Equilibrium moisture content after treatments
EMC varied from 12.3% and 12.6% in the case of reference samples to 11.8% and 12.3% with compressed and fixated wood for beech and oak, respectively. Although the differences are small, the EMC of each sample group was altered by the thermo-hydro-mechanical treatments. Oak possessed most steady structure as its change in EMC was statistically significant only after compression treatment and fixation (Table 1). Changes of EMC in beech were significant already after the steaming process. Its cell lumens are not clogged with tyloses (as in oak) that easies washing effect induced by the steaming treatment.
An important difference between chemical content of beech and oak species lays in composition of hemicelluloses. The share of xylan extracted from hydrolysed beech wood is more than double of the same polymer extracted from oak, assuming the same mass loss. Xylose corresponds therefore to 77% of the total monosugars in beech, compare to 49% extracted from oak (Fišerová et al 2013). It was evidenced that xylan is the least stable polysaccharide when exposing various hemicelluloses (including P-glucan, arabinogalactan, arabinoxylan, galactomannan, glucomannan, xyloglucan, and xylan) to thermal decomposition in inert atmosphere (Werner et al. 2014). The variation of the hemicellulose composition in both species contributes even more to discrepancy of the steaming effects considered here as a mild thermal treatment of wood. Even if the total mass loss caused by this treatment is negligible, some chemical alteration of constitutive polymers (such as dehydratation, deacetylation, re-configuration, cross-linking) occurred. The hemicelluloses are the first structural compounds affected even at relatively low temperatures (Sandak and Sandak 2013). The degradation starts by deacetylation causing releasing of acetic acid. Acetic acid acts as a depolymerization catalyst, leading to further polysaccharide decomposition. Simultaneously hemicelluloses undergo dehydratation having effect in decrease of hydroxyl groups (Esteves and Pereira 2009). It will be evidenced below when discussing infrared reflectance spectra, that such chemical changes were more pronounced in beech than oak, confirming observations of changes noticed in EMC.
Compression resulted in the highest alterations in EMC of beech. It can be explained by variations in the number of available sorption sites (such as -OH groups) caused by the rearrangement of constitutive polymers due to mechanical action (extensive compression) in combination with the proceeding mild thermal treatment (steaming).
IR spectra interpretation in the fingerprint region (1800–600 cm−1)
Figure 3 presents a set of averaged IR spectra collected from beech and oak wood specimens studied in this research (reference, steamed, compressed and released, compressed and fixated). Identified spectral bands are marked and numbered following the literature references as listed in Table 2. A summary of spectral analysis and interpretations induced by visual assessment are also summarized in the same table. Vertical arrows indicate the intensity of alterations (increase or decrease) while changes in the peak position (shifts) toward higher or lower wavenumber are represented by horizontal arrows. In all cases, treated wood spectra were compared to averaged untreated wood (reference) of the corresponding species.
Apparent differences between IR spectra of reference and modified woods are more pronounced in beech than oak, which indicates higher susceptibility of this species for chemical-physical changes induced by the pleating process. It was confirmed by ANOVA testing used for evidencing of the statistical differences between infrared absorbance induced by different treatments. Steaming seems to be a relatively mild process only slightly affecting chemical changes of the wood constitutive polymers. In the case of beech, changes in the functional groups induced by steaming are noticeable for several bands; however, the intensity of alterations is evidently lower than influence of compression and fixation treatments. The most noticeable peak drop as well as shift toward lower wavenumber, resulting from steaming processes, was noticed for band 4 (996) and 5 (1044) assigned to C–O valence vibrations, C–O stretching, C = O stretching as well as to aromatic C–H in-plane deformations. Lowered reflectance values signify the functional groups degradation (polysaccharides) and reduced cross-linking of lignin occurring in these particular bands.
Another drop in spectra reflectance can be observed in band 6 (1086), assigned to C–O deformation in secondary alcohols and aliphatic ethers. Bands 18 (2902) and 19 (2940), linked with symmetric CH2 valence vibrations and asymmetric stretching vibrations of CH related to methyl and methylene in lignin, cellulose and hemicellulose, are also lowered but only after compression and compression followed by fixation. On the contrary, several IR bands exhibit increase of reflectance resulting from the modification processes. This is especially noticeable for bands 1 (670), 2 (834) and 8 to 14 (1147–1505). It can be associated to the condensation of lignin occurring after glass transition induced by elevated temperature in combination with mechanical stresses. Corresponding peaks in the IR spectra of oak wood do not exhibit pronounced visual differences. Moreover, some bands, in the case of oak, seem to not be affected at all by the thermo-hydro-mechanical (pleating) process (e.g., 15 (1598) and 16 (1652) being assigned to C = O stretching and aromatic skeletal vibrations).
It has also been observed that noticeable deviations of IR spectra occurred in the range covering the hydroxy functional groups of wood polymers (peak 20 to 25 (3149–602)), which suggested changes of the hygroscopic properties of treated specimens. Dedicated analyses were therefore performed for these particular spectral bands that are reported in a subsequent chapter.
Principal component analysis
Principal component analysis (PCA) is a powerful chemometric method to reduce highly correlated and multi-dimensional data set to lower dimension. It searches for unique properties of spectral data and separates these into groups of peculiar similarities. Two PCA analysis were performed for the needs of this research. In the first, the whole range of spectra collected from measurements of singular specimens was analyzed, focusing on within batch variation (PCA#1). The second analysis was performed on averaged spectra for each batch, providing a much more generalized spectroscopic description of the studied materials (PCA#2). The data were mean centered before analysis by subtracting the average absorbance spectrum. This pre-processing is a routine procedure performed before PCA computation to minimize the effect of a few dominating peaks on the overall chemometric model.
Results of PCA#1 performed on the spectra of beech and oak wood are presented in Fig. 4. It is evident that treatment batches are better separated in the case of beech where a sequence from reference to compressed and fixated data points follow the PC1 axis direction. This component is dominant in the model and explains nearly 94% of the variance within data. The cluster of reference is clearly separated, confirming hypothesis of the profound effect of steaming and compression on the physical–chemical configuration of beech wood. The cluster of steamed wood slightly overlaps compressed samples and lays around the centre of the PC1. Both compressed sample types are partially overlapped, but some discrimination can be noticed when considering PC2 scores
The spectra corresponding to different batches of oak wood overlapped, creating a single cluster mixing reference, steamed and compressed specimens (Fig. 4). Only compressed and fixated spectra were, to some extent, detached from the rest, creating a separate cluster that slightly overlapped with compressed and released. The first principal component of PCA#1 for oak explains almost 60% of variance while the second more than 20%.
Additional analysis (PCA#2) was performed on the averaged spectra of beech specimens. In that case, each batch was homogenized by computing the average value of absorbance for all wavenumbers, resulting in two data points per treatment type and six points for corresponding reference batches. The resulting score plot is presented in Fig. 5. Improved separability is evident compared to the cloud of data points shown in Fig. 4. Again, first principal component (PC1) separates the reference spectra from those modified, following treatment intensity. Steamed specimens are grouped in a single cluster, clearly separated from other treated woods by means of the second principal component (PC2). Averaged spectra collected from both compression treatments (released and fixated) overlap but, at the same time, are distant from the other clusters.
A great advantage of PCA is the possibility for identification of spectral features supporting discrimination. Two loadings corresponding to PC1 and PC2 are shown in Fig. 5, together with the reference position of identifiable spectral peaks (Table 2). First principal component discriminated reference (negative values) from all the treated samples (positive values). The most noteworthy peaks recorded in the corresponding loading are 8, 9, 13, 14, 16, 17, 21–25 (1174, 1270, 1458, 1505, 1652, 1740, 3280–3602). It reveals that most affected polymers due to treatment are lignin (C–O, C = O, CH2), cellulose (CH2) and all hydroxyl groups affecting hygroscopic properties of wood. Low contribution of PC1 to the steamed wood spectra is related to moderately low absolute values of scores. It suggests the intermediate extent of changes, still indicating more similarity to compressed samples than to untreated wood (positive sign of scores). The second principal component allowed discrimination between steamed and compressed samples, where contribution of peaks 7, 8, 14, 17, 18, 19 (1132, 1174, 1505, 1740, 2902, 2940) are most dominant. It corresponds to the spectral changes particularly in lignin (CH, C–O, C = O, CH2), condensation of guaiacyl as well as in hemicellulose (CH, C = O, CH2). Information recorded in both principal components corresponds to that interpreted by visual assessment with the exception of peak 17 (1740), possessing a high value in PC2 loading. However, the great importance of hydroxy groups (peaks 21 to 25 (3280–3602)) is evidenced, suggesting alteration of water sorption mechanisms in specimens after thermo-hydro-mechanical treatments.
As stated above, the extent of changes recorded in the FTIR of oak was much less profound than of beech. It was confirmed by PCA#2 for oak (not reported here) where discrimination between clusters corresponding to different treatments was limited.
Changes in hydroxy groups (3700–3000 cm−1)
Substantial changes to the FTIR spectra due to thermo-hydro-mechanical treatments are assigned to the modification of wood hygroscopic properties. Even if EMC variation was not very high in specimens after conditioning (Table 1), many differences were statistically significant (ANOVA p < 0.05). Significant difference was detectable between all of the differently treated beech specimens and between oak reference and fixated specimens. Similarly, the outline of the IR spectra in the range corresponding to the absorbed water and –OH groups in general (3700 to 3000 cm−1) was evidently altered following intensity of treatment, especially in the case of beech (Fig. 3). A great limitation of the direct spectra interpretation in that range is presence of several peaks that are broad and highly overlapped. For that reason, a special software has been developed for deconvolution of spectral peaks, assuming that variations of the deconvoluted peak width and its maximum shift are restricted and controlled.
For the needs of this research, six -OH peaks were identified following literature references summarized in Table 2. Each of these are linked to different wood polymers and involved in uptake of moisture at different sorption stages. The result of compressed and fixated spectra deconvolution is shown in Fig. 6 for both beech and oak specimens. Areas and shifts for peaks of interest are plotted as a bar chart to highlight mechanisms of hygroscopic changes to the wood due to treatment. Even if the IR spectra outlines appear to be similar for both studied species, some slight differences are evidenced in the deconvolved peaks. The highest absorbances were noticed for peaks 21 (3280), 22 (3375) and 23 (3496) although the areas below these are rather constant and do not change due to treatments. On the contrary, peak at 24 (3565) (assigned to weakly absorbed water) decreases along treatment intensity. Such change is more monotonic in the case of oak than beech. The peak at 25 at 3602 cm−1 (weakly H-bonded water) follows the opposite trend and its area increases with treatment intensity. Variations of the peak area are associated to peak shifts. It is especially noticeable in the case of beech wood where three peaks (23 (3496), 24 (3565) and 25 (3602)) are monotonously shifted as related to the reference. This was not that evident in the case of more resistant (to the pleating process) oak wood.
The summary of above observations leads to the conclusion that absorbance of the hydroxyl functional groups associated to the weakly bonded wood increases noticeably following treatment intensity. On the contrary, the strongly bonded water is relatively stable and not affected by the treatment. It confirms the trend noticed with visual assessment of spectra (Table 2) as well as alterations of EMC.
It has to be mentioned that the thermo-hydro-mechanical treatment induces morphological changes to the wood that may affect the vapour transport within bulk (diffusion) and in the border of the wood and surrounding air (absorption). Even if it can not be directly evidenced by infrared spectroscopy, the water sorption sites availability changes also due to mechanical re-configuration of constitutive polymers (translocation and partial densification) in combination with micro/nano cracking of cell walls.