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Q4 Permeation Products & Services Newsletter

Monday, January 15, 2024

 
 

Quarter 4 | December 2023

 
 

Sample Prep Is Key to Accurate OTR & WVTR Results

When testing samples with MOCON Permeation Analyzers, there are certain requirements for a successful permeation test. A successful test is both accurate and reproducible. To this end, it's important that the sample be prepared and mounted correctly.

 

Before you begin testing, the following needs to be considered:

Is it representative of the larger piece?

  • Select specimen samples across the web and along the length of the roll. If testing films from a pouch, section pieces from both the front and back panels.
  • Avoid obvious defects, such as wrinkles or coating cracks (unless the purpose of the study is to characterize the effects of these defects.)
  • Analyze the largest sample size possible. A 50cm2 test area is more representative of a roll than a 5cm2 test area as there can be variances across the roll which are magnified with the normalized g/(m2 x day) value.
  • When sectioning the specimen for testing, use the Sample Template, XACTO knife and a clean cutting mat that is free of debris. Some materials are more fragile, such as thin coatings. For these, one can trace the sample around the template with a Sharpie and then cut the piece with scissors.

Take care with mounting the sample into the cell cartridge.

  • Film orientation can be important for moisture sensitive materials, especially if an unbalanced humidity is involved. An example would be a film sample analyzed with dry nitrogen carrier gas and humidified oxygen test gas.
  • Your testing cartridge need to be lightly greased. This is easily over-looked with fast QC-type tests.
  • Film aligned in the cartridge (no gaps that allow the Test Gas and Carrier Gas chambers to mix).
  • Some samples require masking with adhesive-backed foil masks or with special cartridges (such as Edge Effect, Reduced Area and Compression cartridges.) Have standard processes in place for mounting these samples.
  • Ensure cartridge o-rings are in place, before securing into the instrument.
 

What Goes Into Making an Accurate and Repeatable OTR Analyzer?

There are a number of elements that go into obtaining accurate and repeatable OTR measurements. Many of these are specified within the ASTM D3985 Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor standard and some are additional features found on MOCON OTR analyzers.

Key features within the ASTM D3985 standard are designed to minimize oxygen leakage/bias to the test result. This a HUGE concern as the instrument has oxygen (room air is 20.9%) all around it. For perspective, a high OTR film at 100cc/(m2 x day) is transmitting an oxygen equivalent level of 35ppm through a 50cm2 test area. For a high barrier sample (0.01 cc/(m2 x day)) the transmission rate is equivalent to 3.5 ppb! This is an EXTREMELY low level of oxygen permeating through a sample.

The illustration and information below, highlights the integral components that make an accurate, repeatable OTR instrument.




Features to minimize oxygen ingress with the measurement include:

1.) A flat surface “finish” of the diffusion cell, sealing o-rings on the O2 side of the sample and the use of high vacuum sealing grease to minimize edge permeation from ambient air diffusing through the sample. MOCON has improved upon this o-ring seal with the addition of a secondary Truseal sealing method to further minimize edge diffusion.

2.) Tight valving and fittings within the instrument which are especially important for OTR systems that incorporate more than 1 test cell as each connection point is a potential source for oxygen ingress.

3.)The use of forming gas (N2/H2) as the carrier gas and the incorporation of a catalyst bed which together create and essentially oxygen free gas used for measuring the OTR of the sample.

4.) An innately high efficiency coulometric oxygen sensor for accurate test results. Only MOCON’s Coulox detector has a proven 50+ year track record for this test method.

To learn more about testing with ASTM D3985 parameters, read the full guide linked below:

Learn More


Thickness Effect on Transmission Rate transmission rate testing?

How does material thickness influence transmission rate testing?

When the weather gets cold, we quickly learn that adding layers keeps us warmer. Simply put, if you want more barrier, you add more thickness. This same principle applies to gas transmission rate testing. The rule of thumb is that if you double a material’s thickness, the barrier level will also double and the corresponding transmission rate will cut in half.

What few people realize is the amount of test time that is needed to equilibrate the thicker sample. The typical assumption is that doubling the thickness will require doubling the test duration. However, this is not correct. Typically, every time the material thickness is doubled, it takes up to 4X longer for the transmission rate to reach equilibrium. Below is a comparison of 1 and 5 mil thick PET films and their transmission rate levels. These films were chosen as they are known to equilibrate for WVTR in a short amount of time.




In this example the water vapor transmission rate (WVTR) for the 1mil PET film was 10.1 g/(m2 x day). The time required to reach 95% of this value was under 30 minutes. The 5mil PET film had a WVTR of 2.17 g/(m2 x day) and required nearly 450 minutes to reach 95% of the final value. We commonly see that the last 5-10% of TR equilibrium can take a relatively long time for thick samples, especially when measuring higher barrier materials.

With this testing example, it confirms that:

  • When one creates a thicker barrier, the overall transmission rate drops proportionally. The 5X increase in material thickness dropped the measured WVTR from 10.1 to 2.17 g/(m2 x day). With the increased material thickness, more time (more than 5X) was needed to test the sample to equilibrium.
  • As demonstrated, both the transmission rate level AND the time to equilibrium are impacted by material thickness. This is something to keep in mind as you optimize test conditions (such as flow rate for WVTR and CO2TR) and test duration to ensure equilibrium values. A standard test set-up that works well for a thin sample could yield inaccurate / premature results for a thick sample.

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Resources

Updated tech notes available (click images below).



Updated white papers available (click images below).

Curious for more? Uncover a wealth of knowledge on similar topics and industries in our Resource Library linked below:

Resource Library
Events

For U.S.-based customers, we'll be at Converters Expo South in Greenville, SC, for this one-day event - Feb. 20, 2024. Our experts will be on hand to answer all your questions on AMETEK MOCON's best-in-class permeation and package testing instruments.

For customers based in the E.U., we'll be at two shows. We will be at CFIA Rennes 2024 in Rennes, France at stand 2-C3 from March 12th - 14th and Anuga FoodTec from March 19th - 12st in hall 8.1 at stand B020 in Cologne, Germany.

Hope to see you all in 2024!


Our list of 2024 trade shows and events is now final. Check the link below to our events page to see where we will be!

Events