Methods for testing cleanroom garments
Standard tests for comparison of garment systems are still non-existent, and the search continues for better, more repeatable data that demonstrates a real correlation to cleanliness level.
By Susan English-Seaton
Best methodology for cleanroom garment testing has lagged far behind user requirements in the most critical areas of ultraclean manufacturing: semiconductor wafer fabrication, disk drive and electro-optical device. While garment processors who service the pharmaceutical/biotech/medical device sectors are governed by sterile requirements, laundries servicing electronics markets are governed by no such regulatory interests. Yield-driven, semiconductor manufacturers, for example, are primarily interested in managing particles for defect reduction. When it comes to cleanroom garment systems, most manufacturers rely on laundries or cleanroom garment processors for the bulk of the testing. Tests such as those outlined by the Institute of Environmental Science — ASTM 51: Alternate Method, Primary Procedure, body box, liquid extraction, and the Helmke Drum — are among the widely and routinely employed tests to calibrate the elusive balance between comfort and protection from contamination.
However, a lack of uniformity in the application of methods and instrumentation, a lack of repeatability and, according to experts, most significantly, a lack of correlation of the data to actual cleanroom classification means that the research for more sophisticated and reproducible tests continues. The Institute`s own guidelines include a caveat: “No direct correlation has been established between test methods and their relationship to air cleanliness, and none is implied by this document.” Despite recent attempts to promote alternative particulate testing methods, there is still a need for the development of well-thought out and controlled experimental data subject to peer review and publication, says cleanroom consultant Chuck Berndt of C.W. Berndt Associates Ltd. (Highland Park, IL). Test standards and methodology for cleanroom garments are set forth in non-mandatory guidelines published by the Institute of Environmental Sciences, specifically, Working Group 003 “Garments Required in Cleanrooms and Controlled Environments” and its related document Recommended Practice (IES-RP-CC003.2) “Garment System Considerations for Cleanrooms and Other Controlled Environments.” Other relevant standards incorporated in the guidelines are those of the Association for the Advancement of Medical Instrumentation (AAMI), which sets forth test methods and procedures for pharmaceutical and medical cleanrooms, including sterilization; the American Society for Testing and Materials (ASTM); Military Standards; and the EOS/ESD Association.
Bob Spector, chairman of the Institute`s Working Group 003, says: “The end-user can do the same test a mill would perform to confirm the fact that it is not just marketing data they`re looking at. It`s not that you don`t trust somebody else — your environment is different or there may be a detail they haven`t taken into account.” Spector says the group has recently reviewed some wet test methods, but has not agreed to pursue them because they related more to fabric testing than garment testing. The most widely used garment tests are outlined below:
Primary test procedure
A non-destructive particle-count test, the primary test procedure is modified from ASTM F51: Alternate Method (Woven Fabrics). The original ASTM 51 was limited to small area sampling (46.5 cm2) by vacuuming the garment, collecting particles on a 0.8 µm membrane filter and microscopically counting and characterizing particles and fibers equal to or greater than 5.0µm in size. The newest version of the test calls for vacuuming one-square foot of a garment, using the same microscopic counting protocol as the original. It is widely used and still the accepted test method in the pharmaceutical, medical device and other health care-related industries. This version, however, only allows for testing of a coverall — the only piece of apparel that offers the required one-foot-square seamless surface.
Particle containment (body box)
The so-called “body box test” or particle containment test is used to determine relative differences between various sets of apparel while being worn. It measures the ability of the fabric to retain particulate and determines whether garment closure systems, for instance, are effective. Held in a small dispersal test chamber, it is designed to determine particle release rate and size spectrum from the operator/garment system in motion, following a predetermined mobility protocol. Some IES test panelists now feel that the approach should be refined and improved by adding strategically located, multiple probes with aerosol concentrators and automated data acquisition to determine particle release at different locations. This would facilitate real-world assessment of operator/garment system aerosol releases and their location at actual point of entry into the cleanroom, possibly by incorporating it as an integral component of the air shower.
Helmke drum test
A non-destructive particle-counting tumble test designed to measure particles released at 0.5 microns or greater over a tumbling garment, the Helmke drum test is widely used by cleanroom laundries, or garment processors, to determine the efficiency of the cleaning process. The garment is folded and placed in a rotating drum open at one end and rotated at 10 rpm, ۪.1 rpm from the test article. An optical or laser automatic particle counter is used to sample the air within the drum to determine the average particle release rate in particles per minute for particles equal to or greater than 0.3 µm. The particle counts and size spectra are determined at ten-minute intervals during a five-or 10-minute test period. Results are tabulated and analyzed for average, mean and median value as a function of differing size intervals. However, the Helmke Drum Test does not discriminate between particles and fibers and release of contamination from the inside and outside of the garment, unless all garment closures are sealed during the test — an important requirement when testing laminated barrier garments.
Liquid extraction
The liquid extraction test can be used to determine total number of particles on a garment and their chemical composition. A one-foot-square sample of the garment is immersed in boiling solvent for a specified period of time. The solvent is then passed through a membrane filter to remove particles and fibers. Gravimetric techniques are used to determine the amount of extractables in the solvent filtrate. However, because the test is destructive, test samples should be processed together with the garments, so the entire test garment is not destroyed.
ESD testing
IES-RP-CC0022, “Electrostatic Charge in Cleanrooms and other Controlled Environments,” features a real-time, non-destructive test for measuring the static-dissipative characteristics of a garment in real time. Conducted under defined conditions of relative humidity and temperature, the test measures surface resistivity on a one-square-foot swatch, which is vacuum probed and measured for its ability to conduct electricity. The test does not, however, have good repeatability, and in the opinion of testing experts, surface resistivity is a “poor surrogate” for triboelectric charge and static decay measurements, which currently can only be performed destructively on small samples of fabric. The document was recently reopened for the purpose of developing a test with better repeatability. This month, the IES WG22 Committee “Static Decay on Cleanroom Garments” will meet to discuss updating the RP document, according to chairman Edward Davis, who also serves as a liaison between the Working Group and the ESD/EOS Association. The latter also continues to work on the development of a real-time, non-destructive test to measure these parameters.
Sterilization
The two most popular forms of sterilizing garments for use in aseptic processing environments — pharmaceutical, biotech, medical device — are ethylene oxide (EtO) and ionizing radiation (gamma or electron beam). Although cleanroom garments are not regulated as medical devices, when laundered and then sterilized by gamma electron beam, they require validation according to guidelines set forth by the Association for the Advancement of Medical Instrumentation (AAMI). EtO sterilized cleanroom garments require sterility testing of biological indicators — spore strips of Bacillus subtilis at a population of 10 6, as described in the United States Pharmacopeia (USP). A minimum quarantine period of five days is mandated prior to allowing release and certification for sterility, while ionization radiation sterilization (Cobalt 60 and E-Beam) require no quarantine period or USP testing procedures (dosimetric release).
Not all garment processors who service the pharmaceutical/biotech and health care markets validate their processes according to AAMI guidelines, and they turn to local independent laboratories for sterilization. Micron-Clean Uniform, Inc., a garment processor based in Newburgh, NY, has its own testing laboratory and validates its processes. “Because the industry is primarily interested in viable organisms, we do exhaustive bioburden analysis and routine bioburden testing within the cleanroom,” says Brad Whitsel, the company`s vice president of technical operations. “They`re also interested in pyrogens — particles that can cause inflammation. You might just have somebody in there who isn`t wearing their mask properly. It doesn`t mean you`re not cleaning the garment well enough, but letting us look at the entire room as part of the system is important,” Whitsel says.
Whitsel feels that some day, laundries that serve the semiconductor industry will need to address the issue of bioburden and biopermeation, or microbiological grow-through of bacteria generated by the body`s protinaceous microenvironment inside the garment. He likens a cleanroom garment to a “culture,” entraining and trapping bacteria, which can be brushed off as particles — particles that multiply.
End-user testing
At Lucent Technologies in Allentown, PA, senior engineer Joe Rauchut uses some sophisticated testing methods to ensure that his garments — going on their seventh year of use — are not generating contamination. Because he didn`t believe “the experts” who told him that garments should be scrapped every three years, Rauchut decided to push the envelope, using SEM analysis to be sure garment fibers weren`t breaking down. “We make ASIC devices, so we`re very particular about what goes in the cleanrooms. Besides extractables and body box testing, twice a year, we do SEM analysis right down to the fiber.” Rauchut says the tests are actually not very time-consuming and that “once the people are in tune with what you want, it`s fairly simple and cheap. The cost savings of keeping material beyond the so-called three-year lease limit is rather substantial.”
Dave Hope, microcontamination manager at Intel Corp. (Hillsboro, OR) uses the Helmke Drum Test for particles and extractables for ionic testing, although he says ionic testing is probably most applicable during a qualification exercise for a laundry perhaps on an annual basis. The rest of “keeping the laundry honest,” he says, is to verify that it employs a good statistical process control program and is willing to share the data.
On the horizon
“There really is nothing new in the way of testing,” concludes Working Group 003 Chairman Bob Spector. “Companies are looking to go to the next level rather than replace garments.” Craig Woodward, director of communications at Kappler Protective Apparel & Fabrics (Guntersville, AL), says there`s “a whole new generation of fabrics for disposable garments peeking over the horizon that could really change the nature of disposable cleanroom garments.” What he`s referring to is polypropylene fabric — microporous film married to a non-woven fabric — developed by the company for use in the industrial market and now being proven out for cleanrooms (See “Cost, Comfort and Contamination Control Drive Cleanroom Garment Trends,” CleanRooms, Sept. 1996, p. 43). “The benefit is that not only is it a superior barrier to traditional fabrics, but it`s breathable because the technology allows body vapor to escape at a higher rate.”
Editor`s Note: To obtain copies of IES-RP-CC003.2: “Garment System Considerations for Cleanrooms and Other Controlled Environments,” contact the Institute of Environmental Sciences, 940 East Northwest Highway, Mount Prospect, IL 60056, Tel. (847) 255-1561, fax (847) 255-1699.
To become a member or to participate in the work of IES Working Group 22 “Electrostatic Charge in Cleanrooms and Other Controlled Environments,” contact Edward Davis, chairman, (330) 673-7413.
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(TOP PHOTO)
Twice a year SEM analysis and weekly Helmke drum test data assures a polyester garment is wearing well, even after six years. At Lucent TechnologiesU nearby Bell Labs Environmental Laboratory, Ion Chromatography (IC) will
be used to evaluate any particles lodged in the fiber material after laundering.
(MIDDLE PHOTO)
High magnification SEM
of a garment in its second year of use, showing excessive wear. Degradation
can be caused by inferior material, general abuse, laundry solutions that are too harsh, overheating of
material, etc.
(BOTTOM PHOTO)
Representative samples
of a 6-year-old polyester garment obtained by scanning electron microscopy (SEM) at Lucent Technologies. Because it has been carefully maintained and tested, the garment shows little wear, even into its seventh year of wear.
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Reaching protocol performed at W.L. Gore`s test cleanroom on a Gore-Tex garment using a Gorpler aerosol sampler.
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The Motorola team used the data to determine frequency with which the garments needed to be laundered. The technique used was to touch the probe of a Dryden QIII Surface Particle Detector to the fabric, moving it a specific length for six seconds over five frontal areas of the garment and the back. From the garment`s first use after being laundered to its removal after the third day of usage, average particle counts escalated from about 100 to almost 1200.
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To `Helmke` or not to `Helmke`U ?
Nora Sylvestre,product specialist, W. L. Gore & Associates (Elkton, MD) We have our own laundry where we do product development, so we use the Helmke to test whether the laundry is functioning correctly. We don`t generally use it as a test for our garments because we use so many other fabric tests. We will do it as a comparative, first-line test to see what`s happening with a garment as a predictor. Then we use what we call an “in-use test” to test a garment`s performance. The test has its value if you do enough tests and understand what the limitations to the system are. Another thing is that if you want to learn about something that is happening with the garment, you need to look at the whole set of data — not just the final data. A lot of laundry people will take a five-minute sample, then take the total at the end of five minutes and divide it by five. When I worked at AT&T, where the Helmke test was originally developed, we actually did a ten-minute sample. But you recorded each sample after one minute, and then looked at the results. What you would very often find was that the particle counts were very high in the first two minutes. The third minute, the count would start to drop off, and by the end of ten minutes, it would drop down to practically nothing.
What you learned from that sequence was that the particles coming off in those first two or three minutes were probably a residue of the laundry process itself. Then at the end ten minutes, if you leveled out and stabilized to maybe 25 or 30 half-micron size particles, that told you, this is what`s coming off of the garment. No one looks at that sequential data anymore. Let`s say you did a ten-minute sample, and it started to climb and went up to 1,000 particles per minute, and it stayed high for ten minutes: then I`d say my garment has a problem, the idea being that if the particle counts stay high for a long period of time, it`s probably something more related to the composition of the garment itself rather than the laundry process.
Joe Rauchut, senior engineer at Lucent Technologies (Allentown, PA): We obtain Helmke data from the supplier who launders our garments and then test in the Helmke Drum here at our facility just to double-check to make sure things are in line. I have a database that goes back many years on all kinds of consumables — gloves and garments, etc. — and it`s extremely accurate. With Helmke, as long as you stay with the same particle counter, the same set of conditions and the same cleanroom environment, it`s relevant and the information is very accurate.
Philip Mann, manager of technical support, Kappler Protective Apparel and Fabrics (Guntersville, AL): We`re basing everything on the Helmke Drum, but the problem is, there`s no correlation data. No standards for any recognized correlations exist between how many particles a garment sheds and the appropriate classification of cleanroom. People say, “I`m in a Class 100 cleanroom.” Then they say, “How clean is your garment? You ask, “Well, how clean do you need it to be?” “Well, I don`t know.” It`s difficult to know how to go out and market the garment. Say you`ve got a garment that sheds 1,000 particles a half-micron or larger; the next question is, is that garment appropriate for a Class 1,000 cleanroom? If it`s 1,000, it cannot have more than a thousand particles of a half-micron in a cubic foot. But how big is your room? How many people are in it? What are the other contributors to contamination? What`s your airflow? There`s all these other things you can`t control and make consistent from room to room. It makes it very difficult for the user to select the appropriate garment. So what happens in many cases is that it becomes more of a trial-and-error thing.
Brad Whitsel, vice president of technical operations, Micron Clean Uniform Service (Newburgh, NY): The value of that Helmke Tumble Test is that it gives me a baseline of data to work with, and if I make a change to my process it shows me how that affects the cleanliness of the garment. There is some variation with respect to exactly the way the test protocol is executed. But the important thing about the test is that for each laundry over a period of time, their equipment and protocol gives them a marvelous baseline of data, which lets them know if their process is in control. Laundries must have a test that can be routine and reasonably inexpensive; otherwise, it`s cost prohibitive, and will the customer bare that additional cost? No, they won`t, because it`s just so incredibly competitive out there! In addition to using the Helmke Tumble Test as one of our primary cleanliness certification tools, we validate our process — just like any pharmaceutical manufacturer.
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The Helmke Drum Test specified in the IES-RP-CC003.2. a non-destructive particle-counting tumble test designed to measure particles released at 0.5 microns or greater over a tumbling garment. It is widely used by cleanroom laundries to test the efficiency of the cleaning process.