Xenon and Carbon arc weathering standards SAE J1960 & J252

A. Let's tackle your questions one at a time.

Q. Is there any correlation between Xenon weathering and Carbon arc weathering?

A. There are essentially two ways to "answer" the question, or at least skirt around it because there is no "absolute" answer . . . the first is in terms of what environmental conditions each of the test types puts out ("climate", for lack of a more comprehensive term), and secondly the response of materials to those factors("weathering" or "degradation")

First, look at light, both in terms of "quantity" and "quality". If you have access to any of the commercially available information about the Spectral Power Distribution (SPD) of different light sources you can compare the SPD of Sunshine Carbon Arc vs. Sunlight. This will show you that the carbon arc has some UV light energy below the cut-on of sunlight around 300nm wavelength. Since photon energy is inversely proportional to wavelength, this is very powerfully damaging light energy.

You will also see that carbon arc has energy much higher than sunlight in the ~350 to 450nm range and less then sunlight in the visible and infrared (heating) wavelengths. So if I measure and compare the absolute amount of total energy the entire spectrum of carbon arc and sunlight, we can say they are about equal (depending on how you measure). But that's equivalent to saying that an apple and a cucumber are "equal" because they weigh the same. So it's not just "quantity" of light, but also the "quality". And that's just looking at light intensity at each wavelength and ignoring the fact that a photon of low-wavelength UV is much more powerful than visible or infra-red photons. If it weren't, we wouldn't have microwave ovens and would be cooking pot roasts with hand-held flashlights.

So comparing spectral energies (that are very different from) to sunlight is not particularly useful. And it gets even more complicated when trying to compare one non-sunlight source to another, such as carbon to xenon. You might be able to develop some kind of number if you set up enough rules, but would it really mean anything?

You will also notice that the SPD of xenon, depending on the filter combination selected, is closer (but still not an exact match) to sunlight. For example, the SAE J1960 test filter conditions were originally selected to be artificially harsher (meaning more UV and at lower wavelengths than present in sunlight).

Q. Is there a set distinction of hours to the amount of Kilojoules between both types of weathering, or is it dependent on the material type and or exposure time?

A. First, a couple quickie "Laws", greatly oversimplified, about photo-degradation:

1. For materials to degrade and exhibit any change (fade, gloss loss, change in tensile or impact, corrosion, etc.) molecular bonds must be broken and subsequent chemical reactions occur.

2. Materials must absorb light in order to be affected by it (this is absolutely wavelength dependent) and that absorbed light (or other non-ionizing or ionizing radiation) must be of sufficient photon energy (again, wavelength dependent) to break apart chemical bonds. Otherwise nothing happens, game over.

3. Real-world products are not "pure" materials (those exist only in chemistry textbooks) but combinations of the main material, degradation and residual products from sequential processing operations, and intentionally and un-intentionally added contaminants. Each of these is subject to 1 & 2 above and may contribute to the overall "system" degradation or change. The combination of the above 3 factors results in the fact that each and every material, formulation and product will exhibit a unique response to the particular quantity and quality of light, heat, moisture, cycles, etc., that they are exposed to; in short, a unique weathering response to a given environmental stress (e.g., "test"). Environmental stress is more than just "light exposure" and different tests are just that . . . . "different".

While it may be reasonable (at least to some) to argue that a certain material type and formulations will respond with a reasonable similarity, that is about as far as you can dare take it. So a TiO2 pigmented acrylic latex paint will not respond the same as a carbon-black UV-stabilized TPO automotive trim material to the same environmental stress, i.e., different "correlation".

So, determining the response of a particular and unique material to a particular and unique environmental stress can provide a benchmark reference for comparison between tests (e.g., SAE J1960 xenon, Florida outdoor 45-degree south-facing rack exposure, a TPO bumper fascia on a car in Singapore) provided the tests are reasonably similar and DO NOT IN ANY WAY ALTER the degradation chemistry (pathway or kinetics). But, establishing a "correlation" based on the measured material property change, where the reference material exposed to similar stress conditions reaches the same degree of property change can be used to determine "correlation" and in some cases the "acceleration" of one type of test over another, but the caveat it that it is valid only for that particular formulation and can only be reasonably extrapolated for very similar material chemistries and formulations. Such materials are called "actinometers" where the known and measurable response of a material is used for calibrating or measuring an exposure. Unfortunately, most chemical "actinometers" that must be used in weathering work respond in a non-linear fashion to exposure, which makes things very "iffy" indeed.

Q. How do both of these correspond to natural sunlight weathering?

A. Having said all the above, one may integrate the amount of energy deposited on a specimen under the different light sources (sunlight, xenon-arc and carbon-arc) and generate a table of "correlation" based solely on that deposited energy. This will not take any of the issues of photochemistry into account and will, more than likely, lead you in a false direction unless you do a lot of homework to understand your material and its reactions to the different light sources.

Measurements in Florida with a pyrometer show that the average one-year deposition of Total Ultraviolet (TUV, 295 - 385 nanometers) at 26 degrees facing south is 275MJ/m^2. If we integrate the energy deposited in a laboratory xenon-arc device for the same bandwidth with an irradiance level (light output) of the lamp set at 0.55 W/m^2/nm, it will take 1,620 hours to deposit the 275MJ/m^2 (using a Borosilicate Type S inner and outer filter). The Sunshine carbon-arc instrument will take about 1,300 hours to deposit an equivalent amount of energy. Again, these values only take into consideration the amount of light deposited on a surface in a selected bandwidth of the total spectrum, without regard for the spectral sensitivity of the material, the temperature range and amount of moisture (in any form) encountered in the different "climates", and the naturally occurring variability in the outdoor test.

A. SAE J1960, Accelerated Exposure of Automotive Exterior Materials Using a Controlled Irradiance Water Cooled Xenon Arc Apparatus, is a standard for conducting accelerated weathering tests, usually to determine how colored plastics react to UV radiation. You can purchase this standard directly from SAE using their website.

A. A performance version of SAE J1960 was released in 2003. It is SAE J2527. SAE J1960 was a hardware based standard. This means it specified specific equipment to use: Atlas Ci35 and Ci65, only. The performance based standard SAE J2527 describes the test conditions such as the irradiance, temperature, humidity etc. Any machine that meets the test conditions can be used. This is a great improvement and should now make automotive testing more competitive and more affordable.