A phenol-formaldehyde resin adhesive with low free formaldehyde content and a preparation method thereof

Abstract

The present application relates to a kind of low free formaldehyde content phenolic resin adhesive and its preparation method, comprising: S1, choline chloride is mixed with organic acid, heated stirring, obtain binary acid eutectic solvent, then add deionized water or ethylene glycol, continue to obtain ternary acid eutectic solvent stirring;S2, after being treated with alkali lignin, after being mixed with ternary acid eutectic solvent, heating reaction, after cooling, separation purification is carried out, and modified lignin is obtained;S3, the modified lignin, phenol and sodium hydroxide are added to reaction vessel, after first stage reaction is carried out by heating, add formaldehyde aqueous solution, and second stage reaction is carried out by heating;Afterwards, formaldehyde aqueous solution and urea are added, and third stage reaction is carried out, after reaction is finished, cooling is carried out and phenolic resin adhesive is obtained.The method provided by the present application meets the requirements of green environmental protection, the prepared phenolic resin adhesive has low free formaldehyde content, high bonding strength, strong practicality, and has good industrial application prospect.

Description

Technical Field

[0001] This invention relates to the field of high-value utilization of lignin, specifically to a phenolic resin adhesive with low free formaldehyde content and its preparation method. Background Technology

[0002] Phenolic resin adhesives are widely used. However, due to the depletion of petroleum resources and their price fluctuations, as well as concerns about the health problems caused by workers' long-term exposure to phenol, formaldehyde, and other substances during production, researching green process technologies that use bio-based materials to replace petroleum-based phenolic compounds is an important trend in the development of phenolic resin adhesives.

[0003] Lignin is the world's most abundant renewable natural aromatic polymer. With phenylpropane as its backbone, it possesses the structural characteristics to replace petroleum-based phenolic compounds in the preparation of phenolic resins. Using lignin to replace phenol can reduce the production cost and harmfulness of phenolic resin adhesives.

[0004] However, the large molecular weight and high dispersion of lignin, along with the fact that a large number of ortho- and ortho-positioned phenolic hydroxyl groups in its structure that react with formaldehyde are replaced by methoxy groups, resulting in low reactivity, have limited its large-scale application in the production of phenolic resin adhesives.

[0005] Therefore, in order to vigorously develop the industrial application of lignin in the production of phenolic resin adhesives, lignin must be modified and activated. Currently, researchers have conducted extensive work on lignin modification and activation. Commonly used modification methods mainly include chemical methods such as demethylation, oxidation, hydroxymethylation, phenolation, reduction, and hydrolysis. These chemical modification methods generally require the reaction of lignin with modifiers such as phenol, formaldehyde, halogenated acids, and sodium sulfite under the catalysis of acids, alkalis, or metal salts. However, due to the harsh conditions and environmentally unfriendly processes of chemical modification, the cost of lignin modification is high, limiting the industrial application of lignin in the production of phenolic resin adhesives.

[0006] For example, patent application CN 115536793 A uses inorganic acids (sulfuric acid, hydrochloric acid, nitric acid) to catalyze the degradation of lignin, and adds ketone reagents (butanone, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone) during the modification process to inhibit the repolymerization of small molecule lignin into large molecules, thereby improving the reactivity of modified lignin with formaldehyde. However, the addition of inorganic acids in this method not only corrodes equipment but also generates a large amount of waste acid, causing secondary environmental pollution; moreover, the added ketone reagents are low in toxicity and flammable, and can form explosive mixtures with air, meaning that the modification process does not meet the green requirements of safety, hygiene, and environmental protection. The prepared lignin-based phenolic resin adhesive has moderate bonding strength, with a maximum of 1.42 MPa (the basic standard for adhesive strength is ≥0.7 MPa); the free formaldehyde content is moderate, with a minimum of 0.136% (the requirement for free formaldehyde content is ≤0.3%). Although the main performance indicators meet the relevant national standards, the performance is generally average.

[0007] Currently, the free formaldehyde content and bonding strength of lignin-based phenolic resin adhesives in existing technologies are concentrated in the ranges of 0.15–0.30 wt% and 1.0–2.0 MPa, respectively, which limits the application of the products. The free formaldehyde in the adhesives will lead to long-term continuous volatilization during use. Formaldehyde is a carcinogen; excessive inhalation can cause dizziness, chest tightness, nausea, itchy skin, bronchitis, and even respiratory diseases such as edema. Further reducing the free formaldehyde content, even to below 0.17%, is of significant practical importance for environmental protection, safe production, and daily life.

[0008] In order to vigorously develop the industrial application of lignin in the production of phenolic resin adhesives and further reduce dependence on petrochemical resources, the key technical challenge is how to efficiently modify lignin to obtain more reactive sites for formaldehyde condensation and further reduce the free formaldehyde content of phenolic resin adhesives. Summary of the Invention

[0009] The purpose of this invention is to overcome the problems of high free formaldehyde content and low bonding strength of existing phenolic resin adhesives, and to provide a phenolic resin adhesive with low free formaldehyde content and its preparation method. By modifying alkali lignin with a ternary acidic eutectic solvent, the reactivity of lignin with formaldehyde is enhanced, thereby enabling the preparation of a phenolic resin adhesive with low free formaldehyde content.

[0010] In this invention, the ternary acidic eutectic solvent contains organic acids. Compared to inorganic acids, organic acids are weaker, preferably formic acid and oxalic acid. However, both are corrosive. By controlling the pH of the ternary acidic eutectic solvent to 0.5-1, equipment corrosion during the alkali lignin modification process can be mitigated. Furthermore, the third component added to form the ternary eutectic solvent in this invention is water or ethylene glycol, both of which are non-toxic and harmless, and the modification process meets the green requirements of safety, hygiene, and environmental protection. Meanwhile, the synthesis of phenolic resin requires excess formaldehyde, which inevitably leads to a large amount of free formaldehyde in the prepared adhesive. The addition of the third component, such as water or ethylene glycol, in this invention effectively inhibits the repolymerization of lignin molecules during the demethoxylation process, significantly increases the active sites for the reaction between modified lignin and formaldehyde, and prepares a phenolic resin adhesive with low free formaldehyde content. In a specific embodiment, the molar ratio of choline chloride to the organic acid can be 1:1-2, and the binary acidic eutectic solvent accounts for 60%-95% of the total weight. More preferably, the ternary acidic eutectic solvent is composed of the following components in a molar ratio: choline chloride: organic acid: ethylene glycol = 1:1-2:0.3-0.7.

[0011] In this invention, a mixed solution of ethanol and water is used as the antisolvent to dilute the ternary acidic eutectic solvent, allowing the modified lignin to precipitate rapidly and be filtered. Utilizing the significant difference in boiling points between ethanol and the eutectic solvent, the filtrate is evaporated under reduced pressure, and the ethanol and eutectic solvent are collected separately for reuse, essentially achieving zero emissions. In a specific embodiment, in step S2, the volume ratio of ethanol to water is 1:5-10, and the volume ratio of the ethanol-water mixture to the ternary acidic eutectic solvent is 5-10:1. More preferably, the volume ratio of ethanol to water in the ethanol-water mixture is 1:9, and the volume ratio of the ethanol-water mixture to the ternary acidic eutectic solvent is 9:1.

[0012] In this invention, phenolic resin adhesives are prepared by partially replacing phenol with modified lignin and polycondensing it with formaldehyde. Controlling the phenol-to-formaldehyde ratio is crucial for preparing phenolic resin adhesives that meet national standards. The polycondensation reaction process can be effectively controlled by adding formaldehyde in stages. In a specific embodiment, formaldehyde can be added in two stages. In the second stage reaction (S3), the amount of formaldehyde added is 20-40% of the total formaldehyde content. The molar ratio (phenol-to-formaldehyde ratio) of the total amount of modified lignin and phenol in the first stage and the total amount of formaldehyde in the second stage reaction to the third stage reaction is 1:1.5-2.0. More preferably, in the second stage reaction (S3), the amount of formaldehyde added is 30% of the total formaldehyde content. The molar ratio (phenol-to-formaldehyde ratio) of the total amount of modified lignin and phenol in the first stage and the total amount of formaldehyde in the second stage reaction to the third stage reaction is 1:1.6.

[0013] It should be noted that, due to the wide molecular weight distribution of lignin, industrially, the total mass of phenol and lignin is generally divided by the molar molecular weight of phenol to calculate the number of moles of phenol used. This is then compared with the number of moles of formaldehyde to obtain the phenol-formaldehyde ratio. The calculation formula is as follows:

[0014]

[0015] The specific plan is as follows:

[0016] A method for preparing a phenolic resin adhesive with low free formaldehyde content includes the following steps:

[0017] S1. Mix choline chloride with an organic acid, heat and stir to obtain a dibasic acidic eutectic solvent, then add deionized water or ethylene glycol and continue stirring to obtain a ternary acidic eutectic solvent.

[0018] S2. Wash the alkali lignin until neutral, dry it, and mix it evenly with the ternary acidic eutectic solvent. Heat it to 110-130℃ and react for 1-3 hours. After cooling, add a mixed solution of ethanol and water. After the precipitate is formed, filter, wash, and dry it to obtain modified lignin.

[0019] S3. The modified lignin, phenol, and sodium hydroxide are added to a reaction vessel, heated for the first stage reaction, and then an aqueous solution of formaldehyde is added, and the temperature is raised for the second stage reaction. After that, an aqueous solution of formaldehyde and urea are added for the third stage reaction. After the reaction is completed, the mixture is cooled to obtain a phenolic resin adhesive. In the reaction vessel, the amount of modified lignin is 20-50% of the total weight of modified lignin and phenol, that is, the degree of phenol substitution is 20-50%.

[0020] Furthermore, the organic acid in S1 is formic acid or oxalic acid. Preferably, the molar ratio of choline chloride to the organic acid is 1:1-2. Preferably, the heating and stirring in S1 is carried out at a constant temperature of 60-90°C until the mixture becomes clear and transparent, thus obtaining the binary acidic eutectic solvent.

[0021] Furthermore, in the ternary acidic eutectic solvent described in S1, the dibasic acidic eutectic solvent accounts for 60-95% of the total weight, preferably, the dibasic acidic eutectic solvent accounts for 80-90% of the total weight; more preferably, the ternary acidic eutectic solvent is composed of the following components in a molar ratio of choline chloride: organic acid: ethylene glycol = 1:2:0.3-0.7.

[0022] Furthermore, the mass ratio of the alkali lignin and the ternary acidic eutectic solvent in S2 is 1:10-20; preferably, after the alkali lignin and the ternary acidic eutectic solvent in S2 are mixed, they are reacted at 115-125°C for 1.5-2.5 hours, and then cooled to room temperature.

[0023] Further, a mixed solution of ethanol and water, with a volume ratio of 1:5-10, is added to S2. The mixture is left to stand overnight to allow precipitation, and the solid particles are separated by centrifugation and filtration. The particles are washed with deionized water until neutral and then vacuum dried to obtain the modified lignin.

[0024] Furthermore, in the first stage reaction of S3, the ratio of modified lignin to phenol is 0.25-1:1. Preferably, in the first stage reaction of S3, the sodium hydroxide is an aqueous solution of sodium hydroxide with a mass concentration of 10-40%, and the mass of the solute in the aqueous solution of sodium hydroxide accounts for 10-30% of the total mass of modified lignin and phenol. Preferably, in the first stage reaction of S3, the heating temperature is 70-85℃, and the reaction time is 1-3h.

[0025] Furthermore, in the second stage reaction of S3, the formaldehyde mass concentration is 30-37%, the formaldehyde addition is 20-40% of the total formaldehyde amount, and the temperature rise refers to raising the temperature to 90-100℃ and reacting for 30-60 minutes.

[0026] Furthermore, in the third stage reaction described in S3, the amount of formaldehyde added is 60-80% of the total amount of formaldehyde used;

[0027] Preferably, the molar ratio of the total amount of modified lignin and phenol used in the first stage to the total amount of formaldehyde used in the second stage reaction and the third stage reaction is 1:1.5-2.0.

[0028] Preferably, in the third stage reaction described in S3, the amount of urea added is 3-8% of the total mass of the modified lignin and phenol, the reaction temperature is 90-100℃, and the reaction time is 30-60 min.

[0029] The present invention also protects the phenolic resin adhesive prepared by the method for preparing the phenolic resin adhesive with low free formaldehyde content, wherein the bonding strength of the phenolic resin adhesive is 2.1-3.0 MPa.

[0030] Furthermore, the free formaldehyde content of the phenolic resin adhesive is 0.01-0.17 wt%, preferably 0.01-0.10 wt%, and more preferably 0.01-0.07 wt%.

[0031] Beneficial effects:

[0032] In this invention, by adding ethylene glycol and water to a dibasic acidic eutectic solvent, not only are the adverse effects on the equipment during lignin modification reduced, but unexpected technical effects are also achieved, namely, promoting the demethoxylation of the lignin structure, inhibiting repolymerization, significantly increasing the active sites for the reaction between lignin and formaldehyde, thereby significantly reducing the free formaldehyde content of phenolic resin adhesives.

[0033] Furthermore, this invention significantly reduces the free formaldehyde content while simultaneously improving the strength of phenolic resin adhesives. Generally, the bonding strength of phenolic resin adhesives is positively correlated with the amount of phenol used; that is, the higher the amount of phenol, the higher the bonding strength. However, under the condition of a phenol substitution degree of 30%, the phenolic resin adhesive obtained by this invention exhibits significantly higher bonding strength than phenolic resin adhesives using 100% phenol, demonstrating a significant technological advancement.

[0034] In summary, the method for preparing phenolic resin adhesive with low free formaldehyde content provided by this invention uses a ternary acidic eutectic solvent that is easy to recycle and reuse / treat. Its synthetic raw materials, such as choline chloride, formic acid, oxalic acid, and ethylene glycol, are green and low in toxicity, meeting the requirements of green environmental protection. The prepared phenolic resin adhesive has low free formaldehyde content, high bonding strength, strong practicality, and good prospects for industrial application. Detailed Implementation

[0035] Preferred embodiments of the present invention will now be described in more detail. While preferred embodiments of the present invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products. In the following embodiments, unless otherwise specified, "%" refers to weight percentage, and "parts" refers to parts by weight.

[0036] The free formaldehyde content of the lignin-based phenolic resin adhesive in this invention was determined according to the corresponding determination method in the People's Republic of China National Standard GB / T 14074—2017.

[0037] In this invention, lignin-based phenolic resin adhesive is first used to make plywood, and then its bonding strength is measured. The specific method is as follows: a poplar board with a length of 100 mm, a width of 25 mm, and a thickness of 2 mm is taken out, and glue is evenly applied to the poplar board at a rate of 150 g / m². 2 The plywood was hot-pressed at 1.2 MPa and 120°C. After hot pressing, the plywood was first placed at room temperature for 24 hours, then immersed in hot water at 63°C for 3 hours, and finally cooled at room temperature for 10 minutes. The bonding strength of the plywood was then tested as an indicator of the bonding strength of the lignin-based phenolic resin adhesive.

[0038] Example 1

[0039] A phenolic resin adhesive with low free formaldehyde content and its preparation method are disclosed below.

[0040] 1) Mix 102.45g of choline chloride with 67.55g of formic acid (molar ratio 1:2) and stir magnetically at 70℃ until the solution is clear and transparent to obtain a dibasic acid eutectic solvent. Then add 30g of ethylene glycol and continue stirring until clear and transparent to obtain a tribasic acid eutectic solvent.

[0041] 2) Wash the bamboo alkali lignin with 2 mol / L hydrochloric acid until neutral, and after vacuum drying, dissolve 10 g of lignin in the above ternary acidic eutectic solvent, modify it at 120℃ for 2 h, and then quickly cool it to room temperature. Add 9 times the volume of ethanol aqueous solution (volume ratio 1:9), let it stand overnight, precipitate out, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0042] 3) 2.4 g of ternary acidic eutectic solvent-modified lignin, 5.6 g of phenol, and 2.8 mL of 40% sodium hydroxide solution were added to a reaction vessel and stirred at 80 °C for 2 h. Then, 7.168 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) was added, and the mixture was heated to 90 °C and stirred for 50 min. Finally, 0.4 g of urea and 3.072 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) were added, and the mixture was stirred at 90 °C for another 40 min. After the reaction was completed, the reaction system was rapidly cooled to 70 °C and then slowly cooled to room temperature before being discharged to obtain a lignin-based phenolic resin adhesive with a free formaldehyde content of only 0.01 wt%.

[0043] After pressing the obtained phenolic resin adhesive into boards, the bonding strength of the plywood reached as high as 2.94 MPa.

[0044] Example 2

[0045] A phenolic resin adhesive with low free formaldehyde content and its preparation method are disclosed below.

[0046] 1) Mix 108.48g of choline chloride with 71.52g of formic acid (molar ratio 1:2) and stir magnetically at 70℃ until the solution is clear and transparent to obtain a dibasic acid eutectic solvent. Then add 20g of ethylene glycol and continue stirring until clear and transparent to obtain a tribasic acid eutectic solvent.

[0047] 2) Wash the bamboo alkali lignin with 2 mol / L hydrochloric acid until neutral, and after vacuum drying, dissolve 10 g of lignin in the above ternary acidic eutectic solvent, modify it at 120℃ for 2 h, and then quickly cool it to room temperature. Add 9 times the volume of ethanol aqueous solution (volume ratio 1:9), let it stand overnight, precipitate out, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0048] 3) 2.4 g of ternary acidic eutectic solvent-modified lignin, 5.6 g of phenol, and 2.8 mL of 40% sodium hydroxide solution were added to a reaction vessel and stirred at 80 °C for 2 h. Then, 7.168 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) was added, and the mixture was heated to 90 °C and stirred for 50 min. Finally, 0.4 g of urea and 3.072 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) were added, and the mixture was stirred at 90 °C for another 40 min. After the reaction was completed, the reaction system was rapidly cooled to 70 °C and then slowly cooled to room temperature before being discharged to obtain a lignin-based phenolic resin adhesive with a free formaldehyde content of only 0.17 wt%.

[0049] After pressing the obtained phenolic resin adhesive into boards, the bonding strength of the plywood reached as high as 2.74 MPa.

[0050] Example 3

[0051] A phenolic resin adhesive with low free formaldehyde content and its preparation method are disclosed below.

[0052] 1) Mix 111.49g of choline chloride with 73.51g of formic acid (molar ratio 1:2) and stir magnetically at 70℃ until the solution is clear and transparent to obtain a dibasic acid eutectic solvent. Then add 15g of ethylene glycol and continue stirring until clear and transparent to obtain a tribasic acid eutectic solvent.

[0053] 2) Wash the bamboo alkali lignin with 2 mol / L hydrochloric acid until neutral, and after vacuum drying, dissolve 10 g of lignin in the above ternary acidic eutectic solvent, modify it at 120℃ for 2 h, and then quickly cool it to room temperature. Add 9 times the volume of ethanol aqueous solution (volume ratio 1:9), let it stand overnight, precipitate out, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0054] 3) 2.4 g of ternary acidic eutectic solvent-modified lignin, 5.6 g of phenol, and 2.8 mL of 40% sodium hydroxide solution were added to a reaction vessel and stirred at 80 °C for 2 h. Then, 7.168 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) was added, and the mixture was heated to 90 °C and stirred for 50 min. Finally, 0.4 g of urea and 3.072 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) were added, and the mixture was stirred at 90 °C for another 40 min. After the reaction was completed, the reaction system was rapidly cooled to 70 °C and then slowly cooled to room temperature before being discharged to obtain a lignin-based phenolic resin adhesive with a free formaldehyde content of only 0.07 wt%.

[0055] After pressing the obtained phenolic resin adhesive into boards, the bonding strength of the plywood reached as high as 2.40 MPa.

[0056] Example 4

[0057] A phenolic resin adhesive with low free formaldehyde content and its preparation method are disclosed below.

[0058] 1) Mix 81.06g of choline chloride with 52.27g of oxalic acid (molar ratio 1:1) and stir magnetically at 70℃ until the solution is clear and transparent to obtain a dibasic acid eutectic solvent. Then add 66.67g of deionized water and continue stirring until clear and transparent to obtain a tribasic acid eutectic solvent.

[0059] 2) Wash the bamboo alkali lignin with 2 mol / L hydrochloric acid until neutral, and after vacuum drying, dissolve 10 g of lignin in the above ternary acidic eutectic solvent, modify it at 120℃ for 2 h, and then quickly cool it to room temperature. Add 9 times the volume of ethanol aqueous solution (volume ratio 1:9), let it stand overnight, precipitate out, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0060] 3) 2.4 g of ternary acidic eutectic solvent-modified lignin, 5.6 g of phenol, and 2.8 mL of 40% sodium hydroxide solution were added to a reaction vessel and stirred at 80 °C for 2 h. Then, 7.168 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) was added, and the mixture was heated to 90 °C and stirred for 50 min. Finally, 0.4 g of urea and 3.072 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) were added, and the mixture was stirred at 90 °C for another 40 min. After the reaction was completed, the reaction system was rapidly cooled to 70 °C and then slowly cooled to room temperature before being discharged to obtain a lignin-based phenolic resin adhesive with a free formaldehyde content of only 0.15 wt%.

[0061] After pressing the obtained phenolic resin adhesive into boards, the bonding strength of the plywood reached as high as 2.45 MPa.

[0062] Example 5

[0063] A phenolic resin adhesive with low free formaldehyde content and its preparation method are disclosed below.

[0064] 1) Mix 102.45g of choline chloride with 67.55g of formic acid (molar ratio 1:2) and stir magnetically at 60℃ until the solution is clear and transparent to obtain a dibasic acid eutectic solvent. Then add 30g of ethylene glycol and continue stirring until clear and transparent to obtain a tribasic acid eutectic solvent.

[0065] 2) Wash the alkali lignin of bamboo with 2 mol / L hydrochloric acid until neutral, and then vacuum dry. Dissolve 10 g of lignin in the above-mentioned ternary acidic eutectic solvent, wherein the mass ratio of alkali lignin to the ternary acidic eutectic solvent is 1:15. After modification at 115℃ for 2 h, rapidly cool to room temperature, add 10 times the volume of ethanol aqueous solution (volume ratio 1:9), let stand overnight, precipitate, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0066] 3) Add 2.4g of ternary acidic eutectic solvent-modified lignin, 2.4g of phenol, and 2.0mL of 40% sodium hydroxide solution to a reaction vessel and stir at 80℃ for 2h. Then add 7.168mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%), heat to 90℃, and stir for 50min. Finally, add 0.4g of urea and 3.072mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%), and continue stirring at 90℃ for 40min. After the reaction is complete, rapidly cool the reaction system to 70℃ and then slowly cool it to room temperature before discharging to obtain lignin-based phenolic resin adhesive.

[0067] Example 6

[0068] A phenolic resin adhesive with low free formaldehyde content and its preparation method are disclosed below.

[0069] 1) Mix choline chloride and formic acid (molar ratio 1:2) and stir magnetically at 80°C until the solution is clear and transparent to obtain a dibasic acid eutectic solvent. Then add 30g of ethylene glycol and continue stirring until clear and transparent to obtain a tribasic acid eutectic solvent.

[0070] 2) Wash the alkali lignin from bamboo with 2 mol / L hydrochloric acid until neutral, and then vacuum dry. Dissolve 10 g of lignin in the above-mentioned ternary acidic eutectic solvent, wherein the mass ratio of alkali lignin to the ternary acidic eutectic solvent is 1:20. After modification at 125℃ for 2 h, rapidly cool to room temperature, add 5 times the volume of ethanol aqueous solution (volume ratio 1:9), let stand overnight, precipitate, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0071] 3) Add 1.4g of ternary acidic eutectic solvent-modified lignin, 5.6g of phenol, and 2.0mL of 40% sodium hydroxide solution to a reaction vessel and stir at 70℃ for 2h. Then add 7.168mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%), heat to 90℃, and stir for 50min. Finally, add 0.4g of urea and 3.072mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%), and continue stirring at 100℃ for 30min. After the reaction is complete, rapidly cool the reaction system to 70℃ and then slowly cool it to room temperature before discharging to obtain lignin-based phenolic resin adhesive.

[0072] Example 7

[0073] A phenolic resin adhesive with low free formaldehyde content and its preparation method are disclosed below.

[0074] 1) Mix choline chloride and formic acid (molar ratio 1:1) and stir magnetically at 90°C until the solution is clear and transparent to obtain a dibasic acid eutectic solvent. Then add 30g of ethylene glycol and continue stirring until clear and transparent to obtain a tribasic acid eutectic solvent.

[0075] 2) Wash the alkali lignin from bamboo with 2 mol / L hydrochloric acid until neutral, and then vacuum dry. Dissolve 10 g of lignin in the above-mentioned ternary acidic eutectic solvent, wherein the mass ratio of alkali lignin to the ternary acidic eutectic solvent is 1:10. After modification at 120℃ for 2 h, rapidly cool to room temperature, add 9 times the volume of ethanol aqueous solution (volume ratio 1:9), let stand overnight, precipitate, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0076] 3) Add 4.2g of ternary acidic eutectic solvent-modified lignin, 5.6g of phenol, and 3.5mL of 40% sodium hydroxide solution to a reaction vessel and stir at 80℃ for 2h. Then add 7.168mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%), heat to 90℃, and stir for 50min. Finally, add 0.4g of urea and 3.072mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%), and continue stirring at 95℃ for 60min. After the reaction is complete, rapidly cool the reaction system to 70℃ and then slowly cool it to room temperature before discharging to obtain lignin-based phenolic resin adhesive.

[0077] Comparative Example 1

[0078] Referring to Example 1, the difference lies in using a binary acidic eutectic solvent for lignin modification, the specific method of which is as follows:

[0079] 1) Mix 120.53g of choline chloride with 79.47g of formic acid (molar ratio of 1:2) and stir magnetically at 70℃ until the solution is clear and transparent to obtain a dibasic acidic eutectic solvent.

[0080] 2) Wash the bamboo alkali lignin with 2 mol / L hydrochloric acid until neutral, and after vacuum drying, dissolve 10 g of lignin in the above-mentioned binary acidic eutectic solvent, modify it at 120℃ for 2 h, and then rapidly cool it to room temperature. Add 9 times the volume of ethanol aqueous solution (volume ratio 1:9), let it stand overnight, precipitate out, centrifuge and filter to separate the solid particles, wash with deionized water until neutral, and vacuum dry to obtain modified lignin.

[0081] 3) 2.4 g of lignin modified with a dibasic acidic eutectic solvent, 5.6 g of phenol, and 2.8 mL of a 40% sodium hydroxide solution were added to a reaction vessel and stirred at 80 °C for 2 h. Then, 7.168 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) was added, and the mixture was heated to 90 °C and stirred for 50 min. Finally, 0.4 g of urea and 3.072 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) were added, and the mixture was stirred at 90 °C for another 40 min. After the reaction was completed, the reaction system was rapidly cooled to 70 °C and then slowly cooled to room temperature before being discharged to obtain a lignin-based phenolic resin adhesive with a free formaldehyde content as high as 0.47 wt%.

[0082] After pressing the obtained phenolic resin adhesive into boards, the bonding strength of the plywood is 2.12 MPa.

[0083] Comparative Example 2

[0084] Referring to Example 1, the difference being that lignin modification was not performed, the method was as follows: Bamboo alkali lignin was washed to neutral with 2 mol / L hydrochloric acid, vacuum dried, and then 2.4 g of unmodified lignin, 5.6 g of phenol, and 2.8 mL of 40% sodium hydroxide solution were added to a reaction vessel and stirred at 80°C for 2 hours. Then, 7.168 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) was added, and the mixture was heated to 90°C and stirred for 50 minutes. Finally, 0.4 g of urea and 3.072 mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) were added, and the mixture was stirred at 90°C for another 40 minutes. After the reaction was complete, the reaction system was rapidly cooled to 70°C and then slowly cooled to room temperature before being discharged to obtain a lignin-based phenolic resin adhesive with a free formaldehyde content as high as 0.35 wt%.

[0085] After pressing the obtained phenolic resin adhesive into boards, the bonding strength of the plywood is 2.03 MPa.

[0086] Comparative Example 3

[0087] Referring to Example 1, except that lignin was not introduced, 8g of phenol and 2.8mL of 40% sodium hydroxide solution were added to a reaction vessel and stirred at 80°C for 2 hours. Then, 7.168mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) was added, and the mixture was heated to 90°C and stirred for 50 minutes. Finally, 0.4g of urea and 3.072mL of formaldehyde aqueous solution (formaldehyde mass concentration 37%) were added, and the mixture was stirred at 90°C for another 40 minutes. After the reaction was completed, the reaction system was rapidly cooled to 70°C and then slowly cooled to room temperature before being discharged to obtain a lignin-based phenolic resin adhesive with a free formaldehyde content of 0.06wt%.

[0088] After pressing the obtained phenolic resin adhesive into boards, the bonding strength of the plywood was only 1.77 MPa.

[0089] Observations of Example 1 and Comparative Examples 1-3 revealed that the free formaldehyde content of phenolic resin adhesives prepared directly from unmodified lignin and phenolic resin adhesives prepared from lignin modified with a dibasic acidic eutectic solvent without the addition of a third component did not meet the corresponding national standards (≤0.3%). Although the free formaldehyde content of ordinary phenolic resin adhesives prepared without the addition of lignin was low, the bonding strength of the plywood prepared was higher than the corresponding national standard (≥0.7MPa), but significantly lower than the bonding strength of plywood prepared from lignin-based phenolic resin adhesives modified with a ternary acidic eutectic solvent in Example 1 (>2.0MPa). In contrast, the addition of ethylene glycol as a third component in the ternary acidic eutectic solvent process greatly improved the reactivity of modified lignin with formaldehyde.

[0090] As can be seen, the free formaldehyde content of the phenolic resin adhesive prepared by modifying lignin with a ternary acidic eutectic solvent is significantly reduced (basically negligible), reaching the technical standard of commercial phenolic resin adhesives (0.15%). The bonding strength of the plywood prepared is also significantly increased, far exceeding the corresponding national standard, and has great application value.

[0091] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0092] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0093] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing a phenolic resin adhesive with low free formaldehyde content, characterized in that: Includes the following steps: S1. Choline chloride is mixed with an organic acid, heated and stirred to obtain a dibasic acidic eutectic solvent. Deionized water is then added, and stirring continues to obtain a ternary acidic eutectic solvent. The organic acid is formic acid or oxalic acid, and the molar ratio of choline chloride to the organic acid is 1:1-2. The dibasic acidic eutectic solvent accounts for 60-95% of the total weight. S2. Wash the alkali lignin until neutral, dry it, and mix it evenly with the ternary acidic eutectic solvent. Heat it to 110-130℃ and react for 1-3 hours. After cooling, add 5-10 times the volume of a mixed solution of ethanol and water to the reaction system. The volume ratio of ethanol to water in the mixed solution is 1:5-10. Let it stand overnight to precipitate. After the precipitate is precipitated, filter, wash, and dry it to obtain modified lignin. S3. The modified lignin, phenol, and sodium hydroxide are added to a reaction vessel. After heating for the first stage reaction, an aqueous solution of formaldehyde is added, and the temperature is raised for the second stage reaction. Then, an aqueous solution of formaldehyde and urea are added for the third stage reaction. After the reaction is completed, the mixture is cooled to obtain a phenolic resin adhesive. In the reaction vessel, the amount of modified lignin is 20-50% of the total weight of modified lignin and phenol, i.e., the degree of phenol substitution is 20-50%. The bonding strength of the phenolic resin adhesive is 2.1-3.0 MPa.

2. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 1, characterized in that: The heating and stirring described in S1 involves constant temperature stirring at 60-90°C until the mixture becomes clear and transparent, thus obtaining the binary acidic eutectic solvent.

3. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 2, characterized in that: In the ternary acidic eutectic solvent described in S1, the binary acidic eutectic solvent accounts for 80-90% of the total weight.

4. The method for preparing phenolic resin adhesive with low free formaldehyde content according to any one of claims 1-3, characterized in that: The mass ratio of the alkali lignin and the ternary acidic eutectic solvent in S2 is 1:10-20.

5. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 4, characterized in that, The alkali lignin described in S2 and the ternary acidic eutectic solvent are mixed and reacted at 115-125°C for 1.5-2.5 hours, and then cooled to room temperature.

6. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 4, characterized in that: A precipitate was formed in S2, and the solid particles were separated by centrifugation and filtration. The precipitate was washed with deionized water until neutral and then vacuum dried to obtain the modified lignin.

7. The method for preparing phenolic resin adhesive with low free formaldehyde content according to any one of claims 1-3, characterized in that: In the first stage reaction of S3, the ratio of modified lignin to phenol is 0.25-1:

1.

8. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 7, characterized in that: In the first stage reaction of S3, the sodium hydroxide is an aqueous solution of sodium hydroxide with a mass concentration of 10-40%, and the mass of the solute in the aqueous solution of sodium hydroxide accounts for 10-30% of the total mass of the modified lignin and phenol.

9. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 7, characterized in that: In the first stage of reaction in S3, the heating temperature is 70-85℃ and the reaction time is 1-3h.

10. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 7, characterized in that: In the second stage reaction of S3, the formaldehyde mass concentration is 30-37%, the formaldehyde addition is 20-40% of the total formaldehyde amount, and the temperature rise refers to raising the temperature to 90-100℃ and reacting for 30-60 minutes.

11. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 10, characterized in that: In the third stage reaction described in S3, the amount of formaldehyde added is 60-80% of the total amount of formaldehyde used.

12. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 11, characterized in that: The molar ratio of the total amount of modified lignin and phenol used in the first stage reaction to the total amount of formaldehyde used in the second stage reaction and the third stage reaction is 1:1.5-2.

0.

13. The method for preparing phenolic resin adhesive with low free formaldehyde content according to claim 11, characterized in that: In the third stage reaction described in S3, the amount of urea added is 3-8% of the total mass of the modified lignin and phenol, the reaction temperature is 90-100℃, and the reaction time is 30-60 min.

14. The phenolic resin adhesive prepared by the method for preparing phenolic resin adhesive with low free formaldehyde content according to any one of claims 1-13, characterized in that: The bonding strength of the phenolic resin adhesive is 2.1-3.0 MPa.

15. The phenolic resin adhesive with low free formaldehyde content according to claim 14, characterized in that: The free formaldehyde content of the phenolic resin adhesive is 0.01-0.17 wt%.

16. The phenolic resin adhesive with low free formaldehyde content according to claim 15, characterized in that: The free formaldehyde content of the phenolic resin adhesive is 0.01-0.10 wt%.

17. The phenolic resin adhesive with low free formaldehyde content according to claim 16, characterized in that: The free formaldehyde content of the phenolic resin adhesive is 0.01-0.07 wt%.

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