Heat-induced colour changes of pine (Pinus pinaster) and eucalypt (Eucalyptus globulus) wood

Abstract

Heat treatment of Pinus pinaster and Eucalyptus globulus wood was made by hot air in an oven during 2 to 24 h at 170-200 ºC and by steam in an autoclave during 2 to 12 h at 190-210 ºC. The colour parameters L*, a* and b* were determined by the CIELAB method on radial, tangential and transverse sections for untreated and treated wood, and their variation with the treatment (ΔL*, Δa* and Δb*) were calculated in percent.In the untreated woods, for eucalypt wood lightness (L*) varied between 54.1-63.8% with a* between 7. 4-8.5 and b* 15.7-19.9, and for pine wood L* varied between 67.3-76.1%, a* between 6.9 -7.6 and b* 16.3 -24.1. With the heat treatment wood became darker, more for oven treatment (ΔL* about 50% for 4% mass loss), and at the same treatment conditions more for eucalypt wood. In general the contribution of the red colour (a*) and yellow (b*) decreased with the heat treatment. The transverse section darkened less in the two species and for both treatments, with small differences between radial and tangential sections. Lightness decrease was related to chemical changes, with good correlations with glucose (R 2 = 0.96), hemicelluloses (R 2 = 0.92) and lignin (R 2 = 0.86).As regards colour, the heat treatments showed an interesting potential to improve the wood quality for solid timber products from pine and eucalypt.3 IntroductionThe use of heat treatments to modify the properties of wood is not new and Tiemann (1920) already reported that high temperature drying decreased the wood equilibrium moisture content and swelling as well as Stamm and Hansen (1937) with wood heated in several gases. The first studies directed towards improvement of wood dimensional stability were carried out by Stamm et al. (1946) and further continued by Kollmann and Schneider (1963), Kollmann and. Fengel (1965), Fengel (1966, D'Jakonov and Konepleva (1967), Nikolov and Encev (1967), Burmester (1973), Rusche (1973 a, b), Giebeler. (1983, Hillis (1984), Bourgois and Guyonnet (1988), Bourgois et al. (1989) and Dirol and Guyonnet (1993).The development and commercialization of heat treatments to increase wood durability and dimensional stability was stimulated only recently by environmental concerns but there are already several commercial applications in some European countries and North America.The heat treatments reduce the equilibrium moisture content of wood and improve its dimensional stability and durability but may reduce the mechanical resistance, mainly of bending (Kollmann and Schneider 1963; Viitanen et al. 1994;Viitaniemi et al. 1997; Kim et al. 1998; Kubojima et al. 2000;Epmeier et al. 2001; Jämsä and Viitaniemi 2001; Rapp and Sailer 2001; Kamdem et al. 2002; Yildiz 2002;Bengtsson et al. 2002;Bekhta and Niemz 2003; Gosselink et al. 2004; Metsä-Kortelainen et al. 2006; Wang and Cooper 2005; Hakkou et al. 2006;Esteves et al. 2007).The heat treatment of wood changes its chemical composition by degrading both cell wall compounds and extractives. The thermal degradation starts by deacetylation of hemicellulos...

THERMAL MODIFICATION OF Eucalyptus grandis WOOD: VARIATION OF COLORIMETRIC PARAMETERS

“…On the whole, the average values of L*, a* and b* decreased as the temperature and heating time rose. That was also observed by Moura and Brito (2011), Esteves et al (2008) and Unsal et al (2003) in studies with E. grandis, E. globulus and E. camaldulensis wood respectively. According to Hill (2006), physical modifications in thermally modified wood depend heavily on the conditions of the methods applied and the colorimetric modifications are related to temperature and length of treatment.…”

Thermally initiated solvent-free radical modification of beech (Fagus sylvatica) wood

“…Moreover, very low DE values were observed for the respective modified specimens. These observations show the dominant effect of the temperature which had been well documented in the literature (Ahajji et al 2009;Esteves et al 2008;Windeisen et al 2007). …”

The effect of thermal treatment on physical and mechanical properties of Luehea divaricata hardwood

“…Colour changes on wood occur due to the modification of hemicelluloses structure, migration of extractives to surface and formation of oxidative products. Changes in chemical compounds induce darkening of thermally modified wood (Bekhta and Niemz 2003, Esteves et al 2008, Akyildiz et al 2009). …”