Within our contract testing laboratory, we’re often asked, “What temperature should be used for OTR, WVTR or CO2TR testing?” This is difficult to answer, as temperature directly relates to the measured transmission rate value. While mimicking the expected environmental conditions of a given film or package can be a great start for many customers, this data only tells part of the story.
When we think about permeation and how gases diffuse through polymer samples, it’s important to understand that higher temperatures increase the “energy” within the system and permeating molecules move (or diffuse) faster. To put this more simply, transmission rates increase with increasing temperature.
How much does temperature impact transmission rate?
As a rule of thumb, transmission rates tend to double for every 10°C increase in temperature. The actual relationship is unique and will change slightly depending on the polymer and permeant of interest. Below is a transmission rate graph for a polymer that was analyzed with increasingly higher temperatures (from 20°C to 80°C).
By graphing this relationship, we can estimate the transmission rate at other temperatures. However, the curved nature of this relationship makes it difficult to accurately predict transmission rates. As often seen with other reactions in nature, this temperature and transmission rate data follows an Arrhenius relationship. Graphing the data with an Arrhenius plot (lnTR vs 1/Temp K) can be a powerful predictive tool if the data demonstrates a straight line. When this occurs, using the linear equation of the line, one can calculate the OTR for other temperatures.
Arrhenius plots work well for determining transmission rate values that are within the range of the study data. We’ve also seen clients use calculated transmission rate values when extrapolating to lower (freezer or dry ice) temperature conditions.
However, if extrapolating to higher temperatures, it’s important to consider the material’s
glass transition temperature, which changes the diffusion rate of the permeating gas and its Arrhenius relationship. As an example, the graph below includes additional higher temperature OTR results for the same material, up to 120°C.
A close review of the Arrhenius plot illustrates that the material’s transmission rate/temperature relationship did indeed change. This is noted by the new slope to the high temperature values.
The above data reinforces that Arrhenius plots work best when estimating transmission rates below the material’s glass transition temperature.
As a matter of interest, we might consider whether the sample’s barrier properties were impacted by the temperature increase above the glass transition temperature. Once the testing at 120°C was completed, the temperature was reduced to 40°C and then 30°C, where OTR was collected again.
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Temperature
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Pre Heating OTR cc/(m2 x day)
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Post Heating OTR cc/(m2 x day)
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40C
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19.5
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22.8
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30C
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12.9
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15.4
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This testing demonstrated that this material’s OTR barrier property did change by going through its glass transition point. However the change wasn’t drastic and the material recovered well.
Key considerations for predictive models of temperature vs. transmission rate
To understand the impact of temperature on a given material, it’s useful to perform sequential testing at several different temperatures. Once this relationship is understood, Arrhenius plot data helps in calculating transmission rates at more drastic temperatures, which could occur with shipping and distribution.
The Arrhenius plot becomes more limited at temperatures exceeding the material’s glass transition temperature. Some materials, such as the case study seen above, can be impacted if heated to this level. For brand owners and co-packers alike, this may be important to understand, especially for products at risk of high-temperature storage.
What temperature should be used for OTR, WVTR, or CO2TR testing?
Although every customer and material are unique, the below guidelines are a good place to start.
For oxygen- and moisture-sensitive products: The best shelf-life estimate is determined by transmission rate data at real-life temperature and humidity storage conditions. This may be refrigerated, ambient temperature, or a hot warehouse depending on the product’s use.
To understand the TR results at extreme temperature ranges: When the desired data is beyond the temperature limits of your permeation analyzer, an Arrhenius plot can be generated from a series of sequential temperature tests to estimate the desired transition rate at low or high temperatures. This data becomes less reliable above the material’s glass transition temperature.
To compare potential new packaging material against a control material: The most useful data is created by testing materials head-to-head under the same test environment.
Common Industry Test Conditions
Below are common industry test conditions for OTR, WVTR and CO2TR where one will most often see transmission rate specifications cited.
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Temperature
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Test Gas RH
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Carrier Gas RH
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Oxygen
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23°C
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Dry (0%)
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Dry (0%)
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23°C
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50%
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50%
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|
23°C
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50%
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Dry (0%)
|
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Water Vapor
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37.8°C
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100%
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Dry (0%)
|
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23°C
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50%
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Dry (0%)
|
|
Carbon Dioxide
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23°C
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Dry (0%)
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Dry (0%)
|
Ultimately, our clients determine the temperature that best suits the needs for their project.