3.1. Wood Color
Heat treatment causes changes in wood color parameters and their intensity is affected by the extension of treatment time. The results are shown in
Table 1 and
Figure 1. The color of heat-treated wood darkens with extending the heat treatment duration.
Table 1 presents mean values and standard deviations (±σ
n−1) of recorded parameters.
Extension of the heat treatment duration (within the range from 1 to 8 h) leads to an increase in ∆E by approx. 10 units, i.e., by 20%; a decrease in ∆L by approx. 14 units (35%); as well as a reduction in the values of coordinates ∆a and ∆b by approx. 2 and 10 units, i.e., by 17 and 41%, respectively. Color changes are important because ∆E is greater than 3 units, the value at which the variation of wood color is considered perceptible to the human eye [
36]. The results are shown in
Table 1. It was observed that with an extension of heat treatment time the wood color becomes darker (∆E), less red (∆a) and less yellow (∆b), while the lightness decreases, as shown by the difference in lightness (∆L). The decrease in lightness (∆L) resulting in total color difference (∆E) and accelerated darkening when heat treatment exceeded 200 °C was previously reported by González-Peña and Hale [
19], and Bekhta and Niemz [
13].
With an extension of heat treatment duration, an upward trend is observed for ∆E and a downward trend for ∆L, ∆a and ∆b. The level of the total change in wood color (∆E) it determined mainly by the parameter, whose value undergoes the greatest changes, i.e., ∆L. Changes in the total color difference (∆E) and its components in the function of heat treatment duration were approximated by linear functions. The results are shown in
Table 2.
Heat treatment causes an increase in the relative values of the investigated parameters of wood color (relative value = value of the investigated property in heat-treated wood related to the value of this property determined on twin samples of control wood). With an extension of heat treatment duration, the relative values of parameters ∆E and ∆L increase, while those of ∆a and ∆b decrease. After 8 h of heat treatment, the value of color parameter ∆E increases by 100%, ∆L by 340 % and ∆a by 90%, whereas the value of ∆b decreases by 50% in relation to the values of the respective parameter in wood that is not subjected to heat treatment. The results are shown in
Figure 2.
3.2. Mechanical Properties
Heat treatment leads to a reduction in the longitudinal compressive strength of wood (CS
L). The mean value of this strength in heat-treated wood is 62.5 MPa, while in control wood it is 70.2 MPa. The results are given in
Table 3. The decrease in CS
L of heat-treated Scots pine wood was observed by Korkut et al. [
37] and González-Peña and Hale [
11].
Table 3 and the following presents mean values and standard deviations (±σ
n−1) of recorded parameters.
The analysis of changes in the relative compressive strength parallel to the grain in heat-treated wood indicates a slight effect of heat treatment duration. Heat-treated wood, compared to control wood, exhibits on average 12% lower longitudinal compressive strength, regardless of heat treatment duration. The results are given in
Figure 3.
Heat treatment also causes a decrease in the compressive strength of wood perpendicular to the grain. The mean value of CS
T in heat-treated wood is 4.5 MPa, while that of control wood is 7.5 MPa. The mean value of CS
R for heat-treated wood is 2.4 MPa, whereas in control wood it is 3.8 MPa. However, no significant differences were found after an extension of the heat treatment duration. The results are shown in
Table 3.
Compared to control wood, the average reduction in compressive strength in the tangential direction is 41%, while in the radial direction it is 35%. The results are given in
Figure 3.
The analysis of changes in Young’s modulus along the grain indicates a lack of a marked effect of heat treatment duration on the modulus value (
Table 3). The mean value for heat-treated wood is 14.3 GPa, compared to 13.9 GPa for control wood. A lack of any relationship between wood heat treatment duration and the values of the modulus of elasticity parallel to the grain is also confirmed by the analysis of relative values for this parameter (
Figure 3).
The impact strength of heat-treated wood with an extension of heat treatment time decreases from 2.8 to 1.7 J/cm
2. The mean value of impact strength for control wood is 3.9 J/cm
2. Extension of the treatment time causes a decrease in the relative values of impact strength by 24% in the case of a 1-h heat treatment duration, and this is up to 58% for a heat treatment duration of 8 h (
Figure 3). A decrease in the IS after the heat treatment corresponds with other research [
11,
19,
37].
A reduction in strength at dynamic loading indicates an increase in the brittleness of heat-treated wood, while an extension of heat treatment time considerably intensifies the decrease in impact strength. This is confirmed by FTIR results, particularly the recorded increase in the degree of the crystallinity of cellulose. Changes in wood structure that take place at a temperature above 150 °C are significant in terms of mechanical properties [
38]. A considerable reduction in the impact strength of heat-treated wood was also observed in earlier studies [
11,
12].
Tensile strength parallel to the grain in heat-treated wood decreases with an extension of the heat treatment time (
Table 3). An analysis of relative values confirms this observation and indicates a considerable reduction in tensile strength parallel to the grain in the case of heat-treated wood at 220 °C, accounting for 68 to 50% strength of control wood (
Figure 3). This may be explained by structural changes, decomposition and the increased brittleness of wood as a result of heat treatment.
The investigated mechanical properties varied little in the analyzed wood samples. The coefficient of variation (CV) of the control wood ranged from 4% (CST) to 14% (IS), similar to heat-treated wood, where it was from 3% (CST) to 25% (IS).
The testing results show that extension of the heat treatment duration over 2 h at 220 °C does not cause greater color intensity (further darkening) of Scots pine sapwood. An analysis of the effect of heat treatment duration on the mechanical properties of wood shows that reduced compressive strength parallel and perpendicular to grain is more or less identical and does not depend on heat treatment duration within the range of 1 up to 8 h. Moreover, the applied heat treatment has practically no effect on changes in the modulus of elasticity. In contrast, impact strength and tensile strength parallel to grain starting from the 4th h of the heat treatment process are considerably reduced on average by half, compared to the control wood. These results indicate that in the heat treatment of pine sapwood at the temperature of 220 °C to obtain intensive darkening at a relatively small deterioration of impact strength and tensile strength parallel to grain, the treatment duration may be reduced to approx. 2 h.
3.3. Chemical Composition of Heat-Treated Wood
The heat treatment of pine wood caused changes in the contents of the main wood components, as presented in
Table 4.
Extending the heat treatment duration did not significantly affect the content of the extractives soluble in ethanol, used as a solvent. The content of extractives decreased by about 18% in heat-treated wood compared to the control wood and was similar in all variants of heat treatment duration (
Table 4). According to the statistical analysis, wood composition after 6 h of heat treatment was characterized by a lower content of extractives, compared to heat-treated wood in other time variants. The decrease in the extractive contents resulted from the removal of volatile substances, lignin degradation products and carbohydrates in the extraction mixture.
During heat treatment, the content of cellulose in heat-treated wood increased from 45.24% to 57.28% (
Table 4). The apparent relative increase in cellulose content can be attributed to structural changes and the cross-linking reaction of the polymer.
Among all the analyzed wood components, the low molecular weight carbohydrates are the most susceptible to heat treatment. The progress of the degradation depends on the heat treatment duration.
The decrease in hemicelluloses contents in heat-treated wood for 4 h was about 85% of the content in control wood (
Table 4).
Lignin systems were characterized by greater thermal stability compared to the carbohydrates. The lignin content in heat-treated wood in the period from 1 to 8 h showed a relative increase (about 16.9%) compared to the control wood. Moreover, as shown by the statistical analysis, the lignin content in heat-treated wood is related to the duration of the heat treatment. The increase in lignin content could be connected with the condensation reaction [
39,
40], while condensation is the effect of breaking the β-O-4 bonds in lignin and the methoxy groups. The present results of lignin contents at various thermal processing times correspond to the obtained results of wood color changes (
Table 1). The changes in the structural components of wood after heat treatment correspond with those that are presented in other studies [
41,
42].
The ratio of cellulose to lignin (C/L) and of holocellulose to lignin (H/L) in heat-treated wood are indicators of wood decomposition after heat treatment.
The values of the C/L and H/L ratios in heat-treated and control wood are presented in
Table 4. The H/L ratio ranged from 3.25 (for the control sample) to 1.89 (for wood after 8 h heat treatment). A significant reduction in the value of this index is confirmed by the increase in the degradation progress of low molecular weight carbohydrate components in wood during heat treatment.
A slight decrease in the ratio of cellulose to lignin systems (C/L) ranging from 1.68 (control wood) to 1.57 (wood after 8 h treatment) during heat treatment indicates a greater thermal stability of cellulose compared to hemicelluloses. The slight reduction in the C/L ratio is due to the disturbance of the amorphous regions of cellulose during heat treatment.
3.4. Ultimate Analysis of Heat-Treated Wood
The ultimate analysis (nitrogen, carbon, hydrogen and oxygen contents) of the control and heat-treated wood is presented in
Table 5.
The nitrogen and hydrogen concentrations were similar in the control and heat-treated wood, regardless of the heat treatment duration. Heat treatment caused an increase in carbon concentration and a decrease in oxygen concentration when compared to control wood, which was confirmed by the statistical analysis. Moreover, the changes in the C and O contents were especially evident in the case of heat-treated wood for 6 and 8 h. The increase in carbon content and decrease in oxygen concentration in heat-treated wood was also observed in the case of heat-treated oak, poplar, beech, ash, pine or fir wood [
43,
44,
45]. The reduction in O content in heat-treated wood could be connected with decarboxylation (cleavage of acetic acid from hemicelluloses) [
44]. Heat treatment caused a reduction in O/C and H/C ratios compared to control wood, caused mainly by the degradation, decarboxylation and deacetylation of polysaccharide [
46]. The decrease in the O/C and H/C ratios of heat-treated wood was previously reported in the available literature based on X-ray photoelectron spectroscopy analysis and elemental analysis (contents of C, H, N and O) [
43,
47].
3.5. Infrared Spectroscopy of Heat-Treated Wood
Changes in the structure of wood caused by heat treatment carried out at different times were determined using the ATR-FTIR analysis, the results of which are presented in
Figure 4 in the form of spectra.
The heat treatment of pine wood for a period of 1–8 h caused a reduction in the intensity of the band at 3550 cm
−1 ascribed to hydroxyl groups, compared to control wood [
48]. In the spectra of heat-treated wood, the reduction in the intensity bands was observed at 1735 cm
−1 (C=O ester non-conjugated carbonyl group) and at 1655 cm
−1 (C-O in quinones coupled with C=O stretching in various groups), related to hemicelluloses degradation [
42]. The decrease in the intensity of lignin characteristic bands at 1510 cm
−1 (C=C stretching of the aromatic skeleton); 1460 cm
−1 (deformation of the C-H bond of xylan); 1270 cm
−1 (guaiacyl ring breathing); and 1030 cm
−1 (C-O ester stretching vibrations in methoxyl and β-O-4 linkages) was also visible in the spectra of heat-treated wood samples, compared to the spectra of the control wood [
42,
49,
50]. These changes observed in the spectra of heat-treated wood samples in comparison with the spectrum of control wood were connected with lignin degradation during heating. In turn, the increase in band intensity at 1600 cm
−1 in wood after heat treatment was related to the increase in aromatic rings in the wood components after their degradation [
50]. The FTIR results showed that the heat treatment of pine wood caused changes in the structure of wood compared to that in the control sample. On the other hand, extending the time of heat-treated wood had a slight effect on changes taking place in the structure of heat-treated wood.
The applied FTIR analysis is a very simple and useful method of assessing structural changes in wood. The total crystallinity index (TCI), the lateral order index (LOI) and the hydrogen bond intensity (HBI) were determined and are presented in
Table 6.
The LOI index correlated with the overall degree of cellulose order in wood and ranged from 0.62 to 0.68. In turn, the TCI index changed from 1.3 (for control wood) to 1.5 (for wood after 8 h heat treatment), which indicated an increase in the crystallinity of cellulose in wood. However, no significant differences in this parameter were found when extending the time of wood heat treatment. The change in crystallinity may be determined by the reorientation or rearrangement of quasicrystalline amorphous areas of cellulose or may be caused by crystallization in hemicelluloses [
51,
52]. The TCI index that is used to assess changes in the crystallinity of samples is a very useful tool because it correlates well with the results of X-ray analysis [
53]. The increase in crystallinity is a very important parameter, as it indicates greater thermal stability and greater strength of the analyzed materials [
54]. This is explained by changes in the strength of wood after heat treatment in our study.
The hydrogen bond intensity (HBI) ranged from 1.34 to 1.05. This change is related to the crystal system and the degree of intermolecular regularity [
35].