Back coating film, protective film applying back coating film and preparation method of back coating film

Abstract

The invention relates to a back coating film, a protective film applying the back coating film and a preparation method, the back coating film comprises a two-layer structure of a bottom layer and a surface layer, the bottom layer adopts a reaction type slipping agent, the surface layer adopts a non-reaction type slipping agent, and after the protective film is formed after a base material is formed into a film, the bottom layer and the surface layer both form a structure with the friction coefficient gradually increased from inside to outside. The friction coefficient of the surface layer is higher than that of the bottom layer. On the premise of not increasing the total dosage of the slipping agent for the back coating film, the surface friction coefficient of the back coating film is further reduced, and the damage to the bonding performance of the back coating film and a base material is avoided. The protective film is convenient to wind and unwind in the high-speed production process, and the film surface performance of the protective film is not damaged.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of liquid crystal display, in particular to a back coating film and a protective film using the same and a preparation method. BACKGROUND

[0002] A polarizing plate is one of the core components of a liquid crystal display and an organic light emitting diode display. In order to prevent the polarizing plate from being scratched, contaminated or damp during transportation, storage and processing, a layer of optical protective film is usually attached to the outer side of the polarizing plate. The protective film is usually made of a transparent substrate such as polyester (PET), polyolefin (such as PP, PE) and the like.

[0003] During production, the protective film generates a large friction force when being wound or peeled at high speed, which causes scratches on the film surface, uneven peeling and even static electricity, thereby adsorbing dust and affecting the downstream attaching process. Therefore, a back coating film with a low friction coefficient is usually coated on the side away from the PVA film to meet the requirements of high-speed peeling of the protective film. The slip agent is a key component for the back coating film to have a low friction coefficient, but too much slip agent will affect the adhesion, and the adhesion between the back coating film and the protective film substrate is not good.

[0004] Therefore, it is necessary to design a back coating film that takes into account both low friction coefficient and adhesion. SUMMARY

[0005] The application aims to provide a back coating film that takes into account both low friction coefficient and adhesion. When applied to the protective film, it is beneficial to protect the film surface of the protective film and facilitate winding and unwinding during high-speed production.

[0006] To solve the above technical problems, the application can be implemented by the following technical solutions: The application provides a back coating film, which comprises a bottom layer and a surface layer arranged in layers, The bottom layer is composed of the following parts by weight: 85-95 parts of first film-forming resin; 5-15 parts of isocyanate crosslinking agent; 0.5-2 parts of polyether modified polydimethylsiloxane; 150-200 parts of first solvent; The surface layer is composed of the following parts by weight: 5-10 parts of second film-forming resin; 0.1-1.5 parts of perfluoropolyether; 50-100 parts of second solvent.

[0007] As a further improvement of the application, the first film-forming resin is selected from a hydroxyl acrylate resin or a carboxyl acrylate resin, and the second film-forming resin is selected from a hydroxyl acrylate resin or a carboxyl acrylate resin.

[0008] As a further improvement of the application, the hydroxyl acrylate resin has a hydroxyl value of 30-100 mgKOH / g.

[0009] As a further improvement of the application, the first solvent is selected from any one of a mixture of ethyl acetate and toluene at a volume ratio of 1:1, and propylene glycol methyl ether acetate; and the second solvent is selected from any one of a mixture of ethyl acetate and toluene at a volume ratio of 1:1, and propylene glycol methyl ether acetate.

[0010] As a further improvement of the application, the thickness of the bottom layer is 0.5-2 μm, and the thickness of the surface layer is 0.1-0.5 μm.

[0011] As a further improvement of the application, the polyether-modified polydimethylsiloxane has a number average molecular weight of 5000-10000, and the perfluoropolyether has a number average molecular weight of 2000-5000.

[0012] As a further improvement of the application, the isocyanate crosslinking agent is selected from at least one of an HDI trimer and a TDI adduct, and the molar ratio of the NCO groups of the isocyanate crosslinking agent to the hydroxyl groups or carboxyl groups in the first film-forming resin is 1.2:1-1.5:1.

[0013] To achieve the above object, the application provides a protective film composed of a substrate and a back coating film, wherein the back coating film is arranged on any one side surface of the substrate, and the other side surface of the substrate is free of the back coating film, and the back coating film is the back coating film as described above.

[0014] As a further improvement of the application, the substrate is selected from any one of a PET film, a PP film, a PE film, a PMMA film, and a TAC film.

[0015] To achieve the above object, the application provides a preparation method of the protective film as described above, comprising the following steps: S1, dissolving a first film-forming resin in a first solvent, adding a polyether-modified polydimethylsiloxane, uniformly dispersing, adding an isocyanate crosslinking agent, and fully mixing to obtain a bottom layer coating liquid; S2, dissolving a second film-forming resin in a second solvent, adding a perfluoropolyether, and uniformly dispersing to obtain a surface layer coating liquid; S3. Apply the base coat liquid to any side of the substrate surface and bake it at 60℃~100℃ for 15s~45s to obtain the base coat. Then apply the top coat liquid on the base coat and dry it thoroughly at 100℃ for 1min~5min to obtain a protective film including the back coat film.

[0016] The beneficial effects of this application are as follows: This application provides a back-coated film comprising a two-layer structure: a base layer and a top layer. The base layer uses a reactive slip agent, and the top layer uses a non-reactive slip agent. After the film is formed on the substrate to create a protective film, both the base layer and the top layer exhibit a structure where the coefficient of friction gradually increases from the inside out, with the coefficient of friction of the top layer being higher than that of the base layer. Without increasing the total amount of slip agent used in the back-coated film, this further reduces the surface friction coefficient of the back-coated film while avoiding damage to the bonding performance between the back-coated film and the substrate. This facilitates the unwinding and rewinding of the protective film during high-speed production without damaging its surface properties. Detailed Implementation

[0017] As the background technology shows, traditional back-coated films typically reduce their coefficient of friction by adding slip agents during preparation. When applied to the substrate surface, the resulting protective film generates minimal friction during winding and unwinding, thus protecting the substrate surface and stabilizing downstream lamination processes. However, with the development of high-speed automated lamination processes, higher demands have been placed on reducing the coefficient of friction of the back-coated film. Conventional solutions primarily involve increasing the amount of slip agent to lower the coefficient of friction. However, excessive slip agent addition can negatively impact the adhesion performance between the back-coated film and the substrate, thereby failing to protect the substrate surface.

[0018] To address the aforementioned technical problems, this application provides a back coating film, comprising a bottom layer and a top layer stacked together. The bottom layer is composed of the following parts by weight: 85 to 95 parts of the first film-forming resin; 5 to 15 parts of isocyanate crosslinking agent; 0.5 to 2 parts of polyether-modified polydimethylsiloxane; The first solvent is 150 to 200 parts; The surface layer is composed of the following parts by weight: 5 to 10 parts of the second film-forming resin; 0.1 to 1.5 parts of perfluoropolyether; The second solvent is 50 to 100 parts; Wherein: the first film-forming resin is selected from hydroxy acrylate resin or carboxy acrylate resin, and the second film-forming resin is selected from hydroxy acrylate resin or carboxy acrylate resin.

[0019] Based on the above technical solution, the back coating film of this application is designed as a two-layer structure. The bottom layer uses a polyether-modified polydimethylsiloxane slip agent, and the top layer uses a perfluoropolyether slip agent, forming a back coating film with a gradually decreasing coefficient of friction on the outer surface of the substrate. Details are as follows: During the film formation process, the first film-forming resin (hydroxyl or carboxyl acrylate resin) serves as the main component. The hydroxyl or carboxyl groups on its molecular chain react chemically with the isocyanate groups in the isocyanate crosslinking agent under baking conditions, forming a three-dimensional network structure by forming urethane bonds or acylurea bonds, providing the underlying layer with basic mechanical properties and adhesion. The first solvent gradually evaporates after coating, ensuring uniform dispersion of the resin and crosslinking agent and promoting the full progress of the crosslinking reaction. Meanwhile, the polyether-modified polydimethylsiloxane, as a reactive slip agent, contains polydimethylsiloxane segments with low surface energy, which migrate to the surface of the underlying layer during the film formation process. By reducing surface tension, it optimizes the smoothness of the coating and imparts a certain degree of slip to the underlying layer, thus forming preliminary low-friction characteristics on the surface of the underlying layer.

[0020] During the topcoat film formation, the second film-forming resin (hydroxyl or carboxyl acrylate resin) forms a thin film structure after the second solvent evaporates. As a carrier for the perfluoropolyether, the perfluoropolyether, due to its fluorinated segments having lower surface energy than polydimethylsiloxane, is a non-reactive slip agent. It further migrates and accumulates on the topcoat surface. Compared to the bottom layer of polyether-modified polydimethylsiloxane, the fluorine element in its molecular structure imparts stronger surface lubricity and anti-friction capabilities, resulting in a lower coefficient of friction for the topcoat surface. Because the bottom layer achieves basic low-friction characteristics through polyether-modified polydimethylsiloxane, while the topcoat further reduces the coefficient of friction with the lower surface energy of the perfluoropolyether, the two layers form a gradient change in slip agent type from the inside out, ultimately resulting in a gradual decrease in the coefficient of friction from the inside out of the entire backcoat film.

[0021] Meanwhile, the technical solution of this application, through a two-layer structural design, forms a gradient combination of different types of slip agents. Without increasing the total amount of slip agent, it achieves a further reduction in the surface friction coefficient of the back coating film and avoids damage to the bonding performance between the back coating film and the substrate. The bottom layer is mainly composed of a three-dimensional network structure formed by the first film-forming resin and isocyanate crosslinking agent, with only 0.5 to 2 parts of polyether-modified polydimethylsiloxane added as a slip agent. Although this slip agent will migrate to the surface of the bottom layer to impart initial low-friction properties, due to its small amount and moderate compatibility with the resin, it will not excessively penetrate to the interface between the bottom layer and the substrate. In addition, the bottom layer resin forms a tight adhesion with the substrate through chemical crosslinking (especially the hydroxyl / carboxyl groups may have some adhesion to the substrate surface). The polar groups interact to ensure the bonding force between the bottom layer and the substrate. The top layer uses 5-10 parts of the second film-forming resin as a carrier, combined with 0.1-1.5 parts of perfluoropolyether. Because the fluorinated segments of the perfluoropolyether have a lower surface energy than polyether-modified polydimethylsiloxane, it will preferentially migrate to and accumulate on the outermost surface of the top layer. The strong hydrophobicity and low surface energy of fluorine further reduce the surface friction coefficient. The top layer resin and the bottom layer resin have similar chemical structures (both are hydroxyl or carboxyl acrylate resins), and can form a good bond with the bottom layer through intermolecular forces. At the same time, the amount of perfluoropolyether is very small and mainly accumulates on the surface of the top layer, and will not diffuse to the interface between the bottom layer and the substrate, thus avoiding the interface peeling problem caused by excessive slip agent.

[0022] This layered design, which uses a small amount of silicone-based slip agent in the bottom layer to provide basic slip and adhesion assurance, and a small amount of fluorine-based slip agent in the top layer to achieve ultra-low surface friction, achieves a gradient reduction in the coefficient of friction without increasing the total amount through the synergistic effect of different types of slip agents. Furthermore, because the two types of slip agents are confined to their respective layers and their amounts are controllable, it avoids damage to the bonding performance between the bottom layer and the substrate, and between the top layer and the bottom layer, thus balancing the requirements for low friction and adhesion stability.

[0023] In an optional embodiment, the hydroxyl value of the hydroxyl acrylate resin is 30–100 mg KOH / g. This hydroxyl value range is highly compatible with the amount of 5–15 parts of isocyanate crosslinking agent in the underlayer. This ensures that there are enough hydroxyl groups on the resin molecular chain to react with the NCO groups of the crosslinking agent to form a three-dimensional crosslinking network of suitable density, avoiding insufficient crosslinking and poor mechanical properties of the coating due to too few hydroxyl groups, or excessive crosslinking and embrittlement of the coating due to too many hydroxyl groups. At the same time, this range of hydroxyl values ​​can also take into account the solubility of the resin in the first solvent, ensuring uniform and stable coating solution, reducing defects such as pinholes and craters during film formation, and ultimately improving the adhesion between the underlayer and the substrate and the weather resistance of the coating.

[0024] In an optional embodiment, the first solvent is selected from either a 1:1 volume ratio mixture of ethyl acetate and toluene or propylene glycol methyl ether acetate; the second solvent is selected from either a 1:1 volume ratio mixture of ethyl acetate and toluene or propylene glycol methyl ether acetate. Both solvents have good solubility for components such as hydroxyl / carboxyacrylate resins and isocyanate crosslinking agents, which can reduce the viscosity of the coating solution and ensure uniform spreading on the substrate surface during the application of the undercoat and topcoat layers. The 1:1 volume ratio mixture of ethyl acetate and toluene has a moderate evaporation rate, which can match the baking temperature of 60℃ to 100℃, avoiding rapid solvent evaporation that leads to rapid curing and pinhole formation on the coating surface, or slow evaporation that causes sagging. Propylene glycol methyl ether acetate has a higher boiling point, which can prolong the open time of the coating solution, making it suitable for scenarios with high coating precision requirements. The selection of the two solvents can meet the needs of different production processes, while ensuring low solvent residue after film formation, avoiding affecting coating performance and subsequent bonding processes.

[0025] In an optional implementation, the thickness of the bottom layer is 0.5 μm to 2 μm, and the thickness of the top layer is 0.1 μm to 0.5 μm. The 0.5 to 2 μm thickness of the bottom layer ensures that the resin and crosslinking agent react fully to form a complete three-dimensional network, providing sufficient mechanical support and adhesion for the coating. This avoids insufficient protective performance due to excessive thickness, or increased baking time and production costs due to excessive thickness. The thin 0.1 to 0.5 μm thickness of the top layer reduces the amount of the second film-forming resin, lowering costs. At the same time, it allows the perfluoropolyether to migrate and accumulate more easily on the surface of the top layer, fully utilizing its low surface energy characteristics to reduce the coefficient of friction. This avoids uneven dispersion of the perfluoropolyether and unstable surface friction coefficient due to excessive thickness of the top layer. Furthermore, the combination of the two layer thicknesses controls the total thickness of the back coating film, avoiding affecting the overall flexibility of the protective film.

[0026] In an optional embodiment, the number-average molecular weight of the polyether-modified polydimethylsiloxane is 5000-10000, and the number-average molecular weight of the perfluoropolyether is 2000-5000. This molecular weight range of the polyether-modified polydimethylsiloxane balances its dispersibility in the undercoating solution and its migration ability to the coating surface. Too low a molecular weight can lead to excessive migration to the interface between the undercoat and the substrate, affecting the adhesion between them. Too high a molecular weight makes uniform dispersion difficult and fails to effectively reduce the surface tension of the undercoat. The perfluoropolyether's molecular weight of 2000-5000 ensures stable dispersion in the topcoat solution. Simultaneously, its suitable molecular chain length allows it to form a continuous low surface energy layer on the surface, fully utilizing its slip-forming effect to reduce the coefficient of friction. This avoids the problem of the perfluoropolyether being unable to migrate to the surface due to excessively high molecular weight, or easily volatilizing and losing its slip-forming effect during use due to excessively low molecular weight.

[0027] In an optional embodiment, the isocyanate crosslinking agent is selected from at least one of hexamethylene diisocyanate (HDI) trimer and toluene diisocyanate (TDI) adduct, and the molar ratio of the NCO group of the isocyanate crosslinking agent to the hydroxyl or carboxyl group in the first film-forming resin is 1.2:1 to 1.5:1. Both HDI trimer and TDI adduct have multiple NCO groups, which can crosslink with the hydroxyl / carboxyl groups of the resin at multiple points to form a more stable three-dimensional network. Compared with other types of isocyanates, both have moderate reactivity and can react slowly under baking conditions of 60℃~100℃ to avoid the formation of bubbles inside the coating due to excessive reaction. The molar ratio of 1.2:1~1.5:1 can ensure an excess of NCO groups, ensuring that the hydroxyl / carboxyl groups in the resin react fully, reducing the residue of unreacted functional groups, and improving the chemical resistance and water resistance of the coating. At the same time, the excess NCO groups can react with trace moisture in the air to further improve the crosslinking structure and avoid insufficient crosslinking and easy cracking of the coating due to insufficient NCO, ultimately improving the overall performance of the underlying coating.

[0028] This application also provides a protective film comprising a substrate and a back coating film, wherein the back coating film is disposed only on one side surface of the substrate, and the other side surface of the substrate does not have the back coating film, and the back coating film is the back coating film described above.

[0029] In an optional implementation, the substrate is selected from any one of PET (polyethylene terephthalate) film, PP (polypropylene) film, PE (polyethylene) film, PMMA (polymethyl methacrylate) film, and TAC (cellulose triacetate) film. The selected substrates are all types of polymer films with good mechanical properties and weather resistance; and the surface of these substrates can be slightly treated (such as with corona treatment) to enhance adhesion to the underlying coating, forming a good bond with the hydroxyl / carboxylic acrylate resin in the underlying layer, ensuring that the back coating film is not easily peeled off from the substrate surface. Simultaneously, the properties of the substrate itself and the back coating film work synergistically to meet the protective film usage requirements of different fields (such as electronics and packaging).

[0030] This application also provides a method for preparing a protective film, comprising the following steps: S1. Dissolve the first film-forming resin in the first solvent, add polyether-modified polydimethylsiloxane, disperse evenly, add isocyanate crosslinking agent, mix thoroughly and evenly to obtain the bottom coating liquid; S2. Dissolve the second film-forming resin in the second solvent, add perfluoropolyether, and disperse evenly to obtain the surface coating liquid; S3. Apply the undercoating solution to any side of the substrate surface and bake at 60℃~100℃ for 15s~45s to obtain the undercoating layer. Immediately afterwards, apply the topcoating solution onto the undercoating layer and dry it thoroughly at 100℃ for 1min~5min to obtain a protective film including the back coating film. The technical effects of this technical solution are described with reference to the technical effects of the back coating film, and will not be repeated here.

[0031] The present application will now be described in detail. However, these embodiments do not limit the present application, and any structural, methodological, or functional modifications made by those skilled in the art based on these embodiments are included within the scope of protection of this application.

[0032] Example 1 This embodiment provides a protective film, which is prepared by the following steps: 1) Dissolve 85 parts of hydroxy acrylate resin in 150 parts of the first solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 0.5 parts of polyether-modified polydimethylsiloxane and disperse evenly. Then add 5 parts of hexamethylene diisocyanate (HDI) trimer and mix thoroughly to obtain the bottom coating liquid. 2) Dissolve 5 parts of hydroxy acrylate resin in 50 parts of a second solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 0.1 parts of perfluoropolyether and disperse evenly to obtain the surface coating liquid. 3) Apply the bottom coating liquid to any side surface of the substrate. The thickness of the bottom layer is 0.5 μm. After baking at 60°C for 15 seconds, the bottom layer is obtained. Then, apply the top layer coating liquid on the bottom layer. The thickness of the top layer is 0.1 μm. Then, dry it thoroughly at 100°C for 1 minute to obtain a protective film including the back coating film.

[0033] Example 2 This embodiment provides a protective film, which is prepared by the following steps: 1) Dissolve 95 parts of hydroxy acrylate resin in 200 parts of the first solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 2 parts of polyether-modified polydimethylsiloxane and disperse evenly. Then add 15 parts of hexamethylene diisocyanate (HDI) trimer and mix thoroughly to obtain the bottom coating liquid. 2) Dissolve 10 parts of hydroxy acrylate resin in 100 parts of a second solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 1.5 parts of perfluoropolyether and disperse evenly to obtain the surface coating liquid. 3) Apply the bottom coating liquid to any side surface of the substrate. The thickness of the bottom layer is 2μm. After baking at 100℃ for 45s, the bottom layer is obtained. Then, apply the top coating liquid on the bottom layer. The thickness of the top layer is 0.5μm. Then, dry it thoroughly at 100℃ for 5min to obtain a protective film including the back coating film.

[0034] Example 3 This embodiment provides a protective film, which is prepared by the following steps: 1) Dissolve 90 parts of hydroxy acrylate resin in 175 parts of the first solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 1.2 parts of polyether-modified polydimethylsiloxane and disperse evenly. Then add 10 parts of hexamethylene diisocyanate (HDI) trimer and mix thoroughly to obtain the bottom coating liquid. 2) Dissolve 8 parts of hydroxy acrylate resin in 75 parts of a second solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 1 part of perfluoropolyether and disperse evenly to obtain the surface coating liquid. 3) Apply the bottom coating liquid to any side surface of the substrate. The thickness of the bottom layer is 1.2 μm. After baking at 80°C for 30 seconds, the bottom layer is obtained. Then, apply the top layer coating liquid on the bottom layer. The thickness of the top layer is 0.3 μm. Then, dry it thoroughly at 100°C for 3 minutes to obtain a protective film including the back coating film.

[0035] Example 4 The difference between this embodiment and Embodiment 3 is that carboxyacrylate resin is used instead of hydroxyacrylate resin.

[0036] Example 5 The difference between this embodiment and Example 3 is that propylene glycol methyl ether acetate is used instead of the mixed solvent of ethyl acetate and toluene in a volume ratio of 1:1.

[0037] Comparative Example 1 This comparative example provides a protective film prepared by the following steps: 1) Dissolve 90 parts of hydroxy acrylate resin in 175 parts of the first solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 8 parts of polyether-modified polydimethylsiloxane and disperse evenly. Then add 10 parts of hexamethylene diisocyanate (HDI) trimer and mix thoroughly to obtain the bottom coating liquid. 2) Apply the undercoating liquid to any side of the substrate surface. The thickness of the undercoating is 1.5μm. First, bake at 80℃ for 30s, and then dry at 100℃ for 3min to obtain a protective film including the undercoating.

[0038] Comparative Example 2 This comparative example provides a protective film prepared by the following steps: 1) Dissolve 90 parts of hydroxy acrylate resin in 175 parts of the first solvent, which is a mixture of ethyl acetate and toluene in a volume ratio of 1:1. Add 1.2 parts of polyether-modified polydimethylsiloxane and disperse evenly. Then add 10 parts of hexamethylene diisocyanate (HDI) trimer and mix thoroughly to obtain the bottom coating liquid. 2) Apply the undercoating liquid to any side of the substrate surface. The thickness of the undercoating is 1.2μm. First, bake at 80℃ for 30s, and then dry at 100℃ for 3min to obtain a protective film including the undercoating.

[0039] The protective films obtained in Examples 1-5 and Comparative Examples 1-2 were tested for their surface friction coefficient, peel force, and processing stability. 1) Coefficient of friction: Referring to the GB / T10006 standard, the coefficient of friction of the surface is tested using a coefficient of friction meter. The coefficient of friction is controlled within the ideal range of 0.20 to 0.28, which can meet the performance requirements of high-speed winding and unwinding of protective film. There will be no slippage, scratches on the film surface, or uneven peeling.

[0040] 2) Peel Force (Unit: N / 25mm): Using a 2kg manual roller, flatly press 31B tape onto the back coating surface of the protective film (ensuring even adhesion without air bubbles). Fix the protective film sample with the tape applied onto a high-speed tensile testing machine. Set the tensile speed of the machine to 30m / min, start the machine to allow the back coating film and substrate to separate along the bonding surface, and record the force required for separation. This measures the strength of the adhesion between the back coating film and the substrate—the greater the force required for peeling, the stronger the adhesion.

[0041] 3) Processing stability assessment: During actual simulated high-speed (150m / min) unwinding and transport, observe whether the film roll slips or serpentines. Rating: "Excellent" (no abnormalities) or "Poor" (significantly unstable).

[0042] The performance data of the protective films obtained in Examples 1-5 and Comparative Examples 1-2 are shown in Table 1.

[0043] Table 1

[0044] As can be seen from the experimental results in Table 1, by comparing Comparative Document 1 and Example 3, it can be seen that by increasing the amount of slip agent, the friction coefficient of the protective film surface can be reduced, and the adhesion of the protective film during high-speed winding and unwinding can be avoided. However, the adhesion between the back coating film and the substrate is reduced, which makes the back coating film easy to fall off or wrinkle during high-speed winding and unwinding.

[0045] A comparison of document 2 and example 3 shows that without a surface layer structure, the surface friction coefficient of the back coating film formed solely by reactive slip agents does not meet the performance requirements for high-speed winding and unwinding of the protective film.

[0046] A comparison of Comparative Documents 1 and 2 with Examples 1-5 reveals that this application achieves a gradient combination of different slip agents through a two-layer structural design. Without increasing the total amount used, it reduces the surface friction coefficient of the back coating film while avoiding damage to its bonding performance with the substrate. The bottom layer is primarily composed of a three-dimensional network structure formed by the first film-forming resin and an isocyanate crosslinking agent, with only a small amount of polyether-modified polydimethylsiloxane added. Due to its small dosage, moderate compatibility, and tight adhesion to the substrate, the bonding strength is ensured. The top layer uses the second film-forming resin as a carrier, combined with a small amount of perfluoropolyether. Its lower surface energy allows it to preferentially migrate to the surface to further reduce friction. Furthermore, the good bonding between the top and bottom layers prevents the perfluoropolyether from diffusing to the interface, thus avoiding interfacial delamination.

[0047] It should be noted that the above description is only for explaining the preferred embodiments of this application and is not intended to limit this application in any way. Therefore, any modifications or changes made to this application under the same inventive spirit should still be included within the scope of protection intended by this application.

Claims

1. A back coating film, characterized in that, Including the layered bottom and top layers, The bottom layer is composed of the following parts by weight: 85 to 95 parts of the first film-forming resin; 5 to 15 parts of isocyanate crosslinking agent; 0.5 to 2 parts of polyether-modified polydimethylsiloxane; The first solvent is 150 to 200 parts; The surface layer is composed of the following parts by weight: 5 to 10 parts of the second film-forming resin; 0.1 to 1.5 parts of perfluoropolyether; The second solvent is 50 to 100 parts.

2. The back coating film according to claim 1, characterized in that, The first film-forming resin is selected from hydroxy acrylate resin or carboxy acrylate resin, and the second film-forming resin is selected from hydroxy acrylate resin or carboxy acrylate resin.

3. The back coating film according to claim 2, characterized in that, The hydroxyl value of the hydroxyl acrylate resin is 30-100 mg KOH / g.

4. The back coating film according to claim 1, characterized in that, The first solvent is selected from any one of a mixed solvent of ethyl acetate and toluene in a volume ratio of 1:1, or propylene glycol methyl ether acetate; the second solvent is selected from any one of a mixed solvent of ethyl acetate and toluene in a volume ratio of 1:1, or propylene glycol methyl ether acetate.

5. The back coating film according to claim 1, characterized in that, The thickness of the bottom layer is 0.5μm to 2μm, and the thickness of the top layer is 0.1μm to 0.5μm.

6. The back coating film according to claim 1, characterized in that, The polyether-modified polydimethylsiloxane has a number average molecular weight of 5,000 to 10,000, and the perfluoropolyether has a number average molecular weight of 2,000 to 5,000.

7. The back coating film according to claim 1, characterized in that, The isocyanate crosslinking agent is selected from at least one of HDI trimer and TDI adduct, and the molar ratio of the NCO group of the isocyanate crosslinking agent to the hydroxyl or carboxyl group in the first film-forming resin is 1.2:1 to 1.5:

1.

8. A protective film, characterized in that, It consists of a substrate and a back coating film, wherein the back coating film is disposed only on one side surface of the substrate, and the other side surface of the substrate does not have the back coating film, wherein the back coating film is the back coating film according to any one of claims 1-7.

9. The protective film according to claim 8, characterized in that, The substrate is selected from any one of PET film, PP film, PE film, PMMA film, and TAC film.

10. A method for preparing the protective film according to claim 8 or 9, characterized in that, Includes the following steps: S1. Dissolve the first film-forming resin in the first solvent, add polyether-modified polydimethylsiloxane, disperse evenly, add isocyanate crosslinking agent, mix thoroughly and evenly to obtain the bottom coating liquid; S2. Dissolve the second film-forming resin in the second solvent, add perfluoropolyether, and disperse evenly to obtain the surface coating liquid; S3. Apply the base coat liquid to any side of the substrate surface and bake it at 60℃~100℃ for 15s~45s to obtain the base coat. Then apply the top coat liquid on the base coat and dry it thoroughly at 100℃ for 1min~5min to obtain a protective film including the back coat film.

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