Modified tapioca flour adhesive, carton composite material and preparation method of carton composite material

Abstract

The invention discloses a modified tapioca flour adhesive, a carton composite material and a preparation method of the modified tapioca flour adhesive. Aiming at the problems of insufficient bonding strength, low curing speed and the like of a traditional cassava powder adhesive, the performance of the cassava powder adhesive is optimized through the steps of esterification, resin preparation, compound modification and the like. The finished carton is prepared from a modified tapioca flour adhesive and a carton composite material, and the tapioca flour adhesive is a polymer interpenetrating polymer network formed by hexadecanol esterified tapioca flour molecules, polyvinyl formal grafted starch molecules and vinyl acetate. The carton board composite material is prepared by mixing plant fibers and a tapioca flour adhesive according to a certain proportion, the water resistance (the wet-state bonding strength is larger than or equal to 2.5 MPa), the heat resistance (the performance is almost not attenuated at 60 DEG C), the bonding strength (larger than or equal to 4.5 MPa) and the film forming property (the drying speed is smaller than 2 min) of the material are remarkably improved, strong-corrosion raw materials are avoided in the preparation process, and equipment maintenance and worker health are facilitated.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of adhesive preparation, and particularly relates to a modified cassava powder adhesive, a paperboard box composite material and a preparation method thereof. BACKGROUND

[0002] At present, the global ecological environment is becoming increasingly severe, and the pursuit of environmentally friendly materials by various industries is becoming more and more urgent. Adhesives, as the "industrial MSG" in industrial production, have a wide range of applications, from construction, packaging to furniture manufacturing and other industries. Traditional adhesives, such as phenolic resin and urea-formaldehyde resin, are mostly made of petrochemical products, which have many drawbacks such as insufficient water and heat resistance, use of corrosive materials, and so on. According to relevant statistics, the production of traditional adhesives emits a large proportion of global total pollutants every year, which has a serious negative impact on air quality. With the strengthening of environmental awareness, it has become an inevitable trend to use biodegradable and renewable biomass materials to prepare environmentally friendly adhesives. Cassava powder, as a renewable, pollution-free, biodegradable and low-cost biomass material, has become one of the ideal raw materials for preparing adhesives.

[0003] However, the traditional cassava powder adhesive has low solid content and high water content of more than 70%. This high water content directly leads to insufficient bonding strength - the peel strength on paper substrates is often less than 0.3 N / mm, which cannot meet the load-bearing requirements of heavy packaging. At the same time, due to the slow evaporation rate of water, the curing time usually needs 4-6 hours, which seriously restricts the operation efficiency of the production line. More difficultly, the compatibility of cassava powder particles with the water phase system is poor, and obvious stratification occurs after 24 hours of standing. Not only does it need to be stirred frequently for use, but also the bonding effect fluctuates due to uneven composition, which greatly shortens the effective storage period of the product, which is usually only 7-10 days at room temperature. In addition, strong corrosive caustic soda is usually used in the traditional preparation process, which is not conducive to prolonging the service life of production equipment and protecting the health of production workers. In addition, with the increasing requirements for the performance of packaging materials, such as waterproof, thermal insulation, shock absorption, etc., traditional paperboard box composites need to be continuously improved to meet these requirements. SUMMARY

[0004] The purpose of the present application is to solve the problems of low solid content, insufficient bonding strength, long curing time and poor storage of cassava powder adhesives in the prior art. To achieve the purpose of the present application, the following technical solutions are adopted:

[0005] The present application proposes a preparation method of modified cassava powder adhesive, comprising the following steps:

[0006] S1. Preparation of oxidized cassava powder

[0007] The cassava powder is slowly added to a beaker containing 100 mL distilled water and stirred for 10 min; then the above material is added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel, and ferrous sulfate is added at 50°C, and hydrogen peroxide is slowly added dropwise, and oxidized for several times; then a proper amount of sodium thiosulfate is added, and the product is filtered, washed and dried after sand core filtration to obtain oxidized cassava powder; the hydroxyl radicals generated by the Fenton system composed of ferrous ions of ferrous sulfate and hydrogen peroxide oxidize the hydroxyl groups in the cassava powder molecules into carboxyl groups and aldehyde groups, and due to the large steric hindrance, the hydroxyl groups can only be partially oxidized; when the oxidation reaction proceeds to the preset time, sodium thiosulfate is added to terminate the reaction; in addition, the strong oxidizing hydroxyl radicals also attack the glycosidic bond (C-O-C) in the starch molecule, causing the polysaccharide chain to break, thereby preparing for the next esterification reaction;

[0008]

[0009] S2. Esterification reaction

[0010] In a three-necked flask, cetyl alcohol and the oxidized cassava powder obtained in S1 are added, dissolved in xylene, and a water separator, a thermometer and a stirrer are assembled. After heating to completely dissolve the n-hexadecanol, 2-3 g of the catalyst p-toluenesulfonic acid is added, and the heating is continued to 70-80°C to reflux for 4-6 h. The reaction is stopped, cooled to room temperature, washed with methanol, filtered, and the filter cake is collected and added to a three-necked flask. Anhydrous methanol is added, and refluxed at 80°C for 30 min. After the end of the reaction, the mixture is filtered while hot. The filtrate is cooled under magnetic stirring for 5 h, filtered, and dried under vacuum at 65°C for 7 h to obtain a white product, i.e. the esterified cassava powder. The esterification reaction occurs between the carboxyl groups of the oxidized cassava powder and the hydroxyl groups of the cetyl alcohol, and the hydrophobic and flexible alkyl chains are introduced into the cassava powder molecules;

[0011]

[0012] S3. Preparation of polyvinyl formal resin

[0013] Polyvinyl alcohol and distilled water are added to a 250 mL three-necked flask equipped with a spherical condenser, a polytetrafluoroethylene electric stirrer and a constant pressure funnel. After water bubbling for 30 min, the electric stirrer is turned on, and the temperature is maintained at 80-90°C for 30 min until the polyvinyl alcohol is completely dissolved. The temperature is lowered to 65°C, and the pH is adjusted to 3-4 with HCl. Formaldehyde is added dropwise over 30 min. After the addition is completed, the mixture is maintained at the temperature for 1 h, and the pH is adjusted to neutral with NaOH. The temperature is lowered, and the product is discharged to obtain the polyvinyl formal resin. In an acidic environment, the hydroxyl groups (-OH) on the polyvinyl alcohol molecular chain act as nucleophiles to attack the partially positively charged carbonyl carbon in the formaldehyde molecule, forming a cyclic hemiacetal intermediate;

[0014]

[0015] S4. Preparation of modified tapioca powder adhesive

[0016] S401. Tapioca powder grafting: Slowly add tapioca powder into a beaker containing 50 mL distilled water, add 5% dilute sulfuric acid solution, stir well, and then pour into a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, spherical reflux condenser, and constant pressure funnel; turn on the electric stirrer, add polyvinyl formal resin solution, heat to 80°C, and react for 2-3 h under mechanical stirring to obtain a tapioca powder solution of grafted polyvinyl formal resin; under the condition of sulfuric acid as a catalyst, the aldehyde group of the formaldehyde molecule is activated, the hydroxyl group from the polyvinyl alcohol molecule or the primary hydroxyl group in the tapioca powder molecule attacks the carbonyl carbon of the formaldehyde molecule with its lone pair of electrons, an acetalization reaction occurs, one molecule of water is removed, and finally a stable acetal bond (-O-CH2-O-) is formed, successfully grafting polyvinyl formal onto the tapioca powder molecule;

[0017]

[0018] S402. Copolymerization: Add the esterified tapioca powder obtained in S2 and the tapioca powder solution of grafted polyvinyl formal resin obtained in S401 into a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, spherical reflux condenser, and constant pressure funnel, heat to 60°C, and add sodium dodecyl sulfonate and tributyl phosphate, respectively, stir and emulsify for 30 min; add a certain amount of initiator ammonium persulfate, 7 mL of vinyl acetate, and tapioca powder, and react for 30 min; heat to 70°C, start dropping 18 mL of vinyl acetate, control the dropping time within 3 h, heat to 80°C, and keep for 30 min; add sodium thiosulfate, heat to 90°C, keep for 30 min, then cool to 60-70°C, add borax, stir for 15-20 min, cool the system to 50-60°C, and discharge the material. Under the condition of ammonium persulfate as an initiator, esterified tapioca powder, tapioca powder solution of grafted polyvinyl formal resin, and tapioca powder and vinyl acetate (VAc) undergo copolymerization, the remaining VAc self-polymerizes in the solution, the polymerization of VAc and tapioca powder only reacts with the primary hydroxyl group on the tapioca powder molecule due to the large steric hindrance of the six-membered ring, the polymer and the grafted copolymer are stabilized in the emulsion system by the emulsifier sodium dodecyl sulfate (SDS), the hydroxyl group (-OH) and acetal group (-O-R-O-) in the polyvinyl formal resin act as protective colloids, and they interact with starch and polyvinyl formal through hydrogen bonds to stabilize the emulsion, and the hydrophilic group (-SO3 - , Na +) and the hydrophobic group (dodecyl) play an emulsifying role, so that the oil phase VAc is dispersed into stable droplets in the water phase; the ester group of tributyl phosphate acts as a defoaming agent to reduce the foam in the reaction; the reducing group of sodium thiosulfate terminates the remaining free radicals to avoid over-reaction, so that the copolymer of VAc as the main chain and three kinds of cassava powder molecules grafted in a free radical polymerization manner is obtained; finally, the borax grafting is used to form a borate cross-linking structure with the remaining hydroxyl groups on the starch molecules, so that the three polymers of the esterified cassava powder, the polyvinyl acetate grafted cassava powder and the vinyl acetate grafted cassava powder are finally formed, and the interpenetrating polymer network formed by the three polymers and the self-polymer of vinyl acetate is formed.

[0019]

[0020] Preferably, the mass fraction of the cassava powder used in S1 is 45-55 parts; the mass fraction of ferrous sulfate is 0.1 part; the mass fraction of hydrogen peroxide is 0.03-0.05 part; and the mass fraction of sodium thiosulfate is 0.03-0.05 part.

[0021] Preferably, the molar ratio of the oxidized cassava powder to cetyl alcohol used in S2 is 1:(1-1.2).

[0022] Preferably, the mass fraction of the raw materials used in S3 is as follows: the mass fraction of polyvinyl alcohol is 14-16 parts; the mass fraction of distilled water is 130-140 parts; and the mass fraction of formaldehyde is 6-7 parts.

[0023] Preferably, the mass fraction of the raw materials used in S401 is as follows: the mass fraction of the 5% dilute sulfuric acid solution is 10% of the solid mass.

[0024] Preferably, the molar ratio of the raw materials used in the copolymerization reaction in S402 is as follows: the mass fraction of the esterified cassava powder: the mass fraction of the cassava powder: the mass fraction of the grafted polyvinyl formal resin cassava powder = (0.8-1.0):1:0.5; the mass fraction of the oxidized cassava powder used is 20-30 parts; the mass fraction of sodium dodecyl sulfonate is 0.2-0.3 part; the mass fraction of tributyl phosphate is 0.5 mL; the mass fraction of ammonium persulfate is 0.3-0.5 part; the mass fraction of sodium thiosulfate is 0.1-0.2 part; the mass of the borax used is 0.5-2% of the mass of the starch; and the mass of the silane coupling agent is 5-10% of the mass of the starch.

[0025] The modified cassava powder adhesive provided in the application can be used to prepare paperboard boxes with high strength and high water resistance.

[0026] The application further provides a preparation method of the paperboard box composite material, which comprises the following steps:

[0027] (1) At room temperature, the recycled paper pulp and the plant fiber paper pulp are uniformly mixed in a beater, and then the modified cassava powder adhesive obtained in S3 is added, the temperature is increased to 35 DEG C, sodium benzoate is added, and the mixture is fully stirred to obtain a mixed pulp;

[0028] (2) The above-mentioned mixed slurry is fed into the paperboard forming machine for processing. The slurry is first dehydrated and formed on the paperboard forming machine, and then dried 2-3 times until the moisture content of the paperboard is reduced to 5%-10%. Then, the formed paperboard is placed for 12-24 hours for natural aging treatment, and then sanded and cut to finally obtain the paperboard box composite material that meets the requirements.

[0029] Preferably, the solid content of the recycled pulp and plant fiber pulp used in (1) is between 60-72 wt%; the amount of sodium benzoate added is 0.1-0.5% of the total pulp mass.

[0030] Preferably, the amount of modified cassava flour adhesive used in (2) is 6.7-10% of the weight of the molded cardboard.

[0031] The beneficial effects of this invention are as follows:

[0032] 1. Significantly Enhanced Water and Heat Resistance: Unlike traditional starch materials that suffer from poor water resistance due to their high hydroxyl content, this invention introduces the flexible and hydrophobic long alkyl chain of hexadecyl alcohol into cassava flour molecules as a comonomer through esterification, grafting it onto the polyvinyl acetate chain. The resulting adhesive exhibits good flexibility and water resistance, allowing it to maintain stable bonding even in humid environments. The remaining -OH groups in the cassava flour combine with the abundant -OH groups on the pulp, dehydrating to form chemical bonds and improving the adhesive and bonding strength. The grafting of borax forms a borate ester crosslinking structure, significantly increasing the adhesive's viscosity, curing speed, and bonding strength, while also enhancing its water and heat resistance.

[0033] 2. Significantly Improved Bonding Strength: Polyvinyl alcohol formaldehyde (PVA) itself possesses strong intermolecular forces. When grafted onto the cassava flour molecular chain, it enhances the cohesive force of starch molecules and their adhesion to the substrate, particularly showing outstanding bonding effects on porous materials such as paper and wood. The formaldehyde groups (-CH2-O-CH2-) form a partial network structure through a cross-linking reaction, which, when grafted onto the polyvinyl acetate backbone, strengthens the cohesive force of the resin film and stabilizes the intermolecular forces. When the cardboard box is subjected to tension or impact, the adhesive layer is less likely to break, improving the tensile strength and impact resistance of the cardboard box. It also enhances the interfacial adhesion between the adhesive and fibers, ultimately increasing the overall strength of the cardboard composite material, thus increasing the mechanical properties of the adhesive film and improving the bonding strength.

[0034] 3. Excellent film-forming properties: After cetyl alcohol esterification, long-chain alkyl groups are introduced into the cassava flour molecules, which synergistically react with vinyl acetate, significantly accelerating the film-forming speed. After drying, the adhesive forms a continuous and complete film that more tightly encapsulates the fibers, greatly enhancing adhesion. Compared to traditional adhesives, its film layer exhibits superior density and leveling properties, allowing for uniform spreading during coating or mixing, effectively expanding the contact area with the fibers, thereby improving the overall bonding effect.

[0035] 4. Sodium benzoate, added as a preservative to cassava flour binder, can effectively inhibit the growth and reproduction of microorganisms such as bacteria and mold. By disrupting the cell membrane structure of microorganisms and interfering with metabolic processes, it delays the spoilage and deterioration of the binder caused by microbial contamination, extending the product's shelf life and stability. Especially in humid environments or during long-term storage, it can prevent problems such as foul odor, layering, and decreased viscosity in the binder. Detailed Implementation

[0036] The technical solution and effects of the present invention will be further described below with reference to embodiments of the present invention. The following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Simple modifications made to the present invention based on the concept of the present invention are all within the scope of protection claimed by the present invention.

[0037] The sources of the materials used in the following examples and comparative examples are as follows:

[0038] Tapioca flour: Tianjin Guangcheng Chemical Technology Co., Ltd.;

[0039] Ferrous sulfate: Shanghai Maclean Biochemical Technology Co., Ltd.;

[0040] Hydrogen peroxide: Shanghai Maclean Biochemical Technology Co., Ltd.;

[0041] Sodium thiosulfate: Shanghai Aladdin Biotechnology Co., Ltd.;

[0042] Xylene: Shanghai Aladdin Biotechnology Co., Ltd.;

[0043] hexadecyl alcohol: Shanghai Aladdin Biotechnology Co., Ltd.;

[0044] p-Benzylsulfonic acid: Shanghai Aladdin Biotechnology Co., Ltd.;

[0045] Methanol: Shanghai Aladdin Biotechnology Co., Ltd.;

[0046] Anhydrous methanol: Shanghai Aladdin Biotechnology Co., Ltd.;

[0047] Formaldehyde: Shanghai Aladdin Biotechnology Co., Ltd.;

[0048] Sodium dodecyl sulfonate: Shanghai Maclean Biochemical Technology Co., Ltd.;

[0049] Tributyl phosphate: Shanghai Maclean Biochemical Technology Co., Ltd.

[0050] Ammonium persulfate: Shanghai Maclean Biochemical Technology Co., Ltd.;

[0051] Vinyl acetate: Shanghai Maclean Biochemical Technology Co., Ltd.;

[0052] Borax: Tianjin Guangcheng Chemical Technology Co., Ltd.;

[0053] Recycled pulp: Tianjin Guangcheng Chemical Technology Co., Ltd.;

[0054] Plant fiber pulp: Tianjin Guangcheng Chemical Technology Co., Ltd.;

[0055] Sodium benzoate: Shanghai Maclean Biochemical Technology Co., Ltd.

[0056] Distilled water: homemade;

[0057] Example 1

[0058] S1. Oxidation treatment of tapioca flour

[0059] 45g of cassava flour was slowly added to a beaker containing 100mL of distilled water and stirred for 10min. Then, the above material was added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel. 0.1g of ferrous sulfate was added at 50℃, and 0.03g of hydrogen peroxide was slowly added dropwise for oxidation for a certain period of time. Then, 0.03g of sodium thiosulfate was added. The product was filtered through a sintered glass funnel, washed and dried to obtain oxidized cassava flour.

[0060] S2. Esterification reaction

[0061] 81g of hexadecyl alcohol and oxidized cassava flour obtained from S1 were added to a three-necked flask, respectively. Xylene was added to dissolve the cassava flour. A water separator and thermometer were assembled. The mixture was stirred and heated until hexadecyl alcohol was completely dissolved. Then, 2g of p-benzenesulfonic acid catalyst was added. The mixture was heated to 70-80℃ and refluxed for 4-6 hours. The reaction was stopped and cooled to room temperature. Methanol was added to wash the mixture. The mixture was filtered, and the filter cake was collected and added to a three-necked flask. Anhydrous methanol was added, and the mixture was refluxed at 80℃ for 30 minutes. After the reaction was completed, the mixture was filtered while hot. The filtrate was cooled for 5 hours under magnetic stirring, filtered by suction, and dried under vacuum at 65℃ for 7 hours to obtain a white product, namely esterified cassava flour.

[0062] S3. Preparation of polyvinyl alcohol formaldehyde resin

[0063] Add 14g of polyvinyl alcohol and distilled water to a 250mL three-necked beaker equipped with a spherical condenser, a polytetrafluoroethylene electric stirrer, and a constant pressure funnel. After soaking in water for 30 minutes, turn on the electric stirrer and keep the temperature at 95℃ for 30 minutes until the polyvinyl alcohol is completely dissolved. Cool down to 70℃, adjust the pH to 3-4 with HCl, and start adding 6g of formaldehyde dropwise, which is completed within 30 minutes. After the addition is complete, keep the temperature for 1 hour, adjust the pH to neutral with NaOH, cool down and discharge to obtain polyvinyl alcohol formaldehyde resin.

[0064] S4. Preparation of modified cassava flour binder

[0065] S401. Cassava flour grafting: Slowly add 25g of cassava flour to a beaker containing 50mL of distilled water, add 2.5g of 5% dilute sulfuric acid solution, stir evenly, and then pour into a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel; turn on the electric stirrer, add polyvinyl alcohol formaldehyde resin solution, heat to 80℃, and react for 2-3 hours under mechanical stirring to obtain cassava flour slurry grafted with polyvinyl alcohol formaldehyde resin;

[0066] S402. Copolymerization reaction: The esterified cassava flour obtained in S2, the cassava flour mixture with grafted polyvinyl alcohol formaldehyde resin obtained in S401, were added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser, and a constant pressure funnel at a molar ratio of 0.8:1:0.5. The monomer mass was 150g. The temperature was raised to 60℃, and sodium dodecyl sulfonate and tributyl phosphate were added separately. The mixture was stirred and emulsified for 30 minutes. A certain amount of... Ammonium persulfate initiator, 7 mL of vinyl acetate, and cassava flour were reacted for 30 min. The temperature was raised to 70℃, and 18 mL of vinyl acetate was added dropwise, controlled to be completed within 3 hours. The temperature was raised to 80℃ and held for 30 min. Sodium thiosulfate was then added, the temperature was raised to 90℃ and held for 30 min. The temperature was then lowered to 60-70℃, borax was added, and the mixture was stirred for 15-20 minutes. The system was then cooled to 50-60℃, and the modified cassava flour binder was obtained by cooling and discharging.

[0067] Example 2

[0068] S1. Oxidation treatment of tapioca flour

[0069] 50g of cassava flour was slowly added to a beaker containing 100mL of distilled water and stirred for 10min. Then, the above material was added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel. 0.1g of ferrous sulfate was added at 50℃, and 0.04g of hydrogen peroxide was slowly added dropwise for oxidation for a certain period of time. Then, 0.04g of sodium thiosulfate was added. The product was filtered through a sintered glass funnel, washed and dried to obtain oxidized cassava flour.

[0070] S2. Esterification reaction

[0071] 88g of hexadecyl alcohol and oxidized cassava flour obtained from S1 were added to a three-necked flask, respectively. Xylene was added to dissolve the cassava flour. A water separator and thermometer were assembled. The mixture was stirred and heated until hexadecyl alcohol was completely dissolved. Then, 2g of p-benzenesulfonic acid catalyst was added. The mixture was heated to 70-80℃ and refluxed for 4-6 hours. The reaction was stopped, cooled to room temperature, washed with methanol, filtered, and the filter cake was collected and added to a three-necked flask. Anhydrous methanol was added, and the mixture was refluxed at 80℃ for 30 minutes. After the reaction was completed, the mixture was filtered while hot. The filtrate was cooled for 5 hours under magnetic stirring, filtered by suction, and dried under vacuum at 65℃ for 7 hours to obtain a white product, namely esterified cassava flour.

[0072] S3. Preparation of polyvinyl alcohol formaldehyde resin

[0073] Add 14g of polyvinyl alcohol and distilled water to a 250mL three-necked beaker equipped with a spherical condenser, a polytetrafluoroethylene electric stirrer, and a constant pressure funnel. After soaking in water for 30 minutes, turn on the electric stirrer and keep the temperature at 95℃ for 30 minutes until the polyvinyl alcohol is completely dissolved. Cool down to 70℃, adjust the pH to 3-4 with HCl, and start adding 6g of formaldehyde dropwise, which is completed within 30 minutes. After the addition is complete, keep the temperature for 1 hour, adjust the pH to neutral with NaOH, cool down and discharge to obtain polyvinyl alcohol formaldehyde resin.

[0074] S4. Preparation of modified cassava flour binder

[0075] S401. Cassava flour grafting: Slowly add 25g of cassava flour to a beaker containing 50mL of distilled water, add 2.5g of 5% dilute sulfuric acid solution, stir evenly, and then pour into a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel; turn on the electric stirrer, add polyvinyl alcohol formaldehyde resin solution, heat to 80℃, and react for 2-3 hours under mechanical stirring to obtain cassava flour slurry grafted with polyvinyl alcohol formaldehyde resin;

[0076] S402. Copolymerization reaction: The esterified cassava flour obtained in S2, the cassava flour mixture with grafted polyvinyl alcohol formaldehyde resin obtained in S401, and the cassava flour mixture were added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser, and a constant pressure funnel at a molar ratio of 0.9:1:0.5. The monomer mass was 170g. The temperature was raised to 60℃, and sodium dodecyl sulfonate and tributyl phosphate were added separately. The mixture was stirred and emulsified for 30min. A certain amount of... Ammonium persulfate initiator, 7 mL of vinyl acetate, and cassava flour were reacted for 30 min. The temperature was raised to 70℃, and 18 mL of vinyl acetate was added dropwise, controlled to be completed within 3 hours. The temperature was raised to 80℃ and held for 30 min. Sodium thiosulfate was then added, the temperature was raised to 90℃ and held for 30 min. The temperature was then lowered to 60-70℃, borax was added, and the mixture was stirred for 15-20 minutes. The system was then cooled to 50-60℃, and the modified cassava flour binder was obtained by cooling and discharging.

[0077] Example 3

[0078] S1. Oxidation treatment of tapioca flour

[0079] 50g of cassava flour was slowly added to a beaker containing 100mL of distilled water and stirred for 10min. Then, the above material was added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel. 0.1g of ferrous sulfate was added at 50℃, and 0.05g of hydrogen peroxide was slowly added dropwise for oxidation for a certain period of time. Then, 0.05g of sodium thiosulfate was added. The product was filtered through a sintered glass funnel, washed and dried to obtain oxidized cassava flour.

[0080] S2. Esterification reaction

[0081] 99.2g of hexadecyl alcohol and oxidized cassava flour obtained from S1 were added to a three-necked flask, respectively. Xylene was added to dissolve the cassava flour. A water separator and thermometer were assembled. The mixture was stirred and heated until hexadecyl alcohol was completely dissolved. Then, 2g of p-benzenesulfonic acid catalyst was added. The mixture was heated to 70-80℃ and refluxed for 4-6 hours. The reaction was stopped and cooled to room temperature. Methanol was added to wash the mixture. The mixture was filtered, and the filter cake was collected and added to a three-necked flask. Anhydrous methanol was added, and the mixture was refluxed at 80℃ for 30 minutes. After the reaction was completed, the mixture was filtered while hot. The filtrate was cooled for 5 hours under magnetic stirring, filtered by suction, and dried under vacuum at 65℃ for 7 hours to obtain a white product, namely esterified cassava flour.

[0082] S3. Preparation of polyvinyl alcohol formaldehyde resin

[0083] Add 14g of polyvinyl alcohol and distilled water to a 250mL three-necked beaker equipped with a spherical condenser, a polytetrafluoroethylene electric stirrer, and a constant pressure funnel. After soaking in water for 30 minutes, turn on the electric stirrer and keep the temperature at 95℃ for 30 minutes until the polyvinyl alcohol is completely dissolved. Cool down to 70℃, adjust the pH to 3-4 with HCl, and start adding 6g of formaldehyde dropwise, which is completed within 30 minutes. After the addition is complete, keep the temperature for 1 hour, adjust the pH to neutral with NaOH, cool down and discharge to obtain polyvinyl alcohol formaldehyde resin.

[0084] S4. Preparation of modified cassava flour binder

[0085] S401. Cassava flour grafting: Slowly add 30g of cassava flour to a beaker containing 50mL of distilled water, add 3.0g of 5% dilute sulfuric acid solution, stir evenly, and then pour into a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel; turn on the electric stirrer, add polyvinyl alcohol formaldehyde resin solution, heat to 80℃, and react for 2-3 hours under mechanical stirring to obtain cassava flour slurry grafted with polyvinyl alcohol formaldehyde resin;

[0086] S402. Copolymerization reaction: The esterified cassava flour obtained in S2, the cassava flour mixture with grafted polyvinyl alcohol formaldehyde resin obtained in S401, were added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser, and a constant pressure funnel at a molar ratio of 1.0:1:0.5. The monomer mass was 150g. The temperature was raised to 60℃, and sodium dodecyl sulfonate and tributyl phosphate were added separately. The mixture was stirred and emulsified for 30min. A certain amount of... Ammonium persulfate initiator, 7 mL of vinyl acetate, and cassava flour were reacted for 30 min. The temperature was raised to 70℃, and 18 mL of vinyl acetate was added dropwise, controlled to be completed within 3 hours. The temperature was raised to 80℃ and held for 30 min. Sodium thiosulfate was then added, the temperature was raised to 90℃ and held for 30 min. The temperature was then lowered to 60-70℃, borax was added, and the mixture was stirred for 15-20 minutes. The system was then cooled to 50-60℃, and the modified cassava flour binder was obtained by cooling and discharging.

[0087] Comparative Example 1

[0088] It is basically the same as Example 1, except that hexadecyl alcohol is not used for esterification, that is, S1 and S2 are missing.

[0089] Comparative Example 2

[0090] It is basically the same as Example 1, except that cassava flour is not grafted with polyvinyl alcohol formaldehyde resin, that is, S3 and S4 are missing.

[0091] Comparative Example 3

[0092] It is basically the same as Example 1, except that boric acid is not added for crosslinking.

[0093] Comparative Example 4

[0094] It is basically the same as Example 1, except that no additional vinyl acetate is added in step S4.

[0095] Comparative Example 5

[0096] Starch-based adhesive cardboard box composite material sold by Smecta Corporation.

[0097] The following tests were performed on Examples 1-3 and Comparative Examples 1-5:

[0098] (1) Viscosity test: According to GB / T 2794—2013 standard, immerse the rotor in the adhesive to the depth of the rotor mark (avoid the influence of the bottom or wall surface), let it stand for 2 minutes to allow the rotor and sample temperature to equalize, start the viscometer, select an appropriate speed, such as 60 r / min. If the reading exceeds the range, reduce the speed, such as 30 r / min or 10 r / min. Record the reading after the pointer stabilizes. The same sample should be measured at least 3 times, and the average value should be taken as the final viscosity value. The measurement should be performed using a rotational viscometer.

[0099] (2) Initial Tack Test: Fix the coated sample on the inclined plate, ensuring the coated surface is facing upwards and flat without wrinkles. Starting with a steel ball of the smallest diameter (0.8 mm), release the steel ball at the release device at the top of the inclined plate, allowing it to roll along the coated surface. Observe whether the steel ball stops rolling on the coated surface: if the steel ball rolls to the bottom of the plate without stopping, repeat the test with a steel ball of a larger diameter; if the steel ball stops midway, record the number of the steel ball. Roll a steel ball of a certain weight on the coated surface and measure the stopping distance; the shorter the distance, the stronger the initial tack.

[0100] (3) Adhesive strength test:

[0101] According to HG / T 2727—2010 standard, the dry bond strength of small fir wood blocks (40mm×25mm×10mm, double-sided polished, glued on both sides, overlap 25mm×25mm) was determined using an electronic universal testing machine. A 3.5kg weight was applied, and the blocks were pressed for 24 hours. The pressure was then released, and the dry bond strength was measured. The average of five results was used. The wet bond strength was measured immediately after the samples were immersed in water at 30±1℃ for 3 hours and then in water at 60±1℃ for 10 minutes.

[0102] (4) Heat resistance test: Apply cassava flour adhesive evenly to the surfaces of two identical substrates (such as wood, paper, or plastic, depending on the actual application scenario), with the amount of adhesive controlled at (10±1) g / m². 2Two substrates were bonded together according to standard procedures, and a pressure of (0.5±0.1) MPa was applied. The mixture was then cured at room temperature (25±2℃) for 24 hours to prepare a standard bonding sample (100mm×25mm×5mm). Different high-temperature gradients were set: 50℃, 70℃, 90℃, and 110℃, adjusted according to the expected usage environment. The prepared bonding samples were placed in a constant-temperature oven. Each temperature was maintained for 3 hours, during which time the samples were observed for softening, flowing, cracking, or substrate separation. The ratio of the strength after high-temperature treatment to the strength of the untreated sample (retention rate) was calculated. A higher retention rate indicates better heat resistance.

[0103] (5) Drying speed test: Apply the cassava flour adhesive evenly to a glass slide or steel plate, and control the thickness of the adhesive to (0.2±0.02) mm (using a wet film preparation device to control the thickness). Start timing after the adhesive is applied. Every minute, gently touch the adhesive surface with a clean glass slide. Record the time when the glass slide no longer has adhesive on it. This time is the surface skinning time.

[0104] (6) Stability Test: The cassava flour binder was placed in a sealed container and left to stand at room temperature (25±2℃) for 90 days. Observe every 5 days and record whether there is any layering, sedimentation, clumping, mold, or odor. If layering occurs, gently stir and observe whether it can be restored to a uniform state. After 90 days, the viscosity of the binder was measured and compared with the initial viscosity. During storage, there was no layering, sedimentation, or mold growth, and the shelf life met industrial requirements.

[0105] Table 1 shows the performance tests of the modified cassava flour binders prepared in Examples 1-3.

[0106]

[0107] The data in the table show that the modified cassava flour adhesive and cardboard box composite materials demonstrated in Examples 1-3 exhibit good performance with minimal differences. From Example 1 to Example 3, except for the dry and wet adhesive strengths which peaked in Example 2 and then slightly decreased, all other indicators showed an optimization trend. This indicates that with the adjustment of the preparation process, the comprehensive performance of the adhesive is continuously improved. The core reason is that the main component of cassava flour is starch, which contains a large number of hydroxyl groups (-OH) in its molecular chain. These hydroxyl groups easily form hydrogen bonds, giving starch a certain degree of adhesiveness. The grafted polyvinyl acetate molecular chain contains ester groups (-COO-), which are highly polar and can bond with the cassava flour molecular chain. The hydroxyl groups form strong intermolecular forces, and polyvinyl acetate itself has good film-forming properties and flexibility, which can form a continuous film at the bonding interface, enhancing the bonding strength. Methyl polyvinyl acetate is introduced into the cassava flour molecular chain as a comonomer. The -OH in its molecule coordinates with the cassava flour molecules esterified with hexadecyl alcohol, introducing long alkyl chains with both hydrophobicity and flexibility into cassava starch, so that the adhesive can still maintain stable bonding in a humid environment and is not easy to clump. Secondly, cassava flour can form a certain film after heating and gelatinizing, but the uniformity and continuity of the film are poor, and defects such as bubbles and cracks are prone to occur. Polyvinyl acetate has excellent film-forming properties and can form a uniform, continuous and smooth film. When copolymerized with cassava flour, the film-forming properties of cassava flour can be improved, allowing the binder to form a more complete film structure during the drying process. This improves the coverage and uniformity of the binder, ensuring consistent bonding performance. Furthermore, pure cassava flour binders are prone to stratification, precipitation, and gelation during storage due to the retrogradation of starch molecular chains, resulting in poor storage stability. The introduction of polyvinyl acetate can interfere with the regular arrangement of cassava flour molecular chains and inhibit the occurrence of retrogradation. Meanwhile, the interaction between polyvinyl acetate molecular chains and cassava flour molecular chains can form a stable colloidal system, reducing the occurrence of layering and precipitation, extending the shelf life of the adhesive, and enhancing the intermolecular entanglement after starch grafting, thereby improving the cohesive force and bonding strength of the adhesive. Borax used for crosslinking hydrolyzes in the system, where boric acid molecules can act as "bridges" to react with glucose units containing multiple hydroxyl groups and repeating units of polyvinyl alcohol. The three-dimensional network structure formed by crosslinking can disperse external forces, improving the strength, toughness, and tensile strength of the material. Uncrosslinked hydroxyl-containing polymers are highly hydrophilic and easily swell and disintegrate when exposed to water. After borax crosslinking, the network structure hinders the penetration of water molecules, reduces the swelling rate of the material, and greatly improves its water resistance.

[0108] Table 2 shows the performance tests of the modified cassava flour binders prepared in Examples 1-5.

[0109]

[0110] The data in the table show that Comparative Example 1 has the lowest viscosity compared to Examples 1 and Comparative Examples 1-4, with no significant changes in other properties. This is because the cassava flour prepared in Comparative Example 1 lacks hexadecyl alcohol grafting, resulting in a lack of hydrophobicity and flexibility due to the absence of long-chain alkyl groups in the molecule. This leads to excessive molecular rigidity, making the adhesive brittle after curing and prone to cracking under external force, thus affecting the bonding effect. Comparative Example 2 shows a significant decrease in initial tack and dry / wet bonding strength. The core reason for this is the absence of polyvinyl alcohol formal resin grafting, indicating that grafting polyvinyl alcohol formal resin can significantly improve the bonding strength of the adhesive. The hydroxyl groups on the cassava flour molecular chain are hydrophilic, causing the pure cassava flour adhesive to easily swell and disintegrate in water, resulting in poor water resistance. The ester groups in the polyvinyl acetate molecular chain have weak hydrophilicity, and the film structure formed after film formation is relatively dense, which can hinder the penetration of water molecules. When polyvinyl acetate is copolymerized with cassava flour, the hydrophilicity of the cassava flour molecular chains is reduced, and a hydrophobic network structure is formed in the adhesive system, improving the water resistance of the adhesive. Comparative Example 3 shows a significant decrease in tensile properties, while other properties remain unchanged. The core reason is that it is not cross-linked with borax, which greatly reduces the material's viscosity and hydrophobicity. Comparative Example 4 shows a significant difference in drying speed because it does not contain additional vinyl acetate, which slows down the film-forming speed, prolongs the production period, and increases costs. Polyvinyl acetate has excellent film-forming properties and can form a uniform, continuous, and smooth film. When copolymerized with cassava flour, it can improve the film-forming properties of cassava flour, allowing the adhesive to form a more complete film structure during the drying process, thereby improving the adhesive's coverage and uniformity and ensuring consistent bonding effects. Comparative Example 5 uses randomly selected commercially available materials, specifically the starch-based adhesive cardboard box composite material sold by Simida Company, as a comparison. The data in the table clearly show significant performance differences in various aspects.

[0111] The modified cassava flour adhesive and cardboard box composite materials prepared in Examples 1-3 and Comparative Examples 1-5 were used in production. The specific steps included:

[0112] (1) At room temperature, the recycled pulp and plant fiber pulp are mixed evenly in a pulping machine, and the modified cassava flour binder obtained in Example 1 is added. The temperature is raised to 35°C, sodium benzoate is added, and the mixture is stirred thoroughly to obtain a mixed pulp.

[0113] (2) The above-mentioned mixed slurry is fed into a cardboard forming machine for processing. The slurry is first dehydrated and formed on the cardboard forming machine, and then dried 2-3 times until the moisture content of the cardboard is reduced to 5%-10%. Subsequently, the formed cardboard is placed for 12-24 hours for natural aging treatment, followed by sanding and cutting to finally obtain a cardboard box composite material that meets the requirements. The following performance tests are performed on the produced cardboard boxes, and the test results are shown in Tables 3 and 4.

[0114] (1) Bursting strength test: Cut at least 5 100mm×100mm samples from the cardboard box, avoiding creases and areas with dense printing ink. Place the samples in an environment of (23±1)℃ and (50±2)% relative humidity for more than 4 hours. Start the bursting strength tester, ensuring that the upper and lower clamping rings are parallel and well sealed. Clamp the sample between the clamping rings to avoid wrinkles. Start the instrument to allow the rubber membrane to expand slowly until the sample ruptures. Record the maximum pressure value (unit: kPa).

[0115] (2) Edge crush strength test: Cut 100mm×25mm samples (length along the corrugation direction) from the cardboard box, at least 5 samples, ensuring that the edges are flat and undamaged. Place the samples vertically between the upper and lower pressure plates of the edge crush tester, ensuring that the center axis of the sample is aligned with the center of the pressure plate. Apply pressure at a speed of (12.5±2.5)mm / min until the sample is crushed, and record the maximum pressure value (unit: N). Edge crush strength (kN / m) = maximum pressure (N) × 1000 / sample width (mm, i.e. 25mm), and take the average value.

[0116] (3) Puncture strength test: Cut 175mm×175mm samples from the cardboard box, at least 5 samples, avoiding creases and splices, release the weight (with conical puncture head, mass 1000g±1g) and let it fall freely to penetrate the sample, record the energy loss value (unit: J), that is, the puncture strength.

[0117] (4) Moisture resistance test: Samples corresponding to the bursting strength, edge crush strength, etc. tests were taken and divided into two groups (control group and wet treatment group). The wet treatment group samples were placed in an environment of (23±1)℃ and (90±2)% relative humidity for 24h, or immersed in (20±1)℃ distilled water for 30min and then removed and the surface moisture was absorbed with filter paper. The bursting strength, edge crush strength, etc. of the wet treatment samples were tested immediately using the same method; the ratio of the performance after wet treatment to that of the control group (dry state) (retention rate) was calculated. The higher the retention rate, the better the moisture resistance.

[0118] (5) Cushioning test: Take a piece of corrugated cardboard from the carton as a cushioning material sample. Place the sample under the drop hammer of the impact tester to simulate the scenario of the package falling and record the height required for the cardboard box to deform.

[0119] Table 3. Performance test results of cardboard boxes in Examples 1-3

[0120]

[0121] The data in the table show that the modified cassava starch cardboard box composite materials prepared in Examples 1-3 exhibit excellent performance in practical applications, with Examples 2-3 showing the best performance, including improved mechanical strength and moisture resistance. The core reason for this is that the grafted polyvinyl alcohol formal resin can form hydrogen bonds with the -OH groups of the cardboard fibers, while the grafted flexible polyvinyl acetate segments can penetrate deep into the gaps between the cardboard fibers, enhancing the physical entanglement with the fibers through an "anchoring effect." This dual effect of "hydrogen bonds + mechanical anchoring" significantly improves the interfacial adhesion between the adhesive and the cardboard, reduces the risk of interlayer separation, and directly improves the edge crush strength and bursting strength of the cardboard box. Furthermore, the -OH groups in the cassava starch adhesive itself can combine with a large number of -OH groups on the wood surface, dehydrating to form stable chemical bonds. The formalin groups (-CH2-O-CH2-) form a partial network structure through a cross-linking reaction, enhancing the cohesive force of the resin film and making the intermolecular forces more stable. When a cardboard box is subjected to tension or impact, the adhesive layer is less likely to break, thus improving the box's tensile strength and impact resistance, such as during bumpy transport. The formaldehyde reaction replaces some of the hydroxyl groups (-OH) in the cassava flour molecules with ether bonds (-O-), reducing hydrophilicity. Simultaneously, the esterification grafting of hexadecyl alcohol forms a hydrophobic barrier on the membrane surface, reducing the permeation rate of water molecules and extending its service life in humid environments.

[0122] Table 4. Performance test results of cardboard boxes from Comparative Examples 1-5

[0123]

[0124] Analysis of the data in the table shows that the differences in each component have a significant and targeted impact on the performance of the cardboard box. Regarding mechanical strength, compared to Comparative Example 4, which showed the best overall performance, the performance of Comparative Examples 1-3 was unsatisfactory, directly confirming the core role of key components in improving mechanical performance. Specifically, Comparative Example 2's mechanical strength indicators—bursting strength 657 kPa, edge crush strength 3.8 kN / m, and puncture strength 2.9 J—were all at relatively low levels, making it the worst group after Comparative Example 5. Based on the component differences, this is presumably due to the lack of polyvinyl alcohol formal resin in the system. This resin, through the synergistic effect of its rigid molecular chain skeleton and flexible segments, can both enhance the cohesive force of interlayer bonding and disperse stress through continuous film formation. Its absence directly leads to the cardboard box's inability to effectively resist pressure, puncture, and tension, fully demonstrating that polyvinyl alcohol formal resin is the core component for improving mechanical strength. Comparative Example 3's mechanical strength is slightly higher than Comparative Example 2, but still far inferior to Comparative Example 4. This gap reveals the indispensability of borax crosslinking. Borax, by forming coordinate bonds with hydroxyl groups to construct a three-dimensional network, can further enhance the structural stability of the adhesive film and reduce deformation and cracking under stress. The absence or insufficient amount of borax in Comparative Example 3 makes it difficult for the molecular chains to form tight cross-links, resulting in limited improvement in mechanical strength. This indirectly indicates that the cross-linked structure is the backbone supporting mechanical properties, forming a synergistic reinforcing effect with polyvinyl formal resin. Furthermore, the advantages of Comparative Example 3 in moisture resistance (95%) and drying speed suggest that the introduction of long-chain alkyl groups mainly plays a role by increasing hydrophobicity and accelerating moisture evaporation, while only playing a supporting role in improving mechanical strength. Its flexible segments can appropriately improve the toughness of the film, but cannot replace the core bonding and film-forming ability of polyvinyl formal resin. Therefore, the mechanical strength of Comparative Example 3 remains at a relatively low level. It is noteworthy that although the moisture resistance (55%) of Comparative Example 4 is significantly lower than that of Comparative Example 3, its mechanical properties are not significantly affected. This phenomenon indicates that polyvinyl acetate (PVAc) plays a role in balancing the performance of the system. While the ester groups of polyvinyl acetate reduce overall hydrophobicity to some extent, its excellent film-forming properties and compatibility with polyvinyl formal resin ensure the continuity and flexibility of the adhesive layer, maintaining high mechanical strength and achieving a performance orientation prioritizing mechanical strength. Finally, Comparative Example 5 ranked last in all performance aspects, suggesting it may have used unmodified pure starch or other base components. Lacking both the reinforcing effect of polyvinyl formal resin and the cross-linking support of borax, it exhibited weak adhesion, poor water resistance, and insufficient toughness, rendering it essentially unsuitable for practical application.

[0125] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preparing a modified cassava flour adhesive, characterized in that, Includes the following steps: S1. Preparation of oxidized cassava flour Slowly add cassava flour to a beaker containing 100 mL of distilled water and stir for 10 min. Then add the above material to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel. Add ferrous sulfate at 50 °C and slowly add hydrogen peroxide dropwise for oxidation for a certain period of time. Then add an appropriate amount of sodium thiosulfate. After filtering, washing and drying the product through a sintered sand funnel, oxidized cassava flour is obtained. S2. Esterification reaction Cetyl alcohol and S1-derived oxidized cassava flour were added to a three-necked flask, respectively. Xylene was added to dissolve the oxidized cassava flour. A water separator and thermometer were assembled. The mixture was stirred and heated until the cetyl alcohol was completely dissolved. Then, 2-3 g of p-benzenesulfonic acid catalyst was added. The mixture was heated to 70-80℃ and refluxed for 4-6 h. The reaction was stopped, cooled to room temperature, washed with methanol, filtered, and the filter cake was collected and added to a three-necked flask. Anhydrous methanol was added, and the mixture was refluxed at 80℃ for 30 min. After the reaction was completed, the mixture was filtered while hot. The filtrate was cooled for 5 h under magnetic stirring, filtered by suction, and dried under vacuum at 65℃ for 7 h to obtain a white product, namely esterified cassava flour. S3. Preparation of polyvinyl alcohol formaldehyde resin Polyvinyl alcohol and distilled water were added to a 250 mL three-necked beaker equipped with a spherical condenser, a polytetrafluoroethylene electric stirrer, and a constant pressure funnel. After soaking in water for 30 minutes, the electric stirrer was turned on, and the mixture was kept at 80-90℃ for 30 minutes until the polyvinyl alcohol was completely dissolved. The temperature was then lowered to 65℃, and the pH was adjusted to 3-4 with HCl. Formaldehyde was then added dropwise and the addition was completed within 30 minutes. After the addition was completed, the mixture was kept at this temperature for 1 hour, and the pH was adjusted to neutral with NaOH. The mixture was then cooled and discharged to obtain polyvinyl alcohol formaldehyde resin. S4. Preparation of modified cassava flour binder S401. Cassava flour grafting: Slowly add cassava flour to a beaker containing 50 mL of distilled water, add 5% dilute sulfuric acid solution, stir evenly, and then pour into a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel; turn on the electric stirrer, add polyvinyl alcohol formaldehyde resin solution, heat to 80℃, and react for 2-3 hours under mechanical stirring to obtain cassava flour slurry grafted with polyvinyl alcohol formaldehyde resin; S402. Copolymerization reaction: The esterified cassava flour obtained in S2 and the cassava flour slurry grafted with polyvinyl alcohol formaldehyde resin obtained in S401 were added to a four-necked flask equipped with a polytetrafluoroethylene electric stirrer, a spherical reflux condenser and a constant pressure funnel. The temperature was raised to 60°C, and sodium dodecyl sulfonate and tributyl phosphate were added respectively. The mixture was stirred and emulsified for 30 min. A certain amount of initiator ammonium persulfate, 7 mL of vinyl acetate and cassava flour were added and reacted for 30 min. The temperature was raised to 70°C, and 18 mL of vinyl acetate was added dropwise, which was controlled to be completed within 3 h. The temperature was raised to 80°C and held for 30 min. Sodium thiosulfate was then added, the temperature was raised to 90°C and held for 30 min. The temperature was then lowered to 60-70°C, borax was added and stirred for 15-20 minutes. The system was then cooled to 50-60°C and discharged to obtain the modified cassava flour binder.

2. The preparation method of the modified cassava flour adhesive according to claim 1, characterized in that, The cassava flour used in S1 has a mass fraction of 45-55 parts; ferrous sulfate 0.1 parts; hydrogen peroxide 0.03-0.05 parts; and sodium thiosulfate 0.03-0.05 parts.

3. The method for preparing the modified cassava flour adhesive according to claim 1, characterized in that, The molar ratio of oxidized cassava flour to hexadecyl alcohol used in S2 is 1:(1-1.2).

4. The method for preparing the modified cassava flour adhesive according to claim 1, characterized in that, The raw materials used in S3 are as follows: 14-16 parts polyvinyl alcohol; 130-140 parts distilled water; and 6-7 parts formaldehyde.

5. The method for preparing the modified cassava flour adhesive according to claim 1, characterized in that, The raw material used in S401 is as follows: the amount of 5% dilute sulfuric acid solution added is 10% of the solid mass.

6. The method for preparing the modified cassava flour adhesive according to claim 1, characterized in that, The molar ratio of the raw materials used in the S402 copolymerization reaction is esterified cassava flour : cassava flour : cassava flour grafted with polyvinyl alcohol formaldehyde resin = (0.8-1.0) : 1 : 0.5, and the mass parts of the raw materials used are: 20-30 parts of oxidized cassava flour. Sodium dodecyl sulfonate 0.2-0.3 parts, tributyl phosphate 0.5 mL, ammonium persulfate 0.3-0.5 parts, sodium thiosulfate 0.1-0.2 parts; the mass of borax used is 0.5-2% of the starch mass, and the mass of silane coupling agent is 5-10% of the starch mass.

7. A cardboard box composite material, using a modified cassava flour adhesive prepared by the method described in claim 1.

8. The method for preparing the cardboard box composite material according to claim 7, characterized in that, Includes the following steps: (1) At room temperature, recycled pulp and plant fiber pulp are mixed evenly in a pulping machine, and the modified cassava flour binder obtained in claim 1 is added. The temperature is raised to 35°C, sodium benzoate is added, and the mixture is stirred thoroughly to obtain pulp. (2) Input the pulp into the paperboard forming machine for processing; the pulp is first dehydrated and formed on the paperboard forming machine, and then dried 2-3 times until the moisture content of the paperboard is reduced to 5%-10%. Then, the formed paperboard is placed for 12-24 hours for natural aging treatment, and then sanded and cut to finally obtain the paperboard box composite material that meets the requirements.

9. The method for preparing the cardboard box composite material according to claim 8, characterized in that, The recycled pulp and plant fiber pulp used in S5 have a solid content between 60-72 wt%; the amount of sodium benzoate added is 0.1-0.5% of the total pulp mass.

10. The modified cassava flour adhesive, cardboard box composite material, and preparation method thereof according to claim 8, characterized in that, The modified cassava flour adhesive is used in an amount of 6.7-10% of the weight of the molded cardboard.

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