Lignin phenolic resin adhesive, preparation method thereof and method for preparing plywood from lignin phenolic resin adhesive

Abstract

The invention relates to a lignin phenolic resin adhesive which is prepared by the following steps: pretreating biorefinery residues, mixing with water and alkali, separating, repeating, concentrating to obtain lignin concentrated alkali liquor, adding phenol, water and sulfite, pre-reacting, adding formaldehyde and alkali for reacting, finally adding urea, and discharging to obtain the adhesive. According to the method, mild treatment conditions are adopted in lignin extraction, ether bonds can be reserved as many as possible, the activity of lignin can be reserved in subsequent application, the strength of a phenolic resin adhesive is improved, and in the hot pressing process of preparing plywood through the adhesive, lignin can further release active groups and continue to react with residual formaldehyde; sulfite is added for pre-reaction, lignin is further degraded, and a degradation product can directly participate in the reaction in situ in the subsequent process; the synergistic treatment of the two modes is also beneficial to keeping the activity of a small amount of lignin, the lignin reacts with residual formaldehyde in the subsequent hot pressing process, and the formaldehyde emission of a finished product is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of wood adhesive preparation, and particularly relates to a lignin phenolic resin adhesive, a method for preparing the adhesive from biomass residues, and a method for preparing plywood using the adhesive. Background Art

[0002] A large amount of lignin-containing residues will be produced during the biorefining process. These residues are generally burned directly as fuel, which is a low-value utilization. Realizing the high-value utilization of lignin in the residues is of great significance to improving the economic efficiency of the biorefining industry.

[0003] Because the structural units of lignin are similar to phenol, it can be used to replace part of phenol in the synthesis of phenolic resin adhesives. It is generally believed that lignin needs to be activated to increase the amount of lignin that can replace phenol. Patent CN110041481B uses in situ generated Lewis acid to modify alkali lignin at around 150°C to prepare demethylated lignin, which is then used to prepare phenolic resin adhesives. Patent CN111393667B discloses a method for preparing modified lignin, which uses lignin as a substrate and is mixed in a solvent in the presence of a biocatalyst enzyme to undergo a hydroxylation reaction, thereby increasing the phenolic hydroxyl content on the lignin molecule and improving the reaction activity of lignin. Patent CN113174024B adds alkali to lignin-containing biomass hydrolysis residue and refluxes for 3-6 hours to obtain an alkali lignin solution. This alkali lignin solution, sodium hydroxide, and an additive are then mixed into an aqueous solution and added to a microwave digestion vessel to obtain a degraded alkali lignin solution. The degraded alkali lignin solution is then reacted with phenol and sodium hydroxide to obtain a phenolized, prepolymerized alkali lignin solution for use in phenolic resin synthesis. While treating lignin with high temperature, high pressure, or other special treatment methods can increase its activity, the harsh treatment conditions can lead to excessive lignin degradation, thus affecting the performance of the final product. Furthermore, this can increase production costs and inefficiency.

[0004] At the same time, the existing utilization of lignin mostly adopts lignin powder obtained by alkali dissolution and acid precipitation. In this process, sulfuric acid is used to precipitate the lignin, which will produce a large amount of salt-containing wastewater. The wet lignin needs to be obtained by drying and crushing, which consumes a lot of energy. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention provides a lignin-phenolic resin adhesive, a method for extracting lignin from biomass residues and using it to prepare the phenolic resin adhesive, and a method for preparing plywood using the lignin-phenolic resin adhesive. The present invention utilizes relatively mild conditions during the lignin extraction process, ensuring that the majority of the lignin is separated from the biomass and the lignin yield is high while preserving as many ether bonds as possible, thus facilitating the preservation of lignin activity in subsequent applications. Furthermore, the synthesis of the phenolic resin adhesive is performed under alkaline conditions, enabling the coupling of the two.

[0006] In order to achieve the above technical objectives, the technical objective of the first aspect of the present invention is to provide a method for preparing a lignin phenolic resin adhesive, comprising:

[0007] (1) Pretreatment: washing the biorefining residue to remove soluble impurities and obtain washed solid residue;

[0008] (2) Alkali dissolution-separation treatment: the washed solid residue is mixed with water and alkali, and the solid and liquid are separated to obtain a lignin-containing alkali solution; the obtained lignin alkali solution is reused in the alkali dissolution-separation step multiple times until the solid content in the lignin alkali solution reaches more than 10 wt%; and concentrated to obtain a lignin concentrated alkali solution with a solid content of 35%-50 wt%;

[0009] (3) Synthesis of lignin phenolic resin adhesive: The lignin concentrated alkali solution is used for the synthesis of phenolic resin adhesive, which includes the following steps: mixing liquid phenol, water, lignin concentrated alkali solution and sulfite, heating to 60-70°C in a closed reactor, reacting, starting stirring after completion, adding formaldehyde and alkali to react, and polymerizing to a viscosity of 4 cups for more than 18 seconds, adding urea, cooling to 40-45°C and discharging to obtain the lignin phenolic resin adhesive.

[0010] Furthermore, after the formaldehyde is added in step (3), a hydroxymethylation reaction occurs, which releases a large amount of heat. Therefore, it is preferred to add the formaldehyde and alkali in batches, specifically: add 35-45% of the total formaldehyde added in the first batch, raise the temperature to 60-70°C, and release a large amount of heat during the polymerization reaction. Adjust the reaction temperature to 88-95°C and start the first stage polymerization reaction for 50-70 minutes; lower the temperature of the reaction system to 80°C, slowly add the second batch of formaldehyde, the addition amount accounts for 35-45% of the total formaldehyde added, react for 20-30 minutes, add alkali solution, by weight, the addition amount is 10%-20% of the lignin concentrated alkali solution, control the system temperature to 85-93°C, and react for 60-90 minutes; lower the polymerization temperature to 83-85°C, add the remaining formaldehyde, react for 20-30 minutes, and then add liquid alkali.

[0011] Furthermore, the alkali solution is a NaOH solution with a concentration of 20-35 wt%.

[0012] Furthermore, in step (3), the liquid phenol, water, concentrated lignin alkali solution and sulfite are mixed and then heated in a closed reactor for a reaction time of 20-60 minutes.

[0013] Furthermore, the biorefining residue is selected from at least one of xylose residue, furfural residue, bioethanol fermentation residue, biobutanol fermentation residue and biolactic acid fermentation residue.

[0014] Furthermore, in step (1), the water washing is performed by adding water to adjust the solid content to 9wt%-12wt%, stirring and washing, removing the acid and soluble substances in the residue, and performing solid-liquid separation to obtain the pretreated biorefining residue.

[0015] Furthermore, the alkali in step (2) is sodium hydroxide and / or potassium hydroxide, and the alkali concentration added to the alkali adjustment solution is 0.5wt%-2wt%, preferably 0.8wt%-1.5wt%.

[0016] Furthermore, the amount of water added during the alkaline dissolution-separation process in step (2) is 3-20 times, preferably 6-12 times, the weight of the solid residue after washing in step (1) (calculated as the dry weight of the solid residue), the treatment temperature is 60-80°C, and the reaction time is 1-2 hours at normal pressure.

[0017] Furthermore, the concentration treatment in step (2) is selected from one or a combination of multiple-effect evaporation, single-effect evaporation, and mechanical vapor recompression evaporation (MVR).

[0018] Those skilled in the art will appreciate that the solid content in step (2) refers to the content of soluble matter in the solution, which is determined by taking a sample of the solution, evaporating it to dryness, weighing the remaining solids, and calculating the percentage of the total weight of the solution. The concentrated lignin alkali solution obtained in step (2) is primarily composed of lignin and alkali, with a lignin content of approximately 20 wt% to 45 wt% and an alkali content of approximately 5 wt% to 18 wt%.

[0019] The method for determining the lignin content in the concentrated lignin alkali solution is as follows: taking a certain amount of concentrated lignin alkali solution, adjusting the pH to below 2 with 20% sulfuric acid to precipitate the lignin, performing solid-liquid separation by suction filtration, washing with water multiple times, and drying to a constant weight. The lignin content in the concentrated lignin alkali solution is calculated based on the weight of the dried lignin and the weight of the concentrated lignin alkali solution.

[0020] Furthermore, in step (3), the mixing ratio of liquid phenol and concentrated lignin alkali solution is such that the weight ratio of lignin to phenol is 0.4-1.5:1, based on the weight of lignin in the concentrated lignin alkali solution. The amount of sulfite added is 1%-8% of the weight of the lignin, preferably, the sulfite is sodium sulfite. The amount of water added is such that the solid content of the adhesive solution obtained by the final reaction is 40wt%-50wt%. The solid content of the adhesive solution is calculated by dividing the weight of the solid obtained after drying by the weight of the adhesive solution.

[0021] Furthermore, the total amount of formaldehyde added in step (3) is based on a molar ratio of formaldehyde to phenol of 2.3-2.7:1. The total amount of alkali used in step (3) is calculated as 35%-50% by weight of phenol, including the alkali in the lignin concentrated alkali solution. The alkali is preferably sodium hydroxide.

[0022] Furthermore, the amount of urea added in step (3) is 10%-20% of the weight of phenol.

[0023] The technical purpose of the second aspect of the present invention is to provide an adhesive prepared by the above method.

[0024] The third aspect of the present invention is to provide a method for preparing plywood using the above-mentioned adhesive, comprising adding flour to a lignin phenolic resin adhesive to adjust the adhesive viscosity to above 10,000 mPa·s, applying the adhesive to the surface of a veneer, and assembling the veneer after the adhesive application, cold pressing, and hot pressing to form a multilayer plywood.

[0025] Furthermore, the added amount of flour is 5%-20% by weight of the lignin phenolic resin adhesive.

[0026] Furthermore, the double-sided glue application amount is 200-400g / m 3 Apply sizing.

[0027] The plywood prepared by the method of the present invention has a maximum cyclic boiling water resistant bonding strength of 1.9 MPa (GB / T14732-2017 technical requirements for phenolic resins for adhesives: bonding strength ≥ 0.7 MPa), and a formaldehyde emission of less than 0.2 mg / L, meeting the performance requirements of E0 grade Class I plywood.

[0028] The plywood's resistance to cyclic boiling water bonding strength is tested in accordance with the national standard GB / T 17657-2013, "Test Methods for Physical and Chemical Properties of Wood-Based Panels and Faced Wood-Based Panels." The specimens are pretreated using a "boil-dry-boil" cycle: immersed in boiling water for 4 hours, then dried in a forced-air drying oven at (60±3)°C for 16-20 hours, immersed in boiling water for another 4 hours, and then placed in cold water below 30°C for at least 1 hour. This method is a relatively demanding method for measuring plywood strength. Formaldehyde emissions are measured using the desiccator method, in accordance with the group standard T / CNFPIA 1001-2019, "Limits of Formaldehyde Emission from Wood-Based Panels." For plywood, the formaldehyde emission level is ≤0.4 mg / L, achieving the E0 rating.

[0029] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0030] (1) Since the biorefining residue has undergone a series of treatments and its structure is relatively loose, the present invention adopts milder lignin extraction conditions to prevent excessive degradation of lignin and avoid re-condensation, providing conditions for the subsequent in-situ participation of lignin in the reaction. The mild treatment conditions not only ensure the yield of lignin, but also retain as many ether bonds as possible, which is conducive to retaining the activity of lignin in subsequent applications. On the one hand, it can improve the strength of phenolic resin adhesives, and on the other hand, during the hot pressing process of preparing plywood using adhesives, lignin can further release active groups and continue to react with residual formaldehyde, thereby significantly reducing the formaldehyde release of the finished product.

[0031] (2) The present invention extracts lignin from biorefining residue to obtain lignin alkali liquor, which is directly used in the synthesis of phenolic resin adhesive after being concentrated. At the same time, sulfite is introduced during the synthesis process, and a pre-reaction is carried out at 60-70°C. During this process, the addition of sulfite can further degrade the lignin, and the degradation product can directly participate in the reaction in situ in the subsequent process, first forming a lignin-phenol-formaldehyde terpolymer, and then carrying out a subsequent reaction, thereby obtaining a lignin-based phenolic resin adhesive with better performance; at the same time, this coordinated treatment method is also conducive to retaining the activity of a small amount of lignin, reacting with residual formaldehyde in the subsequent hot pressing process, and reducing the formaldehyde release of the finished product.

[0032] (3) Compared with the alkali dissolution and acid precipitation method for preparing lignin powder, directly concentrating the lignin alkali liquor into a high-concentration lignin alkali liquor as a product for downstream applications eliminates the sulfuric acid precipitation step and does not produce a large amount of acid-containing high-salt wastewater.

[0033] (4) In the lignin extraction stage, the lignin alkali liquor obtained by solid-liquid separation is reused, which can increase the lignin content in the lignin alkali liquor and effectively reduce the energy consumption of the concentration process.

[0034] (5) In the preferred technical solution, formaldehyde and sodium hydroxide are added in stages and batches to effectively control the reaction process, so that lignin can effectively participate in the reaction. In addition, the reaction temperature is stable and controllable, preventing the gel problem that may be caused by rapid temperature increase.

[0035] (6) The mild processing conditions during lignin extraction in the present invention are more energy-saving and environmentally friendly. DETAILED DESCRIPTION

[0036] The following examples further illustrate the method for extracting lignin and synthesizing phenolic resin adhesives of the present invention. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the scope of protection of the present invention is not limited to the following examples.

[0037] In the following examples, the cyclic boiling water bond strength of plywood was determined in accordance with the national standard GB / T 17657-2013, "Test Methods for Physical and Chemical Properties of Wood-Based Panels and Veneered Wood-Based Panels." The specimens were pretreated using a "boil-dry-boil" cycle: immersed in boiling water for 4 hours, then dried in a forced-air drying oven at (60±3)°C for 16-20 hours, immersed in boiling water for another 4 hours, and then placed in cold water below 30°C for at least 1 hour.

[0038] The formaldehyde emission is measured using the desiccant method, referring to the group standard T / CNFPIA 1001-2019 "Limits of Formaldehyde Emission from Artificial Boards". For plywood, the formaldehyde emission is ≤0.4mg / L, reaching the E0 level.

[0039] Example 1

[0040] (1) Pretreatment: 20 kg of xylose residue raw material was added to 50 kg of water for washing, and then stirred for 20 minutes and then solid-liquid separation was performed to obtain washed wet xylose residue.

[0041] (2) Alkali dissolution-separation treatment: Weigh 5000 g of water-washed wet xylose residue (water content 67.6%), add 12820 g of water and 162 g of sodium hydroxide, heat to 80°C, react for 1.5 h, perform solid-liquid separation, collect the filtrate, and use it for alkali dissolution of the next batch of water-washed wet xylose residue. After repeating this three times, the solid content of the filtrate reaches 15.4 wt%. The filtrate is concentrated by multi-effect evaporation and single-effect evaporation to achieve a solid content of 35 wt%, of which the lignin content is 24 wt%.

[0042] (3) Synthesis of lignin phenolic resin adhesive: 260g of melted liquid phenol was added to the reactor, stirring was started, 97g of water, 520g of lignin alkali solution, 5.6g of sodium sulfite were added, the temperature was raised to 65°C, and the reaction was carried out for 60 minutes; secondly, 220g of the first batch of 37% formaldehyde solution was added, the reaction temperature was controlled not to exceed 93°C, and the reaction was carried out for 60 minutes; thirdly, the temperature of the reaction system was lowered to 80°C, 220g of the second batch of 37% formaldehyde solution was slowly added, the reaction was carried out for 25 minutes, 69g of 30wt% NaOH liquid alkali was added, the system temperature was controlled at 92°C and the reaction was carried out for 70 minutes; fourthly, the polymerization temperature was lowered to 85°C, 120g of the third batch of 37% formaldehyde solution was added, the reaction was carried out for 30 minutes, and 104g of 30wt% NaOH liquid alkali, when polymerization reaches the target viscosity (coating 4 cups for more than 18s), add 38g of urea, cool to 40-45℃ and discharge to obtain lignin phenolic resin adhesive.

[0043] (4) Preparation of plywood: Add 15% flour to the adhesive to adjust the glue, and the glue amount is 300g / m 2 (double-sided), hot pressing temperature 120℃, hot pressing time 120s / mm.

[0044] The plywood prepared using this adhesive has a cyclic boiling water bonding strength of 1.89 MPa and a formaldehyde emission of 0.167 mg / L.

[0045] Example 2

[0046] (1) Pretreatment: 20 kg of furfural residue raw material was added to 50 kg of water for washing, and after stirring for 20 minutes, solid-liquid separation was performed to obtain washed wet xylose residue.

[0047] (2) Alkali Dissolution-Separation Treatment: 5000 g of washed wet furfural residue (69.5% moisture content) was weighed, 14825 g of water, 61 g of sodium hydroxide, and 61 g of potassium hydroxide were added, and the mixture was heated to 70°C. After reacting for 2 h, solid-liquid separation was performed, and the filtrate was collected and used for the next batch of washed wet xylose residue. After repeating this process three times, the solid content of the filtrate reached 13.1 wt%. The filtrate was concentrated by combining mechanical steam recompression evaporation with single-effect evaporation to a solid content of 40 wt%, including 31 wt% of lignin.

[0048] (3) Synthesis of lignin phenolic resin adhesive: 260g of melted liquid phenol was added to the reactor, stirring was started, 45g of water, 600g of lignin alkali solution, 10.1g of sodium sulfite were added, the temperature was raised to 60°C, and the reaction was carried out for 25 minutes; secondly, 180.6g of the first batch of 37% formaldehyde solution was added, the reaction temperature was controlled not to exceed 95°C, and the reaction was carried out for 60 minutes; thirdly, the temperature of the reaction system was lowered to 80°C, 180.6g of the second batch of 37% formaldehyde solution was slowly added, the reaction was carried out for 30 minutes, 40g of 30wt% NaOH liquid alkali was added, the system temperature was controlled at 92°C and the reaction was carried out for 70 minutes; fourthly, the polymerization temperature was lowered to 85°C, 154.8g of the third batch of 37% formaldehyde solution was added, the reaction was carried out for 20 minutes, and 65g of 37% formaldehyde solution was added. 30wt% NaOH liquid alkali, when polymerization reaches the target viscosity (coating 4 cups for more than 18s), add 26g of urea, cool to 40-45℃ and discharge to obtain lignin phenolic resin adhesive.

[0049] (4) Preparation of plywood: Add 15% flour to the adhesive to adjust the glue, and the glue amount is 300g / m 2 (double-sided), hot pressing temperature 120℃, hot pressing time 120s / mm.

[0050] The plywood prepared using this adhesive has a cyclic boiling water bonding strength of 1.52 MPa and a formaldehyde emission of 0.183 mg / L.

[0051] Example 3

[0052] (1) Pretreatment: 20 kg of bioethanol residue raw material was added to 50 kg of water for washing, and then stirred for 20 minutes and then solid-liquid separation was performed to obtain water-washed wet bioethanol residue.

[0053] (2) Alkali Dissolution-Separation Treatment: 5000 g of water-washed wet bioethanol residue (70.2% moisture content) was weighed, 8410 g of water and 163.9 g of sodium hydroxide were added, and the temperature was raised to 75°C. After reacting for 2 h, solid-liquid separation was performed. The filtrate was collected and used for the next batch of water-washed wet xylose residue. After repeating this process three times, the solid content of the filtrate reached 14.3%. The filtrate was concentrated using a combination of mechanical vapor recompression evaporation and single-effect evaporation to a solid content of 46%, of which the lignin content was 38%.

[0054] (3) Synthesis of lignin phenolic resin adhesive: 260g of melted liquid phenol was added to the reactor, stirring was started, 67g of water, 780g of lignin alkali solution, 8.9g of sodium sulfite were added, the temperature was raised to 60°C, and the reaction was carried out for 40 minutes; secondly, 246.5g of the first batch of 37% formaldehyde solution was added, the reaction temperature was controlled not to exceed 93°C, and the reaction was carried out for 60 minutes; thirdly, the temperature of the reaction system was lowered to 80°C, 246.5g of the second batch of 37% formaldehyde solution was slowly added, and the reaction was carried out for 30 minutes, and then 80g of 30wt% NaOH liquid alkali was added, and the system temperature was controlled at 92°C and the reaction was carried out for 80 minutes; fourthly, the polymerization temperature was lowered to 80°C, 93.9g of the third batch of 37% formaldehyde solution was added, and the reaction was carried out for 25 minutes, and then 125g of 30wt% NaOH liquid alkali, when polymerization reaches the target viscosity (coating 4 cups for more than 18s), add 40g of urea, cool to 40-45℃ and discharge to obtain lignin phenolic resin adhesive.

[0055] (4) Preparation of plywood: Add 15% flour to the adhesive to adjust the glue, and the glue amount is 300g / m 2 (double-sided), hot pressing temperature 120℃, hot pressing time 120s / mm.

[0056] The plywood prepared using this adhesive has a cyclic boiling water bonding strength of 1.77 MPa and a formaldehyde emission of 0.134 mg / L.

[0057] Example 4

[0058] (1) Pretreatment: 20 kg of biobutanol residue raw material was added to 50 kg of water for washing, and then stirred for 30 minutes and then solid-liquid separation was performed to obtain washed wet biobutanol residue.

[0059] (2) Alkali Dissolution-Separation Treatment: Weigh 5000 g of water-washed wet biobutanol residue (69.0% moisture content), add 10500 g of water and 186 g of sodium hydroxide, heat to 80°C, react for 1.5 h, perform solid-liquid separation, collect the filtrate, and use it for the next batch of water-washed wet xylose residue. After repeating this process three times, the solid content of the filtrate reaches 15.9%. The filtrate is concentrated by combining multiple-effect evaporation with single-effect evaporation to a solid content of 50%, of which the lignin content is 43%.

[0060] (3) Synthesis of lignin phenolic resin adhesive: 260g of melted liquid phenol was added to the reactor, stirring was started, 150g of water, 890g of lignin alkali solution, 12.8g of sodium sulfite were added, the temperature was raised to 65°C, and the reaction was carried out for 50 minutes; secondly, 272.3g of the first batch of 37% formaldehyde solution was added, the reaction temperature was controlled not to exceed 93°C, and the reaction was carried out for 70 minutes; thirdly, the temperature of the reaction system was lowered to 80°C, 272.3g of the second batch of 37% formaldehyde solution was slowly added, the reaction was carried out for 30 minutes, 40g of 30wt% NaOH liquid alkali was added, the system temperature was controlled at 91°C and the reaction was carried out for 90 minutes; fourthly, the polymerization temperature was lowered to 80°C, 60.5g of the third batch of 37% formaldehyde solution was added, the reaction was carried out for 30 minutes, and 90g of 37% formaldehyde solution was added. 30wt% NaOH liquid alkali, when polymerization reaches the target viscosity (coating 4 cups for more than 18s), add 52g of urea, cool to 40-45℃ and discharge to obtain lignin phenolic resin adhesive.

[0061] (4) Preparation of plywood: Add 15% flour to the adhesive to adjust the glue, and the glue amount is 300g / m 2 (double-sided), hot pressing temperature 120℃, hot pressing time 120s / mm.

[0062] The plywood prepared using this adhesive has a cyclic boiling water bonding strength of 1.36 MPa and a formaldehyde emission of 0.109 mg / L.

[0063] Example 5

[0064] (1) Pretreatment: 20 kg of biolactic acid residue raw material was added to 50 kg of water for washing, and then stirred for 30 minutes and then solid-liquid separation was performed to obtain washed wet biolactic acid residue.

[0065] (2) Alkali dissolution-separation treatment: Weigh 5000 g of water-washed wet biolactic acid residue with a moisture content of 68.8%, add 13720 g of water and 156 g of sodium hydroxide, heat to 80°C, react for 2 h, perform solid-liquid separation, collect the filtrate, and use it for the next batch of water-washed wet xylose residue. After repeating this process twice, the solid content of the filtrate reaches 10.2%. The filtrate is concentrated by combining multiple-effect evaporation with single-effect evaporation to a solid content of 43%, of which the lignin content is 37%.

[0066] (3) Synthesis of lignin phenolic resin adhesive: 260g of melted liquid phenol was added to the reactor, stirring was started, 120g of water, 780g of lignin alkali solution, 16.4g of sodium sulfite were added, the temperature was raised to 65°C, and the reaction was carried out for 45 minutes; 209.8g of the first batch of 37% formaldehyde solution was added, the reaction temperature was controlled not to exceed 93°C, and the reaction was carried out for 60 minutes; secondly, the temperature of the reaction system was lowered to 80°C, 209.8g of the second batch of 37% formaldehyde solution was slowly added, and the reaction was carried out for 25 minutes, and then 60g of 30wt% NaOH liquid alkali was added, and the system temperature was controlled at 92°C and the reaction was carried out for 70 minutes; thirdly, the polymerization temperature was lowered to 85°C, 132.5g of the third batch of 37% formaldehyde solution was added, and the reaction was carried out for 30 minutes, and then 99g of 37% formaldehyde solution was added. 30wt% NaOH liquid alkali, when polymerization reaches the target viscosity (coating 4 cups for more than 18s), add 35g of urea, cool to 40-45℃ and discharge to obtain lignin phenolic resin adhesive.

[0067] (4) Preparation of plywood: Add 15% flour to the adhesive to adjust the glue, and the glue amount is 300g / m 2 (double-sided), hot pressing temperature 120℃, hot pressing time 120s / mm.

[0068] The plywood prepared using this adhesive has a cyclic boiling water bonding strength of 1.59 MPa and a formaldehyde emission of 0.173 mg / L.

[0069] Comparative Example 1

[0070] (1) Pretreatment: 20 kg of xylose residue raw material was added to 50 kg of water for washing, and then stirred for 20 minutes and then solid-liquid separation was performed to obtain washed wet xylose residue.

[0071] (2) High-temperature cooking to extract lignin: Weigh 5000 g of water-washed wet xylose residue with a moisture content of 67.6%, add 12820 g of water and 320 g of sodium hydroxide, heat to 100°C and cook for 2 h. Then perform solid-liquid separation and collect the filtrate. The filtrate is used for the next batch of water-washed wet xylose residue. After repeating this process three times, the solid content of the filtrate reaches 7.8%. The filtrate is concentrated to a solid content of 45%, of which the lignin content is approximately 36%.

[0072] (3) Synthesis of lignin phenolic resin adhesive: The process is the same as step (3) of Example 1.

[0073] (4) The process is the same as step (4) in Example 1.

[0074] According to the measuring method of the present invention, the plywood prepared by using the adhesive has a cyclic boiling water bonding strength of 0.74 MPa and a formaldehyde emission of 0.372 mg / L.

[0075] The application performance of the adhesive is significantly lower than that of Example 1, mainly because the lignin is cooked under high temperature and high alkaline conditions (relatively intense heating reaction conditions are used), and most of the lignin reaction active sites are exhausted in this process, and cannot effectively participate in the subsequent adhesive synthesis experiment, making it difficult to achieve effective ternary copolymerization of lignin-phenol-formaldehyde.

[0076] Comparative Example 2

[0077] Steps (1) and (2) are the same as those in Example 1. In the synthesis process of the lignin phenolic resin adhesive, the three-step reaction is changed to two steps: 260g of melted liquid phenol is added to the reactor, stirring is started, 97g of water, 520g of lignin alkali solution, and 5.6g of sodium sulfite are added, the temperature is raised to 65°C, and the reaction is carried out for 60 minutes; in the second step, 220g of the first batch of 37% formaldehyde solution is added, the temperature is raised to 65°C, the reaction temperature is controlled not to exceed 93°C, and the reaction is carried out for 60 minutes; in the third step, the temperature of the reaction system is lowered to 80°C, 340g of the second batch of 37% formaldehyde solution is slowly added, and after reacting for 50 minutes, 173g of 30wt% NaOH liquid alkali is added, the system temperature is controlled to react at 92°C, and when the polymerization reaches the target viscosity (coating 4 cups for more than 18 seconds), 38g of urea is added, the temperature is lowered to 40-45°C, and the material is discharged to obtain the lignin phenolic resin adhesive. Step (4) is the same as in Example 1. The plywood prepared using this adhesive has a cyclic boiling water bonding strength of 0.68 MPa and a formaldehyde emission of 0.324 mg / L.

[0078] After the three-step reaction is combined into two steps, the reaction process is shortened, and the amount of formaldehyde and sodium hydroxide added at one time is too high, the reaction releases heat too quickly, the temperature is difficult to control, and local overheating will produce gel, resulting in a decrease in adhesive performance.

[0079] Comparative Example 3

[0080] Steps (1), (2) and (4) are the same as those in Example 1. In the synthesis process of the lignin phenolic resin adhesive in step (3), sodium sulfite is not added. Other operations are the same as those in Example 1. The plywood prepared using the adhesive has a circulating boiling water bonding strength of 0.61 MPa and a formaldehyde emission of 0.416 mg / L.

[0081] The application performance of the adhesive is significantly lower than that of Example 1, mainly because the lignin has relatively few active groups and large steric hindrance. The addition of sodium sulfite can further degrade the lignin, release the active sites, and react with phenol, which helps to achieve effective ternary copolymerization of lignin, phenol, and formaldehyde.

[0082] Comparative Example 4

[0083] Steps (1), (2) and (4) are the same as those in Example 1. In step (3), no pre-reaction is performed. Water, lignin alkali solution, sodium sulfite and the first batch of formaldehyde are directly mixed and reacted. Subsequent operations are the same as those in (3) in Example 1.

[0084] The plywood prepared using this adhesive has a cyclic boiling water bonding strength of 0.89 MPa and a formaldehyde emission of 0.368 mg / L.

[0085] Analysis shows that the main reason is that sodium sulfite does not have time to participate in the further degradation reaction of lignin, which makes lignin unable to effectively participate in the lignin-phenol-formaldehyde ternary copolymerization, resulting in a decrease in bonding strength. In addition, formaldehyde will also react with sodium sulfite. Adding it too early will cause formaldehyde loss and more side reactions.

[0086] Comparative Example 5

[0087] The bonding strength was determined using the factory rapid test method in standard GB / T 14074-2006. This method is relatively mild and involves boiling the specimen in boiling water for 3 hours, taking it out and cooling it at room temperature before measuring.

[0088] The plywood obtained in Example 1 was tested using the above method, and the bonding strength of the plywood was 2.61 MPa.

Claims

1. A method for preparing a lignin phenolic resin adhesive, comprising: (1) Pretreatment: washing the biorefining residue with water to remove soluble impurities and obtain washed solid residue; (2) Alkali dissolution-separation treatment: the washed solid residue is mixed with water and alkali, and the solid and liquid are separated to obtain a lignin-containing alkali solution; the obtained lignin alkali solution is reused in the alkali dissolution-separation step multiple times until the solid content in the lignin alkali solution reaches more than 10wt%; and concentrated to obtain a lignin concentrated alkali solution with a solid content of 35%-50wt%; (3) Synthesis of lignin phenolic resin adhesive: The lignin concentrated alkali solution is used for the synthesis of phenolic resin adhesive, which includes the following steps: mixing liquid phenol, water, lignin concentrated alkali solution and sulfite, heating to 60-70 ° C in a closed reactor, reacting, and after completion, starting stirring, adding formaldehyde and alkali to react, and polymerizing to a viscosity of 4 cups for more than 18 seconds, adding urea, cooling to 40-45 ° C, and discharging to obtain the lignin phenolic resin adhesive.

2. The preparation method according to claim 1, characterized in that In step (3), formaldehyde and alkali are added in batches, specifically: 35-45% of the total amount of formaldehyde is added in the first batch, the temperature is raised to 60-70°C, a large amount of heat is released during the polymerization reaction, the reaction temperature is adjusted to 88-95°C, and the first stage polymerization reaction is started for 50-70 minutes; the temperature of the reaction system is lowered to 80°C, and the second batch of formaldehyde is slowly added, the amount of which accounts for 35-45% of the total amount of formaldehyde added, and the reaction is carried out for 20-30 minutes. Alkali solution is added, and the amount added is 10%-20% of the lignin concentrated alkali solution by weight. The system temperature is controlled to 85-93°C, and the reaction is carried out for 60-90 minutes; the polymerization temperature is lowered to 83-85°C, the remaining formaldehyde is added, the reaction is carried out for 20-30 minutes, and then liquid alkali is added.

3. The preparation method according to claim 1, characterized in that In step (3), the liquid phenol, water, concentrated lignin alkali solution and sulfite are mixed and then heated in a closed reactor for a reaction time of 20-30 minutes.

4. The preparation method according to claim 1, characterized in that The biorefining residue is selected from at least one of xylose residue, furfural residue, bioethanol fermentation residue, biobutanol fermentation residue and biolactic acid fermentation residue.

5. The preparation method according to claim 1, characterized in that The alkali in step (2) is sodium hydroxide and / or potassium hydroxide, and the alkali concentration of the alkali-adjusted solution is 0.5 wt%-2 wt%.

6. The preparation method according to claim 1, characterized in that The amount of water added during the alkaline dissolution-separation process in step (2) is 3-20 times the weight of the solid residue after washing in step (1), the treatment temperature is 60-80°C, and the reaction time is 1-2 hours at normal pressure.

7. The preparation method according to claim 1, characterized in that The concentration treatment in step (2) is selected from one or a combination of multiple-effect evaporation, single-effect evaporation, and mechanical vapor recompression evaporation.

8. The preparation method according to claim 1, characterized in that In step (3), the mixing ratio of liquid phenol and lignin concentrated alkali solution is such that the weight ratio of lignin to phenol is 0.4-1.5:1, based on the weight of lignin in the lignin concentrated alkali solution.

9. The preparation method according to claim 1, characterized in that The added amount of sulfite is 1%-8% of the weight of lignin, and the sulfite is sodium sulfite.

10. The preparation method according to claim 1, characterized in that The amount of water added is calculated so that the solid content of the adhesive solution obtained by the final reaction is 40wt%-50wt%.

11. The preparation method according to claim 2, characterized in that The total amount of formaldehyde added in step (3) is such that the molar ratio of formaldehyde to phenol is 2.3-2.7:

1.

12. The preparation method according to claim 2, characterized in that The total amount of alkali used in step (3) is calculated as 35%-50% of the weight of phenol, wherein the alkali in the lignin concentrated alkali solution should be included, and the alkali is sodium hydroxide.

13. The preparation method according to claim 1, characterized in that The amount of urea added in step (3) is 10%-20% of the weight of phenol.

14. The adhesive prepared by the preparation method according to any one of claims 1 to 13.

15. A method for preparing plywood using the adhesive according to claim 14, characterized in that: Flour is added to the lignin phenolic resin adhesive to adjust the viscosity to above 10,000 mPa·s, and the adhesive is applied to the surface of the veneer. The veneer after adhesive application is assembled, cold pressed, and hot pressed to produce a multi-layer plywood.

16. The method according to claim 15, characterized in that The amount of flour added is 5%-20% of the weight of the lignin phenolic resin adhesive; the double-sided adhesive amount is 200-400g / m 3 Apply sizing.