Effect of Hydrothermal Treatment on the Mechanical and Microscopic Properties of Moso Bamboo

Abstract

In this study, moso bamboo was used as a raw material. To increase the plasticity of bamboo to achieve a greater softening effect, the softening method of hydrothermal treatment was used. Hardness and the flexural elastic modulus were used as the evaluation indices, and the crystallinity and main functional groups of the softened bamboo were analysed using X-ray diffraction and Fourier-transform infrared spectroscopy. Combined with the examination of timber colour, micromorphology, bending strength, and nanomechanical tests, our analysis showed the effects of the hydrothermal treatment on bamboo. The results showed that the hardness and flexural moduli of bamboo decreased with the increase in hydrothermal treatment temperature. However, cracking occurred after 3.5 and 4 h of treatment at 180 °C and 190 °C. This indicated that the softening effect was most pronounced when the treatment temperature and time were 180 ℃ and 3 h, respectively. The cellulose crystallinity of bamboo increased and then decreased with the increase in treatment temperature. Cracks were produced in the cell structure, starch locally disappeared, and the hardness and the elasticity modulus of the thin-walled bamboo cells first increased and then decreased with the increase in treatment temperature.

1. Introduction

Bamboo is the most widely cultivated and most abundant species in China [1]. It has high hardness and sharpness, which makes it difficult to use industrially on a large scale. Hydrothermal treatment can be used to improve the utilization rate of bamboo. Softening refers to the modification of bamboo to reduce its mechanical strength and facilitate subsequent processing and production. Hydrothermal treatment is a safe and non-polluting softening method. To improve the added value of bamboo resources and enrich the variety of bamboo products, bamboo can be softened by hydrothermal treatment.

The stress to which bamboo is subjected when it is spread out is directly proportional to its modulus of elasticity. The method of reducing its modulus of elasticity is known as bamboo softening [2]. Softening is mainly achieved using water as a plasticizer to increase the free volume space within the cell walls. Many researchers have conducted work on bamboo softening. Chen Hong used an alkali treatment to boil bamboo material for a softening treatment [3,4]. They found that the softening effect was better, although the chemical reagents used were expensive and more polluting. Li Yanjun used bamboo tubes as a research object, using the high-temperature steam method for the softening treatment of moso bamboo only, and found that a 6 min softening treatment at 200 °C was the most suitable [5]. Chung M. and Lee C. found that the softening effect of urea and ammonia on bamboo was not as effective as that of a strong alkali solution [6]. M. Stefanidou found that alkali activation was an aggressive treatment that may severely degrade lignocellulosic materials compared to hydrothermal treatment [7]. The strong alkali solution damaged the interior of the bamboo and reduced its quality. Hydrothermal treatment is a softening treatment for bamboo that uses closed containers with high-temperature water as the medium. This has the advantages of no odour and no pollution and is less flammable compared with traditional heat treatment. It can reduce the content of hemicelluloses and obtain the corresponding products required. Hydrothermal treatment can improve the performance of bamboo materials. Meanwhile, it can change the surface colour of bamboo materials from bright yellow to brown, improving their colour singularity [8] and increasing the added value of their products [9]. The temperature of a hydrothermal treatment is the main factor affecting the changes in the chemical composition contents [10]. Hydrothermal treatment can lead to the thermal degradation of hemicelluloses and cellulose in bamboo [11]. The variation in chemical components during hydrothermal treatment is strongly related to the mechanical strength of the material after hydrothermal treatment [12].

In our experiment, the effect of steaming on bamboo was determined using the hardness and the flexural modulus of elasticity of the bamboo to facilitate the subsequent application of this method in bamboo flattening. A hydrothermal steaming treatment was conducted on bamboo slices to investigate the effects of temperature and treatment times on the properties of the bamboo to reduce its strength. A full-factor experimental method was used to study the changes in properties before and after steaming. The properties of the treated bamboo were examined in terms of colour, chemical groups, crystallinity, microscopic morphology, and nanomechanics (MOE and hardness) to provide a theoretical basis for bamboo softening.

2. Materials and Methods

2.1. Materials

Bamboo (Phyllostachys edulis) was collected from Banqiao, Lin’an City, and 5-year-old moso bamboo with a 10–12 cm diameter was selected. Six bamboo tubes without bamboo joints were taken from more than 2.5 m above the ground. The bamboo tubes were then made into 20 mm wide bamboo strips and dried in a drying oven. All the raw materials were dried in the oven at 103 °C for 24 h until their quality was stable. After drying, the bamboo strips were cut crosswise into 100 mm long slices using a circular saw. The outer and inner skins were removed using a thicknesser, and the bamboo was prepared into 100 × 20 × 5 mm (length × width × thickness) slices. Each set of bamboo slices was taken from the same position in the same tube to ensure similar material properties.

2.2. Hydrothermal Treatment Processes

The treated bamboo slices were placed in a hydrothermal reaction kettle (LK-KH-200, Shanghai Lichen Instrument Technology Co., Ltd, Shanghai, China) for treatment. The oven (SEG-021, Shanghai Espec Environmental Equipment Co., Ltd., Shanghai, China) temperature was pre-set, and, when the oven reached the set temperature, the reactor was put into the oven. The temperature and time for the hydrothermal treatment process were 160 °C (3 h, 3.5 h, 4 h), 170 °C (3 h, 3.5 h, 4 h), 180 °C (3 h, 3.5 h, 4 h), and 190 °C (3 h, 3.5 h, 4 h), and one group of bamboo slices was selected from each of the six bamboo tubes, separately, as a sample of one of the groups in the experiment.

2.3. Characterizations

2.3.1. Colour Measurement

In accordance with the 1976 International Commission on Illumination CIE standard chromaticity theory, we used a DP-3-type fully automated colour measurement and colour difference meter to measure the colour parameters of the specimen to characterize its colour change (Konica Minolta, Tokyo, Japan). Three colour measurement points were taken on the bamboo green side of the specimen to measure its L*, a*, and b*, and the difference in lightness (△L*) and the overall colour difference (△E*) was calculated according to the formula of the colourimetric system.

$$△L^{\mathrm{*}}=L^{\mathrm{*}}-L_0^{*}$$

(1)

$$△a^{\mathrm{*}}=a^{\mathrm{*}}-a_0^{*}$$

(2)

$$△b^{\mathrm{*}}=b^{\mathrm{*}}-b_0^{*}$$

(3)

$$△E^{*}=\sqrt{{\left(△L^{\mathrm{*}}\right.)}^2+{\left.\left(△\right.a^{\mathrm{*}}\right)}^2+({△b^{\mathrm{*}})}^2}$$

(4)

where L*, a*, b* are the colour parameters of the hydrothermally treated samples, L₀*, a₀*, b₀* indicate the colour parameters of the untreated samples; △L* indicates the difference in lightness, the positive value indicates brighter than the control sample, and the negative value indicates darker than the control sample; △E* indicates the chromatic aberration, also known as the overall chromatic aberration, and the greater the value, the larger the difference between the colour of the sample under testing and that of the control sample. The larger the value, the greater the difference in colour between the same point measured before and after processing.

2.3.2. Mechanical Performance

According to the standard (GB/T 15780-1995) testing methods for the physical and mechanical properties of bamboo [13], the specimens were 100 mm × 20 mm × 5 mm in size (length × width × thickness); refer to Janka hardness determination method. Hardness sample size is 100 mm × 20 mm × 5 mm (Universal Mechanics Experimental Machine, INSTRON5967, Norwood, MA, USA). The results were obtained by averaging the data of the experiments.

2.3.3. Nanoindentation

The cross-section morphology of moso bamboo slices before and after steaming was analysed for nanomechanics, and samples of bamboo slices of 8 mm × 6 mm × 5 mm were prepared, roughly ground into a four-angled cone with 120-mesh polishing paper, and then the surface was finely ground smooth with 800 mesh, to obtain the fibre area with surface roughness less than 10 nm. The test position was the thin-walled cell tissues in the secondary wall of the cell wall (Hysitron Inc., Minneapolis, MN, USA). The test was performed using a three-phase constant-velocity loading and unloading mode (load/hold/unload for 5 s) with a maximum load of 400 μN. The test position was rescanned at the end of the test to obtain indentation images, 30 valid indentation points were selected, and the final data were averaged.

2.3.4. XRD

XRD analytical tests were carried out using the XRD (Shimadzu, Kyoto, Japan). 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, and the crystalline properties of cellulose were analysed based on the XRD patterns. The crystallinity was calculated according to Formula (5).

Crl = (I₀₀₂ – I_am)/I₀₀₂ × 100%

(5)

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

2.3.5. SEM

The microstructure of moso bamboo material before and after softening was observed using the SEM (TM3030, Hitachi, Tokyo, Japan). An amount of 5 mm × 5 mm × 5 mm moso bamboo slices were taken as test samples using a carving knife. The micromorphology of moso bamboo slices was observed by scanning electron microscope, which was mainly used to observe the destruction of the microstructures of moso bamboo material such as vascular bundles, thin-walled cells, conduits, filler substances, cellular gaps, pores, and so on, after the softening treatment.

2.3.6. FTIR

The FTIR analysis of the moso bamboo material before and after softening was tested using the FTIR (Thermo Scientific, Waltham, MA, USA). A powder sample of less than 100 mesh was made, baked until dry, and mixed with KBr in the ratio of 1:100 to grind homogeneously and then pressed using a tablet press, with a scanning range of 4000~400 cm⁻¹, and the positions and heights of the characteristic peaks of the functional groups were analysed based on the FTIR profiles.

2.3.7. Chemical Compositions

The treated bamboo samples were grounded and screened into powders with the size of 40–60 mesh for chemical composition analysis. The chemical compositions of the samples were tested according to National Renewable Energy Laboratory Standards in the United States. Evaluate the impact by testing the components of cellulose, hemicellulose, and lignin.

2.4. Spss Analysis

Multivariate-way statistical analysis (ANOVA) was performed on the data to evaluate the influence of hydrothermal treatment temperature and time on colour at the 0.05 significance level (p < 0.05), using SPSS software (v.27, SPSS Institute, Cary, NC, USA).

3. Results and Discussions

3.1. Analysis of Variation of Bamboo Colour

A comparison of the colour of the surface of moso bamboo timber after treatment at different hydrothermal treatment temperatures of 160, 170, 180, and 190 °C for 3 h is shown in Figure 1.

During the hydrothermal treatment, the bamboo colour changed from light to dark, from bright yellow to dark brown. With treatment temperatures of 160 °C and 170 °C, the bamboo colour gradually changed to brown. However, there was relatively little difference. At 180 °C and 190 °C, the difference in bamboo colour was greater, and it became dark brown. Through the statistical analysis, judging the impact on material colour based on △L* and △E*, the colour of the material changed significantly with temperature and time. The △L*, △a*, △b*, △E* of hydrothermal treatment materials with different temperatures and times are shown in

Table 1. The △L*, △a*, △b*, △E* of hydrothermal treatment materials with different temperatures and times.

	L*	a*	b*	△L*	△a*	△b*	△E*
Control	72.39	8.42	21.02
160 °C 3 h	48.61	14.72	23.7	−23.78	6.3	2.68	24.75
160 °C 3.5 h	41.99	16.72	21.21	−30.4	8.3	0.19	31.51
160 °C 4 h	37.66	15.16	17.83	−34.73	6.74	−3.19	35.52
170 °C 3 h	44.02	19.11	22.35	−28.37	10.69	1.33	30.35
170 °C 3.5 h	37.87	12.67	10.83	−34.52	4.25	−10.19	36.24
170 °C 4 h	35.6	14.78	18.59	−36.79	6.36	−2.43	37.41
180 °C 3 h	36.23	10.24	13.78	−36.16	1.82	−7.24	36.92
180 °C 3.5 h	34.66	13.41	20.84	−37.73	4.99	−0.18	38.06
180 °C 4 h	18.23	21.48	−0.25	−54.16	13.06	−21.27	59.63
190 °C 3 h	34.48	8.11	11.56	−37.91	−0.31	−9.46	39.07
190 °C 3.5 h	21.02	18.17	−3	−51.37	9.75	−24.02	44.14
190 °C 4 h	4.8	28.18	−20.72	−67.59	19.76	−41.74	81.86

.

3.1.1. Effect of Hydrothermal Treatment on the Brightness of Bamboo

The variation in brightness (L*) of the bamboo after hydrothermal treatment is shown in Figure 2. It has shown a significant decreasing trend with increasing treatment temperature. At 160 °C for 3 h, the L* value of bamboo decreased by 32% compared with that of the untreated material. Meanwhile, at 190 °C for 3 h, the L* value of bamboo decreased by 52% compared with that of the untreated material. Therefore, at the same treatment temperature, the lightness of bamboo decreased with time. With the increase in temperature, there was a greater and more significant temporal effect. At 160 °C for 4 h, the L* of bamboo decreased by 47% compared with that of untreated bamboo, and the effect of time was more significant than that of 3 h. At 160 °C and 4 h, the L* of bamboo decreased by 47% relative to untreated wood and increased by 15% compared to 3 h. At 190 °C, 4 h, the L* of bamboo decreased by 93% compared to untreated wood and increased by 41% compared to bamboo treated for 3 h. The bamboo colour change is caused by a series of complex physical and chemical processes, the most fundamental of which is the increase and decrease in the basic chromophores and chromophore groups [14]. This can be attributed to the increase and decrease in the colour-generating and colour-enhancing groups. Compared with hot-air treatment, cellulose, and hemicelluloses in bamboo, and some other polysaccharides in the state of high temperature and high humidity, are more prone to pyrolysis in the hydrothermal treatment. With an increase in treatment temperature and time, it is more likely to generate furfural and some other phenolic compounds with chromophores, requiring hydrothermal treatment at lower temperatures to change its brightness. Meanwhile, the increase in the relative lignin content allows the absorption spectrum to extend into the visible region, causing the colour of bamboo to become dark [15].

With the hydrothermal treatment temperature at 160 °C, a* increased compared to the untreated material. Hydrothermal treatment of 3 h a* increased from 160 °C to 170 °C, and reached the maximum value at 170 °C, indicating that, at this time, the centre of the red–green axis of the change occurred in the 170 °C to 190 °C, a* with the increase and decrease in temperature. This indicates that at this time it began to change in the direction of the green axis. Hydrothermal treatment of 3.5 h first showed an increase followed by a decrease and then a further increase. Hydrothermal treatment of 4 h a* with a gradual increase to 190 °C, compared to the untreated material, resulted in the greatest increase, of 234%. The a* of hydrothermal treatment for 3.5 h first increased, then decreased, and then increased. The a* of hydrothermal treatment for 4 h gradually increased, and the greatest increase was 234% at 190 °C, compared with the untreated material.

With the increase in temperature, b* showed a general decreasing trend. At 190 °C and 4 h, the b* of treated bamboo decreased by 198% compared with that of untreated material. The effect of treatment time on b* was more significant with the increase in treatment temperature.

3.1.2. Effect of Hydrothermal Treatment on the Overall Colour Difference of Bamboo

The colour change of bamboo after hydrothermal treatment is shown in Figure 3. With the increase in treatment temperature and duration, △E* shows a gradual increase. The higher the temperature, the greater the slope of the increase. Temperature is a significant factor affecting the change of △E*. After treatment, the bamboo gradually deepened and darkened. The analysis showed that, in addition to the role of the three major elements of bamboo, part of the water-soluble extractives will also cause changes in the colour of bamboo [16]. This includes phenols, which contain unsaturated structures. Therefore, it is relatively simple to heat which can lead to discolouration, along with the internal and external moisture at the surface, which can cause surface colour changes [17].

3.2. Spss Analysis

Table 2. The multivariate ANOVA for △L*.

Sources of Divergence	Sum of Squares	Degree of Freedom	Mean Square	F	p-Value
Intercept	56,052.93	1	56,052.93	16,4261.7	0.000
Temperature	2793.16	3	931.053	2728.428	0.000
Time	1702.276	2	851.138	2494.239	0.000
Temperature × Time	515.644	6	85.941	251.847	0.000
Error	8.19	24	0.341
Total	61,072.2	36

and

Table 3. The multivariate ANOVA for △E*.

Sources of Divergence	Sum of Squares	Degree of Freedom	Mean Square	F	p-Value
Intercept	61,370.153	1	61,370.153	57,6155.4	0.000
Temperature	3237.373	3	1079.124	10,131.04	0.000
Time	2863.992	2	1431.996	13,443.87	0.000
Temperature × Time	1661.479	6	276.913	2599.717	0.000
Error	2.556	24	0.107
Total	69,135.553	36

show temperature, time, and interactions (temperature × time) on the △L* and △E* multivariate ANOVA, respectively. As shown in Table 2 and Table 3, the impact on material colour is based on △L* and △E*. The colour of the material changes significantly with temperature and time (p < 0.05).

3.3. Effect of Hydrothermal Treatment on the Mechanical Properties of Bamboo Wood

3.3.1. Effect of Hydrothermal Treatment on the Hardness of Bamboo Wood

As shown in Figure 4, the change in hardness of the bamboo material decreases with the increase in duration and temperature of the hydrothermal treatment. The effect of temperature change on the hardness is more prominent. At 160 °C, and under the 170 °C treatment, a magnitude of decrease of three times is comparable. At 180 °C, the 4 h treatment hardness decreased more than that of the 3 h treatment and was significantly enhanced with 3.5 h of treatment. This is because, at 180 °C, 4 h of treatment of the bamboo material occurred after carbonization, and the surface was cracked. With the 190 °C treatment, the bamboo material cracked, so the hardness decreased substantially [18]. At 190 °C with treatment for 4 h, the hydrothermal treatment has a greater effect on bamboo hardness.

3.3.2. Effect of Hydrothermal Treatment on Flexural Modulus of Elasticity and Flexural Strength of Bamboo Wood

Hydrothermal treatment softens mainly through water molecules in the cellulose amorphous zone, hemicelluloses, and lignin wetting and swelling effects, thereby increasing the space needed for the molecules to move vigorously. Meanwhile, heating promoted an increase in the molecular energy in the non-crystalline region. Under the combined action of water and heat, the cellulose amorphous zone becomes wet and swollen, the lignin is relatively fluid, the hemicelluloses lose their linkage, and bamboo plasticity increases. Therefore, for the modulus of elasticity, a flexural modulus of elasticity was chosen as the evaluation index of the softening effect [19].

As shown in Figure 5, the changes in the flexural modulus of elasticity and flexural strength of bamboo generally decreased with increasing time and temperature of the hydrothermal treatment. The effect of temperature was more significant than that of time [20]. For this experiment, at 160 °C and 170 °C, the changes were relatively small. At 180 °C and 190 °C, the changes increased significantly. Therefore, the hydrothermal treatment can achieve sufficient softening of the bamboo material. Bamboo is mainly composed of cellulose, hemicelluloses, and lignin. As the softening temperature increases, the glass transition points of lignin and cellulose decrease significantly [21]. The plasticity of bamboo was greatly enhanced, the modulus of elasticity decreased [22], and the softening effect was enhanced.

When the softening temperature was relatively low, the bending modulus of elasticity of bamboo was relatively high, and the softening effect was relatively poor. As the temperature increases, the hydrogen bonds in the chemical components are damaged. The bonding force between the molecules is reduced, and the bamboo material becomes more prone to deformation [23]. When a certain temperature is reached, the chemical components of the bamboo material change. When the treatment temperature reaches the glass transition temperature, the modulus of elasticity of the bamboo material is further reduced and the plasticity is enhanced [24]. This indicates that the softening effect is more pronounced. When the treatment temperature reached the glass transition temperature, the modulus of elasticity of the bamboo material decreased and the plasticity increased, indicating that the softening effect improved [25,26]. Except for the specimens that were ruptured on the surface, the modulus of flexural elasticity of bamboo decreased by 17% at 180 °C for 3 h. Therefore, the softening effect of bamboo was found to be the most pronounced with treatment at 180 °C for 3 h [27].

3.4. Effect of Hydrothermal Treatment on XRD Properties

Due to the lack of significant influence over time, studying the influence of temperature is important. Figure 6 shows the XRD patterns of untreated and bamboo materials hydrothermally treated at four different temperatures for 3 h.

Table 4. The crystallinity of hydrothermal treatment materials.

	Processing Temperature (°C)	Processing Time (h)	Crystallinity (%)
1	Untreated	None	53.91
2	160	3	54.86
3	170	3	55.13
4	180	3	51.02
5	190	3	50.11

lists the crystallinity of the hydrothermally treated bamboo materials. As shown in Figure 6 and Table 4, the crystallinity of the untreated material was 53.91%. This increased to 54.86% after hydrothermal treatment at 160 °C for 3 h and continued to increase to 55.13% after hydrothermal treatment at 170 °C for 3 h. After treatment at 180 °C and 190 °C, the crystallinity of the bamboo material decreased to 51.02% and 50.11%, respectively. This is because the hydrothermal treatment at 160 °C and 170 °C made the cellulose quasi-cellulose and quasi-transferable [28]. This occurred because the hydrothermal treatment at 160 °C and 170 °C makes the hydroxyl group in the quasi-crystalline amorphous region of cellulose produce a polycondensation reaction to generate an ether bond. More water molecules can make the microfibrils in the quasi-crystalline region arranged in a more ordered manner. This means that it is close to the crystalline region, and the degree of crystallinity is improved compared to untreated [29]. Therefore, there was an improvement in crystallinity. After treatment at 180 °C and 190 °C, the crystallinity decreased by 5% and 7%, respectively, compared with that of the untreated material [30]. The internal carbonization first hydrolyzed hemicelluloses, and the hydrolysis resulted in the degradation of acetic acid in the crystalline zone. This led to a decrease in crystallinity. This indicates that hydrothermal treatment had a greater effect on the crystallinity of cellulose.

3.5. Effect of Hydrothermal Treatment on the Micro-Morphology of Bamboo Wood

Figure 7 shows the changes in the micromorphology of the cross-section of bamboo before and after hydrothermal treatment at 180 °C for 3 h, the slices after hydrothermal treatment at 180 °C for 3 h, and the changes in vascular bundles and thin-walled cells. Observations were made before and after the hydrothermal treatment at 1 mm, 500 μm, and 200 μm levels at 60-fold, 200-fold, and 500-fold magnifications, respectively. The cell structures of untreated bamboo are shown in Figure 7a–c. The cell structures of untreated bamboo are shown in Figure 7d–f, which show the cell structure of bamboo after treatment at 180 °C for 3 h.

A comparison of Figure 7a,d show that the bamboo cells were subjected to hydrothermal treatment to produce partial dehiscence, and cracks appeared between adjacent thin-walled cells. Figure 7b,e show that bamboo cells were subjected to hydrothermal treatment to rupture the lignin of the vascular bundles. The internal conduits were opened, which facilitated the deep infiltration of the liquid in the bamboo [31]. The conduits then became smaller, and the arrangement of the cells was more tightly arranged, which improved the degree of densification. Figure 7c,f show that starch local disappearance occurred after hydrothermal treatment [20]. This is because under the action of hydrothermal treatment, the cell wall of the osmotic channel was damaged, and part of the cell inclusions was removed [32].

3.6. Effect of Hydrothermal Treatment on the Infrared Spectral Properties of Bamboo Materials

Due to the lack of significant influence over time, studying the influence of temperature is important. Figure 8 shows the infrared absorption spectra of untreated and hydrothermally treated bamboo at different temperatures for 3 h at wave numbers from 4000 to 500 cm⁻¹. The intensity of the hydroxyl stretching vibration absorption peaks at 3425 cm⁻¹ decreased under different temperature treatments compared with untreated bamboo. Treated at 160 °C and 170 °C, given the simultaneous action of high temperature and hydrothermal treatment, the free hydroxyl group of the cellulose molecular chain and glucose in the bamboo processed residue dehydrated to form an ether bond. The intensity of hydroxyl absorption peaks in hydrothermal treatment at 180 °C and, at 190 °C, continued to decrease [33]. This indicated that the carbonization occurred during the carbonization of bamboo and continued to decrease, which showed that dehydration of the bamboo material occurred during carbonization. With an increase in temperature, the C=O vibration peak at 1670 cm⁻¹ was also gradually weakened. The carbonyl absorption peak at 1650 cm⁻¹ corresponded to the lignin-conjugated carbonyl group [34]. After hydrothermal treatment, the absorption peak decreased significantly, mainly because of hydrolysis of the acetyl group after the hydrothermal treatment, resulting in the reduction of the carbonyl group [35]. Meanwhile, the acetyl group produced in the process of the reaction also accelerated the decomposition speed of the carbonyl group to 1270 cm⁻¹ for the C-O-C aromatic ether bond [36]. This is because of the free hydroxyl group between the cellulose molecular chain in the high-temperature condensation reaction, shedding excess water to form the ether bond [37]. An ether bond is formed by the free hydroxyl groups between the cellulose molecular chains under a high-temperature condensation reaction, shedding excess water to form an ether bond. This analysis has shown hydrothermal treatment had no effect on the characteristic cellulose peaks and resulted in a decrease in hemicelluloses and lignin characteristic peaks [28].

3.7. Effect of Hydrothermal Treatment on Nanomechanical Properties of Bamboo Materials

Due to the lack of significant influence over time, studying the influence of temperature. Figure 9 shows that the hardness and modulus of elasticity of untreated bamboo were 0.24 GPa and 5.5 GPa. Hydrothermal treatment had a significant effect on the hardness and modulus of elasticity of the bamboo. The elastic modulus and hardness of the bamboo cell wall showed an increasing trend from 160 °C to 180 °C, and the elastic modulus and hardness decreased at 190 °C. The nanomechanical properties of bamboo cell walls are affected by cellulose crystallinity, chemical composition, water content, density, and lignin content [12,38]. Lignin water content, main chemical composition, cellulose crystallinity, and condensation polymerization affected the mechanical properties of the bamboo cell walls. When the temperature was less than 180 °C, the modulus of elasticity and hardness gradually increased. This may be because of the arrangement of cellulose microfilaments in the bamboo cell walls and changes in the matrix [39]. The decomposition of lignin leads to the condensation reaction of lignin and its cross-linking reaction with byproducts. This is a key reason for the increase in the average elastic modulus and hardness of the bamboo cell walls [40]. At 190 °C, a decrease in elastic modulus and hardness was observed, with a decrease in elastic modulus of up to 52% and a decrease in hardness of up to 59%. This was because of the increase in treatment time. Part of the cellulose chain decomposed, and the compact structure was broken, resulting in a decrease in the modulus of elasticity and hardness of the bamboo cell wall. This trend was consistent with the crystallinity index [31].

3.8. Effect of Hydrothermal Treatment on Chemical Compositions of Bamboo Materials

Figure 10 shows the contents of cellulose, hemicellulose, and lignin. The relative cellulose, hemicellulose, and lignin were 48.9%, 31.2%, and 19.6%, respectively. Hydrothermal treatment had a minimal impact on cellulose content. As the temperature increased and time prolonged, the cellulose content slowly decreased; the maximum decrease was 13.09% compared to untreated. The hydrothermal treatment had a significant impact on hemicellulose, and with the increase in temperature and the extension of time, the hemicellulose content showed a decreasing trend. When subjected to hydrothermal treatment at 180 °C for 4 h, the content of hemicellulose decreased by 75.32% compared to untreated samples. This is because xylan is the main component of hemicellulose, which is more unstable compared to other chemical components. Therefore, they easily degraded at high temperatures [41]. The changes in lignin were opposite to those of cellulose and hemicellulose owning to the good thermal stability of lignin or lignin condensation and cross-linking reactions [12]; as the hydrothermal treatment temperature and time increased, there was an upward trend.

4. Conclusions

(1)

Hydrothermal treatment had a strong softening effect on bamboo. The bamboo hardness decreased with the increase in softening temperature and the extension of softening time. The softening effect was most pronounced at 180 °C and 3 h, and the hardness decreased up to 47%. The flexural strength and flexural elastic modulus of bamboo showed a tendency to increase and then decrease with an increase in temperature, and a tendency to decrease with time. Under the condition of 180 °C and 3 h treatment, the flexural strength decreased by up to 41%, and the flexural elastic modulus decreased by up to 17%.

(2)

Under hydrothermal treatment, with increasing temperature and prolonged time, the colour of the bamboo material changed from bright yellow to dark brown. The brightness showed a decreasing trend, which could decrease by up to 93%, and the overall colour difference increased.

(3)

With the increase in hydrothermal treatment temperature, the crystallinity of bamboo material first increased and then decreased. The infrared spectra of hydrothermally treated bamboo material showed that the hydrothermal treatment had no effect on the characteristic peaks of cellulose. It resulted in changes in the characteristic peaks of hemicellulose and lignin. The cell structure produced part of the cracks, and the starch completely disappeared.

(4)

The hardness and modulus of elasticity of the thin-walled bamboo cells first increased and then decreased with increasing treatment temperature. At 190 °C, the modulus of elasticity and hardness decreased, with the modulus of elasticity decreasing up to 52% and the hardness decreasing up to 59%.

(5)

With the increase in hydrothermal treatment temperature and the extension of time, the cellulose content slowly decreases, the hemicellulose content significantly decreases, and the lignin content increases.

This study provides a new theory for bamboo softening, which is beneficial for subsequent flattening and processing applications of bamboo.