Weathering accelerated exposures

Accelerated exposure equipment may also be used to test for weatherfastness in plastic materials [106], The natural destructive agents inherent in weather are approximated by filtering the radiation emitted by the xenon arc lamp and by spraying the sample with water under standardized conditions [106], Test programs are designed to relate to actual outdoor exposure to rain and humidity. In a standard program, a 3 minute wet cycle typically alternates with a 17 minute dry period. Weatherfastness tests are carried out and evaluated like lightfastness tests the black panel temperature and other parameters are the same in both procedures. 

Standards exist for determining the light fastness of colored pigmented plastics in daylight and in xenon arc light, i.e. in accelerated exposure equipment, and for determining the weathering fastness. 

Figure 18.1 Effect of Weather-Ometer exposure (0.35W/m2/nm) for color of materials [2]. From Weathering of polyester and copolymer sheeting, presentation given by D. R. Fagerburg at the Atlas School for Natural and Accelerated Weathering (ASNAW) course, Miami, FL, May 1999, and reproduced with permission of Atlas Electric Devices Company...

Frequently, pure liquid samples produce charts that are essentially a perfect match for each peak, both by retention and quantity. This allows a quick and certain identification. However, as individual samples of accelerants are "weathered" by exposure to fire and air the more volatile, low-boiling fractions are consumed or lost through evaporation. The compositions of some accelerant mixtures such as gasoline are rapidly altered. With passage of time., "weathered" accelerant mixtures become more difficult to compare with charts of standard mixtures. 

MIL-STD-304 is a commonly used accelerated-exposure technique to determine the effect of weathering and high humidity on adhesive specimens.66 In this procedure, bonded panels are exposed to alternating cold (-54°C) and heat and humidity (71°C, 95 percent RH) for 30 days. The effect of MIL-STD-304 conditioning on the joint strength of common structural adhesives is presented in Table 15.20. However, only relative comparisons can be made with this type of test it is not possible to extrapolate the results to actual service life. 

The 39 materials were subjected to accelerated heat ageing for a series of times and temperatures, artificial weathering and exposure to ozone. This report details the results of the accelerated heat ageing tests and their analysis for the prediction of changes after long-term natural ageing. The accelerated weathering and ozone test results have been published separately [2]. 

The main effects of weathering on LPCs are color fading and loss in mechanical and physical properties. Studies have shown that, for both natural and accelerated weathering, longer exposure times increase the degree of color change and lightness, carbonyl concentrations, and fiber loss on weathered LPC surfaces. 

Typical conditions used to accelerate weathering are exposure to radiant energy, temperature, relative humidity, and sporadic contact with both liquid aud gaseous water along with other relevant elements related to weathering. 

Keywords Reciprocity UV intensity Weathering Acceleration Correlation ORWET Polystyrene Irradiance Temperature Ultra-Accelerated Weathering System UA EMMA Exposure Exhapolation ASTM G90 Color Outdoor Static Dynamic Hypervolume... 

Conversely, the null hypothesis tested for this effort may be stated as there is no difference observed between degradation rates for polystyrene SRMs weathered in un-accelerated exposures and ultra-accelerated exposures when compared (normalized) for UV radiant exposure on the jc-axis of the degradation curves. 

A correlation between natural and artificial weathering was considered for lifetime prediction in a short exposure time. It was found that the confidence level of predicting time on the basis of artificially accelerated exposure trials is dependent on many parameters which include time, material, equipment, etc. 

Bauer, D.R. Interpreting weathering acceleration factors for automotive coatings using exposure models. Polym. Degrad. Stab. 69, 305-316 (2000)... 

In summary, the utilization of accelerated weathering devices can be useful for testing painted plastic parts when data exists relating accelerated exposure data to real long-term field data. Caution should always be exercised and accelerated data should be used in conjunction with other real-time test data. 

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