Comparative Studies on Tensile Mechanical Properties of Water-Saturated Earlywood and Latewood within the Same Growth Ring from Masson Pine

The values of the tensile modulus for the L specimens and T specimens at different temperature levels ranging from 30 °C to 80 °C are depicted in Figure 2a,b, respectively. The tensile modulus of LW was significantly higher than that of EW at each temperature level, irrespective of the grain orientation, which has been observed in some previous studies [8,9,11]. Structurally, the absolute dry densities of LW (0.836 g/cm³) within the same growth ring were 2.2 times larger than those of EW (0.388 g/cm³). Similar observations were made for wood specimens at the macroscopic scale, in which results showed that the tensile modulus values in the L and T directions were positively linear with the wood densities [25,26]. In addition, the MFA of LW was smaller than that of EW at each temperature level, which was also an important contributing factor for the higher tensile modulus values in LW. Furthermore, the tensile modulus of the L specimens was significantly higher than that of the T specimens. In tensile tests, the crystalline cellulose microfibrils are orientated in the L direction, and the matrix of lignin and hemicelluloses is the load-bearing polymer in the T specimens [27,28]. Therefore, it should be noted that the orientation of cellulose microfibrils in the L direction explains the higher tensile modulus of the L specimens.

At the measured temperature of 30 °C, the tensile modulus was determined as 1680 MPa, 5553 MPa, 25 MPa, and 132 MPa in LT-EW, LT-LW, TL-EW, and TL-LW, respectively. It was found that the tensile modulus of LT-LW was 3.3 times greater than that of LT-EW, and the tensile modulus of TL-LW was 5.3 times greater than that of TL-EW. A similar observation was made for EW and LW at all temperature levels. As also seen in Figure 2, the tensile modulus of the wood specimens showed a decreasing trend as the hygrothermal temperature increased. When the temperature increased from 30 °C to 80 °C, the decrement in tensile modulus for LT-EW, LT-LW, TL-EW, and TL-LW was 32.4%, 29.1%, 84.0%, and 76.8%, respectively. Under water-saturated conditions, heat energy is provided for the Brownian motion of polymer molecules and chain segments, resulting in the decrement in tensile modulus with the rising temperature [11,12]. Accordingly, the decrements in tensile modulus for EW were larger than those for LW in both the L and T directions. This result may be related to the quantity of lignin in the wood cell walls, which was the determining factor for the thermal softening properties of water-swollen wood [29]. Compared to LW with a lignin content of 28.31%, EW had a higher lignin content of 33.88% [30]. Consequently, the tensile modulus for EW was more sensitive to temperature changes compared to LW. Furthermore, the decrement in the tensile modulus of the L specimens was significantly lower than that of the T specimens. The cellulose microfibrils dominated the tensile modulus of the L specimens, while the matrix of lignin and hemicelluloses played the more dominating role for the tensile modulus of the T specimens [31]. The cellulose microfibrils are more stable and resistant to heat than the matrix, which is more pliable and sensitive to temperature in the hydrothermal environment [32,33], accounting for the larger values of the decrement in the tensile modulus of the T specimens.

To obtain more detailed information about the relationship between the tensile modulus and the hygrothermal temperature, the parameter of relative tensile modulus was calculated as follows: the tensile modulus at any temperature level (30~80 °C) was divided by the tensile modulus at 30 °C. The relative tensile modulus values of the L specimens and T specimens are visible in Figure 2c,d, respectively. It was seen that the relative tensile modulus of the specimens decreased as the hygrothermal temperature increased, and a clear inflection point was exhibited at 50 °C or 60 °C. In the hydrothermal environment, the softening properties of wood reflect to a large extent the properties of lignin. In general, water-saturated wood specimens were found to soften around 60 °C, corresponding to the relaxation of lignin [34,35,36,37]. Therefore, it should be concluded that the softening of wood played a significantly important role in the decrease in tensile modulus in the hydrothermal environment.