Polymeric materials laboratory-accelerated weathering tests

Laboratory accelerated weathering tests have played an important role in development of polymeric materials with highly improved weatherability. However, since all stresses present in an outdoor exposure cannot be simulated in a laboratory accelerated test, the latter cannot replace natural exposure. It is a complimentary technique, the usefulness of which largely depends on how closely it reproduces the chemistry and weathering effects caused by the slower outdoor exposures. Thus, correlation is a fundamental issue which must be considered when selecting a laboratory accelerated weathering method. It depends on reasonably representing... 

Laboratory accelerated weathering devices have been used for more than 80 years with increasing importance concomitant with the development of more weather-able materials and the need to determine in a short time the effects of natural exposures over prolonged periods. The importance of these devices lies in their ability to accelerate the weathering processes imder controlled and reproducible conditions. They are particularly useful in research and development of new polymeric formulations. They are also used for quality control and specification testing. However, their application to prediction of service life under use conditions is still under development (see section on Laboratory Accelerated Versus Natural Weathering). 

In order to provide a reliable early evaluation of the weatherability of a polymeric material, ideally the laboratory accelerated test should satisfy all correlation criteria when compared with results of natural weathering. However, any accelerated weathering test, either artificial or natural, only approximates the field exposure conditions. Nevertheless, the closer the simulation of the natural balance of all important weathering factors by the accelerated test, the better the... 

The accelerated weathering of polymeric materials in laboratory instruments is covered by ISO Standard 4892. In the US, ASTM standards G23, G26 and G53 cover such testing, and other ASTM standards are derived from these, as shown in Fig. 6.2. Many car manufacturers have their own acceptance specifications. Reviews of these standards and of factors affecting results have been published [4, 9-11]. 

Effect of SPD of the Radiation Source on Weathering. The wavelengths responsible for degradation of a specific polymeric material vary with the SPD of the radiation source. Differences between the laboratory accelerated test source and the natural source in the wavelengths that degrade a material, which are identified by the activation spectra (see section on Spectral Effects of Solar Radiation Activation Spectra), can have the following consequences (53) ... 

Acceleration factors are material dependent and can be significantly different for each material and for different formulations of the same material. Therefore, it is erroneous to attempt to establish a single acceleration factor for a laboratory accelerated test to be used to predict lifetimes under natural weather conditions for a variety of materials and formulations. Because of the complex nature of the interaction of the combined weather stresses with a material, there is presently no simple way to estimate the acceleration factor for a material. Increase in irradiance cannot be equated with acceleration of degradation. For most polymeric materials, the rate of degradation is not simply a linear function of the level of irradiance. Also, it does not take into account the effect of temperature, moisture, and other weather factors. Thus, there is no substitute for determining the acceleration factor for a given material experimentally. 

EMMA devices are successfully used by many manufacturers, particularly in the surface-coating industry, who wish to evaluate rapidly the durability of their products outdoors. However, these devices are also used in the study of accelerated weathering of almost all polymeric materials and plastics. One of the largest testing stations in the world is Heraeus DSET Laboratory Inc. (45601N, 47 Ave., Phoenix, AZ 85027-7042, USA) it employs hundreds of EMMA and EMMAQUA devices. 

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