BR102020003684A2 - Process for obtaining cellulose nanofibrils and obtained cellulose nanofibrils - Google Patents

Abstract

The present invention refers to a process for obtaining cellulose nanofibrils or nanofibrillated cellulose (CNF) from sugarcane bagasse, without the need for the traditional mechanical treatment for defibrillation. An additional object is the obtained cellulose nanofibril. The present invention can be applied in composites and packaging, foams, emulsions, rheological modifiers and drug delivery.

Description

PROCESS FOR OBTAINING CELLULOSE NANOFIBRILLES AND OBTAINED CELLULOSE NANOFIBRILLES Field of Invention

[001] The present invention refers to a process for obtaining cellulose nanofibrils or nanofibrillated cellulose (CNF) from sugarcane bagasse, without the need for the traditional mechanical treatment for defibrillation. An additional object is the obtained cellulose nanofibril.

[002] The present invention can be applied in composites and packaging, foams, emulsions, rheological modifiers and drug delivery.

Fundamentals of the Invention

[003] Nanofibrillated Cellulose (CNF) is the name given to long and flexible cellulose nanoparticles, with nanometer diameter (560 nm) and micrometer length, and which contain alternating amorphous and crystalline regions (EICHHORN et al., 2010; KLEMM et al. ., 2011). CNF belongs to the class of nanocelluloses, which encompasses all cellulosic materials with at least one dimension in the nanometric scale (1-100 nm). This class also includes nanocrystalline cellulose (CNC), which is composed of rigid crystalline particles with a morphology similar to needles and which have diameters between 3 and 5 nm and lengths between 50 and 500 nm (MOON et al., 2011).

[004] Nanocelluloses combine characteristics of nanoscale materials, such as high aspect ratio and high surface area, with the important properties of cellulose, such as hydrophilicity, high mechanical strength, susceptibility to chemical modification, versatility in processing options, low toxicity and biocompatibility. These materials have been studied for different applications, including composites and packaging (GHADERI et al., 2014), foams (FERREIRA; REZENDE, 2018), emulsions (GESTRANIUS et al., 2017), rheological modifiers (LIU et al., 2017 ; SOUZA et al., 2019) and drug delivery (LIN; BRUZZESE; DUFRESNE, 2012; LIU et al., 2018; POONGUZHALI; KHALEEL BASHA; SUGANTHA KUMARIA, 2018; SUPRAMANIAM et al., 2018).

[005] Both CNFs and CNCs can be obtained from cellulose extracted from lignocellulosic plant biomass. In addition to cellulose, lignocellulosic materials are also made up of hemicellulose and lignin. The isolation of cellulose for the production of CNF or CNC requires drastic conditions for treating plant biomass to separate the cellulose from the other components.

[006] These treatments are generally called pre-treatments, as they are carried out before steps that transform the cellulose itself into the final products. They aim at the solubilization and separation of one or more biomass components, in order to increase their accessibility to further processes (SARKAR et al., 2012). Pre-treatments can include physical methods such as grinding (CHANG; HOLTZAPPLE, 2000) and irradiation (LAFITTE-TROUQUÉ; FORSTER, 2002), physical-chemical methods, such as hydrothermal treatment or steam explosion (LASER et al., 2002; SASAKI; SASAKI; ; ADSCHIRI; ARAI, 2003), fiber explosion with ammonia, called AFEX (Ammonia Fiber Explosion) (GOLLAPALLI; DALE; RIVERS, 2002), organic and ionic solvents (ZHANG; ZHAO, 2009), supercritical fluids (MATSUMURA et al ., 2006; SAKA; UENO, 1999), dilute acids or bases (DESCHAMPS; RAMOS; FONTANA, 1996; SUN; LAWTHER; BANKS, 1995; VARGAS BETANCUR; PEREIRA, 2010), sulfite (ZHU et al., 2009) and also biological methods, using fungi and bacteria (KURAKAKE; IDE; KOMAKI, 2007; SRILATHA et al., 1995).

[007] A method of pre-treatment widely used to obtain cellulosic pulps free of lignin and hemicellulose is the Kraft process. This is the traditional method used in the pulp and paper industries and consists of the treatment of lignocellulosic material at a temperature close to 170 °C for approximately 2 h with an aqueous solution of sodium hydroxide and sodium sulfide, leading to the dissolution of lignin (CHAKAR; RAGAUSKAS, 2004). However, this method has as disadvantages the emission of volatile compounds from the reduction of sulfur (GIERER, 1980).

[008] Sulphite pulping, developed in 1867 by Benjamin Tighmann, uses as delignifying agents solutions of sulfurous acid or its salts that generate sulphite ions (SO3-) or bisulfite (HSO3-) during cooking. Compared to the Kraft process, it has the advantages of greater yield and greater whiteness of the pulps, facilitating bleaching and greater yield (BIERMANN, 1993).

[009] Pretreatments using diluted acids (0.5 - 2% m/m) are considered effective in the removal and solubilization of hemicellulose, with low effects on lignin extraction (NASCIMENTO; REZENDE, 2018). In this process, the acid hydrolyses hemicellulose, producing a liquid phase rich in xylose and with smaller amounts of lignin derivatives, being an efficient method for recovering hemicellulose. The acids normally used are hydrochloric, sulfuric and phosphoric at temperatures between 140 and 190 °C (CARVALHEIRO; DUARTE; GÍRIO, 2008; TAHERZADEH; KARIMI, 2008). Under these conditions, the production of cellulose decomposition products is lower. Saha et al., for example, performed a pre-treatment with dilute acid (0.75% v/v) at 121 °C for 1 h and obtained a good yield of sugars after enzymatic saccharification (74%) without forming quantities measurable values of furfural and HMF (SAHA et al., 2005).

[0010] Treatment with diluted base is used for lignin recovery. Causes less degradation of sugars when compared to acid treatment. The bases commonly used are sodium hydroxide, potassium hydroxide and calcium oxide (RACHAKONDA; GANGADHAR, 2016), with sodium hydroxide being the most used, as it presents the best degradation and the best fermentation yield (JACKSON, 1977). The combination of pre-treatment with dilute acid and alkaline treatment are commonly reported in the literature due to the benefits associated with the removal of hemicellulose obtained by the acid step followed by the removal of lignin obtained with the basic treatment. Rezende and collaborators performed sequential pretreatments on sugarcane bagasse using dilute acid and base solutions and obtained a removal of 96% and 85% of hemicellulose and lignin fractions, respectively using 1% H2SO4 and 1% NaOH concentrations (m /v) or greater. (REZENDE et al., 2011)

[0011] The hydrothermal treatment, being autocatalytic, does not require any catalyst or chemical, in addition to being considered a clean technology, as it uses water as the main reactant under pressure so that the water remains in a liquid state at high temperatures (160 - 240 ° C), causing changes in the structure of the lignocellulosic material. This process is based on the solubilization of hemicellulose through hot water, making cellulose more accessible (SILVA et al., 2013)

[0012] The organosolve process emerged in the 1930s as an alternative pulping method. This process uses water and organic solvents, usually alcohols, which act as delignifying agents under heating at high temperatures (160 - 200 °C). Although it does not allow rapid washing of the pulp in water due to the possibility of reprecipitation of lignin on the fibers, it allows for an easier recovery of polioses and lignin as they are less degraded. This process can be carried out using a wide variety of organic solvents, ethanol and methanol being the most commonly used, and allows operation in smaller industrial plants, eliminating the need to recover organic reagents (LANCEFIELD et al., 2017).

[0013] The next step to the pre-treatment is the bleaching of the cellulose which has the function of removing residual lignin and chromophore compounds still present in the material. Conventional bleaching uses chemical reagents containing chlorine or hydrogen peroxide, under alkaline conditions, a combination of reagents and several stages can be used.

[0014] Chlorination is usually the first stage of the bleaching sequences, and its function is to delignify the pulp without changing the color. Chlorine reacts with lignin to form a substance called “chlorolignin”, partially soluble in water and easily removed with basic extraction. The wide use of chlorine as a bleaching agent comes from its low cost. However, its use promotes the formation of dioxins and organochlorine compounds, resistant to biodegradation, responsible for major environmental damage and for toxic, mutagenic and carcinogenic effects. Seeking to reduce this problem, elemental chlorine has been replaced by chlorine dioxide. The use of hydrogen peroxide is also an alternative for providing a reduction in the amount of organochlorine formed, as it is a strong oxidant and does not decompose into polluting products (D’ALMEIDA, 1988).

[0015] After pulp oxidation processes for bleaching, alkaline extraction is performed to remove colored components, which are solubilized in the basic medium. This step does not cause oxidative degradation of the fibers, in addition to improving the physical properties of the pulp. The most used reagent is sodium hydroxide, but calcium hydroxide can also be used. The addition of nucleophilic agents to the process, such as oxygen, hydrogen peroxide or sodium hypochlorite, causes a mild oxidation, obtaining a supplementary delignification that improves the whiteness of the pulp (D’ALMEIDA, 1988).

[0016] Hypochlorite bleaching was traditionally employed as a single-stage bleaching, but with the use of liquid chlorine and alkaline extraction, treatment using only hypochlorite is no longer employed, and is currently used in intermediate and final stages of bleaching sequences .

[0017] Oxygen bleaching was created seeking to develop processes less aggressive to the environment. It is a process that uses oxygen under pressure (7-9 bar) in an alkaline medium. Although oxygen is a cheap reagent, it is less specific in the removal of lignin (D’ALMEIDA, 1988; MCDONOUGH, 1984).

[0018] Enzymes, such as xylanase, have also been widely studied for use in pulp bleaching. Unlike chemical reagents, these enzymes do not delignify the pulp, but act on it to aid in subsequent bleaching, reducing the required amount of chlorinated reagents, for example, or replacing the use of oxidants such as oxygen or hydrogen peroxide (LIN et al., 2018).

[0019] After bleaching, the material goes to mechanical defibrillation, in the case of the production of CNF or for hydrolysis that will lead to the production of CNC. In some cases, it is possible to skip this bleaching step and produce CNC directly from the pretreated pulp. The achievement of CNC occurs through hydrolysis of fibers by concentrated acids (58 to 85% m/m), at temperatures between 45 and 60 °C and reaction time ranging between 25 and 360 min, followed by separation by centrifugation and dialysis or filtration for purification (HABIBI; LUCIA; ROJAS, 2010; KARGARZADEH et al., 2018; NASCIMENTO; REZENDE, 2018). Acid hydrolysis attacks amorphous regions of cellulose, resulting in small particles with high crystallinity and dimensions dependent on the reaction conditions and the cellulose source (HABIBI; LUCIA; ROJAS, 2010).

[0020] Although phosphoric or hydrobromic acids have already been used to prepare CNCs, hydrochloric acid and, mainly, sulfuric acid are the most commonly applied, since the latter causes a greater stability of the colloidal suspension of nanoparticles. The work “The colloidal properties of cellulose micelles” (RANBY, 1951) is considered the pioneer in the production of CNC suspensions by controlled hydrolysis with sulfuric acid. In a 1992 work, "Helicoidal self-ordering of cellulose microfibrils in aqueous suspension" (REVOL et al., 1992), nanocrystalline cellulose was obtained through the hydrolysis of wood Kraft pulp with sulfuric acid 65% m/m at 70 °C for 30 min, followed by washing with distilled water by successive dilutions, centrifugation, dialysis and ultrasound dispersion.

[0021] In the work “Effect of microcrystallite preparation Conditions on the formation of colloid crystals of cellulose'' (DONG; REVOL; GRAY, 1998), the authors investigated in detail the effects of temperature, reaction time and sonication on the properties of nanocrystals produced by from filter paper (98% cotton fiber) and found the use of 64% sulfuric acid (m/v) as the most suitable condition for CNC preparation, temperature of 45 °C, reaction time of 1h and treatment in cutting edge ultrasound for 5 min. Low yields (about 40% from filter paper powder) are expected due to the removal not only of amorphous regions but also amounts of crystalline regions by the hydrolysis reaction.

[0022] The enzymatic hydrolysis can also be used in the extraction of the amorphous regions of the cellulose, being environmentally less aggressive and providing greater yield and less degradation of cellulose for being a more specific reaction. Siqueira et al., in "Morphological investigation of nanoparticles obtained from combined mechanical shearing and enzymatic and acid hydrolysis of sisal fibers" (SIQUEIRA et al., 2010) showed that, depending on the pre-treatments used, the enzymatic hydrolysis of sisal pulp generates CNC suspensions with CNF coexistence.

[0023] Obtaining CNF requires highly energetic mechanical action to detach the nanofibers, which are joined by hydrogen bonds between hydroxyl groups or physically intertwined by isolated polysaccharide chains. Examples of common treatment methods used are: refinement, grinding and homogenization with homogenizers or microfluidizers. (ABDUL KHALIL et al., 2014)

[0024] One of the first production methods of CNF was reported in 1983. The document US4374702A (TURBAK; SNYDER; SANDBERG, 1983) describes the production of CNF through the passage of an aqueous suspension of pre-hydrolyzed cellulose fibers obtained from Hemlock wood kraft pulp by a high pressure (3000 psi) homogenizer. In this type of equipment, the material passes through a small inlet between a homogenizing valve and an impact ring, so that the fibers are subjected to impact and shear forces that lead to fibrillation. In this work, the passage through the homogenizer was repeated about 11 times until a stable suspension was obtained. The authors state that this is a method in which no substantial chemical change or degradation of the starting cellulose was required beyond the cellulose isolation step. Generally, about 10 to 15 passes of the pulp through the homogenizer are necessary to obtain cellulose nanofibers. (DUFRESNE; DUPEYRE; VIGNON, 2000; HABIBI; MAHROUZ; VIGNON, 2009; LEE et al., 2009; LEITNER et al., 2007; SIDDIQUI et al., 2010)

[0025] As an alternative to the homogenizer, Zimmermann et al. (2004) in “Cellulose fibrils for polymer reinforcement” (ZIMMERMANN; POHLER; GEIGER, 2004) reported the use of a microfluidizer for the production of CNF. In this equipment, the sample passes through a small orifice chamber with a specific geometry. With high pressure, shear and impact forces are applied to the suspension against the canal walls, leading to fibrillation. In the cited work, a suspension of sulfite-pretreated softwood cellulose pulp was homogenized with ultra-turrax at 24000 rpm for 8 h, and then passed through the microfluidizer at 1000 bar pressure for 60 min, resulting in nanofibers of 20-100 nm in diameter and tens of micrometers in length.

[0026] The use of grinders (or grinders) for the production of CNFs was reported by Taniguchi and Okamura (1998) in “New Films Produced from Microfibrillated Natural Fibers'' (TANIGUCHI; OKAMURA, 1998), where suspensions of extracted cellulose fibers of different natural resources (kraft pulps of poplar and fir, cotton, tunicate, chitosan, silk and collagen) were subjected to a grinder 10 times, obtaining CNF with diameter in the range of 20-90 nm for subsequent film production.

[0027] Karande et al. in “Nanofibrillation of Cotton Fibers by Disc Refiner and Its Characterization” (KARAND et al., 2011) reported the use of disc refiner for the preparation of CNFs. This equipment uses axial force to make the fibers flow along the blades, to the exit of the refiner. An aqueous dispersion of cotton fibers, pretreated with a solution of sodium hydroxide and nonylphenol ethylene oxide and bleached with sodium silicate and hydrogen peroxide, were passed through 30 cycles in the disc refiner, obtaining fibers with an average diameter of 242±158 nm. Due to the heterogeneity of the CNF formed with the refiner, refinement is usually used as an extra treatment right after the isolation and bleaching of the pulp and before the mechanical step for the production of CNF (NECHYPORCHUK; BELGACEM; BRAS, 2016).

[0028] In the work "The effect of morphological changes from pulp fiber towards nano-scale fibrillated cellulose on the mechanical properties of high-strength plant fiber based composites" (NAKAGAITO; YANO, 2004), for example, the authors obtained cellulose CNF of Kraft pulps of different Pinus species by passing the material through the refiner 2 to 30 times. Part of the material that came out of the refiner was also passed through the high pressure homogenizer (also 2 to 30 times). The authors observed an improvement in the mechanical strength of the refined pulps after passing through the two equipments compared to those that only went through the refiner. Another example is the work “Isolation of cellulose microfibrils—an enzymatic approach” (JANARDHNAN; SAIN, 2006), in which bleached spruce kraft wood undergoes an enzymatic treatment using fungi and then goes through a refiner (125,000 revolutions) and by a cryocrushing process to obtain CNF. Cryocrushing is a method in which the sample is frozen using liquid nitrogen and then ground by applying shear forces. The ice crystals present in the frozen fibers then exert pressure on the cell walls, causing the fibers to break and release. In this work, the cryocrushing process was considered critical for the release of fibers from the cell wall.

[0029] Also using cryocrushing, Bhatnagar and Sain, in "Processing of Cellulose Nanofiber-reinforced Composites" (BHATNAGAR; SAIN, 2005) obtained nanofibers of estimated diameter between 5-60 nm from cellulose obtained from pre hemp and flax fibers -treated with sodium hydroxide and previously peeled kale, ground in a blender and pre-treated with hydrochloric acid and diluted sodium hydroxide.

[0030] Wang et al. in “The Novel Process to Isolate Fibrils from Cellulose Fibers by High-Intensity Ultrasonication, Part 1: Process Optimization' (WANG; CHENG, 2009) proposed the development of a new process that uses high-intensity ultrasonication to isolate cellulosic fibers of different biomasses. Such equipment exposes the cellulose pulp fibers to ultrasound waves, leading to the creation and collapse of small gas bubbles. This treatment creates strong hydrodynamic shear forces, which promote the delamination of the fibers. The authors of this work used four commercial cellulose sources as starting materials (regenerated and pure cellulose fibers, microcrystalline cellulose and cellulose pulp) and studied the use of high-intensity ultrasonication (1500 W), along with six factors that can affect fibrillation efficiency: equipment power, temperature, time, concentration of cellulose dispersion, size and distance of the tip. The study was carried out through the direct application of tip ultrasound in the fibrillation of materials suspended in water. Micro and nanoscale fiber mixtures were obtained. The results showed that only under very severe conditions, such as high power (80%), high temperature (91 °C), process times longer than 30 min and shorter distance between the ultrasound tip and the bottom of the dispersion container (7 mm), a uniform fibrillation was obtained for the pure cellulose fibers. A homogenization procedure using a high pressure homogenizer and a pre-treatment of fibers with NaOH were also tested with the pure and regenerated cellulose fibers to compare fibrillation efficiencies. The authors showed that fibers that were swollen after immersion in 2.5 M NaOH solution for 6 h were more susceptible to fibrillation after a shorter time of ultrasonication (20 min). In addition, the passage of pure cellulose fibers previously sonicated for 30 min through the high pressure homogenizer (110 Mpa, 8 passes) made the suspension more uniform than those treated with ultrasound alone.

[0031] Uetani and Yano (2011) in "Nanofibrillation of wood pulp using a high-speed blender" (UETANI; YANO, 2011) presented the use of a high-speed mixer for the production of NFC with diameters in the range of 15-20 nm of Japanese cedar pulp pretreated with sodium chlorite. The optimal condition found was for the use of samples at a concentration of 0.7% (m/m) at 37000 rpm. Furthermore, they claim that the mixing process for 30 min showed the same degree of fibrillation and less damage to the CNFs obtained compared to the material subjected to a grinding cycle in a grinder.

[0032] As seen in the works cited, the excessive expenditure of energy for the mechanical disintegration of cellulose, which normally requires many passes through the equipment and long process times, explains the great effort currently employed in the search for milder and more sustainable conditions for production of nanocellulose. Chemical or enzymatic treatments performed after the bleaching step and before mechanical disintegration are strategies used to minimize the energy required in the process, favoring the production of CNF on an industrial scale. These pre-treatments can reduce energy consumption from 20000 kWh/ton to 1000 kWh/ton of biomass (SIRÓ; PLACKETT, 2010).

[0033] In document US3041246 (BOLASKI; GALLATIN, 1962), the use of cellulase enzyme to improve fibrillation in the paper production process is described. Pääkkö et al. in “Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels” (PÄÄKKO et al., 2007) developed a new route for preparing CNF using sulfite pulp bleached from refined softwood cellulose. The pulp was subjected to mild enzymatic hydrolysis, followed by a second refinement step and a high pressure homogenization, with 3 passes through the pair of large chambers (400 μm and 200 μm diameter, respectively) and 5 passes through the pair of small chambers (diameters of 200 pm and 100 pm, respectively), obtaining a controlled fibrillation. However, the long reaction times and the high cost of enzymes are disadvantages of using this process.

[0034] Surface modification via addition of loads is another strategy widely employed to facilitate defibrillation. Liimatainen et al. in “Sulfonated cellulose nanofibrils obtained from wood pulp through regioselective oxidative bisulfite pre-treatment” (LIIMATAINEN et al., 2013) developed a new method to promote wood defibrillation to obtain sulfonated nanofibers by means of pre-treatment of bleached cellulose from pulp Birch kraft with periodate and bisulfite followed by 5 cycles of passage in the homogenizer under a pressure of 400-950 bar. In this process, nanofibers with diameters of 10-60 nm and with low charge densities were obtained (0.18 - 0.51 mmol/g).

[0035] The cationization of cellulose using a quaternary ammonium salt (2,3-epoxypropyltrimethylammonium chloride) was applied by Aulin et al. in "Self-Organized Films from Cellulose I Nanofibrils Using the Layer-byLayer Technique" (AULIN et al., 2010). The author performed the cationization of dissolved commercial pulp in order to facilitate the mechanical disintegration, which consisted of 6 passage cycles of the material in high pressure homogenizer.

[0036] A new material based on the production of CNF through the carboxymethylation of commercial softwood sulfite pulp followed by a cycle of homogenization (1650 bar) and ultrasonication for 10 min and centrifugation to remove non-fibrillated residual fibers was reported by Wagberg et al. in “The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationic Polyelectrolytes” (WÅGBERG et al., 2008).

[0037] Tejado et al. in “Energy requirements for the disintegration of cellulose fibers into cellulose nanofibers” (TEJADO et al., 2012) obtained cellulose fibers with different surface charge concentrations through sequential oxidation of bleached softwood kraft pulp with periodate and then chlorite This is a more severe method of oxidation and known in the literature for incorporating high concentrations of carboxylic groups in the fibers. The authors were able to obtain CNF without the need for mechanical energy for fiber disintegration and without the presence of non-disintegrated fibers only when the concentration of surface charges reached 3.5 mmol of carboxylate groups (-COO-) per gram of cellulose.

The oxidation mediated by the catalyst N-oxyl-2,2,6,6-tetramethylpiperidine (TEMPO), as described by Saito et al. in “Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose” (SAITO et al., 2007), it is widely used and reported in the literature. It is a selective catalytic oxidation under moderate aqueous conditions, similar to enzymatic and biological reactions, in order to introduce carboxylate groups (COO-) on the surface of the fibers. After oxidation with TEMPO, nanofibrils are traditionally obtained by mechanical defibrillation by one of the processes described earlier in this text.

[0039] In the work "A comparative study of Eucalyptus and Pinus radiata pulp fibers as raw materials for the production of cellulose nanofibrils" (SYVERUD et al., 2011), the authors verified that samples pretreated with TEMPO were more easily fibrillated and resulted in more homogeneous and transparent films compared to untreated samples. The samples were compared considering a given number of passes of the suspension of Kraft pulp fibers bleached by the homogenizer at the same pressure (1000 bar) and for the same energy consumption (10000 kWh/t per pass).

A work by Zhou et al., entitled “Acid-free preparation of cellulose nanocrystals by TEMPO oxidation and subsequent cavitation” (ZHOU et al., 2018), showed the results of the oxidation of bleached kraft pulp fibers from light wood and microcrystalline cellulose with the TEMPO radical, using NaClO at different concentrations (10 and 20 mmol/g cellulose). After sonication, using tip ultrasound with a power of 300 W, nanocrystalline cellulose was obtained without the need for an acidic hydrolysis step and with a higher yield than CNCs obtained by conventional methods.

[0041] In the present invention, the cellulose for the production of CNF was extracted from sugarcane bagasse. Previous works had already used sugarcane bagasse to obtain nanocellulose. For example, in a work by de Campos et al., “Obtaining nanofibers from curaua and sugarcane bagasse fibers using enzymatic hydrolysis followed by sonication” (DE CAMPOS et al., 2013), CNFs were obtained from bleached sugarcane bagasse with combination of two enzymatic preparations (hemicell/pectinase and endoglucanase), followed by sonication for 20 min. Energy consumption was reduced in this work, but the mechanical process was not completely eliminated.

[0042] In another more recent work, "Characteristics and environmentally friendly extraction of cellulose nanofibrils from sugarcane bagasse" (FENG et al., 2018), the authors combined multiple mechanical treatments and few chemical treatments to reduce energy consumption and pollution associated with the preparation of CNFs. First, steam explosion and dilute base catalyzed hydrothermal pretreatments were applied. Then, bleaching was done with hydrogen peroxide and the CNFs were obtained by mechanical process using a high-speed mixer (48000 rpm) for 5 min, combined with 1000 W ultrasonication of power in an ice bath for 40 min .

Brief Description of the Invention

[0043] The present invention relates to a process for obtaining cellulose nanofibrils or nanofibrillated cellulose (CNF) from sugarcane bagasse, without the need for the traditional mechanical treatment for defibrillation. An additional object is the obtained cellulose nanofibril.

[0044] In the present invention, nanofibrillated cellulose is obtained through the TIME-mediated oxidation of cellulose extracted from biomass. Cellulose is initially extracted from bagasse and isolated by organosolve, thermochemical or acid-base processes (acidic step followed by a basic step). Fibrillation occurs naturally after the oxidation of cellulose without the need for mechanical defibrillation processes (refining, grinding, homogenizers or microfluidizers), which are traditionally used, but which are very expensive in terms of energy. Different NaClO concentrations in the TEMPO oxidation result in different morphologies of the final nanocelluloses. In addition to NFCs, nanocrystals can also be obtained if the oxidized biomass is subjected to a mechanical sonication process, without the need for an acidic step. The process works well on sugarcane bagasse, as this is a low recalcitrant plant biomass.

Brief Description of Figures

[0045] Figure 1 shows the procedure for isolating CNF from sugarcane bagasse, including the steps of pre-treatment, bleaching and oxidation.

[0046] Figure 2 shows photographs of sugarcane bagasse cellulose dispersions (3% m/m) oxidized with TEMPO using 25 mmol NaClO/g cellulose (I and III) and 50 mmol NaClO/g cellulose (II and IV) before (a) and after (b) the tube inversion test.

[0047] Figure 3 shows topography images obtained by atomic force microscopy (AFM) of sugarcane bagasse cellulose samples oxidized with TEMPO (a) using 25 mmol NaClO/g cellulose, where cellulose nanocrystals with average lengths are observed of 379 ± 132 nm and average diameters of 4 ± 1 nm and (b) using 50 mmol NaClO/g cellulose, where cellulose nanocrystals with average lengths of 243 ± 119 nm and average diameters of 4 ± 1 nm are observed. The measurements were obtained with the aid of the Gwyddion 2.48 program of 100 fibers for each sample, taken from a total of four topography images, two images from each sample.

[0048] Figure 4 is a graph containing the amount of carboxylic groups incorporated on the surface of the fibers (black) and the oxygen/carbon ratio (blue) of the oxidized cellulose samples using 25 and 50 mmol NaClO/g cellulose. The data on the amount of carboxylic groups were obtained through conductimetric titration of the oxidized cellulose. This analysis allowed us to understand the dependence of the density of negative charges incorporated in the material with the possibility of disintegration of the fibers already in the washing process after oxidation. This happens because, the greater the amount of charges on the surface of the fibers, the greater the repulsion between them and the easier the dissociation of these fibers into nanofibers. The oxygen/carbon ratio was obtained by photoelectron spectroscopy excited by x-rays (XPS), a surface analysis technique. After oxidation, there will be a conversion of the hydroxyl groups (-OH) of the cellulose into carboxyl groups (-COO-) and this ratio is expected to increase due to the increase in the amount of oxygen, as evidenced by conductimetric titration. However, it is observed that the oxygen/carbon ratios are very similar, that is, there is some other material still present in the fibers that may be being oxidized along with the cellulose. We believe that this material is the lignin remaining in the fibers, since this macromolecule contributes to the adhesion between the fibers and, when removed, facilitates their disintegration.

Detailed Description of the Invention

[0049] The present invention refers to a process for obtaining cellulose nanofibrils or nanofibrillated cellulose (CNF) from sugarcane bagasse, without the need for the traditional mechanical treatment for defibrillation. An additional object is the obtained cellulose nanofibril.

• (a.1) adicionar a biomassa (massa seca) a uma mistura de etanol/água destilada na proporção entre 1:2 e 2:1 (v/v), a uma razão sólido/líquido entre 1:7 e 1:10 em um reator de alta pressão;
• (a.2) aguardar a estabilização da temperatura em 150 a 220°C e manter por 60 a 180 min sob uma pressão de 25 a 35 bar;
• (a.3) adicionar o material resultante da etapa (a.2) com uma solução de NaOH 1% m/v na proporção sólido/líquido 1:4 a 1:8 a um desfibrilador por 3 min;
• (a.4) lavar com água até obter pH neutro;
• (a.5) centrifugar a uma rotação entre 600 e 1800 rpm;
• (a.6) conservar a polpa úmida sob refrigeração;

(b) Branqueamento da polpa:

• (b.1) adicionar o equivalente a massa seca de polpa a uma solução NaOH de 2 a 5% (m/m em água) a uma proporção sólido/líquido de 1:20;
• (b.2) aquecer até 70-80 °C;
• (b.3) adicionar lentamente solução de H2O2 (de 12 a 24% m/m em água) na proporção sólido/líquido de 1:20;
• (b.4) agitar mecanicamente por 40 min a 300-600 rpm;
• (b.5) filtrar usando peneira de 200 mesh;
• (b.6) lavar com água destilada na proporção polpa branqueada/água de 1:20 a 1:50 até obter pH neutro;
• (b.7) repetir as etapas de (b.1) a (b.6);
• (b.8) conservar a polpa branqueada sob refrigeração;

(c) Oxidação mediada pelo radical TEMPO:

• (c.1) adicionar água destilada a uma equivalente a massa seca de polpa branqueada em uma proporção sólido/líquido de 1:50 a 1:100 e manter de 12 a 24 h;
• (c.2) adicionar o reagente TEMPO na proporção TEMPO/polpa branqueada de 1:10 (mmol/g) e brometo de sódio na proporção NaBr/polpa branqueada de 1:1 (mmol/g)
• (c.3) adicionar solução de NaClO 12% (m/v), com pH previamente ajustado para 10 com gotas de solução HCl 0,1 M, sendo a proporção entre 15 mmol NaClO/g celulose e 75 mmol NaClO/g celulose;
• (c.4) agitar a dispersão obtida na etapa (c.3) de 100 a 150 minutos a 100-300 rpm;
• (c.5) manter o pH do meio a 10 com adição de gotas de solução 0,5 M de NaOH;
• (c.6) lavar cerca de 8 a 12 vezes a celulose oxidada obtida na etapa (c.5) com água destilada na proporção massa de polpa oxidada/água destilada de 1:60, por 5 min a 2500-4500 rpm; e
• (c.7) conservar as nanofibrilas de celulose sob refrigeração.

[0050] The process of obtaining cellulose nanofibrils or nanofibrillated cellulose (CNF) comprises the following steps:
(a) Pre-treatment of in natura biomass:

• (a.1) add the biomass (dry mass) to an ethanol/distilled water mixture in a ratio between 1:2 and 2:1 (v/v), at a solid/liquid ratio between 1:7 and 1:10 in a high pressure reactor;
• (a.2) wait for the temperature to stabilize at 150 to 220°C and keep for 60 to 180 min under a pressure of 25 to 35 bar;
• (a.3) add the material resulting from step (a.2) with a solution of NaOH 1% m/v in the solid/liquid ratio 1:4 to 1:8 to a defibrillator for 3 min;
• (a.4) wash with water until neutral pH is obtained;
• (a.5) centrifuge at a rotation between 600 and 1800 rpm;
• (a.6) keep the pulp moist under refrigeration;

(b) Bleaching of the pulp:

• (b.1) add the equivalent of dry pulp mass to a 2 to 5% NaOH solution (m/m in water) at a solid/liquid ratio of 1:20;
• (b.2) heat to 70-80 °C;
• (b.3) slowly add H2O2 solution (12 to 24% m/m in water) in a solid/liquid ratio of 1:20;
• (b.4) mechanically stir for 40 min at 300-600 rpm;
• (b.5) filter using 200 mesh sieve;
• (b.6) wash with distilled water in the proportion bleached pulp/water from 1:20 to 1:50 until obtaining a neutral pH;
• (b.7) repeat steps from (b.1) to (b.6);
• (b.8) keep the bleached pulp under refrigeration;

(c) TIME radical-mediated oxidation:

• (c.1) add distilled water to an equivalent of bleached pulp dry mass in a solid/liquid ratio of 1:50 to 1:100 and keep for 12 to 24 h;
• (c.2) add the TEMPO reagent at the TEMPO/bleached pulp ratio of 1:10 (mmol/g) and sodium bromide at the NaBr/bleached pulp ratio of 1:1 (mmol/g)
• (c.3) add 12% NaClO solution (m/v), with pH previously adjusted to 10 with drops of 0.1 M HCl solution, the proportion being between 15 mmol NaClO/g cellulose and 75 mmol NaClO/g cellulose ;
• (c.4) stir the dispersion obtained in step (c.3) for 100 to 150 minutes at 100-300 rpm;
• (c.5) maintain the pH of the medium at 10 with the addition of drops of 0.5 M NaOH solution;
• (c.6) wash about 8 to 12 times the oxidized cellulose obtained in step (c.5) with distilled water at the oxidized pulp mass/distilled water ratio of 1:60, for 5 min at 2500-4500 rpm; and
• (c.7) keep the cellulose nanofibrils under refrigeration.

[0051] Biomass can be selected from sugarcane bagasse, and other grasses such as corn, rice straw, elephant grass, brachiaria and other grasses.

[0052] In step (a) of pre-treatment, the biomass goes through the pulping process to isolate the cellulose. Pulping must delignify the pulp, which can be carried out through different processes, such as organosolve (using an organic solvent and water), Kraft (using sodium sulfide), sulfite, diluted acid, diluted base and hydrothermal treatment.

[0053] In step (b) of bleaching, the objective is to remove residual lignin and chromophore structures from the pulp. Among the methods that can be used are those that perform an oxidation step, which can use chlorine, chlorine dioxide, or hydrogen peroxide under alkaline conditions (sodium hydroxide). Bleaching using hypochlorite, oxygen or enzymes can also be used.

[0054] In step (c) to obtain CNF, the bleached cellulose is kept hydrated with distilled water for 24 h. Then, the TEMPO reagent (0.1 mmol/g cellulose) and sodium bromide (1 mmol/g cellulose) are added. Oxidation starts when specific amounts (greater than 15 mmol/g) of a 12% (w/v) NaClO solution are added to the reaction (between 15 mmol/g and 75 mmol/g of cellulose). The system remains under mechanical agitation (100 to 300 rpm) at room temperature from 100 to 130 min and the pH is maintained at 10 throughout the reaction by adding 0.5 M NaOH solution until the consumption of NaOH is not most detected. Finally, the oxidized cellulose suspension is then neutralized with addition of 0.1 M HCl solution and washed with water by centrifugation to remove excess reagents until the constant conductivity of the wash water is reached. The final product, nanofibrillated cellulose, is stored in a refrigerator.

Example of the invention

[0055] To obtain CNF, according to the present invention, 300 g of sugarcane bagasse (dry mass) were added to a mixture of 1.5 L of distilled water and 1.5 L of ethanol in a reactor of high pressure. The temperature was stabilized at 190 °C for 40 minutes and the reaction remained for 2 h and under a pressure of approximately 30 bar, reached with the progress of the reaction in the closed flask. At the end of time and after cooling the reactor, the resulting material was added to a defibrillator for 3 min, together with 1200 mL of a 1% m/v NaOH solution, for the defibrillation of the material and for the removal of lignin remaining. Then, the pulp was washed several times with about 10 L of water to neutral pH and centrifuged to remove excess water. The reaction yield, calculated using the moisture balance heated to 105 °C, was 54%. The moist pulp was kept in a refrigerator.

[0056] In the bleaching step, in a 4 L beaker were added 40 g (dry mass) of pulp and 800 mL of 5% NaOH solution (m/m in water). The system was heated to 70 °C and then the slow addition of 800 mL of H2O2 solution (24% m/m in water) was started. At the end of the addition, the system remained under mechanical agitation (500 rpm) for 40 min. At the end of the reaction, the bleached pulp was then separated by filtration using a 200 mesh sieve and washed 4 times with 2L portions of distilled water to neutral pH. The solids content calculated using the moisture balance heated to 105 °C was 25 g of bleached pulp. The bleaching step procedures were then repeated with the resulting pulp, this time using 500 ml of 5% NaOH (m/m in water) and H2O2 (24% m/m in water) solutions and the bleached pulp was washed by 4 times with 1250 mL portions of water. The solids content was recalculated, and the yield of the complete bleaching process was 50%. The bleached pulp was kept wet in a refrigerator.

[0057] For oxidation, 5 g (dry mass) of bleached pulp were placed in a 1 L beaker together with 500 mL of distilled water, where they remained for 24 h. Then 0.08 g of TEMPO reagent and 0.5 g of sodium bromide were added to the beaker. Then, volumes of 12% NaClO solution (m/v) with pH previously adjusted to 10 with drops of 0.1 M HCl solution were added, these volumes being: 78.0 mL (25 mmol NaClO/g cellulose) or 156.0 mL (50 mmol NaClO/g cellulose). The dispersion was stirred at 200 rpm for 130 minutes, the pH of the medium being maintained at 10 with the addition of drops of 0.5 M NaOH solution.

[0058] At the end of the reaction, the oxidized cellulose was washed with 300 mL portions of distilled water for 12 centrifugations at 4500 rpm for 5 min, when then constant conductivity in the wash water was reached. The final material was then stored in a refrigerator.

[0059] The CNFs concentrated at 3% m/m by solvent evaporation obtained with each of the volumes of NaClO used presented the gel appearance shown in Figure 3. The yield of CNF production using 25 mmol NaClO/g cellulose was 57%, while the yield using 50 mmol NaClO/g cellulose was 59%.

[0060] Atomic force microscopy (AFM) images of the CNFs oxidized with 25 and 50 mmol NaClO/g cellulose are shown in Figure 4. The mean diameter and length of the CNFs obtained with 25 mmol NaClO/g cellulose were 4 ± 1 nm and 379 ± 132 nm, respectively. The diameter and mean length of the nanofibers obtained with 50 mmol NaClO/g cellulose were 4 ± 1 nm and 243 ± 119 nm.

[0061] The amount of carboxylic groups incorporated into the surface and oxidized fibers and the oxygen/carbon ratio of the materials are shown in the graph in Figure 6. The CNFs oxidized with 25 and 50 mmol NaClO/g cellulose showed concentration of COO- groups of 1.07 and 1.40 mmol/g cellulose, and oxygen/carbon ratio of 0.82 and 0.83, respectively.

Claims (8)

Process for obtaining cellulose nanofibrils (CNF) characterized by comprising the following steps:

• (a) Pre-treatment of in natura biomass;
• (b) Bleaching of the pulp; and
• (c) Oxidation mediated by the TEMPO radical.

Process according to claim 1, characterized in that step (a) comprises the following sub-steps:

• (a.1) add the biomass (dry mass) to an ethanol/distilled water mixture in a ratio between 1:2 and 2:1 (v/v), at a solid/liquid ratio between 1:7 and 1:10 in a high pressure reactor;
• (a.2) wait for the temperature to stabilize at 150 to 220°C and keep for 60 to 180 min under a pressure of 25 to 35 bar;
• (a.3) add the material resulting from step (a.2) with a solution of NaOH 1% m/v in the solid/liquid ratio 1:4 to 1:8 to a defibrillator for 3 min;
• (a.4) wash with water until neutral pH is obtained;
• (a.5) centrifuge at a rotation between 600 and 1800 rpm; and
• (a.6) keep the pulp moist under refrigeration.

Process according to claim 1, characterized in that step (b) comprises the following sub-steps:

• (b.1) add the equivalent of dry pulp mass to a 2 to 5% NaOH solution (m/m in water) at a solid/liquid ratio of 1:20;
• (b.2) heat to 70-80 °C;
• (b.3) slowly add H2O2 solution (12 to 24% m/m in water) in a solid/liquid ratio of 1:20;
• (b.4) mechanically stir for 40 min at 300-600 rpm;
• (b.5) filter using 200 mesh sieve;
• (b.6) wash with distilled water in the proportion bleached pulp/water from 1:20 to 1:50 until obtaining a neutral pH;
• (b.7) repeat steps from (b.1) to (b.6); and
• (b.8) keep the bleached pulp under refrigeration.

Process according to claim 1, characterized in that step (c) comprises the following sub-steps:

• (c.1) add distilled water to an equivalent of bleached pulp dry mass in a solid/liquid ratio of 1:50 to 1:100 and keep for 12 to 24 h;
• (c.2) add the TEMPO reagent at the TEMPO/bleached pulp ratio of 1:10 (mmol/g) and sodium bromide at the NaBr/bleached pulp ratio of 1:1 (mmol/g)
• (c.3) add 12% NaClO solution (m/v), with pH previously adjusted to 10 with drops of 0.1 M HCl solution, the proportion being between 15 mmol NaClO/g cellulose and 75 mmol NaClO/g cellulose ;
• (c.4) stir the dispersion obtained in step (c.3) for 100 to 150 minutes at 100-300 rpm;
• (c.5) maintain the pH of the medium at 10 with the addition of drops of 0.5 M NaOH solution;
• (c.6) wash about 8 to 12 times the oxidized cellulose obtained in step (c.5) with distilled water at the oxidized pulp mass/distilled water ratio of 1:60, for 5 min at 2500-4500 rpm; and
• (c.7) keep the cellulose nanofibrils under refrigeration.

Process, according to claims 1 to 4, characterized in that the biomass can be selected from sugarcane bagasse, and other grasses, such as corn, rice straw, elephant grass, brachiaria and grasses. Process according to claims 1 to 5, characterized in that it comprises yield between 57 and 59%. Cellulose nanofibrils (CNF) characterized by being obtained according to the process described in claims 1 to 6, comprising a diameter of 4 ± 1 nm and an average length of 243 ± 132 nm. Nanofibrils, according to claim 7, characterized in that they comprise a concentration of COO- groups varying between 1.07 and 1.40 mmol/g cellulose and an oxygen/carbon ratio ranging between 0.82 and 0.83.

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