Abstract Color change associated with significative positive improve in physical properties is a challenge in wood research. This study investigated the changes in the color of the Gmelina arborea wood which underwent a thermomechanical densification process. The process was performed by applying three different temperatures (140 °C, 160 °C and 180 °C) with thickness reduction of 20% and 40% using 2.5 MPa equivalent pressure. The color change of the pieces was analyzed through the CIEL * a * b * system. The relationship between the color and the equilibrium moisture content of the densified material was also analyzed. The process reduced the lightness and yellow hue of the wood, with increased red pigment resulting in darker coloration of the treated pieces. The higher temperature used resulted in more significant changes in wood tone and lower equilibrium moisture. The process proved to be effective to change the color and significantly reduce the equilibrium moisture content in wood samples where the density was increase by 20% and 50% compared to natural wood.

Avaliou-se o efeito da termorretificação nas proprie dades físicas e mecânicas da madeira de Eucalyptus grandis e Eucalyptus saligna. Amostras de madeira foram modificadas termicamente por meio de uma combinação: pré-tratamento em autoclave (127°C - 1h) e tratamento em estufa (180-240°C - 4h); bem como tratamento apenas em estufa a 180-240°C durante 4h. As modificações químicas foram avaliadas por espectroscopia no infravermelho. A avaliação das propriedades físicas foi realizada por meio de ensaios de perda de massa, massa específica básica, teor de umidade de equilíbrio e estabilidade dimensional. Modificações na superfície do material foram analisadas pela técnica de ângulo de contato e o comportamento mecânico foi avaliado com testes de flexão estática. O uso de pré-tratamento em autoclave afetou as propriedades analisadas, no entanto a exposição em estufa denotou as maiores modificações na madeira de ambas a espécies. As modificações químicas foram relacionadas com a degradação das hemiceluloses. Além disso, observou-se redução significativa da higroscopicidade e da resistência mecânica da madeira termorretificada, enquanto que a massa específica básica não possuiu modificações significativas para a madeira das duas espécies estudadas. Os melhores resultados de redução da molhabilidade foram encontrados para as temperaturas mais baixas, enquanto que a estabilidade dimensional elevou-se em função da temperatura de tratamento em estufa. A maior perda de resistência mecânica foi observada no tratamento a 240°C para ambas as espécies.

The effect of thermal treatments on physical and mechanical properties of rose gum and Sydney blue gum wood was evaluated. Wood samples were thermally modified in a combination: pre-treatment in an autoclave (127°C - 1h) and treatment in an oven (180-240°C - 4h); and only treatment in an oven at 180-240°C for 4h. Chemical changes in the structure of woods were evaluated through infrared spectroscopy. Evaluation of physical properties was performed through mass loss, specific gravity, equilibrium moisture content and dimensional stability tests. Surface changes were analyzed through apparent contact angle technique and static bending tests were carried out to evaluate the mechanical behavior. Use of pre-treatment in autoclave affected the properties analyzed, however oven, resulted in the highest changes on wood from both species. Chemical changes were related to the degradation of hemicelluloses. Moreover, a significant decrease of hygroscopicity and mechanical strength of thermally modified woods was observed, while specific gravity did not significantly change for either of the species studied. The best results of decrease of wettability were found in low temperatures, while dimensional stability increased as a function of temperature of exposure in oven. The highest loss of mechanical strength was observed at 240°C for both species.

Colour changes by thermal treatments are a good alternative to increase wood quality, especially for non-structural uses. In this study, rose gum (Eucalyptus grandis) and Sydney blue gum (Eucalyptus saligna) woods from Southern Brazil were thermally treated under different conditions (steam pre-treatment in an autoclave at 127 °C for 1 hour and heat treatment in an oven at 180, 200, 200 and 240 °C for 4 hours) and colour changes were investigated using CIEL*a*b* technique. The main findings show that thermal treatments resulted in darkening of the wood, which was more intense at high temperatures. Colour difference after two-step steam-heat treatments was higher than 45 for both species. In general, the influence of the steam pre-treatment was more visible in lower temperatures. Lightness (L*) was the parameter that best described the colorimetric behaviour of rose gum and Sydney blue gum woods. In conclusion, thermal treatments are a great method to improve the market value of wood, especially for aesthetic uses, but it requires a control of process conditions.