Solvent-free polyurethane adhesive

Abstract

The invention relates to the technical field of adhesives, in particular to a solvent-free polyurethane adhesive. The polyurethane adhesive with excellent steaming and boiling resistance and medium resistance is obtained after the polyester polyol, the reaction type interface adhesion accelerant and the phosphorus-silicon-containing covalent coupling monomer in the component A are matched with the component B for use under the combined action of the polyester polyol, the reaction type interface adhesion accelerant and the phosphorus-silicon-containing covalent coupling monomer in the component A. The component B is an aliphatic isocyanate curing agent, has no problem of primary aromatic amine precipitation, and is suitable for use requirements of cooking-resistant products in food packaging.

Description

Technical Field

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

[0002] With the development of the food packaging industry, the performance requirements for packaging materials are increasing, especially for flexible packaging of meat, poultry, and sauces, which needs to withstand high-temperature sterilization treatments of 121°C or even 135°C and above. Such packaging is often made from materials such as aluminum foil, nylon (PA), and polypropylene through dry lamination or solvent-free lamination processes. Currently, solvent-free polyurethane adhesives have become the market mainstream due to their environmental friendliness, high efficiency, and safety. However, traditional solvent-free adhesives used in retort packaging typically use curing agent components containing aromatic isocyanates (such as MDI and TDI). Under high-temperature and high-humidity retort conditions, these aromatic isocyanates may slowly hydrolyze to generate primary aromatic amines (PAA), which have carcinogenic risks and may migrate into food, thus failing to meet the increasingly stringent food safety regulations (such as the national standard G4806.15-2024). To address the PAA (Patent Adhesion) problem, existing technologies employ aliphatic or alicyclic isocyanates. However, these solutions often result in insufficient heat resistance, initial tack, or final bond strength of the adhesive, particularly poor adhesion to non-polar substrates (such as CPP), making it difficult to meet stringent retort conditions (e.g., above 121 °C for 30 minutes). Furthermore, the adhesive must possess excellent resistance to various media (oil, acid, salt, etc.) to prevent contents from corroding and causing packaging delamination.

[0003] Therefore, developing a solvent-free polyurethane adhesive that can completely eliminate PAA formation while simultaneously possessing excellent high-temperature cooking resistance, high bonding strength, and good resistance to media has become a pressing technical challenge in this field. In existing technologies, to mitigate PAA risks, some solutions use aliphatic isocyanates (such as HDI and IPDI) as curing agents, or partially hydrogenated MDI; others improve adhesion by adding phosphate ester tackifiers or silane coupling agents. While aliphatic isocyanates can prevent PAA formation, they suffer from insufficient heat resistance and bonding strength, poor adhesion to non-polar substrates (such as CPP), and are prone to delamination, especially after high-temperature cooking, requiring special design of the polyol structure. Furthermore, existing silane coupling agents are mostly physically added and are prone to migration and failure under high-temperature and high-humidity environments. Summary of the Invention

[0004] This invention addresses the problem that while aliphatic isocyanates can avoid PAA formation, they suffer from insufficient heat resistance and bonding strength, especially delamination after high-temperature cooking. It provides a solvent-free polyurethane adhesive.

[0005] One of the technical solutions of the present invention is to provide a solvent-free polyurethane adhesive, wherein the solvent-free polyurethane adhesive is composed of a mixture of component A and component B in a certain proportion; by weight, component A includes 50-78 parts of a polyester polyol with stable viscosity and hydroxyl value, 10-30 parts of a silane-grafted polyol, 5-20 parts of a phosphorus-silicon covalent coupling monomer, and 1-2 parts of an additive; by weight, component B is 100 parts of an aliphatic polyisocyanate.

[0006] The solvent-free polyurethane adhesive is composed of component A and component B mixed in an NCO / OH equivalent ratio of 1.2:1.

[0007] Further, the design and synthesis of the polyester polyol used in component A specifically includes: adding aromatic diacid monomers, long-chain diacid monomers, sterically hindered diol monomers, and trimethylolpropane (TMP) in a certain proportion to a reactor equipped with a mechanical stirrer, a tower condenser, a sampling port, a vacuum port, and a nitrogen inlet; adding a catalyst; and slowly heating to 140–160 °C under a nitrogen atmosphere, maintaining the temperature for 30–60 min to achieve uniform mixing and dehydration. The temperature is then further increased to 180–220 °C, while simultaneously purging with stable nitrogen gas to remove the generated water. The reaction water is collected through the tower condenser, and the reaction continues for 2–6 hours until the acid value drops to ≤ 20 mgKOH / g. To obtain hydroxyl-terminated polyester polyols, a small amount of neopentyl glycol (approximately 1–3 mol% of the diacid monomers) is added near the reaction endpoint, and the reaction continues at 180–20 °C for 0.5–1 hour to eliminate residual acid value. After the acid value drops below 3 mgKOH / g, a vacuum is applied to 10–50 mbar, and the mixture is kept at 180–220 °C for 30–60 minutes to remove residual water and low-boiling molecules, yielding a polyester polyol with stable viscosity and hydroxyl value. The mixture is then filtered at 80–100 °C (using a 50–100 μm filter), degassed, and packaged in a container, sealed with nitrogen for storage.

[0008] Furthermore, the diacid monomer is one of the aromatic diacids such as phthalic acid, isophthalic acid, and terephthalic acid; the long-chain diacid monomer is one of dodecanoic acid, dodecanoic acid, or sebacic acid; the diol monomer is one or more of the sterically hindered diol monomers such as 2-methyl-1,3-propanediol, neopentyl glycol, 1,4-cyclohexanediol, hydrogenated bisphenol A diol, and 2,2,4-trimethyl-1,3-pentanediol, used to reduce the hydrolysis sensitivity of the polyester backbone; the catalyst is a titanium ester catalyst, an organic acid catalyst, or a metal-organic catalyst.

[0009] Furthermore, the molar ratio of aromatic diacid to long-chain diacid monomers in the diacid monomers is 1.3:1; to increase the rigidity of the molecular chain, the amount of trimethylolpropane added is 0.2 mol / L of the total amount of diacids. To control the production of -OH-terminated specialty polyester polyols, the molar ratio of total diol monomers to diacid monomers in the system is controlled to be 1.02:1.

[0010] Furthermore, the synthesis of the reactive interfacial adhesion promoter (silane-grafted polyol) in component A specifically includes: A certain amount of polyol was added to a dry four-necked flask, and the water content was removed under vacuum to below 100 ppm under a nitrogen atmosphere. Then, an equimolar amount of propyltriethoxysilane isocyanate (IPTS) was added to the flask, and an organobismuth catalyst was added dropwise. The reaction was stirred at 50–80 °C for 3–5 hours until the characteristic peak of -NCO disappeared as detected by FTIR, indicating the end of the reaction. After cooling and degassing, the silane-grafted polyol was obtained and packaged for later use.

[0011] First add the polyol, then the amount of IPTS added can be calculated based on the amount of polyol added.

[0012] Further, the polyol is a difunctional polyether polyol, polyester polyol, or polycarbonate polyol with a molecular weight of 1000-2000, and the polyol is polypropylene glycol, polyethylene glycol, polytetrahydrofuran ether diol, polypropylene adipate, polyethylene adipate, polyethylene adipate, polyethylene adipate-1,4-butylene adipate diol, polybutylene adipate, polyhexane adipate, neopentyl adipate, or polycarbonate diol.

[0013] Furthermore, the phosphorus-containing silicon covalent coupling monomer is prepared by mixing a silane coupling agent and 2-methyl-2-acrylate-2-hydroxyethyl phosphate in equimolar amounts, then adding a certain amount of initiator, and reacting at 60 °C for 2 h to form a Si-OP structure monomer. The phosphorus-containing silicon covalent coupling monomer is used as an interfacial chemical coupling enhancer, having a phosphate ester structure that can react with aluminum foil and a silane structure that can participate in polyurethane reactions.

[0014] Furthermore, the silane coupling agent is one of the following silane coupling agents containing double bonds: vinyltriethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, allyltrimethoxysilane, p-styryltrimethoxysilane.

[0015] Furthermore, the initiator is one of the thermal initiators such as ammonium persulfate and potassium persulfate, and the amount added is one-thousandth of the amount of silane added.

[0016] Furthermore, the additives include one or more of organosilicon leveling agents, organobismuth catalysts, and polysiloxane defoamers; the aliphatic polyisocyanate is one of HDI trimer and IPDI trimer, with a free NCO content of 22%.

[0017] Furthermore, the solvent-free polyurethane adhesive is coated onto the PET / Al / RCPP composite film.

[0018] Compared with the prior art, the present invention has the following advantages: 1) The polyester polyol used in component A of this invention achieves improved resistance to boiling media through the synergistic design of its molecular structure. Its superior performance mainly stems from the synergistic effect of different structural units: The benzene ring-containing dicarboxylic acid unit serves as the backbone of the polyester main chain, improving the rigidity and thermal stability of the molecular chain. Furthermore, the hydrophobicity of the aromatic ring reduces the diffusion rate of water molecules in the polyester, thereby inhibiting ester bond hydrolysis under high-temperature and humid conditions. The sterically hindered diol unit introduces significant steric hindrance near the ester bonds, effectively protecting the ester bond formation and reducing the probability of nucleophilic attack on the ester bonds by water molecules and chemical media, thus improving hydrolysis and boiling resistance. The microbranched structure introduced by trace amounts of trimethylolpropane improves the cohesive force and network density between polyester molecules without significantly increasing the system viscosity, making the cured polyurethane adhesive layer less prone to chain segment slippage and structural relaxation in high-temperature and humid environments. The synergistic effect of these structural units allows the prepared polyester polyol to significantly improve the hydrolysis resistance, boiling resistance, and thermal stability of the cured adhesive layer while maintaining good processability.

[0019] 2) In this invention, a reactive interfacial adhesion promoter (silane-grafted polyol) is introduced into component A. The terminal siloxy groups and phosphorus-containing silicon covalently coupled monomers in the silane-grafted polyol can form strong covalent or coordination bonds with the inorganic layer (Al film, etc.) after curing under heat / humid conditions, thereby inhibiting interlayer delamination and improving its resistance to media and adhesion strength.

[0020] 3) The curing agent used in this invention is HDI trimer or IPDI trimer. The use of aliphatic isocyanate avoids the precipitation of primary aromatic amines from the source, which complies with environmental regulations and the safety requirements of food contact adhesives. 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 terms are 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: The two-component polyurethane adhesives prepared in each example were mixed and stirred evenly according to the NCO / OH equivalent ratio of components A / B of 1.2:1, and then passed through a solventless laminating machine at 2 g / m 2 The adhesive amount was used to composite a PET / Al / / RCPP three-layer structure, and the T-type peel strength of the composite film was tested after curing at 50 ℃ for 60 h. The specific test method was consistent with GB / T2791-1995.

[0027] Retort resistance test: The peel strength of the composite film after being boiled at 121 ℃ for 40 min is tested. Those that pass the boiling test are marked with ○, and the rest are marked with ※.

[0028] Media resistance test: The composite cured membrane material is made into 12 cm × 12 cm bags, filled with a mixture of 0.5% ethyl maltol aqueous solution and chili oil, and boiled at 121 ℃ for 40 min. Then, the T-type peel strength is tested according to GB / T2791-1995. If it meets the usage requirements, it is marked as ○. If the bag is damaged, leaks or has insufficient strength, it is marked as ※.

[0029] Aromatic amine migration detection: SGS third-party testing.

[0030] Example 1: Step S1: Preparation of polyester polyol. 380 g of terephthalic acid, 360 g of sebacic acid, 308.5 g of neopentyl glycol, 10 g of trimethylolpropane, and 131.9 g of 1,4-cyclohexanediethanol were added to a reactor equipped with a mechanical stirrer, a tower condenser, a sampling port, a vacuum port, and a nitrogen inlet. The mixture was slowly heated to 150 °C under a nitrogen atmosphere and held for 60 min to achieve uniform mixing and dehydration. The temperature was then further increased to 200 °C, while nitrogen was continuously purged at a stable pressure to remove the generated water. The reaction water was collected through the tower condenser. After adding 0.3 g of organic bismuth catalyst and reacting for 5 h, the ester concentration was measured to be 1.5 mg KOH / g. To obtain hydroxyl-terminated polyester polyol, 4 g of neopentyl glycol was added, and the reaction was continued at 200 °C for 1 hour to eliminate residual acid value. The mixture was then evacuated to 20 mbar and kept at 200 °C for 60 minutes to remove residual water and low-boiling molecules. After degassing using a 100 μm filter at 80 °C, it was sealed and stored under nitrogen. The product was a pale yellow, transparent liquid with a viscosity of 2500 mPa‧s at room temperature and a hydroxyl value of 62 mgKOH / g.

[0031] Step S2: Preparation of silane-grafted polyol. 100 g of polytetrahydrofuran ether diol (molecular weight 2000) was added to a 250 mL dry four-necked flask. After vacuuming at 100 °C for 2 h, the water content was measured to be 0.012%. The temperature was then lowered to 60 °C, and 12.37 g of propyltriethoxysilane isocyanate was added. Stirring continued for 4 h. After cooling, the silane-grafted polyol was obtained.

[0032] Step S3: Preparation of phosphorus-containing silicon covalent coupling monomer: 114 g of 2-methyl-2-acrylate-2-hydroxyethyl ester phosphate, 95.2 g of vinyltriethoxysilane and 0.114 g of ammonium persulfate were added to a 250 mL dry four-necked flask, and the temperature was gradually raised to 60 °C and reacted for 2 h to obtain phosphorus-containing silicon covalent coupling monomer.

[0033] The above-obtained polyester polyol (65 parts), silane-grafted polyol (20 parts), phosphorus-silicon covalent coupling monomer (14 parts), organosilicon leveling agent (0.5 parts), polysiloxane defoamer (0.45 parts), and organobismuth catalyst (0.05 parts) were mixed and stirred evenly to obtain component A. Then, this mixture was mixed evenly with HDI trimer (component B) at an NCO / OH equivalent ratio of 1.2:1. The mixture was then processed through a solventless laminating machine at a speed of 2 g / m³. 2 The adhesive application amount was adjusted to form a PET / Al / / RCPP three-layer structure, and the T-peel strength of the composite film was tested after curing at 50 ℃ for 60 h.

[0034] Example 2: Step S1: Preparation of polyester polyol. 380 g of terephthalic acid, 360 g of sebacic acid, 308.5 g of neopentyl glycol, 10 g of trimethylolpropane, and 131.9 g of 1,4-cyclohexanediethanol were added to a reactor equipped with a mechanical stirrer, a tower condenser, a sampling port, a vacuum port, and a nitrogen inlet. The mixture was slowly heated to 150 °C under a nitrogen atmosphere and held for 60 min to achieve uniform mixing and dehydration. The temperature was then further increased to 200 °C, while nitrogen was continuously purged at a stable pressure to remove the generated water. The reaction water was collected through the tower condenser. After adding 0.3 g of organic bismuth catalyst and reacting for 5 h, the ester concentration was measured to be 1.5 mg KOH / g. To obtain hydroxyl-terminated polyester polyol, 4 g of neopentyl glycol was added, and the reaction was continued at 200 °C for 1 hour to eliminate residual acid value. The mixture was then evacuated to 20 mbar and kept at 200 °C for 60 minutes to remove residual water and low-boiling molecules. After degassing using a 100 μm filter at 80 °C, it was sealed and stored under nitrogen. The product was a pale yellow, transparent liquid with a viscosity of 2500 mPa‧s at room temperature and a hydroxyl value of 62 mgKOH / g.

[0035] Step S2: Preparation of silane-grafted polyol. 100 g of polytetrahydrofuran ether diol (molecular weight 2000) was added to a 250 mL dry four-necked flask. After vacuuming at 100 °C for 2 h, the water content was measured to be 0.012%. The temperature was then lowered to 60 °C, and 12.37 g of propyltriethoxysilane isocyanate was added. Stirring continued for 4 h. After cooling, the silane-grafted polyol was obtained.

[0036] Step S3: Preparation of phosphorus-containing silicon covalent coupling monomer: 114 g of 2-methyl-2-acrylate-2-hydroxyethyl ester phosphate, 95.2 g of vinyltriethoxysilane and 0.114 g of ammonium persulfate were added to a 250 mL dry four-necked flask, and the temperature was gradually raised to 60 °C and reacted for 2 h to obtain phosphorus-containing silicon covalent coupling monomer.

[0037] The above-obtained polyester polyol (64 parts), silane-grafted polyol (30 parts), phosphorus-silicon covalent coupling monomer (5 parts), organosilicon leveling agent (0.5 parts), polysiloxane defoamer (0.45 parts), and organobismuth catalyst (0.05 parts) were mixed and stirred evenly to obtain component A. Then, this mixture was mixed evenly with HDI trimer (component B) at an NCO / OH equivalent ratio of 1.2:1. The mixture was then passed through a solventless laminating machine at a speed of 2 g / m³. 2 The adhesive application amount was adjusted to form a PET / Al / / RCPP three-layer structure, and the T-peel strength of the composite film was tested after curing at 50 ℃ for 60 h.

[0038] Example 3: Step S1: Preparation of polyester polyol. 380 g of isophthalic acid, 406 g of dodecanoic acid, 308.5 g of neopentyl glycol, 10 g of trimethylolpropane, and 139.9 g of 1,4-cyclohexanediol were added to a reactor equipped with a mechanical stirrer, a tower condenser, a sampling port, a vacuum port, and a nitrogen inlet. The mixture was slowly heated to 150 °C under a nitrogen atmosphere and held for 60 min to achieve uniform mixing and dehydration. The temperature was further increased to 200 °C, while nitrogen was continuously purged at a stable pressure to remove the generated water. The reaction water was collected through the tower condenser. After adding 0.3 g of organic bismuth catalyst and reacting for 4 h, the acid ester concentration was measured to be 2.0 mg KOH / g. To obtain hydroxyl-terminated polyester polyol, 4 g of neopentyl glycol was added, and the reaction was continued at 200 °C for 1 hour to eliminate residual acid value. The mixture was then evacuated to 20 mbar and kept at 200 °C for 60 minutes to remove residual water and low-boiling molecules. After degassing using a 100 μm filter at 80 °C, it was sealed and stored under nitrogen. The product was a pale yellow, transparent liquid with a viscosity of 2800 mPa‧s at room temperature and a hydroxyl value of 59 mgKOH / g.

[0039] Step S2: Preparation of silane-grafted polyol. 100 g of polyethylene adipate (molecular weight 2000) was added to a 250 mL dry four-necked flask. After vacuuming at 100 °C for 2 h, the water content was measured to be 0.015%. The temperature was then lowered to 60 °C, and 12.37 g of propyltriethoxysilane isocyanate was added. The reaction was continued with stirring for 4 h. After cooling, the silane-grafted polyol was obtained.

[0040] Step S3: Preparation of phosphorus-containing silicon covalent coupling monomer: 114 g of 2-methyl-2-acrylate-2-hydroxyethyl ester phosphate, 81.1 g of vinyltriethoxysilane and 0.114 g of ammonium persulfate were added to a 250 mL dry four-necked flask, and the temperature was gradually raised to 60 °C and reacted for 2 h to obtain phosphorus-containing silicon covalent coupling monomer.

[0041] The above-obtained polyester polyol (50 parts), silane-grafted polyol (28 parts), phosphorus-silicon covalent coupling monomer (20 parts), organosilicon leveling agent (1 part), polysiloxane defoamer (0.95 parts), and organobismuth catalyst (0.05 parts) are mixed and stirred evenly to obtain component A. Then, this mixture is mixed evenly with HDI trimer (component B) at an NCO / OH equivalent ratio of 1.2:1. The mixture is then processed through a solventless laminating machine at a speed of 2 g / m³. 2 The adhesive application amount was adjusted to form a PET / Al / / RCPP three-layer structure, and the T-peel strength of the composite film was tested after curing at 50 ℃ for 60 h.

[0042] Example 4: Step S1: Preparation of polyester polyol. 380 g of phthalic acid, 406 g of dodecanoic acid, 308.5 g of neopentyl glycol, 10 g of trimethylolpropane, and 87.42 g of 2-methyl-1,3-propanediol were added to a reactor equipped with a mechanical stirrer, a tower condenser, a sampling port, a vacuum port, and a nitrogen inlet. The mixture was slowly heated to 150 °C under a nitrogen atmosphere and held for 60 min to achieve uniform mixing and dehydration. The temperature was then increased to 200 °C while purging with stable nitrogen gas to remove generated water. The reaction water was collected through the tower condenser. After adding 0.3 g of organic bismuth catalyst and reacting for 6 h, the ester concentration was measured to be 1.2 mgKOH / g. To obtain hydroxyl-terminated polyester polyol, 4 g of neopentyl glycol was added, and the reaction was continued at 200 °C for 1 hour to eliminate residual acid value. The mixture was then evacuated to 20 mbar and kept at 200 °C for 60 minutes to remove residual water and low-boiling molecules. After degassing using a 100 μm filter at 80 °C, it was sealed and stored under nitrogen. The product was a pale yellow, transparent liquid with a viscosity of 1900 mPa‧s at room temperature and a hydroxyl value of 61 mgKOH / g.

[0043] Step S2: Preparation of silane-grafted polyol. 100 g of polyethylene adipate-1,4-butanediol diol (molecular weight 1000) was added to a 250 mL dry four-necked flask. After vacuuming at 100 °C for 2 h, the water content was measured to be 0.018%. The temperature was then lowered to 60 °C, and 24.74 g of propyltriethoxysilane isocyanate was added. The reaction was continued with stirring for 4 h. After cooling, the silane-grafted polyol was obtained.

[0044] Step S3: Preparation of phosphorus-containing silicon covalent coupling monomer: 114 g of 2-methyl-2-acrylate-2-hydroxyethyl ester phosphate, 124.18 g of γ-(methacryloyloxy)propyltrimethoxysilane and 0.114 g of ammonium persulfate were added to a 250 mL dry four-necked flask, and the temperature was gradually raised to 60 °C and reacted for 2 h to obtain phosphorus-containing silicon covalent coupling monomer.

[0045] The above-obtained polyester polyol (78 parts), silane-grafted polyol (10 parts), phosphorus-silicon covalent coupling monomer (11 parts), organosilicon leveling agent (0.5 parts), polysiloxane defoamer (0.45 parts), and organobismuth catalyst (0.05 parts) were mixed and stirred evenly to obtain component A. Then, this mixture was mixed evenly with IPDI trimer (component B) at an NCO / OH equivalent ratio of 1.2:1. The mixture was then processed using a solventless laminating machine at a speed of 2 g / m³. 2 The adhesive application amount was adjusted to form a PET / Al / / RCPP three-layer structure, and the T-peel strength of the composite film was tested after curing at 50 ℃ for 60 h.

[0046] Comparative Example 1: Step S1: Preparation of polyester polyol. 380 g of terephthalic acid, 360 g of sebacic acid, 308.5 g of neopentyl glycol, 10 g of trimethylolpropane, and 131.9 g of 1,4-cyclohexanediethanol were added to a reactor equipped with a mechanical stirrer, a tower condenser, a sampling port, a vacuum port, and a nitrogen inlet. The mixture was slowly heated to 150 °C under a nitrogen atmosphere and held for 60 min to achieve uniform mixing and dehydration. The temperature was further increased to 200 °C, while nitrogen was continuously purged at a stable pressure to remove the generated water. The reaction water was collected through the tower condenser. After adding 0.3 g of organic bismuth catalyst and reacting for 5 h, the ester concentration was measured to be 1.5 mg KOH / g. To obtain hydroxyl-terminated polyester polyol, 4 g of neopentyl glycol was added, and the reaction was continued at 200 °C for 1 hour to eliminate residual acid value. The mixture was then evacuated to 20 mbar and kept at 200 °C for 60 minutes to remove residual water and low-boiling molecules. After degassing using a 100 μm filter at 80 °C, it was sealed and stored under nitrogen. The product was a pale yellow, transparent liquid with a viscosity of 2500 mPa‧s at room temperature and a hydroxyl value of 62 mgKOH / g.

[0047] Step S2: Preparation of phosphorus-containing silicon covalent coupling monomer: 114 g of 2-methyl-2-acrylate-2-hydroxyethyl ester phosphate, 95.2 g of vinyltriethoxysilane and 0.114 g of ammonium persulfate were added to a 250 mL dry four-necked flask, and the temperature was gradually raised to 60 °C and reacted for 2 h to obtain phosphorus-containing silicon covalent coupling monomer.

[0048] The above-obtained polyester polyol (78 parts), phosphorus-containing silicon covalent coupling monomer (20 parts), organosilicon leveling agent (1 part), polysiloxane defoamer (0.95 parts), and organobismuth catalyst (0.05 parts) were mixed and stirred evenly to obtain component A. Then, this mixture was mixed evenly with HDI trimer (component B) at an NCO / OH equivalent ratio of 1.2:1. The mixture was then passed through a solventless laminating machine at a speed of 2 g / m³. 2 The adhesive application amount was adjusted to form a PET / Al / / RCPP three-layer structure, and the T-peel strength of the composite film was tested after curing at 50 ℃ for 60 h.

[0049] Comparative Example 2: Step S1: Preparation of polyester polyol. 380 g of terephthalic acid, 360 g of sebacic acid, 308.5 g of neopentyl glycol, 10 g of trimethylolpropane, and 131.9 g of 1,4-cyclohexanediethanol were added to a reactor equipped with a mechanical stirrer, a tower condenser, a sampling port, a vacuum port, and a nitrogen inlet. The mixture was slowly heated to 150 °C under a nitrogen atmosphere and held for 60 min to achieve uniform mixing and dehydration. The temperature was then further increased to 200 °C, while nitrogen was continuously purged at a stable pressure to remove the generated water. The reaction water was collected through the tower condenser. After adding 0.3 g of organic bismuth catalyst and reacting for 5 h, the ester concentration was measured to be 1.5 mg KOH / g. To obtain hydroxyl-terminated polyester polyol, 4 g of neopentyl glycol was added, and the reaction was continued at 200 °C for 1 hour to eliminate residual acid value. The mixture was then evacuated to 20 mbar and kept at 200 °C for 60 minutes to remove residual water and low-boiling molecules. After degassing using a 100 μm filter at 80 °C, it was sealed and stored under nitrogen. The product was a pale yellow, transparent liquid with a viscosity of 2500 mPa‧s at room temperature and a hydroxyl value of 62 mgKOH / g.

[0050] Step S2: Preparation of silane-grafted polyol. 100 g of polytetrahydrofuran ether diol (molecular weight 2000) was added to a 250 mL dry four-necked flask. After vacuuming at 100 °C for 2 h, the water content was measured to be 0.012%. The temperature was then lowered to 60 °C, and 12.37 g of propyltriethoxysilane isocyanate was added. Stirring continued for 4 h. After cooling, the silane-grafted polyol was obtained.

[0051] The above-obtained polyester polyol (78 parts), silane-grafted polyol (20 parts), organosilicon leveling agent (1 part), polysiloxane defoamer (0.95 parts), and organobismuth catalyst (0.05 parts) were mixed and stirred evenly to obtain component A. Then, this mixture was mixed evenly with HDI trimer (component B) at an NCO / OH equivalent ratio of 1.2:1. The mixture was then passed through a solventless laminating machine at a speed of 2 g / m³. 2 The adhesive application amount was adjusted to form a PET / Al / / RCPP three-layer structure, and the T-peel strength of the composite film was tested after curing at 50 ℃ for 60 h.

[0052] Table 1: Performance test data of Examples 1-3 and Comparative Examples 1-2: As can be seen from Examples 1-4, introducing silane-grafted polyols into the polyurethane adhesive system allows the organosilicon structure to participate in the polyurethane curing reaction in a covalent form, significantly improving the chemical bonding strength between the adhesive layer and the aluminum foil interface. The phosphorus-containing silicon covalently coupled monomer forms a Si-OP covalent coupling structure during curing, maintaining stable interfacial bonding even under high-temperature and high-humidity conditions, which is a key factor in achieving retortability. The synergistic design of the aromatic structure, steric hindrance structure, and microbranched structure in the polyester polyol improves the heat resistance and hydrolysis resistance of the adhesive layer. As can be seen from Comparative Examples 1 and 2, simply using a mixture of special polyester polyols and single silane-grafted polyols or phosphorus-containing silicon covalently coupled monomers cannot achieve the ideal strength and retortability. The above components are not simply physically compounded, but form a multiple synergistic reaction network during curing, thus enabling the solvent-free polyurethane adhesive to possess excellent resistance to retortability and long-term stability without generating primary aromatic amines.

[0053] 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.

[0054] 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 solvent-free polyurethane adhesive, characterized in that, The solvent-free polyurethane adhesive is composed of component A and component B mixed in a certain proportion; by weight, component A includes 50-78 parts of polyester polyol with stable viscosity and hydroxyl value, 10-30 parts of silane-grafted polyol, 5-20 parts of phosphorus-silicon covalent coupling monomer and 1-2 parts of additives; component B is an aliphatic polyisocyanate.

2. The solvent-free polyurethane adhesive according to claim 1, characterized in that, The polyester polyol is prepared by adding aromatic diacid monomers, long-chain diacid monomers, sterically hindered diol monomers, and trimethylolpropane to a reaction vessel in a specific ratio, adding a catalyst, and slowly heating to 140–160 °C under a nitrogen atmosphere, maintaining the temperature for 30–60 min, then further heating to 180–220 °C and reacting for 2–6 hours until the acid value drops to ≤ 20 mgKOH / g. Neopentyl glycol is then added, and the reaction continues at 180–220 °C for 0.5–1 hour to eliminate residual acid value. A vacuum of 10–50 mbar is then applied, and the reaction continues at 180–220 °C for 30–60 min to remove residual water and low-boiling molecules, yielding the polyester polyol with stable viscosity and hydroxyl value. The polyol is then filtered at 80–100 °C, degassed, canned, and sealed for storage.

3. The solvent-free polyurethane adhesive according to claim 2, characterized in that, The diacid monomer is one of phthalic acid, isophthalic acid, and terephthalic acid; the long-chain diacid monomer is one of dodecanoic acid, dodecanoic acid, or sebacic acid; the diol monomer is one of 2-methyl-1,3-propanediol, neopentyl glycol, 1,4-cyclohexanediol, hydrogenated bisphenol A glycol, and 2,2,4-trimethyl-1,3-pentanediol; and the catalyst is a titanium ester catalyst, an organic acid catalyst, or a metal-organic catalyst.

4. The solvent-free polyurethane adhesive according to claim 1, characterized in that, The silane-grafted polyol is prepared by adding a polyol to a reaction vessel, removing water to below 100 ppm under vacuum in a nitrogen atmosphere, then adding propyltriethoxysilane isocyanate to the reaction vessel, and adding an organobismuth metal catalyst dropwise. The reaction is stirred at 50–80 °C for 3–5 hours until the -NCO characteristic peak disappears. After cooling and degassing, the silane-grafted polyol is obtained and packaged for later use.

5. The solvent-free polyurethane adhesive according to claim 4, characterized in that, The polyol is a difunctional polyether polyol, polyester polyol, or polycarbonate polyol with a molecular weight of 1000-2000. The polyol is polypropylene glycol, polyethylene glycol, polytetrahydrofuran ether diol, polypropylene adipate, polyethylene adipate, polyethylene adipate, polyethylene adipate-1,4-butylene adipate diol, polybutylene adipate, polyhexane adipate, neopentyl adipate, or polycarbonate diol.

6. The solvent-free polyurethane adhesive according to claim 1, characterized in that, The phosphorus-containing silicon covalent coupling monomer is prepared by mixing a silane coupling agent and 2-methyl-2-acrylate-2-hydroxyethyl phosphate in a certain proportion, then adding an initiator to react and form a Si-OP structure monomer, thus obtaining the phosphorus-containing silicon covalent coupling monomer as an interfacial chemical coupling enhancer.

7. The solvent-free polyurethane adhesive according to claim 6, characterized in that, The silane coupling agent is one of vinyltriethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, allyltrimethoxysilane, and p-styryltrimethoxysilane.

8. The solvent-free polyurethane adhesive according to claim 6, characterized in that, The initiator is ammonium persulfate or potassium persulfate.

9. The solvent-free polyurethane adhesive according to claim 1, characterized in that, The additives include one or more of organosilicon leveling agents, organobismuth catalysts, and polysiloxane defoamers; the aliphatic polyisocyanate is an HDI trimer or an IPDI trimer.

10. The solvent-free polyurethane adhesive according to any one of claims 1 to 9, characterized in that, The solvent-free polyurethane adhesive is coated onto the PET / Al / RCPP composite film.

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