3.2. Color Changes of Rubberwood
displays the visual observation of the color changes of the rubberwood after being treated in different buffered media and temperatures.
The most visible effect of treatment was a darkening of the color of the wood. Figure 3
depicts photographs of wood samples treated with hydrothermal treatment at various temperatures in different buffered media (acidic, water and alkaline). As can be seen in Figure 3
, increased treatment temperatures increased the intensity of discoloration. Different media exhibited color differences at low temperatures, but there were no discernible changes in color at high temperatures. Lower temperatures (160 and 180 °C) caused less color changes in pH 6, 8 and 10 compared to acidic and water media, while high temperatures (200 °C) caused the most color changes. The hydrothermal treatment changed the color of the sample significantly as the treatment temperature increased [31
]. This finding was consistent with the research findings in [34
]. Color changes (wood darkening) are frequently attributed to the formation of oxidation and degradation elements from wood components [35
]. The decrease in lightness caused by thermal treatment primarily indicates the formation of several components that absorb visible light [37
]. According to Chen et al. [36
], the condensation reactions of lignin and some extractives, as well as the formation of biproducts, contribute to an increase in the intensity of red tone in wood samples. Table 1
shows the chromatic value changes of treated and control rubberwood samples.
The chromatic values L*, a* and b* were measured for evaluating the overall color changes (ΔE*). Table 1
shows that the lightness (L*) values of all treated wood samples decreased when compared to untreated wood, indicating that the wood darkened after treatment. The L* value of the untreated wood was 76.91. The L* value on the wood treated by hydrothermal treatment with different media decreased as the temperature increased. In general, treatment temperature had a greater impact on the darkening of wood than the buffered media. Rubberwood treated with pH 6 at 160 °C had the smallest reduction in L* value (52.43). Meanwhile, the most severe reduction in lightness was observed in rubberwood treated in tap water at 200 °C. The decrease in lightness with increasing temperature during the hydrothermal treatment in different buffered media in this work could be attributed to extractive migration on the wood surface [38
Green–red coordinate (a*) values of the treated rubberwood increased after treatment at 160 °C and 180 °C in all buffered media (acidic, water and alkaline), indicating that the samples turned redder. However, when the samples were treated at 200 °C, the a* value decreased, indicating that the samples were beginning to turn green. The findings agreed with those of Cai et al. [39
] and Lee et al. [40
]. The samples treated at 160 °C in pH 6 had the highest a* value, while the samples treated at 200 °C in pH 4.0 had the lowest a* value. The treatment, on the other hand, reduced the yellow–blue coordinate (b*) values. A decrease in b* indicated that the samples became bluer after treatment. Similarly, the highest b* value was recorded in pH 6 media at 160 °C, while the lowest b* value was recorded in tap water media at 200 °C.
Previous studies found that treating the wood surface in different buffered media resulted in color changes due to the removal of extractives, hydrolysis of hemicellulose and oxidation of these components [38
]. To validate the color changes of the wood, the total color differences ΔE* were calculated, which involved changing all color coordinates (ΔL*, Δa*, Δb*). The E* value gradually increased with increasing treatment temperature, as shown in Table 1
. Different media displayed different values of ΔE*. The greater color changes caused by hydrothermal treatments in the water media compared to other media (acidic, alkaline) were due to a greater contribution from the chromaticity coordinates, namely ΔL*, Δa* and Δb*. Overall, E* increased as the treatment temperature increased and the effect of the buffered media was less noticeable.
3.3. Physical Properties of Rubberwood
Equilibrium moisture content (EMC %), wood density ρ (kg/m3
), mass loss (ML%), water absorption (WA%) and volumetric swelling coefficient (VSC%) were evaluated in this study. The equilibrium moisture content of treated and untreated rubberwood samples are presented in Table 2
shows that the highest and lowest EMC values of the treated rubberwood samples were 9.05% in pH 8 at 160 °C and 7.03% in pH 4.0 at 200 °C, respectively, while the untreated sample (control) had a value of 10%. There was a significant difference between the samples treated in buffered media with different pH values. As the reduction in EMC of the wood was directly proportional to the intensification of temperature [43
], treatment temperatures were found to be a more influential factor in reducing the EMC of the treated samples. On the other hand, alkaline media (pH 8, 10) reduced EMC the least. The main reason for the decrease in EMC was the loss of hydroxyl groups after heat treatment. Due to the lack of these hydroxyl groups, the cell walls were able to absorb less water [44
]. Furthermore, the decrease in EMC could be caused by increased cellulose crystallinity, which reduces the availability of hydroxyl (OH-) groups to water molecules [30
]. The hydrothermal treatment might have an effect on the chemical constituents of the rubberwood samples. When compared to cellulose and lignin, the degradation of OH- groups in hemicellulose occurred even at low temperatures [44
]. As a result of the degradation of OH- groups from hemicelluloses, the treated samples had lower EMCs than the untreated samples. As hemicelluloses contain a greater number of OH- groups than cellulose and lignin components, they are more hygroscopic. Table 3
shows the effects of the hydrothermal treatment in various media (acidic, water, alkaline) on the content of holocellulose, cellulose, hemicellulose, lignin and extractives in the rubberwood. A reduction in hemicellulose content after treatment was observed, ranging from 18.78% to 39.50% compared to that of 39.97% in the control samples.
shows the density of the rubberwood before and after treatment in different buffered media. Before treatment, the average density of the rubberwood was 644.94 kg/m3
. After treatment, the density of the rubberwood decreased to a different extent. Generally, the reduction in density increased along with increasing temperature. It was observed that the rubberwood treated with the acidic medium (pH 4) had the highest density reduction, particularly those treated at 200 °C, which experienced a 16.3% reduction in density compared to that of untreated samples. Samples treated with an alkaline medium (pH 8 and pH 10) experienced a lower reduction in density. The lowest density reduction of 0.5% was recorded in the samples treated in pH 8 at 160 °C. These findings were in line with previous work where the authors reported density losses that ranged from 1.89%–5.67% in different media (water, acidic, neutral, alkaline) at 160 °C and 2.96%–7.84% at 180 °C, respectively [20
Mass loss (ML%) of hydrothermally treated rubberwood samples is presented in Figure 4
. Mass loss increased along with the increasing treatment temperature.
Temperatures of 200 °C in pH 4 (25.39%) and 160 °C in pH 8 (2.58%) resulted in the highest and lowest percentage of mass loss, respectively. According to Tjeerdsma et al. [26
], acidic hydrolysis at various hydrothermal treatments can significantly increase the degradation rate of wood polymers. According to Theander and Nelson [19
], the degradation rate of carbohydrates is high in acidic conditions, which is facilitated by the high availability and low crystallinity of hemicelluloses. In general, the exclusion of hydrogen (H+) and hydroxide (-OH) ions in hydrothermal treatment has no effect on the pH of the hydrothermally treated medium. Furthermore, in hydrothermal treatments using buffered media, the pH can be kept constant at a certain level and the destructive effects of the released acid can be controlled [29
Untreated samples had a water absorption (WA) of 20.5%. The samples treated in pH 8 at 180 °C had the highest WA value of 25.02%, while the samples treated in pH 4 at 200 °C had the lowest WA value of 22.42% (Table 5
). It is worth noting that WA increased as the treatment temperature increased from 160 °C to 180 °C, but began to decrease as the temperature was raised to 200 °C. This phenomenon might be explained by heat-induced damage within the wood structure. This study’s findings were consistent with previous studies [5
]. Figure 5
shows the cracks that occurred on the samples after being treated at a higher temperature of 200 °C.
shows the effects of pH and treatment temperature on the VSC of untreated and treated rubberwood. The VSC of the hydrothermally treated rubberwood was lower than that of the untreated samples (8.09%). When the samples were treated in pH 8 at 200 °C, the lowest VSC value of 6.97% was observed. Similarly, as the treatment temperature increased, the VSC values decreased. Lower VSC values indicated that the samples are more dimensionally stable. In general, alkaline-treated samples had lower VSC values and, thus, better dimensional stability.
TS and ASE of the treated and untreated rubberwood samples are presented in Table 7
TS and ASE values varied according to pH and temperature. The hydrothermally treated rubberwood samples had lower TS values than the control samples. The highest TS value for the treated samples was 3.47% at 160 °C in pH 4, while the lowest TS (2.95%) was recorded in the samples treated at 200 °C in pH 8. According to Table 7
, the ASE values of the samples ranged from 7.23 to 13.91% (pH 4.0, 160–200 °C), 12.85–18.17% (pH 6.0, 160–200 °C), 8.33 to 16.82% (tap water, 160–200 °C), 16.05 to 21.18% (pH 8.0, 160–200 °C) and 15.13 to 20.06% (pH 10.0, 160–200 °C). Furthermore, both the treatment temperature and the buffered media had a significant effect on ASE values. The samples treated in alkaline media had a lower TS than the samples treated in acidic and water media. These observations were in agreement with the finding that was revealed by Ebadi et al. [18