Weathering is generally understood to be the influence of radiation, temperature and humidity/water. A difference is made between natural weathering (outdoor weathering) and artificial weathering (laboratory weathering). 

Natural weathering

Natural weathering is the most realistic test of material properties, but it is dependent on the location and, above all, on seasonal variations (the weather is not the same every year). Typical storage locations with higher exposure to extreme climates are, for example, in Florida and Arizona. In the case of outdoor weathering, local influencing factors such as possible air pollutants from industry or traffic or atmospheric components such as salt near the sea are also taken into account. Exceptional weather conditions, such as strong winds or hail, are also taken into account in outdoor weathering.

Artificial weathering

Artificial weathering is carried out in special test chambers under defined and reproducible conditions in the laboratory. Radiant power, temperature and humidity are modelled on those locations on earth that are to be simulated during storage. Extreme climatic conditions with high solar radiation are often chosen for this purpose, such as those found in Florida (high humidity and temperature) or Kalahari / Arizona (dry desert climate). Compared to outdoor weathering, periods of lower radiation exposure at night can be omitted, resulting in a significantly shorter time for artificial weathering (time-lapse). However, depending on the weathering programme, it is also possible to include specific periods of darkness or spray rain.

Analytik Service Obernburg uses several weathering devices with Xenon arc lamps (Xenotest devices). These emit the full solar spectrum from the short-wave UV to the long-wave IR range. The lamp is located vertically in the centre of the devices, while the samples, which are also oriented vertically, rotate around the lamp in special holders. After weathering cycles specified by the customer, temperature and humidity are adjusted or the sample surface is sprayed with water. The general procedure for weathering is described in ISO 4892-2 (Plastics - Methods of exposure to laboratory light sources - Part 2: Xenon arc lamps).  In the automotive sector, the conditions and duration of the weathering cycles, as well as the number of repetitions, are specified in the individual OEM standards and are based on particularly critical parameters for the material/paint to be tested. 

Component inspection after weathering

The inspection of a component after weathering is initially performed by means of a visual assessment with regard to discoloration, blistering, cracks or other visual changes. In addition, the adhesion of a coating can be checked by cross-cut tests. Mechanical tests before and after weathering provide indications of material degradation.

Failure symptoms after weathering

A common damage pattern after weathering is a fading of colour pigments or a yellowing of the component. However, material degradation can also occur, resulting in microcracks in the surface or a significant reduction in the strength of the base material (embrittlement). With unsuitable lacquers, the binder matrix can be destroyed, causing fillers of the lacquer to reach the surface. The result is increased light scattering and thus a brighter appearance. This effect, known as chalking, can be reduced by wiping the surface. Our failure analysis department can help you if damage occurs.

Christopher Wolf
Automotive Testing
Xenotest 440 zur Bewitterungsprüfung
Verfärbung des Kotflügels durch Kreidung. Die Bindermatrix ist zerstört und der Lack erscheint durch Reflexionen an den frei liegenden Füllstoffpartikeln heller.
Lackablösung an einem Außenspiegel durch natürliche Bewitterung

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