Vivilon

Accelerated Weathering Tests For Improved Paints Formulation
by William Rice

Coatings formulators need to know how variables in a coating will effect the coatings durability under long term exposure to the sun. How do they get the answers to the question of extended outdoor durability? There are three basic testing methods available.

The first is to place sample panels coated with a given product out on stationary racks in a position and at an angle to get maximum sun exposure. Two states have ideal places for such tests. They are Arizona and Florida. To find out how the products will stand up in real world conditions, you simply monitor the panels for whatever period of time you wish to test. This will give you solid test results but at the cost of having to wait for a very long time, usually measured in multiple years, to get the test data. Of course, we used a stationary angled exposure rack in Galveston, TX for long term testing that had the additional harmful elements of exposure to salt spray, acid rain from chemical refineries in the area, bird droppings (seagulls) and blowing sand. (See the History section for details.)

A second method is a unique variation on static outdoor exposure. It is a test rack called EMMAQUA¨ (Equatorial Mount with Mirrors for Acceleration with Water). EMMAQUA is unique in that it concentrates natural sunlight via 10 highly reflective, specially coated mirrors onto the specimen target area with an intensity of approximately eight suns. The device tracks the sun and exposes specimens to the full spectrum of sunlight, making it one of the most realistic accelerated tests available. You get real world conditions in a fraction of the time.

For even more rapid results, you can use one of the accelerated weathering boxes with a light source (either fluorescent, Xenon Arc or metal halide lamps) to simulate solar radiation, combined with a moisture (rain/dew) source and a heat source. There are of course other variables like pollution, abrasion, solvents, biological contaminants, chlorides, pH, etc. that can occur in the real world conditions. However, Light, Temperature and Moisture are the big three environmental variables in weathering research because they can be easily and precisely measured in a controlled environment. (See the websites of both major providers of weathering services and accelerated weathering devices for further details and excellent articles that give you a wealth of information: http://q-lab.com and http://www.atlas-mts.com). For short term testing of additives and resins to determine if they could improve the coatings performance, we used a xenon arc chamber.

Polymers are composed of different atoms joined by chemical bonds which are susceptible to some form of degradation over time. All chemical bonds may be sites for degradation, and these sites are influenced by their surrounding environment. Thermal, photo, mechanical, chemical, and biological degradation bring about changes in physical properties on polymers. Changes resulting from these degradations are generally classified as cross-linking, chain scission, side group elimination, hydrolysis, substitution, and dissociation reactions. This is a complex way of saying something simple but dramatic. A polymer chain is very much a chain like you might use to pull something heavy. It has links that make up its strength. It is only strong as long as those links are undisturbed. When any environmental variable attacks these links, performance is degraded. (See the analogy to an umbrella with tiny holes in the article "Don't Fade Away".) Weathering testing is a comparably quick way to test how much damage the chain can take before it begins to break.

How is the testing done? What can the testing tell you?

Metal panels coated with the material to be tested (approximately 4X 6" or 5" X 8") are placed inside an enclosed chamber which has lights, a water sprayer and a heat control. They are kept in there for a given period of hours and removed at given time intervals to be visually checked for signs of chalking, cracking, blistering, peeling or corrosion. They are also tested by instruments that measure precisely any changes in gloss, distinctness of image or color change. Depending on the results, they can be placed back in the chamber for another interval of time after which the tests are conducted again. This cycle continues until the answer(s) the tester is looking for is achieved.

So, what can the testing tell you? What we would ideally like to know is, how many hours in the chamber equal a year of real outdoor exposure. But, unfortunately that's the one answer we cannot get. The reason is not that the "perfect" weathering tester has not been developed yet. The reason is the inherent variability and complexity of outdoor exposure situations. So, no one number of hours of "tester time" can be used to compute one year of outdoor exposure because the former is a constant and the later is a variable. Looking for a conversion factor requires pushing the data beyond the limits of its validity.

But, that doesn't stop those vendors who prefer to sell the sizzle instead of the steak, because there is no steak. Be aware that whenever anyone tells you that the product they sell has "passed" testing of a given number of hours in a given weather chamber (whether from QUV or Atlas) and that PROVES it will last for a given number of years, they are not telling you the truth. One competitor said that their product passed a test of 500 hours in a QUV chamber and that "proved" it lasted for four years. The common accepted standard is that at most 500 hours "might" roughly equate to 6 months outdoor exposure, not four years. Figures don’t lie but unfortunately, liars can figure.

However, while you can't get ABSOLUTE data you still can get excellent COMPARATIVE durability data from accelerated weathering testers. For instance, if you compare one coating to say two others and you find that at 1000 hours in a weathering chamber the first coating has lost about 3 points on its gloss readings from the measurement taken at the beginning of the test, while the second has lost 5 and the third has lost 7; then, your assumption that the first coating is superior in gloss retention to the others is probably correct.

The comparisons that are of most interest to us are ones between our coating, as is (with the readings for given hours in the weather chamber already known), and, our coating with different levels of new resins, UVA's, or Hals, or other additives incorporated into the formula. This tells us if the new material has either a positive, negative or neutral effect on performance. Obviously, if there is a positive effect, tests are conducted with higher amounts of the additive to determine where the positive effect levels out. 

To give you a sense of reference, a 1,000-hour test takes 42 days in a weathering test chamber. It takes time and money to get the right answer to the question of whether a new additive can improve performance, particularly since such tests might take 2,000 or 3000 hours before a definitive result is achieved.

The additives are usually quite expensive and increasing their quantity in a formulation adds to the finished cost of the product. But, if you want to honestly say your coating is the most durable of the hundreds of coatings out there, you do what needs to be done. So, whenever an additive or even a different resin becomes available, and is proven in testing to improve the product, we add it to our formulation, whatever the cost.