Dynamic Wiping Efficiency and Particle Removal Ability tests set out to solve the illusive “in use” conditions need
By J.M. Oathout and Russell L. Bromley
While wipers and other consumable products are critical to the maintenance and function of an effective cleanroom environment, these products have the potential to contaminate the very environment they're meant to protect.
Fully understanding this old adage, test methods developed in the mid-1980s were aimed at determining the potential of consumables to compromise the cleanroom's integrity. In most cases, the methods used to evaluate cleanroom compatibility were originally designed to monitor product quality and consistency. These methods provided information about the wiper materials' physical characteristics or, in some cases, the product's maximum potential to generate contamination when subjected to extreme test conditions.
Rarely, if ever, did the test conditions attempt to simulate the product's actual use conditions. Given the broad range of applications for cleanroom consumables, it's understandable why a set of reproducible “in use” conditions had not been developed.
But without test methods and conditions that simulate actual “in use” conditions, test data cannot predict how a product will actually perform.
It was this complexity that led to the development of two new testing procedures: Dynamic Wiping Efficiency (DWE) and Wet Particle Removal Ability (WPRA). Both aim to simulate how efficiently a wiper removes liquid from a surface, and how effectively that wiper removes particles contained in liquid.
The Dynamic Wiping Efficiency test
One of the critical functions of a cleanroom wiper is to clean up small volume spills. This function is common across virtually all industries that use wet processes in a cleanroom environment.
The purpose of the DWE test procedure is to determine the ability of various wiping materials to sorb and remove spilled liquids.
Nine cleanroom wiper materials were selected for the development of this test. The composition, basis weight and construction of each can be found in Table 1. These wipers represent nearly the full gamut of materials and fabrics commonly used for cleanroom wipers today, and represent a spectrum of composition, construction and cost.
Also shown is the inherent “Po static” particle burden of easily releasable particles, 1.0 to 3.0 µm (microns) per unit area as determined by IEST-RP-CC004.2.3
This size range was reported because particles in this same size range were chosen to create a known challenge to the wiping materials.
DWE test method:The test, conducted in an ISO Class 4 or better laminar flow, utilizes a mechanical apparatus and procedure designed to simulate how a cleanroom wiper is commonly employed:
- The maximum sorptive capacity of each material is determined by weighing a sample of the fabric, immersing the sample in filtered de-ionized (DI) water, allowing it to drip for 60 seconds, weighing the sample again and subtracting the dry weight from the wet mass.
- Individual samples of each wiping material are quarter-folded and attached with a clip to the convex leading edge of a one-kilogram test fixture—a stainless steel sled. The sled's dimensions and mass were selected to simulate the surface area and pressure applied by a human hand during wiping.
- Each wiper is challenged with increasing amounts of 0.2-µm filtered reverse osmosis DI water in a range of volumes up to 130 percent of the wiper's capacity as determined above. This is done to challenge each wiper fabric over a broad range of potential use conditions. For accuracy and reproducibility, the liquid is dispensed with a digital burette and placed directly in front of the sled and wiper, resting in a stainless steel pan, 50-cm outside the longest dimension.
- The sled is pulled at a speed of ~25 centimeters per second, through the “spill” and across a previously cleaned stainless steel surface (36-cm free area in front of sled). The speed and distance were selected to simulate the wiping action when cleaning a tabletop or bench.
- The wetted wiper is then carefully removed and promptly weighed. The mass of the liquid picked up is calculated by subtracting the mass of the dry wiper. The volume of water picked up by the wiper can be calculated by dividing the mass of water retained by the density of water (0.997g/mL).
Results: Figures 1 and 2 reveal how “thirsty” each wiper material is and how effective it is in picking up the entire spill. In other words, it compares the wipers' abilities to pick up a given volume of liquid from a surface, regardless of the mass of the wiper itself (a “mattress,” for instance, will pick up more than a “tissue paper”).
Figures 3 and 4 demonstrate which materials maintain good “pick up” ability even when nearing their saturation point. It compares a wiper's ability to remove liquid from a surface versus its individual sorbency potential.
Materials constructed from hydrophilic fibers demonstrate superior DWE or “wipe-dry” than those constructed from hydrophobic fibers (with residual surfactant on fabric left from a laundering operation). In addition, they maintain better “wipe-dry” characteristics, up to their sorptive capacity, than those made from hydrophobic fibers. This is not surprising, since it is certainly possible to “squeeze” the water out of the matrix of fibers in much the same way water is squeezed from a sponge.
Wet Particle Removal Ability (WPRA)
As an enhancement to the DWE test, particles were added to the liquid challenge to determine the effectiveness of each wiper material in retaining and removing a known quantity of particles.
Polystyrene latex spheres (PSL) of 1.59-µm diameter were selected, since they are readily available in a known concentration, uniform in size and shape, and can be easily and reproducibly deposited with a microliter syringe and counted using a liquid particle counter. In fact, this consistency appears to be a quick reliable way to validate particle counter stability over time.
Wet Particle Removal Ability test method: The same test procedure used for the DWE, except that 10 million polystyrene latex spheres are added to the liquid in front of the sled. This large number of particles was selected to ensure that contamination remaining after wiping was well above the blank, thus improving both reproducibility and particle counting accuracy.
In addition, this large quantity was selected in an attempt to challenge the capacity of the wiper fabric to retain and remove particles (Note: Dynamic Wiping Efficiency and Wet Particle Removal Ability should be conducted simultaneously.):
- Once the wiper and sled are removed from the stainless steel surface, the surface is rinsed with a known volume of filtered DI water. The liquid is then analyzed using a liquid particle counter and the number of particles remaining on the surface is determined.
- Once again, each wiper is tested using increasing volumes of water, up to and including 130 percent of the wipers' sorptive capacity.
Results: In Figures 5 and 6, data is plotted to show WPRA versus the volume of liquid. Almost every wiper fabric demonstrates fairly good ability to remove particles up to a certain point. Beyond that point, however, the ability to retain and remove particles falls off rapidly.
This is further illustrated in Figures 7 and 8 where WPRA is plotted against the percent of the wipers' capacity. What becomes evident is that the ability of all cleanroom wipers to retain and remove particles is compromised when the volume of solution exceeds 80 to 90 percent of their sorptive capacity. Some wipers exhibit relatively poor WPRA at much lower levels of their sorbent capacity. This has implications both regarding spill clean-up and when solutions are applied to dry wiper materials for cleaning purposes.
Figures 9 and 10 show the number of particles left on the surface as a function of DWE. The data shows that the higher the percentage of liquid removed, the lower the number of particles left behind.
What becomes evident is that the number of particles left on a surface is largely dependent on the quantity of liquid left behind by the wiping process. Thus, to maximize particle removal, it becomes critical not to overload the wiper with liquid.
Wipe dry, wipe clean
The selection of a cleanroom wiper is based largely on its perceived initial cleanliness, not how well it cleaned a surface. It was somewhat surprising to see the high percentage of particles that are retained by the wiper fabric in spite of the enormous challenge of 10 million particles.
Every cleanroom wiper tested removed significantly more particles than would be predicted using DWE alone.
For example, if the DWE is 80 percent, then 20 percent of the liquid would be left behind. With a particle challenge of 10 million, one would expect 2 million particles to be left on the surface. In fact, for most cleanroom wipers, the actual quantity left is 10 percent or less of that number.
The exception is polyester knits, which left a much higher percentage of the particle challenge than the other fabrics, under equivalent test conditions. If this disproportionate removal of particles is viewed as sort of a filtration process where particles are trapped in the wiper's fibrous structure, it is likely that the knits' poorer performance may be due to their more compact yarn structure and to the larger denier of filaments found in the knits versus the non-woven fabrics tested.
When considering the data from WPRA testing, it's also important to understand the amount of contamination contributed by the wiper in the wiping process. This can't be accomplished by conventional test methods that soak the wiper in a pool of water (with or without agitation) or surfactant, since they do not simulate use.
With this in mind, we modified the WPRA test to measure the amount of contamination generated by each wiper fabric in the wiping process.
Particle Contribution test method: In this test, the same procedure is followed as in the DWE test, again using increasing quantities of filtered DI water, up to 130 percent of each wiper's capacity. But no particles are deliberately added to the liquid challenge, as done in the WPRA test.
Once the wiper and sled are removed, filtered DI water is added to the stainless steel tray and the particles are counted as in the WPRA test. In this case, the only particles collected are those coming from the wiper material itself. This method is derived from one presented by Mattina, McBride, Nobile and Turner.2
Results: These data, taken together, reveal:
- The particle release from actual dynamic wiping does not parallel the data from conventional (static) methods;
- The particle contribution to wetted surfaces is much lower than suggested by current recommended practices (or conventional tests), which attempt to enumerate the particle extracted from a wiper at various levels of mechanical stress.
The data demonstrated that cleanroom wipers made from fabrics that have an exceptional ability to “wipe the surface dry” leave the surface cleaner than those that do not, since the residual contamination resulting from a spill always lies suspended in the liquid phase left behind on the wiped surface.
The overwhelming conclusion from these data is that wipe-dry is not merely a desirable feature in a cleanroom wiping material from a housekeeping point of view, but is a critical feature in wiping up spills of contaminated liquids.
James Marshall Oathout is a Senior Research Associate with DuPont in Old Hickory, Tenn. He serves on Working Group 4 (Wipers) of the Institute of Environmental Sciences, the Standard Test Committee of INDA (Association of Nonwoven Manufacturers), and ASTM Textile sub-committee D13.64. He has authored three standard test methods for INDA and four for ASTM. He can be reached at: [email protected] RUSSELL L. BROMLEY is founder of Regent Technology, Redwood City, Calif.—a management consulting practice specializing in technology and market assessment, strategic planning, channel development and technology transfer. He is a senior member of IEST, having served on numerous RP committees. He can be reached at: [email protected]
- Oathout, J. M. “Determining the Dynamic Efficiency of Cleanroom Wipers for Removal of Liquids and Particles from Surfaces, Journal of the IEST, V. 42, No. 3, p. 17 (1999)
- Mattina, C. F., McBride, J., Nobile, D. and Turner, K., “The Cleanliness of Wiped Surfaces: Particles Left Behind as a Function of Wiper and Volume of Solvent Used,” Proceedings, CleanRooms '96 East, 183 (1996).
- “Evaluating Wiping Materials Used in Cleanrooms and Other Controlled Environments”, IEST-RP-CC004.2, Institute of Environmental Sciences and Technology, 940 East Northwest Highway, Mount Prospect, IL 60056, 1992.