KR102347896B1 - A method of extracting cellulose nanofibers from bamboo fibers - Google Patents

Abstract

The present invention relates to a method for extracting cellulose nanofibers from bamboo fibers in an eco-friendly manner by combining microwave liquefaction, bleaching treatment, and low-energy defibrillation processes of the bamboo fibers, comprising: a first step of liquefying a bamboo-containing solution by microwave irradiation; a second step of bleaching the residue generated through the liquefaction using hydrogen peroxide; a third step of subjecting the bleached residue to alkali treatment and acid treatment to obtain cellulose fibers; and a fourth step of defibrillating the cellulose fibers using a low-energy blender.

Description

A method of extracting cellulose nanofibers from bamboo fibers

The present invention relates to a method for environmentally friendly extraction of cellulose nanofibers from bamboo fibers by combining the microwave liquefaction, bleaching, and low-energy non-fibrillation processes of bamboo fibers.

Lignocellulosic biomass is emerging as a natural-based resource due to its low abundance, mass production potential, renewable potential, biodegradability, biocompatibility, greenhouse gas emissions and carbon-neutral emissions.

The main constituent materials of lignocellulosic biomass are cellulose, hemicellulose and lignin, which are the most commonly used renewable polymers in the biosphere due to their relatively low content of hemicellulose and lignin.

On the other hand, studies to utilize cellulose as a nano material due to the hierarchical nature, self-assembly characteristics, crystal structure, anisotropic behavior and high mechanical properties of cellulose are also being actively conducted. , reinforcing materials, thin films, energy devices, catalysts, etc.

In order to utilize the cellulose as a nano material, a process of removing the amorphous component from the raw material, obtaining high-purity cellulose fibers, and then extracting the nano-cellulose fibers is required.

To this end, conventionally, a method of separating a lignin polymer from a raw material using an acid-chloric acid delignification process has been performed. However, when the process is used, it has a detrimental effect on the oxidative decomposition of polysaccharides, and these chemicals are environmentally and There are problems that negatively affect human health.

One object of the present invention is to provide a method for environmentally friendly extraction of cellulose nanofibers from bamboo fibers by combining microwave liquefaction, bleaching treatment, and low-energy non-fibrillation processes.

Another object of the present invention is to provide a cellulose nanofiber extracted through the above method.

A method of extracting cellulose nanofibers from bamboo fibers according to an embodiment of the present invention comprises: a first step of liquefying a bamboo-containing solution by irradiating microwaves; a second step of bleaching the residue generated through the liquefaction using hydrogen peroxide; a third step of subjecting the bleached residue to alkali treatment and acid treatment to obtain cellulose fibers; and a fourth step of defibrillating the cellulose fibers using a low-energy blender.

In an embodiment, the bamboo-containing solution may include diethylene glycol (DEG) as a solvent and sulfuric acid as a catalyst.

In one embodiment, the weight ratio of the bamboo and DEG is preferably 1: 4 (w/w).

In an embodiment, the first step may be performed for 6 minutes to 10 minutes at a power of 200 to 300 W.

In one embodiment, after the first step, vacuum filtration of the solution after the completion of microwave liquefaction, and drying the residue obtained after the vacuum filtration may be further included.

In an embodiment, the method may further include separating the liquid mixture obtained after vacuum filtration with a solvent to obtain a biopolyol.

In one embodiment, the second step is preferably performed at a temperature of 70 to 100 ℃ under the conditions of pH 4 to 5.

In an embodiment, delignification of bamboo fibers may be performed in the first and second steps.

In one embodiment, the alkali treatment may use a KOH solution, and the acid treatment may use a sulfuric acid solution.

In an embodiment, the fourth step may be performed by blending for 20 minutes to 1 hour at an operating speed of 23000 rpm to 25000 rpm and an output of 1600 to 2000 W using a low energy blender.

In an embodiment, in the third step, separation of hemicellulose from bamboo fibers may be performed.

On the other hand, the cellulosic nanofiber according to another embodiment of the present invention is extracted by the above method, the crystallinity according to the XRD analysis method is 80% or more, and the cross-sectional diameter of the fiber may be 5 to 30 nm.

The present invention can effectively separate delignification and hemicellulose from cellulose fibers by microwave liquefaction, bleaching, and chemical treatment of bamboo, a renewable eco-friendly non-wood biomass, and additional low-energy non-fibrillation treatment without the generation of toxic substances Cellulose nanofibers with high crystallinity can be extracted in an environmentally friendly way.

1 is an image showing the change of the residue and supernatant according to the extraction method of the present invention.
2 shows FTIR spectra of samples obtained according to the extraction step of the present invention. ((a) raw bamboo fiber, (b) microwave liquefaction residue, (c) bleached fiber, (d) cellulose fiber and (e) cellulose nanofiber)
3 shows XPS irradiation spectra of raw bamboo fibers, microwave liquefaction residues and cellulose nanofibers obtained according to the extraction step of the present invention.
_{4 shows C 1s} and O _1s XPS irradiation spectra of raw bamboo fibers, microwave liquefaction residues and cellulose nanofibers obtained according to the extraction step of the present invention.
5 shows an X-ray diffraction graph of samples obtained according to the extraction step of the present invention. ((a) raw bamboo fibers, (b) microwave liquefaction residues, (c) bleached fibers, (d) cellulose fibers and (e) cellulose nanofibers)
6 is an FE-SEM image of samples obtained according to the extraction step of the present invention. ((a,b) raw bamboo fiber, (c) microwave liquefaction residue, (d) bleached fiber, (e) cellulosic fiber)
7 shows a TEM image (ac) of the cellulose nanofibers obtained according to the extraction step of the present invention, and the diameter distribution (d) of the cellulose nanofibers.
8 shows the zeta potential (ae) of the samples obtained according to the extraction step of the present invention and the DLS measurement result (f) of the cellulose nanofiber.
9 shows the TG(a) and DTG(b) curves of samples obtained according to the extraction step of the present invention.
10 shows the measurement results of activation energy of samples obtained according to the extraction step of the present invention. ((a) raw bamboo fibers, (b) microwave liquefaction residues, (c) bleached fibers, (d) cellulose fibers and (e) cellulose nanofibers)
Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In this specification for purposes of explanation, various descriptions are presented to provide an understanding of the present invention. However, it will be apparent that these embodiments may be practiced without these specific descriptions.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Since the present invention can have various changes and can have various forms, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention. In describing each figure, like reference numerals have been used for like elements.

The terms used in the present application are only used to describe specific embodiments and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as “comprise” or “have” are intended to designate that a feature, step, operation, component, part, or combination thereof described in the specification is present, and includes one or more other features or steps. , it should be understood that it does not preclude the possibility of the existence or addition of an operation, a component, a part, or a combination thereof.

The method of extracting cellulose nanofibers from bamboo fibers according to an embodiment of the present invention includes a first step of liquefying a bamboo-containing solution by irradiating microwaves, and bleaching the residue generated through the liquefaction using hydrogen peroxide. a second step, a third step of subjecting the bleached residue to alkali treatment and acid treatment to obtain cellulosic fibers, and a fourth step of defibrillating the cellulosic fibers using a low energy blender.

First, a first step of liquefying the bamboo-containing solution by irradiating microwaves is performed.

Bamboo is a non-woody biomass containing lignin, hemicellulose and cellulose as major components, which are complex and strongly chemically interconnected. Therefore, in the present invention, in order to effectively remove lignin from bamboo fibers, the bamboo-containing solution is liquefied by irradiating microwaves.

In one embodiment, the bamboo-containing solution contains DEG (diethylene glycol) as a solvent and sulfuric acid as a catalyst, and the weight ratio of the bamboo and DEG is preferably 1: 4 (w/w), However, the present invention is not limited thereto.

In an embodiment, the first step may be performed for 6 minutes to 10 minutes at a power of 200 to 300 W. Preferably, it can be performed for 8 minutes with a power of 250 W. At this time, in order to prevent boiling of the mixture, it can be carried out by pausing for 5 minutes after running for 4 minutes.

Meanwhile, the present invention may further include, after the first step, vacuum filtering the solution after microwave liquefaction and drying the residue obtained after the vacuum filtration.

Specifically, after the first step, the solution after microwave liquefaction is cooled, diluted, and vacuum filtered to separate a solid residue and a liquid mixture. Thereafter, the residue obtained after vacuum filtration may be dried in an oven at 80 to 100° C. for about 24 hours for further processing.

On the other hand, the liquid mixture obtained after the vacuum filtration can be separated from the solvent to obtain a biopolyol.

Next, a second step of bleaching the residue generated through the liquefaction using hydrogen peroxide is performed.

In one embodiment, the second step, under the conditions of pH 4 to 5, may be performed at a temperature of 70 to 100 °C, preferably at a temperature of 85 °C for about 2 hours.

As described above, when the bamboo fibers are subjected to microwave liquefaction treatment and bleaching treatment using excessive hydrogen, delignification of bamboo fibers proceeds, and separation and decomposition of hemicelluloses from cellulose are also partially progressed.

Thereafter, a third step of obtaining cellulose fibers by alkali treatment and acid treatment of the bleached residue is performed.

In one embodiment, the alkali treatment uses a KOH solution, and the acid treatment uses a sulfuric acid solution, but is not limited thereto.

During the alkali treatment and acid treatment, separation and decomposition of hemicellulose from bamboo fibers proceeds to obtain cellulose fibers (CFs) from which lignin and hemicellulose are separated.

Finally, in order to extract the cellulose nanofibers from the cellulose fibers, a fourth step of defibrillating the cellulose fibers using a low energy blender may be performed.

In one embodiment, the fourth step is performed by blending for 20 minutes to 1 hour at an operating speed of 23000 rpm to 25000 rpm, and an output of 1600 to 2000 W using a low energy blender, and through this non-fibrillation process Cellulose nanofibers can be finally extracted from bamboo fibers by splitting cellulose fibers (CFs) into cellulose nanofibers (CNFs).

According to the present invention, it is possible to effectively separate delignification and hemicellulose from cellulose fibers by microwave liquefaction, bleaching, and chemical treatment of bamboo, a renewable eco-friendly non-wood biomass, and additional low-energy non-fibrillation treatment to remove toxic substances. Cellulose nanofibers with high crystallinity can be extracted in an environmentally friendly manner without generation.

On the other hand, the present invention, as another embodiment, the cellulose nanofibers extracted by the above method may be mentioned.

The cellulose nanofibers of the present invention extracted by the above method have a crystallinity of 80% or more according to XRD analysis, and a cross-sectional diameter of the fibers is 5 to 30 nm.

Therefore, the cellulose nanofibers of the present invention have high crystallinity and can be used as a promising reinforcing material for composite materials and nanocomposite materials to improve mechanical properties.

Hereinafter, the content of the present invention will be described in more detail with specific examples.

[Example]

1. Microwave liquefaction of bamboo

The microwave liquefaction reaction of bamboo was carried out in a household microwave oven (SK Magic, MWO-18M1, 700W, 2.45GHz, Korea).

First, 250 mL of bamboo powder and a liquefaction reagent with a weight ratio of DEG of 1/4 (w/w), and sulfuric acid (95%) as a catalyst at a concentration of 3% by weight of DEG, a liquefaction solvent, covered with a glass cap placed in an Erlenmeyer flask.

The microwave liquefaction reaction was accelerated at a fixed power of 250 W and a total operating time of 8 minutes. At this time, in order to prevent boiling of the reaction mixture, it was carried out by pausing for 5 minutes after running for 4 minutes.

After a period of 8 min, the reaction mixture was immediately cooled to room temperature and diluted with 1,4-dioxane/water solution (8/2, v/v) with stirring for 2 h, then filtered under vacuum to a solid residue The water and liquid mixture were separated. The solid residue was oven dried at 90° C. for 24 h under constant weight for further processing. Then, a biopolyol was obtained by removing the diluted solvent to a biomass conversion rate of 74.81%, whereas the microwave liquefaction residue was 25.19%.

2. Chemical Pretreatment of Microwave Liquefaction Residue and Extraction of Cellulose Nanofibers (CNFs)

Thereafter, the oven-dried residue from microwave liquefaction was subjected to further chemical treatment to extract cellulose fibers (CFs) and cellulose nanofibers (CNFs).

The microwave liquefied residue was immersed in a bleaching solution containing hydrogen peroxide (30%) under the condition of pH = 4-5 adjusted with acetic acid diluted to a solid content of 5% by weight to perform bleaching treatment. The reaction was carried out in an oil bath at 85 °C for 2 hours. Then, the solid residue was filtered and washed with deionized water until neutralization.

Next, the bleached fibers were alkali-treated with a 5% (w/v) KOH solution at 85° C. for 2 hours. After this process, the product was diluted with cold water to stop the reaction, filtered and rinsed with deionized water to obtain cellulose fibers (CFs).

Thereafter, CFs were acid hydrolyzed with sulfuric acid solution (2%, v/v) at 75 °C for 1 hour, and then centrifuged several times at 10000 rpm for 20 minutes to remove residual acid and collect the precipitate.

Finally, the precipitate was diluted with deionized water of the desired concentration, added to a high-speed blender (Koselig, YB-S03G, Philips, Netherlands), and then blended at a constant operating speed of 24000 rpm and an output of 1800 W.

The defibrillation process was performed for 30 min to split the cellulose fibers (CFs) into cellulose nanofibers (CNFs). The CNFs yield was measured to be 48.64% for the microwave liquefied residue and 16.13% for the raw bamboo fiber.

Visual observation of raw bamboo fibers and chemically modified bamboo fibers

The effect of the detailed chemical pretreatment process related to the extraction of purified CFs and CNFs can be clearly seen through the discoloration of raw bamboo fibers, and the related results are shown in FIG. 1 .

As can be seen in FIG. 1 , the initial color of the natural bamboo fibers was creamy tan and showed a dramatic change to dark brown after microwave liquefaction.

After microwave liquefaction treatment, the discoloration of raw bamboo fibers is caused by the widespread β-1,4-glycosidic bonds in carbohydrate molecules, β-O-4-aryl ethers, 4-O-5- This is due to the cleavage of various dominant bonds in bamboo fibers, such as diaryl ether and dibenzodioxosine bonds.

Further discoloration occurred by subjecting the microwave liquefied residue to a chlorine-free bleaching treatment with hydrogen peroxide.

Delignification using hydrogen peroxide is the most effective method to damage the chromophore structure of lignin through oxidation of carbonyl bonds and quinoid bonds of lignin side chains, which are the main causes of brown color of raw bamboo fibers.

In addition, the bleaching treatment helped change the solubility of lignin by reacting with the benzoquinone structure to further degrade the carbonyl side chain and carbon-carbon double bond of lignin to form functional hydroxyl, carboxyl and carbonyl groups that promote lignin removal, followed by Helps in the separation of chemically purified cellulose in the step.

Pure white fibers were obtained by alkali treatment and acid hydrolysis. Alkali treatment is used to cleave the lignin-carbohydrate bond between lignin and hemicellulose. Further hydrolysis of hemicellulose and further depolymerization of lignin in alkali treatment helped release partially water-soluble sugar derivatives and phenolic compounds as the purified cellulose content increased.

FTIR spectral analysis

The chemical structure changes of raw bamboo fibers, CFs, CNFs and their intermediates obtained through various chemical pretreatment steps were investigated by FTIR spectroscopy, and the results are shown in FIG. 2 .

As can be seen in FIG. 2 , the FTIR spectrum shows the main absorbance peaks indicating the presence of numerous functional groups such as alcohols, alkanes, esters, aromatics and ketones in cellulose, hemicellulose, and lignin, which are the main components of raw bamboo fibers.

The disappearance of some characteristic peaks for lignin and hemicellulose indicates that impurities were removed after each chemical treatment to obtain purified CFs and CNFs.

In the high wavenumber region of 2700-3700 cm ^-1 , it shows a broad absorption band with an intermediate peak at ^{3423 cm -1} due to the OH stretching vibration of hydroxyl groups in cellulose, hemicellulose and lignin molecules observed in all spectra.

The intensity peak at 2902 cm ^-1 is due to the CH stretching oscillations of the methyl and methylene groups in cellulose, hemicellulose and lignin, which are also found in all samples.

The prominent peak at 1635 cm ^-1 indicates the stretching oscillations of the absorbed water due to the abundant functional hydroxyl groups of cellulose. This peak intensity was dramatically enhanced with CNFs in the raw bamboo fiber, showing that the chemical structure of the cellulose was not affected even after chemical treatment.

The effect of chemical treatment on the chemical composition of bamboo fibers and purified samples was clearly demonstrated in the low wavenumber range from ^{600 to 1750 cm -1 .}

The intensity peak detected at 1735 cm ^-1 indicates a carbonyl C=O stretching oscillation, which is identical to the acetyl and sugar ester groups of hemicellulose. The presence of this peak in the FTIR spectrum of raw bamboo fibers can also be considered as an ester linkage of the carboxylic acid groups of ferulic acid and p-coumaric acid of lignin or hemicellulose (Julie et al., 2016; Khawas and Deka, 2016). ; Xie et al., 2016).

The intensity of this peak shows a decrease after microwave liquefaction. This indicates that microwave liquefaction promotes the removal of hemicellulose and lignin by the formation of a water-soluble substance that stimulates the breakdown of the ester bond of the carboxyl group in the lignin-carbohydrate complex and the decomposition of lignin and hemicellulose into a solvent.

Nevertheless, ^{the peak at 1735 cm −1} still remained in the spectrum of the bleached fibers after hydrogen peroxide treatment, indicating that the hemicellulose molecules were not completely removed after the chemical extraction process.

Therefore, liquefaction and bleaching using microwaves is an efficient method to remove the most lignin compounds from bamboo fibers.

The disappearance of the intensity peak at 1735 cm ^-1 indicates that hemicellulose was effectively decomposed by treatment with a diluted potassium hydroxide solution.

The vibrational frequencies at 1597, 1511, 1461 and 1243 cm ⁻¹ are aromatic ring skeleton vibrations, breathing vibrations of guaiacyl rings, C=C vibrations of methoxy groups of lignin or hemicellulose and/or lignin. It is characterized by a CH modification associated with

1597 cm ^-1 peak disappeared and the 1511, 1461 and 1243cm ^-1 band shows a small peak after liquefying microwave.

This result indicates that most of the lignin functional components such as aromatic rings were almost completely decomposed and dissolved in the solvent. ^{The small shoulder band of 1205 cm -1} caused by the S=O oscillations shows that sulfate groups were attached during microwave liquefaction and acid hydrolysis because sulfuric acid was used as the catalyst, or the esterification reaction between sulfate and hydroxyl groups occurred after sulfuric acid. show the results

The shape of this peak can be observed in all FTIR spectra except for the initial bamboo fiber.

The peaks appearing at 1167, 1114 and 1032 cm ⁻¹ are due to the COC stretching oscillations of the β-1,4-glycosidic ring linkages between the D-glucose units of cellulose, which are more pronounced after each chemical treatment. In addition, the shoulder peak in the ^{897 cm -1} _{region corresponds to the C 1} -H glycosidic modification of cellulose.

These results indicate that the cellulose component did not change during the chemical pretreatment process.

Elemental composition analysis using XPS method

In order to investigate the effect of elements present on the bamboo surface and chemical treatment on the production of CNFs, samples of the present invention were analyzed using XPS analysis, and the results are shown in FIG. 3 and Table 1.

Bamboo is composed of cellulose, hemicellulose and lignin, so carbon, oxygen and hydrogen are the main elements of raw bamboo fiber.

Figure 3 shows the entire XPS spectrum, and Table 1 shows the surface elemental composition of raw bamboo fibers, microwave liquefaction residues and CNFs.

As can be seen in Table 1, it can be seen that the O/C ratio from the raw bamboo material to the cellulose nanofibers (CNFs) after the treatment of the present invention is increased to effectively remove lignin, hemicellulose and other extracts from the biomass.

The O/C theoretical values of lignin and cellulose were 0.33 and 0.8, respectively (Wei et al., 2018; De et al., 2020).

In the case of raw bamboo fiber, an O/C value of 0.36 is shown, which is close to the reference value of lignin, confirming the presence of lignin on the surface of the bamboo.

After microwave liquefaction, the O/C value of the residue was found to be 0.41 due to the effective removal of lignin and/or hemicellulose.

On the other hand, after acid hydrolysis, a maximum O/C ratio of 0.69 was obtained, indicating complete decomposition of lignin and hemicellulose, indicating that only cellulose was detected on the fiber surface.

_{The presence of a new peak at 168.0 eV indicates the S 2p} binding energy of the microwave liquefaction residue and CNFs because sulfuric acid was used as the solvent.

_{High-resolution C 1s} (298-279 eV) and O 1s (545-525 eV) scans of raw bamboo fibers, microwave liquefaction residues and CNFs were measured, and the results are shown in FIG. 4 .

In addition, the quantitative atomic concentration in the peak region was calculated using the deconvolution method through the XPS elemental scan, and the results are shown in Table 2.

As can be seen in Figure 4, all C1 spectra show three peaks at 283.6, 285.2 and 287.5 eV, which correspond to three categories of carbon atoms (C1-C3).

The C1 peak shows carbon atoms linked only with carbon and hydrogen (C-C/C-H) due to the presence of lignin, extract and impurities. The C2 peak is associated with carbon atoms with ether groups (C5-O-C1 and C1-O-C4) and unmodified C-OH groups of the cellulose chain.

The C3 peak is assigned to the contribution of the acetal moieties of anhydroglucose units and/or ester groups associated with hemicellulose and lignin (Espino-Pxrez et al., 2014; Wei et al., 2018; De et al., 2020 ; Xu et al., 2013).

Meanwhile, looking at Table 2, it can be seen that the intensity of the C1 peak decreased from 48.30% to 45.76% and 14.97% for raw bamboo fibers, microwave liquefaction residues and CNFs.

It can be observed that hemicellulose and lignin are present on the surface of the biomass, and these components are effectively removed, so that the intensity of the C3 peak (reduced from 5.84% to 3.20% and 2.34%) has a similar tendency to that of the C1 peak intensity.

On the other hand, after alkali and acid treatment, the intensity of the C2 peak improved from 45.76% to 51.04% and 82.69% due to the acceptance of the cellulosic component.

The O _1s spectrum shows two distinct peaks at 530.8 and 531.8 eV. The O1 peak represents the oxygen atoms of the lignin and/or hemicellulose structure (OC=O), whereas the O2 peak comes from the oxygen atoms bonded to carbon with single bonds (COC and CO), which are the hemicellulose and cellulose of the raw bamboo fiber. corresponds to

_{In the results of the O 1s} spectra presented in Table 2, the O1 peak decreased while the O2 peak was enhanced after each chemical treatment. These results were consistent with the FTIR analysis results.

Crystal structure analysis using XRD method

The crystal structures of various types of cellulose fibers and cellulose nanofibers were analyzed using XRD analysis.

Intermolecular and intermolecular hydrogen bonding occurred between functional hydroxyl groups on the cellulose surface, which play an important role in the various ordered crystal arrangements representing the chemical and thermal properties of nanocellulose.

The X-ray diffractograms of raw bamboo fibers, CFs, CNFs, and intermediate products generated during chemical pretreatment are shown in FIG. 5 .

, (110), (200) 및 (004)의 셀룰로오스 I 격자의 결정 다형체를 나타냈고, 이는 각각 1,4-무수-D-글루코피라노스(1,4-anhydro-D-glucopyranose) 단위의 빌딩 블록을 반복하여 달성되었다. 즉, 대나무 섬유의 원래 셀룰로오스 결정 구조는 고순도 CFs와 CNFs를 분리하는 동안 그대로 유지되었다.5, all samples are

, (110), (200) and (004) showed the crystalline polymorph of the cellulose I lattice, which is a 1,4-anhydro-D-glucopyranose unit, respectively. This was achieved by iterating the building blocks. That is, the original cellulose crystal structure of bamboo fibers was maintained during the isolation of high-purity CFs and CNFs.

However, the difference between each chemical treatment step is clearly observable through the change in peak intensity, which accounts for the change in the CrI value and average crystal size of each sample. (See Table 3 below)

The crystallinity index (CrI) value was calculated as follows according to the Segal method.

[Equation 1]

In Equation 1, I ₀₀₂ is the maximum intensity of the diffraction peak 200 at 2θ = 22.3 °, and I _am is the minimum intensity of the diffraction peak at 2θ = 18 °.

Referring to Table 3, in the case of untreated raw bamboo fibers, a very broad diffraction peak was observed at 2θ = 22.3˚. This peak increases in strength and becomes narrower for chemically treated fibers, indicating that CFs and CNFs have a higher degree of crystallinity compared to raw bamboo fibers.

In general, the estimated CrI values of raw bamboo fiber, microwave liquefaction residue, bleached fiber, CFs and CNFs were 62.38%, 78.08%, 78.46%, 78.62% and 80.82%, respectively, indicating that the crystallinity of both chemically treated samples was shows a significant improvement.

For raw bamboo fibers, the crystal domains are densely packed in a matrix of disordered components in which lignin and hemicellulose are intertwined, resulting in a low crystallinity of 62.38%.

After 8 minutes of microwave liquefaction, the apparent crystallinity of the microwave liquefaction residue increased by 125%, indicating that non-cellulosic components such as lignin and hemicellulose derivatives were gradually removed from the amorphous region and these components were dissolved in the solvent. Therefore, microwave liquefaction can be a very effective pretreatment process for the separation of purified cellulose and CNFs.

Using chlorine-free bleaching with hydrogen peroxide, a crystallinity increase of 78.46% can be obtained. This is mainly because the lignin remaining in the liquefied residue has been removed.

On the other hand, alkali treatment with KOH solution can further improve the crystallinity to 78.62% by completely removing lignin and hemicellulose compared with the raw bamboo material.

CNFs with a higher crystallization index of 80.82% can then be released in further acid hydrolysis treatment and blending. In general, CNFs are used as promising reinforcements for composites and nanocomposites to improve stiffness due to the high modulus of longitudinally aligned crystal regions. The high crystallinity of the CNFs of the present invention can improve the mechanical properties of the target material.

On the other hand, the CrI of CNFs calculated from raw bamboo fibers was obtained from CNFs from Helicteres isora plants (90%) (Chirayil et al., 2014), and CNFs isolated from arecanut hull fibers (73%) (Julie et al., 2016). ), and CNFs (78%) extracted from removed sugar beet pulp (Li et al., 2014).

As shown in Table 3, the average cross-sectional crystal sizes of bamboo fibers and refined fibers were the full width at half maximum of the most intense crystalline peak 200 at an increased 2θ = 22.3˚ of 6.49 nm with a maximum value at 5.61 nm after chemical pretreatment. is directly related to

This is mainly due to the effective decomposition of the non-cellulosic component, which promotes the rearrangement of a significant portion of the crystalline region through hydrogen bonding.

Intermolecular and intramolecular hydrogen bonding disrupts the free vibrations of cellulosic chains, which are closely organized and packed together to form highly ordered regions that tend to have larger crystal sizes.

Morphological analysis by scanning electron microscopy (SEM)

The morphologies of raw bamboo fibers, CFs and CNFs were analyzed through the SEM image shown in FIG. 6 .

Referring to FIG. 6 , the raw bamboo fiber has some small fragments on the surface corresponding to the presence of amorphous components such as lignin, hemicellulose, and extract, so that numerous fiber bundles and aggregates are formed. (Fig. 6a, b).

The morphological structure of bamboo fibers after microwave liquefaction treatment shows an intact structure with a rough surface. (FIG. 6C) The image in FIG. 6C shows significant removal of lignin, hemicellulose and impurities known as bonding binders on the bamboo fiber surface.

In addition, microwave liquefaction can enhance the surface area and promote the ability of chemical reagents to trigger the inner portion of the fiber.

In Fig. 6d, it can be confirmed that the bleaching treatment induces effective delignification and can contribute to splitting the fiber bundle into fine fibers. Each microfibrils can be considered as nanocellulose bundles attached along the microfibrils by amorphous regions.

Additional fiber fragmentation can be observed in the SEM image of CFs after alkali treatment (Fig. 6e). This step progressively accelerates the cleavage of the hemicellulose adhered together with the semi-crystalline cellulose, resulting in a softer fiber surface.

On the other hand, in the subsequent treatment by acid hydrolysis, these amorphous cellulose components were exfoliated to obtain highly crystalline cellulose. Due to the high hydroxyl content of the cellulose surface, a blender process was applied to limit the temporary hydrogen bonds formed between the cellulose molecules to release individual CNFs. The morphological characteristics of CNFs were investigated by TEM images.

A TEM image of CNFs after the defibrillation process is shown in FIG. 7 .

In Figure 7, chemical pretreatment combined with mechanical treatment can productively remove amorphous components (lignin, hemicellulose and extract) and separate cellulose microfibrils into individual nano-sized fibrils.

Wire-like CNFs with some aggregates can appear as shown in Figs. 7a,b,c when sonication is applied, which is a reasonable and cost-effective method to break hydrogen bonds and fragment purified cellulose fibers into needle-like CNFs.

Figure 7d shows that the overall cross-sectional diameter of CNFs is in the range of 5 - 30 nm with an average diameter of 13 nm and a length of several micrometers.

Zeta potential and dynamic light scattering properties

Zeta potential is an important parameter in determining the colloidal stability of CNFs dispersions, and the results were analyzed and presented in Figs. 8a-e and Table 4.

Zeta potential measurements showed that the average values of the surface charge were -17.5 ± 0.5 mV for raw bamboo material, -26.7 ± 1.1 mV for microwave liquefied residues, -28.4 ± 0.2 mV (bleached fibers), and -29.5 for CFs. ± 0.4 mV, and CNFs (-31.6 ± 1.8 mV) showed the highest zeta potential.

It was found that the zeta potential increased towards the final material due to the effective removal of non-cellulosic components including lignin and hemicellulose.

The low negative surface charge of raw bamboo fibers is due to the presence of abundant functional groups such as hydroxyl groups and carboxyl groups on the surfaces of cellulose, hemicellulose and lignin. The zeta potential value of -31.6 ± 1.8 mV for CNFs is due to the presence of negatively charged sulfate groups on the surface of the cellulose nanofibers resulting from the esterification of hydroxyl groups and sulfate groups by the acid hydrolysis step.

These groups on the CNFs surface have high electrostatic repulsion, limiting individual CNFs from accumulating together. Therefore, the treatment of the present invention can produce a stable CNFs suspension as the apparent surface charge exhibits a value greater than ± 30 mV.

Meanwhile, the statistical particle size and distribution of nanocellulose in suspension was measured by dynamic light scattering (DLS).

Specifically, the size measured by the DLS technique, which can be considered as the diameter of the particle at the same diffusion coefficient, is shown in Fig. did.

Looking at the intensity according to the size distribution in Fig. 8f, the CNFs suspension showed two distribution regions. The small peak at 80~200nm with the most intense size of 106nm showed an intensity percentage of about 4.7%, and the most intense size in the particle size distribution range of 500~1000nm showed a value of 615nm (17.8% intensity percent). Overall, the average size of CNFs was measured to be about 643 ± 21 nm.

Thermal Stability Analysis

The thermal performance of raw bamboo fibers, CFs, CNFs and their intermediates was performed by thermogravimetry (TGA) for comparison, showing the effect of different chemical treatment steps on depolymerization properties.

9 shows the TG and DTG curves of unmodified bamboo fibers and modified fibers. Table 5 presents the TGA data including the 5% mass loss temperature (T ₅ ), the maximum decomposition temperature (T _max ) exported from the DTG curve, and the carbonized residue content at 550 and 750 °C.

Unmodified bamboo fibers contain various chemical compositions such as hemicellulose, lignin, cellulose and a small amount of extracts, and the pyrolysis of the fibers shows various stages of weight loss.

In addition, hemicellulose begins to break down rapidly into random polysaccharides, while lignin is considered the most difficult component to break down. It can be seen that delignification occurred slowly at a temperature from room temperature to 800 °C, but the weight loss rate was very low.

Unmodified bamboo fibers showed a slight weight loss in the initial stage when heated from 50 °C to 150 °C. This is mainly due to the vaporization of absorption-bound water and the removal of small amounts of extract.

Figure 9a shows that the raw bamboo fiber mainly exhibits two stages of pyrolysis. The first stage of decomposition of raw bamboo fiber occurred between 226 and 400 °C, corresponding to the onset of carbohydrate decomposition and lignin decomposition. The DTG curve of FIG. 9b showed a very small shoulder peak at 278 °C, indicating the presence of some hemicellulose.

On the other hand, the disappearance of this peak after alkali treatment proves that the hemicellulose component was completely removed by the alkali chemical treatment.

The second stage occurred at about 400-700 °C corresponding to the lignin component. Of the three major components of lignocellulosic biomass, lignin is the most difficult component to decompose, and delignification pyrolysis occurred due to the presence of numerous oxygen functions in the lignin structure with unique thermal properties indicating the lignin decomposition temperature at various temperatures. .

Therefore, this amorphous component was effectively removed by bleaching the microwave liquefied residue.

Table 5 shows that the initial decomposition temperature of the fibers treated according to the embodiment of the present invention is higher than that of the untreated bamboo fibers. This is because non-cellulosic components are removed during chemical treatment. The raw bamboo fiber started decomposition at 226 °C, and the microwave liquefied residue and bleached fiber were pyrolyzed at 265 °C and 269 °C, respectively.

The higher the decomposition initiation temperature, the more efficiently hemicellulose and lignin, which were recognized as materials with low thermal stability, were removed after liquefaction and bleaching.

Also, the decomposition temperature of the residue after microwave liquefaction (365 °C) was higher than that of raw bamboo fiber (346 °C). However, for bleached fibers (310 °C), CFs (307 °C) and CNFs (305 °C), the T _max values were reduced compared to the microwave liquefied residue (365 °C). This is because the bleaching treatment improves the ability of heat to separate fiber bundles and separate them.

Specifically, the remaining components of the fibers after the bleaching treatment mainly consist of hemicellulose and cellulose, and therefore, additional thermal decomposition easily occurred in the modified fibers after alkali and acid hydrolysis treatment, thereby reducing _{T max .}

On the other hand, thermal stability can be investigated through the kinetic parameters of pyrolysis.

Activation energy (AE) was calculated by TG curve using Broido's method as shown in Equation 2 below. (Maiti et al., 2013):

[Equation 2]

In Equation 2, y = w _t /w _O , w _t = weight remaining at time 't', w ₀ = initial weight, T (K) = temperature, R is a universal gas constant.

The activation energy can be calculated from the slope of a linear plot of ln[ln[(1/y)] versus 1/T, and the result is shown in FIG. 10 .

Referring to FIG. 10 , it can be seen that the activation energy values of the chemically treated fibers, CFs and CNFs, were improved compared to the raw bamboo fibers, and the raw bamboo fibers had lower thermal stability than the chemically treated fibers.

For microwave liquefied fiber (28.78 kJ/mol), the highest AE value is due to the higher stability compared to raw bamboo fiber (24.18 kJ/mol) because a small amount of lignin remains in the fiber.

A decrease in AE values was obtained for bleached fibers (26.38 kJ/mol), CFs (26.25 kJ/mol) and CNFs (25.89 kJ/mol) due to the presence of residual hemicellulose and sulfate groups after acid hydrolysis. Hemicellulose activated the initiation of the pyrolysis process of the residue as an initial stage of decomposition.

In the process of liquefaction and acid hydrolysis, the introduction of sulfate groups caused a dehydration reaction, which reduced the AE value and lowered the thermal stability of the fibers.

Characteristics of biopolyols obtained by microwave liquefaction of bamboo

Microwave liquefaction of bamboo yielded a black biopolyol liquid and residue.

The microwave liquefaction residue is subjected to further chemical treatment to extract CNFs, while the biopolyol can be used as a starting material for polyurethane foam production.

Therefore, the chemical composition of the biopolyol ^{was analyzed by FTIR and 1} H NMR spectroscopy, and is shown in FIG. 11 .

As shown in FIG. 11 , the biopolyol ^{exhibits a broadened absorption peak at about 3368 cm −1} indicating that the black liquid mixture mainly contains hydroxyl-rich groups.

In general, quantification of the structural properties of biopolyols was investigated by atomic (proton) NMR, and the results are shown in FIG. 12 .

12, the peak in the 0.5-3.0 ppm region is due to the aliphatic protons (methyl and methylene) of the DEG structure.

The resonance region between 3.0 and 4.4 ppm represents the proton of the carbon atom next to the aliphatic alcohol, ether, ester and/or methylene group linking the two benzene rings.

For example, in the 4.5 to 6.0 ppm region, the prominent signal is generated by the carbohydrate functionality and/or aromatic ether protons of the lignin structure.

The description of the presented embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the scope of the invention. Thus, the present invention is not to be limited to the embodiments presented herein, but is to be construed in the widest scope consistent with the principles and novel features presented herein.

Claims (12)

A first step of liquefying a bamboo-containing solution containing DEG (diethylene glycol) as a solvent and sulfuric acid as a catalyst by irradiating microwaves;
a second step of bleaching the residue generated through the liquefaction using a mixed solution of hydrogen peroxide and acetic acid;
a third step of subjecting the bleached residue to alkali treatment and acid treatment to obtain cellulose fibers; and
A fourth step of defibrillating the cellulose fibers using a low-energy blender;
The second step, under the conditions of pH 4 to 5, characterized in that it is performed at a temperature of 70 to 100 ℃,
A method for extracting cellulose nanofibers from bamboo fibers.
delete According to claim 1,
The weight ratio of the bamboo and DEG is 1: 4 (w / w) characterized in that,
A method for extracting cellulose nanofibers from bamboo fibers.
According to claim 1,
The first step, characterized in that performed for 6 minutes to 10 minutes, with a power of 200 to 300 W,
A method for extracting cellulose nanofibers from bamboo fibers.
According to claim 1,
After the first step,
vacuum filtering the solution after completion of microwave liquefaction; and
Drying the residue obtained after the vacuum filtration; characterized in that it further comprises,
A method for extracting cellulose nanofibers from bamboo fibers.
6. The method of claim 5,
Separating the liquid mixture obtained after the vacuum filtration with a solvent to obtain a biopolyol; characterized in that it further comprises,
A method for extracting cellulose nanofibers from bamboo fibers.
delete According to claim 1,
In the first and second steps, delignification of bamboo fibers is characterized in that
A method for extracting cellulose nanofibers from bamboo fibers.
According to claim 1,
The alkali treatment uses a KOH solution, and the acid treatment uses a sulfuric acid solution,
A method for extracting cellulose nanofibers from bamboo fibers.
According to claim 1,
The fourth step is
characterized in that blending is performed for 20 minutes to 1 hour at an operating speed of 23000 rpm to 25000 rpm, and an output of 1600 to 2000 W using a low-energy blender,
A method for extracting cellulose nanofibers from bamboo fibers.
According to claim 1,
In the third step, characterized in that the separation of hemicellulose from bamboo fibers proceeds,
A method for extracting cellulose nanofibers from bamboo fibers.
Claims 1, 3 to 6, as a cellulose nanofiber extracted by the method according to any one of claims 8 to 11,
Characterized in that the crystallinity according to XRD analysis is 80% or more, and the cross-sectional diameter of the fiber is 5 to 30 nm,
Cellulose nanofibers extracted from bamboo fibers.

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