Sodium bicarbonate stripping processes are used as alternatives to traditional chemical paint stripping. Bicarbonate of soda (or sodium bicarbonate) is a soft blast medium with a heavier specific gravity and less hardness than most plastic abrasives. The bicarbonate of soda stripping process can be used with or without water. It is most frequently used with water, which acts as a dust suppressant. In this form, compressed air delivers sodium bicarbonate media from a pressure pot to a nozzle, where the medium mixes with a stream of water. The soda/water mixture impacts the coated surface and removes old coatings from the substrate. The water dissipates the heat generated by the abrasive process, reduces the amount of dust in the air, and assists in the paint removal by hydraulic action. Workers need to mask the surface of the material being stripped to prevent intrusion of bicarbonate of soda blast media. Blast media begins to decompose at 140° Fahrenheit and may become corrosive if left entrapped within a structure. Settling or filtration can separate the solid residue from the wastewater generated from this process.
The effectiveness of bicarbonate of soda stripping depends on a number of operating parameters, including: nozzle pressures, standoff distance, angle of impingement, flow rate, water pressure, and traverse speed. In general, bicarbonate of soda stripping systems remove paint slower than most methods (other than chemical paint stripping) currently used. The type of equipment used in this stripping process may also influence results (e.g., Aqua Miser® vs. the Accustrip System®).
Use of sodium bicarbonate in its dry form (or when not fully mixed with water) can create a cloud of dust that will require monitoring and may require containment to meet air standards. The dust generated is not an explosive hazard, nor is sodium bicarbonate toxic in this form. However, the airborne particulates generated from the stripping operation can contain toxic elements from the paint being removed. This stripping process should be performed in areas where exhaust particulates can be contained and/or exhaust ventilation system controls are present to remove hazardous airborne metals. If bicarbonate of soda stripping is operated outdoors, air monitoring of dust (e.g., for metals) may be necessary to ensure that air standards are met. However, tests have shown that lead will adhere to the sodium bicarbonate, thus reducing the risk.
The waste generated from bicarbonate of soda stripping systems using the wet process is a slurry consisting of sodium bicarbonate media, water, paint chips, and miscellaneous residues such as dirt and grease. Some installations are employing centrifuges to separate the water from the contaminated waste stream, thus reducing the amount of hazardous waste being disposed. Filtered wastewater containing dissolved sodium bicarbonate may be treated at an industrial wastewater treatment plant. In the dry stripping process, waste generated includes nuisance dust, paint chips, and miscellaneous residues such as dust and grease. The solid waste may be suitable for disposal in a sanitary landfill. Analysis of wastewater and waste solids is required prior to disposal. Wastewater and bicarbonate residue disposal requirements will depend on the toxicity of the coatings and pigments to be removed. The sodium bicarbonate media can not be recycled. The paint chip and miscellaneous residue wastes may be considered a hazardous waste.
Currently, bicarbonate of soda stripping is not approved by NAVAIR for depainting aircraft. NAVAIR’s primary concern is that at temperatures of 140 to 160 degrees Fahrenheit, sodium bicarbonate may convert to sodium carbonate, which is corrosive.
The Air Force has expressed the same concerns with the sodium bicarbonate stripping process as NAVAIR for depainting aircraft. The process is currently only being utilized as a supplementary process to chemical paint stripping for C-130 and C-141 aircraft. With regard to component stripping, where intrusion is not a significant factor, this process offers a viable alternative to chemical stripping.
This technology has been tested at the USMC Logistics Base, Albany, Georgia. They tested sodium bicarbonate media in three types of commercial off-the-shelf equipment. The cost of these systems ranged from $15,000 (Accustrip 16W) to $40,000 (Aqua Miser E25 and Jet Stripper DP-1). The Aqua Miser and Jet Stripper use medium-pressure water to remove paint with sodium bicarbonate injected to enhance removal effectiveness (with the Aqua Miser system, the sodium bicarbonate flow can be turned on and off as necessary). Accustrip uses a stream of 30 to 90-psi air and sodium bicarbonate, and is combined with a stream of blast water at the nozzle to try to eliminate the dust. In both cases, the sodium bicarbonate medium is then propelled against the surface, and the sharp edges of the media blast paint away from the base metal.
Use of sodium bicarbonate paint stripping as a replacement for chemical paint strippers results in the following compliance benefits:
- Elimination of volatile organic compounds (VOCs) used as strippers which are associated with the formation of smog typically regulated by federal and state agencies as well as local air pollution control districts.
- Elimination of hazardous air pollutants (HAPs) in strippers which are regulated by federal, state, and local regulations including the National Emissions Standards for Hazardous Air Pollutants (NESHAPs) (40 CFR 63).
- Elimination of all SARA Title III reporting substances from coating process (by the Navy) (40 CFR 300, 355, 370, and 372).
- Reduced volumes of organic wastes, which must be managed as hazardous waste under 40 CFR 260 and related sections.
- Reduced hazardous materials usage as required of federal facilities by Executive Order (EO) 13148, Greening the Government Through Leadership in Environmental Management.
- Reduced occupational exposures, which are regulated under 29 CFR 1910.
Compliance benefits include: 1) reduction or elimination of recordkeeping and reporting requirements under the Title V Operating Permits Program, NESHAP Program and SARA programs; 2) reduce administrative burden associated with hazardous waste (i.e., tracking, plans, reports, training); and 3) reduced administrative burden associated with OSHA (i.e., training and recordkeeping).
The compliance benefits listed here are only meant to be used as general guidelines 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.
Material compatibility must be evaluated with respect to the item being cleaned. Uninhibited sodium bicarbonate and water residue can corrode substrates; however, current testing indicates that the corrosion potential of inhibited formulations is similar to that of organic solvent strippers. Results from an Air Force test program (Tasking Directive I-90) indicated that there was excessive corrosion of aluminum cladding materials when sodium bicarbonate and water slurry were used.
Safety and Health
Health concerns are dependent on the variety of paint to be removed. Inhalation of lead- and zinc chromate-based paints can lead to irritation of the respiratory system. Some lead compounds are carcinogenic. Solvent-based paints can irritate the lungs and mucous membranes. Prolonged exposure can affect respiration and the central nervous system.
Because of the noise and dust produced (the amount of dust produced will vary from system to system), a sodium bicarbonate stripping system should only be operated in an isolated area outdoors or indoors in a confined or remote area. Operators must wear double hearing protection – an air-hood blast helmet with an air-supplied respirator and optional half mask (for those blasting systems that generate large amounts of dust), or a full-faced air purification respirator with HEPA filters – and protective clothing (e.g., rain suits, rubber gloves, and safety-toed rubber boots, depending upon the substrate coating).
Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing this technology.
- Significantly reduces in the amount of hazardous waste generated compared to chemical stripping.
- Reduces the number of hours required for paint stripping in comparison to chemical stripping.
- Selectively removes individual coating layers.
- Prewashing and masking is not required in most applications.
- No size limitations for parts being stripped.
- Wastewater stream may be centrifuged to reduce its volume or treated (if required) at industrial wastewater treatment plants.
- Blast media is usually less expensive than PMB, wheat starch, and CO2 pellets.
- Requires subsequent washing of the item; thus, electrical components cannot be exposed to this stripping process.
- The sodium bicarbonate solution can not be recycled for stripping, although the water can be separated for disposal.
- Process may require monitoring.
- Containment may be required.
- Not approved by NAVAIR for depainting aircraft.
|Annual operational costs for depainting 228 aircraft wheels with bicarbonate of soda stripping system (Accustrip) compared to chemical paint stripping were evaluated at the Lyndon B. Johnson Space Center, Houston, Texas, as shown below.Assumptions:
- Accustrip System cost: $20,000.
- Compressor, trailer mounted: $20,000.
- Materials and installation: $125,000.
- Other significant costs (engineering, contingency, startup): $60,000.
Annual Operating Cost Comparison for Bicarbonate of Soda Stripping and Chemical Paint Stripping
Bicarbonate of Soda Stripping
|Total Operational Costs:
|Total Recovered Income:
|Net Annual Cost/Benefit:
* Cost is based on the material cost of $1,600 for chemical paint remover at $10/gallon, the material consumption is 160 gallons. Paint/solvent disposal is $2.10 to $14.00 per gallon, depending on the specific solvents and paint in the waste. Using the high estimate of $14.00 per gallon, the cost for waste disposal is $2,240. For waste water treatment an extrapolation was based on the $8.24/1000 gallons figure from the Plastic Media Blasting Paint Stripping data sheet, a conservative estimate of 70,000 gallons of waste water, and a waste water treatment cost of $580. Therefore, the total waste disposal cost is $2,800.
Economic Analysis Summary:
- Annual Savings for Bicarbonate of Soda Stripping System: $13,500
- Capital Cost for Equipment/Process: $225,000
- Payback Period for Investment in Equipment/Process: < 17 years
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|High Pressure Cleaner
|High Pressure Cleaner
|Waste Water Centrifuge
|BOSS BLAST Blast Media
||49/50 lb Bags
|Blast Cleaning Contain. Room
*There are multiple MSDSs for most NSNs. The MSDS is only meant to serve as an example.
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|Appropriate authority for making process changes should always be sought and obtained prior to procuring or implementing any of the technologies identified herein.
This process is contained in T.O.1-1-8, but the use of it requires approval by the engineering authority of the specific Weapon System Manager or Equipment Item Manager with the Air Force. Approval has not been granted by NAVAIR for application on aircraft and aircraft components without pretreatment of aircraft substrate to remove all blast media.