| Abstract: | CONCLUSION. A recent review about service life of exterior wood coatings gives a fairly good picture of the current situation in the field of accelerated light ageing tests in the polymer industry in its wish to control the durability of products.92 Although it is widely recognized that more efforts should be put into understanding the underlying chemical processes occurring at the molecular scale, responsible for the macroscopic evolution of an object, durability studies still rely on correlating the evolution of physical properties between natural weathering and accelerated photoageing, like colour or gloss measurement.93 The photochemical degradation of macromolecules in general and of paper more specifically, appears to be fairly complex. Variations of physical properties and of chemical composition can easily be observed during photo-degradation in ambient conditions. However since long periods of time are involved, the variations in environmental parameters (temperature, humidity, light quality, light intensity) are not accurately known. At the laboratory level, a second difficulty is encountered if light-ageing of paper is performed in accelerated conditions. Each photoageing unit has its own specific characteristics depending on the light source used and the type of temperature and humidity control. The relevance of accelerated laboratory experiments to real ageing conditions cannot be reliably assessed on the variation of macroscopic properties like mechanical properties or aspect changes since the same variation can have several different origins. Reliability is gained when relevance is assessed by a more detailed survey of the various chemical evolutions i.e. oxidation, formation and bleaching of absorbing yellowing moieties, formation and photooxidation of fluorescent species. Due to our limited understanding of chemical changes at the molecular level, the control of relevance of accelerated light ageing tests is still difficult and more knowledge of paper photooxidation mechanisms is needed. |
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