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topic 31701

SAE J1960: How does it correlate to Florida exposure


2004

Many sources site that Florida's radiant energy exposure for 1 year is approximately 2800 KJ/M2. In the automotive industry, they use the SAE J1960 standard to evaluate weathering of exterior plastic materials by accelerated means. Most engineers involved with this test state that 2500 KJ/M2 is approximately 2 years of Florida testing. Why?

Kenneth Baraw
Rain Bird Corporation - Tucson, Arizona, USA


2004

10-year rolling average measurements of Miami solar radiation, south facing 45-degree inclination, are indeed ~2800 kiloJoules/m^2 @ 340nm per calendar year. It is also true that anecdotal conventional wisdom equates 2500 kJ/m^2 @ 340nm to produce the weathering effects seen in two and sometimes even 3 years of Miami weathering.

First, it is important to understand that the spectral power distribution (light intensity vs. wavelength) of the SAE J1960 test method does not match actual Miami sunlight. The test, which uses a quartz/borosilicate-S filter combination, was developed in the 1970's when "harsher is better" was the prevailing testing philosophy. This test has UV energy at wavelengths significantly below the ~300nm cut-on of terrestrial sunlight (thank the ozone layer), in fact down to about 275nm. Since Planck's law says that photon energy is inversely proportional to wavelength, this low wavelength UV has sufficient energy to cause much more damage to organic materials than sunlight.

While you might think that this would just result in a shorter test time, it has frequently been shown that the SAE J1960 test as written has the potential to change the photochemiisty of degradation. While this sometimes results in faster same-type degradation (hence the 2-3 yr Miami "equivalent") it can both cause failures not seen in outdoor exposures as well as cause different kinds of changes (some actually less severe!) than will happen in the outdoor exposure. Either way, it can be a non-predictive test for some materials. Some automotive companies have moved away from this test as written and incorporated better optical filter combinations (Boro-S/Boro-S or CIRA/Soda Lime) that more closely match true Miami solar radiation. So when comparing J1960 to outdoors, you're comparing apples to oranges and they don't equate.

One other factor is that this type of comparison, based on a single factor (solar radiant energy) does not take into account the other weathering factors of heat, moisture, etc., and their synergistic effects, which magnify the effects of solar radiation.

While the automotive OEM's and suppliers use J1960 (as-written or modified) for preliminary material approval, they still conduct outdoor testing to validate the interim results and develop models for materials that "correlate" the lab exposure to real outdoor weathering.

Allen Zielnik
Atlas Material Testing Technology LLC - Chicago, Illinois, USA


2006

The SAE J1960 test does include heat ageing (83C) and water spray cycles so it should account for some of these other effects you mention. You make a good point about the light source though. Has there been any activity focused on changing the light source and filters to more accurately predict real life exposure?

We use SAE J1960 heavily for automotive and non-automotive weathering as a defacto test to predict long term performance. Automotive OEM's require this. Outside of automotive, perhaps we should be more careful about the tests we choose and the light source/filter combinations.

Still, it's a conundrum when considering accelerated tests like Xenon Arc or even oven aging, to test under more severe requirements to predict performance over very long periods of time (i.e. 1 - 10 years) with a test that is done within 3 - 4 months or sooner. These tests assume the time-temperature superposition principle without taking into account thermooxidative effects at higher temperatures, or the deleterious effects of shorter wavelength light (and thus higher energy). We see the same thing in tests like UL 1581 for wire and cable.

The other issue I have with SAE J1960 is that it does require oven aging at 83C for most of the test cycle. Some polymers undergo heat degradation that cause yellowing or darkening not at all due to UV. When this test temperature is lowered to 63C, the effect of discoloration is much less pronounced and we can focus mostly on the UV effects.

Paul Zwick
Advanced Polymer Alloys - League City, Texas



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