Physical and Mechanical Properties and Microstructure Characterization of Thermally Modified Flattened Bamboo (Phyllostachys edulis) Material

Abstract

This study investigated the effects of thermal modification treatment on flattened bamboo lumber by using temperature (180 °C, 190 °C, 200 °C) and duration (2, 3, 4 h) as experimental variables. The physicochemical properties, crystallinity, bending deformation, chemical composition, and microstructural evolution of the material before and after treatment were systematically analyzed using universal mechanical testing, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and nanoindentation. This comprehensive approach aimed to achieve high-performance flattened bamboo lumber. The results revealed that thermal modification significantly reduced the flexural modulus of elasticity and hardness of the flattened bamboo lumber, which reached their minimum values of 4479 MPa and 786.71 N, under the treatment at 190 °C/3 h. Conversely, it enhanced the longitudinal compressive strength of flattened bamboo lumber, achieving a maximum value of 57.28 MPa at 180 °C/2 h. At the microscale, the nanomechanical strength decreased under 190–200 °C treatments, accompanied by a tighter cell arrangement and evident shrinkage and deformation of the parenchyma cells. Dimensional stability tests combined with FTIR and crystallinity analyses demonstrated a reduction in the number of hydrophilic groups and improved dimensional stability after thermal modification. Notably, the material treated at 200 °C for 4 h retained its dimensional stability and exhibited no deformation.

1. Introduction

Bamboo flattening is an effective method for enhancing bamboo utilization efficiency. This process involves softening arc-shaped bamboo strips by using high-temperature and high-humidity treatments, followed by mechanical pressing to transform them into flat sheet materials [1,2,3]. The resulting flattened bamboo can replace small-sized bamboo slivers or strips in the production of engineered bamboo composites, significantly reducing adhesive consumption. Furthermore, this technology lays the foundation for large-scale automated manufacturing of raw bamboo strips and novel engineered bamboo composites.

Systematic investigations of bamboo-flattening technology have been conducted, focusing on process optimization (e.g., softening temperature, pressure, duration) and performance evaluation to enhance the softening efficiency and utilization rates of flattened bamboo materials. Studies have also explored the multi-scale microstructural evolution and stress–strain response mechanisms of flattened bamboo under hygro-thermal–mechanical coupling effects, aiming to mitigate hygroscopic deformation and broaden its applications. Thermal modification, relative to chemical modification, is an ecologically environmentally friendly modification method, and is typically conducted by using conventional heat treatment at 160–220 °C to improve dimensional stability, decay resistance, and moisture resistance [4,5,6]. Additionally, advanced thermal treatments utilizing saturated steam, thermal oil, microwaves, or inert gas as media have been employed to effectively reduce the hygroscopicity, swelling ratio, equilibrium moisture content, and mechanical degradation of bamboo [7,8,9].

This study explores the inherent structural challenges in flattened bamboo lumber: its wall-layer structure exhibits an externally compressed and internally tensioned stress state, where the dense green outer bamboo layer and porous yellow inner bamboo layer induce flattening recovery during moisture sorption and desorption cycles. To resolve these limitations, this study developed thermal modification for deformation fixation. The aim was to develop flattened bamboo materials with dimensional stability superior to that of current flattened bamboo materials.

This study addresses the structural challenges in flattened bamboo lumber: its wall-layer structure exhibits an externally compressed and internally tensioned stress state, where the dense green outer bamboo layer and porous yellow inner bamboo layer induce flattening recovery during moisture sorption/desorption cycles. To resolve this, thermal modification is proposed for deformation fixation, aiming to develop flattened bamboo materials with superior dimensional stability.

2. Materials and Methods

2.1. Materials

Moso bamboo (Phyllostachys edulis) was supplied by China Jiangxi Benbo Technology Development Co., Ltd. Four-year-old fresh bamboo culms were selected. Middle sections with comparable density (0.90–1.04 g/cm³) were cut into 1050 mm long culm segments. Each segment was split into 3–4 arc-shaped strips, processed in a high-temperature steam vessel at 190 °C for 6 min, and flattened and dried using a groove-free flattening machine. The final specimens were prepared as flattened bamboo sheets with dimensions of 500 × 80 × 5 mm (length × width × thickness).

2.2. Thermal Modification Process

The flattened bamboo sheets were thermally modified in a 1 m³ pilot-scale drying kiln. A full factorial experimental design was implemented, with processing temperature (180 °C, 190 °C, 200 °C) and duration (2, 3, 4 h) as variables, totaling nine experimental groups. Two sheets per group were selected from the two culm segments with comparable material properties. The thermal modification process is shown in Figure 1.

Carbonization protocol: initial drying at 60 °C for 6 h.

Gradual heating: 70–80 °C (30 min per interval), 90–100 °C (2 h per 10 °C increment), and 110–150 °C (1 h per 10 °C increment).

Final stabilization: 160 °C for 2 h and 170–180 °C for 30 min each, and then an increase to the target temperature for specified durations. Post-modified, specimens were equilibrated in a climate chamber (20 ± 2 °C, 65 ± 5% RH) until the moisture content reached 10%–12% for subsequent testing.

2.3. Characterizations

2.3.1. Mechanical Performance

The mechanical properties were evaluated according to GB/T 15780-1995 (“Test Methods for Physical and Mechanical Properties of Bamboo”) [10]. The specimen dimensions were standardized as follows: flexural modulus of elasticity, 160 × 10 × 5 mm (length × width × thickness); longitudinal compressive strength, 20 × 20 × 5 mm (length × width × thickness); and Janka hardness [11], 100 × 10 × 5 mm (length × width × thickness). All tests were conducted using a universal mechanical testing machine (INSTRON 5967, Norwood, MA, USA). The results were obtained by obtaining the average of six samples for each group of samples.

2.3.2. Dimensional Stability

Thickness Swelling Rate and Water Absorption Rate

The specimens (20 × 20 × 5 mm, length × width × thickness) were prepared by measuring the thickness at the midpoints of all four edges and the mass, with the values averaged. Grouped specimens, secured with spacers, were immersed in 80 °C water for 2 h and 20 °C water for 24 h. The results were obtained by obtaining the average of five samples for each group of samples. The post-immersion thickness and mass were measured to calculate the thickness swelling rate ($W$) and water absorption rate (${\alpha }_D$) using the following formulas:

$$W={\displaystyle \frac{m_1-m_0}{m_0}}\times 100\%$$

(1)

$${\alpha }_D={\displaystyle \frac{D_1-D_0}{D_0}}\times 100\%$$

(2)

where $W$ = water absorption rate (%); $m_1$ = wet mass after immersion (g); $m_0$ = oven-dry mass (g); ${\alpha }_D$ = thickness swelling rate (%); $D_1$ = post-immersion average thickness (mm); $D_0$ = initial average thickness (mm).

Bending Height

The thermally modified flattened bamboo lumber specimens (50 × 80 × 6 mm, length × width × thickness) were subjected to an 80 °C water bath treatment for 2 h. The test was conducted using an electric thermostatic water bath (HH-M6, Shanghai, China). The maximum warpage height—defined as the peak vertical displacement of the specimens after treatment—was measured to quantify bending deformation. The results were obtained by repeating the average of five samples for each group of samples.

2.3.3. FTIR

Fourier transform infrared spectroscopy (FTIR) analysis was performed using an FTIR spectrometer (THysitron Inc., Minneapolis, MN, USA). The samples were prepared by grinding bamboo material into powder (<100 mesh), oven-drying them, and homogeneously mixing them with KBr at a 1:100 mass ratio. The mixture was pressed into transparent pellets by using a hydraulic pellet press and scanned across a wavenumber range of 4000–400 cm⁻¹ with 32 scans at a resolution of 4 cm⁻¹.

2.3.4. XRD

A powder sample of less than 100 mesh was prepared, baked until dry, evenly spread onto a metal sheet with circular grooves, and scanned at angles ranging from 5°to 60°at intervals of 0.02° at a frequency of 3°/min. The crystalline properties of cellulose were analyzed using XRD (Shimadzu, Kyoto, Japan) patterns. The crystallinity was calculated according to Formula (3).

$$C_{r}I={\displaystyle \frac{\left(I_{002}-I_{am}\right)}{I_{002}}}\times 100\%$$

(3)

where $C_{r}I$ is the crystallinity index, $I_{am}$ denotes the minimum intensity of the amorphous region, and $I_{002}$ is the maximum intensity of the diffraction.

2.3.5. SEM

Scanning electron microscopy (SEM) specimens were prepared by extracting 5 × 5 × 5 mm sections from flattened bamboo lumber by using a precision scalpel. The observation surfaces were smoothed and flattened using a microtome, mounted on SEM stubs by using a conductive adhesive, and sputter-coated with a 10 nm gold layer to enhance conductivity. The microstructural morphology was analyzed using a tabletop SEM (Hysitron Inc., Minneapolis, MN, USA) operated at an accelerating voltage of 15 kV in high-vacuum mode.

2.3.6. Nanoindentation

Samples with dimensions of 8 × 6 × 5 mm (length × width × thickness) were polished using an ultramicrotome equipped with a diamond blade to achieve a surface roughness below 10 nm in the fiber regions. The test locations were the parenchymal tissue within the secondary cell walls. The cellular regions were imaged using the built-in atomic force microscope (AFM) of a nanoindentation system (Ti800, Hysitron Inc., Irvine, CA, USA), with six cells randomly selected per specimen for analysis. A three-stage constant-rate protocol (5 s each for the loading, holding, and unloading phases) was implemented with a maximum load of 400 μN. Post-test rescanning of the indented areas enabled the acquisition of residual impression images. Thirty valid indentation points that fulfilled the quality criteria were subsequently analyzed to evaluate their mechanical properties.

3. Results

3.1. Effect of Thermal Modification Treatment on Mechanical Properties of Flat Bamboo Lumber

As shown in Figure 2a, the hardness of the thermally modified flattened bamboo exhibited a temperature-dependent decline, ranging from 786.71 N to 1486.9 N. This represents a reduction of 7.36%–50.98% compared with untreated specimens (1605 N), with processing temperature exerting a more pronounced influence on hardness than on treatment duration. The average hardness values of thermally modified bamboo specimens treated for 2 h, 3 h, and 4 h were found to be 1133.6 N, 956.3 N, and 1031.6 N. A notably pronounced hardness decrease was observed in specimens subjected to the 3 h treatment. Across all temporal conditions, specimens treated at 180 °C, 190 °C, and 200 °C exhibited progressive hardness deterioration, with values of 1279.3 N, 971.0 N, and 871.3 N, respectively. The 200 °C treatment group demonstrated the maximum hardness loss. The minimum hardness of 786.71 N was observed at 190 °C/3 h treatment. Notably, hardness variation stabilized in specimens treated above 190 °C for ≥2 h. This is similar to the conclusion obtained by Zhang M [7] after four heat treatments (1~4 h) for seven temperature classes (100~220 °C), and this phenomenon was attributed to the thermal degradation of the internal microstructure at 190 °C, where hemicellulose degradation and cellulose chain scission or depolymerization occurred, ultimately compromising mechanical integrity through embrittlement and structural weakening mechanisms [12].

As illustrated in Figure 2c, the longitudinal compressive strength of thermally modified flattened bamboo specimens generally exceeded that of their untreated counterparts (36.01 MPa), ranging from 34.35 to 57.28 MPa, except for the 180 °C/4 h treatment. The longitudinal compressive strength of thermally modified bamboo specimens treated for 2 h, 3 h, and 4 h demonstrated average values of 54.3 MPa, 44.0 MPa, and 36.3 MPa, respectively, with more pronounced elevation observed at the 2 h treatment duration. Across all temporal conditions, specimens processed at 180 °C, 190 °C, and 200 °C exhibited average longitudinal compressive strengths of 45.3 MPa, 45.0 MPa, and 44.3 MPa, respectively. The most significant strength enhancement was recorded at 180 °C thermal treatment. The maximum strength of 57.28 MPa, achieved under 180 °C/2 h conditions, demonstrated a 59.1% enhancement compared with that of untreated material. This improvement was attributed to volumetric shrinkage and reduced porosity during thermal modification, which enhanced the stress distribution and load-bearing capacity [2,12]. Notably, the longitudinal compressive strength exhibited a progressive decline with extended treatment duration. While strength decreased with increasing temperature in the 2 h and 3 h treatments, an inverse correlation emerged at 4 h of exposure, where strength increased proportionally with temperature elevation.

As depicted in Figure 2b, the flexural modulus of the thermally modified flattened bamboo (4479–8986 MPa) was consistently worse than that of the untreated specimens (12,145 MPa), exhibiting reductions of 26.01%–63.12%. The flexural modulus of elasticity in thermally modified bamboo specimens treated for 2 h, 3 h, and 4 h exhibited average values of 8585.5 MPa, 7029.1 MPa, and 6075.8 MPa, respectively. A progressive stiffness degradation pattern was observed, with the most marked reduction occurring for the 4 h treatment duration. Under all temporal conditions, specimens processed at 180 °C, 190 °C, and 200 °C demonstrated flexural moduli of 7915.3 MPa, 6525.2 MPa, and 8150.1 MPa, respectively. Notably, the 190 °C treatment group displayed a distinctive stiffness minimum. This demonstrates the effective thermal softening effect of thermal modification, with the minimum modulus (4479 MPa) observed at 190 °C/3 h. For identical treatment durations, the flexural modulus initially decreased, followed by an increase with increasing temperature. The initial reduction likely stemmed from the thermal decomposition of lignin during the early stage of thermal modification, which weakens the interfacial bonding between the cellulose and hemicellulose matrices [13]. This is consistent with the results of Zhu R [14] upon treating bamboo in high-temperature steam. Conversely, temperatures exceeding 190 °C induced progressive cellular wall densification through structural contraction, enhancing stress homogeneity under flexural loading and consequently improving the modulus values [15].

3.2. Effect of Thermal Modification Treatment on Dimensional Stability of Flat Bamboo Wood

3.2.1. Thickness Swelling Rate and Water Absorption Rate

As shown in Figure 3a,b, the water absorption of the thermally modified flattened bamboo specimens was consistently lower than that of their untreated counterparts. At fixed temperatures, prolonged treatment durations increased the water absorption across all hydrothermal conditions. The water absorption capacity of thermally modified flattened bamboo specimens processed at 180 °C, 190 °C, and 200 °C exhibited average values of 39.1%, 33.3%, and 26.8%. The average water absorption rates after 2 h, 3 h, and 4 h treatments were 26.8%, 31.8%, and 35.6%. The minimum absorption values of 20.79% (20 °C/24 h bath) and 21.29% (80 °C/2 h bath) were achieved in the 200 °C/2 h treatment, representing reductions of 59.29% and 51.01% relative to the untreated material (20 °C: 51.07%; 80 °C: 43.49%), respectively. Specimens treated at 180 °C, 190 °C, and 200 °C exhibited average water absorption rates of 33.8%, 33.9%, and 26.2%, respectively. Those treated for 2 h, 3 h, and 4 h demonstrated average water absorption rates of 26.8%, 31.8%, and 35.6%. This demonstrates that thermal modification significantly improves dimensional stability through two mechanisms: (1) lignin degradation reduces hydrophilic hydroxyl groups, and (2) cellular densification decreases interchain spacing in cellulose matrices while enhancing hydrogen bonding among residual hydroxyls. Concurrently, the improved molecular orientation in the amorphous regions further restricts water molecule penetration pathways [16,17].

As shown in Figure 3c,d, the thickness swelling rate of the thermally modified flattened bamboo exhibited substantial reductions compared with that of its untreated counterparts, with progressive decreases observed with elevated processing temperatures and extended durations. Specimens treated at 200 °C/4 h demonstrated minimal swelling rates, achieving reductions of 87.61% (20 °C/24 h bath) and 86.82% (80 °C/2 h bath) relative to untreated material. This enhanced dimensional stability is due to the thermally induced cross-linking between lignin derivatives and residual cellulose/hemicellulose networks during thermal modification [18]. The covalent bonding restrains hygroscopic expansion–contraction behavior by structurally reinforcing cell walls, limiting the water-mediated plasticization of microfibril aggregates [19,20]. In a study by Zhang Y [21], heat treating bamboo at 180 °C, the degradation of polysaccharide, and the thermal crosslinking of lignin led to a decrease in hydrophilic -OH, C=O, and C-O groups. This structural modification consequently decreased the hygroscopicity of the treated material, which aligns with the findings of the present study.

3.2.2. Bending Height

Figure 4 illustrates the flexural deformation of thermally modified flattened bamboo under 80 °C hydrothermal treatment for 1–3 h. Bending height decreased progressively with elevated treatment temperatures and durations, with temperature exerting a greater influence than time. Specimens treated at 200 °C/2 h achieved near-complete deformation fixation (<0.1 mm residual curvature) after 1 h of immersion, exhibiting bending heights of 1.88 mm (84.5% reduction vs. untreated: 12.15 mm) and 4.56 mm (65.8% reduction vs. untreated: 13.36 mm) at exposures of 2 h and 3 h, respectively. The 200 °C/3 h treatment group achieved deformation stabilization within 1–2 h of immersion, displaying a 90.9% reduction (1.21 mm vs. 13.36 mm) at 3 h. Complete dimensional fixation occurred in specimens treated at 200 °C/4 h.

As shown in Figure 4d, bending deformation decreased with increasing thermal modification temperature, becoming negligible above 200 °C/2 h. This stabilization mechanism primarily stems from hemicellulose degradation during thermal modification [21], which reduces the number of hydrophilic hydroxyl groups and suppresses moisture-induced plasticization. Notably, all specimens exhibited preferential curvature recovery toward the inner culm surface (huang side) during swelling. This anisotropic behavior originates from the residual tensile stresses on the huang side and compressive stresses on the outer culm surface (qing side) induced during flattening. Moisture absorption triggers stress relaxation, enabling the partial recovery of the original curved morphology through differential hygroexpansion [22,23,24].

3.3. Effect of Thermal Modification Treatment on Infrared Spectral Properties of Flat Bamboo Wood

As shown in Figure 5, the positions and intensities of the characteristic absorption bands evolved with increasing thermal modification temperature. A broad band near 3450 cm⁻¹, assigned to O-H stretching vibrations of hydrophilic hydroxyl groups in cellulose, hemicellulose, and lignin [25], exhibited reduced intensity at elevated temperatures. This attenuation suggests either diminished hydroxyl content or enhanced hydrogen bonding interactions due to thermal modification [26]. Therefore, the treated material showed a decrease in the water absorption rate and water absorption thickness expansion rate, and a decrease in bending height. The prominent peak at 1737 cm⁻¹, attributed to C=O stretching vibrations from esterified carboxyl groups in hemicellulose and coumaric acid moieties in lignin [27,28], progressively weakened with temperature elevation, indicating thermal degradation of hemicellulosic components. The 1420 cm⁻¹ band retained its position but displayed altered lineshape profiles, likely reflecting environmental changes in carbonyl group configurations. Enhanced absorption in the 1328 cm⁻¹ region corresponded to intensified C-O stretching vibrations, consistent with lignin enrichment during thermal modification [29]. Concurrently, the 1043 cm⁻¹ band associated with C-O-C/C-O stretching in cellulose/hemicellulose polysaccharide backbones [30] demonstrated progressive intensity loss, confirming the progressive depolymerization of carbohydrate networks. This can be clearly seen in subsequent SEM tests, where the starch granules almost completely disappear.

3.4. Effect of Thermal Modification Treatment on XRD Properties

As evidenced in Figure 6a, all XRD patterns exhibited distinct diffraction peaks, confirming the retention of crystallinity in thermally modified flattened bamboo despite thermal processing. However, variations in the peak intensity and profile with increasing treatment temperature reflect structural reorganization, including crystallinity evolution and crystallite size modification [12,23]. Specifically, specimens treated at 180 °C/3 h displayed a 2.62% increase in crystallinity relative to untreated material (Figure 6b). This enhancement was due to partial hemicellulose decomposition at 180 °C, which reduces the amorphous matrix content and promotes tighter molecular packing of cellulose chains into ordered configurations [31]. Conversely, crystallinity decreased to 51.08% (190 °C) and 50.02% (200 °C), representing reductions of 2.78% and 3.84% compared with that of the untreated counterparts. These declines correlate with accelerated pyrolysis at elevated temperatures, where chain scission in cellulose and the degradation of amorphous domains progressively disrupt crystalline regions [6,32], thus showing a decrease in hardness and elastic modulus at the macro level, which is also reflected in the study of Dan L’s heat treatment of bamboo under low pressure [4].

3.5. Effect of Thermal Modification Treatment on Micro-Morphology of Flat Bamboo Wood

Figure 7a–c present the cellular microstructure of untreated specimens. Figure 7d–f display the corresponding features of thermally modified flattened bamboo treated at 190 °C/3 h. Observations were conducted at magnifications of 60×, 100×, and 250×, focusing on structural modifications in vascular bundles and parenchyma cells across hierarchical scales.

As shown in Figure 7a,b, untreated specimens exhibited distinct annular arrangements of vascular bundles surrounded by parenchyma cells, with clearly defined cell walls and lumina. After thermal modification, the cellular packing density increased significantly. The overall morphology of vascular bundles remained intact, and adjacent parenchyma cells displayed pronounced contraction and structural distortion (Figure 7d,e). This deformation was due to hemicellulose degradation during thermal treatment, which reduces cell wall porosity to enhance material densification while simultaneously inducing localized buckling [33]. Partial fissures observed in the treated cellular structures are probably due to the synergistic effects of cell wall compositional changes and moisture evaporation during pyrolysis [34]. Notably, xylem vessels within vascular bundles retained their structural integrity, with sieve tubes, companion cells, and small parenchyma cells showing no detectable deformation. Vessels exhibited negligible morphological alteration, although perivascular tissues demonstrated localized delamination and disaggregation [35].

3.6. Effect of Thermal Modification Treatment on Nanomechanical Properties of Flat Bamboo Wood

As shown in Figure 8, untreated specimens exhibited an elastic modulus of 4.86 GPa and hardness of 0.22 GPa. The nanomechanical strength of thermally modified flattened bamboo displayed temperature-dependent behavior: specimens treated at 180 °C demonstrated enhanced strength relative to untreated material, and those processed at 190 °C and 200 °C showed significant reductions. At 190 °C, the elastic modulus and hardness decreased by 43.79% and 31.32%, respectively. This trend paralleled the biphasic evolution of cellulose crystallinity observed in prior analyses: initially increasing at 180 °C due to amorphous matrix removal and molecular chain realignment and then decreasing at elevated temperatures (190–200 °C) as crystalline domains underwent thermal degradation. The progressive disruption of structural integrity at higher temperatures directly correlated with the diminished nanoscale mechanical performance because pyrolysis-induced chain scission compromised load-bearing capacity [36,37,38].

4. Conclusions

This study systematically investigated the effects of thermal modification on flattened bamboo by conducting a comprehensive analysis of its mechanical properties, chemical composition, and microstructure. The main findings are as follows:

(1) Chemical Modifications and Nanomechanical Behavior

The FTIR spectra revealed temperature-dependent alterations in the characteristic absorption bands of cellulose, hemicellulose, and lignin. Crystallinity exhibited a biphasic response—peaking at 55.27% (180 °C) before declining at temperatures higher than 180 °C. Microstructural observations confirmed preserved vascular bundle morphology, with contracted parenchyma cells and increased cellular packing density. Parenchyma cell hardness and elastic modulus peaked at 180 °C, exceeding the untreated levels by 18.6% and 22.3%, respectively. Subsequent thermal degradation at 190–200 °C reduced these values below baseline levels, correlating with cellulose crystallinity loss and amorphous matrix disruption. The above chemical mechanism analysis and its microscopic level all reflect the changes in the macroscopic mechanical properties, dimensional stability, and bending height of the modified materials.

(2) Mechanical Performance Evolution and Enhanced Dimensional Stability

Thermal modification reduced the hardness and flexural modulus while enhancing the longitudinal compressive strength. Specimens treated at 190 °C/3 h exhibited minimum values for hardness (50.98% reduction) and flexural modulus (63.12% reduction) relative to untreated material. Conversely, the maximum longitudinal compressive strength (59.1% increase) was achieved using the 180 °C/2 h treatment. Thermally modified specimens demonstrated significantly improved hygroscopic stability, with the water absorption and thickness swelling rates reduced by up to 30.29% and 86.37%, respectively. Flexural deformation was effectively suppressed at elevated temperatures, achieving complete dimensional fixation with the 200 °C/4 h treatment.