Solventless polyurethane adhesive and its application

Abstract

The present application relates to the technical fields of polyurethane adhesive, in particular to a kind of solventless polyurethane adhesive and application thereof.The solventless polyurethane adhesive is prepared by preparing PET-A component, PET-B component, PET-A component and polyurethane curing agent respectively, polyurethane curing agent is used as A component, PET-B and ECR are mixed to be used as B component, the R value between A component and B component is 2.2-3, A component and B component are mixed uniformly to obtain epoxy cinnamic castor oil acid polyurethane adhesive, the solventless polyurethane adhesive is prepared by biological base material, and has the advantages of high temperature and high humidity resistance, environmental protection and low cost.

Description

Technical Field

[0001] This invention relates to the field of polyurethane adhesives, specifically to a solvent-free polyurethane adhesive and its applications. Background Technology

[0002] Retort-resistant flexible packaging composite films are widely welcomed in the market because they can significantly extend the shelf life of food and preserve its original flavor. However, Figure 1 As shown, most adhesives currently used for this type of film lamination are solvent-based polyurethane adhesives, which are prone to hydrolysis in high-temperature and high-humidity environments, leading to decreased peel strength and, in severe cases, even delamination of the composite film or rupture of the retort pouch. Currently, this can be addressed by modifying polyurethane adhesives. Modification methods include silicone modification, polyacrylate modification, and epoxy resin modification. However, the modification of raw materials for traditional polyurethane adhesives largely relies on petrochemical resources, which is detrimental to sustainable development.

[0003] For example, Chinese patent CN202111607212.2 discloses a flexible two-component polyurethane adhesive for flexible packaging composite films. The polyol component of this patent includes an aliphatic-aromatic copolymer polyol modified with polyepoxides, ethyl acetate, and additives. Although this polyurethane adhesive can improve bond strength and tensile toughness, the components used are all environmentally unfriendly materials. For instance, ethyl acetate, as an ester solvent, has slight toxicity, and its emission process poses certain health risks, hindering sustainable development. Furthermore, bio-based polyurethane adhesives also exist, but their performance still lags behind solvent-based polyurethane adhesives. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a solvent-free polyurethane adhesive. This solvent-free polyurethane adhesive is made from bio-based materials and has the advantages of high temperature and high humidity resistance, environmental friendliness and low cost.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A solvent-free polyurethane adhesive is characterized by being prepared by the following steps:

[0007] Preparation of PET-A component:

[0008] 1,6-Adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol were mixed thoroughly, with the molar ratio of 1,6-adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol being 6:5:2:2 to 6:1:4:4. The mixture was stirred and heated to 220–235°C, followed by the addition of tetrabutyl titanate. The reaction was maintained at this temperature for 2–2.5 hours to obtain a final mixture.

[0009] The acid value of the mixture is monitored. When the acid value stabilizes, the mixture is vacuum-treated, and the acid value and hydroxyl value of the mixture are detected at set intervals. After the detected acid value and hydroxyl value reach the standard, the temperature is lowered to 120-150°C and maintained under negative pressure to separate excess water from the mixture, obtaining the PET-A component, whose chemical structure is as follows:

[0010]

[0011] Preparation of PET-B component:

[0012] 1,6-Adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol were mixed thoroughly, with the molar ratio of 1,6-adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol being 5:1:4:4 to 4:4:6:6. The mixture was stirred and heated to 220–235°C, followed by the addition of tetrabutyl titanate. The reaction was maintained at this temperature for 2–2.5 hours to obtain a final mixture.

[0013] The acid value of the mixture is monitored. When the acid value stabilizes, the mixture is vacuum-treated, and the acid value and hydroxyl value of the mixture are detected at set intervals. After the detected acid value and hydroxyl value reach the standard, the temperature is lowered to 120-150°C and maintained under negative pressure to separate excess water from the mixture, obtaining the PET-B component, whose chemical structure is as follows:

[0014]

[0015] Preparation of bio-based polyol ECR: Epoxidized soybean oil, cinnamic acid, and ricinoleic acid were mixed evenly, with a molar ratio of 1:2:0.5. Triethylamine was added as a catalyst, and the mixture was reacted at 120–150 °C until the acid value of the reactants stabilized, yielding bio-based polyol ECR. Its chemical structural formula is as follows:

[0016]

[0017] Preparation of polyurethane curing agent: PET-A and diphenylmethane diisocyanate are mixed evenly and reacted at 65–80°C for 5–2.5 h. When the isocyanate mass fraction ω of the reactants… NCO =15%~18% and stop the reaction after it stabilizes to obtain a polyurethane curing agent, the chemical structure of which is as follows:

[0018]

[0019] Preparation of adhesive: Polyurethane curing agent is used as component A, and PET-B and ECR are mixed to form component B. The R value between components A and B is 2.2–3. Components A and B are mixed evenly to obtain epoxy cinnamon ricinoleic acid polyurethane adhesive, the chemical structural formula of which is as follows:

[0020]

[0021] In some embodiments, when preparing the PET-A component, the amount of tetrabutyl titanate used is 0.004% to 0.006% of the total mass of the PET-A components.

[0022] In some embodiments, when preparing bio-based polyol ECR, the amount of triethylamine used is 0.5% to 1.5% of the total mass of the bio-based polyol ECR.

[0023] In some embodiments, when preparing the adhesive, the weight ratio of the ECR to the PET-B is 5 to 1:5 to 1.

[0024] The beneficial effects of the solvent-free polyurethane adhesive of the present invention are as follows:

[0025] (1) The solvent-free polyurethane adhesive of the present invention synthesizes two different polyesters, PET-A and PET-B, through monomer molecular structure design. PET-A reacts with diphenylmethane diisocyanate to form a polyurethane curing agent Pre-PUA with curing function, while PET-B is mixed with bio-based polyol ECR to form a polyurethane curing agent with bio-groups. The bio-based polyol ECR is obtained from renewable bio-resources, including epoxidized soybean oil, castor oil acid, and cinnamic acid. The benzene ring structure of cinnamic acid provides thermal stability to the adhesive. The aliphatic long chains of soybean oil and ricinoleic acid provide high hydroxyl values ​​and hydrophobicity, effectively improving the bonding strength and heat-sealing strength of the composite film adhesive before and after boiling. Therefore, the epoxy cinnamon ricinoleic acid polyurethane adhesive prepared by mixing bio-based polyol ECR with polyurethane polyol PET-B and then reacting it with Pre-PUA as a curing agent is both resistant to high-temperature boiling and has the properties of a bio-based polyurethane adhesive. It not only maintains the high-temperature and boiling resistance of solvent-based polyurethane adhesives, but also has environmentally friendly properties, overcoming the problems of environmental pollution and dependence on petroleum associated with traditional solvent-based polyurethane adhesives.

[0026] (2) The solvent-free polyurethane adhesive of the present invention introduces biomass raw materials into the B component of the two-component polyurethane adhesive, thereby reducing the synthesis cost and alleviating the dependence of polyurethane adhesive on petroleum-based raw materials and environmental pollution.

[0027] (3) The solvent-free polyurethane adhesive of the present invention, which is a bio-based two-component polyurethane adhesive prepared from epoxidized soybean oil, castor oil, and cinnamic acid, has high hydrophobicity and heat resistance, and successfully improves the peel strength before and after boiling and the heat sealing strength of the composite film. The peel strength of the AI / RCPP layer is much higher than the requirements of the national standard GB / T41168-2021.

[0028] Applications of solvent-free polyurethane adhesives are also provided, wherein the solvent-free polyurethane adhesive is mixed with a silane coupling agent, and the mixture is used for composite films. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the flexible packaging composite film structure in a specific implementation method.

[0030] Figure 2 These are peel force diagrams of the polyurethane adhesive before and after boiling in water, based on a specific implementation method.

[0031] Figure 3 This is a peel force diagram of the inner layer of the adhesive after being boiled at 121°C according to a specific embodiment.

[0032] Figure 4 This is the DSC curve of the bio-based polyurethane adhesive in a specific implementation.

[0033] Figure 5 This refers to the static water contact angle of the bio-based polyurethane adhesive in a specific implementation.

[0034] Figure 6 These are infrared spectra of the components of the polyurethane adhesive in a specific embodiment. Detailed Implementation

[0035] Preferred embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0036] The terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular forms “a” and “the” as used in this invention and the appended claims are intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0037] It should be understood that although the terms "first," "second," "third," etc., may be used in this invention to describe various information, this information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, without departing from the scope of this invention, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0038] Example 1

[0039] This embodiment discloses a solvent-free polyurethane adhesive, which is prepared by the following steps:

[0040] Preparation of PET-A component:

[0041] 1,6-Adipic acid (AA), neopentyl glycol (NPG), ethylene glycol (EG), and 1,2-propanediol (1,2-PG) were mixed thoroughly, with the molar ratio of 1,6-adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol being 6:5:2:2 to 6:1:4:4. The mixture was stirred and heated to 220–235°C, followed by the addition of tetrabutyl titanate. The reaction was maintained at this temperature for 2–2.5 hours to obtain a final mixture.

[0042] The above-mentioned mixture of 1,6-adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol undergoes an esterification reaction between acid and alcohol. In the early stages of the reaction, no catalyst is needed for the reaction to proceed smoothly. However, in the later stages of the reaction, the acidity in the system decreases, and the autocatalytic efficiency becomes too low. It takes tens of hours for the product acid value to meet the standard, which seriously affects the production efficiency. At this point, tetrabutyl titanate is added as a catalyst to accelerate the reaction.

[0043] If the acid value of the product is too high, it will lead to excessive side reactions during subsequent compounding with the curing agent. Since the ring-opening reaction is the reaction of carboxyl groups with epoxy groups, a constant acid value indicates that the carboxyl groups are unlikely to undergo further ring-opening reactions. The acid value at this point represents the content of excess carboxyl groups. Therefore, the acid value of the mixture is monitored. When the acid value stabilizes, it indicates that the reaction is nearing completion. Subsequently, the mixture is vacuum-treated. Vacuuming will not remove unreacted alcohols, thus avoiding a decrease in the alcohol-acid ratio of the reactants. The acid value and hydroxyl value of the mixture are measured at set intervals. Once the acid and hydroxyl values ​​reach the standard, the temperature is lowered to 120–150°C and maintained under negative pressure to separate excess water from the mixture, yielding the PET-A component. Its reaction process and chemical structure are as follows:

[0044]

[0045] In the esterification stage, the diacid and diol undergo a catalytic esterification reaction to form ester bonds, yielding a low-molecular-weight esterified product. In the subsequent polycondensation stage, under high temperature and vacuum conditions, the small-molecule diol is removed from the polymerization system via transesterification, ultimately obtaining a high-molecular-weight polymer.

[0046] Preparation of PET-B component:

[0047] 1,6-Adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol were mixed thoroughly, with the molar ratio of 1,6-adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol being 5:1:4:4 to 4:4:6:6. The mixture was stirred and heated to 220–235°C, followed by the addition of tetrabutyl titanate. The reaction was maintained at this temperature for 2–2.5 hours to obtain a final mixture.

[0048] The acid value of the mixture is monitored. When the acid value stabilizes, the mixture is vacuum-treated, and the acid value and hydroxyl value of the mixture are detected at set intervals. After the detected acid value and hydroxyl value reach the standard, the temperature is lowered to 120-150°C and maintained under negative pressure to separate excess water from the mixture, obtaining the PET-B component, whose chemical structure is as follows:

[0049]

[0050] The only difference between PET-B and PET-A is the content of raw materials. By adjusting the content of raw material components, PET components with different molecular chain amounts can be obtained.

[0051] Preparation of bio-based polyol ECR: Epoxidized soybean oil, cinnamic acid, and ricinoleic acid were mixed evenly, with a molar ratio of 1:2:0.5. Triethylamine was added as a catalyst, and the mixture was reacted at 120–150 °C until the acid value of the reactants stabilized, yielding bio-based polyol ECR. The reaction process and chemical structural formula are as follows:

[0052]

[0053] Preparation of polyurethane curing agent: PET-A and diphenylmethane diisocyanate were mixed evenly and reacted at 65–80°C for 5–2.5 h. The reaction was stopped when the isocyanate mass fraction ωNCO of the reactants was 15%–18% and stable, yielding the polyurethane curing agent. The reaction process and chemical structural formula are as follows:

[0054]

[0055] The theoretical addition amount of diphenylmethane diisocyanate is calculated by pre-designing the isocyanate mass fraction. When no side reactions occur, the quantitative relationship of the above variables satisfies the following equation:

[0056]

[0057] In the formula, m is the theoretical amount (g) of diphenylmethane diisocyanate required per 100g of curing agent polyester, and ω NCO To design the theoretical isocyanate group mass fraction (%) to be synthesized, OH v The value is the hydroxyl value of the polyester (mgKOH / g).

[0058] Preparation of adhesive: Polyurethane curing agent is used as component A, and PET-B is mixed with ECR to form component B. The R value between components A and B is 2.2–3. Components A and B are mixed evenly to obtain epoxy cinnamon ricinoleic acid polyurethane adhesive. The reaction process and chemical structural formula are as follows:

[0059]

[0060] The isocyanate exponent R values ​​are related as follows:

[0061] In the formula: m p m B The values ​​are: mass (g) of component A and component B, respectively; R is the molar ratio of isocyanate groups to hydroxyl groups in the two components; ω NCO The mass fraction (%) of isocyanate hydroxyl groups in component A; OH represents the hydroxyl value (mg KOH / g) of component B.

[0062] Polyurethane adhesives with different structural properties were prepared by adjusting the ratio of the two adhesive components to reduce the impact of subsequent adhesive formulation.

[0063] In this embodiment, when preparing the PET-A component, the amount of tetrabutyl titanate used is 0.004% to 0.006% of the total mass of all PET-A components, and this amount can be adjusted according to actual needs.

[0064] In this embodiment, when preparing bio-based polyol ECR, the amount of triethylamine used is 0.5% to 1.5% of the total mass of bio-based polyol ECR, and this amount can be adjusted according to actual needs.

[0065] In this embodiment, when preparing the adhesive, the weight ratio of ECR ​​to PET-B is 5-1:5-1, and this amount can be adjusted according to actual needs.

[0066] In this embodiment, the solvent-free polyurethane adhesive is mixed with a silane coupling agent, and the mixture is used to composite film.

[0067] Effect verification

[0068] To further illustrate the effect of the solvent-free polyurethane adhesive obtained by the present invention, the following experiments were conducted.

[0069] Experimental Example 1

[0070] Preparation of polyester polyol PET-A: The corresponding raw materials AA, NPG, EG, and 1,2-PG were added to a four-necked flask in a molar ratio of 6:3:3:3. A reaction apparatus was set up, stirring was started, and the mixture was heated from room temperature to 230°C, maintaining the top thermometer temperature at 100±2°C. Once the temperature reached 230°C, 0.004% tetrabutyl titanate was added to the four-necked flask, and the mixture was kept at this temperature for 2 hours. The acid value was tested; when the acid value no longer changed, a vacuum was applied at a rate of -0.01 MPa / 30 min, and the changes in acid value and hydroxyl value of the reaction product were tested at regular intervals. After the test results were satisfactory, the temperature was lowered to 130°C and maintained at -0.1 MPa for 2 hours to remove excess moisture from the polyester. Finally, the reaction was stopped, and the product was discharged after cooling. Polyester PET-B and polyester PET-A were synthesized using the same preparation process, with a raw material molar ratio of AA:PTA:EG:DEG = 5:2:5:5.

[0071] Synthesis of bio-based polyol ECR: ESO, CA, and RA were added in a 500 mL four-necked flask equipped with a stir bar, thermometer, and reflux condenser at a feed ratio of 1:2:0.5. TEA was added as a catalyst at a total mass of 1%. The reaction was carried out at 140 °C until the acid value of the reactants no longer changed.

[0072] Synthesis of polyurethane curing agent: Preparation of 17% ω NCO The polyurethane curing agent content was calculated, and 100g of dehydrated polyester PET-A and 153g of MDI were added to a four-necked flask. The reaction temperature was maintained at 75℃, and the reaction time was 2 hours. During this period, samples were taken at regular intervals to test the isocyanate mass fraction of the reactants. The reaction was stopped when the isocyanate mass fraction of the reaction product reached the theoretical value and tended to stabilize. The product was then sealed and stored for later use. The polyurethane curing agent sample was recorded as Pre-PU-A.

[0073] PET-B and ECA were mixed in mass ratios of 5:0, 4:1, 3:2, 2:3, 1:4, and 0:5 as component B, and Pre-PU-A was used as component A. Components A and B were mixed with an R value of 2.5 to prepare epoxy cinnamon-castor oil-based polyurethane adhesive PU-ECR. Then, 0.5% of the total mass of the adhesive was added as a silane coupling agent for film lamination.

[0074] like Figure 1The schematic diagram of the flexible packaging composite film shown is used to test the peel strength, heat seal strength and retort resistance of the composite film according to the test methods in GB / T41168-2021 "Plastic and Aluminum Foil Retort Composite Films and Bags for Food Packaging". The testing instrument is the XLW tensile testing machine of Jinan Langguang Electromechanical Technology Co., Ltd.

[0075] Figure 2 The figures show the peel force diagrams of the inner and outer layers before and after boiling. It can be seen that with increasing ECR mass fraction, the peel force initially increases and then decreases before boiling, reaching a maximum of 7.03 N for the AI / RCPP layer when the ECR content is 60%. This is because ECR molecules have multiple functional groups and high functionality, which can increase the degree of cross-linking between adhesive molecules. However, when the ECR content exceeds a certain amount, larger molecular clusters are formed, causing the -OH groups to be encapsulated within the molecular clusters, making complete reaction difficult and leading to a decrease in peel force. The heat-sealing strength shows the same trend as the peel force of the AI / RCPP layer, reaching a maximum of 53.4 N / 15 cm when the ECR content is 80%. After boiling, the decrease in peel force of the AI / RCPP layer decreases with increasing ECR mass fraction. This is because the benzene ring structure of cinnamic acid enhances its heat resistance, and the long-chain structure of ricinoleic acid enhances its hydrolysis resistance. The synergistic effect of these two factors improves the hydrothermal resistance of the adhesive, reaching a maximum peel force of 5.59 N when the addition amount is 80%. The peel strength of the AI / RCPP layer increased to 4.95 N after boiling with 100% ECR. This is because, with increasing temperature, the ECR molecular clusters effectively expand and flatten, reacting with the substrate surface and exposing the encapsulated hydroxyl groups, allowing them to react with the small amount of unreacted -NCO groups, thus enhancing the hydrogen bonding with the substrate and increasing the peel strength. When the boiling conditions were increased to 121℃ for 30 minutes, the peel strength of the AI / RCPP layer remained as follows: Figure 3 As shown, even with the addition of terephthalic acid, the peel strength of the polyurethane adhesive PU-ECR-0 is only 2.1 N. However, as the ECR content increases, its peel strength gradually increases, reaching a maximum of 6.2 N at a mass fraction of 100%. This is because when the temperature is further increased, the ECR molecular groups expand more completely, exposing more -OH groups, and the reaction with -NCO is more thorough, further enhancing the surface interaction with the substrate and thus increasing the peel strength.

[0076] Preparation of flexible packaging composite film

[0077] The preparation of flexible packaging bags involves two processes: film lamination and heat sealing. Aluminum foil (AI), polyethylene terephthalate film (PET), and retortible polypropylene cast film (RCPP) are cut into rectangles of 30cm × 19cm. PET, as the outer layer, is fixed on a coating platform. A mixed polyurethane adhesive is evenly coated onto the PET surface at a rate of 1.2–1.5 g / m². The aluminum foil is then pressed against the PET. The above steps are repeated to laminate the RCPP film with the aluminum foil to obtain a PET / AI / RCPP composite film. The laminated film is placed in a 40℃ oven for 72 hours to cure. After the adhesive is fully cured, it is cut into 15cm wide strips for peel strength testing. The cured composite film is then hot-pressed using a heat-sealing tester to prepare retort pouches. The retort pouches are filled with tap water to more than half their volume, and all air is removed. The upper and lower temperatures of the heat-sealing tester are set to 160℃.

[0078] Experimental Example 2

[0079] The procedures are the same as in Example 1, except that: the raw material ratio for preparing polyester PET-A is AA:NPG:EG:1,2-PG in a molar ratio of 6:2:3.5:3.5. The raw material ratio for preparing polyester PET-B is AA:PTA:EG:DEG = 5:3:6:6. The reaction temperature for the synthesis of bio-based polyol ECR is 150℃. The ω in the polyurethane curing agent... NCO =16%, MDI addition amount is 138g. Silane coupling agent addition amount is 1% of total mass.

[0080] Figure 4 The DSC curves for bio-based polyurethane adhesives with different ECR mass fractions are shown in the figure. The glass transition temperature (Tg) of the polyurethane is marked in the table in the figure. The glass transition temperature is related to the composition and structure of the polyester polyol and is greatly affected by the rigidity of the soft segment. With the increase of ECR ​​mass fraction, the rigidity of the polyurethane adhesive increases, and the glass transition temperature rises. This is because the cinnamic acid content in ECR is higher, and the proportion of rigid benzene ring structure is larger, resulting in a significantly higher glass transition temperature compared with adhesives without added ECR.

[0081] Experimental Example 3

[0082] The steps are the same as in Implementation Case 2. However, the control group (AA:PTA:EG:DEG = 4:4:6:6) uses polyester polyols synthesized without the addition of terephthalic acid, and its raw material ratio is AA:PTA:EG:DEG = 8:0:6:6. The ECR reaction temperature for the bio-based polyol is 135℃, and the triethylamine addition is 1.5% of the total mass. The ω in the polyurethane curing agent... NCO =18%, MDI addition amount is 168g. Silane coupling agent addition amount is 0.5% of total mass.

[0083] Figure 5 The figure shows the surface water contact angles of different polyurethane adhesives. The hydrophobicity of polyurethane adhesives is characterized by static water contact angles. As shown, all polyurethane adhesives exhibit high hydrophobicity. The contact angle of the adhesive without benzene rings is 89.3°, while that of the polyurethane adhesive with terephthalic acid is 82.7°. This may be because as the terephthalic acid content increases, the content of adipic acid, which has a long-chain aliphatic structure, decreases, thus reducing hydrophobicity. Furthermore, the contact angles of adhesives with added ECR are all larger than those of PU-ECR-0 adhesives. The contact angles tend to stabilize after the addition amount exceeds 20%. This is because the addition of ECR ​​increases the degree of cross-linking of the adhesive, and the increase in aliphatic long chains enhances the hydrophobicity of the adhesive. However, as the mass fraction of ECR ​​increases, the hydrophilic groups introduced by unreacted cinnamic acid and ricinoleic acid also increase accordingly, which reduces its hydrophobicity. The two effects interact with each other, resulting in a contact angle that tends to be in the range of 89.8° to 91.6°. Therefore, it can be concluded that polyurethane adhesives prepared by bio-based polyol ECR have high hydrophobicity.

[0084] Comparative Example 1

[0085] Other conditions were the same as in Experiment 1, but the polyester polyol synthesized without the addition of terephthalic acid served as the control group, with a raw material ratio of AA:PTA:EG:DEG = 8:0:6:6.

[0086] Figure 6 The infrared spectra of the components of the polyurethane adhesive are shown in the figure, at 3500 cm⁻¹. -1 The broad peak at 2950 cm⁻¹ corresponds to the stretching vibration of the hydroxyl group (-OH), indicating that the polyester polyol contains abundant hydroxyl groups. -1 and 2870cm -1 The peaks at 3350 cm⁻¹ represent the characteristic absorption peaks of the CH₂ and CH₃ stretching vibrations. In the spectrum of the curing agent Pre-PU-A, the -OH characteristic peak disappears, and the peak at 3350 cm⁻¹ disappears. -1 and 1530cm -1 The stretching and bending vibration peaks of the -NH group in the urethane group appear at 1610 cm⁻¹, the characteristic peak of C=O in the urethane group is at 2260 cm⁻¹. -1 The new peak appearing at [location] is attributed to the C=N stretching vibration peak in -NCO. The peak at 1740 cm⁻¹ is the C=O stretching vibration peak of the ester group, and the peak at 729 cm⁻¹ is the peak of the C=O stretching vibration of the ester group. -1 The peak at 1690 cm⁻¹ represents the out-of-plane bending vibration peak of the =CH group on the benzene ring. -1 The peak at 1640 cm⁻¹ is a characteristic peak of the carbonyl group (C=O) in the carboxylic acids of cinnamic acid and ricinoleic acid. -1 The absorption peak at 829 cm⁻¹ represents the double bond C=C. The characteristic peak of the epoxide group in epoxidized soybean oil appears at 829 cm⁻¹.-1 At that location, and in the product 829cm -1 The peak value weakens significantly at 3500cm. -1 The presence of the -OH characteristic peak is due to the ring-opening reaction between the epoxy groups on epoxidized soybean oil and the carboxyl groups on cinnamic acid and ricinoleic acid, which consumes the epoxy groups and generates hydroxyl groups.

[0087] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A solvent-free polyurethane adhesive, characterized in that, It is prepared by the following steps, including: Preparation of PET-A component: 1,6-Adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol were mixed thoroughly, with the molar ratio of 1,6-adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol being 6:5:2:2 to 6:1:4:

4. The mixture was stirred and heated to 220-235°C, followed by the addition of tetrabutyl titanate. The reaction was maintained at this temperature for 2-2.5 hours to obtain a final mixture. The acid value of the mixture is monitored. When the acid value is stable, the mixture is vacuumed and the acid value and hydroxyl value of the mixture are detected at set intervals. After the acid value and hydroxyl value reach the standard, the temperature is lowered to 120~150℃ and maintained under negative pressure to separate excess water in the mixture to obtain PET-A component. Preparation of PET-B component: 1,6-Adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol were mixed thoroughly, with the molar ratio of 1,6-adipic acid, neopentyl glycol, ethylene glycol, and 1,2-propanediol being 5:1:4:4 to 4:4:6:

6. The mixture was stirred and heated to 220-235°C, followed by the addition of tetrabutyl titanate. The reaction was maintained at this temperature for 2-2.5 hours to obtain a final mixture. The acid value of the mixture is monitored. When the acid value is stable, the mixture is vacuumed and the acid value and hydroxyl value of the mixture are detected at set intervals. After the acid value and hydroxyl value reach the standard, the temperature is lowered to 120~150℃ and maintained under negative pressure to separate excess water in the mixture to obtain PET-B component. Preparation of bio-based polyol ECR: Epoxidized soybean oil, cinnamic acid and ricinoleic acid are mixed evenly, wherein the molar ratio of epoxidized soybean oil, cinnamic acid and ricinoleic acid is 1:2:0.

5. Triethylamine is added as a catalyst, and the reaction is carried out at 120~150℃ until the acid value of the reactants is stable to obtain bio-based polyol ECR. Preparation of polyurethane curing agent: The PET-A and diphenylmethane diisocyanate are mixed evenly and reacted at 65~80℃ for 5~2.5h. The reaction is stopped when the isocyanate mass fraction ωNCO of the reactants is 15%~18% and stable, and the polyurethane curing agent is obtained. Preparation of adhesive: Polyurethane curing agent is used as component A, and PET-B and ECR are mixed to form component B. The R value between component A and component B is 2.2~3. Component A and component B are mixed evenly to obtain epoxy cinnamon ricinoleic acid polyurethane adhesive.

2. The solvent-free polyurethane adhesive according to claim 1, characterized in that, When preparing PET-A components, the amount of tetrabutyl titanate used is 0.004% to 0.006% of the total mass of all PET-A components. When preparing PET-B components, the amount of tetrabutyl titanate used is 0.004% to 0.006% of the total mass of each component of PET-B.

3. The solvent-free polyurethane adhesive according to claim 1, characterized in that, When preparing bio-based polyol ECR, the amount of triethylamine used is 0.5% to 1.5% of the total mass of the bio-based polyol ECR.

4. The solvent-free polyurethane adhesive according to claim 1, characterized in that, When preparing the adhesive, the weight ratio of the ECR to the PET-B is 5~1:5~1.

5. The application of the solvent-free polyurethane adhesive according to any one of claims 1 to 4, characterized in that, The solvent-free polyurethane adhesive is mixed with a silane coupling agent, and the mixture is used to create a composite film.

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