Whole-Package versus Spot Tests of Porosity
Whole-package testing of bags and pouches may help packagers predict package sterilizability and ease package validation.
Grey Tilley, West Coast Representative, Tolas Health Care Packaging (Feasterville, PA)
Currently, porosity of porous packaging material is measured with a Gurley densitometer. A similar device, called the Bendtsen tester, is also used. The Gurley tester determines the time for 100 cm3 of air to flow through a 1-sq-in. area of test material, under a pressure gradient of 1.22 kPa. The Bendtsen tester determines the volume of air that will flow through a 10-cm2 area of test material in 1 minute, under a pressure gra-dient of 1.47 kPa. Both instruments test relatively small areas hence the designation, spot test.
In practice, porosity specifications cover both average values and indi-vidual values. This control method is generally regarded as satisfactory. The perceived need to specify both average values and individual values arose because Tyvek, one of the most widely used packaging materials, exhibits considerable spot-to-spot variation in porosity test results. Variation in the porosity of Tyvek raises questions about the merits of spot testing with Gurley and Bendtsen instruments, versus whole-package testing.
There are obvious differences be-tween spot testing and whole-package testing of porosity, such as:
- A whole-package test is, by definition, package specific. The package is tested after fabrication.
- A whole-package test is nondestructive. Test samples can be used for other purposes, like package validation.
- Whole-package porosity test results as measured are area averaged. One whole-package test has an averaging effect equivalent to multiple spot tests; the exact multiple is obviously dependent on package size.
Popular usage has expanded the definition of porosity. Both the Gurley and the Bendtsen testers actually measure flow resistance, but it is popularly called porosity. Porosity is more narrowly defined as percentage of void space in a porous material. In this article, porosity and flow resistance are used synonymously.
Importance of Porosity
Flow resistance (porosity) affects both the initial sterilizability of a package and the maintenance of sterility. If sterilization involves the flow of gas into and out of the package, flow resistance plays a part in sterilization. EtO, steam, and hydrogen peroxide plasma are porosity-dependent sterilization methods. This dependent relationship, combined with the variable results obtained through spot testing, would seem to favor whole-package testing.
Porosity is also an important consideration in maintenance of sterility for reasons unrelated to initial sterilizability. While porosity is not a direct measurement of bacterial barrier, porosity measurements are used to establish lot-to-lot consistency in the manufacturing process for Tyvek, from which an inference is drawn regarding bacterial barrier properties. A few, relatively larger flow channels can produce the same flow resistance as more numerous, relatively smaller, flow channels. This difference is of no consequence to initial sterilizability, but it may be of great significance to maintenance of sterility.
Whole-package testing would better satisfy any concerns with initial sterilizability, but has no obvious advantage over spot testing regarding bacterial barrier concerns.
Whole-Package Test Equipment
|Schematic diagram of tester. (Click to enlarge).|
An experimental tester was built to measure the flow resistance of pouches. See Figure 1 for a schematic diagram on the tester. The tester resembles a pouch burst tester, modified to measure flow rate. Flow rate was measured with two rotameters covering a flow-rate range of 0ï¿½60,000 cm3/min. This range was expected to cover pouches up to 100 sq in. in area. In practice, a test pouch was clamped in the same fixture used in burst testing. Airflow was started, and flow rate was recorded when the pressure inside the pouch reached 4.9 in. H2O, the same pressure used in the Gurley test.
This tester is an experimental de-vice, manually operated and suitable for testing pouches only. It would not be suitable for routine lab work. The tester was assembled from components, several of which were scavenged from an instrument used to measure burst pressure of pouches. Burst testers, similar to one that was scavenged, are currently available from Carleton Technologies, Test-A-Pack Systems (Orchard Park, NY). The pouch porosity tester could be modified for testing tray/lid packages, but such a tester has not been demonstrated. Such a tray/lid tester would be manually operated and not well suited for routine lab work.
Relationship of Whole- Pouch Porosity to Gurley Porosity
The Gurley tester measures time in seconds. The Gurley value is the time needed for 100 cm3 of air to flow through a 1-sq-in. test area. The experimental tester measures flow rate, in cubic centimeters per minute, for an entire pouch. Test area is dependent on the size of the pouch. Flow rate data on pouches was converted to Gurley units as follows.
Qp = pouch flow rate (cm3/min).
AG = Gurley test area = 1 sq in.
Q = time (seconds).
AP = breathable area of test pouch (sq in.).
qG = Gurley flow rate per unit area (cm3/min/sq in.).
qP = pouch flow rate per unit area (cm3/min/sq in.).
Flow through the testers can be expressed as shown in the two equations below.
Some typical test results are shown in Table I on page 40. The whole-package test, a nondestructive test, was run first. Gurley testing of the same set of pouches followed.
Flow rate was measured with precalibrated rotameters with a reported reading accuracy of +/-2%. Pressure was measured with a vertical manometer with a reported accuracy of +/-2%.
Agreement of Test Results
|Porosity test data. (Click to enlarge).|
Spot testing provides the only historical basis for comparison. However, statistical comparison of whole-package porosity with Gurley porosity is of limited value since the test material exhibits spot-to-spot variation in porosity, and the same spot cannot be tested by both methods. To partially resolve this dilemma, multiple spot tests were run on each test specimen. Spot testing was done on the same test specimen that had previously been subjected to whole-package testing. Test results agreed within 5%. See results shown in Table I for 3 X 12.25-in. test pouches. Five different spot tests, each in a different location, were run on each 3 X 12.25-in. pouch.
Reuse of Test Specimens for Package Validation
Reuse of test specimens for package validation would, of course, not be valid if the test itself altered the flow resistance or the seal integrity of the pouch.
Flow resistance remained constant, within the limitations of the test, after three repetitions of the test. See previous section on test accuracy. The effect of the test on seal integrity was not studied experimentally, but a review of Tolas internal quality control data on a wide variety of pouch sizes revealed that the test pressure for the porosity test was less than 50% of the pressure required to produce seal failure. One-time exposure to an internal pressure of 4.9 in. H2O would not be expected to affect seal integrity.
The experimental data confirm the predicted averaging effect, as evidenced by the lower range and standard deviation for whole-package porosity versus Gurley (spot) porosity. And, the standard deviation and range are both lower for larger pouches
versus smaller pouches.
Whole-package testing of pouch porosity can be done with a modified burst tester.
Whole-package porosity test results agree with Gurley (spot) test results within 5%.
Whole-package testing, as compared with spot testing, would better satisfy concerns with initial sterilizability but has no obvious advantage over spot testing as regards sterile barrier.
The main advantage of whole-package testing of porosity would be realized in package validation. Test samples would be available for package validation since the test is nondestructive. Whole-package testing would provide a means for screening and selecting test samples for package validation.