Lignin-based adhesive material, method for its preparation and adhesive and glued product

Abstract

The present application relates to the technical field of adhesives, and particularly relates to a lignin-based adhesive material, a preparation method thereof, an adhesive and a glued product. The present application uses a natural small-molecule aldehyde-based deep eutectic solvent as a solvent and an aldehyde donor at the same time, can react with lignin to obtain a lignin-based adhesive material, uses lignin to replace phenol, uses a food-grade non-toxic and harmless natural hydroxyl aromatic aldehyde to replace formaldehyde, the obtained lignin-based adhesive material can be directly used as an adhesive, and has strong bonding performance and bending strength. The lignin and the natural small-molecule hydroxyl aromatic aldehyde in the deep eutectic solvent in the present application can be derived from biomass raw materials, are low in cost, simple to obtain, avoid the introduction of phenol and formaldehyde and other substances, and the product is more green and environmentally friendly. The glued product prepared from the lignin-based adhesive material in the present application has excellent water resistance and mildew resistance, is green and environmentally friendly, has stronger performance, and is lower in cost.

Description

Technical Field

[0001] This invention relates to the field of adhesive technology, specifically to lignin-based adhesive materials, their preparation methods, adhesives, and bonding products. Background Technology

[0002] Currently, adhesives are widely used in the market, especially in furniture, which people frequently come into contact with. Common adhesives used for MDF and plywood are resin-based adhesives such as urea-formaldehyde resin and phenol-formaldehyde resin, which are also the most commonly used adhesives on the market. These adhesives contain a large amount of volatile aldehyde compounds, such as formaldehyde. This substance is very harmful to human health and the environment; therefore, when using MDF, plywood, and particleboard, the formaldehyde emitted may cause harm to human health. Furthermore, the manufacturing process of these adhesives is complex, the raw material cost is high, and the raw materials used are toxic and harmful, making them less than environmentally friendly. Therefore, there is a need for an adhesive with a simple manufacturing process, environmental friendliness, and low cost to replace them.

[0003] To address these challenges, related technologies use lignin to replace phenol in traditional phenolic resins. However, most lignin-based adhesives require various modifications to the lignin itself, or partial substitution of phenol with lignin, due to the low reactivity of lignin. This is not only cumbersome but also increases processing costs. Furthermore, most adhesives in these technologies still use formaldehyde to react with lignin, with some using less toxic furfural or glyoxal to replace formaldehyde. While using low-toxicity aldehydes to replace formaldehyde is indeed more environmentally friendly, these aldehyde compounds still possess some toxicity and are relatively expensive. Summary of the Invention

[0004] In view of this, the technical problem to be solved by the present invention is to provide lignin-based adhesive materials, their preparation methods, adhesives and bonding products. The lignin-based adhesive materials provided by the present invention use lignin to replace phenol, do not require modification of lignin, can be used directly as adhesives, and have strong bonding properties and bending strength.

[0005] This invention provides a lignin-based adhesive material obtained from a eutectic solvent and lignin;

[0006] The eutectic solvent includes choline chloride and natural small-molecule hydroxy aromatic aldehydes.

[0007] The eutectic solvent of this invention is a natural small-molecule aldehyde-based eutectic solvent, comprising choline chloride and a natural small-molecule hydroxy aromatic aldehyde. Choline chloride acts as a hydrogen bond acceptor, and the natural small-molecule hydroxy aromatic aldehyde acts as a hydrogen bond donor. When mixed in a specific ratio, they form stable hydrogen bonds, resulting in a clear and transparent eutectic solvent. The molar ratio of choline chloride to the natural small-molecule hydroxy aromatic aldehyde in this invention is 1:(1~3), with a most preferred ratio of 1:2. Preliminary experiments show that a higher or lower number of hydrogen bond donors prevents the formation of stable hydrogen bond structures.

[0008] In some embodiments of the present invention, the natural small-molecule hydroxy aromatic aldehyde is selected from at least one of vanillin, ethyl vanillin, o-vanillin, salicylaldehyde, or protocatechuic aldehyde. The natural small-molecule hydroxy aromatic aldehyde in the eutectic solvent of the present invention is preferably selected from vanillin, as vanillin has lower cost, higher reactivity, and better performance in adhesives prepared with lignin. Compared with hydroxy aromatic aldehydes such as ethyl vanillin, o-vanillin, salicylaldehyde, and protocatechuic aldehyde, vanillin is the best substitute for formaldehyde.

[0009] In some embodiments of the present invention, both the lignin and the natural small-molecule hydroxy aromatic aldehydes are derived from biomass raw materials, which are inexpensive, readily available, and avoid the introduction of substances such as phenol and formaldehyde, making the product more environmentally friendly. In some embodiments of the present invention, the lignin compound is sourced from one or more of straw, rice husks, and corn cobs; the natural small-molecule hydroxy aromatic aldehydes are sourced from one or more of vanilla beans or beets.

[0010] This application uses a natural small-molecule aldehyde-based eutectic solvent as both a solvent and an aldehyde donor, which can react with lignin to prepare lignin-based adhesive materials. Lignin is used to replace phenol without modifying lignin, and food-grade, non-toxic, and harmless natural hydroxy aromatic aldehydes are used to replace formaldehyde. The resulting lignin-based adhesive materials can be used directly as adhesives and have strong bonding properties and flexural strength.

[0011] This invention also provides a method for preparing the lignin-based adhesive material according to any of the above-described technical solutions, comprising the following steps: reacting a eutectic solvent and lignin to obtain the lignin-based adhesive material. The eutectic solvent and lignin used in this invention are the same as described above and will not be repeated here.

[0012] The eutectic solvent of this invention is obtained by mixing choline chloride with a natural small-molecule hydroxy aromatic aldehyde; specifically, the eutectic solvent is obtained by mixing choline chloride and a natural small-molecule hydroxy aromatic aldehyde in a molar ratio of 1:(1~3), preferably in a molar ratio of 1:2. The mixing temperature of this invention is 80℃~100℃, and the mixing time is 20 min~40 min.

[0013] This invention first reacts a eutectic solvent with lignin; specifically, the eutectic solvent and lignin are dispersed before the reaction. In some embodiments of this invention, the lignin is uniformly dispersed in the eutectic solvent before the reaction. The dispersion temperature in this invention is 80℃~100℃, preferably 90℃, and the dispersion time is 6h~12h. This invention uniformly disperses lignin in the eutectic solvent, allowing for more complete contact between lignin and the natural small-molecule hydroxy aromatic aldehydes in the eutectic solvent, thus resulting in a more complete reaction. Simultaneously, it eliminates the use of organic solvents, ensuring no waste is generated during the reaction, achieving a "one-pot, three-step" method for preparing adhesives.

[0014] This invention involves reacting the eutectic solvent with lignin at a temperature of 120°C to 140°C for 8 to 10 hours. The temperature of 120 to 140°C is raised to achieve the polymerization temperature of lignin with the natural small-molecule hydroxy aromatic aldehydes in the eutectic solvent. Upon completion of the reaction, a lignin-based adhesive material with optimal performance is obtained.

[0015] This invention involves reacting a eutectic solvent with lignin, specifically by reacting the eutectic solvent, lignin, and an alkaline catalyst in a mass ratio of (1.9~2.1):1:(0~0.05). In some embodiments of this invention, (19~21) g of eutectic solvent, 10 g of lignin, and (0~0.5) g of alkaline catalyst are reacted. The alkaline catalyst of this invention includes at least one of sodium hydroxide, sodium carbonate, or sodium bicarbonate. If the eutectic solvent is too small, the lignin cannot be completely dispersed, resulting in an incomplete reaction. If the eutectic solvent is too large, the viscosity of the entire system will decrease, causing the resulting adhesive to be immersed in the eutectic solvent, affecting its bonding strength. Therefore, when the mass ratio of eutectic solvent to lignin is close to 2:1, the resulting lignin-based adhesive material exhibits the best performance.

[0016] This invention also provides an adhesive, which is the lignin-based adhesive material described in any of the above-described technical solutions; or, it comprises the lignin-based adhesive material described above and additives. The lignin-based adhesive material of this invention can be used directly as an adhesive, or it can be used in combination with additives as an adhesive.

[0017] This invention also provides an adhesive product obtained from wood raw materials and an adhesive; said adhesive is a lignin-based adhesive material as described in any of the above-mentioned technical solutions; or, said adhesive comprises a lignin-based adhesive material as described in any of the above-mentioned technical solutions and additives; the wood raw material is selected from bamboo powder or poplar wood boards. The adhesive of this invention is applicable to MDF, plywood, particleboard, and bamboo-wound products.

[0018] This invention involves heat-treating wood raw materials and the adhesive to obtain a glued product. The glued product prepared from the lignin-based adhesive material of this invention exhibits excellent water resistance. In one embodiment of this invention, the glued product is plywood, whose water resistance is far superior to that of bamboo fiberboard (MBB), bamboo powder board (BFB), and commercially available high-density fiberboard (HDF). Furthermore, due to the presence of choline chloride and natural small-molecule hydroxy aromatic aldehydes in the eutectic solvent, it also possesses excellent mildew resistance. Therefore, the glued product obtained by this invention is not only environmentally friendly but also has stronger performance and lower cost.

[0019] In one embodiment of the present invention, bamboo powder and any of the lignin-based adhesives described above are thoroughly mixed and then hot-pressed. After cooling, a high-performance MDF is obtained. The hot pressing of the present invention is carried out at 110℃~130℃ and 20MPa~40MPa for 20 min~40 min. The mass ratio of bamboo powder to lignin-based adhesive is 1:(1~3), preferably 1:2; the bonding strength of the lignin-based adhesive to the bamboo board at room temperature is 4.24 (±0.26) MPa. Preliminary experiments showed that if there is too much bamboo powder, its performance will decrease, while if there is too little bamboo powder, its performance will not improve much, but the cost will increase. The MDF obtained by the present invention has a flexural strength of 43.21 (±2.45) MPa and an internal bond strength of 6.8 (±0.4) MPa. The commercially available high-density fiberboard has a flexural strength of 21.76 MPa and an internal bond strength of 1.17 MPa. In comparison, the MDF of the present invention has superior performance, is more environmentally friendly, and has a lower cost.

[0020] In another embodiment of the present invention, multiple poplar boards are coated with the lignin-based adhesive material described in any of the above technical solutions and then hot-pressed. After cooling, a high-strength plywood is obtained. The lignin-based adhesive material of the present invention has a bonding strength of 5.08 (±0.3) MPa to poplar wood at room temperature. The coating amount of the present invention is 100 g / m². 2 ~150g / m 2The hot-pressing pressure described in this invention is 2 MPa to 4 MPa, and the hot-pressing time is 20 min to 40 min. Under the 2 MPa hot-pressing condition, the plywood bending strength is 68.14 (±4.2) MPa; under the 4 MPa hot-pressing condition, the plywood bending strength is 78.77 (±4.8) MPa. The bending strength of this plywood is significantly greater than the 27.86 MPa of commercially available poplar plywood.

[0021] Based on the above technical solution, the lignin-based adhesive material provided by the present invention has at least one of the following beneficial effects:

[0022] (1) In this invention, choline chloride, the hydrogen bond acceptor, is mixed with natural small-molecule hydroxy aromatic aldehydes, the hydrogen bond donor, in a certain proportion to form stable hydrogen bonds. A clear and transparent eutectic solvent is then obtained as a dispersant. Lignin is added to the eutectic solvent, and after uniform dispersion, the temperature is increased to allow the lignin to polymerize with the natural small-molecule hydroxy aromatic aldehydes in the eutectic solvent, thus obtaining a lignin-based adhesive. The lignin and natural small-molecule hydroxy aromatic aldehydes in this invention can both be derived from biomass raw materials, making them inexpensive and readily available. The preferred natural small-molecule hydroxy aromatic aldehyde is vanillin, which is a non-toxic and harmless food additive and an excellent substitute for formaldehyde. Lignin is also an excellent substitute for phenol. Therefore, this lignin-based adhesive is more environmentally friendly, has a simpler process, and is less expensive than phenolic resin and urea-formaldehyde resin adhesives.

[0023] (2) This invention uses a eutectic solvent as a dispersant, which reduces the damage to the lignin structure caused by the solvent and allows for sufficient contact between lignin and vanillin, resulting in a more complete reaction. Furthermore, this invention uses a "one-pot, three-step" method to prepare lignin-based adhesives, which is simple, efficient, generates little waste, and is easily industrialized. In addition, no toxic or harmful substances such as phenol or formaldehyde are introduced during the preparation process, making it a green and environmentally friendly adhesive processing technology.

[0024] (3) The high-performance MDF obtained by combining the lignin-based adhesive material of the present invention with bamboo powder has performance far exceeding that of currently available MDF and is an excellent substitute for MDF and plastic boards. The high-performance plywood obtained by combining the lignin-based adhesive material of the present invention with poplar plywood also exceeds the performance of commonly available poplar plywood. The lignin-based adhesive material of the present invention has high bonding strength to both poplar and bamboo boards, far exceeding the national standard for phenolic resin wood adhesives, which requires a bonding strength ≥0.7 MPa (GB / T 14732-2017). Furthermore, the preparation process is simple, the raw materials are non-toxic and harmless, the cost is low, and no by-products are generated during the production process, making it green and environmentally friendly. Especially in furniture and industry, it can be used as a safe, reliable, and highly stable adhesive, improving the quality of adhesives used in wood, and has good economic and social benefits with a broad market prospect. Attached Figure Description

[0025] Figure 1 This is a schematic diagram showing the bonding strength of the lignin-based adhesive prepared in Example 1 of the present invention at room temperature;

[0026] Figure 2 This is an appearance diagram of the MDF board prepared in Example 2 of the present invention;

[0027] Figure 3 The graph shows the test results of bending strength and internal bond strength of the MDF prepared in Example 2 of the present invention.

[0028] Figure 4 A comparison chart of the bending strength test results of the plywood prepared in Example 3 of the present invention;

[0029] Figure 5 This is a comparison chart of the mass expansion rate and thickness expansion rate of the MDF prepared in Example 2 of the present invention. Detailed Implementation

[0030] This invention discloses a lignin-based adhesive material, its preparation method, and the adhesive and bonded products thereof. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art will clearly be able to modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0031] The present invention will be further described below with reference to the embodiments:

[0032] Example 1

[0033] Preparation of lignin-based adhesive: 3.0 g of choline chloride and 6.5 g of vanillin were mixed at a molar ratio of 1:2 and stirred at 90 °C for 20 min to obtain a clear and transparent eutectic solvent. 4.5 g of lignin was added to the eutectic solvent, along with 0.2 g of sodium carbonate catalyst. After stirring for 6 h, when the lignin was completely dispersed in the eutectic solvent, the temperature was raised to 120 °C and stirred for 8 h to obtain the lignin-based adhesive. The mass ratio of the eutectic solvent to lignin was 2:1.

[0034] The bonding strength of the lignin adhesive obtained above to poplar boards and bamboo boards was tested. The specific testing process is as follows:

[0035] 1) Sample preparation: Take 0.2 g of the lignin-based adhesive prepared in Example 1 and apply it while hot to about one-quarter (15 × 20 mm) of the sample. 2 The sample was coated onto poplar wood chips and bamboo board samples (20 mm × 60 mm × 3 mm). A portion of the coated sample was placed in a hot press and hot-pressed at 1.5 MPa and 120℃ for 10 min, then slowly cooled to room temperature and continued to be cold-pressed for 10 h.

[0036] 2) Room temperature (25℃) bonding performance test: After cold pressing, the sample was placed at room temperature for 10 h, and then the bonding performance was tested using a tensile tester at a speed of 5 mm / min. The bonding strength was recorded as the maximum tensile strength.

[0037] like Figure 1 As shown, Figure 1 This is a schematic diagram of the bonding strength of the lignin-based adhesive prepared in Example 1 of the present invention at room temperature. The lignin-based adhesive has strong bonding strength. At room temperature, its bonding strength to poplar wood board can reach 5.08 MPa, and its bonding strength to bamboo board can reach 4.24 MPa. This far exceeds the bonding strength (≥0.7 MPa) required by the national standard GB / T 14732-2017 for phenolic resin wood adhesives, indicating that the lignin-based adhesive provided by the present invention meets the application needs of daily life.

[0038] Example 2

[0039] Preparation of high-performance MDF: The lignin-based adhesive prepared in Example 1 was mixed with bamboo powder at a mass ratio of 1:2 and then hot-pressed at 110°C and 20 MPa for 20 min. After natural cooling, high-strength MDF was obtained. The bending strength was tested using the three-point bending method, while the internal bond strength was tested according to GB / T31765-2015.

[0040] like Figures 2-3As shown, Figure 2 This is an appearance diagram of the MDF board prepared in Example 2 of the present invention. Figure 3 The figure shows the test results of the bending strength and internal bond strength of the MDF prepared in Example 2 of this invention. Its bending strength is 43.21 (±2.45) MPa, and its internal bond strength is 6.8 (±0.4) MPa. The internal bond strength of this MDF far exceeds the industry requirement of 1 MPa. This indicates that the high-performance MDF provided by this invention can serve as an excellent alternative to high-density fiberboard.

[0041] Example 3

[0042] Preparation of high-performance plywood: The lignin-based adhesive prepared in Example 1 can be used to prepare plywood with poplar boards, with an adhesive application rate of 100 g / m². 2 The hot-pressing conditions are 2 MPa or 4 MPa for 20 min, followed by natural cooling to obtain high-performance plywood. The bending strength of this plywood is still tested using the three-point bending method.

[0043] like Figure 4 As shown, Figure 4 The image shows a comparison of the bending strength test results of the plywood prepared in Example 3 of this invention. Under a hot-pressing condition of 2 MPa, the bending strength of the plywood is 68.14 (±4.2) MPa; under a hot-pressing condition of 4 MPa, the bending strength is 78.77 (±4.8) MPa. The bending strength of this plywood is significantly greater than the 27.86 MPa of commercially available poplar plywood. It meets the national standard GB / T 9846-2015, indicating that this plywood meets the requirements for daily use. Furthermore, the plywood of this invention is formaldehyde-free and has superior performance compared to commercially available poplar plywood, making it an excellent alternative.

[0044] Example 4

[0045] Preparation of lignin-based adhesive: Choline chloride and ethyl vanillin were mixed at a molar ratio of 1:2 and stirred at 90°C for 20 min to obtain a clear and transparent eutectic solvent. 4.5 g of lignin was added to the eutectic solvent, along with 0.1 g of sodium hydroxide catalyst. After stirring for 6 h, when the lignin was completely dispersed in the eutectic solvent, the temperature was raised to 120°C and stirred for 8 h to obtain the lignin-based adhesive. The mass ratio of the eutectic solvent to lignin was 2:1.

[0046] Example 5

[0047] Water resistance and mildew resistance tests. The MDF prepared in Example 2 was immersed in ultrapure water for 24 hours, then removed, dried, and its weight gain (WA) and thickness gain (TS) were tested. Figure 5 As shown, Figure 5This is a comparison chart of the mass expansion rate and thickness expansion rate of the MDF prepared in Example 2 of the present invention. Figure 5 As can be seen, the water resistance of this MDF is far superior to that of Moso Bamboo Board (MBB), Bamboo Powder Board (BFB), and commercially available High-Density Fiberboard (HDF).

[0048] Furthermore, due to the presence of choline chloride and at least one of the hydroxy aromatic aldehydes such as vanillin, ethyl vanillin, o-vanillin, salicylaldehyde, and protocatechuic aldehyde, it also exhibits excellent anti-mold properties. Bamboo boards and bamboo powder boards will develop mold in 4-7 days in a humid environment, while commercially available high-density fiberboard will develop mold in 7-10 days. The density board in Example 2, however, showed no mold growth after 15 days.

[0049] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. Lignin-based adhesive material, characterized in that, The lignin-based adhesive material is obtained from a deep eutectic solvent and lignin; The deep eutectic solvent comprises choline chloride and a natural small-molecule hydroxyl aromatic aldehyde; The natural small-molecule hydroxyl aromatic aldehyde is at least one selected from vanillin, ethyl vanillin, o-vanillin, salicylaldehyde or protocatechuic aldehyde; The molar ratio of the choline chloride to the natural small-molecule hydroxyl aromatic aldehyde is 1:(1-3); The preparation method of the lignin-based adhesive material comprises the following steps: The deep eutectic solvent, the lignin and the alkaline catalyst are reacted at a mass ratio of (1.9-2.1):1:(0-0.05) to obtain the lignin-based adhesive material.

2. The lignin-based adhesive material according to claim 1, characterized in that, The lignin and the natural small-molecule hydroxyl aromatic aldehyde are both derived from biomass raw materials.

3. The method for producing a lignin-based adhesive material according to any one of claims 1 to 2, characterized by, The preparation method of the lignin-based adhesive material comprises the following steps: The deep eutectic solvent, the lignin and the alkaline catalyst are reacted at a mass ratio of (1.9-2.1):1:(0-0.05) to obtain the lignin-based adhesive material.

4. The production method according to claim 3, characterized by, The reaction temperature is 120-140 DEG C, and the reaction time is 8-10 h.

5. The preparation method according to claim 3, characterized in that, The deep eutectic solvent is obtained by mixing the choline chloride and the natural small-molecule hydroxyl aromatic aldehyde; The mixing temperature is 80-100 DEG C, and the mixing time is 20-40 min.

6. Adhesive, characterized in that The lignin-based adhesive material is the lignin-based adhesive material according to any one of claims 1-2. Alternatively, the lignin-based adhesive material comprises the lignin-based adhesive material according to any one of claims 1-2 and an additive.

7. A glued product, characterized in that The wood material is obtained from a wood raw material and an adhesive; The adhesive is the lignin-based adhesive material according to any one of claims 1-2. Alternatively, the adhesive comprises the lignin-based adhesive material according to any one of claims 1-2 and an additive.

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