Abstract

Wood materials

Specimens were obtained from a 44-year-old C. bungei tree. Heartwood and sapwood zones were identified through visual inspection, and the average air-dry densities were about 0.49 and 0.42 g/cm³, respectively. Clear wood specimens without any visible defects and knots were cut from the heartwood and sapwood parts along the two wood directions across the grain, i.e., the radial and tangential directions. Wood specimens were rectangular, with a cross-section of 6 by 1.4 mm² and a length of 35 mm. Specimen nomenclature indicates the location and direction, with heartwood specimens in the radial and tangential directions named as H-R and H-T specimens, and sapwood specimens across the grain named as S-R and S-T specimens, respectively. All wood specimens were stored in distilled water at 25°C for more than 3 months until a constant mass was achieved.

Experimental methods

Tests were performed in tensile mode on a dynamic mechanical analyzer (DMA Q800; TA Instruments) equipped with submersible configuration. The submersion clamp was fixed in such a way that water-saturated specimens were put into a stainless-steel cup that maintained the specimens immersed and tested directly in water. During the DMA test, the tensile stress was applied along the length of wood specimens. The parameters of the storage modulus (E′) and loss modulus (E″) were automatically recorded, and the Tg of the specimen was defined as the temperature corresponding to the maximum E″. Temperature scans were performed in the range from 25°C to 79°C with heating rates of 0.5°C/min, 1°C/min, and 2°C/min. The span between the supporting points was fixed at 15 mm. A preload force of 0.01 N and a force track of 125 percent were applied. To ensure that all tests were carried out within the linear viscoelastic region, a sinusoidal displacement was applied with an amplitude of 15 μm at a frequency of 1 Hz. Three replicates were performed for each condition.

Storage modulus in heartwood and sapwood

Figure 1 shows E′ as function of temperature in heartwood and sapwood specimens across the grain at different heating rates. It shows that the E′ appears to decline gradually with increasing temperature in heartwood and sapwood specimens, regardless of the grain orientation. The wood cell wall polymers are provided with heat energy for segmental motion, resulting in the decrement of wood stiffness (Engelund and Salmén 2012, Li et al. 2018). In addition, the reduction of wood rigidity may be explained by the partial degradation of hemicelluloses during the hydrothermal treatment (Assor et al. 2009, Furuta et al. 2010, Salmén et al. 2016).

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Table 1 provides the initial and final values of E′ in heartwood and sapwood specimens across the grain at different heating rates. For each wood specimen, the initial value of E′ decreased slightly with increasing heating rate, while the final value of E′ was very close at any heating rate. Under hydrothermal conditions, the various changes in the amorphous polymers led to substantial reorganization within wood cell walls, so that the rigidity of wood was eventually almost restored to its initial value (Assor et al. 2009). As expected, the E′ of specimens showed a clear difference in the radial and tangential directions, irrespective of the sampling location and heating rate. These findings were similar to previous reports (Backman and Lindberg 2001, Placet et al. 2007, Redman et al. 2011) in which results for the tangential direction revealed lower E′ values than the radial direction.

Table 1.— Values of the storage modulus (E′) in heartwood and sapwood specimens in the radial and tangential directions at different heating rates.^a

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In order to compare the changes of E′ in heartwood and sapwood specimens, the parameters of ΔE′ and ΔE′/Δt were calculated as where E′₂₅ and E′₇₉ indicate E′ at the beginning and end of the measured temperature range, respectively; t represents the time of experimentation; and v represents the heating rate: 0.5°C/min, 1°C/min, and 2°C/min. As can be seen in Table 1, at a heating rate of 0.5°C/min, the ΔE′ values of H-R, H-T, S-R, and S-T specimens were 57.9, 60.5, 58.9, and 64.8 percent, respectively, and the ΔE′ values of H-R, H-T, S-R, and S-T specimens were 56.1, 57.0, 57.7, and 63.5 percent at 1°C/min, respectively. Clearly, the ΔE′ of heartwood and sapwood specimens decreased with increasing heating rate, regardless of the grain orientation. In the same range of measured temperatures, the lower the heating rate, the longer the heating time. The values of E′ in wood specimens decreased with the prolonged heating time in water (Assor et al. 2009, Jiang et al. 2009, Engelund and Salmén 2012), resulting in the reduction of ΔE′ with an increasing heating rate. Interestingly, at each heating rate, the ΔE′/Δt values of heartwood and sapwood specimens were very similar, regardless of the grain orientation. Also, the value of ΔE′/Δt was directly proportional to heating rate. The greater the provided heating energy for segmental motions per unit time, the greater the value of ΔE′/Δt.

At any heating rate, the ΔE′ values in heartwood specimens were slightly lower than those in sapwood specimens (Table 1). Accordingly, the ΔE′ values of H-R specimens were slightly lower than those of S-R specimens, and the ΔE′ values of H-T specimens were slightly lower than those of S-T specimens. The differences in ΔE′ between heartwood and sapwood might be explained by the different chemical compositions. Li et al. (2019a) pointed out that increasing amounts of tyloses were found from sapwood to heartwood in C. bungei. Therefore, abundant distribution of tyloses in heartwood might provide higher mechanical support and reduce wood permeability (De Micco et al. 2016), thus resulting in the lower ΔE′ of heartwood specimens.

The ΔE′ values of wood specimens in the tangential direction were different from the radial direction, regardless of the sampling location and heating rate. Obviously, the ΔE′ in the tangential direction was slightly higher than that in the radial direction, which is in agreement with the results of Placet et al. (2007) and Redman et al. (2011). The differences in ΔE′ of wood specimens across the grain might be related to variation in the chemical composition and anatomical structure (Backman and Lindberg 2001, Reiterer et al. 2002, Placet et al. 2007, Redman et al. 2011). It was assumed that greater amounts of tyloses were found in vessels in the radial plane of C. bungei wood studied here, resulting in a lower ΔE′ in the radial direction. Furthermore, wood specimens in the tangential direction were found to be more sensitive to hydrothermal treatment compared with specimens in the radial direction (Placet et al. 2007, Redman et al. 2011, Li et al. 2019b).

Loss modulus in heartwood and sapwood

The loss modulus (E″) as function of temperature in heartwood and sapwood specimens in the radial and tangential directions at different heating rates is depicted in Figure 2. The results clearly show that water-saturated wood specimens were found to soften in the temperature range of 40.9°C to 60.2°C, corresponding to the relaxation of lignin (Salmén 1984, Olsson and Salmén 1997, Placet et al. 2007, Redman et al. 2011, Furuta et al. 2014). A relaxation of E″ was observed in the H-R and H-T specimens at any heating rate, whereas relaxation of E″ occurred in sapwood specimens only at 0.5°C/min. Yang and Li (2020) confirmed that sapwood has a lower lignin content and a higher benzyl alcohol extractive content as compared to heartwood of C. bungei, indicating that sapwood needed more energy to activate the softening process or the chain mobility of lignin. In addition, the prolonged heating time provides more heating energy for segmental motions in the wood cell wall, resulting in the observation of a relaxation in sapwood at lower heating rate.

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Table 2 lists the Tg of lignin in heartwood and sapwood specimens across the grain at different heating rates. The variability of results was calculated based on the standard deviations obtained from the average of three specimens for each test condition. The Tg values of lignin in the H-R and H-T specimens with heating rates of 0.5°C/min, 1°C/min, and 2°C/min were 57.3°C and 52.2°C; 54.8°C and 43.5°C; and 51.3°C and 40.9°C, respectively. Clearly, the Tg of lignin in heartwood decreased with increasing heating rate, irrespective of the grain orientation. With hemicelluloses partially removed, a more direct stress connection between cellulose and lignin becomes possible, and lignin was enabled to play a more direct role in the stress transfer in the matrix of the wood cell wall, resulting in a lower lignin softening temperature (Salmén et al. 2016). In addition, the higher the heating rate, the greater the instability of internal structure of wood specimens (Zhan et al. 2018a, 2018b), resulting in wood softening in the lower temperature range.

Table 2.— The glass transition temperature of lignin in heartwood and sapwood specimens in the radial and tangential directions at different heating rates.^a

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At the heating rate of 0.5°C/min, the Tg of lignin in the H-R, H-T, S-R, and S-T specimens were 57.3°C, 52.2°C, 60.2°C, and 54.1°C, respectively. Accordingly, the Tg of lignin in heartwood was slightly lower than that in sapwood, irrespective of the grain orientation. Differences in the lignin softening of heartwood and sapwood might be related to the chemical compositions of C. bungei wood. It is well known that the extractives are strongly associated with the wood cell wall components, which would restrict wood softening (Masahiro et al. 2000, Kampe and Magel 2013). As discussed previously, the greater extractive content in sapwood obstructed its access to the plasticizer of water, and therefore more energy was needed to activate the chain mobility of its lignin. Furthermore, the higher lignin content of heartwood compared with sapwood is another reason that the lignin Tg in heartwood occurred at a lower temperature range. Also, the Tg of lignin in wood specimens showed a minor difference in the radial and tangential directions, in agreement with previous studies (Backman and Lindberg 2001, Redman et al. 2011, Li et al. 2019b). More tyloses were found in vessels in the radial plane of C. bungei wood, which may be a plausible explanation a higher lignin Tg in the radial direction relative to the tangential direction.

Loss modulus versus storage modulus in heartwood and sapwood

Cole-Cole diagrams represent a convenient way to summarize the viscoelastic properties of wood materials and to visualize the relaxation mechanisms (Placet et al. 2007, Zhan et al. 2015, Li et al. 2019b). The viscoelastic data of E′ and E″ for the water-saturated heartwood and sapwood specimens in the transverse directions were plotted as Cole-Cole diagrams to show the glass transitions of lignin (Fig. 3). Clearly, a circular arc was observed in heartwood specimens, irrespective of heating rate and the grain orientation, while only one circular arc was found in sapwood specimens, at 0.5°C/min, irrespective of the grain orientation. As for C. bungei wood, sapwood has a lower lignin content and a higher extractive content as compared with heartwood. Therefore, sapwood specimens needed more energy to activate the wood softening process and resulted the circular arc losing its integrity at higher heating rates.

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