Physical properties of Tectona grandis (Lamiaceae) juvenile wood thermally modified in a closed system

This paper reports the first results of the effect of an industrial closed-system thermal modification process on the physical properties of fast-growth teak wood (Tectona grandis L.f.). The wood (juvenile) came from thinning a 15-18-year-old plantation located in Mato Grosso state, Midwest Brazil, broken down into plainsawn battens measuring 25 x 70 x 1 070 mm (radial x tangential x longitudinal). The battens were from material close to the pith which was combined heartwood (approximate to 75%) and sapwood. We sampled untreated and thermally modified battens, which were transformed into specimens for testing. The thermal modification was carried out at 160 degrees C in an industrial scale reactor, according to the parameters of the TanWood process, which is classified as a closed hygrothermal system. Twelve specimens by treatment (25 x 60 x 50 mm3 - radial x tangential x longitudinal) were conditioned in a climatic chamber set up at 35 degrees C and the relative humidity (RH) was raised stepwise (31%, 49%, 69% and 81%). Apparent density and equilibrium moisture content of the specimens were measured at every step. The latter was used as an index of wood hygroscopicity. For the statistical analysis, only the results of apparent density and moisture content at 35 degrees C and 69% RH were compared. At the end of the conditioning, the specimens were saturated in water, followed by oven-drying and then the total swelling (from saturated to oven-dried) was calculated. Thermally modified wood became less dense, less hygroscopic and more dimensionally stable than untreated wood. The apparent density and the equilibrium moisture content of wood were reduced by 20% and 18% respectively. With the continuous increase of RH from 31% to 81%, the thermally modified wood proportionally adsorbed less water than the untreated wood. The radial, tangential and volumetric total swelling were reduced by 25%, 5% and 15% respectively.