Low-formaldehyde environment-friendly adhesive and preparation method thereof

Abstract

The invention relates to the technical field of adhesives, and particularly discloses a low-formaldehyde environment-friendly adhesive and a preparation method thereof. The invention relates to a low-formaldehyde environment-friendly adhesive which is prepared from the following raw materials in parts by mass: 30-50 parts of modified starch, 15-25 parts of vegetable protein, 5-10 parts of modified nano cellulose, 2-5 parts of a composite cross-linking agent and 80-150 parts of water, the composite cross-linking agent comprises amino silane and polyaspartic acid; the modified starch is prepared by performing graft copolymerization on starch subjected to enzymolysis by alpha-amylase and composite anhydride; the vegetable protein is subjected to alkalization and heat treatment before being used; the modified nano-crystalline cellulose is prepared by performing ring-opening etherification reaction on nano-crystalline cellulose, epoxypropane and ethylene oxide in sequence. The low-formaldehyde environment-friendly adhesive prepared by the invention has relatively good bonding strength and relatively low formaldehyde emission.

Description

Technical Field

[0001] This application relates to the field of adhesive technology, and more specifically, to a low-formaldehyde environmentally friendly adhesive and its preparation method. Background Technology

[0002] Adhesives, as an important class of industrial chemicals, play a crucial role in numerous fields such as wood processing, packaging, construction, textiles, and papermaking, and their market demand continues to grow. Traditional adhesives such as phenolic resins, urea-formaldehyde resins, and polyurethanes, while exhibiting excellent bonding performance, rely on non-renewable resources and release harmful gases such as formaldehyde during production and use, seriously threatening human health and the ecological environment.

[0003] Biomass resources offer advantages such as wide availability, controllable cost, environmental friendliness, and biodegradability, making them increasingly attractive for the development of biomass-based adhesives both domestically and internationally. However, existing biomass-based adhesives generally suffer from poor water resistance, insufficient bonding strength, and complex preparation processes or the need for high-temperature curing, limiting their large-scale application. For example, the synthesis of sucrose-based adhesives mainly relies on esterification and oxidative crosslinking techniques. Strong acid conditions or highly reactive oxidants often lead to excessive degradation of sucrose molecules, affecting the target crosslinking and impairing water resistance.

[0004] Patent application CN111100581A discloses a method for preparing a fully bio-based adhesive, comprising: preparation of demethylated nano-lignin: nano-sizing demethylated lignin to obtain demethylated nano-lignin; preparation of oxidized nanocellulose: oxidizing nanocellulose to obtain aldehyde-rich oxidized nanocellulose; preparation of the fully bio-based adhesive: pre-crosslinking polymerization of demethylated nano-lignin, oxidized nanocellulose, and pectin; wherein the mass fraction of galacturonic acid in the pectin is 70%–74%.

[0005] In this scheme, pectin molecules, as the key "molecular bridge" for building the network, have strong hydrophilicity. The carboxyl and hydroxyl groups they contain are prone to forming hydrogen bonds with water molecules in a humid environment, causing the adhesive layer to swell. At the same time, the chemical bonds they participate in forming are prone to hydrolysis and breakage under humid and hot conditions, ultimately leading to the destruction of the cross-linked network structure, which restricts the further improvement of the adhesive's water resistance and bonding strength. Summary of the Invention

[0006] To improve the water resistance and bonding strength of adhesives, this application provides a low-formaldehyde environmentally friendly adhesive and its preparation method.

[0007] In the first aspect, this application provides a low-formaldehyde environmentally friendly adhesive, which adopts the following technical solution: A low-formaldehyde, environmentally friendly adhesive is prepared from the following raw materials in parts by weight: 30-50 parts modified starch, 15-25 parts plant protein, 5-10 parts modified nanocellulose, 2-5 parts composite crosslinking agent, and 80-150 parts water; The composite crosslinking agent includes aminosilane and polyaspartic acid; The modified starch is obtained by enzymatic hydrolysis of starch with α-amylase, followed by graft copolymerization with a composite acid anhydride. The plant protein undergoes alkalization and heat treatment before use; The modified nanocellulose was prepared by sequentially reacting nanocellulose with propylene oxide and ethylene oxide in a ring-opening etherification reaction.

[0008] In this scheme, a flexible matrix is ​​constructed using enzymatically hydrolyzed and composite anhydride-modified starch to provide basic adhesion; the abundant functional groups of plant proteins are used as an active reinforcing phase to enhance cohesion; modified nanocellulose with a "hydrophobic core-hydrophilic shell" structure is introduced to achieve nanoscale interface reinforcement; finally, with the help of a composite crosslinking agent system of aminosilane and polyaspartic acid, multiple crosslinking reactions occur during the subsequent hot-pressing curing process, and the carboxyl groups of polyaspartic acid react with the active groups of the system to construct an organic network framework; the silanol groups generated by the hydrolysis of aminosilane under alkaline conditions can both condense with each other to form a strong Si-O-Si inorganic network and combine with the hydroxyl groups on the substrate surface to achieve organic-inorganic hybridization and interface anchoring, thereby constructing a stable three-dimensional covalent network.

[0009] Preferably, the mass ratio of aminosilane to polyaspartic acid is (0.8~1.2):1.

[0010] Preferably, the aminosilane is a silane coupling agent KH550 or KH792.

[0011] Preferably, the plant protein undergoes the following pretreatment steps before use: Disperse the plant protein powder in water, adjust the pH to 10-11, heat to 50-70℃, mix for 60-90 minutes, neutralize, separate the solid and liquid, wash, dry, and set aside.

[0012] In this scheme, alkalization and heat treatment can destroy the tight higher-order structure of plant proteins, causing their molecular chains to unfold and exposing more encapsulated amino and carboxyl groups and other active functional groups. This significantly improves their solubility and dispersibility in water systems and their reactivity with crosslinking agents, thereby more effectively playing their role as an active reinforcing phase and enhancing cohesion.

[0013] Preferably, the method for preparing the modified starch includes the following steps: Starch is uniformly dispersed in water, heated to 60-65℃, α-amylase is added, and the mixture is stirred for 40-50 minutes. The temperature is then raised to 95-100℃ and kept at that temperature for 8-12 minutes. After cooling and solid-liquid separation, the starch is redispersed in anhydrous ethanol under an inert atmosphere. Composite acid anhydride and catalyst are added and mixed evenly. The temperature is raised to 60-70℃ and the reaction is carried out for 2-3 hours. After cooling, solid-liquid separation is performed, and the mixture is dried to obtain modified starch.

[0014] Preferably, the amount of α-amylase used is 0.2% to 1.0% of the starch mass.

[0015] More preferably, the amount of α-amylase used is 0.4% to 0.6% of the starch mass.

[0016] Preferably, the amount of the composite anhydride is 10% to 30% of the starch mass.

[0017] More preferably, the amount of the composite anhydride used is 15% to 25% of the starch mass.

[0018] Preferably, the amount of catalyst used is 0.1% to 0.5% of the starch mass.

[0019] More preferably, the amount of catalyst used is 0.3% to 0.5% of the starch mass.

[0020] Preferably, the catalyst is p-toluenesulfonate pyridine salt.

[0021] In this scheme, α-amylase catalyzes the hydrolysis of starch into small dextrin fragments, significantly increasing the reactive sites and specific surface area of ​​starch molecules. This is followed by short-term inactivation at 90-100℃ to ensure the termination of enzymatic hydrolysis. Then, under an inert atmosphere, the composite anhydride and the enzymatically hydrolyzed starch undergo synergistic esterification: maleic anhydride constructs multi-branched ester bonds between starch hydroxyl groups through a dicarboxylic anhydride structure, utilizing steric hindrance to reduce the contact between water molecules and hydrophilic groups; acetic anhydride, on the other hand, modifies the residual hydroxyl groups through monoesterification, precisely controlling the hydrophobic / hydrophilic balance of the starch molecular chain.

[0022] Preferably, the composite anhydride is selected from at least two of maleic anhydride, acetic anhydride, succinic anhydride, and phthalic anhydride.

[0023] More preferably, the composite anhydride includes maleic anhydride and acetic anhydride.

[0024] In this scheme, maleic anhydride provides carboxyl groups that can be crosslinked, while acetic anhydride and other substances are used to acetylate the hydrophilic hydroxyl groups of starch. Without sacrificing too many reaction sites, the hydrophilicity of starch itself is significantly reduced, thereby improving water resistance from the source.

[0025] Preferably, the method for preparing the modified nanocellulose includes the following steps: Under an inert atmosphere, nanocellulose was dispersed in an aqueous isopropanol solution, potassium hydroxide was added, and the temperature was lowered to 10-15°C. Propylene oxide was added first, and the temperature was raised to 55-65°C for 3-5 hours. The temperature was then lowered to -20 to -15°C, ethylene oxide was added, and the temperature was raised to 55-65°C for another 2-3 hours. The mixture was then cooled, neutralized, and subjected to solid-liquid separation, washing, and drying to obtain modified nanocellulose.

[0026] Preferably, the mass ratio of the nanocellulose, propylene oxide and ethylene oxide is 1:(0.3~1):(0.3~0.8).

[0027] More preferably, the mass ratio of the nanocellulose, propylene oxide, and ethylene oxide is 1:(0.5~0.8):(0.5~0.6).

[0028] In this scheme, under the catalysis of potassium hydroxide, nanocellulose first forms a hydrophobic polyoxypropylene segment as a core through ring-opening grafting with propylene oxide, reducing agglomeration; then, it forms a hydrophilic polyoxyethylene segment as a shell through ring-opening grafting with ethylene oxide, enhancing compatibility with the matrix; together, they construct a block polyether structure of hydrophobic core and hydrophilic shell, simultaneously achieving nano-reinforcement and interfacial compatibility. The modified nanocellulose can be uniformly dispersed in the adhesive system, its hydrophobic core effectively blocks water penetration, while its hydrophilic shell forms strong hydrogen bonds with biomass matrices such as starch and protein, thereby significantly improving the water resistance and mechanical properties of the adhesive.

[0029] Preferably, the low-formaldehyde environmentally friendly adhesive further includes 8-10 parts by weight of natural latex powder.

[0030] In this solution, the high molecular weight polyisoprene chains in natural latex act as a flexible phase, effectively absorbing and dispersing external stress, significantly improving the flexibility and impact resistance of the adhesive film. Simultaneously, the active groups on the surface of the natural latex powder particles provide additional sites for the composite crosslinking agent, allowing it to form chemical bonds with the matrix during final curing, preventing phase separation and achieving a balance between improved toughness and enhanced initial tack.

[0031] Preferably, the low-formaldehyde environmentally friendly adhesive further includes 3-5 parts by weight of chitosan.

[0032] In this solution, the amino groups on the surface of chitosan molecules can undergo strong ionic interactions with the carboxyl groups of modified starch, plant protein, and polyaspartic acid, forming a dense polyelectrolyte complex network. This multi-linking mechanism greatly enhances the density and stability of the crosslinked network, while significantly improving the dry and wet bonding strength of the adhesive. Furthermore, the inherent positive charge and antibacterial properties of chitosan provide natural antibacterial and antifungal protection for the nutrient-rich starch-protein system, extending the adhesive's shelf life and post-use durability. Its excellent film-forming properties and gas-phase barrier properties also form a barrier within the adhesive layer, indirectly improving water resistance and substrate protection.

[0033] Secondly, this application provides a method for preparing a low-formaldehyde environmentally friendly adhesive, comprising the following steps: S1: Mix the composite crosslinking agent with some water until homogeneous, adjust the pH to 8.5~9.5, mix for 1.5~2 hours to obtain a premixed solution; S2: Under an inert atmosphere, mix the modified starch, plant protein and remaining water evenly, heat to 50~65℃, mix for 20~40 min, add the modified nanocellulose, continue mixing for 15~30 min, add the premixed liquid, continue mixing for 20~40 min, cool, and obtain a low formaldehyde environmentally friendly adhesive.

[0034] In this scheme, alkaline premixing aims to pre-hydrolyze aminosilanes and activate the carboxyl groups of polyaspartic acid, preparing for subsequent crosslinking. Modified starch and plant protein are pre-formed into a continuous phase matrix at 50-65℃. Subsequently added modified nanocellulose can form hydrogen bonds with the matrix through a hydrophilic shell, achieving initial anchoring. The final addition of the premix ensures that the crosslinking agent is uniformly dispersed in the system, resulting in preliminary pre-crosslinking. This step aims to ensure that the adhesive has good storage stability and application rheology. The final, complete three-dimensional network crosslinking reaction is triggered by externally supplied energy (heat) during the hot pressing process after adhesive application.

[0035] Preferably, in step S2, after adding plant protein, the step of adding natural latex powder is also included.

[0036] Preferably, in step S2, after adding the plant protein, the step of adding the chitosan dispersion is also included; The chitosan dispersion is prepared by mixing chitosan with acetic acid at a mass fraction of 3% to 5%.

[0037] In summary, this application has the following beneficial effects: In this application, by enzymatic hydrolysis of starch and synergistic modification with composite anhydrides, hydrophobic properties are endowed from the molecular source while retaining high reactivity; by using modified nanocellulose with a "hydrophobic core-hydrophilic shell" structure, interface enhancement and uniform dispersion at the nanoscale are achieved; and by using a composite crosslinking system of aminosilane and polyaspartic acid, multiple reactions occur with each component during the curing stage (subsequent hot pressing), constructing a stable three-dimensional network in the matrix. Detailed Implementation

[0038] The present application will be further described in detail below with reference to the embodiments.

[0039] Unless otherwise specified, the raw materials used in the embodiments and comparative examples of this application are all commercially available.

[0040] Preparation Examples 1-3: Modified Starch Preparation Example 1 The method for preparing the modified starch in this preparation example includes the following steps: 5 kg of corn starch was added to 15 L of deionized water and stirred at 500 rpm for 30 min. While stirring, the mixture was heated to 60 °C, and 0.025 kg of α-amylase was added. After incubation for 45 min, the mixture was heated to 95 °C and incubated for 12 min. After cooling to room temperature, the mixture was centrifuged, washed twice with anhydrous ethanol, and dried at 60 °C to constant weight. Under nitrogen protection, the mixture was redispersed in 20 L of anhydrous ethanol and stirred for 30 min. Then, 0.5 kg of maleic anhydride, 0.5 kg of acetic anhydride, and 0.015 kg of p-toluenesulfonic acid pyridine salt were added and stirred until homogeneous. The mixture was heated to 65 °C and stirred for 2.5 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, washed three times with anhydrous ethanol, and dried at 60 °C to constant weight to obtain modified starch.

[0041] Preparation Example 2 The method for preparing the modified starch in this preparation example includes the following steps: Add 5 kg of corn starch to 15 L of deionized water and stir at 500 rpm for 30 min. While stirring, heat to 65 °C, add 0.03 kg of α-amylase, keep warm and mix for 50 min, then heat to 100 °C and keep warm for 10 min. After cooling to room temperature, centrifuge and wash twice with anhydrous ethanol. Dry at 60 °C to constant weight. Under nitrogen protection, redisperse in 20 L of anhydrous ethanol and stir for 30 min. Add 0.5 kg of maleic anhydride, 0.5 kg of acetic anhydride, 0.25 kg of phthalic anhydride, and 0.025 kg of p-toluenesulfonic acid pyridine salt. Stir until homogeneous, heat to 70 °C, and stir for 2 h. After the reaction is complete, cool to room temperature, filter the product, wash three times with anhydrous ethanol, and dry at 60 °C to constant weight to obtain modified starch.

[0042] Preparation Example 3 The method for preparing the modified starch in this preparation example includes the following steps: Add 5 kg of corn starch to 15 L of deionized water and stir at 500 rpm for 30 min. While stirring, heat to 65 °C, add 0.02 kg of α-amylase, keep warm and mix for 40 min, then heat to 100 °C and keep warm for 8 min. After cooling to room temperature, centrifuge and wash twice with anhydrous ethanol. Dry at 60 °C to constant weight. Under nitrogen protection, redisperse in 20 L of anhydrous ethanol and stir for 30 min. Add 0.5 kg of maleic anhydride, 0.25 kg of succinic anhydride, and 0.015 kg of p-toluenesulfonic acid pyridine salt. Stir until homogeneous, heat to 60 °C, and stir for 3 h. After the reaction is complete, cool to room temperature, filter the product, wash three times with anhydrous ethanol, and dry at 60 °C to constant weight to obtain modified starch.

[0043] Preparation Examples 4-6: Modified Nanocellulose Preparation Example 4 The preparation method of the modified nanocellulose in this example includes the following steps: Under nitrogen protection, 2 kg of nanocellulose was added to 8 L of isopropanol aqueous solution (isopropanol and deionized water volume ratio 4:1), transferred to an ultrasonic device, and set to 300 W, 40 kHz. After ultrasonication for 30 min, it was transferred to a reaction vessel. Under nitrogen atmosphere, 0.15 kg of potassium hydroxide (pre-dissolved in 1 L of deionized water) was added and stirred for 30 min. The mixture was cooled to 10 °C, and 1 kg of propylene oxide was slowly added dropwise. The temperature was raised to 65 °C and reacted for 3 h. The mixture was then cooled to -15 °C, and 1 kg of ethylene oxide (pre-cooled to liquid state) was slowly added. The temperature was raised to 65 °C and the reaction was continued for 2 h. After the reaction was completed, the system was cooled to room temperature, and the pH was adjusted to neutral using 10% glacial acetic acid. The solid product was obtained by centrifugation, washed three times each with methanol and acetone, and dried at 50 °C to constant weight to obtain modified nanocellulose.

[0044] Preparation Example 5 The preparation method of the modified nanocellulose in this example includes the following steps: Under nitrogen protection, 2 kg of nanocellulose was added to 8 L of isopropanol aqueous solution (isopropanol and deionized water volume ratio 4:1), transferred to an ultrasonic device, and set to 300 W, 40 kHz. After ultrasonication for 30 min, it was transferred to a reaction vessel. Under nitrogen atmosphere, 0.2 kg of potassium hydroxide (pre-dissolved in 1 L of deionized water) was added and stirred for 30 min. The mixture was cooled to 15 °C, and 1.6 kg of propylene oxide was slowly added dropwise. The temperature was raised to 60 °C and reacted for 5 h. The temperature was lowered to -15 °C, and 1 kg of ethylene oxide (pre-cooled to liquid state) was slowly added. The temperature was raised to 60 °C and the reaction was continued for 2 h. After the reaction was completed, the system was cooled to room temperature, and the pH was adjusted to neutral using 10% glacial acetic acid. The solid product was obtained by centrifugation, washed three times each with methanol and acetone, and dried at 50 °C to constant weight to obtain modified nanocellulose.

[0045] Preparation Example 6 The preparation method of the modified nanocellulose in this example includes the following steps: Under nitrogen protection, 2 kg of nanocellulose was added to 8 L of isopropanol aqueous solution (isopropanol and deionized water volume ratio 4:1), transferred to an ultrasonic device, and set to 300 W, 40 kHz. After ultrasonication for 30 min, it was transferred to a reaction vessel. Under nitrogen atmosphere, 0.25 kg of potassium hydroxide (pre-dissolved in 1 L of deionized water) was added and stirred for 30 min. The mixture was cooled to 10 °C, and 1.2 kg of propylene oxide was slowly added dropwise. The temperature was raised to 55 °C and reacted for 4 h. The temperature was lowered to -20 °C, and then 1.2 kg of ethylene oxide (pre-cooled to liquid state) was slowly added. The temperature was raised to 55 °C and the reaction was continued for 3 h. After the reaction was completed, the system was cooled to room temperature, and the pH was adjusted to neutral using 10% glacial acetic acid. The solid product was obtained by centrifugation, washed three times each with methanol and acetone, and dried at 50 °C to constant weight to obtain modified nanocellulose.

[0046] Example 1 The preparation method of the low-formaldehyde environmentally friendly adhesive in this embodiment includes the following steps: S1: Mix 0.045 kg of silane coupling agent KH550, 0.045 kg of polyaspartic acid and 0.8 kg of deionized water evenly, adjust the pH to 9.0 with triethylamine, stir and mix at 25°C for 1.5 h to obtain a premixed solution; S2: Under a nitrogen atmosphere, 1.2 kg of modified starch, 0.6 kg of soybean protein and 2.8 kg of deionized water are mixed evenly, heated to 60°C, and stirred for 30 min. 0.24 kg of modified nanocellulose is added and the mixture is stirred for another 20 min. Under stirring, the premixed liquid is added and the mixture is stirred for another 30 min. The mixture is then stirred and cooled to below 40°C and discharged to obtain a low-formaldehyde environmentally friendly adhesive.

[0047] Soy protein undergoes the following pretreatment steps before use: Add soybean protein powder to water at a solid-liquid ratio of 1g:10mL, stir and mix at 500rpm for 20min, adjust the pH to 10 with 10% sodium hydroxide solution, heat to 50℃, stir and mix at 200rpm for 90min, adjust the pH to neutral with 10% glacial acetic acid, centrifuge, wash three times with deionized water, dry at 45℃ to constant weight, grind and disperse for later use.

[0048] The modified starch was prepared in Example 1; the modified nanocellulose was prepared in Example 4.

[0049] Example 2 The preparation method of the low-formaldehyde environmentally friendly adhesive in this embodiment includes the following steps: S1: Mix 0.067 kg of silane coupling agent KH550, 0.083 kg of polyaspartic acid and 1 kg of deionized water evenly, adjust the pH to 9.5 with triethylamine, stir and mix at 25°C for 1.5 h to obtain a premixed solution; S2: Under a nitrogen atmosphere, mix 1.5 kg of modified starch, 0.75 kg of soybean protein and 3.5 kg of deionized water evenly, heat to 65°C, stir and mix for 20 min, add 0.3 kg of modified nanocellulose, continue mixing for 30 min, add premixed liquid while stirring, continue mixing for 20 min, stir and cool to below 40°C, and discharge to obtain a low formaldehyde environmentally friendly adhesive.

[0050] Soy protein undergoes the following pretreatment steps before use: Add soybean protein powder to water at a solid-liquid ratio of 1g:10mL, stir and mix at 500rpm for 20min, adjust the pH to 11 with 10% sodium hydroxide solution, heat to 70℃, stir and mix at 200rpm for 60min, adjust the pH to neutral with 10% glacial acetic acid, centrifuge, wash three times with deionized water, dry at 45℃ to constant weight, grind and disperse for later use.

[0051] The modified starch was prepared in Example 2; the modified nanocellulose was prepared in Example 5.

[0052] Example 3 The preparation method of the low-formaldehyde environmentally friendly adhesive in this embodiment includes the following steps: S1: Mix 0.033 kg of silane coupling agent KH550, 0.027 kg of polyaspartic acid and 0.5 kg of deionized water evenly, adjust the pH to 8.5 with triethylamine, stir and mix at 25°C for 2 hours to obtain a premixed solution; S2: Under a nitrogen atmosphere, mix 0.9 kg of modified starch, 0.45 kg of soybean protein and 2.0 kg of deionized water evenly, heat to 50°C, stir and mix for 40 min, add 0.15 kg of modified nanocellulose, continue mixing for 15 min, add premixed liquid while stirring, continue mixing for 40 min, stir and cool to below 40°C, discharge to obtain low formaldehyde environmentally friendly adhesive.

[0053] Soy protein undergoes the following pretreatment steps before use: Add soybean protein powder to water at a solid-liquid ratio of 1g:10mL, stir and mix at 500rpm for 20min, adjust the pH to 10.5 with 10% sodium hydroxide solution, heat to 60℃, stir and mix at 200rpm for 70min, adjust the pH to neutral with 10% glacial acetic acid, centrifuge, wash three times with deionized water, dry at 45℃ to constant weight, grind and disperse for later use.

[0054] The modified starch was prepared in Example 3; the modified nanocellulose was prepared in Example 6.

[0055] Example 4 The preparation method of the low-formaldehyde environmentally friendly adhesive in this embodiment includes the following steps: S1: Mix 0.06 kg of silane coupling agent KH792, 0.06 kg of polyaspartic acid and 0.8 kg of deionized water evenly, adjust the pH to 9.0 with triethylamine, stir and mix at 25°C for 2 hours to obtain a premixed solution; S2: Under a nitrogen atmosphere, 1.2 kg of modified starch, 0.6 kg of pea protein and 2.8 kg of deionized water are mixed evenly, heated to 60°C and stirred for 30 min. 0.24 kg of modified nanocellulose is added and the mixture is stirred for another 30 min. Under stirring, the premixed liquid is added and the mixture is stirred for another 30 min. The mixture is then stirred and cooled to below 40°C and discharged to obtain a low-formaldehyde environmentally friendly adhesive.

[0056] Pea protein undergoes the following pretreatment steps before use: Add pea protein powder to water at a solid-liquid ratio of 1g:10mL, stir and mix at 500rpm for 20min, adjust the pH to 10 with 10% sodium hydroxide solution, heat to 50℃, stir and mix at 200rpm for 90min, adjust the pH to neutral with 10% glacial acetic acid, centrifuge, wash three times with deionized water, dry at 45℃ to constant weight, grind and disperse for later use.

[0057] The modified starch was prepared in Example 1; the modified nanocellulose was prepared in Example 6.

[0058] Example 5 The difference between this embodiment and embodiment 4 is that: S2: Under a nitrogen atmosphere, 1.2 kg of modified starch, 0.6 kg of pea protein, 0.24 kg of natural latex powder and 3.2 kg of deionized water are mixed evenly, heated to 60°C, and stirred for 30 min. 0.24 kg of modified nanocellulose is added and the mixture is stirred for another 30 min. Under stirring, the premixed liquid is added and the mixture is stirred for another 30 min. The mixture is then stirred and cooled to below 40°C, and discharged to obtain a low-formaldehyde environmentally friendly adhesive.

[0059] The rest is the same as in Example 4.

[0060] Example 6 The difference between this embodiment and embodiment 5 is as follows: S2: Under a nitrogen atmosphere, 1.2 kg of modified starch, 0.6 kg of pea protein, 0.3 kg of natural latex powder and 3.3 kg of deionized water are mixed evenly, heated to 60°C, and stirred for 30 min. 0.24 kg of modified nanocellulose is added and stirred for another 30 min. Under stirring, the premixed liquid is added and stirred for another 30 min. The mixture is then stirred and cooled to below 40°C. The product is then discharged to obtain a low-formaldehyde environmentally friendly adhesive.

[0061] The rest is the same as in Example 5.

[0062] Example 7 The difference between this embodiment and embodiment 5 is as follows: S2: Under a nitrogen atmosphere, 1.2 kg of modified starch, 0.6 kg of pea protein, 0.24 kg of natural latex powder, chitosan dispersion (containing 0.09 kg of chitosan) and 2.26 kg of deionized water are mixed evenly, heated to 60°C, and stirred for 30 min. 0.24 kg of modified nanocellulose is added, and mixing continues for 30 min. Under stirring, premixed liquid is added, and mixing continues for 30 min. The mixture is stirred and cooled to below 40°C, and then discharged to obtain a low-formaldehyde environmentally friendly adhesive.

[0063] The chitosan dispersion is prepared by mixing 0.09 kg of chitosan and 0.9 kg of acetic acid with a mass fraction of 5% evenly.

[0064] The rest is the same as in Example 5.

[0065] Example 8 The difference between this embodiment and embodiment 4 is that: S2: Under a nitrogen atmosphere, 1.2 kg of modified starch, 0.6 kg of pea protein, chitosan dispersion (containing 0.15 kg of chitosan) and 1.6 kg of deionized water are mixed evenly, heated to 60°C, and stirred for 30 min. 0.24 kg of modified nanocellulose is added, and mixing continues for 30 min. Under stirring, premixed liquid is added, and mixing continues for 30 min. The mixture is stirred and cooled to below 40°C, and then discharged to obtain a low-formaldehyde environmentally friendly adhesive.

[0066] The chitosan dispersion is prepared by mixing 0.15 kg of chitosan and 1.5 kg of acetic acid with a mass fraction of 3% evenly.

[0067] The rest is the same as in Example 4.

[0068] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: The modified starch was replaced with an equal mass of corn starch.

[0069] Everything else is the same as in Example 1.

[0070] Comparative Example 2 The difference between this comparative example and Example 1 is as follows: The modified nanocellulose was replaced with an equal mass of nanocellulose.

[0071] Everything else is the same as in Example 1.

[0072] Comparative Example 3 The difference between this comparative example and Example 1 is as follows: S1: Mix 0.09 kg of silane coupling agent KH550 and 0.8 kg of deionized water evenly, adjust the pH to 9.0 with triethylamine, stir and mix at 25°C for 1.5 h to obtain a premixed solution; Everything else is the same as in Example 1.

[0073] Comparative Example 4 The difference between this comparative example and Example 1 is as follows: Soy protein powder is untreated.

[0074] Everything else is the same as in Example 1.

[0075] Performance testing The low-formaldehyde environmentally friendly adhesives prepared in Examples 1-8 and Comparative Examples 1-4 were subjected to performance tests according to GB / T 17657-2013 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels" and GB 18583-2008 "Limits of Hazardous Substances in Adhesives for Interior Decoration and Renovation Materials". The test results are shown in Table 1. The specific steps are as follows: Preparation of glued specimens: The adhesive to be tested was evenly applied to a poplar veneer (200mm×200mm×2mm), and the amount of adhesive applied (total amount applied to both sides) was controlled to be approximately 300~320g / m². 2 After applying the adhesive, the material is left to age at 25±2℃ for 10 minutes. Then, the material is assembled into three-layer plywood, which is then hot-pressed and cured on a hot press. The process parameters are: temperature 125℃, pressure 1.2MPa, and hot-pressing time calculated based on the total thickness of the three-layer plywood, which is approximately 50~60s / mm.

[0076] Specimen handling and testing: The hot-pressed specimen was sawn into standard samples and treated and tested under the following conditions: Dry bond strength: The sample was conditioned in a standard climate chamber at 20±2℃ and 65±5% relative humidity for 7 days and then tested immediately. Wet bond strength: The sample was soaked in hot water at 63±2℃ for 3 hours, then removed and cooled at 25±2℃ for 10 minutes, and then tested immediately.

[0077] Table 1. Performance test results of the low-formaldehyde environmentally friendly adhesives prepared in Examples 1-8 and Comparative Examples 1-4

[0078] Analysis of Examples 1-4 shows that within the technical system constructed in this application, by adjusting the dosage of each component and process conditions, the comprehensive performance of the adhesive can be effectively controlled, maintaining good dry strength while possessing excellent water resistance.

[0079] Analysis of Example 1 and Comparative Examples 1-4 shows that the water resistance of Comparative Example 1 decreased significantly, indicating that the hydrophobic modification of starch is the basis for improving water resistance; Comparative Examples 2 and 4 show that modified nanocellulose and pretreated plant protein powder are indispensable for enhancing mechanical properties and network density; Comparative Example 3 highlights the synergistic advantages of the hybrid network constructed by polyaspartic acid and aminosilane.

[0080] Analysis of Examples 5-8 shows that the introduction of natural latex powder and chitosan, either individually or in combination, leads to a synergistic improvement in both dry and wet bonding strength, particularly achieving a high level of wet strength. This indicates that the flexibility and toughening effect of natural latex powder and the polyelectrolyte complexing effect of chitosan can positively interact with the basic three-dimensional network, optimizing the overall performance of the adhesive layer.

[0081] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A low-formaldehyde environmentally friendly adhesive, characterized in that, It is prepared from the following raw materials in parts by weight: 30-50 parts modified starch, 15-25 parts plant protein, 5-10 parts modified nanocellulose, 2-5 parts composite crosslinking agent, and 80-150 parts water; The composite crosslinking agent includes aminosilane and polyaspartic acid; The modified starch is obtained by enzymatic hydrolysis of starch with α-amylase, followed by graft copolymerization with a composite acid anhydride. The plant protein undergoes alkalization and heat treatment before use; The modified nanocellulose was prepared by sequentially reacting nanocellulose with propylene oxide and ethylene oxide in a ring-opening etherification reaction.

2. The low-formaldehyde environmentally friendly adhesive according to claim 1, characterized in that, The aminosilane is a silane coupling agent KH550 or KH792.

3. The low-formaldehyde environmentally friendly adhesive according to claim 1, characterized in that, The plant protein undergoes the following pretreatment steps before use: Disperse the plant protein powder in water, adjust the pH to 10-11, heat to 50-70℃, mix for 60-90 minutes, neutralize, separate the solid and liquid, wash, dry, and set aside.

4. The low-formaldehyde environmentally friendly adhesive according to claim 1, characterized in that, The method for preparing the modified starch includes the following steps: Starch is uniformly dispersed in water, heated to 60-65℃, α-amylase is added, and the mixture is stirred for 40-50 minutes. The temperature is then raised to 95-100℃ and kept at that temperature for 8-12 minutes. After cooling and solid-liquid separation, the starch is redispersed in anhydrous ethanol under an inert atmosphere. Composite acid anhydride and catalyst are added and mixed evenly. The temperature is raised to 60-70℃ and the reaction is carried out for 2-3 hours. After cooling, solid-liquid separation is performed, and the mixture is dried to obtain modified starch.

5. The low-formaldehyde environmentally friendly adhesive according to claim 4, characterized in that, The amount of the composite acid anhydride used is 10% to 30% of the starch mass.

6. The low-formaldehyde environmentally friendly adhesive according to claim 4, characterized in that, The composite anhydride is selected from at least two of maleic anhydride, acetic anhydride, succinic anhydride, and phthalic anhydride.

7. The low-formaldehyde environmentally friendly adhesive according to claim 1, characterized in that, The method for preparing the modified nanocellulose includes the following steps: Under an inert atmosphere, nanocellulose was dispersed in an aqueous isopropanol solution, potassium hydroxide was added, and the temperature was lowered to 10-15°C. Propylene oxide was added first, and the temperature was raised to 55-65°C for 3-5 hours. The temperature was then lowered to -20 to -15°C, ethylene oxide was added, and the temperature was raised to 55-65°C for another 2-3 hours. The mixture was then cooled, neutralized, and subjected to solid-liquid separation, washing, and drying to obtain modified nanocellulose.

8. The low-formaldehyde environmentally friendly adhesive according to claim 1, characterized in that, The low-formaldehyde environmentally friendly adhesive also includes 8-10 parts by weight of natural latex powder.

9. A method for preparing a low-formaldehyde environmentally friendly adhesive as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1: Mix the composite crosslinking agent with some water until homogeneous, adjust the pH to 8.5~9.5, mix for 1.5~2 hours to obtain a premixed solution; S2: Under an inert atmosphere, mix the modified starch, plant protein and remaining water evenly, heat to 50~65℃, mix for 20~40 min, add the modified nanocellulose, continue mixing for 15~30 min, add the premixed liquid, continue mixing for 20~40 min, cool, and obtain a low formaldehyde environmentally friendly adhesive.

10. The method for preparing the low-formaldehyde environmentally friendly adhesive according to claim 9, characterized in that, In step S2, after adding plant protein, the step of adding natural latex powder is also included.

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