Formaldehyde-free adhesive based on activated lignin and preparation method thereof

Abstract

The invention discloses a formaldehyde-free adhesive based on activated lignin and a preparation method of the formaldehyde-free adhesive. The formaldehyde-free adhesive is prepared from the following components in parts by mass: 80 to 120 parts of activated lignin dispersion liquid, 5 to 15 parts of waterborne polyacrylate emulsion, 10 to 25 parts of curing agent, 15 to 30 parts of filler and 2 to 5 parts of additive, the preparation method of the activated lignin solution comprises the following steps: mixing a lignin raw material, sodium sulfite, a first solvent and a catalyst, adding alkali to adjust the pH value to obtain a reaction system, then heating to react, and collecting filtrate after the reaction is finished; and adding an acid solution into the filtrate, collecting separated insoluble substances, and dispersing the insoluble substances in a second solvent to obtain the activated lignin dispersion liquid. Under the alkaline condition, sodium sulfite serves as a nucleophilic reagent, the catalyst is added, the demethylation efficiency of lignin is improved, the hydroxyl content and reaction activity of activated lignin are improved, steric hindrance is reduced, then the utilization rate of the activated lignin is increased, and compounding use of the activated lignin and water-based polyacrylate emulsion is facilitated.

Description

Technical Field

[0001] This invention relates to the field of lignin-based formaldehyde-free adhesives, and particularly to a formaldehyde-free adhesive based on activated lignin and its preparation method. Background Technology

[0002] Lignin molecules contain numerous phenolic hydroxyl groups, aliphatic hydroxyl groups, aldehyde groups, and carboxyl groups, enabling them to react with aldehydes or phenolic compounds. The reaction process is similar to that of phenolic resin synthesis, thus lignin has the potential to replace phenol or formaldehyde. Lignin-based adhesives are not only low-cost but also have lower levels of free formaldehyde and free phenol after the reaction. Furthermore, they can lower the curing temperature and shorten the curing time. The high molecular weight and complex structure of lignin also make the adhesive less prone to permeation. However, the low activity and instability of industrial lignin severely limit its large-scale production and application in adhesives.

[0003] Because unmodified lignin has low reactivity, adhesives made directly from lignin can only achieve a phenol substitution rate of less than 60% to meet application requirements. Furthermore, lignin's complex three-dimensional structure results in significant steric hindrance, making it difficult for many active sites to participate in the reaction. Therefore, unactivated lignin is unsuitable for producing high-performance adhesives. Numerous studies have attempted to increase the content of phenolic and aliphatic hydroxyl groups in lignin through appropriate chemical modification methods, thereby improving the adhesive's bonding strength and other properties. For example, CN119371934A discloses a formaldehyde-free adhesive, its preparation method, and its application. By weight, the formaldehyde-free adhesive is prepared from the following raw materials: 80-150 parts modified lignin solution, 10-30 parts polyurethane emulsion, 15-35 parts curing agent, 6-12 parts modified filler, and 5-15 parts flame retardant. The modified lignin solution is obtained by hydroxyethylating straw lignin in a sodium hydroxide and glyoxal solution. This formaldehyde-free adhesive is highly environmentally friendly, possesses certain flame retardancy, and also exhibits excellent mechanical properties. However, the addition of polyurethane requires an additional curing agent, increasing costs. CN118085811A discloses a high-performance soybean meal protein powder adhesive and its preparation method. The adhesive, by weight, comprises 35-45 parts plant protein, 2-10 parts lignin epoxidized modified powder, and 0.6-1.5 parts activator, with the remainder being auxiliary materials. These auxiliary materials consist of fillers and thickeners, with a filler-to-thickener mass ratio of 5-10:1. This adhesive raw material does not contain benzene or formaldehyde, making it safe and environmentally friendly; however, its low bonding strength limits its application.

[0004] There is still a need to provide low-cost, safe, environmentally friendly, and high-strength activated lignin-based formaldehyde-free adhesives. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention provides a formaldehyde-free adhesive based on activated lignin, comprising, by weight, 80-120 parts of activated lignin dispersion, 5-15 parts of water-based polyacrylate emulsion, 10-25 parts of curing agent, 15-30 parts of filler, and 2-5 parts of additives; The method for preparing the activated lignin solution includes, The reaction system is prepared by mixing lignin raw material, sodium sulfite, first solvent and catalyst, adjusting the pH with alkali, then heating the reaction, and collecting the filtrate after the reaction is completed. Add acid to the filtrate, collect the precipitated insoluble matter, disperse the insoluble matter in a second solvent, and obtain an activated lignin dispersion.

[0006] In common industrial lignins, such as corn stalk lignin, a byproduct of corn stalk biorefining, unlike traditional lignin derived from papermaking waste (such as sulfate lignin, sulfonate lignin, and alkali lignin) which have not undergone salt or alkali cooking, the lignin molecule contains a large number of methoxy groups. The presence of methoxy groups results in significant steric hindrance, thus affecting the lignin's activity. Demethylation of lignin can effectively increase its hydroxyl content and reactivity, reduce steric hindrance, and thereby improve its utilization rate. This is of great significance for the preparation of biomass-based wood adhesives using lignin as a raw material.

[0007] Therefore, this invention uses sodium sulfite as a nucleophile to demethylate lignin. Furthermore, the demethylation reaction of lignin mainly occurs at the ortho and para positions of the lignin benzene ring, specifically at the carbon atom on the lignin methoxy group reacting with SO3. 2- Nucleophilic substitution reactions occur in the lignin groups. An alkaline environment can promote the dissolution of lignin and has a certain catalytic effect on the reaction, but this effect is limited. Therefore, this invention also incorporates a catalyst to promote the reaction and increase the hydroxyl content of the activated lignin.

[0008] Furthermore, the mass ratio of the lignin raw material, sodium sulfite, first solvent, and catalyst is 1:0.1~0.3:5-20:0.005~0.015; The alkali includes at least one of sodium hydroxide, potassium hydroxide, and tetrapropylammonium hydroxide, and the pH of the reaction system obtained by adding the alkali is 11-13; The temperature is raised to 50~80℃; The concentration of the acid solution is 0.05~0.2 mol / L.

[0009] Furthermore, the method for preparing the catalyst includes, Ammonium salt, metal salt and ammonia water were added to a silica dispersion to obtain a mixed system, and then a solvothermal reaction was carried out to collect the insoluble matter; The insoluble material was thermally reduced to obtain the catalyst.

[0010] Under solvothermal high-temperature and high-pressure conditions, ammonia water micro-etches silica, generating silicate ions. Ammonium salts can inhibit the dissociation of ammonia water, preventing the formation of hydroxide precipitates from metal salts. Free metal ions combine with silicate ions to form metal oxyacid salt precursors. In the thermal reduction process, the metal elements are reduced and thus modified in situ onto the silica surface, yielding a metal species-modified silica catalyst.

[0011] Furthermore, the solution for dispersing silica need not be strictly limited, and can be, for example, at least one of water, ethanol, acetone, dimethyl sulfoxide, and N,N-dimethylformamide.

[0012] Furthermore, the concentrations of silica, ammonium salt, metal salt, and ammonia in the mixed system are 0.5-2 g / L, 0.05-0.5 mol / L, 0.005-0.03 mol / L, and 0.05-0.3 mol / L, respectively. The particle size of the silica is 200~800nm; The metal salt includes at least one of iron, cobalt, and nickel salts.

[0013] Furthermore, the solvothermal reaction is carried out at 80-220℃ for 10-48 hours; The thermal reduction treatment is carried out in a reducing gas atmosphere at 600-900℃ for 1-6 hours.

[0014] Furthermore, the reducing gas can be at least one of ammonia, hydrogen, carbon monoxide, etc., with a concentration of 5%-20%, and the remaining gas is a rare gas or nitrogen.

[0015] The role of the curing agent is to react with the active groups (such as phenolic hydroxyl groups and aldehyde groups) on lignin to form a three-dimensional network structure, which changes the adhesive layer from thermoplastic to thermosetting, thus giving it strength and insoluble and infusible properties.

[0016] Furthermore, the curing agent includes at least one of polyphenylmethane polyisocyanate, polyethyleneimine, triethylenediamine, epichlorohydrin, polyethylene glycol diglycidyl ether, and tannin. Although lignin itself is activated, its curing speed and crosslinking density are often insufficient to achieve ideal bonding strength, especially water resistance.

[0017] Furthermore, the filler includes at least one of calcium carbonate, barium sulfate, kaolin, and talc powder with a mesh size of 200-500.

[0018] Furthermore, the additives include at least one of thickeners, defoamers, leveling agents, and preservatives.

[0019] Furthermore, the first solvent and the second solvent include at least one of water, ethanol, acetone, dimethyl sulfoxide, and N,N-dimethylformamide; The activated lignin concentration in the activated lignin dispersion is 40%-70%.

[0020] This invention also provides a method for preparing the above-mentioned formaldehyde-free adhesive based on activated lignin, comprising: The activated lignin dispersion was added to the aqueous polyacrylate emulsion and stirred until homogeneous. Then, the curing agent, filler and additives were added and stirred until homogeneous to obtain a formaldehyde-free adhesive based on activated lignin.

[0021] Compared with the prior art, the present invention has the following beneficial effects: This invention improves the demethylation efficiency of lignin, increases the content and reactivity of activated lignin hydroxyl groups, and reduces steric hindrance under alkaline conditions by using sodium sulfite as a nucleophile and adding a catalyst, thereby increasing the utilization rate of activated lignin and facilitating its compounding with waterborne polyacrylate emulsions.

[0022] The formaldehyde-free adhesive provided by this invention has a high proportion of activated lignin dispersion, which effectively reduces costs. The preparation method of the activated lignin dispersion is simple and the conditions are easy to achieve, showing good application prospects. Attached Figure Description

[0023] Figure 1 A scanning electron microscope image of silicon dioxide is shown; Figure 2 A scanning electron microscope image of the catalyst prepared in Example 1 is shown. Detailed Implementation

[0024] The following describes some of the raw materials used in the embodiments and comparative examples of this invention: Aqueous polyacrylate emulsion, model RESIN HF-05A, with a solid content of approximately 40%, purchased from Dow Chemical Company; Silica, model SS-S500JK, spherical, with a particle size of approximately 500nm, purchased from Hangzhou Jikang New Materials Co., Ltd. Defoamer, model BYK-051, purchased from BYK Chemical GmbH, Germany; Leveling agent, model BYK-333, purchased from BYK Chemical GmbH, Germany; Corn stalk cellulose, with a purity of over 90%, was purchased from Songyuan Yuanhe Chemical Co., Ltd.

[0025] All other unmentioned raw materials are common raw materials. The above content is only for illustrative purposes and should not be construed as a strict limitation of the present invention. Those skilled in the art can directly purchase commercially available raw materials or prepare the same / similar raw materials themselves. These contents will not be repeated in the embodiments.

[0026] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Example 1: A method for preparing a formaldehyde-free adhesive based on activated lignin, comprising the following steps, S1. Weigh out 9 kg of activated lignin dispersion, 1 kg of aqueous polyacrylate emulsion, 2 kg of epichlorohydrin, 2 kg of 300 mesh calcium carbonate, 0.1 kg of defoamer, and 0.1 kg of leveling agent; S1. Add the activated lignin dispersion to the aqueous polyacrylate emulsion and stir at 450 rpm for 20 min. Then add epichlorohydrin, 300 mesh calcium carbonate, defoamer and leveling agent, and continue stirring at 450 rpm for 30 min to obtain a formaldehyde-free adhesive based on activated lignin.

[0029] The preparation method of activated lignin dispersion includes, 10 kg of corn stalk cellulose, 1.5 kg of sodium sulfite, 150 kg of water, and 0.1 kg of catalyst were mixed and stirred at 350 rpm for 10 min. During the process, tetrapropylammonium hydroxide was added to adjust the pH to 12.5 to obtain the reaction system. The temperature was then raised to 65 °C and stirred at 350 rpm for 2 h. After the reaction was completed, the filtrate was collected and the catalyst was separated by magnetic adsorption. 0.1 mol / L hydrochloric acid aqueous solution was added to the filtrate, and the precipitated insoluble matter was collected, washed with water until neutral, filtered, and dried to obtain activated lignin. The activated lignin was mixed with water and stirred at 500 rpm for 15 min to obtain an activated lignin dispersion with a mass concentration of 60%.

[0030] Methods for preparing catalysts include, 200g of silica and 200L of water were stirred at 450rpm for 30min to obtain a silica dispersion. 10mol of ammonium bicarbonate, 4mol of nickel nitrate hexahydrate and 2L of ammonia (mass concentration 25%, initial concentration 13.4mol / L) were added to the silica dispersion to obtain a mixed system. The mixture was then solvothermal reacted at 180℃ for 15h. After natural cooling, the insoluble matter was collected, washed three times each with water and ethanol, and dried to constant weight. The mixture was then transferred to a tube furnace at 750℃ and calcined for 2h in a reducing gas atmosphere of 10% hydrogen and 90% argon. Finally, it was naturally cooled to room temperature to obtain the catalyst.

[0031] Example 2: Compared with Example 1, the difference lies in the method for preparing the activated lignin dispersion, including: 10 kg of corn stalk cellulose, 1.5 kg of sodium sulfite, 150 kg of water, and 0.1 kg of catalyst were mixed and stirred at 350 rpm for 10 min. Sodium hydroxide was added during the process to adjust the pH to 12.5 to obtain the reaction system. The temperature was then raised to 65 °C and stirred at 350 rpm for 2 h. After the reaction was completed, the filtrate was collected and the catalyst was separated by magnetic adsorption. 0.1 mol / L hydrochloric acid aqueous solution was added to the filtrate, and the precipitated insoluble matter was collected, washed with water until neutral, filtered, and dried to obtain activated lignin. The activated lignin was mixed with water and stirred at 500 rpm for 15 min to obtain an activated lignin dispersion with a mass concentration of 60%.

[0032] Comparative Example 1: A method for preparing a formaldehyde-free adhesive based on activated lignin, comprising the following steps, S1. Weigh out 9 kg of activated lignin dispersion, 1 kg of aqueous polyacrylate emulsion, 2 kg of epichlorohydrin, 2 kg of 300 mesh calcium carbonate, 0.1 kg of defoamer, and 0.1 kg of leveling agent; S1. Add the activated lignin dispersion to the aqueous polyacrylate emulsion and stir at 450 rpm for 20 min. Then add epichlorohydrin, 300 mesh calcium carbonate, defoamer and leveling agent, and continue stirring at 450 rpm for 30 min to obtain a formaldehyde-free adhesive based on activated lignin.

[0033] The preparation method of activated lignin dispersion includes, 10 kg of corn stalk cellulose, 1.5 kg of sodium sulfite, and 150 kg of water were mixed and stirred at 350 rpm for 10 min. During the process, tetrapropylammonium hydroxide was added to adjust the pH to 12.5 to obtain the reaction system. The temperature was then raised to 65 °C and stirred at 350 rpm for 2 h. After the reaction was completed, the filtrate was collected. 0.1 mol / L hydrochloric acid aqueous solution was added to the filtrate, and the precipitated insoluble matter was collected. The precipitate was washed with water until neutral, filtered, and dried to obtain activated lignin. The activated lignin was mixed with water and stirred at 500 rpm for 15 min to obtain an activated lignin dispersion with a mass concentration of 60%.

[0034] Comparative Example 2: The difference from Example 1 is that silica microspheres were used instead of the catalyst.

[0035] Test Example: The microstructure of the silica raw material and the catalyst prepared in Example 1 were observed using a scanning electron microscope, as shown below. Figure 1 and Figure 2 As shown, the catalyst surface of Example 1 is significantly rougher than that of silica, demonstrating the successful modification of the silica surface by nickel species. The catalyst exhibits a uniform morphology, with nickel species well-distributed on the silica surface and no obvious agglomeration. The excellent structure of the catalyst is beneficial in increasing the active sites for interaction with corn straw cellulose, thereby promoting the demethylation reaction of lignin.

[0036] The total hydroxyl content in the activated lignin prepared in the examples and comparative examples was tested using acetylation titration: Approximately 40 mg of activated lignin was weighed into a sealable ground glass joint test tube, 0.5 g of acetylation reagent was added, and the tube was sealed and heated at 50 °C for 24 h until the lignin and acetylation reagent were uniformly mixed and reacted. After cooling, 5 mL of acetone solution was added. A dioxane aqueous solution (dioxane to water volume ratio of 4:1) was used to wash the reaction solution into a 25 mL volumetric flask and brought to volume. Then, 5 mL of the solution was accurately transferred using a pipette and titrated to the endpoint with 0.1 mol / L NaOH standard solution. The endpoint was determined using a potentiometric titrator, with phenolphthalein indicator used for observation. A blank experiment was also performed; the endpoint changed from colorless to pale pink. The phenolic hydroxyl content in the activated lignin prepared in the examples and comparative examples was determined by spectrophotometry: Approximately 15 mg of sample was dissolved in 10 mL of dioxane, and then two 2 mL aliquots of the solution were taken and diluted to 50 mL with buffer solutions of NaHPO4 and NaOH (pH=6) and 0.2 mol / L NaOH solution, respectively. The samples were analyzed using a UV-Vis spectrophotometer. The absorbance of the 0.2 mol / L NaOH lignin sample solution at wavelengths of 300 nm and 360 nm was measured using the pH=6 lignin sample solution as a reference. The phenolic hydroxyl content was calculated from the absorbance. The test results are shown in Table 1. Table 1: Total hydroxyl and phenolic hydroxyl content in activated lignin

[0037] As can be seen from the test results in Table 1, due to the lack of salt or alkali cooking, the lignin molecular structure contains a large number of methoxy groups, and the total hydroxyl content and phenolic hydroxyl content of corn straw cellulose raw material are the lowest. Comparative Example 1 uses sodium sulfite as a nucleophile to demethylate lignin in an alkaline liquid environment provided by tetrapropylammonium hydroxide. The alkaline environment promotes the dissolution of lignin and has a certain catalytic effect on the reaction. The carbon atoms on the methoxy groups of lignin react with SO3... 2- Nucleophilic substitution of functional groups effectively increases the hydroxyl content of lignin. The results from Example 1 and Comparative Example 2 show that silica has virtually no catalytic effect on the reaction. The superior structure of nickel-modified silica increases the active sites for interaction with corn straw cellulose, significantly promoting the demethylation reaction of lignin. This results in the activated lignin prepared in Example 1 having the highest total hydroxyl and phenolic hydroxyl content. In Example 2, compared to Example 1 where sodium hydroxide was used instead of tetrapropylammonium hydroxide, the total hydroxyl and phenolic hydroxyl content decreased. This may be because tetrapropylammonium hydroxide has a certain coordinating effect, further enhancing the interaction between the catalyst and tetrapropylammonium hydroxide.

[0038] The adhesives prepared in the examples and comparative examples were applied to 270mm×270mm×1.6mm poplar veneers at a double-sided application rate of 400g / m². The veneers were then hot-pressed at 135℃ and 1.5MPa for 6 minutes to obtain three-layer plywood. The bond strength was tested according to the national standard GB / T17657-2022 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels," and the results are shown in Table 2. Table 2: Bond Strength Test Results

[0039] As can be seen from the test results in Table 2, the formaldehyde-free adhesive plywood based on activated lignin provided by this invention has good mechanical properties and high water resistance.

[0040] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An activated lignin-based formaldehyde-free adhesive, characterized in that, comprising, by mass parts, an activated lignin dispersion 80-120 parts, an aqueous polyacrylate emulsion 5-15 parts, a curing agent 10-25 parts, a filler 15-30 parts, and an auxiliary agent 2-5 parts; a preparation method of the activated lignin solution comprises, mixing a lignin raw material, sodium sulfite, a first solvent, and a catalyst, adding alkali to adjust pH to obtain a reaction system, then heating to react, collecting filtrate after the reaction is completed; adding acid liquor to the filtrate, collecting insoluble substances precipitated, and dispersing the insoluble substances in a second solvent to obtain an activated lignin dispersion.

2. The activated lignin-based formaldehyde-free adhesive according to claim 1, characterized in that, The mass ratio of the lignin raw material, sodium sulfite, the first solvent, and the catalyst is 1:0.1-0.3:5-20:0.005-0.015; The alkali includes at least one of sodium hydroxide, potassium hydroxide, and tetrapropylammonium hydroxide, and the pH of the reaction system obtained by adding alkali is 11-13; The temperature is raised to 50-80℃; The concentration of the acid liquor is 0.05-0.2 mol / L.

3. The activated lignin-based formaldehyde-free adhesive according to claim 1, characterized in that, A preparation method of the catalyst comprises, adding an ammonium salt, a metal salt, and ammonia water to a silica dispersion to obtain a mixed system, then performing a solvothermal reaction to collect insoluble substances; heat-reducing the insoluble substances to obtain the catalyst.

4. The activated lignin-based formaldehyde-free adhesive according to claim 3, characterized in that, The concentrations of silica, the ammonium salt, the metal salt, and ammonia water in the mixed system are 0.5-2 g / L, 0.05-0.5 mol / L, 0.005-0.03 mol / L, and 0.05-0.3 mol / L, respectively; The particle size of the silica is 200-800 nm; The metal salt includes at least one of iron, cobalt, and nickel salts.

5. The activated lignin-based formaldehyde-free adhesive according to claim 3, characterized in that, The solvothermal reaction is performed at 80-220℃ for 10-48 h; The heat-reducing treatment is performed in a reducing gas atmosphere at 600-900℃ for 1-6 h.

6. The activated lignin-based formaldehyde-free adhesive according to claim 1, wherein, The curing agent includes at least one of polyphenylmethane polyisocyanate, polyethyleneimine, triethylenediamine, epoxy chloropropane, polyethylene glycol diglycidyl ether, and tannin.

7. The activated lignin-based formaldehyde-free adhesive according to claim 1, wherein, The filler includes at least one of 200-500 mesh calcium carbonate, barium sulfate, kaolin, and talc powder.

8. The activated lignin-based formaldehyde-free adhesive according to claim 1, wherein, The auxiliary agent includes at least one of a thickening agent, a defoaming agent, a leveling agent, and a preservative.

9. The activated lignin-based formaldehyde-free adhesive according to claim 1, wherein, The first solvent and the second solvent include at least one of water, ethanol, acetone, dimethyl sulfoxide, and N,N-dimethylformamide; The mass concentration of activated lignin in the activated lignin dispersion is 40%-70%.

10. A process for the preparation of a formaldehyde-free adhesive based on activated lignin according to any one of claims 1 to 9, characterized in that, comprising, adding the activated lignin dispersion to the aqueous polyacrylate emulsion and stirring until uniform, then adding the curing agent, the filler, and the auxiliary agent and continuing to stir until uniform to obtain the formaldehyde-free adhesive based on activated lignin.

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