Resistance to artificial weathering of four tropical woods: the effect of the extractives

As madeiras tropicais: garapeira ( Apuleia leiocarpa ), tauari ( Couratari stellata ), ita\ufaba ( Mezilaurus itauba ) e marup\ue1 ( Simarouba amara ) foram selecionadas para examinar o efeito da presen\ue7a dos extrativos na varia\ue7\ue3o de cor e no processo de fotodecomposi\ue7\ue3o das suas superf\uedcies. Amostras com e sem extrativos foram submetidas a ciclos de radia\ue7\ue3o ultravioleta (350 nm) e \ue1gua at\ue9 totalizar 2.000 horas de irradia\ue7\ue3o e 400 horas de lixivia\ue7\ue3o. A mudan\ue7a da cor natural das madeiras foi monitorada por espectrocolorimetria de reflex\ue3o difusa. Amostras de marup\ue1 e tauari com teores de extrativos menores que 2% exibiram varia\ue7\ue3o de cor com o aumento do tempo de tratamento quase iguais \ue0s amostras sem extrativos. A resist\ueancia anormal do tauari \ue0 altera\ue7\ue3o de cor n\ue3o pode ser atribu\uedda aos seus extrativos. Para a garapeira e para a ita\ufaba, a presen\ue7a de 11,7 e 7,42% de extrativos respectivamente acelerou a velocidade de varia\ue7\ue3o de cor. A an\ue1lise dessas varia\ue7\uf5es de cor indica que os extrativos, nesse caso, agiram acentuando a cor natural da madeira. Adicionalmente, verificou-se que o tauari foi a madeira mais resistente \ue0 varia\ue7\ue3o de cor do grupo estudado, seguido pela garapeira, marup\ue1 e ita\ufaba todas com extrativos.The tropical woods Apuleia leiocarpa , Couratari stellata , Mezilaurus itauba and Simarouba amara were selected to examine the effect of the presence of the extractives in the color variation and in the photodecomposition process of their surfaces. Samples with and without extractives were submitted to cycles of ultraviolet radiation (350 nm) and immersion in water until totalizing 2,000 hours of irradiation and 400 hours of water leaching. Changings in the natural wood colors were monitored by diffuse reflectance spectroscopy. Samples of Simarouba amara and Couratari stellata with extractives content of 2% or less showed color variation with time of treatment almost similar to the samples without extractives. The abnormal resistance of Couratari stellata to color changing can not be attributed to extractives. For the Apuleia leiocarpa and Mezilaurus itauba woods, the presence of 11,7% and 7,42% of extractives compounds, respectively, accelerated the color variation rate. Analysis of the color variation indicated that the extractives, in this case, acted by intensifying the natural wood color. Additionally, it was verified that Couratari stellata was the more resistant wood to color variation in this studied group of woods, followed by Apuleia leiocarpa, Simarouba amara and Mezilaurus itauba, all with extractives