Paints and coatings come in an endless variety of physical properties, appearances, and durability. They can protect all kinds of product surfaces. Coatings are applied onto metal and polymer parts in transportation applications, on indoor and outdoor furniture, on wood and concrete, electronic displays, optical glasses, and many more.
Environmental Degradation
All coatings undergo irreversible physical and chemical changes caused by ultraviolet (UV) radiation outdoors or through window glass, heat, and moisture. In the presence of oxygen, UV radiation typically initiates photo oxidation. Temperature drives the chemical degradation. Water causes physical and chemical stress.
The property changes caused by the effects of weathering can be:
• Yellowing, especially of transparent and white coatings
• Color change
• Cracking
• Gloss loss caused by micro cracks
• Delamination
Different Moisture and Water Spray Cycles
Why are different and specific test cycles needed for coatings? First of all, we need to cover different types of coatings with different sensitivity to weather conditions. Second, the applications can be very different.
Swelling and drying causes physical stress. Water uptake of coatings increases with the number of polar groups they include. Furthermore, water absorption decreases the glass transition temperature Tg. Important for weathering testing: coatings in their amorphous state typically show a significantly higher degradation rate due to increased diffusion of water and oxygen. Therefore water spray cycles are integrated in different ways into weathering standards to address specific coating types and applications.
Select the Right Test Method
There is a variety of xenon-arc and UV fluorescent standard test methods for outdoor and indoor coating applications. The table lists 10 laboratory tests methods commonly used: 8 of those simulate outdoor and 2 indoor conditions.
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A fair question: which is the right test for my product? The answer is easy when you are dealing with a coating application for which a tailored standard exists. For example, the coating for an automotive exterior part, for wood, or an outdoor powder coating. The answer is a little bit more difficult for other applications. First choice to test other coating types are typically the generic weathering tests ISO 16474-2 cycle 1/2, or ASTM G154 cycle 2.
More Information
Weathering testing of coatings is a vast subject that is not only consisting of laboratory weathering, but also natural weathering. Atlas has posted additional articles and resources on the website to help you getting the most out of your coating testing, e.g.:
Download free Atlas Guides:
• Weathering standards for coatings
• Applications for coatings
• Using SUNTEST to determine the service life extension of polymers through UV stabilizers
Listen to recorded online seminars:
• Weathering testing of coatings
• Photooxidation and stabilization for polymers
• Time scales in environmental aging of polymers
• Design of weathering test methods