The effect of the hydrothermal modification (HTM) of the deciduous woods birch and aspen on their sorption behavior has been investigated by the vapor sorption method. An analysis of the experimental results was carried out based on the concept of Hansen solubility parameters (HSP), which takes into account the contribution of different forces – dispersion forces, dipole action, and hydrogen bonding – to the total cohesion energy. Sorption isotherms were measured concerning the vapors of water, methanol, and ethanol with unmodified and HTM woods at 160°C and 170°C during 3 and 1 h, respectively. The choice of sorbates was based on the parts of the hydrogen bonding and dispersion force to cohesion energy, in decreasing order of the former and increasing order of the latter. As a criterion of sorption, the value of the monolayer capacity was used, which was derived from the Brunauer-Emmett-Teller equation. Vapor sorption with unmodified and modified wood increased with increasing dispersion force component of the HSP of the sorbate. However, more substantial increase occurred for HTM wood, that is, wood surface became more hydrophobic. The reason for this observation is the change in the decreasing ratio holocellulose/lignin upon HTM. However, the chemical structure of lignin is also changed by HTM.
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 anti-swelling 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.
The chemical changes in birch wood occurring at thermo-hydro treatment (THT) was studied at temperatures (T) of 150, 160 and 170°C by analytical pyrolysis [Py-gas chromatography/mass spectrometry/flame ionisation detector (GC/MS/FID)], elemental analysis and traditional wet-chemical analysis. THT wood (THTW) was also extracted with acetone. Mass losses (ML) due to THT and acetone extraction of THTW were considered for material balance calculations. The holocellulose and hemicellulose (HC) contents decrease with increasing THT temperature (THTT), thus the apparent lignin content is elevated by ca. 20%. The HC degradation begins at 150°C, while that of α-cellulose modification at 170°C. Compared to unmodified birch, the THT170°C material contains ca. 10% less α-cellulose and up to 40% less HC. The Py-GC/MS also indicates decreasing amounts of volatile products from polymeric carbohydrates (CHs) and lignin origin as a function of increasing THTT. The identified CH-based Py products of THT170°C of non-extracted (ne) and extracted (e) materials resulted in 13 and 22% weight decrements, respectively, while the lignin-type Py products were reduced by 13 and 49%, respectively. With increasing THTT, the total content of CO2, water and methanol decreases, and the amount of unidentified compounds increases by 30%.
The effect of thermo-hydro treatment (THT) on the properties of birch (Betula spp.) wood veneers has been studied. THT was carried out in a multi-functional pilot scale wood modification device of wood treatment technology (WTT, Latvia) under elevated water vapor pressure conditions at four combinations of temperature and treatment time (°C/min): 150/10; 150/50; 160/10 and 160/50. After THT, the following veneer properties were examined: mass loss (ML), chemical composition, bending strength (BS), tensile strength (TS), equilibrium moisture content (EMC), resistance to decay by mould and blue stain fungi, and surface contact angle (CA). The chemical components were changed by THT. Increased THT temperature and time resulted in hydrophobization of veneers as indicated by decreasing EMC and increasing CA data. All THT were effective against wood discoloring fungi, although insufficient decay resistance was observed. The mechanical strength properties of THT veneers were also deteriorated.
The microstructural changes in a selection of softwoods and hardwoods resulting from thermo-hydro treatment (THT) at 160°C were examined by means of a state-of-the-art micro X-ray computed tomography. A dedicated X-ray scanning and volumetric processing protocol was developed. All reconstructed volumes had an approximate voxel pitch between 0.8 and 1.2 μm3. The microstructures of the same needle-shaped specimens before and after THT were visualized, and the individual parameters (maximum opening and lumen volume) for various cell types were quantified and compared. The highest values of substance volume were recorded for the ash sapwood (81%) and spruce specimens (72%). After THT, a significant correlation was found between the mass loss determined by gravimetry and the X-ray volume loss. The largest change occurred in the lumen volume of several tissue components, such as libriform fibers, tracheids, and ray parenchyma. The average aspen fiber volume reduction after THT was 31%, a value 2.6 times higher than the volume reduction of the average vessels. The porosity of ash sapwood increased from 41 to 56%, whereas the porosity of birch decreased from 34 to 29%.
Thermal modification (TM) of wood has occupied a relatively narrow but stable niche as an alternative for chemical wood protection. There are different technological solutions for TM and not all details of their effects on wood tissue have been understood. The one-stage hydrothermal modification (HTM) at elevated vapour pressure essentially changes the wood’s composition and structure. In the present paper, the changes in three hardwood lignins (alder, aspen, and birch) were observed within the cell wall by means of cellular UV microspectrophotometry. The lignin absorbances in the compound middle lamella (CML) of unmodified wood are 1.7- to 2.0-fold higher than those in the fibre S2 layer. The woods were modified in the temperature range from 140 to 180°C, while in the lower temperature range (140°C/1 h), the UV absorbances are little affected. Essential changes occur in the range of 160–180°C and the UV data reflect these by absorbtion changes, while the absorbances at 278 nm rise with factors around 2 more in the S2 layer than in the CML. The absorbance increments are interpreted as polycondensation reactions with furfural and other degradation products of hemicelluloses with the lignin moiety of the cell wall.