Assessment of Wood Photodegradation by Microtensile Testing

The susceptibility to weathering of wood surfuces and their consequential high maintenance demands discourage the exterior use of wood and highlight the need for methods of enhancing the resistance of wood to photodegradation. Weathering processes in the surface layers of wood have been studied by measurement of the tensile strength changes occurring in 75 μm thick softwood strips during exposure to natural weathering or several regimes of artificial weathering. Microtensile testing at zero and 10 mm span enabled the relative changes in cellulose microfibril strength and the lignin-dependent properties of the matrix to be distinguished. The technique was found to offer a rapid, accurate and reproducible means of evaluating the chemical und structural changes involved in the weathering of wood and coated wood surfaces during weathering. Tensile strength curves indicated three phases in the degradation process. The initial phase was characterised by a slow rate of strength loss, and at high humidity levels even an increase in strength. During the second and third phases believed to be associated with the successive degradation of lignin and cellulose, strangth losses were more rapid. Differences in the weathering behaviour of three softwood species were distinguished, and temperature, moisture and radiation source shown to exert strong influences on degradation rate. Artificial weathering was found to provide a valid alternative to natural weathering in systematic investigation of degradation mechanisms. SEM studies revealed that structured changes in the wood are associated with the eurly stages of photodegradation. Fractography showed that the progression of degradation involves the development of brittleness und reduction of stress transfer capabilities through lignin degradation, followed by reductions in microfibril strength resulting from cellulose degradation.