Composite paperboard having flame retardant and cushioning properties

Abstract

The present application relates to the technical field of paperboard, in particular to a composite paperboard with flame-retardant and cushioning properties. The composite paperboard comprises, from top to bottom, a surface layer paper, a cushioning core layer and a bottom layer paper, which are compounded by a self-repairing interface adhesive. The surface layer paper and the bottom layer paper contain plant fibers, bamboo fibers, dynamic flame-retardant monomers, etc. The cushioning core layer contains plant fibers, dynamic flame-retardant monomers, PCL elastic microspheres and sodium-based montmorillonite. The self-repairing interface adhesive is compounded by dynamic flame-retardant monomers and oxidized sodium alginate. The dynamic flame-retardant monomers contain acylhydrazone bonds, phosphoric acid groups and cinnamoyl double bonds, which, in combination with modified nano zinc oxide and sodium-based montmorillonite, construct a flame-retardant system. The PCL elastic microspheres and sodium-based montmorillonite realize the cushioning function. The composite paperboard has excellent flame-retardant, cushioning and self-repairing properties, is mechanically stable and environmentally friendly, does not need to rely on halogen-based flame retardants, and can be widely used in the transportation and protection of electronic devices, precision instruments and the like.

Description

Technical Field

[0001] This invention relates to the field of paperboard technology, and more particularly to a composite paperboard with flame-retardant and cushioning properties. Background Technology

[0002] With the upgrading of transportation packaging and application scenarios for electronic devices, precision instruments, furniture, and home appliances, the market is placing higher demands on the functional integration of composite paperboard. It must not only possess reliable flame-retardant properties to cope with accidental fire risks (such as short circuits or combustion caused by high temperatures), but also have excellent cushioning properties to mitigate impacts and vibrations during transportation, preventing damage to the internal products. Especially in the electronics packaging sector, it is necessary to meet stringent flame-retardant standards such as UL-94 V-0 to prevent the release of toxic gases or molten drips during combustion, while also reducing transportation losses of precision components through cushioning structures. Traditional single-function paperboard is no longer sufficient to meet these dual requirements of "flame retardancy and cushioning."

[0003] Current mainstream flame-retardant composite paperboards have significant technical limitations: On the one hand, most products rely on adding halogenated or traditional phosphorus-based flame retardants to achieve flame-retardant effects. Halogenated flame retardants easily release toxic gases such as hydrogen chloride and hydrogen bromide when burning, which does not meet environmental regulations and health requirements. Traditional phosphorus-based flame retardants require high addition amounts to achieve the desired effect, which can easily lead to embrittlement of the paperboard's mechanical properties, a significant decrease in tensile strength and tear strength, and a loss of the packaging material's inherent toughness. On the other hand, existing flame-retardant paperboards mostly focus on optimizing flame-retardant performance, neglecting the synergistic design of cushioning function. Some products use high-density pressing processes or add rigid flame-retardant fillers to improve flame retardancy, resulting in a paperboard cushioning rebound rate of less than 60%, which cannot meet the impact absorption requirements in the transportation of precision products. A few flame-retardant paperboards that also provide cushioning suffer from poor compatibility between flame retardants and cushioning components (such as foam and loose fiber structures), leading to problems such as flame retardant migration and cushioning structure collapse, resulting in significant performance degradation after long-term use.

[0004] In the field of cushioning composite paperboard, existing technologies also have functional shortcomings: traditional cushioning paperboards mostly achieve cushioning effects by introducing materials such as pearl cotton, EPS foam, and fluffy fibers. However, pearl cotton and EPS foam are flammable and release molten drips when burning, failing to meet flame retardant requirements. While fluffy fibers can improve cushioning, they are difficult to effectively combine with flame retardant components—adding flame retardants reduces the bonding force between fibers, making the cushioning core layer prone to delamination and causing a sharp drop in compression resilience. In addition, the existing support structure of the cushioning core layer is mostly a single component (such as using only foam or only fiber), lacking synergistic design, with weak stress dispersion capabilities, making it prone to localized crushing under impact, and unable to achieve long-term stable cushioning protection.

[0005] More importantly, existing composite paperboards generally lack self-healing capabilities. During repeated transportation and handling, paperboards are prone to interlayer cracks or surface damage due to collisions and compression. Once damaged, traditional paperboards cannot be repaired and must be scrapped, which not only shortens their service life but also increases the replacement cost of packaging materials. Although a few studies have applied self-healing technology to polymer materials, they mostly focus on plastics and rubber. Moreover, self-healing systems (such as those based on DA addition and hydrogen bonding) are difficult to integrate with the fiber matrix and flame-retardant / cushioning components of paperboards—either the self-healing efficiency is low, or the introduced self-healing components destroy the flame-retardant effect or cushioning structure. There is still no composite paperboard technology solution that can simultaneously achieve the triple functions of "flame retardancy-cushioning-self-healing".

[0006] In summary, current composite paperboard technology faces three core problems: "difficulty in synergistic effect between flame retardancy and cushioning", "contradiction between environmental protection and performance stability", and "lack of self-healing function". It cannot meet the demand for multifunctional integrated materials in high-end packaging, electronic protection, logistics and transportation and other fields. Developing a composite paperboard with high efficiency in flame retardancy, excellent cushioning and reliable self-healing performance has become a technical pain point that the industry urgently needs to solve. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the existing technology and to propose a composite paperboard with flame retardant and cushioning properties.

[0008] To achieve the above objectives, the present invention provides a composite paperboard with flame retardant and cushioning properties, comprising a top paper, a cushioning core layer, and a bottom paper, which are bonded together from top to bottom by a self-healing interface adhesive. The surface paper and the bottom paper comprise the following raw materials in parts by weight: plant fiber: 90-110 parts, bamboo fiber: 5-8 parts, dynamic flame retardant monomer: 22-28 parts, photoinitiator: 1.2-1.4 parts, modified nano zinc oxide: 6-7 parts, and ethanol / water mixed solution: 220-240 parts. The buffer core layer comprises the following raw materials in parts by weight: 33-37 parts plant fiber, 7-13 parts dynamic flame retardant monomer, 0.8-1.1 parts photoinitiator, 4-5 parts modified nano zinc oxide, 16-18 parts PCL elastic microspheres, 9-11 parts organomontmorillonite, 10-14 parts waterborne polyurethane adhesive, and 80-100 parts ethanol / water mixed solution. The self-healing interface adhesive is made by mixing dynamic flame retardant monomers and sodium alginate in a weight ratio of 3:1, and then adding deionized water to adjust the solid content to 30-40%. The chemical structural formula of the dynamic flame-retardant monomer is: .

[0009] Preferably, the photoinitiator is prepared by mixing 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:1.

[0010] Preferably, the modified nano zinc oxide refers to nano zinc oxide grafted with silane coupling agent KH-550 on the surface, model: KND-NG550, purchased from Changzhou Xinnada New Material Technology Co., Ltd.

[0011] Preferably, the volume ratio of ethanol to water in the ethanol / water mixed solution is 1:4.

[0012] Preferably, the particle size of the PCL elastic microspheres is 20-40 μm.

[0013] Preferably, the organo-montmorillonite refers to sodium-based montmorillonite, model: XFI44, purchased from Jiangsu Xianfeng Nanotechnology Co., Ltd., with an interlayer spacing of 1.2-1.4 nm.

[0014] Preferably, the preparation method of the dynamic flame retardant monomer is as follows: (1) In a reaction vessel equipped with a reflux condenser, add adipyl dihydrazide and ethanol, stir at room temperature for 10-20 min, then add 2,3,4-trihydroxybenzaldehyde and heat to 75-85℃, react for 6-8 h, cool to room temperature, and a solid precipitates out. Collect the solid by filtration, recrystallize with an ethanol / water mixture to obtain intermediate A; the chemical reaction equation is as follows: ; The product was characterized by ¹H NMR; this reaction was a nucleophilic addition-elimination reaction, the core of which was the acylation of the hydrazine group of adipicohydrazide with the aldehyde group of 2,3,4-trihydroxybenzaldehyde. At room temperature, after adipicohydrazide was dissolved in ethanol and fully dispersed, the carbonyl carbon of the aldehyde group in the added 2,3,4-trihydroxybenzaldehyde was electron-deficient due to the high electronegativity of the oxygen atom. This carbonyl carbon was nucleophilically attacked by the nitrogen atom (containing a lone pair electron) of the hydrazine group, initially forming a hemiacetal intermediate. Upon heating to 75-85℃, the intermediate dehydrated, eliminating one molecule of water to form a stable acylation bond (-CH=N-NH-). (2) Under nitrogen protection, L-phosphotyrosine and triethylamine were added to anhydrous dichloromethane in a three-necked flask. The mixture was stirred and cooled to 0-5°C. Cinnamyl chloride was prepared into anhydrous dichloromethane solution with a concentration of 1 mol / L and then added dropwise to the three-necked flask. During the dropwise addition, the reaction temperature was controlled not to exceed 5°C. After the dropwise addition was completed, the mixture was brought back to room temperature and stirred for 6-8 hours. After washing three times with saturated brine, the organic phase was taken, dried with anhydrous sodium sulfate, and the solvent was removed by rotary evaporation. Then, the mixture was subjected to silica gel column chromatography with gradient elution to obtain intermediate B. The chemical reaction equation is as follows: ; The product was characterized by ¹H NMR. This step involved the N-acylation of the amino group of L-phosphotyrosine with the acyl group of cinnamoyl chloride. Triethylamine was used as an acid-binding agent to neutralize the byproduct HCl. Low temperature controlled the reaction selectivity, and nitrogen protection prevented oxidation of the raw materials. After the L-phosphotyrosine molecule dissolved in anhydrous dichloromethane under nitrogen protection, triethylamine first formed a weak coordination with the amino group, further enhancing the lone pair electron density of the amino nitrogen atom and increasing its nucleophilic activity. Cooling to 0-5℃ not only suppressed the hydrolysis side reaction of cinnamoyl chloride but also reduced the reaction rate, preventing excessive acylation of the amino group. At this temperature, the electron-deficient acyl carbon in cinnamoyl chloride was preferentially attacked by the nitrogen atom of the amino group, resulting in a nucleophilic substitution reaction. The chlorine atom was released as a leaving group, ultimately forming a stable amide bond. After the reaction, the generated triethylamine hydrochloride was removed by washing with saturated brine. The organic phase was dried with anhydrous sodium sulfate, the solvent was removed by rotary evaporation, and then eluted with a gradient of ethyl acetate and petroleum ether to obtain intermediate B. (3) Under nitrogen protection, intermediates A and B were added to anhydrous dichloromethane and stirred for 10-20 min. Then, dicyclohexylcarbodiimide and 4-dimethylaminopyridine were added. The mixture was stirred at room temperature for 10-14 h. The mixture was filtered, and the filtrate was collected and washed successively with 10% citric acid solution, saturated sodium bicarbonate solution, and saturated sodium chloride solution. After drying with anhydrous sodium sulfate, the solvent was removed by rotary evaporation. Then, the mixture was recrystallized from an N,N-dimethylformamide / water mixed solution to obtain the dynamic flame retardant monomer. The chemical reaction equation is as follows: ; The product was characterized by ¹H NMR. This reaction step is an esterification reaction of carboxyl and phenolic hydroxyl groups. Dicyclohexylcarbodiimide (DCC) (condensing agent) and 4-dimethylaminopyridine (DMAP) (catalyst) synergistically enhance the reaction efficiency. Under nitrogen protection, the carboxyl group of intermediate B first reacts with DCC to form a highly active O-acylisourea intermediate. This intermediate readily combines with DMAP to form a more reactive acyl-DMAP complex, significantly enhancing the electrophilicity of the acyl carbon. Subsequently, the phenolic hydroxyl group in intermediate A that did not participate in the previous reaction acts as a nucleophile, attacking the acyl carbon of the acyl-DMAP complex, undergoing a nucleophilic addition-elimination reaction. DMAP is regenerated as a leaving group, ultimately forming an ester bond. This covalently links the acylhydrazone structure of intermediate A with the phosphate ester and cinnamic yl structures of intermediate B, resulting in a dynamic compound containing acylhydrazone bonds, phosphate ester groups, and cinnamic yl double bonds. Flame retardant monomer; In addition, in the molecular structure of intermediate A, the three phenolic hydroxyl groups of the 2,3,4-trihydroxybenzaldehyde fragment have significant activity differences: the phenolic hydroxyl group directly adjacent to the acylhydrazone bond (-CH=N-NH-) has a significantly higher electron cloud density of the hydroxyl oxygen atom due to the conjugation electron donation effect of the acylhydrazone bond, and its nucleophilicity is significantly stronger than that of the other two non-adjacent phenolic hydroxyl groups. In the DCC / DMAP catalytic system, the "acyl-DMAP complex" formed by the activation of the carboxyl group of intermediate B has extremely strong electrophilicity and will preferentially bind to the ortho-phenolic hydroxyl group with the highest nucleophilicity. Therefore, the ortho-phenolic hydroxyl group becomes the absolutely preferred reaction site.

[0015] Preferably, in (1), the molar ratio of adipamide dihydrazide and 2,3,4-trihydroxybenzaldehyde is 1:0.5-0.6, the weight ratio of adipamide dihydrazide and ethanol is 1:8-12, and the volume ratio of ethanol to water in the ethanol / water mixed solution is 1:1.

[0016] Preferably, in step (2), the molar ratio of L-phosphotyrosine, triethylamine, and cinnamoyl chloride is 1:0.1-0.2:1-1.2, the weight ratio of L-phosphotyrosine and anhydrous dichloromethane is 1:8-12, and gradient elution refers to the volume ratio of ethyl acetate and petroleum ether in each eluent step being successively from 0:1, 1:20, 1:10 to 1:1.

[0017] Preferably, in step (3), the molar ratio of intermediate A, intermediate B, dicyclohexylcarbodiimide and 4-dimethylaminopyridine is 1:0.5-0.6:1-1.2:0.01-0.03, the weight ratio of intermediate A and anhydrous dichloromethane is 1:8-12, and the volume ratio of N,N-dimethylformamide to water in the N,N-dimethylformamide / water mixed solution is 1:2.

[0018] Furthermore, the present invention also provides a method for preparing a composite paperboard with flame-retardant and cushioning properties, comprising the following steps: S1. Preparation of the surface paper: Plant fibers are added to an ethanol / water mixture and stirred for 10-20 minutes. Then, a photoinitiator is added, and the mixture is exposed to light at a wavelength of 365 nm and an intensity of 10-12 mW / cm². 2 Pre-irradiate with ultraviolet light for 20-25 minutes; then add dynamic flame retardant monomer and modified nano zinc oxide, heat to 60-65℃, and stir for 5-6 hours; then add bamboo fiber and pulp in a beater to a freeness of 51-53°SR; transfer to a fourdrinier paper machine, control the papermaking speed at 30-35 m / min to obtain a wet paper sheet; finally, irradiate at 105-110℃ with a wavelength of 365 nm and a light intensity of 15-18 mW / cm². 2 The surface paper is obtained by cross-linking, curing and drying under ultraviolet light for 30-35 minutes and then cooling to room temperature. S2. Preparation of the base paper: The preparation method of the base paper is the same as step S1; S3. Preparation of the buffer core layer: Add plant fibers to an ethanol / water mixture and stir for 10-20 minutes. Then add a photoinitiator and apply light at a wavelength of 365 nm and an intensity of 10-12 mW / cm². 2 The mixture is pre-irradiated with ultraviolet light for 20-25 minutes; then dynamic flame retardant monomers and modified nano zinc oxide are added, the temperature is raised to 60-65℃, and the mixture is stirred for 5-6 hours. After filtration, the mixture is washed with deionized water until the filtrate is neutral, then dried and mixed with PCL elastic microspheres, organomontmorillonite and waterborne polyurethane adhesive, and stirred for 20-25 minutes to obtain a mixture. The mixture is then put into a flat vulcanizing machine and pressed for 5-8 minutes at a pressure of 0.3-0.4MPa and a temperature of 75-80℃ to form the core layer. After forming, the core layer is transferred to a hot air drying oven and dried to obtain the buffer core layer. S4. Composite Molding: Mix the dynamic flame-retardant monomer and sodium alginate oxide, add deionized water to adjust the solid content to 30-40%, and obtain a self-healing interface adhesive. Using a doctor blade coating method, evenly coat the self-healing interface adhesive on the inner sides of the surface paper and the bottom paper, with a coating thickness of 0.2-0.4 mm. Place the buffer core layer between the surface paper and the bottom paper, ensuring the three layers are aligned without misalignment. Place the laminated three-layer structure into a hot press laminator and hot press for 15-25 minutes at a pressure of 0.5-0.6 MPa and a temperature of 75-85℃. After hot pressing, allow it to cool naturally to room temperature, and then apply a light irradiation at a wavelength of 365 nm and an intensity of 12-15 mW / cm². 2 Irradiate with ultraviolet light for 5-10 minutes to obtain composite paperboard with flame retardant and cushioning properties.

[0019] Preferably, the mechanism of action of the composite paperboard with flame-retardant and cushioning properties of the present invention is as follows: The flame-retardant performance of composite paperboard relies on a dual protection system of physical barriers and chemical inhibition constructed synergistically by multiple components. Dynamic flame-retardant monomers are the core flame-retardant components. The phosphate ester groups in their molecules preferentially decompose at high temperatures to generate acidic substances such as phosphoric acid and polyphosphoric acid. These substances catalyze the dehydration and carbonization of substrates such as plant fibers and bamboo fibers, forming a dense char layer structure. This char layer not only isolates oxygen from the flame but also prevents heat transfer to the interior of the paperboard, reducing the release of combustible gases. Simultaneously, the acylhydrazone bonds (-CH=N-NH-) in the dynamic flame-retardant monomers undergo endothermic decomposition at high temperatures, absorbing heat from the surrounding environment, lowering the local temperature, and slowing the spread of combustion. Modified nano-zinc oxide, due to its better dispersibility after surface modification, can evenly distribute the flame. Evenly distributed in the surface, bottom, and buffer core layers, it can catalyze the formation of the char layer and improve its density. On the other hand, it can capture free radicals generated during combustion and inhibit chain combustion reactions. The sodium-based montmorillonite in the buffer core layer utilizes its layered structure to expand at high temperatures and form a physical flame-retardant barrier, further hindering the transfer of oxygen and heat. In synergy with dynamic flame-retardant monomers and modified nano zinc oxide, the composite paperboard achieves excellent flame-retardant effects as a whole. In addition, the content of dynamic flame-retardant monomers in the surface and bottom layers is higher than that in the buffer core layer, which can preferentially form a flame-retardant protective layer on the outer surface of the paperboard, reducing the damage of external flames to the internal buffer core layer. The cushioning performance is mainly achieved through the synergistic effect of the special structure and components of the cushioning core layer, while the surface and bottom layers provide auxiliary support. The PCL elastic microspheres in the cushioning core layer are the core cushioning unit, possessing a low glass transition temperature and strong elastic deformation capability. When the cardboard is subjected to external impact or pressure, the PCL microspheres undergo elastic deformation, converting impact energy into deformation energy, and then recover their original shape through self-rebound, achieving energy absorption and cushioning. The plant fibers, after being modified with dynamic flame-retardant monomers, retain good fiber interweaving ability, forming a three-dimensional support structure of fiber-microsphere-layered clay together with the PCL microspheres and sodium montmorillonite. The layered structure of sodium montmorillonite can disperse local stress, avoiding localized crushing caused by stress concentration. The water-based polyurethane adhesive tightly bonds these components, ensuring the structural stability of the cushioning core layer under stress, preventing component detachment or delamination. The bamboo fibers in the surface and bottom layers can enhance the overall toughness of the paperboard, prevent external impacts from causing the surface / bottom layers to crack, and thus protect the internal cushioning core layer. At the same time, the dense structure of the surface and bottom layers can also initially disperse external pressure, help improve the overall cushioning effect, and ultimately make the composite paperboard have both good cushioning performance and structural stability. Composite paperboard also possesses self-healing properties. The self-healing interface adhesive is composed of a dynamic flame-retardant monomer and sodium alginate oxide. The acylhydrazone bonds contained in the dynamic flame-retardant monomer have dynamic reversibility. When micro-cracks appear between layers due to impact or aging, the broken acylhydrazone bonds can undergo acylhydrazone exchange reactions with the aldehyde groups of sodium alginate oxide during hot pressing or ultraviolet irradiation, thus achieving chemical bond recombination. At the same time, the cinnamyl double bonds in the dynamic flame-retardant monomer undergo a [2+2] cycloaddition reaction under ultraviolet light irradiation to form a cross-linked structure, bridging the crack gaps. The aldehyde groups of sodium alginate oxide can also undergo hemiacetal / acetal reactions with the hydroxyl groups of plant fibers in the surface, bottom, and buffer core layers, assisting in repairing the interlayer bonding force. Ultimately, a stable adhesive structure is re-formed at the crack, restoring the mechanical properties and interlayer bonding strength of the paperboard, thus achieving a self-healing effect.

[0020] The beneficial effects of this invention are: 1. This invention constructs a flame-retardant system of "chemical inhibition + dual physical barriers" through the synergistic effect of dynamic flame-retardant monomers, modified nano-zinc oxide, and sodium-based montmorillonite, exhibiting excellent flame-retardant performance and outstanding environmental friendliness. The phosphate ester groups in the dynamic flame-retardant monomer molecules catalyze the dehydration and carbonization of the substrate to form a dense char layer, isolating oxygen and heat; the acylhydrazone bonds can endothermally decompose to lower the local temperature and inhibit flame spread; the modified nano-zinc oxide further catalyzes the densification of the char layer and captures combustion free radicals; and the sodium-based montmorillonite expands at high temperatures to form an additional physical barrier. Simultaneously, this flame-retardant system does not rely on halogenated flame retardants, avoiding the release of toxic gases during combustion. Furthermore, the dynamic flame-retardant monomers are chemically bonded to fibers, making them less prone to migration and loss, and maintaining stable flame-retardant performance even after long-term use. In addition, the higher content of dynamic flame-retardant monomers in the surface and bottom layers allows for the preferential formation of a protective layer on the outer surface of the cardboard, reducing the damage of external flames to the internal buffer core layer and further ensuring the overall flame-retardant effect.

[0021] 2. The cushioning core layer of this invention adopts a composite structure of "PCL elastic microspheres + sodium montmorillonite + modified plant fibers," exhibiting excellent cushioning performance and strong structural stability. PCL elastic microspheres, due to their low glass transition temperature, possess excellent elastic deformation capabilities, converting impact energy into deformation energy under stress and rapidly rebounding to their original shape after unloading. The layered structure of sodium montmorillonite effectively disperses localized stress, preventing irreversible crushing of the PCL microspheres due to stress concentration. The plant fibers, modified with dynamically flame-retardant monomers, retain good interweaving ability, working together with water-based polyurethane adhesive to tightly bond the various cushioning components, forming a three-dimensional support system of "fiber-microsphere-clay," ensuring that the components do not detach or delaminate under stress. Simultaneously, the bamboo fibers in the surface and bottom layers enhance the overall toughness of the cardboard, preventing surface damage from external impacts and assisting in dispersing external pressure. This allows the composite cardboard to effectively absorb impact energy while maintaining a high compression resilience, meeting the cushioning requirements of precision product transport packaging.

[0022] 3. This invention utilizes a self-healing interlayer adhesive formulated with "dynamic flame-retardant monomer + sodium alginate oxide," endowing composite paperboard with excellent interlayer self-healing capabilities, effectively solving the problem of traditional paperboard's inability to be repaired after damage. When micro-cracks appear in the interlayer of the paperboard due to impact or aging, the acylhydrazone bonds in the dynamic flame-retardant monomer can undergo acylhydrazone exchange reactions with the aldehyde groups of sodium alginate oxide under hot pressing or ultraviolet irradiation, reorganizing broken chemical bonds; the cinnamoyl double bonds undergo cycloaddition reactions under ultraviolet light, forming a cross-linked structure to bridge the cracks; the aldehyde groups of sodium alginate oxide can also form hemiacetal / acetal bonds with cellulose hydroxyl groups, further enhancing the interlayer bonding strength after repair. Through this triple repair mechanism, a stable adhesive structure can be reformed at the crack, restoring the mechanical properties and interlayer bonding strength of the paperboard, reducing scrap due to damage, significantly extending the service life of the paperboard, and reducing the replacement and usage costs of packaging materials.

[0023] 4. This invention significantly improves the mechanical properties of composite paperboard through the synergistic effect of chemical crosslinking and physical interweaving, while also possessing excellent adaptability to various application scenarios. In terms of mechanical properties, the dynamic flame-retardant monomers can crosslink with plant fibers and bamboo fibers via ester bonds and double bonds. The high tensile strength of bamboo fibers combined with the physical interweaving force between fibers significantly enhances the tensile strength of the paperboard. In the self-healing interface adhesive, the acylhydrazone bonds and phenolic hydroxyl groups of the dynamic flame-retardant monomers form a chemical bond with the aldehyde groups of oxidized sodium alginate. Combined with the synergistic adsorption of polar groups, this enhances interlayer peel strength and prevents delamination. Regarding application scenario adaptability, the plant fibers and oxidized sodium alginate in the composite paperboard raw materials are bio-based components with a certain degree of biodegradability, aligning with the trend of environmentally friendly packaging. Furthermore, the overall structure combines flame retardancy, cushioning, and self-healing functions, eliminating the need for additional functional layers. With controllable thickness, it can be widely used in transport packaging or protective applications for electronic equipment, precision instruments, furniture, and home appliances, meeting diverse usage needs. Attached Figure Description

[0024] Figure 1 The H NMR spectrum of intermediate A prepared in Example 2 of this invention; Figure 2 The H NMR spectrum of intermediate B prepared in Example 2 of this invention; Figure 3 The H NMR spectrum of the dynamic flame retardant monomer prepared in Example 2 of this invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0026] Preparation Example 1: The specific synthesis method of dynamic flame retardant monomer includes the following steps: (1) In a reaction vessel equipped with a reflux condenser, 10 g of adipyl dihydrazide and 80 g of ethanol were added and stirred at room temperature for 10 min. Then, 4.42 g of 2,3,4-trihydroxybenzaldehyde was added and the temperature was raised to 75 °C. The reaction was carried out for 6 h and then cooled to room temperature. A solid precipitated out. The solid was collected by filtration and recrystallized with an ethanol / water mixture (ethanol and water volume ratio of 1:1) to obtain intermediate A. (2) Under nitrogen protection, 10g of L-phosphotyrosine and 0.32g of triethylamine were added to 80g of anhydrous dichloromethane in a three-necked flask. The mixture was stirred and cooled to 0-5℃. 5.25g of cinnamyl chloride was prepared into anhydrous dichloromethane solution with a concentration of 1mol / L. The solution was then added dropwise to the three-necked flask. During the dropwise addition, the reaction temperature was controlled not to exceed 5℃. After the dropwise addition was completed, the mixture was brought back to room temperature and stirred for 6h. The organic phase was washed three times with saturated saline solution, dried with anhydrous sodium sulfate, and the solvent was removed by rotary evaporation. The mixture was then subjected to silica gel column chromatography with gradient elution. The volume ratio of ethyl acetate to petroleum ether in each eluent was 0:1, 1:20, 1:10 to 1:1, respectively, to obtain intermediate B. (3) Under nitrogen protection, 5g of intermediate A and 2.51g of intermediate B were added to 40g of anhydrous dichloromethane and stirred for 10min. Then, 2.31g of dicyclohexylcarbodiimide and 0.01g of 4-dimethylaminopyridine were added. The mixture was stirred at room temperature for 10h. The mixture was filtered and the filtrate was collected. The filtrate was washed successively with 10% citric acid solution, saturated sodium bicarbonate solution and saturated sodium chloride solution. After drying with anhydrous sodium sulfate, the solvent was removed by rotary evaporation. The mixture was then recrystallized from a N,N-dimethylformamide / water mixed solution (N,N-dimethylformamide and water in a volume ratio of 1:2) to obtain the dynamic flame retardant monomer.

[0027] Preparation Example 2: The specific synthesis method of dynamic flame retardant monomer includes the following steps: (1) In a reaction vessel equipped with a reflux condenser, 50 g of adipyl dihydrazide and 500 g of ethanol were added and stirred at room temperature for 15 min. Then, 24.33 g of 2,3,4-trihydroxybenzaldehyde was added and the temperature was raised to 80 °C. The reaction was carried out for 7 h and then cooled to room temperature. A solid precipitated out. The solid was collected by filtration and recrystallized with an ethanol / water mixture (ethanol and water volume ratio of 1:1) to obtain intermediate A. (2) Under nitrogen protection, 50g of L-phosphotyrosine and 2.39g of triethylamine were added to 500g of anhydrous dichloromethane in a three-necked flask. The mixture was stirred and cooled to 0-5℃. 28.88g of cinnamyl chloride was prepared into anhydrous dichloromethane solution with a concentration of 1mol / L. The solution was then added dropwise to the three-necked flask. During the dropwise addition, the reaction temperature was controlled not to exceed 5℃. After the dropwise addition was completed, the mixture was brought back to room temperature and stirred for 7h. The organic phase was washed three times with saturated saline solution, dried with anhydrous sodium sulfate, and the solvent was removed by rotary evaporation. The mixture was then subjected to silica gel column chromatography with gradient elution. The volume ratio of ethyl acetate to petroleum ether in each eluent was 0:1, 1:20, 1:10 to 1:1, respectively, to obtain intermediate B. (3) Under nitrogen protection, 50g of intermediate A and 27.56g of intermediate B were added to 500g of anhydrous dichloromethane and stirred for 15min. Then, 25.42g of dicyclohexylcarbodiimide and 0.27g of 4-dimethylaminopyridine were added. The mixture was stirred at room temperature for 12h. The mixture was filtered and the filtrate was collected. The filtrate was washed successively with 10% citric acid solution, saturated sodium bicarbonate solution and saturated sodium chloride solution. After drying with anhydrous sodium sulfate, the solvent was removed by rotary evaporation. Then, the mixture was recrystallized from a N,N-dimethylformamide / water mixed solution (N,N-dimethylformamide and water in a volume ratio of 1:2) to obtain the dynamic flame retardant monomer.

[0028] Preparation Example 3: The specific synthesis method of dynamic flame retardant monomer includes the following steps: (1) In a reaction vessel equipped with a reflux condenser, 100g of adipyl dihydrazide and 1.2kg of ethanol were added and stirred at room temperature for 20min. Then, 53.08g of 2,3,4-trihydroxybenzaldehyde was added and the temperature was raised to 85℃. The reaction was carried out for 8h and then cooled to room temperature. A solid precipitated out. The solid was collected by filtration and recrystallized with an ethanol / water mixture (ethanol and water volume ratio of 1:1) to obtain intermediate A. (2) Under nitrogen protection, 100g of L-phosphotyrosine and 6.38g of triethylamine were added to 1.2kg of anhydrous dichloromethane in a three-necked flask. The mixture was stirred and cooled to 0-5℃. 63.01g of cinnamyl chloride was prepared into anhydrous dichloromethane solution with a concentration of 1mol / L. The solution was then added dropwise to the three-necked flask. During the dropwise addition, the reaction temperature was controlled not to exceed 5℃. After the dropwise addition was completed, the mixture was brought back to room temperature and stirred for 8h. The organic phase was washed three times with saturated saline solution, dried with anhydrous sodium sulfate, and the solvent was removed by rotary evaporation. The mixture was then subjected to silica gel column chromatography with gradient elution. The volume ratio of ethyl acetate to petroleum ether in each eluent was 0:1, 1:20, 1:10 to 1:1, respectively, to obtain intermediate B. (3) Under nitrogen protection, 100g of intermediate A and 60.13g of intermediate B were added to 1.2kg of anhydrous dichloromethane and stirred for 20min. Then, 55.46g of dicyclohexylcarbodiimide and 0.82g of 4-dimethylaminopyridine were added. The mixture was stirred at room temperature for 14h. The mixture was filtered and the filtrate was collected. The filtrate was washed successively with 10% citric acid solution, saturated sodium bicarbonate solution and saturated sodium chloride solution. After drying with anhydrous sodium sulfate, the solvent was removed by rotary evaporation. Then, the mixture was recrystallized from a N,N-dimethylformamide / water mixed solution (N,N-dimethylformamide and water volume ratio of 1:2) to obtain the dynamic flame retardant monomer.

[0029] Comparative Preparation Example 1: The difference between Comparative Preparation Example 1 and Preparation Example 2 is that adipicohydrazide is replaced with adipicoamide.

[0030] Comparative Preparation Example 2: The difference between Comparative Preparation Example 2 and Preparation Example 2 is that L-phosphotyrosine is replaced with L-tyrosine.

[0031] Comparative Preparation Example 3: The difference between Comparative Preparation Example 3 and Preparation Example 2 is that cinnamyl chloride is replaced with hydrogenated cinnamyl chloride.

[0032] Example 1: A specific method for preparing a composite paperboard with flame-retardant and cushioning properties, comprising the following steps: S1. Preparation of the surface paper: 900g of plant fiber was added to 2.2kg of ethanol / water mixed solution (ethanol to water volume ratio of 1:4), stirred for 10min, and then 12g of photoinitiator (prepared by mixing 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:1) was added. The solution was then heated with light at a wavelength of 365nm and an intensity of 10mW / cm². 2 The sample was pre-irradiated with ultraviolet light for 20 min; then 220 g of the dynamic flame retardant monomer prepared according to Preparation Example 1 and 60 g of modified nano zinc oxide were added, the temperature was raised to 60 °C, and the mixture was stirred and reacted for 5 h; then 50 g of bamboo fiber was added, and the sample was put into a pulper and pulped to a freeness of 51-53°SR; the sample was transferred to a wire paper machine, and the papermaking speed was controlled at 30-35 m / min to obtain a wet paper sheet; finally, the sample was irradiated with ultraviolet light at 105-110 °C with a wavelength of 365 nm and a light intensity of 15 mW / cm. 2 The surface paper was obtained by cross-linking and curing by irradiation with ultraviolet light for 30 minutes and then drying. After cooling to room temperature, the surface paper was obtained. S2. Preparation of the base paper: The preparation method of the base paper is the same as step S1; S3. Preparation of the buffer core layer: 330g of plant fiber was added to 800g of an ethanol / water mixture (ethanol to water volume ratio of 1:4), stirred for 10min, and then 8g of photoinitiator (prepared by mixing 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:1) was added. The mixture was then exposed to light at a wavelength of 365nm and an irradiation intensity of 10mW / cm². 2 The mixture was pre-irradiated with ultraviolet light for 20 minutes; then 70g of the dynamic flame retardant monomer prepared according to Preparation Example 1 and 40g of modified nano zinc oxide were added, the temperature was raised to 60℃, and the mixture was stirred for 5 hours. After filtration, the mixture was washed with deionized water until the filtrate was neutral, and then dried. It was then mixed with 160g of PCL elastic microspheres, 90g of sodium montmorillonite and 100g of waterborne polyurethane adhesive, and stirred for 20 minutes to obtain a mixture. The mixture was then put into a flat vulcanizing machine and pressed for 5 minutes at a pressure of 0.3MPa and a temperature of 75℃. After molding, it was transferred to a hot air drying oven and dried to obtain a buffer core layer. S4. Composite Molding: The dynamic flame-retardant monomer prepared according to Preparation Example 1 and sodium alginate oxide were mixed at a weight ratio of 3:1. Deionized water was added to adjust the solid content to 30% to obtain a self-healing interface adhesive. The self-healing interface adhesive was evenly coated on the inner side of the surface paper and the inner side of the bottom paper using a doctor blade coating method, with a coating thickness of 0.2 mm. The buffer core layer was sandwiched between the surface paper and the bottom paper to ensure that the three-layer structure was aligned and without misalignment. The stacked three-layer structure was placed in a hot press laminating machine and hot-pressed for 15 min at a pressure of 0.5 MPa and a temperature of 75 °C. After hot pressing, the structure was allowed to cool naturally to room temperature, and then subjected to a light intensity of 12 mW / cm² at a wavelength of 365 nm. 2 After irradiating with ultraviolet light for 5 minutes, a composite paperboard with flame-retardant and cushioning properties was obtained.

[0033] Example 2: A specific method for preparing a composite paperboard with flame-retardant and cushioning properties, comprising the following steps: S1. Preparation of the surface paper: 1 kg of plant fiber was added to 2.3 kg of ethanol / water mixed solution (ethanol to water volume ratio of 1:4), stirred for 15 min, and then 13 g of photoinitiator (prepared by mixing 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:1) was added. The solution was then heated with light at a wavelength of 365 nm and an irradiation intensity of 11 mW / cm². 2 The paper was pre-irradiated with ultraviolet light for 22 min; then 250 g of the dynamic flame retardant monomer prepared according to Preparation Example 2 and 65 g of modified nano zinc oxide were added, the temperature was raised to 63 °C, and the reaction was stirred for 5.5 h; then 65 g of bamboo fiber was added, and the paper was put into a pulper and pulped to a freeness of 51-53°SR; the paper was transferred to a fourdrinier paper machine, and the papermaking speed was controlled at 30-35 m / min to obtain a wet paper sheet; finally, the paper was irradiated at 105-110 °C with a wavelength of 365 nm and a light intensity of 16 mW / cm². 2The surface paper was obtained by cross-linking and curing by irradiation with ultraviolet light for 32 minutes and then drying. After cooling to room temperature, the surface paper was obtained. S2. Preparation of the base paper: The preparation method of the base paper is the same as step S1; S3. Preparation of the buffer core layer: 350g of plant fiber was added to 900g of an ethanol / water mixture (ethanol to water volume ratio of 1:4), stirred for 15min, and then 9.5g of photoinitiator (prepared by mixing 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:1) was added. The mixture was then subjected to a light intensity of 11mW / cm² at a wavelength of 365nm. 2 The mixture was pre-irradiated with ultraviolet light for 22 min; then 100 g of the dynamic flame retardant monomer prepared according to Preparation Example 2 and 45 g of modified nano zinc oxide were added, the temperature was raised to 63 °C, and the mixture was stirred for 5.5 h. After filtration, the mixture was washed with deionized water until the filtrate was neutral, and then dried. It was then mixed with 170 g of PCL elastic microspheres, 100 g of sodium montmorillonite and 120 g of waterborne polyurethane adhesive, and stirred for 22 min to obtain a mixture. The mixture was then put into a flat vulcanizing machine and pressed for 6 min at a pressure of 0.35 MPa and a temperature of 77 °C. After molding, it was transferred to a hot air drying oven and dried to obtain a buffer core layer. S4. Composite Molding: The dynamic flame-retardant monomer prepared according to Preparation Example 2 and sodium alginate oxide were mixed at a weight ratio of 3:1, and deionized water was added to adjust the solid content to 35% to obtain a self-healing interface adhesive. The self-healing interface adhesive was evenly coated on the inner side of the surface paper and the inner side of the bottom paper using a doctor blade coating method, with a coating thickness of 0.3 mm. The buffer core layer was sandwiched between the surface paper and the bottom paper to ensure that the three-layer structure was aligned and without misalignment. The stacked three-layer structure was placed in a hot press laminating machine and hot-pressed at a pressure of 0.55 MPa and a temperature of 80 °C for 20 min. After hot pressing, the structure was allowed to cool naturally to room temperature, and then subjected to a light intensity of 13 mW / cm² at a wavelength of 365 nm. 2 After irradiating with ultraviolet light for 8 minutes, a composite paperboard with flame-retardant and cushioning properties was obtained.

[0034] Example 3: A specific method for preparing a composite paperboard with flame-retardant and cushioning properties, comprising the following steps: S1. Preparation of the surface paper: 1.1 kg of plant fiber was added to a 2 / 4 kg ethanol / water mixture (ethanol to water volume ratio of 1:4), stirred for 20 min, and then 14 g of photoinitiator (prepared by mixing 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:1) was added. The mixture was then subjected to a light intensity of 12 mW / cm² at a wavelength of 365 nm. 2The sample was pre-irradiated with ultraviolet light for 25 min; then 280 g of the dynamic flame retardant monomer prepared according to Preparation Example 3 and 70 g of modified nano zinc oxide were added, the temperature was raised to 65 °C, and the mixture was stirred and reacted for 6 h; then 80 g of bamboo fiber was added, and the sample was put into a pulper and pulped to a freeness of 51-53°SR; the sample was transferred to a wire paper machine, and the papermaking speed was controlled at 30-35 m / min to obtain a wet paper sheet; finally, the sample was irradiated with ultraviolet light at 105-110 °C with a wavelength of 365 nm and a light intensity of 18 mW / cm. 2 The surface paper was obtained by cross-linking and curing by irradiation with ultraviolet light for 35 minutes and then drying. After cooling to room temperature, the surface paper was obtained. S2. Preparation of the base paper: The preparation method of the base paper is the same as step S1; S3. Preparation of the buffer core layer: 370g of plant fiber was added to 1kg of ethanol / water mixed solution (ethanol to water volume ratio of 1:4), stirred for 20min, and then 11g of photoinitiator (prepared by mixing 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:1) was added. The solution was then heated with light at a wavelength of 365nm and an irradiation intensity of 12mW / cm². 2 The mixture was pre-irradiated with ultraviolet light for 25 min; then 130 g of the dynamic flame retardant monomer prepared according to Preparation Example 3 and 50 g of modified nano zinc oxide were added, the temperature was raised to 65 °C, and the mixture was stirred for 6 h. After filtration, the mixture was washed with deionized water until the filtrate was neutral, and then dried. It was then mixed with 180 g of PCL elastic microspheres, 110 g of sodium montmorillonite and 140 g of waterborne polyurethane adhesive, and stirred for 25 min to obtain a mixture. The mixture was then put into a flat vulcanizing machine and pressed for 8 min at a pressure of 0.4 MPa and a temperature of 80 °C. After molding, it was transferred to a hot air drying oven and dried to obtain a buffer core layer. S4. Composite Molding: The dynamic flame-retardant monomer prepared according to Preparation Example 3 and sodium alginate oxide were mixed at a weight ratio of 3:1, and deionized water was added to adjust the solid content to 40% to obtain a self-healing interface adhesive. The self-healing interface adhesive was evenly coated on the inner side of the surface paper and the inner side of the bottom paper using a doctor blade coating method, with a coating thickness of 0.4 mm. The buffer core layer was sandwiched between the surface paper and the bottom paper to ensure that the three-layer structure was aligned and without misalignment. The stacked three-layer structure was placed in a hot press laminating machine and hot-pressed at a pressure of 0.6 MPa and a temperature of 85°C for 25 min. After hot pressing, the structure was allowed to cool naturally to room temperature, and then subjected to a light intensity of 15 mW / cm² at a wavelength of 365 nm. 2 After irradiating with ultraviolet light for 10 minutes, a composite paperboard with flame-retardant and cushioning properties was obtained.

[0035] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the dynamic flame retardant monomer prepared according to Preparation Example 2 is replaced with the dynamic flame retardant monomer prepared according to Comparative Preparation Example 1.

[0036] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that the dynamic flame retardant monomer prepared according to Preparation Example 2 is replaced with the dynamic flame retardant monomer prepared according to Comparative Preparation Example 2.

[0037] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the dynamic flame retardant monomer prepared according to Preparation Example 2 is replaced with the dynamic flame retardant monomer prepared according to Comparative Preparation Example 3.

[0038] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that the self-healing interface adhesive is replaced with epoxy resin adhesive, model E-44.

[0039] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that sodium-based bentonite is not added. Performance Testing: 1. Limiting Oxygen Index (LOI) Test: Referring to GB / T 2406.2-2009 "Determination of Combustion Behavior by Oxygen Index Method for Plastics - Part 2: Room Temperature Test", 80mm×10mm×1.6mm samples were cut from the composite paperboards prepared in Examples 1-3 and Comparative Examples 1-5. The samples were equilibrated for 24 hours at 23℃ and 50% relative humidity. Using an oxygen index meter, the mixed flow rate of oxygen and nitrogen was adjusted (total flow rate 10L / min). Starting from a low oxygen concentration (18%), the samples were placed vertically into the combustion chamber and ignited at the top with an igniter (flame height 5mm). The combustion of the samples was observed: if the combustion time was >3min or the combustion length was >50mm, the oxygen concentration was reduced; if the combustion time was <3min or the combustion length was <50mm, the oxygen concentration was increased until the lowest oxygen concentration was found where "the sample just maintained combustion for 3min or 50mm", which is the LOI value. The experimental results are shown in Table 1.

[0040] 2. UL-94 rating test: based on GB / T 2408-2021, from the composite paperboard prepared in Examples 1-3 and Comparative Examples 1-5, strips with dimensions of 125mm × 13mm × 1.6mm were cut. Before testing, the strips were placed in an environment of 23℃ and 50% relative humidity for 24 hours. During testing, the strips were fixed vertically with the lower end 300mm away from the absorbent cotton. A Bunsen burner was used, with the flame height adjusted to 20mm. The center of the flame was aligned with the lower edge of the strip (5mm from the bottom of the strip). After ignition for 10 seconds, the flame was removed. The self-extinguishing time of the strip was observed and recorded (the time from removing the flame to the complete extinguishing of the flame on the strip, denoted as t1). If the flame on the strip did not extinguish and burned to the 100mm mark line from the fixture, the flame was immediately removed and the burning status was recorded. After the strip had completely cooled (or 1 minute after self-extinguishing), a second ignition was performed (again, igniting for 10 seconds), and the second self-extinguishing time was recorded (denoted as t2). At the same time, it was observed whether any molten drips ignited the absorbent cotton. The result was determined to be UL-94. V-0 level: The self-extinguishing time t1 and t2 of the two ignitions are both ≤10s, the total self-extinguishing time of the two ignitions is ≤50s, and there are no molten drips that ignite the degreased cotton. UL-94 V-1 rating: The self-extinguishing time t1 and t2 of the two ignitions are both ≤30s, and the total self-extinguishing time of the two ignitions is ≤250s. No molten drips ignite the degreased cotton. UL-94 V-2 rating: The self-extinguishing time of the two ignitions meets the requirements of V-1 rating, but molten drips ignite the degreased cotton. Non-compliant: Self-extinguishing time exceeds 30 seconds, or the flame burns to the clamp, or there is no self-extinguishing phenomenon; The experimental results are shown in Table 1.

[0041] 3. Compression rebound rate test: Referring to GB / T 1041-2008 "Determination of compression properties of plastics", the composite paperboards prepared in Examples 1-3 and Comparative Examples 1-5 were used as samples with a cross-sectional size of only 25mm × 25mm × actual thickness. The universal testing machine was set to compression mode with a compression speed of 5mm / min. The "pressure-displacement" monitoring mode was set, and the pressure was applied to 0.35MPa and held for 30s. The compression amount L1 (original thickness - compressed thickness) of the composite paperboard was recorded when the pressure was held. After unloading, the paperboard was allowed to stand for 10min, and the recovered thickness was recorded. The rebound amount L2 (recovered thickness - compressed thickness) was calculated. The compression rebound rate = (L2 / L1) × 100%. The experimental results are shown in Table 1.

[0042] 4. Interlayer self-healing rate test: Referring to GB / T 2791-1995 "Test method for T-peel strength of adhesives, flexible materials to flexible materials", take the composite paperboards prepared in Examples 1-3 and Comparative Examples 1-5, and cut two identical samples of 200mm×25mm. For one sample, pre-peel the surface paper from the cushioning core layer by 50mm, and test the T-peel strength using a universal testing machine at a speed of 50mm / min, recording the original peel strength P0. For the other sample, use a blade to make a straight crack with a depth of 0.5mm and a length of 100mm at the "surface paper / cushioning core layer" interface, and then expose it to 365nm ultraviolet light at 13mW / cm². 2 Irradiate for 8 minutes, process the cracked sample, and retest the peel strength. Record the peel strength P1 after repair and calculate the self-healing efficiency = (P1 / P0) × 100%. The experimental results are shown in Table 1.

[0043] 5. Tensile strength test: Referring to GB / T 12914-2018 "Determination of tensile strength of paper and paperboard - Part 2: Constant speed tensile test", composite paperboards prepared in Examples 1-3 and Comparative Examples 1-5 were used. The tensile testing machine was set to constant speed tensile mode with a tensile speed of 10 mm / min. The two ends of the sample were clamped (clamping distance 200 mm). The machine was started until the sample broke. The maximum tensile strength was recorded. The tensile strength was calculated as maximum tensile strength / sample width. The experimental results are shown in Table 1.

[0044] Table 1 Performance Test Results Performance Analysis: As can be seen from the experimental data in Table 1, the composite paperboards prepared in Examples 1-3 all exhibited excellent flame retardancy, cushioning, self-healing and mechanical properties, with Example 2 showing the best overall performance.

[0045] The high flame retardant performance of Example 2 stems from the microscopic synergy of its core components: the phosphate groups in the dynamic flame retardant monomer decompose at high temperatures to generate phosphoric acid and polyphosphoric acid, which catalyze the dehydration and carbonization of plant and bamboo fibers through proton transfer, forming a dense carbon layer (physical barrier) that blocks oxygen from contacting combustible gases; the acylhydrazone bonds undergo endothermic decomposition, absorbing surrounding heat to lower the local temperature and inhibit flame spread (chemical inhibition); the layered structure of sodium-based montmorillonite (interlayer spacing 1.2-1.4 nm) expands at high temperatures, further constructing a physical barrier layer, forming a flame retardant system of "chemical inhibition + dual physical barrier" with the former two; at the same time, the high content of dynamic flame retardant monomers in the surface / bottom layers preferentially forms flame retardant protection on the outer layer of the cardboard, reducing the damage of the flame to the buffer core layer, ultimately achieving a high LOI and V-0 rating; Comparative Example 1... The acylhydrazone bond was replaced with an amide bond. The amide bond has no endothermic decomposition function and loses its chemical inhibition effect. It relies only on the phosphate ester group and montmorillonite, resulting in a decrease in flame retardant efficiency. In Comparative Example 2, because the dynamic flame retardant monomer does not contain phosphorus, it cannot generate acidic substances that catalyze carbonization. There is no dense carbon layer, and a large amount of combustible gas is released, resulting in a flame retardant rating of V-2. In Comparative Example 3, because the cinnamoyl double bond is missing, the dynamic flame retardant monomer cannot cross-link through cyclization. It is easy to migrate at high temperatures, resulting in poor carbon layer continuity and a decrease in flame retardant efficiency. In Comparative Example 5, because there is no sodium-based montmorillonite, the expansion barrier layer is missing. Oxygen and heat can easily penetrate the buffer core layer, resulting in a decrease in LOI and a rating of V-1. Only Comparative Example 4, because the dynamic flame retardant monomer is unchanged (the flame retardant core is not destroyed), still maintains the V-0 rating. However, the poor interface bonding leads to uneven local flame retardancy, and the LOI is slightly lower than that of Example 2.

[0046] Example 2's high compression resilience relies on the synergistic microstructure of the buffer core layer: PCL elastic microspheres, due to their low glass transition temperature, possess excellent elastic deformation capabilities. Under stress, they can absorb impact energy (deformation energy) through molecular chain sliding, and recover their original shape after unloading via their own cross-linked network; the layered structure of sodium-based montmorillonite can disperse local stress, preventing irreversible crushing of PCL microspheres due to stress concentration; the plant fibers modified with dynamic flame-retardant monomers retain good fiber interweaving ability, tightly bonding PCL microspheres, montmorillonite, and water-based polyurethane adhesive to form a three-dimensional support system of "fiber-microsphere-clay," ensuring coordinated deformation of each component under stress. In Comparative Example 1, the acylhydrazone bond was changed to an amide bond, resulting in decreased interfacial flexibility and uncoordinated deformation transfer between the buffer core layer and the surface / bottom layer, leading to a slightly lower resilience. In Comparative Example 2, the absence of phosphorus did not affect the buffer structure (phosphorus only acts on flame retardancy), resulting in a smaller decrease in resilience. In Comparative Example 3, the absence of double bond crosslinking weakened the bonding force between the fiber and the PCL microspheres, making the microspheres prone to misalignment under stress, thus reducing the resilience. In Comparative Example 4, the absence of oxidized sodium alginate only affected interlayer bonding (not acting on the core layer buffer), resulting in a resilience close to that of Example 2. In Comparative Example 5, the absence of sodium-based montmorillonite prevented stress dispersion, causing the PCL microspheres to easily deform excessively or even rupture, resulting in a significant decrease in resilience.

[0047] The high self-healing efficiency of Example 2 stems from the triple microscopic repair mechanism of the self-healing interface adhesive: the acylhydrazone bonds (reversible bonds) in the dynamic flame-retardant monomer can undergo acylhydrazone exchange reactions with the aldehyde groups of oxidized sodium alginate under hot pressing or ultraviolet irradiation, recombining broken chemical bonds; the cinnamoyl double bonds undergo cycloaddition reactions under 365nm ultraviolet light, forming cross-linked structures to bridge interlayer cracks; the aldehyde groups of oxidized sodium alginate can also form hemiacetal / acetal bonds with the hydroxyl groups of plant fibers in the surface / bottom layer and buffer core layer, assisting in enhancing the interlayer bonding force after repair. The three work synergistically to achieve efficient crack repair; in Comparative Example 1, the acylhydrazone bonds are converted into amide bonds (irreversible), resulting in the loss of... The dynamic exchange pathway relies solely on a small number of double bonds for repair, resulting in a significant decrease in efficiency. Comparative Example 2, lacking phosphorus, does not affect the repair mechanism, but the weak bonding between the dynamic flame-retardant monomer and the fiber leads to micro-gaps at the crack after repair, resulting in slightly lower efficiency. Comparative Example 3, lacking double bonds, lacks a cyclization bridging pathway, and acylhydrazone exchange alone cannot completely repair the crack, leading to reduced efficiency. Comparative Example 4, lacking oxidized sodium alginate, loses both the acylhydrazone exchange and hemiacetal repair pathways, relying solely on a small number of unmigrated double bonds, resulting in a sharp drop in efficiency. Comparative Example 5, lacking montmorillonite, does not affect the interface repair (montmorillonite acts on the core layer), but the slightly poor flatness of the core layer reduces the repair contact area, resulting in a slight decrease in efficiency.

[0048] The high peel strength and tensile strength of Example 2 stem from the microscopic synergy of chemical crosslinking and physical interweaving: Regarding peel strength, the acylhydrazone bonds and phenolic hydroxyl groups of the dynamically flame-retardant monomer in the self-healing interface adhesive form a chemical bond with the aldehyde groups of oxidized sodium alginate. Simultaneously, the phosphate groups of the dynamically flame-retardant monomer form a synergistic adsorption with the hydroxyl groups of the surface / bottom fiber layers and the water-based polyurethane adhesive in the core layer, enhancing interlayer adhesion. Regarding tensile strength, the bamboo fiber (high tensile strength) in the surface paper crosslinks with the ester bonds and double bonds of the dynamically flame-retardant monomer through hydroxyl groups. Simultaneously, the physical interweaving force of the bamboo fiber and plant fiber superimposed with the chemical crosslinking force enhances the tensile strength of the fiber network. Comparative Example 1 shows the transformation of acylhydrazone bonds into amides. Due to the absence of phosphorus, the chemical bonding force at the interface weakens, resulting in a decrease in both peel strength and tensile strength. In Comparative Example 2, the absence of phosphorus reduces the esterification reaction between the dynamic flame retardant monomer and the fiber, lowering the degree of crosslinking and causing a decrease in both strengths. In Comparative Example 3, the absence of double bond crosslinking weakens the bonding force of the fiber network, reducing tensile strength, and the inability to form a three-dimensional crosslinked structure at the interface further reduces peel strength. In Comparative Example 4, the absence of oxidized sodium alginate results in a sharp drop in peel strength due to physical adhesion (without chemical bonding) at the interface, while tensile strength is slightly lower due to interlayer slippage. In Comparative Example 5, the absence of montmorillonite reduces the surface smoothness of the core layer, decreases the interlayer bonding area, reduces peel strength, and weakens the bonding force between the core layer and the surface layer, resulting in slightly lower tensile strength.

[0049] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A composite paperboard with flame-retardant and cushioning properties, characterized in that, It consists of a top layer paper, a cushioning core layer, and a bottom layer paper, which are bonded together from top to bottom with a self-healing interface adhesive. The surface paper and the bottom paper comprise the following raw materials in parts by weight: plant fiber: 90-110 parts, bamboo fiber: 5-8 parts, dynamic flame retardant monomer: 22-28 parts, photoinitiator: 1.2-1.4 parts, modified nano zinc oxide: 6-7 parts, and ethanol / water mixed solution: 220-240 parts. The buffer core layer comprises the following raw materials in parts by weight: 33-37 parts plant fiber, 7-13 parts dynamic flame retardant monomer, 0.8-1.1 parts photoinitiator, 4-5 parts modified nano zinc oxide, 16-18 parts PCL elastic microspheres, 9-11 parts organomontmorillonite, 10-14 parts waterborne polyurethane adhesive, and 80-100 parts ethanol / water mixed solution. The photoinitiator is a mixture of 2-hydroxy-2-methyl-1-phenyl-1-propanone and benzoin ether in a weight ratio of 3:

1. The self-healing interface adhesive is made by mixing dynamic flame retardant monomers and sodium alginate in a weight ratio of 3:1, and then adding deionized water to adjust the solid content to 30-40%. The chemical structural formula of the dynamic flame-retardant monomer is: 。 2. The composite paperboard with flame-retardant and cushioning properties according to claim 1, characterized in that, The modified nano zinc oxide refers to nano zinc oxide whose surface has been grafted with silane coupling agent KH-550.

3. The composite paperboard with flame-retardant and cushioning properties according to claim 1, characterized in that, The volume ratio of ethanol to water in the ethanol / water mixture is 1:

4.

4. The composite paperboard with flame-retardant and cushioning properties according to claim 1, characterized in that, The PCL elastic microspheres have a particle size of 20-40 μm; the organic montmorillonite refers to sodium-based montmorillonite.

5. The composite paperboard with flame-retardant and cushioning properties according to claim 1, characterized in that, The preparation method of the dynamic flame retardant monomer is as follows: (1) Add adipicohydrazide and ethanol to a reaction vessel equipped with a reflux condenser, stir at room temperature for 10-20 min, then add 2,3,4-trihydroxybenzaldehyde and heat to 75-85℃, react for 6-8 h, cool to room temperature, and a solid precipitates out. Collect the solid by filtration, recrystallize with an ethanol / water mixture to obtain intermediate A. (2) Under nitrogen protection, L-phosphotyrosine and triethylamine were added to anhydrous dichloromethane in a three-necked flask. The mixture was stirred and cooled to 0-5℃. Cinnamyl chloride was prepared into anhydrous dichloromethane solution with a concentration of 1mol / L and then added dropwise to the three-necked flask. During the dropwise addition, the reaction temperature was controlled not to exceed 5℃. After the dropwise addition was completed, the mixture was restored to room temperature and stirred for 6-8 hours. After washing three times with saturated saline, the organic phase was taken, dried with anhydrous sodium sulfate, and the solvent was removed by rotary evaporation. Then, the mixture was subjected to silica gel column chromatography with gradient elution to obtain intermediate B. (3) Under nitrogen protection, intermediate A and intermediate B were added to anhydrous dichloromethane and stirred for 10-20 min. Then, dicyclohexylcarbodiimide and 4-dimethylaminopyridine were added. The mixture was stirred at room temperature for 10-14 h. The mixture was filtered and the filtrate was collected. It was washed successively with 10% citric acid solution, saturated sodium bicarbonate solution and saturated sodium chloride solution. After drying with anhydrous sodium sulfate, the solvent was removed by rotary evaporation. Then, the mixture was recrystallized from N,N-dimethylformamide / water mixed solution to obtain the dynamic flame retardant monomer.

6. The composite paperboard with flame-retardant and cushioning properties according to claim 5, characterized in that, In (1), the molar ratio of adipamide dihydrazide and 2,3,4-trihydroxybenzaldehyde is 1:0.5-0.6, the weight ratio of adipamide dihydrazide and ethanol is 1:8-12, and the volume ratio of ethanol to water in the ethanol / water mixed solution is 1:

1.

7. The composite paperboard with flame-retardant and cushioning properties according to claim 5, characterized in that, In step (2), the molar ratio of L-phosphotyrosine, triethylamine, and cinnamoyl chloride is 1:(0.1-0.2):(1-1.2), and the weight ratio of L-phosphotyrosine and anhydrous dichloromethane is 1:8-12. Gradient elution refers to the volume ratio of ethyl acetate and petroleum ether in each eluent step from 0:1, 1:20, 1:10 to 1:

1.

8. The composite paperboard with flame-retardant and cushioning properties according to claim 5, characterized in that, In (3), intermediate A, intermediate B, dicyclohexylcarbodiimide and 4-dimethylaminopyridine are in a molar ratio of 1:(0.5-0.6):(1-1.2):(0.01-0.03), intermediate A and anhydrous dichloromethane are in a weight ratio of 1:8-12, and the volume ratio of N,N-dimethylformamide and water in the N,N-dimethylformamide / water mixed solution is 1:

2.

9. The method for preparing the composite paperboard with flame-retardant and cushioning properties according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Preparation of the surface paper: Plant fibers are added to an ethanol / water mixture and stirred for 10-20 minutes. Then, a photoinitiator is added, and the mixture is exposed to light at a wavelength of 365 nm and an intensity of 10-12 mW / cm². 2 Pre-irradiate with ultraviolet light for 20-25 minutes; then add dynamic flame retardant monomer and modified nano zinc oxide, heat to 60-65℃, and stir for 5-6 hours; then add bamboo fiber and pulp in a beater to a freeness of 51-53°SR; transfer to a fourdrinier paper machine, control the papermaking speed at 30-35 m / min to obtain a wet paper sheet; finally, irradiate with ultraviolet light at 105-110℃ with a wavelength of 365 nm and a light intensity of 15-18 mW / cm². 2 The surface paper is obtained by cross-linking, curing and drying under ultraviolet light for 30-35 minutes and then cooling to room temperature. S2. Preparation of the base paper: The preparation method of the base paper is the same as step S1; S3. Preparation of the buffer core layer: Add plant fibers to an ethanol / water mixture and stir for 10-20 minutes. Then add a photoinitiator and apply light at a wavelength of 365 nm and an intensity of 10-12 mW / cm². 2 The mixture is pre-irradiated with ultraviolet light for 20-25 minutes; then dynamic flame retardant monomers and modified nano zinc oxide are added, the temperature is raised to 60-65℃, and the mixture is stirred for 5-6 hours. After filtration, the mixture is washed with deionized water until the filtrate is neutral, then dried and mixed with PCL elastic microspheres, organomontmorillonite and waterborne polyurethane adhesive, and stirred for 20-25 minutes to obtain a mixture. The mixture is then put into a flat vulcanizing machine and pressed for 5-8 minutes at a pressure of 0.3-0.4MPa and a temperature of 75-80℃ to form the core layer. After forming, the core layer is transferred to a hot air drying oven and dried to obtain the buffer core layer. S4. Composite Molding: Mix the dynamic flame-retardant monomer and sodium alginate oxide, add deionized water to adjust the solid content to 30-40%, and obtain a self-healing interface adhesive. Using a doctor blade coating method, evenly coat the self-healing interface adhesive on the inner sides of the surface paper and the bottom paper, with a coating thickness of 0.2-0.4 mm. Place the buffer core layer between the surface paper and the bottom paper, ensuring the three layers are aligned without misalignment. Place the laminated three-layer structure into a hot press laminator and hot press for 15-25 minutes at a pressure of 0.5-0.6 MPa and a temperature of 75-85℃. After hot pressing, allow it to cool naturally to room temperature, and then apply a light irradiation at a wavelength of 365 nm and an intensity of 12-15 mW / cm². 2 Irradiate with ultraviolet light for 5-10 minutes to obtain composite paperboard with flame retardant and cushioning properties.

Images