleaning applications. The same can be said for vapor degreasing with chlorinated solvents. But only degreasing with chlorinated chemicals has the combination of high solvency, high-precision cleaning, cost effectiveness, low flammability, and a small footprint. Existing health and environmental regulations in North America and Western Europe call for careful control of emissions and low levels of human exposure. Waste products from the process are considered hazardous materials and must be disposed of in accordance with local and federal regulations.
When used with properly designed equipment and following the existing health, safety, and environmental standards, vapor degreasing with chlorinated solvents cannot be matched by the alternative technologies.