This column reports on three unrelated outcomes about which you need know to manage critical cleaning work, and points out their significance.
IT’S NOT OVER UNTIL WE SAY IT’S OVER… IT’S OVER!1
Trichloroethylene (TCE) has been found to be a human carcinogen.
In early October, EPA released its final health hazard assessment categorizing TCE as “…carcinogenic to humans and as a human non-cancer health hazard…” This assessment is part of EPA’s Integrated Risk Information System (IRIS) which conducts scientific reviews of laboratory and epidemiological data upon which some agency decisions are based.
Many had thought it was so, for years, and managed their affairs accordingly.
Many others had chosen to believe that it was not so, and used it per NESHAP equipment requirements because of the absence of other options. That dearth of options hasn’t been returned to abundance by this news.
The practical meaning of this assessment is substantial. EPA will now have a stronger basis to:
- Revise the Maximum Contaminant Level for TCE as part of the carcinogenic volatile organic compounds group in drinking water,
- Establish benchmarks and methodology for cleanup of the 761 Superfund sites where TCE has been identified as a contaminant,
- Provide legal reference in personal injury lawsuits, and
- Revise downward the workplace exposure limit of this Hazardous Air Pollutant (HAP).
Make no mistake, those using TCE in cleaning operations would do well to view the common alternative to TCE (n-propyl bromide) as if this finding applied to it five-to-ten years from now,2 and consider other alternatives such as binary azeotropes and solvent blends.
Materials scientists at the University of Birmingham (UK) have devised a way of making stainless steel surfaces resistant to bacteria. Their technique not only kills bacteria but makes the material very hard and resistant to wear during cleaning. It involves infusing Silver (or Copper), Nitrogen, and Carbon within the steel surface (rather than coating it on to the surface).
They have replicated the FDA process for cleaning medical instruments,3 applied it 120 times, and found that the antibacterial properties of the stainless steel were still intact and the surface still resistant to wear.
Intended applications are: hospitals, food service, and dentistry. To my mind, current proper sanitary practices will be more cost effective until (and if) volume applications are developed. It’s too soon to know if this technology is a curiosity, or can be later accepted by the FDA.
An interesting and recent paper4 describes experiments in cleaning of Silicon wafers (to be used in photovoltaic cells) of nanometer-sized particles without use of either of the customary cleaning chemistry systems: SC1 (H2O/H2O2/NH4OH: 40/2/1) or piranha (3:1 volume solution of H2SO2 and 30% H2O2).
The method was to use acoustic streaming pressure waves produced at super-megasonic frequencies in a hot (80°C) bath of distilled water. Test objectives were to: (1) minimize surface damage, (2) avoid use of corrosive chemicals, and (3) learn if/how #1 and #2 could actually be done with organic contamination and silica/silicon nitride particles between 10 and 100 nm in size.
Results were not defining of a commercial process. But removal of organic contamination was equivalent to that obtained with the piranha solution. However, the removal mechanism was unclear (though it was not direct oxidation). At short contact times of cleaning (20 min.) wafer performance in photovoltaic service was acceptable; at longer contact times (60 and 120 min.) surface degradation had taken place.
Clearly, if this process could be understood and made repeatable it could have substantial value in critical cleaning in the photovoltaic and microelectronic industries.
- Actually, it may not be over–thought it should be so. Problems with EPA’s methodology in their IRIS have been identified by the Government Accounting Office, and the National Academy of Sciences (NAS). Some industry participants feel that a recent study completed by the NAS of use, disposal, and groundwater contamination by TCE at Camp Lejeune produced evidence called “convincing” by EPA was at most “limited or suggestive of an association.”
- See http://ntp.niehs.nih.gov/go/34797.
- Not the one proposed for comment in May 2011, which can be studied at http://www.fda.gov/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/ucm252999.htm., but the procedure described in 21 CFR Part 801.
- Podoliana, A., Nadtochiya, A., Kuryliuka, V., Korotchenkova, O., Schmidt, J., Drapalik, M., and Schlosser, V., “The Potential of Sonicated Water in the Cleaning Processes of Silicon Wafers,” Solar Energy Materials and Solar Cells, Vol. 95, No. 2, February 2011, pages 765 to 777.
John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0-0804-48887). He is the author of the forthcoming book Solvent Cleaning for the 21st Century, also to be published by Elsevier, and is an independent consultant specializing in critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612- 677-3170; or email@example.com.