Green, low-cost and high-performance lignin adhesive as well as preparation method and application thereof

Abstract

The invention discloses a green, low-cost and high-performance lignin adhesive as well as a preparation method and application thereof, and belongs to the technical field of efficient and comprehensive utilization of lignin. According to the preparation method, a deep eutectic solvent (DES) composed of choline chloride, urea, acetic acid and glycerol is adopted to efficiently dissolve, degrade and modify lignin, and the reaction activity of lignin is improved. And in combination with the curing and crosslinking effects of citric acid, a compact network structure is constructed, and the bonding strength and water resistance are remarkably enhanced. The dry strength of the adhesive reaches 4.45 MPa, the wet strength of the adhesive reaches 1.55 MPa, and the dry strength and the wet strength exceed the industrial standard, so that the adhesive is expected to replace formaldehyde-based resin. The raw materials are all renewable biomass, the process is simple, the method is suitable for industrial production, and the product is low in toxicity, degradable and carbon-negative in emission, conforms to the green manufacturing concept and is suitable for the fields of packaging, furniture, buildings and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of efficient comprehensive utilization of lignin, and specifically relates to a green, low-cost, high-performance lignin adhesive and a preparation method and application thereof. Background Art

[0002] Adhesives are widely used in many fields such as building decoration, product packaging, wood processing and home life due to their excellent adhesion and sticking properties. At present, the annual global usage of adhesives is about 22 million tons, and with the booming development of the takeaway, express delivery and decoration industries, its consumption is still on the rise. However, the mainstream adhesives in the market are mostly made of petroleum-based raw materials, which not only consume non-renewable fossil energy, but also involve the release of toxic formaldehyde in their preparation and use. At the same time, the adhesives themselves are difficult to recycle, and natural degradation is difficult and the degradation cycle is long, posing a serious threat to soil, water quality and biological habitats.

[0003] Although some adhesives are made from sustainable raw materials, such as bio-based epoxy resins made from vegetable oils and natural phenolic compounds, polylactic acid-based adhesives derived from biodegradable polymers obtained from renewable resources (such as corn sugar), and natural rubber and resins, these products meet the sustainability of raw materials and meet performance standards, but due to high costs and low yields, they may even cause competition for food resources, making it difficult to achieve large-scale engineering production, market popularization and promotion, and widespread application in multiple fields.

[0004] As a natural resource with abundant reserves and low cost, lignin has the advantages of wide availability, renewability and biocompatibility. Its aromatic compound structure rich in phenolic hydroxyl and methyl oxygen makes it an ideal substitute for petroleum-based material phenol. However, the high degree of polymerization of lignin leads to significant steric hindrance effect, limited reaction activity, poor solubility, and low activity and degree of participation in chemical reactions. Although the current methods for modifying lignin, such as chemical oxidation, hydroxyalkylation, demethylation, phenolation, etc., can improve the performance of lignin to a certain extent, they all require the participation of chemical reagents, the modification effect is poor, and most of the reagents are derived from fossil energy.

[0005] Therefore, exploring an environmentally friendly and efficient means to improve the solubility and reactivity of lignin so that it can achieve large-scale and efficient bonding with adhesive components has become the key to optimizing the performance of lignin adhesives. At the same time, in-depth research on the curing mechanism of adhesive components after lignin has improved solubility and reactivity, as well as overcoming the problem of improving the water resistance of lignin adhesives, is the core and focus of the successful preparation of new green and durable lignin adhesives.

[0006] Existing adhesives have exposed many problems in application that need to be improved, as follows:

[0007] Petroleum-based wood adhesives have obvious unsustainability. It encompasses diverse synthetic polymers such as polyurethanes, epoxy resins, and phenolic resins, all of which are based on non-renewable petroleum resources. This inevitably consumes a vast amount of energy during their extraction and use, having an adverse impact on environmental friendliness and the long-term stable utilization of resources. More severely, when petroleum-based adhesives decompose, they are extremely likely to generate microplastic particles. Such microplastics are difficult to degrade, and they will continuously accumulate and ultimately infiltrate into water bodies and soil, impacting the balance of the ecosystem and even potentially threatening human health. Moreover, in actual use scenarios, this type of adhesive will release toxic volatile substances such as toluene and formaldehyde, which will directly erode human health and seriously pollute indoor air, resulting in a significant decline in air quality.

[0008] Although sustainable wood adhesives adhere to the concept of sustainability, they face the bottlenecks of high cost and poor wet adhesion strength. Their raw material pool includes vegetable oils, epoxy soybean oil, natural proteins (such as soy protein), starches, celluloses, as well as natural rubbers and resins, etc. However, the extraction and processing procedures of these raw materials are quite complex and often require the use of special enzymes or chemicals for purification and modification operations, which undoubtedly increases the complexity of the production process and cost investment. In terms of wet adhesion strength, especially in humid and variable environmental conditions, some bio-based adhesives have their bonding efficacy significantly weakened due to lack of water resistance, making it difficult to meet the requirements for a durable and stable bonding effect in actual applications.

[0009] Lignin-based wood adhesives are confronted with the dilemmas of poor solubility and insufficient reactivity of lignin itself. The lignin structure is complex, with a high degree of polymerization resulting in significant steric hindrance, and at the same time, the scarcity of active functional groups directly reduces its reactivity. Currently, common lignin activation and modification methods, such as demethylation, phenolation, carboxymethylation, acetylation, oxidation, sulfidation, and esterification, etc., have mediocre actual application effects and often require the consumption of a large amount of chemical reagents and fossil energy to achieve the modification purpose.

[0010] In summary, existing adhesives are facing a series of intractable challenges. Among them, issues such as how to conceive a sustainable production formula, accurately select biomass raw materials that can be stably sourced, effectively reduce preparation costs, and comprehensively improve wet adhesion strength are particularly prominent. Regrettably, the current state-of-the-art technical means have not been able to provide a satisfactory answer. This not only severely restricts the expansion path of the high-value utilization of lignin but also greatly hinders the process of using lignin as a breakthrough to replace petroleum-based materials to create green and environmentally friendly adhesives. Therefore, successfully developing a wood adhesive with the advantages of sustainability, purchasability, low cost, and high performance has become an urgent and extremely practical problem to be solved in this field. Summary of the Invention

[0011] The present invention is committed to providing a green, low-cost, and high-performance lignin adhesive and its preparation method. This method uses sustainable and procured green components such as choline chloride, urea, acetic acid, glycerol, and citric acid as core raw materials. Through the dissolution, degradation, and chemical modification of lignin by deep eutectic solvents (DES), the solubility and reactivity of lignin are enhanced. At the same time, the curing and cross-linking effects of citric acid are utilized to construct a firm network structure, enhancing the water resistance and bonding strength of the adhesive.

[0012] The technical problems solved by the present invention and its technical solutions are as follows:

[0013] (1) The high degree of polymerization and hydrophobic insolubility of lignin result in low reactivity and insufficient binding sites, restricting the performance of the adhesive. To address the existing problems, it is proposed to prepare a deep eutectic solvent (DES). Choline chloride provides ions and forms a liquid, while glycerol, acetic acid, and urea provide hydrogen bond binding sites. Through dissociation, changing the secondary structure of lignin, reducing the cluster effect, and constructing a hydrogen bond network structure, etc., DES significantly enhances the solubility and reactivity of lignin.

[0014] (2) In a humid environment, the exposed hydrophilic groups such as hydroxyl and carboxyl groups in the lignin adhesive form hydrogen bonds with water molecules, weakening the bonding strength between the adhesive and the substrate. To address the existing problems, it is proposed to use citric acid as a curing agent. Its multiple carboxyl groups can further construct a hydrogen bond network structure to enhance the bonding strength. At the same time, the hydrophilic end of citric acid participates in the curing and adhesion processes, while its hydrophobic end reduces the water absorption of the adhesive due to its low surface energy, improving the stability in a humid environment.

[0015] The present invention provides a green, low-cost, and high-performance lignin adhesive, which uses lignin as the adhesive raw material, a deep eutectic solvent prepared by mixing at least two of choline chloride, acetic acid, urea, and glycerol as the lignin solvent, and citric acid as the curing agent. First, at least two of choline chloride, acetic acid, urea, and glycerol are mixed in proportion and stirred evenly to prepare a deep eutectic solvent. Then, lignin is dissolved in the deep eutectic solvent to prepare a lignin solution. Finally, citric acid is added to the lignin solution and heated to prepare the green, low-cost, and high-performance lignin adhesive.

[0016] The present invention also provides a preparation method for the above-mentioned green, low-cost, and high-performance lignin adhesive, including the following steps:

[0017] S1. Mix at least two of choline chloride, acetic acid, urea, and glycerol in proportion, stir and mix evenly to prepare a deep eutectic solvent;

[0018] S2. Add lignin to the deep eutectic solvent obtained in step S1, stir to fully dissolve the lignin, and prepare a lignin solution;

[0019] S3. Add citric acid to the lignin solution obtained in step S2 and stir to make it react fully, then the green, low-cost and high-performance lignin adhesive can be obtained.

[0020] Further, the temperature in step S1 is 90 °C, the stirring speed is 300 r / min, and the stirring time is 10 min.

[0021] Further, in step S1, when preparing the deep eutectic solvent with choline chloride and acetic acid, the molar ratio of choline chloride to acetic acid is 1:2; when preparing the deep eutectic solvent with choline chloride and glycerol, the molar ratio of choline chloride to glycerol is 2:3; when preparing the deep eutectic solvent with choline chloride and urea, the molar ratio of choline chloride to urea is 1:2; when preparing the deep eutectic solvent with choline chloride, glycerol and acetic acid, the molar ratio of choline chloride, glycerol and acetic acid is 2:2:1.

[0022] Further, in step S2, the mass ratio of lignin to the deep eutectic solvent is 1:1; in step S3, the mass ratio of the lignin solution to citric acid is 3:1.

[0023] Further, the temperature in step S2 is 105 °C, the stirring speed is 300 r / min, and the stirring time is 6 hours.

[0024] Further, the temperature in step S3 is 120 °C, the stirring speed is 300 r / min, and the reaction time is 6 hours.

[0025] In addition, the present invention also provides the application of the green, low-cost and high-performance lignin adhesive as a wood adhesive. Coat the green, low-cost and high-performance lignin adhesive on the wood surface, and the best adhesion effect can be achieved through hot pressing; the coating amount is 150 - 200 g / m 2 , and the hot pressing conditions are: 170 °C, 1.0 MPa, 60 min.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) Green environmental protection and resource sustainability: All components of the raw materials used are biomass materials, which have the characteristics of being cheap, easy to obtain, renewable and sustainable, effectively reducing the dependence on non-renewable resources and conforming to the global green environmental protection development trend. This not only helps to protect the environment but also promotes the sustainable use of resources.

[0028] (2) Excellent adhesive properties: The prepared lignin-based adhesive has excellent properties, comparable to or even exceeding commercial fossil-based adhesives, meeting industrial standards. Its dry strength of 4.45 MPa and wet strength of 1.55 MPa fully exceed the industrial threshold of 0.7 MPa and are comparable to the standards of commercially available urea-formaldehyde resins, providing a practical alternative to formaldehyde-based resins and promising to replace traditional adhesives in many fields.

[0029] (3) Mild process and industrial compatibility: Different from previous methods that required a large amount of pretreatment or irritating chemicals, the strategy of this invention is carried out under mild conditions and is compatible with existing manufacturing infrastructure. The process is simple, easy to operate and control, very suitable for industrial scale production, helping to reduce production costs and improve production efficiency.

[0030] (4) Environmentally friendly and healthy safety: The prepared lignin adhesive has environmental degradability, extremely low toxicity and carbon negative emissions, fully meeting the standards of green manufacturing and healthy materials. It has minimal harm to the environment and human health during use, helping to promote sustainable development and the application of healthy and safe materials.

[0031] (5) Interdisciplinary integration and industry transformation: Integrating green chemistry, materials engineering and circular economy design interdisciplinaryly, it successfully bridges the long-term gap between lignin research and industrial deployment. This innovative measure not only promotes the high-value utilization of lignin, but also promotes the systematic transformation of the adhesive industry. This invention provides a set of scalable, cost-controllable and environmentally friendly biomass materials engineering solutions, promising to be widely applied in many fields such as packaging, furniture, and construction, providing new ideas and ways for the sustainable development of related industries. Detailed implementation manners

[0032] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0033] Example 1

[0034] A preparation method and application of a green, low-cost and high-performance lignin adhesive are as follows:

[0035] (1) Preparation of choline chloride - acetic acid combined deep eutectic solvent (DES): According to the molar ratio of choline chloride to acetic acid being 1:2, with the molecular weight of choline chloride being 139.6 and that of acetic acid being 60.05, 100 g of choline chloride and 86 g of acetic acid (about 82 mL in volume) were placed in a 1000 mL conical flask, and magnetic stirring was carried out using a 4 cm C - type rotor. After heating in an oil bath at 90 °C at a speed of 300 r / min for 10 min, a colorless transparent solution was obtained;

[0036] (2) Dissolving lignin to obtain a lignin solution: Industrial lignin was weighed according to the mass ratio of lignin to the deep eutectic solvent being 1:1 and added to the DES. It was placed in an oil bath at 105 °C and stirred at a speed of 300 r / min for 6 hours to obtain a brown lignin solution;

[0037] (3) Preparation of viscous adhesive: Citric acid was weighed according to the mass ratio of the lignin solution to citric acid being 3:1 and added to the above - obtained lignin solution. It was placed in an oil bath at 120 °C and reacted at a speed of 300 r / min for 6 hours to obtain a viscous adhesive;

[0038] (4) Adhering wood chips: The wood was cut into sheet specifications of 1.5 mm × 25 mm × 100 mm. Before use, the viscous adhesive was appropriately heated to reduce viscosity and enhance fluidity, and it was coated at a dosage of 150 g / m 2 and placed in a vulcanizer for hot pressing. The parameter conditions were: 170 °C, 1.0 Mpa, 60 min; The prepared adhered wood chips were left at room temperature for 24 hours for subsequent testing;

[0039] (5) Mechanical property testing: First, the thickness of the glue was measured with a thickness gauge, and the length and width of the adhesion area were measured. Then, the adhered wood chips were soaked in warm water at 60 °C for 3 hours, taken out and air - dried at room temperature for 10 minutes, and then the mechanical properties were tested using an electronic universal material testing machine. Multiple data were tested for replicated samples. If testing the dry adhesion strength, no soaking treatment was required and it could be directly measured. The dry adhesion strength was: 1.936 ± 0.247 MPa; The wet adhesion strength was: 1.095 ± 0.126 Mpa.

[0040] The obtained lignin adhesive can be completely degraded after 120 days when landfilled in the natural environment (humid soil).

[0041] Example 2

[0042] A preparation method and application of a green, low - cost, and high - performance lignin adhesive are as follows:

[0043] (1) Preparation of choline chloride and glycerol combined deep eutectic solvent (DES): According to the molar ratio of choline chloride to glycerol of 2:3, with the molecular weight of choline chloride being 139.6 and the molecular weight of glycerol being 92.09, 100 g of choline chloride and 98.95 g of glycerol (about 77.3 mL in volume) were placed in a 1000 mL conical flask, and magnetic stirring was carried out using a 4 cm C-type rotor. After heating in an oil bath at 90 °C for 10 min at a rotation speed of 300 r / min, a colorless transparent solution was obtained;

[0044] (2) Dissolving lignin to obtain a lignin solution: Industrial lignin was weighed according to the mass ratio of lignin to deep eutectic solvent of 1:1 and added to DES. It was placed in an oil bath at 105 °C and stirred at a rotation speed of 300 r / min for 6 hours to obtain a brown lignin solution;

[0045] (3) Preparation of viscous adhesive: Citric acid was weighed according to the mass ratio of lignin solution to citric acid of 3:1 and added to the above-obtained lignin solution. It was placed in an oil bath at 120 °C and reacted at a rotation speed of 300 r / min for 6 hours to obtain a viscous adhesive;

[0046] (4) Adhering wood chips: The wood was cut into sheet specifications of 1.5 mm × 25 mm × 100 mm. Before use, the viscous adhesive was appropriately heated to reduce viscosity and enhance fluidity, and it was applied at a quantitative rate of 175 g / m 2 and placed in a vulcanizer for hot pressing. The parameter conditions were: 170 °C, 1.0 Mpa, 60 min; The prepared adhered wood chips were placed at room temperature for 24 hours for testing;

[0047] (5) Mechanical property testing: First, the thickness of the glue was measured with a thickness gauge, and the length and width of the adhesion area were measured. Then, the adhered wood chips were immersed in warm water at 60 °C for 3 hours, taken out and air-dried at room temperature for 10 minutes, and then the mechanical properties were tested using an electronic universal material testing machine. Multiple data were tested for the prepared repeated samples. If testing the dry adhesion strength, no immersion treatment was required and it could be directly measured. The dry adhesion strength was: 2.517 ± 0.328 MPa; The wet adhesion strength was: 1.127 ± 0.172 Mpa.

[0048] The obtained lignin adhesive can be completely degraded after 120 days when landfilled in the natural environment (humid soil).

[0049] Example 3

[0050] A preparation method and application of a green, low-cost, and high-performance lignin adhesive are as follows:

[0051] (1) Preparation of a choline chloride / urea combined deep eutectic solvent (DES): According to the molar ratio of choline chloride:urea of ​​1:2, and the molecular weight of choline chloride is 139.6, and the molecular weight of urea is 60.06, 100 g of choline chloride and 86 g of urea are placed in a 1000 mL conical flask, and a 4 cm C-type rotor is used for magnetic stirring. After heating at 300 r / min in a 90°C oil bath for 10 min, a colorless and transparent solution is obtained;

[0052] (2) Dissolving lignin to obtain a lignin solution: weigh industrial lignin and add it to DES in a mass ratio of lignin to low eutectic solvent of 1:1, place it in an oil bath at 105°C and stir at 300 r / min for 6 hours to obtain a brown lignin solution;

[0053] (3) Preparation of viscous adhesive: citric acid was weighed and added to the lignin solution obtained above in a mass ratio of lignin solution to citric acid of 3:1, and the mixture was placed in an oil bath at 120°C and reacted at a speed of 300 r / min for 6 hours to obtain a viscous adhesive;

[0054] (4) Adhesion of wood chips: Cut the wood into sheets of 1.5 mm × 25 mm × 100 mm. Before use, heat the viscous adhesive appropriately to reduce viscosity and enhance fluidity. 2 The prepared adhesive wood chips were coated with a certain amount of the adhesive and placed in a vulcanizer for hot pressing at 170°C, 1.0 Mpa, and 60 min. The prepared adhesive wood chips were placed at room temperature for 24 hours before being tested.

[0055] (5) Mechanical properties test: First use a thickness gauge to test the glue thickness, measure the length and width of the adhesive area, and then soak the adhesive wood pieces in 60°C warm water for 3 hours. After taking them out, air-dry them at room temperature for 10 minutes. Then use an electronic universal material testing machine to test their mechanical properties. Prepare repeated samples to test multiple data. If testing their dry adhesion strength, there is no need for soaking treatment and they can be measured directly. The dry adhesion strength is: 3.068 ± 0.372 MPa; the wet adhesion strength is: 0.805 ± 0.103 Mpa.

[0056] The obtained lignin adhesive can be completely degraded after 120 days when landfilled in the natural environment (moist soil).

[0057] Example 4

[0058] A preparation method and application of a green, low-cost, high-performance lignin adhesive are as follows:

[0059] (1)Prepare the choline chloride, glycerol, acetic acid combined deep eutectic solvent (DES): According to the molar ratio of choline chloride: glycerol: acetic acid being 2:2:1, and the molecular weight of choline chloride being 139.6, the molecular weight of glycerol being 92.09, and the molecular weight of acetic acid being 60.05. Then, place 100 g of choline chloride, 65.97 g of glycerol, and 21.51 g of urea into a 1000 mL conical flask, use a 4 cm C-type rotor for magnetic stirring, and heat in an oil bath at 90 °C at a rotation speed of 300 r / min for 10 min to obtain a colorless transparent solution;

[0060] (2)Dissolve lignin to obtain a lignin solution: Weigh industrial lignin according to the mass ratio of lignin to the deep eutectic solvent being 1:1, add it to the DES, place it in an oil bath at 105 °C, and stir at a rotation speed of 300 r / min for 6 hours to obtain a brown lignin solution;

[0061] (3)Prepare a viscous adhesive: Weigh citric acid according to the mass ratio of the lignin solution to citric acid being 3:1, add it to the above-mentioned obtained lignin solution, and place it in an oil bath at 120 °C and react at a rotation speed of 300 r / min for 6 hours to obtain a viscous adhesive;

[0062] (4)Adhere wood chips: Cut the wood into sheet specifications of 1.5 mm × 25 mm × 100 mm. Before use, appropriately heat the viscous adhesive to reduce its viscosity and enhance its fluidity, apply it quantitatively at 200 g / m 2 and place it in a vulcanizer for hot pressing. The parameter conditions are: 170 °C, 1.0 Mpa, 60 min; Let the prepared adhered wood chips stand at room temperature for 24 hours for testing use;

[0063] (5)Mechanical property test: First, use a thickness gauge to measure the thickness of the glue, measure the length and width of the adhesion area, then soak the adhered wood chips in warm water at 60 °C for 3 hours, take them out and air dry at room temperature for 10 minutes, and then use an electronic universal material testing machine to test their mechanical properties. Prepare repeated samples to test multiple data. If testing its dry adhesion strength, no soaking treatment is required and it can be directly measured. The obtained dry adhesion strength is: 4.416 ± 0.532 MPa; The wet adhesion strength is: 1.543 ± 0.277 Mpa.

[0064] The obtained lignin adhesive can be completely degraded after 120 days when landfilled in the natural environment (humid soil).

Claims

1. A green, low-cost, and high-performance lignin adhesive, characterized in that, Using lignin as the raw material of the adhesive, a deep eutectic solvent prepared by mixing at least two of choline chloride, acetic acid, urea and glycerol as the lignin solvent, and citric acid as the curing agent; first, at least two of choline chloride, acetic acid, urea and glycerol are mixed to prepare a deep eutectic solvent, then lignin is dissolved in the deep eutectic solvent to prepare a lignin solution, and finally citric acid is added to the lignin solution and heated to prepare the green, low-cost and high-performance lignin adhesive.

2. A preparation method of a green, low-cost and high-performance lignin adhesive, characterized in that, It includes the following steps: S1. Mix at least two of choline chloride, acetic acid, urea and glycerol in proportion, stir and mix evenly to prepare a deep eutectic solvent; S2. Add lignin to the deep eutectic solvent obtained in step S1, stir to fully dissolve the lignin, and prepare a lignin solution; S3. Add citric acid to the lignin solution obtained in step S2, stir to make it react fully, and the green, low-cost and high-performance lignin adhesive can be obtained.

3. The preparation method according to claim 2, characterized in that, The temperature of step S1 is 90 °C, the stirring speed is 300 r / min, and the stirring time is 10 min.

4. The preparation method according to claim 2, characterized in that, In step S1, when preparing the deep eutectic solvent with choline chloride and acetic acid, the molar ratio of choline chloride to acetic acid is 1:2; when preparing the deep eutectic solvent with choline chloride and glycerol, the molar ratio of choline chloride to glycerol is 2:3; when preparing the deep eutectic solvent with choline chloride and urea, the molar ratio of choline chloride to urea is 1:2; when preparing the deep eutectic solvent with choline chloride, glycerol and acetic acid, the molar ratio of choline chloride, glycerol and acetic acid is 2:2:

1.

5. The preparation method according to claim 2, characterized in that, In step S2, the mass ratio of lignin to the deep eutectic solvent is 1:1; in step S3, the mass ratio of the lignin solution to citric acid is 3:

1.

6. The preparation method according to claim 2, characterized in that, The temperature of step S2 is 105 °C, the stirring speed is 300 r / min, and the stirring time is 6 hours.

7. The preparation method according to claim 2, characterized in that, The temperature of step S3 is 120 °C, the stirring speed is 300 r / min, and the reaction time is 6 hours.

8. Application of the green, low-cost and high-performance lignin adhesive described in claim 1 or the green, low-cost and high-performance lignin adhesive prepared by the preparation method described in claims 2-7 as a wood adhesive.

9. The application according to claim 8, wherein Coat the green, low-cost and high-performance lignin adhesive on the wood surface, and achieve the best adhesion effect through hot pressing.

10. The application according to claim 8, wherein The coating amount of the green, low-cost and high-performance lignin adhesive is 150-200 g / m 2 , and the hot pressing conditions are: 170 °C, 1.0 MPa, 60 min.