One-stage thermo-hydro treatment (THT) of hardwoods: an analysis of form stability after five soaking-drying cycles

Abstract

A newly developed thermo-hydro treatment (THT) for use in a one-stage heat treatment process was examined by focusing on the form stability-related properties of European aspen (Populus tremula), birch silver (Betula pendula), and gray alder (Alnus incana). In particular, wood specimens were subjected to THT in a saturated steam atmosphere in a pilot-scale autoclave heated between 140 and 180°C for 1-3 h. Several parameters of untreated and treated samples after several soaking and drying cycles were compared, namely, the changes in the volumetric swelling, swelling in the radial and tangential directions, cell wall total water capacity, and antiswelling efficiency (ASE). Due to repeated wetting in the cyclic water submersion-drying test, the original ASE of 73% decreased to 65% (180°C for 1 h), and the original ASE of 33% decreased to 5% (140°C for 1 h). Wood modified at 170°C presented good results that were not significantly lower than wood treated at higher temperatures while consuming less energy to deliver ASE improvement and was selected as optimum. To increase the ASE by 1%, the amount of energy consumed was decreased by 41%, 39%, and 17% compared with the treatment regimes of 160°C for 1 h, 160°C for 3 h, and 180°C for 1 h, respectively. The new THT regime led to improved long-term dimensional stability due to the cross-linking of cell wall polymers, which resulted in increased cell wall rigidity.

Wood moisture content during the thermal modification process affects the improvement in hygroscopicity of Scots pine sapwood

“…This reduction is higher for dry sample dimensions than for wet sample dimensions because nanopores previously occupied by WSC within the cell wall close in dry conditions, while they are partly filled with water in wet conditions. The results thus confirm a cell wall bulking effect of degradation products on the dimensional stabilization, as suggested by Biziks et al (2015) and Cermak et al (2015). Furthermore, it is also evident from Fig.…”

“…They argued that TM increases the cell wall modulus and that this increase in matrix stiffness further restricts the expansion of the cell wall nanopores, leading to reduced sorption. Furthermore, additional mechanisms which are not directly caused by chemical changes, such as hornification as a result of severe drying (Borrega and Kärenlampi 2010) or cell wall bulking effects caused by remaining degradation products (Biziks et al 2015;Cermak et al 2015), need to be considered, as they can affect the hygroscopicity and the dimensional stability of thermally modified wood.…”

Wood modification with tricine

“…Moreover, the EMC of the modified wood decreased significantly in agreement with previous observations (t-test: P ≤ 0.05); that is, the EMC of the reference oak was 10.5 ± 0.2% and that of the treated oak decreased to 6.6 ± 0.4% (data based on eight determinations). It can be assumed that the readily accessible amorphous carbohydrates were modified with tricine in such a way that their hydrophilicity is decreased in a similar fashion as it occurs during thermal modification at 170°C-250°C (Jämsä et al 2000;Biziks et al 2014). …”

Retificação térmica, termorretificação, tratamento térmico, tratamento com calor ou modificação térmica?

“…A revista Holzforschung ficou na segunda posição, com 11 artigos publicados (BIZIKS et al, 2015;HILL;PFRIEM, 2015;JAVED et al, 2015;KIM et al, 2015a;2015b;LI et al, 2015;PEARSON;ORMARSSON;GABBITAS, 2015;SONDEREGGER et al, 2015;TRCALA;ČERMÁK, 2015;WANG et al, 2015;WILLEMS et al, 2015).…”