Lignocellulosic nanofibres, thermosetting adhesive comprising same and use thereof for wood composites

Abstract

The present invention relates to lignocellulosic nanofibres (LNFC) or biopolymeric reinforcing nanoadditive as a mechanical reinforcement of low-molar-ratio phenol formaldehyde (PF) and urea formaldehyde (UF) thermosetting adhesives, wherein the use of natural nanocompounds based on lignin and cellulose makes the nanoadditive or nanofibre a "green" or environmentally friendly additive, said additive being useful in the production of wood-based composites. The LNFCs are made by means of a process that does not involve chemical additives and which is based on thermomechanical wood pulp, using microshredding and high-pressure microfluidsation. The present natural nanoadditive or natural reinforcement fibre is intended for the industry of manufacturing plywood, medium density fibreboard (MDF) and particleboard.

Description

[0001] LIGNOCELLULOSIC NANOFIBERS, THERMOSETTING ADHESIVE COMPRISING THEM, USEFUL FOR WOOD COMPOSITES.

[0002] Technical Sector

[0003] The present invention relates to lignocellulosic nanofibers (LNFCs) or biopolymeric reinforcing nanoadditives as mechanical reinforcement for low molar ratio thermosetting adhesives, phenol-formaldehyde (FF) and urea-formaldehyde (UF). The use of natural compounds based on lignin and cellulose, in nanoscale, gives it the character of a "green" or environmentally friendly additive, making it useful in the production of wood-based composites. The LNFCs are manufactured from thermomechanical wood pulp using a process without chemical additives, through microdefibration and microfluidization at high pressures. This natural reinforcing additive or fiber is intended for the plywood, medium-density fiberboard (MDF), and particleboard manufacturing industries.

[0004] Previous Technique

[0005] Wood-based panels and composite materials are a vital component of the global timber and furniture industries. A wide variety of wood-based materials are available for diverse applications. In Chile, the panel and veneer industry has seen significant growth and increased production over the years. In 2021, the panel and veneer industry's production level reached its highest point on record, at 3.57 million m³. 3(Soto Aguirre, DA, & Gysling Caselli, AJ (2016). Primary Forestry Industry in Chile. Period 2006-2015. Primary Forestry Industry in Chile. Period 2006-2015. https: / / doi.org / 10.52904 / 20.500.12220 / 21347) with plywood leading the way at 38.7% of the total, followed by MDF (medium-density fiberboard), representing 28.3% of total production for the respective year. This is mainly due to their excellent workability, light weight, and high rigidity, which makes them attractive to the furniture market and interior design in general.

[0006] Specifically, MDF boards are a type of engineered wood composite made from wood fibers bonded with synthetic resins, such as urea-formaldehyde (UF) or melamine-urea-formaldehyde (MUF). They are manufactured by subjecting the wood fibers to pressure and heat, resulting in a dense, homogeneous, and smooth material. Some of their main characteristics are: Smooth surface: Ideal for painting, varnishing, or applying laminates; Easy to work with: Can be easily cut, routed, drilled, or shaped; Durable: Although less resistant to moisture than solid wood or plywood, MDF is denser and has uniform strength; Economical: Typically less expensive than natural wood or plywood. It is commonly used in the manufacture of furniture, moldings, doors, shelving, and other woodworking products.However, its main disadvantage is that it is not very resistant to moisture, so it may not be suitable for outdoor or humid areas unless it is properly treated.

[0007] Particleboard, or MDP (Medium Density Particleboard), is a panel made from wood particles, such as wood chips and sawdust, combined with synthetic resins (UF, MUF) and subjected to pressure and heat to form a compact material. It is similar to MDF, but with some key differences in its structure and properties.Key characteristics of MDP: Composition: It is made from larger wood particles or chips compared to the fine fibers of MDF; Strength: Although lighter than MDF, MDP has good strength for applications requiring flat surfaces and stability, but it is not as strong as MDF for fine or detailed work; Surface: Its finish may not be as smooth as MDF, making it less ideal for projects requiring paint or fine finishing details, although laminates and melamine can be applied to it; Cost: It is typically less expensive than MDF, making it a popular choice for low-cost furniture such as shelving or cabinets. Like MDF, MDP is not very resistant to moisture and can deteriorate if exposed to it without proper treatment.

[0008] On the other hand, plywood panels are made from thin sheets of wood (called veneers) bonded together with synthetic resins, UF and phenol-formaldehyde (FF). The wood layers are arranged so that the grain of one layer is perpendicular to the grain of the next, giving plywood greater strength and stability.Main characteristics of plywood: Structure: It is composed of several layers (generally an odd number) arranged in a cross-laminated pattern, which gives it strength in both directions; Strength: It is very strong, especially in bending, making it ideal for structural applications such as furniture, flooring, ceilings, and walls; Lightness: Compared to other types of boards, such as MDF, plywood is lighter, making it easier to handle and transport; Stability: Due to its cross-laminated structure, it has high dimensional stability, meaning it does not easily warp with changes in temperature or humidity; Variety: There are different types of plywood, depending on the type of wood used and the treatment it receives, for example, marine plywood (moisture-resistant), for structural or decorative use.

[0009] Plywood is a highly versatile and durable option, used in a wide range of applications, from construction to furniture making, paneling, and decoration. Thermosetting resins, or synthetic resins, such as urea-formaldehyde (UF), phenol-formaldehyde (PF), and melamine-urea-formaldehyde (MUF), are commonly used in plywood manufacturing, applied to the particles, fibers, or veneers depending on the type of board. However, concerns regarding the use of formaldehyde-based resins have been increasing due to their negative health and environmental effects, which has led producers to reduce the amount of this compound by lowering the F / U or F / P molar ratio in the aforementioned adhesives.

[0010] Urea-formaldehyde resins are fundamental to the production of wood-based panels or composites due to their high reactivity, good performance, and low cost (Dunky, M. (March 1998). Urea-formaldehyde (UF) adhesive resins of wood. Science Direct, 18(2), 95-107. doi:https: / / doi.org / 10.1016 / 80143-7496(97)00054-7), and are considered the most important and widely used class of amino resin adhesives. These resins are polymer condensation products of the reaction of aldehydes with compounds carrying amino groups, with formaldehyde being the main aldehyde used. This complex reaction between urea and formaldehyde results in linear and branched polymers, as well as three-dimensional networks, in the cured resin (Pizzi A. Handbook of Adhesive Technology, Revised and Expanded, 2003. Engineering ad technology, physical sciences).Fiberboards bonded with this type of resin are characterized by their high mechanical strength, dimensional stability, hardness, resistance to microorganisms and abrasion (Riquelme-Valdés, J., Ramírez, E., Contreras, D., Freer, J., & Rodríguez, J. (December 2008). Fiberboard manufactured without resin by means of a Fenton reaction. Journal of the Chilean Chemical Society (JCCS), 53(4), 1722-1725. doi:http: / / dx.doi.org / 10.4067 / S0717-.

[0011] 97072008000400019). However, one of the main sources of formaldehyde emissions is related to the strong presence of particleboard manufactured with UF adhesives in constructions such as homes, which has led to the establishment of restrictions on formaldehyde exposure for this use (Estévez, P. (2012). Development, characterization and optimization of urea-formaldehyde (UF) based resins, as adhesives for particleboard with low formaldehyde emissions. Doctoral Thesis, University of Burgos, Burgos, Spain. https: / / hubu.ubu.eS / bitstream / handle / 10259 / 196 / Est%c3%a9vez_Bol%c3%advar.pdf?sequence=2&isAllowed=y). Several investigations have focused on mitigating formaldehyde emissions through different techniques, one of which is the reduction of the molar ratio between Urea-formaldehyde by adding additional urea in the resin manufacturing process (Castelblanco Benítez, L. (2021).Reduction of Formaldehyde Emissions in MDF Board Manufacturing. Medellin, Antioquia, Colombia: Engineering Documentation Center (CENDOI). The proportions of approximately 1.6 that were common 10 to 15 years ago have been reduced to values ​​as low as 1.0, and in some cases, lower (Conner, A. (1996). Urea-Formaldehyde-Adhesive Resins. Université de Nancy I. https: / / www.researchgate.net / publication / 266266974_Urea-Formaldehyde_Adhesives). However, reducing this molar ratio can affect the physical and mechanical properties of the resin and the products manufactured with it. Melamine combined with urea-formaldehyde (MUF) has been used to reduce formaldehyde emissions in the panel; however, this thermosetting material has a high production cost (Diop, C., Tajvidi, M., Bilodeau, M., W. Bousfield, D., & Hunt, J. (06 of 05, 2017).Isolation of lignocellulose nanofibhls (LCNF) and application as adhesive replacement in wood composites: example of fiberboard. Cellulose, 24, 3037-3050. doi:https: / / doi.org / 10.1007 / s10570-017-1320-z). Other researchers have opted for more natural methods by incorporating nanocellulose into adhesives. In a study by Rigg Aguilar (2018). Effect of the addition of crystalline micro-nanocellulose in adhesives and its application in wood agglomerates of tropical forest species. https: / / repositoriotec.tec.ac.cr / bitstream / handle / 2238 / 9783 / efecto_adici%c3%b3 n_micro-nanocelular_cristal¡na_adhesivos.pdf?sequence=1 &¡sAllowed=y), where 1% of crystalline micro-nanocellulose was incorporated into PVAc and UF adhesives, it was observed that reinforcement with cellulose nanocrystals (CNC) in three tropical forest species increased the mechanical resistance of the glue line.However, the biggest obstacle to the production of this material is the high energy consumption and the chemical or enzymatic pretreatments (Siró, I., & Plackett, D. (February 21, 2010). Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose, 17, 459–494. doi:https: / / doi.org / 10.1007 / s10570-010-9405-y), since the pulp must undergo a chemical process for bleaching and lignin extraction. Alternatively, lignonanocellulose (LNFC) production has emerged, which has properties similar to nanocellulose (NFC) but requires fewer chemical processes, and therefore lower costs, since it is obtained from thermomechanical pulp (TMP).

[0012] Phenol-formaldehyde resin or adhesive is a synthetic thermosetting resin obtained as a product of the reaction of phenols (P) with formaldehyde (F). In this reaction, phenol readily condenses with aldehydes, releasing water molecules and linking the molecules together to form an extensive network (Covarrubias, H., Sáenz, A., & Castañeda, A. (2016). Thermosetting Phenol-Formaldehyde Resins. Rev. Iberoam. Polímeros, 17(6), 266-276. https: / / reviberpol.files.wordpress.com / 2019 / 07 / 2016-covarrubias.pdf, Gutierrez Elizondo, J. (1978). Production of phenolic phenol-formaldehyde resins. 1-46). One way to classify phenolic resins is by whether their curing process is hot or cold. Hot-curing adhesives are used in the presence of alkaline solutions, while cold-curing adhesives require the addition of strong acids as a hardening agent (Bilurbina, L., Liesa, F., & Bilurbina Alter, L. (1990).Industrial Adhesives. Marcombo). Among the advantages of this adhesive is its good resistance, even when curing is done at low temperatures.

[0013] Phenol-formaldehyde also possesses excellent mechanical properties, good resistance to high temperatures, chemical agents and humidity, low shrinkage, good adhesion, resistance to shock, fatigue, and vibrations (Bilurbina, L, Liesa, F., & Bilurbina Alter, L. (1990). Industrial Adhesives. Marcombo). Some of the applications of this adhesive are in the production of brake linings, abrasive wheels, foundry molds, thermal insulation, and mainly in the wood panel industry as an adhesive in the manufacture of plywood panels, oriented strand board (OSB), and particleboard (Covarrubias, H., Sáenz, A., & Castañeda, A. (2016). Thermosetting Phenol-Formaldehyde Resins. Rev. Iberoam. Polímeros, 17(6), 266-276. https: / / reviberpol.files.wordpress.com / 2019 / 07 / 2016-covarrubias.pdf).In 2004, various institutions around the world classified formaldehyde-based adhesives as a carcinogen for humans (International Agency for Research on Cancer. (2006). INTERNATIONAL AGENCY FOR RESEARCH ON CANCER IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Formaldehyde, 2-Butoxyethanol). Since then, the scientific community has conducted numerous studies focused on reducing the use of this substance in various industrial sectors.

[0014] The publication by Diop et al. (2017), titled "Evaluation of the incorporation of lignocellulose nanofibers as a sustainable adhesive replacement in medium-density fiberboards," published in Industrial Crops and Products 109 (2017) 27-36, uses lignocellulose nanofibers (LCNF) as a substitute for adhesive or resin in MDF fiberboards; that is, MDF boards were manufactured without UF adhesive. The physical-mechanical properties of the panels were affected by the LCNF content and the press temperature. The modulus of rupture (MOR) and the internal bond strength (IB) of the MDF showed a linear relationship with the increase in LCNF, where the percentages of LNFC used were very high, up to 20%, based on the weight of the board.Optimal processing conditions were achieved at 180°C with an LCNF content of 20%, resulting in a MOR value of 12.1 MPa, close to the minimum recommended for commercial fiberboards, thus limiting its application range. The IB, thickness swelling, and modulus of elasticity met standard values. The authors mention that press temperatures above 180°C could cause material degradation, reducing the mechanical properties of the fiberboards; this limitation on press temperature increases prevents reductions in press cycle times related to this variable. This technology of completely replacing the adhesive with LNFC is extremely expensive, considering that the cost of nanofibers exceeds US$20 per kilogram.Furthermore, the final board without adhesive has lower dimensional stability in high humidity environments, greatly limiting the board's end applications.

[0015] Patent PL245452B1 (Research Network Lukasiewicz Institute of Wood Technology) relates to a lignocellulosic compound and a method for producing it from crushed lignocellulosic raw materials, an aminoresin-based binder, and a nanobiomodifier of the adhesive resin, which provides the finished product with reduced formaldehyde content and emission and increased strength.The lignocellulosic composite contains a lignocellulosic raw material, in an amount of up to 50%, and a binder based on an amino resin modified with a nanobiopolymer of an adhesive resin, wherein the nanobiomodifier of the adhesive resin is a nanocellulose modifier functionalized with organosilicon compounds and contains a hardener, preferably in the form of ammonium nitrate, wherein the binder based on an amino resin is a resin selected from a melamine-urea-formaldehyde MUF resin or a urea-formaldehyde UF resin, and wherein the nanobiomodifier functionalized with organosilicon compounds selected from: MS5 - methacryloxypropyltrimethoxysilane, BS5 - isobutylthmethoxysilane and FS5 - (3,3,3-thfluoropropyl)trimethoxysilane, is contained in an amount not exceeding 3% of the dry weight of the adhesive resin.The hardener is a 40% solution of ammonium nitrate in an amount of at least 2%, preferably 2-3%. It is a compound with reduced formaldehyde content and formaldehyde emission, in which the formaldehyde content in the compound is reduced.

[0016] Patent application US20230167607A1 (WR Meadows) describes fiberboard sheet materials containing cellulosic fibers in which cellulose nanofibrils are used as a binder to adhere the cellulosic fibers together. The cellulose nanofibrils are present in an amount of 0.5% to 7.5% by weight based on the dry weight of the fiberboard sheet. The fiberboard sheet materials have strength properties that meet or exceed the requirements of ASTM C208 and are useful for structural sheathing and roofing board applications. A process for manufacturing the fiberboard sheet is also disclosed.

[0017] But there remains a need for a thermosetting adhesive reinforced with natural nano-additives that eliminates the use of high-cost and environmentally harmful chemicals in its preparation; reduces the energy consumption required in its preparation; requires a low amount of natural reinforcing nano-additives to achieve significant reinforcement of the UF and FF resin or adhesive for wood composites, and thereby noticeably improves the mechanical properties of UF and FF adhesives reinforced with the natural nano-additive in MDF, particleboard and plywood; and reduces the time for the curing or setting reaction of the reinforced UF and FF adhesive.and that it does not alter the production processes of wood-based boards by maintaining, after its application, the production process conditions, for example, the drying process of the wood (veneers, fibers or particles), the adhesive application technologies and / or the hot pressing conditions of the board and the final product finish.;

[0018] Brief description of the invention

[0019] The present invention relates to a thermosetting Urea-Formaldehyde (UF) and Phenol-Formaldehyde (FF) adhesive with lignocellulose nanofibers (LNFC) or natural nanoadditive as a mechanical reinforcing agent, of low UF / FF ratio, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, preferably 31:69.

[0020] This thermosetting adhesive differs from those that only include cellulose-based nanofibers because, by additionally including lignin, it eliminates the use of expensive and environmentally harmful chemicals in its preparation, as well as the energy consumption required to prepare nanofibers based solely on cellulose. Thus, the preparation of this LFNC-reinforced thermosetting adhesive does not use chemicals such as strong acids, which are commonly employed in lignin extraction to purify cellulose from natural lignocellulosic fibers. Therefore, there are no liquid effluents associated with these lignin removal processes. Furthermore, the reinforcing nanoadditive, or LNFC, is used in small quantities, approximately...1% (LNFC dry mass / adhesive solids mass) achieves significant reinforcement in UF and FF resins or adhesives for wood composites, noticeably improving the mechanical properties of these UF and FF adhesives in MDF, particleboard, and plywood, increasing the strength of the adhesive and the manufactured board by 20% to 60%. The reinforcing nano-additive or LNFC also accelerates the setting reaction, reducing the curing time of UF and FF by 10% to 35% compared to the curing time of conventional resins. This latter characteristic allows for increased speed in the production lines of wood-based boards (MDP, MDF, Plywood).Finally, the use of the reinforcing nanoadditive or LNFC, in aqueous suspension, does not alter the production processes of wood-based boards since the incorporation of LNFC does not change the production process conditions, for example, it does not change: the drying process of the wood (veneers, fibers or particles), the adhesive application technologies and / or the hot pressing conditions of the board and the final product finish.

[0021] Brief description of the figures

[0022] Figure 1: LNFC in wet state (suspension) at the outlet of the microdefibrator.

[0023] Figure 2: LNFC after passing through the ultracentrifugation equipment.

[0024] Figure 3: 4-point bending test.

[0025] Figure 4: Shear test specimen, according to APA PS-1.

[0026] Figure 5: Wood-adhesive-wood laminate joint for DMA analysis.

[0027] Figure 6: AFM view of the LNFC.

[0028] Figure 7: Storage module of FF and FF + LNFC at different charging frequencies; analysis with dynamic nanoindentations, nanoDMA on set adhesives. Figure 8: Evaluation of the setting of FF and FF + LNFC using DMA.

[0029] Figure 9: Response curves of the Storage Module (E') during the setting reaction for the microfiberized UF urea formaldehyde resin.

[0030] Detailed description of the invention

[0031] The present invention relates to lignocellulosic nanofibers (LNFCs), or natural nanoadditives, as mechanical reinforcements for low molar ratio adhesives, phenol-formaldehyde (FF) and urea-formaldehyde (UF). The use of natural compounds based on lignin and cellulose gives it the character of a "green" or environmentally friendly additive, making it useful in the production of wood-based composites. The nanofibers are manufactured from thermomechanical wood pulp using a process without chemical additives, through microdefibration and microfluidization at high pressures.

[0032] This natural reinforcing fiber or additive is intended for the manufacturing industry of: a) plywood, allowing: FF adhesives reinforced with LNFC to set faster, reducing pressing time; the use of a lower quantity (basis weight) of adhesive compared to other known adhesives; a high cost-benefit ratio; a higher percentage of adhesion than required in international standards applicable to plywood; greater strength of UF-reinforced plywood compared to boards using conventional resins; less adhesive use (lower basis weight) in veneers; and the use of wetter veneers in production processes, which substantially reduces production costs; and b) medium-density fiberboard (MDF) and particleboard, allowing: boards using this adhesive to significantly increase their rigidity when the temperature is increased during pressing;a faster setting of the UF adhesive, increasing the speed of continuous production lines; the use of a minimal amount (grammage) of nano-additive compared to other solutions, for example, melamine, to obtain the same reinforcing effect, particularly in low molar ratio resins, increasing the rigidity of the new adhesive system.

[0033] The present biopolymeric reinforcing nanoadditive for thermosetting adhesive, with a low molar ratio of phenol-formaldehyde (FF) and urea-formaldehyde (UF), for wood composites, including plywood, medium-density fiberboard (MDF), or particleboard, comprising lignocellulose nanofibers (LNFC) obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, preferably 31:69. The diameter of the lignocellulose nanofiber is less than 100 nm and it has a length of less than 5 microns.

[0034] The present method of preparing the present biopolymer reinforcing nanoadditive for thermosetting adhesive, with a low molar ratio of phenol formaldehyde (FF) and urea formaldehyde (UF), for wood composites, including plywood, medium density fiberboard (MDF) or particleboard, wherein the reinforcing nanoadditive comprises lignocellulose nanofibers obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately40:60 to 20:80, preferably 31:69, comprising: a) pretreating said thermodynamic pulp containing lignin and cellulose by immersing it in a 0.5% v / v NaOH solution until a suspension with swollen lignocellulose fibers is obtained, wherein preferably said thermodynamic pulp comes from pulp treated in a high pressure (8 to 10 bar) and high temperature (140 to 150°C) digester, b) microdefibrillating the swollen fibers obtained in the step by means of a microdefibrillator, continuously repeating the microdefibrillation until a suspension of lignin and cellulose nanofibers with a biopolymer solids content of approx. 3% to 4%, preferably, the micro defibrillation is repeated 6 to 7 times with a disc opening in the micro defibrillator equipment of 0.5 to 1.0 pm and 900 to 1000 rpm of rotation, c) subject the lignin and cellulose nanofiber suspension obtained in step b) to high pressure microfluidization between 28,000 to 30.000 psi, to homogenize and reduce the size of the nanofibers to diameters less than 100 nm and repeating the microfluidization continuously, preferably the microfluidization is repeated 4 to 5 times, using in-line interaction chambers of 200 and 100 Dm in diameter; and optionally, d) reducing the water content of the microfluidized lignin and cellulose fiber suspension obtained in step c), preferably by ultracentrifugation, preferably said ultracentrifugation is conducted at 12,000 to 15,000 rpm and at a temperature of 14 to 16°C.

[0035] The present method for preparing a low molar ratio phenol-formaldehyde (FF) and urea-formaldehyde (UF) thermosetting adhesive reinforced with a biopolymeric reinforcing nanoadditive comprising lignocellulose nanofibers obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, preferably 31:69, for wood composites, including plywood, medium-density fiberboard (MDF), or particleboard, comprises: mixing, under continuous stirring, a low molar ratio phenol-formaldehyde (FF) and urea-formaldehyde (UF) adhesive with a biopolymeric reinforcing nanoadditive comprising lignocellulose nanofibers (LNFC) obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80.40:60 to 20:80, preferably 31:69, where the amount of LNFC is 0.8% to 1.2%, preferably 1%, based on the mass of the LNFC solids and thermosetting adhesive solids, until a homogeneous mixture is achieved having an effective dispersion of the reinforcing nanofibers in the thermosetting adhesive.

[0036] This reinforced thermosetting adhesive is used as an adhesive in the manufacturing processes of wood-based composite materials, including plywood, medium-density fiberboard (MDF), and particleboard. Depending on the type of process or composite material being manufactured, this LNFC-reinforced FF / UF thermosetting adhesive is applied by roller or spray to wood veneers or fiber-particle boards, respectively.

[0037] In nanoindentation tests on the glue line for control boards (made with unreinforced thermosetting adhesive) and boards manufactured with the present reinforced thermosetting adhesive (1% LNFC), values ​​for the modulus of elasticity were obtained from the initial slope of the unloading zone of the load-displacement curve, as well as the adhesive hardness of the applied force relative to the projected area of ​​the quasi-static and dynamic nanoindentations, using a nominal load of 100 pN. See Table 1 and Figure 7. These tests established that the UF adhesive significantly increases the mechanical properties of the laminate at the nanoscale when 1% LNFC is added; this increase is approximately 28%. Furthermore, the storage modulus of the reinforced adhesive increases exponentially and significantly as the frequency of the load applied to the adhesive system increases.

[0038] From bending tests on small samples (microtests) of plywood boards bonded with unreinforced and LNFC-reinforced thermosetting adhesive, performed in a Microtest machine mounted inside the chamber of a scanning electron microscope (SEM), numerical data and in situ images of the fracture process of the adhesive joints were extracted. These joints were subjected to bending at four different points at test speeds of 0.2 mm / min and 1.0 mm / min (Figure 3). A notch was induced in the specimens used and analyzed in the test to induce fracture and thus obtain a better image of the fracture and crack propagation at the adhesive-wood interface. The maximum force applied at fracture was recorded, and SEM images of the failure zone of the specimen were captured.

[0039] The microtest calculates the bending breaking strength (o) using the following formula: where F is the breaking force, L is the distance between the support supports, and b and d are the width and thickness of the specimen for the micro bending test.

[0040] Table 2 shows the flexural tests on laminates with unreinforced thermosetting adhesive as a control and reinforced thermosetting adhesive, where an increase of 100% and 58% in the modulus of rupture is evident when two different loading application speeds are used, 0.2 and 1.0 mm / min.

[0041] In shear tests on plywood boards bonded with the unreinforced thermosetting adhesive and the present LNFC-reinforced thermosetting adhesive, the shear strength at the adhesive line and the percentage of wood failure at the joint were determined, where the minimum requirement is 85% wood failure (15% adhesive failure). The results are shown in Table 3, and the adhesion in laminates or plywood with the reinforced thermosetting adhesive far exceeds the requirements of the international standard PS1-APA (85%), that is, in terms of the percentage of adhesion when the LNFC nano-additive is used in the UF / FF thermosetting adhesive. Similarly, the shear strength in the laminate joints increases by 57% when the LNFC nano-additive is used in the UF / FF thermosetting adhesive.

[0042] In the mesoscale (DMA) and nanoscale (nanoDMA) dynamic mechanical thermal analysis, in LNFC-reinforced thermosetting adhesive (UF / FF), laminar joints were prepared as shown in Figure 5, in radiata pine sheets of dimensions 0.5 x 5.0 x 19 mm, where an amount of adhesive corresponding to 300 g / m² was applied. 2 Using DMA, the setting curve of the unreinforced thermosetting adhesive (control) and the LNFC-reinforced thermosetting adhesive were determined, as well as the viscoelastic responses of the wood-adhesive-wood bond after setting, using a multifrequency analysis of applied load. The mechanical properties of each adhesive at the bond line were also evaluated by quasi-static and dynamic nanoindentation, as previously described.

[0043] For the curing of the unreinforced and LNFC-reinforced thermosetting adhesives, a DMA analysis was performed using a Perkin Elmer DMA 7e instrument in three-point bending mode on the specimens shown in Figure 5 at a frequency of 1 Hz. The static and dynamic loads used were 275 mN and 250 mN, respectively. The load was applied perpendicular to the wood grain. The DMA analysis for the curing of the unreinforced thermosetting adhesive was performed over a temperature range of 25°C to 180°C. The heating rate was 10°C / min.

[0044] The curing reaction was determined for both the reinforced thermosetting adhesive (FF-LNFC) and the unreinforced thermosetting adhesive (FF resin, control) using dynamic mechanical analysis (DMA), as shown in Figure 8. It was found that the reinforced adhesive (FF + LNFC) significantly increased its stiffness as a function of the test temperature, exceeding the setting behavior of the unreinforced thermosetting adhesive. At the end of the curing process, the stiffness or storage modulus increased by 30% compared to the control (Table 4). Furthermore, the complete setting time for the reinforced thermosetting adhesive (FF + LNFC) was reduced by 25% to 35% compared to the control adhesive.

[0045] The same behavior of FF (Figure 8) was observed in the LNFC reinforced adhesive (Figure 9), where it was also shown that the reinforced thermosetting adhesive sets 10% to 15% faster and at lower temperatures (Table 5) than the control adhesive without reinforcement, a condition that would allow accelerating the production speed in an MDF board or particle board line.

[0046] The samples in Figure 5, with the thermosetting adhesive cured, were left to rest in a desiccator for one day. Subsequently, viscoelastic responses such as loss modulus, storage modulus, and tangent of delta were evaluated by DMA analysis in a frequency range of 1 Hz to 50 Hz at a constant temperature of 25°C.

[0047] To determine the nanomechanical properties evaluated with quasi-static and dynamic nanoindentation, small specimens were set and cut from the DMA samples shown in Figure 5 (control samples and sample with LNFC-reinforced thermosetting adhesive), a Hysitron Triboindenter TI-900 nanoindentation device equipped with nanoDMA, and a Berkovich-type diamond indenter, applying a dynamic load of 100 pN. Since the test was performed at multiple load application frequencies, a loading cycle was used in which the frequencies increased from 0.1 to 220 Hz.

[0048] Application examples

[0049] Example 1: Preparation of lignonan cellulose (LNFC)

[0050] Lignonanocellulose consists of fibrous elements with nanometric dimensions, less than 100 nm in diameter and less than 5 micrometers in length. These nanofibers are composed of the biopolymers lignin and cellulose, in percentages of 31% w / w and 69% w / w, respectively. Lignonanocellulose is obtained using wood pulp produced through a thermomechanical process that involves its treatment in a digester at high pressure (8 to 10 bar) and temperature (140 to 150°C).

[0051] First, the thermomechanical pulp, containing lignin and cellulose, is immersed in a 0.5% v / v NaOH stock solution for 24 hours to achieve swelling and easy / rapid disintegration of the lignocellulose fibers. After this pre-processing, the swollen fiber suspension is processed in a microdefibrator, which can be adjusted to the following operating conditions: disc opening of 0.5 to 1.0 µm, disc rotation speed of 900 to 1000 rpm, and 6 to 7 continuous passes of the lignin- and cellulose-based fibrous suspension through the equipment. These conditions correspond to the optimal range in terms of energy consumption, material residence time during micro-milling, and the morphology of the nanofibers obtained from the naturally assembled lignin- and cellulose-based biopolymers.The output of this process is a suspension of lignin + cellulose nanofibers with a solids content of approximately 3% to 4%, based on the anhydrous mass of the LNFCs in the aqueous suspension (see figure 1).

[0052] Subsequently, the lignin + cellulose nanofiber suspension undergoes a second process that further reduces the nanofiber size and homogenizes its final dimensions to fiber diameters below 100 nm and lengths below 5 micrometers. This second process consists of high-pressure microfluidization with the following process conditions: microfluidization pressure of 28,000 to 30,000 psi, and 4 to 5 passes, using in-line interaction chambers with pass diameters of 200 and 100 µm for the nanofiber suspension.

[0053] For the application of nanofibers as a reinforcing element in UF and FF adhesives, it is possible to reduce the water content in the suspensions by performing an ultracentrifugation process. Reducing the water content in the suspensions can help obtain new adhesive mixtures with lower water content, which can help decrease vapor pressure inside the boards during the hot pressing processes in the production of MDF, particleboard, veneer board, and plywood. Thus, the moist LNFC sample can be ultracentrifuged under the following conditions: 12,000 to 15,000 rpm, 12 to 15 minutes, and a temperature of 14 to 16°C. This yields the LNFC nanofibers (Figure 2) that are subsequently used in the manufacture of wood-based panels.

[0054] Example 2: Determination of Lignonanocelluloses, LNFC

[0055] Nanofiber morphology, atomic force microscopy (AFM)

[0056] For sample preparation for atomic force microscopy (AFM), 10 g of the wet LNFC sample was centrifuged using a Nanosurf Naio AFM instrument. The supernatant was then filtered through 0.25 µm filter paper. A small aliquot of the filtered sample was added to the sample holder in the AFM instrument. The sample was dried with nitrogen gas and then observed using the AFM instrument (see results in Figure 6). The dimensions of the lignin-cellulose nanofibers were less than 100 nm in diameter and less than 5 micrometers in length (Figure 6).

[0057] Example 3: Reinforcement of adhesive mixtures: Preparation of FF / UF thermosetting adhesive reinforced with LNFC and analysis of adhesive bonds.

[0058] The reinforcement of low FF and UF ratio thermosetting adhesives is achieved by mixing the thermosetting adhesive with a nanoadditive containing 0.8% to 1.2% LNFC (anhydrous LNFC mass / adhesive solids mass), based on the mass of the nanoadditive solids and the thermosetting adhesive solids. The mixture is prepared using a homogenizer at 4,000 to 5,000 rpm for 7 to 9 minutes. This ensures uniform mixing of the adhesive and LNFCs, resulting in effective dispersion of the reinforcing nanofibers within the adhesive matrix. This dispersion is fundamental to the properties of the resulting nanocomposite, which is then used as an adhesive in the manufacturing processes of wood-based composite materials.

[0059] Depending on the type of process or composite material to be manufactured, this LNFC-reinforced FF / UF thermosetting adhesive is applied by roller or spray to wood veneers and fiber-particles, respectively. Example 4: Determination of LNFC-reinforced wood adhesives (UF and FF) i) Nanoindentations

[0060] Experimental laminated boards bonded with a thermosetting resin / adhesive (UF and FF, or unreinforced or UF only) were manufactured as a control, and the same thermosetting adhesive was reinforced with the present LNFC nanoadditive (reinforced UF, or reinforced, 1% LNFC, anhydrous mass of LNFC / mass of adhesive solids). Small specimens measuring 5 mm long and 5 mm wide were cut for the nanoindentation test at the glue line. Two specimens were obtained for the control board (UF) and two specimens for the laminates manufactured with a UF adhesive reinforced with 1% LNFC (anhydrous mass of LNFC / mass of adhesive solids). The specimens were cut to a smaller size and into a pyramidal shape using a Leica EZ4 stereomicroscope, and then the nanoindentation test was performed. The test was carried out by applying two nanoindentations at the glue line on each of the specimens. The values ​​of elastic modulus and adhesive hardness were obtained from each nanoindentation.Nanoindentations in the hardened adhesive after pressing the laminates were performed using the Hysitron Thboindenter TI-900; an aesthetic-dynamic loading cycle with a nominal load of 100 pN was used. The elastic modulus was obtained from the initial slope of the unloading zone of the load-displacement curve, and the hardness was obtained from the applied force relative to the projected area of ​​the nanoindentation.

[0061] Quasi-static and dynamic nanoindentations in UF and FF.

[0062] Table 1: Average and 95% confidence interval for the reduced modulus of elasticity (E r ) of the UF thermosetting adhesive evaluated with nanoindentations; control and UF + LNFC specimens.

[0063] The nanomechanical determination of the control versus the thermosetting adhesive reinforced with the present LNFC nanoadditive, along with the thermal, dynamic, and mechanical analyses, are presented in Table 1 and Figure 7, respectively. These analyses establish that the UF adhesive significantly increases its nanoscale mechanical properties when 1% LNFC is added; this increase is approximately 28%. Furthermore, the storage modulus of the reinforced adhesive increases exponentially and significantly as the frequency of the load applied to the adhesive system increases.

[0064] i) Flexural tests on small samples (microtests), plywood boards with UF adhesive. Small-dimension laminates or plywood are manufactured using LNFC-reinforced UF adhesive and subsequently subjected to microflexural tests with the aid of a Microtest machine mounted inside the chamber of a scanning electron microscope (SEM). This testing method yields numerical data and in situ images of the fracture process of the adhesive joints. The fracture of the joints was evaluated on samples subjected to bending at four points at different test speeds: 0.2 mm / min and 1.0 mm / min (Figure 3). A notch was induced in the analyzed specimens to induce fracture and thus obtain a better image of the fracture and crack propagation at the adhesive-wood interface. The maximum force applied at fracture was recorded, and SEM images of the failure zone of the specimen were captured.The microtest calculates the bending breaking strength (o) from the formula eq. 1 indicated above.

[0065] Flexural tests on laminates with UF, control, and reinforced UF, are shown below using microtest analysis, in Table 2.

[0066] Table 2: Flexural breaking stresses of laminates with UF and UF + LNFC.

[0067] Table 2 shows that small-scale bending tests revealed a 100% and 58% increase in the modulus of rupture when two different load application rates of 0.2 and 1.0 mm / min were used. iii) Shear tests, plywood boards with UF

[0068] Plywood panels were manufactured using UF adhesive and UL adhesive reinforced with LNFC. Specimens measuring 75 cm long and 2.54 cm wide were cut from these panels according to the American standard APA PS-1 (Figure 4). This test determined the shear strength at the adhesive line and the percentage of wood failure at the joint, where the minimum requirement is 85% wood failure (15% adhesive failure). The results of the shear and adhesion tests on UF laminates are shown in Table 3. Table 3: Shear Test Results.

[0069] The adhesion of UF laminates or plywood far exceeds the requirements of the international standard PS1-APA (85%), that is, in terms of the percentage of adhesion when LNFCs are used in the UF adhesive. Similarly, the shear strength of the laminate joints increases by 57% when LNFCs are used. iv) Mesoscale (DMA) and nanoscale (nanoDMA) dynamic mechanical thermal analysis of UF and FF adhesives with LNFCs.

[0070] For the mesoscale analysis of the present reinforced adhesive, laminated joints were prepared as shown in Figure 5, using two radiata pine sheets measuring 0.5 x 5.0 x 19 mm. Each wood sample was treated with an amount of adhesive corresponding to 300 g / m². 2

[0071] Using DMA, the setting curve of control adhesives (UF and FF) and adhesives reinforced with LNFC was determined, as well as the viscoelastic responses of the wood-adhesive-wood bond after setting, using a multifrequency analysis of applied load. From the same sample in Figure 5, smaller samples were cut to evaluate the mechanical properties of the adhesive at the glue line using quasi-static and dynamic nanoindentation, as described in point i).

[0072] Setting of FF and UF under control and with LNFC, DMA analysis: Samples with the unset adhesive were subjected to the DMA test using the Perkin Elmer DMA7e instrument. The DMA analysis was performed in three-point bending mode on the specimens shown in Figure 5 at a frequency of 1 Hz. The static and dynamic loads used were 275 mN and 250 mN, respectively. The load was applied perpendicular to the wood grain. The DMA analysis for the setting of FF and UF was performed over a temperature range of 25°C to 180°C. The heating rate was 10°C / min.

[0073] Determination of the curing reaction of the reinforced adhesive (FF-LNFC) and the FF resin by mechanical-dynamic analysis (DMA).

[0074] Using DMA (Figure 8), it was verified that the reinforced adhesive (FF + LNFC) significantly increased its stiffness as a function of the test temperature, exceeding the setting behavior of the unreinforced FF. At the end of the setting process, the stiffness or storage modulus increased by 30% compared to the control resin (Table 4). Furthermore, the complete setting time for the reinforced adhesive (FF+LNFC) was reduced by 25% to 35% compared to the control adhesive.

[0075] Table 4: Minimum and maximum storage module values ​​(E min and E max) during the curing reaction of the reinforced adhesive (FF-LNFC) and the FF adhesive, control.

[0076] The same behavior of FF (Figure 8) was observed in the LNFC reinforced adhesive (Figure 9), where it was also shown that the reinforced adhesive sets 10% to 15% faster and at lower temperatures (Table 5) than the unreinforced control UF adhesive, a condition that would allow accelerating the production speed in an MDF or particleboard line.

[0077] Table 5: Minimum and maximum storage module values ​​(E' m ! n and E' max ) during the curing reaction of the reinforced adhesive, UF-LNFC, and the UF adhesive, control.

[0078] Viscoelastic properties of FF with LNFC, DMA analysis: The samples in Figure 5, with the adhesive set, were kept at rest in a desiccator for one day. Subsequently, viscoelastic responses such as loss modulus, storage modulus, and tangent of delta were evaluated by DMA analysis in a frequency range of 1 Hz to 50 Hz at a constant temperature of 25°C.

[0079] Nanomechanical properties evaluated with quasi-static and dynamic nanoindentation:

[0080] For this treatment, the small test specimens set and cut from the DMA samples shown in Figure 5 were used; these samples were cut to 6 mm in length in the direction of the fiber, using ultra-sharp blades.

[0081] Quasi-static and dynamic nanoindentation tests were performed on a Hysitron Triboindenter TI-900 nanoindentation machine equipped with nanoDMA, on UF and FF control and LNFC-enhanced samples.

[0082] Each sample was placed in the nanoindenter so that the wood fibers and adhesive were positioned parallel to the indenter. A Berkovich-type diamond indenter was used for the tests, and a dynamic load of 100 pN was applied. Since the test was performed at multiple load application frequencies, a load cycle was used in which the frequencies increased from 0.1 to 220 Hz.

Claims

Claims 1. Biopolymeric reinforcing nanoadditive for thermosetting adhesive, with low molar ratio phenol formaldehyde (FF) and urea formaldehyde (UF), for wood composites, including plywood, medium density fiberboard (MDF) or particleboard, CHARACTERIZED in that it comprises lignocellulose nanofibers (LNFC) obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, and the size of the diameter of the lignocellulose nanofiber is less than 100 nm and lengths less than 5 micrometers.

2. The biopolymeric reinforcing nanoadditive of claim 1, CHARACTERIZED in that the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, preferably 31:

69.

3. Method of preparing the present biopolymeric reinforcing nanoadditive for thermosetting adhesive, with low molar ratio phenol formaldehyde (FF) and urea formaldehyde (UF), for wood composites, including plywood, medium density fiberboard (MDF) or particleboard, wherein the reinforcing nanoadditive comprises lignocellulose nanofibers obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approx.40:60 to 20:80, preferably 31:69, CHARACTERIZED in that it comprises: a) pretreating thermodynamic pulp containing lignin and cellulose by immersing it in a 0.5% v / v NaOH solution until a suspension with swollen lignocellulose fibers is obtained, b) microdefibrillating the swollen fibers obtained in the step using a microdefibrillator, continuously repeating the microdefibrillation until a suspension of lignin and cellulose nanofibers with a biopolymer solids content of approximately 3% to 4% is obtained, c) subjecting the suspension of lignin and cellulose nanofibers obtained in step b) to high-pressure microfluidization between 28,000 and 30,000 psi, to homogenize and reduce the size of the nanofibers to diameters less than 100 nm and continuously repeating the microfluidization.

4. The method of claim 3, CHARACTERIZED in that said thermodynamic pulp comes from pulp treated in a high pressure digester, between 8 to 10 bar, and high temperature, between 140 to 150°C.

5. The method of claim 3, CHARACTERIZED in that said microdefibrillation is repeated 6 to 7 times with a disc opening in the microdefibrillator equipment of 0.5 to 1.0 pm and 900 to 1000 rpm of rotation.

6. The method of claim 3, CHARACTERIZED in that the microfluidization is repeated 4 to 5 times, using in-line interaction chambers of 200 and 100 pm diameter.

7. The method of claim 3, CHARACTERIZED in that it further comprises reducing the water content of the microfluidized lignin and cellulose fiber suspension obtained in step c).

8. The method of claim 7, CHARACTERIZED in that the water content is reduced by ultracentrifugation.

9. The method of claim 8, CHARACTERIZED in that the ultracentrifugation is conducted at a speed of 12,000 to 15,000 rpm and a temperature of 14 to 16°C.

10. Method for preparing a low molar ratio phenol-formaldehyde (FF) and urea-formaldehyde (UF) thermosetting adhesive reinforced with lignocellulose nanofibers obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, preferably said ratio is 31:69, for wood composites, including plywood, medium-density fiberboard (MDF) or particleboard, CHARACTERIZED in that it comprises mixing under continuous stirring, a low molar ratio phenol-formaldehyde (FF) and urea-formaldehyde (UF) thermosetting adhesive with a biopolymer reinforcing nanoadditive comprising lignocellulose nanofibers (LNFC) obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately40:60 to 20:80, where the amount of LNFC is 0.8% to 1.2%, based on the mass of the LNFC solids and the thermosetting adhesive solids, until a homogeneous mixture is achieved, having an effective dispersion of the reinforcing nanofibers in the thermosetting adhesive.

11. The method of claim 10, CHARACTERIZED in that said low molar ratio phenol formaldehyde (FF) and urea formaldehyde (UF) thermosetting adhesive is mixed with a biopolymer reinforcing nanoadditive comprising lignocellulose nanofibers (LNFC) obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, wherein the amount of LNFC is 1%, based on the mass of the LNFC solids and thermosetting adhesive solids.

12. A method for preparing wood composites, including plywood, medium-density fiberboard (MDF), or particleboard, with a low molar ratio phenol-formaldehyde (FF) and urea-formaldehyde (UF) thermosetting adhesive reinforced with lignocellulose nanofibers obtained from thermodynamic pulp comprising lignin and cellulose, wherein the weight ratio of lignin to cellulose is approximately 40:60 to 20:80, preferably 31:69, CHARACTERIZED in that it comprises applying said reinforced thermosetting adhesive as an adherent in the manufacturing process of said wood composites or wood-based composite materials, wherein said reinforced thermosetting adhesive is applied with roller or spray on wood veneers or fiber-particles, respectively.

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