An educated consumer will choose the wiper that's appropriate for the application
By Howard Siegerman
Editor's note: Part one of this two-part feature (see CleanRooms, November 2004, page 22) laid the groundwork for recognizing the importance of using wipers and outlined and defined the various types of wipers available. Here in part two, the author continues with the discussion of wiper contamination characteristics and highlights the key physical parameters of wipers.
Nonvolatile residue (NVR )—NVR is the quantity of molecular matter remaining after the filtration of a solvent containing contaminants and evaporation of the solvent at a specified temperature.35 More simply, NVR is a measure of contamination that will not evaporate. NVR is a residue of indistinct dimensions that can consist of high molecular weight organic moieties (e.g., hydrocarbons, silicones, alkyl phthalates, oligomers) or ionic species present in the wiper matrix. The composition of the residue will depend on the extractant solution used. Organic solvents such as alcohol will favor the extraction of organic residues, whereas water will be more efficient at extracting ionic constituents.
Low-NVR wipers are specified for cleaning cleanrooms,36 so that the cleaning agents will not dissolve appreciable amounts of contaminants from the wiper to deposit on the wiped surface. Cleanroom wipers are commonly tested for NVR using 100 percent isopropyl alcohol (IPA) or deionized water as the extractant.
An ASTM procedure37 and the RP4.3 document provide a comprehensive description of testing wipers for NVR. The RP4.3 document suggests either of two extraction conditions—boiling a quantity of wipers in the solvent of interest for five minutes or steeping the wipers in the solvent at ambient temperature for 10 minutes. After extraction, the solution is filtered and then evaporated. The weight of the residue remaining after evaporation is the NVR and is expressed either in g/m2 or g/g of fabric. To avoid weighing errors, at least 50 mg of residue should be available after extraction of the fabric and evaporation of the solvent. This dictates the number of wipers that must be collectively extracted.
The aerospace industry has stringent requirements on NVR in wiping materials. Prior to wiping down an aerospace payload in a cleaning operation or sampling a surface to ensure minimal residual NVR, the wipers are extracted multiple times with an appropriate solvent. Of the various fabrics tested, only woven cotton can withstand the multiple extractions and exhibit the minimum levels of NVR required. To avoid environmental restrictions, a solvent mixture consisting of 53 percent (vol) cyclohexane and 47 percent (vol) ethyl acetate can be used to replace the 3:1 (v/v) mixture of 1,1,1, trichloro-ethane:ethanol normal-ly recommended.37
 Figure 1. FTIR spectrum of hexane extract of a polyester knit wiper indicating the presence of silicone contamination. |
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The testing of wipers for NVR is similar to NVR testing of components used in the disk drive and the aerospace industries. A procedure for determining the NVR content of swabs38 also is available.
Organics—Silicones and amides are two organic contaminants that are sometimes found in wiping materials. These materials are of great concern to the disk drive industry because of the problems they can cause with the disk reading and writing processes. Silicones and other organics such as diooctyl phthalate (DOP) are of concern to the semiconductor industry because they are considered airborne molecular contaminants that will interfere with wafer processing. Silicones and DOP are readily extracted from wipers with hexane while IPA is used to extract amides. After extraction, the solvents are evaporated to dryness and the residues identified by Fourier Transform Infrared Spectroscopy (FTIR). Figure 1 illustrates the FTIR spectrum of the hexane extract of a polyester knit wiper contaminated with silicone oil.
The marked peaks are clear fingerprints for the presence of silicone.
Particles—Not surprisingly, particles in wipers are of greatest concern, because wipers are used extensively for cleaning activities in particle-controlled environments, i.e., cleanrooms. Particles have a variety of shapes and sizes—spherical, cylindrical, irregular—and range in length from sub-micrometer to hundreds of micrometers. They act as killer defects in the semiconductor and data storage industries, and as carriers of bacteria and other viables in the pharmaceutical and medical device industries.
Testing of wipers for particles can be as simple as visual examination of lint released from wipers shaken in air, flexing of dry wipers using air particle counters, liquid particle counting (LPC) of the solutions used to wet the wipers, or the use of optical or scanning electron microscopy to count particles filtered from the wetting solutions. The various particle-testing techniques are discussed in an article by Paley.39 Air particle measurement techniques, which suffer from a high degree of non-reproducibility, are not included in the new RP4.3 document for estimating releasable particles on wipers. The optical microscopy techniques18-20 referenced earlier for fiber testing are also useful for counting large particles (i.e., particles > 5µm) in wipers.
In addition to incorporating low-surface tension liquids such as a dilute surfactant solution or an alcohol-water mixture as referenced in the fiber section in part one (see CleanRooms, November 2004, page 22), the RP4.3 document also replaced the so-called “zero stress” test (simply immersing the wiper into the test liquid) with either orbital shaking or biaxial shaking to encourage the particles to leave the wiper matrix.
Paley40 reviewed the use of a surfactant-based particle release solution and the methodology for counting small particles in the range 0.5 – 5 µm, large particles in the range 5 – 100 µm, and fibers >100 µm. A brief description of the method17 for releasing and enumerating particles and fibers in wipers follows.
In an ISO Class 5 or cleaner environment, 500 ml of DI water is added to a clean photographic tray. The tray is placed on the platform of an orbital shaker. A 25 ml aliquot of stock 0.1 percent surfactant solution is added to the water in the tray. To facilitate complete mixing, the tray is shaken for one minute. Using gloved hands and clean forceps, a sample wiper is removed from the bag and gently draped onto the surface of the liquid in the tray. The shaker is run at 150 rpm for five minutes. The wiper is slowly lifted from the tray by forceps and the excess liquid is allowed to drip into the tray. The extract is poured from the tray into a beaker. About 25 ml of clean DI water is used to rinse the tray and is combined with the extract in the beaker. The area of the wet wiper is calculated by multiplying the actual length by the actual width, measured in milli-meters to two significant figures. The area in mm2 is divided by 106 to obtain the area in m2.
The extract is filtered through a pre-cleaned 25 mm diameter, 0.45µm etched pore polycarbonate filter under vacuum at a rate of about 50 ml/minute. The filter is allowed to air dry before being affixed to the specimen stub by applying several spots of conductive paint. The stub is transferred to the vacuum sputtering unit where a gold coating is applied to the filter. A background filter stub is also prepared following the same procedures, but with no wiper added.
Using a stereobinocular optical microscope, the entire surface of the filter is inspected for uniform particle distribution. Fibers larger than 100 µm are counted at 20X magnification. The fibers counted in the blank are subtracted from the sample total. The adjusted fiber count is divided by the area of the wiper to give results as fibers per m2. This procedure is similar to the methods described above for fiber testing, except that, in this case, non-gridded filters are used.
 Figure 2. Small particles (0.5 – 5 µm) released from two different polyester knit wipers. Magnification 3000x. |
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After counting fibers using OM, particles on the same filter are counted using SEM. Counts at 200X magnification include all particles between 5 µm and 100 µm. Smaller particles in the 0.5 µm to 5 µm range are counted at 3000X magnification. When particle contamination is low, counting can be done manually. Higher particle counts utilize computer-assisted particle recognition software to perform counts automatically. Again, counts from the blank are subtracted and the results are reported as particles per m2. Figure 2 shows small particles released from two different sealed-edge polyester knit wipers, trapped on membrane filters and counted at 3000X magnification. At this magnification it is possible to see the 0.45µm holes (shown as black objects) of the filter.
Liquid particle counting (LPC) as an enumeration technique for the liquid used to saturate the wiper may be less than adequate for the following reasons:
- LPC is an indirect, relative method.
- The instrument must be calibrated with latex spheres, but particles released from wipers are often not spherical in shape.
- Particles can be missed because of refractive index issues and coincidence errors.
- Bubbles in the liquid can be erroneously counted as particles.
- Films released from the wiper and bacteria are not detected.
- Fibers are not counted by this technique.
Total Organic Carbon (TOC)—TOC is an instrumental technique used for verifying the absence of contaminants in cleaning validation activities, typically in pharmaceutical or biotechnology aseptic filling areas. TOC measurements are usually performed on swabs used to sample surfaces that have been previously cleaned, to prove that carbon-based product residues or carbon-based cleaning agent residues are absent, or are below acceptable minimums.41-51 TOC is considered here as a contamination characteristic because wipers have been used as sampling agents and the testing for TOC applies equally well to wipers and swabs. Swabs are generally preferred because the contaminants removed from the surface are concentrated into a small area on the swab head and the resulting TOC signal-to-background ratio is optimized.
Typically, a defined surface area of 25 cm2 is swabbed, the swab is extracted with an appropriate solution (deionized water or an inorganic acid or base), and then the solution is injected into a TOC instrument. The higher the TOC reading in ppm, the more organic carbon can be ascribed to being present on the swabbed surface. Obviously, swabs and vials with low extractable TOC content must be used to avoid high blank levels. It is feasible to obtain commercially available vials and swabs that are certified to <10 ppb and <50 ppb TOC, respectively. It is important to note that the TOC level of a substrate refers to its extractable TOC not the total organic content of the fabric as might be measured in an elemental analyzer.
Physical parameters (arranged alphabetically)
Absorbent capacity and rate&— Absorbency characteristics for wipers tend to be important for pharmaceutical and biotechnology environments where large amounts of liquids must be applied for cleaning and disinfection of production surfaces and equipment and where highly absorbent wipers can speed up the process.
In the microelectronics area, wipers are used damp, not saturated, and generally, absorbency is less of an issue. In fact, highly absorbent wipers can be a disadvantage in microelectronics, because it will take correspondingly larger amounts of liquids to bring the wiper to a given degree of dampness. This can result in increased solvent costs, and if the wetting agents are organic and volatile, can result in increased costs for volatile organic carbon (VOC) emissions.
Absorption capacity and rate tests are described in RP4.3 document. The following is a wiper capacity test method that is similar to the RP4.3 method.
1. The absorbency of a fabric is a measure of its sorptive capacity (SC), typically expressed in ml/m2. SC is determined by measuring the volume of water absorbed by the fabric at saturation, then dividing by the area of the fabric tested.
2. Measure the length and width of a wiper in millimeters (mm). Calculate the area, A, and express A in units of m2. (Divide the area in mm2 by 106 to obtain m2).
3. Using a top loading balance accurate to 0.01 g, determine the gram weight, m1, of a dry wiper.
Immerse the wiper in a tray of water to fully saturate the wiper. Remove the wiper from the water and allow excess water to drip from the wiper for a period of 60 seconds. Determine the gram weight, m2, of the wet wiper. Calculate the weight of water, m3 = m2 – m1 absorbed by the wiper. Assuming the density of water is 1 gm/ml, the volume of water, V, (in ml) absorbed by the wiper is equal to the numerical value of m3.
4. Calculate SC = V/A in ml/m2.
Note that hydrophobic wipers such as hydroentangled polyester or untreated polypropylene will not absorb enough water to become saturated; they cannot be tested in this manner without some modification of the test method. Either sufficient surfactant must be added to the water to wet the hydrophobic wipers, or the liquid changed to one that will cause these wipers to be wetted.
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The RP4.3 document defines sorbency rate as the time required for the disappearance of specular reflectance from a droplet of water dispensed onto a wiper, and the document provides a procedure for reproducibly dispensing droplets of water onto the wiper surface from a fixed height.
A useful test method52 originally published by the Association of the Nonwoven Fabrics Industry (INDA) and now included in the RP4.3 document provides the ability to simultaneously measure basis weight (see below), absorbent capacity and sorbency rate. This is all based on the time required to wet out a stack of wipers. The test assumes that the absorption rate and sorption capacity are independent of the number of wiper plies and that the absorption rate is independent of the area of the wipers. Also, this test may be inappropriate for wipers with low absorption rates (<200 ml/m2/s). For occasional measurements of sorption rate, this test method is somewhat more complicated than the reflection loss method cited above. However, if frequent testing for sorption rate is required, then this method may be valuable.
Basis weight&—The basis weight of a fabric is its areal density, typically expressed in g/m2.52, 53 To calculate basis weight, first determine the gram weight, m1, of a wiper on analytical balance to the nearest milligram. Then measure the length and width of the wiper in millimeters (mm). Calculate the area, A, and express A in units of m2. (Divide the area in mm2 by 106 to obtain m2.) Calculate basis weight as the ratio of m1/A in g/m2. As indicated earlier, basis weight information can be used to convert reported units from other measurements to a weight basis. As an example, if the absorbency value of a wiper (normally expressed in ml/m2) is divided by its basis weight (expressed in g/m2), the absorbency value in ml/g is obtained.
Tensile strength&—For textiles such as wipers, a tensile test involves stretching the material in one direction to determine the load-elongation characteristics, the breaking load, or the breaking elongation.54, 55 Tensile testing machines test at a constant rate of extension, constant rate of traverse, or constant rate of loading.56 For knitted fabrics, which incorporate a high degree of stretch, a test method utilizing a constant rate of extension may be most appropriate.57 The common wiping materials utilized in cleanrooms, such as knit polyester wipers or hydroentangled polyester-cellulose wipers, are sufficiently strong for most cleaning operations that the tensile strength of the fabric is rarely an issue, especially if the wiper is used in the recommended quarter-folded fashion.
Acknowledgements
This article makes frequent reference to the IEST RP4.3 document. It is a pleasure to acknowledge the contributions of the many dedicated individuals on the IEST Working Group on Wipers that produced the RP4.3 document. I would also like to acknowledge the efforts of many here at ITW Texwipe–Kathy Miscioscio, Wendy Hollands, John Skoufis, Ram Sivakumar, Allen Spivey–as well as numerous previous co-workers whose work is incorporated here.
Howard Siegerman is director of technology for ITW Texwipe (Mahwah, NJ). Contact him via e-mail at [email protected].
References
17. “Standard Test Method for Size-Differentiated Counting of Particles and Fibers Released from Clean Room Wipers Using Optical and Scanning Electron Microscopy,” E2090-00, ASTM International, West Conshohocken, PA.
18. “Standard Method for Measuring and Counting Particulate Contamination on Surfaces,” F24—00, idem.
19. “Standard Test Method for Sizing and Counting Particulate Contaminant In and On Clean Room Garments,” F51-00(2002), idem.
20. “Standard Method for Sizing and Counting Airborne Particulate Contamination in Clean Rooms and Other Dust-Controlled Areas Designed for Electronic and Similar Applications,” F-25-68 (1999), idem.
35. “Standard Practice for Tests of Cleanroom Materials,” E2312-04, ASTM International, West Conshohocken, PA.
36. “Standard Practice for Cleaning and Maintaining Controlled Areas and Clean Rooms,” E2042-04, idem.
37. “Standard Test Method for Gravimetric Determination of Nonvolatile Residue From Cleanroom Wipers,” E1560-95(2001), idem.
38. “Procedure for Determining The Non-Volatile Residue (NVR) Extractable from Swabs in a Given Solvent,” Test Method 10, ITW Texwipe, Mahwah, NJ.
39. S. Paley, “The Development of Scientific Particle Testing for Cleanroom Wipers,” A2C2, March 1999, p. 13.
40. S. Paley, “New Standard for Wiper Testing Developed by ASTM,” A2C2, August 2000, p. 23.
41. W. K. Gavlick et al., “Analytical Strategies for Cleaning Agent Residue Determination,” Pharmaceutical Technology, March 1995.
42. K. M. Jenkins et al., “Application of Total Organic Carbon Analysis to Cleaning Validation,” PDA Journal of Pharmaceutical Science & Technology, Vol. 50 (1), January-February 1996, p. 6.
43. M. A. Strege et al., “Total Organic Carbon Analysis of Swab Samples for the Cleaning Validation of Bioprocess Fermentation Equipment,” BioPharm, April 1996, p. 42.
44. R. Hwang et al., “Process Design and Data Analysis for Cleaning Validation,” Pharmaceutical Technology, January 1997.
45. K. Miscioscio and A. Thorpe, “Choosing the Correct Swab for Cleaning Validation,” CleanRooms, January 1997.
46. D. W. Cooper, “Cleaning, Validating and Monitoring Aseptic Fill Areas,” Pharmaceutical Technology Asia, September/October 1997.
47. D. W. Cooper et al., “How Clean is Clean,” Pharmaceutical Formulation and Quality, July/August 1999, p. 24.
48. D. W. Cooper, “Cleaning Validation Using Total Organic Carbon (TOC) Analysis,” A2C2, Vol. 3, March 2000, p. 30.
49. B. Kanegsberg and M. Chawla, “TOC Analysis for Monitoring Surface Cleanliness,” A2C2, December 2001, p. 29.
50. D. W. Cooper, “Using Swabs for Cleaning Validation: A Review,” Cleaning Validation: An Exclusive Publication, July 1976, p. 74; available from the Institute of Validation Technology, Royal Palm Beach, FL.
51. D.W. Cooper, “Swab Sampling for Cleaning Validation,” The Cleanroom Resource, Volume 3, Issue 1, 1999, p. 4.
52. INDA, Association of the Nonwoven Fabric Industry, “Standard Test Method for Measuring the Rate of Sorption of Wiping Materials,” INDA Standard Test IST 10.2, Cary, NC, 1998.
53. “Standard Test Method for Mass Per Unit Area (Weight) of Fabric,” D3776-96 ASTM International, West Conshohocken, PA.
54. “Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Grab Test),” D5034-95 (2003), idem.
55. “Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method),” D5035-95 (2003), idem.
56. “Standard Test Methods for Bonded, Fused, and Laminated Apparel Fabrics,” D2724-03, idem.
57. “Standard Test Method for Tension and Elongation of Elastic Fabrics (Constant-Rate-of-Extension Type tensile Testing Machine),” D4964-96, idem.
For a complete list of references, please refer to “Know what's in your cleanroom wipers—Part one” (CleanRooms, November 2004, page 22). Available online at www.cleanrooms.com.