Weathering
.JPG)
The weathering is an environmental simulation and describes the testing of materials , materials or products through targeted Wetterexponierung . It is used for quality assurance in terms of weather resistance and durability. The aim is to determine the service life of products that are exposed to weather conditions as precisely as possible.
The degree of retention of the properties investigated is referred to as weather resistance or weathering stability . According to ISO 2810 , weather resistance is defined as resistance to all changes caused by the interaction of all factors of a physical and chemical nature contained in the weather. The weather fastness differs from the light fastness , which is a pure exposure test. This represents the effects of radiation , while the weather resistance also relates to the effects of moisture , temperature and temperature changes, as well as industrial exhaust gases and other atmospheric components such as salts .
The tests are carried out either in the open air or in the laboratory . Depending on the type of exposure , a distinction is made between natural weathering ( outdoor weathering ) and artificial weathering . In natural weathering, a distinction is made between real-time weathering and accelerated weathering ( rapid weathering ). The artificial weathering methods are usually accelerated weathering methods.
Influencing variables
The following factors determine the weathering process:
| Impact factor | Climatic variables |
|---|---|
| Global radiation | Irradiance |
| warmth | Ambient temperature |
| water | relative humidity , wetting |
| Air oxygen | Oxygen concentration (constant) |
In addition, artificial environmental influences such as acid rain and exhaust gases can have an influence.
Methods
Natural weathering
Natural weathering methods represent the most realistic testing of material properties. Weathering stations offer the possibility of exposing the test objects to complex environmental conditions in a practical manner. They are either in places with extreme climatic conditions (e.g. arid climate ) or with a climate that is as constant as possible (e.g. maritime climate ). The former enable a shortening of the time of outdoor weathering due to increased stress, the latter a relatively precise repeatability and reproducibility of test series. Typical relocation sites with greater stress are, for example, in Florida and Arizona . The exposure to sunlight is very high at both locations. While in Florida there is a high level of humidity and a high salt load due to the exposure to the sea, Arizona has a desert climate .
In the stations, all test items are exposed to the same climatic factors. The result of the outdoor exposure essentially depends on the storage location and the prevailing conditions there. However, there is a strong dependency on the time of outsourcing. On the one hand, the weathering results of different years differ, since the exposure to sunlight, moisture and other framework conditions are not the same in every year, on the other hand, the type of exposure in the first months of exposure plays a major role. The aim is a direct comparison and documentation of the shelf life or changes in properties.
Artificial weathering
Artificial weathering tests are more reproducible, since seasonal fluctuations and fluctuations between the years are not to be expected. Stochastic fluctuations and long test times during outdoor weathering are avoided by the shorter, repeatable and reproducible artificial weathering. There may be differences between the results delivered, for example caused by different device designs and geometries or radiation sources. Typical test devices for accelerated mechanical weathering are Weather-Ometer , QUV , Xenotest and Suntest .
application
Weathering tests are among the most important material tests for paints , plastics and textiles . In addition, the weathering test is used in the area of high-speed printing inks . There are numerous standards and quality seals that relate to weathering data. Which property is tested depends on the application of the sample to be tested. Frequently tested properties are, for example, gloss retention , color stability and stability against chalking in paints or the mechanical strength in plastics.
Damage patterns
Weathering can result in a number of damage patterns depending on the impact factors and the structure of the weathered system.
A distinction must be made between the initially similar looking damage patterns of chalking and fading . When chalking, the binder matrix is broken down so that the pigments and fillers incorporated in the binder are exposed on the paint surface and can be rubbed off. Chalking shows up visually as a result of lightening due to increased scattering on the pigment and filler particles. Fading also shows up in the form of a lightening. This is generated during the fading through the degradation of the colorant in the system, whereby the binding agent is not necessarily destroyed and thus there is no difference when rubbed off. Since pigments and binders are often matched to one another, these damage patterns overlap in practice.
The splitting of the polymer chains of the binder, followed by cross-linking of the fragments, creates harder and more brittle plastic surfaces and paint films. The embrittlement produced in this way leads to the formation of cracks in the event of further aging .
In the case of lacquers, a detachment from the substrate can occur, which is known as delamination or peeling . If, in the case of multicoat paint systems, there is insufficient coordination between the paint layers, in particular between the clearcoat and the layer below, binder degradation takes place in the lower paint layer, which is referred to as underchalking . Since the binder used in the lower layer is broken down, the pigments and fillers used lie between the layers of paint. This causes the top layer, usually a clear lacquer, to peel off. Delamination can also occur when ionic reaction products are formed at the interface between the lacquer layers. These effects are intensified when the adhesive strength is difficult to ensure, for example when painting plastic surfaces.
literature
- Ulrich Schulz: Short-term weathering: natural and artificial weathering in paint chemistry . Vincentz Network, Hannover 2007, ISBN 978-3-86630-899-2 .
- A. Goldschmidt, H. Streitberger: BASF Handbook Painting Technology . Vincentz Network, Hannover 2002, ISBN 3-87870-324-4 .