Revision: 9/96Dimethylketone, 2-propanone, CH3COCH3; 1-butanol, butyric

Process Code: Navy and Marine Corps: ID-01-99; Air Force: ST01; Army: DPT

Usage List: Navy: Low; Marine Corps: Low; Army: Low; Air Force: Medium

Substitute for: Chemical Cleaning and Stripping CH2ClCOOH; methylene dichloride,

Applicable EPCRA Targeted Constituents: Acetone (67-64-1), n-Butyl Alcohol (CAS: 71-36-3), Methyl Ethyl Ketone (CAS: 78-93-3), Toluene (CAS: 108-88-3), Xylenes (CAS: 1330-20-7), Lead (CAS: 7439-92-1), Chromium (CAS: 7440-47-3), Zinc Compounds, Phenols (CAS: 108-95-2), Chloroacetic Acids (CAS: 79-11-8), Methylene Chloride (CAS: 75-09-2) CH3(CH2)2CH2OH; ethyl methyl ketone, 2-butanone, MEK, CH3COCH2CH3; methyl

Overview:
acid, phenylic acid, benzophenol, hydroxybenzene, C6H5OH; chloracetic acid, MCA,

Carbon dioxide (CO₂) blasting is an alternative process to chemical cleaning and stripping. The obvious advantage of CO₂ blasting over chemical stripping is the introduction of inert media that dissipates, in this case CO_2.. There are two basic types of CO2 blasting systems: pellet blasting for heavy cleaning, and snow blasting for precision cleaning.

CO₂ Pellet Blasting:

CO₂ pellets are uniform in shape and the effectiveness of the pellets as a blast media is similar to abrasive blasting media. However, the pellets do not affect 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 allow 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. Furthermore, since CO₂ pellet blasting is not an abrasive operation, it is excellent for components with tight tolerances. 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.

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 can not 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.

Materials Compatibility:

CO₂ as a completely oxidized compound is a non-reactive gas, and thus 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. This problem can be mitigated by heating the component in some manner so that its temperature remains above the surrounding atmosphere's dew point after blasting. If components can not take heat, then blasting can be done in an enclosed space purged with a dry gas to lower or eliminate the dew point problem.

Safety and Health:

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 protection equipment (PPE) is also required when blasting.

Static energy can build up if grounding is not provided. CO₂ blasting should not be done in flammable or explosive atmospheres.

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.

Benefits:

• 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

Disadvantages:

• 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 coating debris may redeposit on substrate
• Nonautomated system fatigues workers quickly because of cold temperature, weight, and thrust of the blast nozzle
• Potential hazard from compressed air or high velocity CO₂ pellets

Economic Analysis:

CO₂ Pellet Blasting: Units come in several different configurations. The blasting unit alone can be:

1. Purchased- $25,000 to $50,000 or
2. Rented- $1,500 to $2,500 per month.
3. Units that combine pelletizing and blasting are also available, but generally are not economical unless the blasting operation is performed 24 hours/day, seven days/week
4. Pellet blasting jobs can be done on a contract basis for a cost between $200 to $300 per hour including labor, pellets, and equipment (not including travel time or travel expenses).
5. Pellet cost:
   • Made by a stand-alone pelletizer that can be purchased for a cost between $50,000 to $130,000 (cost to make pellets from delivered liquid carbon dioxide is about $0.10-0.15/lb), or
   • Purchased directly from a manufacturer for a cost between $0.10/lb and $0.50/lb delivered, depending on the purity and the distance from the manufacturer (pelletizer purchase is reported to be economical only if blasting is done more than 40 hours/week).

CO₂ Snow Blasting: Units are much lower in cost and operation, as compared to CO₂ pellet blasting, and again there are several different configurations to choose from:

1. All manual units cost about $2,000.
2. Semi-automated units (can also be used in assembly applications) cost between $3,000 to $5,000.
3. For highest quality precision cleaning with substantial volume requirements, CO2 purifiers are also available. Units that can purify commercial grade liquid CO2 start at a cost of about $5,000.

Some of the following data was obtained from US Air Force for paint stripping of FX Fighter Aircraft using CO₂ blasting.

Assumptions:

• CO₂ blasting equipment cost: $50,000
• Aircraft skin area: 3,100 ft²
• Paint removal and cleaning area: 2,410 ft²
• Labor rate: $60/hr
• CO₂ cost: $0.50/lb
• Paint and solvent sludge disposal cost: $2/gal
• Dry paint waste disposal cost: $2/lb
• Water treatment/disposal cost: $8.24/1,000 gal
• Chemical procurement cost: $11/gal
• One aircraft is de-painted per month
• CO₂ blasting
• Paint removal average rate: 11 ft2/hr
• Paint removal and cleaning time: 219 hr/aircraft or 2,628 hr/yr
• Total CO₂ usage (including training): 101,430 lb/aircraft or 1,217,160 lb/yr
• Dry paint waste residue: 27 lb/aircraft or 324 lb/yr
• Chemical stripping
• Total chemical usage: 1,373 gal/aircraft or 16,476 gal/yr
• Paint removal and cleaning time: 1,500 hr/aircraft or
  18,000 hr/yr
• Wet chemical waste residue: 1,373 gal/aircraft or 16,476 gal/yr
• Contaminated wastewater residue: 308,700 gallons
  

	CO2 Blasting	Chemical Stripping
Operational Costs:
Labor	$157,700	$1,080,000
Material	$608,600	$181,200
Waste Disposal	$650	$33,000
Wastewater Treatment	$0	$2,500
Total Operational Costs:	$766,950	$1,296,700
Total Recovered Income:	$0	$0
Net Annual Cost/Benefit:	-$766,950	-$1,296,700

Economic Analysis Summary

• Annual Savings for CO₂ Blasting: $529,750
• Capital Cost for Diversion Equipment/Process: $50,000
• Payback Period for Investment in Equipment/Process: <1 year
  

Approving Authority:

Navy: Approval is controlled locally and should be implemented only after engineering approval has been granted. Major claimant approval is not required.

Points of Contact:

Mr. Alan Dailey
Mechanical Engineer
SM-ALC/TIELE
McClellan AFB, CA 95652
Phone: (916) 643-6151, Fax: (916) 643-2869;
DSN: 633-6151, Fax: 633-2869

For information on design, procurement, and use of a mobile CO₂ pellet blasting system primarily for cleaning and degreasing operations.

Mr. Mike Patry
Production Engineer
OC-ALC/LPPEE
Tinker AFB, OK 73145
Phone: (405) 736-5185, Fax: (405) 736-2501
DSN: 336-5185, Fax: 336-2501

For information on procurement and use of CO₂ pellet blasting systems for cleaning and some de-painting operations.

Vendors:

Alpheus Cleaning Technologies
9119 Milliken Ave.
Rancho Cucamonga, CA 91730
Phone: (909) 944-0055, Fax: (909) 980-5696
Manufacturer of carbon dioxide pelletizers and blasting equipment.
Mr. Howard Bartley, Sales Engineer

Cold Jet Inc.
455 Wards Corner Road, Suite 100
Loveland, OH 45140
Phone: (800) 337-9423, (513) 831-3211, Fax: (513) 831-1209
Manufacturer of carbon dioxide pelletizing and blasting equipment.
Mr. Michael A. Henderson

Va-Tran Systems, Inc.
677 Anita St., Suite A
Chula Vista, CA 91911-4661
Phone: (619) 423-4555, Fax: (619) 423-4604
Manufacturer of the SNO-GUN™ carbon dioxide precision cleaning system.
Mr. Jim Sloan

Source(s):

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.