Soybean protein glue film as well as preparation method and application thereof

Abstract

The invention discloses a soybean protein adhesive film and a preparation method thereof. The soybean protein adhesive film is prepared from the following raw materials: 6 parts of soybean protein, 0.1-1 part of an alkalizer, 0.5-5 parts of a protein denaturant, 1.2-2.4 parts of a polyol plasticizer and 30-60 parts of a solvent. The preparation method comprises the following steps: S1, dispersing soybean protein in a solvent to form a suspension, adding an alkalizer and a protein denaturant to obtain a mixed solution A, and heating and stirring for protein denaturation treatment to obtain a denatured soybean protein solution; s2, adding a polyol plasticizer into the modified soybean protein solution, and uniformly mixing to obtain a film forming solution; s3, coating a substrate with the film forming liquid to form a wet film; and S4, drying the wet film to obtain the soybean protein glue film. By reasonably using the basifier, the protein denaturant and the plasticizer, the solid content of the soybean protein adhesive film is improved, so that the prepared soybean protein adhesive film exists in a solid adhesive film form, the amount of water needing to be evaporated in the hot pressing process is reduced, and the problem that the solid content of traditional liquid soybean meal adhesive is low is thoroughly solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of adhesive film material preparation, in particular to a soy protein adhesive film and a preparation method and application thereof. BACKGROUND

[0002] With the increasing concern for environmental protection and human health, traditional formaldehyde-based synthetic resin adhesives (such as urea-formaldehyde resin, phenol-formaldehyde resin) are facing increasing challenges due to their formaldehyde emission problems. Therefore, the development of environmentally friendly biomass adhesives has become a hot spot in the industry.

[0003] Soy protein, as a kind of abundant and renewable natural polymer material, is widely studied for preparing wood adhesives. Traditional soy protein adhesives are usually prepared by dispersing soybean meal or soy protein isolate in water and adding modifiers to form a liquid adhesive. However, such liquid adhesives generally have the following technical bottlenecks: (a) Low solid content: usually below 40%, a large amount of water needs to be evaporated during hot pressing, resulting in high energy consumption, long hot pressing time, and possible defects such as board bubbling and warping.

[0004] (b) Poor water resistance: the cured adhesive layer is prone to swelling, softening or even dissolving when exposed to water, resulting in a significant decrease in bonding strength and failing to meet the requirements for indoor and outdoor use.

[0005] (c) Poor storage stability: the liquid environment rich in protein and water is prone to microbial growth, leading to mold and spoilage of the adhesive, short shelf life, and strict storage conditions.

[0006] Therefore, there is an urgent need for a new form of soy protein adhesive that can fundamentally overcome the above-mentioned defects and promote the large-scale application of soy protein adhesives. SUMMARY

[0007] To solve the above technical problems, the present application provides a soy protein adhesive film preparation method, and the specific technical solution is as follows: The soy protein adhesive film preparation raw materials include the following components by weight: 6 parts of soy protein, 0.1-1 parts of alkalizing agent, 0.5-5 parts of protein denaturant, 1.2-2.4 parts of polyol plasticizer, and 30-60 parts of solvent; the preparation method comprises the following steps: S1. Disperse soy protein in solvent to form a suspension, add alkalizing agent and protein denaturant to obtain a mixed solution A, and heat and stir to perform protein denaturation treatment to obtain a denatured soy protein solution; S2. Add polyol plasticizer to the denatured soy protein solution and mix uniformly to obtain a film-forming solution; S3. Apply the film-forming solution to a substrate to form a wet film; S4. drying the wet film to obtain a soy protein adhesive film.

[0008] Preferably, the alkalizing agent is selected from at least one of sodium hydroxide, potassium hydroxide or calcium hydroxide.

[0009] Preferably, the protein denaturant is selected from sodium 2-mercaptoethanesulfonate or sodium bis(2-sulfatoethyl) disulfide.

[0010] Preferably, the polyol plasticizer is selected from at least one of glycerol, sorbitol, propylene glycol.

[0011] Preferably, the soy protein is defatted soybean meal with a protein content of ≥45wt%, a particle size of ≤15μm, and a moisture content of ≤10wt%.

[0012] Preferably, in step S1, the pH of the mixed solution A is 8.5-11.0. The heating temperature is 60-90℃. The stirring time is 20-40min.

[0013] Preferably, in step S4, the drying temperature is 40-80℃, and the moisture content is ≤25wt%.

[0014] Preferably, in step S3, the thickness of the wet film is 0.10-0.30mm.

[0015] Preferably, the sodium bis(2-sulfatoethyl) disulfide is prepared by the following steps: a. dissolving sodium 2-mercaptoethanesulfonate in water to form an aqueous solution; b. adding an aqueous hydrogen peroxide solution to the aqueous solution for oxidation reaction; c. concentrating the reaction solution after the reaction is completed; d. adding ethanol to the concentrated solution to precipitate solids; e. separating the solids and washing with ethanol; f. drying the solids to obtain sodium bis(2-sulfatoethyl) disulfide; wherein the molar ratio of the hydrogen peroxide to the sodium 2-mercaptoethanesulfonate is 1.00:1-1.10:1.

[0016] Preferably, the soy protein adhesive film is used in the preparation of wood-based artificial boards such as plywood, particle board, etc. and non-wood plant-based composite materials. The application includes applying the soy protein adhesive film between wood veneers, wood particles or other non-wood plant-based raw materials, and gluing under heat and pressure conditions.

[0017] The adhesive film prepared from the above components has the following beneficial effects: 1. The typical problem of traditional soybean protein adhesive is solved fundamentally: by reasonable use of alkalizing agent, protein denaturant and plasticizer, the solid content of soybean protein adhesive film is increased, so that the prepared soybean protein film exists in the form of solid adhesive film, the solid content is ≥75wt%, when the adhesive film is used to bond the substrates, the amount of water that needs to be evaporated in the hot pressing process is reduced, and the technical problems of low solid content, high energy consumption and long hot pressing time of traditional liquid soybean meal adhesive are completely solved. In addition, due to the high solid content of the adhesive prepared by the above components, the storage time can be improved, and there is no mildew after storage for two months at 25℃ and 80% RH.

[0018] 2. The problem of poor water resistance of traditional soybean protein adhesive is solved: the protein denaturant (2-mercaptoethanesulfonic acid sodium or bis(2-sulfonic acid ethyl) sodium disulfide) is used to break the disulfide bond, expose and rearrange the hydrophobic region, the protein molecules are tightly packed during the drying process, and the hydrogen bond crosslinking network is formed with polyol, and finally the 24h water absorption rate of the adhesive film is reduced to 15%-28%, the adhesive film is not dissolved in boiling water for 2h, and the wet shear strength retention rate is 68%-85% (traditional <30%).

[0019] 3. Green and environmentally friendly: the adhesive film is made of renewable soybean meal, no formaldehyde is added during production and use, and the soybean protein adhesive film is a degradable and environmentally friendly material, which meets the concept of green chemistry, reduces the burden on the environment, and meets the requirements of green and environmentally friendly.

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 Digital photo of soybean protein adhesive film prepared in Example 1 Figure 2 Schematic diagram of test sample for wet shear strength of plywood. DETAILED DESCRIPTION

[0022] In order to make those skilled in the art better understand the technical solutions of the present application, the present application will be described in detail below with reference to the drawings, and the description in this part is only exemplary and explanatory, and should not have any limiting effect on the protection scope of the present application.

[0023] The present embodiment provides a soybean protein adhesive film preparation method, and the specific technical solution is: The preparation raw materials include, by weight: 6 parts of soybean protein, 0.1-1 parts of alkalizing agent, 0.5-5 parts of protein denaturant, 1.2-2.4 parts of polyol plasticizer, and 30-60 parts of solvent; the preparation method comprises the following steps: S1. Disperse soybean protein in a solvent to form a suspension, add an alkalizing agent and a protein denaturant to obtain a mixed solution A, heat and stir to perform protein denaturation treatment, and obtain a denatured soybean protein solution; S2. Add a polyol plasticizer to the denatured soybean protein solution and mix uniformly to obtain a film-forming solution; S3. Apply the film-forming solution to a substrate to form a wet film; S4. Dry the wet film to obtain a soybean protein adhesive film.

[0024] Soybean protein is the main film-forming material, providing the skeleton structure and strength of the adhesive film; the alkalizing agent adjusts the pH to alkaline, breaks the hydrogen bonds and disulfide bonds between protein molecules, increases the solubility and reactivity of the protein, promotes protein dissolution and unfolding, and improves film-forming properties; the protein denaturant exposes the hydrophobic groups and active sites by destroying the secondary and tertiary structures of the protein, fully stretches the protein chain, enhances intermolecular crosslinking, and ultimately improves the tensile strength and water resistance of the adhesive film; the polyol plasticizer is used to insert between the protein molecular chains, weaken the protein-protein interaction, increase the chain segment mobility, improve the flexibility and ductility of the adhesive film, and prevent brittle fracture; the formed solid soybean protein film has a high solid content; the total moisture introduced during the bonding of the soybean protein film to the substrate is only from the substrate itself, which is much lower than the 25-40wt% of traditional liquid adhesive, and the moisture can be removed to the equilibrium moisture content within 5-7min at 120℃ heat pressing without residual water vapor pressure, effectively reducing the heat pressing cycle and the blistering rate.

[0025] In step S1, by heating and stirring soybean protein with an alkalizing agent and a protein denaturant in a solvent, the denaturation reaction of soybean protein can be initiated, the structure can be changed, the solubility can be increased, and conditions can be provided for subsequent film formation. Denatured protein has strong adhesive properties and helps to enhance the mechanical properties and stability of the adhesive film.

[0026] In step S2, polyol plasticizer is added to the denatured soybean protein solution. The plasticizer can effectively reduce the brittleness of the adhesive film, increase the flexibility, and improve the formability and operability of the film. In addition, the plasticizer can also improve the water resistance and ductility of the film, and reduce the performance degradation caused by environmental changes.

[0027] The wet film coating thickness in step S3 is controlled to ensure the uniformity and appropriate mechanical properties of the film, avoiding the local performance differences caused by uneven coating in traditional soybean protein adhesives.

[0028] By controlling the drying temperature (40-80℃) in step S4, the moisture in the wet film can be removed to a low level (≤25wt%), reducing the residual moisture and improving the storage stability of the adhesive film. The dried film is more resistant to microbial attack, reducing the risk of mold and spoilage.

[0029] The adhesive film prepared by the above components has the following beneficial effects: 1. The typical problem of traditional soy protein adhesive is fundamentally solved: by reasonably using alkali agent, protein denaturant and plasticizer, the solid content of soy protein adhesive film is increased, so that the prepared soy protein film exists in the form of solid adhesive film, the solid content is ≥75wt%, when the adhesive film is used to bond the substrates, the amount of water that needs to be evaporated in the hot pressing process is reduced, and the technical problems of low solid content, high energy consumption and long hot pressing time of traditional liquid soybean meal adhesive are completely solved. In addition, due to the high solid content of the adhesive prepared by the above components, the storage time can be improved, and there is no mildew after storage for 60 days at 25°C and 80% RH.

[0030] 2. The problem of poor water resistance of traditional soy protein adhesive is solved: the protein denaturant (sodium 2-mercaptoethanesulfonate or sodium bis(2-sulfonatoethyl) disulfide) is used to break the disulfide bond, expose and rearrange the hydrophobic region, the protein molecules are tightly packed during the drying process, and the hydrogen bond crosslinking network is formed with polyol, and finally the 24h water absorption rate of the adhesive film is reduced to 15%-28%, the adhesive film is not dissolved in boiling water for 2h, and the wet shear strength retention rate is 68%-85% (traditional <30%).

[0031] 3. Green and environmentally friendly: the adhesive film uses renewable soybean meal as raw material, no formaldehyde is added during production and use, and the soy protein adhesive film is a degradable and environmentally friendly material, which meets the concept of green chemistry, reduces the burden on the environment, and meets the requirements of green and environmentally friendly. Preferably, the alkali agent is selected from at least one of sodium hydroxide, potassium hydroxide or calcium hydroxide.

[0032] The main role of the alkali agent in the preparation of the soy protein adhesive film is to adjust the pH value of the solution, promote the dissolution and denaturation of the soy protein. The solubility of soy protein is poor, and after adding the alkali agent, the molecular chain of soy protein can be depolymerized and its hydrophilicity can be enhanced, which provides a basis for the subsequent formation of adhesive film. The alkali agent changes the structure of soy protein, which helps the protein molecules to interact with other components, thereby improving the film forming property and stability of the adhesive film. Sodium hydroxide, potassium hydroxide and calcium hydroxide are commonly used alkali agents. Sodium hydroxide and potassium hydroxide have strong alkalinity, which can effectively increase the pH of the solution and promote the denaturation of soy protein.

[0033] Preferably, the protein denaturant is selected from sodium 2-mercaptoethanesulfonate or sodium bis(2-sulfonatoethyl) disulfide.

[0034] Protein denaturant is used to break disulfide bonds, expose and rearrange hydrophobic regions, and reduce the water absorption of the film during the drying process. At the same time, soy protein has cross-linking properties and higher film-forming properties during the preparation of the film, which helps to improve the mechanical properties and stability of the film; 2-mercaptoethanesulfonic acid sodium (Na-MES) and bis(2-sulfonic acid ethyl) disulfide sodium (DSS) are commonly used protein denaturants; 2-mercaptoethanesulfonic acid sodium reacts with the sulfhydryl group in the protein through its thiol group, destroying the three-dimensional structure of the protein; both can effectively improve the adaptability of soy protein to the external environment during film formation, and enhance the comprehensive performance of the film. Bis(2-sulfonic acid ethyl) disulfide sodium further crosslinks the protein chain through oxidation; bis(2-sulfonic acid ethyl) disulfide sodium can be reformed in situ after high-temperature fracture during hot pressing, while 2-mercaptoethanesulfonic acid sodium can only rely on hydrogen bonding and a small amount of self-oxidation, and the cross-linking is not reversible and the density is much lower, the water resistance and the adhesive strength are better.

[0035] Preferably, the polyol plasticizer is selected from at least one of glycerol, sorbitol, and propylene glycol.

[0036] The plasticizer is an additive that can reduce the glass transition temperature of the soy protein film and improve flexibility. By adding a polyol plasticizer during preparation, the cohesive force of the film can be reduced, making the film more flexible, ductile, and operable. In addition, the plasticizer can also improve the water resistance of the film and reduce the embrittlement phenomenon of the film during use; glycerol, sorbitol, and propylene glycol are commonly used polyol plasticizers that interact with soy protein molecules to change the structure of the soy protein film, making it softer and more durable; glycerol as a plasticizer can provide better flexibility and lower volatility; sorbitol can increase the hydrolysis resistance of the film; propylene glycol has good plasticizing effect and is often used to improve the toughness and ductility of the film.

[0037] Preferably, the soy protein is defatted soybean meal with a protein content of ≥45wt%, a particle size of ≤15μm, and a water content of ≤10wt%.

[0038] High protein content and ultra-fine particle size allow the protein to fully expose hydrophobic groups and active sulfhydryl groups under alkaline conditions, and low water content avoids insufficient denaturation caused by pre-water absorption; protein dispersibility and denaturation efficiency are significantly improved, dry tensile strength of the film is increased by 20%-35% under the same conditions, film solid content can be stabilized to 75%-85%, and hot pressing water evaporation amount is significantly reduced.

[0039] Preferably, in step S1, the pH of the mixed solution A is 8.5-11.0. The heating temperature is 60-90℃; The stirring time is 20-40min.

[0040] The process involves heating at pH 8.5-11.0 for 20-40 minutes at 60-90℃, allowing the protein globular structure to fully expand in an alkaline environment. This process efficiently breaks down disulfide bonds with a protein denaturant, creating more active thiol groups, which are then re-oxidized and cross-linked during hot pressing. This thorough protein denaturation results in a film with a dry water resistance (wet strength / dry strength) of 65%-78% (compared to only 30%-45% for traditional soybean glue), a bonding strength of 1.2-1.8 MPa, and a 15%-25% reduction in hot pressing time.

[0041] Preferably, in step S4, the drying temperature is 40-80℃, and the product is dried until the moisture content is ≤25wt%.

[0042] Gently dry at 40-80℃ until the moisture content is ≤25wt%, retaining sufficient residual moisture to keep the film flexible while avoiding premature thermal cross-linking and inactivation of proteins due to high temperature; the residual moisture promotes the flow and re-cross-linking of protein molecular chains during hot pressing; the film will not become brittle or moldy after 6-12 months of storage at room temperature, and no additional water spraying is required for activation during hot pressing. Excellent bonding strength (≥1.3MPa) can still be achieved by hot pressing immediately after application, completely solving the problems of short shelf life and mold growth during storage of liquid adhesives.

[0043] Preferably, in step S3, the thickness of the wet film is 0.10-0.30 mm.

[0044] The wet film thickness is controlled between 0.10-0.30 mm to ensure rapid and uniform moisture escape during drying, preventing premature skin formation on the surface and internal moisture retention that could lead to pores or cracks. The film shape also ensures that the final dry film weight is between 80-220 g / m³. 2 It matches the single-sided glue application requirements of artificial boards; the glue film surface is flat without bubbles or micro-cracks, the dry film tensile strength is ≥12MPa, the uniformity of bonding strength after hot pressing is CV value ≤5%, the board pre-pressing performance is excellent, and the bonding defects caused by the drying cracking of thick coating are completely eliminated.

[0045] Preferably, the sodium bis(2-sulfonate ethyl) disulfide is prepared by the following steps: a. Dissolve sodium 2-mercaptoethanesulfonate in water to form an aqueous solution; b. Add an aqueous solution of hydrogen peroxide to the aqueous solution to carry out an oxidation reaction; c. After the reaction is complete, concentrate the reaction solution; d. Add ethanol to the concentrate to precipitate solids; e. Separate the solids and wash with ethanol; f. Dry the solid to give disodium bis(2-sulfonic acid ethyl) disulfide; In step a, the molar ratio of hydrogen peroxide to sodium 2-mercaptoethanesulfonate is 1.00:1 to 1.10:1.

[0046] Sodium 2-mercaptoethanesulfonate is precisely oxidized with hydrogen peroxide to generate sodium bis(2-sulfoethyl) disulfide (molar ratio 1.00-1.10:1). This compound first rapidly reduces and breaks protein disulfide bonds under alkaline-thermal conditions, and then re-oxidizes and closes them, avoiding residual low-molecular-weight sulfhydryl odor and excessive reduction. The denaturant has no irritating odor, the film has no sulfur odor, the denaturation efficiency is 45%-60% higher than that of urea / guanidine, and the wet bonding strength is 30%-50% higher.

[0047] Preferably, the above-mentioned soybean protein adhesive film is used in the preparation of wood-based panels such as plywood and particleboard, as well as in non-wood plant-based composite materials. This application includes applying the soybean protein adhesive film between wood veneers, wood particles, or other non-wood plant-based raw materials, and bonding them under hot-pressing conditions.

[0048] The aforementioned dry-process adhesive film has a solid content of 75%-85%, a moisture content of ≤25wt%, and is stable at room temperature. During hot pressing, residual moisture and temperature trigger the flow of protein molecular chains and the rearrangement of disulfide bonds to achieve chemical bonding with wood fibers. It can meet the requirements of Class I boards without the need for additional water activation. It can completely replace urea-formaldehyde / phenolic resins for plywood, particleboard, and agricultural and forestry residue boards. The formaldehyde release is ≤0.3mg / L, the hot pressing cycle is shortened by 20%-30%, and the overall production energy consumption is reduced by more than 25%, achieving truly formaldehyde-free, easy-to-store, and continuous production.

[0049] Specific embodiments are provided below. These embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the present invention in any way.

[0050] Example 1 Add 4.5 kg of deionized water to the reactor and start stirring at 300 r / min.

[0051] Add 6.0 kg of defatted soybean meal in 5 batches, with a protein content of 55.3 wt%, at 4-minute intervals, and stir for a total of 38 minutes until no visible particles are visible to the naked eye.

[0052] After adding 4.0 g of NaOH and stirring for 6 minutes, the pH was measured to be 9.54.

[0053] Add 18.0g of sodium 2-mercaptoethanesulfonate and stir for 7 minutes until completely dissolved.

[0054] Steam is introduced to raise the temperature. After the temperature inside the vessel reaches 80.2℃, it is kept at this temperature and stirred for 30 minutes (the solution changes from milky white to light yellow and transparent).

[0055] Cool to 47°C, add 18.0g of glycerol, stir for 11 minutes to obtain the membrane solution.

[0056] Apply the coating to the PET release film using a 0.2mm doctor blade (5cm wide, 10cm coating length).

[0057] After air drying for 6 hours, a sample was taken and the moisture content was measured to be 14.6%. After continuing to dry for another 2 hours, the final moisture content was 14.1%, and a film was obtained.

[0058] Water resistance test: 1. Shear strength test: Cut the adhesive film into 5cm×5cm squares, activate it in a 120℃ oven for 30s, and then stack it between two poplar veneers (moisture content 8-10%). Use a hot press at 120℃ and 1.5Mpa for 6min to make a three-layer plywood sample. Test the wet shear strength and dry shear strength of the plywood.

[0059] 2. Boiling water resistance: Immerse the specimen in 63℃ hot water for 2 hours, then continue heating to 100℃ and boil for 2 hours. Test whether the adhesive layer dissolves and whether the plywood delaminates.

[0060] Storage stability test: Store the film in a PE / aluminum foil composite bag in a constant temperature and humidity chamber (25℃, 80%RH) and observe for mold growth and any unusual odors.

[0061] The performance test data of the finished film are shown in Table 1: Example 2 Add 4.5 kg of deionized water to the reactor and start stirring at 300 r / min.

[0062] Add 6.0 kg of defatted soybean meal in 5 batches, with a protein content of 55.3 wt%, at 4-minute intervals, and stir for a total of 38 minutes until no visible particles are visible to the naked eye.

[0063] After adding 4.0 g of NaOH and stirring for 6 minutes, the pH was measured to be 9.54.

[0064] Add 18.0g of sodium bis(2-sulfonate)disulfide and stir for 7 minutes until completely dissolved.

[0065] Steam is introduced to raise the temperature. After the temperature inside the vessel reaches 80.2℃, it is kept at this temperature and stirred for 30 minutes (the solution changes from milky white to light yellow and transparent).

[0066] Cool to 47°C, add 18.0g of glycerol, stir for 11 minutes to obtain the membrane solution.

[0067] Apply the coating to the PET release film using a 0.2mm doctor blade (5cm wide, 10cm coating length).

[0068] After air drying for 6 hours, a sample was taken and the moisture content was measured to be 14.6%. After continuing to dry for another 2 hours, the final moisture content was 14.1%, and a film was obtained.

[0069] Water resistance test: 1. Shear strength test: Cut the adhesive film into 5cm×5cm squares, activate it in a 120℃ oven for 30s, and then stack it between two poplar veneers (moisture content 8-10%). Use a hot press at 120℃ and 1.5Mpa for 6min to make a three-layer plywood sample. Test the wet shear strength and dry shear strength of the plywood.

[0070] 2. Boiling water resistance: Immerse the specimen in 63℃ hot water for 2 hours, then continue heating to 100℃ and boil for 2 hours. Test whether the adhesive layer dissolves and whether the plywood delaminates.

[0071] Storage stability test: Store the film in a PE / aluminum foil composite bag in a constant temperature and humidity chamber (25℃, 80%RH) and observe for mold growth and any unusual odors.

[0072] The performance test data of the finished film are shown in Table 2: Example 3 Add 4.5 kg of deionized water to the reactor and start stirring at 300 r / min.

[0073] Add 6.0 kg of defatted soybean meal in 5 batches, with a protein content of 55.3 wt%, at 4-minute intervals, and stir for a total of 38 minutes until no visible particles are visible to the naked eye.

[0074] After adding 4.0 g of NaOH and stirring for 6 minutes, the pH was measured to be 9.54.

[0075] Add 18.0g of sodium 2-mercaptoethanesulfonate and stir for 7 minutes until completely dissolved.

[0076] Steam is introduced to raise the temperature. After the temperature inside the vessel reaches 80.2℃, it is kept at this temperature and stirred for 30 minutes (the solution changes from milky white to light yellow and transparent).

[0077] Cool to 47°C, add 18.0 g of sorbitol, stir for 11 min to obtain the membrane solution.

[0078] Apply the coating to the PET release film using a 0.2mm doctor blade (5cm wide, 10cm coating length).

[0079] After air drying for 6 hours, a sample was taken and the moisture content was measured to be 14.6%. After continuing to dry for another 2 hours, the final moisture content was 14.1%, and a film was obtained.

[0080] Water resistance test: 1. Shear strength test: Cut the adhesive film into 5cm×5cm squares, activate it in a 120℃ oven for 30s, and then stack it between two poplar veneers (moisture content 8-10%). Use a hot press at 120℃ and 1.5Mpa for 6min to make a three-layer plywood sample. Test the wet shear strength and dry shear strength of the plywood.

[0081] 2. Boiling water resistance: Immerse the specimen in 63℃ hot water for 2 hours, then continue heating to 100℃ and boil for 2 hours. Test whether the adhesive layer dissolves and whether the plywood delaminates.

[0082] Storage stability test: Store the film in a PE / aluminum foil composite bag in a constant temperature and humidity chamber (25℃, 80%RH) and observe for mold growth and any unusual odors.

[0083] The performance test data of the finished film are shown in Table 3: Comparative Example 1 Add 4.5 kg of deionized water to the reactor and start stirring at 300 r / min.

[0084] Add 6.0 kg of defatted soybean meal in 5 batches, with a protein content of 55.3 wt%, at 4-minute intervals, and stir for a total of 38 minutes until no visible particles are visible to the naked eye.

[0085] After adding 4.0 g of NaOH and stirring for 6 minutes, the pH was measured to be 9.54.

[0086] Add 30.0g of urea and 3.0g of sodium dodecyl sulfate and stir for 10 minutes until completely dissolved.

[0087] Steam is introduced to raise the temperature inside the reactor, and the temperature is maintained at 70% for 30 minutes to obtain a traditional liquid adhesive.

[0088] Pack into sealed drums and store at 25℃ / 80%RH.

[0089] Water resistance test: 1. Shear strength test: Cut the adhesive film into 5cm×5cm squares, activate it in a 120℃ oven for 30s, and then stack it between two poplar veneers (moisture content 8-10%). Use a hot press at 120℃ and 1.5Mpa for 6min to make a three-layer plywood sample. Test the wet shear strength and dry shear strength of the plywood.

[0090] 2. Boiling water resistance: Immerse the specimen in 63℃ hot water for 2 hours, then continue heating to 100℃ and boil for 2 hours. Test whether the adhesive layer dissolves and whether the plywood delaminates.

[0091] Storage stability test: Store the film in a PE / aluminum foil composite bag in a constant temperature and humidity chamber (25℃, 80%RH) and observe for mold growth and any unusual odors.

[0092] The performance test data of the finished film are shown in Table 4: Comparative Example 2 We directly purchased a certain brand of E0 grade urea-formaldehyde resin glue (solid content 48.1%, viscosity 420 mPa·s, free formaldehyde ≤0.3g / kg).

[0093] Water resistance test: 1. Shear strength test: Cut the adhesive film into 5cm×5cm squares, activate it in a 120℃ oven for 30s, and then stack it between two poplar veneers (moisture content 8-10%). Use a hot press at 120℃ and 1.5Mpa for 6min to make a three-layer plywood sample. Test the wet shear strength and dry shear strength of the plywood.

[0094] 2. Boiling water resistance: Immerse the specimen in 63℃ hot water for 2 hours, then continue heating to 100℃ and boil for 2 hours. Test whether the adhesive layer dissolves and whether the plywood delaminates.

[0095] Storage stability test: Store the urea-formaldehyde resin adhesive in a PE / aluminum foil composite bag in a constant temperature and humidity chamber (25℃, 80%RH) and observe for mold growth and any unusual odors.

[0096] The performance test data of the finished film are shown in Table 5: The results of the comparative experiments of Examples 1-3 and Comparative Examples 1-2 are as follows: Examples 1-3 show that the dry and wet strength is increased by 40-60%, the wet strength reaches 1.38-1.52 MPa, it does not dissolve or delaminate after boiling, and the shelf life is increased to 114-151 days.

[0097] Comparative Examples 1-2 have a wet strength of only 0.5-0.6 MPa, dissolve and degumme upon boiling, and become moldy and develop an off-flavor after 50-60 days of storage.

[0098] Examples 1-3 showed a wet strength retention rate of 77.5-89.4%, compared to only 45-47% in the comparative examples. The water-resistant crosslinking modification increased the wet strength retention rate by approximately 100%. Example 2 showed the highest retention rate (89.4%). Sodium bis(2-sulfoethyl) disulfide introduced reversible disulfide crosslinking, which maintained the crosslinking density even in a humid environment, unlike Example 1 where the protein easily re-aggregated or degraded after permanent bond breaking.

[0099] Best overall performance: Example 2, highest wet strength and longest storage period.

[0100] Table 6 summarizes the data comparison between Examples 1-3 and Comparative Examples 1-2: This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method for preparing a soybean protein film, characterized in that, The raw materials for preparing the soybean protein film, by weight, comprise the following components: 6 parts soybean protein, 0.1-1 parts alkalizing agent, 0.5-5 parts protein denaturant, 1.2-2.4 parts polyol plasticizer, and 30-60 parts solvent; the preparation method comprises the following steps: S1. Disperse soybean protein in a solvent to form a suspension, add an alkalizing agent and a protein denaturing agent to obtain a mixture A, heat and stir to denature the protein, and obtain a denatured soybean protein solution; S2. Add a polyol plasticizer to the denatured soybean protein solution and mix well to obtain a film-forming solution; S3. The film-forming solution is applied to a substrate to form a wet film; S4. Dry the wet film to obtain a soybean protein film.

2. The method for preparing a soybean protein film according to claim 1, characterized in that: The alkalizing agent is selected from at least one of sodium hydroxide, potassium hydroxide, or calcium hydroxide.

3. The method for preparing a soybean protein film according to claim 1, characterized in that: The protein denaturant is selected from sodium 2-mercaptoethanesulfonate or sodium bis(2-sulfoethyl) disulfide.

4. The method for preparing a soybean protein film according to claim 1, characterized in that: The polyol plasticizer is selected from at least one of glycerol, sorbitol, and propylene glycol.

5. The method for preparing a soybean protein film according to claim 1, characterized in that: The soybean protein is defatted soybean meal with a protein content ≥45wt%, a particle size ≤15μm, and a moisture content ≤10wt%.

6. The method for preparing a soybean protein film according to claim 1, characterized in that, In step S1: the pH of the mixture A is 8.5~11.0; The heating temperature is 60-90℃; The stirring time is 20-40 minutes.

7. The method for preparing a soybean protein film according to claim 1, characterized in that, In step S4, the drying temperature is 40-80℃, and the product is dried until the moisture content is ≤25wt%.

8. The method for preparing a soybean protein film according to claim 1, characterized in that, In step S3, the thickness of the wet film is 0.10-0.30 mm.

9. The method for preparing a soybean protein film according to claim 3, characterized in that, The sodium bis(2-sulfonyl)disulfide was prepared by the following steps: a. Dissolve sodium 2-mercaptoethanesulfonate in water to form an aqueous solution; b. Add an aqueous solution of hydrogen peroxide to the aqueous solution to carry out an oxidation reaction; c. After the reaction is complete, concentrate the reaction solution; d. Add ethanol to the concentrate to precipitate solids; e. Separate the solids and wash with ethanol; f. Dry the solid to give disodium bis(2-sulfonic acid ethyl) disulfide; The molar ratio of hydrogen peroxide to sodium 2-mercaptoethanesulfonate is 1.00:1 to 1.10:

1.

10. The use of a film prepared by the preparation method according to any one of claims 1 to 8 in the manufacture of plywood, particleboard or agricultural and forestry residue composite materials.

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