Monday, 1 June 2015

Cleaning - Using Dry Ice

                [Carbon dioxide (chemical formula CO2) is typically known as a gas, though it exists as a solid when it reaches temperatures below −78.5 °C (−109.3 °F). This process is called deposition, and goes directly from gas to solid. When temperatures warm up, it sublimates and goes directly back into a gas]

Carbon dioxide is a colourless, odourless gas, this chemical compound is composed of two oxygen atoms each covalently double bonded to a single carbon atom. Using dry ice (a solid form of carbon dioxide) is a form of abrasive blasting, similar to sand blasting, plastic bead blasting, or soda blasting but substitute’s dry ice as the media, where it involves propelling pellets at extremely high speeds in a pressurized air stream and directed at a surface in order to clean it.

How does it work?
Dry ice pellets are accelerated in a jet of compressed air and strike the coating to be removed at velocities up to the speed of sound. Cleaning results from three effects:

Kinetic Effect - when dry ice pellets strike a surface at the speed of sound, any coating on the surface is cracked and loosened.

Thermal Effect - the low temperature of dry ice pellets makes the coating brittle, cracks it and loosens it as a result of reduced bonding between the coating and the underlying surface. This allows dry ice to permeate the coating.

Sublimation - dry ice penetrates the coating and immediately sublimes (passes directly from solid to vapour state). This results in a 700-fold increase in volume, an explosive effect that lifts the coating off the surface.

Cleaning with dry ice
Most system uses small rice size pellets of dry ice ejecting them from a jet nozzle with compressed air. It works somewhat like sandblasting, with superior results. The frigid temperature of the dry ice -109.3°F (-78.5°C) "blasting" against the material to be removed, causes it to shrink and lose adhesion from its sub surface. 

Additionally when some of dry ice penetrates through the material to be removed, it comes in contact with the underlying surface. The warmer sub surface causes the dry ice to convert back into carbon dioxide gas. The gas has 800 times greater volume and expands behind the material speeding up its removal. Paint, oil, grease, asphalt, tar, decals, soot, dirt, ink, resins, and adhesives are some of the materials removed by this procedure. Only the removed material must be disposed of, as the dry ice sublimes into the atmosphere.

The actual dry-ice pellets are quite soft, and much less dense than other media used in blasting-cleaning (i.e. sand or plastic pellets). Upon impact, the pellet sublimates almost immediately, transferring minimal kinetic energy to the surface on impact and producing minimal abrasion and without damage to the surface or its substrate. Dry-ice leaves no chemical residue as it sublimates at room temperature.

This is assumed to improve cleaning as the top layer of dirt or contaminant is expected to transfer more heat than the underlying substrate and flake off more easily. The efficiency and effectiveness of this process depends on the thermal conductivity of the substrate and contaminant. The rapid change in state from solid to gas also causes microscopic shock waves, which are also thought to assist in removing the contaminant.

The EPA recommends dry ice blasting as an alternative to many types of solvent-based cleaning. Dry ice blasting can clean numerous objects with differing, complex geometries at once.
These type of systems are used in semi-conductor clean rooms, as there is no moisture involved with Co2 as it does not exist in a liquid form at room temperature and any moisture (condense) will flash-off immediately.

Ideal for use on – grilles, wheel surfaces, engine bay or intricate or limited space

Safety and PPE

Carbon dioxide is increasingly toxic starting at concentrations above 1% and can also displace oxygen resulting in asphyxia if equipment is not used in a ventilated area. In addition, because carbon dioxide is heavier than air, exhaust vents are required to be at or near ground level to efficiently remove the gas.

 Dry Ice temperature is extremely cold at -109.3°F (-78.5°C) Always handle dry Ice with care and wear protective cloth or leather gloves whenever touching it. An oven mitt or towel will work. If touched briefly it is harmless, but prolonged contact with the skin will freeze cells and cause injury similar to a thermal burn.

Eye and ear protection are required to safely use dry ice cleaning equipment. Compared to other blasting-cleaning methods, dry ice blasting produces fewer waste products and does not require clean-up of a the cleaning media#

Handling and Use
Compressed air requirement – 87 PSI (6 bar) pressure amount of air delivery would depend upon machine size.

Replaces Sandblasting
This method is superior to sandblasting because the dry ice is soft enough not to pit or damage the underlying surface. The dry ice sublimes quickly into the air and only the removed material must be cleaned up. 

Dry Ice blasting eliminates equipment damage in two ways. First, dry ice does not erode or wear away the targeted surface as traditional grit media and even wire brushes do. This means that surface integrity and critical tolerances are preserved and equipment will not have to be replaced due to surface erosion common with sand, glass beads, and other abrasive media. 

Second, with traditional cleaning methods, equipment is often damaged (bumped, dropped, etc.) while in transit to or from the dedicated cleaning area. Instead most equipment and machinery can be cleaned while in place.

Replaces Steam
1.        Dry ice blasting is also an improvement over steam and water blasting for several reasons:
2.        Electrical parts and generators can be immediately put back into service used without waiting to dry.
3.        Radioactive contaminate removal doesn't require large storage containers for contaminated water.
4.        Mildew and mould removal are far more complete with less chance of regeneration because of water vapour or moisture.
5.        Dry ice blasting removes more algae, sea slime, and mussels on boat hulls, than water blasting, which allows the organic matter to reattach sooner.

Replaces Damaging Solvents

Finally Dry Ice blasting is used in place of many environmentally damaging solvents. These chemicals include trichloroethane, methylene chloride, perchloroethyleneortho dichlorobenzene, cresylic acid, and caustic solutions. Since dry ice evaporates completely as a gas it leaves no wastes. 

Only the material being removed must be disposed of. When dry ice cleaning replaces hazardous chemical cleaners the disposal cost of that chemical is eliminated. Tinker Air Force Base has reported it eliminated hazardous waste disposal associated with 17,000 gallons of chemicals they no longer need to use each year because of dry ice cleaning.
Dry ice is completely non-toxic. In contrast to cleaning methods that use water or high-pressure hosing, there is no emission of polluted water to the environment. The dislodged coating can be swept or vacuumed up and disposed of as appropriate. Dry ice blasting produces no toxic fumes from solvents and other chemicals during cleaning.

1.        The loud noise produced requires ear protection equipment and may cause irritation to other people nearby although newer machines are much quieter.
2.        Effective cleaning can only occur in a straight line of sight from the Dry Ice jet nozzle. Sometimes parts can be dissembled to help.
3.        Large amounts of carbon dioxide are released which can be harmful if not ventilated out of the space.

Typical equipment Vendors

PolarJet -

I would like to think that these articles become an asset to anyone who is new to detailing and to professionals alike, as well as industry experts who seek to advance their knowledge.
I hope the above article was informative. By having some understanding of the ‘What’ and ‘Why’ as well as the ‘How’ along with a little science to help you understand how the chemicals we use react, you can achieve the results you desire.

I would appreciate it if you would share this article as it helps other detailers further their knowledge.
Questions and/ or constructive comments are always appreciated.

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