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First Published in EOS/ESD Technology April/May 1992 Evaluating ESD Smocks An effective smock depends not
only on the fabric used to make it, but also Gary L. Johnson Dan D. Steffe There are many questions and concerns that come up when evaluating static-control smocks, including garment comfort, durability and the effectiveness of static control. This paper will describe how to properly evaluate smocks, by using various test methods and the help of a cleaning vendor. Smock Evaluation Smock evaluation can be a very confusing process, especially considering there are as many test methods and test results as there are garment manufacturers. Most of the testing addresses fabric performance and not coat performance. One exception is the current EOS/ESD Draft Standard No. 2 on smock testing, paired with a point-to-point resistance check. This method allows smock performance and fabric performance to be analyzed and tested as separate items. Test results showed that a good-performing fabric did not necessarily translate into a good-performing smock, unless certain modifications were made. Once an acceptable smock was designed, the issue of smock preservation should be addressed, along with how different cleaning procedures will affect smock performance and life. Test Methods Many different test methods were evaluated. While a few appeared to offer a correct approach to testing, they did not yield repeatable results. The desired test would be one that the material and smock manufacturers could set up for in-house testing, as well as one that end-users could perform if needed. The four test methods we found to provide consistent
results from one sample to the next Decay testing. Many versions of decay tests were evaluated, including some as simple as rubbing a static generator on the smock material and observing the total voltage generated and the decay time. The test we found to be the most consistent was the method described in EOS/ESD Draft Standard No. 2. Figure 1 illustrates the test setup.
The test requires isolating the smock and charging the whole garment to 1500V DC. The voltage is applied to a contact point on one sleeve, while the voltage from the other sleeve is monitored with a non-contacting meter. The voltage is removed and this same point is grounded. The voltage's decay time is monitored by the meter, which is connected to a computer system to allow the decay curve to be recorded and plotted at a later time. Resistance testing. Two resistance tests were evaluated: ASTM D257 surface receptivity and surface-to-surface. The ASTM D257 test is probably the most used and misused test method on the market today. It is very dependent on the type o weave for the conductive thread, and can produce misleading results. The surface-to-surface test was run with two 5-lb. probes. With this test, the resistance can be checked across seams or from proint-to-point on the same section of the garment. Visual testing. The main purpose of this test was to look for flaws in the garments or for broken conductive threads. Visual tests were performed using a microscope Voltage retention. This test was run with the same setup as the decay test. A charge was applied to the garment and monitored for an even distribution throughout. Then the garment is discharged and retested for any voltage remaining on the garment. The garment at this point should measure zero volts. A handheld static voltmeter was used to run this test. Material Types The history. The material used to make ESD smocks has gone through many changes in the last 40 years, from 100% cotton to rayon to carbon-suffused nylon. The 100% cotton lab coat, with its neutral triboelectric position, was used in the early clean room stage because of its hygroscopic properties. As technology moved forward, the use of 65% polyester, 34% cotton, and 1% stainless steel was used in the operating room where flammable gases were used in surgery. This material was also used in the electronic production areas to control static charges generated by the operator's clothing. The problem with stainlesssteel fibers falling from the coat became disastrous as electronic parts became smaller and more sensitive. Carbon-suffused nylon. The fiber manufacturers reacted with the development of a carbon-suffused nylon which could be woven into fabric. When the fabric was woven, the weave formed a stripe, diamond or check pattern. This carbon-suffused nylon made a circuit path for the static charge to travel. The carbon-suffused nylon fiber can be woven into a number of materials and combination of materials, including:
Evaluating the material is the first step in the smock evaluation process. If the material doesn't test consistently, the end product will be erratic. Material testing. The decay test method used to test finished smocks can also be used for material evaluation. Decay testing can be used to verify the decay time of the material and set a standard to be used after the smock is made. Voltage retention tested at the same time as decay will let you know if the material is charging and discharging uniformly (typically referred to as looking for hot spots in the fabric). Resistance measurements of the material will give some guidelines for future testing on the smocks. Grounding. One of the most critical items to check on material is its ability to ground the finished smock. The most common method of grounding is contact at the wrist by the inside of the material. If the conductive threads do not make contact with the bare skin, the material will not bleed a charge away. A visual check with a microscope will show how the conductive thread is woven into the fabric. Also, a visual check can reveal if there are any broken threads or potential problems where the thread can snag and break. Comfort. Weight of the material and its ability to allow air to pass through are very critical for comfort, an important material selection factor. Some of the material that tested high on the scale for ESD properties was disqualified for comfort reasons. Design and Testing of Smocks Several different types of smocks that were available, varying both in material and design, were selected for evaluation (since the completion of our testing, newer types of smocks have become available). The samples included both new and used smocks, with samples of both types from the same manufacturer being used when possible to show the effects of wear on different properties. Initial testing. An initial test was run on all smocks to provide a baseline from which to work. Initially, decay and surface-to-surface resistance tests were run on all smocks. Resistance testing proved to be very erratic. When testing from sleeve to sleeve, a number of smocks showed very high resistance (above 10 tera ohms). These same smocks, if tested from point-to-point on a seamless piece of fabric, showed much lower resistance. The decay test showed much the same results (Fig. 2). If done from sleeve to sleeve, the garment would charge to only a fraction of the 1500V applied and then would slowly discharge to a point well above zero. These same smocks will charge to the full 1500V and discharge rapidly if the probes are placed on the same piece of fabric, for example, on the back and not across a seam (Fig. 3).
Life cycle testing. A number of the samples passed both the decay and resistance tests with no problem. At this point, only the samples that passed went on to the next phase, life cycle testing. To start with, the smocks were sent through only one cleaning cycle and then tested. A few more of the smocks dropped out. The remaining samples were sent through the equivalent of one year's worth of cleaning. Decay and resistance testing were again run. All but one style failed the testing. The style remaining was the lowest on the list for comfort. At this point, none of the smocks would be satisfactory for our needs. Different types of material currently not used to make smocks were evaluated and subsequently made into smocks. The results were basically the same. Cleaning problems. The next step was to look into the cause of failures. The cleaning cycle was investigated first. After examining the seams on these smocks, it was concluded that the seams were separating, allowing detergent to build up and become an insulator between sections of the garment. Additional rinse cycles were added to the cleaning process and the garments retested. We saw a considerable difference in the results but they still were not at an acceptable level. Other cleaning options. Other means of cleaning were investigated. Dry cleaning solvents coated the conductive threads and slowed the decay to an unacceptable level. Steam tunnel cleaning was too hot for the conductive fiber and melted the fiber attached to the conductive thread. Cleaning at home proved to be as bad a method, if not worst. Home dryers are not temperature controlled and the washers do not rinse the smocks sufficiently to remove the laundry detergent. Seam contact. We worked on a number of ways to improve the seam contact. Conductive thread was added across the seam, but it would not hold tight through the cleaning process. Other methods tried included adding a more conductive piece of fabric across the seams, and increasing the contact area at the seams. Some of these methods worked, but were unrealistic to design into the smock. The one method we considered most consistent was using a metal grommet in the armpit to tie all the sections together. Metal grommets. The next step was to run a controlled test on the metal grommet idea. Decay and resistance tests were run on two new identical garments. Next, grommets were put in the armpits of one garment, leaving the other in the original condition. Tests were run on the modified smock again and the results recorded. After that, both smocks went through a one-year cleaning cycle. The smock with the grommets passed with no problem. The smock without grommets failed (Fig. 4). To see if the material itself was the problem or if the in-seams were causing it, we added grommets to the smock and retested. The smock passed (Fig. 5). A three-year cleaning cycle was run on these two smocks; the smocks were retested and both passed.
Convincing evidence. At this point, we felt that the design of the smock was realistic and would to the job, but a number of people, generally those who believe smocks are not needed at all, were still not convinced. To prove our point, we tried a real life experiment. An operator wearing a sweater was measured with a static voltmeter for the voltage level present on the sweater. A voltage in excess of 2000V was measured. Next, the operator put on a smock and a wrist strap. When another measurement was taken, the level was below 50V. The test was redone with a smock without grommets. The results were similar. This showed both smocks would shield in this case, but would they both bleed off if they became charged? To test this, a volunteer was isolated from ground and the smock charged to 1500V The volunteer was then grounded and the smock measured for voltage retention. The smock with the grommets discharged and the one without grommets ended up with hot sections that had not discharged, proving that the grommets were a necessary modification. Unlike clean room garments that have a set changing time because of particulate generation, ESD smocks outside of clean room areas do not have a set change interval. Working with different end users showed the following problems with long intervals between changes:
These are basic findings but do show a problem. We recommend that the smocks be cleaned a minimum of once a week. The testing done during the design process highlighted the need for a cleaning process that could be controlled. Visual testing performed on smock samples taken from various cleaning processes indicated that conductive fibers were broken on most of the samples. In one case, whole sections of the smock were missing the conductive fibers. Looking into the reasons this was happening, we surmised that: the wash loads were too heavy, the washing/drying temperatures were too high, the cleaning process coated conductive fibers, and the detergent was not rinsed out. Controlling the cleaning process can be just as important as the design of the smock. The following items were controlled during the cleaning process:
Unless smocks are checked on a regular basis, you will not know whether you have an acceptable smock or not. As an ongoing test, the decay test would be the most accurate, but it is also the most difficult to do on a steady basis. The logical test to run, preferably at the cleaners, would be the sleeve-to-sleeve resistance test. We have set a resistance top end of 10 gigaohm. An smock over this reading has a decay test run on it before discarding it.
Figure 6 shows, in a simple block diagram, the process we feel should be followed to ensure smocks are providing the static control and other properties necessary. We recommend that the smock manufacturers incorporate the test method from EOS/ESD Draft Standard No. 2 into their process. Without a reliable test it is just guesswork whether the smock is acceptable or not.
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were:
decay, resistance (point to point), visual, and voltage retention.
65% cotton, 34% polyester, 1% carbon-suffused nylon
