A salt spray resistant composite corrugated cardboard and its preparation method

Abstract

This application discloses a salt spray resistant composite corrugated cardboard and its preparation method. The salt spray resistant composite corrugated cardboard is made by bonding a facing paper and a corrugated core paper together with an adhesive. The adhesive comprises the following raw materials in parts by weight: 40-60 parts of metal ion modified soybean protein adhesive; 10-20 parts of epoxy silane oligomer; the raw materials for preparing the metal ion modified soybean protein adhesive include polyvinylpyrrolidone, soybean protein, metal ions, and water in a mass ratio of (8-12):(15-25):(0.01-0.05):(70-100); the metal ions include at least one selected from magnesium ions, iron ions, sodium ions, copper ions, and aluminum ions. The corrugated cardboard obtained by this application exhibits good stability in a humid salt spray environment.

Description

Technical Field

[0001] This application relates to the field of corrugated cardboard, and in particular to a salt spray corrosion resistant composite corrugated cardboard and its preparation method. Background Technology

[0002] Corrugated cardboard is a sheet-like material made by bonding linerboard and corrugated core paper with a corrugated structure. It is usually used as an upstream raw material for corrugated boxes, which play a very important role in industrial product packaging. Starch is widely used as an adhesive in the production of corrugated cardboard due to its advantages such as low cost, eco-friendliness, good bonding properties, and ability to improve the hardness and impact strength of corrugated cardboard.

[0003] During the transportation of goods using corrugated cardboard boxes, they often encounter humid environments. Starch, a natural polysaccharide found in plants, has strong hydrophilicity and is easily eroded by water in humid environments, affecting its adhesive properties. On one hand, salt spray can penetrate the starch adhesive, causing swelling and physical damage to the adhesive. On the other hand, in humid salt spray environments, salt can easily react with pollutants such as nitrogen oxides in the air to form highly acidic substances. Starch has weak resistance to acids and is easily eroded in acidic environments, damaging the structure of the corrugated cardboard. The cardboard and core themselves are also corroded due to insufficient protection.

[0004] In summary, due to starch's insufficient resistance to water and acid, adhesives are easily corroded in humid and acidic environments such as salt spray, which can damage the composite structure of corrugated cardboard, reduce its stability and durability, and limit its usability. Summary of the Invention

[0005] To address the problem that insufficient resistance of adhesives in corrugated cardboard to moisture and salt spray affects the stability and durability of the cardboard, this application provides a salt spray resistant composite corrugated cardboard and its preparation method.

[0006] In a first aspect, this application provides a salt spray corrosion resistant composite corrugated cardboard, which is made by bonding a facing paper and a corrugated core paper together with an adhesive, wherein the adhesive comprises the following raw materials in parts by weight:

[0007] 40-60 parts of metal ion modified soybean protein binder;

[0008] 10-20 parts of epoxy silane oligomer;

[0009] The raw materials for preparing the metal ion modified soybean protein adhesive include polyvinylpyrrolidone, soybean protein, metal ions and water in a mass ratio of (8-12):(15-25):(0.01-0.05):(70-100); the metal ions include at least one of magnesium ions, iron ions, sodium ions, copper ions and aluminum ions.

[0010] This application uses soybean protein as the main adhesive material to achieve bonding and lamination of corrugated paper. It achieves excellent adhesion through hydrogen bonds, chemical bonds, and the presence of abundant natural fibers in the corrugated cardboard. However, soybean protein adhesives are rich in hydrophilic groups and lack chemical cross-linking between molecules, resulting in poor water resistance. This application incorporates metal ions into the soybean protein adhesive. Because metal ions carry a large positive charge, through electrostatic neutralization or shielding, the negative charge on the surface of the protein colloid dissolved in the adhesive changes, reducing the electrostatic repulsion between proteins and making them more prone to aggregation or even coagulation. This increases the adhesion and bonding strength of the adhesive after curing.

[0011] On the other hand, due to its unique spherical molecular structure, most of the active and hydrophobic groups of soybean protein are located inside the molecule, making it difficult to react. This results in very few sites for cross-linking, preventing the formation of effective cross-linked structures with cross-linking agents. The addition of metal ions can alter the surface charge of protein particles, thereby changing the structure and position of secondary bonds such as ionic bonds, hydrogen bonds, and hydrophobic bonds that construct the spherical structure of soybean protein. The aggregation state of the spherical soybean protein thus undergoes complex changes, generally manifested as an extension of the spherical structure, exposing more internal groups and increasing the number of active and hydrophobic groups on the surface of soybean protein. While improving the hydrophobicity of soybean protein, it also possesses more cross-linkable active groups. Among them, carboxyl and amino groups can more easily cross-link with the nitrogen-containing heterocyclic butyl groups in polyvinylpyrrolidone to form a three-dimensional network structure, enhancing the cross-linking strength within the adhesive, increasing the viscosity of the adhesive, and the cross-linked network structure is very compact, possessing good barrier effects, improving the hydrophobicity of the adhesive, and enabling the adhesive to stably perform its bonding function in humid environments, thus improving the stability and safety of corrugated cardboard in humid environments.

[0012] However, the addition of metal ions cannot improve the acid resistance of adhesives. In fact, under acidic conditions, some metal ions can even induce non-enzymatic polymerization of proteins, leading to protein degradation and reducing the stability of the adhesive in acidic environments. This application incorporates epoxy silane oligomers into a metal ion-modified soybean protein adhesive, thereby enhancing the adhesive's acid resistance. Epoxy silane oligomers exhibit excellent stability in acidic environments due to the highly stable silicon-oxygen bonds within them. Furthermore, the abundant hydroxyl groups in epoxy silane oligomers can undergo esterification condensation reactions with the carboxyl groups in proteins, effectively linking them together and enhancing the adhesive's acid resistance. This allows the adhesive to remain stable and perform its bonding function under acidic conditions, improving the stability and safety of corrugated cardboard in salt spray environments.

[0013] Although the viscosity of the adhesive increases with the addition of more metal ions, the entanglement and hydrogen bonding between protein molecular chains increase, the water resistance of the adhesive shows a trend of first increasing and then decreasing. Therefore, when the amount of metal ions added is 1-5 wt% of soybean protein, the adhesive can maintain good adhesion while having excellent hydrophobic properties.

[0014] Preferably, the preparation method of the metal ion modified soybean protein binder includes the following steps:

[0015] Adhesive preparation: Dissolve polyvinylpyrrolidone in water, add soy protein, and stir at 20-30℃ for 5-10 minutes to obtain soy protein adhesive;

[0016] Metal ion modification: Add metal ion solution to soybean protein binder and stir at 20-30℃ for 10-20 minutes to obtain metal ion modified soybean protein binder.

[0017] By adopting the above technical solution, soybean protein is first mixed with polyvinylpyrrolidone, and then metal ion solution is added to introduce metal ions, thereby changing the secondary structure of soybean protein, increasing its surface active groups and hydrophobic groups, and the active groups crosslink with polyvinylpyrrolidone, further improving the adhesive's bonding ability and hydrophobic ability.

[0018] Preferably, the metal ion solution added in the metal ion modification step is a metal chloride solution.

[0019] By adopting the above technical solution, since the soybean adhesive system itself contains chloride ions, the introduction of other anions can be avoided and prevent any adverse effects.

[0020] Preferably, the metal ion is at least one of magnesium ions and iron ions.

[0021] Different metal ions have significantly different effects on soy protein. When metal ions with high charge strength, such as aluminum, enter soy protein, they easily cause the soy protein to form a network aggregate through ion attraction, reducing the dispersibility of soy protein in polyvinylpyrrolidone solution and inhibiting the cross-linking reaction. This negatively impacts the hydrophobicity and adhesive strength of the resulting metal ion-modified soy adhesive. Magnesium and iron ions, on the other hand, possess suitable charge strength. They open the soy protein structure, releasing hydrophobic and active groups without excessively agglomerating the soy protein, ensuring good dispersibility. Furthermore, magnesium and iron ions also have good dispersibility in solution, better enabling them to open the soy protein structure, strengthen the cross-linking strength within the adhesive, improve the hydrophobicity of the adhesive, and enhance the stability of corrugated cardboard in humid environments.

[0022] Preferably, the raw materials for preparing the epoxy silane oligomer include an epoxy silane coupling agent, a short-chain alcohol, and an alcohol-water mixed solution in a mass ratio of (2-3):(8-10):(3-4); the alcohol solution includes an alcohol and water in a mass ratio of (1-2):(1-2).

[0023] This application enhances the acid resistance of adhesives by linking the hydroxyl groups in epoxy silanes and the carboxyl groups in soybean protein through an esterification condensation reaction. Furthermore, self-polymerizing epoxy silanes into oligomers reduces the proportion of hydroxyl groups, increases the proportion of silicon-oxygen bonds in the epoxy silane linked to soybean protein, improves the acid resistance of the adhesive, and consequently enhances the salt spray corrosion resistance of corrugated cardboard.

[0024] Preferably, the preparation process of the epoxy silane oligomer includes the following operations:

[0025] Hydrolysis: Dissolve the epoxy silane coupling agent in a short-chain alcohol, add a catalyst, and heat to 50-60℃ for 1-2 hours to obtain the prepolymer;

[0026] Polymerization: While maintaining the temperature, add an alcohol-water mixture dropwise to the prepolymer, raise the temperature to 70-80℃ and react for 2-3 hours. Recover the short-chain alcohol by vacuum distillation to obtain the epoxysilane oligomer.

[0027] By adopting the above technical solution, epoxy silane can be stably polymerized into oligomers, increasing the proportion of silicon-oxygen bonds in the adhesive, increasing the acid resistance of the adhesive, and improving the stability of corrugated cardboard in acidic environments.

[0028] Preferably, the short-chain alcohol is methanol or ethanol; the alcohol-water mixed solution is an aqueous solution of methanol or ethanol.

[0029] Preferably, the catalyst is at least one of boric acid and sodium fluoride; the amount of catalyst added is 0.2-1 wt% of the epoxy silane coupling agent.

[0030] In the preparation of conventional epoxy silane oligomers, acid catalysis or base catalysis is generally used. Acid catalysis uses sulfuric acid or hydrochloric acid solution to catalyze the reaction, which can easily cause ring-opening damage to the epoxy groups in the epoxy silane, affecting the product's performance. Base catalysis has a fast hydrolysis rate, and hydrolysis can occur simultaneously with polymerization; however, the simultaneous occurrence of hydrolysis and polymerization makes reaction control difficult, resulting in uneven polymerization degrees in the obtained epoxy silane oligomers, affecting product quality. This application uses at least one of boric acid and sodium fluoride as a catalyst, which can promote the hydrolysis of epoxy silanes and control the rate of the hydrolysis reaction, increasing the homogeneity of product polymerization without damaging other groups, improving the homogeneity of the epoxy silane oligomers, enhancing the acid resistance of adhesives, and increasing the stability of corrugated cardboard.

[0031] Preferably, the adhesive raw material further includes a terminal sulfonic acid anionic compound; the amount of the terminal sulfonic acid anionic compound added is 1-5 wt% of the metal ion modified soybean protein adhesive.

[0032] After ionization in water, the sulfonic acid groups at the ends of the anionic compounds can effectively shear soybean protein, break the disulfide bonds in soybean protein, increase the degree of expansion of soybean protein, and expose more internal active and hydrophobic groups. This increases the adhesive strength and salt spray corrosion resistance of the adhesive, and improves the stability of corrugated cardboard. On the other hand, the sulfonic acid groups can promote the self-crosslinking of the nitrogen-containing heterocyclic butyl group in polyvinylpyrrolidone, increase the hydrophobicity of the adhesive, and improve the stability of corrugated cardboard in humid environments.

[0033] As the amount of terminal sulfonic acid anionic compounds increases, the number of ring-opened aziridine groups also increases, leading to a deeper degree of self-crosslinking of polyvinylpyrrolidone. This self-crosslinking synergistically enhances the crosslinking strength of the adhesive with that of soybean protein and aziridine groups. However, both types of crosslinking require aziridine groups, creating a competitive relationship. Excessive addition of terminal sulfonic acid anionic compounds results in excessive ring-opening self-crosslinking of aziridine groups, reducing the degree of crosslinking with the active groups on the soybean protein surface and consequently decreasing the crosslinking strength of the adhesive. When the amount of terminal sulfonic acid anionic compounds is 1-5 wt% of the soybean protein adhesive, it ensures that the number of aziridine groups crosslinked with the active groups on the soybean protein surface is not reduced, while also enabling the ring-opening self-crosslinking of uncrosslinked aziridine groups. This improves the hydrophobicity of the adhesive and increases the stability of corrugated cardboard in humid environments.

[0034] Preferably, the terminal sulfonic acid anionic compound is aminosulfonic acid.

[0035] Aminosulfonic acid has excellent water solubility and can ionize well in water, which enhances the cleavage effect of the sulfonic acid group on the disulfide bond. Furthermore, the amino group in aminosulfonic acid can react with the epoxy group in epoxy silane oligomers to form a bond, thereby increasing the compatibility and stability of aminosulfonic acid in adhesives.

[0036] Secondly, this application provides a method for preparing salt spray corrosion resistant composite corrugated cardboard, comprising the following steps: adhesive preparation: mixing and stirring a metal ion modified soybean protein adhesive, an epoxy silane oligomer and a terminal sulfonic acid anionic compound for 20-40 minutes to obtain an adhesive;

[0037] Corrugated board preparation: The paperboard and paper core are bonded together with adhesive and dried at 90-110℃ for 1-3 minutes to obtain salt spray corrosion resistant composite corrugated board.

[0038] By adopting the above technical solution, adhesives can be used to bond corrugated cardboard. The adhesives themselves not only have good stability in humid and salt spray environments, but also maintain the bonding structure of the corrugated cardboard, increasing the stability and safety of the corrugated cardboard.

[0039] When the raw materials do not contain terminal sulfonic acid anionic compounds, terminal sulfonic acid anionic compounds are not added in the adhesive preparation step.

[0040] In summary, this application has the following beneficial effects:

[0041] 1. This application modifies the structure of soybean protein by adding metal ions. Under the action of the positive charge of the metal ions, the spherical structure extends, exposing a large number of active groups and hydrophobic groups inside the spherical structure. This promotes the cross-linking of the active groups and the nitrogen-containing heterocyclic butyl groups in polyvinylpyrrolidone. Together with the hydrophobic groups, this increases the hydrophobic properties of the adhesive. The adhesive can remain stable in a humid environment to maintain the bonding structure of corrugated cardboard, thus improving the stability and safety of corrugated cardboard in a humid environment.

[0042] 2. This application introduces epoxy silane oligomers into the adhesive, which are tightly linked to the carboxyl groups in soybean protein through epoxy groups. The siloxane bonds give the adhesive excellent acid resistance. Furthermore, the addition of boric acid as a catalyst during the preparation of the epoxy silane oligomers improves the uniformity of the obtained product and enhances the acid resistance of the adhesive. The adhesive can remain stable in a salt spray environment, maintaining the bonding structure of corrugated cardboard and improving the stability and safety of corrugated cardboard in a salt spray environment.

[0043] 3. This application enhances the stretching effect on the spherical structure of soybean protein by adding aminosulfonic acid to the adhesive, exposing more hydrophobic and active groups. At the same time, it promotes the ring-opening self-crosslinking of nitrogen heterocyclic butyl groups in polyvinylpyrrolidone, improves the degree of crosslinking inside the adhesive, enhances the hydrophobicity of the adhesive, and improves the resistance of corrugated cardboard to humid salt spray environment. Detailed Implementation

[0044] The polyvinylpyrrolidone used in the preparation examples or embodiments of this application has a number-average molecular weight of 58,000, and the polyvinyl alcohol used is a product with a degree of polymerization of 1700 and a degree of alcoholysis of 87-89%. The epoxy silane coupling agent used is silane coupling agent KH-563.

[0045] Preparation example of metal ion modified soybean protein binder

[0046] Preparation Example 1: A metal ion-modified soybean protein binder was prepared according to the following method:

[0047] Adhesive preparation: Dissolve 10g of polyvinylpyrrolidone in 90g of water, add 40g of defatted soybean flour (protein content 50%), and stir at 25℃ for 10 minutes to obtain soybean protein adhesive.

[0048] Metal ion modification: Add 2.5g of 1% magnesium chloride solution to 140g of soybean protein binder and stir at 25℃ for 15 minutes to obtain metal ion modified soybean protein binder.

[0049] Preparation Example 2: A metal ion modified soybean protein binder was prepared according to the following method:

[0050] Adhesive preparation: Dissolve 8g of polyvinylpyrrolidone in 72g of water, add 50g of defatted soybean flour (protein content 50%), and stir at 25℃ for 10 minutes to obtain soybean protein adhesive.

[0051] Metal ion modification: Add 5g of 1% magnesium chloride solution to 130g of soybean protein binder and stir at 25℃ for 20 minutes to obtain metal ion modified soybean protein binder.

[0052] Preparation Example 3: A metal ion modified soybean protein binder was prepared according to the following method:

[0053] Adhesive preparation: Dissolve 12g of polyvinylpyrrolidone in 108g of water, add 30g of defatted soybean flour (protein content 50%), and stir at 25℃ for 5 minutes to obtain soybean protein adhesive.

[0054] Metal ion modification: Add 1g of 1% magnesium chloride solution to 150g of soybean protein binder and stir at 25℃ for 10 minutes to obtain metal ion modified soybean protein binder.

[0055] Preparation Example 4, a metal ion modified soybean protein binder, differs from Preparation Example 1 in that, in the metal ion modification step, an equal concentration and equal amount of ferric chloride solution is used instead of magnesium chloride solution.

[0056] Preparation Example 5, a metal ion modified soybean protein binder, differs from Preparation Example 1 in that the amount of magnesium chloride solution added in the metal ion modification step is 10g.

[0057] Preparation Example 6, a metal ion modified soybean protein adhesive, differs from Preparation Example 1 in that, in the adhesive preparation step, an equal amount and concentration of polyvinyl alcohol solution is used instead of polyvinylpyrrolidone solution.

[0058] Example

[0059] Example 1: A salt spray resistant composite corrugated cardboard was prepared according to the following method:

[0060] Hydrolysis: Dissolve 100g of epoxy silane coupling agent in 450g of methanol, add 0.5g of boric acid, heat to 55℃ and react for 1.5 hours to obtain the prepolymer.

[0061] Polymerization: While maintaining the temperature, add a mixed solution of 25g methanol and 25g water dropwise to the prepolymer, heat to 75℃ and react for 2.5 hours. After recovering the methanol by vacuum distillation, epoxy silane oligomers are obtained.

[0062] Adhesive preparation: Mix 50g of metal ion modified soybean protein adhesive, 15g of epoxy silane oligomer and 1.25g of aminosulfonic acid and stir for 30 minutes to obtain corrugated paper adhesive.

[0063] Corrugated paperboard preparation: The linerboard and the corrugated core paper are bonded together with an adhesive and dried at 100°C for 2 minutes to obtain salt spray corrosion resistant composite corrugated paperboard.

[0064] Example 2: A salt spray resistant composite corrugated cardboard was prepared according to the following method:

[0065] Hydrolysis: Dissolve 100g of epoxy silane coupling agent in 500g of methanol, add 1g of boric acid, heat to 55℃ and react for 1.5 hours to obtain the prepolymer.

[0066] Polymerization: While maintaining the temperature, a mixed solution of 34g methanol and 17g water was added dropwise to the prepolymer. The temperature was raised to 75℃ and the reaction was carried out for 2.5 hours. After recovering the methanol by vacuum distillation, the epoxy silane oligomer was obtained.

[0067] Adhesive preparation: Mix 40g of metal ion modified soybean protein adhesive, 20g of epoxy silane oligomer and 2g of aminosulfonic acid for 30 minutes to obtain corrugated paper adhesive.

[0068] Corrugated paperboard preparation: The linerboard and the corrugated core paper are bonded together with an adhesive and dried at 100°C for 2 minutes to obtain salt spray corrosion resistant composite corrugated paperboard.

[0069] Example 3: A salt spray resistant composite corrugated cardboard was prepared according to the following method:

[0070] Hydrolysis: Dissolve 100g of epoxy silane coupling agent in methanol, add 0.1g of boric acid, heat to 55℃ and react for 1 hour to obtain the prepolymer.

[0071] Polymerization: While maintaining the temperature, a mixed solution of 17g methanol and 34g water was added dropwise to the prepolymer. The temperature was raised to 75℃ and the reaction was carried out for 2 hours. After recovering the methanol by vacuum distillation, the epoxy silane oligomer was obtained.

[0072] Adhesive preparation: Mix 60g of metal ion modified soybean protein adhesive, 10g of epoxy silane oligomer and 0.6g of aminosulfonic acid and stir for 20 minutes to obtain corrugated paper adhesive.

[0073] Corrugated paperboard preparation: The linerboard and the corrugated core paper are bonded together with an adhesive and dried at 100°C for 2 minutes to obtain salt spray corrosion resistant composite corrugated paperboard.

[0074] Example 4, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that an equal amount of the metal ion modified soybean protein adhesive prepared in Example 4 is used instead of the metal ion modified soybean protein adhesive prepared in Example 1 in the adhesive preparation step.

[0075] Example 5, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that an equal amount of the metal ion modified soybean protein adhesive obtained in Example 5 is used instead of the metal ion modified soybean protein adhesive obtained in Example 1 in the adhesive preparation step.

[0076] Example 6, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that an equal amount of the metal ion modified soybean protein adhesive prepared in Example 6 is used instead of the metal ion modified soybean protein adhesive prepared in Example 1 in the adhesive preparation step.

[0077] Example 7, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that, in the hydrolysis step, an equal amount of acetic acid is used instead of boric acid.

[0078] Example 8, a salt spray resistant composite corrugated cardboard, differs from Example 1 in that aminosulfonic acid is not added in the adhesive preparation step.

[0079] Example 9, a salt spray resistant composite corrugated cardboard, differs from Example 1 in that the amount of aminosulfonic acid added in the adhesive preparation step is 5g.

[0080] Example 10, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that, in the adhesive preparation step, an equal amount of epoxy silane coupling agent is used instead of epoxy silane oligomer. The preparation method includes the following steps:

[0081] Adhesive preparation: Mix 50g of metal ion modified soybean protein adhesive, 15g of epoxy silane coupling agent and 1.25g of aminosulfonic acid and stir for 30 minutes to obtain corrugated paper adhesive.

[0082] Corrugated paperboard preparation: The linerboard and the corrugated core are bonded together with an adhesive and dried at 100°C for 2 minutes to obtain salt spray corrosion resistant composite corrugated paperboard.

[0083] Comparative Example

[0084] Comparative Example 1, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that, in the adhesive preparation step, an equal amount of soybean protein adhesive is used instead of the metal ion modified soybean protein adhesive obtained in Example 1. The preparation method includes the following steps:

[0085] Adhesive preparation: Dissolve 10g of polyvinylpyrrolidone in 90g of water, add 40g of defatted soybean flour (protein content 50%), and stir at 25℃ for 10 minutes to obtain soybean protein adhesive.

[0086] Hydrolysis: Dissolve 100g of epoxytrimethoxysilane in 450g of methanol, add 0.5g of boric acid, heat to 55℃ and react for 1.5 hours to obtain the prepolymer.

[0087] Polymerization: While maintaining the temperature, add a mixed solution of 25g methanol and 25g water dropwise to the prepolymer, heat to 75℃ and react for 2.5 hours, then recover the methanol by vacuum distillation to obtain the epoxy silane oligomer.

[0088] Adhesive preparation: Mix 50g of soybean protein adhesive, 15g of epoxy silane oligomer and 1.25g of aminosulfonic acid and stir for 30 minutes to obtain corrugated paper adhesive.

[0089] Corrugated board preparation: The linerboard and the corrugated core paper are connected with adhesive and dried at 100°C for 2 minutes to obtain salt spray resistant composite corrugated board.

[0090] Comparative Example 2, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that epoxy silane oligomers are not added in the adhesive preparation step.

[0091] Comparative Example 3, a salt spray corrosion resistant composite corrugated cardboard, differs from Example 1 in that, in the corrugated cardboard preparation step, an equal amount of starch adhesive is used instead of the corrugated cardboard adhesive of this application. The starch adhesive is prepared according to the following steps: 20g of corn starch is dissolved in 100g of water, stirred evenly, 5g of 10% sodium hypochlorite solution and 100g of 20% sodium hydroxide solution are added, and stirred for 50 minutes to obtain the starch adhesive.

[0092] Performance testing

[0093] Test 1: Water resistance test method for corrugated cardboard adhesive: The water resistance of the corrugated cardboard in the examples and comparative examples was tested according to the method in GB / T 22873-2008 "Determination of water resistance of adhesive for corrugated cardboard (water resistance method)". The results were characterized by the time it takes for the adhesive lines to peel off. The test results are shown in Table 1.

[0094] Test 2: Acid resistance test method for corrugated cardboard adhesives: According to the method in GB / T 22873-2008 "Determination of water resistance of corrugated cardboard adhesives (water resistance method)", the liquid in which the sample is immersed is replaced with 1 mol / L dilute hydrochloric acid solution. The acid resistance of the examples and comparative examples is tested, and the results are characterized by the time it takes for the adhesive line to peel off. The test results are shown in Table 1.

[0095] Table 1. Water resistance and acid resistance of corrugated cardboard

[0096] Underwater stripping time (h) Acid stripping time (h) Example 1 31.2 13.2 Example 2 33.4 13.5 Example 3 30.8 13.1 Example 4 28.7 12.7 Example 5 25.3 12.8 Example 6 24.6 12.1 Example 7 30.5 10.2 Example 8 24.3 12.2 Example 9 26.1 12.4 Example 10 30.2 8.9 Comparative Example 1 8.2 5.7 Comparative Example 2 28.4 2.8 Comparative Example 3 11.6 3.3

[0097] Analysis of experimental results:

[0098] 1. As can be seen from Examples 1-3 and Comparative Examples 1-3, and Table 1, this application increases the stability of corrugated cardboard in humid environments by adding metal ions to the soybean protein adhesive. This may be because metal ions have abundant positive charges on their surface, which can alter the surface charge of protein particles and penetrate the spherical protein structure. This alters the spherical structure of soybean protein, opening and expanding it to expose the active and hydrophobic groups within. This increases the cross-linking active sites on the soybean protein surface, promoting cross-linking with the nitrogen-containing heterocyclic butyl groups in polyvinylpyrrolidone to form a network structure. Together with the hydrophobic groups, this enhances the hydrophobic properties of the adhesive, improving the stability and safety of the corrugated cardboard in humid environments.

[0099] 2. As can be seen from Examples 1-3, 7, 10, and Comparative Example 2, and in conjunction with Table 1, this application improves the stability of corrugated cardboard in acidic environments by adding epoxy silane oligomers to soybean protein adhesive. This may be because the silicon-oxygen bonds in the epoxy silane oligomers possess excellent acid resistance, and they are linked to the carboxyl groups in soybean protein through hydroxyl groups, thus endowing the soybean protein adhesive with acid resistance. Furthermore, the epoxy silane oligomers obtained by using boric acid to catalyze the polymerization of epoxy silanes exhibit better uniformity and stability, thereby enhancing the stability and safety of corrugated cardboard in salt spray environments.

[0100] 3. As can be seen from Examples 1-6 and Table 1, this application improves the water resistance of corrugated cardboard by using magnesium ions as a metal ion added to soybean adhesive and polyvinylpyrrolidone as a crosslinking agent. The reason may be that, compared to other metal ions, magnesium ions have a moderate charge strength, sufficient to open and extend the spherical structure of soybean protein, but not so strong that it causes soybean protein to form a network aggregate through ion attraction. This reduces the dispersion of soybean protein in the polyvinylpyrrolidone solution, inhibits the crosslinking reaction between the two, and reduces the stability of the corrugated cardboard in water.

[0101] 4. As can be seen from Examples 1-3 and 8-9 and Table 1, this application improves the hydrophobicity of corrugated cardboard by adding aminosulfonic acid to soybean adhesive. This may be because the sulfonic acid groups in aminosulfonic acid can shear the soybean protein, increasing the degree to which the spherical structure of soybean protein is opened, exposing more hydrophobic and active groups, increasing the crosslinking strength between soybean protein and polyvinylpyrrolidone, and improving the hydrophobicity of the adhesive. Simultaneously, aminosulfonic acid promotes the ring-opening self-crosslinking of the nitrogen-containing heterocyclic butyl groups in polyvinylpyrrolidone, which also improves its water resistance to a certain extent. Furthermore, aminosulfonic acid can also be stably dispersed in the adhesive through the amino groups and carboxyl groups in soybean protein, improving the stability of corrugated cardboard in water.

[0102] 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 salt spray resistant composite corrugated cardboard, made by bonding facing paper and corrugated core paper together with an adhesive, characterized in that, The adhesive comprises the following raw materials in parts by weight: 40-60 parts of metal ion modified soybean protein binder; 10-20 parts of epoxy silane oligomer; The raw materials for preparing the metal ion modified soybean protein adhesive include polyvinylpyrrolidone, soybean protein, metal ions, and water in a mass ratio of (8-12):(15-25):(0.01-0.05):(70-100); wherein the metal ion is at least one of magnesium ions and iron ions. The raw materials for preparing the epoxy silane oligomer include an epoxy silane coupling agent, a short-chain alcohol, and an alcohol-water mixture in a mass ratio of (2-3):(8-10):(3-4); the alcohol-water mixture includes an alcohol and water in a mass ratio of (1-2):(1-2). The adhesive raw materials also include terminal sulfonic acid anionic compounds; the amount of the terminal sulfonic acid anionic compounds added is 1-5 wt% of the metal ion modified soybean protein adhesive.

2. The salt spray resistant composite corrugated cardboard according to claim 1, characterized in that, The preparation method of the metal ion modified soybean protein binder includes the following steps: Adhesive preparation: Dissolve polyvinylpyrrolidone in water, add soy protein, and stir at 20-30℃ for 5-10 minutes to obtain soy protein adhesive; Metal ion modification: Add metal ion solution to soybean protein binder and stir at 20-30℃ for 10-20 minutes to obtain metal ion modified soybean protein binder.

3. The salt spray resistant composite corrugated cardboard according to claim 2, characterized in that, The metal ion solution used in the metal ion modification step is a metal chloride solution.

4. The salt spray resistant composite corrugated cardboard according to claim 1, characterized in that, The preparation process of the epoxy silane oligomer includes the following steps: Hydrolysis: Dissolve the epoxy silane coupling agent in a short-chain alcohol, add a catalyst, and heat to 50-60℃ for 1-2 hours to obtain the prepolymer; Polymerization: While maintaining the temperature, add an alcohol-water mixture dropwise to the prepolymer, raise the temperature to 70-80℃ and react for 2-3 hours. Recover the short-chain alcohol by vacuum distillation to obtain the epoxysilane oligomer.

5. The salt spray resistant composite corrugated cardboard according to claim 4, characterized in that, The catalyst is at least one of boric acid and sodium fluoride; the amount of catalyst added is 0.2-1 wt% of the epoxy silane coupling agent.

6. The salt spray corrosion resistant composite corrugated cardboard according to claim 1, characterized in that, The terminal sulfonic acid anionic compound is aminosulfonic acid.

7. A method for preparing a salt spray resistant composite corrugated cardboard according to any one of claims 1-6, characterized in that, Includes the following steps: Adhesive preparation: Mix and stir the metal ion modified soybean protein adhesive, epoxy silane oligomer and aminosulfonic acid for 20-40 minutes to obtain the adhesive; Corrugated board preparation: The paperboard and paper core are bonded together with adhesive and dried at 90-110℃ for 1-3 minutes to obtain salt spray corrosion resistant composite corrugated board.