The evolving cleanroom wiper

Market Survey

Design, technology, monitoring and testing are to be considered as you choose the wiper that best fits your application

by Ken O'Connor

Wiping cloths or wipers are used extensively in a range of industrial, commercial and household processes. Through thoughtful product design, the humble wiper can be employed effectively in both critical and non-critical cleaning processes. For example, we routinely use towels made of paper and other sorbent materials to clean-up household spills and leaks. Through personal experience and word of mouth, we have learned that cotton T-shirts work best as wipers for cleaning and polishing cars. Through trial and error, we have developed cleaning liquids that work best with pieces of used clothing for washing windows.

Wiper production evolution: Texwipe’s proprietary Vectra process is completely automated, utilizing minienvironments to produce ultraclean wipers in successive steps.
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A broad range of wiping products have been designed for personal and household use, and these products are marketed based on the convenience and quality results they provide. We have all used one or more of these products for hand cleaning, lens cleaning, make-up removal or changing diapers. More recently, products have become available (in presaturated form) to simplify floor mopping and waxing. Other presaturated wipers have been developed as waterless bath systems for use in hospitals and nursing homes.

Not surprisingly, the application of wiper technology in critical environments has paralleled developments in household and personal-care products. We have seen that the technology, though deceptively uncomplicated, can be effectively deployed in a broad spectrum of industrial processes.

Wiping products—dry and presaturated—are regularly used in semiconductor, medical device, biotech and automotive/aerospace manufacturing processes.

Back in the “white room” days
The earliest form of a wiper for critical environments was a cotton twill wiper first used in “white rooms” that housed “clean” processes for the NASA space program. “White rooms” were controlled environments that were the precursors of the critical environments known today as cleanrooms.

The cleanliness of the cotton wiper was improved through calendaring, bleaching and, in some cases, commercial laundering. As a result, the twill wiper was useful in the NASA process and is still in use today in less-critical applications. However, this wiper was not suitable for use in most semiconductor and pharmaceutical processes. By the same token, the limitations of “white rooms” necessitated advances in cleanroom design to provide manufacturing space more suitable for the manufacture of complex microelectronic devices and sophisticated biotechnology products.

The smaller size, increased complexity and processing speeds of today's semiconductors have been achieved through the development of cleaner manufacturing processes. Biotechnology products are manufactured in similar environments that require control of both particle and microbial levels.

In turn, wiper manufacturers have had to respond with products designed to work effectively in clean and ultra-clean processes. To design cleaner wipers, efforts have focused on the selection of cleaner fabrics and more effective cleaning and laundering methods. The desired result is a wiper that is cleaner in terms of particle levels, organic and inorganic residues and bioburden. Additionally, the wiper fabrics that were selected had to be compatible with the various liquids and solvents used in cleanroom manufacturing processes.

Sealed-edge wipers
Most would agree that the cleanest wiper commercially available today is an ISO Class 4 (Class 10) cleanroom-laundered, sealed-edge, knit-polyester wiper. The fabric used to make sealed-edge wipers is knitted from texturized yarn—usually 70 to 85 denier—on circular knitting machines set up in either a jersey knit or double-knit configuration.

This wiper type is available from a number of wiper manufacturers and is widely used in semiconductor facilities worldwide. While generally similar to each other, there are sealed-edge products on the market that incorporate proprietary cleaning processes or specialty coatings as added refinements to improve wiper performance. Still other sealed-edge wipers have unique shapes to shorten edge perimeter where it is thought that the larger particles or fibers originate. Using wipers differentiated by shape or textured surface patterns simplifies wiper identification when different wiper types are used in close proximity.

In developing sealed-edge wipers, manufacturers have focused on reducing particle and residue levels to levels acceptable for semiconductor processes. Also, because they are processed in cleanroom laundries, sealed-edge wipers have extremely low bioburden levels making them prime candidates for medical device and pharmaceutical applications. However, the absolute measurement of wiper cleanliness remains one of the significant challenges in choosing which of the sealed-edge wipers to use.

Just as the sealed-edge wiper is considered the cleanest, it is also the most expensive wiper option.

Don't forget the non-wovens
Based on the intended application, cost pressures may dictate that less-expensive wipers made from non-woven fabric be selected. Non-woven fabrics are manufactured from chopped fibers using processes similar to those in the papermaking industry. Non-woven fabrics are made from a variety of fiber sources, including polyester, polyester/cellulose blends and polyolefin resins. The cellulose fibers come from a variety of sources such as wood pulp, synthetic Rayon or Tencel.

There are a number of fabrication methods—mechanical, chemical or thermal—to form fibers into non-woven fabrics. Chemical methods that rely upon binder coatings are the least popular due to the inherent cleanliness issues and the decreased solvent resistance observed for non-wovens that contain binders.

For cleanroom use, non-wovens with fibers that have been mechanically intertwined are the better option. Although not nearly as clean as sealed-edge wipers, non-wovens without binders work well in many of the less critical cleanroom processes (ISO Class 6 Class 1000) and less critical.

Thermally bonded polyolefin non-wovens are equally effective and in wide use in presaturated wiper products. Because of their more consistent quality, non-woven wipers are rapidly displacing the use of rags in noncleanroom, industrial processes, especially in painting and powder coating operations.

Pre-saturated convenience
Within the last ten years, wiper companies have attempted to integrate both knit and non-woven wipers with cleaning solvents that previously had been applied using spray bottles. The resultant prewetted or presaturated wipers are more convenient and result in a more reproducible cleaning process.

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Prewetted wipers for critical environments are available today in many different forms and packaging options. In many instances, packaging options, such as buckets, are designed to be re-sealable providing savings in time and money for labor-intensive operations such as tool maintenance. Semiconductor processes dictate that clean polyester knit wipers with sealed edges be presaturated with semiconductor-grade solutions. Based on the nature of the different cleaning processes in the semiconductor industry, presaturation solutions are generally aqueous solutions of isopropyl alcohol.

The humble wiper can be employed effectively in both critical and noncritical cleaning processes. Here, workers on an automobile assembly line use wipes for final wipe-down before the painting process begins.
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In recent years, the concept of mass customization has been introduced through innovative methods in the manufacture of presaturated wipers. Specifically, the saturation level is varied so that wipers with different levels of wetness can be produced cost effectively. This is particularly beneficial for the more difficult cleaning applications that require wetter wipers. Similarly, maximizing the wiper saturation level is important for applications that require a heavy, but uniform application of disinfectants.

Prewetted wipers are used to solve the process problems routinely encountered in medical device and pharmaceutical facilities. Wipers that contain different blends of isopropyl alcohol and water (or ethyl alcohol and water) are used to clean process work surfaces and product surfaces and to remove particles from workers' gloves.

Typically, the presaturation solutions contain 70 percent alcohol by volume. However, for the more rigorous processes, presaturated wipers containing as much as 100 percent alcohol are readily available. For sterile, aseptic processes, alcohol presaturated wipers are marketed in cleanroom-compatible packaging sterilized by electron beam or gamma radiation.

Monitoring effectiveness
Periodically, medical device and biotech firms must monitor product contacting work surfaces for the presence of harmful, resistant bacteria. This process is performed using a widely accepted procedure, wherein sterile plates are placed in contact with the surface being evaluated.

Through this physical contact, microbial contamination is transferred to the culture media contained in the monitoring plates. However, during the sampling process, a small amount of the culture media is deposited from the surface plates onto the surface being sampled. If not removed, the residual culture media would serve as an ideal site for microbial growth and future contamination problems.

Wipers continue to play a key role in small component assembly.
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Presaturated wipers that contain alcohol and water afford a simple, yet effective method for removing residual culture media contamination. A 70/30 blend of alcohol and water works well for thoroughly removing the media and the aqueous alcoholic solution readily evaporates at room temperature. The quality history of both the sterile alcohol and sterile wipers can be traced using one product code and lot number provided for the presaturated wipers. This is a bit more convenient than keeping track of individual spray bottles and dry wipers to meet the FDA requirements specified in the current good manufacturing practices (cGMPs) regulations.

In addition to offering better control over cleaning processes, presaturated wipers minimize worker exposure to solvents that are no longer atomized as occurs with spray bottles. Presaturated wipers designed for critical environments are marketed in packaging that minimizes solvent evaporation while maintaining wiper quality and integrity. This is not the case when spray bottles are used. Lastly, the disposal of empty solvent bottles and drums is eliminated with the use of presaturated wipers.

Wipers put to the test
In response to all of the developments in wiper design, significant refinements in testing methodologies have occurred. Previously, wipers were evaluated for visual defects and, in the case of the cotton twill wiper, for visual fibers and lint.

Measurements of wiper thickness (loft) and the basis weight per square meter of fabric were used to define wiper quality as well. These tests, though still in use today, provide insufficient information for differentiating between the many wiper options. In particular, test methods were needed to better quantify the specialized performance requirements of high-end sealed-edge wipers.

Most efforts have focused on the development of test methods and instruments to measure wiper cleanliness or particle level. In fact, the measurement of particles, while receiving the most emphasis, is also the most difficult of all parameters to measure. Related measurements, such as extractable organic and inorganic materials, can be measured much more accurately, but are generally regarded as less important than wiper particle levels. In the past, decisions about which wiper to use were made primarily on the basis of which wiper had the lowest particle level.

To measure inorganic and organic contaminants, wipers are extracted in suitable solvents (water, alcohol, hydrocarbons, chlorinated solvents) at room temperature or in Soxhlet extraction devices. The resultant extract is analyzed by standard chemical methods such as ion chromatography, atomic absorption, or inductively coupled plasma for inorganic substances and Fourier transform infrared spectroscopy for organic species. The methods are widely accepted and yield results that can be correlated from lot to lot and between different laboratories.

Wiper performance is also evaluated through measurements of sorbent rate and capacity. Sorbent rate testing compares wipers on the basis of which wiper adsorbs the greatest amount of liquid, usually water, in the shortest amount of time. Capacity is a measurement of total volume in milliliters of liquid adsorbed normalized per unit weight in grams or per unit surface area in square meters of wiper fabric.

Sorbent testing, while almost always performed when comparing wipers, is not regarded as important as particle and extractable ion testing. Both sorbent rate and capacity can be improved through the addition of hydrophilic materials to the wiper fabric. With non-woven wipers, this is usually done by blending cellulosic fibers during the non-woven fabric forming process. Knit wipers are treated with wetting or wicking agents on the surface of the wiper fabric to improve the hydrophilic nature of the wiper.

Mesuring particle levels
Different approaches for measuring particle levels have been proposed. Most of the methods have focused on wiper particle generation during use. Methods have been developed to test wipers in both dry and wet states using laser particle counters to enumerate particle levels and sizes.

Dry tests are performed by tumbling wipers in a stainless steel drum (like the Helmke Drum) while particle counts are taken in the air inside the drum using an airborne particle counter. Wet testing is conducted in water with little mixing—the p-sub zero test—and with agitation to simulate use conditions called the biaxial shake test. The later method is now considered the better approach and is being adopted by the Institute of Environmental Sciences and Technology (IEST) as recommended practice for wet testing.

Wiper performance is evaluated through measurements of sorbent rate and capacity. Sorbent rate testing compares wipers on the basis of which wiper adsorbs the greatest amount of liquid, usually water, in the shortest amount of time.
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Problems persist, however, with calibration of laser particle counters and, especially with establishing correlations between counters. As a result, two new methods for assessing wiper cleanliness have emerged. One method substitutes the use of scanning electron microscopy (SEM) (with computerized image analysis and counting capability) in place of laser particle counters. The other method attempts to characterize the ability of wipers to remove liquid from a surface and to measure the particles left on the surface after wiping. Measurement of particles remaining on the surface after wiping still relies on the use of laser particle counters. It is anticipated that the IEST will revise its recommended practice for wet testing to specify the biaxial shake test and the newer method using SEM.

Despite the recent advances in particle measurement, no consensus exists for the single method to be used. Additional methods development must occur and, perhaps, there must be additional improvements in the reproducibility and calibration of test instrumentation. Indeed, the development of particle solution standards traceable to the National Institiute of Standards and Testing (NIST) standards and representative of the particles typically released from wiper substrates will need to be developed. Perhaps the newer approaches such as the SEM method will provide greater insight. Until then, wipers will be compared side by side on the same tester and even on the same day to obtain the best comparisons.

Selection is in your hands
Now and in the future, wiper selection will include consideration of other parameters such as ionic and organic residues, and on intangibles such as the long-term stability of the wiper manufacturer.

The ability to provide technical support for problem resolution and new product development will be used to differentiate one wiper source from another. For global customers, the ability to deliver wipers worldwide will be used to screen wiper suppliers. In the current economic climate the industry is seeing interest in wiper products engineered to meet both required perfomance and cost parameters.

Increasingly, wipers are being viewed as tools developed for specific applications and priced accordingly. In many instances, the wiper chosen will be based not only on the tests results of the wiper, but also on the perceived ability of the wiper supplier to be an effective vendor partner.

Ken O'Connor is a technical sales consultant for Protocol Inc. He holds three US Patents with a fourth application pending. Ken is a graduate of the University of Vermont with experience in medical device, sterilization and cleanroom companies.


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