A tannic acid-modified lignin-phenolic resin antibacterial adhesive and its preparation method

Abstract

This invention discloses a tannic acid-modified lignin-phenolic resin antibacterial adhesive and its preparation method, belonging to the field of bio-based adhesive technology. The invention uses alkali lignin as a raw material to prepare lignin nanoparticles, then modifies the lignin nanoparticles with ferric chloride and tannic acid to obtain tannic acid / iron complexed lignin nanoparticles. The tannic acid / iron complexed lignin nanoparticles, phenol, formaldehyde, and alkaline solution are mixed, and the lignin-phenolic resin adhesive is prepared through an addition condensation reaction. The lignin-phenolic resin adhesive prepared by this invention can achieve a high proportion of phenol substitution and also has good photothermal antibacterial effects, providing a new approach for the development of antibacterial and environmentally friendly adhesives.

Description

Technical Field

[0001] This invention belongs to the field of bio-based adhesive technology, and particularly relates to a tannic acid-modified lignin phenolic resin antibacterial adhesive and its preparation method. Background Technology

[0002] Phenolic resin adhesives are polymer adhesives formed by the reaction of phenol and its derivatives (such as xylenol and resorcinol) and formaldehyde as the main raw materials under the action of a catalyst. The cured products exhibit excellent heat resistance, high bonding strength, good aging resistance, and excellent electrical insulation, and are widely used in the manufacture of plywood, engineered wood products, and high-grade wood products. However, the preparation of phenolic resins heavily relies on fossil fuels, resulting in high production costs and causing resource depletion and environmental pollution. Furthermore, traditional phenolic resin adhesives, limited by their molecular structure, are prone to bacterial and fungal growth in humid environments and during prolonged use, severely affecting the adhesive's effectiveness and lifespan, ultimately leading to delamination of the bonding carrier.

[0003] Lignin, the most abundant aromatic compound in nature, has an annual production of up to 50 million tons. Lignin molecules contain numerous phenolic structures, making it a relatively ideal biomass raw material for the production of phenolic resins, replacing phenol. However, due to its complex molecular structure, high heterogeneity, and few active sites, lignin exhibits low reactivity, making it difficult to fully utilize. To improve the reactivity of lignin, strategies such as hydroxymethylation, amination, and oxidation are commonly employed for modification. However, these modification strategies involve complex reaction steps and require large amounts of chemical reagents, easily causing serious environmental pollution.

[0004] Therefore, there is an urgent need to develop a low-cost adhesive with antibacterial properties. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a tannic acid-modified lignin-phenolic resin antibacterial adhesive and its preparation method. The lignin nanoparticles prepared using this method possess a high specific surface area, fully exposing the active sites of alkali lignin. The tannic acid / iron complex layer further enriches the reaction sites of alkali lignin, achieving a high proportion of phenol substitution. Simultaneously, the tannic acid / iron complex layer imparts excellent photothermal antibacterial effects to the adhesive.

[0006] To achieve the above objectives, the present invention provides the following technical solutions: One of the technical solutions of the present invention:

[0007] A method for preparing a tannic acid-modified lignin-phenolic resin antibacterial adhesive includes: using alkali lignin as raw material, obtaining lignin nanoparticles through dialysis; modifying the lignin nanoparticles with tannic acid and ferric chloride to obtain tannic acid / iron complexed lignin nanoparticles; mixing the tannic acid / iron complexed lignin nanoparticles with phenol, formaldehyde, and alkaline solution, and preparing the tannic acid-modified lignin-phenolic resin antibacterial adhesive through an addition condensation reaction.

[0008] Furthermore, the preparation method of the tannic acid-modified lignin-phenolic resin antibacterial adhesive includes the following steps:

[0009] Alkali lignin was dissolved in an organic solvent, water was added and stirred to obtain a solution, and the solution was dialyzed and freeze-dried to obtain lignin nanoparticles.

[0010] The lignin nanoparticles were dispersed in water, and ferric chloride and tannic acid were added. After the reaction was completed, the mixture was centrifuged and freeze-dried to obtain tannic acid / iron complexed lignin nanoparticles.

[0011] The tannic acid / iron complexed lignin nanoparticles, phenol, a portion of formaldehyde solution, and a portion of alkali solution are mixed and reacted at 65–75°C for 30–60 min. Then, a portion of formaldehyde solution and a portion of alkali solution are added, and the mixture is heated at 75–85°C for 30 min. The remaining formaldehyde solution and the remaining alkali solution are then added, and the temperature is raised to 85–95°C and maintained for 60–90 min. After the phenolic resin reaches the required viscosity, it is cooled and discharged to obtain the tannic acid-modified lignin phenolic resin antibacterial adhesive.

[0012] This invention involves nano-sizing of alkali lignin, which effectively reduces its heterogeneity and increases its specific surface area, exposing more active sites and effectively enhancing the reactivity of alkali lignin. To improve the antibacterial effect of the adhesive, an antibacterial agent is introduced during its preparation. Tannic acid is a natural plant-derived polyphenol compound with antioxidant, free radical scavenging, and antibacterial properties. Its complexes with metal ions exhibit excellent near-infrared photothermal conversion capabilities. Under near-infrared light irradiation, these tannic acid-metal complexes can efficiently convert light energy into heat energy, generating a localized high-temperature effect that disrupts the integrity of bacterial cell membranes, leading to bacterial death. In summary, this invention uses inexpensive and renewable alkali lignin as raw material, performs nano-processing, and further introduces tannic acid / iron complexes to replace part of the phenol, thus preparing phenolic resin adhesives. This process offers several unique advantages: firstly, the nano-processing and the introduction of tannic acid effectively increase the reactive sites of lignin, achieving a high proportion of phenol substitution; secondly, the tannic acid / iron complexes possess excellent near-infrared photothermal conversion capabilities, imparting photothermal antibacterial effects to the adhesives. This is of great significance for promoting the resource utilization of lignin and expanding the application range of phenolic resin adhesives.

[0013] The organic solvent is one of tetrahydrofuran, methanol, and acetonitrile; and / or

[0014] The volume ratio of the organic solvent to water is 1:(1-9).

[0015] The mass ratio of lignin nanoparticles, ferric chloride, and tannic acid is (10–20):(1–2):(2–4). This mass ratio affects the loading capacity and stability of the lignin nanoparticles on the tannic acid / iron complex, thereby influencing the antibacterial effect of the adhesive.

[0016] The mass of the tannic acid / iron complexed lignin nanoparticles is 30% to 70% of the mass of phenol.

[0017] The total amount of formaldehyde solution added is 80% to 100% of the total mass of phenol and tannic acid / iron-complexed lignin nanoparticles, and the mass ratio of formaldehyde solution added sequentially is 1:1:2. The synthesis of phenolic resin is a condensation polymerization process, in which formaldehyde and phenol polymerize stepwise under the action of a catalyst. Adding formaldehyde in batches makes the reaction more stable, avoiding violent reactions caused by excessive formaldehyde, thus facilitating control of the reaction temperature and rate. Furthermore, adding formaldehyde in batches also makes the reaction more uniform, avoiding uneven product properties caused by excessively rapid local reactions.

[0018] The formaldehyde solution is a formaldehyde solution with a mass concentration of 35% to 40%, preferably a formaldehyde solution with a mass concentration of 37%.

[0019] The total amount of alkali solution added is 30%–50% of the total mass of phenol and tannic acid / iron-complexed lignin nanoparticles, with the mass ratio of alkali solution added sequentially being 1:2:2. The alkali solution acts as a catalyst, accelerating the synthesis reaction of phenolic resin. Adding the alkali solution in batches allows the reaction rate to gradually increase, avoiding excessively rapid reaction and temperature runaway caused by excess alkali solution. During the synthesis of phenolic resin, various side reactions may occur; adding the alkali solution in batches can make the reaction more inclined to produce the desired phenolic resin, reducing the formation of byproducts. Adding the alkali solution in batches can also improve the stability of the product. During the reaction, if the alkali solution is excessive, it may lead to structural instability of the resin, making it prone to decomposition or volatilization at high temperatures. Adding it in batches allows the pH value in the reaction system to gradually increase, thus promoting the stability of the resin structure.

[0020] The mass concentration of the alkaline solution is 40%.

[0021] The alkaline solution is one of sodium hydroxide solution, potassium hydroxide solution, and barium hydroxide solution.

[0022] The required viscosity is determined based on actual needs. For example, the required viscosity refers to a viscosity of 60–200 mPa·s.

[0023] The second technical solution of the present invention:

[0024] The present invention also provides a tannic acid-modified lignin phenolic resin antibacterial adhesive prepared according to the above preparation method.

[0025] The third technical solution of the present invention:

[0026] The present invention also provides the application of the tannic acid-modified lignin phenolic resin antibacterial adhesive in the preparation of plywood.

[0027] This invention relates to an antibacterial lignin-phenolic resin adhesive modified with tannic acid, which is widely used in the wood processing industry, especially in the production of engineered wood products, due to its excellent bonding properties. Utilizing tannic acid-modified lignin-phenolic resin can reduce the amount of phenol used and decrease formaldehyde emissions, achieving environmental protection goals. Alkali lignin, as a natural and renewable resource, contains active groups such as hydroxyl, methoxy, and carboxyl groups in its chemical structure, making it easy to functionalize and an ideal raw material for synthesizing environmentally friendly wood adhesives. The development of this tannic acid-modified lignin-phenolic resin antibacterial adhesive not only reduces dependence on fossil resources but also lowers environmental pollution. The introduction of alkali lignin can improve adhesive properties, such as increasing bonding strength and reducing curing time. Alkali lignin can replace some of the phenol in phenolic resin adhesives without affecting the performance of the board. Since alkali lignin is an inexpensive raw material, its application in adhesives helps reduce production costs. The tannic acid-modified lignin-phenolic resin antibacterial adhesive can reduce residual formaldehyde during preparation, thereby reducing formaldehyde emissions and improving product safety. The tannic acid-modified lignin-phenolic resin antibacterial adhesive of this invention can be applied in the wood industry for the preparation of high-end synthetic boards. Simultaneously, the cured phenolic resin is also a good carbon source and can be used to prepare energy storage electrode materials for supercapacitors. This tannic acid-modified lignin-phenolic resin antibacterial adhesive can also be used in other industrial fields, such as friction materials and the casting industry, and is valued for its high-temperature resistance and bonding strength. For example, the tannic acid-modified lignin-phenolic resin antibacterial adhesive of this invention can bond wood, such as wood flooring, cabinet panels, particleboard, etc.

[0028] Compared with the prior art, the present invention has the following advantages and technical effects:

[0029] (1) The present invention performs nano-processing on alkali lignin, which can fully expose the active sites of alkali lignin and significantly enhance its reactivity. The tannic acid / iron complex layer can further enrich the active sites of alkali lignin, achieve a high proportion of phenol substitution, and the reaction process is simple.

[0030] (2) The tannic acid / iron complex layer has good near-infrared photothermal conversion ability, which can give the adhesive a photothermal antibacterial effect. Without the need to introduce additional antibacterial agents, the tannic acid modified lignin phenolic resin antibacterial adhesive can have a good antibacterial effect. Attached Figure Description

[0031] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0032] Figure 1 This is a scanning electron microscope image of the lignin nanoparticles prepared in Example 1;

[0033] Figure 2 This is a scanning electron microscope image of the tannic acid / iron complexed lignin nanoparticles prepared in Example 1. Detailed Implementation

[0034] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0035] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0036] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0037] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0038] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0039] Unless otherwise specified, the room temperature in this invention is 25±2℃.

[0040] All raw materials used in the embodiments of this invention were obtained through commercial purchase.

[0041] It should be noted that all aspects not described in detail in this invention are conventional operating methods in the field and are not the focus of this invention. For example, specific methods such as dialysis and freeze-drying are all completed using conventional methods.

[0042] This invention prepares lignin nanoparticles from alkali lignin, then modifies them with ferric chloride and tannic acid to obtain tannic acid / iron complexed lignin nanoparticles. The tannic acid / iron complexed lignin nanoparticles, phenol, formaldehyde, and alkaline solution are mixed, and a lignin-phenolic resin adhesive is prepared through an addition condensation reaction. The lignin-phenolic resin adhesive prepared by this invention can achieve a high proportion of phenol substitution and also has good photothermal antibacterial effects, providing a new approach for the development of antibacterial and environmentally friendly adhesives.

[0043] The term "alkali lignin" refers to an industrial byproduct produced by spray drying alkaline pulping wastewater using the latest production technology; therefore, it will also be referred to as "industrial alkali lignin" below. Alkali lignin is a brownish-yellow powder with a distinctive odor; it is soluble in alkaline solutions and some organic solvents. Alkali lignin can be purchased directly or prepared according to literature. In the embodiments of this invention, the alkali lignin was purchased from Huatai Group Co., Ltd.

[0044] The technical solution of the present invention will be further illustrated by the following embodiments.

[0045] Example 1

[0046] First, 20g of industrial alkali lignin was dissolved in 20mL of tetrahydrofuran, and 180mL of deionized water was added and stirred for 30min. After the reaction was completed, the solution was dialyzed and freeze-dried to obtain lignin nanoparticles. The scanning electron microscope image is shown below. Figure 1 It can be seen that the lignin nanoparticles prepared in this step are spherical and have a high specific surface area;

[0047] In the second step, 20g of lignin nanoparticles were dispersed in deionized water, 1g of ferric chloride and 2g of tannic acid were added and stirred for 20 minutes. After centrifugation and freeze-drying, tannic acid / iron complexed lignin nanoparticles were obtained. The scanning electron microscope image is shown below. Figure 2 It can be seen that the tannic acid / iron complexed lignin nanoparticles prepared in this step have a rough surface and particulate matter attached, indicating that the tannic acid / iron complex is formed and attached to the surface of the lignin nanoparticles.

[0048] The third step involves adding 6g of tannic acid / iron-chelated lignin nanoparticles, 20g of phenol, 5.2g of 37wt.% formaldehyde solution, and 1.6g of sodium hydroxide solution (concentration 40wt.%) to a reaction vessel and reacting at 65℃ for 30min. Then, 5.2g of 37wt.% formaldehyde solution and 3.2g of potassium hydroxide solution are added, and the mixture is heated at 75℃ for 30min. Next, 10.4g of 37wt.% formaldehyde solution and 3.2g of sodium hydroxide solution are added, the temperature is raised to 85℃, and the mixture is kept at this temperature for 60min. The mixture is then cooled and discharged to obtain a lignin-tannic acid-modified phenolic resin antibacterial adhesive.

[0049] According to section 4.17 of GB / T 17657-2013 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels", poplar veneer was used to prepare three-layer plywood, and the strength of the plywood was tested. The strength was measured at an adhesive application rate of 120 g / m². 2 Plywood prepared under the conditions of hot pressing temperature of 190℃, hot pressing pressure of 2.0MPa, and hot pressing time of 10 minutes, has a bonding strength of 1.5MPa as measured by the Class I plywood testing method, which is greater than the national standard of 0.7MPa.

[0050] Example 2

[0051] The first step is to dissolve 20g of industrial alkali lignin in 20mL of acetonitrile, add 140mL of deionized water and stir for 30min. After the reaction is completed, the solution is dialyzed and freeze-dried to obtain lignin nanoparticles.

[0052] The second step is to take 20g of lignin nanoparticles and disperse them in deionized water, add 1g of ferric chloride and 3g of tannic acid and stir for 20min. After centrifugation and freeze drying, tannic acid / iron complexed lignin nanoparticles are obtained.

[0053] The third step involves adding 8g of tannic acid / iron-chelated lignin nanoparticles, 20g of phenol, 6.3g of 40wt.% formaldehyde solution, and 2.2g of sodium hydroxide solution (concentration 40wt.%, the same below) to a reaction vessel and reacting at 70℃ for 40min. Then, 6.3g of 40wt.% formaldehyde solution and 4.4g of sodium hydroxide solution are added, and the mixture is heated at 80℃ for 40min. Next, 12.6g of 40wt.% formaldehyde solution and 4.4g of sodium hydroxide solution are added, the temperature is raised to 90℃, and the mixture is kept at that temperature for 70min. The mixture is then cooled and discharged to obtain tannic acid-modified lignin phenolic resin antibacterial adhesive.

[0054] The adhesive strength of the adhesive prepared in this embodiment was tested using the same method as in Example 1, and the adhesive strength was measured to be 1.4 MPa.

[0055] Example 3

[0056] The first step is to dissolve 20g of industrial alkali lignin in 20mL of methanol, add 100mL of deionized water and stir for 30min. After the reaction is completed, the solution is dialyzed and freeze-dried to obtain lignin nanoparticles.

[0057] The second step is to take 20g of lignin nanoparticles and disperse them in deionized water, add 1g of ferric chloride and 4g of tannic acid and stir for 30min. After centrifugation and freeze drying, tannic acid / iron complexed lignin nanoparticles are obtained.

[0058] The third step involves adding 10g of tannic acid / iron-chelated lignin nanoparticles, 20g of phenol, 7.5g of 35wt.% formaldehyde solution, and 3g of sodium hydroxide solution (concentration 40wt.%, the same below) to a reaction vessel and reacting at 75℃ for 50min. Then, another 7.5g of 35wt.% formaldehyde solution and 3g of sodium hydroxide solution are added, and the mixture is heated at 85℃ for 50min. Finally, 15g of 35wt.% formaldehyde solution and 3g of sodium hydroxide solution are added, the temperature is raised to 95℃, and the mixture is kept at that temperature for 80min. The mixture is then cooled and discharged to obtain tannic acid-modified lignin phenolic resin antibacterial adhesive.

[0059] The adhesive strength of the adhesive prepared in this embodiment was tested using the same method as in Example 1, and the adhesive strength was measured to be 1.4 MPa.

[0060] Example 4

[0061] The first step is to dissolve 20g of industrial alkali lignin in 20mL of methanol, add 60mL of deionized water and stir for 30min. After the reaction is completed, the solution is dialyzed and freeze-dried to obtain lignin nanoparticles.

[0062] The second step is to take 15g of lignin nanoparticles and disperse them in deionized water, add 2g of ferric chloride and 4g of tannic acid and stir for 40min. After centrifugation and freeze drying, tannic acid / iron complexed lignin nanoparticles are obtained.

[0063] The third step involves adding 12g of tannic acid / iron-complexed lignin nanoparticles, 20g of phenol, 8g of 37wt.% formaldehyde solution, and 3.2g of potassium hydroxide solution (concentration 40wt.%, the same below) to a reaction vessel and reacting at 75℃ for 60min. Then, another 8g of 37wt.% formaldehyde solution and 3.2g of potassium hydroxide solution are added, and the mixture is heated at 85℃ for 60min. Finally, 16g of 37wt.% formaldehyde solution and 6.4g of potassium hydroxide solution are added, the temperature is raised to 95℃, and the mixture is kept at that temperature for 90min. The mixture is then cooled and discharged to obtain tannic acid-modified lignin-phenolic resin antibacterial adhesive.

[0064] The adhesive strength of the adhesive prepared in this embodiment was tested using the same method as in Example 1, and the adhesive strength was measured to be 1.6 MPa.

[0065] Example 5

[0066] The first step is to dissolve 20g of industrial alkali lignin in 20mL of methanol, add 20mL of deionized water and stir for 30min. After the reaction is completed, the solution is dialyzed and freeze-dried to obtain lignin nanoparticles.

[0067] The second step is to take 15g of lignin nanoparticles and disperse them in deionized water, add 1g of ferric chloride and 3g of tannic acid and stir for 60min. After centrifugation and freeze drying, tannic acid / iron complexed lignin nanoparticles are obtained.

[0068] In the third step, 14g of tannic acid / iron-complexed lignin nanoparticles, 20g of phenol, 8.5g of 37% formaldehyde solution, and 4.3g of barium hydroxide solution were added to a reaction vessel and reacted at 75℃ for 60min. Then, 8.5g of 37% formaldehyde solution and 4.3g of sodium hydroxide solution were added, and the mixture was heated at 85℃ for 60min. Next, 17g of 37% formaldehyde solution and 8.6g of barium hydroxide solution were added, the temperature was raised to 95℃, and the mixture was kept at that temperature for 90min. After cooling, the mixture was discharged to obtain tannic acid-modified lignin phenolic resin antibacterial adhesive.

[0069] The adhesive strength of the adhesive prepared in this embodiment was tested using the same method as in Example 1, and the adhesive strength was measured to be 1.5 MPa.

[0070] Comparative Example 1

[0071] The first step is to dissolve 20g of industrial alkali lignin in 20mL of methanol, add 60mL of deionized water and stir for 30min. After the reaction is completed, the solution is dialyzed and freeze-dried to obtain lignin nanoparticles.

[0072] In the second step, 12g of lignin nanoparticles, 20g of phenol, 8g of 37% formaldehyde solution, and 3.2g of potassium hydroxide solution were added to a reaction vessel and reacted at 75℃ for 60min. Then, 8g of 37% formaldehyde solution and 3.2g of sodium hydroxide solution were added, and the mixture was heated at 85℃ for 60min. Next, 16g of 37% formaldehyde solution and 6.4g of potassium hydroxide solution were added, the temperature was raised to 95℃, and the mixture was kept at that temperature for 90min. After cooling, the mixture was discharged to obtain lignin phenolic resin adhesive.

[0073] The adhesive strength of the adhesive prepared in this comparative example was tested using the same method as in Example 1, and the adhesive strength was measured to be 0.9 MPa.

[0074] Comparative Example 2

[0075] The first step is to dissolve 20g of industrial alkali lignin in 20mL of methanol, add 20mL of deionized water and stir for 30min. After the reaction is completed, the solution is dialyzed and freeze-dried to obtain lignin nanoparticles.

[0076] In the second step, 14g of lignin nanoparticles, 20g of phenol, 8.5g of 37% formaldehyde solution, and 4.3g of barium hydroxide solution were added to a reaction vessel and reacted at 75℃ for 60min. Then, 8.5g of 37% formaldehyde solution and 4.3g of sodium hydroxide solution were added, and the mixture was heated at 85℃ for 60min. Next, 17g of 37% formaldehyde solution and 8.6g of barium hydroxide solution were added, the temperature was raised to 95℃, and the mixture was kept at that temperature for 90min. After cooling, the mixture was discharged to obtain lignin phenolic resin adhesive.

[0077] The adhesive strength of the adhesive prepared in this comparative example was tested using the same method as in Example 1, and the adhesive strength was measured to be 1.1 MPa.

[0078] Comparative Example 3

[0079] The first step is to dissolve 20g of industrial alkali lignin in 20mL of methanol, add 20mL of deionized water and stir for 30min. After the reaction is completed, the solution is dialyzed and freeze-dried to obtain lignin nanoparticles.

[0080] The second step is to take 15g of lignin nanoparticles and disperse them in deionized water, add 1g of ferric chloride and 3g of tannic acid and stir for 60min. After centrifugation and freeze drying, tannic acid / iron complexed lignin nanoparticles are obtained.

[0081] The third step involves adding 14g of tannic acid / iron-complexed lignin nanoparticles, 20g of phenol, 37g of 37% formaldehyde solution, and 17.2g of barium hydroxide solution directly into a reaction vessel. The mixture is reacted at 75°C for 60 minutes, then heated to 85°C for 60 minutes, and finally heated to 95°C and held at that temperature for 90 minutes. After cooling, the mixture is discharged to obtain tannic acid-modified lignin phenolic resin antibacterial adhesive.

[0082] The adhesive strength of the adhesive prepared in this comparative example was tested using the same method as in Example 1, and the adhesive strength was measured to be 1.2 MPa.

[0083] Referring to the national standard GB / T 14732-2017, the solid content, pH, viscosity, free aldehyde content, and free phenol content of the adhesives prepared in the above examples and comparative examples were tested. At the same time, the antibacterial properties of the adhesives were tested using the inhibition zone method under 808nm near-infrared light irradiation. The test results are shown in Tables 1 and 2.

[0084] Table 1 Main properties of adhesives

[0085]

[0086] Note: In Table 1, the lignin / phenol mass ratio refers to the percentage of lignin nanoparticles or lignin nanoparticles in the mass of phenol that are tannic acid / iron complexed.

[0087] Table 2 Antibacterial properties of adhesives

[0088]

[0089]

[0090] As shown in Table 1, the adhesives prepared in all embodiments and comparative examples of this invention conform to the relevant parameters specified in the national standard GB / T14732-2017. Although the adhesives prepared in each comparative example conform to the viscosity requirements of the national standard GB / T 14732-2017, under the same preparation process, compared with Comparative Example 1 and Comparative Example 2, due to the polyphenolic structure of tannic acid, the reactivity is enhanced, resulting in increased viscosity of the adhesives in the examples, reduced content of free aldehydes and free phenols, and improved bonding strength. Furthermore, under the same preparation process, compared with Comparative Example 3 where formaldehyde and alkali solution are added all at once, the batch addition of formaldehyde and alkali solution in each example effectively increases the viscosity of the adhesive, reduces the content of free aldehydes and free phenols, and improves bonding strength.

[0091] As shown in Table 2, under the same preparation process, compared with Comparative Example 1 and Comparative Example 2, the adhesives in the examples exhibited a larger antibacterial zone under near-infrared irradiation due to the presence of tannic acid / iron, indicating that they have a better antibacterial effect; compared with Comparative Example 3, which added formaldehyde and alkali solution all at once, the adhesives prepared by adding formaldehyde and alkali solution in batches in each example showed improved antibacterial properties.

[0092] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a tannic acid-modified lignin-phenolic resin antibacterial adhesive, characterized in that, Includes the following steps: Alkali lignin was dissolved in an organic solvent, water was added and stirred to obtain a solution, and the solution was dialyzed and freeze-dried to obtain lignin nanoparticles. The lignin nanoparticles were dispersed in water, and ferric chloride and tannic acid were added. After the reaction was completed, the mixture was centrifuged and freeze-dried to obtain tannic acid / iron complexed lignin nanoparticles. The tannic acid / iron complexed lignin nanoparticles, phenol, a portion of formaldehyde solution, and a portion of alkali solution are mixed and reacted at 65-75°C for 30-60 min; then a portion of formaldehyde solution and a portion of alkali solution are added, and the mixture is heated at 75-85°C for 30 min; then the remaining formaldehyde solution and the remaining alkali solution are added, the temperature is raised to 85-95°C, and the mixture is kept at this temperature for 60-90 min. The mixture is then cooled and discharged to obtain the tannic acid-modified lignin phenolic resin antibacterial adhesive. The mass ratio of the lignin nanoparticles, ferric chloride and tannic acid is (10~20):(1~2):(2~4). The total amount of formaldehyde solution added is 80% to 100% of the total mass of phenol and tannic acid / iron complexed lignin nanoparticles, and the mass ratio of formaldehyde solution added sequentially is 1:1:

2. The formaldehyde solution is a formaldehyde solution with a mass concentration of 35% to 40%. The total amount of alkali solution added is 30% to 50% of the total mass of phenol and tannic acid / iron complexed lignin nanoparticles, and the mass ratio of the alkali solution added sequentially is 1:2:2, and the mass concentration of the alkali solution is 40%. The viscosity of the tannic acid-modified lignin-phenolic resin antibacterial adhesive is 60~200 mPa·s.

2. The preparation method of the tannic acid-modified lignin-phenolic resin antibacterial adhesive according to claim 1, characterized in that, The organic solvent is one of tetrahydrofuran, methanol, and acetonitrile; and / or In the preparation of lignin nanoparticles, the volume ratio of the organic solvent to water is 1:(1~9).

3. The method for preparing the tannic acid-modified lignin-phenolic resin antibacterial adhesive according to claim 1, characterized in that, The mass of the tannic acid / iron complexed lignin nanoparticles is 30% to 70% of the mass of phenol.

4. A tannic acid-modified lignin-phenolic resin antibacterial adhesive, characterized in that, It is prepared according to any one of claims 1 to 3.

5. The application of the tannic acid-modified lignin-phenolic resin antibacterial adhesive according to claim 4 in the preparation of plywood.

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