Effect of Acetylation on the Physical and Mechanical Performances of Mechanical Densified Spruce Wood

Abstract

Inherent drawbacks (e.g., loose structures, dimensional instabilities, and poor mechanical performances) restrict the applications of fast-growing wood species. In this study, a thermal compression treatment was carried out to densify acetylated spruce wood. The aim of acetylation was to improve the plasticity and water resistance of spruce wood. The water absorption, set-recovery, surface hardness, modulus of rupture, modulus of elasticity, and microstructure of the resulting wood were analyzed. The results show that acetylation can improve the plasticity of wood and reduce the interaction between wood and water, significantly reducing the set recovery of the compressed wood. When the water immersion time reaches 168 h, the water absorption rate of wood is reduced by 37% after acetylation, and the densification can further reduce the water absorption (55% for AD-40 and 70% for AD-60). The hardness of the densified wood is significantly higher than that of control wood and increases with the increase of the compression ratio. The cell wall of acetylated wood is thicker than that of control wood, which could increase the compression density of the wood. As a result, the hardness and MOR of acetylated densified wood are remarkably higher than that of unacetylated densified wood. However, a high compression ratio (60%) could lead to structural damage and, thus, reduce the mechanical properties.

1. Introduction

Wood is an indispensable and important material that has been used by societies for thousands of years. Due to its advantages (e.g., renewability, processability, outstanding visual appearance, and high strength-to-weight ratio), wood is widely used in furniture, musical instruments, handicrafts, finishes, construction engineering, and other industries [1]. To meet the growing demands of the wood processing industries, fast-growing forest plantations are (globally) becoming important in forestry development and the furniture industry [2,3]. However, loose structures, dimensional instabilities, low mechanical strengths, and susceptibility to fungal attacks greatly limit their application ranges [4,5,6]. It is common knowledge that the strength of wood is positively related to its density [7]. When the strength of fast-growing wood artificially increases, it can be used to replace scarce and expensive high-quality wood, and its application range will significantly expand.

Wood densification is a well-known strengthening method used to improve the physical and mechanical properties of fast-growing wood [8,9,10]. Wood densification methods include resin impregnation, compression, and a combination of the two [11,12,13,14,15]. Thermomechanical compression is a promising approach used in wood densification (with no chemicals added to the process) [16]. By interlocking the wood cell walls at the micro-level, the volume of the wood cell cavity is reduced; therefore, the density is proportionately multiplied [17]. As a result, the mechanical properties (e.g., modulus of rupture, modulus of elasticity, and hardness) are significantly enhanced [18,19]. However, when subjected to renewed climate stress (high relative humidity or immersion in water), densified wood tends to spring back to its original shape due to its hydrophobicity and high density [20,21], which is the main challenge of the thermomechanical compression.

Wood mechanical densification is an effective method used to improve wood performance, expand the field of wood product applications, and provide insight into the development of new wood-based products. However, controlling the set recovery of densified wood with high efficiency and minimal negative effects (e.g., brittleness, components degradation, and color deepening) is still a challenge. Efforts have been made to surmount this problem. For example, with the addition of steam during compression, the increased water content of wood can reduce the glass transition temperature, thus softening wood at lower temperatures and causing a rapid rise in the temperature inside the wood [22]. As a result, the mechanical performance and dimensional stability of wood can be significantly improved with the addition of steam densification. Gong et al. [23] reduced the set recovery of the densified wood by secondary treatment at high temperatures, while Schwarzkopf et al. [24] used a resin impregnation method to improve the dimensional stability. Increasing the plasticity of wood can promote wood compression, resulting in high stability.

In addition to the strategy using water and heat, chemical treatments, such as ammonia [19,20], acetylation [25], and resin impregnation [11,26,27], were also shown to improve the plasticity of the wood. Wood acetylation is an esterification reaction that substitutes hydrophilic hydroxyl groups in wood for hydrophobic acetyl groups, which can significantly improve the dimensional stability of wood by bulking wood cell walls, with slight changes in color and the mechanical properties of wood [28,29]. Moreover, acetylation changes the composition of the wood and significantly improves its plasticity. The introduction of bulky acetyl groups reduces the cohesive forces between the main chains of the lignocellulosic polymers and facilitates their backbone motions [30]. Jebrane et al. found that acetylation caused an energy reduction, initiating chain mobility due to the improved internal plasticity of wood [31]. Therefore, acetylation could be carried out as a pretreatment process before densification to reduce the set recovery of the densified wood [32].

Hence, the effect of acetylation on the densification efficiency and physical and mechanical properties of the resulting wood were investigated in this study. The acetylation of spruce wood was conducted prior to thermomechanical densification to increase the plasticity of the wood. The set recovery, water absorption, and mechanical performances, such as surface hardness, modulus of rupture, and modulus of elasticity of the resulting wood were evaluated. The microstructures of the wood samples were also observed via a scanning electron microscope.

2. Materials and Methods

2.1. Experimental Materials

Spruce lumber (Picea asperata Mast.) with dimensions of 20 (radial) × 100 (tangential) × 1000 (longitudinal) mm³ was obtained from Guangxi Province of China, and machined into dimensions of 20 × 50 × 120 mm. The density of the wood samples was about 0.45 g/cm³. Prior to treatment, all samples were oven-dried at 103 °C to a constant weight and divided into 6 groups. The groups were labeled as C (control), A (acetylated), D-40 (40% densified), D-60 (60% densified), AD-40 (acetylated + 40% densified), and AD-60 (acetylated + 60% densified), respectively.

Acetic anhydride (chemical grade 98%) was purchased from the China National Pharmaceutical Group Co., Ltd., and was used without further purification.

2.2. Acetylation Process of Spruce Wood

Three groups of wood samples were impregnated with acetic anhydride under vacuum (0.1 MPa) for 30 min followed by atmospheric pressure for 12 h. After immersion, all samples were taken out, wrapped in aluminum foil, and placed in an oven at 120 °C for 5 h. The foil was then removed and the samples were oven-dried at 103 °C to a constant weight. The weight percent gain (WPG) of the acetylated wood was calculated as follows:

WPG (%) = (M₂ − M₁)/M₁ × 100%

(1)

where M₁ and M₂ are the oven-dried weights of a sample before and after acetylation.

2.3. Densification Process of Spruce Wood

Two groups of untreated wood and two groups of acetylated wood samples were chosen for the thermal-mechanical compression treatment in an open system. The samples were placed in the center of the plate of the thermal compression machine (HCD-600F, Tianjin Hench Technology Co., Ltd., Tianjin, China) and compressed in the radial direction to the target thickness, which was controlled by metal stops, resulting in different target compression ratios of 40% and 60%. The temperature of the hot pressing plate was 150 °C. When reaching 150 °C, the sample stayed for 10 min (to be heated uniformly), and then was pressed for 3 h. Before releasing the load, the press was cooled by cooling water to room temperature to minimize the immediate spring back.

2.4. Performance Testing

2.4.1. Water Absorption, Thickness Swelling Rate, and Cyclic Set Recovery

The set recovery was tested according to the study by Laine et al. [19]. The densified wood samples were machined into small samples with dimensions of 20 × 20 mm and a compression thickness, immersed in deionized water for 24 h; the thickness was measured before and after oven-drying. Two soaking-drying cycles were carried out. The thickness swelling rate and set recovery were calculated as follows:

Thickness swelling rate (%) = (H₂ − H₁)/H₁ ×100%

(2)

Set recovery (%) = (H₃ − H₁)/(H₁ − H₀) × 100%

(3)

where the H₂ is the thickness after 24-h immersion in water. H₃ is the oven-drying thickness after 24-h immersion in water. H₁ is the thickness after densification. H₀ is the original thickness of the sample before densification.

The samples with dimensions of 20 × 20 mm (and compression thickness) were immersed in deionized water for one week. During immersion, their weights were measured at interval times and used to calculate the water absorption rate as follows:

Water absorption rate (%) = (m₂ − m₁)/m₁ × 100%

(4)

where m₁ and m₂ are the weights of the wood sample before and after water immersion.

2.4.2. Microstructure Observation

The cross-sections of the untreated, acetylated, and acetylated densified samples were smoothed with a wood slicing microtome, pasted on a stub with conductive adhesive, and then observed by a scanning electron microscope (FEI Quanta 200, UK) with an accelerating voltage of 15 kV.

2.4.3. Mechanical Performance Testing

Three-point bending tests and surface hardness were carried out in a universal mechanical testing machine (AGS-X10KN, Shimadzu, Kyoto, Japan). The modulus of rupture (MOR) and modulus of elasticity (MOE) of the samples in the radial direction were tested according to the Chinese standards of GB/T 1939.1-2009 and GB/T 1936.2-2009, respectively. The loading rate was 10 mm/min.

$$\mathrm{Bending} \mathrm{strength} (\mathrm{MPa})=\frac{3P_{max}L}{2b{h}^2}$$

(5)

$$\mathrm{Modulus} \mathrm{of} \mathrm{elasticity} (\mathrm{MPa})=\frac{(P_2-P_1)L^3}{4b{h}^2(d_2-d_1)}$$

(6)

where P_max is the maximum load; L is the span between two supports; b and h are the width and height (thickness) of the sample, respectively; $(P_2-P_1)$ are the increments of the load on the straight-line portion of the curve. $(d_2-d_1)$ is the increment of the sample deformation corresponding to $(P_2-P_1)$.

The surface hardness of the samples was evaluated by the force corresponding to the indentation depth of 1 mm on the surface of the sample by a spherical indenter, according to the Chinese standard of GB/T 1941-2009. The average values were obtained from three different positions.

3. Results and Discussion

3.1. Set Recovery and Water Absorption Rate Analysis

After acetylation, the average weight percent gain of spruce wood is 25.7%. The photo images of untreated, acetylated, and acetylated densified wood samples are shown in Figure 1. No obvious changes were found between untreated and acetylated wood samples from the macro level. After densification, the thickness of the wood in the radial direction significantly reduced.

To analyze the dimensional stability of the densified wood samples, the thickness swelling rate and set recovery of the densified wood samples after 24-h water immersion were tested; the results are shown in Figure 2. Remarkably, the swelling rate of the acetylated wood (1.36%) was lower than the control sample (6.32%), indicating the efficient impact of acetylation on the dimensional stability of the wood. The unacetylated densified wood had a high thickness swelling rate after water absorption, e.g., D-40 was 73% and D-60 was 89%, indicating that a notable dimensional instability and direct heat compression are not feasible for the preparation of stable densified wood. Compared to densified wood without acetylation, the acetylated densified wood exhibited a lower thickness swelling rate. Meanwhile, the set recoveries of acetylated densified wood samples (18.2% for AD-40 and 26.1% for AD-60) were obviously lower than those of unacetylated samples (95.36% for D-40 and 96.3% for D-60). These results indicate that the densified wood can recover under the effect of water and acetylation can reduce this set recovery. Similar results were also reported by Laine et al. [33], who reduced the set recovery of surface-densified wood by acetylation. The effect of acetylation on the set recovery was due to the substitution of acetyl groups for hydroxyl groups of wood, which decreases the interaction between wood and water and improves the plasticity of wood [28,31]. In addition, the substitution of acetyl groups decreases the glass transition temperature of wood [34], which could be beneficial to the immobilization of wood at low temperatures. As the inner stress remained in the densified wood, the second set recovery of all densified wood samples slightly increased. Compared to the 40% compression ratio, the 60% ratio had a higher thickness swelling rate and set recovery for unacetylated and acetylated densified wood. The possible reason for this increment is that a higher compression ratio leads to higher inner stress, which will release during the water immersion.

The water absorptions of each group of spruce wood samples, as functions of immersion times, were displayed (Figure 3). It can be seen that the acetylated wood exhibited a much lower water absorption rate than the control sample. The dimensional stability of wood was closely related to its water absorption. In general, the lower the water absorption rate of the wood, the better the dimensional stability of the wood. After densification, the water absorption rate of acetylated wood further decreased with the increase in the compression ratio. When the immersion time reached 168 h, the water absorption rate of the control sample was 108%, while that of acetylated wood was 68% (37% lower than the control); acetylated wood with a 40% compression ratio was 49%, and acetylated wood with a 60% compression ratio was 32%. This decrease in the water absorption rate indicates that acetylation can significantly reduce the water absorption of wood due to the reduction of wood hydroxyl groups and the swelling effects of cell walls [35,36]. In addition, densification reduces the interior space of wood and, thus, decreases the water uptake.

3.2. Microstructural Analysis

The microstructures of the control, acetylated, and AD-60 samples were observed via a scanning electron microscope, as shown in Figure 4. The control sample exhibited a typical softwood structure with earlywood (larger diameter and thinner cell walls, Figure 4c) and latewood (smaller diameter and thicker cell walls, Figure 4b). All of these cells were aligned adjacent to each other, longitudinally, to form a porous wood structure. After acetylation, the wood cell walls were obviously thickened (Figure 4d–f) due to the addition of acetyl groups to the cell walls [37]. Due to the high set recovery of D-40 and D-60 wood samples, their microstructures were not observed. For the AD-60 sample, the cell lumens, regardless of region (earlywood or latewood), were significantly compressed. During the compression, high temperature improves the mobility of molecular chains, resulting in the plastic deformation of wood cells. Since the structure of earlywood is looser than that of latewood, the degree of change in earlywood after compression is greater than that of latewood. Remarkably, the cellular structure of earlywood is basically crushed and stacked together in a tile-like pattern. Therefore, the excessive compression ratio could be higher than the withstanding ability of wood cells and, thus, damage the cell wall structure, which could have negative effects on the mechanical properties of wood.

The thickened cell walls caused by acetylation could result in the compression density of acetylated wood being higher than that of unacetylated wood at the same compression ratio, which will also affect the physical and mechanical performances of wood. To indicate the effect of acetylation on the density of densified wood, the densities of all samples were measured and are displayed in Figure 5. Clearly, the increased density of acetylated wood resulted in the density of densified wood being much higher than that of unacetylated densified wood. For example, the density of the D-60 sample was 0.92 g/cm³, while that of AD-60 reached 1.12 g/cm³ at the same compression ratio.

3.3. Mechanical Performance Analysis

The surface hardness of each group of spruce wood samples is shown in Figure 6. Compared to the control sample, the hardness of the acetylated wood improved from 0.46 to 0.54 kN, indicating that acetylation has a slightly positive impact on the hardness of wood. The hardness of the densified wood also improved and increased with the increase in the compression ratio. This is because the high compression ratio increases the surface density of the wood, thereby increasing the surface hardness. Compared to the unacetylated densified wood, the surface hardness of the acetylated densified wood improved higher with the same compression ratio. For example, the value of AD-40 improved from 0.73 to 1.60 kN (119% increment), and the value of AD-60 improved from 2.95 to 4.9 kN (66% increment). These significant improvements could be due to the effect of densification, which amplifies the positive impact of acetylation.

The three-point bending tests of all wood samples were carried out; the results are shown in Figure 7. It can be seen that acetylation could increase the MOR and MOE of wood to some extent. Compared to the control sample, the MOR and MOE of D-40 increased by 60.5% and 96.5%, respectively, resulting from the high density caused by densification. Nevertheless, the high set recovery of these samples restricted their applications. The MOR of AD-40 further improved by 28.0% compared to D-40. This improvement could be due to the high compression density caused by acetylation. Similar results were also reported by Pelit, who obtained improvements of 119% (fir wood) and 154% (aspen wood) through the densification of styrene-pretreated wood and attributed it to the decreased water absorption and set recovery and increased density [38]. However, the MOR value of AD-60 had less improvement than that of AD-40. This phenomenon could be because part of the structure was damaged by the high compression ratio (an obvious radial gap formed, as shown in Figure 4h), thus reducing the bending strength. The differences in MOE for all compressed samples were not remarkable, indicating that acetylation had a slight impact on the MOE of densified wood.

4. Conclusions

In this study, the effects of acetylation on the densification performance of spruce wood were evaluated. The set recovery of densified wood was significantly reduced by acetylation due to the low water absorption and high plasticity caused by acetylation. In addition, the cell wall thickness of acetylated wood increased due to the bulking effect of acetylation, which could increase the compression density of the wood. As a result, the surface hardness and MOR of AD-40 improved, respectively, by 119% and 28.0% compared to that of D-40. However, these improvements of AD-60 were less than that of AD-40. A microstructure observation indicated that a high compression ratio could lead to structural damage and, thus, reduce the mechanical properties. The MOE of wood was also increased by densification, but the acetylation had a slight impact on the MOE of densified wood. To summarize, the combination of acetylation and densification can improve the performance of spruce wood in furniture, floor, and outdoor applications.