Method for preparing low-dispersible lignin composite nanoparticles by heat induction and its application in modified wood adhesive

Abstract

The application discloses a method for preparing low-dispersion lignin composite nanoparticles by a heat induction mode and application of the lignin composite nanoparticles in modified wood adhesives, and belongs to the technical field of nanomaterials. The method comprises the following steps: dissolving lignin in water, adjusting the pH of the solution to 10-12; using polyamide epoxy chloropropane (PAE) as a modifier, mixing the lignin with the PAE under the condition of pH=10-12 to avoid the formation of precipitates in the composite process, then adjusting the pH of the solution to 4.5-9.5, and heating to 80 DEG C for 3h; and finally preparing lignin composite nanoparticles. The preparation method is simple, water is used as a solvent, the method is green and environment-friendly, the prepared lignin composite nanoparticles have uniform and adjustable particle size distribution, and have the characteristics of regular shape, uniform size, low dispersion and high pH stability. The lignin composite nanoparticles can be used in modified wood adhesives, and can improve the bonding performance and water resistance of the adhesives.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterials technology, specifically relating to a method for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent by thermal induction and its application in modified wood adhesives. Background Technology

[0002] Lignin, along with cellulose and hemicellulose, is a major component of the plant skeleton, primarily found in woody and herbaceous plants. Lignin is the second most abundant natural biomass resource after cellulose, and it is also a non-petroleum resource that can provide renewable aromatic compounds. As the second largest natural polymer, lignin is an environmentally friendly renewable biomass resource, and its high-value applications, as a byproduct of the paper industry, have received widespread attention.

[0003] Lignin, a major aromatic polymer found in nature, has shown great potential as an alternative to traditional synthetic organic materials due to its biocompatibility, biodegradability, and low cost. Currently, global pulp and paper facilities produce 500,000-700,000 tons of lignin annually, most of which is used for low-value incineration power generation, with only about 2% primarily used for the production of specialty chemicals. This is mainly because lignin's complex and variable structure, high polydispersity, and immiscibility with the host polymer matrix limit its applications. One way to overcome these limitations is to prepare lignin-based nanoparticles with regular structures and uniform sizes, which can improve the blending properties of lignin with the host matrix and provide morphological and structural control of lignin. Furthermore, nanostructured lignin offers possibilities for higher-value applications, such as drug delivery, catalyst supports, and antibacterial materials. Lignin nanoparticles, due to their inexpensive raw materials and biodegradability and non-toxicity, show great promise for applications in biocatalysis, drug delivery, and nanocomposites.

[0004] Lignin nanoparticles refer to nanoparticles with regular shapes (usually spherical) prepared from highly polydisperse and irregular lignin through various methods. Currently reported methods for preparing lignin-based nanoparticles include antisolvent methods, interfacial crosslinking, acid precipitation, and ultrasonic treatment. Among the existing methods for preparing lignin-based nanoparticles, antisolvent methods have received widespread attention, not only because of their relatively simple preparation process, but more importantly because the nanoparticles prepared by this method are usually of regular shape and uniform size. For example, Chinese patent application number 201010281868.5 discloses a method for preparing nano-lignin using supercritical antisolvent technology. The lignin used in this method comes from papermaking black liquor. The refined industrial lignin obtained from the papermaking black liquor is dried at low temperature, dissolved in dioxane, and the insoluble lignin is recovered and reused by centrifugation to obtain a lignin-dioxane solution. Nano-lignin is then prepared using a supercritical carbon dioxide equipment via the supercritical antisolvent method. However, the antisolvent method also has many limitations, such as: 1. The unavoidable use of organic solvents leads to additional environmental risks and a high-energy-consuming recovery process. 2. This method cannot prepare small-diameter nanoparticles under high-concentration conditions, while the particle size of nanoparticles for rapid intracellular drug delivery typically needs to be 50-200 nm. 3. Due to the lack of covalent cross-linking, the lignin nanoparticles prepared by this method lack stability under extreme solvent conditions, such as water with pH > 10. Therefore, there is an urgent need to develop a green, environmentally friendly, economical, simple, and easy-to-implement method to prepare lignin-based nanoparticles with uniform size, regular shape, low dispersion, and high stability. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing low-dispersion lignin composite nanoparticles by thermal induction, addressing the shortcomings of existing technologies, and its application in modified wood adhesives. This preparation method is simple and easy to implement, uses water as a solvent, is environmentally friendly, and is suitable for preparing high-concentration composite nanoparticles. Using this method, lignin composite nanoparticles with regular shapes, uniform sizes, low dispersion, and high stability can be obtained.

[0006] The technical solution adopted in this invention is as follows: A method for preparing low-dispersion lignin composite nanoparticles by thermal induction includes the following steps: (1) Dissolve lignin in water at a ratio of 1g:20-300ml for lignin:water, and then adjust the pH of the solution to 10-12 with a 20% sodium hydroxide solution. (2) Add 10% to 100% of the lignin mass of polyamide epichlorohydrin (PAE), stir evenly, and gradually adjust the pH to between 4.5 and 9.5 with hydrochloric acid. Then raise the temperature to 80°C and keep it at that temperature for 3 hours. (3) Wash the lignin composite nanoparticles 5 to 6 times by centrifugation or ultrafiltration.

[0007] Furthermore, in the above method for preparing low-dispersion lignin composite nanoparticles by thermal induction, the hydrochloric acid used in step (2) is 0.5 mol / L hydrochloric acid.

[0008] The method of this invention is applicable to lignin from different sources, including but not limited to alkali lignin, sulfate lignin, kraft paper lignin, etc.

[0009] Appendix Figure 1 This invention demonstrates the entire preparation process of a method for preparing low-dispersion lignin composite nanoparticles via thermal induction. The invention uses polyamide epichlorohydrin (PAE) as a modifier. First, lignin is mixed with a weakly cationic polyelectrolyte—polyamide epichlorohydrin (PAE)—at pH 10-12 to prevent precipitation during the composite process. Then, the charge stoichiometry is controlled by adjusting the pH, thereby controlling the properties of the lignin-PAE composite (L-PEC).

[0010] The results showed that L-PEC reached a very delicate equilibrium state within a certain pH range. Within this equilibrium state, increasing the temperature enhanced the dehydration and hydrophobicity of the amide and aromatic rings of L-PEC. Under the influence of hydrophobicity, hydrogen bonding, and ionic bonding, L-PEC self-assembled into well-defined spherical nanoparticles. The results also indicated that the pH of the equilibrium state was related to factors such as the type of lignin, the concentration of the solution, and the doping ratio of PAE.

[0011] Another object of the present invention is to provide the application of the lignin composite nanoparticles prepared by the aforementioned method for preparing low-dispersion lignin composite nanoparticles via thermal induction in modifying wood adhesives and improving the adhesive properties and water resistance of the adhesives. Further, the wood adhesive is a urea-formaldehyde resin adhesive. The present invention demonstrates through experiments that the lignin composite nanoparticles prepared by the method of the present invention can be used to prepare Pickering emulsions and for modifying urea-formaldehyde resin adhesives. The lignin composite nanoparticles prepared by the present invention can modify urea-formaldehyde resin adhesives, improving the adhesive properties and water resistance of the adhesives.

[0012] The beneficial effects of this invention are as follows: 1. This invention provides a novel method for preparing low-dispersion (polydispersity index (PDI) ≤ 0.2) and highly stable spherical lignin-based nanoparticles in water via thermal induction. The thermal responsiveness of lignin is improved by combining it with a cationic polyelectrolyte-polyamide epichlorohydrin. Compared to the previously reported polyionic method, the entropy loss caused by polyelectrolyte composite is smaller, resulting in a more stable composite that maintains its composite state even at extremely low concentrations, making it a more economical and efficient modification method. Polyamide epichlorohydrin (PAE) is used as the modifier because it is inexpensive, provides stable ononium groups for covalent crosslinking, is low in toxicity, and has abundant heat-sensitive imide groups. Results show that this simple composite method has significant modification effects on lignin from different sources and can be used to prepare spherical nanoparticles.

[0013] 2. The method of this invention simply combines lignin with PAE by controlling reaction conditions to prepare a thermoresponsive lignin-based polyelectrolyte composite, and successfully prepares regularly shaped and uniformly sized spherical nanoparticles using thermal induction. This method is applicable not only to lignin from different sources but also to high concentrations (20 g / L). The entire process is carried out in an environmentally friendly aqueous solvent, avoiding expensive equipment investment and energy-intensive organic solvent recovery processes. This method is green, environmentally friendly, economical, low-cost, simple, and easy to implement, possessing strong market potential. Furthermore, the lignin composite nanoparticles prepared by this method exhibit high pH stability due to covalent interactions, remaining stable in extreme solvents, and possessing controllable pH responsiveness and a porous structure. These unique properties will greatly expand the application prospects of the lignin-based nanomaterials prepared by this invention, such as smart nanomaterials and polymeric porous media (ppm).

[0014] 3. This invention provides a method for preparing low-dispersion lignin composite nanoparticles in a single water-based solvent via thermal induction. The entire preparation process does not use organic solvents, using water as the solvent, making it environmentally friendly and simple. It offers advantages such as ease of operation, low cost, low cationic doping ratio, and suitability for preparing high-concentration composite nanoparticles. The lignin composite nanoparticles prepared using this method have a clear spherical structure, uniform and adjustable particle size distribution, and are characterized by regular shape, uniform size, low dispersion, and high stability. These lignin composite nanoparticles can also undergo covalent surface modification under extreme solvent conditions and exhibit pH responsiveness, making them suitable for a wide range of applications. The lignin composite nanoparticles prepared using this method can modify wood adhesives (such as urea-formaldehyde resin adhesives), improving their adhesive properties and water resistance. Attached Figure Description

[0015] Figure 1 The present invention provides a process flow diagram for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent via thermal induction. Figure 2 Changes in absorbance of lignin-PAE complex solutions under different pH conditions before and after heating; Figure 2 In the table, AL represents the lignin solution without the addition of 20% PAE modification, AL-P20 represents the lignin-PAE complex solution before heating, and HAL-P20 represents the lignin-PAE complex solution after heating. Figure 3 Photographs of lignin-PAE complex solutions before and after heating at different pH values; Figure 4 TEM image of composite nanoparticles prepared by thermal induction at pH=6; Figure 5 The hydrodynamic diameter distribution of nanoparticles prepared under different pH conditions was detected by a dynamic laser particle size analyzer; Figure 6 Hydrodynamic diameter distribution of DAL-P20 (after adjusting the pH of the lignin solution to 6.0 and then directly compounding it with PAE) and HDAL-P20 (after adjusting the pH of the lignin solution to 6.0 and then directly compounding it with PAE, and then reacting at 80℃ for 3h) (a) TEM image of HDAL-P20 (b). Figure 7 Dh and PDI of AL-P20 composite nanoparticles under different pH conditions. In the figure, solid: pH from 6 to 12, hollow: pH from 12 to 6. Figure 8 FTIR spectra of AL-P20 before and after the reaction; Figure 9 The Dh and ζ potentials of NCHAL-P20 composite nanoparticles modified by non-covalent cationization at different pH values; Figure 10 The Dh and ζ potentials of covalently cationized CHAL-P20 composite nanoparticles at different pH values; Figure 11 Comparison of Pickering emulsion morphologies prepared from different cationic lignin nanoparticles; Figure 12 Bonding strength of plywood pressed from lignin-modified urea-formaldehyde resin with lignin-composite nanoparticles. Figure 13 A comparison of unmodified urea-formaldehyde resin adhesive and urea-formaldehyde resin adhesive modified by adding 5% of the lignin composite nanoparticles of this invention for pressing plywood. Detailed Implementation

[0016] Example 1 A method for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent via thermal induction includes the following steps: (1) Dissolve lignin in water according to the ratio of alkaline lignin to water of 1g:50ml, and then adjust the pH of the solution to 12 with a 20% sodium hydroxide solution. (2) Add 20% polyamide epichlorohydrin (PAE) by weight of lignin, stir evenly, and gradually adjust the pH to 7.5 with 0.5 mol / L hydrochloric acid. Then raise the temperature to 80℃ and keep it at that temperature for 3 hours. (3) The lignin composite nanoparticles were obtained by washing five times by centrifugation.

[0017] Example 2 A method for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent via thermal induction includes the following steps: (1) Dissolve the lignin in water according to the ratio of 1g:100ml of kraft paper lignin:water, and then adjust the pH of the solution to 11 with a 20% sodium hydroxide solution. (2) Add 20% polyamide epichlorohydrin (PAE) by weight of lignin, stir evenly, and gradually adjust the pH to 8.2 with 0.5 mol / L hydrochloric acid. Then raise the temperature to 80℃ and keep it at that temperature for 3 hours. (3) The lignin composite nanoparticles were obtained by washing the nanoparticles six times by ultrafiltration.

[0018] Example 3 A method for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent via thermal induction includes the following steps: (1) Dissolve lignin in water according to the ratio of alkali lignin: water of 1g: 300ml, and then adjust the pH of the solution to 12 with a 20% sodium hydroxide solution. (2) Add 30% polyamide epichlorohydrin (PAE) by weight of lignin, stir evenly, and gradually adjust the pH to 6.3 with 0.5 mol / L hydrochloric acid. Then raise the temperature to 80℃ and keep it at that temperature for 3 hours. (3) The lignin composite nanoparticles were obtained by washing five times by centrifugation.

[0019] Example 4 A method for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent via thermal induction includes the following steps: (1) Dissolve lignin in water according to the ratio of sulfate lignin: water of 1g: 300ml, and then adjust the pH of the solution to 10.5 with a 20% sodium hydroxide solution. (2) Add 20% polyamide epichlorohydrin (PAE) by weight of lignin, stir evenly, and gradually adjust the pH to 8.0 with 0.5 mol / L hydrochloric acid. Then raise the temperature to 80℃ and keep it at that temperature for 3 hours. (3) The lignin composite nanoparticles were obtained by washing the nanoparticles six times by ultrafiltration.

[0020] Example 5 A method for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent via thermal induction includes the following steps: (1) Dissolve the lignin in water according to the ratio of 1g:250ml of kraft paper lignin:water, and then adjust the pH of the solution to 11 with a 20% sodium hydroxide solution. (2) Add 60% polyamide epichlorohydrin (PAE) by weight of lignin, stir evenly, and gradually adjust the pH to 9.5 with 0.5 mol / L hydrochloric acid. Then raise the temperature to 80℃ and keep it at that temperature for 3 hours. (3) The lignin composite nanoparticles were obtained by washing five times by centrifugation.

[0021] Example 6 A method for preparing low-dispersion lignin composite nanoparticles in a single aqueous solvent via thermal induction includes the following steps: (1) Dissolve lignin in water according to the ratio of alkali lignin: water of 1g: 300ml, and then adjust the pH of the solution to 11 with a 20% sodium hydroxide solution. (2) Add 20% polyamide epichlorohydrin (PAE) by weight of lignin, stir evenly, and gradually adjust the pH to 6.0 with 0.5 mol / L hydrochloric acid. Then raise the temperature to 80℃ and keep it at that temperature for 3 hours. (3) The lignin composite nanoparticles were obtained by washing five times by centrifugation.

[0022] Example 7 Test Data 1. Changes in the thermal response properties of lignin before and after modification with polyamide epichlorohydrin (PAE) as a modifier. Alkali lignin was dissolved in water at a ratio of 1g:300ml, and the pH of the solution was adjusted to 11 using a 20% sodium hydroxide solution. Then, 20% (by weight) of polyamide epichlorohydrin (PAE) was added, and the mixture was stirred until homogeneous. The pH was then gradually adjusted with 0.5mol / L hydrochloric acid to obtain the lignin-PAE composite solution before heating (AL-P20). The solution was then heated to 80℃ and maintained at this temperature for 3 hours to obtain the heated lignin-PAE composite solution (HAL-P20). The solution was washed five times by centrifugation or ultrafiltration to obtain the lignin composite nanoparticles. The absorbance of the lignin-PAE composite solution under different pH conditions before and after heating was measured, and the color change of the lignin-PAE composite solution before and after heating was observed. The results are shown in [Figure number missing]. Figures 2-3 .

[0023] Figure 2 The absorbance changes of lignin-PAE complex solutions under different pH conditions before and after heating; Figure 2 In the table, AL represents the lignin solution without the addition of 20% PAE modification, AL-P20 represents the lignin-PAE complex solution before heating, and HAL-P20 represents the lignin-PAE complex solution after heating. Figure 2 The changes in the thermal response properties of lignin before and after modification with 20% PAE are shown. It can be seen that alkali lignin does not exhibit thermal response before modification, but shows significant thermal response behavior at pH ≤ 7 after the addition of PAE.

[0024] Figure 3 These are photographs of lignin-PAE complex solutions under different pH conditions before and after heating; Figure 3 The color changes of the AL-P complex solution before and after heating are shown, with significant color changes observed between pH 7 and 6. These phenomena indicate that lignin exhibits thermally responsive behavior after PAE modification, which forms the basis for the thermally induced preparation of lignin-based nanoparticles.

[0025] 2. Transmission electron microscopy (TEM) images of the prepared lignin composite nanoparticles and hydrodynamic diameter distribution of nanoparticles prepared under different pH conditions. Following the method for preparing lignin composite nanoparticles in Example 6, the transmission electron microscope (TEM) image of the lignin composite nanoparticles prepared by thermal induction with hydrochloric acid adjusted to pH=6.0 is shown below. Figure 4 . Figure 4 The images show lignin-based composite nanoparticles prepared by thermal induction at pH 6.0. These particles exhibit a clear spherical structure with a uniform and tunable particle size distribution. A loose fractal structure is observed on the magnified surface of the nanoparticles. Figure 4 This may mean that the nanoparticles themselves contain voids.

[0026] On the other hand, the hydrodynamic diameter of the composite nanoparticles prepared under different pH conditions was detected using a dynamic laser particle size analyzer, and the results are shown in [Figure number missing]. Figure 5 The results showed that the hydrodynamic diameter distribution of the prepared HAL-P20-NPs composite nanoparticles exhibited a single peak between pH 7.0 and 6.0, and the PDI (polydispersity index) was below 0.2, comparable to that of lignin nanoparticles synthesized by the antisolvent method. Some studies have reported methods for preparing lignin-based composite nanoparticles by directly combining lignin with cationic polyelectrolytes. This invention also conducted research experiments, and the results showed that the directly combined HDAL-P20 formed a large number of non-spherical aggregates with a bimodal hydrodynamic diameter distribution. Figure 6 The PDI was greater than 0.2, which is consistent with previously reported lignin-based nanoparticles prepared by the polyelectrolyte composite method. On the other hand, the same hydrodynamic particle size and PDI variation trends with pH were observed in different lignin composite nanoparticles: Dh gradually increased as pH decreased, while PDI first decreased and then increased, indicating that excessively high or low pH values ​​are unfavorable for preparing composite nanoparticles with uniform size. This invention also successfully synthesized narrowly distributed nanoparticles with hydrodynamic diameters below 200 nm in lignin solutions up to 20 g / L.

[0027] 3. Stability HAL-P20 nanoparticles, prepared by composite modification of lignin with 20% PAE, exhibit high pH stability. For example... Figure 7 As shown, the hydrodynamic diameter of the composite nanoparticles under different pH conditions was detected using a dynamic laser particle size analyzer. The results showed that the average hydrodynamic diameter (Dh) of the nanoparticles remained stable within the pH range of 6-11 after heating for 180 min. Only at pH 12 did Dh increase, but the PDI (polydispersity index) remained at a low level, indicating that the composite nanoparticles only swelled rather than disintegrated at this point. Furthermore, this swelling was reversible; as the pH decreased from 12 to 6, the average hydrodynamic diameter of the nanoparticles gradually returned from 243 nm to 194 nm. On the other hand, the swelling and shrinkage processes of HAL-P20 were not consistent, possibly due to the difference between pH-induced and thermally induced processes. The high pH stability of HAL-P20 nanoparticles is mainly attributed to the covalent reaction between the carboxyl and hydroxyl groups. Figure 8 These are the FT-IR spectra of AL-P20 before and after the reaction. The shoulder peak of AL (lignin) at 1632 cm⁻¹ is attributed to the stretching vibration peak of the C=O group of the carboxyl group. Due to the addition of PAE, the peak of AL-P20 at this location is masked. The infrared peak of the reaction between the carboxyl group and the ononium group to form the ester group redshifts to 1710 cm⁻¹.

[0028] 4. Surface modification The inherent negative charge of lignin nanoparticles allows for surface modification through simple adsorption of opposite charges, and the composite nanoparticles prepared in this invention are no exception. The composite nanoparticle aqueous dispersion was gradually added dropwise to a 1 wt% polydimethyldiallylammonium chloride (PDADMAC) solution for non-covalent surface modification. Figure 9 The zeta potential and average hydrodynamic diameter of the modified HAL-P20 nanoparticles (prepared by composite modification of lignin with 20% PAE at pH=6) are shown. The cationic particles exhibit pH-switching surface charges, carrying a positive charge at pH ≤ 5 and a negative charge at pH ≥ 6, with a rapid transition between pH 5 and 6. Furthermore, the Dh of the modified nanoparticles shows a very interesting change; as the pH decreases to 3, the Dh of the nanoparticles shows a significant increase, which may be due to partial desorption of PDADMAC segments caused by electrostatic repulsion from the non-cationized PAE within the nanoparticles.

[0029] Thanks to the stabilizing effect of covalent bonds, HAL-P20 can undergo covalent modification in more extreme solvent environments, in addition to non-covalent surface modification. By adjusting the pH of the nanoparticle aqueous dispersion to 11.7 and adding 50% (by weight) glycidyltrimethylammonium chloride (GMC) to the nanoparticles, covalent modification was performed at 70°C for 3 hours. The results showed that the covalently modified HAL-P20 also exhibited switchable surface charges, but a metastable state appeared in the pH range of 4-7. Figure 10 This is similar to previous reports. Furthermore, the average particle size of the covalently surface-modified nanoparticles remained stable in the pH range of 2–4, confirming that the desorption of PDADMAC led to an increase in the particle size of the non-covalent cationic nanoparticles.

[0030] In summary, this experiment successfully prepared a thermoresponsive lignin-based polyelectrolyte composite by simply combining lignin with 20 wt% PAE, and successfully fabricated regularly shaped, uniformly sized spherical nanoparticles using thermal induction. This method is applicable not only to lignin from various sources but also to high concentrations (20 g / L). The entire process is carried out in an environmentally friendly aqueous solvent, avoiding expensive equipment investment and energy-intensive organic solvent recovery processes. We believe this method has strong market potential. Furthermore, the composite nanoparticles remain stable in extreme solvents due to covalent interactions and exhibit controllable pH responsiveness and a porous structure. These unique properties will greatly expand the application prospects of lignin-based nanomaterials, such as smart nanomaterials and polymeric porous media (ppm).

[0031] Example 8: Application Effect of Lignin Composite Nanoparticles Lignin nanoparticles have important prospects in drug delivery, nanocomposite materials, Pickering emulsions and other fields. The following are two applications of lignin composite nanoparticles prepared by the method of this invention.

[0032] 1. Preparation of Pickering emulsion using composite nanoparticles Pickering emulsion was prepared using the lignin composite nanoparticles obtained by the method in Example 1 of this invention. Olive oil was used as the oil phase, mixed with water at a ratio of 1:9, and then 0.5% (by weight of olive oil) of lignin composite nanoparticles was added. The mixture was homogenized for 1 min to obtain the Pickering emulsion. The pH responsiveness of Pickering emulsions prepared with different cationic addition ratios was compared. The results showed that the particle size of the Pickering emulsion modified with 1% cationic lignin composite nanoparticles decreased with pH change, which is beneficial for the sustained release of the drug in human gastric and intestinal fluids. A comparison of the morphologies of Pickering emulsions prepared with different cationic lignin nanoparticles is shown in the figure below. Figure 11 .

[0033] 2. Composite nanoparticle modified urea-formaldehyde resin The lignin composite nanoparticles prepared by combining lignin with 60% by mass of PAE according to the method of Example 5 of this invention were used to modify urea-formaldehyde resin adhesive. The results showed that after adding 5% by mass of nanoparticles to modify the urea-formaldehyde resin, the wet strength of the plywood pressed using the modified adhesive was significantly enhanced. The bonding strength results of the plywood pressed with urea-formaldehyde resin modified by lignin composite nanoparticles are shown in the figure. Figure 12 A comparison chart of unmodified urea-formaldehyde resin adhesive and urea-formaldehyde resin adhesive modified with 5% lignin composite nanoparticles of this invention used for pressing plywood is shown below. Figure 13 , Figure 13 The first example uses unmodified urea-formaldehyde resin adhesive, which is transparent. After tensile fracture, the bonded surfaces between the wood chips show no wood breakage, indicating that the fracture occurs through the adhesive layer. This demonstrates the low bonding strength of the unmodified urea-formaldehyde resin. The second example uses urea-formaldehyde resin adhesive modified with 5% lignin composite nanoparticles (as described in this invention). This modified adhesive is yellowish, and after tensile fracture, the bonded surfaces between the wood chips show severe wood breakage, indicating that the fracture occurs through the wood itself. This demonstrates the high bonding strength of the modified adhesive. Therefore, the lignin composite nanoparticle-modified urea-formaldehyde resin adhesive prepared using the method of this invention can improve the adhesive's bonding performance and water resistance.

[0034] As can be seen from the above, the lignin composite nanoparticles prepared by the method of the present invention have a good modifying effect on urea-formaldehyde resin adhesives.

Claims

1. A method for preparing low-dispersion lignin composite nanoparticles by thermal induction, characterized in that, Includes the following steps: (1) Dissolve lignin in water at a ratio of 1g:20-300ml for lignin:water, and then adjust the pH of the solution to 10-12 with a 20% sodium hydroxide solution. (2) Add 10% to 100% of the lignin mass of polyamide epichlorohydrin, stir evenly, and gradually adjust the pH to between 4.5 and 9.5 with hydrochloric acid. Then raise the temperature to 80°C and keep it at that temperature for 3 hours. (3) Wash the lignin composite nanoparticles 5 to 6 times by centrifugation or ultrafiltration.

2. The method for preparing low-dispersion lignin composite nanoparticles by thermal induction according to claim 1, characterized in that, The hydrochloric acid used in step (2) is 0.5 mol / L hydrochloric acid.

3. The application of lignin composite nanoparticles prepared by the method of preparing low-dispersion lignin composite nanoparticles by thermal induction as described in claim 1 in modified wood adhesives.

4. The application according to claim 3, characterized in that, The wood adhesive mentioned is a urea-formaldehyde resin adhesive.

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