5-17 UNICARB® Supercritical CO2 Coating Spray System
UNICARBŪ spray system, patented by Union Carbide (now part of Dow Chemical
Corporation), uses carbon dioxide (CO2) in its supercritical form as a solvent
for coating materials. As a result, a portion of the organic solvents that
are currently present in coating formulations can be removed; thereby, lowering
volatile organic compound (VOC) emissions, while retaining coating sprayability.
Theoretically, coatings applied by the UNICARB system are no different than conventionally applied coatings. In a cured coating film, the delivery solvent rarely plays a role. Its only purpose is to lower the viscosity so that the material can be sprayed. After application, the delivery solvent evaporates and leaves the film. Supercritical CO2 is used by the UNICARB system to replace the organic delivery solvents. Other solvents and materials such as the resin system, pigment package, and other additives remain unchanged. The CO2 supplies an atomization mechanism and is vaporized from the coating during the atomization process.
For material to be used in the UNICARB system, the coating supplier must know of the intent to use supercritical CO2 as the solvent. Testing is required to ensure the CO2 will properly dissolve in the organic coating material, provide adequate solvent properties, and is compatible with the other materials. Other major components of the coating system remain unchanged.
The UNICARB equipment is more complex than conventional spray systems although its operation is as simple. Operators and facility maintenance personnel would require three days of training with the equipment. Operators would need to be trained on the aspects of airless sprayers, specifically the effect of nozzle selection on spray pattern and application rate. Typical spray practices will need to be followed. The operator would be able to select the correct nozzle (spray pattern) for different parts, and personnel spraying preferences.
Once the ratio of paint to CO2 is set for a given coating, it will not require changing. However, regular maintenance is necessary to ensure correct calibration of critical measurement components. If these components are neglected, an improper mixture of coating and CO2 could occur, resulting in application problems. One 50-lb CO2 cylinder should supply the needs of 19 trailers.
Use of the UNICARB
CO2 system results in the following compliance benefits:
Compliance benefits include: 1) elimination of recordkeeping and reporting requirements under the Title V, NESHAP Program and SARA programs, 2) reduced administrative burden associated with hazardous waste (i.e., tracking, plans, reports, and training), and 3) reduced administrative burden associated with OSHA (i.e., training and recordkeeping).
According to an Environmental Protection Agency (EPA) legal opinion dated April 10, 1998, the agency has the authority to regulate CO2 as a pollutant under the Clean Air Act. Currently the EPA has no plans to do this. However, the UNICARB process does not produce any new CO2 emissions that would be regulated. Rather, existing CO2 is used for the process and returned to the environment; additionally, the volume emitted from a UNICARB spray operation would be considered infinitesimal in comparison to other major CO2 emitters (such as power plants).
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.
No materials compatibility issues were identified. However, as with all coatings, coatings used by the UNICARB system require a well prepared, clean surface for optimum adhesion and protection performance.
|Safety and Health:||
The UNICARB system may be used in conjunction with an electrostatic paint spray system. Electrostatic spray guns operate at high voltages (30 to 150 kV). Hence, operator safety is a major concern. All items in the work area must be grounded, including the operators, the paint booth, the application equipment (unless applying conductive coatings), and conveyors. Ungrounded items should be removed from the work area. Removing paint buildup from the paint booth helps ensure that workpieces are grounded. Workers should never wear rubber- or corked-soled shoes (special shoe-grounding devices are available). Adequate skin contact is required when using hand-held guns. Painters should grasp the gun with bare hands or use gloves with fingertips and palms cut out.
Proper design, operation, and maintenance of the equipment is required for its safe use. The spray booth must be well ventilated. Additional health concerns depend on the coating being applied. Solvent-based coatings can irritate the lungs and mucous membranes. Prolonged exposure can affect the respiration system and/or the central nervous system. Proper personnel protective equipment should be worn, if required.
Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies.
Material costs are shown in Table 1.
Equipment costs range from $40,000 to 90,000. This range takes into account
other factors that affect costs, including coating recirculation, application
rate, number of spray guns used simultaneously per system, spray gun (electrostatic/manual),
and operator interface/operations data requirements. Recent technological
advancements in equipment design have firmed the cost for the equipment
to spray this coating at $50,000.
|Operational Cost:|| The
operational cost per trailer is $58. This figure is based on the following
Note: Utility costs
were not included, and could be considered insignificant.
The calculated payback period for investment in the equipment/process: 0.8
years, using an equipment cost of $40,000 and a production rate of 12 trailers
per week (624 per year).
potential advantages/savings of transitioning to a UNICARB® system for
a production of 12 trailers per week were as follows:
Approval is controlled locally, and the technology should be implemented only after engineering approval has been granted. Major claimant approval is not required.
|Points of Contact:||
James E. Tardoni
Dow Chemical Corporation is the patent owner of the system, and Linden EMB is a manufacturer of the equipment.
Concurrent Technologies Corporation. UNICARB CO2 Painting Demonstration for Rock Island Arsenal (RIA). Demonstration Data Analysis/Report. October 16, 1998.
|Supplement(s) to the Data Sheet:||
A simplified schematic of the Linden system is presented below: