Wood specimens
Specimens of ABW were obtained from logs over 24 cm in diameter at breast height, harvested in 2018 at the Forest Stewardship Council (FSC)-certified forest located in the Kilwa District, Lindi Region, Tanzania. Two types of specimens were prepared for this study. Disk-shaped specimens of ABW heartwood obtained from tangential sections of the wood with 15-mm diameter (longitudinal, L × tangential, T) and 2-mm thick in the radial direction were prepared for the free compression test with 20 replicates. Rectangular specimens of ABW (heartwood and sapwood), measuring 30 mm (L; longitudinal direction) × 1 mm (R; radial direction) × 5 mm (T; tangential direction), were prepared for the dynamic mechanical analysis with 15 replicates per specimen. All specimens were cut from air-dried timber conditioned for over 3 months at room temperature, and specimens were kept in a controlled chamber (KCL-2000, Tokyo Rikakikai Co. Ltd., Tokyo, Japan) conditioned at 22 ± 2 °C and 60% relative humidity (RH) for over 30 days.
Pretreatment prior to the tests
Figure 1a, b shows the experimental procedures for the disk-shaped and rectangular specimens, respectively. For both, four types of treatment [air-drying (AD), water extraction (WT), ethanol/benzene extraction (EB), and oven-drying (OD)], were prepared with 5 replicates according to the experimental procedures (Fig. 1a, b). All specimens were oven-dried at 105 °C for over 24 h, and their oven-dried weights (W0) were measured with an electronic scale (GH-252, A&D Company Ltd., Tokyo, Japan).
The extraction processes were applied for the WT and EB specimens (Fig. 1). Water extraction was performed as follows. Oven-dried specimens were soaked in 150 mL of distilled water using a sealed Erlenmeyer flask. The soaked specimens were stirred for 10 min in the water bath at 40 °C with ultrasonic treatment (Branson 5510JDTH, Yamato Scientific Co., Ltd., Tokyo, Japan), and then kept in the controlled chamber at 45 ± 5 °C for 48 h. For the extraction, specimens from different sampling parts (heartwood, sapwood) were placed in different flasks to prevent the migration of extractives between parts. For the EB specimens, extraction was performed in the same way using an (1:2, v/v) ethanol/benzene solution instead of water.
After extraction, both the WT and EB specimens were stored at room temperature for over 1 week, and then oven-dried at 105 °C for over 24 h to measure the extracted weight (We) with the electric scale (Fig. 1a, b). The extraction rate was calculated by Eq. 1 using W0 and We:
$${\text{Extraction rate}} = \frac{{W_0}-{W_{\text{e}}}}{W_0} \times 100 (\%).$$
(1)
The AD and extracted WT and EB specimens were conditioned at 22 ± 2 °C and 60% RH for over 3 weeks. The conditioned weight (W1) was then measured with the electric scale. In addition, the dimensions of each specimen were measured after the conditioning process, as described later. The moisture content (MC) of each specimen was calculated before the tests using the following Eqs. 2a and 2b:
$$\text{MC} = \frac{{{{W}}_{1}}-{{{W}}_{0}}}{{{{W}}_{0}}} \times 100 (\%),$$
(2a)
$$\text{MC } = \frac{{{{W}}_{1}}-{{{W}}_{\text{e}}}}{{{{W}}_{\text{e}}}}\times 100 (\%).$$
(2b)
The MC of AD specimens was calculated using Eq. 2a, while that of extracted specimens (WT and EB) was calculated using Eq. 2b.
The dimensions of AD, WT, EB, and OD specimen were measured just before the tests (Fig. 1a, b). For the specimens provided to free compression test (Fig. 1a), the dimension of radial direction (R-direction, h0) was measured at the center point of specimens with a micrometer (OMV-25MX, Mitutoyo Corp., Kawasaki, Japan); the dimensions of longitudinal (L-direction) and tangential directions (T-direction) were measured at the centerline of each direction with a digital caliper (CD-15CP, Mitutoyo Corp., Kawasaki, Japan). The cross-sectional area was calculated using the image processing software ImageJ [26, 27]. For dynamic mechanical analysis (DMA) specimens (Fig. 1b), R-direction and T-direction dimensions were measured at the centerline of each with the above-noted digital caliper.
Free compression test
The free compression test was performed with a universal testing machine (Instron 5582, Instron Co., MA, USA) as illustrated in Fig. 2. Specimens were placed on the lower punch controlled at 120 °C, and held in place with the upper punch without loading for the pre-heating time of 60 s. (Fig. 2). They were then compressed at a constant speed (0.02 mm/s), while both compressive stress (P) and gap displacement caused by deformation of specimen (hs) were measured. Compression was also performed without specimens, the P and the gap displacement caused by deformation of punches (hb) were measured. The actual displacement (h) was calculated using Eq. 3:
$${{h}} = {{h}}_{\text{s}}+ {{h}}_{\text{b}}.$$
(3)
The stress–strain curve was described using nominal strain (ε) and nominal stress (σ) calculated using Eqs. 4 and 5:
$$\varepsilon = 1-(h/{h}_{0}),$$
(4)
$$\sigma = P/(\pi {d}^{2}/4),$$
(5)
where h0 is the initial specimen thickness (in the R-direction), π is the circular constant, and d is the diameter of the punch (d = 15 mm). Specimens were compressed to a maximum compressive load of 20 kN, equivalent to 113 MPa in compressive stress. In this study, water vapor pressure, caused by heating air-dried specimens, was neglected due to the small specimen size.
After the test, all specimens except for the OD were placed in a controlled chamber for 1 week at 22 ± 2 °C and 60% RH for conditioning, and the parameters of specimen weight, dimensions (L-direction, R-direction, and T-direction) and cross-sectional area, were measured (Fig. 1a). The parameters of OD were measured immediately after the test. Dimensional changes (Dc) caused by the test (L-direction, T-direction and cross-sectional area) were calculated by Eq. 6:
$${{D}}_{\text{c}} = \frac{{{{D}}_{\text{a}}}-{{{D}}_{\text{b}}}}{{{{D}}_{\text{b}}}}\times 100 (\%),$$
(6)
where Db and Da are the dimensional values of specimens before and after the test, respectively.
The physical parameters, Young’s modulus and maximum strain, were determined from the stress–strain curve collected through the test results. Young’s modulus (E) was calculated from the angle of elastic deformation area in the curve. The stress at the flow-starting point (σf) was defined as the inflexion point of the stress–strain curve (Fig. 3), where the first peak of the derivative stress with respect to the strain (dσ/dε). The strain at the inflection point was defined as the flow-starting strain (εf). The maximum strain (εm) was defined as the compressive strain value at the maximum compressive stress, σm = 113 MPa in the test.
Dynamic mechanical analysis
The DMA was performed using a rheometer (ARES-G2, TA Instruments, New Castle, USA). The complex dynamic modulus (G*) of viscoelastic materials generally represents the relation between the storage modulus (G′) and loss modulus (Gʺ), which are calculated from the dynamic performance with oscillation stress and strain by Eqs. 7 and 8:
$${{G}}^{*}={{G}}^{^{\prime}}\left(\omega\right)+{{i}}{{G}}^{^{\prime\prime}}\left(\omega\right)=\left|{{G}}^{*}\right|\left(\cos\delta + {{i}}\sin\delta\right),$$
(7)
$$\tan\delta={{G}}^{^{\prime\prime}}\left(\omega\right)/{{G}}^{^{\prime}}\left(\omega\right),$$
(8)
where i is the imaginary number, ω is the angular frequency, δ is the phase angle, and tanδ is the loss factor. In this study, G′, Gʺ and tanδ were calculated from the amplitude and phase difference (δ) of the oscillation curve for torque using the analysis software (TRIOS, TA Instruments, New Castle, USA).
The temperature-ramp test was conducted under a controlled environment by N2 purge, from – 50 to 250 °C at a constant temperature ramp rate (5 °C/min). Both edges of specimens were cramped at 20 mm in the L-direction, and loaded with dynamic torsion, 0.5% oscillation shearing strain at a constant frequency of 1.0 Hz (Fig. 4).
Statistical analysis
The Tukey–Kramer test at 1% critical difference (p < 0.01) was used to analyze statistical differences between values (BellCurve for Excel, Social Survey Research Information Co. Ltd., Tokyo, Japan).