Faraji, Farshid (2005) Study on the natural durability of mixed plywood panels composed the durable and non-durable veneers. PhD thesis Sciences forestières et du bois, ENGREF.

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Abstract

Nowadays, under growing environmental pressure, it is necessary to find alternatives to reduce the environmental impact of chemically treated wood and wood-based panels. In respect to this necessity we have evaluated the natural durability of mixed plywood composed of naturally durable plies (cypress, cedar and chestnut heartwood) and non-durables plies (poplar, beech, cypress and cedar sapwood) against wood destroying fungi (Coriolus versicolor and Coniophora puteana) and termites (Reticulitermes santonensis).
The thickness of plies (1.3 and 2.6 mm), the percentage of durable plies and non durable plies (40% and 60%), the number of plies (5 and 9) and finally the modality of integration of durable plies to non durable plies (11 different symmetrical combinations) in each mixed panel were studied as variable factors. Ultrasonic (non destructive) and indentation (semi destructive) methods were assessed to evaluate the presence and the extend of decay in pure and mixed plywood both as rapid methods in comparison with the biological assessment based on mass loss (according to the ENV12038). Shear test was also performed before and after exposition to fungi as a reference mechanical property of plywood in order to evaluate the relationship between the mass loss and the level of shear strength decrease after fungal degradation.
The results obtained have shown that mixing durable plies with non durable plies improves the natural durability of plywood against fungi. Nevertheless, the shear test after fungal degradation revealed that the decrease of shear strength in mixed plywoods is closer to the decrease observed for the non durable pure plywoods than for the pure durable plywoods. These results signify that there is a localised attack in the non durable plies of the mixed plywoods. The localised mass loss of non durable plies of mixed plywoods is better correlated to the decrease of shear strength than the classical mass loss (total) of the plywoods.
The plywoods the most resistant against fungi were found amongst the different mixed plywoods made of cypress sapwood and heartwood, and some models were found resistant to termite attack as well. The resistant panels to both fungi and termites had durable plies in exterior faces. The presence of intact thick sapwood plies in mixed cypress plywoods shows a protective effect (against termite attack) from the durable plies.
The indentation test (the measurement of energy before and after degradation) in comparison with the ultrasonic test (the measurement of velocity before and after degradation) showed the better correlation to the mass loss. In addition the indentation test can also be used to determine the gradient of mass loss and the threshold of rupture when decay takes place in the non durable plies situated under the durable exterior plies.

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