When talking about solvent substitution most people immediately begin thinking of substitute solvents, some type of liquid. This line of thought bypasses some very important questions such as "Do I really need to clean?", "Why are my parts dirty?" and "Is there a non-liquid way to clean my parts?". There are a few problems inherent to ALL liquid solvent substitutes (i.e. disposal) so it is prudent to investigate non-solvent cleaning first.
Blast cleaners are the most popular of the non-liquid cleaning systems. Sandblasting? Not quite. Over the last several years new blast cleaning systems have been developed which use media such as ice, steel shot, foam balls and dry ice. Although a number of these were developed for paint stripping, they may also be viable options for parts cleaning. Below are descriptions of some of these new media along with advantages and disadvantages of each.
One of the many uses for CFC-113 is to remove dust and particulates from sensitive assemblies. In many applications filtered, compressed air can be used as a substitute. There are no emissions and no excess waste is generated. If surface charge accumulation on non-metallic surfaces is a problem, ionized air can be used. One used oil recycler in Jacksonville is now using compressed air instead of solvent to clean sludge from his system filters. KACHING!!$$$$$
Water blast systems are now available that can do everything from removing soils to removing paint. Water is easy on substrates and easily separated from the soil after cleaning, but you may not want wet parts and you may end up with a lot of contaminated water, depending on your soil. Ultra high pressure water jets are available that can be used for cutting steel and concrete. Let's see, I could eliminate a machining oil waste stream AND a cleaning step ...?
The Canadian Navy has developed a system that extrudes and blasts ice crystals to remove paint in submarines. The pros and cons for this system are about the same as for water but the price is probably higher.
By far the most archaic of the blast media, it is rough on the substrate, nearly impossible to segregate from the soils after cleaning, produces hazardous dust, and, if the soil is hazardous (i.e. lead based paint), you end up with a LOT of hazardous waste on your hands. One advantage: sand is cheap.
Similar to bird shot, it is rough on a substrate but not nearly as rough as sand. Advantages are that it can be magnetically separated from the soils and reused and it does not create a hazardous dust. Wastes are kept to a minimum.
A popular system for paint stripping PMB is relatively easy on substrates and can be reused. PMB can be tailored to a lot of paint stripping applications by using plastic beads of varying size and hardness. Systems are relatively inexpensive.
Oils, greases, dirt, even paint can be removed from parts by blasting with little bits of urethane foam. If the soil is moist (i.e. oil & grease), the foam bits will absorb it. The foam can then be washed, dried and reused. Waste streams are similar to a parts washer with the advantage that the part itself does not get wet. For paint or scale removal, the foam can be wet to reduce dust generation.
Developed for aircraft paint stripping, this system has some interesting characteristics. First, there is no excess waste generated. The CO2 pellets sublimate after contact with the part, dissipating as CO2 gas; paint and soil falls to the ground. Second, the sublimation of pellets on impact helps to lift paint from the surrounding substrate. First generation systems were slow but recent modifications have improved the speed. CO2 blasting is gentle on substrates but, as you might guess, it is not cheap.
This relatively inexpensive media was also developed for aircraft paint stripping. The crystalline wheat starch used in this system can be reused and actually becomes more aggressive as it breaks down. Wheat starch is very forgiving of error and is easy on substrates. Crystalline wheat starch molecules are different from the naturally occurring polymer so explosion potential is not a problem. Wheat starch media has also been successfully used in fluidized beds for parts cleaning.
These unusual media are used, among other things, to clean carbon deposits from engine parts. Ground walnut shells are gentle on the substrate and inexpensive, dependent, of course, on the futures market for walnut shells. Peanut shells and ground corn cobs are also used as blast media.
Baking soda is a very effective media for paint stripping without being destructive to substrates and is inexpensive. Unfortunately, there is a high potential for subsequent corrosion, especially on aluminum parts. The media is not recyclable and disposal (usually sewering) requires copious amounts of water. Care must be taken to keep contaminants (i.e. heavy metals) out of the waste stream.
This is actually a blast system that can use a variety of media. A special vacuum head is held against the substrate. The media is accelerated to the head using a vacuum instead of compressed air. After impact the vacuum pulls the media back to the holding bin where it is continuously reused. This reduces environmental impact and worker exposure to hazardous substances and allows other nearby operations to continue uninterrupted.
This system uses multiple, reciprocating needles, to pummel paint, dirt and scale from surfaces. The needles are contained in a vacuum head that is held up against the part during cleaning. The vacuum system captures the removed soil, thus eliminating worker and environmental exposure. A major advantage is that no excess wastes are generated. Popular in applications such as removing lead-based paint from bridges.
Consider some of the old fashioned ways of cleaning. Scraping, brushing, wiping. These can be labor intensive but considering the problems and cost involved with other cleaning systems, you may want to reconsider them. Pyrolysis and fluidized bed furnaces can be a good option for soil removal if parts can be heated but there are safety and emission considerations.
Plasma cleaning uses ionized gases, blown through a cleaning chamber to vaporize organic contaminants. UV/Ozone uses a combination of ultraviolet light and ozone in a cleaning chamber to achieve the same result. Both systems are high cost, high-tech and only effective against organic soils. Neither system will remove gross contamination, but these systems can precision clean the whole part, inside and out, to levels unattainable with chemicals. Other "Rocket Science" entries include Xenon Flash Lamp and Laser Ablation, developed by the military for aircraft paint stripping. With recent defense budget cut-backs, even the military can hardly afford them.
Fact Sheets available in the Industrial Cleaning series:
- in the Jacksonville area, call 904-448-4300
- in the Tampa area, call 813-744-6100.
Last Updated: October 17, 1995