materials testing | PVC weatherability
Figure 3. High exposure temperature catastrophic effects
observe inappropriately high exposure temperatures in thick vinyl composite decking where the materials’ low thermal conductivity and thick dimensions prevent sufficient heat exchange with convective cooling air under concentrated solar irradiance. Excessive exposure temperatures can skew the results of exposure testing for some temperature sensitive materials resulting in unnatural appearance and mechanical property changes. Effect of temperature on appearance: A simple
weathering exposure of white rigid vinyl illustrates the dramatic effect of exposure temperature variation on weathering appearance2
. To compare the effect of
Figure 4. New low tempera- ture EMMA device with ‘cold mirrors’
backing, researchers mounted single continuous strips of white vinyl siding so that some areas of the strip were backed with plywood while other areas of the strip remained unbacked. A slightly accelerated track rack exposure kept the PVC strips oriented directly towards the sun during the day in Arizona for 24 months. Colour data correlated colour change from the weathering exposure with exposure temperature. The colour change (delta b*) appeared greater for backed areas. Thermocouples attached to the unexposed surface of the strips measured exposure temperature. The data
indicated a high degree of correlation between tem- perature difference and colour change difference for the backed and unbacked areas of the PVC strips as shown in Figure 2. The moderate temperature differences of this slightly accelerated exposure had significant effects on the weathering appearance for the rigid PVC material. Clearly, exposure temperature represents a critical consideration in accelerated weathering testing. Effect of temperature on mechanical properties:
High exposure temperature can have even more catastrophic effects on temperature sensitive materials. For example, rigid vinyl compounds often have heat deflection temperatures only slightly above some accelerated weathering test exposure temperatures. If the test specimens approach the heat deflection temperature in an accelerated exposure like ASTM G90, test specimens can deform out of the path of cooling air flow. If this happens, exposure temperature further increases, resulting in more deformation and ‘runaway’ temperatures ruining the exposure test specimens. Examples of this are shown in Figure 3. If exposure temperatures rise to high enough levels, actual burning of the temperature sensitive test specimen surface can occur ruining the test as also shown in Figure 3. Clearly the need exists to supplement the convective cooling of the G90 accelerated weathering device to accommodate temperature sensitive materials and problematic specimen geometries which otherwise may result in unacceptably high exposure temperatures for temperature sensitive materials.
The goal and the solution The goal of this project, therefore, became how to further reduce exposure temperature of the ASTM G90 devices. Specifically, to enable temperature sensitive specimen exposure under high irradiance, with high fidelity to natural sunlight UV spectral distribution without temperature specimens exceeding critical exposure temperatures that cause unnatural weather- ing degradation, heat deformation or burning. Current G90 devices rely on convective cooling to
46 COMPOUNDING WORLD | June 2014
www.compoundingworld.com
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