Industrial lignin environment-friendly adhesive as well as preparation method and application thereof

Abstract

The invention provides an industrial lignin environment-friendly adhesive as well as a preparation method and application thereof. According to the mass percentage content, the lignin-cellulose composite material comprises 1.8-7.2% of industrial lignin, 1.8-3.6% of cellulose derivatives, formic acid aqueous solution and the balance of water, wherein the total mass percentage content is 100%; the content of formic acid in the adhesive is 24-48%; the total solid content of the industrial lignin and the cellulose derivative is 3.6-10.8%; the raw materials are uniformly mixed by stirring to obtain a uniform and stable dark viscous liquid, namely the lignin environment-friendly adhesive. The technical problem to be solved is how to prepare the environment-friendly industrial lignin adhesive which does not contain formaldehyde and phenol substances and is healthy and environment-friendly, and a pure biomass system is constructed by taking a papermaking byproduct industrial lignin (waste) and a natural cellulose derivative as core raw materials so as to realize high-value utilization of resources. Meanwhile, the preparation process of the adhesive can be completed at normal temperature and normal pressure, excellent bonding strength can be achieved through mild hot pressing when the adhesive is used for bonding wood, and the water resistance is good.

Description

Technical Field

[0001] This invention relates to the field of lignin-based formaldehyde-free and phenol-free adhesive manufacturing technology, and in particular to an industrial lignin-based environmentally friendly adhesive, its preparation method, and its application. Background Technology

[0002] In the modern home furnishing and packaging industries, composite engineered wood products are widely used due to their low cost and good processability, and adhesives are the core raw material for engineered wood product production. Currently, traditional petrochemical-based adhesives such as urea-formaldehyde resin, phenolic resin, and melamine-formaldehyde resin dominate the global market. However, these adhesives continuously release formaldehyde during production and use, posing a threat to human health, and rely on non-renewable petrochemical raw materials, which is inconsistent with environmental protection and sustainable development trends. With increasing consumer demand for healthy home furnishings and tightening environmental policies, the development of formaldehyde-free, phenol-free, and environmentally friendly bio-based adhesives has become an urgent industry need.

[0003] Lignin is the second most abundant natural polymer in nature and a major byproduct of pulping waste in the paper industry. Its molecular structure is rich in phenylpropane units and has active functional groups such as hydroxyl, methoxy, and carbonyl groups distributed on its surface. Theoretically, it has the potential to be used as a raw material for adhesives. If it can be utilized in a high-value manner, it can not only solve the problem of industrial waste treatment but also replace petrochemical raw materials. However, the development of lignin and biomass adhesives in existing technologies faces many bottlenecks: conventional cellulose and other biomass adhesives generally have poor water resistance due to the large number of hydrophilic groups in their molecular structure, making it difficult to meet the water resistance requirements of Class I plywood in the national standard GB / T9846-2015, thus limiting their promotion in the mainstream engineered wood products sector; more than 90% of industrial lignin is converted into heat energy only through incineration in alkali recovery furnaces, and its rich aromatic ring structure and active functional groups are not used in high-value-added fields such as adhesives, resulting in serious waste and misallocation of resources; most current lignin-based adhesives require complex chemical modifications such as phenolation and hydroxymethylation of industrial lignin to improve reactivity, which is cumbersome, energy-intensive, consumes a large amount of chemicals, significantly increases costs, and poses potential environmental risks; moreover, most high-performance lignin adhesives require high temperature, high pressure, and long-term curing, relying on specialized equipment, which is complex to operate and difficult to mass-produce. Summary of the Invention

[0004] The main objective of this invention is to provide an environmentally friendly industrial lignin adhesive, its preparation method, and its application. The technical problem to be solved is how to prepare an environmentally friendly industrial lignin adhesive that does not contain formaldehyde or phenolic substances, making it healthy and environmentally friendly. Furthermore, it uses industrial lignin (waste) from papermaking byproducts and natural cellulose derivatives as core raw materials to construct a pure biomass system to achieve high-value utilization of resources. At the same time, the adhesive preparation process can be completed at room temperature and pressure, and when bonding wood, it can achieve excellent bonding strength through gentle hot pressing. It also has good water resistance, making it more suitable for practical use.

[0005] The objective of this invention and the technical problem it solves are achieved by the following technical solution. The industrial lignin-based environmentally friendly adhesive proposed in this invention comprises, by mass percentage: 1.8-7.2% industrial lignin, 1.8-3.6% cellulose derivatives, and an aqueous solution of formic acid, made up to 100% with water; the formic acid content in the adhesive is 24-48% by mass; and the total solids content of the industrial lignin and cellulose derivatives is 3.6-10.8%.

[0006] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0007] Preferably, in the aforementioned industrial lignin-based environmentally friendly adhesive, the industrial lignin is selected from at least one of alkali lignin, sulfate lignin, formic acid lignin, enzymatically hydrolyzed lignin, and hydrolyzed lignin.

[0008] Preferably, in the aforementioned industrial lignin-based environmentally friendly adhesive, the cellulose derivative is selected from at least one of hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, ethyl cellulose, and carboxymethyl cellulose.

[0009] Preferably, in the aforementioned industrial lignin-based environmentally friendly adhesive, the content of industrial lignin is 3.6-7.2%; and the total solid content of the industrial lignin and cellulose derivatives is 5.4-10.8%.

[0010] Preferably, in the aforementioned industrial lignin-based environmentally friendly adhesive, the cellulose derivative is carboxymethyl cellulose.

[0011] The objective of this invention and the technical problem it solves are achieved through the following technical solution. A method for preparing an industrial lignin-based environmentally friendly adhesive according to this invention includes the following steps: S1 Weigh out industrial lignin, cellulose derivatives, and formic acid aqueous solution; by mass percentage, industrial lignin 1.8~7.2%, cellulose derivatives 1.8~3.6%, and formic acid 24~48%, with water to make up the balance; the total solid content of the industrial lignin and cellulose derivatives is 3.6~10.8%; Stirring S2 mixes the raw materials evenly to obtain a uniform and stable dark viscous liquid, which is a lignin-based environmentally friendly adhesive.

[0012] The objective of this invention and the technical problem it solves are achieved through the following technical solution. According to this invention, the application of the aforementioned industrial lignin-based environmentally friendly adhesive in the preparation of engineered wood panels includes the following steps: S1 Glue application: Apply the lignin-based environmentally friendly adhesive evenly to the upper and lower surfaces of the wood board; S2 Hot Press Molding: Stack three layers of wood boards, align the edges, and place them in a hot press. Press them for 3-15 minutes under a pressure of 1-2 MPa and a temperature of 140-180℃, and then cool them down at room temperature. The three-layer stacking of wood boards involves placing the wood board coated with adhesive between two other wood boards. The amount of adhesive applied per square meter of artificial board is 150-350g. S3 Post-processing: Cut the cooled three-layer plywood to the design dimensions.

[0013] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0014] Preferably, in the aforementioned application, the hot pressing time is 8 to 15 minutes.

[0015] Preferably, in the aforementioned application, the amount of adhesive applied is 250~350g / m²; the hot pressing parameters are: pressure 1.5~2MPa, temperature 160~180℃, and hot pressing time 8~10min.

[0016] Preferably, in the aforementioned applications, the engineered wood product is one of plywood, particleboard, or fiberboard.

[0017] By employing the above technical solution, the present invention proposes an industrial lignin-based environmentally friendly adhesive, its preparation method, and its application. This invention comprehensively overcomes existing technological bottlenecks in terms of raw material selection, preparation process, and product application, and has at least the following advantages: Firstly, regarding adhesive products, this invention uses unmodified industrial lignin (1.8~7.2%) as the core raw material, combined with cellulose derivatives (1.8~3.6%) and formic acid (24~48%) to construct a pure biomass raw material system. This system is completely free of toxic substances such as formaldehyde and phenol, meeting consumers' demand for healthy homes and effectively avoiding the formaldehyde release risk of traditional petrochemical-based adhesives. It also upgrades industrial lignin, a byproduct of papermaking, from low-value incineration fuel to a high-value-added adhesive component, solving the industry pain point of lignin resource misallocation and waste. At the same time, the total solid content of industrial lignin and cellulose derivatives in the adhesive is only 3.6~10.8%, far lower than the existing technology level. This ensures that the product is a uniform and stable dark viscous liquid with good coating and rolling flowability, significantly reducing raw material consumption, directly reducing production costs, and balancing environmental protection and economy. Furthermore, through the synergistic effect of industrial lignin, cellulose derivatives, and formic acid, the core process involves formic acid first providing H⁺ to catalyze the protonation activation of CMC carboxylate groups and lignin phenolic hydroxyl groups, simultaneously forming an interfacial bond with the wood board. Based on this, the activated lignin constructs a hydrophobic framework with aromatic rings, cross-linking with the CMC cellulose backbone through ester and ether bonds. CMC, in turn, forms a dense three-dimensional network with flexible chains linked to lignin, buffering water absorption and swelling stress while preventing lignin aggregation and porosity. Ultimately, a stable adhesive layer is formed, exhibiting resistance to water penetration, chemical bond hydrolysis, and interfacial delamination, thus achieving excellent water resistance. The formic acid volatilized during plywood pressing is recyclable and imparts antibacterial properties to the plywood, facilitating high bonding strength, water resistance, and antibacterial properties in various applications. This completely overcomes the shortcomings of existing single-cellulose adhesives, which suffer from poor water resistance and are unsuitable for high-performance applications, enabling the product to meet the requirements of GB / T9846-2015 Class I plywood standard.

[0018] Secondly, the adhesive preparation method of the present invention further amplifies the product advantages. This method only requires weighing the raw materials according to the formula and stirring to obtain the target product. The entire process does not require high-temperature reaction, complex chemical modification (such as phenolation, hydroxymethylation) or additional additives. This not only greatly simplifies the preparation process and reduces energy consumption and operating threshold, but also allows ordinary production line workers to get started with simple training. It also reduces the cost of chemical procurement and consumption, and avoids the potential risk of toxic substance residues during the modification process. Moreover, the stirring and mixing preparation method is highly efficient, with a short single-batch production cycle, and can be directly adapted to large-scale continuous production.

[0019] Finally, in the application scenarios of engineered wood products, this invention achieves a balance between performance and economy by rationally designing the glue application amount (150~350g / m²) and hot-pressing conditions (pressure 1~2MPa, temperature 140~180℃, time 3~15min). This glue application amount ensures that the adhesive fully covers the surface of the wood board to guarantee bonding strength, while avoiding waste caused by excessive raw materials and reducing the raw material cost of engineered wood product production. The solution is applicable to a wide range of raw materials, and the relatively mild hot-pressing conditions not only reduce energy consumption in the hot-pressing process, but also eliminate the need for special adhesive reaction equipment, allowing direct adaptation to existing engineered wood product production lines without additional equipment modifications, significantly lowering the industrialization threshold. Furthermore, the simplified process from glue application and hot pressing to cooling and cutting can be efficiently matched with the existing engineered wood product production rhythm, avoiding a decline in production efficiency due to complex processes, and facilitating engineered wood product companies to quickly achieve formaldehyde-free transformation.

[0020] In summary, this invention comprehensively solves the problems of insufficient environmental friendliness, high cost, complex processes, and difficulty in industrialization of existing lignin-based adhesives, from raw materials and processes to applications, and provides a feasible solution for the industrial promotion of biomass adhesives.

[0021] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0022] Figure 1 XPS analysis of samples of lignin, lignin aqueous solution applied and hot-pressed, and adhesive of the present invention applied and hot-pressed, wherein 1 is a lignin sample, 2 is a lignin aqueous solution applied and hot-pressed sample, and 3 is an adhesive of the present invention applied and hot-pressed sample. Detailed Implementation

[0023] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following detailed description, in conjunction with preferred embodiments, provides a detailed explanation of the specific implementation methods, structures, features, and effects of an industrial lignin-based environmentally friendly adhesive, its preparation method, and its application. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable manner.

[0024] This invention proposes an environmentally friendly industrial lignin adhesive and its preparation method. The adhesive comprises, by mass percentage, 1.8-7.2% industrial lignin, 1.8-3.6% cellulose derivatives, and an aqueous formic acid solution, with water added to make up to 100%. The formic acid content in the adhesive is 24-48% by mass. By coordinating the proportions of the above raw material components, the total solid content of the industrial lignin and cellulose derivatives is 3.6-10.8%. In preparing the adhesive, after weighing the industrial lignin, cellulose derivatives, and aqueous formic acid solution according to the designed formula, only room temperature stirring is required to mix the raw materials uniformly, resulting in a homogeneous, stable, dark-colored viscous liquid, which is the environmentally friendly lignin adhesive.

[0025] It should be noted that the above-mentioned solid content refers to the percentage of non-volatile solid components in the adhesive by mass. Its core function is to measure the proportion of effective components actually remaining after the adhesive dries. Formic acid is a volatile liquid and mainly serves as a solvent or reaction medium in the formulation, rather than an effective solid component that provides adhesive properties; it will completely evaporate after the adhesive dries. Water is a solvent and also completely evaporates after drying. The solid content mentioned above in this invention actually refers to the total content of industrial lignin and cellulose derivatives in the adhesive.

[0026] As attached Figure 1The figures show X-ray photoelectron spectroscopy (XPS) analyses of samples of lignin, lignin aqueous solution sizing and hot pressing, and the adhesive of this invention sizing and hot pressing. The horizontal axis represents binding energy (eV), and the vertical axis represents the percentage of peak area of ​​each functional group (%). Curve 1 represents the lignin sample, curve 2 represents the lignin aqueous solution sizing and hot pressing sample, and curve 3 represents the adhesive of this invention sizing and hot pressing sample. The XPS analysis shows that formic acid in the formulation exists as a catalyst, which promotes the esterification, etherification, or dehydration condensation reaction between the hydroxyl groups (-OH) of lignin and the hydroxyl / carboxyl groups of cellulose derivatives. This consumes a large number of hydrophilic hydroxyl groups and reduces the binding sites of water molecules. Therefore, the CO percentage in sample 3 in the figure decreased from 32.56% in sample 1 and 24.48% in sample 2 to 11.38%, while the O=CO percentage in sample 3 (16.95%) was significantly higher than that in sample 1 (8.97%) and sample 2 (11.28%). Simultaneously, during the hot-pressing process, lignin and cellulose derivatives cross-link through covalent bonds such as ether and ester bonds, forming a larger molecular network. This increases the relative proportion of hydrophobic aromatic ring carbons (lignin benzene rings) and alkyl carbons (long-chain alkyl groups of cellulose derivatives), enhancing overall hydrophobicity. Therefore, in the figure, the CC / CH ratio of No. 3 increases from 57.31% in No. 1 and 54.28% in No. 2 to 63.28%. Furthermore, the proportion of "C=C (OCO)" in No. 3 is 8.4%, slightly lower than 9.96% in No. 2. However, combined with the formic acid, cellulose derivatives, and hot-pressing process, a deep cross-linking reaction occurs between lignin and cellulose derivatives, such as the formation of aromatic ring conjugated structures and covalent connections of ether / ester bonds, causing the molecular chains to be tightly intertwined, forming a dense network structure. This structure can physically block the penetration channels of water molecules, further improving water resistance. In other words, this invention achieves a dual water-resistant mechanism of chemical hydrophobicity and physical water blocking through the formic acid system, greatly improving the water resistance of the adhesive.

[0027] In some specific embodiments of the present invention, the selection range of industrial lignin is clearly defined, and it is selected from at least one of alkali lignin, sulfate lignin, formic acid lignin, enzymatically hydrolyzed lignin, and hydrolyzed lignin. The core technical purpose of this selection setting is to balance the performance, production cost, and environmental protection requirements of adhesives by flexibly adjusting the component characteristics while ensuring the stability of raw material supply, and to accurately adapt to the target preparation process. These industrial lignins are all by-products of industrial production, which are not only widely available and have a stable supply, but also show good industrial compatibility with the formic acid system. Among them, alkali lignin and sulfate lignin mainly come from black liquor in the papermaking industry, and the output is very large; formic acid lignin comes from the process of formic acid pretreatment of plant fibers (such as pulping and biorefining); and hydrolyzed lignin comes from the hydrolysis waste liquid produced by cellulosic ethanol, furfural, etc. The present invention, through the selection range of at least one, can effectively avoid dependence on a single raw material. In actual operation, it can be flexibly adjusted according to the raw material supply situation in different regions and at different times, which significantly reduces supply chain risks. Meanwhile, using these industrial lignins as raw materials also has significant environmental and cost advantages. On the one hand, it can avoid pollution caused by the direct discharge of industrial waste and realize the resource utilization of waste. On the other hand, compared with the monomers of artificially synthesized polymer adhesives, such as phenolic resin and urea-formaldehyde resin, industrial lignin is cheaper and can significantly reduce the production cost of adhesives. In addition, lignin itself is biodegradable, which can reduce the environmental burden of adhesives after use or disposal.

[0028] In some specific embodiments of the present invention, the selection range of cellulose derivatives is also defined, specifically selected from at least one of hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, ethyl cellulose, and carboxymethyl cellulose. The technical objective is to utilize the structural differences of different derivatives to specifically optimize the rheological properties, compatibility, bonding assistance effect, and water resistance of adhesives, while taking into account process adaptability and environmental costs, ultimately achieving controllable adjustment of the overall performance of the adhesive. One of the core functions of cellulose derivatives is thickening and viscosity adjustment. This characteristic directly determines the coatability, flowability, and wettability of adhesives. Different derivatives have significant differences in thickening ability and viscosity characteristics, which can meet the needs of diverse construction scenarios. For example, hydroxymethyl cellulose and carboxymethyl cellulose have high thickening efficiency and can adjust the low viscosity formic acid-lignin system to a suitable viscosity for adhesive application, effectively avoiding the problem of flow during the adhesive application process. Methyl cellulose has a relatively mild viscosity and is more suitable for scenarios requiring thin coating or rapid wetting of the bonded materials, such as wood fibers. Based on the availability of at least one option, the viscosity can be flexibly adjusted according to the specific construction method, such as brushing, spraying, or scraping, to ensure a uniform adhesive layer and avoid material waste.

[0029] Considering that the adhesive system of this invention contains solid lignin, liquid formic acid, and water, the components are prone to stratification and precipitation due to differences in solubility. Cellulose derivatives can act as a compatibility bridge to ensure the uniformity and stability of the system. On the one hand, most cellulose derivatives possess both hydrophilic hydroxyl groups and a certain degree of polarity, enabling them to form hydrogen bonds or polar interactions with polar solvents such as formic acid, water, and hydroxyl-containing lignin, effectively preventing lignin particle aggregation and sedimentation. On the other hand, these cellulose derivatives exhibit excellent solubility under acidic conditions, such as in the formic acid environment of the system, and are not prone to adverse reactions with lignin, thus maintaining the homogeneity of the system for a long time and avoiding stratification failure in the short term after adhesive preparation. It is worth noting that although cellulose derivatives are not the main adhesive component of adhesives, they can supplement adhesive strength, improve the toughness and water resistance of the adhesive layer through their own structural characteristics, and compensate for the performance shortcomings of a single lignin adhesive layer: the hydroxyl groups (-OH) in the derivative molecules can form hydrogen bonds with lignin and the hydroxyl groups on the surface of the bonded materials, such as wood and fibers, to enhance the interfacial adhesion between the adhesive layer and the substrate, and indirectly improve the overall adhesive strength; the substituents of some hydrophobic derivatives can reduce the hydrophilicity of the adhesive layer and reduce the penetration of water into the adhesive layer. Even carboxymethyl cellulose can moderately improve water resistance while thickening by adjusting the degree of substitution, avoiding the adhesive layer from easily swelling and failing when exposed to water; in addition, the long-chain molecules of cellulose derivatives can also form an elastic network in the adhesive layer, alleviating the problem of easy cracking of the lignin adhesive layer after curing, and enhancing the impact resistance and durability of the adhesive layer. At the same time, the selected cellulose derivatives are all industrially mass-produced products with mature production processes and natural sources and biodegradable characteristics, which can form a synergistic effect with the waste recycling properties of industrial lignin, further strengthening the environmental and cost advantages of the adhesive.

[0030] This invention also proposes an application of the aforementioned industrial lignin-based environmentally friendly adhesive in the preparation of engineered wood panels, comprising the following steps: First, the adhesive is applied: Based on the designed amount of adhesive, the lignin-based environmentally friendly adhesive is evenly applied to the top and bottom surfaces of a portion of the wood panels and / or one surface of another portion. Then, the panels are hot-pressed: three layers of wood panels are stacked, edges aligned, and placed in a hot press. The press is then heated for 3-15 minutes at a pressure of 1-2 MPa and a temperature of 140-180℃, followed by cooling at room temperature. The three-layer stacking involves placing a wood panel with adhesive applied to two sides between two other wood panels that are not coated with adhesive or have adhesive applied to only one surface. Each square meter... The amount of adhesive applied to each square meter of engineered wood is 150-350g. In this invention, the amount of adhesive applied per square meter of engineered wood is based on the area of ​​the three-layer plywood after hot pressing (i.e., the surface area of ​​a single sheet of plywood, consistent with the area of ​​the core board / face board), rather than the total surface area of ​​the core board coated with adhesive. When the three-layer stacked plywood is placed in the hot press, no adhesive is applied to the two surfaces in contact with the hot press. Finally, post-processing is performed: the cooled three-layer plywood is cut according to the design dimensions to obtain engineered wood, including but not limited to plywood, particleboard, or fiberboard.

[0031] The engineered wood panels prepared by the above process have better bonding strength and water resistance. Performance tests on poplar boards show that the adhesives prepared by the technical solution of this invention can meet the Class III plywood quality standards when used in engineered wood panel preparation.

[0032] Under the preferred process conditions, the content of industrial lignin is 3.6-7.2%, cellulose derivatives 1.8-3.6%, and formic acid 24-48%, with a total solid content of lignin and cellulose derivatives of 5.4-10.8%. The adhesive is applied evenly to the surface of the wood board at an application rate of 150-350 g / m². Then, the board is hot-pressed: the adhesive-coated wood board is placed in a hot press and hot-pressed for 8-15 minutes at a pressure of 1-2 MPa and a temperature of 140-180°C. Under these preferred process conditions, the adhesive prepared by the technical solution of this invention can meet the Class II board quality standard when used in the preparation of engineered wood panels.

[0033] Under more preferred process conditions, the content of industrial lignin is 3.6-7.2%, the cellulose derivative is carboxymethyl cellulose, the formic acid content is 48%, and the total solid content of industrial lignin and cellulose derivative is 5.4-10.8%; the total solid content of industrial lignin and cellulose derivative is 7.2-10.8%; the adhesive is evenly applied to the surface of the wood board at an application rate of 250-350 g / m²; then hot-pressed: the wood board with the adhesive is placed in a hot press and hot-pressed for 8-10 minutes under a pressure of 1.5-2 MPa and a temperature of 160-180°C; under these more preferred process conditions, the adhesive prepared by the technical solution of the present invention can meet the Class I board quality standard when used in the preparation of artificial boards.

[0034] The specific testing method is as follows: First, prepare three-layer poplar plywood according to the designed process; then cut the prepared three-layer poplar plywood into standard samples of 100mm×25mm, and groove the samples to ensure that the test area of ​​the samples is 25mm×25mm; finally, test the dry and wet shear strength of the plywood according to the national standard GB / T 9846-2015.

[0035] Dry shear strength test standard: The tensile shear strength of the specimen shall be ≥1.1MPa when tested using a universal tensile testing machine.

[0036] Standard for wet shear strength test of Class I plywood: Boil the sample in 100℃ boiling water for 3 hours, let it stand at room temperature for 10 minutes, and then use a universal tensile testing machine to test the tensile shear strength of the sample. It must be ≥0.7MPa.

[0037] Standard for wet shear strength test of Class II plywood: Boil the sample in water at 63℃ for 3 hours, let it stand at room temperature for 10 minutes, and then use a universal tensile testing machine to test the tensile shear strength of the sample. It must be ≥0.7MPa.

[0038] Standard for wet shear strength test of Class III plywood: Immerse the sample in cold water for 24 hours, let it stand at room temperature for 10 minutes, and then use a universal tensile testing machine to test the tensile shear strength of the sample. It must be ≥0.7MPa.

[0039] The present invention will be further described below with reference to specific embodiments, but this should not be construed as a limitation on the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention still fall within the scope of protection of the present invention.

[0040] Unless otherwise specified, all materials and reagents mentioned below are commercially available products well known to those skilled in the art; unless otherwise specified, all methods described are methods known in the art. Unless otherwise defined, the technical or scientific terms used should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0041] Example 1 This embodiment describes the preparation of an industrial lignin-based environmentally friendly adhesive and the application of this adhesive to the fabrication of plywood. The specific steps are as follows: 1) Adhesive preparation: Weigh 1.8wt% alkali lignin and 1.8wt% hydroxymethyl cellulose by mass percentage and add them to formic acid aqueous solution, the formic acid content is 48wt%; stir at 250r / min for 20min at room temperature, mix evenly and then let stand at room temperature for storage; 2) Plywood preparation: Apply 10g of adhesive evenly to both surfaces of a poplar board with dimensions of 200mm×200mm×1.5mm. Take two more poplar boards of the same size, use the board with adhesive as the core and the board without adhesive as the veneer, stack the three boards, align the edges, and use a hot press at a pressure of 2MPa and a temperature of 140℃ for 8 minutes. After cooling at room temperature, cut and groove to ensure that the test area is 25mm×25mm. 3) Performance testing: Tested according to GB / T9846-2015, the dry shear strength is 1.23±0.11MPa, the wet shear strength of Class III plywood is 0.99±0.15MPa, and the wet shear strength of Class II plywood is 0.66±0.03MPa, meeting the Class III plywood standard.

[0042] Example 2 Same as Example 1, except that: 1) the industrial lignin is formate lignin, with a content of 3.6 wt%; 2) the cellulose derivative is hydroxyethyl cellulose; 3) the dry shear strength is 1.61 ± 0.14 MPa, the wet shear strength of Class II plywood is 0.95 ± 0.16 MPa, and the wet shear strength of Class I plywood is 0.56 ± 0.06 MPa, meeting the Class II plywood standard.

[0043] Example 3 Same as Example 1, except that: 1) the industrial lignin is formate lignin with a content of 5.4 wt%; 2) the cellulose derivative content is 3.6 wt%; 3) the hot pressing temperature is 160℃; 4) the dry shear strength is 1.87±0.21 MPa, the wet shear strength of Class II plywood is 1.53±0.19 MPa, and the wet shear strength of Class I plywood is 0.92±0.18 MPa, meeting the Class I plywood standard.

[0044] Example 4 Same as Example 1, except that: 1) the industrial lignin is hydrolyzed lignin with a content of 5.4 wt%; 2) the cellulose derivative is hydroxypropyl cellulose with a content of 3.6 wt%; 3) the formic acid content is 24 wt%; 4) the hot pressing temperature is 160℃; 5) the dry shear strength is 1.34 ± 0.12 MPa, the wet shear strength of Class III plywood is 0.79 ± 0.14 MPa, and the wet shear strength of Class II plywood is 0.53 ± 0.13 MPa, meeting the Class III plywood standard.

[0045] Example 5 Same as Example 1, except that: 1) the industrial lignin is sulfate lignin with a content of 3.6 wt%; 2) the cellulose derivative is hydroxyethyl cellulose; 3) 6 g of adhesive, hot pressing pressure of 1 MPa, hot pressing temperature of 160°C, and hot pressing time of 15 min; 4) the dry shear strength is 1.44 ± 0.24 MPa, the wet shear strength of Class II plywood is 1.03 ± 0.06 MPa, and the wet shear strength of Class I plywood is 0.63 ± 0.03 MPa, meeting the Class II plywood standard.

[0046] Example 6 Same as Example 1, except that: 1) the industrial lignin is hydrolyzed lignin with a content of 3.6 wt%; 2) the cellulose derivative is methylcellulose; 3) the formic acid content is 24 wt%; 4) the adhesive is 12 g, the hot pressing pressure is 1 MPa, and the hot pressing temperature is 180℃; 5) the dry shear strength is 1.57 ± 0.15 MPa, the wet shear strength of Class II plywood is 0.86 ± 0.09 MPa, and the wet shear strength of Class I plywood is 0.34 ± 0.03 MPa, meeting the Class II plywood standard.

[0047] Example 7 Same as Example 1, except that: 1) the industrial lignin is formic acid lignin, with a content of 5.4 wt%; 2) the cellulose derivative is carboxymethyl cellulose, with a content of 3.6 wt%; 3) the formic acid content is 24 wt%; 4) the adhesive is 12 g, the hot pressing pressure is 1 MPa, and the hot pressing time is 3 min; 5) the dry shear strength is 1.14 ± 0.17 MPa, the wet shear strength of Class III plywood is 0.85 ± 0.11 MPa, and the wet shear strength of Class II plywood is 0.54 ± 0.02 MPa, meeting the Class III plywood standard.

[0048] Example 8 Same as Example 1, except that: 1) the industrial lignin content is 5.4 wt%; 2) the cellulose derivative is ethyl cellulose with a content of 3.6 wt%; 3) the hot pressing temperature is 160℃; 4) the dry shear strength is 1.78 ± 0.15 MPa, the wet shear strength of Class II plywood is 0.83 ± 0.23 MPa, and the wet shear strength of Class I plywood is 0.21 ± 0.04 MPa, meeting the Class II plywood standard.

[0049] Example 9 Same as Example 1, except that: 1) the industrial lignin is formic acid lignin, with a content of 3.6 wt%; 2) the cellulose derivative is hydroxypropyl cellulose; 3) the formic acid content is 24 wt%; 4) 12 g of adhesive, hot pressing temperature of 180℃; hot pressing time of 3 min; 5) the dry shear strength is 1.23 ± 0.25 MPa, the wet shear strength of Class III plywood is 0.76 ± 0.13 MPa, and the wet shear strength of Class II plywood is 0.23 ± 0.08 MPa, meeting the Class III plywood standard.

[0050] Example 10 Same as Example 1, except that: 1) the industrial lignin is formate lignin; 2) the cellulose derivative is carboxymethyl cellulose, with a content of 3.6 wt%; 3) the formic acid content is 24 wt%; 4) the adhesive is 12 g, and the hot pressing time is 15 min; 5) the dry shear strength is 1.34 ± 0.16 MPa, the wet shear strength of Class III plywood is 0.98 ± 0.04 MPa, and the wet shear strength of Class II plywood is 0.58 ± 0.18 MPa, meeting the Class III plywood standard.

[0051] Example 11 Same as Example 1, except that: 1) the cellulose derivative is methylcellulose; 2) the formic acid content is 24wt%; 3) the adhesive is 12g, and the hot pressing temperature is 160℃; 4) the dry shear strength is 1.45±0.13MPa, the wet shear strength of Class III plywood is 0.91±0.15MPa, and the wet shear strength of Class II plywood is 0.66±0.09MPa, meeting the Class III plywood standard.

[0052] Example 12 Same as Example 1, except that: 1) the industrial lignin is formate lignin, with a content of 3.6 wt%; 2) the cellulose derivative is carboxymethyl cellulose, with a content of 3.6 wt%; 3) 12 g of adhesive, hot pressing temperature of 180℃; 4) the dry shear strength is 2.05 ± 0.22 MPa, the wet shear strength of Class II plywood is 1.67 ± 0.19 MPa, and the wet shear strength of Class I plywood is 1.23 ± 0.12 MPa, meeting the Class I plywood standard.

[0053] Example 13 Same as Example 1, except that: 1) the industrial lignin is enzymatically hydrolyzed lignin (purity 92%, phenolic hydroxyl content 5.2 mmol / g), with a content of 7.2 wt%; 2) the cellulose derivative is carboxymethyl cellulose, with a content of 3.6 wt%; 3) 14 g of adhesive, hot pressing temperature 180℃; 4) dry shear strength 1.94±0.25 MPa, wet shear strength of Class II plywood 1.57±0.13 MPa, wet shear strength of Class I plywood 0.78±0.10 MPa, meeting the Class I plywood standard.

[0054] Comparative Example 1 Same as Example 1, except that: 1) the industrial lignin is formate lignin with a content of 3.6 wt%; 2) no cellulose derivatives were added; 3) the hot pressing temperature was 160℃; 4) the dry shear strength was 1.29±0.22MPa, and the wet shear strength of Class III plywood was 0.14±0.05MPa, which did not meet the national plywood standard.

[0055] Comparative Example 2 Same as Example 1, except that: 1) no industrial lignin was added; 2) the cellulose derivative was carboxymethyl cellulose, with a content of 3.6 wt%; 3) the hot pressing temperature was 160℃; 4) the dry shear strength was 1.45±0.22MPa, and the wet shear strength of Class III plywood was 0.33±0.04MPa, which did not meet the national plywood standard.

[0056] Comparative Example 3 Same as Example 1, except that: 1) the industrial lignin content is 3.6wt%; 2) the cellulose derivative is hydroxymethyl cellulose; 3) no formic acid is added; 4) the hot pressing temperature is 160℃; 5) the dry shear strength is 0.45±0.12MPa, and the wet shear strength of Class III plywood is 0.07±0.03MPa, which does not meet the national plywood standard.

[0057] As can be seen from the test data of the examples and comparative examples, in the technical solution of the present invention, industrial lignin is used as the core adhesive substrate, and its content directly determines the intermolecular forces and cross-linking density, with the data showing a clear positive correlation. For example, in Examples 1 to 3 and Example 13, as the content of industrial lignin increases from 1.8% to 7.2%, the performance and compliance level of the prepared plywood also gradually improve. In Example 1, due to insufficient number of lignin molecules and sparse cross-linking network, it only meets the Class III plywood standard. In Example 2, the intermolecular hydrogen bonds and covalent bonds are enhanced, meeting the Class II plywood standard. In Examples 3 and 13, the lignin aromatic ring structure is dense and the hydrophobic region is expanded, meeting the Class I plywood standard. Moreover, Example 13, as an example of the upper limit of lignin content, still maintains the performance of Class I plywood, proving that high lignin content does not lead to performance degradation due to agglomeration.

[0058] As can be seen from the test data of the examples, in the technical solution of the present invention, the content of industrial lignin is generally better at higher contents than at lower contents. When the types of cellulose derivatives and the content of formic acid are within the conventional range, such as 1.8~3.6wt% for cellulose derivatives and 24~48wt% formic acid, as the content of industrial lignin increases from 1.8wt% to 5.4~7.2wt%, the bonding strength, including dry shear strength and wet shear strength, shows an overall upward trend, and the compliance level is better. In Examples 1, 10, and 11, the lignin content is low, and they only meet the Class III plywood standard. In Examples 2-9 and 12, the lignin content is medium to high. Under similar conditions of other parameters, the plywood can meet the Class II plywood standard. With optimization of other parameters, it can even meet the Class I plywood standard. The strength upper limit of the high-content cases is significantly higher. In addition, under the premise of similar lignin content and other parameters, the type of lignin has a significant impact on performance. The strength performance of formic acid lignin is generally better than that of alkali lignin, sulfate lignin, and hydrolyzed lignin. In cases with similar lignin content, formic acid content, and process parameters, the type of cellulose derivative shows a clear trend in its influence on strength. Carboxymethyl cellulose (CMC) generally exhibits better water resistance and dry shear strength than other types. Under similar lignin and cellulose derivative parameters, formic acid content has a significant impact on strength. When the formic acid content is 48%, the plywood generally meets the Class II plywood standard, and after some parameter optimization, it can reach the Class I standard. Furthermore, Class II plywood generally has higher wet shear strength, as seen in Examples 2, 3, 5, 8, and 12. However, when the formic acid content is 24%, the plywood generally only meets the Class III plywood standard, and the wet shear strength of Class II plywood is generally lower, as seen in Examples 4, 7, 9, 10, and 11. With similar raw material ratios, the strength of an application rate of 250~350g / m² is generally better than that of 150g / m². High-temperature hot pressing at 160-180℃ and medium-long-term hot pressing for 8-10 minutes can better balance crosslinking efficiency and adhesive layer density.

[0059] As can be seen from the comparative test data, in the technical solution of this invention, although the dry shear strength meets the standard when no cellulose derivatives are added, the wet shear strength of Class III plywood is 0.14 MPa, which is far below the national standard (≥0.7 MPa). Even with a lignin content of 3.6 wt%, an effective hydrophobic network cannot be formed, proving that an effective hydrophobic network cannot be formed without cellulose derivatives, and the adhesive layer is easily loosened when exposed to water, as shown in Comparative Example 1; although the dry shear strength meets the standard when no industrial lignin is added, the wet shear strength of Class III plywood is 0.14 MPa. 0.33 MPa, without the support of the aromatic ring structure of lignin, the adhesive layer lacks rigidity, and the bonding strength drops sharply, as shown in Comparative Example 2; without the addition of formic acid, the dry shear strength is only 0.45 MPa, and the wet shear strength of Class III plywood is only 0.07 MPa, indicating that without the catalytic cross-linking effect of formic acid, ester / ether bonds cannot be formed, and the adhesive layer is loose, as shown in Comparative Example 3; indicating that cellulose derivatives, industrial lignin, and formic acid are all indispensable components of the technical solution of this invention, and the synergistic effect of the three components is the key to the invention overcoming the bottleneck of water resistance.

[0060] The gluing and hot-pressing processes for particleboard and fiberboard are consistent with the core logic of plywood. The parameters can be finely adjusted according to the density of the board. The recommended gluing amount for particleboard is 110-120 kg / m³, and the recommended gluing amount for fiberboard is 200-220 kg / m³. The hot-pressing parameters can be directly adopted from the plywood parameters. Those skilled in the art can implement this based on existing experience in wood-based panel production.

[0061] The technical features in the claims and / or specification of this invention can be combined, and the combination is not limited to the combinations obtained through reference in the claims. Technical solutions obtained by combining the technical features in the claims and / or specification are also within the scope of protection of this invention.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. An environmentally friendly industrial lignin adhesive, characterized in that, The adhesive comprises 1.8-7.2% of industrial lignin, 1.8-3.6% of cellulose derivative and formic acid aqueous solution by mass percentage, and water is added to make up 100%; the mass content of formic acid in the adhesive is 24-48%; and the total solid content of the industrial lignin and the cellulose derivative is 3.6-10.8%.

2. The environmentally friendly industrial lignin adhesive according to claim 1, characterized in that, The industrial lignin is at least one selected from alkali lignin, kraft lignin, formic acid lignin, enzymatic hydrolysis lignin and hydrolysis liquor lignin.

3. The environmentally friendly industrial lignin adhesive according to claim 1, characterized in that, The cellulose derivative is at least one selected from hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, ethyl cellulose and carboxymethyl cellulose.

4. The environmentally friendly industrial lignin adhesive according to any one of claims 1 to 3, characterized in that, The content of the industrial lignin is 3.6-7.2%, and the total solid content of the industrial lignin and the cellulose derivative is 5.4-10.8%.

5. A method for preparing an environmentally friendly industrial lignin adhesive, characterized in that, The method comprises the following steps: S1: weighing industrial lignin, cellulose derivative and formic acid aqueous solution; the industrial lignin is 1.8-7.2% by mass percentage, the cellulose derivative is 1.8-3.6% by mass percentage, formic acid is 24-48% by mass percentage, and water is added to make up the balance; and the total solid content of the industrial lignin and the cellulose derivative is 3.6-10.8%; S2: stirring to uniformly mix the raw materials, obtaining a uniform and stable dark viscous liquid, and obtaining the lignin environmental protection adhesive.

6. Use of the environmentally friendly industrial lignin adhesive according to any one of claims 1 to 4 in the production of wood-based panels, characterized in that, The method comprises the following steps: S1: gluing: uniformly applying the lignin environmental protection adhesive on the upper and lower surfaces of the wood board; S2: hot pressing: stacking three layers of wood boards with aligned edges, and then placing the wood boards in a hot press under the conditions of a pressure of 1-2 MPa and a temperature of 140-180 ℃ for hot pressing for 3-15 min, and then cooling at room temperature; the three layers of wood boards are stacked by placing the wood board with the applied adhesive between the other two wood boards; and the amount of the adhesive applied per square meter of the artificial board is 150-350 g; S3: post-processing: cutting the cooled three-layer plywood according to the designed size.

7. Use according to claim 6, characterized in that, The hot pressing time is 8-15 min.

8. Use according to claim 6, characterized in that, The amount of the adhesive applied is 250-350 g / m².

9. Use according to claim 6, characterized in that, The hot pressing parameters are: a pressure of 1.5-2 MPa, a temperature of 160-180 ℃, and a hot pressing time of 8-10 min.

10. Use according to any one of claims 6 to 9, characterized in that, The artificial board is one of plywood, particle board or fiber board.

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