CO₂ Pellet Blasting:

CO₂ pellets are uniform in shape and the effectiveness of the pellets as a blast medium is similar to abrasive blasting media. However, the pellets do not abrade the substrate; therefore, CO₂ pellet blasting is technically not an abrasive operation. This process can be used for cleaning, degreasing, some de-painting applications, surface preparation, and de-flashing (flashing is the excess material formed on the edges of molded parts).

The process starts with liquid CO₂ stored under pressure (~850 psig). The liquid CO2 is fed to a pelletizer, which converts the liquid into solid CO₂ snow (dry ice flakes), and then compresses the dry ice flakes into pellets at about -110^o F. The pellets are metered into a compressed air stream and applied to a surface by manual or automated cleaning equipment with specially designed blasting nozzles. The CO₂ pellets are projected onto the target surface at high speed. As the dry ice pellets strike the surface, they induce an extreme difference in temperature (thermal shock) between the coating or contaminant and the underlying substrate, weakening the chemical and physical bonds between the surface materials and the substrate. Immediately after impact, the pellets begin to sublimate (vaporize directly from the solid phase to a gas), releasing CO₂ gas at a very high velocity along the surface to be cleaned. The high velocity is caused by the extreme density difference between the gas and solid phases. This kinetic energy dislodges the contaminants (coating systems, contaminants, flash, etc.), resulting in a clean surface. Variables that affect process optimization include the following: pellet density, mass flow, pellet velocity, and propellant stream temperature.

CO₂ pellet blasting is effective in removing some paints, sealants, carbon and corrosion deposits, grease, oil and adhesives, as well as solder and flux from printed circuit board assemblies. This process also provides excellent surface preparation prior to application of coatings or adhesive and is suitable for most metals and some composite materials. However, thin materials may be adversely affected. Blasting efficiency is approximately equal to that of other blasting operations and can approach 1 ft²/minute after optimization. CO₂ blasting can be done at various velocities: subsonic, sonic, and even supersonic. Therefore, equipment noise levels are high (between 95 and 130 dB). This operation always requires hearing protection.

Waste cleanup and disposal are minimized because only the coating or contaminant residue remains after blasting. There is no liquid waste because CO₂ pellets disintegrate. They pass from liquid to gaseous state, leaving no spent media residue. With regard to toxic air control, small quantities of coating particles are emitted to the air. A standard air filtration system should be provided.

Higher pellet velocities and a more durable pellet are required to effect paint removal on military coatings. This "more aggressive" process showed the potential to cause peening, warping, and an increased cold working. This was especially evident on sheet aluminum less than 0.060 inches thick. The paint removal rate was still deemed too slow for economical use. The more durable pellets were achieved using liquid nitrogen injection to cool the blasting air, which transports the pellets to the blast nozzle.

CO₂ Snow Blasting:

In contrast to CO₂ pellet blasting, CO₂ snow blasting is a low impact process. Applications for this process are primarily in the precision cleaning domain. A typical precision cleaning operation must clean small contaminant particles that attach to surfaces and/or surface layers of adsorbed moisture or soil due to electrostatic attraction. These particles are so small that they have a large
fraction of their surface area attached to the surface layers. CO₂ snow blasting is most effective in breaking the adhesive forces and dislodging particles from the substrate surface. Small flakes of dry ice transfer their kinetic energy to sub-micron particulate contaminants, then sublimate, lifting the particulate matter from the substrate surface as the adhesive bonds are broken. This process is often used as a final cleaning process for sub-micron particulate and light soils removal.

CO₂ snow is generated from liquid CO₂, and is discharged directly from the nozzle of the blasting device. The liquid CO₂ is partially vaporized as it passes through the nozzle, while the rest of the stream solidifies as pressure is reduced. The "snow", fine solid particles, is propelled by the fraction of CO₂ that vaporizes. No compressed air or other inert gas is needed to propel the snow.

Most media cannot be used in precision cleaning because they are too aggressive or they contaminate the component with media residue. CO₂ snow, however, is ideal for this application, since it is relatively gentle in application, leaves no media residue, and is highly purified, introducing no new contaminants. CO₂ snow blasting is often done in a clean room or cabinet purged with nitrogen to provide a dry atmosphere, minimizing moisture buildup on the component.

Carbon dioxide blasting operations generate less hazardous waste than chemical stripping since solvents are not used. The decrease in hazardous waste helps facilities meet the requirements of waste reduction under RCRA, 40 CFR 262, Appendix, and may also help facilities reduce their generator status and reduce their regulatory burden (e.g., recordkeeping, reporting, inspections, transportation, accumulation time, emergency prevention and preparedness, emergency response) under RCRA, 40 CFR 262. In addition, the decrease in the amount of solvents on site decreases the possibility that a facility will meet any of the reporting thresholds of SARA Title III for solvents (40 CFR 300, 355, 370, and 372; and EO 12856).

The compliance benefits listed here are only meant to be used as a general guideline and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g. the amount of workload involved.

CO₂ as a completely oxidized compound is a non-reactive gas, and thus is compatible with most metals and non-metals.

Dry ice processes are cold and can cause thermal fracture of a component. In addition, prolonged use on a component in one spot will cause condensation and ice buildup. However, this is rarely a problem, because CO₂ blasting is a fast-acting, non-stationary process. Particulate and organic contamination is either quickly removed or unable to be removed by continued blasting at a single point. Therefore, the component temperature does not change much, since contact time is short. Nevertheless, should component temperature drop below the dew point of the surrounding atmosphere, moisture will accumulate on the component. Heating the component in some manner so that its temperature remains above the surrounding atmosphere’s dew point after blasting can mitigate this problem. If components cannot take heat, then blasting can be performed in an enclosed space purged with a dry gas to lower or eliminate the dew point problem.

CO₂ does not support combustion and it is non-toxic; however, it is an asphyxiant. CO₂ will displace air since its density is greater than that of air, causing it to accumulate at the lowest level of enclosed spaces. When blasting with CO₂ pellets, additional ventilation should be provided for enclosed spaces. Personal protective equipment (PPE) is also required when blasting.

Due to the ergonomics involved, robotics should be considered for full time (continuous) depainting operations.

Static energy can build up if grounding is not provided. CO₂ blasting should not be conducted in a flammable or explosive atmosphere.

High pressure gases should be handled with great care. Always chain or secure high pressure cylinders to a stationary support such as a column, prior to their use.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS for CO₂ prior to implementing this technology.

• Significant reduction in the amount of hazardous waste and hazardous air emissions generated compared to chemical stripping. 
• Time required for cleaning/stripping processes is reduced by 80-90%. 
• Leaves no residue on the component surface. 
• Effective in precision cleaning. 
• Introduces no new contaminants. 

• CO₂ blasting is not always a one-pass operation; an effective blasting operation usually requires multiple passes to achieve the desired effect. 
• Requires operator training. 
• Can have high capital costs. 
• Fixed position blasting operation can damage the component’s surface. 
• Generates solid waste containing coating chips that are potentially hazardous; media does not add to the volume of solid waste. 
• Rebounding pellets may carry coating debris and contaminate workers and work area. 
• Some soils (in cleaning operations) may redeposit on substrate. 
• Non-automated system fatigues workers quickly because of cold temperature, weight, and thrust of the blast nozzle. Automation (robotics) are required for full aircraft stripping operations. 
• Potential hazard from compressed air or high velocity CO₂ pellets. 
• Carbon dioxide (CO₂) blasting is not an effective paint removal process for aircraft. A production rate of 219 hours per aircraft (27 shifts) is not acceptable for the Air Force. The Air Force developed a liquid nitrogen injection system to enhance the depaint operation which improved strip rate. However cost, reliability and complexity of the operation renders it unsuitable for production operation. 

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The Air Force Corrosion Program Office does not approve of this process for paint removal and will not provide technical guidance for this process.  Any implementation of this process for paint removal in the Air Force would require approval of the engineering authority for specific Weapon System Managers or Equipment Item managers.

None identified.

Air Force:
Randy Ivey
Chief
Materials Engineering Section
WR-ALC/TIEDM
420 2nd Street, Suite 100
Robins AFB, GA 31098
Phone: (478) 926-4489 
DSN: 468-4489 
FAX: (478) 926-1743 

Air Force Corrosion Prevention and Control Office
AFRL/MLS-OLR (Bldg. 165)
325 Richard Ray Boulevard
Robins AFB, GA 31098-1640
Phone: (478) 926-3284 
DSN: 468-3284
Email:  AFCORR@ROBINS.MIL

CAE Alpheus
9119 Milliken Ave.
Rancho Cucamonga, CA 91730
Phone: (800) 445-6131 
FAX: (909) 980-5696
Service: Manufacturer of carbon dioxide pelletizers and blasting equipment

Cold Jet Inc.
455 Wards Corner Road
Suite 100
Loveland, OH 45140
Phone: (800) 337-9423 
or (513) 831-3211
FAX: (513) 831-1209
Contact: Mr. Jerry Raschau
Service: Manufacturer of carbon dioxide pelletizing and blasting equipment

Va-Tran Systems, Inc.
677 Anita Street
Suite A
Chula Vista, CA 91911-4661
Phone: (619) 423-4555 x 102 
FAX: (619) 423-4604
URL: http://www.vatran.com
Contact: Mr. Jeff Sloan
Service: Manufacturer of the “Sno-Gun” carbon dioxide pelletizing and blasting equipment.

This is not meant to be a complete list, as there may be other suppliers of this type of equipment.

EPA SAGE 2.0 "Solvent Alternative Guide."
Cold Jet® product literature and video.
Va-Tran Systems, Inc. product literature.
Hill, E. A., "Carbon Dioxide Snow Examination and Experimentation," Precision Cleaning, p. 36-39, February 1994.
Sloan, J., "Dry Ice Snow Surface Cleaning of Electronics, Optics and Metal Parts,"
MICROCONTAMINATION 93 Conference Proceedings, p. 671-676, 1993.
Randy Ivey, Robins AFB, 2/00.