A high-viscosity, high-toughness wheat starch-based paperboard adhesive and its preparation method

Abstract

This invention discloses a high-viscosity, high-toughness wheat starch-based paperboard adhesive and its preparation method. The adhesive uses wheat starch as the main raw material, which is divided into cooked wheat starch and raw wheat starch. The cooked wheat starch undergoes slight esterification modification, is mixed with a film-forming aid and gelatinized, and then a rigid micro-crosslinking agent is added to form a cooked adhesive carrier system. The raw wheat starch is dispersed with an alkaline regulator at a constant temperature of 25±2℃ to form a raw adhesive matrix system. After mixing the cooked adhesive carrier system and the raw adhesive matrix system, a flexible dynamic crosslinking agent is added when the system is cooled to 30-40℃, and the adhesive is then subjected to pH adjustment, filtration, and maturation to obtain the finished product. This invention improves the problems of low viscosity, strong settling, and brittle adhesive film of wheat starch adhesive solution under conditions of lower alkali and lower modifier dosage, thereby improving the bonding strength, storage stability, and adhesive layer toughness of paperboard.

Description

Technical Field

[0001] This invention relates to the field of adhesives for paperboard production, specifically to a high-viscosity, high-toughness wheat starch-based paperboard adhesive and its preparation method. Background Technology

[0002] Starch-based adhesives are typically used in the production of corrugated cardboard, carton board, and other paper-based substrate materials. Among existing cardboard manufacturers, cassava starch is often used as the main starch raw material for cardboard adhesives due to its stable gelatinization properties, high viscosity, and good flowability.

[0003] However, the domestic supply of cassava starch is significantly affected by factors such as production location, import channels, international market prices, transportation cycles, and inventory turnover. For paperboard manufacturers with continuous production processes, long-term reliance on imported cassava starch increases the uncertainty of raw material supply and production costs.

[0004] Domestic wheat starch has a relatively stable source and ample supply, providing the basic conditions for replacing imported cassava starch in paperboard adhesives. However, wheat starch and cassava starch have different properties, and directly using wheat starch in paperboard adhesives can easily lead to the following problems: First, wheat starch adhesive has a lower viscosity. The viscosity of ordinary wheat starch after gelatinization is lower than that of cassava starch, resulting in insufficient initial tack when used directly, which can easily affect the bonding strength of paperboard. Second, wheat starch adhesive has strong settling properties. After gelatinization, the molecular chains of wheat starch easily recombine, and the adhesive is prone to stratification, sedimentation, or changes in flowability during storage, transportation, or recycling. Additionally, wheat starch adhesive films have a shorter adhesive toughness. The adhesive layer is prone to brittleness after drying, and cracks can easily form during paperboard folding, handling, stacking, or external impact, affecting the durability of the paperboard bond.

[0005] In existing technologies, to increase the viscosity of wheat starch adhesives, methods such as increasing starch concentration, increasing caustic soda dosage, or increasing the degree of gelatinization are commonly used. While these methods can increase the apparent viscosity of the adhesive to some extent, they also have drawbacks. Increasing starch concentration increases the resistance to adhesive transport and the burden on the adhesive application process; adding large amounts of caustic soda can easily cause starch molecular chain degradation and increase the pressure on subsequent wastewater treatment; simply increasing the degree of gelatinization cannot effectively solve the problems of wheat starch adhesive sedimentation and brittle dry adhesive film.

[0006] Therefore, there is a need to provide a paperboard adhesive suitable for the characteristics of domestically produced wheat starch materials and its preparation method, so that wheat starch can achieve better bonding strength, storage stability and adhesive layer toughness with lower alkali and lower modifier dosages. Summary of the Invention

[0007] The purpose of this invention is to provide a high-viscosity and toughness wheat starch-based paperboard adhesive and its preparation method, which improves the problems of low viscosity, strong settling, brittle film and insufficient stability when wheat starch is directly used as a paperboard adhesive.

[0008] This invention divides wheat starch into a cooked glue portion and a raw glue portion, and combines it with shallow esterification modification, constant temperature pulping, low alkali dispersion, rigid micro-crosslinking and flexible dynamic crosslinking, so that wheat starch adhesive can meet the bonding needs of paperboard with a lower amount of chemical additives.

[0009] The above-mentioned technical objective of the present invention is achieved through the following technical solution: a high-viscosity and toughness wheat starch-based paperboard adhesive, wherein the raw materials for preparing the adhesive, based on 100 parts by total weight of wheat starch, include: 100 parts wheat starch, 300-450 parts water, 0.5-1.5 parts alkaline regulator, 0.5-3.0 parts film-forming aid, and 0.4-0.8 parts core active modifying / crosslinking component; The core active modifying / crosslinking components include a shallow esterification modifier, a rigid micro-crosslinking agent, and a flexible dynamic crosslinking agent; wherein, the shallow esterification modifier mainly acts on wheat starch for cooking rubber, the rigid micro-crosslinking agent mainly acts on the cooking rubber carrier system, and the flexible dynamic crosslinking agent mainly forms a reversible connection structure after the cooking rubber carrier system and the raw rubber main body system are mixed. The wheat starch is divided into two parts: 15-25 parts are used to react with the shallow esterification modifier to form modified wheat starch for mature gum, and 75-85 parts are used as raw wheat starch for raw gum. The adhesive includes a cooked rubber carrier system constructed from the modified wheat starch for cooked rubber, the film-forming aid, and the rigid micro-crosslinking agent, and a raw rubber host system constructed from the original wheat starch for raw rubber and the alkaline regulator; The rigid micro-crosslinking agent is added after the cooked rubber is gelatinized with modified wheat starch and forms a micro-crosslinking connection structure in the cooked rubber carrier system; the flexible dynamic crosslinking agent is added after the cooked rubber carrier system and the raw rubber main body system are mixed and cooled, and dispersed in the mixture of the cooked rubber carrier system and the raw rubber main body system.

[0010] Furthermore, the amount of the core active modifying / crosslinking component is 0.55 to 0.65 parts; wherein, the amount of the shallow esterification modifier is 0.12 to 0.18 parts, the amount of the rigid micro-crosslinking agent is 0.18 to 0.25 parts, and the amount of the flexible dynamic crosslinking agent is 0.18 to 0.25 parts. The shallow esterification modifier is one or more of succinic anhydride, octenyl succinic anhydride, acetic anhydride, citric acid, or citric anhydride. The rigid micro-crosslinking agent is one or more of the following: polyisocyanate, polymeric MDI, PAPI, epoxy crosslinking agent, glyoxal crosslinking agent, or citric acid crosslinking agent; The flexible dynamic crosslinking agent is one or more of borax, boric acid, metaborate, or organic borates.

[0011] Furthermore, the degree of substitution of the modified wheat starch in the cooked gum is 0.005 to 0.050.

[0012] Furthermore, the film-forming aid is one or more of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl methyl cellulose, aqueous polyurethane emulsion, or starch-grafted film-forming components. The alkalinity regulator is one or more of sodium hydroxide, sodium carbonate, potassium hydroxide, and borax-carbonate composite alkali.

[0013] Furthermore, the adhesive also includes a nano / bio-based reinforcing component, wherein the amount of the nano / bio-based reinforcing component is 0.05% to 0.60% of the total weight of wheat starch; The nano / bio-based reinforcing components include one or more of the following: surface-modified lignin, cellulose nanofibers, nano-silica, nano-montmorillonite, nano-cellulose crystals, or starch-grafted nanoparticles.

[0014] Furthermore, the cooked rubber carrier system is formed by reacting the cooked rubber with modified wheat starch, film-forming aid, and rigid micro-crosslinking agent at 55-65°C; the flexible dynamic crosslinking agent is added after the cooked rubber carrier system and the raw rubber main body system are mixed and the system temperature drops to 30-40°C.

[0015] The present invention also provides a method for preparing the above-mentioned high-viscosity and high-toughness wheat starch-based paperboard adhesive, comprising the following steps: S1. Wheat starch is divided into 15-25 parts of cooked wheat starch for gelatin and 75-85 parts of raw wheat starch for gelatin. S2. The wheat starch for cooking gum is lightly esterified using a light esterification modifier to obtain modified wheat starch for cooking gum. S3. Provide pulping water and control the temperature of the pulping water at 25±2℃; S4. Take a portion of the pulping water, mix the cooked gum with modified wheat starch and film-forming aid, heat and gelatinize, and add a rigid micro-crosslinking agent to react, to obtain a cooked gum carrier system. S5. Disperse the raw rubber with raw wheat starch in the remaining pulping water at a temperature of 25±2℃, and add an alkaline regulator to obtain the raw rubber main system. S6. Mix the cooked rubber carrier system with the raw rubber main body system to obtain a cooked rubber-raw rubber mixed system; S7. After the cooked-raw rubber mixture is cooled, a flexible dynamic crosslinking agent is added. After pH adjustment, filtration and aging, the high-viscosity and toughness wheat starch-based paperboard adhesive is obtained.

[0016] Furthermore, in step S2, the reaction temperature for shallow esterification modification is 45–60°C, the pH of the reaction system is 8.0–9.5, the reaction time is 20–90 min, and the degree of substitution of the modified wheat starch in the resulting cooked gum is 0.005–0.050.

[0017] Furthermore, in step S4, the film-forming aid is a cold water-soluble polyvinyl alcohol, or a polyvinyl alcohol aqueous solution that has been dissolved and cooled at 80-95°C in advance. The gelatinization temperature of the modified wheat starch used for the cooked gum is 55-65℃; The reaction temperature after adding the rigid micro-crosslinking agent is 55-65℃, and the reaction time is 10-30 min.

[0018] Furthermore, in step S6, the raw rubber main body system is added to the cooked rubber carrier system for mixing, with a mixing temperature of 35-50°C and a mixing time of 10-40 minutes.

[0019] Furthermore, in step S7, the flexible dynamic crosslinking agent is added after the cooked rubber-raw rubber mixture is cooled to 30-40°C; After adding the flexible dynamic crosslinking agent, the pH of the system is adjusted to 7.5-9.0, and the mixture is then filtered and cured to obtain the high-viscosity, high-toughness wheat starch-based paperboard adhesive.

[0020] Compared with the prior art, the present invention has at least the following beneficial effects: 1. This invention divides wheat starch into a cooked gum portion and a raw gum portion, and modifies the cooked gum portion of wheat starch through shallow esterification to form a cooked gum carrier system. This can improve the problem of easy coagulation and stratification of wheat starch glue solution, and improve the stability of the glue solution during storage, transportation and machine operation.

[0021] 2. By adding film-forming aids and rigid micro-crosslinking agents to the adhesive carrier system, this invention can improve the initial tack of the adhesive and the cohesive strength of the adhesive layer, enabling domestically produced wheat starch adhesives to meet the bonding strength requirements of paperboard.

[0022] 3. This invention improves the problem of wheat starch film becoming brittle after drying by adding a flexible dynamic crosslinking agent after mixing the cooked glue carrier system with the raw glue body system, and enhances the crack resistance and adhesive toughness of the glue layer during paperboard folding, handling and stress.

[0023] 4. By controlling the temperature of the water used for pulping at 25±2℃ and using a low-alkali dispersion process, this invention can reduce the impact of seasonal temperature differences and water temperature fluctuations on the wheat starch pulping process, improve the fluidity and production stability of different batches of adhesive solution, and reduce the amount of caustic soda used.

[0024] 5. This invention uses domestically produced wheat starch as the main starch raw material, which, while ensuring the adhesive performance of paperboard, helps to reduce dependence on imported cassava starch and lower the raw material cost of paperboard adhesives. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of the preparation method of the high-viscosity and toughness wheat starch-based paperboard adhesive of the present invention. Detailed Implementation

[0026] The following is combined Figure 1 The technical solution of the present invention will be further described below. The following examples are used to illustrate the implementation of the present invention. Those skilled in the art can make adjustments within the component range and process conditions disclosed in the present invention. In each example, "parts" refers to parts by weight. Unless otherwise specified, the wheat starch used is domestic wheat starch, the water used is softened water or deionized water, and the stirring equipment used is a slurry tank with jacket temperature control.

[0027] like Figure 1 As shown, this invention provides a high-viscosity, high-toughness wheat starch-based paperboard adhesive, using wheat starch as the main raw material. The wheat starch is divided into cooked wheat starch and raw wheat starch. The cooked wheat starch is first lightly esterified and modified, then combined with film-forming aids and rigid micro-crosslinking agents to form a cooked adhesive carrier system. The raw wheat starch is dispersed in constant-temperature pulping water and forms a raw adhesive main system under the action of an alkaline regulator. Subsequently, the cooked adhesive carrier system and the raw adhesive main system are mixed, and after cooling, a flexible dynamic crosslinking agent is added. After pH adjustment, filtration, and curing, the finished adhesive is obtained.

[0028] The reason for adopting the above method is that when wheat starch is used directly as a paperboard adhesive, it is prone to problems such as low viscosity, strong settling, and brittle adhesive film. Pre-preparing a portion of the wheat starch into a cooked adhesive carrier system can provide initial tack and suspension stability; retaining the majority of the wheat starch as the raw adhesive body system can maintain the ability to further gelatinize and bond during paperboard production. After mixing the cooked adhesive carrier system and the raw adhesive body system, the adhesive stability, machine flowability, and paperboard bonding strength can be balanced.

[0029] In this invention, the amount of wheat starch used for cooking rubber is controlled at 15-25 parts, and the amount of raw wheat starch used for raw rubber is controlled at 75-85 parts. If the amount of wheat starch used for cooking rubber is too low, the cooking rubber carrier system will be insufficient, and the initial viscosity and anti-coagulation ability of the rubber solution will decrease; if the amount of wheat starch used for cooking rubber is too high, the viscosity of the rubber solution will easily be too high, which is not conducive to pipeline transportation and uniform coating. Controlling the amount of wheat starch used for cooking rubber to 15-25 parts allows the cooking rubber carrier system to play a stabilizing and supporting role, while ensuring that the raw rubber main body system still accounts for the majority.

[0030] The temperature of the water used for pulping is controlled at 25±2℃. Wheat starch is sensitive to changes in water temperature; if the water temperature is too high, it is prone to premature swelling or localized gelatinization, while if the water temperature is too low, dispersion and wetting are insufficient. Controlling the water temperature for pulping at 25±2℃ can reduce the impact of seasonal temperature differences and workshop water temperature variations on the flowability and batch stability of the glue solution. This temperature control is mainly used for the dispersion, alkalization, and initial preparation stages of cooked and raw glues of wheat starch; during the gelatinization stage of cooked glues, the temperature still needs to be raised to a suitable temperature for gelatinization and micro-crosslinking reactions.

[0031] Wheat starch for gelatin production is modified using a shallow esterification method. Shallow esterification does not involve deep modification of the wheat starch; rather, it introduces a small number of ester groups into the wheat starch molecular chains. This weakens the excessive hydrogen bonding between starch chains, thereby reducing the tendency for retrogradation and sedimentation after gelatinization. Suitable shallow esterification modifiers include succinic anhydride, octenyl succinic anhydride, acetic anhydride, citric acid, or citric anhydride. The degree of substitution in the modified wheat starch is controlled between 0.005 and 0.050. Too low a substitution level results in little improvement in anti-sealation properties; too high a substitution level may affect the starch's own gelatinization and adhesive properties and increase modification costs.

[0032] Film-forming aids are used to improve the continuity of the adhesive layer. Suitable film-forming aids include polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose (HMC), waterborne polyurethane emulsions, or starch-grafted film-forming components. When using PVA, cold-water soluble PVA can be used, or ordinary PVA can be pre-dissolved in an aqueous solution at 80–95°C and then added to the adhesive system after cooling. This avoids incomplete dissolution of ordinary PVA in low-temperature water, which could lead to undissolved particles in the adhesive solution.

[0033] Rigid micro-crosslinking agents are used to improve the cohesive strength of the gluten carrier system. Rigid micro-crosslinking agents can be selected from polyisocyanates, polymeric MDI, PAPI, epoxy crosslinking agents, glyoxal crosslinking agents, or citric acid crosslinking agents. When using PAPI, its isocyanate groups can react with the hydroxyl groups in the starch and polyvinyl alcohol molecular chains to form certain linkage structures; under aqueous phase conditions, some isocyanate groups may also form urea bonds. These linkage structures can provide good support for the gluten carrier system, but the amount added needs to be controlled within the micro-crosslinking range to avoid the formation of localized gels, which would affect delivery and machine use.

[0034] Flexible dynamic crosslinking agents are used to improve the toughness of dry adhesive films. These agents can be borax, boric acid, metaborates, or organic borates. For example, borax can form a reversible linkage structure with the hydroxyl groups in starch and polyvinyl alcohol molecular chains under weakly alkaline conditions. This structure can disperse stress when the adhesive layer is folded, impacted, or stretched, thereby reducing the problem of the film becoming brittle and cracking after drying. The flexible dynamic crosslinking agent is preferably added after mixing the cooked adhesive carrier system and the raw adhesive matrix system and cooling to 30–40°C, which can reduce the problem of premature local gelation at high temperatures.

[0035] Alkaline regulators are used to assist in the dispersion, swelling, and pH adjustment of wheat starch. Sodium hydroxide, sodium carbonate, potassium hydroxide, or a borax-carbonate compound alkali can be selected. The dosage of the alkaline regulator should be controlled at 0.5–1.5 parts. This dosage ensures a relatively stable dispersion of the raw wheat starch used for gum production, while avoiding excessive caustic soda that could cause starch chain degradation and increased wastewater treatment pressure.

[0036] In applications requiring further improvement in adhesive layer toughness, moisture resistance, or storage stability, nano / bio-based reinforcing components can be added. These components can be surface-modified lignin, cellulose nanofibers, nano-silica, nano-montmorillonite, nano-cellulose crystals, or starch-grafted nanoparticles. These components can form support points or bridging structures in the adhesive solution, helping to disperse stress on the adhesive layer. They can also reduce the tendency for starch chains to rearrange themselves tightly through spatial barrier effects. The dosage of the nano / bio-based reinforcing component should be controlled at 0.05%–0.60% of the total weight of wheat starch. Too low a dosage will result in insignificant reinforcing effect; too high a dosage may lead to excessively high adhesive viscosity and affect coating uniformity.

[0037] Example 1 is a basic wheat starch-based paperboard adhesive. Based on 100 parts by total wheat starch weight, 100 parts of domestically produced wheat starch were used, of which 20 parts were used as cooked wheat starch and 80 parts were used as raw wheat starch. 380 parts of water, 1.0 part of sodium hydroxide, 1.5 parts of polyvinyl alcohol, and 0.60 parts of a core active modifying / crosslinking component were also used. The core active modifying / crosslinking component contained 0.16 parts of succinic anhydride, 0.22 parts of PAPI, and 0.22 parts of borax.

[0038] First, 20 parts of cooked rubber wheat starch were dispersed in water, and the pH was adjusted to 8.5. Succinic anhydride was then slowly added at 55°C to initiate a shallow esterification reaction for 45 minutes. After the reaction, the mixture was washed with water, filtered, and dried to obtain modified cooked rubber wheat starch with a degree of substitution of approximately 0.025. This step gives the cooked rubber wheat starch a lower tendency for retrogradation and sedimentation, providing a stable foundation for the subsequent cooked rubber carrier system.

[0039] Water for pulping was added to a jacketed pulping tank, and the water temperature was controlled at 25±2℃ using cooling water or a heat exchanger. 150 parts of the pulping water were used to prepare the gluten carrier system, and the remaining water was used to prepare the raw rubber matrix system. Polyvinyl alcohol was pre-dissolved at 90℃ to prepare an aqueous solution with a mass fraction of approximately 10%, cooled to below 35℃, and then added to the gluten preparation tank. Modified wheat starch for gluten preparation was then added, stirred evenly, and heated to 60℃, maintaining stirring to ensure complete gelatinization. After gelatinization, PAPI was slowly added, and the mixture was reacted at 60℃ for 20 minutes to obtain the gluten carrier system. During this process, polyvinyl alcohol improved the film-forming continuity of the gluten carrier system, and PAPI formed a small number of micro-crosslinking points, giving the gluten carrier system good cohesive strength.

[0040] Eighty parts of raw wheat starch were added to the remaining pulping water at 25±2℃ and stirred for 15 minutes. Then, sodium hydroxide aqueous solution was slowly added, and stirring was continued for 20 minutes to obtain the main raw rubber system. This main raw rubber system remained in a flowable state and did not undergo significant premature gelatinization. Low-alkali dispersion wetted and moderately swelled the raw wheat starch granules, while retaining their ability to further gelatinize and bond during the paperboard thermal processing stage.

[0041] The raw rubber matrix system was slowly added to the cooked rubber carrier system, with the mixing temperature controlled at around 40℃, and stirred for 20 minutes to obtain a cooked rubber-raw rubber mixture. This mixing method facilitates the dispersion and coating of the raw wheat starch by the cooked rubber carrier system, reducing the sedimentation of raw rubber particles. The mixture was then cooled to 35℃, and a pre-prepared borax aqueous solution was added. Stirring continued for 20 minutes, and the pH of the system was adjusted to 8.2. Finally, the mixture was filtered through an 80-mesh sieve and allowed to stand for 30 minutes to mature, yielding a basic high-viscosity, high-toughness wheat starch-based paperboard adhesive.

[0042] The adhesive solution obtained in Example 1 was uniform and showed no obvious lumps, making it suitable for ordinary corrugated cardboard production. In this example, the gluten carrier system mainly provides initial tack and suspension stability, while the raw adhesive matrix system mainly maintains the gelatinization matrix during the thermal bonding process of the cardboard. Borax was added later to form a flexible dynamic connection structure, which improves the toughness of the adhesive film.

[0043] Example 2 is a high-tack, high-strength reinforced wheat starch-based paperboard adhesive. Based on 100 parts by total wheat starch weight, 100 parts of domestically produced wheat starch were used, of which 22 parts were used as cooked wheat starch and 78 parts were used as raw wheat starch. 400 parts of water, 0.9 parts of sodium hydroxide, 2.0 parts of polyvinyl alcohol, and 0.55 parts of a core active modifying / crosslinking component were also included. The core active modifying / crosslinking component contained 0.15 parts of octenyl succinic anhydride, 0.20 parts of PAPI, and 0.20 parts of borax. Additionally, 0.07 parts of surface-modified lignin and 0.10 parts of nano-silica were used as nano / bio-based reinforcing components.

[0044] Twenty-two portions of wheat starch for gelatinization were reacted with octenyl succinic anhydride at pH 8.8 and 58°C for 60 min. After washing, dehydration, and drying, modified wheat starch for gelatinization was obtained, with a degree of substitution of approximately 0.030. The hydrophobic side groups introduced by octenyl succinic anhydride can reduce the tight bonding between molecular chains after wheat starch gelatinization, which is beneficial to reducing the tendency of sedimentation during storage.

[0045] The temperature of the pulping water was controlled at 25±2℃. 160 parts of the pulping water were used for the gluten carrier system, and the remainder for the raw rubber body system. Polyvinyl alcohol was pre-dissolved at 90℃ and cooled, then mixed with modified wheat starch for gluten production, and the mixture was heated to 62℃ for gelatinization. After gelatinization, PAPI was added, and the mixture was reacted at 62℃ for 18 minutes to obtain the gluten carrier system. Surface-modified lignin was first prepared into an aqueous dispersion and then added to the gluten carrier system, and stirring was continued for 10 minutes to ensure uniform dispersion.

[0046] 78 parts of raw rubber were dispersed in residual pulping water at 25±2℃ using raw wheat starch, and sodium hydroxide aqueous solution was added to obtain the raw rubber main body system. Nano-silica was first pre-dispersed in a small amount of polyvinyl alcohol aqueous solution, and then added to the raw rubber main body system and stirred for 20 min. The pre-dispersion treatment can reduce the agglomeration of nano-silica, so that it forms a more uniform stress dispersion point in the rubber solution.

[0047] The raw rubber matrix system was added to the cooked rubber carrier system, and the mixing temperature was controlled at 38℃. The mixture was stirred for 25 minutes. After the system temperature dropped to 35℃, a borax aqueous solution was added, and the mixture was stirred for another 20 minutes. The pH was adjusted to 8.3, filtered through an 80-mesh sieve, and cured for 40 minutes to obtain a high-viscosity, tough, reinforced wheat starch-based paperboard adhesive.

[0048] In Example 2, surface-modified lignin and nano-silica work together to provide support and stress dispersion. The polyhydroxyl structure in lignin can form hydrogen bonds with starch and polyvinyl alcohol, while nano-silica provides micro-support points. Combined with the flexible dynamic bonding structure formed by borax, these two components can further improve the crack resistance and moisture stability of the adhesive layer after drying.

[0049] Example 3 is a low-alkali stabilized wheat starch-based paperboard adhesive. Based on 100 parts by total wheat starch weight, 100 parts of domestically produced wheat starch were used, of which 18 parts were used as cooked wheat starch and 82 parts were used as raw wheat starch. 390 parts of water, 0.65 parts of sodium hydroxide, 1.8 parts of cold water-soluble polyvinyl alcohol, and 0.58 parts of core active modifying / crosslinking components were also used. The core active modifying / crosslinking components included 0.14 parts of succinic anhydride, 0.22 parts of glyoxal crosslinking agent, and 0.22 parts of borax.

[0050] First, 18 parts of cooked wheat starch were reacted with succinic anhydride at pH 8.5 and 55℃ for 40 min to obtain modified wheat starch for cooked rubber with a degree of substitution of approximately 0.020. This low degree of substitution is mainly used to reduce the tendency to gel and at the same time reduce the consumption of modifiers.

[0051] The temperature of the water used for pulping was controlled at 25±2℃. A portion of the pulping water was added to the gluten preparation tank, followed by the addition of cold water-soluble polyvinyl alcohol and stirring until dissolved. Then, modified wheat starch for gluten preparation was added, and the mixture was heated to 60℃ for gelatinization. After gelatinization, a glyoxal-based crosslinking agent was added, and the mixture was reacted at 60℃ for 20 minutes to obtain the gluten carrier system.

[0052] 82 parts of raw rubber were dispersed in the remaining pulping water at 25±2℃ using raw wheat starch, and 0.65 parts of an aqueous solution prepared with sodium hydroxide were added. The mixture was stirred for 25 minutes to obtain the raw rubber main body system. This example reduced the amount of sodium hydroxide used and compensated for the insufficient adhesion performance under low-alkali conditions through the cooked rubber carrier system and subsequent dynamic crosslinking.

[0053] The raw rubber matrix system was added to the cooked rubber carrier system and mixed at 40°C for 20 minutes. After cooling to 34°C, borax aqueous solution was added and stirring was continued for 20 minutes. The pH was adjusted to 8.1, and after filtration and curing, a low-alkali stable wheat starch-based paperboard adhesive was obtained.

[0054] The adhesive obtained in Example 3 is characterized by a low amount of alkaline modifier, making it suitable for production scenarios with high requirements for wastewater treatment pressure and circulating water reuse. Through shallow esterification, a cured adhesive carrier system, and flexible dynamic crosslinking, it can maintain good adhesive stability and bond strength under low-alkali conditions.

[0055] Example 4 is a wheat starch-based paperboard adhesive with high storage stability. Based on 100 parts by total wheat starch weight, 100 parts of domestically produced wheat starch were used, of which 25 parts were used as cooked wheat starch and 75 parts were used as raw wheat starch. 430 parts of water, 1.1 parts of sodium hydroxide, 2.2 parts of polyvinyl alcohol, and 0.65 parts of core active modifying / crosslinking components were also included. The core active modifying / crosslinking components included 0.18 parts of succinic anhydride, 0.22 parts of PAPI, and 0.20 parts of borax. Additionally, 0.05 parts of nano-montmorillonite were used as a nano / bio-based reinforcing component.

[0056] First, 25 parts of cooked gum were lightly esterified with wheat starch at a reaction temperature of 58°C, a reaction system pH of 8.7, and a reaction time of 60 min, yielding modified wheat starch for cooked gum with a degree of substitution of approximately 0.035. Due to the high proportion of modified wheat starch for cooked gum, the cooked gum carrier system in this example exhibits strong continuity, which is beneficial for improving storage stability.

[0057] The temperature of the water used for pulping was controlled at 25±2℃. Ordinary polyvinyl alcohol was dissolved at 90℃ and cooled before being added to the gelatin preparation tank. Modified wheat starch for gelatin preparation was then added, and the mixture was heated to 60℃ for gelatinization. After adding PAPI, the mixture was reacted at 60℃ for 20 minutes to obtain the gelatin carrier system. Nano-montmorillonite was pre-dispersed in a small amount of gelatin solution and added to the gelatin carrier system, with stirring continued for 15 minutes.

[0058] 75 parts of raw rubber were dispersed in the remaining pulping water at 25±2℃ using virgin wheat starch. Sodium hydroxide aqueous solution was added, and the mixture was stirred for 20 min to obtain the raw rubber matrix system. The raw rubber matrix system was then added to the aging carrier system and mixed at 42℃ for 25 min. After cooling to 35℃, borax aqueous solution was added to adjust the pH to 8.4. The mixture was filtered and aged for 45 min to obtain a wheat starch-based paperboard adhesive with high storage stability.

[0059] In Example 4, the higher proportion of the cooked rubber carrier system and the spatial barrier effect of nano-montmorillonite can slow down the sedimentation of raw rubber particles and the retrogradation of starch chains, which is beneficial to maintaining the uniformity of the rubber solution under long-term storage or cyclic supply conditions.

[0060] To verify the beneficial effects of the present invention, Comparative Examples 1 to 5 were set up. Comparative Example 1 used imported cassava starch to prepare a conventional paperboard adhesive, serving as a commonly used reference sample in production. Comparative Example 2 used domestic wheat starch to directly replace cassava starch, without shallow esterification modification, and without adding rigid micro-crosslinking agents or flexible dynamic crosslinking agents. Comparative Example 3 used domestic wheat starch and increased the amount of sodium hydroxide for strong alkaline pulping, without using the cooked glue-raw glue stepwise construction method. Comparative Example 4 used a similar raw material ratio to Example 1, but instead of controlling the pulping water at 25±2℃, it used room temperature water in the workshop for pulping, with the room temperature water temperature fluctuating between 18 and 32℃ during testing. Comparative Example 5 used a shallow esterification and rigid micro-crosslinking method similar to Example 1, but without adding borax as a flexible dynamic crosslinking agent.

[0061] All embodiments and comparative examples used the same batch of paper for board fabrication tests, preparing three-layer corrugated cardboard samples. The amount of adhesive applied was controlled within the same range, and the heating temperature of the hot plate and the production speed were kept consistent. Adhesive strength was tested using common methods for corrugated cardboard adhesive strength, and the average value was recorded. Adhesive flowability was characterized by outflow time, recording the initial outflow time and the difference in outflow time after 24 hours of standing. Storage stability was assessed by observing whether the adhesive exhibited visible delamination, hard sedimentation, or irreversible clumping when placed in a sealed container at room temperature. Film crack resistance was assessed by applying the adhesive to form a film, drying it, and then bending it 180° to observe and record the crack patterns. At least five samples were tested for each data set, and the data in the table are average values.

[0062]

[0063] Table 1 Results of adhesive flowability and storage stability tests As shown in Table 1, the direct wheat starch gum exhibited significant flowability deviation after 24 hours of standing and showed obvious sedimentation after 30 days of storage. While the high-alkali wheat starch gum had a higher initial viscosity, its flowability changed considerably in the later stages, with localized gelation and sedimentation. The gum produced without constant-temperature pulping also showed significant batch and storage fluctuations. The flowability deviation of Examples 1 to 4 was controlled within the range of 2.6–4.2 seconds after 24 hours, and no obvious stratification or hard sedimentation was observed after 30 days of storage. This indicates that shallow esterification, constant-temperature pulping at 25±2℃, stepwise construction of cooked and raw gums, and the addition of a flexible dynamic crosslinking agent after cooling after mixing can improve the storage stability of the wheat starch gum solution.

[0064]

[0065] Table 2. Test results of cardboard adhesive strength As shown in Table 2, the average adhesive strength of the direct wheat starch adhesive was significantly lower than that of the imported cassava starch adhesive and the example samples, with more peeling of the adhesive layer in the failure mode. The adhesive strength of the high-alkali wheat starch adhesive was improved, but it was still lower than that of the imported cassava starch adhesive and the example samples. The average adhesive strength of Example 1 reached 582 N·m. - ¹, The average adhesive strength of Example 2 reached 675 N·m - ¹, The average adhesive strength of Example 4 reached 642 N·m - ¹ This indicates that the combination of the cooked adhesive carrier system, rigid micro-crosslinking agent, and film-forming aid can improve the bonding strength of wheat starch adhesive on paperboard. In Example 2, the addition of surface-modified lignin and nano-silica further improved the bonding strength, demonstrating that the reinforcing components help improve the cohesive strength of the adhesive layer and the stability of the interfacial bonding.

[0066]

[0067] Table 3. Adhesive film toughness and adhesion retention after folding As shown in Table 3, the dry adhesive films of direct wheat starch adhesive and high-alkali wheat starch adhesive exhibited obvious cracks after bending, and the retention rate of adhesive strength after folding was low. Comparative Example 5 used a shallow esterification and rigid micro-crosslinking method, but did not add a flexible dynamic crosslinking agent. Its initial adhesive strength was close to that of Example 1, but the retention rate of adhesive strength after folding decreased to 78.7%, and more fine cracks appeared in the dry adhesive film. In contrast, the retention rates of adhesive strength after folding in Examples 1 to 4 were all close to or higher than 89%, indicating that the addition of the flexible dynamic crosslinking agent after cooling the cooked-raw rubber mixture can improve the crack resistance and strength retention of the dry adhesive film when bent.

[0068]

[0069] Table 4 Results of Low-Alkali Pulping and Production Adaptability Tests Table 4 shows that although high-alkali wheat starch adhesive can increase the initial viscosity, the amount of sodium hydroxide required is relatively high, occasional clumping occurs during the adhesive supply process, and the circulating water is highly alkaline, requiring further adjustment. Examples 1 to 4, with lower sodium hydroxide dosages, did not experience pipe blockage, exhibited good adhesive uniformity, and showed no significant sediment in the circulating water. Example 3, by reducing the sodium hydroxide dosage to 0.65 parts, still maintained good production adaptability, indicating that by combining shallow esterification, a cooked adhesive carrier system, rigid micro-crosslinking, and flexible dynamic crosslinking, the dependence on strong alkali for thickening can be reduced.

[0070] As can be seen from Tables 1 to 4, this invention improves the storage stability, adhesive strength, film toughness, and production adaptability of domestically produced wheat starch adhesive by using wheat starch in stages, shallow esterification of wheat starch for gluten production, constant temperature pulping at 25±2℃, low-alkali dispersion, mixing of gluten and raw starch, rigid micro-crosslinking, and flexible dynamic crosslinking. Compared with direct wheat starch adhesive, the 24-hour flowability deviation of the sample in the examples is significantly reduced, the paperboard bonding strength is improved, and the cracks after film folding are reduced. Compared with high-alkali wheat starch adhesive, the sample in the examples maintains good adhesive performance and machine stability at a lower alkali dosage. Compared with imported cassava starch adhesive, the adhesive strength and storage stability of Examples 2 and 4 are similar to or higher than those of imported cassava starch adhesive.

[0071] The shallow esterification modifier, rigid micro-crosslinking agent, flexible dynamic crosslinking agent, film-forming aid, alkaline regulator, and nano / bio-based reinforcing components listed in the above embodiments can be selected within the range specified based on the type of paperboard, production line speed, coating amount, hot plate temperature, and storage time requirements. Within the raw material dosage range, temperature range, pH range, and mixing sequence defined by this invention, by using wheat starch in stages, mixing the cooked glue carrier system with the raw glue main system, constant temperature pulping, low-alkali dispersion, and adding the flexible dynamic crosslinking agent later, similar improvements in glue stability, adhesive strength, and film toughness can be obtained as in the above embodiments.

[0072] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still make various modifications and equivalent substitutions to the technical solutions of the present invention, and such modifications and equivalent substitutions should not depart from the spirit and scope of protection of the present invention.

Claims

1. A high-viscosity, high-toughness wheat starch-based paperboard adhesive, characterized in that, Based on a total weight of 100 parts wheat starch, the raw materials for preparing the adhesive include: 100 parts wheat starch, 300-450 parts water, 0.5-1.5 parts alkaline regulator, 0.5-3.0 parts film-forming aid, and 0.4-0.8 parts core active modifying / crosslinking component; The core active modifying / crosslinking components include a shallow esterification modifier, a rigid micro-crosslinking agent, and a flexible dynamic crosslinking agent; The wheat starch is divided into two parts: 15-25 parts are used to react with the shallow esterification modifier to form modified wheat starch for mature gum, and 75-85 parts are used as raw wheat starch for raw gum. The adhesive includes a cooked rubber carrier system constructed from the modified wheat starch for cooked rubber, the film-forming aid, and the rigid micro-crosslinking agent, and a raw rubber host system constructed from the original wheat starch for raw rubber and the alkaline regulator; The rigid micro-crosslinking agent is added after the cooked rubber is gelatinized with modified wheat starch and forms a micro-crosslinking connection structure in the cooked rubber carrier system; the flexible dynamic crosslinking agent is added after the cooked rubber carrier system and the raw rubber main body system are mixed and cooled, and dispersed in the mixture of the cooked rubber carrier system and the raw rubber main body system.

2. The high-viscosity, high-toughness wheat starch-based paperboard adhesive according to claim 1, characterized in that, The amount of the core active modifying / crosslinking component is 0.55 to 0.65 parts; wherein, the amount of the shallow esterification modifier is 0.12 to 0.18 parts, the amount of the rigid micro-crosslinking agent is 0.18 to 0.25 parts, and the amount of the flexible dynamic crosslinking agent is 0.18 to 0.25 parts. The shallow esterification modifier is one or more of succinic anhydride, octenyl succinic anhydride, acetic anhydride, citric acid, or citric anhydride. The rigid micro-crosslinking agent is one or more of the following: polyisocyanate, polymeric MDI, PAPI, epoxy crosslinking agent, glyoxal crosslinking agent, or citric acid crosslinking agent; The flexible dynamic crosslinking agent is one or more of borax, boric acid, metaborate, or organic borates.

3. The high-viscosity, high-toughness wheat starch-based paperboard adhesive according to claim 1, characterized in that, The degree of substitution of the modified wheat starch in the cooked gum is 0.005 to 0.

050.

4. The high-viscosity, high-toughness wheat starch-based paperboard adhesive according to claim 1, characterized in that, The film-forming aid is one or more of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl methyl cellulose, aqueous polyurethane emulsion, or starch-grafted film-forming components. The alkalinity regulator is one or more of sodium hydroxide, sodium carbonate, potassium hydroxide, and borax-carbonate composite alkali.

5. The high-viscosity, high-toughness wheat starch-based paperboard adhesive according to claim 1, characterized in that, The adhesive further includes a nano / bio-based reinforcing component, wherein the amount of the nano / bio-based reinforcing component is 0.05% to 0.60% of the total weight of wheat starch; The nano / bio-based reinforcing components include one or more of the following: surface-modified lignin, cellulose nanofibers, nano-silica, nano-montmorillonite, nano-cellulose crystals, or starch-grafted nanoparticles.

6. A method for preparing a high-viscosity, high-toughness wheat starch-based paperboard adhesive as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Wheat starch is divided into 15-25 parts of cooked wheat starch for gelatin and 75-85 parts of raw wheat starch for gelatin. S2. The wheat starch for cooking gum is lightly esterified using a light esterification modifier to obtain modified wheat starch for cooking gum. S3. Provide water for pulping and control the temperature of the water for pulping at 25±2℃; S4. Take a portion of the pulping water, mix the cooked gum with modified wheat starch and film-forming aid, heat and gelatinize, and add a rigid micro-crosslinking agent to react, to obtain a cooked gum carrier system. S5. Disperse the raw rubber with raw wheat starch in the remaining pulping water at a temperature of 25±2℃, and add an alkaline regulator to obtain the raw rubber main system. S6. Mix the cooked rubber carrier system with the raw rubber main body system to obtain a cooked rubber-raw rubber mixed system; S7. After the cooked-raw rubber mixture is cooled, a flexible dynamic crosslinking agent is added. After pH adjustment, filtration and aging, the high-viscosity and toughness wheat starch-based paperboard adhesive is obtained.

7. The preparation method according to claim 6, characterized in that, In step S2, the reaction temperature for shallow esterification modification is 45–60℃, the pH of the reaction system is 8.0–9.5, the reaction time is 20–90 min, and the degree of substitution of the modified wheat starch in the resulting cooked gum is 0.005–0.

050.

8. The preparation method according to claim 6, characterized in that, In step S4, the film-forming aid is cold water-soluble polyvinyl alcohol, or a polyvinyl alcohol aqueous solution that has been dissolved and cooled at 80-95°C in advance. The gelatinization temperature of the modified wheat starch used for the cooked gum is 55-65℃. The reaction temperature after adding the rigid micro-crosslinking agent is 55-65℃, and the reaction time is 10-30 min.

9. The preparation method according to claim 6, characterized in that, In step S6, the raw rubber main body system is added to the cooked rubber carrier system for mixing at a temperature of 35-50°C for 10-40 minutes.

10. The preparation method according to claim 6, characterized in that, In step S7, the flexible dynamic crosslinking agent is added after the cooked rubber-raw rubber mixture is cooled to 30-40°C; After adding the flexible dynamic crosslinking agent, the pH of the system is adjusted to 7.5-9.0, and the mixture is then filtered and matured to obtain the high-viscosity, high-toughness wheat starch-based paperboard adhesive.

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