A temperature-resistant single-component solvent-free polyurethane adhesive for paper-to-paper bonding, and a preparation method and application thereof

Abstract

This invention discloses a heat-resistant, single-component, solvent-free polyurethane adhesive for paper-plastic composites, its preparation method, and its application. It relates to the technical field of adhesives for paper-based flexible packaging composites. The adhesive's structural formula includes structural units derived from polyester polyols, structural units derived from polyether polyols, and end-capping units derived from isocyanates. Based on a total weight of 100 wt% for the structural units derived from polyester polyols, polyether polyols, and isocyanates, the following proportions are observed: 50-59 wt% for the polyester polyol-derived structural units; 8-12 wt% for the polyether polyol-derived structural units; and 31-40 wt% for the isocyanate-derived end-capping units. This heat-resistant, single-component, solvent-free polyurethane adhesive, applicable to composite paper-plastic-aluminum structures, features a low application temperature while meeting the requirements for solvent-free composites in terms of curing time and bond strength.

Description

Technical Field

[0001] This invention relates to the field of adhesives for paper-layer flexible packaging composites, and more specifically, to a heat-resistant, single-component, solvent-free polyurethane adhesive for paper lamination, its preparation method, and its application. Background Technology

[0002] With the strict implementation of national environmental protection policies, adhesives that do not involve volatile organic solvents and have almost no VOC emissions during the production, manufacturing and use process, such as one-component waterborne polyurethane adhesives and one-component solvent-free polyurethane adhesives, are favored by a wide range of manufacturers engaged in paper-layer flexible packaging lamination (including manufacturers of flexible packaging sealing bags in various fields such as milk, snacks, and ointments).

[0003] As Li Gongliang's 2014 article "Application of Single-Component Waterborne Polyurethane Adhesives in Paper-Plastic and Paper-Aluminum Composites" published in *Packaging Frontiers* magazine indicates, when using single-component waterborne polyurethane adhesives to laminate plastic / paper / aluminum foil (or plastic) / plastic structures, considering the issue of solvent water penetrating the paper, after laminating two layers, the adhesive must be fully cured and allowed to evaporate completely before laminating other layers. Otherwise, a large amount of residual moisture will affect the appearance of the laminated packaging. This makes the operation cumbersome and reduces lamination efficiency. Therefore, solvent-free polyurethane adhesives have become one of the hot development directions in the adhesive industry.

[0004] Packaging bags containing paper composite layers offer advantages such as good sturdiness, excellent stain and moisture resistance, printable outer layers, and lower cost. There are many types of composite paper, with white cardboard, white corrugated card, and kraft paper being the most common paper-plastic composite materials on the market. Regardless of the type of paper, its porous structure and hygroscopic nature mean it contains a certain amount of moisture. Two-component solvent-free polyurethane adhesives have lower viscosity and smaller molecular weight, but the paper material has many fibers and numerous tiny micropores, allowing the adhesive to penetrate the paper, causing it to harden and discolor. In contrast, single-component solvent-free polyurethane adhesives have a larger molecular weight and can utilize the moisture in the paper for cross-linking and curing, making them more suitable for use as adhesives for composite paper materials.

[0005] Taking the single-component polyurethane adhesive for paper-plastic composites disclosed in Chinese patent CN114774064A as an example, most commercially available single-component polyurethane adhesives can only effectively bond paper / plastic structures. For paper / aluminum composite structures, their bonding strength is low, which seriously affects the sealing performance of packaging bags containing paper-plastic-aluminum composite structures, thus limiting their application range. Furthermore, their bonding temperature needs to be as high as 90℃, resulting in high energy consumption and a risk of burns to the adhesive application operators.

[0006] Due to the low thermal conductivity of paper, some flexible packaging composite manufacturers tend to shorten the heat-sealing time to improve production efficiency, which necessitates increasing the heat-sealing temperature to ensure the airtightness of the packaging bag. However, most solvent-free polyurethane adhesives for paper-plastic composites on the market suffer from poor temperature resistance, primarily manifested as delamination of the composite layers or blistering and delamination at the heat-sealed packaging area. For example, most patents, such as Chinese Patent CN106497493A and Chinese Patent CN106883807A, disclose single-component solvent-free polyurethane adhesives for paper / plastic and paper / aluminum composites, but none describe their resistance to high-temperature heat sealing.

[0007] To expand the application range of single-component solvent-free polyurethane adhesives and meet the process requirements of most manufacturers as much as possible, it is particularly important to develop a solvent-free polyurethane adhesive with low coating temperature, which can also be used in composite paper / plastic, paper / aluminum, and aluminum / plastic structures and has good temperature resistance. Summary of the Invention

[0008] To address the problems existing in the prior art, this invention provides a heat-resistant, single-component, solvent-free polyurethane adhesive for paper lamination, its preparation method, and its application. This heat-resistant, single-component, solvent-free polyurethane adhesive, applicable to composite paper-plastic-aluminum structures, features a low coating temperature while simultaneously meeting the requirements for solvent-free lamination in terms of curing time and bond strength.

[0009] One objective of this invention is to provide a heat-resistant, single-component, solvent-free polyurethane adhesive for paper lamination.

[0010] The heat-resistant single-component solvent-free polyurethane adhesive for paper lamination of the present invention comprises structural units derived from polyester polyols, structural units derived from polyether polyols, and end-capping units derived from isocyanates.

[0011] Based on a total weight of 100 wt% of the structural units derived from polyester polyols, structural units derived from polyether polyols, and end-capping units derived from isocyanates:

[0012] The structural units derived from polyester polyols comprise 50-59 wt%;

[0013] The structural units derived from polyether polyols are 8-12 wt%;

[0014] The isocyanate-derived end-capping unit is 31-40 wt%.

[0015] In a preferred embodiment of the present invention:

[0016] Based on a total weight of 100 wt% of the structural units derived from polyester polyols, structural units derived from polyether polyols, and end-capping units derived from isocyanates:

[0017] The structural units derived from polyester polyols comprise 51-55 wt%;

[0018] The structural units derived from polyether polyols comprise 9-10 wt%;

[0019] The isocyanate-derived end-capping unit is 36-39 wt%.

[0020] In a preferred embodiment of the present invention:

[0021] The adhesive has an NCO value of 6.00%-8.50%, preferably 6.75-8.00%; and / or a viscosity of 6000-15000 mPa·s at 65°C, preferably 7500-10000 mPa·s.

[0022] The present invention relates to a heat-resistant, paper-aluminum composite, single-component, solvent-free polyurethane adhesive, which is a polyurethane prepolymer end-capped with isocyanate (-NCO) groups.

[0023] A second objective of this invention is to provide a method for preparing a heat-resistant, single-component, solvent-free polyurethane adhesive for paper lamination, as described in one objective of this invention.

[0024] The method for preparing the heat-resistant single-component solvent-free polyurethane adhesive for paper lamination according to the present invention includes:

[0025] The adhesive is prepared by mixing polyester polyol and polyether polyol, then reacting them with isocyanate at a heated temperature; preferably,

[0026] Based on a total weight of 100 wt% of the polyester polyol, polyether polyol, and isocyanate,

[0027] Polyester polyol 50-59 wt%, preferably 51-55 wt%;

[0028] 8-12 wt% of polyether polyol, preferably 9-10 wt%;

[0029] Isocyanate 31-40 wt%, preferably 36-39 wt%.

[0030] In a preferred embodiment of the present invention:

[0031] The polyester polyol is prepared by esterification and dehydration reaction of diacid and polyol under the protection of antioxidant, followed by polycondensation reaction under the catalysis of catalyst.

[0032] In a preferred embodiment of the present invention:

[0033] The molar ratio of the polyol to the diacid is (1.15-1.30):1, preferably (1.20-1.25):1; and / or,

[0034] The antioxidant is 100-200 ppm, preferably 100-120 ppm, of the total weight of the polyol and diacid; and / or,

[0035] The catalyst is 10-100 ppm of the total weight of polyols and dicarboxylic acids, preferably 20-50 ppm.

[0036] In a preferred embodiment of the present invention:

[0037] The dicarboxylic acid is a dicarboxylic acid, preferably at least one of succinic acid, adipic acid, and sebacic acid; and / or

[0038] The polyol is an aliphatic polyol, preferably at least one selected from ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, and diethylene glycol; and / or,

[0039] The antioxidant is a triphenyl phosphate derivative and / or a triphenyl phosphite derivative; and / or,

[0040] The catalyst is at least one of dibutyltin dilaurate, stannous octoate, tetrabutyl titanate, and tetraisopropyl titanate.

[0041] In a preferred embodiment of the present invention:

[0042] The esterification and dehydration reaction includes: heating a diacid, a polyol, and an antioxidant to a temperature under a protective gas atmosphere until cooling water is collected in the reaction apparatus; stopping the supply of the protective gas; and then carrying out a first esterification reaction and a second esterification reaction to obtain an intermediate product; preferably,

[0043] The heating rate of the first esterification reaction is 8-12 °C / h, and / or the reaction temperature is 190-200 °C, and / or the reaction time is 1-2 h; and / or,

[0044] The second esterification reaction is carried out at a heating rate of 8-12 °C / h, and / or at a reaction temperature of 220-240 °C, and / or for a reaction time of 2-3 h; and / or,

[0045] The acid value of the intermediate product after the esterification and dehydration reaction is less than 30 mg KOH / g.

[0046] In a preferred embodiment of the present invention:

[0047] The polycondensation reaction includes: adding a catalyst under a protective gas atmosphere, carrying out a first depressurization reaction and a second depressurization reaction, and stopping the reaction after sampling and testing to ensure that a preset acid value and a preset hydroxyl value are reached; preferably,

[0048] The vacuum degree of the first decompression reaction is -0.075 to -0.085 MPa; and / or, the reaction time is 15-60 min; and / or,

[0049] The vacuum degree of the second decompression reaction is -0.095 to -0.100 MPa; and / or,

[0050] The preset acid value is 0.2-2.0 mgKOH / g, preferably 0.2-1.5 mgKOH / g, and / or the preset hydroxyl value is 45-68 mgKOH / g, preferably 50-56 mgKOH / g.

[0051] In a preferred embodiment of the present invention:

[0052] The polyether polyol is at least one selected from polyethylene oxide glycol, polypropylene oxide glycol, polypropylene oxide polyol, polytetrahydrofuran glycol, and tetrahydrofuran-propylene oxide copolyether alcohol; preferably, the average molecular weight of the polyether polyol is 200-2000 g / mol; and / or,

[0053] The isocyanate is a diisocyanate, preferably at least one of toluene diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate, 1,6-hexanediisocyanate, and isophorone diisocyanate.

[0054] In a preferred embodiment of the present invention:

[0055] The mixing of the polyester polyol and the polyether polyol is carried out under a protective gas atmosphere, and / or the moisture content after mixing is <0.05%; if the moisture content is ≥0.05%, a dehydration treatment is performed; preferably,

[0056] The dehydration treatment is carried out at a temperature of 110-130℃, and / or with a vacuum degree below -0.095MPa, and / or for a treatment time of 2-4 hours; and / or,

[0057] The heating reaction is carried out under a protective gas atmosphere, and / or the reaction temperature is 75-95°C, and / or the reaction time is 2-3 hours.

[0058] The following solutions can be adopted:

[0059] (I) Synthesis of polyester polyols:

[0060] 1. Esterification stage: According to the formula, add the polyol, diacid, and antioxidant to the reactor used for polyester synthesis. Under a nitrogen atmosphere, heat until cooling water begins to collect through the long-spined column and condenser. Immediately stop supplying nitrogen and raise the temperature to 190-200℃ at a rate of 8-12℃ per hour, holding the temperature for 1-2 hours. Then continue raising the temperature to 220-240℃ at a rate of 8-12℃ per hour, holding the temperature for 2-3 hours. After sampling and testing, once the acid value is less than 30 mg KOH / g, the polycondensation stage can begin. Note that the peak temperature must be kept below 105℃ throughout the entire process.

[0061] 2. Polycondensation reaction stage: Maintain a constant temperature of 220-240℃ throughout the process, and perform vacuum distillation at a rate of -0.02 MPa per hour until the vacuum degree reaches -0.075 to -0.085 MPa. Inject nitrogen to atmospheric pressure, slowly add the catalyst, and stir for 15 minutes until a homogeneous state is reached. Stop supplying nitrogen, allowing the vacuum degree to reach -0.075 to -0.085 MPa and maintain this vacuum degree within this range for 15-60 minutes. Then, control the top temperature below 100℃ at a certain decompression rate until the vacuum degree reaches -0.095 to -0.100 MPa. Continue the reaction and distill off excess alcohol and trace amounts of water until the sample test shows that the preset acid value and hydroxyl value are reached. Stop the reaction and inject the synthesized polyester polyol into a storage tank. Allow it to cool naturally to 50-60℃ before proceeding to the next step.

[0062] (II) Mixing of polyols:

[0063] The formulated amounts of polyester polyol and polyether polyol are injected into a reactor for synthesizing adhesives. The mixture is stirred for 30 minutes under a nitrogen atmosphere until homogeneous. A sample is taken to test the moisture content. If the moisture content is less than 0.05%, the next step can be carried out. If the moisture content is greater than or equal to 0.05%, the moisture needs to be removed for 2-4 hours under an internal temperature of 110-130℃ and a vacuum degree below -0.095MPa until the moisture content of the sample meets the standard before the next step can be carried out.

[0064] (III) Synthesis of polyurethane adhesives:

[0065] Cold water is circulated through the reactor wall to rapidly cool the mixture with the required moisture content as described in (II) to below 40°C. The prescribed amount of diisocyanate is added under a nitrogen atmosphere, and the internal temperature is slowly raised to 75-95°C. The reaction is carried out within this temperature range for 2-3 hours until the sample test shows that the preferred NCO value and viscosity (65°C) are achieved.

[0066] A third objective of this invention is to provide an application of a heat-resistant single-component solvent-free polyurethane adhesive for composite paper as described in one objective of this invention, or a heat-resistant single-component solvent-free polyurethane adhesive for composite paper prepared by the method described in another objective of this invention, in the field of flexible packaging with composite paper-plastic-aluminum structures.

[0067] When using a solventless laminating machine, the adhesive is preheated to 75-80℃ before application; during application, the temperatures of the metering roller, transfer roller, and application roller are set to 70-80℃, and the temperature of the pressing roller is set to 30-50℃; the application rate is controlled at 2-6 g / m². 2 The laminating machine speed is 100-200 m / min; during lamination, all rollers are kept at a constant temperature within the set temperature range, and the relative humidity is controlled within the range of 40-75%. If the ambient humidity is too low, the air environment of the laminating plant can be moderately humidified; possible lamination structures are: plastic / paper / plastic, plastic / paper / aluminum / plastic, and specific lamination structures can be PET / paper / PE, PET / paper / aluminum / PE; the curing time in the drying room after lamination is 20-48 hours, and the curing temperature is 40-55℃.

[0068] Because paper has a microporous structure and strong adsorption capacity, commonly used two-component polyurethane adhesives have too low viscosity and react very slowly with moisture, easily penetrating the paper before curing, causing the paper to harden and discolor, affecting the appearance of flexible packaging containing paper materials. However, this invention provides a heat-resistant, single-component, solvent-free polyurethane adhesive for paper lamination with only one component and a higher viscosity. In an environment with a certain relative humidity, it rapidly undergoes a cross-linking reaction with moisture, allowing the adhesive to cure and form a stable adhesive layer before penetrating the paper after lamination with other materials using a solvent-free laminating machine. This enables it to provide good adhesion between paper and other types of substrates without excessive penetration. The mechanism of this adhesion is that the products of its cross-linking reaction with moisture in the environment generate hydrogen bonds and mechanical forces with the substrate being bonded, thereby enhancing molecular cohesion and achieving a good adhesive effect.

[0069] Furthermore, due to the low thermal conductivity of paper, some flexible packaging composite manufacturers tend to shorten the heat-sealing time in order to improve production efficiency, which necessitates increasing the heat-sealing temperature to ensure the airtightness of the packaging bag. Compared with most single-component adhesive products on the market, the heat-resistant single-component solvent-free polyurethane adhesive for paper lamination of this invention has a certain cross-linked network structure, which is beneficial to improving its temperature resistance and meeting the high-temperature heat-sealing requirements of most flexible packaging composite manufacturers for paper-containing flexible packaging.

[0070] The beneficial effects of this invention are:

[0071] (1) The heat-resistant single-component solvent-free polyurethane adhesive for paper lamination of the present invention has a simple composition, can be mass-produced industrially, has good bonding strength, and can be widely used in the flexible packaging industry for paper / plastic, paper / aluminum and aluminum / plastic composites.

[0072] (2) The heat-resistant single-component solvent-free polyurethane adhesive for paper lamination of the present invention can meet the basic requirements of the composite paper-containing material layer structure by controlling the preheating temperature before application and the set temperature of each wire roller of the solvent-free laminating machine at 70-80℃, which is lower than the 90-100℃ of other commercially available single-component solvent-free polyurethane adhesives.

[0073] (3) The curing time of the heat-resistant single-component solvent-free polyurethane adhesive for paper lamination of the present invention is 20h-48h after sizing, and the curing temperature is 40-55℃.

[0074] (4) After the heat-resistant single-component solvent-free polyurethane adhesive for paper composites of the present invention is fully cured, the peel strength of each composite layer is greater than 2.0 N / (15 mm); when peeling the paper / plastic and paper / aluminum structures, the paper fibers are easily transferred to the surface of the plastic and aluminum foil layers respectively; when peeling the plastic / aluminum structure, the aluminum foil can be pulled to the surface of the plastic structure.

[0075] (5) After the heat-resistant single-component solvent-free polyurethane adhesive for paper composite of the present invention is fully cured, when making flexible packaging bags, the heat-resistant conditions that the composite structure PET / paper / PE can withstand are 250-260℃ for 0.5s and the heat-resistant conditions that the composite structure PET / paper / aluminum / PE can withstand are 290-300℃ for 0.5s. Detailed Implementation

[0076] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0077] The raw materials used in the embodiments and comparative examples of this invention are all commercially available products.

[0078] The test methods and standards involved in the embodiments and comparative examples of this invention are as follows:

[0079] Acid value: Tested according to HG / T 2708-1995 standard;

[0080] Hydroxyl value: Tested according to HG / T 2709-1995 standard;

[0081] Viscosity: Tested according to GB / T 2794-2013 standard (rotational viscometer method);

[0082] Moisture content: Tested according to GB / T 6283-2008 standard;

[0083] NCO value: Tested according to HG / T 2409-1992 standard.

[0084]

Example 1

[0085] A temperature-resistant, one-component, solvent-free polyurethane adhesive is prepared using the following method:

[0086] 1. Esterification stage: With a feed ratio of 1.25:1 (alkyd to acid molar ratio), 1,2-propanediol, adipic acid, and 115 ppm of antioxidant 168 (total weight of alkyd) were added to the reactor for polyester synthesis. Under a nitrogen atmosphere, the temperature was raised until cooling water began to be collected through the long-spiked column and condenser. Nitrogen supply was immediately stopped, and the temperature was increased to 190°C at a rate of 10°C per hour, and held for 2 hours. The temperature was then increased to 220-240°C at a rate of 10°C per hour, and held for 3 hours. The peak temperature was controlled below 105°C throughout the process.

[0087] 2. Polycondensation reaction stage: The entire process is carried out at a constant temperature of 220-240℃, with vacuum distillation performed at a rate of -0.02 MPa per hour until the vacuum degree reaches -0.08 MPa. Nitrogen gas is injected to atmospheric pressure, and 15 ppm of tetrabutyl titanate catalyst (total weight of alkyd and acid) is slowly added. After stirring for 15 minutes until homogeneous, the nitrogen supply is stopped, and the vacuum degree reaches -0.08 MPa and is maintained for 30 minutes. Then, the top temperature is controlled below 100℃ at a certain decompression rate until the vacuum degree reaches -0.1 MPa. The reaction continues, and excess alcohol and trace amounts of water are distilled off. Finally, a polyester polyol with an acid value of 1.1 mg KOH / g and a hydroxyl value of 56 mg KOH / g is obtained.

[0088] 3. Mixing of polyols: 51 parts by weight of the above-mentioned polyester polyol and 10 parts by weight of polyoxypropylene polyol with an average molecular weight of 400 g / mol were injected into a reactor for synthesizing adhesives. The mixture was stirred for 30 minutes under a nitrogen atmosphere until homogeneous. The moisture content was measured to be 0.023%.

[0089] 4. Synthesis of polyurethane adhesive: Cool water is circulated through the reactor wall to rapidly cool it to below 40°C. Under a nitrogen atmosphere, 39 parts by weight of diphenylmethane diisocyanate are added, and the internal temperature is slowly raised to 75-95°C for 3 hours.

[0090] The obtained polyurethane adhesive has an NCO value of 7.53% and a viscosity of 7900 mPa·s at an internal temperature of 65℃.

[0091]

Example 2

[0092] A temperature-resistant, one-component, solvent-free polyurethane adhesive is prepared using the following method:

[0093] 1. Esterification stage: With a feed ratio of 1.23:1 (alkyd to acid molar ratio), diethylene glycol, succinic acid, and 150 ppm of antioxidant 168 (total weight of alkyd) were added to the polyester synthesis reactor. Under a nitrogen atmosphere, the temperature was raised until cooling water began to collect through a long-spiked column and condenser. Nitrogen supply was immediately stopped, and the temperature was increased to 190°C at a rate of 10°C per hour, and held for 2 hours. The temperature was then increased further at a rate of 10°C per hour to 220-240°C, and held for 3 hours. The peak temperature was controlled below 105°C throughout the process.

[0094] 2. Polycondensation reaction stage: Maintain a constant temperature of 220-240℃ throughout, and perform vacuum distillation at a rate of -0.02 MPa per hour until the vacuum reaches -0.08 MPa. Inject nitrogen to atmospheric pressure, then slowly add 20 ppm of tetraisopropyl titanate as a catalyst. Stir for 15 minutes until homogeneous, then stop supplying nitrogen, allowing the vacuum to reach -0.08 MPa and maintaining this pressure for 30 minutes. Then, control the top temperature to be below 100℃ at a controlled decompression rate until the vacuum reaches -0.1 MPa. Continue the reaction and distill off excess alcohol and trace amounts of water. The final product is a polyester polyol with an acid value of 0.9 mg KOH / g and a hydroxyl value of 55 mg KOH / g.

[0095] 3. Mixing of polyols: 52 parts by weight of the above-mentioned polyester polyol and 9 parts by weight of polytetrahydrofuran diol with an average molecular weight of 400 g / mol were injected into a reactor for synthesizing adhesives. The mixture was stirred for 30 minutes under a nitrogen atmosphere until it was homogeneous. The moisture content was measured to be 0.018%.

[0096] 4. Synthesis of polyurethane adhesive: Cool water is circulated through the reactor wall to rapidly cool it to below 40°C. 39 parts by weight of toluene diisocyanate are added under a nitrogen atmosphere, and the internal temperature is slowly raised to 75-95°C for 3 hours.

[0097] The obtained polyurethane adhesive has an NCO value of 7.76% and a viscosity of 6100 mPa·s at an internal temperature of 65℃.

[0098]

Example 3

[0099] A temperature-resistant, one-component, solvent-free polyurethane adhesive is prepared using the following method:

[0100] 1. Esterification stage: With a feed alkyd-acid molar ratio of 1.20:1, diethylene glycol, 1,3-propanediol (weight ratio of diethylene glycol to 1,3-propanediol is 3:2), sebacic acid, and 165 ppm of antioxidant 168 (total weight of alkyd) were added to the reactor for polyester synthesis. Under a nitrogen atmosphere, the temperature was raised until cooling water began to collect through a long-spiked column and condenser. Nitrogen supply was immediately stopped, and the temperature was increased to 190°C at a rate of 10°C per hour, and held for 2 hours. The temperature was then increased to 220-240°C at a rate of 10°C per hour, and held for 3 hours. The peak temperature was controlled below 105°C throughout the process.

[0101] 2. Polycondensation reaction stage: The entire process is carried out at a constant temperature of 220-240℃, with vacuum distillation performed at a rate of -0.02 MPa per hour until the vacuum degree reaches -0.08 MPa. Nitrogen gas is injected to atmospheric pressure, and 50 ppm of stannous octoate catalyst (total weight of alkyd and acid) is slowly added. After stirring for 15 minutes until homogeneous, the nitrogen supply is stopped, and the vacuum degree is maintained at -0.08 MPa for 30 minutes. Then, the top temperature is controlled below 100℃ at a certain decompression rate until the vacuum degree reaches -0.1 MPa. The reaction continues, and excess alcohol and trace amounts of water are distilled off. Finally, a polyester polyol with an acid value of 1.1 mg KOH / g and a hydroxyl value of 51 mg KOH / g is obtained.

[0102] 3. Mixing of polyols: 54 parts by weight of the above-mentioned polyester polyol and 9 parts by weight of tetrahydrofuran-propylene oxide copolyether alcohol with an average molecular weight of 330 g / mol were injected into a reactor for synthesizing adhesives. The mixture was stirred for 30 min under a nitrogen atmosphere until it was homogeneous. The moisture content was measured to be 0.045%.

[0103] 4. Synthesis of polyurethane adhesive: Cool water is circulated through the reactor wall to rapidly cool it to below 40°C. Under a nitrogen atmosphere, 15 parts by weight of toluene diisocyanate and 22 parts by weight of diphenylmethane diisocyanate are added, and the internal temperature is slowly raised to 75-95°C for 3 hours.

[0104] The obtained polyurethane adhesive has an NCO value of 7.03% and a viscosity of 10350 mPa·s at an internal temperature of 65℃.

[0105] Comparative Example 1

[0106] A one-component solvent-free polyurethane adhesive is prepared using the following method:

[0107] 1. Esterification stage: With a feed ratio of 1.20:1 (alkyd to acid molar ratio), diethylene glycol, adipic acid, and 125 ppm of antioxidant 168 (total weight of alkyd) were added to the polyester synthesis reactor. Under a nitrogen atmosphere, the temperature was raised until cooling water began to collect through a long-spiked column and condenser. Nitrogen supply was immediately stopped, and the temperature was increased to 190°C at a rate of 10°C per hour, and held for 2 hours. The temperature was then increased to 220-240°C at a rate of 10°C per hour, and held for 3 hours. The peak temperature was controlled below 105°C throughout the process.

[0108] 2. Polycondensation reaction stage: Maintain a constant temperature of 220-240℃ throughout, and perform vacuum distillation at a rate of -0.02 MPa per hour until the vacuum reaches -0.08 MPa. Inject nitrogen to atmospheric pressure, then slowly add 20 ppm of tetrabutyl titanate as a catalyst. Stir for 15 minutes until homogeneous, then stop supplying nitrogen, allowing the vacuum to reach -0.08 MPa and maintaining this pressure for 30 minutes. Then, control the top temperature to be below 100℃ at a controlled decompression rate until the vacuum reaches -0.1 MPa. Continue the reaction and distill off excess alcohol and trace amounts of water. The final product is a polyester polyol with an acid value of 0.9 mg KOH / g and a hydroxyl value of 55 mg KOH / g.

[0109] 3. Synthesis of adhesive: 74 parts by weight of polyester polyol were injected into a reactor for synthesizing adhesive. Cold water was circulated to rapidly cool the reactor wall to below 40°C. Under a nitrogen atmosphere, 26 parts by weight of diphenylmethane diisocyanate were added, and the internal temperature was slowly raised to 75-95°C for 3 hours.

[0110] The obtained adhesive has an NCO value of 5.81% and a viscosity of 5650 mPa·s at an internal temperature of 65℃.

[0111] The adhesives prepared in the above examples and comparative examples were applied and laminated using a solventless laminating machine. Before application, the adhesives were preheated to 75-80°C. During application, the temperatures of the metering roller, transfer roller, and coating roller were set to 70-80°C, and the temperature of the pressing roller was set to 30-50°C. The application amount was controlled at 2-6 g / m³. 2 The laminating machine speed is 100-200m / min; during lamination, all rollers are kept at a constant temperature and the relative humidity is controlled at 40-75%; after lamination, the curing temperature in the drying room is 40-55℃, and the relative humidity of the air outside the drying room is controlled at 40-75%.

[0112] The composite structure is: PET / paper / PE; PET / paper / aluminum / PE; wherein the PET film is 12μm thick, the paper is printable yellow kraft paper with a specification of 7g*m, the aluminum foil is 6.5μm thick, and the PE film is 60μm thick.

[0113] The above-mentioned PET / paper / PE and PET / paper / aluminum / PE composite structures were removed after curing for 2 hours and 20 hours and cooled to room temperature. The peel strength of each layer of the composite structure was tested. When no paper fiber transfer or aluminum foil breakage was observed when peeling by hand or testing on a tensile testing machine, the average range of the T-shaped peel strength was recorded. The test results of the samples cured for 2 hours are shown in Table 1 below, and the test results of the samples cured for 20 hours are shown in Table 2 below.

[0114] Table 1

[0115] Testing items PET / paper (or paper / PE) Paper / Aluminum Aluminum / PE Example 1 Paper fiber transfer Paper fiber transfer 1.7-2.5N / (15mm) Example 2 Paper fiber transfer 1.5-2.0N / (15mm) 1.0-1.5N / (15mm) Example 3 Paper fiber transfer Paper fiber transfer 2.0-2.5N / (15mm) Comparative Example 1 Paper fiber transfer 0.2-0.6 N / (15 mm) Unable to test

[0116] As shown in Table 1 above, after 2 hours of curing, Examples 1-3 and Comparative Example 1 can all transfer paper fibers from the PET / paper composite structure. Comparative Example 1, lacking a certain cross-linking network structure due to the absence of polyether polyol in its molecular design, has a lower efficiency in bonding the paper / aluminum composite structure. The "unable to test" mentioned in Table 1 is because the peel strength of the paper / aluminum structure composited using Comparative Example 1 after 2 hours of curing is only 0.2-0.6 N / (15 mm), which is significantly different from the peel strength of other composite layers. This results in the paper / aluminum layer always peeling off during the peeling of the aluminum / PE layer.

[0117] Table 2

[0118] Testing items PET / paper (or paper / PE) Paper / Aluminum Aluminum / PE Example 1 Paper fiber transfer Paper fiber transfer Aluminum foil breaks Example 2 Paper fiber transfer Paper fiber transfer Aluminum foil breaks Example 3 Paper fiber transfer Paper fiber transfer Aluminum foil breaks Comparative Example 1 Paper fiber transfer 1.0-1.3N / (15mm) Unable to test

[0119] As shown in Table 2 above, after 20 hours of curing, Examples 1-3 and Comparative Example 1 all enabled the transfer of paper fibers in the PET / paper composite structure and the breakage of the aluminum foil in the aluminum / PE composite structure. Comparative Example 1, lacking a certain cross-linking network structure due to the absence of polyether polyol in its molecular design, resulted in lower efficiency in bonding the paper / aluminum composite structure. The "unable to test" mentioned in Table 2 is because the peel strength of the paper / aluminum structure composited using Comparative Example 1 after 20 hours of curing was only 1.0-1.3 N / (15 mm), significantly different from the peel strength of other composite layers, causing it to always peel down to the paper / aluminum layer when peeling the aluminum / PE layer.

[0120] After the above PET / paper / PE and PET / paper / aluminum / PE composite structures were cured for 20 hours, samples were taken for a constant temperature heat sealing test for 0.5 seconds. The test results are shown in Table 3 below. The "tolerable" performance is characterized by the absence of delamination of the composite layer in the composite structure, or the presence of poor temperature resistance such as blistering and delamination at the heat-sealed packaging area.

[0121] Table 3

[0122]

[0123] As shown in Table 3 above, after 20 hours of curing, the PET / paper / PE and PET / paper / aluminum / PE composite structures underwent a continuous temperature resistance test for 0.5 seconds. Examples 1-3 consistently withstood temperatures 10-30°C higher than Comparative Example 1. This indicates that the adhesive contains polyether polyols in its molecular structure, which can form a certain cross-linked network structure. This not only ensures the bonding effect of the paper / aluminum composite structure but also improves the temperature resistance of the adhesive to a certain extent.

[0124] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A heat-resistant, single-component, solvent-free polyurethane adhesive for paper lamination, characterized in that... The adhesive has a structural formula including structural units derived from polyester polyols, structural units derived from polyether polyols, and end-capping units derived from isocyanates. Based on a total weight of 100 wt% of the structural units derived from polyester polyols, structural units derived from polyether polyols, and end-capping units derived from isocyanates: The structural units derived from polyester polyols comprise 50-59 wt%; The structural units derived from polyether polyols are 8-12 wt%; The isocyanate-derived end-capping unit is 31-40 wt%.

2. The adhesive according to claim 1, characterized in that: Based on a total weight of 100 wt% of the structural units derived from polyester polyols, structural units derived from polyether polyols, and end-capping units derived from isocyanates: The structural units derived from polyester polyols comprise 51-55 wt%; and / or, The structural units derived from the polyether polyol comprise 9-10 wt%; and / or, The isocyanate-derived end-capping unit is 36-39 wt%; and / or, The adhesive has an NCO value of 6.00%-8.50%, preferably 6.75%-8.00%; and / or a viscosity of 6000-15000 mPa·s at 65°C, preferably 7500-10000 mPa·s.

3. A process for the preparation of a one-part solvent-free polyurethane adhesive for temperature-resistant laminated paper, according to claim 1 or 2, characterized in that The method includes: The adhesive is prepared by mixing polyester polyol and polyether polyol, then reacting them with isocyanate at a heated temperature; preferably, Based on a total weight of 100 wt% of the polyester polyol, polyether polyol, and isocyanate, Polyester polyol 50-59 wt%, preferably 51-55 wt%; 8-12 wt% polyether polyol, preferably 9-10 wt%; Isocyanate 31-40 wt%, preferably 36-39 wt%.

4. The method according to claim 3, characterized in that: The polyester polyol is prepared by esterification and dehydration of a diacid and a polyol under the protection of an antioxidant, followed by polycondensation under the catalysis of a catalyst; preferably, The molar ratio of the polyol to the diacid is (1.20-1.30):1, preferably (1.23-1.25):1; and / or, The antioxidant is 100-200 ppm, preferably 100-120 ppm, of the total weight of the polyol and diacid; and / or, The catalyst is 10-100 ppm of the total weight of polyols and dicarboxylic acids, preferably 20-50 ppm.

5. The method according to claim 4, characterized in that: The dicarboxylic acid is a dicarboxylic acid, preferably at least one of succinic acid, adipic acid, and sebacic acid; and / or The polyol is an aliphatic polyol, preferably at least one selected from ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, and diethylene glycol; and / or, The antioxidant is a triphenyl phosphate derivative and / or a triphenyl phosphite derivative; and / or, The catalyst is at least one of dibutyltin dilaurate, stannous octoate, tetrabutyl titanate, and tetraisopropyl titanate.

6. The method according to claim 4, characterized in that: The esterification and dehydration reaction includes: heating a diacid, a polyol, and an antioxidant to a temperature under a protective gas atmosphere until cooling water is collected in the reaction apparatus; stopping the supply of the protective gas; and then carrying out a first esterification reaction and a second esterification reaction to obtain an intermediate product; preferably, The heating rate of the first esterification reaction is 8-12 °C / h, and / or the reaction temperature is 190-200 °C, and / or the reaction time is 1-2 h; and / or, The second esterification reaction is carried out at a heating rate of 8-12 °C / h, and / or at a reaction temperature of 220-240 °C, and / or for a reaction time of 2-3 h; and / or, The acid value of the intermediate product after the esterification and dehydration reaction is less than 30 mg KOH / g.

7. The method according to claim 4, characterized in that: The polycondensation reaction includes: adding a catalyst under a protective gas atmosphere, carrying out a first depressurization reaction and a second depressurization reaction, and stopping the reaction after sampling and testing to ensure that a preset acid value and a preset hydroxyl value are reached; preferably, The vacuum degree of the first decompression reaction is -0.075 to -0.085 MPa; and / or, the reaction time is 15-60 min; and / or, The vacuum degree of the second decompression reaction is -0.095 to -0.100 MPa; and / or, The preset acid value is 0.2-2.0 mgKOH / g, preferably 0.2-1.5 mgKOH / g, and / or the preset hydroxyl value is 45-68 mgKOH / g, preferably 50-56 mgKOH / g.

8. The method according to claim 3, characterized in that: The polyether polyol is at least one selected from polyethylene oxide glycol, polypropylene oxide glycol, polypropylene oxide polyol polytetrahydrofuran glycol, and tetrahydrofuran-propylene oxide copolyether alcohol; preferably, the average molecular weight of the polyether polyol is 200-2000 g / mol; and / or, The isocyanate is a diisocyanate, preferably at least one of toluene diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate, 1,6-hexanediisocyanate, and isophorone diisocyanate.

9. The method according to claim 3, characterized in that: The mixing of the polyester polyol and the polyether polyol is carried out under a protective gas atmosphere, and / or the moisture content after mixing is <0.05%; if the moisture content is ≥0.05%, a dehydration treatment is performed; preferably, The dehydration treatment is carried out at a temperature of 110-130℃, and / or with a vacuum degree below -0.095MPa, and / or for a treatment time of 2-4 hours; and / or, The heating reaction is carried out under a protective gas atmosphere, and / or the reaction temperature is 75-95°C, and / or the reaction time is 2-3 hours.

10. The application of a heat-resistant single-component solvent-free polyurethane adhesive for composite paper as described in claim 1 or 2, or a heat-resistant single-component solvent-free polyurethane adhesive for composite paper prepared by the method described in any one of claims 3-9, in the field of flexible packaging with composite paper-plastic-aluminum structures.

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