Adjusting timing of weathering test to account for seasonal variations in UV exposure A. Heikkila ¨ a, * , A. Tanskanen a , P. Ka ¨rha ¨ b , K. Hanhi c a Finnish Meteorological Institute, R&D/Earth Observation, P.O. Box 503, FI-00101 Helsinki, Finland b Helsinki University of Technology, Metrology Research Institute, P.O. Box 3000, FI-02015 TKK, Finland c Tampere University of Technology, Laboratory of Plastics and Elastomer Technology, P.O. Box 589, FI-33101 Tampere, Finland Received 10 October 2006; received in revised form 17 November 2006; accepted 10 December 2006 Available online 27 January 2007 Abstract Satellite-derived UV climatology has been used to design a timetable for the outdoor UV exposure of polymeric material specimens. By pre- exposure computation, fixed time increments are transformed into a schedule with a predicted average accumulation of UV dose. The method was applied to produce a timetable for an ongoing exposure programme in a network of seven European test sites over the latitude range 28.5e 67.4  N. The effect of the average seasonal variability of solar UV irradiance on the predicted accumulated UV exposure is shown to be notably diminished by the method. Relative underestimations of 5e82% of cumulative UV stress are estimated to be avoided by extensions of 6e167 days exposure durations as suggested by the adjustment procedure. Hence, more reproducible degradation data are expected to be obtainable for exposures that are not multiples of the full annual cycle of UV, especially for those with a duration <1 year. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Weathering; Outdoor; Timing; UV radiation 1. Introduction Solar UV radiation is one of the key factors causing the degradation, and thus limiting the service life, of many mate- rials. For some materials, UV radiation has been assessed to be actually the most important determinant of durability [1]. Starting from the 1980s, the depletion of the atmospheric col- umn ozone has led to increase in the levels of UV reaching the Earth’s surface [2]. In contrast to other atmospheric factors, e.g., temperature, humidity, and precipitation, terrestrial UV radiation has thus enhanced its deteriorative power within a rel- atively short period of time. Recent scenarios have predicted the elapse of several decades before the recovery of column ozone abundances. Even then the eventual stabilized state of the ozone layer will probably not be at pre-1980 levels, but be- low those [3]. It is therefore most likely that solar UV radia- tion will remain as an important issue, not only in the field of atmospheric science, but also within research on material degradation. Prediction of the lifetime of materials basically employs two different weathering strategies complementary to each other: artificial and outdoor weatherings. Accelerated ageing offers a profitable means of shortening the time required for a new product to enter the market. Long-term outdoor weath- ering, on the other hand, is needed to establish a correlation between the laboratory and real service environments. Several sites for extensive outdoor weathering tests have been founded worldwide (see Ref. [4]), of which especially Florida (US) has served for decades as the de facto standard weathering site. Compared to sites at higher latitudes, the advantages of Flor- ida (26  N) are obvious. UV radiation levels are relatively high in the subtropics. Testing conditions in Florida are hence * Corresponding author. Tel.: þ358 9 19294151; fax: þ358 9 19293146. E-mail address: anu.heikkila@fmi.fi (A. Heikkila ¨). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.12.015 Polymer Degradation and Stability 92 (2007) 675e683 www.elsevier.com/locate/polydegstab