Solvent-free adhesive for flexible package printing and preparation method thereof

Abstract

The invention belongs to the technical field of solvent-free adhesives, and discloses a solvent-free adhesive for flexible package printing and a preparation method thereof.The adhesive is composed of a component A and a component B. The component A comprises prepolymer polyisocyanate, a fluorine modified chain segment and a hydroxyl-containing reactive diluent; the component B comprises multifunctional polyether polyol, epoxy modified polyester polyol and an adhesion promoter with hydrogen bond donor / acceptor groups. Through a unique slow release mechanism, gradual release under the curing condition of 40-50 DEG C is ensured, insufficient leveling caused by too fast initial reaction is avoided, and the overall curing period is shortened to 12-24 hours. Meanwhile, the nano hybrid filler with the surface modified by isocyanate or epoxy groups can enhance chemical bonding with a polyurethane matrix, a compact three-dimensional cross-linked network is formed, the curing process is further accelerated, and the mechanical property is improved; the synergistic effect fundamentally solves the bottleneck of production efficiency.

Description

Technical Field

[0001] This invention belongs to the field of solvent-free adhesive technology, specifically a solvent-free adhesive for flexible packaging printing and its preparation method. Background Technology

[0002] Solvent-free lamination technology is one of the most widely used and mature lamination processes in the flexible packaging industry. This technology uses solvent-free adhesives, mostly two-component reactive polyurethanes, consisting of a component A (containing isocyanate groups) and a component B (containing hydroxyl and other active hydrogen groups) curing agent. The process involves using specialized solvent-free lamination equipment to mix the component and curing agent in precise proportions, then uniformly coating the first substrate layer using multiple heated coating rollers, and immediately laminating it with the second substrate layer using hot-press rollers. After winding, the composite film is placed in a curing chamber at a specific temperature, typically 40-50 degrees Celsius, to undergo a chemical cross-linking reaction, ultimately forming a strong bond. Compared to traditional dry lamination processes, solvent-free lamination contains no organic solvents, emits no volatile organic compounds, and offers faster production speeds, lower energy consumption, and controllable unit costs. These significant advantages have made it dominant in food, pharmaceutical, and daily chemical packaging production, and it is widely recognized as an environmentally friendly and efficient lamination solution.

[0003] Despite the significant advantages of solvent-free lamination technology, its inherent process sensitivity and initial performance defects have revealed a series of substantial problems in actual production. One of the most prominent bottlenecks is the low production efficiency caused by the excessively long curing cycle. The curing of solvent-free adhesives relies on the chemical addition reaction of polyurethane, which is very slow at room temperature. It requires 24 to 72 hours or even longer of heating and curing to achieve the final composite strength required for slitting or bag making. This long wait not only greatly extends the production cycle but also occupies a large amount of storage space, leading to inventory backlog and decreased capital turnover. Furthermore, it makes it impossible to assess product quality in a timely manner, resulting in huge losses if problems arise. Another key issue is the low initial adhesion of the adhesive. The peel strength between the two substrate layers of the newly produced composite film is extremely low, and they can almost easily separate. This low initial tack, when tension control is improper or the internal stress of the substrate is high, easily leads to quality defects such as tunneling, wrinkles, and edge curling, which is particularly severe when processing rigid materials such as aluminum foil or aluminized materials. At the same time, the control window of this process is extremely narrow. The final performance of adhesives is highly sensitive to parameters such as the mixing ratio of the main agent and the curing agent, the coating temperature, the lamination pressure, the surface tension of the substrate, and even the ambient temperature and humidity. Any slight fluctuation may cause defects such as white spots, crystal spots, orange peel texture, and uneven coating, which poses a severe challenge to the skills of operators and the stability of the production environment.

[0004] Beyond the challenges of the manufacturing process, existing solvent-free adhesives also present deeper risks regarding the final product performance and chemical safety. Firstly, there is the food safety risk posed by chemical migration. Because the stoichiometry of the two-component reaction is difficult to achieve with absolute precision, and the reaction process is easily disrupted and cannot proceed completely, unreacted monomers, such as isocyanates, may remain in the cured adhesive layer. More seriously, these residual monomers can hydrolyze under certain conditions, generating primary aromatic amines, which have potential carcinogenic risks. These small molecules, when in prolonged contact with the packaged contents, may migrate and permeate into the food, posing a serious threat to consumer health and exposing flexible packaging companies to strict regulatory oversight and brand reputation risks. Secondly, the products lack adaptability to specific applications. Traditional general-purpose solvent-free adhesives have limited heat and media resistance, making it difficult to meet the growing demand for high-performance packaging, such as packaging meat products that need to withstand high-temperature cooking exceeding 121 degrees Celsius, or seasonings used to package acidic or oily contents. Under these harsh conditions, the adhesive layer is prone to hydrolysis, swelling, or strength reduction, which in turn leads to packaging delamination and reduced barrier properties, making it impossible to effectively guarantee the shelf life and safety of the contents. Summary of the Invention

[0005] The purpose of this invention is to provide a solvent-free adhesive for flexible packaging printing and its preparation method, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a solvent-free adhesive for flexible packaging printing, wherein the adhesive is composed of component A and component B: Component A includes prepolymer polyisocyanate, fluorinated segments, and hydroxyl-containing reactive diluent; Component B includes multifunctional polyether polyols, epoxy-modified polyester polyols, and adhesion promoters with hydrogen bond donor / acceptor groups. The A / B components, after being mixed, form a three-dimensional cross-linked network through the synergistic reaction of nano-hybrid fillers and slow-release catalysts, thereby achieving rapid curing and high initial adhesion.

[0007] Component A is a key component of the adhesive system, and its core component includes prepolymer polyisocyanate, which is the main source of crosslinking reactivity. To impart excellent heat resistance and media resistance to the final composite film, fluorine-modified segments are specifically introduced into Component A. These segments can be fluorinated end-group isocyanates or fluorinated polyether polyols, with the addition amount controlled at 2-10% of the total mass of Component A. The introduction of fluorine atoms significantly reduces the surface energy of the polyurethane segments, thereby improving the composite material's resistance to high temperatures and chemical corrosion.

[0008] Component B provides the polyol components required for adhesive curing and introduces special adhesion promoters. Among them, multifunctional polyether polyols and epoxy-modified polyester polyols form the main body of the three-dimensional cross-linked network. The special feature of the epoxy-modified polyester polyol is that it can further cross-link with the isocyanate in component A during subsequent curing, thereby improving the density and migration resistance of the adhesive layer. This means that the cured adhesive layer is more compact and less prone to the migration of small molecules.

[0009] As a further technical solution of the present invention, the fluorinated modified segment is a fluorinated terminal isocyanate or a fluorinated polyether polyol, and its introduction amount is 2-10% of the total mass of component A, which is used to improve the heat resistance and media resistance of the composite film.

[0010] As a further technical solution of the present invention, the active diluent is a low molecular weight polyol or carbonate compound containing hydroxyl groups, with a molecular weight of 100-500 g / mol, used to adjust the viscosity of the system and provide a fast reaction site.

[0011] As a further technical solution of the present invention, the adhesion promoter is selected from compounds containing amide groups, urea groups or phenolic hydroxyl groups, so as to form reversible forces with the substrate surface through hydrogen bonds in the early stage of composite bonding, thereby significantly improving the initial tack.

[0012] As a further technical solution of the present invention, the nano-hybrid filler is selected from nano-silica, graphene oxide or organomontmorillonite, and its surface is modified by isocyanate or epoxy groups to enhance the chemical bonding with the polyurethane matrix and improve the coating uniformity and mechanical properties.

[0013] As a further technical solution of the present invention, the slow-release catalyst is composed of a metal-organic catalyst and a microporous support, which can achieve gradual release under the curing conditions of 40-50℃, avoiding insufficient leveling caused by the initial reaction being too fast, and shortening the overall curing cycle to 12-24 hours.

[0014] A method for preparing a solvent-free adhesive for flexible packaging printing includes the following steps: 1) Preparation of component A: Polyisocyanate and fluorinated segments are reacted under an inert atmosphere to obtain a prepolymer, and then an active diluent and surface-modified nanofiller are added and uniformly dispersed. 2) Preparation of component B: Mix polyether polyol with epoxy-modified polyester polyol, and add adhesion promoter and stabilizer; 3) Pre-disperse the slow-release catalyst in component B; 4) Use solventless lamination equipment to dynamically mix component A and component B at a mass ratio of 100:(80-120), coat and laminate, then roll up and cure at 40-50℃ for 12-24 hours.

[0015] As a further technical solution of the present invention, the nanofiller in step 1 is treated by a dual process of ultrasonic dispersion and high shear mixing to ensure that it presents a stable dispersion state in component A.

[0016] As a further technical solution of the present invention, the epoxy-modified polyester polyol in step 2 further crosslinks with isocyanate during the curing process, thereby improving the density and migration resistance of the adhesive layer.

[0017] As a further technical solution of the present invention, the final composite film has a peel strength of not less than 1 N / 15 mm within 2 hours after being removed from the machine, and a peel strength of ≥3 N / 15 mm after 24 hours of curing. It can also withstand boiling at 121°C for 30 minutes without delamination or loss of adhesion.

[0018] The beneficial effects of this invention are as follows: 1. This invention significantly shortens the curing time of adhesives and greatly improves production efficiency through the synergistic effect of nano-hybrid fillers and slow-release catalysts. Traditional solvent-free adhesives require 24-72 hours of heating and curing to reach their final strength, which greatly prolongs the production cycle, leading to inventory backlog and reduced capital turnover. The slow-release catalyst in this invention is a composite of a metal-organic catalyst and a microporous carrier. Its unique slow-release mechanism ensures gradual release under curing conditions of 40-50℃, avoiding insufficient leveling caused by excessively rapid initial reaction and shortening the overall curing cycle to 12-24 hours. Simultaneously, the nano-hybrid fillers, modified with isocyanate or epoxy groups, enhance the chemical bonding with the polyurethane matrix, forming a dense three-dimensional cross-linked network, further accelerating the curing process and improving mechanical properties. This synergistic effect fundamentally solves the production efficiency bottleneck, enabling companies to respond to market demands more quickly and reduce operating costs.

[0019] 2. This invention addresses the problem of low initial tack in traditional solvent-free adhesives, which easily leads to quality defects such as "tunneling effect," wrinkles, and edge lifting. The invention introduces an adhesion promoter with hydrogen-bonding donor / acceptor groups into component B. This adhesion promoter can form reversible forces with the substrate surface through hydrogen bonds in the early stages of lamination, thereby significantly improving initial tack. According to the embodiments, the final composite film has a peel strength of no less than 1 N / 15 mm within 2 hours of production, effectively solving various quality problems caused by the low initial tack of traditional adhesives. Its effect is particularly significant when producing rigid composite structures such as aluminum foil or aluminized metallized films. This high initial tack provides a stable foundation for the subsequent curing process, ensuring the appearance quality and production yield of the composite film.

[0020] 3. This invention comprehensively enhances the overall performance of the composite film through the synergistic effect of multiple components, while simultaneously addressing food safety hazards caused by chemical migration. Firstly, the introduction of fluorine-modified segments into component A effectively improves the heat resistance and media resistance of the composite film. This allows the final product to withstand boiling at 121°C for 30 minutes without delamination or loss of adhesion, meeting the demands of high-performance packaging. Secondly, the epoxy-modified polyester polyol in component B can further crosslink with isocyanate during the curing process, improving the density and migration resistance of the adhesive layer. Furthermore, the rapid curing and high crosslinking density of this invention fundamentally reduce the residue of unreacted monomers, resulting in an isocyanate residue in the final composite film below 0.5 mg / kg. This not only significantly reduces the food safety risks caused by chemical migration but also ensures that the adhesive of this invention meets stringent regulatory requirements while protecting consumer health. Attached Figure Description

[0021] Figure 1 This is a flowchart of the preparation method of the present invention; Figure 2 This is a schematic diagram showing the composition ratio of the solvent-free adhesive components of the present invention; Figure 3 A schematic diagram illustrating the contribution ratio of the synergistic reaction in this invention; Figure 4 This is a line graph showing the performance test results of the composite membrane of this invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] like Figures 1 to 4 As shown in the embodiment of the present invention, a solvent-free adhesive for flexible packaging printing is composed of component A and component B: Component A includes prepolymer polyisocyanate, fluorinated segments, and hydroxyl-containing reactive diluent; Component B includes multifunctional polyether polyols, epoxy-modified polyester polyols, and adhesion promoters with hydrogen bond donor / acceptor groups. The A / B components, after being mixed, form a three-dimensional cross-linked network through the synergistic reaction of nano-hybrid fillers and slow-release catalysts, thereby achieving rapid curing and high initial adhesion.

[0024] Component A is a key component of the adhesive system, and its core component includes prepolymer polyisocyanate, which is the main source of crosslinking reactivity. To impart excellent heat resistance and media resistance to the final composite film, fluorine-modified segments are specifically introduced into Component A. These segments can be fluorinated end-group isocyanates or fluorinated polyether polyols, with the addition amount controlled at 2-10% of the total mass of Component A. The introduction of fluorine atoms significantly reduces the surface energy of the polyurethane segments, thereby improving the composite material's resistance to high temperatures and chemical corrosion.

[0025] To effectively adjust the viscosity of the system and provide rapid reaction sites, component A also includes a hydroxyl-containing reactive diluent. These diluents are typically low-molecular-weight polyols or carbonate compounds with a molecular weight ranging from 100 to 500 g / mol. They not only dilute the high-viscosity prepolymer, reducing the difficulty of application, but the hydroxyl groups on their molecular chains also participate in the curing reaction of the isocyanate, accelerating the curing speed and ensuring high initial adhesion.

[0026] During the preparation of component A, the introduction of surface-modified nano-hybrid fillers, including nano-silica, graphene oxide, or organomontmorillonite, undergoes surface modification treatment with isocyanates or epoxy groups, enabling stronger chemical bonding with the polyurethane matrix. Through a dual process of ultrasonic dispersion and high-shear mixing, the nano-fillers are ensured to form a stable dispersion in component A. This uniformly dispersed nano-filler not only acts as a nano-hybrid filler, but also synergizes with the slow-release catalyst during subsequent curing to form a dense three-dimensional cross-linked network, significantly improving the mechanical properties and coating uniformity of the adhesive layer. Furthermore, it can fill microscopic defects on the substrate surface to a certain extent, further enhancing adhesive strength.

[0027] Component B provides the polyol components required for adhesive curing and introduces special adhesion promoters. Among them, multifunctional polyether polyols and epoxy-modified polyester polyols form the main body of the three-dimensional cross-linked network. The special feature of the epoxy-modified polyester polyol is that it can further cross-link with the isocyanate in component A during subsequent curing, thereby improving the density and migration resistance of the adhesive layer. This means that the cured adhesive layer is more compact and less prone to the migration of small molecules.

[0028] To achieve significant adhesion in the early stages of bonding, an adhesion promoter with hydrogen bond donor / acceptor groups is added to component B. These promoters are typically compounds containing amide, urea, or phenolic hydroxyl groups. They can form reversible forces between the adhesive and the substrate surface through hydrogen bonds. This reversible force enables rapid adhesion establishment within a short time, known as initial tack, thus providing sufficient strength for subsequent processing and use even before the adhesive is fully cured. This short-term action mechanism based on hydrogen bonds effectively solves the problem of insufficient initial tack in traditional adhesives.

[0029] Components A and B are dynamically mixed in a solventless lamination device at a mass ratio of 100:(80-120). After mixing, the adhesive is applied to the substrate and lamination is performed. At this point, the nano-hybrid filler and the slow-release catalyst begin to exert their synergistic effect. The slow-release catalyst is composed of a metal-organic catalyst and a microporous support, and its ingenious feature is its ability to gradually release the catalyst. This slow-release mechanism ensures that the reaction is not too vigorous in the initial stage under curing conditions of 40-50℃, avoiding insufficient leveling due to excessively rapid reaction and ensuring a uniform and smooth adhesive layer. At the same time, it ensures that moderate catalytic activity is maintained throughout the curing cycle, thereby shortening the overall curing cycle to 12-24 hours and improving production efficiency.

[0030] The fluorinated modified segment is a fluorinated terminal isocyanate or a fluorinated polyether polyol, and its introduction amount is 2-10% of the total mass of component A, which is used to improve the heat resistance and media resistance of the composite film.

[0031] The reactive diluent is a low-molecular-weight polyol or carbonate compound containing hydroxyl groups, with a molecular weight of 100-500 g / mol, used to adjust the viscosity of the system and provide a fast reaction site.

[0032] The adhesion promoter is selected from compounds containing amide, urea, or phenolic hydroxyl groups, which form reversible forces with the substrate surface through hydrogen bonds in the early stage of composite bonding, thereby significantly improving initial tack.

[0033] The nano-hybrid filler is selected from nano-silica, graphene oxide or organomontmorillonite, and its surface is modified with isocyanate or epoxy groups to enhance the chemical bonding with the polyurethane matrix and improve coating uniformity and mechanical properties.

[0034] Among them, the slow-release catalyst is composed of a metal-organic catalyst and a microporous support. It can be gradually released under the curing conditions of 40-50℃, avoiding insufficient leveling caused by the initial reaction being too fast, and shortening the overall curing cycle to 12-24 hours.

[0035] A method for preparing a solvent-free adhesive for flexible packaging printing includes the following steps: 1) Preparation of component A: Polyisocyanate and fluorinated segments are reacted under an inert atmosphere to obtain a prepolymer, and then an active diluent and surface-modified nanofiller are added and uniformly dispersed. 2) Preparation of component B: Mix polyether polyol with epoxy-modified polyester polyol, and add adhesion promoter and stabilizer; 3) Pre-disperse the slow-release catalyst in component B; 4) Use solventless lamination equipment to dynamically mix component A and component B at a mass ratio of 100:(80-120), coat and laminate, then roll up and cure at 40-50℃ for 12-24 hours.

[0036] The core of component A lies in the combination of prepolymer polyisocyanate and fluorinated modified segments. The fluorinated modified segments, comprising 2-10% of the total mass of component A, such as fluorinated end-group isocyanates or fluorinated polyether polyols, significantly enhance the heat resistance and media resistance of the composite film by introducing fluorine atoms, ensuring stability even in harsh environments. Simultaneously, the added hydroxyl-containing reactive diluents, with a molecular weight of 100-500 g / mol, such as low-molecular-weight polyols or carbonate compounds, not only effectively adjust the system viscosity for easy coating but also provide abundant rapid reaction sites, laying the foundation for rapid curing of the adhesive layer. Component B integrates multifunctional polyether polyols and epoxy-modified polyester polyols. The latter can further crosslink with isocyanates during curing, significantly improving the density and migration resistance of the adhesive layer, ensuring the safety of the packaged contents.

[0037] Component B incorporates adhesion promoters with hydrogen bond donor / acceptor groups, including compounds containing amide, urea, or phenolic hydroxyl groups. These compounds can form reversible forces with the substrate surface through hydrogen bonds in the early stages of lamination, greatly improving initial tack and ensuring rapid subsequent production.

[0038] In step 1, the nanofiller is treated with a dual process of ultrasonic dispersion and high-shear mixing to ensure that it is stably dispersed in component A.

[0039] In step 2, the epoxy-modified polyester polyol further crosslinks with isocyanate during the curing process, improving the density and migration resistance of the adhesive layer.

[0040] The final composite film has a peel strength of no less than 1 N / 15 mm within 2 hours of being removed from the machine, and a peel strength of ≥3 N / 15 mm after 24 hours of curing. It can also withstand boiling at 121℃ for 30 minutes without delamination or loss of adhesion.

[0041] Example 1: Preparation of the solvent-free composite adhesive of the present invention 1. Preparation of Component A 100 parts of HDI trimer, NCO content 23% trimer isocyanate, were stirred at 60°C under nitrogen protection. Add 8 parts of fluorinated modified polyether polyol with Mn≈800 and fluorine mass fraction of 8%, and react for 2 hours to obtain the prepolymer; Add 10 parts of low molecular weight reactive diluent and 1,4-butanediol carbonate with a molecular weight of approximately 200 to the prepolymer; Three parts of nano-silica with isocyanate functionalized surface were added, and the mixture was uniformly dispersed by ultrasonic dispersion and high shear mixing. The high shear mixing parameters were 6000 rpm and 10 min to obtain component A.

[0042] 2. Preparation of Component B Mix 80 parts of polyether polyol with Mn≈2000 (number average molecular weight of 2000) with 20 parts of epoxy modified polyester polyol; Add 2 parts of an adhesion promoter containing an amide group, such as N-hydroxyethylacrylamide; Add 0.3 parts of antioxidant stabilizer; Add 1 part of dibutyltin dilaurate slow-release catalyst supported on porous silica, stir evenly, and obtain component B.

[0043] The number of portions mentioned refers to the number of parts by weight.

[0044] 3. Compound operation Components A and B are dynamically mixed in a solventless compounding machine at a mass ratio of 100:95. The coating amount is 2.5 g / m²; The composite structure is PET12 / Al7 / CPP60; After winding, it is cured at 45℃.

[0045] Example 2: Comparative Experiment To verify the effectiveness, Example 1 was compared with a traditional commercial two-component solvent-free polyurethane adhesive (free of fluorinated segments, diluents, and nanofillers). The results are shown in the table below:

[0046] Example 3: Applicability to different substrates Two common structures, PET / PA / CPP and PET / Al / PE, were selected for composite testing, and the composite conditions were the same as in Example 1. The results are as follows: PET / PA / CPP: Peel strength 1.0 N / 15mm after 2 hours off the machine, 3.2 N / 15mm after 24 hours of curing, with no edge curling or tunneling effect.

[0047] PET / Al / PE: Strength 1.5 N / 15mm after 2 hours of drying, strength 3.8 N / 15mm after 24 hours of curing, no delamination after cooking at 121℃.

[0048] Example 4: Validation of Catalyst Release Rate Component B from Example 1 was placed at a constant temperature of 45°C, and the reaction rate was monitored by conductivity. The results showed that the catalyst release rate was low in the first 4 hours, and the system was well leveled; the catalytic activity gradually increased from 4 to 12 hours, and the reaction rate accelerated; cross-linking was basically completed within 24 hours. This demonstrates that the slow-release catalyst effectively shortens the curing period and avoids surface defects.

[0049] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A solvent-free adhesive for flexible packaging printing, characterized in that: The adhesive is composed of component A and component B: Component A includes prepolymer polyisocyanate, fluorinated segments, and hydroxyl-containing reactive diluent; Component B includes multifunctional polyether polyols, epoxy-modified polyester polyols, and adhesion promoters with hydrogen bond donor / acceptor groups. The A / B components, after being mixed, form a three-dimensional cross-linked network through the synergistic reaction of nano-hybrid fillers and slow-release catalysts, thereby achieving rapid curing and high initial adhesion.

2. The solvent-free adhesive for flexible packaging printing according to claim 1, characterized in that: The fluorinated modified segment is a fluorinated terminal isocyanate or a fluorinated polyether polyol, and its introduction amount is 2-10% of the total mass of component A, used to improve the heat resistance and media resistance of the composite film.

3. The solvent-free adhesive for flexible packaging printing according to claim 1, characterized in that: The active diluent is a low-molecular-weight polyol or carbonate compound containing hydroxyl groups, with a molecular weight of 100-500 g / mol, used to adjust the viscosity of the system and provide a fast reaction site.

4. The solvent-free adhesive for flexible packaging printing according to claim 1, characterized in that: The adhesion promoter is selected from compounds containing amide, urea, or phenolic hydroxyl groups, which form reversible forces with the substrate surface through hydrogen bonds in the early stage of composite bonding, thereby significantly improving initial tack.

5. The solvent-free adhesive for flexible packaging printing according to claim 1, characterized in that: The nano-hybrid filler is selected from nano-silica, graphene oxide, or organomontmorillonite, and its surface is modified with isocyanate or epoxy groups to enhance the chemical bonding with the polyurethane matrix and improve coating uniformity and mechanical properties.

6. The solvent-free adhesive for flexible packaging printing according to claim 1, characterized in that: The slow-release catalyst is composed of a metal-organic catalyst and a microporous support. It can be gradually released under aging conditions of 40-50℃, avoiding insufficient leveling caused by excessively rapid initial reaction, and shortening the overall aging period to 12-24 hours.

7. The solvent-free adhesive for flexible packaging printing according to any one of claims 1-6, characterized in that: Its preparation method includes the following steps: 1) Preparation of component A: Polyisocyanate and fluorinated segments are reacted under an inert atmosphere to obtain a prepolymer, and then an active diluent and surface-modified nanofiller are added and uniformly dispersed. 2) Preparation of component B: Mix polyether polyol with epoxy-modified polyester polyol, and add adhesion promoter and stabilizer; 3) Pre-disperse the slow-release catalyst in component B; 4) Use solventless lamination equipment to dynamically mix component A and component B at a mass ratio of 100:(80-120), coat and laminate, then roll up and cure at 40-50℃ for 12-24 hours.

8. The solvent-free adhesive for flexible packaging printing according to claim 7, characterized in that: The nanofiller in step 1 is treated with a dual process of ultrasonic dispersion and high-shear mixing to ensure that it exhibits a stable dispersion state in component A.

9. The solvent-free adhesive for flexible packaging printing according to claim 7, characterized in that: In step 2, the epoxy-modified polyester polyol further crosslinks with isocyanate during the curing process, improving the density and migration resistance of the adhesive layer.

10. The solvent-free adhesive for flexible packaging printing according to claim 7, characterized in that: The final composite film has a peel strength of no less than 1 N / 15 mm within 2 hours of being removed from the machine, and a peel strength of ≥3 N / 15 mm after 24 hours of curing. It can also withstand boiling at 121℃ for 30 minutes without delamination or loss of adhesion.

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