ResumoA termorretificação é um processo de agregação de valor, que confere à madeira colorações semelhantes àquelas de espécies tropicais de maior valor econômico, além de melhorar sua estabilidade dimensional e resistência a fungos. A termorretificação deve ser estudada visando obter seus benefícios com o mínimo de perdas referentes às propriedades mecânicas da madeira. Madeiras consideradas de menor valor econômico (Eucalyptus grandis e Pinus caribaea var. hondurensis) foram submetidas a diversos tratamentos de termorretificação, em presença de oxigênio do ar (140, 160 e 180 ºC) e escassez de oxigênio (160, 180 e 200 ºC), sendo analisadas suas propriedades mecânicas. O eucalipto, além de ter sido mais susceptível à perda de massa com o aumento da temperatura, também manifestou maior propensão à perda de resistência em função do tratamento térmico, quando comparado ao pínus. A termorretificação resultou em queda da resistência ao cisalhamento, para ambas as madeiras testadas, e à flexão estática apenas para a madeira de eucalipto. A temperatura de 200 ºC não foi suficiente para provocar perda da resistência à compressão paralela. Não foi possível determinar com clareza o efeito das condições de presença e escassez de oxigênio do ar durante a termorretificação sobre as propriedades mecânicas da madeira. Palavras-chave: Tratamento térmico; resistência mecânica; madeira. Abstract Effects of thermal rectification on mass loss and mechanical properties of Eucalyptus grandis and Pinus caribaea var. hondurensis woods.The thermal rectification is an adding-value process, which provides to wood colors similar to those observed in high-valued tropical species, as well as higher dimensional stability and better resistance against fungi. The thermal rectification process must be studied seeking to obtain its benefits with minimum losses in mechanical properties of wood. Comparatively low-valued woods (Eucalyptus grandis and Pinus caribaea var. hondurensis) had been submitted to different thermal rectification treatments, in presence of oxygen from the air (140, 160 and 180 ºC) and scarcity of oxygen (160, 180 and 200 ºC), and their mechanical properties were evaluated. Eucalyptus was more susceptible to mass loss with increasing temperature, and was more prone to loss of mechanical resistance during thermal treatments, compared to pinus. The thermal rectification reduced shear strength, for both species tested, as well as bending strength, for eucalyptus only. The treatment at 200 ºC was not sufficient to cause losses in longitudinal compressive strength. It was not possible to clearly determine effects of presence or scarcity of air oxygen during thermal rectification on the mechanical properties of wood.

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Thermal treatment (thermal rectification) is a process in which technological properties of wood are modified using thermal energy, the result of which is often value-added wood. Thermally treated wood takes on similar color shades to tropical woods and offers considerable resistance to destructive microorganisms and climate action, in addition to having high dimensional stability and low hygroscopicity. Wood samples of Eucalyptus grandis were subjected to various thermal treatments, as performed in presence (140ºC; 160ºC; 180ºC) or in absence of oxygen (160ºC; 180ºC; 200ºC) inside a thermal treatment chamber, and then studied as to their chemical characteristics. Increasing the maximum treatment temperatures led to a reduction in the holocellulose content of samples as a result of the degradation and volatilization of hemicelluloses, also leading to an increase in the relative lignin content. Except for glucose, all monosaccharide levels were found to decrease in samples after the thermal treatment at a maximum temperature of 200ºC. The thermal treatment above 160ºC led to increased levels of total extractives in the wood samples, probably ascribed to the emergence of low molecular weight substances as a result of thermal degradation. Overall, it was not possible to clearly determine the effect of presence or absence of oxygen in the air during thermal treatment on the chemical characteristics of the relevant wood samples.

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Table 1 Average roughness average (R a ) of sugar maple sanded surfaces measured along and across the grain for three sanding programs, four feed speeds and two abrasive minerals

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The condition of superficial tissues plays an important role in the behavior of adhesive/wood interfaces. Tissues at the wood surface should be the least distorted possible during surfacing in order to avoid the formation of mechanical weak boundary layers. These layers cause significant loss of adhesion of coating films and gluelines during utilization of wood. In this study, sugar maple (Acer saccharum Marsh.), northern red oak (Quercus rubra L.), and eastern white pine (Pinus strobus L.) were surfaced by oblique cutting and face milling, while paper birch (Betula papyrifera Marsh.) was face milled. Cell damage was considerably higher in face milling than in oblique cutting. This damage was manifested in the form of lateral distortion, bending, and rupture of tissues. In general, superficial cell damage presented similar patterns for all species studied, but its severity was quite variable as a function of species and cutting conditions. The present study describes damage patterns and discusses the possible implication of cell damage in wood finishing and gluing.

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