P2 Tech

Technical Issues and
CO2-based Cleaning Systems


dot TECHNICAL FEASIBILITY OF SCCO2 CLEANING

dot COMPATIBILITY OF SCCO2 CLEANING WITH POLYMERIC MATERIALS

dot EFFECT OF MIXING ON SCCO2 CLEANING

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dot TECHNICAL FEASIBILITY OF SCCO2 CLEANING

SCCO2 cleaning has been tested on both a laboratory and pilot scale fairly extensively over the past decade. Published results of these tests have been primarily generated by United States federal government sources in the Department of Defense, Department of Energy, and Environmental Protection Agency, although a few published results that were located came from the private sector. Collectively, these tests have proven that SCCO2 has the potential to be a viable technical alternative for many cleaning applications found in the manufacturing environment — particularly in precision cleaning applications where the parts being cleaned have very small cracks or crevices that must be cleaned, as is the case with gyroscopes and instrument bearings. SCCO2 has proven to be especially suited to removing nonpolar, hydrophobic contaminants, which include many of the oils used in industrial and commercial applications. Some examples include:

While test results show the technology works quite effectively in many cases, the few published case studies from industry that were located during the literature search resulted in decisions not to pursue the technology for full-scale application. For example: As can be seen from these examples, while SCCO2 cleaning appears to be effective in a number of instances from a technological standpoint, some technical limitations combined with the economies of the process, which will be discussed more below, have resulted in a slow rate of implementation in the private industrial sector. Informal discussions with several vendors of SCCO2 systems that took place during this technology review indicated that none of them "mass manufacture" SCCO2 cleaning systems (as is the case with aqueous cleaning systems and vapor degreasers) because the demand for the product is not high enough. This prevents SCCO2 cleaning-system prices from being able to realize the economies of scale that take place with mass manufacturing and, subsequently, the systems are expensive.

Some of the reasons for lack of demand in the private sector for SCCO2 cleaning were identified by researchers at Pacific Northwest National Laboratory during a SCCO2 cleaning market assessment completed in 1994. (Ref. 3) Potential barriers to acceptance identified include:

The market assessment identified the following circumstances where SCCO2 cleaning would be superior: Additional technical findings in the published literature located during this review include 1) information related to the compatibility of SCCO2 with polymeric materials and 2) the effect of mixing on SCCO2 cleaning. Each topic is discussed below.

dot COMPATIBILITY OF SCCO2 CLEANING WITH POLYMERIC MATERIALS

As discussed above, SCCO2 cleaning is compatible with a fairly wide range of substrates, including most metals and glass, and many plastics. A recent study by researchers at the University of Massachusetts's Toxics Use Reduction Institute investigated the interactions between supercritical and subcritical carbon dioxide and a number of different polymers to explore in detail the applicability of using the SCCO2 for the precision cleaning of polymers. This study was performed to address the concern that using SCCO2 to clean polymers may negatively affect the polymers since many polymers are known to undergo significant absorption of gases and vapors. The absorption of CO2 into polymers can alter a number of the properties of the polymer, including increasing the melting temperature or plasticizing the material, or other changes that affect the desired physical properties of a polymer product. (Ref. 8)

An initial series of tests were performed on nine crystalline polymers, including high-density polyethylene, polypropylene, Teflon, Mylar, and Kynar. A second series of tests were completed on 11 amorphous polymers, including Plexiglas, ABS, polyurethane, and PVC. Results of the tests showed that SCCO2 cleaning can, in most cases, be adjusted to have no detrimental effect on the crystalline polymers by optimizing parameters such as the treatment or decompression time, pressure, and/or temperature. In general, it was found that the amorphous polymers absorb CO2 to a greater extent than crystalline polymers and, therefore, experience a greater amount of plasticization, making these amorphous polymers less favorable for CO2 cleaning. The amorphous polymers showed visible bending and/or bubbling of the surface in many of the samples.

dot EFFECT OF MIXING ON SCCO2 CLEANING

Two studies located during the review that discuss the effect of mixing on SCCO2 cleaning resulted in somewhat different conclusions. As mentioned earlier, a manufacturer of metal discs was not able to determine that mixing had any effect on cleaning efficiency, but that a larger sample size may have given a more definitive result. (Ref. 6)

A more in-depth study specifically designed to examine the effect of fluid turbulence on SCCO2 cleaning concluded that mixing has an effect. The study was completed by researchers at Pacific Northwest National Laboratory. A SCCO2-cleaning chamber equipped with an impeller was used in the experiment. Tests were done using stainless steel coupons. One series of tests were done with the coupons contaminated with silicone oil, and the other with high-temperature oil. In both tests, the percent of contaminant removed from the coupons increased as the speed of the impeller was increased until a maximum percent removal was reached. Further increases in impeller speed did not have a significant effect on cleaning efficiency. The researchers recommended that agitation be used whenever possible for SCCO2 cleaning applications to help maximize cleaning efficiency, and that mixing rates can be optimized to minimize power costs. (Ref. 9)

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