Special binder for PP (polypropylene) aluminum-plastic composite film and preparation method thereof

Abstract

The invention discloses a special binder for a PP (polypropylene) aluminum-plastic composite film and a preparation method thereof, and belongs to the technical field of laminating adhesives, the binder mainly comprises a modified polyester polyol component and an isocyanate-terminated prepolymer component; wherein the modified polyester polyol component comprises modified polyester polyol with the solid content of 20%-50% and ethyl acetate; and the isocyanate group terminated prepolymer component is prepared from an isocyanate group terminated prepolymer and ethyl acetate. In addition, the invention also discloses a preparation method of the modified polyester polyol component and the isocyanate-terminated prepolymer component, the palm oil modified polyester polyol component is used as a main agent, the isocyanate-terminated prepolymer component is used as a curing agent, and the main agent and the curing agent are compounded to obtain the solvent-free double-component laminating adhesive. By utilizing the double-component laminating adhesive, the peel strength of the PP aluminum-plastic composite film can be obviously improved.

Description

Technical Field

[0001] The invention relates to the technical field of laminating adhesives, in particular to a special adhesive for PP aluminum-plastic composite films and a preparation method thereof. Background Art

[0002] Laminating adhesive can also be called adhesive for composite film, which is mainly used on paper-plastic, plastic-plastic and aluminum-plastic substrates. Laminating adhesive has the advantages of low temperature curing, strong adhesion, water resistance, solvent resistance and low temperature resistance; currently, it has been widely used in the production process of pp aluminum-plastic composite film.

[0003] Specifically, laminating adhesives can be classified into solvent-based laminating adhesives, water-based laminating adhesives, and solvent-free laminating adhesives according to the solvents used. Among them, water-based laminating adhesives use water to replace organic solvents, which fundamentally solves the problem of solvent residues; however, water-based laminating adhesives have problems such as slow drying speed, poor water resistance, and low composite strength. The solvents used in solvent-based laminating adhesives are mostly toluene, gasoline, ethyl acetate, etc.; during the coating process, these solvents will evaporate into the air, which will not only waste resources, but also cause serious pollution to the environment, and have safety hazards such as flammability and explosion; at the same time, it will also easily cause odor and solvent residue problems in composite materials. In today's increasingly environmentally friendly awareness, it has been gradually replaced. Solvent-free laminating adhesives do not have the addition of solvents in production. Its biggest advantages are low dosage, high-speed curing, and no residual solvents in the composite film. However, there are some problems with domestic solvent-free polyurethanes, such as insufficient R&D investment, equipment problems, high viscosity, and poor coating properties.

[0004] As people pay more and more attention to the sanitation and safety of packaging materials and environmental protection, some developed countries have formulated relevant laws and regulations to limit the use of solvents. All of these factors have prompted researchers to develop environmentally friendly adhesives. Therefore, green, environmentally friendly, safe and economical solvent-free laminating adhesives will be rapidly and widely used in the composite flexible packaging industry. Therefore, the research and development of low viscosity, high peel strength and boiling resistance products is the development direction of solvent-free laminating adhesives.

[0005] Based on this, Chinese patent CN110305283A discloses a polyurethane adhesive for composite polylactic acid film and aluminum foil, which includes two components A and B, a polyurethane prepolymer solution terminated with isocyanate groups obtained by reacting polyester polyol, polyether polyol and isocyanate in a solvent, and component B is a mixture of polyester polyol, polyether polyol, resin, chain extender and solvent. Among them, the mass percentage content of isocyanate groups in component A is 6-12%. The polyurethane adhesive for composite polylactic acid film and aluminum foil disclosed in the patent document can improve the wetting effect on polylactic acid film and aluminum foil by using polyester polyol synthesized from long-chain carbon atom polyacid to prepare polyurethane prepolymer, and it has excellent bonding effect on polylactic acid film and aluminum foil; it uses polyether polyol with amine as initiator, which can increase the reaction speed of component A and component B, further accelerate the formation of initial adhesion, and catalyze the curing reaction.

[0006] However, the above-mentioned polyurethane adhesive disclosed still has the technical problem that the peel strength cannot meet the use requirements. Specifically, the functional soft packaging products used today, for example, the use of packaging materials such as PP aluminum-plastic composite films cannot be separated from high-functional adhesives. The evaluation of the packaging industry has been increasingly closely related to the development of new adhesive materials and their production processes. With the advancement of science and technology and the enhancement of environmental awareness, the laminating adhesive has gradually developed from solvent-based to solvent-free and water-based. The advantages of solvent-based laminating adhesives such as high initial viscosity and final adhesion, good appearance of composite products and tensile strength resistance make it still retain a considerable share in the composite film market: but its disadvantages are that there is solvent release, the working environment is bad, and the residual solvent in the composite film is harmful to human health. The solvent-free type often has low initial viscosity and slow formation of bonding strength, and the composite process needs to be extended. Therefore, it is also necessary to develop a solvent-free adhesive that can form high bonding strength quickly so that it can meet the actual process requirements. Summary of the invention

[0007] Based on this, it is necessary to provide a special adhesive for PP aluminum-plastic composite film and a preparation method thereof in order to improve the peel strength of polyurethane adhesive.

[0008] A special adhesive for PP aluminum-plastic composite film comprises the following steps: it comprises: a modified polyester polyol component and an isocyanate-terminated prepolymer component; wherein the modified polyester polyol component comprises a modified polyester polyol with a solid content of 20%-50% and ethyl acetate; and the isocyanate-terminated prepolymer component comprises an isocyanate-terminated prepolymer and ethyl acetate.

[0009] Specifically, the ingredients of the modified polyester polyol include: isophthalic acid, terephthalic acid, sebacic acid, ethylene glycol, neopentyl glycol, antimony glycol and palm oil.

[0010] Specifically, the components of the isocyanate-terminated prepolymer include: polypropylene glycol 2000, polypropylene glycol 400, diphenylmethane diisocyanate and benzoyl chloride.

[0011] Further, a method for preparing the aforementioned special adhesive for PP aluminum-plastic composite film comprises the following steps: S1: first preparing a modified polyester polyol component: weighing a preset amount of modified polyester polyol by mass, chopping the modified polyester polyol and placing it in a container, adding a preset amount of ethyl acetate into the container, and sealing the container; heating and stirring the container to prepare a uniform and stable modified polyester polyol solution; S2: preparing an isocyanate-terminated prepolymer component: weighing a preset amount of isocyanate-terminated prepolymer in a glass container, adding a preset amount of ethyl acetate to dissolve it, and then diluting it into a curing agent solution of 1:5; S3: Compounding into a binder: After measuring a preset amount of the modified polyester polyol component and the isocyanate-terminated prepolymer component by mass, drop the isocyanate-terminated prepolymer component into the modified polyester polyol component, seal the container, stir thoroughly, and then place the mixed components at room temperature for a preset time.

[0012] Further, the preparation method of the modified polyester polyol is as follows: S11: First, close the esterification and polycondensation channel valves, and add solid mixed powders of terephthalic acid, isophthalic acid, sebacic acid, neopentyl glycol and ethylene glycol antimony into the polyester reactor according to the preset ratio; then, add ethylene glycol and start stirring at a frequency of 25Hz; after the power of the stirrer is stable, increase the stirring power to 50Hz; and when the material adding process is completed, close the feed port, evacuate and fill with nitrogen; then turn on the heating device of the reactor to heat the material, set the jacket heating temperature to 210℃, and gradually increase the temperature to 230℃ and 250℃; monitor the temperature and air pressure in the reactor; when the temperature in the reactor reaches 150℃, set the gas phase outlet temperature to 160℃, and control the tower top temperature to 130℃; when the temperature at the end of the condensation system drops below 100℃, the esterification reaction is complete; S12: After the esterification reaction is completed, the polycondensation stage is entered, the reaction temperature is increased to 275°C, the vacuum pump is turned on, the small molecules in the reaction system are extracted, and the polyester molecular chain continues to grow; the small molecules enter the cold trap through the pipeline, the phase state changes, and remain in the cold trap; when the reaction pressure is less than 100Pa, the small molecule content of the system is extremely low, and the valve in front of the by-product receiving tank is closed to prevent the small molecules therein from volatilizing and reducing the vacuum degree in the kettle; when the stirring paddle power reaches the specified power, the stirring frequency is reduced to 25Hz, the kettle body heating is turned off, and the reaction is stopped; N2 is filled into the kettle to normal pressure, the heating device is turned off, and when the temperature in the reactor drops below 180°C, the discharge hole at the bottom of the reactor is opened to discharge the material to prepare polyester polyols; S13: In a reaction kettle equipped with a stirrer, a condenser and a thermometer, a preset amount of catalyst methanesulfonic acid, polyester polyol and palm oil are added, and nitrogen is passed through to protect the mixture. The temperature is raised to 140 to 160°C while stirring, and an esterification reaction occurs with water flowing out. After the temperature is kept for dehydration for a preset time, the temperature is further raised to 210°C, and the head temperature is maintained below 100°C to carry out a condensation reaction; thereby preparing a modified polyester polyol.

[0013] Further, the preparation method of the isocyanate-terminated prepolymer is: S21: preparing polyether polyol, firstly placing polypropylene glycol 2000 and polypropylene glycol 400 in a molar ratio of 1:1 in a reaction kettle equipped with a stirrer and a thermometer, heating, stirring, heating and evacuating, dehydrating at 110 to 120° C. and a vacuum degree of 133.3 Pa, until no obvious bubbles are generated in the system, that is, the dehydration is completed, pouring the product into a drying bottle, filling it with nitrogen and sealing it for later use; S22: In a reaction kettle equipped with a stirrer, a condenser, a constant pressure dropping funnel and a thermometer, first add diphenylmethane diisocyanate of the same mass as the polyether polyol, then add the pre-dehydrated polyether polyol dropwise at 45°C for 1 hour, pass nitrogen protection, and keep the reaction warm for 2 hours. Then, add benzoyl chloride accounting for 1% of the total amount of diphenylmethane diisocyanate, stir evenly, and cool to room temperature to obtain the isocyanate-terminated prepolymer.

[0014] In summary, the present invention provides a special adhesive for PP aluminum-plastic composite film, which mainly includes a modified polyester polyol component and an isocyanate-terminated prepolymer component; wherein the modified polyester polyol component includes a modified polyester polyol with a solid content of 20%-50% and ethyl acetate; the isocyanate-terminated prepolymer component includes an isocyanate-terminated prepolymer and ethyl acetate. In addition, the present invention also discloses a preparation method of the aforementioned modified polyester polyol component and the isocyanate-terminated prepolymer component, and uses a palm oil-modified polyester polyol component as a main agent, and an isocyanate-terminated prepolymer component as a curing agent, and compounding the main agent and the curing agent to obtain a solvent-free two-component laminating adhesive; the two-component laminating adhesive can significantly improve the peel strength of the PP aluminum-plastic composite film. Therefore, the present invention provides a special adhesive for PP aluminum-plastic composite film and a preparation method thereof to solve the technical problem of how to improve the peel strength of polyurethane adhesives. DETAILED DESCRIPTION

[0015] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with specific embodiments. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0016] Specifically, the present invention provides a special adhesive for PP aluminum-plastic composite film, which includes: a modified polyester polyol component and an isocyanate-terminated prepolymer component; wherein the modified polyester polyol component includes a modified polyester polyol with a solid content of 20%-50% and ethyl acetate; the isocyanate-terminated prepolymer component includes an isocyanate-terminated prepolymer and ethyl acetate.

[0017] More specifically, the preparation method of the modified polyester polyol component is as follows: weigh a preset amount of modified polyester polyol by mass, chop the modified polyester polyol and place it in a container, add a preset amount of ethyl acetate into the container, and seal the container; heat and stir the container to prepare a uniform and stable modified polyester polyol solution, so as to prepare the modified polyester polyol component.

[0018] More specifically, the preparation method of the isocyanate-terminated prepolymer component is as follows: weigh a preset amount of isocyanate-terminated prepolymer in a glass container, add a preset amount of ethyl acetate to dissolve it, and then dilute it to a 1:5 curing agent solution to prepare the isocyanate-terminated prepolymer component.

[0019] The method for preparing a special adhesive for PP aluminum-plastic composite film of the present invention using a modified polyester polyol component and an isocyanate-terminated prepolymer component is as follows: after measuring a preset amount of the modified polyester polyol component and the isocyanate-terminated prepolymer component by mass, the isocyanate-terminated prepolymer component is dripped into the modified polyester polyol component, the container is sealed, and after sufficient stirring, the mixed components are placed at room temperature and allowed to stand for a preset time.

[0020] Furthermore, the modified polyester polyol comprises, by mass, 42 parts of isophthalic acid, 19 parts of terephthalic acid, 39 parts of sebacic acid, 86 parts of ethylene glycol, 14 parts of neopentyl glycol, 0.002 parts of ethylene glycol antimony and 30 parts of palm oil.

[0021] Specifically, the preparation method of the modified polyester polyol is as follows: Step 1: First, close the esterification and polycondensation channel valves, and add solid mixed powders of terephthalic acid, isophthalic acid, sebacic acid, neopentyl glycol and ethylene glycol antimony into the polyester reactor according to the preset ratio; then, add ethylene glycol and start stirring at a frequency of 25Hz; after the power of the agitator is stable, increase the stirring power to 50Hz; and when the material addition process is completed, close the feed port, vacuumize and fill with nitrogen; then turn on the heating device of the reactor to heat the material, set the jacket heating temperature to 210℃, and gradually increase the temperature to 230℃ and 250℃. Monitor the temperature and air pressure in the reactor. When the temperature in the reactor reaches 150℃, set the gas phase outlet temperature to 160℃ and control the tower top temperature to 130℃. When the temperature at the end of the condensation system drops below 100℃, the esterification reaction is complete.

[0022] Step 2: After the esterification reaction is completed, it enters the polycondensation stage. The reaction temperature is increased to 275°C, and the vacuum pump is turned on to extract the small molecules in the reaction system so that the polyester molecular chain continues to grow. The small molecules enter the cold trap through the pipeline, change phase, and remain in the cold trap. When the reaction pressure is less than 100Pa, the small molecule content of the system is extremely low. Close the valve in front of the by-product receiving tank to prevent the small molecules from volatilizing and reducing the vacuum degree in the kettle. When the power of the stirring paddle reaches the specified power, reduce the stirring frequency to 25Hz, turn off the kettle heating, and stop the reaction. Fill the kettle with N2 to normal pressure, turn off the heating device, and when the temperature in the reactor drops below 180°C, open the discharge hole at the bottom of the reactor to discharge the material to prepare polyester polyols.

[0023] Step 3: In a reaction kettle equipped with an agitator, a condenser and a thermometer, a preset amount of catalyst methanesulfonic acid, polyester polyol and palm oil are added, and nitrogen is passed through to protect the mixture. The temperature is raised to 140 to 160°C while stirring, and an esterification reaction occurs with water flowing out. After the temperature is kept for dehydration for a preset time, the temperature is further raised to 210°C, and the head temperature is maintained below 100°C to carry out a condensation reaction; thereby preparing a modified polyester polyol.

[0024] Specifically, natural vegetable oil - palm oil can be used to modify polyester polyols to increase the degree of biodegradation, thereby obtaining a composite film adhesive with moderate viscosity and transparent appearance, thereby solving the current technical problems of insufficient viscosity and low peel strength of solvent-free adhesives obtained from polyester polyols.

[0025] Furthermore, the components of the isocyanate-terminated prepolymer include: polypropylene glycol 2000, polypropylene glycol 400, diphenylmethane diisocyanate and benzoyl chloride.

[0026] Specifically, the preparation method of the isocyanate-terminated prepolymer is: Step 1: Prepare polyether polyols. First, place polypropylene glycol 2000 and polypropylene glycol 400 in a molar ratio of 1:1 in a reaction kettle equipped with a stirrer and a thermometer, heat them, start stirring, increase the temperature and evacuate, dehydrate at 110 to 120°C and a vacuum degree of 133.3 Pa, and dehydrate until no obvious bubbles are generated in the system. Pour the product into a drying bottle, fill it with nitrogen, seal it and store it for later use.

[0027] Step 2: In a reaction kettle equipped with a stirrer, a condenser, a constant pressure dropping funnel and a thermometer, first add diphenylmethane diisocyanate of the same mass as the polyether polyol, and then add the pre-dehydrated polyether polyol dropwise at 45°C for 1 hour. Pass nitrogen protection, keep the reaction warm for 2 hours, then add benzoyl chloride accounting for 1% of the total amount of diphenylmethane diisocyanate, stir evenly, and cool to room temperature to obtain an isocyanate-terminated prepolymer.

[0028] Furthermore, the T-peel strength test can be used to test the actual peel strength of the adhesive specifically for PP aluminum-plastic composite film of the present invention, specifically: Step 1: Preparation of composite film: take a preset amount of the mixed adhesive for PP aluminum-plastic composite film of the present invention on the aluminum foil, and evenly apply the adhesive on the surface of the aluminum foil with a scraper of preset thickness; place the coated aluminum foil in an oven at 80°C for 3 minutes to remove the solvent in the adhesive solution; then, use a laminating machine to composite the dried aluminum foil and BOPET film. Repeat the above operation to composite the CPP film and aluminum foil, and place them in an oven to complete curing.

[0029] Step 2: According to GB / T8808-1998 standard, use an electronic peel tester to measure. Perform T-type peel strength measurement on the electronic peel tester at a speed of 300 mm / min, read and record, perform parallel testing 5 times, and take the average value.

[0030] Step 3: Two commercially available adhesives were compared with the embodiment of the present invention in which the solid content of the modified polyester polyol was 50% and the results were shown in Table 1.

[0031] Table 1: Comparison data of peel strength

[0032] It can be seen from the comparison data that the composite film bonded by using the special adhesive for PP aluminum-plastic composite film of the present invention has a more obvious improvement in the peeling strength.

[0033] Furthermore, the solid content of the modified polyester polyol of the present invention refers to the solid content of the modified polyester polyol in ethyl acetate. For example, m1g of the modified polyester polyol is weighed and dissolved in m2g of ethyl acetate. The solid content is calculated as follows: C=m1 / (m1+m2)*100% Among them, the solid content of C-modified polyester polyol is %, % m1-mass of modified polyester polyol, g; m2- represents the mass of ethyl acetate, g.

[0034] Add excess modified polyester polyol to ethyl acetate, heat and stir to fully dissolve, filter out insoluble matter with a 200-mesh filter, and weigh. Pour the remaining solution into a dry and clean crucible, place it in a 70°C oven and dry it to constant weight. When the temperature of the residue drops to room temperature, weigh it again to calculate the maximum solid content.

[0035] Furthermore, the modified polyester polyol was dissolved in ethyl acetate to prepare polyester solutions with solid contents of 20%, 25%, 30%, 35%, 40%, 45%, and 50%, and the aforementioned film-making process was repeated to obtain composite films, which were then subjected to peeling tests. The corresponding peeling strengths were 1.49, 1.53, 1.55, 1.57, 1.60, 1.68, and 1.75, respectively. It can be seen that the effect of solid content on the peeling strength of the composite film is as follows: when the solid content of the modified polyester polyol is high, the effect of solvent volatilization on the adhesive layer structure is small, the probability of chemical reaction between the reactive groups on the surface of the adhesive and the substrate increases, and the peeling strength is high. However, when the solid content exceeds 50%, the polyester is difficult to dissolve, the adhesive has high viscosity and poor fluidity, and the adhesive is unevenly dispersed after coating, which will form a large number of defects. Therefore, the optimal solid content of the modified polyester polyol is 50%.

[0036] Further, the same mass of modified polyester polyol solution was weighed, and 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 2.0%, 3.0%, and 4.0% of the mass of the polyester solution was added to the cyanate-terminated prepolymer component curing agent for curing. The above composite film preparation process was repeated to obtain a composite film, and the peeling test was performed, and the peeling force data obtained were as follows: 1.33, 1.36, 1.42, 1.58, 1.75, 1.70, 1.71, and 1.68. It can be seen that the effect of the amount of curing agent on the peeling strength of the composite film is as follows: when the amount of curing agent is less than 1w%, the peeling strength of the adhesive is significantly improved as the amount of curing agent increases; when the amount of curing agent exceeds 1w%, the peeling strength of the adhesive decreases as the amount of curing agent increases. In order to accurately weigh the mass of the curing agent, the curing agent was diluted with ethyl acetate, and the polyester solution and the curing agent solution were mixed to reduce the solid content of the adhesive. The amount of curing agent is affected by the humidity of the environment, because isocyanate reacts with water. The reactivity of active hydrogen in water is smaller than that of primary hydroxyl groups, and is equivalent to the activity of active hydrogen in primary hydroxyl groups. Therefore, the optimal amount of curing agent is 1w%.

[0037] In summary, the present invention provides a special adhesive for PP aluminum-plastic composite film, which mainly includes a modified polyester polyol component and an isocyanate-terminated prepolymer component; wherein the modified polyester polyol component includes a modified polyester polyol with a solid content of 20%-50% and ethyl acetate; the isocyanate-terminated prepolymer component includes an isocyanate-terminated prepolymer and ethyl acetate. In addition, the present invention also discloses a preparation method of the aforementioned modified polyester polyol component and the isocyanate-terminated prepolymer component, and uses a palm oil-modified polyester polyol component as a main agent, and an isocyanate-terminated prepolymer component as a curing agent, and compounding the main agent and the curing agent to obtain a solvent-free two-component laminating adhesive; the two-component laminating adhesive can significantly improve the peel strength of the PP aluminum-plastic composite film. Therefore, the present invention provides a special adhesive for PP aluminum-plastic composite film and a preparation method thereof to solve the technical problem of how to improve the peel strength of polyurethane adhesives.

[0038] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0039] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A special adhesive for PP aluminum-plastic composite film, characterized in that: It includes: A modified polyester polyol component and an isocyanate-terminated prepolymer component; wherein the modified polyester polyol component comprises a modified polyester polyol with a solid content of 20%-50% and ethyl acetate; and the isocyanate-terminated prepolymer component comprises an isocyanate-terminated prepolymer and ethyl acetate.

2. The special adhesive for PP aluminum-plastic composite film according to claim 1, characterized in that: The ingredients of the modified polyester polyol include isophthalic acid, terephthalic acid, sebacic acid, ethylene glycol, neopentyl glycol, antimony glycol and palm oil.

3. The special adhesive for PP aluminum-plastic composite film according to claim 3, characterized in that: The components of the isocyanate-terminated prepolymer include polypropylene glycol 2000, polypropylene glycol 400, diphenylmethane diisocyanate and benzoyl chloride.

4. A method for preparing a special adhesive for PP aluminum-plastic composite film as claimed in claim 3, characterized in that: It includes the following steps: S1: first prepare a modified polyester polyol component: weigh a preset amount of modified polyester polyol by mass, chop the modified polyester polyol and put it in a container, add a preset amount of ethyl acetate into the container, and seal the container; The container is heated and stirred to prepare a uniform and stable modified polyester polyol solution; S2: preparing an isocyanate-terminated prepolymer component: weighing a preset amount of isocyanate-terminated prepolymer in a glass container, adding a preset amount of ethyl acetate to dissolve it, and then diluting it into a curing agent solution of 1:5; S3: Compounding into a binder: After measuring a preset amount of the modified polyester polyol component and the isocyanate-terminated prepolymer component by mass, drop the isocyanate-terminated prepolymer component into the modified polyester polyol component, seal the container, stir thoroughly, and then place the mixed components at room temperature for a preset time.

5. The method for preparing a special adhesive for PP aluminum-plastic composite film according to claim 4, characterized in that: The preparation method of modified polyester polyol is as follows: S11: First, close the esterification and polycondensation channel valves, and add solid mixed powders of terephthalic acid, isophthalic acid, sebacic acid, neopentyl glycol and ethylene glycol antimony into the polyester reactor according to the preset ratio; then, add ethylene glycol and start stirring at a frequency of 25Hz; after the power of the stirrer is stable, increase the stirring power to 50Hz; and when the material adding process is completed, close the feed port, evacuate and fill with nitrogen; then turn on the heating device of the reactor to heat the material, set the jacket heating temperature to 210℃, and gradually increase the temperature to 230℃ and 250℃; monitor the temperature and air pressure in the reactor; when the temperature in the reactor reaches 150℃, set the gas phase outlet temperature to 160℃, and control the tower top temperature to 130℃; when the temperature at the end of the condensation system drops below 100℃, the esterification reaction is complete; S12: After the esterification reaction is completed, the polycondensation stage is entered, the reaction temperature is increased to 275°C, the vacuum pump is turned on, the small molecules in the reaction system are extracted, and the polyester molecular chain continues to grow; the small molecules enter the cold trap through the pipeline, the phase state changes, and remain in the cold trap; when the reaction pressure is less than 100Pa, the small molecule content of the system is extremely low, and the valve in front of the by-product receiving tank is closed to prevent the small molecules therein from volatilizing and reducing the vacuum degree in the kettle; when the stirring paddle power reaches the specified power, the stirring frequency is reduced to 25Hz, the kettle body heating is turned off, and the reaction is stopped; N2 is filled into the kettle to normal pressure, the heating device is turned off, and when the temperature in the reactor drops below 180°C, the discharge hole at the bottom of the reactor is opened to discharge the material to prepare polyester polyols; S13: In a reaction kettle equipped with a stirrer, a condenser and a thermometer, a preset amount of catalyst methanesulfonic acid, polyester polyol and palm oil are added, and nitrogen is passed through to protect the mixture. The temperature is raised to 140 to 160°C while stirring, and an esterification reaction occurs with water flowing out. After the temperature is kept for dehydration for a preset time, the temperature is further raised to 210°C, and the head temperature is maintained below 100°C to carry out a condensation reaction; thereby preparing a modified polyester polyol.

6. The method for preparing a special adhesive for PP aluminum-plastic composite film according to claim 4, characterized in that: The preparation method of the isocyanate-terminated prepolymer is as follows: S21: preparing polyether polyol, firstly placing polypropylene glycol 2000 and polypropylene glycol 400 in a molar ratio of 1:1 in a reaction kettle equipped with a stirrer and a thermometer, heating, stirring, heating and evacuating, dehydrating at 110 to 120° C. and a vacuum degree of 133.3 Pa, until no obvious bubbles are generated in the system, that is, the dehydration is completed, pouring the product into a drying bottle, filling it with nitrogen and sealing it for later use; S22: In a reaction kettle equipped with a stirrer, a condenser, a constant pressure dropping funnel and a thermometer, first add diphenylmethane diisocyanate of the same mass as the polyether polyol, then add the pre-dehydrated polyether polyol dropwise at 45°C for 1 hour, pass nitrogen protection, and keep the reaction warm for 2 hours. Then, add benzoyl chloride accounting for 1% of the total amount of diphenylmethane diisocyanate, stir evenly, and cool to room temperature to obtain the isocyanate-terminated prepolymer.

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