Micro-tensile behavior of Scots pine sapwood after heat treatments in superheated steam or pressurized hot water

A total of six blocks of Scots pine (Pinus sylvestris L.) with dimensions of 15 × 40 × 30 mm³ (tangential × radial × longitudinal) were prepared from a single slat (Fig. 1). The annual ring orientation deviated by ca. 10° from the radial plane to avoid stretching of wood rays over the micro-veneer surface. The blocks were vacuum-impregnated with 10% aqueous ethanol at ca. 50 mbar for 2 h and left to soak in fresh aqueous ethanol for 3 days. Micro-veneers with a thickness of 60 µm were cut from the soaked blocks using a rotary microtome. The wet micro-veneers were fixed between two glass slides in bundles of forty and stored at 20 °C and 65% RH until HT. The thickness of each veneer was measured on a micrometer (SE250, Lorentzen & Wettre, Sweden) and veneers that deviated from the average thickness by more than 15% were discarded.

Two different HT techniques were applied and the treatment conditions were chosen based on previous studies to ensure that the resulting decrease in hemicelluloses content was in a similar range [6, 26]. For treatments in superheated steam at atmospheric pressure (Dry-HT), the micro-veneers were fixed between two stainless steel plates and placed into a steam-oven that was pre-heated to 105 °C. The temperature in the oven was increased by 15 °C every 30 min until reaching a treatment temperature of 210 °C, which was held for 1, 3, 5 or 7 h, before the oven heating was switched off to decrease the temperature below 100 °C within 1 h. Steam was continuously inserted into the oven throughout the treatment while atmospheric pressure maintained. For treatments in pressurized hot water (Wet-HT), the micro-veneers were soaked in deionized water overnight and filled into small stainless steel vessels together with 200 ml of deionized water. The vessels were closed tightly and heated in an oil bath at 140 °C for 1, 3, 5 or 7 h, before cooling in cold tap water for ca. 10 min.

After the treatments, all samples were stored in deionized water for a minimum of 3 days. A total of 24 veneers per sample group (four veneers per block) was kept in deionized water at 25 °C with regular water changes for a maximum of 2 weeks until micro-tensile testing. Another set of 24 veneers per sample group was fixed between two glass slides and dried at 60 °C for ca. 24 h, before conditioning at 23 °C and 50% RH for a minimum of 2 weeks until the micro-tensile testing. For samples that were treated in pressurized hot water, another set of 24 veneers per sample group was first dried at 60 °C for 24 h, which was followed by soaking in deionized water at 25 °C for a minimum of 48 h before the micro-tensile testing (“rewet”). Veneers with visible defects due to handling of the heat-treated veneers were discarded prior to micro-tensile testing, leading to small deviations in the number of veneers that were tested for each sample group.

The tensile tests were performed on a MTS 400/M tensile tester (MTS Systems Corporation) using a 200 N load cell. The distance between the two clamps was set to 25 mm (Fig. 1). Preliminary tests with clamping forces between 0.15 and 0.3 MPa showed the highest average maximum load at a force of 0.25 MPa. This clamping force was considered as the optimal compromise between slippage of the veneers at low clamping forces and crushing of the veneers at high clamping forces and was applied during the actual test series. The veneers were tested at a speed of 1 mm min⁻¹ and elongation was set to zero and at a pre-load of ca. 3 N. The work in traction was determined by integration of the load-elongation curve until maximum load. The stiffness was calculated as the slope of the load-deformation curve between 10 and 40% of the maximum load. The tensile properties of the treated veneers were calculated as residual stiffness, residual maximum load and residual work in traction by relating the stiffness, maximum load or work in traction of the treated samples to the corresponding average value of the reference samples that were measured in the same moisture state (conditioned or water-saturated).

Micro-veneers were milled in a Wiley mill to pass through a 30 mesh screen and extracted in a Soxhlet apparatus with acetone for 6 h. Lignin and carbohydrates were determined by acid hydrolysis according to NREL/TP-510-42618 [27], as described previously [6]. The ash content was determined according to TAPPI 211 on-02 by exposing oven-dried samples to 525 °C for 5 h. The chemical composition was calculated on an extractive-free, oven-dry basis. The lignin content was calculated as the sum of the acid-soluble and insoluble fraction. The contents of cellulose, hemicelluloses, xylan and glucomannan were calculated according to Janson [28]. All measurements were done in duplicate.

Fourier transform infrared (FT-IR) spectra of wood micro-veneers were measured using a FT-IR spectrometer (Spectrum Two, PerkinElmer, USA) equipped with an ATR unit and a diamond crystal. Spectra were acquired within the wavenumber region 4000–750 cm⁻¹ at a resolution of 4 cm⁻¹ and 8 accumulations. The spectra were baseline corrected and normalized to the absorbance at 1509 cm⁻¹.

The water sorption behavior within the hygroscopic range (0–95% RH) was analyzed in a dynamic vapor sorption (DVS) apparatus (DVS intrinsic, Surface Measurement Systems, London, UK) at a temperature of 25 °C and a gas flow of 200 sccm. Approx. 15 mg of micro-veneers were placed on a sample pan and exposed to a dry nitrogen flow (~ 0% RH) to determine the dry mass of the sample. This was followed by the exposure to 50% and finally 95% RH. Each RH-step, including the drying step at 0% RH, was kept until the mass change per minute (dm/dt) was less than 0.0005% min⁻¹ over a 10 min period. The dm/dt was calculated using a 10 min regression window. The chosen dm/dt value was lower than the value recommended by the manufacturer for the sample mass used in this study (dm/dt 0.002% min⁻¹) in order to reduce the deviation from the equilibrium state. The wood MC (in %) was quantified as the mass of water related to the dry mass of the wood. In addition, sorption rates (in % mg⁻¹ min⁻¹) were calculated as described by Himmel and Mai [29].

After the tensile tests, fracture surfaces of selected micro-veneers were observed by scanning electron microscopy (SEM). Besides untreated veneers, only micro-veneers that were heat-treated for 7 h were analyzed. Small pieces from several veneers were glued to aluminum stubs using carbon tape with the fracture surface facing upwards. They were coated with gold–palladium and observed in a SEM (Zeiss Sigma VP, Zeiss, Germany) using a beam acceleration voltage of 2 kV and a detector for secondary electrons.

Micro-tensile data were analyzed using Welch-ANOVA with Games-Howell post hoc analysis. For each tensile property and HT technique, data were separated into groups depending on the treatment duration and the moisture state during tensile testing. Normal distribution was assessed by the Shapiro–Wilk test (p > 0.05) and logarithmic (Log₁₀) transformation was applied when necessary. Furthermore, linear correlations between residual tensile properties and chemical composition were evaluated by Pearson correlation coefficients that were calculated based average values.

Both HT techniques led to a loss of hemicelluloses and a consequent accumulation of cellulose and lignin (Table 1). However, Wet-HT resulted in a faster decrease in hemicelluloses content than Dry-HT, despite the lower treatment temperature applied during Wet-HT. After a treatment duration of 7 h, the initial hemicelluloses content of ca. 25% had decreased to 19 and 13% after Dry-HT and Wet-HT, respectively. The increase in cellulose and lignin content approximately followed the decrease in hemicelluloses content. Therefore, a faster increase in lignin and in cellulose content over time was recorded for Wet-HT. The difference in cellulose content between the HT techniques was large, which was in line with the differences in hemicelluloses content. The cellulose content of wet heat-treated wood reached up to 125% of the reference value, while the cellulose content of dry heat-treated wood did not exceed 110%. Differences in lignin content were smaller and the lignin content was nearly identical for both HT techniques after a treatment duration of 7 h. The HT techniques also differed in the removal of the two main hemicelluloses in Scots pine. Wet-HT was particularly effective in removing glucomannan and less than 50% of the initial glucomannan content remained after a treatment duration of 7 h, whereas ca. 88% still remained after Dry-HT. The decrease in xylan content over time was similar for both HT techniques, and ca. 66% of the initial xylan content remained after a treatment duration of 7 h.

Chemical changes during HT were further analyzed by FT-IR spectroscopy (Fig. 2). The assignments of the FT-IR bands in the wavenumber region 1850–750 cm⁻¹ are listed in Supplementary Table S.1. Both HT techniques showed losses in absorbance at bands that were assigned to adsorbed water (1369 and 1643 cm⁻¹). Furthermore, bands assigned to C–H and C–O absorbance (i.e., at 1029, 1053, 1105 and 1158 cm⁻¹) decreased compared to the lignin-related band at 1509 cm⁻¹ that was used for normalization of the spectra, which showed the preferential degradation of carbohydrates during HT. In line with the chemical composition analysis (Table 1), wet heat-treated wood showed a decrease at 809 and 870 cm⁻¹ due to the removal of glucomannan, which was not observed to the same extent in dry heat-treated wood. However, the FT-IR spectra also revealed chemical changes that could not be derived from the chemical composition data. Wet-HT resulted in a stronger decrease at 1264 cm⁻¹ than Dry-HT, which indicated the hydrolytic cleavage of ether linkages in lignin. Such pronounced hydrolytic action during Wet-HT was also in line with losses in absorbance at 1730 and 1231 cm⁻¹, which was caused by the cleavage of acetyl groups from the hemicelluloses. Dry-HT did not cause a decrease at these two bands, but a shift toward lower wavenumbers.

The MC of the micro-veneers was measured in a DVS apparatus after conditioning at 25 °C and two different RH levels, 50 and 95% (Table 2). Conditioning at 25 °C and 50% RH was very similar to the conditioning temperature and RH of the veneers before the micro-tensile measurements in conditioned state (23 °C and 50% RH). However, the DVS apparatus cannot maintain a stable RH close to 100%. Hence, the MC measured at 95% RH was not an estimate for the amount of water within the cell walls of the veneers during the micro-tensile tests in water-saturated sate. Instead, the DVS measurements provided information on differences in the effectiveness of HT in reducing the MC at intermediate and high RH.

The MC of the wood was reduced by both HT techniques, but their effectiveness in reducing the MC differed, particularly at 95% RH. Despite higher residual hemicelluloses contents in dry heat-treated wood, both HT techniques resulted in similar MCs at 50% RH when the same treatment duration was applied. The deviation in MC at the same treatment duration did not exceed 0.3%-points. However, Dry-HT was particularly efficient in reducing the MC at 95% RH. At this RH level, Dry-HT for 1 h already decreased the MC to ca. 84% of the reference MC, while the MC of wet heat-treated wood did not decrease below 92% even for a treatment duration of 7 h. Furthermore, Dry-HT increased the sorption rate during the conditioning at 50% RH compared to the reference, whereas the sorption rate remained nearly unchanged for wet heat-treated wood. Fewer differences in the sorption rate were found during the conditioning at 95% RH. For both HT technologies, the longest treatment duration resulted in the lowest sorption rate.

There was a large difference in the tensile behavior of the reference micro-veneers between the tests in conditioned and in water-saturated state. Tensile stiffness, maximum load and work in tension were roughly twice as high when tested in conditioned state compared to the tests in water-saturated state. However, to assess the effect of HT, the tensile properties of the heat-treated veneers were related to the corresponding average value of the reference veneers that were measured in the same moisture state. Hence, the average values of the reference sample group were set to 100% for all tensile properties and both moisture states.

Figure 3 shows the pair-wise comparison of the residual tensile properties in water-saturated and conditioned state for each treatment duration. In case of Wet-HT, the results of the tests in water-saturated state were limited to the rewetted veneers, which were oven-dried after the treatment followed by re-soaking in water. This is more comparable to the tensile tests of dry-heat-treated veneers in water-saturated state, which also required the soaking of dry veneers in water. In most cases, the treatment duration had a significant effect (p < 0.05) on the tensile properties, i.e., on the residual maximum load and work in traction. For Wet-HT, however, nearly identical residual tensile properties were determined in water-saturated and in conditioned state (Fig. 3a–c). Significant differences (p < 0.05) were only found for a treatment duration of 1 h, at which the tensile tests in water-saturated state resulted in a larger residual stiffness, but a lower residual maximum load and a lower residual work in tension compared to the tests in conditioned state. Therefore, the change in micro-tensile behavior by Wet-HT was nearly unaffected by the moisture state during testing. In contrast, the residual tensile properties of dry heat-treated wood were always higher when tested in water-saturated state compared to the tests in conditioned state. For most treatment durations, these differences were statistically significant (p < 0.05; Fig. 3d–f). Hence, the change in tensile behavior of dry heat-treated veneers was not only dependent on the treatment duration, but also on the moisture state during testing. Figure 3 also shows that the stiffness decreased slightly or increased after the treatments, whereas the maximum load and the work in tension decreased to less than 45 and 20% of the corresponding reference value. However, it is not sensible to compare the effect of the two HT techniques on the basis of the treatment duration, because significantly different treatment conditions were applied. Instead, the tensile behavior of Dry-HT and Wet-HT is compared on the basis of the chemical composition changes.

Correlations between the residual tensile properties and the chemical composition are shown by the scatter matrixes in Supplementary Fig. S.1 and Fig. S.2 for the tensile tests in conditioned and water-saturated state, respectively. As a general trend, the residual maximum load and the residual work in traction decreased with decreasing hemicelluloses contents and increasing cellulose and lignin contents. However, except for correlations between the residual maximum load and residual work in traction and between cellulose and hemicelluloses contents, the correlations were not uniform but dependent on the HT technique. Accordingly, Pearson correlation coefficients, which indicate linear relationships, were higher when calculated separately for dry and wet heat-treated wood (Tables 3, 4). While linear correlations that included residual stiffness and/or lignin were often weak (0.25 ≤ r ≤ − 0.97), residual maximum load and residual work in traction were negatively correlated with the cellulose content (r ≤ − 0.94) and positively correlated with the hemicelluloses content (r ≥ 0.80). Separating the hemicelluloses in glucomannan and xylan did not improve the correlation coefficients. Overall, linear correlations between tensile properties and chemical composition were slightly better for wet heat-treated than for dry heat-treated wood.

In view of the proposed, dominant role of hemicellulose degradation on the initial strength loss of heat-treated wood [24, 25], the changes in micro-tensile properties are shown in detail as functions of the residual hemicelluloses content (Fig. 4). When tested in conditioned state, the residual stiffness, determined as the initial slope of the load-deformation curves, was not affected much by HT (Fig. 4a). Dry-HT resulted in a small increase in stiffness to a maximum of ca. 118% of the reference value, while the stiffness of wet heat-treated wood remained in the range between 87 and 110%. In contrast to the stiffness, HT affected the maximum load (Fig. 4b) and the work in traction (Fig. 4c) in conditioned state to considerable extent. Wet-HT resulted in a linear decrease of the residual maximum load and the residual work in traction as a function of the hemicelluloses content. When the hemicelluloses content was reduced to ca. 52% of the reference, a residual maximum load and a residual work in traction of ca. 39 and 17% were measured for wet heat-treated wood, respectively. Similar losses in maximum load and work in traction were also determined for dry heat-treated wood, but at a much higher residual hemicellulose content of ca. 84%. A further loss in hemicelluloses did not lead to a further loss in tensile properties. The rates at which the residual maximum load and the residual work in traction decreased as functions of the residual hemicellulose content were approximately the same for wet and dry heat-treated wood. Instead, maximum load and work in traction of dry heat-treated were approximately decreased by a constant factor compared to wet heat-treated wood within the observed range of residual hemicelluloses. The additional decrease in tensile properties by Dry-HT was especially notable after a treatment duration of 1 h. Although 97% of the initial hemicelluloses content remained after this treatment, the residual maximum load and the residual work in traction decreased to less than 70 and 40%, respectively.

The differences in the micro-tensile behavior between the two HT techniques were also observed by SEM. The fractured tracheid surfaces showed morphological differences between wet and dry heat-treated wood that were both treated for 7 h, although the residual maximum load and the residual work in traction were nearly identical (Figs. 3, 4). The fracture surfaces of wet heat-treated wood (Fig. 5c, d) resembled the fractured tracheid cells of the reference veneers (Fig. 5a, b). The fracture surfaces were not completely smooth and had a number distorted and broken cell wall pieces or fibrillar structures still attached to the cells. After Dry-HT, however, the fractured surfaces were smooth and appeared almost as cut with a blade perpendicular to the fiber direction in most regions of the micro-veneers with nearly no cell wall pieces or fibrillar structures attached to the cells (Fig. 5e, f).

The course of the residual tensile properties measured in water-saturated state in dependence on the residual hemicellulose content did not differ much from the results of the micro-tensile tests in conditioned state (Fig. 4). Furthermore, the residual tensile properties of wet heat-treated wood in water-saturated state after the process and in water-saturated state after intermediate oven-drying (rewet) were nearly identical. The main difference compared to the tests in conditioned state was the continuous increase in residual stiffness with decreasing hemicelluloses content for dry heat-treated wood. Furthermore, tensile testing in water-saturated state reduced the differences in residual maximum load and the residual work in traction as functions of the hemicelluloses content between Dry- and Wet-HT (Fig. 4e, f). This was caused by the dependence of the tensile behavior of dry heat-treated wood on the moisture state during the tensile tests, which resulted in an increase in the residual tensile properties when tested in water-saturated state compared to the tests in conditioned state (Fig. 3d–f).

SEM observations of fractured tracheid cells also revealed differences between wet and dry heat-treated wood when the micro-tensile tests were performed with water-saturated veneers (Fig. 6). For reference and wet heat-treated veneers, the fractured cell walls were uneven with a number of broken cell wall pieces still attached to them (Fig. 6a–d). Often, lamellar, open structures were seen in the secondary cell wall, which differed from the appearance after the tensile tests of conditioned veneers. In contrast, the fracture surfaces in dry heat-treated wood were nearly identical to those observed after testing the veneers in conditioned state (Fig. 6e, f). The majority of tracheids had cleanly split perpendicular to the fiber direction with barely any broken cell wall pieces attached to the fractured surfaces.