Polyurethane glue with stable high-temperature stripping force and OLED (Organic Light Emitting Diode) protective film prepared from polyurethane glue

Abstract

The invention relates to the technical field of polyurethane polymers, in particular to polyurethane glue with stable high-temperature stripping force and an OLED (Organic Light Emitting Diode) protective film prepared from the polyurethane glue. A composition for forming the polyurethane glue with the stable high-temperature stripping force comprises branched polyether polyol with the functionality degree of 3-6 and aliphatic diisocyanate, and in the process of preparing the polyurethane glue, mono-alcohol is used for end capping, and a catalyst is not added; in the composition of the polyurethane glue, the molar ratio of NCO to hydroxyl is 0.5 to 1. The glue with low stripping force, low viscosity, stable process window and low stripping force climbing after high temperature and high humidity is synthesized through a catalyst-free temperature control segmented solution polymerization method of a diisocyanate compound and polyhydric alcohol, so that the strict requirements of high-end flexible display manufacturing on an OLED (Organic Light Emitting Diode) protective film adhesive are met.

Description

Technical Field

[0001] This invention relates to the field of polyurethane polymer technology, specifically to a polyurethane adhesive with stable high-temperature peel strength and the OLED protective film prepared therefrom. Background Technology

[0002] OLED, as a next-generation display technology, boasts advantages such as self-emissiveness, wide viewing angle, high contrast, fast response speed, and flexibility, and has been widely used in smartphones, televisions, wearable devices, and other fields in recent years. OLED displays typically consist of a multi-layered structure, including an anode, an organic light-emitting layer, a cathode, and an encapsulation layer. The top protective film (TPF) is a crucial protective material in the OLED module manufacturing process, primarily serving to encapsulate, protect the internal structure, and enhance optical performance.

[0003] The adhesive for the protective film is a key material that bonds the protective film to other layers of the OLED module (such as polarizers and cover plates). The performance of the adhesive directly affects the overall performance, reliability, and lifespan of the OLED display. For OLED protective film adhesives, the following requirements are typically met:

[0004] Low peel force: Ensures that the protective film can be easily peeled off during the manufacturing process without damaging the internal structure of the OLED module.

[0005] Low viscosity: Facilitates coating process, ensures uniform adhesive layer, and avoids defects such as bubbles.

[0006] Stable process window: The adhesive performance remains stable under different production conditions (such as temperature and humidity), ensuring the controllability and consistency of the production process.

[0007] High temperature resistance: It can withstand the high temperature environment that the OLED module may encounter during packaging and use without performance degradation, especially the peel force should not increase.

[0008] Traditional OLED protective film adhesives are mainly based on materials such as epoxy resin, silicone, and polyurethane. While these materials have excellent properties in some aspects, they also have some shortcomings:

[0009] Peel strength increase: Under high temperature conditions, traditional adhesives may undergo further wetting, cross-linking or curing reactions, resulting in increased peel strength, which makes subsequent repairs or replacements difficult.

[0010] High viscosity: Traditional adhesives have high viscosity, which is not conducive to the coating process and is prone to defects such as bubbles.

[0011] Narrow process window: Traditional adhesives are highly sensitive to production conditions and are easily affected by factors such as temperature and humidity, leading to performance fluctuations.

[0012] Catalyst usage: Traditional polyurethane adhesive synthesis usually requires the use of catalysts (such as organotin catalysts), which may introduce impurities and cause post-crosslinking in polyfunctional hydroxyl systems, affecting the purity and performance of the adhesive.

[0013] Polyurethane adhesives are made by compounding polyurethane glue with curing agents, additives, etc. To solve problems such as peel force increase at high temperature, high viscosity, and purity issues caused by the use of catalysts, we need to find solutions from the synthesis of polyurethane glue. Summary of the Invention

[0014] The purpose of this invention is to provide a polyurethane adhesive with stable high-temperature peel strength. It is synthesized by catalyst-free temperature-controlled segmented solution polymerization of diisocyanate compounds and polypolyols, resulting in an adhesive with low peel strength, low viscosity, stable process window, and low peel strength creep after high temperature and high humidity, so as to meet the stringent requirements of high-end flexible display manufacturing for OLED protective film adhesives.

[0015] To achieve the above objectives, the following technical solution is adopted:

[0016] The first aspect of the present invention provides a polyurethane adhesive with stable high-temperature peel strength. The composition forming the polyurethane adhesive includes a branched polyether polyol with a functionality of 3-6 and an aliphatic diisocyanate. Monool end-capping is used in the preparation of the polyurethane adhesive without the addition of a catalyst. In the composition of the polyurethane adhesive, the molar ratio of NCO to hydroxyl groups is 0.5-1.

[0017] By completely eliminating metal catalysts and chemical sealants and relying solely on physical temperature control methods, the problems of low product purity, poor storage stability, and potential damage to OLED devices caused by catalyst residues have been solved. At the same time, the process has been simplified and the process window has been broadened.

[0018] Furthermore, the branched polyether polyol with a functionality of 3-6 is a monopentaerythritol polypropoxylate and / or a dipentaerythritol polypropoxylate.

[0019] This invention goes beyond traditional linear polyurethane, introducing a hyperbranched polymer structure. Based on advanced molecular topology design, it achieves a decoupling of low viscosity and high performance. These three-dimensional spherical or radial molecular structures, compared to linear long chains, have fewer intermolecular entanglement points because more entanglement occurs within the self-entanglement of the side chains, resulting in lower solution viscosity at the same molecular weight. Simultaneously, the numerous end-group functional groups ensure sufficient cross-linking reaction, forming a cured network with both high cohesive strength and uniform stress distribution.

[0020] Branched polyether polyols with a functionality of 3-6 are used as the core "soft segment" building blocks for polyurethane synthesis. Their unique three-dimensional topology reduces intermolecular chain entanglement at the molecular level, solving the problem of excessively high viscosity in traditional linear polymers when pursuing high cohesive strength, and achieving a balance between low viscosity (<1500 mPa·s) and high reliability.

[0021] Furthermore, the aliphatic diisocyanate is a linear aliphatic diisocyanate and / or an alicyclic diisocyanate.

[0022] The present invention uses aliphatic diisocyanates, which have higher CH bond energies than aromatic diisocyanates containing benzene ring structures, and do not contain conjugated structures that are easily oxidized to form quinone chromophores under ultraviolet or blue light irradiation.

[0023] Furthermore, the monool is one or more of methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, n-pentanol, and isopentanol.

[0024] By strictly controlling the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH), i.e., the R value ([-NCO] / [-OH]) is kept below 0.5 to 1.0, and employing a segmented, temperature-controlled, catalyst-free polymerization process terminated with a monool compound, the complete reaction of the -NCO groups is strictly ensured. The prepolymer molecular chains end only with hydroxyl groups, allowing for efficient reaction with the NCO-containing curing agent in the subsequent curing stage to form the final cross-linked network. This prevents residual -NCO groups from undergoing uncontrollable reactions after prolonged exposure to high temperature and humidity, fundamentally solving the problem of peel strength degradation after high temperature and humidity, and ensuring high stability of peel strength. If the R value is below 0.5, a large amount of polyol will not participate in the polymerization, leading to turbidity and layering of the adhesive.

[0025] Furthermore, the composition forming the polyurethane adhesive also includes a linear polyether polyol with a functionality of 3-6.

[0026] By compounding linear polyols, the average functionality of the polyols in the composition can be reduced, the crosslinking density can be further decreased, and the viscosity can be further reduced.

[0027] Further, the mass ratio of the linear polyether polyol to the branched polyether polyol with a functionality of 3-6 is (1-10):1;

[0028] Furthermore, without affecting the performance of the present invention, the composition of the polyurethane adhesive may also include polyester polyols, such as one or more of conventional polyester polyols, polycaprolactone polyols, and polycarbonate diols.

[0029] The second aspect of this invention provides a method for preparing a polyurethane adhesive with stable high-temperature peel strength. In an organic solvent, branched polyether polyols or / and linear polyether polyols with a functionality of 3-6, possibly polyester polyols, and aliphatic diisocyanates are first polymerized at a first temperature of 60-90°C, then polymerized at a second temperature of 80-120°C, followed by the addition of a monool for end-capping, cooling, and discharge to complete the preparation. The second temperature is at least 5°C higher than the first temperature.

[0030] Precise and controllable catalyst-free solution polymerization is the core process for ensuring high purity and stability of the product. Through a deep understanding of reaction kinetics, we utilize temperature gradients to guide the polymerization process. Due to the difference in NCO reactivity between the two stages (the two NCOs may have the same or different reactivity at the beginning of the reaction; after the first functional group reacts preferentially in the initial stage, the reactivity of the remaining second functional group decreases), stepwise reaction control is possible. Furthermore, the reaction rate between the isocyanate group (-NCO) and the hydroxyl group (-OH) is highly sensitive to temperature; through staged temperature control, the timing and extent of the reaction can be precisely controlled. Catalyst-free reaction conditions are milder, which is beneficial for controlling the reaction process and obtaining a more uniform molecular structure.

[0031] Furthermore, the preparation method of the high-temperature peel strength stable polyurethane adhesive is as follows: in an organic solvent, the branched polyether polyol with a functionality of 3-6 and the aliphatic diisocyanate are first polymerized at a first temperature of 60-90℃, then polymerized at a second temperature of 80-120℃, and then monool is added for end capping, followed by cooling, discharge, and completion of preparation; the second temperature is higher than the first temperature, and the temperature difference is ≥5℃.

[0032] Furthermore, the preparation method of the high-temperature peel strength stable polyurethane adhesive is as follows: in an organic solvent, the branched polyether polyol, linear polyether polyol, and aliphatic diisocyanate with a functionality of 3-6 are first polymerized at a first temperature of 60-90℃, then polymerized at a second temperature of 80-120℃, and then monool is added for end capping, followed by cooling, discharge, and completion of preparation; the second temperature is higher than the first temperature, and the temperature difference is ≥5℃.

[0033] Furthermore, the preparation method of the high-temperature peel strength stable polyurethane adhesive is as follows: in an organic solvent, the branched polyether polyol, linear polyether polyol, polyester polyol and aliphatic diisocyanate with a functionality of 3-6 are first polymerized at a first temperature of 60-90℃, and then polymerized at a second temperature of 80-120℃. Then, a monool is added for end capping, the temperature is lowered, the material is discharged, and the preparation is completed; the second temperature is higher than the first temperature, and the temperature difference is ≥5℃.

[0034] Furthermore, the amount of organic solvent added results in a solid content of 60%-80% for the polyurethane adhesive.

[0035] Furthermore, the molar ratio of the monool to the aliphatic diisocyanate is 0.01%-3%.

[0036] Furthermore, the first temperature is 60-80℃, and the second temperature is 90-120℃.

[0037] When the difference between the second temperature and the first temperature is ≥10℃, it is beneficial to effectively control the occurrence of side reactions, thereby making the prepared polyurethane adhesive stable.

[0038] A third aspect of the present invention provides an OLED protective film, comprising the above-mentioned high-temperature peel strength stable polyurethane adhesive or the high-temperature peel strength stable polyurethane adhesive prepared according to the above preparation method, wherein the OLED protective film, after aging at 65°C and 90% humidity for 240 hours, has a 180° peel strength creep rate ≤20%.

[0039] Furthermore, the method for preparing the OLED protective film includes the following steps:

[0040] S1: Add curing agent and additives to polyurethane adhesive, stir evenly to obtain polyurethane adhesive;

[0041] S2: Coat the PET film with the above-mentioned high-temperature peel strength stable polyurethane adhesive or the high-temperature peel strength stable polyurethane adhesive prepared according to the above preparation method.

[0042] S3: After curing at 150-180℃ for 2-10 minutes, an OLED protective film is obtained.

[0043] Furthermore, the mass ratio of the polyurethane adhesive, curing agent, and additives is (50-100): (0.1-10): (0.0001-0.01).

[0044] Furthermore, in step S2, the amount of adhesive applied during coating is such that the dry adhesive thickness of the protective film is 50-100 micrometers.

[0045] Beneficial effects:

[0046] 1. The problem of increased peel force of OLED protective film after exposure to high temperature and humidity has been solved, ensuring the stability and repeatability of the process.

[0047] Arguments and Analysis: The fundamental reason for the increase in peel force after high temperature is that the residual active functional groups (mainly -NCO) in the system undergo an uncontrollable post-curing reaction under thermal action, leading to an increase in crosslinking density. This technical solution solves this problem through three aspects: ① Stoichiometric control: Strictly control the -NCO / -OH molar ratio (R value) to be less than 1.0 to ensure that the -NCO groups are completely reacted during the prepolymerization stage; ② Kinetic process control: Employ a segmented temperature-controlled catalyst-free polymerization method to complete the prepolymerization reaction gently and fully; ③ Use monool end-capping to strictly ensure that the -NCO groups react completely without any trace residue, and that the prepolymer molecular chain ends only have hydroxyl groups, so that they can react efficiently and accurately with the NCO-containing curing agent in the subsequent curing stage to form the final crosslinking network.

[0048] Quantitative Effect: Since there are no residual -NCO active groups in the polymer, the final peel force of the adhesive is determined solely by its inherent cross-linking network and interfacial forces, and no longer changes with thermal history. The adhesive prepared using this method, after lamination and initial curing of the PET substrate, exhibits an initial peel force at 180°C that can be precisely controlled within an ideal range of less than 2.0 gf / in. After 240 hours of exposure to 65°C and 90% humidity, the peel force change rate of this sample is less than 20%. In contrast, existing technology products typically show a peel force creep rate greater than 30% under the same conditions, demonstrating superior high-temperature post-performance stability.

[0049] 2. It achieves ultra-low solution viscosity, significantly optimizing the coating process.

[0050] Arguments and Analysis: The core raw materials used in this scheme are branched polyether polyols with a functionality of 3-6 and linear polyether polyols with a functionality of 3-6. According to polymer physics theory, the viscosity of a polymer solution is directly related to the degree of extension of its molecular chains and the density of physical entanglement in the solution. The higher the molecular weight of traditional linear polymers, the more severe the chain entanglement, leading to an exponential increase in viscosity. However, the three-dimensional topological structure used in this scheme has a compact molecular morphology, preventing the formation of effective physical entanglement between molecules. Specifically, by controlling the R value to make the polyurethane end with hydroxyl groups, and the polyols including branched polyether polyols with a functionality of 3-6, the polyurethane forms large side chains ending with hydroxyl groups. This gives the molecules a stronger tendency for self-entanglement or internal hydrogen bonding, thus greatly reducing the kinetics of intermolecular entanglement and hydrogen bonding.

[0051] Quantitative Effect: While achieving the same high molecular weight (ensuring cohesive strength), the solution viscosity of the adhesive in this solution is significantly reduced. With a solid content of 70% (ethyl acetate solvent), the viscosity of the polyurethane adhesive prepared using this method can be stably controlled below 1500 mPa·s. This low viscosity characteristic greatly improves the leveling properties of the adhesive, enabling it to adapt to high-speed, precision scraping or slot coating processes, effectively avoiding coating defects such as pinholes, bubbles, impurities, and bumps, thereby improving production efficiency and product yield.

[0052] 3. The product has extremely high purity and excellent storage stability, making it safer for OLED devices.

[0053] Arguments and Analysis: The preparation method of this scheme completely eliminates the addition of any heavy metal catalysts such as organotin and chemical blocking agents such as phenols and oximes.

[0054] Quantification effect:

[0055] High storage stability: Because there is no catalyst in the system to activate the reaction of -NCO groups with trace amounts of moisture in the environment, the polyurethane adhesive prepared by this method exhibits a viscosity increase of less than 10% after 3 months of storage at 25°C and 55% humidity. In contrast, traditional catalytic single-component systems often show a viscosity increase of over 50% under the same conditions, and may even gel. This significantly extends the product's shelf life and reduces storage and usage costs.

[0056] 4. The adhesive layer has excellent weather resistance and optical stability.

[0057] Argument and Analysis: In terms of raw material selection, this scheme specifies the use of straight-chain aliphatic / alicyclic diisocyanates, which avoids the defect of aromatic isocyanates containing benzene rings being prone to yellowing.

[0058] Quantitative Results: Therefore, the polyurethane adhesive layer cured using this method exhibits outstanding resistance to light and thermal oxidation. In the QUV accelerated aging test, using a UVA-340 lamp to simulate sunlight, after 168 hours of irradiation, the yellowing index (ΔYI) change of the adhesive film using this method was less than 2.0, maintaining excellent transparency and a colorless appearance. In contrast, the comparative samples using aromatic isocyanates typically had a yellowing index exceeding 10.0, severely impacting optical performance. This characteristic is crucial for OLED protective film applications that require maintaining high light transmittance and color neutrality over long periods. Attached Figure Description

[0059] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0060] Figure 1 This is a schematic diagram of the polyurethane structure designed for this invention;

[0061] Figure 2 This is a schematic diagram of the structure of monopentaerythritol polypropylene oxide;

[0062] Figure 3 This is a schematic diagram of the structure of dipentaerythritol polypropylene oxide. Detailed Implementation

[0063] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0064] 1. Polyurethane adhesive with stable peel strength at high temperatures

[0065] The polyurethane adhesive of this invention is a one-component thermosetting polyurethane solution, which is a prepolymer with terminal -OH groups obtained by reacting the following raw materials in an organic solvent:

[0066] (A) Branched polyether polyols with a functionality of 3-6

[0067] The branched polyether polyol with a functionality of 3-6 has a specific three-dimensional topological structure, with an OH value of 50-150 mg KOH / g and a number-average molecular weight (Mn) of 1000-8000 g / mol. Specifically, it can be one or more blends of mono / dipentaerythritol polypropoxylates with a functionality of 3-6. This component is the core of this invention. Its three-dimensional spherical or radial molecular conformation, compared to traditional low-functionality linear polyols, can provide sufficient functional groups to form a high-strength cross-linked network while effectively reducing physical entanglement between molecular chains, thus fundamentally solving the contradiction between high molecular weight and low viscosity. This allows the final adhesive to maintain excellent flowability while possessing high cohesive strength.

[0068] (B) Aliphatic diisocyanates

[0069] The aliphatic diisocyanate is a linear aliphatic diisocyanate and / or an alicyclic diisocyanate. As a building block of the polyurethane rigid segment, the selected aliphatic diisocyanate ensures excellent resistance to yellowing and weathering of the adhesive, meeting the optical stability requirements of OLED applications.

[0070] Examples of linear aliphatic diisocyanates include 1,4-diisocyanate 4-methylpentane, 1,5-diisocyanate 5-methylhexane, and hexamethylene diisocyanate.

[0071] As alicyclic diisocyanates, such as isophorone diisocyanate and dicyclohexylmethane diisocyanate;

[0072] Isophorone diisocyanate (IPDI) or hexamethylene diisocyanate (HDI), or a blend of the two, are preferred.

[0073] (C) Solvent components

[0074] A non-protic polar solvent, such as methyl isobutyl ketone (MIBK), toluene, or a mixture thereof, is used to dissolve all reactants, provide the medium required for the reaction, and ultimately adjust the adhesive solids content to 60%-80% to obtain a viscosity suitable for the coating process.

[0075] (D) Monools

[0076] The monool is one or more selected from methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, n-pentanol, and isopentanol. Termination with a monool compound strictly ensures the complete reaction of the -NCO groups, resulting in prepolymer molecular chains ending only in hydroxyl groups. This allows for efficient reaction with the NCO-containing curing agent in the subsequent curing stage, forming the final cross-linked network. The monool content is 0.01%-3% of the molar amount of the aliphatic diisocyanate, for example, 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, etc.

[0077] (E) Linear polyether polyols

[0078] The linear polyether polyol, when combined with the branched polyether polyol with a functionality of 3-6, can reduce the average functionality of the polyol in the composition, further lower the crosslinking density, and achieve a further reduction in viscosity. Examples of linear polyether polyols include, for instance, one or more of glycerol-initiated polyoxypropylene-polyoxyethylene triols (functionality 3, Mn = 3000-8000) and erythritol-initiated polyoxypropylene-polyoxyethylene tetraols (functionality 4, Mn = 3000-8000).

[0079] (F) Polyester polyol

[0080] It is understood that, without impairing the effects of the present invention, the polyurethane adhesive may also contain polyester polyol; the addition of the polyester polyol appropriately increases the main chain stiffness to enhance the cohesive energy of the resin.

[0081] (G) Key proportion control

[0082] This invention also requires strict control of the molar ratio of isocyanate groups (-NCO) in component (B) to hydroxyl groups (-OH) in components (A), (E), and (F), i.e., the R value ([-NCO] / [-OH]) should be controlled below 0.5 to 1.0. This design ensures that after all -NCOs have completely reacted, the prepolymer molecular chain ends only have hydroxyl groups, so that it can react efficiently with the NCO-containing curing agent in the subsequent curing stage to form the final crosslinked network.

[0083] The mass ratio of the branched polyether polyol with a functionality of 3-6 to the linear polyether polyol is (1-10):1.

[0084] 2. Polyurethane adhesive with stable peel strength at high temperatures

[0085] Step 1: Raw material preparation and feeding

[0086] In a clean, dry 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser, and nitrogen protection port, add the above-mentioned amount of raw material (A) and the compounded linear polyether polyol (E), start stirring (100-150 rpm), and vacuum at 100-120 degrees Celsius for 1-3 hours until the water content is below 200 ppm.

[0087] Step 2: Prepolymerization reaction

[0088] Slowly cool the reaction system to 60-80℃. Add the prescribed amount of component (B) dropwise at a uniform rate over 1 hour using a constant pressure dropping funnel. After the addition is complete, raise the reaction temperature to 90-120℃ and maintain the reaction at this temperature for 3-6 hours.

[0089] Process control: Samples were taken every hour, and the content of -NCO groups in the system was detected by di-n-butylamine titration. The measured NCO% was close to 0, which was regarded as the endpoint of the prepolymerization reaction.

[0090] Reaction termination: Add the prescribed amount of monool (D) to the system and stir continuously for 1-3 hours; cool down to terminate the reaction.

[0091] Step 3: Filtration and Discharge

[0092] The clear, transparent glue is filtered through a 200-mesh filter and then placed into a sealed, dry container to complete the preparation.

[0093] First, the raw materials used in this application are shown in the table below:

[0094]

[0095] Next, the synthesis of the polyurethane adhesive of the present invention will be described.

[0096] Example 1

[0097] Step 1: Raw material preparation and feeding

[0098] In a clean, dry 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser, and nitrogen protection port, add monopentaerythritol polypropoxylate (chemical structure shown in [reference missing]) at a mass ratio of 1:1. Figure 2 (n=2) Trifunctional linear polyether polyol BASF / Lupranol® 3300, turn on the stirrer (120 rpm), and vacuum at 110 degrees Celsius for 1-3 hours until the water content is below 200 ppm.

[0099] Step 2: Prepolymerization reaction

[0100] The reaction system was slowly cooled to 70°C. Hexamethylene diisocyanate dissolved in methyl isobutyl ketone was added dropwise over 1 hour using a constant-pressure dropping funnel. The molar ratio of NCO to OH in the branched polyether polyol and the linear polyether polyol was 0.5. After the addition was complete, the reaction temperature was raised to 100°C and the reaction was maintained at this temperature for 5 hours.

[0101] Process control: Samples were taken every hour, and the content of -NCO groups in the system was detected by di-n-butylamine titration. The measured NCO% was close to 0, which was regarded as the endpoint of the prepolymerization reaction.

[0102] Reaction termination: Add methanol to the system, the molar amount of methanol being 0.1% of the molar amount of aliphatic diisocyanate, and stir continuously for 2 hours; add an appropriate amount of methyl isobutyl ketone to make the solid content 70%; cool down to terminate the reaction.

[0103] Step 3: Filtration and Discharge

[0104] The clear, transparent glue is filtered through a 200-mesh filter and then placed into a sealed, dry container to complete the preparation.

[0105] Example 2

[0106] Step 1: Raw material preparation and feeding

[0107] In a clean, dry 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser, and nitrogen protection port, add tetrafunctional linear polyether polyol erythritol-initiated polyoxypropylene-polyoxyethylene tetraol (functionality 4, Mn=3000) and monopentaerythritol polypropoxylate (chemical structure shown in [reference missing]) in a mass ratio of 5:1. Figure 2 (n=2), start stirring (120 rpm), vacuum at 110 degrees Celsius for 1-3 hours until the water content is below 200 ppm.

[0108] Step 2: Prepolymerization reaction

[0109] The reaction system was slowly cooled to 70°C. Dicyclohexylmethane diisocyanate dissolved in methyl isobutyl ketone was added dropwise over 1 hour at a constant pressure dropping funnel. The molar ratio of NCO to OH in the branched polyether polyol and the linear polyether polyol was 0.7. After the addition was complete, the reaction temperature was raised to 100°C and the reaction was maintained at this temperature for 5 hours.

[0110] Process control: Samples were taken every hour, and the content of -NCO groups in the system was detected by di-n-butylamine titration. The measured NCO% was close to 0, which was regarded as the endpoint of the prepolymerization reaction.

[0111] Reaction termination: Add ethanol to the system, the molar amount of ethanol being 1% of the molar amount of aliphatic diisocyanate, and stir continuously for 2 hours; add an appropriate amount of methyl isobutyl ketone to make the solid content 70%; cool down to terminate the reaction.

[0112] Step 3: Filtration and Discharge

[0113] The clear, transparent glue is filtered through a 200-mesh filter and then placed into a sealed, dry container to complete the preparation.

[0114] Example 3

[0115] Step 1: Raw material preparation and feeding

[0116] In a clean, dry 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser, and nitrogen protection port, add hexafunctional linear polyether polyol JEFFOL at a mass ratio of 10:1. ® S-490, dipentaerythritol polypropoxylate (chemical structure see below) Figure 3 (n=2), start stirring (120 rpm), vacuum at 110 degrees Celsius for 1-3 hours until the water content is below 200 ppm.

[0117] Step 2: Prepolymerization reaction

[0118] The reaction system was slowly cooled to 70°C. Hexamethylene diisocyanate dissolved in methyl isobutyl ketone was added dropwise over 1 hour using a constant-pressure dropping funnel. The molar ratio of NCO to OH in the branched polyether polyol and the linear polyether polyol was 0.9. After the addition was complete, the reaction temperature was raised to 100°C and the reaction was maintained at this temperature for 5 hours.

[0119] Process control: Samples were taken every hour, and the content of -NCO groups in the system was detected by di-n-butylamine titration. The measured NCO% was close to 0, which was regarded as the endpoint of the prepolymerization reaction.

[0120] Reaction termination: Add methanol to the system, the molar amount of methanol being 3% of the molar amount of aliphatic diisocyanate, and stir continuously for 2 hours; add an appropriate amount of methyl isobutyl ketone to make the solid content 70%; cool down to terminate the reaction.

[0121] Step 3: Filtration and Discharge

[0122] The clear, transparent glue is filtered through a 200-mesh filter and then placed into a sealed, dry container to complete the preparation.

[0123] Comparative Example 1 (without branched polyether polyols)

[0124] The preparation method of the polyurethane adhesive is the same as in Example 1, except that the branched polyether polyol pentaerythritol polypropoxylate is replaced with an equal molar amount of trifunctional linear polyol BASF / Lupranol. ® 3300.

[0125] Comparative Example 2 (Isocyanate is an aromatic isocyanate)

[0126] The preparation method of the polyurethane adhesive is the same as in Example 1, except that the isocyanate used is the aromatic isocyanate diphenylmethane diisocyanate (MDI).

[0127] Comparative Example 3 (prepolymerization reaction not segmented and controlled)

[0128] The preparation method of the polyurethane adhesive is the same as in Example 1, except that the polymerization process is not carried out in stages and is conducted at 100°C throughout.

[0129] Comparative Example 4 (without monool termination treatment)

[0130] The preparation method of the polyurethane adhesive is the same as in Example 1, except that no monool is added at the end of the polymerization process to terminate the polymerization.

[0131] Comparative Example 5 (a catalyst was added to the system)

[0132] The preparation method of the polyurethane adhesive is the same as in Example 1, except that 200 ppm of dibutyltin dilaurate catalyst is added directly after the polyol dehydration process in step one.

[0133] The polyurethane adhesives of Examples 1-3 and Comparative Examples 1-6 were tested for viscosity using a rotational viscometer at 25°C and with a #3 rotor. The viscosity was measured after 60 seconds. The adhesives were then left to stand at 25°C and 55% humidity for 3 months, and the viscosity was tested again. The average of the three tests was taken, and the unit is mPa·s. The viscosity values ​​are recorded in Table 1.

[0134] HDI trimer curing agent (CAS No.: 3779-63-3) and BYK-320 additive (main component: polyether-modified polymethylalkylsiloxane solution) were added to the polyurethane adhesives of Examples 1-3 and Comparative Examples 1-6. The mass ratio of the polyurethane adhesive, HDI trimer curing agent, and BYK-320 additive was 100:0.5:0.005. The mixture was stirred evenly to obtain a polyurethane adhesive, which was then coated onto a PET film and cured at 160°C for 3 minutes to obtain an OLED protective film with an adhesive layer thickness of 75 μm. Appearance observation, peel strength testing, and yellowing testing were then performed and recorded in Table 1.

[0135] 1. Observe the appearance and record it.

[0136] Pinholes, bubbles, impurities, bumps, and other coating defects are called pinholes. Based on the number of pinholes, the appearance of the protective film can be visually inspected and categorized into the following five types:

[0137] Good: No obvious defects

[0138] Requirements: 10cm*10cm, number of defects ≤ 1

[0139] General size: 10cm*10cm, 1 < number of missing spots ≤ 5

[0140] Poor: 10cm*10cm, number of defects ≤10

[0141] Very poor: 10cm*10cm, number of punctures > 10

[0142] 2. Peel force test

[0143] The initial peel force test was conducted according to the standard ASTM D3330. The substrate was a glass plate, the peel rate was 300 mm / min, and the peel force was measured in gf / in.

[0144] Referring to the standard ASTM D3330, the protective film strip was placed under conditions of 65°C and 90% humidity for 240 hours, and then the peel strength was tested. The adhered material was a glass plate, the peel rate was 300 mm / min, and the peel strength unit was gf / in.

[0145] 3. Yellowing test

[0146] Yellowing (yellowing) performance test: The yellow index (YI) of the sample was determined according to ASTM E313 standard. The film to be tested was prepared as a transparent film with a thickness of 75 μm (the thickness was measured and recorded using a thickness gauge). The test was conducted using a spectrophotometer in transmission mode, with a D65 standard light source used for measurement (the yellow index was calculated according to ASTM E313). After calibrating the instrument, the yellow index YI_0 of the sample before aging was measured.

[0147] UV aging treatment: The above-mentioned film samples were placed in a QUV fluorescent ultraviolet aging test chamber and accelerated aging was performed using a UVA-340 fluorescent ultraviolet lamp for 168 hours (the aging program was executed and recorded according to the equipment settings). After aging, the samples were placed in a specified environment until temperature equilibrium was reached, and then the yellow index YI_168 was retested according to the above ASTM E313 conditions. The degree of yellowing before and after aging was characterized by ΔYI = YI_168 − YI_0.

[0148] The test results of the protective films prepared with polyurethane adhesives in each embodiment and comparative example were recorded in Table 1 after the above tests.

[0149] Table 1

[0150]

[0151] Experimental conclusion:

[0152] Examples 1-3, under mild conditions without a catalyst, exhibit moderate reaction rates and easily controllable single temperature variables. This ensures that the -NCO and -OH groups react fully, uniformly, and with minimal side reactions, resulting in a more homogeneous molecular structure. This step replaces the catalytic effect of a catalyst. Furthermore, after monool termination treatment, no active NCO groups remain, guaranteeing the stability of the exfoliation force.

[0153] Comparative Example 1 used only linear polyols and did not use branched polyether polyols with a functionality of 3-6. The resulting polyurethane adhesive had a significantly higher viscosity than the example and a peel strength higher than 2 gf / in, indicating that linear polyols cannot achieve the effect of low peel strength.

[0154] Comparative Example 2 used aromatic isocyanate MDI, and the resulting polyurethane adhesive had high viscosity, high peel strength, and obvious yellowing.

[0155] Comparative Example 3 did not perform segmented temperature control during the reaction process, resulting in polyurethane adhesive with high viscosity and high peel strength, indicating that the goal of low viscosity and low peel strength cannot be achieved without segmented temperature control.

[0156] The reaction of Comparative Example 4 was not terminated with monool, resulting in trace amounts of NCO residue in the system, which is not conducive to the stability of the product. After being placed at 25°C and 55% humidity for 3 months, the viscosity value increased significantly. After the protective film sample was placed at 65°C and 90% humidity for 240 hours, the peel strength also increased significantly.

[0157] The polyurethane adhesive prepared by adding a catalyst in Comparative Example 5 has high viscosity and gels well. After a period of time, the peel strength increases significantly and yellowing occurs in the later stage.

[0158] The above description is merely an optional embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the contents of the present invention's specification and drawings under the inventive concept of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A polyurethane adhesive with stable high-temperature peel strength, characterized in that, The composition forming the polyurethane adhesive includes a branched polyether polyol with a functionality of 3-6 and an aliphatic diisocyanate. Monool end-capping is used and no catalyst is added during the preparation of the polyurethane adhesive. In the composition of the polyurethane adhesive, the molar ratio of NCO to hydroxyl groups is 0.5-1.

2. The high-temperature peel strength stable polyurethane adhesive according to claim 1, characterized in that, The branched polyether polyol with a functionality of 3-6 is a monopentaerythritol polypropoxylate and / or a dipentaerythritol polypropoxylate.

3. The high-temperature peel strength stable polyurethane adhesive according to claim 1, characterized in that, The aliphatic diisocyanate is a linear aliphatic diisocyanate and / or an alicyclic diisocyanate.

4. The high-temperature peel strength stable polyurethane adhesive according to claim 1, characterized in that, The monool is one or more of methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, n-pentanol, and isopentanol.

5. The high-temperature peel strength stable polyurethane adhesive according to claim 1, characterized in that, The composition forming the polyurethane adhesive also includes a linear polyether polyol having a functionality of 3-6.

6. The high-temperature peel strength stable polyurethane adhesive according to claim 5, characterized in that, The mass ratio of the linear polyether polyol to the branched polyether polyol with a functionality of 3-6 is (1-10):

1.

7. The method for preparing the high-temperature peel strength stable polyurethane adhesive according to any one of claims 1-4, characterized in that: In an organic solvent, the branched polyether polyol with a functionality of 3-6 and the aliphatic diisocyanate are first polymerized at a first temperature of 60-90℃ and then polymerized at a second temperature of 80-120℃. Then, a monool is added for end-capping, the temperature is lowered, the material is discharged, and the preparation is completed. The second temperature is higher than the first temperature, and the temperature difference is ≥5℃.

8. The method for preparing the high-temperature peel strength stable polyurethane adhesive as described in any one of claims 5-6, characterized in that: In an organic solvent, the branched polyether polyol, linear polyether polyol, and aliphatic diisocyanate with a functionality of 3-6 are first polymerized at a first temperature of 60-90℃, then polymerized at a second temperature of 80-120℃, and then end-capped with a monool, cooled, discharged, and the preparation is completed; the second temperature is higher than the first temperature, and the temperature difference is ≥5℃.

9. The method for preparing the high-temperature peel strength stable polyurethane adhesive according to claim 7, characterized in that, The first temperature is 60-80℃, and the second temperature is 90-120℃.

10. An OLED protective film, characterized in that, Including the high-temperature peel strength stable polyurethane adhesive as described in any one of claims 1-6, the OLED protective film, after aging at 65°C and 90% humidity for 240 hours, has a 180° peel strength creep rate ≤20%.

11. The OLED protective film according to claim 10, characterized in that, Its preparation method includes the following steps: S1: Add curing agent and catalyst to polyurethane adhesive, stir evenly to obtain polyurethane adhesive; S2: Apply the high-temperature peel strength stable polyurethane adhesive onto the PET film; S3: After curing at 150-180℃ for 2-10 minutes, an OLED protective film is obtained.

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