The cleaner consumable

Cleanroom consumables, such as gloves, wipers and facemasks, are often tested for microcontamination levels and certified prior to use in cleanrooms and other controlled environments. Traditionally, items such as gloves and wipers are tested for subvisible particle levels by rinsing with DI water, followed by particle enumeration using either an electronic Particle Counter or by filtration and microscopy. This liquid extraction method is generally considered to provide "worst-case" data, since it breaks the static charge that can hold particles on the surface of the dry test item. Some liquid test methods, such as the Biaxial Shake for wipers, incorporate mechanical energy into the extraction, which can help release particles trapped within a fibre weave and possibly even generate particles from breakdown of the material. While liquid test methods are undoubtedly effective for particle removal, some would argue that they do not realistically represent the particulate cleanliness of the item during actual use. For items that are used in a dry environment, a dry test method for evaluating cleanroom consumables should be an option.

A dry particle test that has been used for many years to evaluate cleanroom garments is known as the Helmke Drum Test. It is described in the Institute of Environmental Sciences and Technology recommended practice IEST-RP-CC0-03.2 and is based on the equipment and method originally developed by George Helmke and Dick Yeich. In this test, a garment is placed in a rotating stainless steel drum and tumbled for ten minutes while airborne particles larger than 0.5 microns are counted in the drum's interior (Fig. 1). While cleanroom garment system components, such as coveralls, hoods and frocks, are routinely tested using the Helmke Drum, consumable cleanroom items can also be tested in this manner. This article presents the results of Helmke Drum particle testing of a variety of cleanroom consumables and examines the variables and factors that can affect test data.

Table 1 lists several types of cleanroom garments, wipers, facemasks, gloves, bouffants and shoe covers, along with the particle counts obtained using the Helmke Drum test method. Data is reported as the number of particles greater than 0.5 microns per item per minute. Since the Airborne Particle Counter samples one cubic foot of air per minute (1 cfm), the data also indicates the number of particles that the item sheds into one cubic foot of air. Results of the Helmke Drum test can be considered when evaluating the potential effect of using a particular item in the cleanroom. For example, a Brand A disposable coverall (2,137 particles/cubic foot) would be a better choice than a Brand B disposable coverall (37,262 particles/cubic foot) in a Class 100 cleanroom. However, neither would be clean enough for a Class 10 cleanroom, in which a laundered reusable polyester coverall would probably be a better choice.

While the concept of Helmke Drum testing of cleanroom consumables has not yet been subjected to the same level of scrutiny as garments, preliminary results indicate that this method does have merit. Table 1 lists a variety of cleanroom wiper products, which have varying degrees of particle cleanliness as a result of both their base material and their manufacturing processes. Someone with a basic knowledge of cleanroom wipers would expect foam and laundered sealed edge polyester knit wipers to be the cleanest, laundered and unlaundered knife-cut polyester wipers to be of marginal cleanliness, and cotton wipers to be extremely dirty. The Helmke Drum test results reflect this ranking. Dried presaturated wipers and nonwovens, which can vary significantly in particle cleanliness, were fairly clean when tested by the Helmke Drum method.

Garment testing using the Helmke Drum has historically been a controversial matter. Several round robin studies coordinated by IEST and ASTM working groups have been unable to validate the method for garments due to extreme statistical variation in the data. This variation is probably due to a combination of factors including different drum designs, different brands of Particle Counters, tendency of the garment to roll up during tumbling, sleeves or legs falling out of the drum during tumbling, the garment getting stuck on the sampling probe, and the effects of electrostatic charging. Many of these problems arise during the test because there is simply too much material in the drum. It would make sense that smaller items might tumble better, leading to less statistical variation.

Cleanroom consumables, such as wipers and facemasks, are small enough to prevent complications like probe obstruction and items falling out of the drum during tumbling. Rolling up of certain types of samples can still occur, especially with products such as knit wipers. Testing of multiple samples simultaneously can reduce the tendency for rolling. Preliminary studies suggest that a sample size of three pieces is optimum for overcoming physical problems without preventing particle release.

While traditional wet test methods provide an absolute or worst-case evaluation of the particle cleanliness of an item, the Helmke Drum test offers data that may be more realistic to those interested in an item's potential for causing contamination in a dry environment. Repeated testing of a variety of products indicates that the method is appropriate in most cases, but may need to be refined for products made of certain materials or that have certain physical characteristics. There are also cases where a product may be too clean to provide statistically usable data.