Roger W. Welker, R.W. Welker Associates; and Peter G. Lehman, Ansell Healthcare
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Cleanroom gloves are critical to high-technology manufacturing. A brief search of the recent literature on contamination and electrostatic discharge (ESD) shows a surprising lack of published research. One study examined alcohol extracts using infrared spectroscopy for organic species and optical emission spectroscopy for trace element cations. Auger electron spectroscopy was also performed on glove fingerprints. This study showed that a high quantity of plasticizer can be extracted from polyvinyl chloride gloves. Another study dealt with both functional and quantitative laboratory evaluations of gloves. Yet another study demonstrated the effectiveness of aqueous isopropyl alcohol (IPA), pure deionized (DI) water, and DI water plus 0.5% of a surfactant for removing loose particles from the surface of gloves. A correlation between the amount of extractable contamination and the amount of contamination that could be transferred by contact was found in another study.
Considering that gloves are among the most expensive cleanroom consumables, more published work would have been expected. A 1994 study illustrated at least one company's cost estimate, as shown in Table I. In addition to their high relative cost, gloves are among the most likely sources of contamination, primarily because of contact transfer.
Many different types of gloves are used in cleanrooms. Dipped film gloves are one of the most popular types, because they provide a continuous barrier protecting the cleanroom environment and products from contamination by the wearer. Other types of cleanroom gloves include knitted and sewn fabric gloves, some of which are also available with barrier films. Other gloves are intended for specialty applications, such as chemical safety gloves or gloves to provide personal protection from heat or cold. The discussion in this article will be limited by dipped film gloves intended for product protection from contamination or ESD.
Many different parameters can be considered when selecting a glove. Among these are mechanical properties, such as length and thickness, the absence of pinholes, and puncture and wear-and-tear resistance. Contamination considerations fall into two broad categories: functional and nonfunctional tests. Examples of functional contamination tests are contact and near-contact stain, while nonfunctional tests include extractable particles, anions, cations, viable organisms, and organic contaminants. In some applications, functional and nonfunctional tests for ESD properties may also be important. This article concentrates on functional and nonfunctional contamination and ESD tests appropriate for the qualification of cleanroom gloves. Future articles in this series will explore in detail the contamination and ESD behavior of cleanroom gloves in real-world conditions.
Jumpsuit, changed 2.5 times per week
Hood, changed 3.5 times per week
Knee-high booties, changed 1 time per week
Cost = $0.67/operator/day
Frock, changed 2.5 times per week
Hood, changed 3.5 times per week
No shoe covers
Cost = $0.57/operator/day
|Swabs and wipers
||Cost = $0.52/operator/day
||Cost = $0.67/operator/day
4.8 pairs of natural latex per day
Cost = $2.32/operator/day
4.8 pairs of natural latex per day
Cost = $2.32/operator/day
|Table I: Costs associated with consumable cleanroom supplies.5
Functional Contamination Tests: Contact and Near-Contact Stain
As withmany materials used in cleanrooms, gloves or their extraction products sometimes come in contact with, or are in close proximity to, chips, disks, or other products being manufactured. Two types of tests can be used to evaluate the functional suitability of glove materials for cleanroom applications in these two circumstances-contact and near-contact stain. Other tests may be specified, depending on the users' functional requirements.
In a contact stain test, an apparatus to hold the test material and product is prepared in such a way that the apparatus's contribution to the test is negligible. Several strips of the material being tested are held against the product. The apparatus is then sealed within a polyethylene plastic bag to prevent gases from adjacent bags that contain test specimens from interacting. The bags are then placed in a temperature/relative humidity (TRH) chamber for conditioning. Many different companies use this test. Typical conditions are 70 to 80 C and 70 to 85% RH for a period of 4 to 7 days. At the end of the test, the TRH chamber is returned to ambient temperature and humidity under noncondensing conditions. The product is removed from the chamber and inspected for signs of stains, discoloration, or corrosion. This may be done by unaided eye inspection or inspection using magnification.
The near-contact stain test is virtually identical to the contact test, except that the material being evaluated is held close to, but not in contact with, the product. Testers must make sure the material being tested does not drip or sag onto the product. The test material is placed beneath the product. Spacing between the material and the product ranges from 250 to 1270.
There is one other consideration in both stain tests. The glove material may come in contact with water, IPA, or other chemicals that extract damaging substances from the gloves. If this is the case, extracts obtained by soaking the gloves in appropriate solvents are used as the challenge material in the functional tests, often as dried residues.
Nonfunctional Tests: Objective Laboratory Measurements
Materials qualified under functional tests are then characterized with objective laboratory tests. The results of these tests are then used to specify the desired properties to the supplier. The tests will quantify parameters such as extractable particles, anions, cations, organic, and viable contaminants. Electrostatic charge can be considered a form of contaminant for some applications. These tests are done because glove suppliers seldom have access to the users' end products to conduct either kind of stain tests.
The extractable particles test, which was originally developed for characterization of natural rubber latex gloves, was one of the earliest test methods to be used. But it was soon discovered that 40-kHz ultrasonic extraction was unsuitable since natural rubber latex was extremely sensitive to damage by ultrasonic waves. Ultrasonic extraction has been replaced by the orbital shaker to remove particles. A glove is filled with filtered DI water spiked with approximately 200 ppm by volume of a surfactant and dropped into a beaker containing the same solution. The beaker is oscillated for 10 minutes, then the shaker is turned off, the glove retrieved, and its liquid contents drained back into the beaker.
After 10 minutes of oscillation, considerable air in the form of tiny bubbles can be entrained into the liquid. Liquid particle counters (LPCs) typically count air bubbles as if they are particles. Thus, a procedure had to be developed to degas the resulting suspension. Two different procedures for degassing are available. One uses ultrasonic degassing. The beaker containing the suspension is immersed in an ultrasonic tank. The power to the tank is pulsed on and off rapidly. This procedure is repeated 10 to 20 times until the suspension no longer effervesces. An alternative procedure allows the suspension to stand, undisturbed, for 20 minutes. The 20-minute stand results typically in a 5x to 10x reduction in particle count versus ultrasonic degassing. Following degassing the suspension is counted using an LPC; current practice is to count using a 0.5 um-resolution particle counter.
Extractable Ionic Content
Ironic contamination is usually extracted in DI water with no detergent. In one company's method, a glove is turned inside out, filled with DI water, and placed on a hot plate at 80 C. This is often called an outside-only leach. Another procedure calls for known-surface-area pieces of the glove to be immersed in 80 C water for 1 hour.
Parameters for selecting a glove are length, thickness, and wear-and-tear resistance
This is an inside and outside leach test. Ionic extraction for shorter times (typically 10 minutes) at ambient temperature is called extraction to differentiate it from leaching tests.
Following extraction, samples are analyzed using anion chromatography for anionic species and atomic absorption spectroscopy for cations. Anions of interest generally are chloride, nitrate, and sulfate, although some end-users also specify phosphate. Cations of interest include aluminum, copper, iron, magnesium, silicon, sodium, and zinc.
Other Contamination Tests
Several other contamination tests are available, including nonvolatile residue (NVR), organic extractable, and viable organisms. In the NVR test the glove is washed with a suitable solvent, often IPA, and the solvent is left to evaporate in a preweighed weighing dish. The resulting added mass is reported in milligrams per square foot of surface area. The NVR test is time-consuming and procedurally difficult and occasionally results in gross errors. There is direct linear correlation between the count of cumulative particles >0.5 um per unit area and the NVR results, as Figure 1 illustrates. The strong correlation between NVR results and LPC tests indicates the NVR test may be redundant.
Organic materials can be extracted from certain types of cleanroom gloves by various organic solvents. Again, IPA, commonly used in cleanrooms, might be a good starting point for extracting organic residues. In other cases, it might be desirable to extract with more aggressive solvents, such as acetone, methylene chloride, or hexane, to enhance recovery of hydrocarbons, soluble oligomers, plasticizers, siloxanes, or other molecules considered undesirable.
After recovery of the soluble material, the samples can be concentrated by evaporation, as in the NVR procedure. However, instead of weighing the concentrate, some of it is analyzed by Fourier transform infrared spectroscopy, or in the case of extremely complex mixtures, gas chromatography with mass spectrometry detection. Many organic compounds are so detrimental to products or processes that the acceptance criterion is "none detected."
Viable contaminants may be detected by contacting the surface of a culture medium or by pipetting a wash from the glove onto the medium. The medium is then incubated to develop colonies of the viable organisms, which can be identified and counted.