On the surface: Selecting the right cleanroom wiper
11/01/2006
Understanding the wiping application enables cleanroom professionals to select the optimum wiper for their task
By Kimberly Dennis MacDougall, Kimberly-Clark Professional
Cleanrooms today need to be cleaner than ever. As a result, contamination and quality controls have become stricter and more complex. One example of this trend is the way cleanroom professionals now use wipers when cleaning. In the past, cleanroom wipers were used primarily to absorb spills and for general cleaning. Today, cleanroom wipers are engineered to accomplish those tasks and many more.
 In the past, cleanroom wipers were used primarily to absorb spills and for general cleaning. Today, cleanroom wipers are engineered to accomplish those tasks and many more. |
Advanced wiper technology offers both performance and physical attributes to meet specific cleanroom needs. By carefully understanding and comparing the requirements of the wiping application (what the wiper needs to do) with the ability of the wiper to meet those requirements, cleanroom professionals will be in a position to select the optimum wiper for their task.
Wiper material and technology selection
For most cleanroom environments, synthetic fibers and blends are the materials of choice. The development and expansion of polymer technology, including polypropylene, polyester, and polyethylene, have improved to meet the ever-restricting requirements for cleanroom wipers. In combination with manufacturing process advancements (such as surface treatments that help make wipers more hydrophilic), these technological achievements have led to the development of new fabrics for use in wiper base sheets. In many ways, these new fabrics provide superior performance compared to traditional textile alternatives. Step-change advances in attributes such as purity and absorbency have become critical in the development of materials suitable for state-of-the-art cleanrooms.
Purity and extractables
The materials used to create a cleanroom wiper base sheet affect the wiper’s physical properties and performance. In cleanrooms, it is important for wipers to work without leaving residuals, which are materials or chemical residues originating from the wiping substrate that remain behind on a surface after wiping. Residuals can be divided into solid particles, classified as “lint,” and chemical elemental components, labeled as “extractables.”
Solid particles, or lint, are commonly quantified by measuring particles collected in fluid when the wiper is saturated and subjected to mechanical energy (agitation) to simulate use. This practice is known as a biaxial shake lint test, and it measures the number of particulates greater than 0.5 micron in size collected from the wiper in the fluid. Another way to evaluate lint is to measure the particles released into the air in response to mechanical energy input (flexing or twisting). The particles are usually quantified by weight or through automated counting of particles per unit of air. When the counting is completed, the particle sizes are also measured and grouped by size distribution. In general, a wiper made of 100 percent synthetic material will provide lower particulates than cellulose-based wipers because the synthetic single-filament fibers are less likely to shed.
Extractables are the second residual contaminant of concern in cleanrooms. These elemental materials-such as sodium, potassium and chloride-can be leached out of both disposable and woven wipers by solvents or other fluids being used on the wiper. They are subsequently deposited on the surface being cleaned or prepped. Elemental materials can cause various issues in cleanrooms: Electronics can be affected, corrosion can occur, and defects can be caused by ion transfer to critical surfaces. To avoid any of these issues, it is important to evaluate wipers for extractables with various solvents prior to purchasing. Normally, extractables data are provided for both water and a solvent, such as isopropyl alcohol (IPA). Cleanroom wipers made of synthetic fibers that are thermally bonded or are woven using processes that avoid glues, surfactants, binders or other additives typically have low extractables.
When evaluating different cleanroom wiper options, be sure to ask for test results from an independent laboratory that follows a recognized testing protocol, such as the IES-RP-CC004, Evaluating Wiping Materials Used in Cleanrooms and other Controlled Environments, from the Institute of Environmental Sciences.
Absorbency and residual fluid
As discussed above, because disposable cleanroom wipers are used to absorb fluid spills, they must be absorbent and have the ability to wipe a surface clean without leaving a residue and without generating lint particles and extractables that can contaminate the cleanroom.
Traditional test methods to measure the absorbent characteristics of cleanroom wipers include:
Dynamic wiping efficiency (DWE): This test method measures both the dry weight of a wiper and its weight after being manually pulled through a liquid insult at a predetermined speed. These two measurements, along with the density of the liquid and the volume of the insult, are used to calculate wiping efficiency. Because the wiping motion is performed manually, slight variances in wiping speed and method among different testers can lead to variable test results. Test results also do not give any indication of how the fluid is distributed on the surface after the wiping motion has been completed. Therefore, while DWE is a good indicator of the absorbent capacity of the wiper, it does not accurately represent a wiper’s ability to retain fluid and leave the surface free of residue.
Absorbent capacity and rate: This method, as measured by IEST-RP-CC004.3, places a wiper in a tray containing a selected liquid and allows the wiper to absorb as much of that liquid as possible. Once the wiper has absorbed to its full capacity, it is removed from the tray and suspended to allow excess liquid to drip into the tray. After 60 seconds of suspension, the mass of the wetted wiper is recorded. This measurement, along with the dry mass of the wiper, area of the wiper, and density of the liquid, can be used to calculate the absorbent capacity per unit area of the wiper. This method is sufficient for determining how much liquid the wiper can hold. However, the wiper is stationary throughout the test procedure, and therefore, the method does not indicate how the wiper would handle the fluid insult in a real-world wiping application. The second portion of this method tests the rate of absorption, measured by recording the time required for the disappearance of a drop of water dispensed onto the wiper from a fixed height. Again, while this method allows one to determine how quickly the wiper absorbs liquid, it does not address the wiper’s ability to remove fluid residue from the wiped surface.
A new test method has been introduced which is specifically designed to measure the residual fluid left on the surface after wiping:
Clean wiping efficiency test method: This test is more closely representative of the wiper’s actual use in a cleanroom. It displays a greater consistency in measuring the performance of a wiper and minimizes tester error. In the test, a wiper sample is quarter-folded and mounted so the folded edge is the first to come in contact with the surface to be wiped. A specified volume of fluid containing a fluorescent solution is applied to the wiping surface through a fixed syringe applicator. The system is then enclosed and the wiper is passed over the rotating wiping surface using a traverse arm, in a steady and fixed rate while applying a specified pressure. The moment the test movement is stopped, the remaining fluid on the platter is quantified via ultraviolet light and a computerized imaging system. The image is then analyzed to calculate the area of the surface that is free of the fluorescent solution, which is recorded in square centimeters.
The right wiper for the task
Choosing the right wiper for the task is a function of understanding a number of variables that can exist in a cleanroom, including:
■ the contaminant you are cleaning from the surface
■ the nature of any chemicals you are cleaning with or applying to the surface
■ the nature of the surface being wiped (texture, electrical properties, sensitivity, etc.)
■ the duration of the task (single-use or extended/multiple-use)
■ the risk involved (if work in progress is damaged)
■ the specific objective of the wiping task (what you want to accomplish by wiping the surface)
Cleanroom environments have very specific requirements in terms of the number of particles (equal to or larger than 0.5 micron in size) that can be present in the air. For example, under the old Federal Standard 209 classification system, a Class 10 cleanroom could have only 10 particles equal to or larger than 0.5 micron per cubic foot of air. Today, those limits are equivalent to those in an ISO 4 cleanroom. It is therefore crucial that wipers selected for any class of cleanroom be thoroughly assessed to understand their ability to minimize lint generation.
For cleanroom environments, the choice of wiper will generally be from one of the following base sheet options:
Woven: Usually made with 100 percent continuous filament double-knit polyester fibers, these wipers are most suited for high-end cleanroom applications. Some wipers in this class offer sealed edges, low linting and other attributes like controlled extractables and low ion contamination, as well as solvent and abrasion resistance, which are needed in ISO 3 and cleaner cleanrooms.
Spunlace: Usually made with polyester and cellulose, but sometimes made with a blend of rayon and polyester, these cloth-like wipers are ideal for higher classification cleanrooms, for virtually any surface cleaning/preparation task (with water and/or solvents). These materials are hydrophilic and offer fast absorption of water, solvents, bio fluids, and oil/grease. As with meltblown-based wipers, some spunlace wipers may be packaged specifically for use in controlled cleanrooms, while other versions are designed for use in clean manufacturing environments.
Meltblown: Usually made from polypropylene, these wipers are ideal for virtually any surface cleaning/preparation task or for use as surface liners. These wipers offer better resistance to acids, bases and solvents than many other wiping materials, while providing low extractables. Some meltblown-based wipers may be used for critical tasks in certain classes of cleanrooms, while others are designed for use in clean manufacturing environments. Meltblown-based wipers are also excellent for surface preparation tasks, due to their ability to efficiently carry and release solvents to surfaces. Because of their chemical resistance, meltblown base sheets are also commonly used for presaturated wipes.
In addition to absorbency and the other performance factors listed above, other questions for the selection process include:
Can the wiper be sterilized? While it is preferable to use a wiper that is sterile “out-of-the-box,” some users may want the option of sterilizing on premises using their existing equipment. If that’s the case, make sure the wiper can stand up to high temperatures (up to 280°F).
Will the wiper be used in contact with pharmaceutical, biotechnology or medical products? If so, it may need to meet the contact-safe requirements of the U.S. Food and Drug Administration (FDA). Some meltblown-based wipers (made of polypropylene) that contain no glues or binders meet USDA requirements for food processing establishments.
Is absorbency the most important attribute? If it is, look for wipers that absorb liquids on contact while still delivering low particle and extractables levels. Or, if the wiper is being used to deposit a solution to prepare a surface, perhaps fluid release is the more critical attribute. Here, you’ll want to make sure the wiper stands up to commonly used cleanroom solvents and releases few extractables when wet.
How durable is the wiper? A continuous filament fiber can provide an outer surface that helps to impart a high level of strength and durability, as well as a clothlike feel. However, if the expectation for the duration of use is minimal (single pass), a lighter weight, less durable wiper may meet the user’s needs.
What is the edge finish of the wiper? Some wipers have a sealed-seam edge as a result of passing the wiper through an ultrasonic welding device. About 1⁄4 inch around the outer edge of the product is melted together. This reduces lint, but it also makes the outer edge of the wiper non-absorbent and very harsh to the touch. Scratching from the edges may also be an issue. Other wipers use laser-sealed seaming technology to fuse the ends of the fiber bundles. This technology reduces the possibility of lint while leaving the entire wiper able to absorb to its capacity without the complication of dealing with a thick, harsh edge.
Are the wipers available in different packaging formats to meet various needs throughout the facility? If so, does the packaging properly protect the wipers from in-transit contamination and from static build-up during the dispensing process? Does the packaging allow the product to be conveyed in such a way that it arrives uncontaminated to the clean environment? Double-bag packaging helps to protect the wipers from static and other contaminants during transit and storage.
Is the wiper going to be used with a common solvent like isopropyl alcohol? Consider moving to a one-step application using a resealable, presaturated wiping product. Commonly available, these wipers are typically presaturated with a 70 percent isopropyl alcohol/30 percent deionized water solution.
Do you need to disinfect and/or sanitize surfaces? Make sure the base sheet is compatible with bleach and/or quaternary amine disinfectants and provides the highest possible levels of actives concentration during prolonged periods of use.
Conclusion
Cleanroom wipers can be highly designed, sophisticated tools capable of significantly improving processing, quality and contamination control. To select the correct wiper for cleanroom environments, use the information discussed here to be sure the wiper meets your needs. For more information on cleanroom wipers, visit www.kimtech.com.
Kimberly Dennis MacDougall is a research scientist with Kimberly-Clark Professional (Roswell, GA). She can be reached at [email protected].