Table 1. Listing of materials selected for evaluation.

How Can We Compare These Materials?

Since mechanical properties are related to the structure of the material, tensile strength was the chosen comparative response. Over time, the material may begin to degrade through mechanisms such as oxidation or radiation (i.e. thermal, ultraviolet) exposure. This degradation leads to changes at many scales including the macroscale, microscale and the molecular level, as depicted in Figure 1.

Experimental Test Set-Up

Four groups of samples were tested, all differing by the amount of exposure to outdoor conditions in arid Phoenix, Arizona. See Figure 2 for the breakdown of groupings. Tensile strength was determined by a modified version of ASTM D638.

Results

It should be noted that typically fabric, elastomeric and non-elastomeric samples follow different protocols when measuring tensile strength. The values here should only be regarded as qualitative values for comparison, rather than actual values of measured tensile strength.

From the results obtained after measuring tensile strength, the polymer films (63-20, FEP-5, PFR-10, FSR-16) did not show much change throughout the test. The other 4 remaining materials, there were some changes that occurred over the duration of the test. Trevira, which was the only fabric in this test, fared the worst. The tensile strength decreased linearly after 31 and 62 days. At 93 days, the rate of decreased tensile strength seemed to have slowed slightly. PVL and RPH showed a decrease at 31 days and then slight increases at 62 and 93 days. VNHLT-23 showed fluctuations in tensile strength over the duration of the test.

While there are many reasons why tensile strength is changing in these materials, it is clear that Trevira was the only material that consistently showed a decrease in tensile strength. This would be an indication of material degradation. The changes in the measured tensile strength of the other materials are a good indication of structural changes occurring when exposed to outside conditions. Further analysis would need to be done to better understand how the materials have changed. Techniques such as Fourier Transform Infrared Spectroscopy (FTIR) for changes in structure (i.e. functional groups), Differential Scanning Calorimetry (DSC) for polymeric structural changes (i.e. glass transition temperature, melting temperature or crystallinity changes) or Scanning Electron Microscopy (SEM) for visualization of the microstructure are three possibilities.

Below in Figure 4, is a plot of the measured conditions for this testing. Table 2, breaks down the data to show how each interval on average compared to the other intervals.

Table 2. Averages of the tracked weather conditions at different time intervals.

Based on the data collected from this comparative study, we can conclude how these materials behave in a short-term weathering test. Below in Table 3, the materials are compared by the color fading seen at each interval and the overall percent change in tensile strength after 93 days.

Table 3. Comparison summary of the result from this study. The percent change of tensile strength from the baseline measurement to day 93. Ranking is from best (1) to worst (8).

The ranking indicates how the material tensile behavior changed with respect to the baseline tensile strength. Rankings were determined by ordering the absolute values of the percent change in tensile strength by smallest to largest. Ideally, a material selected for an application should not have a drastic change in its properties. When these changes do occur, such as increasing tensile strength, another property for instance, elongation, is given up. This would lead to a more rigid material over time.

Final Thoughts

Color fading was an observed phenomenon for most of these samples. FEP-5 was the only sample to not exhibit a noticeable color change. The others did start to change at 62 and 93 days. The most prominent color change was with Trevira. At every interval, the color change was very apparent. If cosmetic appearance is important, this is factor that should be considered during material selection.

Materials that ranked 1-4, generally did not perform poorly, so from a tensile strength perspective, these could be plausible materials for short term outdoor use. A more ideal choice would be rankings 1 or 2, as these exhibited a very small change in tensile strength, which is ideally what you would want to aim for. Keep in mind, the conditions of the outdoor environment were in an arid environment, where the temperature and UV were intense, with not much precipitation. These materials may perform differently if subjected to a different environment, where there is more precipitation or less intense weather.

 It is generally accepted that decreases in tensile strength indicate degradation of the material. While increases in tensile strength do indicate a change in the structure, it is not always considered degradation. However, if a material is exhibiting increases in tensile strength over time, the material is losing another property, like flexibility. If your specific application needs to maintain flexibility, that should be factored into your decision while selecting materials.

References

1. UV data: Chandler UV Index, AZ 85225 - WillyWeather
2. Daily forecast: Personal Weather Station Dashboard | Weather Underground (wunderground.com)


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