Carbon dioxide cleaning

Dry-ice blasting used to clean bakery equipment

In pellet cleaning ("dry-ice blasting"), relatively large pellets of solid CO₂ are fired at the surface to be cleaned. These pellets impinge on the surface, mechanically dislodging contaminant particles. Pellet cleaning is only appropriate for surfaces sturdy enough to withstand significant impacts.[1][4]^: 276

In CO₂ snow cleaning, compressed liquid or gaseous carbon dioxide is expelled from a nozzle, condensing into a mixture of solid particles and gas, which impact the surface to be cleaned.[1][4]^: 276 Jet velocities are frequently supersonic.[8] Snow cleaning works by a combination of momentum transfer (mechanically dislodging contaminant particles) and solvent action.[1][4]^: 273 The CO₂ sublimates on contact, increasing in volume up to 800 times, thereby generating pressure to sweep particles away.[8] The CO₂ also dissolves hydrocarbon contaminants, and its low temperature embrittles residues such as fingerprints, making them easier to blow away.[2][9]

Snow cleaning has found application in the aeronautical, automotive, medical, optical, semiconductor, and space industries. It can provide a gentle cleaning, appropriate for delicate surfaces.[2][4]^: 270[9] The effectiveness of carbon dioxide snow cleaning has been demonstrated via light microscopy, particle counting, scanning electron microscopy, microprobing, X-ray photoelectron spectroscopy, atomic-force microscopy,[10][11] and mass spectroscopy.[4]^: 279

Equipment costs for a carbon dioxide snow cleaning system can range from US$1500 for a basic system to $50,000 for a high-end automated unit.[4]^: 292 Material costs are comparatively low, although ultra-pure CO₂ must often be used to avoid the introduction of new contaminants.

At temperatures and pressures above its critical point, CO₂ can be maintained as a supercritical fluid, exhibiting extremely low viscosity and high solvency. To apply this method, parts to be cleaned are enclosed in a pressure vessel that is then filled with supercritical CO₂. This method is appropriate for small and delicate parts such as microelectronics, and is not ideal for particulate removal.[12][1] Aside from cleaning, applications of supercritical carbon dioxide include targeted chemical supercritical fluid extraction and materials processing.

Liquid CO₂ washing, like supercritical fluid CO₂ washing, relies on the high solvent power of CO₂,[4]^: 275 but at lower temperatures and pressures, the latter making it simpler to implement. Because liquid CO₂ does not have the solvent power of the supercritical fluid, agitation and surfactants may be added to improve the effectiveness of the method.[1] Liquid CO₂ has been used in dry cleaning and machined parts degreasing.

CO₂ cleaning may present certain safety risks. If the process is used to remove hazardous materials, precautions must be taken to avoid exposure to these materials in the vent stream. Because the CO₂ stream is cryogenic, it may cause injury with direct skin contact. In addition, care must be taken to prevent the concentration of carbon dioxide in the work area from exceeding safe levels.[4]^: 272[15]

Some commercial grades of carbon dioxide may contain traces of heavy hydrocarbons, which can be left behind on the surface being cleaned. Abrasive particles originating in the cleaning equipment itself may need to be filtered out as well. The low temperature of the carbon dioxide stream can also induce moisture condensation on the part, which may be mitigated with hot plates, heat guns, heat lamps, or dry boxes.[4]^{: 292–294}

Ionization caused by the flowing gas can result in potentially damaging static charge buildup on non-conductive parts. This can be mitigated by grounding or positive ionization sources.[4]^: 294