What about aqueous cleaning?
Aqueous cleaning systems need to be very large to accomplish the cleaning and drying processes. The equipment is much more expensive, consumes more energy, takes up more floor space, and takes longer to process parts than the typical DuPont™ Vertrel® specialty fluids cleaning system. So, when looking at the total cost per part cleaned, the Vertrel® specialty fluids system usually is less expensive to buy and operate.
In general, aqueous cleaning systems are used most often for (a) cleaning parts that are not very complicated in form; (b) the cleaning requirement is not very stringent; or (c) the parts are very large or the production volume is extremely high.
Aqueous cleaning usually is not successful for one or more of the following reasons:
What about HCFC-141b?
- Size & Spacing: Aqueous specialty fluids typically have difficulty getting into and cleaning in and around parts that are extremely small or have extremely small spaces. This can be somewhat ameliorated by additives in the water or high pressures, but these work-arounds cause problems of their own (residues, damage, higher costs).
- Entrapment: Aqueous cleaners resist coming out of tiny spaces. Typical work-arounds include extra processing, such as baking the products in ovens to drive the water out of the traps.
- Spots: Water often leaves unacceptable water spots. Common work-arounds include more additives to reduce the surface tension or enhance drying, more aggressive heating and powerful air knives. Less commonly, a final rinse in a solvent cleaner is often used.
- Compatibility: Water cleaning often is not suitable because some components or products are sensitive to the high pressures of water cleaning, the heat of washing and/or drying, or the minor surface residues mentioned above. There are no common work-arounds that can solve compatibility problems.
- Effectiveness: Many common types of contamination are not soluble in water, so no amount of water, pressure, and heat can remove them. Common work-arounds include more additives, higher pressures, and higher temperatures.
- Environmental Problems: Water cleaners require a great deal of water and electricity, and produce a continuous discharge of contaminated water. The most common work-around is an expensive water treatment system to be installed alongside the cleaning system.
- Costs: While water is usually cheaper than specialty fluids on a per-pound basis, the costs of buying, installing, and operating the machines can be far higher than a solvent-cleaning system.
HCFC-141b is a great cleaner, and for a decade it has provided companies around the world a reasonable choice for vapor cleaning. It's a powerful cleaner, it forms azeotropes with other additives, it works great in vapor degreasers, it has relatively good toxicity ratings, and it is nonflammable.
HCFC-141b is a Class II Ozone-Depleting Substance under the U.S. Clean Air Act. Because of this, the Montreal Protocol required that for most of the world the manufacture of HCFC-141b be phased out at the end of 2002.
Vertrel® specialty fluids are completely ozone-safe (zero ODP) and are not regulated by the U.S. Environmental Protection Agency. Vertrel® specialty fluids are recommended as an excellent replacement for HCFC-141b in cleaning applications.
What about trichloroethylene?
Trichloroethylene (TCE) is generally used for coarse cleaning—the removal of heavy, thick oils and greases where precision and reliability of the part is not a high priority. But TCE is not suitable for precision cleaning applications. The main appeal for TCE is that they are far, far less expensive than more modern solvent choices.
Traditional chlorinated specialty fluids such as TCE are very aggressive cleaners. This means they will be incompatible with most modern elastomers and plastics.
They boil at a higher temperature (which means they use a lot more energy) and must be monitored daily for acid build-up (which means higher labor costs, more complex operations, and a system that is not as resilient to errors).
There is also a pervasive and lingering concern over the toxicity of many of the chlorinated specialty fluids such as TCE, which has been classified as a suspected carcinogen for years.
What about nPB?
"nPB" is a chemical abbreviation for one member of the family of brominated specialty fluids, also known as normal-Propyl Bromide or n-Propyl Bromide. These specialty fluids are manufactured by many companies around the world and are an intriguing choice for many customers.
nPB is a powerful degreaser with very familiar handling, similar to 1,1,1-trichloroethane that was popular 30 years ago. Specialty fluids based on nPB will boil at about 71°C/160°F so it can be used to remove waxy residues that other specialty fluids cannot touch. It is very aggressive so cleaning is fast and easy.
nPB is a volatile organic compound (VOC). It has the potential to contribute to low-level smog and will be unsuitable for use in localities that have strict VOC rules, such as southern California. Interestingly, some people also have claimed nPB is an ozone-depleting solvent. Recent analyses suggest this is not true. The nPB molecule has an atmospheric life of only 11 to 17 days. With such a short atmospheric life, the molecule does not have enough time to migrate to the stratosphere to cause a problem.
Toxicity is another issue for many people looking at nPB. While some nPB producers are claiming a 100 ppm threshold limit value is proper for nPB, ACGIH issued on January 30, 2005, a final report that set the TLV at 10 ppm. Because of this relatively low value, brominated specialty fluids only will be safe to use in tightly engineered cleaning machines that limit worker exposure to the solvent fumes, such as vapor degreaser systems designed under the NESHAP regulations.
Another problem is that the aggressive nature of the cleaner can require complicated work-arounds when working with softer plastics and elastomers. nPB is not plastic-safe.
In a poorly monitored system nPB can turn acidic. Chemically speaking, the solvent can hydrolyze with water to form acidic compounds. Sometimes the water comes from a drying process, but most often the water comes as a by-product of condensation on the cooling coils. In a busy machine, this can be a real problem. So users of nPB will have to monitor every machine on a weekly or even daily basis with an "acid acceptance test" to confirm that the solvent is still safe and effective. None of these issues are insurmountable; they just take time and add to the operating costs.
nPB also has a distinct and pungent odor that is very pervasive but desensitizing. In a poorly maintained degreaser, the smell often is noticeable even far from the vapor degreaser.
What about HCFC-225?
HCFC-225 is a moderately powerful cleaner. It works well in vapor degreasers. It is nonflammable and not as aggressive on plastics as HCFC-141b.
A major problem with HCFC-225 is that it is an ozone-depleting substance. Specifically, it is a Class II Ozone-Depleting Substance under the U.S. Clean Air Act. Because of this, the Montreal Protocol requires that for most of the world the manufacture of HCFC-225 must be phased out at the end of 2015.