Methods for Assessing Degrading Effects of Uv Radiation on Materials

UV radiation is one of the key factors ageing materials and thus limiting their service life time. Damages induced by UV radiation may be observed as changes in the properties of the material. Various types of weathering experiments are used to study the chemical and physical mechanisms behind the degradation process. Material samples are exposed to UV radiation and other environmental stress agents in natural outdoor and artificial conditions in laboratory. Quantification of the material’s response to the exposure yields knowledge on the ageing behaviour of the material. This knowledge is essential when selecting materials to comply with the service requirements, as well as developing materials with extended durability. The focus of this study has been on certain methods used in investigation of the deleterious effects of UV radiation on materials. Various aspects of weathering and analysis of material property changes are considered. As regards weathering, three main topics unfold: sampling of data from outdoor weathering experiments; construction of continuous time series of UV radiation exposures over the weathering periods; and comparability of artificial UV radiation conditions to those encountered under the Sun. Within the scope of the property change analysis, the study has focused on the wavelength dependencies found in damage induction. A new method is proposed for planning the timetable of a natural outdoor weathering experiment in a way that the seasonal and geographical variations are accounted for. The method simplifies the post-exposure analysis of the material properties and improves the comparability of test results obtained at different weathering sites. A new approach is adopted to reconstruct the solar spectral UV irradiance at the Earth’s surface through radiative transfer modelling. The method provides a means for deriving UV irradiance in cases where no direct measurements of the quantity are available. As a result, a continuous UV radiation time series is obtained, as required for the determination of the UV radiation exposure accumulated on the materials. To improve the comparability of accelerated ageing to that naturally occurring outdoors, a method for characterizing the UV radiation conditions used in a UV chamber is presented. Knowledge on the exposure is further used to establish a correlation between the artificial and natural ageing environments, facilitating predictions on how the materials would survive in their real service environments. The study aiming at improvements in the analysis of material property changes is two-fold. A novel facility designed and constructed for exposing materials with spectrally resolved UV radiation is reported. The instrument opens new pathways for the investigations on wavelength specific damages in materials. The facility has been exploited in a study on the wavelength sensitivity of newspaper to photoyellowing. A new approach in deriving the action spectrum of photoyellowing of a material is proposed. The resulting action spectrum may be used in optimizing the chemical and/or physical protection of the material against UV radiation. Publishing unit Finnish Meteorological Institute, Research and Development, Climate Research Classification (UDC)