A Faster Real-World Porous Package Testing Method

DuPont Protection Technologies is expected to begin transitioning to a new Tyvek material for medical packaging later this year. Manufacturers who use Tyvek in their packaging systems will be affected by this change. Once the transition is complete, the current material will be discontinued, and no longer available for purchase. While DuPont is doing extensive testing with Nelson Laboratories to show functional equivalency between the two materials, and will be submitting the data to the U.S. Food and Drug Administration and other worldwide regulatory bodies for functional equivalency approval, each individual Tyvek user is responsible for performing a risk assessment in conjunction with the change. DuPont has made controlled sales of the new Tyvek available for validation purposes, giving companies access to the material for their individual risk assessments.

The lesser known, but promising ASTM F2638 test method was used as part of the transition test protocol to show microbial barrier equivalency on both sheet goods and final packages of various types. Manufacturers who anticipate comparing their test data to the equivalency data published by DuPont will need a basic understanding of how this newer test works, and how to interpret the data.

Introduction & Advantages
The ASTM International Committee’s 2007 vote to adopt the testing methods detailed in ASTM F2638  “Standard Test Method for Using Aerosol Filtration in Measuring the Performance of Porous Packaging Materials” as a surrogate microbial barrier offers an efficient test, which endeavors to more accurately mimic real-world packaging exposures. This test also provides manufacturers more options when it comes to choosing the right package testing method for their product.

ASTM F2638 is intended for porous packaging testing and provides quick test results (same day as test) in a wide range of flow rates. Traditional procedures used in porous packaging materials testing include ASTM F1608 and DIN 58953-6 to perform the packaging and sterile barrier testing. In contrast, ASTM F2638 eliminates the microbial element (where it may take days to incubate the organism), and does not use dyes, but instead uses synthetic particles. The shift from live organisms to synthetic particles is one of the primary reasons ASTM F2638 offers accelerated test results. In addition to faster test outcomes, another potential advantage of the ASTM F2638 method is the opportunity to test multiple, as well as lower flow rates which in theory is intended to more accurately mimic real-world exposures. ASTM F2638 is designed to challenge porous materials using flow rates the package would normally encounter as part of its life cycle. Using filtration mechanics, this test method finds the point where the filtration of the material is the least effective, also known as the point of maximum penetration.

One of the main disadvantages of the ASTM F2638 standard is the large range of particle concentrations it recommends in the testing process.

“ASTM F2638 is an option for porous packaging materials but only when the testing facility has the equipment and experience needed to conduct the test,” said Wendy Mach, packaging section leader for Nelson Laboratories. “As the world’s only contract facility with the experience and resources to execute the F2638 test, Nelson Laboratories has worked through the standard’s otherwise challenging process. We understand the correct collection of testing variables. We know how to use the testing equipment and we are working closely with the ASTM F2638 standards committee to implement necessary changes based on our experience.”

Manufacturers considering the ASTM F2638 test should be fastidious in their testing facility selection. With the ASTM F2638 test the acceptable range of values is large. Accordingly, the testing facility and manufacturer need to decide, which parameters to run. This requires selection of parameters, running the test, and comparison of test data.

Process
The ASTM F2638 test requires a sample size of 120 millimeter (mm) diameter circles with exposed surface of 100 mm diameter circles to a known titer of 1.0 micrometer latex spheres. The spheres become aerosolized and are drawn via vacuum through the material at known flow rates. The flow rates range from approximately zero to two liters per minute. The particles are counted both before and after they have passed through the material over the range of multiple flow rates. With this data, a percent penetration can be calculated for each flow rate. Using a logarithmic scale for both the penetrations and flow rates, the flow rates and percent penetrations are graphed to find a point of maximum penetration. The filtration efficiency curve provides the value for how well the packaging materials filter out the particles.

ASTM F2638 uses the science of filtration mechanics to find the point where neither impaction nor diffusion is taking full effect thereby providing the point where the most particles get through.

Impaction occurs at higher flow rates where the air and particles are passing through the material at a high velocity and they impact and adhere to the porous material.

Diffusion occurs at lower flow rates where the air and particles are passing through the material at a very low velocity and they will be drawn into the material by electrostatic charges or Brownian motion.

The maximum penetration value is the point where these filtration mechanics are least effective.

The current process recommends testing the materials by progressing from a high flow rate to a low flow rate, which may artificially lower the penetration value. At a higher flow rate, a large number of particles will try to pass through the same pore in the material potentially blocking the pore. By the time lower flow rate testing begins, fewer pores are available for the particles to travel through, thus creating an artificially lower penetration value. If the process were to run from a low to a high rate instead, fewer particles would pass through the pores (lower velocity) initially, giving the particles a better opportunity to pass through the material in both the low and high flow rate testing phases. Due to the short test duration of each flow rate the opportunity for the pores to become blocked when operating from a low to high flow rates is not as likely. Thus, beginning with the lower flow rate and then increasing to the higher flow rates will give a more accurate representation of the true penetration value.

The standard also recommends a large range of particles to be used in the testing process (200,000 to 8,000,000 particles per liter). The maximum percent penetration can be influenced by the concentration of particles; therefore, it is essential that the correct concentration is selected in relation to the material being tested.

Test Curves
The results of a typical ASTM F2638 test will generally be represented in the form of a curve. The standard indicates that best practice is to establish two points on either side of the maximum penetration. Figure 1 (on page 54) demonstrates this practice and shows the penetration points grouped closely along the arc allowing a percent penetration maximum to be established.

Curves may present in a variety of formations, each indicative of different test material issues. For instance, some curves may be upside down indicating that the maximum penetration has not yet been established. An upside-down arc may also indicate the material is an excellent filter, only permitting a small amount of particle penetration. Curves can also plateau meaning there is no indication that the maximum penetration has been established. This occurs when the testing equipment’s testable range has been exhausted and no definitive maximum point has been established. Regardless of whether or not a maximum penetration point is established, this test still demonstrates what the material will filter under realistic conditions.

A plateauing or inverted arc may not always indicate a high flow rate. Some materials are a highly effective filter and will block out most of the challenge presented to it. These very low penetration values will contribute to a plateau or inverted arc and can be determined to be within the noise of the instrument.

Currently, no guidance is provided by the standard regarding the best course of action when interpreting results where a maximum penetration point is not established from the resulting filtration curve. Retesting a new sheet of the same material is a potential option. The retest could yield a curve but most often will not. The test must be performed within the tolerances and ranges of the test equipment and occasionally maximum points must be estimated when data plateaus or upside down curves are observed.

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Selecting the right porous packaging materials testing method presents a unique challenge. But if you are looking for rapid test results that mimic real-world environmental material exposure, you may consider using ASTM F2638. ASTM F2638 is a nice option for porous packaging materials when performed by a lab with an established ASTM F2638 practice. This test is not widely known and the standard’s relatively wide range of test parameters means the exactness of results depends on the lab’s knowledge of the best collection of testing variables. While still in need of standard updates including irregular curve guidance, suggested flow rate amendments, and a narrower particle testing range, ASTM F2638 is a nice addition to the porous package test family. 

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Craig Fisch is a study director at Salt Lake City, Utah-based Nelson Laboratories. Fisch is an expert in ASTM F2638 material qualifications and container closure integrity (CCI) testing. Since 2012, he has overseen integrity testing (ASTM F1929, ASTM F2096, CCI by Dye Immersion), microbial barrier testing (ASTM F2638), and the accelerated and real-time aging processes (ASTM F1980) in Nelson Laboratories’ packaging department.