Testing the integrity of glovebox gloves for sub-viral size defects
12/01/2001
GLOVES
by Gregory J. Gormley
New electron-beam technology could help users identify voids and holes in an effort to keep the glovebox the cleanest cleanroom available
There is a global concern among world health organizations and regulatory agencies regarding the quality of nonconductive materials, including protective-barrier materials such as glovebox gloves. Disease-causing viruses, such as AIDS and hepatitis B, can pass through small holes or voids in these materials, thus infecting the user with the virus or contaminating the glovebox with a hazardous and contagious material.
These holes or voids are formed during the manufacturing of the nonconductive products or pursuant to anomalies present in the nonconductive materials. Accordingly, the electron-beam technology presented in this article has been developed for the testing and measuring of the porosity and anomalies of these nonconductive materials. This technology can be used for the on-line, real-time, non-destructive, non-contact, non-abrasive, dry testing and measuring of nonconductive materials, including thin-film protective-barrier materials, for voids, holes or anomalies having a diameter as little as one nanometer.
Anomilies detected include contamination, blisters, bubbles, uncatalyzed or unblended resin, low-density material, high-density material, overlapping material, stress fractures, formulation defects and other structural and non-void anomalies in the material.
Products and materials used to screen viral-size viruses must have their porosity and anomaly presence determined in order to ensure that no imperfections are present or may be formed by anomalies, which would permit the passage of a virus. These viruses may be as small as 20 nm in diameter.
Goods and materials that may act as viral barriers include glovebox gloves, medical-grade gloves, thin-film membranes, filtration media, materials used in electronic applications, gowns and aprons used in the medical field and operating room environments. Goods and materials that may act as filter media include medical and scientific membranes, fiber and cloth-like filled devices and microporous analytical and diagnostic membranes, as well as various polymer combinations.
Before we examine the electron-beam test, let's consider several of the current, available tests for porosity and anomalies.
The water test
One primary test for determining the porosity of nonconductive thin-film materials is the water or electrical hydraulic test. Using this test, the product or material to be tested, such as a glove, is placed on a conformal-shaped electrode or mandrel and is submersed in a water bath or electrolyte solution.
An electrical potential is applied between the mandrel and the water solution. If there is a void of material in the thin-film material, the water will pass from the charged container water bath to the electrode, causing a short circuit. A current reading will be displayed on a connected ammeter indicating that a defective void exists in the material, and the material will be rejected.
However, this test can only determine if there is a sizable hole in the material. It cannot reveal the presence of an anomaly, such as a blister or bubble. In this case, the blister or bubble will not break in the water and, in turn, fails to allow the water to pass from the water bath to the ground.
A blister or bubble could easily break in the use of a glove, and the glove would fail during use.
In the water test, only holes of approximately 50 microns or greater will be detected due to the surface tension of the water. It has been proposed to add soap or alcohol to the water in order to decrease the surface tension. However, even with a lower surface tension, this test will not detect viral-size holes.
The dry test
A second primary test used to determine the porosity of materials is called a dry test, or spark test. This method involves an electrically charged brush with 1,300 to 1,500 volts AC, 60 Hz. The conductive brush rubs against the mandrel on which the glove is placed.
During the test, the brush and mandrel rotate. When there is a large void, about 50 to 100 microns, the voltage from the brush will spark through as a straight-forward, thick spark. Because it's a strong discharge, it creates large holes, This method is destructive because the brush touches and breaks the glove. Furthermore, during this process, a strong current flows that can destroy the glove.
The "wet tester"
A third test is disclosed in U.S. Patent No. 5,196,799 entitled "Method and apparatus for testing a protective barrier material for pinholes and tear strenght," and issued to Beard et al. This is a water test conducted at different frequencies, not just DC or 60 Hz. This method permits the discovery of holes, bubbles and blisters in the material being tested. This is a capacitive test in which distance, environment and thickness of the product are critical to the repeatability and calibration of the test. It is an integral measurement, meaning it measures relatively large areas as one "gray" measurement.
 Figure 3. The sensor might be positioned using an automatic positioning device such as a servomotor, a stepper motor or a programmable positioning robot. |
It is also a wet tester. One of the inherent problems with wet testing is that after the material has been wet it must be dried, and usually dried with hot air. The hot air, containing ozone, can weaken the nonconductive material and increase or enhance the number of voids that might be present in the final product.
These tests are used in conjunction with more extensive destructive test methods to statistically sample lots of finished product. Laboratory testing has shown that in a given, finished production lot, there may be a few, if any, defective products. Thus, more accurate and reliable process testing of each manufactured product is needed.
Electron-sensor testing
In this new technology, the porosity or presence of anomalies of a nonconductive material is determined by using an electron sensor in an open atmosphere under a fluid cover gas, or a flow of a cover gas. The cover gas is directed on the material and, if there is a small aperture, hole or anomaly in the material, a change in the electric discharge or "corona" (also known as an electron beam, an electrostatic corona or a corona discharge) occurs, which is measured by a sensor.
The electron sensor comprises an electrode and a sensing mechanism that records electrons that are sent through the hole or anomaly in the nonconductive material. The occurrence of this change in discharge is due to the Griebel-Gormley Aperture Effect, which is sometimes referred to as the "Aperture Effect."
The Griebel-Gormley Aperture Effect is shown by the use of a smooth, rounded grounded cathode (approximately cylindrical) in proximity to a tip of an anode (a point). Very few electrons (or corona) are discharged if the voltage is low enough. But when the cathode is masked with a dielectric material containing a very small void of material (or hole or anomaly), an electrical point is masked out on the grounded cathode.
A point-to-point effect would be created and electrons would flow from the cathode through the hole or anomaly in the dielectric material to the anode tip without increasing the applied voltage. This flow of electrons is detected as a change in the electric discharge (See Fig.2).
Note that the diameter of the anode tip in the electron sensor, the quality of the plating material (barium, platinum, gold, silver) and the heating of the anode and cathode tips are factors relating to the quality and length of the electric discharge (the corona beam) detected.
Other important factors are the dielectric quality of the material being tested, the type of defect being tested and the operating parameters of the testing equipment, such as the frequency, amplitude, waveshape and voltage.
The proper combination of these factors leads to the ability to detect subnanometer-size apertures, holes or anomalies in the material being tested.
A cover gas is also important in achieving the Aperture Effect. Typical cover gases include nitrogen, noncombustible gases, noble gases and dehydrated air. The results vary with the particular cover gases used. It makes a dramatic difference whether nitrogen is used opposed to air, neon or other noble gases.
The flow rate and gas pressure are other important factors; the higher the gas pressure, the more gas flows and the beam lengthens. As the pressure increases, the gas becomes denser, and the electrons flowing from the cathode to the anode move more slowly. For example, with a pressure of about 1 atmosphere in a cover gas of air, the electrons will move at about 1/10 the speed of light. If the pressure is increased, the speed will decrease.
The beam may move and wander in the cover gas environment. The beam is self-seeking within the focus of the fluid cover gas. Thus, the beam moves in the area of the material bounded by the fluid cover gas in order to locate properly sized aperture or anomaly and creates a focal area on the surface of the test material that can vary based on the setting's parameters.
The sensor might be positioned using an automatic positioning device such as a servomotor, a stepper motor or a programmable positioning robot (See Fig.-3).
Other important factors in the creation of the Griebel-Gormley Aperture Effect are the power supply voltage, the frequency of the pulsed DC and the distance from the cathode to the anode. Moreover, the distance between the cathode and the material being tested is an important factor in obtaining the Aperture Effect.
If the material being tested is too far from the cathode, the Aperture Effect will be lost. However, this can be alleviated when a conductive noble gas is grounded and used to supplement the difference in the conformal space required between the material and the mandrel.
Applications
This technology, as it relates to the glovebox industry, can be applied in three different formats. The first format would be to test and certify the gloves at the manufacturing facility by the approach that has been mentioned in the previous sections. This would be a conformal mandrel similar to the technique used in the dipping process.
The mandrel would be made from a conductive material. The test could be performed as a secondary operation if the material has to be cured in a secondary process. However, if the material is fully cured at the end of the dipping process line, the product could be tested with a series of sensors that would account for the entire area of the product's surface by rotating the mandrel one full revolution before glove removal. The method of removal from the mandrel is critical. The removal process has to ensure that damage does not occur.
Some removal methods are too harsh and may potentially relegate on-line testing useless. The basic idea is to eliminate the labor factors of re-handling the product to keep the cost as low as possible and test on-line.
The second method of testing is to test the deployed product in the glovebox. This could be accomplished by having a small series of sensors in the glovebox accessible to the operator's reach so that the critical-wear areas of the glove could be tested on a regular basis. This type of test is limited to the fingertips, finger shafts and nest areas between the fingers.
A light signal could indicate the acceptability of the integrity of the glove and an area that is flawed. The beam for this application would be very forgiving in that the beam would articulate from the flaw, because the C-Beam will follow its potential by bending to eliminate line-of-sight problems associated with the complex geometry of the glove.
The third method would be to test a deployed glove from outside the glovebox. This would be accomplished by turning the glove inside out so that it would hang out. Then the test procedure would place a tourniquet-type of device on the upper portion of the arm section of the glove above the location of a valve that would be built into the glove.
The glove would be filled with a conductive gas, such as a neon or xenon, through the valve with a conductive needle to a prescribed level to extend the test area of the fingers, hand and arm. The needle for the valve would be left in place during the test to provide the necessary ground. A handheld device could then scan the test area to determine the integrity of the glove prior to use. This may be a more favorable method because a single device is relatively inexpensive compared to building a series of sensors over every portal of the glovebox. In the second method, multiple sensors would be required for each portal access location if total integrity were to be achieved.
The "Achilles' heel"
The glove within the glovebox has been labeled the "Achilles' heel" for many different reasons, "since they are often the weakest link in the containment barrier," as described in the article by Rodney B. Smith, "A Glovebox-The Ultimate Cleanroom?"
The corona beam technology proposes is an alternative to the current status-quo testing. Integrity can now be assured at every step of the glove's life cycle all the way to its retirement. The real "will" to make this change will have to be driven by the needs of the end users who pride themselves on making the glovebox the cleanest and safest of all cleanrooms for the protection of their customers and their employees.
Gregory J. Gormley is president and founder of Newtown, PA-based ConverTec Corporation. He can be reached at [email protected]