WEATHERING | TESTING
fundamentals have changed. “Companies still perform accelerated testing with UV fluorescent and xenon arc instruments using the same suite of test standards they have for some time. There is some interest in metal halide testing and poten- tially LED in the future, and some exploration of new accelerated test cycles, but most users stick to basic, established tests,” he says. “Testing is usually around colour change and physical property degradation caused by exposure to light. A lot of it is for quality control and for supplier qualification, though some more advanced users are performing R&D testing and correlating to outdoors. Nevertheless, there is always a desire to accelerate results even further. We caution users that over-accelerating can lead to unrealistic results, but managers are frequently pushing for faster test cycles with higher irradiances to get results in shorter time frames,” he says. “In order to meet these demands, we have introduced our UVA-340+ and UVB-313EL+ lamps, which offer the highest maximum irradiances of any UV fluorescent tester, plus extended guaranteed lifetime of 1500 hours at popular high irradiance levels like 1.55 W/m2
/nm.
Some customers are turning to these for highly durable products.” One area of study that Q-Lab has been involved
in recently concerns replacement of inorganic colour additives in plastics, such as PVC, with organic additives. “While organic pigments are often better from a safety perspective, they often have decreased lightfastness performance,” Francis says. “Performance of these coloured materials in outdoor environments can be studied with natural and accelerated weathering testing. Sunlight can cause different weathering phenomena, such as yellowing and colour fade, depending on the amounts of incident UV or visible light. This is
Figure 1: Correlation of accelerated versus outdoor exposure for different coloured PVC compounds Source: Q-Lab Corporation
where accelerated weathering testing, in conjunc- tion with outdoor exposure, is vital.” The PVC weathering programme included outdoor exposure, UV fluorescent and xenon arc testing. Accelerated weathering testing of different coloured PVC plastics was performed and colour change (ΔE) measured. This involved outdoor exposure for two months (Florida) and accelerated laboratory testing for 200 hours (UV fluorescent and xenon arc). Significant differences in the extent of colour change were observed among the nine different colours. The correlation for colour change was compared for accelerated tests versus outdoor tests (Figure 1). Very good rank order correlation was observed for xenon (daylight or extended UV filter) and UV fluorescent (UVA-340 lamps) testing, however, there was poor correlation for UV fluorescent UVB-313 lamps (Figure 2). This matches typical expectations from these light sources, as UVB-type lamps are intended more for quality control purposes than for matching outdoor results. Different degradation was also observed for pigments and base plastics. There was darkening from plastic yellowing resulting from shortwave UV, with fade from the breakdown of pigments from visible light. Differences were most pronounced for pink and red specimens, illustrating the need for thorough colour characterisation beyond colour change. “There is no ‘magic number’ for accelerated
Figure 2: Rank order correlation of accelerated versus outdoor exposure for different coloured PVC compounds under different test methods Source: Q-Lab Corporation
www.compoundingworld.com
testing,” says Francis. “Weathering testing is strongly material-dependent. Good correlation for colour change, for instance, does not necessarily mean good correlation for physical properties. Under- standing your failure mode is key and retesting must be done any time materials’ chemistry is modified. Outdoor weathering data is critical for
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