Reproducibility weathering tests

However, outdoor weathering tests are not quite reproducible due to the instability of the climate. Moreover, they are invariably slow, longer and longer exposure periods being required for the evaluation of new polymers with improved stability. This gradual increase in exposure time and a tendency towards more standardized methods has led industries to develop more powerful light sources and artificial weathering devices. 

Aging by artificial means to obtain an indication on how a material will behave under normal conditions over long period. Also tests in which conditions are intensified to reduce the time required to obtain deteriorating effects, similar to these resulting from normal service conditions. Duplicating or reproducing weather conditions by machine-made means. Test in which the normal weathering conditions are accelerated by means of a device. 

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... 

For example, a film of 100 pm thickness can become totally saturated after one day, whereas this can take up to ten days for a 2 mm roof film. This wide range of influences is the reason why it is often difficult to reproduce the results obtained by weathering tests. Often, the we dry cycles are so short that total saturation of the test material cannot be achieved [202]. 

In the fluorescent UV weathering tester, control of irradiance is simplified by the inherent spectral stabihty of its fluorescent UV lamps. All light sources decline in output as they age. However, unlike most other lamp types, fluorescent lamps experience no shift in spectral power distribution over time. This enhances the reproducibility of test results and is a major advantage of testing with the fluorescent UV weathering tester. 

Stanners have designed a test rig (Fig. 19.12) which provides results that can be correlated with actual atmospheric exposure data. The rig has been designed to investigate a wide range of alloying elements in a development programme on slow-weathering steels for which it was essential to have a rapid, reliable and reproducible test that incorporated the specific atmospheric factors responsible for rust formation. 

Since weather varies from day to day, year to year, and place to place, no precise correlation exists between artificial laboratory weathering and natural outdoor weathering. However, standard laboratory test conditions produce results with acceptable reproducibility and in general agreement with data obtained from long-time outdoor exposures. 

Very light and weather resistant systems require very long outdoor exposure times. In order to accelerate the testing of pigmented systems, test methods have been developed which simulate outdoor conditions. The evaluation of such a test is not only conveniently accelerated, but the resulting data are easy to reproduce and independent of location, climate, date, and time. 

Figure 18.2 Effect of Weather-Ometer reciprocity testing at high (0.70 W/m2/nm) and low (0.35 W/m2/nm) for color of Spectar copolymer and Spectar UV sheeting [7a]. From Fagerburg, D. R. and Donelson, M. E., Effect of water spray and irradiance level on changes in copolyester sheeting with xenon arc exposure, ANTEC 98 Conference Proceedings, Paper 808, Atlanta, GA, April 30, 1998, and reproduced with permission of the Society of Plastics Engineers...

Field tests have the best validity, but lack control and therefore reproducibility is a problem. The weather that has a big impact on clothing evaluation is out of control and thus makes the results hard to interpret for other situations. 

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). 

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