Improving Biomass-Degradation Properties and Nano-Mechanics of Moso Bamboo via a Simple Nitrogen Heat Treatment

Abstract

Nitrogen is generally used as a protective gas to provide an oxygen-free environment for the heat treatment of biomaterials. In order to indicate the effect of nitrogen heat treatment of bamboo, the changes in terms of the chemical composition, chemical functional groups, cellulose crystallinity index, surface color, micro-mechanics and anti-mildew properties of bamboo, and the interaction relationship among the properties, were analyzed. The mass loss ratio of treated bamboo samples increased significantly during the process of thermal modification. In detail, the hemicellulose exhibited a decreasing tendency from 23.7% to 16.6%, while the lignin content presented an increasing tendency. The decreased hemicellulose and cellulose contents are a benefit to enhancing lignin content and crystallinity degree, thus increasing the modulus of elasticity and hardness of treated bamboo cell walls. The obtained bamboo sample treated at 190 °C/3 h displayed the best micro-mechanical properties. It presented a maximum modulus of elasticity of 22.1 GPa and a hardness of 0.97 GPa. Meanwhile, the lignin and cellulose content was proven to increase in the bamboo surface in chemical composition analysis, resulting in lower free-hydroxyl groups on the bamboo surface. Thus, the contact angle value of bamboo increased. Furthermore, nitrogen thermal modification positively contributed to the mildew resistance of bamboo specimens.

1. Introduction

Moso bamboo has attracted attention in recent decades because of its high yield, rapid growth, sustainability, and excellent mechanical properties. However, traditional engineering bamboo products, such as bamboo plastic composites, bamboo scrimber, bamboo plywood, and wood–bamboo composites have lost their original appearance [1,2,3]. In addition, the grain and elegant texture of the natural surface cannot be found in these traditional engineering products and the large natural bamboo surface is lost [4]. Therefore, bamboo and bamboo products in their natural and original appearance have recently attracted more attention. Additionally, natural bamboo tubes have been used as potential candidates for traditional engineering bamboo products in some construction and decoration areas. Unfortunately, natural bamboo tubes are easily affected by UV radiation, moisture content, and fungi due to their abundance of starches and carbohydrates [5]. To increase the outdoor utilization of natural bamboo tubes, improving the resistance to fungi and mechanical properties of natural bamboo tubes is important [6,7,8].

It is well known that thermal modification (TM) is a useful and feasible method to increase the durability and decrease the hygroscopicity of bamboo and bamboo-based products. As is known, bamboo consists of three main chemical compositions (hemicellulose, cellulose, and lignin), ash, and extractives. When the treatment temperature is above 150 °C, thermal modification can effectively decrease the hygroscopicity and dimensional stability of bamboo products [9,10]. When the treatment temperature increases from 150 °C to 180 °C, the anti-fungal properties of bamboo are enhanced due to the decomposition of hemicellulose and a large amount of starches. In the past, heat-treatment modification, hot oil, hot air, hot water, saturated steam, and so on were usually used as treatment mediums. For example, Wang et al. treated Moso bamboo under different temperatures and durations and reported that the modulus of elasticity, density, and EMC showed a decreasing tendency [11]. The decrement of density can be attributed to the mass loss due to bamboo decomposition during saturated steam heat treatment. Similar conclusions have also been reported by Yuan et al., Wang et al., and Lou et al. [12,13,14]. Hao et al. investigated the effects of oil heat treatment on Moso bamboo [15]. The mass loss of treated bamboo specimens increased as a function of treatment severity. At the same time, the dimensional stability of the bamboo samples was enhanced due to the lower moisture absorption. However, the above-mentioned studies focus on the effects of heat treatment on the macro-mechanical properties of bamboo such as density, mass loss, modulus of elasticity, and modulus of rupture. Few studies have revealed the effects of thermal modification on bamboo on a nano-scale. In addition, bamboo usually gives off a toxic and unpleasant smell after heat treatment due to the chemical reaction of volatile by-products with oxygen during the thermal decomposition process. Additionally, these traditional treatment mediums can lead to the loss of MC in bamboo, resulting in obvious cracks occurring on the bamboo surface.

Even if many studies have reported the effect of traditional heat treatment mediums (oil, saturated steam, hot water, hot air, and so on) on the macro-physical/mechanical properties of wood and bamboo, no data have been reported on the effect of such thermal modification on bamboo chemical components and micro-mechanical properties under a nitrogen environment. In fact, it is obvious that an inert atmosphere used during thermal modification can directly affect thermo-decomposition reactions and finally affect the properties of the bamboo or woody materials. Recently, some studies have reported a new thermo-decomposition process based on the use of a vacuum [16,17,18]. In these studies, it was found that the vacuum environment can effectively remove volatile by-products and accelerate the decomposition of chemical composition in woody cell walls. Finally, the materials do not emit unpleasant gases after treatment. The existence of the chemical composition of bamboo determines the micro-mechanical properties of biomass materials. A nanoindentation is a useful approach for investigating the change of micro-mechanical properties of woody material, bamboo, or biomass materials on a nano-scale. It can help readers who want to deeply understand the thermal modification mechanism at the cell-wall level. In addition, in outdoor applications, bamboo is often attacked by fungi in natural environments, resulting in serious reduction of mechanical properties and shortened service life [19]. Thus, the anti-fungal property of the bamboo specimens, investigated through incubate samples with Aspergilus niger, is important [20].

Here, in this paper, we aimed to investigate the effect of nitrogen thermal treatment on the chemical composition, chemical functional groups, cellulose crystallinity, and micro-mechanical properties on bamboo. The bamboo strips were heat-treated under nitrogen heat treatment at different temperatures (150, 170, 190, and 210 °C) and times (1, 2, 3, and 4 h). This paper aims to explore the effect of heat treatment on the macro-/micro-properties of bamboo samples in an oxygen-free environment through Fourier-transform infrared (FTIR), X-ray diffraction (XRD), the wet chemistry method, nanoindentation (NI) and the anti-mildew property test. This newer heat treatment strategy provides a innovative and feasible solution for bamboo-processing industries.

2. Materials and Methods

2.1. Materials and Thermal Modification

Natural six-year-old bamboo was collected from Fujian, China. Nitrogen was used as a protective gas to provide an oxygen-free environment for the heat treatment. Firstly, the moso bamboo was split and cut into bamboo strips with an average size of 100 mm × 20 mm × 6 mm (length × width × thickness). The initial density and moisture content of moso bamboo were 0.75 g/cm³ and 85%. The heat parameters were conducted at 150, 170, 190, and 210 °C for time periods of 1, 2, 3, and 4 h. A tube furnace (GSL-1700X-80-HNG, Jingmi Technology., Ltd., Shanghai, China) was used as heat treatment equipment to provide a nitrogen environment. The experimental process of sample preparation and thermal modification is shown in Figure 1.

2.2. Mass Loss

The mass loss of untreated and treated bamboo samples was measured by weighing scales. The mass loss can be calculated as below:

$${\mathit{M}}_{\mathbf{0}}= \frac{\left({\mathit{m}}_{\mathbf{0}}-{\mathit{m}}_{\mathbf{1}}\right)}{{\mathit{m}}_{\mathbf{0}}}\times \mathbf{100} \%$$

(1)

where M₀ represents the oven-dried samples’ mass before the thermal modification, and M₁ represents the oven-dried samples’ mass after the thermal modification.

2.3. X-ray Diffraction (XRD) Test

X-ray diffraction (XRD) measurements were performed on an X-ray diffraction (XRD) diffractometer (D8 ADVANCE, BRUKER, Germany). The scan range was from 5° to 60°. In addition, the crystallinity index of the samples were calculated by Segal’s formula [19,20,21]. Each experiment was repeated 3 times to obtain the average value. Segal’s formula can be calculated as below:

$${\mathit{C}}_{\mathit{r}}\mathit{I}=\frac{\left({\mathit{I}}_{\mathbf{002}}-{\mathit{I}}_{\mathit{a}\mathit{m}}\right)}{{\mathit{I}}_{\mathbf{002}}}\times \mathbf{100} \%$$

(2)

where I₀₀₂ and Iam represent both crystalline and amorphous parts, and only the intensity of amorphous parts, respectively.

2.4. Fourier-Transform Infrared Spectroscopy (FTIR) Test

Fourier-transform infrared spectroscopy (FTIR, Escalab 250XI, Thermo Fisher, Waltham, MA, USA) was applied to determine the change in functional groups in bamboo samples after nitrogen heat treatment at different temperatures and durations. The tested wavenumber range was 400–4000 cm⁻¹, and the resolution was 4 cm⁻¹. Each experiment was repeated 3 times to obtain the average value.

2.5. Chemical Composition Test

A total of 300 mg of the sample was weighed and placed at the bottom of the glass tube, 3 mL of 72% H₂SO₄ was added, and the mixture stirred thoroughly with a glass rod. The test tubes were placed in a water bath for 60 min. Then, 84 mL of deionized water was added to dilute the concentrated sulfuric acid to 4%. The solution was washed to neutral with water through a vacuum pump. The hydrolysate and the sample residue after reaction were used to measure the hemicellulose and lignin content, respectively. The cellulose content was obtained from the known hemicellulose and lignin content. The main chemical compositions were cellulose, hemicellulose, and lignin [21]. The methods of wood or bamboo chemistry were performed according to NREL’S LAPS [22]. Liquid chromatography (Agilent 7890A Series, Thermo Fisher) was applied to determine the content of hemicellulose and lignin. The content of cellulose was obtained from the known hemicellulose and lignin content. Each experiment was repeated 3 times to obtain the average value.

2.6. Nanoindentation Test

Nanoindentation (NI) is a useful technology that can analyze the nano-mechanics of bamboo from the cell-wall level. To analyze the effects of heat treatment on the nano-mechanics of bamboo samples, NI was used. The detailed nanoindentation equipment (IMicro Inc., Minneapolis, MN, USA) and corresponding bamboo samples are shown as Figure 2:

The test location of bamboo cell wall and typical load-depth nanoindentation curves are shown in Figure 3A,B. The bamboo specimens with an average size of 5 mm × 3 mm × 3 mm (L × W × H) were prepared according to the quasi-static nanoindentation procedure reported by the previous literature [19]. Secondly, the cross-section for the nanoindentation test was polished with a diamond knife and then placed in a constant temperature humidity chamber under 25 °C and 65% (humidity) for 48 h. During the nanoindentation test, the bamboo specimens were fixed on a sample holder. The hardness and modulus of elasticity of bamboo specimens can be calculated by obtaining load–displacement curves [23]:

$$\mathbf{H}=\frac{{\mathbf{P}}_{\mathbf{m}\mathbf{a}\mathbf{x}}}{\mathbf{A}}$$

(3)

where Pmax is the maximum loading force, and A is the area of the indents at maximum loading force;

$${\mathit{E}}_{\mathit{r}}=\frac{\sqrt{\mathit{\pi }}}{\mathbf{2}\mathit{\beta }}\frac{\mathit{S}}{\sqrt{\mathit{A}}}$$

(4)

where Er is the combined elastic modulus of the sample and indenter, and S is the slope of the initial unloading stiffness.

2.7. Anti-Mildew Property Test

All the tests were carried out according to the Chinese National Standard (GB/T 18261-2013). The untreated and treated bamboo samples were cut to an average size of 50 mm × 20 mm × 5 mm (L × W × H), followed by being placed on cultured molds for the mildew-resistance test. Finally, the Petri dishes were put into an incubator at 25 °C and 85% (humidity) for 30 days. The changes in the surface of the bamboo samples and the infection ratio during the anti-mildew test were recorded by camera every two days.

2.8. SPSS Analysis

SPSS software was applied to determine the significant difference between the groups. The characteristic values of normality and homogeneity of variances were applied to analyze the results of the measured data [24]. Duncan multiple range tests were conducted to determine the significance between samples. The randomized block design was used in this experiment to minimize the effect of raw material variation on the test results.

3. Results and Discussion

3.1. SEM Analysis

The microstructure of the untreated and treated bamboo specimens were further observed by SEM, as shown in Figure 4A–C. Before thermal modification, the pure bamboo samples exhibited a natural porous three-dimensional structure. When the treatment temperature was increased to 170 °C, many cracks were generated in the sclerenchyma cell, which may be attributed to the decomposition of chemical composition between the sclerenchyma cells [25]. When the treatment temperature was increased to 190 °C, the three-dimensional structure network structure still remained, but the thin-wall cells distorted and deformed because of the thermo-decomposition reaction. Hemicellulose acts as a matrix in the cell structure and fills in between the cellulose [26]. Thus, more surface area of the cellulose is exposed, leading to the interfibrillar hydrogen-bonding interaction among the cellulose fibers, thus resulting in the deformation of parenchyma cells. Because hemicellulose acts as a substrate filled with cellulose, as the hemicellulose degrades, less hemicellulose is attached to the cellulose skeleton, so more area is generated on the cellulose surface [27]. The connection between cellulose is mainly based on the intermolecular force. With the reduction of hemicellulose, the matrix filling between cellulose is reduced, which leads to the reduction of hydrogen bonding. Thus, the parenchyma cell structure shrinks and deforms sharply during thermal modification after the decomposition of hemicellulose and cellulose content, resulting in the production of more gaps between sclerenchyma cells and parenchyma cells [28].

3.2. Mass Loss

Figure 5 shows the mass loss of the different bamboo specimens. As shown in Figure 5, the mass loss positively corresponded to the treatment severity. However, no significant difference can be observed between groups under 150 °C. With the increasing severity of treatment parameters, the mass loss ratio exhibited the highest value (7.8%) when the treatment parameters were set at 210 °C and 4 h, which was determined by the chemical composition. After the heat treatment, the main chemical compositions decreased, and thus the mass was lower than those of the control [28].

3.3. Main Chemical Components

The method reported by NERL’S LAPs was applied for the determination of cellulose, hemicellulose, and lignin in bamboo.

Table 1. Chemical composition proportions of treated bamboo samples after different temperature parameters.

Time	Hemicellulose (%)				Cellulose (%)				Lignin (%)
	150 °C	170 °C	190 °C	210 °C	150 °C	170 °C	190 °C	210 °C	150 °C	170 °C	190 °C	210 °C
Control		25.1				46.2				28.7
1 h	23.7	24.8	23.2	22.4	44.6	45.2	44.4	44.2	32.0	30.7	30.3	32.3
2 h	23.7	23.4	21.9	19.8	46.0	45.5	45.3	44.1	31.1	32.5	33.4	34.1
3 h	23.9	22.8	21.6	20.1	45.6	44.4	44.0	43.8	31.4	33.1	32.9	36.2
4 h	24.1	22.2	20.1	16.6	45.6	44.1	43.6	43.6	28.9	32.8	36.1	36.1

shows the chemical compositions of the different bamboo specimens. The difference between the relative content of the main chemical composition was not too obvious when the bamboo sample was treated at 150 °C. Therefore, these results confirmed the previous literature that reported on thermal modification above 150 °C. Namely, the increment of treatment parameters induce negative effects on hemicellulose and cellulose content. Table 1 shows that hemicellulose and cellulose content decreased to 16.6% and 43.6% after 210 °C/4 h treatment. The lignin content increased from 29.0% to 36.1%. This is due to lignin recondensation [28,29,30]. Accordingly, the lignin increased from 28.7 (control) to 36.1% (210 °C/4 h), which is a co 24.9% increment, indicating that the increment of treatment temperature further promoted the decomposition of hemicellulose’s by-products. According to the previous literature [31], lignin condensation can also contribute to the incrementation of lignin content. Among the structural units of lignin, the carbon atom of the side chain of one lignin molecule is C-linked with the carbon atom of the benzene ring of another structural unit, thus forming the lignin condensation with large molecular weight [32]. As a result, the lignin becomes difficult to degrade and the lignin content increases. The results from the changes in chemical components confirmed the observation of mass loss [33].

3.4. Fourier-Transform Infrared Spectroscopy (FTIR) and X-ray Diffractor (XRD) Analysis

Fourier-transform infrared spectroscopy (FTIR) was used to investigate the effect of thermal modification on the chemical functional groups in bamboo. The FTIR curves of untreated and treated bamboo specimens from 500 cm⁻¹ to 4500 cm⁻¹ are shown in Figure 6. The spectra of bamboo present the typical peaks of bamboo: 1731 cm⁻¹ (C=O strength vibrations of hemicellulose), 1230 cm⁻¹ (C-O strength vibration peaks), 1590 cm⁻¹ (stretching of carboxylic acid), 1425 cm⁻¹ (strengthening of acetyl acid). It can be seen from Figure 7 that there was no significant difference in the FTIR spectra between untreated and treated bamboo specimens. The relative intensity of the peak at 1370 cm⁻¹ did not change too much due to the stability of C-H, indicating that C-H remains unchanged during the thermal modification. The relative intensities of the peaks at 1230 cm⁻¹ and 1730 cm⁻¹ showed a decreasing trend in comparison to those of untreated bamboo samples, which can be attributed to the decomposition of the cellulose and hemicellulose in bamboo samples. During the heat treatment process, the relative intensities of the peaks at 1630 cm⁻¹ and 1590 cm⁻¹ increased with the increase of the treatment temperature. This can be attributed to the increase in lignin content. The condensation reaction of lignin contributes to the increase in relative lignin content and enhances the dimensional stability of bamboo specimens. Lastly, the relative intensity of the peak at 898 cm⁻¹ obviously decreases, which possibly results from the acid environment provided by the decomposition of hemicellulose.

The cellulose crystallinity index was investigated by XRD and is shown in Figure 7 and Figure 8. The average grain size and 002 half-peak width of bamboo specimens did not show a significant difference. Interestingly, the crystallinity index increased with increasing temperature and time. Figure 8 presented that the initial crystallinity index of untreated bamboo specimens was 54.9%. The crystallinity degree of the bamboo sample increased from 54.9% to 57.6% (210 °C/3 h). These results demonstrated that high temperature had a positive effect on the crystallinity index. It is well known that the hemicellulose and paracrystalline parts of cellulose determine the crystallinity index [33,34]. During the heat treatment process, the paracrystalline parts of cellulose and hemicellulose decreased, leading to the increment of the crystallinity index. However, when the treatment time was prolonged to 4 h, the crystallinity showed a decreasing tendency. The cellulose structure breaking under high temperature and pressure contributed to this observation.

3.5. Contact Angle Analysis

The water contact angle of the bamboo surface is an important property for outdoor bamboo applications, which is determined by its chemical composition change. The surface hydrophobicity of thermally modified Moso bamboo was characterized by a contact angle test. Figure 9 presents the contact angle of untreated bamboo specimens as 48.5°. After nitrogen thermal modification, the contact angle of the treated bamboo specimens increased from 48.5° (control) to 105.3° (170 °C/3 h). These results may be attributed to the decomposition of a free hydroxyl group (-OH), which is mainly existed in hemicellulose. During the heat treatment process, the force between hydroxyl groups and hemicellulose weaken, thus the hydroxyl group decreased. The increment of water contact angle value represented the increment of bamboo surface hydrophobicity, it can be attributed to the thermal modification. For detail, dense bamboo cell walls with low surface energy is formed on the heat-treated bamboo samples [35,36]. Meanwhile, the lignin is proved to increase in the bamboo surface in chemical composition analysis, resulting in lower free-hydroxyl groups on the bamboo surface. Thus, the contact angle value of bamboo increased.

3.6. Micro-Mechanics of Bamboo

Figure 10 shows the nano-mechanics of different bamboo specimens. Generally, the treated bamboo samples exhibited higher hardness and modulus of elasticity. Namely, with increasing treatment temperature, the nano-mechanics of bamboo specimens showed an increasing tendency. For instance, when treated under 190 °C/3 h, the hardness of bamboo nano-mechanics increased from 0.58 GPa to 0.96 GPa. The hardness and modulus of elasticity of bamboo cell walls are affected by many factors, such as moisture content, cellulose microfibril angle, cellulose crystallinity degree, hemicellulose, and lignin. As the tested bamboo samples were prepared from the same layer of bamboo, so the microfibril angle and the chemical composition of the bamboo specimens were almost the same before the thermal modification. According to the previous literature [33,34], the formation of carbonic acids, the cleavage of acetyl groups in hemicellulose, and lignin re-condensation can affect the micro-mechanical properties of the bamboo specimens. The increased lignin content can contribute to an increase in bamboo micro-mechanical properties.

3.7. Anti-Fungal Properties

Bamboo is often attacked by fungi in outdoor applications, resulting in a serious reduction of mechanical properties and short service life. Thus, the anti-fungal property of the bamboo specimens was investigated through incubating samples with Aspergilus niger. Figure 11 shows the mildew resistance of the hydrothermally modified bamboo. Figure 11 shows that the infection ratio of the control reached 100% after only 8 days of incubation. However, after hydrothermal modification, the mildew resistance property of bamboo samples increased. In other words, thermal modification is a positive influence on the mildew-resistance property of bamboo. For example, after treatment at 190 °C, the mildew proportion of bamboo specimens remained 0% in the first 8 days, while the bamboo samples which were treated at 150 °C presented a very poor anti-fungal property in comparison to the bamboo samples (190 °C). These changes indicate that the anti-mildew property of bamboo samples positively corresponded to the treatment parameters. This is due to the decomposition of polysaccharide and starch in bamboo samples and the enhanced surface hydrophobicity. In detail, the enhanced surface hydrophobicity inhibits the bamboo samples from absorbing water, and thus effectively reduces the adhesion between Aspergilus niger and bamboo samples [35,36]. In addition, the decrement of polysaccharide and starch hinders the growth of Aspergilus niger. A schematic diagram of the thermal modification mechanism is presented in Figure 12.

4. Conclusions

The surface morphology, chemical components, crystallinity index, nano-mechanics and anti-mildew property of bamboo after heat treatment were characterized in this paper. The results showed that the thin-wall fiber cell deformed and distorted due to the decomposition of the main chemical components in bamboo cell walls. In detail, the hemicellulose exhibited an decreasing tendency from 23.7% to 16.6%. The obtained bamboo sample treated at 190 °C/3 h displayed the best micro-mechanical properties. It presented a maximum modulus of elasticity of 22.1 GPa and the hardness was 0.97 GPa. Meanwhile, the lignin was proven to increase in the bamboo surface in chemical composition analysis, resulting in lower free-hydroxyl groups on the bamboo surface. Thus, the contact angle value of bamboo increased. Nitrogen thermal modification made positive contributions to the mildew resistance of bamboo specimens. Therefore, the biomass degradation properties and nano-mechanics of moso bamboo can be enhanced through a simple nitrogen heat treatment.