Fiber morphology and chemical composition analysis
Light-delignification can cause changes in fiber morphology and chemical composition, which in turn affected the performance of HMFM. The average and general value of the dimensions are shown in Table 1.
Table 1: Average and general value of the fiber dimensions.
The general value of STP fiber lengths was between 495.5 to 808.3 μm, which was smaller compared to that of UTP fibers. However, the general value of STP fiber diameters ranging from 14.3 to 19.3 μm, which was greater than that of UTP fibers. It resulted in a decrease in aspect ratio of STP fibers. The average value of fiber dimensions also showed the same changes as the general value. The fiber length decreased, the diameter increased, and the aspect ratio decreased after the light-delignification. The decrease of aspect ratio may weaken the mechanical cross-linking between fibers. However, the increase of diameter provided more contact area between fibers, which was beneficial to the improvement of inter-fiber bonding strength. The general value of STP fiber wall thickness ranged from 3.5 to 3.9 μm, which was smaller than that of UTP fibers. It may be related to the removal of lignin on the fiber surface, resulting in the decrease of fiber wall thickness and the increase of fiber softness, which was beneficial to the increase in inter-fiber compactness. Table 2 shows the yields and chemical composition.
The yield of STP decreased by 20.3% compared to that of UTP. In addition to the decomposition and dissolution of lignin and other compositions, it might be affected by the changes of some small fibers, which resulted in an increase in mass loss during washing. After the light-delignification, the ratio of holocellulose to lignin content varied from 2.8 to 7.0, indicating a significant change in fiber composition. The content of lignin was significantly decreased by 54.0%, and the contents of holocellulose and α-cellulose were increased correspondingly. However, the content of pentosan did not increase significantly. The Light-delignification reduced the amount of pentosan in holocellulose from 22.0% to 19.9%, showing a certain degree of degradation of hemicellulose.
Physical and mechanical properties of HMFM
Mechanical strength is one of the most important indexes to characterize the performance of HMFM. The density and mechanical properties are shown in Table 3 and Fig. 1.
After the light-delignification, the density increased, the tensile strength and bending strength improved significantly, and the corresponding strain also increased slightly. The density of STS increased by 6.0%, the tensile strength of STS increased by 22.0% and the bending strength of STS increased by 23.9% compared to those of UTS. The light-delignification increased the softening degree of fibers. The fibers were pressed more densely, resulting in an increase in density. The formation of adhesive material between fibers, as well as the compaction of the fiber cell lumens, promoting the tensile strength and bending strength of HMFM, which was consistent with the XPS and SEM results.
Surface chemical composition analysis of HMFM
The Light-delignification caused the changes of the outer surface chemical composition of HMFM. XPS provides quantitative information of different bonded carbon atoms on the HMFM surface besides the chemical composition19, which are shown in Fig. 2 and Table 4. Carbon (~285 eV) and oxygen (~532 eV) were the main elements detected in the fibers in XPS survey scan, and a small amount of nitrogen (~399 eV) was also found. The outer surface nitrogen atom concentration decreased slightly after light-delignification. The outer surface lignin content of HMFM was calculated using Eq. (1), and the results are presented in Table 4. However, the outer surface lignin concentration of STS was increased. The reason may be that the light-delignification made the lignin on the outer fiber surface dissolved out in the form of debris, which was enriched on the material surface during the hot-pressing process, resulting in the increased outer surface coverage of lignin.
The O/C ratios can be used to characterize the outer surface carbohydrate, lignin and extractives contents. Due to the removal of acetone-extracted extractives, the increase in the O/C ratio can represent a higher carbohydrate concentration on the material surface20. The decreased O/C ratio of the STS indicated that the oxygen-rich composition on the material surface was relatively reduced, which was consistent with the results of outer surface lignin content. The theoretical O/C ratios of cellulose and lignin are 0.83 and 0.33, respectively21. The O/C ratios of STS and UTS were between0.33 and 0.83, and close to 0.33, which was consistent with the chemical composition of HYP fibers.
According to the classification of carbon atoms in wooden materials, the C1s peak was deconvoluted into four subpeaks: C1 corresponds to C–C or C–H, and C1 is considered to only lignin (extractives are removed); C2 and C3 refer to the C–O and C=O or O–C–O respectively, existing in carbohydrate; C4 refers to O–C=O, which represents carboxylic acids, resins and other substances22.
Figure 3 presents the deconvoluted C1s signals of STS and UTS. After the light-delignification, the relative amount of C1 and C4 increased obviously as C2 and C3 decreased. After the light-delignification, the degree of polymerization of lignin on the fiber surface was decreased, and more phenolic hydroxyl radicals were exposed. During the hot-pressing process, phenolic hydroxyl radicals and the degradation products of carbohydrates were polymerized, producing resin polymers useful for inter-fiber bonding, thereby the C4 relative content was increased. Additionally, the hydrophobic properties of the material surface can be expressed as the C1/C2 ratio23. A C1/C2 value of 0.91 was obtained for UTS, while it increased to 1.48 for STS (Table 4), indicating that the hydrophobic properties of HMFM were improved.
Thermal properties analysis of HMFM
The light-delignification caused changes in the fiber chemical composition, which may lead to changes in the thermal properties of the material. Figure 4 shows the TG, DTG and DSC curves of UTS and STS. Important data derived from TG curves are explained briefly in Table 5.
TG (a), DTG (b) and DSC (c) curves of UTS and STS.
Table 5: TG results of UTS and STS. aT i values for initial decomposition temperature.
As shown in Fig. 4(a) and (b), UTS and STS had similar weight loss behavior. From 26 to 115 °C, the weight loss was slight corresponding to the first endothermic process, which was due to the evaporation of water. But the weight loss of STS was greater, indicating that the moisture absorption was larger. From 115 to 240 °C, the samples were almost without weight loss. From 240 to 395 °C, there was a great weight loss with apparently different weight loss rates. As seen in Fig. 4(c), the characteristic peaks of the DSC curves were basically the same, indicating that similar chemical reactions occurred during the thermal decomposition process. But the intensities of the characteristic peaks were different, which was due to the difference of the content of the chemical composition.
The decomposition temperature ranges of hemicellulose, cellulose, and lignin in wood fiber materials were 180–240 °C, 230–310 °C, and 300–400 °C, respectively24. As seen in Table 5, The T i values of STS and UTS were 238.2 °C and 242.8 °C, respectively, indicating that the hemicellulose began to undergo thermal decomposition. The T m values of STS and UTS were 345.6 °C and 370.0 °C, respectively, which was due to the depolymerization of most of the cellulose and a portion of the lignin25. The T f values of STS and UTS were 389.5 °C and 394.5 °C, respectively, indicating that the residual lignin decomposed gradually.
Compared to UTS, the T i value and T f value of STS decreased slightly, while the T m value of STS decreased significantly. These results indicate that the STS was less thermally stable than UTS. The lignin polymerization degree of the fiber surface was reduced by the light-delignification, which weakened the barrier effect of lignin on the thermal degradation of fiber chemical composition26, so that the thermal stability was reduced. Compared to UTS, the RW of STS decreased at 450 °C, which could be attributed to the decrease of lignin polymerization degree accelerating more decomposition of lignin.
As shown in Fig. 4(c), the peak temperature values of major endothermic peaks appeared in the DSC thermogram of UTS and STS were 369.1 °C and 348.3 °C, respectively, and the corresponding enthalpy values were 110.8 J/g and 68.0 J/g, respectively. The results showed that the temperature of thermal decomposition decreased, the enthalpy required for thermal decomposition decreased and the thermal stability of material decreased after the light-delignification, which was consistent with the TG results. A weak endothermic peak in the DSC thermogram of UTS was detected at 398 °C, which was assigned to the thermal decomposition of lignin27. However, it did not appear in the DSC thermogram of STS, which may be related to the decrease in lignin content28.
Micro morphology analysis of HMFM
The light-delignification caused changes in the microscopic morphology of fibers, resulting in changes in material properties. The SEM micrographs of the material surface, cross section, and inner surface are showed in Figs 5 and 6.
SEM micrographs of the surface of UTS (a,c) and STS (b,d).
SEM micrographs of the cross section (a UTS, b STS) and inner surface (c UTS, d STS).
As shown in Fig. 5, UTS fibers appeared stiffer, and had a lower fibrillation extent. The fibers of UTS were intertwined mechanically with smaller contact area, looser binding degree, and more surface holes. On the contrary, STS fibers showed a lower hardness. The fibers of STS bonded together by some adhesive substances with increased contact area and improved binding compactness.
As shown in Fig. 6a and b, compared to UTS, the fibers of STS appeared more compaction, more regular and neat arrangement, and increased tightness of the combination. As shown in Fig. 6c and d, compared to UTS, the fibers of STS showed more melted fragments spreading between fibers, which caused more agglomerations of fibers and contributed to the improvement of the inter-fiber bonding strength.
Hydrophobic property analysis of HMFM
Hydrophobic property is an important index influencing the application of HMFM. The water contact angle is an effective method to evaluate the surface hydrophobicity of material. Figure 7 shows the curve of surface water contact angle of HMFM over time.
Water contact angles of the UTS and STS at different contact times.
As seen in Fig. 7, the water contact angle of UTS and STS decreased gradually over time. The water contact angle of UTS varied from 72.7° to 64.3° in 60 s, and the water contact angle of STS varied from 84.3° to 80.8° in 60 s. The significantly increased water contact angle values indicated the increase in hydrophobic property of HMFM. The stability of water contact angle over time is a very important parameter for a hydrophobic surface29. The water contact angle of UTS decreased 8.4° in 60 s, while the water contact angle of STS only decreased 3.5° in 60 s, which indicated that the hydrophobic stability was improved. The light-delignification significantly improved the hydrophobic property of the HMFM surface, which was consistent with the XPS results.