- TECHNICALBRIEF #9 THE POWDER COATING INSTITUTE 2121 EISENHOWER AVENUE, SUITE 401, ALEXANDRIA, VIRGINIA 22314 WEATRERIN G One of the most important evaluations made by powder manufacturers, applicators and ultimately the user of powder coated products is the changes in appearance that occur when these products are exposed to various types of exterior exposure conditions. Some examples are light, moisture, temperature variation, chemicals and combinations of these. Because change in appearance is important, the degree of change caused by exposure to various deleterious conditions needs to be measured in order to guide formulators toward development of materials with improved performance. Two general types of weathering tests often used are exterior exposure and exposure to artificial weathering conditions. EXTERIOR EXPOSURE Ultimately, one needs to test the coated product or examples representing the product (often exterior exposure test panels) under the conditions of exposure proposed for the product. Typical choices for test areas are Florida or Arizona where extremes in temperature, light and moisture are common and higher in degree than in northern climates. The theory behind these accelerated exterior exposures is that if a coating can withstand a given time of exposure under Florida or Arizona conditions, it should easily withstand a greater time in less severe environments. PRTIFICIAT* EXPOSURX Because exterior exposure often takes several years to cause significant changes in appearance, artificial means are used to accelerate the weathering conditions as well as to provide more controlled conditions. Some types of instruments available for this artificial weathering are: Fade-Ometer Weather-Ometer QW Common types of light sources are: Carbon Arc Fluorescent Xenon The material provided in XI Technical Briefs is for general informational purposes only. Always consult an expert in powder coating before attempting actual applications.
i Although artificial exposure does usually cause quicker changes in appearance than exterior exposure, it can not always be used to accurately predict durability under actual exposure conditions. FACTORS THAT AFFECT WEATHERING There are numerous factors that affect weathering. Radiation, including visible light, is a primary factor. When a sample is said to be lightfast, this typically means that artificial testing has been done where no moisture is present. Weatherfastness typically involves artificial exposure to light and moisture and/or exposure to actual exterior conditions. The wavelength of light available can have an impact upon the type of degradation that can occur. The presence or absence of moisture is also important. Sample placement is another important factor (i.e., 45 degrees vs. 5 degrees South Florida exposure). The spectrum typically of interest is from 200 to 1,000 nanometers. In simple terms, the ranges of interest are ultraviolet (200-400), visible (400-700) and near infrared (700-1000 nanometers). It is important to know the wavelength distribution of the light source, especially if one is talking about artificial exposure. The size of the photon, i.e., the real energy punch, is inversely proportional to the wavelength. Therefore, the shorter the wavelength, the greater the energy. If the radiation energy is high enough, an electron can be displaced from its normal orbit and a chemical reaction occurs. These reactions initiate the degradation process. -- HOW TO EVALUATE WEATHERING EFFECTS VISUAL Visual assessment is always of ultimate importance in the evaluation of any sample. However, because we use our eyes to perform the visual assessment, our mind integrates a variety of factors that have occurred and cannot analytically separate all the components of change. INSTRUMENTAL Instrumental evaluation of exposure changes can range from the simple quantification of color (colorimeter or spectrophotometer) to a detailed quantification of all changes in the appearance of materials under test including gloss.
WHAT TO EVALUATE Performance Several changes can occur to exposed coatings which affect changes in appearance. Some of these are: Gloss Pigment fading or darkening Chalking Dirt accumulation Vehicle yellowing Internal separation of pigment and binder Cracking and crazing Corrosion ComDonents All the components contained in the coatings need to be considered to understand their individual, as well as their composite behavior. GENERAL COMMENT Ideally, one would like to perform accelerated weathering testing and have excellent correlation with outdoor exposure. Unfortunately, there is no correlation in absolute terms. Control formulations of known performance should be included in any I weathering study. Some studies have concluded that while there is no correlation of surface degradation between artificial and exterior exposure, there is a correlation of the magnitude of color change and a relative ranking appears valid. However, in doing this type of work, it is critical that the light source provide consistent reproducible energy distribution as well as total luminescence. Artificial weathering devices that emit a larger amount of shorter wavelength of light cause accelerated failure, but often do not correlate as well as instruments which emit wavelength closer to the distribution of sunlight.