Hydrophobic nanocellulose, process for preparing same and uses thereof

Abstract

The present invention relates to a one-pot process for producing hydrophobic nanocellulose using a deep eutectic solvent (DES) and acetic anhydride. The method is sustainable, recyclable and efficient, achieving a high degree of substitution (DS) and improving the hydrophobicity of nanocellulose. It is also scalable, maintaining good performance and thermal properties in large-scale production.

Description

[0001] Hydrophobic nanocellulose, its preparation process and uses

[0002] DESCRIPTIVE MEMORANDUM

[0003] SCOPE

[0004] The present invention relates to the field of nanocellulose modification, in particular, to a process for obtaining hydrophobic nanocellulose with improved hydrophobic properties, with potential for use in sustainable coatings, biomedical devices, marine vehicles, textiles, paints, and other composite materials requiring compatibility with hydrophobic polymer matrices.

[0005] BACKGROUND

[0006] Nanocellulose is a renewable and versatile material with applications in diverse areas, such as coatings, polymer composites, biomedical devices, and packaging products. Significant progress has been made in modifying nanocellulose to enhance its properties for specific applications. In particular, modifying the surface of nanocellulose is crucial for improving its compatibility with nonpolar polymer matrices and its barrier and hydrophobicity properties.

[0007] Several methods have been developed to modify nanocellulose, as shown in publication W02022027150, which proposes obtaining nanocellulose using "green" solvents such as ionic liquids (ILs) and deep eutectic solvents (DESs). These solvents allow for an acid hydrolysis process and a subsequent improvement in the physical properties, such as the thermal stability of nanocellulose. However, the technology described in this document focuses on improving thermal stability without directly modifying the material's surface to alter its hydrophobic properties, which limits its usefulness in certain applications. On the other hand, document W02020044210 describes a method for the hydrophobic modification of cellulose microfibril (MFC) films using a deep eutectic solvent combined with a hydrophobic modifying agent.The technology described in this document seeks to improve the hydrophobicity and barrier properties of MFC films, but it is limited to the modification of preformed films, not providing a suitable process for the direct production of hydrophobic nanocellulose in a simple manner.

[0008] Publication CN104004101A also describes a method for preparing acetylated nanocellulose using an anhydrous phosphoric acid system in a one-step process. This approach has the advantage of simplifying the modification process by not requiring the separation of intermediate products. However, the use of anhydrous phosphoric acid presents limitations in terms of corrosion and process sustainability, and the possibility of recycling the solvent used is not emphasized.

[0009] Nanocellulose exhibits a high affinity for water due to the abundance of hydroxyl groups on its surface, with a contact angle of 28.1°. This limits its application in composite materials and coatings that require hydrophobic properties and good compatibility with nonpolar polymer matrices. Although methods have been developed to improve the properties of nanocellulose, these methods primarily focus on enhancing its thermal and physical properties, without addressing the need for hydrophobicity and compatibility with nonpolar materials.

[0010] In view of the above, there is a need to develop an efficient and sustainable method for the production of hydrophobic nanocellulose that combines the simplicity of a single-stage process, the use of recyclable green solvents, and the optimal hydrophobic functionalization of the nanocellulose. BRIEF DESCRIPTION OF THE INVENTION

[0011] The present invention relates to an efficient and sustainable one-pot process for obtaining hydrophobic nanocellulose using a deep eutectic solvent (DES) composed of choline chloride, p-toluenesulfonic acid, and phosphoric acid, in combination with acetic anhydride as an acetylating agent. The process aims to impart hydrophobic properties to the nanocellulose, facilitating its use in applications requiring compatibility with hydrophobic polymer matrices, such as coatings and composite materials.

[0012] This method offers the advantage of using a recyclable solvent system that acts as both a dispersion medium and a catalyst, eliminating the need for intermediate separation or drying steps found in conventional methods. Modifying the surface of the nanocellulose with acetyl groups significantly improves its hydrophobicity, resulting in greater performance and functionality in industrial applications.

[0013] BRIEF DESCRIPTION OF THE FIGURES

[0014] Figure 1: Comparative FTIR spectra of commercial cellulose (CFII) (top line) and hydrophobic cellulose of the present invention (bottom line).

[0015] Figure 2: (A) FESEM image of hydrophobic nanocellulose obtained with DES ChCl:pTSA:PA and 3 ml of AcA; (B) STEM image of hydrophobic NC obtained with DES ChCl:pTSA:PA and 3 ml of AcA; (C) and (D) Histograms obtained from the FESEM and STEM analyses; respectively.

[0016] Figure 3: Contact angle obtained for a) commercial cellulose (CFII); b) nanocrystalline cellulose obtained using ternary eutectic mixtures ChCl:pTSA:PA (1:1:1.35); c) hydrophobic nanocellulose according to the invention using ChCl:pTSA:PA (1:1:1.35) and AcA (3 ml). Figure 4: Graph with yields of five cycles to obtain A-NC using DES ChCl:pTSA:PA (1:1:1.35) and 3 ml of AcA, at 80 °C and 3 h reaction time.

[0017] DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to the production of hydrophobic nanocellulose (NC) prepared by a single-stage process employing a system of deep eutectic solvents (DES) comprising choline chloride, p-toluenesulfonic acid, and phosphoric acid, which enables the simultaneous dispersion and acetylation of the cellulose. The process is carried out in a single reactor, eliminating the need to separate intermediate products and utilizing the DES as a catalyst, thus contributing to the efficiency and sustainability of the process.

[0019] In this process, cellulose, preferably in the form of cotton fibers or microcrystals, is dispersed in the eutectic solvent at a suitable temperature (e.g., 80°C). Acetic anhydride is then added dropwise, causing direct acetylation of the hydroxyl groups of the cellulose. The reaction is carried out for a specific period of time to ensure complete acetylation, thus achieving the desired hydrophobic modification.

[0020] The DES system employed in this invention offers several advantages, such as its low toxicity and the possibility of being recycled and reused in successive reaction cycles, which significantly reduces the environmental impact of the process. Furthermore, the crystalline structure of the nanocellulose is maintained during acetylation, thus preserving the physical and mechanical properties of the material.

[0021] The resulting products, acetylated nanocellulose nanocrystals, are evaluated to determine their degree of substitution, hydrophobicity (measured by the water contact angle), performance, crystallinity, and thermal stability, ensuring they meet specifications for industrial applications in coatings, polymer composites, and other areas requiring hydrophobic and sustainable materials. Definitions

[0022] The parameters that allow the determination of the functional and performance properties of the hydrophobic nanocellulose obtained according to the present invention are defined below.

[0023] Degree of substitution (DS): This parameter measures how many hydroxyl groups in the cellulose structure have been replaced by acetyl groups. A higher DS indicates greater acetylation.

[0024] Water contact angle (WCA): This angle indicates the hydrophobicity of the material's surface. The larger the WCA, the more hydrophobic the nanocellulose. Similar to DS, WCA increases with the volume of AcA.

[0025] Yield (%): Amount of acetylated nanocellulose obtained in relation to the amount of initial cellulose used in the process.

[0026] Dimensions (Length and Width): These dimensions correspond to the average measurements of the nanocellulose nanoparticles obtained.

[0027] Thermal properties (Tmax): Maximum temperature before thermal decomposition, which allows evaluation of the thermal stability of the modified material.

[0028] PREFERRED EMBODIMENTS OF THE INVENTION

[0029] The present invention relates to a process for preparing hydrophobic nanocellulose, comprising: a) preparing a deep eutectic system (DES); b) dispersing powdered cellulose in said DES system at a temperature between 60°C and 100°C; c) adding acetic anhydride to the mixture obtained in step b) and allowing the acetylation of the cellulose.

[0030] Where said deep eutectic system (DES) is prepared by mixing choline chloride, p-toluenesulfonic acid and phosphoric acid.

[0031] Preferably, the ratio of choline chloride, toluenesulfonic acid, and phosphoric acid is in the range of 1:1:0.3 to 1:2:2. The process further comprises the steps of: d) stopping the reaction by adding cold water, followed by centrifugation and sonication to remove solvent residues; and e) drying the acetylated nanocellulose obtained to produce hydrophobic nanocellulose in powder form.

[0032] Optionally, the deep eutectic system (DES) is recovered for reuse. This involves adding activated carbon with agitation at a temperature between 45°C and 80°C. This process removes sugars that may be produced by the complete hydrolysis of cellulose due to excessive acidity in the medium, as well as other impurities, typically hydrophobic nanocellulose fragments suspended in the solvent to be recycled. The filtered liquid is then distilled to remove excess water and acetic anhydride.

[0033] The DES system can be reused up to 8 times without problems.

[0034] Furthermore, the present invention relates to hydrophobic nanocellulose obtained by the process described above.

[0035] The hydrophobic nanocellulose obtained comprises cellulose nanocrystals, in which the surface hydroxyl groups of the cellulose are acetylated, and in which the hydrophobic nanocellulose has a water contact angle (WCA) greater than 46.2°.

[0036] The present invention also relates to the use of hydrophobic nanocellulose for the manufacture of water-resistant protective coatings or as a reinforcing agent in non-polar polymer composite materials, intended for applications in the packaging, construction, textile, and biomedical device industries. EXAMPLES

[0037] Example 1: Preparation of Hydrophobic Nanocellulose using a Ternary Deep Eutectic System (DES)

[0038] In a round-bottom flask, choline chloride, β-toluenesulfonic acid, and phosphoric acid were mixed in a molar ratio of 1:1:1.35 to prepare the deep eutectic system (DES). The mixture was stirred at 60°C for 4 hours until a homogeneous liquid was obtained. Then, powdered cellulose (3% w / w) was added to the DES, and the mixture was heated to 80°C with constant stirring at 500 rpm. Acetic anhydride was added dropwise at a rate of 1 mL / min, allowing the acetylation reaction to occur for 3 hours.

[0039] After the reaction was complete, cold deionized water was added to the mixture to stop the reaction, followed by centrifugation at 10,000 rpm for 15 minutes to separate the residues. This washing procedure was repeated three times to ensure complete removal of the DES and any remaining acetic anhydride. The dispersion was sonicated and then centrifuged. The suspended phase was dried at 60°C for 24 hours to obtain hydrophobic nanocellulose in the form of a white powder.

[0040] Analysis of the Degree of Hydrophobicity and Performance

[0041] The degree of substitution (DS) was determined by Fourier transform infrared spectroscopy (FTIR), normalizing the peak area of ​​the ester carbonyl group band (1742 cm′) to the COC stretching peak of the cellulose ring (1162 cm′). The results showed that the DS value varied between 0.2 and 2.9, depending on the amount of acetic anhydride used and the reaction conditions.

[0042] The water contact angle (WCA) was measured to evaluate the hydrophobicity of the nanocellulose. The results indicated that when 3 mL of acetic anhydride were used, a contact angle greater than 64° was obtained, while with smaller volumes of anhydride, the contact angle decreased, indicating lower hydrophobicity. The process yield was calculated based on the amount of hydrophobic nanocellulose obtained.

[0043] Table 1 summarizes the results obtained by varying the volume of acetic anhydride (AcA) in the acetylation process of nanocellulose according to the present invention.

[0044] Table 1

[0045] *: P7 was performed at 100°C for 4 hours.

[0046] Table 1 shows that the SD increases as the volume of AcA used increases. For example, with 3.0 ml of AcA (P1), an SD of 2.4 was achieved, while with smaller volumes, such as 1.5 ml (P4), the SD was only 0.4.

[0047] Like DS, WCA increases with AcA volume. For example, the highest WCA value, 66.62°, was reached when 3 ml of AcA at 100°C was used (P7).

[0048] The maximum yield was 77% when 3 mL of AcA were used at 100°C (P7). In general, larger volumes of AcA produce better yields.

[0049] Regarding the dimensions obtained, corresponding to the average measurements of the nanocellulose nanoparticles, as the volume of AcA increases, the average dimensions also vary, although there is no clear trend. In P1, with 3 ml of AcA, the nanoparticles had an average length of 193 nm and a width of 43 nm, while in P5, with 1 ml of AcA, the nanoparticles were somewhat larger.

[0050] Table 2 presents the results of the scalability of the acetylation process of the nanocellulose obtained according to the present invention, as the amount of DES used increases. Table 2

[0051] In all cases, good acetylation was achieved. However, as the reaction scale increases (from x20 to x800), the DS tends to decrease slightly. This is common in industrial-scale processes, where efficiency can decrease due to the difficulties in homogeneously controlling the reaction at larger volumes. At EO (original scale), the DS is 2.41, while at E4 (x800), it decreases to 2.26.

[0052] Regarding the WCA, it also shows a slight decrease as the scale increases. The highest value of 69.3° was obtained at the x20 scale (E1), while the lowest value was 63.5° at the x800 scale (E4). This indicates that the hydrophobicity of nanocellulose is more difficult to maintain at larger scales.

[0053] The maximum thermal decomposition temperature also decreases slightly with increasing scale. This suggests that the thermal properties of the material are not significantly affected. Regarding the dimensions of the nanoparticles, at larger scales, the dimensions of the nanocellulose nanoparticles remain within the nanometric range. At E4 (x800 scale), the average particle dimensions were 35 nm wide and 246 nm long, which is comparable to the results obtained at the original scale.

[0054] The process yield remains relatively high across all scales, ranging from 61.5% to 72.5%. Surprisingly, the yield remains fairly constant even with significant scale-up, which is a good indication of the process's viability for large-scale production. The nanocellulose acetylated by the one-pot process using the deep eutectic system (DES) and acetic anhydride of the present invention exhibits a significantly higher degree of substitution than those reported for conventional methods, reaching up to 2.9, which suggests more efficient acetylation of the hydroxyl groups. Furthermore, the water contact angles (WCA) are higher, reaching up to 66.6°, indicating greater hydrophobicity.This is due to the use of a DES that acts as both a solvent and a catalyst, allowing a more effective surface modification of the nanocellulose in a single step, which not only improves the hydrophobic properties, but also increases the process yield, reaching up to 77%.

[0055] In contrast, acetylated nanocellulose, obtained through conventional two-stage acetylation methods, exhibits a degree of substitution (DS) ranging from 1.4 to 1.7, as described in the literature. These methods typically involve a hydrolysis step followed by acetylation in the presence of toxic agents such as sulfuric acid. Regarding hydrophobic properties, the water contact angles (WCA) of commercially available acetylated nanocellulose are usually below 50°, indicating limited hydrophobicity.

[0056] In summary, the acetylated nanocellulose obtained according to the present invention not only surpasses commercial acetylated nanocellulose in its degree of substitution, but also exhibits better hydrophobicity and a more sustainable and efficient production process.

[0057] Example 2: Reuse of Deep Eutectic Solvent

[0058] The DES system (choline chloride, β-toluenesulfonic acid, and phosphoric acid) was recovered from the residual mixture after nanocellulose production. To do this, activated carbon was added to the recycled DES, and the mixture was stirred at 60°C for 1 hour to remove sugars, which can be produced by the complete hydrolysis of cellulose due to excessive acidity in the medium, and other impurities, typically hydrophobic nanocellulose fragments remaining suspended in the solvent to be recycled. Vacuum distillation was then performed to remove excess water and acetic anhydride. This recycling process was repeated five times, and the DES yield in hydrophobic nanocellulose production remained constant throughout all cycles. Figure 4 shows the yield results obtained in each cycle.

[0059] Crystallinity and Thermal Stability Analysis

[0060] The crystallinity of the acetylated nanocellulose was evaluated by X-ray diffraction (XRD) analysis. The results showed that the crystalline structure of the nanocellulose remained intact after acetylation. Furthermore, thermal stability was determined by thermogravimetric analysis (TGA), demonstrating that acetylation did not negatively affect the thermal resistance of the material. ma The x of the hydrophobic nanocellulose was 391°C, higher than that of unmodified cellulose (366°C).

[0061] These examples demonstrate that the developed process allows obtaining hydrophobic nanocellulose in a single stage and a single reactor, maintaining its structural properties and improving its hydrophobicity and thermal stability, while employing a sustainable and recyclable solvent system.

[0062] The preceding specification is considered to be merely illustrative of the principles of the invention. The scope of the claims should not be limited by the exemplary embodiments set forth in the preceding section, but should be given the broadest interpretation consistent with the description as a whole.

Claims

CLAIMS 1. Process for preparing hydrophobic nanocellulose, CHARACTERIZED in that it comprises: a) preparing a deep eutectic system (DES); b) dispersing powdered cellulose in said DES system at a temperature between 60°C and 100°C; and c) adding acetic anhydride to the mixture obtained in step b) and allowing the acetylation of the cellulose; 2. The preparation process according to claim 1, CHARACTERIZED in that said deep eutectic system (DES) is prepared by mixing choline chloride, toluenesulfonic acid and phosphoric acid.

3. The preparation process according to claim 2, CHARACTERIZED in that the ratio of choline chloride, toluenesulfonic acid and phosphoric acid is in the range of 1:1:0.3 to 1:2:2.

1.

4. The preparation process according to claim 1, CHARACTERIZED in that it further comprises the steps of: d) stopping the reaction by adding cold water, followed by centrifugation and sonication to remove solvent residues; and e) drying the acetylated nanocellulose obtained to produce hydrophobic nanocellulose in powder form.

5. The preparation process according to claim 1, CHARACTERIZED in that the deep eutectic system (DES) is recovered for reuse.

6. The preparation process according to claim 5, CHARACTERIZED in that it comprises contacting the system with activated carbon at a temperature between 45°C and 80°C.

7. The preparation process according to claim 5, CHARACTERIZED in that the DES system can be reused up to 8 cycles.

8. Hydrophobic nanocellulose, CHARACTERIZED in that it is obtained by the process according to claim 1.

9. Hydrophobic nanocellulose obtained by the process according to claim 1, CHARACTERIZED in that it comprises cellulose nanocrystals, wherein the surface hydroxyl groups of the cellulose are acetylated, and wherein the hydrophobic nanocellulose has a water contact angle (WCA) greater than 46.2°.

10. Use of hydrophobic nanocellulose according to claim 8 or 9, CHARACTERIZED in that it serves for the manufacture of water-resistant protective coatings or as a reinforcing agent in non-polar polymer composite materials, intended for applications in the packaging, construction, textile and biomedical device industries.

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