Determination of the Effects of Superheated Steam on Microstructure and Micromechanical Properties of Bamboo Cell Walls Using Quasi-Static Nanoindentation

Abstract

In this paper, quasi-static nanoindentation was applied for investigating the influence of superheated steam on microstructure and micromechanical properties of Moso bamboo cell walls. The changes of mico-morphology, chemical composition, cellulose crystallinity index, micro-mechanical properties of bamboo were analyzed via scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and nanoindentation. As expected, the content of hemicellulose and cellulose showed a downward trend, whereas the relative lignin content increased. Elastic modulus and hardness of the cell wall increased compared with that of the untreated sample. The elastic modulus and hardness of bamboo increased from 11.5 GPa to 19.5 GPa and from 0.35 GPa to 0.59 GPa. Furthermore, results showed that the creep resistance positively correlated to treatment severity.

1. Introduction

Due to high yield, sustainability, rapid growth, adaptability, and great mechanical properties, Moso bamboo has been considered as an important economic in China [1,2]. However, multiple products such as bamboo plywood, bamboo scriber, bamboo plastic composites, and wood-bamboo composite lose their natural appearance [3,4]. Furthermore, no large original bamboo surface can be found on these products and the elegant texture and grain of the original surface is lost. As a result, bamboo in its natural state has recently received more attention. Furthermore, when compared to existing bamboo-based goods, natural bamboo culms are becoming one of the most promising non-conventional sustainable construction materials. However, because of the abundance of carbohydrates and starches, bamboo culms are easily impacted by fungus and UV radiation. Improving the mechanical properties and resistance to fungi is essential for moving toward an increasing outdoor utilization of bamboo culms [5].

It is well known that bamboo is a natural polymer composite that consists of hemicellulose, cellulose, lignin, and ash. Thermal modification (TM) is an environmentally benign and cost-effective method of decreasing the hygroscopicity and increasing the durability of bamboo and bamboo products. At 150 °C, the TM can effectively minimize hygroscopicity, shrinkage, and swelling qualities. The considerable breakdown of hemicellulose at 180 °C or above positively contributes to anti-fungi capabilities [6,7]. Conventional heat treatment media, such as oil, heat air, and high frequencies, can lead to the loss of moisture content in Moso bamboo, which can cause cracks on the bamboo surface. Furthermore, high temperatures reduce the macro characteristics of bamboo owing to the breaking of hydrogen bonds and the disintegration of chemical compositions. Because of the interplay of high-pressure and water vapor, superheated steam may effectively minimize the needed duration for heat treatment while also increasing the moisture content of bamboo culms. Due to the elastic solids and viscous liquids features of bamboo cell walls, the deformation of bamboo cell walls increased with increasing time of quasi-static force. Furthermore, conventional research concentrated on the macroscale physical and mechanical characteristics of bamboo and seldom examined the influence of heat alteration on the micro-mechanical properties of bamboo at the micro-scale. Many variables influence the mechanical characteristics of bamboo cell walls, including microstructure and chemical composition. As a result, understanding the mechanical characteristics of bamboo cell walls at the cellular level is important [8,9,10].

Nanoindentation is a useful method to study the micromechanical properties of woody materials. Toward a more sustainable and efficient bamboo use, it is of great importance to investigate the changes in mechanical properties of bamboo cell walls. In this work, the micro-morphology and nanomechanical properties of bamboo were analyzed by SEM, the wet chemistry method, and nanoindentation. Mechanical properties were studied at the cell wall scale, and the creep ratio was analyzed to further explore the relationship between thermal modification parameters and mechanical properties of bamboo cell walls.

2. Materials and Methods

2.1. Materials and Thermal Modification

Untreated and superheated steam-treated moso bamboo (Phyllostachy Heterocycle) was collected from JiangXi province in China. Firstly, bamboo samples with the same dimensions of 1050 (L) × 200 (T) × 10 mm (wall thickness) were taken. Then, the bamboo culms were placed in a pressure tank (RDW-1.5/0.5-5-D, Rongda Boiler Container Co., Nanjing Ltd., Nanjing, China) for heat treatment at different temperatures (160 °C, 170 °C, and 180 °C) and times (90 min, 120 min, and 150 min). The temperature of superheated steam at 160 °C, 170 °C, and 180 °C corresponds to air pressure of 6.1 MPa, 7.92 Mpa, and 10.027 MPa, respectively. The temperature in the factory was room temperature, and it was summer when the experiments were conducted.

2.2. Bamboo Cellulose Crystallinity Calculation

The crystallinity of cellulose was determined by X-ray diffraction (Ultima IV, Japan). The bamboo powders were exposed to X-ray radiation. The 2-theta and scan rate were set from 5° to 45° and 2° min⁻¹, respectively. The crystallinity of cellulose can be calculated according to the Segal method as below:

C_rI = (I₀₀₂ − I_am)/I₀₀₂ × 100%,

(1)

where C_rI represents the crystallinity index of cellulose, I_am is the minimum intensity of the amorphous, and I₀₀₂ denotes the maximum intensity of the diffraction.

2.3. Mass Loss (ML) Analysis

To calculate the mass loss, the changes in mass of bamboo samples before and after treatment were calculated as follows:

%ML = 100 (m₀ − m₁)/m₀

(2)

where m₀ and m₁ represent the oven-dried specimen mass before and after heat treatment.

2.4. Measurement of Chemical Groups in Bamboo Samples

Specimen powder of 200 mg was tested with range 500–4000 cm⁻¹ on a VERTEX 80 V FTIR spectrometer (BrukerCorporation, Karlsruhe, Germany). Fourier transform infrared (FTIR) spectroscopy was obtained to analyze the chemical groups in bamboo samples. The untreated bamboo samples and treated bamboo samples were milled to powders of 100 mesh and then pressed together with KBr powders into transparent film. The resolution of the device was 2 cm⁻¹ with 32 accumulations. The range of the wavelength was recorded from 400–4000 cm⁻¹.

2.5. Chemical Compositions Analysis

The treated bamboo samples were grounded and screened into powders with the size of 40–80 mesh for chemical composition analysis. The main chemical composition (cellulose, hemicellulose, and lignin) of the samples were tested according to NREL’S LAPS [11,12]. NREL’S LAPS method could not measure the content of ash and extractive in bamboo samples so that the content of chemical composition in the tables does not sum to 100%.

2.6. Measurement of Dimensional Changes

These physical factors were tested according to the Chinese standard GB/T 15780-1995. Specimens were oven-dried at 103 °C for 24 h until constant mass reached. Then, the bamboo samples were placed in constant humidity and temperature (25 °C and 65% relative humidity) chamber for 2 weeks until constant mass. When the samples had been oven-dried at 103 °C for 12 h, we recorded the mass. Then, we recorded the mass every 3 h, and when the mass remained constant, we considered the specimens oven-dried. The approximate mass of each sample was 0.72 g. When the mass was constant, we considered the specimens as having reached the EMC. The moisture absorption (equilibrium moisture content) can be measured as bellow:

EMC (%) = 100 × (m₁ − m₀)/m₀

(3)

where m₀ is the oven-dried mass of the sample and m₁ is the constant mass after moisture absorption.

The length, width, and thickness of bamboo samples were measured by micrometer screw (accurate 0.01 mm). The initial volume V₀ can be calculated by length (mm) × width (mm) × thichness (mm). Anti-swelling efficiency (ASE) can be calculated as below for investigating the dimension stability.

ASE (%) = 100 × (S_c − S_t)/S_c

(4)

where S_c and S_i represent the volumetric swelling coefficient of untreated specimens and treated specimens, respectively.

Volumetric swelling coefficient can by calculated by following formula:

S (%) = 100 × (V − V₀)/V₀

(5)

where V₀ and V is the initial volume and volume after moisture absorption, respectively.

2.7. Nanoindentation (NI)

First, bamboo samples were cut into 0.5 × 0.5 × 1 cm³ average dimensions. A diamond knife was used to clean the cross-sections of bamboo samples. All samples were kept at a constant temperature (21 °C) and humidity (65%) for 24 h before to the experiments. The bamboo specimens were then bonded to stubs. To achieve at least 30 valid indentations, NI was performed in a load-controlled mode (Figure 1). Loading took 5 s, holding time was 5 s, and discharging took 5 s. On all indents, the maximum load (400 μN) was applied. Figure 1 depicts an image of a bamboo cell wall, typical load-depth curves, and typical time-depth curves.

2.8. Measurement of Bamboo Cell Wall Mechanical Properties

The hardness and elastic modulus can be calculated based on the method according to Oliver (1992) reported by previous literature [11,12,13,14]:

$$H=\frac{P_{max}}{A}$$

(6)

where P_max is the peak load and A is that the projected contact space of the indents at peak load.

$$E_r=\frac{\sqrt{\pi }}{2\beta }\frac{S}{\sqrt{A}}$$

(7)

where E_r is the combined elastic modulus of both the sample and indenter; S is initial unloading stiffness; and β is a correction factor correlated to indenter geometry (β = 1.034).

2.9. Creep Behavior

For determining the creep behavior of the samples over an extended period, the load-holding time was extended to 100 s. Creep behavior can be calculated according to the method Konnerth and Gindl reported in previous literature [4].

$$C_{IT}(\%)=\frac{h_2-h_1}{h_1}\times 100$$

(8)

where h₂ and h₁ are represented the final and first penetration depth of segment, respectively.

2.10. Statistical Analysis

The significance of the data of chemical composition, relative crystallinity of cellulose, and micro-mechanical characteristics of bamboo cell walls was investigated using Analysis of Variance (ANOVA). The significant difference between groups was expressed by different capital letters (p < 0.05).

3. Results and Discussion

3.1. Mass Loss (ML)

Figure 2 presents the results of mass loss of moso bamboo after superheated steam treatment. ML as a function of treatment severity can be seen in Figure 2. The different letters represent significant differences between treatments (p < 0.05). To be specific, when the treatment temperature is at 160 °C, there is no significant change of mass loss in comparison to no treatment (p > 0.05). However, the treatment severity makes a positive contribution to the mass loss of moso bamboo when the treatment temperature is over 170 °C. According to previous literature [4,5,6,7], the decomposition of hemicellulose begins at 180 °C, and with the increasing decomposition of hemicellulose, more acetic acid is released. The increment in concentration of acetic acid has a considerable effect on the hydrolysis of carbonrate polymers in bamboo structure. In addition, during the process of superheated steam treatment, the decrement of extractives may also lead to this conclusion [13,14].

3.2. Micro-Morphology of Bamboo Cell Walls

The micro-morphology of the cross-section of the untreated bamboo and superheated steam treated bamboo were observed via SEM, as shown in Figure 3. The SEM images denote the obvious deformation of the parenchyma cell after treated by superheated steam. The cell cavity of parenchyma cells changed from an oval shape to a flat shape. These SEM images show that the parenchyma cell of bamboo became loose and fragile with the increment of treatment severity. This can be attributed to the degradation of carbohydrates in the bamboo samples [15].

3.3. Main Chemical Compositions

The purpose of this study was to see how superheated steam affected the major chemical makeup of bamboo specimens.

Table 1. Relative content of chemical composition in bamboo samples.

Time	Cellulose			Hemicellulose			Lignin
	160 °C	170 °C	180 °C	160 °C	170 °C	180 °C	160 °C	170 °C	180 °C
Untreated		54.6			18.1			21.8
90	54.1	53.5	49.2	17.5	15.1	14.5	23.1	25.0	26.1
120	54.4	53.7	47.9	17.0	14.3	13.4	23.2	25.8	26.3
150	54.7	52.5	46.8	16.4	14.1	13.2	23.5	25.3	26.7

displays the hemicellulose, cellulose, and lignin content values. The relative cellulose, hemicellulose, and lignin contents were 18.1%, 54.6%, and 21.8%, respectively. Heat treatment had a minor effect on the cellulose content, as expected. Hemicellulose content correlated adversely with treatment intensity. The hemicellulose content, in particular, exhibited a decreased trend. When the samples were treated at 180 °C for 150 min, the relative level of hemicellulose was 13.2% This represents a decrease of around 27% compared to untreated samples. This observation is consistent with earlier research [15,16,17]. The major component of hemicellulose is xylan, which is susceptible to high temperatures and dehydration processes. The lignin level increased as hemicellulose and cellulose levels decreased. This phenomenon might be caused by the condensation of hemicellulose byproducts with lignin.

3.4. Crystallinity Index of Bamboo Fiber

As is visible in Figure 4, the cellulose crystallinity index can be calculated by Equation (2). The mean crystallinity index values of the control was 39.9%. The most significant change in the crystallinity index was in samples treated at 180 °C for 90 min. Superheated steam treatment at 180 °C for 90 min had a higher CrI than that of the untreated samples. This is an increment from 39.9% to 57.1%, which is co. 43.1%, which can be related to the degradation of the hemicellulose. Additionally, under the acidic environment, the paracrystalline part of cellulose decreased. As such, the crystallinity of bamboo may be further increased. However, with the increment of treatment time, the crystallinity of cellulose showed a decreasing trend, which can be attributed to the broken cellulose structure under the superheated steam thermal modification [16].

3.5. FTIR Analysis

Figure 5 shows the change in chemical composition of bamboo samples after thermal modification. The intensity of 3400 cm⁻¹ (hydrogen bond O–H stretching vibrations) decreased, which confirmed the reduction in hydroxyl groups. The peak at 2900 cm⁻¹ showed visible displacement and indicated the structure of cellulose was affected by high-pressure and high-temperature. The peak at 1731 cm⁻¹ represented C=O stretching and carboxyl groups in hemicelluloses and lignin. A reduction in the intensity of 1731 cm⁻¹ may be attributed to the degradation of acetyl groups in hemicelluloses during the process of hydrothermal treatment. With the increasing treatment temperature, more acetic acid and formic acid was released, accelerating the decrease of hemicellulose. The band at 1604 cm⁻¹ originated from the conjugated C–O in quinines. The intensity of 1604 cm⁻¹ decreased with an increase in treatment temperature. Similarly, the band at 1425 cm⁻¹ (CH₂ bending in hemicellulose), 1328 cm⁻¹ (CH₂ wagging vibration in cellulose), and 1162 cm⁻¹ (C–O stretching vibration in xylan) showed the same decreasing tendency. The conclusion of FTIR analysis is in line with the analysis of the change of the content of cellulose, hemicellulose, and lignin.

3.6. EMC and ASE Analysis

The results of EMC and ASE are shown in Figure 6A. As expected, the EMC decreasing with an increase in hygrothermal treatment severity. The EMC of the untreated sample decreased from 11.8% to 6.5% in 180 °C and 150 min. As discussed previously, this is due to the reduction of OH groups in hemicellulose. The ASE data are shown in Figure 6B. The ASE changes are shown as a function of treatment parameters. In Figure 6B, it is obvious that the ASE of the control is lower than the treated bamboo sample. This could be due to the decomposition of hemicellulose in bamboo cell walls [17,18].

3.7. Elastic Modulus and Hardness

Figure 7 depicts the average values of the mechanical characteristics of bamboo cell walls. As previously observed in the literature, moisture content has a detrimental impact on the rigidity of bamboo cell walls. As the temperature is raised to 160 °C/150 min, the elastic modulus and hardness rise by 28% and 31%, respectively, when compared to the control, which may be attributed to the lower EMC. Increased treatment temperature and time improve the elastic modulus and hardness of bamboo cell walls. For example, increasing the treatment temperature and duration to 180 °C and 90 min enhanced the average results by approximately 42% and 45%, respectively. Previous literature illustrated that the moisture content, the main chemical composition, the crystallinity of cellulose, and condensation polymerization of lignin affect the bamboo cell wall’s mechanical properties [19]. The condensation of lignin and its cross-linking reactions with by-products arose from the decomposition of hemicellulose are important reasons for the increasing average bamboo cell wall modulus of elasticity and hardness [20]. However, the increasing trend of cell wall mechanical properties are a function of temperature and time under 170 °C. At 180 °C or higher, the effects on this regard were weakened [21]. For 180 °C and 150 min, the mechanical properties of bamboo cell walls decreased compared with that of bamboo samples treated at 180 °C/120 min. This is due to the increment of treatment time, as parts of the cellulose chain decomposed and the tight structures broke, so that the elastic modulus and hardness of bamboo cell walls decreased. This trend is consistent with the change of crystallinity index [22,23,24].

3.8. Heat Treatment on Creep Behaviors

The effects of thermal modification parameters on creep behaviour of bamboo specimen were investigated and the results are presented in Figure 8. It is obvious that the average creep ratio of the untreated bamboo cell wall was 13.1%. After treatment at 180 °C/150 min, the results from statistical analysis show that the creep ratio of bamboo samples decreased significantly. For instance, the creep ratio of bamboo samples decreased from 13.1%% to 6.2% (treated at 180 °C/150 min). Heat treatment, as a complex process of physical change, led to the decomposition of hemicellulose, and the decomposition and crystallization of morphous cellulose [25,26,27]. The decreased hemicellulose content and increased cellulose index positively enhance the creep resistance. In addition, the condensation polymerization of lignin also contributed to the reduction of creep ratio [28,29,30].

4. Conclusions

The bamboo samples were treated by superheated steam and then analyzed by different methods. SEM results showed that the parenchyma cell distorted due to the pressure of superheated steam and decomposition of chemical composition in the cell wall. The content of hemicellulose decreased by ca. 20% compared to the untreated bamboo, while the relative content of lignin and crystallinity of cellulose increased. In the end, nanoindentation was used to analyze the mechanical properties of bamboo specimens. As expected, compared with the untreated samples, the elastic modulus and hardness values of bamboo cell walls increased by ca. 42% and 45% at 180 °C for 90 min. However, the elastic modulus and hardness decreased with the increment of treatment time due to the broken of cellulose structure. Fortunately, the creep resistance of bamboo specimens increased with the increasing of treatment temperature and treatment time.