Closed type waterborne polyurethane vegetable protein composite adhesive and preparation method thereof

Abstract

The invention relates to the technical field of adhesives, in particular to a closed type waterborne polyurethane vegetable protein composite adhesive and a preparation method thereof. Comprising the following steps: step 1, uniformly mixing castor oil and isophorone diisocyanate, and then heating to obtain a first mixture; 2, adding 2, 2-dimethylolpropionic acid and triethylamine into the first mixture to obtain a second mixture, and 3, adding imidazole into the second mixture to obtain a third mixture, and adding second deionized water into the third mixture for emulsification to obtain closed waterborne polyurethane; adding deionized water into the protein powder, uniformly stirring to obtain protein powder slurry, regulating the pH value of the protein powder slurry by using sodium hydroxide, and dropwise adding the closed waterborne polyurethane into the protein powder slurry to obtain the closed waterborne polyurethane vegetable protein composite adhesive. According to the method, the closed type waterborne polyurethane vegetable protein composite adhesive with proper deblocking temperature and good bonding performance can be prepared.

Description

Technical Field

[0001] The invention relates to the technical field of adhesives, and in particular to a closed waterborne polyurethane plant protein composite adhesive and a preparation method thereof. Background Art

[0002] Polyurethane adhesives are adhesives made from polymer materials, and the main repeating units in their molecular chains are urethane groups (-NHCOO-) or isocyanate groups (-NCO). Since polyurethane is a soft and hard segment block compound, it has both rigidity and flexibility. Various molecular structures bring different properties to polyurethane adhesives, so polyurethane adhesives are widely used.

[0003] Among them, waterborne polyurethane is a polyurethane product that uses water as a solvent or dispersion. Since waterborne polyurethane does not contain any organic solvent or contains only a small amount of organic solvent, it is more environmentally friendly than solvent-based polyurethane, has low VOC emissions, is pollution-free, and is non-flammable, so it is widely used in packaging, construction, electronic components, printing and other fields.

[0004] Waterborne polyurethane adhesives include blocked waterborne polyurethane adhesives. In the preparation process of blocked waterborne polyurethane adhesives, firstly, a terminal isocyanate polyurethane prepolymer containing a carbamate structural unit is obtained by reaction, then reacted with a blocking agent to obtain a blocked polyurethane prepolymer, and finally neutralized with a neutralizing agent and emulsified with water to obtain a fully waterborne blocked polyurethane adhesive. When in use, the blocked waterborne polyurethane is heated to a certain temperature to unblock the NCO group, and the active NCO group cross-links with the active hydrogen of the substrate to form a chemical bond, thereby achieving the bonding strength and water resistance of the adhesive.

[0005] Soy protein polyurethane has good adhesive properties, is widely used and sustainable, and is becoming an important research direction in the field of adhesives due to its environmental protection and sustainability.

[0006] Among them, the current preparation method of soybean protein polyurethane composite adhesive is:

[0007] First, dehydrated polyethylene glycol, isophorone diisocyanate and catalyst dibutyltin dilaurate are reacted to form a mixture, and then dimethyl butynedioate, dimethyl oxalate and butadiene are added as chain extenders and dripped into the system in sequence until the isocyanate groups in the polymer are completely reacted to obtain CWPOU crosslinker emulsion. The crosslinker emulsion and soybean protein powder are evenly mixed, and then 1,2,3-propanetriol diglycidyl ether curing agent is added to obtain soybean protein polyurethane.

[0008] The above-mentioned preparation method of soybean protein polyurethane composite adhesive still has shortcomings:

[0009] It requires the use of catalysts, chain extenders DMAD, DMO, BD and other toxic reagents to extend the chain to obtain a cross-linking agent, and then a curing agent (PTGE) is used to connect the cross-linking agent to the soybean protein. The synthesis process is complicated and not environmentally friendly. Summary of the invention

[0010] The purpose of the present invention is to avoid the shortcomings of the prior art and provide a method for preparing a closed water-based polyurethane plant protein composite adhesive. The method for preparing a closed water-based polyurethane plant protein composite adhesive can prepare a closed water-based polyurethane plant protein composite adhesive with suitable unsealing temperature and good bonding performance in an environmentally friendly and low-cost manner.

[0011] To achieve the above object, the present invention provides the following technical solutions:

[0012] Provided is a method for preparing a closed waterborne polyurethane plant protein composite adhesive, comprising the following steps:

[0013] Step 1, castor oil and isophorone diisocyanate are mixed evenly, then heated to 60-80° C., stirred and reacted for 1.5-3 hours to obtain a first mixture;

[0014] Step 2: adding 2,2-dihydroxymethylpropionic acid and triethylamine to the first mixture to obtain a transparent and viscous second mixture;

[0015] Step 3, adding imidazole to the transparent and viscous second mixture and stirring the mixture at 50° C. to 70° C. for 1.5 h to 3 h to obtain a third mixture, and adding deionized water to the third mixture for emulsification to obtain a closed waterborne polyurethane;

[0016] Step 4: Use protein powder, add deionized water to the protein powder, stir evenly to obtain protein slurry, use sodium hydroxide solution to adjust the pH of the protein slurry to 9-10, and then add the blocked waterborne polyurethane dropwise to the protein powder slurry, wherein the amount of the blocked waterborne polyurethane added is 10% to 70% of the amount of the protein slurry, stir evenly, and obtain a plant protein composite adhesive.

[0017] In some embodiments, in step 2, the weight ratio of the castor oil, the isophorone diisocyanate, the 4-dimethylaminopyridine, and the triethylamine is 8-10:3-7:1-2:1.5-3.

[0018] In some embodiments, in step 3, the weight ratio of the second mixture to the imidazole is 15 to 30:1.

[0019] In some embodiments, the protein powder is waste plant protein.

[0020] In some embodiments, the plant is soybean or cottonseed bean.

[0021] In some embodiments, in step 1, the heating environment is a nitrogen atmosphere.

[0022] The beneficial effects of the preparation method of a closed waterborne polyurethane plant protein composite adhesive of the present invention are as follows:

[0023] (1) The method for preparing a closed waterborne polyurethane plant protein composite adhesive of the present invention fully utilizes the protein structure and the waterborne polyurethane structure to prepare an environmentally friendly closed waterborne polyurethane plant protein composite adhesive. Specifically, waterborne polyurethane is first prepared, and the waterborne polyurethane is combined with protein molecules through physical bonding, which effectively strengthens the protein multiple structure and improves the cross-linking tightness. The long-chain fatty acid structure of the cross-linking system serves as a soft segment, which provides the adhesive with good wettability and flexibility, thereby avoiding the problem of insufficient bonding performance caused by rigid structural defects in traditional cross-linking modification systems, which in turn affects the bonding performance. In addition, the protein structure is unfolded by alkali solution to expose the reactive groups. When the water-based polyurethane is introduced into the protein system, the water-based polyurethane can better bind to the protein, inducing the system to produce a microphase separation structure, and providing bonding strength through the covalent bond cross-linking network. The physical hydrogen bond cross-linking network and microphase separation structure between the water-based polyurethane and the protein molecules jointly assist the protein glue system in dissipating energy when it is under stress, effectively improving the toughness of the adhesive, jointly ensuring the structural integrity of the adhesive, and effectively improving the cohesive strength and water and heat resistance of the adhesive, with more excellent performance.

[0024] (2) The preparation method of a closed waterborne polyurethane plant protein composite adhesive of the present invention contains protein and waterborne polyurethane structure. When the closed waterborne polyurethane plant protein composite adhesive is heated, the active groups can be unblocked, so that they can react with proteins. Under the multiple bonding effects of chemical bonds and hydrogen bonds, a cross-linked network is formed, so that the surface hydrophobicity of the adhesive is enhanced, and the wet bonding strength of the protein adhesive is improved.

[0025] (3) The preparation method of a closed waterborne polyurethane protein composite adhesive of the present invention does not require the use of complex process toxic reagents such as catalyst DBTDL, chain extender DMAD, DMO and BD to synthesize the crosslinking agent, and does not require the use of a curing agent to combine the crosslinking agent and the protein. It has the advantages of being environmentally friendly, low cost and easy to operate.

[0026] (4) The preparation method of a closed water-based polyurethane plant protein composite adhesive of the present invention does not use organic reagents as solvents, does not use toxic and harmful catalysts, and the main raw materials are derived from waste biomass. By adding an appropriate amount of protein modifier and water-based polyurethane, heating and stirring at a certain temperature, the operation is simple, and NaOH is added as a modifier to strengthen the effective cross-linking between protein and protein, polyurethane and protein. There is no need to prepare additional cross-linking agents, and the cross-linking tightness is improved. The prepared protein-based adhesive is green and environmentally friendly, formaldehyde-free, and low in cost. It has a simple structure, uses less materials, and is suitable for large-scale production and application.

[0027] Also provided is a closed type waterborne polyurethane plant protein composite adhesive, which is prepared by the preparation method of the closed type waterborne polyurethane plant protein composite adhesive. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the infrared spectrum test chart of Test Example 1.

[0029] Figure 2 This is the infrared spectrum test chart of Test Example 2.

[0030] Figure 3 This is the XPS test chart of Experimental Example 3.

[0031] Figure 4 This is the adhesive strength test of Test Example 4. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the preferred embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0033] Example

[0034] The preparation method of the closed waterborne polyurethane plant protein composite adhesive disclosed in this embodiment comprises the following steps:

[0035] Step 1, castor oil and isophorone diisocyanate are mixed uniformly, and then heated to 60-80° C., preferably 70° C., under a nitrogen environment, and stirred for reaction for 1.5-3 h, preferably 2 h, to obtain a first mixture;

[0036] Step 2: adding 2,2-dihydroxymethylpropionic acid and triethylamine to the first mixture to obtain a transparent and viscous second mixture, and adding first deionized water for emulsification. The amount of the first deionized water added can be selected according to the actual situation to obtain a waterborne polyurethane;

[0037] Step 3: adding imidazole to the transparent and viscous second mixture and stirring the mixture at 50° C. to 70° C. for 1.5 to 3 hours to obtain a third mixture; and adding 80 g of the second deionized water to the third mixture for emulsification to obtain a closed waterborne polyurethane;

[0038] Step 4: Use protein powder, add deionized water to the protein powder, stir evenly to obtain protein slurry, use sodium hydroxide solution to adjust the pH of the protein slurry to 9-10, preferably pH 9, then add the water-based polyurethane dropwise to the protein powder slurry, the addition amount of the blocked water-based polyurethane is 10% to 70% of the amount of the protein slurry, stir evenly, and obtain a plant protein composite adhesive.

[0039] In this embodiment, in step 2, the weight ratio of the castor oil, the isophorone diisocyanate, the 4-dimethylaminopyridine, and the triethylamine is 8-10:3-7:1-2:1.5-3, preferably 9.3:6.7:1.3:1.1, and the weight of the raw materials can be adjusted according to actual needs.

[0040] In this embodiment, in step three, the weight ratio of the second mixture to the imidazole is 19:1, and the weight of the raw materials can be adjusted according to actual needs.

[0041] In this embodiment, the protein powder is waste plant protein, preferably the plant is soybean or cottonseed meal. The specific type of protein powder can be selected according to the actual situation.

[0042] Performance Testing

[0043] In order to illustrate the performance of the closed waterborne polyurethane plant protein composite adhesive prepared by the present invention, the following experimental performance tests were performed:

[0044] The abbreviations represent the following:

[0045] CPC Purified Cottonseed Protein Powder Slurry

[0046] BWPU Blocked Waterborne Polyurethane

[0047] WPU Waterborne Polyurethane

[0048] CPC / BWPU closed waterborne polyurethane protein composite adhesive

[0049] CPC / WPU water-based polyurethane protein composite adhesive

[0050] Test Example 1

[0051] Sample preparation:

[0052] Step 1, 9.32 g of castor oil and 6.66 g of isophorone diisocyanate were mixed evenly, and then heated at 70° C. and stirred in a nitrogen environment for reaction for 2 h to obtain a first mixture;

[0053] Step 2: Add 1.38 g of 2,2-dimethylolpropionic acid and 1.1 g of triethylamine to the first mixture to obtain a transparent and viscous second mixture, wherein deionized water is added to emulsify the mixture to obtain a waterborne polyurethane WPU, which is subsequently used for comparative testing;

[0054] Step 3: add 1.1 g of imidazole to the transparent and viscous second mixture, stir and react at 60° C. for 2 h to obtain a third mixture, add 80 g of the second deionized water to the third mixture for emulsification, and obtain a blocked waterborne polyurethane BWPU;

[0055] The obtained WPU and BWPU were subjected to the following tests:

[0056] Test results such as Figure 1 As shown, the infrared spectrum of BWPU ( Figure 1 a) Medium, 3320~3620cm -1 The broad absorption peak at 2933 cm corresponds to the -NH stretching vibration peak of carbamate and allophanate. -1 The stretching vibration peak of methyl group is 2857cm -1 The peak at 1710cm is the bending vibration peak of methylene. -1 The peak can be attributed to the stretching vibration absorption peak of -C=O of carbamate. Before the end-capping, part of the -NCO reacts with the -OH in DMPA to form a carbamate bond. After the end-capping, part of the -NCO continues to react with imidazole to form a carbamate carbonyl and a urea carbonyl. Therefore, after the end-capping, a peak of 1710 cm -1 and 1640cm -1 There is a sharp strong absorption peak at Figure 1 It can be clearly seen that before closing 2275cm -1 There is an obvious absorption peak at 2840~2930cm after closure, which belongs to the stretching vibration absorption peak of -NCO. After closure, the absorption peak intensity gradually decreases until it disappears. -1 The peak at 40° was enhanced, indicating that there was no -NCO group in the system after blocking, and the imidazole-terminated water-based polyurethane emulsion was successfully prepared.

[0057] When the BWPU sample was placed in a 130℃ environment for 400s, its infrared spectrum ( Figure 1 b) at 2270cm -1 The characteristic absorption peak attributed to isocyanate appears at 1530cm -1The intensity of the characteristic absorption peak attributable to the -NH bending vibration decreases, indicating that the blocking and unblocking of isocyanate has been achieved. The unblocking temperature of BWPU was characterized by DSC test. The type of blocking agent and isocyanate affects the unblocking temperature of blocked waterborne polyurethane. The unblocking temperature of imidazole blocking agents is generally between 110 and 130 °C. Figure 1 In the DSC test of c, the BWPU curve shows an obvious deblocking endothermic peak, and the starting position of the endothermic peak is at 100°C, indicating that the blocker begins to deblock at this temperature; when the temperature rises to 105°C, the deblocking reaction endothermic peak reaches the maximum value, indicating that the deblocking reaction is fastest at this time; then the endothermic peak gradually decreases until there is no endothermic peak at 115°C, indicating that the deblocking reaction has been completed. The wider curve range in the DSC graph shows that the deblocking reaction of imidazole and isocyanate is a process in which the active -NCO is gradually released as the temperature rises, and Figure 1 d shows the unsealing process of blocked waterborne polyurethane under hot pressing. The maximum unsealing temperature is 115°C, which is suitable to meet the hot pressing temperature during the use of adhesives. It overcomes the problem that the unsealing temperature of traditional capping agents is too high, resulting in difficulty in unsealing and bonding, or the unsealing temperature is too low, resulting in bonding during transportation.

[0058] Test Example 2

[0059] To further illustrate the relationship among CPC, WPU, BWPU, CPC / WPU, and CPC / BWPU, the following tests were performed:

[0060] Sample preparation:

[0061] Step 1, 9.32 g of castor oil and 6.66 g of isophorone diisocyanate were mixed evenly, and then heated at 70° C. and stirred in a nitrogen environment for reaction for 2 h to obtain a first mixture;

[0062] Step 2: Add 1.38 g of 2,2-dihydroxymethylpropionic acid and 1.1 g of triethylamine to the first mixture to obtain a transparent and viscous second mixture, wherein the first deionized water is added for emulsification to obtain a waterborne polyurethane WPU for subsequent comparative testing.

[0063] Step 3: add 1.1 g of imidazole to the transparent and viscous second mixture, stir and react at 60° C. for 2 h to obtain a third mixture, add 80 g of the second deionized water to the third mixture for emulsification, and obtain a blocked waterborne polyurethane BWPU;

[0064] Step 4: Use protein powder, add deionized water to the protein powder, stir evenly to obtain protein slurry, use sodium hydroxide solution to adjust the pH of the protein slurry to 9, and then add the blocked waterborne polyurethane dropwise to the protein slurry. The amount of blocked waterborne polyurethane added is 50% of the amount of protein slurry. Stir evenly to obtain a plant protein composite adhesive sample CPC / BWPU.

[0065] To further illustrate the comparative effect, protein powder was also used, deionized water was added to the protein powder, and the mixture was stirred evenly to obtain a protein slurry. The pH of the protein slurry was adjusted to 9 using a sodium hydroxide solution, and then the water-based polyurethane WPU was added dropwise to the protein slurry. The amount of the water-based polyurethane added was 50% of the amount of the protein slurry. The mixture was stirred evenly to obtain a non-enclosed plant protein composite adhesive sample CPC / WPU.

[0066] Infrared spectra of CPC, WPU, BWPU, CPC / WPU, CPC / BWPU adhesives Figure 2 As shown in the FTIR spectrum of WPU, -1 The -NH stretching vibration peak is shown at 1723 cm -1 The peaks are -C=O stretching vibration and 1532cm -1 -NH bending vibration peak. 3000-3500cm in CPC -1 The peak corresponding to the stretching vibration of -OH and -NH in proteins is 2800-3000cm -1 The symmetric and asymmetric stretching vibrations of CH. -1 、1543cm -1 、1215cm -1 The corresponding peaks are the stretching vibration peaks of -C=O in the amide I region, -NH stretching vibration in the amide II region, and the characteristic peak CN stretching vibration in the amide III region. These characteristic absorption peaks are red-shifted or blue-shifted under the influence of multiple hydrogen bonds. Under heat and pressure, the blocked isocyanate groups in BWPU will be unblocked and react with polar groups such as amino or hydroxyl groups in the protein to further form a chemical cross-linking network. However, the carbamate or urea groups generated by the reaction will be covered by the amide bonds of the protein itself, and the 2270cm -1 Therefore, we further proved through Experimental Example 3 that its isocyanate group was unblocked and formed a chemical cross-linking network with the protein.

[0067] Test Example 3

[0068] In order to further verify the interaction mechanism between BWPU and plant protein powder, XPS was used to study the composition of chemical bonds in BWPU in Experimental Example 1. Figure 3 As shown in the figure, from the spectrum of C1s peak fitting of the protein composite adhesive, it can be obtained that the characteristic peaks at 284.7eV, 286.4eV, and 288.2eV belong to C1 (-CC / -CH), C2 (-CO / -CN), and C3 (-C=O), respectively. After adding BWPU, it is found that the proportion of C2 in the CPC / BWPU composite adhesive increases from 9.09% to 12.29%, and the proportion of C3 increases from 12.29% to 14.64%. The reason for the increase in the proportion of C2 and C3 is that the isocyanate groups unblocked by BWPU react with the amino and hydroxyl groups in the protein to form carbamate or urea groups. The spectrum of the protein composite adhesive N1S peak fitting provides key evidence for the isocyanate group formed by thermal excitation to consume the amino group to form a chemical cross-linking network. From the figure, we can see that the N1s region is located at 399.8eV and 401.1eV. The two characteristic peaks belong to N1 (-R-NH-) and N2 (R-NH2), among which the proportion of N2 is related to the number of free amino groups in the adhesive. After adding BWPU, the proportion of N2 dropped from 9.61% to 5.73%, which proves that BWPU does release the isocyanate group and consume the amino group under thermal excitation.

[0069] Test Example 4

[0070] Sample preparation:

[0071] Step 1, 9.32 g of castor oil and 6.66 g of isophorone diisocyanate were mixed evenly, and then heated at 70° C. and stirred in a nitrogen environment for reaction for 2 h to obtain a first mixture;

[0072] Step 2: Add 1.38 g of 2,2-dihydroxymethylpropionic acid and 1.1 g of triethylamine to the first mixture to obtain a transparent and viscous second mixture, wherein the first deionized water is added for emulsification to obtain a waterborne polyurethane WPU for subsequent comparative testing.

[0073] Step 3: add 1.1 g of imidazole to the transparent and viscous second mixture, stir and react at 60° C. for 2 h to obtain a third mixture, add 80 g of the second deionized water to the third mixture for emulsification, and obtain a blocked waterborne polyurethane BWPU;

[0074] Step 4: Use protein powder, add deionized water to the protein powder, stir evenly to obtain protein slurry, use sodium hydroxide solution to adjust the pH of the protein slurry to 9, and then add the blocked waterborne polyurethane dropwise to the protein slurry. The amount of blocked waterborne polyurethane added is 50% of the amount of protein slurry. Stir evenly to obtain a plant protein composite adhesive sample CPC / BWPU.

[0075] To further illustrate the comparative effect, protein powder was also used, deionized water was added to the protein powder, and the mixture was stirred evenly to obtain a protein slurry. The pH of the protein slurry was adjusted to 9 using a sodium hydroxide solution, and then the water-based polyurethane WPU was added dropwise to the protein slurry. The amount of the water-based polyurethane added was 10%, 30%, 50%, and 70% of the amount of the protein slurry. The mixture was stirred evenly to obtain a non-enclosed plant protein composite adhesive sample CPC / WPU.

[0076] Test results such as Figure 4 As shown in the figure, in the dry glue strength test, when the WPU addition amount is 0%, the dry glue strength of the adhesive is 1.78MPa. When the WPU addition amount is 0-50%, the dry glue strength gradually increases with the increase of the addition amount. When the addition amount is 10% and 30%, the dry glue strength of the adhesive is 1.97MPa and 2.42MPa respectively; and when the addition amount is 50%, the dry glue strength is 2.65MPa; in the wet glue strength test, when the WPU addition amount is 0%-10%, the wet bonding strength is around 0.9MPa. After continuing to increase its addition amount, the wet glue strength has been significantly increased. When the WPU addition amount is 50%, the strength reaches 1.23MPa. When WPU is replaced by BWPU, the dry glue and wet glue strength of CPC / BWPU adhesive are further improved, indicating that the CPC / BWPU adhesive has better bonding strength. The bonding strength of the adhesive prepared in this experiment is much higher than the requirements for Class II plywood in GB / 17657-2013 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels".

[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a closed waterborne polyurethane plant protein composite adhesive, characterized in that: The following steps are involved: Step 1, castor oil and isophorone diisocyanate are mixed evenly, then heated to 60° C. to 80° C., and stirred for reaction for 1.5 h to 3 h to obtain a first mixture; Step 2: adding 2,2-dihydroxymethylpropionic acid and triethylamine to the first mixture to obtain a transparent and viscous second mixture; Step 3, adding imidazole to the transparent and viscous second mixture and stirring the mixture at 50° C. to 70° C. for 1.5 h to 3 h to obtain a third mixture, and adding deionized water to the third mixture for emulsification to obtain a closed waterborne polyurethane; Step 4: Use protein powder, add deionized water to the protein powder, stir evenly to obtain protein slurry, use sodium hydroxide solution to adjust the pH of the protein slurry to 9-10, and then add the blocked waterborne polyurethane dropwise to the protein powder slurry, wherein the amount of the blocked waterborne polyurethane added is 10% to 70% of the amount of the protein slurry, stir evenly, and obtain a plant protein composite adhesive.

2. The method for preparing a closed waterborne polyurethane plant protein composite adhesive according to claim 2, characterized in that: In step 2, the weight ratio of the castor oil, the isophorone diisocyanate, the 4-dimethylaminopyridine and the triethylamine is 8-10:3-7:1-2:1.5-3.

3. The method for preparing a closed waterborne polyurethane plant protein composite adhesive according to claim 1, characterized in that: In step 3, the weight ratio of the second mixture to the imidazole is 15-30:

1.

4. The method for preparing a closed waterborne polyurethane plant protein composite adhesive according to claim 1, characterized in that: The protein powder is waste plant protein.

5. The method for preparing a closed waterborne polyurethane plant protein composite adhesive according to claim 1, characterized in that: The plants are soybean and cottonseed meal.

6. The method for preparing a closed waterborne polyurethane plant protein composite adhesive according to claim 1, characterized in that: In step 1, the heating environment is a nitrogen atmosphere environment.

7. A closed waterborne polyurethane plant protein composite adhesive, characterized in that: The adhesive is prepared by the method for preparing the closed waterborne polyurethane plant protein composite adhesive according to any one of claims 1 to 6.

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