A green, low-cost, high-performance lignin adhesive and a preparation method and application thereof

Abstract

The application 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 comprehensive utilization of lignin. The preparation method adopts a deep eutectic solvent (DES) composed of choline chloride, urea, acetic acid and glycerol to efficiently dissolve, degrade and modify lignin and improve the reaction activity of the lignin. In combination with the solidification and crosslinking effects of citric acid, a dense network structure is constructed, and the bonding strength and water resistance are significantly enhanced. The dry strength of the adhesive reaches 4.45 MPa, and the wet strength reaches 1.55 MPa, which exceeds the industrial standard and is expected to replace formaldehyde-based resins. The raw materials used are all renewable biomass, the process is simple, and the product is suitable for industrial production. The product is low-toxic, degradable and carbon-negative, conforms to the green manufacturing concept, and is suitable for fields such as packaging, furniture and building.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of efficient comprehensive utilization of lignin, and particularly relates to a green, low-cost and high-performance lignin adhesive as well as a preparation method and application thereof. BACKGROUND

[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 pasting performance. At present, the global annual consumption of adhesives is about 22 million tons, and with the vigorous development of take-out, express delivery and decoration industries, the consumption is still on the rise. However, most of the mainstream adhesives on the market are prepared from petroleum-based raw materials, which not only consumes non-renewable fossil energy, but also involves the release of toxic substances such as formaldehyde in the preparation and use process. In addition, the adhesives themselves are difficult to recycle, and natural degradation is difficult and the degradation period is long, which poses a serious threat to the soil, water quality and biological habitat.

[0003] Although some adhesives are prepared from sustainable raw materials, such as bio-based epoxy resins using plant oils and natural phenolic compounds as raw materials, and polylactic acid-based adhesives derived from biodegradable polymers obtained from renewable resources (such as corn sugar), natural rubber and resin, etc. These products meet the requirements of sustainable raw materials and performance, but due to high cost, low yield, and even the possibility of causing food resource competition, it is difficult to achieve engineering scale production, market popularization and promotion, and wide application in many fields.

[0004] Lignin is a natural resource with abundant reserves and low cost, and has advantages such as wide availability, renewability and biocompatibility. Its aromatic compound structure rich in phenolic hydroxyl and methyl oxygen makes it an ideal substitute for petroleum-based phenol. However, the high degree of polymerization of lignin leads to significant steric hindrance effect, limited reactivity, poor solubility, and low activity and degree of participation in chemical reactions. Current methods for modifying lignin, such as chemical oxidation, hydroxyalkylation, demethylation, and phenolation, can improve the performance of lignin to some extent, but they all require chemical reagents, and the modification effect is not good, 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 be bonded with adhesive components in large quantities and efficiently is the key to optimizing the performance of lignin adhesives. At the same time, in-depth study of the curing mechanism of adhesive components after lignin solubility and reactivity is improved, and the difficulty of improving the water resistance of lignin adhesives, is the core and key to successfully preparing new green and durable lignin adhesives.

[0006] The existing adhesives have exposed many problems in application that need to be improved, which are as follows:

[0007] Oil-based wood adhesives have obvious unsustainability. They cover a variety of synthetic polymers such as polyurethane, epoxy resin, and phenolic resin, all of which are based on non-renewable petroleum resources. This inevitably consumes a large amount of energy in the process of its mining and use, and has a negative impact on environmental friendliness and long-term stable use of resources. More seriously, oil-based adhesives are extremely easy to generate microplastic particles when decomposed. Such microplastics have the characteristics of being difficult to degrade, and they will continue to accumulate and eventually penetrate into water and soil, causing impact on the balance of the ecosystem, and even potential risks to human health. Moreover, in actual use scenarios, such adhesives release toxic volatile substances such as toluene and formaldehyde, which directly erode human health and seriously pollute indoor air, causing a significant decline in air quality.

[0008] Sustainable wood adhesives, although adhering to the concept of sustainability, face the bottleneck of high cost and poor wet adhesion strength. The raw material pool includes plant oil, epoxy soybean oil, natural protein (such as soybean protein), starch, cellulose, and natural rubber and resin, etc. However, the extraction and processing procedures of these raw materials are quite complicated, often requiring the use of special enzymes or chemicals for purification and modification operations, which undoubtedly increases the complexity and cost of the production process. In terms of wet adhesion strength, especially in humid and variable environmental conditions, some bio-based adhesives lack water resistance, and their bonding effectiveness is greatly weakened, making it difficult to meet the requirements of long-term stable bonding effect in actual applications.

[0009] Lignin-based wood adhesives are hindered by the poor solubility and insufficient reactivity of lignin itself. Lignin has a complex structure, high polymerization degree, significant steric hindrance, and a lack of active functional groups, which directly reduces its reactivity. Current common lignin activation and modification methods, such as demethylation, phenolation, carboxymethylation, acetylation, oxidation, sulfuration, and esterification, have unsatisfactory practical application results, often requiring a large amount of chemical reagents and fossil energy to achieve the modification purpose.

[0010] In summary, existing adhesives are facing a series of challenging problems, including how to conceive a sustainable production formula, accurately select stablely procurable biomass raw materials, effectively reduce production costs, and improve wet adhesion strength in all directions. Unfortunately, current technical means have not yet been able to provide satisfactory answers. This not only severely restricts the expansion path of lignin high-value utilization, but also greatly hinders the process of replacing petroleum-based materials with lignin as a breakthrough to create green and environmentally friendly adhesives. Therefore, successfully developing a wood adhesive with the advantages of sustainability, procurability, low cost, and high performance has become a difficult problem that needs to be overcome and has great practical significance in the field. SUMMARY

[0011] The present application is devoted to providing a green, low-cost and high-performance lignin adhesive and its preparation method. The method takes the green components of sustainable procurement, such as choline chloride, urea, acetic acid, glycerol and citric acid, as the core raw materials, and through the dissolution, degradation and chemical modification of the low eutectic solvent (DES) on the lignin, the solubility and reactivity of the lignin are improved; at the same time, the solidification and crosslinking effect of citric acid is used to build a firm network structure, and the water resistance and adhesive strength of the adhesive are enhanced.

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

[0013] (1) The high polymerization degree of lignin, the hydrophobic and difficult solubility, lead to low reactivity and insufficient binding sites, which limit the performance of the adhesive. In view of the existing problems, the low eutectic solvent (DES) is prepared, choline chloride provides ions and forms a liquid, glycerol, acetic acid and urea provide hydrogen bonding sites. DES can significantly improve the solubility and reactivity of lignin by dissociation, changing the secondary structure of lignin, reducing the cluster effect and building hydrogen bond network structure.

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

[0015] The present application provides a green, low-cost and high-performance lignin adhesive, which is prepared by mixing choline chloride with at least one of acetic acid, urea and glycerol to prepare a low eutectic solvent, dissolving industrial lignin in the low eutectic solvent to prepare a lignin solution, and then adding citric acid to the lignin solution and heating to obtain the green, low-cost and high-performance lignin adhesive.

[0016] The present application also provides a preparation method of the green, low-cost and high-performance lignin adhesive, which comprises the following steps:

[0017] S1. Mix choline chloride with at least one of acetic acid, urea and glycerol in proportion, and stir to mix uniformly to prepare a low eutectic solvent;

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

[0019] S3. Adding citric acid to the lignin solution obtained in step S2, and stirring to fully react, to obtain the green, low-cost and high-performance lignin adhesive.

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

[0021] Further, in step S1, when the eutectic solvent is prepared by using choline chloride and acetic acid, the molar ratio of choline chloride to acetic acid is 1:2; when the eutectic solvent is prepared by using choline chloride and glycerol, the molar ratio of choline chloride to glycerol is 2:3; when the eutectic solvent is prepared by using choline chloride and urea, the molar ratio of choline chloride to urea is 1:2; when the eutectic solvent is prepared by using 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 industrial lignin to eutectic solvent is 1:1; and in step S3, the mass ratio of lignin solution to citric acid is 3:1.

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

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

[0025] In addition, the application further provides the use of the green, low-cost and high-performance lignin adhesive as a wood adhesive. The green, low-cost and high-performance lignin adhesive is applied to the surface of wood, and the best adhesion effect is achieved through hot pressing; the application amount is 150-200 g / m 2 , and the hot pressing conditions are 170 DEG C, 1.0 MPa and 60 min.

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

[0027] (1) Green environmental protection and resource sustainability: all components of the raw materials used are biomass materials, which are cheap, renewable and sustainable, effectively reducing the dependence on non-renewable resources, and meeting 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 performance: The prepared lignin-based adhesive has excellent performance, which is comparable or even superior to commercial fossil-based adhesives, meeting industrial standards. It exhibits a dry strength of 4.45 MPa and a wet strength of 1.55 MPa, which fully exceeds the industrial threshold of 0.7 MPa, is comparable to the standard of commercially available urea-formaldehyde resin, and provides a feasible alternative to formaldehyde-based resins, which is expected to replace traditional adhesives in many fields.

[0029] (3) Mild process and industrial compatibility: Unlike previous methods that require extensive pretreatment or irritating chemicals, the present strategy is carried out under mild conditions and is compatible with existing manufacturing infrastructure. The process is simple, easy to operate and control, and is very suitable for industrial scale production, which helps to reduce production cost and improve production efficiency.

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

[0031] (5) Interdisciplinary integration and industry transformation: The interdisciplinary integration of green chemistry, materials engineering and circular economy design successfully bridges the long gap between lignin research and industrial deployment. This innovative initiative not only promotes the high-value utilization of lignin, but also promotes the systematic transformation of the adhesive industry. The present application provides an expandable, cost-controlled and environmentally friendly solution for biomass material engineering, which is expected to be widely used in packaging, furniture, building and other fields, and provides a new idea and approach for the sustainable development of related industries. DETAILED DESCRIPTION

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

[0033] Example 1

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

[0035] (1) Preparation of choline chloride, acetic acid combination deep eutectic solvent (DES): according to the molar ratio of choline chloride: acetic acid is 1:2, and the molecular weight of choline chloride is 139.6, and the molecular weight of acetic acid is 60.05, so 100 g of choline chloride and 86 g of acetic acid (about 82 mL volume) are placed in a 1000 mL conical flask, and a 4 cm C-shaped rotor is used for magnetic stirring, and a colorless transparent solution is obtained after heating at 300 r / min for 10 min in a 90°C oil bath;

[0036] (2) Dissolution of lignin to obtain lignin solution: according to the mass ratio of lignin to deep eutectic solvent is 1:1, industrial lignin is weighed and added into DES, and placed in a 105°C oil bath with a stirring speed of 300 r / min for 6 hours, a brown lignin solution is obtained;

[0037] (3) Preparation of viscous adhesive: according to the mass ratio of lignin solution to citric acid is 3:1, citric acid is weighed and added into the above obtained lignin solution, and placed in a 120°C oil bath with a stirring speed of 300 r / min for 6 hours, a viscous adhesive is obtained;

[0038] (4) Adhesion of wood pieces: cut the wood into 1.5 mm × 25 mm × 100 mm pieces, before use, the viscous adhesive is appropriately heated to reduce the viscosity and enhance the flowability, and is coated with a dosage of 150 g / m 2

[0039] (5) Mechanical property test: first, test the adhesive thickness with a thickness gauge, measure the length and width of the adhesion area, then place the adhesion wood pieces in 60°C warm water for 3 hours, take them out and place them in room temperature air for 10 minutes, then test their mechanical properties using an electronic universal material testing machine, repeat the sample preparation to test multiple data, if the dry adhesion strength is tested, no soaking treatment is needed, and the dry adhesion strength can be directly measured, the dry adhesion strength is 1.936 ± 0.247 MPa, and the wet adhesion strength is 1.095 ± 0.126 Mpa.

[0040] The obtained lignin adhesive can be completely degraded in a natural environment (wet soil) after 120 days.

[0041] Example 2

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

[0043] ​(1) Preparation of choline chloride, glycerol combined eutectic solvent (DES): according to the molar ratio of choline chloride: glycerol is 2:3, and the molecular weight of choline chloride is 139.6, and the molecular weight of glycerol is 92.09, so 100 g of choline chloride and 98.95 g of glycerol (about 77.3 mL volume) are placed in a 1000 mL conical flask, and a 4 cm C-shaped rotor is used for magnetic stirring, and a colorless transparent solution is obtained after heating at 300 r / min for 10 min in a 90°C oil bath;

[0044] (2) Dissolution of lignin to obtain lignin solution: according to the mass ratio of lignin to eutectic solvent is 1:1, industrial lignin is weighed and added into the DES, and placed in a 105°C oil bath with a stirring speed of 300 r / min for 6 hours, a brown lignin solution is obtained;

[0045] (3) Preparation of viscous adhesive: according to the mass ratio of lignin solution to citric acid is 3:1, citric acid is weighed and added into the above obtained lignin solution, and placed in a 120°C oil bath with a stirring speed of 300 r / min for 6 hours, a viscous adhesive is obtained;

[0046] (4) Adhesion of wood pieces: cut the wood into 1.5 mm × 25 mm × 100 mm pieces, before use, the viscous adhesive is heated appropriately to reduce the viscosity and enhance the flowability, and coated with a dosage of 175 g / m 2

[0047] (5) Mechanical property test: first, test the adhesive thickness with a thickness gauge, measure the length and width of the adhesion area, then place the adhesion wood pieces in 60°C warm water for 3 hours, take them out and place them in room temperature air for 10 minutes, then test their mechanical properties using an electronic universal material testing machine, repeat the sample preparation to test multiple data, if the dry adhesion strength is tested, no soaking treatment is needed, and the dry adhesion strength can be directly measured, the dry adhesion strength is 2.517 ± 0.328 MPa, and the wet adhesion strength is 1.127 ± 0.172 Mpa.

[0048] The obtained lignin adhesive can be completely degraded in a natural environment (wet soil) after 120 days.

[0049] Example 3

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

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

[0052] (2) Dissolution of lignin to obtain lignin solution: according to the mass ratio of lignin to deep eutectic solvent is 1:1, the industrial lignin is weighed and added into the DES, and placed in a 105°C oil bath with a stirring speed of 300 r / min for 6 hours, and a brown lignin solution is obtained;

[0053] (3) Preparation of viscous adhesive: according to the mass ratio of lignin solution to citric acid is 3:1, citric acid is weighed and added into the above obtained lignin solution, and placed in a 120°C oil bath with a stirring speed of 300 r / min for 6 hours, and a viscous adhesive is obtained;

[0054] (4) Adhesion of wood pieces: the wood is cut into a piece with a size of 1.5 mm × 25 mm × 100 mm, the viscous adhesive is appropriately heated to reduce the viscosity and enhance the flowability before use, and is coated with a dosage of 180 g / m 2

[0055] (5) Mechanical property test: first, the thickness of the adhesive is tested with a thickness gauge, the length and width of the adhesion area are measured, then the adhesion wood piece is soaked in 60°C warm water for 3 hours, and after taking it out, it is placed in room temperature air for 10 minutes, and then the mechanical properties are tested with an electronic universal material testing machine, and multiple data are tested by preparing repeated samples, if the dry adhesion strength is tested, no soaking treatment is needed, and the dry adhesion strength can be directly measured, and the dry adhesion strength is 3.068 ± 0.372 MPa, and the wet adhesion strength is 0.805 ± 0.103 Mpa.

[0056] The obtained lignin adhesive can be completely degraded in a natural environment (wet soil) after 120 days.

[0057] Example 4

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

[0059] ​(1) Preparation of choline chloride, glycerol, acetic acid combination deep eutectic solvent (DES): according to the molar ratio of choline chloride: glycerol, acetic acid is 2:2:1, and the molecular weight of choline chloride is 139.6, the molecular weight of glycerol is 92.09, and the molecular weight of acetic acid is 60.05, so 100 g of choline chloride, 65.97 g of glycerol and 21.51 g of urea are placed in a 1000 mL conical flask, a 4 cm C-shaped rotor is used for magnetic stirring, and a colorless transparent solution is obtained after heating at 300 r / min for 10 min in a 90°C oil bath;

[0060] (2) Dissolution of lignin to obtain lignin solution: according to the mass ratio of lignin to deep eutectic solvent is 1:1, industrial lignin is weighed and added into DES, and placed in a 105°C oil bath with a stirring speed of 300 r / min for 6 hours, a brown lignin solution is obtained;

[0061] (3) Preparation of viscous adhesive: according to the mass ratio of lignin solution to citric acid is 3:1, citric acid is weighed and added into the above obtained lignin solution, and placed in a 120°C oil bath with a stirring speed of 300 r / min for 6 hours, a viscous adhesive is obtained;

[0062] (4) Adhesion of wood pieces: wood is cut into 1.5 mm × 25 mm × 100 mm pieces, the viscous adhesive is appropriately heated to reduce the viscosity and enhance the flowability before use, and is coated at a rate of 200 g / m 2 , and is placed in a vulcanizing machine for hot pressing with the following parameters: 170°C, 1.0 Mpa, 60 min; the prepared adhesion wood pieces are placed at room temperature for 24 hours before testing;

[0063] (5) Mechanical property test: first, test the adhesive thickness with a thickness gauge, measure the length and width of the adhesion area, then place the adhesion wood pieces in a 60°C water bath for 3 hours, take them out and place them in a room temperature air dryer for 10 minutes, then test their mechanical properties using an electronic universal material testing machine, repeat the sample preparation to test multiple data, if the dry adhesion strength is tested, no soaking treatment is needed, and the dry adhesion strength can be directly measured, which 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 in a natural environment (wet soil) after 120 days.

Claims

1. A green, low-cost, high-performance lignin adhesive, characterized in that, Industrial lignin is used as an adhesive raw material, a eutectic solvent composed of choline chloride and urea is used as a dissolving-activating agent, and citric acid is used as a subsequent curing-crosslinking agent; first, the eutectic solvent is prepared by mixing choline chloride and urea, industrial lignin is mixed and dissolved with the eutectic solvent at 105 DEG C, without separation and purification, citric acid is directly added and reacted at 120 DEG C, so that the eutectic solvent and lignin form a homogeneous fluid adhesive together; The dry adhesion strength of the adhesive to wood chips is 3.068±0.372 MPa, the wet adhesion strength is 0.805±0.103 MPa under the hot-pressing conditions of 170 DEG C, 1.0 MPa and 60 min, and the adhesive can be completely degraded in moist soil for 120 days.

2. A method for preparing green, low-cost, high-performance lignin adhesive, characterized in that, The method comprises the following steps: S1. choline chloride and urea are mixed in proportion, stirred and uniformly mixed to prepare a eutectic solvent; S2. industrial lignin is added to the eutectic solvent obtained in step S1, and stirred to fully dissolve the industrial lignin, to prepare a lignin solution; S3. citric acid is added to the lignin solution obtained in step S2, and stirred to fully react, to obtain the green, low-cost and high-performance lignin adhesive; The temperature in step S1 is 90 DEG C, the stirring speed is 300 r / min, and the stirring time is 10 min; In step S1, when the eutectic solvent is prepared from choline chloride and urea, the molar ratio of choline chloride to urea is 1:2; In step S2, the mass ratio of industrial lignin to eutectic solvent is 1:1; in step S3, the mass ratio of lignin solution to citric acid is 3:1; The temperature in step S2 is 105 DEG C, the stirring speed is 300 r / min, and the stirring time is 6 hours; The temperature in step S3 is 120 DEG C, the stirring speed is 300 r / min, and the reaction time is 6 hours.

3. The use of the green, low-cost, high-performance lignin adhesive of claim 1 or the green, low-cost, high-performance lignin adhesive prepared by the preparation method of claim 2 as a wood adhesive, characterized in that, The green, low-cost and high-performance lignin adhesive is coated on the surface of wood, and the best adhesive effect is achieved through hot pressing; the coating amount of the green, low-cost and high-performance lignin adhesive is 150-200 g / m 2 , and the hot pressing condition is: 170 DEG C, 1.0 MPa, 60 min.