Two-component solventless polyurethane adhesive

Abstract

This invention relates to the field of packaging materials technology, and particularly to a two-component solvent-free polyurethane adhesive. Traditional "ethyl maltol migration resistance" solutions mostly rely on physical barriers, lacking a solution to the problem of ethyl maltol precipitation at the molecular interaction level. The two-component solvent-free polyurethane adhesive of this invention forms a dual network structure of covalent crosslinking and metal-coordinated crosslinking after the A / B components react, inhibiting the diffusion of small-molecule ethyl maltol at the molecular structure level. After the A / B components are mixed and cured, without the presence of primary aromatic amines, the migration risk of ethyl maltol can be significantly reduced, while also exhibiting good adhesive strength and stability, making it suitable for solvent-free lamination of food flexible packaging materials.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, and in particular to a two-component solvent-free polyurethane adhesive. Background Technology

[0002] Two-component solvent-free polyurethane adhesives are widely used in composite packaging due to their advantages such as high solids content, convenient construction, good environmental performance, high bond strength, and adjustable curing speed. Component A is typically an isocyanate prepolymer, and component B is a hydroxyl-containing polyol. In the food packaging field, adhesives must possess good migration resistance, contamination resistance, and excellent barrier properties to ensure that harmful low-molecular-weight substances are not released into packaged food. Ethyl maltol, a commonly used food additive, is widely found in fruits, beverages, milk tea, meat products, and baked goods. It can reduce the inherent bitterness and fishy smell of products and enhance their freshness and aroma. However, in traditional polyurethane adhesive systems, ethyl maltol is highly susceptible to contamination through the following pathways: complexation or side reactions with primary aromatic amines, hydrogen bond adsorption with "-NH-CO-" in PU segments, adsorption by polar soft segments, and migration along the polymer free volume. Therefore, when using solvent-free polyurethane adhesives to laminate different substrates to make bags, there is a high risk of reduced peel strength or even interlayer separation during the storage and packaging of contents containing ethyl maltol.

[0003] Furthermore, the A component of traditional two-component adhesives commonly contains aromatic isocyanates such as MDI and TDI. These are not only highly susceptible to forming complexes or reaction byproducts with ethyl maltol, but their reaction products with water vapor during post-curing (primary aromatic amines) can also be harmful to human health, leading to problems such as excessive levels of primary aromatic amines, ethyl maltol leaching, and unpleasant odors in the composite film. Therefore, existing adhesives can no longer meet the requirements for products containing ethyl maltol without the release of primary aromatic amines. Summary of the Invention

[0004] This invention addresses the issue that traditional "ethyl maltol migration resistance" solutions mostly rely on physical barriers and lack a solution to the ethyl maltol precipitation problem at the molecular interaction level, by providing a two-component solvent-free polyurethane adhesive.

[0005] This invention belongs to the technical field of composite adhesives for flexible food packaging. The adhesive of this invention consists of component A and component B. Component A is an alicyclic isocyanate prepolymer, and component B contains a low-adsorption polyester polyol, which is mainly a compound composed of functional monomers containing β-diketone structures that form stable coordination structures with metal ions. After the A / B components are mixed and cured, a dual network structure is formed in the adhesive layer, consisting of a polyurethane covalent crosslinking network and a metal ion coordination network. This coordination structure can compete for coordination with ethyl maltol or occupy potential adsorption sites, inhibiting the adsorption and migration of ethyl maltol in the adhesive layer at the molecular interaction level. Compared with the prior art, this invention, without the presence of primary aromatic amines, can significantly reduce the migration risk of ethyl maltol while also possessing good adhesive strength and stability, making it suitable for solvent-free composite applications in flexible food packaging materials.

[0006] This invention primarily employs a two-component solvent-free polyurethane adhesive, wherein component A is an isocyanate prepolymer system free of primary aromatic amines, and component B is a mixture system composed of a special low-adsorption polyester polyol and additives. It mainly includes the following aspects: Step 1: Preparation of Component A (Prepolymer without Primary Aromatic Amine Isocyanates) Aliphatic isocyanate was added to a dry reactor and then degassed under reduced pressure at 40–60 °C for 30 min. The temperature was then raised to 60–85 °C, and a low-polarity polyol was added with stirring. The reaction was maintained under nitrogen-sealed conditions for 2–4 h to obtain an isocyanate prepolymer with an NCO content of 6–18 wt%. The system was then cooled to 45–65 °C, and 0.1–1 wt% of an alicyclic amine or secondary amine regulator was added to adjust the reaction rate of subsequent components and component B. After stirring for 0.5–1 h, the NCO content and viscosity were measured, and the mixture was sealed and stored under nitrogen.

[0007] Furthermore, the aforementioned aliphatic isocyanates are HDI (hexamethylene diisocyanate), HDI trimer, IPDI (isophorone diisocyanate), and H... 12 One or more of MDI (hydrogenated MDI) and CHDI (1,4-cyclohexane diisocyanate).

[0008] Furthermore, the aforementioned low-polarity polyols are aliphatic polyester polyols, low-polarity polycarbonate diols, and polyether diols containing cyclic aliphatic structures, specifically including one or more of the following: poly(ethylene adipate), poly(hexanediol adipate), poly(neopentyl adipate), poly(1,4-butanediol adipate), polycarbonate diol type polyurethane (PCDL), neopentyl glycol type polycarbonate diol, and cyclohexanediethanol type polyester diol.

[0009] Furthermore, the aforementioned alicyclic amine or secondary amine modifier is one of isophorone diamine (IPDA), CHDA (1,4-cyclohexanediamine), or N,N-dimethylethanolamine.

[0010] Step 2: Preparation of Component B: Step S1: Preparation of low-adsorption polyester polyol A diacid, a diol, and trimethylolpropane (TMP) were added to a reaction vessel and stirred until homogeneous under nitrogen protection. The mixture was heated to 160 °C for a pre-esterification reaction for 1.5 h. Subsequently, a catalyst was added, and the temperature was slowly increased to 220 °C while continuously removing the water generated during the reaction. Heating was stopped when the acid value of the system dropped to ≤ 1.0 mgKOH / g. The temperature was then lowered to 185 °C, and the hydroxyl value was adjusted by adding a small amount of 2-methyl-1,3-propanediol. Furthermore, in molar ratio, the aforementioned dicarboxylic acids include: neopentanoic acid (40 mol%), adipic acid (40 mol%), and sebacic acid (20 mol%); in molar ratio, the aforementioned diols include: 1,4-cyclohexanediol (45 mol%), 2-methyl-1,3-propanediol (40 mol%), and 1,6-hexanediol (13 mol%), and the amount of trimethylolpropane added is 2 mol%. The selected catalyst is tetrabutyl titanate, accounting for 0.03 wt% of the total raw material mass.

[0011] The above-mentioned polyester polyol intermediate was cooled to 110 °C, and ethyl acetoacetate modified monomer was added in an amount of 2.0 wt% of the polyester polyol mass. The reaction was then maintained at this temperature for 2 h, allowing the β-diketone structure to be introduced into the polyester side chain via transesterification. In the system, the β-diketone structure binds to the polyol backbone through transesterification, forming uniformly distributed multidentate coordination sites. The temperature was then further reduced to 80 °C, and a metal ion salt was added with stirring in an amount of 0.5 wt% of the polyester polyol mass. The reaction was maintained at this temperature for 2 h, allowing the metal ion to fully form a stable coordination complex with the β-diketone ligand. The product was then heated to 100 °C and degassed under vacuum for 1 h. The solution was filtered through a 100 μm filter to obtain a colorless and transparent low-adsorption polyester polyol solution. The hydroxyl value and room temperature viscosity of the product were tested.

[0012] Furthermore, the aforementioned ethyl acetoacetate modified monomer refers to a compound whose molecule simultaneously contains an acetoacetic acid structural unit (β-diketone structure) and at least one functional group that can chemically react with polyester polyols. Specifically, it can be either ethyl acetoacetate or neopentyl glycol diacetate.

[0013] Furthermore, the aforementioned metal ion salt can be either zinc acetylacetonate Zn(C5H7O2)2 or aluminum acetylacetonate Al(C5H7O2)3.

[0014] Step S2: Compatibility of Component B The low-adsorption polyester polyol and additives obtained above are mixed and stirred evenly at a mass ratio of 120-130:1 to obtain component B.

[0015] Furthermore, the aforementioned additives include silicone leveling agents from Shanghai Hesheng; defoamers from Shanghai Hesheng; and curing speed regulators: organic bismuth catalysts from Shanghai Ruicai Chemical Technology Co., Ltd.

[0016] Step 3: Composite substrate The A / B components prepared above were mixed and stirred evenly at a ratio of R value of 1.1 to 1.5:1. Then, a solvent-free laminating machine was used to laminate a two-layer structure of NY / / RCPP or a three-layer structure of PET / / Al / / RCPP. After lamination, the laminated material was placed in a curing chamber for curing to obtain a film substrate. Subsequently, the film substrate was made into bags and filled with an ethyl maltol aqueous solution with a mass concentration of 0.02%, sealed, and boiled. The peel strength of the composite film bags was then tested.

[0017] Compared with the prior art, the present invention has the following advantages: 1) This invention relates to a solvent-free two-component polyurethane adhesive resistant to ethyl maltol. It introduces fixed coordination sites into a polyester-based polyol to form a metal-ligand complex. By competing with the reaction sites of ethyl maltol through metal ions, the migration of ethyl maltol is reduced. Through the construction of a dual network structure—a covalent cross-linked network formed after the reaction of adhesive components A and B, and a metal-coordinated "cross-linked" network—the diffusion of small-molecule ethyl maltol is inhibited at the molecular structural level.

[0018] 2) This invention introduces a fixed metal ion coordination structure into the polyurethane adhesive system, preventing ethyl maltol from reacting with the active sites in the adhesive layer, thereby reducing its migration to the adhesive surface. Furthermore, the metal ions are fixed within the polymer network through a multidentate ligand structure, preventing them from existing as free ions and avoiding metal ion migration, precipitation, or adverse effects on adhesive layer performance, thus ensuring the stability and safety of the adhesive during long-term use.

[0019] 3) The two-component adhesive of the present invention has both a polyurethane covalent crosslinking network and a metal ion coordination network after curing. The resulting dual network structure improves the density and molecular constraint of the adhesive layer without significantly increasing brittleness, ensuring excellent bonding strength and effectively inhibiting the diffusion of small molecules such as ethyl maltol.

[0020] 4) The A / B components of this invention avoid the use of aromatic isocyanates in both raw material selection and reaction design, fundamentally eliminating the risk of complexation or side reactions between ethyl maltol and aromatic amines, effectively reducing the potential for the formation and migration of primary aromatic amines, and meeting the safety requirements for food contact materials. Detailed Implementation

[0021] The following examples are provided to further illustrate the present invention and are intended to explain the invention, not to limit its scope. Unless otherwise specified, all figures are expressed in parts by weight and weight percentages.

[0022] Unless otherwise specified, the raw materials used in this invention are all conventional commercially available products; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0023] The embodiments of the present invention will be further described below with reference to several examples.

[0024] It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0025] The terminology used in the embodiments of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of this invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0026] Performance testing: NCO content determination: NCO groups can react with excess di-n-butylamine to form urea. The excess di-n-butylamine is titrated with hydrochloric acid solution using bromocresol green as an indicator. The amount of di-n-butylamine consumed by the NCO groups is calculated, and then the percentage content of NCO groups in the test sample is calculated.

[0027] Composite performance testing: After the two-component adhesives prepared in each example were stirred evenly according to a fixed ratio, they were laminated into a two-layer structure of NY / / RCPP or a three-layer structure of PET / / NY / / RCPP using a solventless laminating machine. The adhesive application rate was 1.8–2.2 g / m³. 2 After lamination, the material is placed in a curing chamber at 40–50 °C and cured for 48–60 h. The peel strength between different layers is then measured. The specific test method is consistent with GB / T2791-1995.

[0028] Media resistance test: The composite film was made into food packaging bags, which were then filled with food containing ethyl maltol or a solution containing ethyl maltol. After packaging, the bags were placed in a retort (100 ℃, 60 min) for high-temperature cooking. Afterward, the bags were rinsed with cold water, and the appearance was observed and the peel strength between different layers was tested.

[0029] Example 1: Step S1: Preparation of Component A 1.68 kg of HDI and 1.68 kg of CHDI were added to a dry reactor, followed by degassing under reduced pressure at 50 °C for 30 min. The temperature was then raised to 75 °C, and 7.8 kg of poly(2000) adipate-neopentyl glycol (molecular weight 2000) was added with stirring. The reaction was then maintained under nitrogen-sealed conditions for 3 h. The system was cooled to 55 °C, and 0.1 wt% of isophorone diamine was added to adjust the reaction rate with component B. After stirring for 0.5–1 h, the NCO content was measured to be 13.22%, and the viscosity at 25 °C was 3206 mPa·s. The reactor was then sealed and stored under nitrogen.

[0030] Step S2: Preparation of Component B The dicarboxylic acid monomers: neopentanoic acid (1.18 kg), adipic acid (1.46 kg), and sebacic acid (1.01 kg); and the diol monomers: 1,4-cyclohexanediol (3.57 kg), 2-methyl-1,3-propanediol (1.98 kg), 1,6-hexanediol (844.5 g), and trimethylolpropane (147.6 g) were added to a reaction vessel and stirred until homogeneous under nitrogen protection. A pre-esterification reaction was carried out at 160 °C for 1.5 h. Subsequently, 25 g of tetrabutyl titanate was added, and the temperature was slowly increased to 220 °C while continuously removing the water generated in the reaction. After continuing the reaction for 3 h, the acid value of the system was measured to be 0.2 mgKOH / g, and the heating was stopped. The temperature was then lowered to 185 °C, and 0.05 kg of 2-methyl-1,3-propanediol was added to adjust the hydroxyl value, ultimately yielding a product with a hydroxyl value of 90 mgKOH / g.

[0031] Subsequently, the obtained polyester polyol was cooled to 110 °C, and ethyl acetoacetate was added at a rate of 2.0 wt% of the polyester polyol mass. After reacting at this temperature for 2 hours, the temperature was further lowered to 80 °C, and zinc acetylacetone (Zn(C5H7O2)2)2 was added with stirring at a rate of 0.5 wt% of the polyester polyol mass. The reaction was maintained at this temperature for 2 hours. Afterward, the temperature was raised to 100 °C, and the product was degassed under vacuum for 1 hour. The solution was filtered through a 100 μm filter to obtain a colorless and transparent low-adsorption polyester polyol solution. The hydroxyl value of the product was tested to be 85 mgKOH / g, and the viscosity at room temperature was 2180 mPa‧s. Component B was obtained by mixing 99.18 parts of the low-adsorption polyester polyol, 0.6 parts of the leveling agent, 0.1265 parts of the defoamer, and 0.1 parts of the organic bismuth catalyst until homogeneous.

[0032] Step S3: Composite substrate After mixing the A / B components prepared above at a ratio of R=1.1 and stirring evenly, a solventless laminating machine was used to laminate a two-layer structure of NY / / RCPP, with an adhesive application rate of 2 g / m². 2 After lamination, the film was placed in a curing chamber at 40 ℃ and cured for 48 h. Then, bags were made and filled with an ethyl maltol aqueous solution with a mass concentration of 0.02%, sealed, and boiled in water at 100 ℃ for 60 min. The peel strength of the composite film bags was then tested, and the average peel strength was found to be 4.5 N / 15 mm. The bags showed normal appearance with no delamination.

[0033] Example 2: Step S1: Preparation of Component A 2.62 kg of H 12 MDI and 1.68 kg of CHDI were added to a dry reactor, followed by degassing under reduced pressure at 50 °C for 30 min. The temperature was then raised to 75 °C, and 7.8 kg of poly(ethylene adipate) with a molecular weight of 2000 was added with stirring. The reaction was then maintained under nitrogen-sealed conditions for 3 h. The system was cooled to 55 °C, and 0.1 wt% N,N-dimethylethanolamine was added to adjust the reaction rate with component B. After stirring for 0.5–1 h, the NCO content was measured to be 14.12%, and the viscosity at 25 °C was 2920 mPa·s. The reactor was then sealed and stored under nitrogen.

[0034] Step S2: Preparation of Component B The dicarboxylic acid monomers: neopentanoic acid (1.18 kg), adipic acid (1.46 kg), and sebacic acid (1.01 kg); and the diol monomers: 1,4-cyclohexanediol (3.57 kg), 2-methyl-1,3-propanediol (1.98 kg), 1,6-hexanediol (844.5 g), and trimethylolpropane (147.6 g) were added to a reaction vessel and stirred until homogeneous under nitrogen protection. A pre-esterification reaction was carried out at 160 °C for 1.5 h. Subsequently, 25 g of tetrabutyl titanate was added, and the temperature was slowly increased to 220 °C while continuously removing the water generated in the reaction. After continuing the reaction for 3 h, the acid value of the system was measured to be 0.2 mgKOH / g, and the heating was stopped. The temperature was then lowered to 185 °C, and 0.05 kg of 2-methyl-1,3-propanediol was added to adjust the hydroxyl value, ultimately yielding a product with a hydroxyl value of 90 mgKOH / g.

[0035] Subsequently, the obtained polyester polyol was cooled to 110 °C, and neopentyl glycol diacetate was added at a concentration of 2.0 wt% of the polyester polyol. After reacting at this temperature for 2 hours, the temperature was further lowered to 80 °C, and zinc acetylacetone (Zn(C5H7O2)2)2 was added with stirring at a concentration of 0.5 wt% of the polyester polyol. The reaction was then maintained at this temperature for 2 hours. Afterward, the temperature was raised to 100 °C, and the product was degassed under vacuum for 1 hour. The solution was filtered through a 100 μm filter to obtain a colorless and transparent low-adsorption polyester polyol solution. The hydroxyl value of the product was 82 mgKOH / g, and the viscosity at room temperature was 2310 mPa‧s. Component B was obtained by mixing 99.25 parts of the low-adsorption polyester polyol, 0.6 parts of the leveling agent, 0.114 parts of the defoamer, and 0.08 parts of the organic bismuth catalyst until homogeneous.

[0036] Step S3: Composite substrate The solvent-free adhesive prepared in Example 2 was mixed and stirred evenly at a ratio of A / B components with an R value of 1.2. Then, a three-layer structure of PET / Al / RCPP was laminated using a solvent-free laminating machine, with an adhesive application rate of 2 g / m². 2 After lamination, the film was placed in a curing chamber at 50 ℃ and cured for 60 h. Then, bags were made and filled with a 0.02% (w / w) ethyl maltol aqueous solution, sealed, and boiled in water at 100 ℃ for 60 min. The peel strength of the composite film bags was then tested. The average peel strength of PET / / Al was 3.9 N / 15 mm, and the average peel strength of Al / / RCPP was 4.4 N / 15 mm. The appearance was normal with no delamination.

[0037] Example 3: Step S1: Preparation of Component A 2.62 kg of H 12MDI and 1.68 kg of CHDI were added to a dry reactor, followed by degassing under reduced pressure at 50 °C for 30 min. The temperature was then raised to 75 °C, and 7.8 kg of poly(1,6-hexanediol adipate) with a molecular weight of 2000 was added with stirring. The reaction was then maintained under nitrogen-sealed conditions for 3 h. The system was cooled to 55 °C, and 0.1 wt% N,N-dimethylethanolamine was added to adjust the reaction rate with component B. After stirring for 0.5–1 h, the NCO content was measured to be 13.98%, and the viscosity at 25 °C was 2430 mPa·s. The reactor was then sealed and stored under nitrogen.

[0038] Step S2: Preparation of Component B The dicarboxylic acid monomers: neopentanoic acid (1.18 kg), adipic acid (1.46 kg), and sebacic acid (1.01 kg); and the diol monomers: 1,4-cyclohexanediol (3.57 kg), 2-methyl-1,3-propanediol (1.98 kg), 1,6-hexanediol (844.5 g), and trimethylolpropane (147.6 g) were added to a reaction vessel and stirred until homogeneous under nitrogen protection. A pre-esterification reaction was carried out at 160 °C for 1.5 h. Subsequently, 25 g of tetrabutyl titanate was added, and the temperature was slowly increased to 220 °C while continuously removing the water generated in the reaction. After continuing the reaction for 3 h, the acid value of the system was measured to be 0.18 mgKOH / g, and the heating was stopped. The temperature was then lowered to 185 °C, and 0.05 kg of 2-methyl-1,3-propanediol was added to adjust the hydroxyl value, ultimately yielding a product with a hydroxyl value of 86 mgKOH / g.

[0039] Subsequently, the obtained polyester polyol was cooled to 110 °C, and neopentyl glycol diacetate was added at a concentration of 2.0 wt% of the polyester polyol mass. After reacting at this temperature for 2 h, the temperature was further lowered to 80 °C, and aluminum acetylacetonate Al(C5H7O2)3 was added with stirring at a concentration of 0.5 wt% of the polyester polyol mass. The reaction was then maintained at this temperature for 2 h. Afterward, the temperature was raised to 100 °C, and the product was degassed under vacuum for 1 h. The solution was filtered through a 100 μm filter to obtain a colorless and transparent low-adsorption polyester polyol solution. The hydroxyl value of the product was tested to be 81 mgKOH / g, and the viscosity at room temperature was 2360 mPa‧s. Component B was obtained by mixing 99.32 parts of the low-adsorption polyester polyol, 0.6 parts of the leveling agent, 0.1 parts of the defoamer, and 0.064 parts of the organic bismuth catalyst until homogeneous.

[0040] Step S3: Composite substrate The solvent-free adhesive components A and B prepared in Example 3 were mixed and stirred evenly at a ratio of R value 1.5. Then, a three-layer structure of PET / Al / RCPP was laminated using a solvent-free laminating machine, with an adhesive application rate of 2 g / m². 2After lamination, the film was placed in a curing chamber at 50 ℃ and cured for 60 h. Then, bags were made and filled with a 0.02% (w / w) ethyl maltol aqueous solution, sealed, and boiled in water at 100 ℃ for 60 min. The peel strength of the composite film bags was then tested. The average peel strength of PET / / Al was 5.2 N / 15 mm, and the average peel strength of Al / / RCPP was 4.8 N / 15 mm. The appearance was normal with no delamination.

[0041] Comparative Example 1: Step S1: Preparation of Component A 2.62 kg of H 12 MDI and 1.68 kg of CHDI were added to a dry reactor, followed by degassing under reduced pressure at 50 °C for 30 min. The temperature was then raised to 75 °C, and 7.8 kg of poly(ethylene adipate) with a molecular weight of 2000 was added with stirring. The reaction was then maintained under nitrogen-sealed conditions for 3 h. The system was cooled to 55 °C, and 0.1 wt% of N,N-dimethylethanolamine was added to adjust the reaction rate with component B. After stirring for 0.5–1 h, the NCO content was measured to be 14.12%, and the viscosity at 25 °C was 2920 mPa·s. The reactor was then sealed and stored under nitrogen.

[0042] Step S2: Preparation of Component B The dicarboxylic acid monomers: neopentanoic acid (1.18 kg), adipic acid (1.46 kg), and sebacic acid (1.01 kg); and the diol monomers: 1,4-cyclohexanediol (3.57 kg), 2-methyl-1,3-propanediol (1.98 kg), 1,6-hexanediol (844.5 g), and trimethylolpropane (147.6 g) were added to a reaction vessel and stirred until homogeneous under nitrogen protection. The mixture was heated to 160 °C for a pre-esterification reaction for 1.5 h. Subsequently, 25 g of tetrabutyl titanate was added, and the temperature was slowly increased to 220 °C while continuously removing the water generated in the reaction. After continuing the reaction for 3 h, the acid value of the system was measured to be 0.2 mgKOH / g, and the heating was stopped. The temperature was then lowered to 185 °C, and 0.05 kg of 2-methyl-1,3-propanediol was added to adjust the hydroxyl value, ultimately yielding a product with a hydroxyl value of 90 mgKOH / g. Component B is obtained by mixing and stirring 99.21 parts of the obtained polyester polyol, 0.6 parts of the leveling agent, 0.1 parts of the defoamer and 0.09 parts of the organic bismuth catalyst evenly.

[0043] Step S3: Composite substrate The solvent-free adhesive prepared in Comparative Example 1 was mixed and stirred evenly with components A and B having an R value of 1.2. Then, a two-layer NY / RCPP structure was laminated using a solvent-free laminating machine, with an adhesive application rate of 2 g / m². 2After lamination, the film was placed in a curing chamber at 50 ℃ and cured for 60 h. Then, bags were made and filled with an ethyl maltol aqueous solution with a mass concentration of 0.02%, sealed, and boiled in water at 100 ℃ for 60 min. The peel strength of the composite film bags was then tested, and the average peel strength was found to be 1.6 N / 15 mm. The film showed delamination.

[0044] As can be seen from Example 1 and Comparative Example 1, the introduction of coordination structures and metal ions can significantly improve the adhesive's resistance to ethyl maltol, increasing the peel strength from 1.6 N / 15 mm to 4.5 N / 15 mm, while maintaining a better appearance without delamination. Examples 2 and 3 show that, with the introduction of coordination structures, changing the types of metal ions and polyols can achieve the same resistance to ethyl maltol, demonstrating that this approach can suppress the side effects of ethyl maltol at the molecular level and effectively improve the peel strength of the composite film when filled with contents containing ethyl maltol.

[0045] The above embodiments describe in detail the structure, features, and effects of the present invention. The above description is only a preferred embodiment of the present invention. Any changes made in accordance with the concept of the present invention, or equivalent embodiments modified to have equivalent changes, shall still fall within the scope of protection of the present invention if they do not exceed the scope covered by the specification.

[0046] Although embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the scope of the invention is not limited to the contents disclosed in the embodiments.

Claims

1. A two-component solvent-free polyurethane adhesive, characterized in that, The two-component solvent-free polyurethane adhesive is composed of component A and component B mixed in a certain proportion; component A is an aliphatic isocyanate prepolymer, and component B includes low-adsorption polyester polyol and additives, wherein the mass ratio of low-adsorption polyester polyol to additives is 120-130:

1.

2. The two-component solvent-free polyurethane adhesive according to claim 1, characterized in that, The aliphatic isocyanate prepolymer is prepared by adding aliphatic isocyanate to a reaction vessel, followed by degassing under reduced pressure and heating, adding a low-polarity polyol under stirring, and maintaining the reaction under nitrogen-sealed conditions to obtain an isocyanate prepolymer with an NCO content of 6–18 wt%. The reaction system is then cooled to 45–65 °C, and 0.1–1 wt% of a regulator is added. After stirring and reacting, the NCO content and viscosity are measured, and the mixture is sealed and stored under nitrogen.

3. The two-component solvent-free polyurethane adhesive according to claim 2, characterized in that, The aliphatic isocyanate is one or more of hexamethylene diisocyanate, HDI trimer, isophorone diisocyanate, hydrogenated MDI, and 1,4-cyclohexane diisocyanate.

4. The two-component solvent-free polyurethane adhesive according to claim 2, characterized in that, The low-polarity polyol is an aliphatic polyester polyol, a low-polarity polycarbonate diol, or a polyether diol containing an alicyclic structure. The low-polarity polyol is one or more of the following: poly(ethylene adipate), poly(hexanediol adipate), poly(neopentyl adipate), poly(1,4-butanediol adipate), polycarbonate diol type polyurethane, neopentyl glycol type polycarbonate diol, and cyclohexanediethanol type polyester diol.

5. The two-component solvent-free polyurethane adhesive according to claim 2, characterized in that, The regulator is one or more of isophorone diamine, 1,4-cyclohexanediamine, or N,N-dimethylethanolamine.

6. The two-component solvent-free polyurethane adhesive according to claim 1, characterized in that, The low-adsorption polyester polyol comprises adding a diacid, a diol, and trimethylolpropane together into a reaction vessel, stirring evenly under nitrogen protection, heating and then carrying out a pre-esterification reaction, followed by the addition of a catalyst, slow heating, and continuous removal of water generated in the reaction. When the acid value of the system drops to ≤ 1.0 mgKOH / g, the heating is stopped; after cooling, a low-polarity diol is added to adjust the hydroxyl value to obtain a polyester polyol intermediate. The polyester polyol intermediate was cooled and then modified with ethyl acetoacetate monomer. The reaction was then maintained at a certain temperature, allowing the β-diketone structure in the ethyl acetoacetate monomer to be introduced into the polyester side chain via transesterification, forming uniformly distributed multidentate coordination sites in the polyol system. The temperature was then further reduced, and a metal ion salt was added under stirring. The reaction was maintained at a certain temperature, allowing the metal ion to fully form a stable coordination complex with the β-diketone ligand. Afterward, the temperature was raised and the mixture was degassed under vacuum, filtered, and a colorless and transparent low-adsorption polyester polyol solution was obtained.

7. The two-component solvent-free polyurethane adhesive according to claim 6, characterized in that, The dicarboxylic acid includes neoglutaric acid, adipic acid and sebacic acid mixed in proportion; the diol includes 1,4-cyclohexanediol, 2-methyl-1,3-propanediol, 1,6-hexanediol and trimethylolpropane mixed in proportion; the selected catalyst is tetrabutyl titanate, and the selected catalyst accounts for 0.03 wt% of the total raw material mass.

8. The two-component solvent-free polyurethane adhesive according to claim 6, characterized in that, The ethyl acetoacetate modified monomer includes a compound that contains both an acetoacetic acid structural unit and at least one functional group that can chemically react with a polyester polyol; the metal ion salt may be zinc acetylacetonate Zn(C5H7O2)2 or aluminum acetylacetonate Al(C5H7O2)3.

9. The two-component solvent-free polyurethane adhesive according to any one of claims 1 to 8, characterized in that, The additives include leveling agents, defoamers, and curing speed regulators.

10. The two-component solvent-free polyurethane adhesive according to claim 9, characterized in that, The adhesive is prepared by mixing component A and component B in a ratio of R value of 1.1 to 1.5:

1. The adhesive is then laminated with a film substrate. The adhesive is applied to the field of solvent-free lamination of food flexible packaging materials.