Method for green gluing native wood for outdoor use

Abstract

The present invention relates to a method for producing partially acetylated cellulose nanofibres, CNF, which have a low re-agglomeration tendency and can be used as a reinforcing agent for thermoplastic and thermosetting resins. The method comprises the steps of fibre processing, acetylation and washing. This is a simple and low-cost method that does not require the use of a catalyst or acetic anhydride as a reagent.

Description

[0001] GREEN GLUING PROCEDURE FOR NATIVE WOODS FOR EXTERIOR USE

[0002] Technical Sector

[0003] The present invention relates to the forestry industry, more particularly to a process for making a reinforcing additive based on cellulose nanofibers with a low tendency to reagglomerate and whose homogeneous distribution in different types of resins is feasible with traditional mixing methods.

[0004] Previous Technique

[0005] Cellulose nanofibrils (CNFs) and microfibrils are an industrial additive of growing interest to the paper, adhesives, concrete, and plastics industries, among others. Their incorporation into various matrices improves the strength properties of the resulting composite material because a network of minute fibrils acts as a mechanical reinforcement. However, a fundamental requirement for this to occur is that the CNFs be homogeneously distributed, without forming fibril clusters, which would have the opposite effect, weakening the material.

[0006] Non-cellulose fibers (NCFs) are composed of cellulose, which comprises glucose monomeric units linked by glycosidic bonds. Each glucose monomer, in turn, possesses three hydroxyl groups that exert hydrogen bonding forces, causing the cellulose chains to attract each other. For this reason, the cellulose fibers are tightly intertwined, resulting in highly crystalline regions (characterized by strong intermolecular interactions) and amorphous areas (where these interactions are more limited). Given their high polarity, NCFs are typically used in hydrophilic media, preferably aqueous; however, high shear forces are necessary for their dispersion.

[0007] Numerous studies have established the possibility of incorporating NFC as a reinforcing agent in both thermoplastic and thermoset materials. The results are highly variable in terms of the properties of the resulting composite materials, primarily due to the difficulty of homogeneously dispersing the NFC within the material.

[0008] In response, surface modification of NFCs has been proposed, including partial acetylation of the hydroxyl groups. Partially acetylated NFCs behave differently from unmodified NFCs. On the one hand, acetyl groups are less polar than hydroxyl groups, thereby increasing the compatibility between cellulose and hydrophobic materials. On the other hand, the acetyl groups separate the remaining intermolecular hydroxyl groups in the cellulose fibers, reducing the attractive forces and, consequently, the tendency for reagglomeration. Therefore, acetylated NFCs are a suitable additive for reinforcing the mechanical properties of materials, including resins used as adhesives, paints, and plastics, among others.

[0009] Cellulose acetylation is a well-known process that has been carried out industrially for over a century. It is used to produce cellulose acetate, a thermoplastic material with high-value applications. The acetylation reaction is a type of esterification reaction that is generally performed with acetic anhydride in concentrated acetic acid in the presence of a catalyst (in the laboratory, other organic solvents are also used). Acetylation usually occurs preferentially on the surface, substituting the three hydroxyl groups of glucose; therefore, the final product can be very heterogeneous in terms of the distribution of acetyl groups along the cellulose chain: surface and amorphous chains can have a high degree of substitution, while crystalline parts may remain unacetylated.Therefore, the industrial practice for producing cellulose acetate is to completely replace the cellulose fibers with acetyl groups (substitution degree 3) and then partially and in a controlled manner hydrolyze the material until the desired substitution level is reached (typically a substitution degree between 2 and 2.5). While this practice yields a product with a homogeneous distribution of acetyl groups along the cellulose chains, it is associated with complex processes, sophisticated solvent recovery systems, and high production costs.

[0010] The acetylation of NFC has been investigated for the last few decades, as the process not only modifies its polarity but also significantly influences its dispersion ease and, consequently, its ability to enhance the mechanical properties of various materials. Some variables that affect the properties and performance of acetylated NFCs are the following:

[0011] Degree of substitution: the extent to which hydroxyl groups are replaced by acetyl groups along the glycosidic chain of cellulose. This is expressed in terms of the average number of acetyl groups per glucose monomer in cellulose. Since the glucose monomers that make up cellulose have three hydroxyl groups, the degree of substitution will range from 0 to 3, where 0 corresponds to unacetylated cellulose and 3 to fully acetylated cellulose. The polarity of non-cellulose fibers (NCFs) can be decreased by increasing the degree of substitution. However, the maximum is reached at a degree of substitution of 0.6–0.8, as higher substitutions transform the fibers into a thermoplastic mass.

[0012] Acetyl group distribution: the degree of homogeneity with which the acetylation reaction occurs. The degree of substitution refers to the average number of esterified hydroxyl groups, but this can occur heterogeneously, which is undesirable because it leads to the formation of agglomerates that weaken the materials. Since acetylation reactions occur at accessible points in cellulose, preferably on its outer surface, some glucose monomers may be triacetiated, while others still retain their original hydroxyl groups.

[0013] Fibrillation degree: a measure of the extent to which cellulose is transformed into smaller fibrils by the application of shear forces. A higher degree of fibrillation results in smaller fibrils. This measure depends on the mechanical stress to which the cellulose is subjected, the way in which this stress is applied, the type of cellulose used, and any pretreatments it may have undergone. Generally, the smaller the fibrils, the greater their surface area and mechanical reinforcement capacity. On the other hand, a higher degree of fibrillation requires more energy during manufacturing. Therefore, there is an optimum that depends on the material in which the fibrils are dispersed and the application of the resulting composite material, among other factors.

[0014] Some studies on the use of NFC to improve its dispersion in polymer matrices include: Tingaut et al. (2010), who evaluated the preparation of polylactic acid (PLA) matrices reinforced with acetylated cellulose microfibrils. By varying both the acetic acid content and the amount of incorporated microfibrils, it was possible to adapt the properties of these composite materials, obtaining products with better dispersion, greater thermal stability, and lower hygroscopicity than those generated with unmodified microfibrils. The acetylated microfibrils also exhibited better compatibility with PLA. Bolta et al. (2012) performed acetylation of MFC in toluene, carrying out solvent exchange after fibrillation, and then incorporating these acetylated microfibrils into a polylactic acid solution in chloroform.Achieving improved dispersion of the microfibrils within the matrix and a maximum acetyl group content of 10.3% (a degree of substitution of 0.43), Tingaut et al. (2011) evaluated the acetylation of cellulose microfibrils in acetic anhydride and determined their tensile strength and hydrophobicity. The barrier properties of the microfibrils were improved, resulting in a product with an oxygen transmission rate comparable to that of conventional packaging materials and a vapor transmission rate lower than that of non-acetylated microfibrils, without significantly affecting their mechanical properties. Park et al. (2017) studied the effect of incorporating unmodified cellulose microfibrils into urea-formaldehyde resin formulations of varying quality. The effects of their incorporation varied depending on the quality of the resins analyzed. In general, the addition resulted in increased curing temperatures.In particular, the presence of MFCs in the resins decreased the degradation temperature of the lower quality resins, while increasing the thermal stability of the higher quality sample.

[0015] Some patents related to this technology are detailed below:

[0016] 1. US Patent 6,117,545 (Cavaille et al.) protects a product and process consisting of cellulose microfibrils for structural and reinforcing applications, wherein at least 25% of the hydroxyl groups present on their surface are esterified with at least one organic compound possessing at least one functional group capable of reacting with said hydroxyl groups. It also protects composite materials containing these materials.

[0017] 2. Application US2009 / 0298976(A1) (Yano et al.) disclosing a composite material made of fiber-reinforced resins. In particular, the fibers may be cellulose, derived from plant species, bacterial cellulose, or cellulose microfibrils obtained by refining cellulose obtained from plant or bacterial species; these may subsequently be subjected to a chemical reaction to increase their compatibility with the resins.

[0018] 3. US Patent 8,030,375(62) (Yano et al.) protects a highly transparent composite material consisting of a polymer matrix reinforced with modified NFCs. The microfibrils are modified through a reaction with at least one esterifying and / or etherifying agent. The modifying agents may be acids, alcohols, halogenated reagents, acid anhydrides, or isocyanates. The modifications must be carried out under conditions that allow for a substitution of between 5 and 40% of the hydroxyl groups.

[0019] 4. US Patent 9,512,304(62) (Yano et al.) describes a methodology for producing a highly hydrophobic resin (thermoplastic or thermoset) reinforced with a homogeneous dispersion of microfibrinated vegetable fibers, modified with alkyl succinic anhydride or alkenyl succinic anhydride to increase their hydrophobicity. Materials molded with these resins acquire improved mechanical properties. Furthermore, the resins produced possess thermal resistance and a low coefficient of linear thermal expansion.

[0020] 5. US Patent 6,184,373(61) (Bernard et al.) describes a methodology for obtaining a fibrillated cellulose ester in a controlled manner. The method involves fibrillating the cellulose ester in its esterifying medium in the presence of a coagulating agent. The process is subdivided into multiple stages in which these materials are subjected to various mechanical processes.

[0021] 6. Application WO2023 / 148504(A1) (Onyianta et al.) disclosing a method for producing surface-modified nanocellulose, which considers the use of a mixture of solvents, one of which is an organic acid of the type dicarboxylic acid, tricarboxylic acid or a mixture of both, and treatment at a temperature below 100°C and subjecting the material to high shear forces.

[0022] 7. Application EP3272812(A1) (Takeshi et al.) protects a method for the production of chemically modified NFCs using organic acids and mechanical methods such as refining and milling. This method is energy-intensive and does not offer an alternative for separating the acetiated NFCs.

[0023] 8. Application EP2196478(A1) (Bordeanu et al.) proposes a process for the production of surface-modified cellulose nanostructures using alkoxysilanes. This is a relatively complex and expensive process.

[0024] 4 Referencias:

[0025] . Tingaut et al.: “Synthesis and Characterization of Bionanocomposites with Tunable Properties from Poly(lactic acid) and Acetylated Microfibrillated Cellulose”, Biomacromolecules 11 , 454-464 (2010).

[0026] . Bolta et al.: “Acetylated microfibrillated cellulose as a toughening agent in poly(lactic acid)”, Journal of Applied Polymer Science 126, E448-E457 (2012).

[0027] . Tingaut et al.: “Surface chemical modification of microfibrillated cellulose: improvement of barrier properties for packaging applications”. Rodionova et al., Cellulose 18, 127-134 (2011).

[0028] . Parket al.: “Effect of microfibrillated cellulose addition on thermal properties of three grades of urea-formaldehyde resin”, International Journal of Adhesion and Adhesives 72, 75-79 (2017). Breve descripción de las figuras

[0029] Figura 1: imagen SEM con aumento de 85x (A) e imagen SEM con aumento de 1.500x.

[0030] Figure 2: Graph on the degree of substitution of acetylated MFCs.

[0031] Figure 3: ABES tests (at 160 °C for 90 s) of reinforced UF adhesive resins, with acetylated NFC, with dosages of (a) 100g / m 2 ; (b) 150g / m 2 ; and (c) 200g / m 2 .

[0032] Figure 4: Tensile strength of particleboards manufactured with NFC-reinforced UF adhesives in aqueous and acetic media; for bonding ratios of (a) 8%; (b) 10%; and (c) 12%. Figure 5: Mechanical properties of PIA, PEG, and NFC with and without acetylation. A) Modulus of elasticity; B) Maximum stress; and C) Maximum elongation; where (a) corresponds to PLA+PEG; (b) PLA + PEG + NFC; and (c) acetylated PLA + PEG + NFC. Disclosure of the Invention

[0033] This technology describes a process for manufacturing a reinforcing additive based on cellulose nanofibers with a low tendency to reagglomerate and whose homogeneous distribution in different types of resins is feasible using traditional mixing methods. Advantageously, this process avoids the use of a catalyst and acetic anhydride as a reagent.

[0034] This surprisingly simple and low-cost process begins with the defibration of cellulose in a concentrated acetic acid medium. The operation is typically performed using high-shear equipment to separate and reduce the size of the cellulose fibers, preferably, though not exclusively, in a refiner, defibrator, homogenizer, or similar device. Through this process, the acetic acid penetrates the amorphous interstices of the cellulose. Subsequently, the fibrillated cellulose suspension in acetic acid is subjected to a sufficiently high temperature to produce partial functionalization of the cellulose.

[0035] Specifically, this process for obtaining partially acetylated cellulose nanofibers comprises at least the following stages: a. Fiber processing: A suspension of wood fibers from coniferous trees, hardwoods, or annual plants obtained from Kraft, sulfite, organosolv, thermochemical, mechanical, or other processes is prepared. These fibers may contain between 0.1 and 15% lignin, as well as recycled fibers, in acetic acid with a consistency between 0.5 and 12% by mass and an acetic acid concentration of 90 to 100% by mass. This suspension is processed in a refiner, defibrator, homogenizer, colloid mill, or other equipment that exerts shear forces on the cellulose fibers to separate and reduce their size. The processing may occur in batch mode, circulating a specific volume of the fiber suspension in concentrated acetic acid for a predetermined time using one of the aforementioned pieces of equipment.Continuous or semi-continuous processing is also possible, using several units in series, of either one or several types. The severity and processing time depend primarily on the type of fibers being processed, the consistency of the suspension (fiber mass / suspension mass ratio), and the desired degree of fibrillation. In a preferred embodiment, when processing in a discontinuous manner in a disc refiner, the typical operating conditions for bleached eucalyptus fiber are: a consistency of 0.5–10 and a disc spacing of 0.01–1 mm. The suspension is circulated through the refiner between 1 and 40 times until the desired degree of fibrillation is achieved, which depends on the intended application of the NFCs. The resulting NFCs typically have a thickness between 10 and 100 nm and a length of less than 50 micrometers, although they are frequently an interconnected network of fibrils rather than individual fibrils.It is also feasible to use more than one piece of equipment, typically processing the cellulose macrofibers first in a refiner and then in a homogenizer. Through this operation, the acetic acid penetrates the amorphous interstices of the micro- and nanofibers, causing intimate contact between the cellulose chains and the acid; b. Acetylation: the temperature of the NFC suspension in acetic acid is increased to 120–200°C for a period of 5–240 minutes to cause a partial substitution of the cellulose's hydroxyl groups with acetyl groups. The higher the temperature and the longer the time, the greater the degree of substitution. Typically, a degree of substitution of between 0.15 and 0.6 is obtained. Due to their high specific surface area, NFCs are obtained in the form of an acetic acid gel, the consistency of which depends on the fiber / concentrated acetic acid ratio processed, with a typical range of 0.5 to 3%; c.Pressing or centrifugation (optional): The fiber suspension is mechanically drained by pressing or centrifugation to a consistency of 5–35% by mass, depending on the degree of fibrillation and the type of drainage; d. Evaporation (optional): Partial or total evaporation of the acetic acid is carried out by heating in suitable equipment; typically a paddle, rotary, or tray dryer, either at ambient pressure or under vacuum. The operating temperature should be between 40 and 118°C, the absolute pressure between 0.04 and 1 bar, and the drying time typically between 10 and 120 minutes; and e.Washing: The fibers are washed with water to displace the residual acetic acid they contain; where an organic solvent, preferably an alcohol such as hexanol, pentanol or cyclohexanol, an aliphatic ester, polyethylene glycol or a paraffin wax with a preferred chain length of C8 to C60, may be added during the evaporation operations or replacing the water during washing.

[0036] The technology allows for the production of partially acetylated nanocellulose without the need for acetic anhydride as a reagent; it also does not require a catalyst. Therefore, it is a simple and low-cost process for obtaining cellulose nanofibers with a low tendency to reagglomerate and whose homogeneous distribution in different types of resins is feasible using traditional mixing methods. Consequently, the acetylated NFCs can be easily dispersed in thermoset and thermoplastic resins and have an excellent reinforcing effect.

[0037] Application example

[0038] Example 1. Distribution of acetyl groups in NFC fibrillated in water and acetic acid.

[0039] The objective of this test is to establish the degree of NFC substitution with acetyl groups that can be achieved by treating nanofibers with glacial acetic acid at 160°C and different times.

[0040] Raw materials:

[0041] . Bleached eucalyptus kraft pulp;

[0042] distilled water; and

[0043] . glacial acetic acid (99.99%).

[0044] Teams:

[0045] A 12-inch disc refiner; a centrifugal pump; a 3000 L reactor; a 500 L agitated tank; and a centrifuge.

[0046] Procedure: NFCs were produced in a disk refiner, under the following conditions:

[0047] Disc separation: 0.05 mm;

[0048] Pump flow: 12 m3 / h;

[0049] Consistency: 2%; and

[0050] Number of recirculations through the refiner: 40 passes.

[0051] The pulp was fibrillated in glacial acetic acid, and the sheets made from the processed cellulose can be seen in Figure 1, which shows two SEM images with different magnifications. In (A), various intertwined nanofibers are visible, highlighting their elongated morphology and a high degree of intertwining; meanwhile, in (B), high fibrillation and a fiber diameter of approximately 10 nanometers are observed.

[0052] In a subsequent step, the fibrillated pulp was treated under the following conditions:

[0053] Table 1. Reaction conditions with acetic acid

[0054] The acetyl groups of the NFCs were determined according to ASTM D871-96. Figure 2 shows the acetyl groups (expressed as the degree of substitution of each glucose monomeric unit) of the NFCs after treatment with glacial acetic acid at 160°C. It can be observed that the degree of substitution increased with treatment time, reaching a maximum after 100 minutes. This maximum corresponds to a degree of substitution of 0.56, a sufficiently high value for the NFCs to remain homogeneously distributed in a suspension.

[0055] Example 2. Fortification of UF resins on a laboratory scale.

[0056] This example evaluates the effect of adding partially acetylated NFCs to urea-formaldehyde resins on the internal cohesion of the adhesive.

[0057] Raw materials:

[0058] . Non-acetylated and acetylated NFCs obtained according to Example 1 (degree of substitution 0.51).

[0059] Commercial urea-formaldehyde adhesive resin (UF).

[0060] Maple wood veneers.

[0061] Teams:

[0062] Ultraturrax.

[0063] . ABES (Automated Bonding Evaluation System) team.

[0064] Procedure:

[0065] In two parallel tests, 0.2% acetylated NFC was added to UF resin with a 30% solids content in the first case, and 0.2% non-acetylated NFC in the second case (percentages expressed relative to the resin solids). Both components were homogeneously mixed in an Ultraturrax mixer for 6 minutes at 8,000 rpm.

[0066] The adhesion tests were performed using an ABES apparatus to determine the mechanical behavior of the adhesive mixtures, as this apparatus allows for the evaluation of adhesion kinetics and the determination of bond strength. The wooden specimens used had the characteristics indicated in Table 2.

[0067] Table 2. Specifications of test specimens in ABES equipment.

[0068] The resins tested in the ABES equipment were added with the following dosages to the wooden specimens, represented in Figure 3:

[0069] 200 g / m³ 2 (0.02 g of resin in 1 mm 2 of area) (a);

[0070] 150 g / m 2 (0.015 g of resin in 1 mm2 of area) (b); and

[0071] 100 g / m 2 (0.01 g of resin in 1 mm 2 of area) (c).

[0072] The operating temperature was 160°C and the curing time was 90 seconds.

[0073] Figure 3 shows the results obtained with UF resin without NFC addition and with 0.2% (by mass on a dry basis) NFC. On the one hand, it can be observed that, with and without NFC, as the resin dosage decreases, the adhesion strength, expressed as force, also decreases. On the other hand, comparing the results obtained with and without NFC addition reveals that for all three dosages, the strength was significantly higher with the fortified resin. In fact, even the strength of the NFC-fortified specimens at the minimum dosage (100 g / m²) was higher. 2 ) was superior to the resistance of the test specimen without NFC fortification at a maximum dosage (200 g / m 2 This demonstrates that partially acetylated NFCs are an effective reinforcing agent for urea-formaldehyde type adhesives, significantly increasing their internal cohesion.

[0074] Example 3. Fortification of UF resins on a pilot scale.

[0075] This example evaluates the strengthening effect of partially acetylated NFC resins in urea-formaldehyde resins during the manufacture of particleboard. Raw materials:

[0076] . Non-acetylated NFC.

[0077] . Acetylated NFCs (substitution degree 0.51).

[0078] Commercial urea-formaldehyde adhesive resin (UF).

[0079] Pine wood particles.

[0080] Teams:

[0081] Ultraturrax.

[0082] Hydraulic press.

[0083] . Universal testing equipment.

[0084] Procedure:

[0085] Analogous to Example 2, two parallel tests were conducted using UF resin with and without acetylated NFC. Then, single-layer particleboards were manufactured at pilot scale using each of the two resins, at three bonding ratios: 8%, 10%, and 12%, with: (i) UF resin, (ii) UF resin with unacetylated NFC, and (iii) UF resin with acetylated NFC. The specifications of the manufactured boards were as follows:

[0086] Density: 0.65 (g / cm³) 3 ); . Board dimensions: Width 30 cm; length 30 cm; thickness 1.4 cm;

[0087] Tabletop volume: 1260 cm³ 3 ; and

[0088] . Board mass: 819 g.

[0089] The boards were manufactured in a hydraulic press, according to the EN 312 standard "Particleboard". The pressing process was carried out under the following conditions: . Pressing factor: 0.4 min / mm. . Specific pressure: 25 kg / cm² 2

[0090] . Temperature: 180°C.

[0091] Hydraulic pressure: 109.7 bar.

[0092] Total pressing time: 5.4 min.

[0093] The boards were cut into test specimens, according to EN 319 “Determination of tensile strength perpendicular to the faces of the board”.

[0094] The tensile strength measurement of the test specimens was carried out in accordance with EN 312 “Particle board, specifications”, in which the test specimens are analyzed on the KARG Industrietechnik smarTens 00 universal testing equipment.

[0095] Figure 4 shows the mechanical strength, expressed as stress, of board samples manufactured with: (i) UF adhesive resin, (ii) UF adhesive resin fortified with unacetylated NFC, and (iii) adhesive resin fortified with acetylated NFC; at three bonding ratios: 8% (a), 10% (b), and 12% (c). Looking at each block, it is clear that the mechanical strength increased in all three cases as the bonding ratio increased. Furthermore, comparing the results of the three blocks—that is, the adhesive with and without fortification—it is observed that the UF resin fortified with unacetylated NFC shows better performance than the unfortified resin at all three bonding ratios tested. However, when the fortification is done with acetylated NFC, the improvement in mechanical strength is even greater.In fact, if the block of results of the resin without fortification is compared with the block of results manufactured with resin fortified with acetylated NFC, it is possible to establish that the resistance of a board with the maximum gluing ratio of 12% manufactured with resin without fortification (1.16 N / mm. 2 ) is less than the resistance of the board made with resin fortified with acetylated NFC, with the lowest gluing ratio, of 8% (1.25 N / mm 2 Example 4. Fortification of PLA resins.

[0096] In this example, the effect of partially acetylated NFCs on the mechanical properties of thermoplastic resins of the polylactic acid type is evaluated.

[0097] Raw materials:

[0098] . Non-acetylated NFC.

[0099] . Acetylated NFCs (substitution degree 0.51).

[0100] Polylactic acid (PLA).

[0101] . Polyethylene glycol 1500 (PEG).

[0102] Teams:

[0103] Ultraturrax.

[0104] Torque rheometer.

[0105] HAAKE mini jet injector.

[0106] . Universal testing equipment.

[0107] Procedure: PLA was mixed with 7.5% by mass of PEG in a torque rheometer. Subsequently, 1% dry basis of acetylated NFC was added. The mixing conditions used were:

[0108] . Temperature: 175 °C.

[0109] Rotation speed: 90 rpm.

[0110] Time: 8 min.

[0111] Type IV test specimens were manufactured using a composite material of PLA, PEG, and acetylated NFC in a mini injection molding machine. The parameters used were as follows:

[0112] Mixture weight: 2.5 g. Piston temperature: 175°C. Mold temperature: 100°C.

[0113] . Warm-up time: 5 min.

[0114] Injection pressure: 400 bar.

[0115] Injection time: 30 s.

[0116] The tensile strength of the resulting test specimens was measured, according to ASTM D638, using a universal testing machine. The parameters and conditions used were as follows:

[0117] . Distance between jaws: 100 mm. . Machine speed: 5 mm / min.

[0118] Figure 5 shows graphs on the mechanical properties of PLA, PEG and NFC with and without acetylation, specifically, it shows the modulus of elasticity (A), the maximum stress (B) and the maximum elongation (C) of specimens of plastic materials composed of: (a) PLA+PEG; (b) PLA+PEG+NFC and (c) acetylated PLA+PEG+NFC.

[0119] If the samples without the reinforcing agent and with the non-acetylated NFC reinforcing agent are compared, it is observed that the mechanical properties, modulus of elasticity and maximum stress, are maintained or tend to decrease slightly, and the percentage tends to increase from 6 to 8 in the case of maximum elongation.

[0120] On the other hand, if the samples mentioned above are compared with those reinforced with acetylated NFC, a significant increase is achieved in the three mechanical properties evaluated.

[0121] Finally, based on all the tests carried out, the improvement in the mechanical properties of modulus of elasticity, maximum stress and maximum elongation can be noted when adding partially acetiated NFC to polylactic acid type resins.

Claims

Claims 1. A process for producing partially acetylated cellulose nanofibers (CNFs) useful as a reinforcing agent for thermoplastic and thermoset resins, CHARACTERIZED in that it comprises at least the following steps: a. fiber processing: preparing a suspension of wood fibers in acetic acid, with a solids content of between 0.5 and 10% by mass and with an acetic acid concentration of 90 to 100% by mass; this suspension is fibrillated to separate and reduce its size; b. acetylation: the temperature of the NFC suspension in acetic acid is increased to 120-200°C for 5 to 240 min to cause a partial substitution of cellulose hydroxyl groups by acetyl groups, achieving a degree of substitution between 0.15 and 0.6; and c. washing: the fibers are extracted to remove the residual acetic acid they contain.

2. The process for producing partially acetylated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that the wood fiber suspension contains between 0.1 and 10% lignin and recycled fibers.

3. The process for producing partially acetylated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that after fibrillation the NFCs reach a thickness between 10 and 100 nm and a length of less than 50 micrometers.

4. The process for producing partially acetylated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that in stage “a” the wood fibers are from conifers, from hardwood or annual plants obtained from Kraft, Sulfite, Organosolv, thermochemical or mechanical processes.

5. The process for producing partially acetylated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that in stage “a”, the wood fiber suspension is processed in one or more pieces of equipment that subject the suspension to high shear, to separate and reduce the size of the cellulose fibers, including, but not limited to: refiners, defibrators, homogenizers, colloid mills, being able to operate and be arranged in various possible modes and configurations.

6. The process for producing partially acetylated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that in step “a” an interconnected network of fibrils is frequently obtained.

7. The process for producing partially acetylated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that, optionally, after the acetylation step A pressing stage is carried out, where a mechanical drainage of the fiber suspension is performed by pressing or centrifugation until a consistency of 5 - 35% by mass, depending on the degree of fibrillation and the type of drainage.

8. The process for producing partially acetiated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim I, CHARACTERIZED in that, optionally, after the acetylation stage, a partial or total evaporation stage of the acetic acid is carried out, through heating in equipment at ambient pressure or vacuum, operating between 40 and 118°, at an absolute pressure between 0.04 and 1 bar and during a drying time between 10 and 120 min.

9. The process for producing partially acetiated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claims 1 and 8, CHARACTERIZED in that it is carried out in a paddle, rotary, or tray dryer.

10. The process for producing partially acetiated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that, in step “c”, it is extracted with water or an organic solvent.

11. The process for producing partially acetiated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claim 1, CHARACTERIZED in that in step “c” an organic solvent of the alcohol type, such as hexanol, pentanol or cyclohexanol, is replaced with an aliphatic ester, polyethylene glycol or a paraffin wax with a preferred chain length of C8 to C60.

12. The process for producing partially acetiated cellulose nanofibers useful as a reinforcing agent for thermoplastic and thermoset resins, according to claims 1 and 8, CHARACTERIZED in that in the evaporation step the organic solvent is an alcohol such as hexanol, pentanol or cyclohexanol, an aliphatic ester, polyethylene glycol or a paraffin wax with a preferred chain length of C8 to C60.

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