Tuesday, March 31, 2009

Understanding Cleanroom Wiper Test Data and The Role of Product Data/Information Sheets

Just as the materials and performance requirements have changed since the first cleanroom wipers were introduced in the 1960s, so too have the data reporting requirements and test methods utilized to support those requirements. Early testing was primarily based on TAPPI (Technical Association of the Pulp and Paper Industry) and ASTM (American Society for Testing and Materials) methods created for other materials. As contamination cause and control knowledge have evolved, new test methods and measurement equipment have evolved. Today, most cleanroom wiper testing is performed in accordance with the recommended practices developed by the Contamination Control Division of IEST (Institute for Environmental Sciences and Technology) in conjunction with cleanroom wiper manufacturers and end-users. The cleanroom wiper manufacturer's data is gathered internally and then presented along with pertinent narrative information in the form a product data/information sheet for use by customers as an aid in cleanroom wiper selection. However, variations do exist within some of the test methods dependent on the section of the method applied; this is particularly true for the methods measuring particles, fibers, and extractions. Furthermore, some cleanroom wiper manufacturers still use the older methodologies for testing and reporting purposes. This combination of factors combined with slight differences in the analytical equipment utilized by the labs performing the testing will cause differences in the data reported on supplier's product data/information sheets. And finally, cleanroom wiper manufacturers often present the information and data in different formats.

How then does the customer understand the cleanroom wiper data presented and the degree of comparability between data presented by cleanroom wiper manufacturers?
Let's begin with a basic primer on the critical wiper data reported by most cleanroom wiper manufacturers in product data/information sheets:

Basis Weight is the actual material weight of the product and is generally reported in the format of g/m². The determination of basis weight is a simple technique and therefore should be comparable among different product data/information sheets.

Sorbency, sometimes referred to as absorbency, is related to the amount of solution a wiping material can take up either through absorption or adsorption. Absorption is typically related to naturally hydrophilic materials, such as cellulose non-wovens, while adsorption typically relates to synthetic materials, such as polyester knits, which without treatment are hydrophobic. Sorbency is measured in terms of capacity, efficiency, and rate with fairly straightforward calculations which should be comparable among different product data/information sheets:

Capacity is the term typically used to express the total volume of liquid that can be held by a wiper and is expressed as cc/m² or mL/m². Other terms used include absorbency and extrinsic sorbency.

Efficiency is related to how effective the material is at absorbing or adsorbing a liquid and is expressed as cc/g or mL/g. Other terms used are specific absorbency or intrinsic sorbency.

Rate is related to how fast a water drop's specular reflection can disappear after dropped onto a wiper surface and is expressed in seconds. Other terms used are time to ½ and time to sorption.

Non-Volatile Residues are the unspecified extractable matter that can be extracted from a wiper under a certain set of circumstances related to time, temperature, and solution and is generally reported as % by weight or g/m². Other commonly used terms are NVRs and extractables. Typically non-volatile residues are extracted in DI water and in IPA. Other solutions, such as Acetone, may be used based on customer requirements or process. There are two basic methods for performing the extractions:

  1. The first method exhaustively extracts all non-volatile matter using boiling solvent.
  2. The second method is a short-term or ambient temperature extraction that more closely relates to how actual wipers are used in most processes.

Most wiper manufacturers typically report short term or ambient temperature extraction data; however, some do report using the full exhaustion method.

Ions are the quantified species of matter that can be extracted from a wiper and are generally expressed as ppm or ppb. While the extraction method could use elevated temperature most, if not all, wiper manufacturers use an ambient temperature soak for the extraction. Most, if not all, wiper manufacturers also utilize Ion Chromatography (IC) to measure the extracted ion content. Due to the consistency in technique and measurement equipment, ionic content is typically comparable among competitor product data/information sheets.

Fibers and Particles state the amount of burden released from a wiper under a given set of conditions and are composed of releasable particles, those present on a wipers surface, and generated particles, those that are created by exposing the wiper to mechanical energy. The typical data reported is for fibers and particles released under mechanical energy and generally expressed as fibers and particles/cm2 or fibers and particles/ m2. Fibers and particles can also be reported in different size ranges. Typically fibers are reported in the 100 µm size. Particles are typically reported in the 0.5 µm size; however, other sizes such as 0.3 µm and 5.0 µm may also be reported.

There are three main methods that can be used in fiber and particle testing: zero stress, orbital shake, and biaxial shake. Zero stress measures readily releasable fibers and particles with no mechanical energy applied. Orbital shake is used to impart moderate mechanical energy to the surface of the wiper at 150 rpm (rotations per minute). Biaxial shake is used to impart vigorous mechanical energy to the surface of the wiper at approximately 280 opm (oscillations per minute). Both the orbital and biaxial shake methods produce a combination of readily releasable and generated fibers and particles. As part of the testing methodology, wipers are immersed in either a 100% DI water solution or a surfactant/ DI water solution. Fibers and particles are typically measured by LPC (Liquid Particles Counter) with SEM (Scanning Electron Microscopy) used as an alternate method. Due to the multiple combinations allowed within the particle and fiber testing methodologies for shake equipment, immersion solutions, and analytical measurement devices, trying to compare fiber and particle data presented on product data/information sheets is most often not an apples to apples comparison and should only be used with caution and a complete understanding of the exact differences in the product test methods.

What role does the product data/information sheet play in wiper selection?
Now that you have a basic understanding of wiper test methods, data, and terminology, let's focus on how to properly use the product data/information sheets provided by cleanroom wiper manufacturers in the wiper selection process.

The product data/information sheet is generally formatted to include sections that provide general information about the product including composition, attributes, benefits, and applications in addition to the specific test data about the particular product. Generally, standard product offerings will be listed with alternative versions, such as pre-wetted and sterile, sometimes referenced. The role of the product data/information sheet is to provide the customer with a basic understanding of the wiper material and product format and data related to that product for a first cut general comparison to a current or competitive product used for a similar application. In using the product data/information sheet as a tool, the following basic guideline steps should be used:

Step 1: Compare the composition, attributes, and benefits presented in the narrative section to the composition, attributes, and benefits of the current or competitive product. If the wipers are similar in the above factors, then they may be comparable. For example:

  • Are the products all sealed edge, cleanroom laundered, 100% polyester knits?
  • Are the products all compatible with all solutions used in the process?
  • Are the products all abrasion resistant?
  • Is the packaging of the products comparable?
  • Are the products all validated sterile?

Step 2: Compare the types of applications that the wipers are recommended for in the narrative. If the wipers are similar in recommended applications or used for higher end applications, then they may be comparable. For example:

  • Are the recommended ISO classification uses the same?
  • Are they recommended for aseptic applications?
  • Are they recommended for semiconductor applications?
  • Are they recommended for sensitive or optical applications?

Step 3: Compare the technical data using the information provided in the above primer on test methods, data, and terminology remembering the following key facts:

  • Basis weight data is generally comparable.
  • Absorbency data is generally comparable.
  • Ionic data is generally comparable.
  • NVR data is generally comparable by method: Short term or ambient extractions are comparable to each other and exhaustive extractions are comparable to each other; however, the two different methods are not comparable.
  • Fiber and particle data are the most complicated and contain the most variation. If the test methods are fully explained and the same, are the numbers in the same general range? If so, then they may be comparable. If the test methods are different, then the results are not comparable. Fiber and particle data is at best a general indicator and not an absolute determinant.
  • Is the data presented an average of all data tested or is the data presented "typical." "Typical" data may or may not be a statistical average and should be defined by the manufacturer as to how the data is derived.
  • Valid technical data comparisons can only be gained through side-by-side testing of multiple samples in the same facility utilizing the same methods and the same equipment under the same conditions.

Cleanroom wiper testing is based on recommended practices, methods, and analytical equipment that have been developed over many years by cleanroom wiper manufacturers, end-users, and testing organizations. While standardization has been the goal, variations do exist in the practices, equipment, and reporting formats between cleanroom wiper manufacturers. For this reason, product data/information sheets should only be used as an aid in cleanroom wiper selection and not as an absolute measure of the true performance comparability. Valid comparisons can only be accomplished through side-by-side testing performed in the same test facility utilizing the same method and the same equipment under the same conditions with the ultimate indicator being actual internal process validation.

Selecting the correct cleanroom wiping product is critical for the most effective operation of the customer's process. While knowledge can be gained from Cleanroom Wiper Product Data/Information Sheets, applying that knowledge effectively to make the best choice can be difficult at best. For these reasons, it can be extremely beneficial to consult the cleanroom wiper manufacturer and draw on their expertise directly. With extensive knowledge of cleanroom wiping materials and applications, the cleanroom wiper manufacturer can enhance the selection process by fully evaluating the application and presenting a total value package with technical comparisons, insights, and options that may not otherwise be gained prior to final selection and process validation.

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