Crosslinking modification process for degradable starch-based adhesive

Abstract

The invention discloses a crosslinking modification process of a degradable starch-based adhesive, and belongs to the technical field of adhesive preparation. The process comprises the following steps: firstly, compounding corn starch and cassava starch, and pre-gelatinizing to obtain uniform starch paste; adjusting the pH value step by step through diluted hydrochloric acid and a buffer solution, and sequentially adding citric acid, boric acid and epichlorohydrin for cross-linking reaction; then adding small-molecular polyol subjected to dehydration pretreatment for intercalation modification, and locking moisture by matching with trehalose; then adding modified carboxymethylated chitosan and natural lecithin to form a compact interfacial film and improve the compatibility of the system; and finally, adding modified carboxylated polyvinyl alcohol for toughening, and carrying out vacuum degassing, corrosion prevention, filtration and low-temperature aging to obtain a finished product. The problems that an existing starch-based adhesive is poor in water resistance, insufficient in toughness and low in structure and storage stability are solved, and the prepared adhesive has the advantages of high bonding strength, excellent wet retention rate and toughness, good system compatibility, uniform performance and biodegradability.

Description

Technical Field

[0001] This invention relates to the field of adhesive preparation technology, specifically to a crosslinking modification process for a biodegradable starch-based adhesive. Background Technology

[0002] Starch-based adhesives have become an important alternative to petroleum-based adhesives due to their advantages such as renewable, biodegradable, and environmentally friendly raw materials, and are widely used in packaging, wood processing, and paper bonding. However, natural starch molecules contain a large number of hydroxyl groups, which easily lead to excessive hydrogen bonding between molecules, resulting in inherent defects in the adhesives. For example, a single cross-linking method is difficult to build a stable network structure, resulting in poor structural stability of the adhesive. The bonding strength in the dry state is low, and it is particularly prone to brittleness and lacks toughness, making it unable to withstand impacts in actual use. In addition, starch molecules themselves are highly hydrophilic, and existing starch-based adhesives have poor water resistance. They easily swell and delaminate when exposed to water, resulting in poor bonding strength in the wet state, which limits their application to dry environments. Furthermore, the compatibility between various modified components is also poor, easily leading to agglomeration and delamination, resulting in inconsistent performance across different parts of the adhesive.

[0003] To address these issues, existing technologies often employ single crosslinking agent modification or simple addition of toughening agents or water-resistant agents. While these methods can improve certain properties to some extent, they can easily lead to the degradation of other properties, making it difficult to achieve synergistic improvement of multiple properties. At the same time, existing processes lack sufficient control over the preparation and addition conditions of functional components. For example, during the carboxylation modification of polyvinyl alcohol, the catalyst has low catalytic efficiency, uneven distribution of carboxyl groups, poor binding effect with starch network, and the toughening effect cannot be fully realized.

[0004] Therefore, it is necessary to provide a crosslinking modification process for biodegradable starch-based adhesives to solve the above-mentioned technical problems. Summary of the Invention

[0005] The purpose of this invention is to provide a crosslinking modification process for biodegradable starch-based adhesives to solve the problems of poor structural stability, insufficient water resistance, and low toughness of existing starch-based adhesives.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a crosslinking modification process for a biodegradable starch-based adhesive, comprising the following steps: (1) Starch pretreatment: Mix starch raw materials with plasticizer, add deionized water and stir evenly, and then pregelatinize to form a uniform starch paste; (2) Cross-linking reaction: After adjusting the pH of the starch paste, three cross-linking agents are added in sequence to react and form cross-linking products; (3) Polyol intercalation: Add pretreated small molecule polyol to the crosslinking product and stir to obtain a polyol intercalation modified crosslinking system; (4) Film formation and compatibility control: Modified carboxymethylated chitosan and natural lecithin were added to the polyol intercalation modified crosslinking system, and the mixture was stirred and dispersed to obtain a mixed system; (5) Toughening and post-treatment: Add modified carboxylated polyvinyl alcohol to the mixture obtained in step (4), heat and stir, and after anti-corrosion and filtration treatment, obtain a biodegradable starch-based adhesive.

[0007] Preferably, in step (1): the starch raw material is a mixture of corn starch and cassava starch, with a mass ratio of 7:3-8:2; the plasticizer is glycerol, and the amount added is 3%-5% of the total mass of the starch raw material; the pregelatinization treatment temperature is 65-75℃, the stirring rate is 150-200r / min, and the temperature is lowered to 40-50℃ after the treatment is completed.

[0008] Preferably, in step (2): the three crosslinking agents are citric acid, boric acid, and epichlorohydrin in sequence; during the crosslinking reaction, the initial pH of the starch paste is first adjusted to 4.0-4.5 with dilute hydrochloric acid at a concentration of 0.1-0.5 mol / L, and 2%-4% of the total mass of starch raw materials is added dropwise with citric acid at a dropping rate of 2.5-3.5 mL / min; then the pH of the system is adjusted to 5.0-6.0 with citric acid-borax buffer solution, and 0.5%-1.5% of the total mass of starch raw materials is added dropwise with boric acid at a dropping rate of 1.0-2.0 mL / min; subsequently, the pH is adjusted to 7.0-7.5 with sodium hydroxide-borax composite weak alkaline buffer solution, and 1%-3% of the total mass of starch raw materials is added dropwise with a dropping rate of 0.8-1.2 mL / min; the total incubation reaction time is 150-180 min, and the pH of the system is adjusted to 6.5-7.5 after the reaction is completed.

[0009] Preferably, in step (3): the small molecule polyol is selected from glycerol or xylitol, and the amount added is 2%-3% of the total mass of starch raw material; the pretreatment conditions of the small molecule polyol are vacuum dehydration at 75-85℃ and -0.08~-0.10MPa for 1.5-2.5h, and the moisture content after treatment is ≤0.8%.

[0010] Preferably, the preparation steps of the modified carboxymethylated chitosan are as follows: 1a) Chitosan with a degree of deacetylation ≥90% is added to isopropanol and dispersed evenly. Then, a sodium hydroxide aqueous solution with a mass fraction of 25%-35% is added, and the mixture is kept at 40-50℃ for 1.5-2.5 hours to obtain an alkalized system. 2a) Add 2%-3% by weight of chitosan glycidyltrimethylammonium chloride to the alkalization system, stir and react for 1-1.5 h, dissolve chloroacetic acid in isopropanol, slowly add it dropwise to the alkalization system, and keep it at 55-65℃ for 3.5-4.5 h to obtain the intermediate system; 3a) Adjust the pH of the intermediate system to 6.5-7.5 with 8%-12% dilute hydrochloric acid, collect the solid product by filtration, wash it successively with anhydrous ethanol and deionized water, vacuum dry it to constant weight at 55-65℃, and pulverize it to 70-90 mesh to obtain modified carboxymethylated chitosan with a degree of carboxymethyl substitution of 0.8-1.0 and a water solubility of ≥95% in the neutral system.

[0011] Preferably, the preparation steps of the modified carboxylated polyvinyl alcohol are as follows: 1b) Add polyvinyl alcohol with a degree of hydrolysis of 88% to deionized water, heat to 85-95℃ and stir until completely dissolved to form a polyvinyl alcohol aqueous solution with a mass fraction of 8%-12%, and cool to 45-55℃. 2b) Add ferric chloride-copper chloride composite catalyst to a polyvinyl alcohol aqueous solution, wherein the mass ratio of ferric chloride to copper chloride in the composite catalyst is 1:1. After stirring and dissolving, slowly add hydrogen peroxide with a mass fraction of 25%-35% and keep the reaction at 45-55℃ for 2.5-3.5h. 3b) Add sodium sulfite to the system, stir and mix, then vacuum dry at 55-65℃ to constant weight, and pulverize to 50-70 mesh to obtain modified carboxylated polyvinyl alcohol with a carboxyl content of 1%-2%.

[0012] Preferably, in step (3): the dried modified carboxymethylated chitosan, modified carboxylated polyvinyl alcohol and small molecule polyol are mixed and placed in a vacuum drying oven at 80-90℃ and -0.09~-0.11MPa for 1.2-1.8h to dehydrate, and the total moisture content after mixing is controlled to be ≤0.8%; after dehydration, 0.3%-0.5% of the total mass of starch raw materials of natural trehalose are added and stirred and mixed.

[0013] Preferably, in step (4): the amount of modified carboxymethylated chitosan added is 0.2%-0.4% of the total mass of starch raw materials; the amount of natural lecithin added is 0.8%-1.2% of the total mass of starch raw materials, and it is added after being dissolved in anhydrous ethanol accounting for ≤3% of the total mass of the system, the stirring rate is 100-160 r / min, and the stirring time is 20-30 min.

[0014] Preferably, in step (5): the amount of modified carboxylated polyvinyl alcohol added is 8%-10% of the total mass of starch raw material, and the stirring time is 30-40 min; vacuum degassing is introduced during the stirring process, and the vacuum degree is -0.07~-0.09 MPa.

[0015] Preferably, in step (5): the preservative is sodium dehydroacetate or ε-polylysine, and the amount added is 0.2%-0.3% of the total mass of the system; the filtration adopts a multi-stage filtration method of 70-90 mesh coarse filtration and 190-210 mesh fine filtration, and after filtration, the adhesive is aged at low temperature at 23-27℃ for 20-28h; in step (5): the heating and stirring temperature is 58-62℃.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention first combines corn starch and cassava starch and then pregelatinizes them to overcome the shortcomings of single starches. Corn starch has a high amylose content, resulting in a compact structure and stable cross-linking sites after gelatinization, which improves the structural strength and adhesion of the adhesive. Cassava starch has a high branched chain content, a low gelatinization temperature, good fluidity and film-forming properties, and soft molecular chains with many reaction sites, which increases the toughness of the adhesive and allows for a more uniform cross-linking reaction. The combination of the two starches forms a uniform starch paste, preventing raw material clumping and uneven dispersion, thus ensuring that the subsequent cross-linking reaction proceeds evenly throughout the entire system.

[0017] 2. This invention adjusts the pH value of the system stepwise to match the optimal reaction conditions of the three crosslinking agents: citric acid, boric acid, and epichlorohydrin. This process forms ionic bonds, dynamic bonds, and covalent bonds step by step, ultimately building a complete three-dimensional crosslinked network of covalent-ionic-dynamic bonds. These three types of chemical bonds complement each other. Covalent bonds act as a strong framework for the adhesive, making the overall structure more stable and improving adhesive strength. Ionic bonds make the network denser. Dynamic bonds can be reversibly reconstructed, dispersing internal stress when subjected to external forces. This solves the problem that adhesives made using traditional single crosslinking methods are either brittle or too soft and easily deformed.

[0018] 3. This invention intercalates small-molecule polyols into the molecular gaps of a three-dimensional cross-linked network, effectively breaking excessive hydrogen bonds between starch molecules and disrupting the state of molecules being squeezed together. This allows for reasonable adjustment of the system's rheological properties, preventing coating difficulties due to excessively high system viscosity. Simultaneously, the small-molecule polyol, modified carboxymethylated chitosan, and modified carboxylated polyvinyl alcohol undergo joint dehydration, controlling the system's moisture content at the source. Combined with the moisture-locking effect of natural trehalose, the remaining free water in the system is firmly captured, preventing water from seeping in and damaging the cross-linked network and the bonding structure between components. This significantly improves the adhesive's storage stability, preventing problems such as viscosity fluctuations, stratification, and water separation during storage.

[0019] 4. The modified carboxymethylated chitosan in this invention forms a continuous and dense hydrophobic interface layer on the adhesive surface, which greatly reduces the permeability of water molecules and significantly improves the water resistance of the adhesive. Natural lecithin is an amphiphilic component. Its polar groups can form ionic complexes with polar polymers such as chitosan and polyvinyl alcohol, while its non-polar chains can fill the gaps between molecules. This effectively improves the compatibility between different polar polymers, avoids component aggregation and stratification in the system, and allows each functional component to be uniformly dispersed in the system and fully exert its function. The overall performance uniformity of the adhesive will also be improved.

[0020] 5. This invention uses a ferric chloride-copper chloride composite catalyst to prepare modified carboxylated polyvinyl alcohol. The combined action of the two metal ions allows for the stable generation and directional transfer of hydroxyl radicals, effectively controlling the degree of carboxylation in polyvinyl alcohol and ensuring uniform distribution of carboxyl groups on the molecular chain. The modified polyvinyl alcohol forms stable chemical bonds with the hydroxyl groups on the starch crosslinking network through the carboxyl groups. Its flexible chains can also be uniformly embedded in the three-dimensional crosslinking network, working together with the dynamic bonds within the network to achieve a toughening effect. When subjected to external force, the flexible chains absorb stress through molecular chain slippage, while the dynamic bonds release stress through reversible reconstruction, effectively achieving dual dispersion and stress absorption. This significantly improves the toughness and impact resistance of the adhesive without compromising the structural stability of the crosslinking network. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1 This embodiment provides a crosslinking modification process for a biodegradable starch-based adhesive, specifically including the following steps: 1. Starch pretreatment: Corn starch and tapioca starch are mixed at a mass ratio of 7:3 to obtain starch raw material. The starch raw material and 3% of the total mass of starch raw material glycerol plasticizer are added to the reaction vessel. Deionized water is added to the reaction vessel and stirred thoroughly. The mixture is heated to 65℃ for pregelatinization. During the pregelatinization process, the stirring speed is maintained at 150r / min. The stirring is continued until a homogeneous starch paste is formed. After the pregelatinization is completed, the system is cooled to 40℃ for later use.

[0023] 2. Crosslinking reaction: Add 0.1 mol / L dilute hydrochloric acid to the prepared homogeneous starch paste to adjust the initial pH of the starch paste to 4.0. Then, add 2% (by weight of the total starch raw material) of citric acid to the system at a dropping rate of 2.5 mL / min. After the citric acid is added, adjust the pH of the system to 5.0 using a citric acid-borax buffer solution. Then, add 0.5% (by weight of the total starch raw material) of boric acid at a dropping rate of 1.0 mL / min. After the boric acid is added, adjust the pH of the system to 7.0 using a sodium hydroxide-borax composite weak alkaline buffer solution. Continue to add 1% (by weight of the total starch raw material) of epichlorohydrin at a dropping rate of 0.8 mL / min. The total incubation time for the three crosslinking agents to be added and reacted sequentially is 150 min. After the crosslinking reaction is completed, adjust the pH of the system to 6.5 to form the crosslinking product.

[0024] 3. Polyol Intercalation: Glycerol was selected as the small molecule polyol. Glycerol was first subjected to vacuum dehydration at 75℃ and -0.08MPa for 1.5 hours until the moisture content was ≤0.8%. The pretreated glycerol was added to the crosslinking product at 2% of the total starch raw material mass, along with dried modified carboxymethylated chitosan and modified carboxylated polyvinyl alcohol. The mixture was then placed in a vacuum drying oven at 80℃ and -0.09MPa for 1.2 hours for combined dehydration, controlling the total moisture content to ≤0.8%. After combined dehydration, 0.3% of the total starch raw material mass of natural trehalose was added to the system, and the mixture was thoroughly stirred to obtain the polyol-intercalated modified crosslinking system.

[0025] 4. Film formation and compatibility control: Add 0.2% of modified carboxymethylated chitosan (total mass of starch raw material) to the above-mentioned polyol intercalation modified crosslinking system. After stirring and mixing evenly, take 0.8% of natural lecithin (total mass of starch raw material) and dissolve it in anhydrous ethanol (1% of total mass of system) to prepare lecithin alcohol solution. Add this solution to the system and stir continuously at a stirring rate of 100 r / min for 20 min to disperse the components evenly and obtain a mixed system.

[0026] 5. Toughening and Post-treatment: Add 8% (by weight of starch raw material) of modified carboxylated polyvinyl alcohol to the above mixture, heat the system to 58℃ and stir continuously for 30 min, introducing a vacuum degassing treatment of -0.07 MPa during stirring; after degassing, add 0.2% (by weight of system total mass) of sodium dehydroacetate as a preservative to the system, first coarsely filter the system using a 70-mesh sieve, and then finely filter it using a 190-mesh sieve to remove unreacted impurities; after filtration, place the adhesive in an environment of 23℃ for low-temperature aging treatment for 20 h to finally obtain a biodegradable starch-based adhesive.

[0027] The preparation steps of modified carboxymethylated chitosan are as follows: 1a) Add chitosan with a degree of deacetylation ≥90% to isopropanol, stir thoroughly to disperse the chitosan evenly, add 25% sodium hydroxide aqueous solution to the system, and keep it at 40℃ for 1.5h for alkalization treatment to obtain the alkalized system. 2a) Add 2% by weight of chitosan glycidyltrimethylammonium chloride to the above alkalinization system, stir and react for 1 h, dissolve chloroacetic acid in isopropanol to prepare chloroacetic acid alcohol solution, slowly add the solution dropwise to the alkalinization system, and keep it at 55°C for 3.5 h for etherification treatment to obtain intermediate system; 3a) Adjust the pH of the above intermediate system to 6.5 with 8% hydrochloric acid by mass fraction. Collect the solid product in the system by vacuum filtration. Wash the solid product with anhydrous ethanol and deionized water in sequence. After washing, vacuum dry it to constant weight at 55°C. Finally, pulverize the dried product to 70 mesh to obtain modified carboxymethylated chitosan.

[0028] The preparation steps for modified carboxylated polyvinyl alcohol are as follows: 1b) Add polyvinyl alcohol with a degree of hydrolysis of 88% to deionized water, heat to 85°C and stir continuously until the polyvinyl alcohol is completely dissolved to form a polyvinyl alcohol aqueous solution with a mass fraction of 8%. Cool the aqueous solution to 45°C for later use. 2b) Add the ferric chloride-copper chloride composite catalyst to the prepared polyvinyl alcohol aqueous solution, wherein the mass ratio of ferric chloride to copper chloride is 1:1. After stirring to completely dissolve the composite catalyst, add 25% hydrogen peroxide by mass dropwise to the system at a constant rate and keep the reaction at 45°C for 2.5 h. 3b) Add sodium sulfite to the system after the above reaction, stir and mix thoroughly, then vacuum dry the system at 55°C to constant weight, and finally pulverize the dried product to 50 mesh to obtain modified carboxylated polyvinyl alcohol.

[0029] Example 2 This embodiment provides a crosslinking modification process for a biodegradable starch-based adhesive, specifically including the following steps: 1. Starch pretreatment: Corn starch and tapioca starch are mixed at a mass ratio of 7.5:2.5 to obtain starch raw material. The starch raw material and 4% of the total mass of starch raw material glycerol plasticizer are added to the reaction vessel. Deionized water is added to the reaction vessel and stirred thoroughly. The mixture is heated to 70℃ for pregelatinization. During the pregelatinization process, the stirring speed is maintained at 175r / min. The stirring is continued until a homogeneous starch paste is formed. After the pregelatinization is completed, the system is cooled to 45℃ for later use.

[0030] 2. Crosslinking reaction: Add 0.3 mol / L dilute hydrochloric acid to the prepared homogeneous starch paste to adjust the initial pH of the starch paste to 4.2. Then, add 3% (by weight of the total starch raw material) of citric acid to the system at a dropping rate of 3.0 mL / min. After the citric acid is added, adjust the pH of the system to 5.5 using a citric acid-borax buffer solution. Then, add 1.0% (by weight of the total starch raw material) of boric acid at a dropping rate of 1.5 mL / min. After the boric acid is added, adjust the pH of the system to 7.2 using a sodium hydroxide-borax composite weak alkaline buffer solution. Continue to add 2% (by weight of the total starch raw material) of epichlorohydrin at a dropping rate of 1.0 mL / min. The total incubation time for the entire process of adding and reacting the three crosslinking agents is 165 min. After the crosslinking reaction is completed, adjust the pH of the system to 7.0 to form the crosslinking product.

[0031] 3. Polyol Intercalation: Glycerol was selected as the small molecule polyol. Glycerol was first subjected to vacuum dehydration at 80℃ and -0.09MPa for 2.0h until the moisture content of glycerol was ≤0.8%. The pretreated glycerol was added to the above crosslinking product at an addition rate of 2.5% of the total mass of starch raw materials. Simultaneously, dried modified carboxymethylated chitosan and modified carboxylated polyvinyl alcohol were added. The mixture was then placed in a vacuum drying oven at 85℃ and -0.10MPa for joint dehydration for 1.5h, controlling the total moisture content of the mixture to ≤0.8%. After joint dehydration, 0.4% of the total mass of starch raw materials of natural trehalose was added to the system, and the mixture was thoroughly stirred to obtain the polyol-intercalated modified crosslinking system.

[0032] 4. Film formation and compatibility control: Add 0.3% of modified carboxymethylated chitosan (total mass of starch raw material) to the above polyol intercalation modified crosslinking system. After stirring and mixing evenly, take 1.0% of natural lecithin (total mass of starch raw material) and dissolve it in 2% anhydrous ethanol (total mass of system) to prepare lecithin alcohol solution. Add this solution to the system and stir continuously at a stirring rate of 130 r / min for 25 min to disperse the components evenly and obtain a mixed system.

[0033] 5. Toughening and Post-treatment: Add 9% (by weight of starch raw material) of modified carboxylated polyvinyl alcohol to the above mixture, heat the system to 60℃ and stir continuously for 35 min, introducing a vacuum degassing treatment at -0.08 MPa during stirring; after degassing, add 0.25% (by weight of system total mass) of ε-polylysine as a preservative to the system, first coarsely filter the system using an 80-mesh sieve, then finely filter it using a 200-mesh sieve to remove unreacted impurities; after filtration, place the adhesive in a 24℃ environment for low-temperature aging treatment for 24 h to finally obtain a biodegradable starch-based adhesive.

[0034] The preparation steps of modified carboxymethylated chitosan are as follows: 1a) Add chitosan with a degree of deacetylation ≥90% to isopropanol, stir thoroughly to disperse the chitosan evenly, add 30% sodium hydroxide aqueous solution to the system, and keep it at 45℃ for 2.0h for alkalization treatment to obtain the alkalized system. 2a) Add 2.5% (by weight of chitosan) of glycidyltrimethylammonium chloride to the above alkalinization system, stir and react for 1.25 h, dissolve chloroacetic acid in isopropanol to prepare chloroacetic acid alcohol solution, slowly add the solution dropwise to the alkalinization system, and keep it at 60 °C for 4.0 h for etherification treatment to obtain intermediate system; 3a) Adjust the pH of the above intermediate system to 7.0 with 10% hydrochloric acid by mass fraction. Collect the solid product in the system by vacuum filtration. Wash the solid product with anhydrous ethanol and deionized water in sequence. After washing, vacuum dry it to constant weight at 60°C. Finally, pulverize the dried product to 80 mesh to obtain modified carboxymethylated chitosan.

[0035] The preparation steps for modified carboxylated polyvinyl alcohol are as follows: 1b) Add polyvinyl alcohol with a degree of hydrolysis of 88% to deionized water, heat to 90°C and stir continuously until the polyvinyl alcohol is completely dissolved to form a 10% polyvinyl alcohol aqueous solution. Cool the aqueous solution to 50°C for later use. 2b) Add the ferric chloride-copper chloride composite catalyst to the prepared polyvinyl alcohol aqueous solution, wherein the mass ratio of ferric chloride to copper chloride is 1:1. After stirring to completely dissolve the composite catalyst, add 30% hydrogen peroxide by mass dropwise to the system at a constant rate and keep the reaction at 50°C for 3.0 h. 3b) Add sodium sulfite to the system after the above reaction, stir and mix thoroughly, then vacuum dry the system at 60°C to constant weight, and finally pulverize the dried product to 60 mesh to obtain modified carboxylated polyvinyl alcohol.

[0036] Example 3 This embodiment provides a crosslinking modification process for a biodegradable starch-based adhesive, specifically including the following steps: 1. Starch pretreatment: Corn starch and tapioca starch are mixed at a mass ratio of 8:2 to obtain starch raw material. The starch raw material and 5% of the total mass of starch raw material glycerol plasticizer are added to the reaction vessel. Deionized water is added to the reaction vessel and stirred thoroughly. The mixture is heated to 75℃ for pregelatinization. During the pregelatinization process, the stirring speed is maintained at 200r / min. The stirring is continued until a homogeneous starch paste is formed. After the pregelatinization is completed, the system is cooled to 50℃ for later use.

[0037] 2. Crosslinking reaction: Add 0.5 mol / L dilute hydrochloric acid to the prepared homogeneous starch paste to adjust the initial pH of the starch paste to 4.5. Then, add 4% (by weight of the total starch raw material) of citric acid to the system at a dropping rate of 3.5 mL / min. After the citric acid is added, adjust the pH of the system to 6.0 using a citric acid-borax buffer solution. Then, add 1.5% (by weight of the total starch raw material) of boric acid at a dropping rate of 2.0 mL / min. After the boric acid is added, adjust the pH of the system to 7.5 using a sodium hydroxide-borax composite weak alkaline buffer solution. Continue to add 3% (by weight of the total starch raw material) of epichlorohydrin at a dropping rate of 1.2 mL / min. The total incubation time for the three crosslinking agents to be added and reacted sequentially is 180 min. After the crosslinking reaction is completed, adjust the pH of the system to 7.5 to form the crosslinking product.

[0038] 3. Polyol Intercalation: Xylitol was selected as the small molecule polyol. First, xylitol was subjected to vacuum dehydration at 85℃ and -0.10MPa for 2.5h until the moisture content of xylitol was ≤0.8% to complete the pretreatment. The pretreated xylitol was added to the above crosslinking product at an addition rate of 3% of the total mass of starch raw materials. At the same time, dried modified carboxymethylated chitosan and modified carboxylated polyvinyl alcohol were added. The mixture was placed in a vacuum drying oven at 90℃ and -0.11MPa for joint dehydration treatment for 1.8h, and the total moisture content of the mixture was controlled to be ≤0.8%. After the joint dehydration was completed, 0.5% of the total mass of starch raw materials of natural trehalose was added to the system and stirred thoroughly to obtain the polyol intercalation modified crosslinking system.

[0039] 4. Film formation and compatibility control: Add 0.4% of modified carboxymethylated chitosan (total mass of starch raw material) to the above-mentioned polyol intercalation modified crosslinking system. After stirring and mixing evenly, take 1.2% of natural lecithin (total mass of starch raw material) and dissolve it in 3% anhydrous ethanol (total mass of system) to prepare lecithin alcohol solution. Add this solution to the system and stir continuously at a stirring rate of 160 r / min for 30 min to disperse the components evenly and obtain a mixed system.

[0040] 5. Toughening and Post-treatment: Add 10% (by weight of starch raw material) of modified carboxylated polyvinyl alcohol to the above mixture, heat the system to 62℃ and stir continuously for 40 min, and introduce vacuum degassing treatment at -0.09 MPa during stirring; after degassing, add 0.3% (by weight of system total mass) of sodium dehydroacetate as a preservative to the system, first coarsely filter the system with a 90-mesh sieve, and then finely filter it with a 210-mesh sieve to remove unreacted impurities in the system; after filtration, place the adhesive in an environment of 27℃ for low-temperature aging treatment for 28 h to finally obtain a biodegradable starch-based adhesive.

[0041] The preparation steps of modified carboxymethylated chitosan are as follows: 1a) Add chitosan with a degree of deacetylation ≥90% to isopropanol, stir thoroughly to disperse the chitosan evenly, add 35% sodium hydroxide aqueous solution to the system, and alkalize at 50°C for 2.5h to obtain an alkalized system. 2a) Add 3% by weight of chitosan glycidyltrimethylammonium chloride to the above alkalinization system, stir and react for 1.5 h, dissolve chloroacetic acid in isopropanol to prepare chloroacetic acid alcohol solution, slowly add the solution dropwise to the alkalinization system, and keep it at 65°C for 4.5 h for etherification treatment to obtain intermediate system; 3a) Adjust the pH of the above intermediate system to 7.5 with 12% hydrochloric acid by mass fraction. Collect the solid product in the system by vacuum filtration. Wash the solid product with anhydrous ethanol and deionized water in sequence. After washing, vacuum dry it to constant weight at 65°C. Finally, pulverize the dried product to 90 mesh to obtain modified carboxymethylated chitosan.

[0042] The preparation steps for modified carboxylated polyvinyl alcohol are as follows: 1b) Add polyvinyl alcohol with a degree of hydrolysis of 88% to deionized water, heat to 95°C and stir continuously until the polyvinyl alcohol is completely dissolved to form a 12% polyvinyl alcohol aqueous solution. Cool the aqueous solution to 55°C for later use. 2b) Add the ferric chloride-copper chloride composite catalyst to the prepared polyvinyl alcohol aqueous solution, wherein the mass ratio of ferric chloride to copper chloride is 1:1. After stirring to completely dissolve the composite catalyst, add 35% hydrogen peroxide dropwise to the system at a constant rate and keep the reaction at 55°C for 3.5 h. 3b) Add sodium sulfite to the system after the above reaction, stir and mix thoroughly, then vacuum dry the system at 65°C to constant weight, and finally pulverize the dried product to 70 mesh to obtain modified carboxylated polyvinyl alcohol.

[0043] Comparative Example 1 The only difference between this comparative example and Example 2 is that only citric acid was used as a single crosslinking agent in the crosslinking reaction step, and boric acid and epichlorohydrin were not added to the system. The remaining process steps, raw material ratios and process parameters are completely consistent with those of Example 2.

[0044] Expected performance: Relying solely on citric acid to form ionic crosslinks, it is impossible to construct a three-dimensional crosslinked network of covalent-ionic-dynamic bonds. The overall structural stability of the adhesive is extremely poor, with low bonding strength and insufficient toughness in the dry state, making it prone to brittleness. Water resistance is significantly reduced, and the adhesive layer is prone to swelling and detachment after immersion in water, failing to meet the water resistance requirements for conventional use. During system storage, problems such as viscosity fluctuations and stratification are likely to occur.

[0045] Comparative Example 2 The only difference between this comparative example and Example 2 is that the order of adding the three crosslinking agents in the crosslinking reaction step is adjusted to epichlorohydrin, boric acid, and citric acid, and they are not added in the order appropriate for the pH value in the example. The remaining process steps, raw material ratios, and process parameters are completely consistent with Example 2.

[0046] Expected performance: The order of crosslinking agent addition does not match the pH value of the system, and each crosslinking agent cannot crosslink under the optimal reaction environment. The formation efficiency of covalent bonds, ionic bonds and dynamic bonds is greatly reduced, and the network structure of the crosslinking product is loose and uneven. The adhesive strength and toughness are significantly reduced, a large number of unreacted sites exist inside the adhesive layer, the weather resistance is deteriorated, and the performance is prone to decay after long-term use. At the same time, the system has poor compatibility and is prone to component aggregation.

[0047] Comparative Example 3 The only difference between this comparative example and Example 2 is that the polyol intercalation step was not performed, and the cross-linked product obtained from the cross-linking reaction was directly subjected to the subsequent film formation and compatibility control steps. All other process steps, raw material ratios and process parameters are completely consistent with Example 2.

[0048] Expected performance: The lack of intercalation modification with small molecule polyols results in a large number of excessive hydrogen bonds in the starch crosslinking network, leading to excessively high system viscosity and poor coatability. The dense crosslinking network structure lacks buffer space, resulting in insufficient adhesive toughness and easy cracking. Furthermore, the interfacial properties of the system cannot be controlled by polyols, leading to uneven chitosan film formation, reduced water resistance and adhesion of the adhesive layer, and extremely low retention of bonding strength after immersion in water.

[0049] Comparative Example 4 The only difference between this comparative example and Example 2 is that natural lecithin was not added in the film formation and compatibility control steps; only modified carboxymethylated chitosan was added to the system. The remaining process steps, raw material ratios, and process parameters are completely consistent with Example 2.

[0050] Expected performance: Without natural lecithin as a molecular bridge, the modified carboxymethylated chitosan has extremely poor compatibility with other polymer components in the system, and is prone to agglomeration and stratification. Chitosan cannot form a dense and uniform interfacial barrier film on the surface of the system, and the water resistance and air permeability of the adhesive layer are greatly reduced. At the same time, the components are unevenly dispersed, and the performance of different regions of the adhesive varies greatly. The toughness and bonding strength are both poor, and the storage stability is poor.

[0051] Comparative Example 5 The only difference between this comparative example and Example 2 is that in the preparation step of modified carboxylated polyvinyl alcohol, a single ferric chloride catalyst is used instead of the ferric chloride-copper chloride composite catalyst. The remaining process steps, raw material ratios and process parameters are completely consistent with Example 2.

[0052] Expected performance: A single ferric chloride catalyst cannot achieve stable generation and directional transfer of free radicals, making it difficult to control the degree of carboxylation of polyvinyl alcohol, and the carboxyl groups are unevenly distributed on the molecular chain; the modified carboxylated polyvinyl alcohol obtained has low binding efficiency with the starch crosslinking network, and cannot achieve uniform toughening, resulting in insufficient toughness and excessively high viscosity in local areas of the adhesive; at the same time, polyvinyl alcohol is prone to excessive oxidative degradation, and its toughening function of flexible chains is greatly reduced, resulting in a significant tendency for the adhesive to become brittle and poor impact resistance.

[0053] Comparative Example 6 The only difference between this comparative example and Example 2 is that the system was not subjected to vacuum degassing in the toughening and post-treatment steps, and no low-temperature aging treatment was performed after filtration. All other process steps, raw material ratios and process parameters are completely consistent with Example 2.

[0054] Expected performance: Due to the lack of vacuum degassing, a large number of air bubbles remain in the system. After the adhesive layer cures, there are pore defects, resulting in a significant decrease in bonding strength and water resistance. The air bubbles are prone to becoming stress concentration points, and the adhesive layer is prone to cracking from the pores when subjected to external forces. Due to the lack of low-temperature aging, the intermolecular interactions of the components in the system have not reached equilibrium, and the adhesive performance is not fully stable. After leaving the factory, problems such as viscosity fluctuations and hardness changes are likely to occur, the shelf life is greatly shortened, and the performance consistency between different batches of products is poor.

[0055] To compare the performance differences of the crosslinking modification processes of the biodegradable starch-based adhesives provided in Examples 1-3 and Comparative Examples 1-6, the present invention provides the following test methods: 1. Bond strength test Referring to GB / T17517-2017 "Determination of Peel Strength of Adhesives - Flexible Materials vs. Rigid Materials", a universal electronic tensile testing machine was used to test the dry peel strength of the adhesive to wood-wood substrates at room temperature of 25℃ and a tensile rate of 50 mm / min. The bonded samples were immersed in distilled water at room temperature for 48 hours, and after being removed and dried, the wet peel strength was tested under the same conditions. The wet bond strength retention rate was calculated.

[0056] 2. Toughness test Referring to GB / T528-2019 "Determination of Tensile Stress-Strain Properties of Vulcanized Rubber or Thermoplastic Rubber", the adhesive was prepared into standard tensile specimens. The elongation at break was tested under the conditions of room temperature 25℃ and tensile rate 200mm / min. The higher the elongation at break, the better the toughness of the adhesive. At the same time, the impact strength of the specimens was tested, referring to GB / T1843-2008 "Determination of Impact Strength of Plastic Cantilever Beams", to evaluate the impact resistance of the adhesive.

[0057] 3. Water resistance test Apply the adhesive on a clean glass slide to form a film with uniform thickness. After drying, weigh the initial mass of the film. Immerse the film in normal-temperature distilled water for 72 hours. After taking it out, absorb the surface moisture with filter paper and weigh the mass of the film after immersion. Calculate the water absorption rate of the film. At the same time, observe the appearance change of the film after immersion and record whether phenomena such as swelling, cracking, delamination, etc. occur.

[0058] 4. Storage Stability Test Seal the prepared adhesive and store it in a constant-temperature environment at 25°C for 6 months. Take samples at 1 month, 3 months, and 6 months respectively, and test the viscosity change rate of the adhesive (refer to GB / T 2794-2022 "Determination of Viscosity of Adhesives"), and observe whether phenomena such as stratification, agglomeration, mildew, etc. occur in the adhesive; if the viscosity change rate ≤ 5% and there is no abnormal appearance, it is judged that the storage stability is qualified.

[0059] 5. Film-Forming Property and Compatibility Test Apply the adhesive on a polytetrafluoroethylene plate. After drying, observe the surface state of the film and record whether defects such as pinholes, cracks, agglomeration spots, etc. occur; use a scanning electron microscope (SEM) to observe the microscopic structure inside the adhesive and evaluate the dispersion uniformity of each component. If there is no obvious agglomeration and the dispersion is uniform, it is judged that the compatibility is qualified.

[0060] 6. Degradability Test Refer to GB / T19277.1-2011 "Determination of the Ultimate Aerobic Biodegradation Ability of Materials under Controlled Composting Conditions - Method by Measuring the Released Carbon Dioxide - Part 1: General Method", and test the biodegradation rate of the adhesive under composting conditions. After 60 days of cultivation, if the biodegradation rate ≥ 90%, it is judged that the degradability performance is qualified. The experimental data are as follows: Table 1 Performance Test Results of Examples and Comparative Examples

[0061] Based on the experimental data in Table 1, it can be seen that, in terms of adhesive strength and water resistance, the dry peel strength of Examples 1-3 is ≥12.0 N / 25 mm, and Example 2 even reaches ≥14.5 N / 25 mm. The wet peel strength is ≥9.5 N / 25 mm, the wet adhesive strength retention rate is ≥77%, and the water absorption rate is ≤8.5%. However, the performance of each comparative example is significantly reduced. Comparative Example 1, which uses only citric acid as a single crosslinking agent, has a dry peel strength ≤5.0 N / 25 mm, a wet peel strength ≤1.5 N / 25 mm, a wet retention rate ≤30%, and a water absorption rate ≥25.0%. Even after changing the order of crosslinking agent addition, the wet retention rate of Comparative Example 2 is only ≤53%, and the water absorption rate is ≥18.0%. This data difference fully demonstrates that the covalent-ionic-dynamic bond three-dimensional cross-linked network constructed by stepwise pH adjustment in this invention can significantly improve the structural stability and bonding strength of the adhesive. The hydrophobic interface layer formed by modified carboxymethyl chitosan can effectively block water molecules, greatly improve the water resistance of the adhesive, and solve the problems of poor water resistance and severe wet strength decay of existing starch-based adhesives.

[0062] In terms of toughness, the elongation at break of Examples 1-3 was ≥150%, and Example 2 reached ≥180%, demonstrating excellent toughness and impact resistance. In contrast, the elongation at break of Comparative Examples 1-5 was ≤70%, with Comparative Example 1 even ≤40%, exhibiting obvious hard and brittle characteristics. These results indicate that the modified carboxylated polyvinyl alcohol prepared by the ferric chloride-copper chloride composite catalyst effectively improves the system's toughness through the synergistic effect of the flexible chains and dynamic bonds of the three-dimensional crosslinked network. Conversely, single crosslinking, lack of polyol intercalation, and preparation of toughening agents using a single catalyst all lead to insufficient adhesive toughness, failing to meet the impact resistance requirements of practical applications.

[0063] In terms of storage stability and film-forming compatibility, the viscosity change rate of Examples 1-3 over 6 months was ≤3.5%, and that of Example 2 was only ≤2.0%, with excellent film-forming properties, uniform dispersion of all components, and no defects such as agglomeration or pinholes. In contrast, the viscosity change rate of Comparative Examples 1-4 over 6 months was ≥10.0%, and that of Comparative Example 4, due to the lack of natural lecithin, was ≥18.0%, and exhibited severe agglomeration. This indicates that the combined dehydration and trehalose moisture-locking process in the examples can effectively stabilize the system viscosity and improve storage stability, while the compatibility regulation effect of natural lecithin is key to ensuring uniform dispersion of all components and improving film-forming properties and performance uniformity.

[0064] Furthermore, the biodegradability of Examples 1-3 was ≥92%, Example 2 reached ≥95%, and the biodegradability of each comparative example was also ≥90%, indicating that the examples did not sacrifice the biodegradable properties of the starch-based adhesive, and retained its environmental advantages while achieving high performance. Meanwhile, Comparative Example 6, lacking vacuum degassing and low-temperature aging steps, although outperforming other comparative examples in various aspects, had a dry peel strength ≤9.0 N / 25 mm, a viscosity change rate ≥9.0%, and pinhole defects in the adhesive layer, demonstrating that the post-treatment process plays a crucial role in the final stabilization and optimization of the adhesive's performance.

[0065] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A crosslinking modification process for a biodegradable starch-based adhesive, characterized in that, Includes the following steps: (1) Starch pretreatment: Mix starch raw materials with plasticizer, add deionized water and stir evenly, and then pregelatinize to form a uniform starch paste; (2) Cross-linking reaction: After adjusting the pH of the starch paste, three cross-linking agents are added in sequence to react and form cross-linking products; (3) Polyol intercalation: Add pretreated small molecule polyol to the crosslinking product and stir to obtain a polyol intercalation modified crosslinking system; (4) Film formation and compatibility control: Modified carboxymethylated chitosan and natural lecithin were added to the polyol intercalation modified crosslinking system, and the mixture was stirred and dispersed to obtain a mixed system; (5) Toughening and post-treatment: Add modified carboxylated polyvinyl alcohol to the mixture obtained in step (4), heat and stir, and after anti-corrosion and filtration treatment, obtain a biodegradable starch-based adhesive.

2. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 1, characterized in that, In step (1): the starch raw material is a mixture of corn starch and cassava starch, with a mass ratio of 7:3-8:2; the plasticizer is glycerol, and the amount added is 3%-5% of the total mass of the starch raw material; the pregelatinization treatment temperature is 65-75℃, the stirring rate is 150-200r / min, and the temperature is reduced to 40-50℃ after the treatment is completed.

3. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 1, characterized in that, In step (2): the three crosslinking agents are citric acid, boric acid, and epichlorohydrin in sequence; during the crosslinking reaction, the initial pH of the starch paste is first adjusted to 4.0-4.5 with dilute hydrochloric acid at a concentration of 0.1-0.5 mol / L, and 2%-4% of the total mass of starch raw materials with citric acid is added dropwise at a dropping rate of 2.5-3.5 mL / min; then the pH of the system is adjusted to 5.0-6.0 with citric acid-borax buffer solution, and 0.5%-1.5% of the total mass of starch raw materials with boric acid is added dropwise at a dropping rate of 1.0-2.0 mL / min; subsequently, the pH is adjusted to 7.0-7.5 with sodium hydroxide-borax composite weak alkaline buffer solution, and 1%-3% of the total mass of starch raw materials with epichlorohydrin is added dropwise at a dropping rate of 0.8-1.2 mL / min; the total incubation reaction time is 150-180 min, and the pH of the system is adjusted to 6.5-7.5 after the reaction is completed.

4. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 1, characterized in that, In step (3): the small molecule polyol is selected from glycerol or xylitol, and the amount added is 2%-3% of the total mass of starch raw material; the pretreatment conditions of the small molecule polyol are vacuum dehydration at 75-85℃ and -0.08~-0.10MPa for 1.5-2.5h, and the moisture content after treatment is ≤0.8%.

5. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 4, characterized in that, The preparation steps of the modified carboxymethylated chitosan are as follows: 1a) Chitosan with a degree of deacetylation ≥90% is added to isopropanol and dispersed evenly. Then, a sodium hydroxide aqueous solution with a mass fraction of 25%-35% is added, and the mixture is kept at 40-50℃ for 1.5-2.5 hours to obtain an alkalized system. 2a) Add 2%-3% by weight of chitosan glycidyltrimethylammonium chloride to the alkalization system, stir and react for 1-1.5 h, dissolve chloroacetic acid in isopropanol, slowly add it dropwise to the alkalization system, and keep it at 55-65℃ for 3.5-4.5 h to obtain the intermediate system; 3a) Adjust the pH of the intermediate system to 6.5-7.5 with 8%-12% dilute hydrochloric acid, collect the solid product by filtration, wash it successively with anhydrous ethanol and deionized water, vacuum dry it to constant weight at 55-65℃, and pulverize it to 70-90 mesh to obtain modified carboxymethylated chitosan with a degree of carboxymethyl substitution of 0.8-1.0 and a water solubility of ≥95% in the neutral system.

6. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 5, characterized in that, The preparation steps of the modified carboxylated polyvinyl alcohol are as follows: 1b) Add polyvinyl alcohol with a degree of hydrolysis of 88% to deionized water, heat to 85-95℃ and stir until completely dissolved to form a polyvinyl alcohol aqueous solution with a mass fraction of 8%-12%, and cool to 45-55℃. 2b) Add ferric chloride-copper chloride composite catalyst to a polyvinyl alcohol aqueous solution, wherein the mass ratio of ferric chloride to copper chloride in the composite catalyst is 1:

1. After stirring and dissolving, slowly add hydrogen peroxide with a mass fraction of 25%-35% and keep the reaction at 45-55℃ for 2.5-3.5h. 3b) Add sodium sulfite to the system, stir and mix, then vacuum dry at 55-65℃ to constant weight, and pulverize to 50-70 mesh to obtain modified carboxylated polyvinyl alcohol with a carboxyl content of 1%-2%.

7. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 6, characterized in that, In step (3): the dried modified carboxymethylated chitosan, modified carboxylated polyvinyl alcohol and small molecule polyol are mixed and placed in a vacuum drying oven at 80-90℃ and -0.09~-0.11MPa for 1.2-1.8h to dehydrate, and the total moisture content after mixing is controlled to be ≤0.8%; after dehydration, 0.3%-0.5% of natural trehalose by weight of starch raw material is added and stirred and mixed.

8. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 7, characterized in that, In step (4): the amount of modified carboxymethylated chitosan added is 0.2%-0.4% of the total mass of starch raw materials; the amount of natural lecithin added is 0.8%-1.2% of the total mass of starch raw materials, and it is added after being dissolved in anhydrous ethanol accounting for ≤3% of the total mass of the system. The stirring rate is 100-160 r / min and the stirring time is 20-30 min.

9. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 7, characterized in that, In step (5): the amount of modified carboxylated polyvinyl alcohol added is 8%-10% of the total mass of starch raw material, and the stirring time is 30-40 min; vacuum degassing is introduced during the stirring process, and the vacuum degree is -0.07~-0.09 MPa.

10. The crosslinking modification process for the biodegradable starch-based adhesive according to claim 9, characterized in that, In step (5): the preservative is sodium dehydroacetate or ε-polylysine, and the amount added is 0.2%-0.3% of the total mass of the system; the filtration adopts a multi-stage filtration method of 70-90 mesh coarse filtration and 190-210 mesh fine filtration, and after filtration, the adhesive is aged at low temperature at 23-27℃ for 20-28h; In step (5): the temperature for heating and stirring is 58-62℃.

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