Leather transfer film and method of making
By constructing a micro-nano structure release layer and a thermally reversible adhesive layer in the leather transfer film, and combining this with a self-healing elastic layer preparation method, the problems of flexibility and bending resistance of the transfer film were solved, achieving a highly efficient and stable leather decoration effect.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- 湖南壹鑫科技有限公司
- Filing Date
- 2026-03-13
- Publication Date
- 2026-08-04
AI Technical Summary
Existing leather transfer films have poor flexibility and bending resistance, and are prone to cracking and delamination after transfer, making it difficult to meet the needs of long-term use and large-scale production.
By anodizing, pore-expanding, and silanizing a rigid carrier substrate, a micro-nano structure release layer is constructed. A Diels-Alder reversible reaction thermally reversible adhesive layer and a self-healing elastic layer are then coated. Combined with UV curing and two-stage heat treatment, a dynamically repairable elastic network is formed, controlling the reactive primer in a semi-cured state.
It significantly improves the bending resistance and self-healing ability of the transfer film, ensures the integrity of the transfer, reduces material waste and production costs, and enhances the durability and operational stability of leather products.
Smart Images
Figure CN122501076A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of composite films and surface treatments, and more particularly to a leather transfer film and its preparation method. Background Technology
[0002] In leather processing, using transfer film to decorate the leather surface is a highly efficient process and a standard practice in the industry. Currently, the main technology involves using release paper or polymer film coated with a functional coating as the transfer film, and transferring the coating to the leather surface through heat pressing.
[0003] In existing technologies, transfer films have poor flexibility and bending resistance. After being transferred to the leather surface, the pattern is prone to cracking and damage as the leather is bent and stretched, making it difficult to meet the needs of long-term use of leather products. Some transfer films have poor bonding stability with the leather substrate after transfer, and are prone to problems such as delamination and peeling after long-term use. In addition, the operability of the transfer process is poor, making it difficult to meet the high-efficiency requirements of large-scale production. Summary of the Invention
[0004] The first aspect of this application provides a method for preparing a leather transfer film, the method comprising:
[0005] The surface of the rigid carrier substrate is sequentially subjected to anodizing, pore-expanding and silanizing treatments to obtain the release layer carrier substrate. A thermally reversible adhesive layer coating based on the Diels-Alder reversible reaction is applied to the release layer carrier substrate and cured at 80~110℃ for 3~10 minutes to obtain a thermally reversible adhesive layer. A pattern is printed on the thermally reversible adhesive layer to obtain a patterned decorative layer; The self-healing elastic layer coating is applied to the patterned decorative layer, and then subjected to UV curing and two heat treatments in sequence to obtain the elastic layer. The two heat treatments include treatment at 60~80℃ for 5~15 minutes, followed by treatment at 90~110℃ for 3~8 minutes. A reactive primer containing a terminal isocyanate group prepolymer and a leather penetrant is applied to the elastic layer and pre-cured at 70~90°C for 60~120 seconds to obtain a semi-cured primer layer with an isocyanate group reaction degree of 30%~50%. A composite protective film is applied to the semi-cured primer layer to obtain the initial transfer film; The initial transfer film is aged at 40-60°C and 30-50% relative humidity to obtain the finished transfer film.
[0006] Optionally, the step of sequentially performing anodizing, pore-expanding, and silanizing treatments on the surface of the rigid carrier substrate to obtain the release layer carrier substrate includes: The rigid carrier substrate is pretreated with degreasing and alkaline washing to obtain a clean substrate surface; The cleaned substrate is immersed in a 15% sulfuric acid solution at 15±2℃ and anodized at 16±1V DC voltage for 40~50 minutes to obtain a substrate with a porous alumina layer on the surface. The substrate with the porous alumina layer was immersed in a 4% phosphoric acid solution and subjected to a pore-expanding treatment at 25±5℃ for 30±5 minutes to obtain a micro-nano structured substrate with a nanopore array on the surface. The micro / nanostructured substrate was placed in a vacuum reactor and evacuated to 10 °C. -3 Pa, heated to 150±5℃, and heat-treated at 180±5℃ for 60±10 minutes under nitrogen protection to obtain the release layer carrier substrate.
[0007] Optionally, the pretreatment of the rigid carrier substrate with degreasing and alkaline washing to obtain a clean substrate surface includes: The substrate is immersed in an alkaline degreasing solution at 70-80°C and ultrasonically treated for 8-12 minutes. The alkaline degreasing solution contains 3-5% sodium hydroxide and 2-4% sodium carbonate by mass. Rinse the degreased substrate with deionized water and immerse it in a sodium hydroxide solution at 60-70°C and a concentration of 50-80 g / L for 3-6 minutes for alkaline washing. The substrate after alkaline washing is dried at 100~120℃ for 10~20 minutes to obtain a clean substrate surface.
[0008] Optionally, the thermally reversible adhesive coating comprises thermoplastic polyurethane, furan-terminated flexible segments, and bismaleimide crosslinking agent in a mass ratio of 50:(10~15):(3~5).
[0009] Optionally, the step of applying the self-healing elastic layer coating onto the patterned decorative layer, followed by UV curing and two stages of heat treatment to obtain the elastic layer includes: The self-healing elastic layer coating is applied to the patterned decorative layer to obtain a wet film coating with a thickness of 35~45μm; The wet film coating was placed under an LED ultraviolet light source with a wavelength of 395nm for ultraviolet curing to obtain a pre-cured coating; The pre-cured coating is placed in the first heat treatment environment and kept at 60~80℃ for 5~15 minutes; The coating that has undergone the first heat treatment is placed in the second heat treatment environment and kept at 90~110℃ for 3~8 minutes to obtain the elastic layer.
[0010] Optionally, the reactive primer comprising a terminal isocyanate group prepolymer and a leather penetrant is applied to the elastic layer and pre-cured at 70-90°C for 60-120 seconds to obtain a semi-cured primer layer with an isocyanate group reaction degree of 30%-50%, comprising: A reactive primer is applied to the elastic layer to obtain a coating with a wet film thickness of 30~40μm; Place the coated layer in a hot air circulating oven and treat it at a temperature of 70~90℃ for 60~120 seconds; By controlling the intensity decay of the isocyanate characteristic peak in the coating to a preset range, a semi-cured primer layer with an isocyanate group reaction degree of 30%~50% is obtained.
[0011] Optionally, the terminal isocyanate prepolymer is a prepolymer obtained by reacting isophorone diisocyanate with polycaprolactone triol, and the mass percentage of isocyanate groups is 5.0%~7.0%; the leather penetrant is N-methylpyrrolidone.
[0012] Optionally, the surface of the release layer carrier substrate is an anodized aluminum plate with a nanopore array, wherein the average pore diameter of the nanopores is 70~90nm and the pore depth is 10~20μm.
[0013] The second aspect of this application provides a leather transfer film prepared by the aforementioned preparation method.
[0014] Optionally, the dry film thickness of the thermally reversible adhesive layer is 5~8μm; The dry film thickness of the self-healing elastic layer is 10~15μm; The dry film thickness of the semi-cured primer layer is 6~10μm.
[0015] As can be seen from the above technical solutions, this application has the following advantages: By anodizing, pore-expanding, and silanizing the rigid carrier substrate, a permanent release layer with both micro-nano structure and chemical inertness is constructed on its surface. A Diels-Alder reversible reaction thermally reversible adhesive layer is then coated and cured on this layer. During hot pressing at 80-110℃, it forms a strong bond with the upper coating, ensuring complete transfer. After cooling, it loses its adhesiveness, allowing the rigid carrier to be peeled off without damage and reused dozens of times, significantly reducing per-use costs and material waste while improving operational stability. The self-healing elastic layer, through a combination of UV curing and two-stage heat treatment, forms a dynamically repairable elastic network, enabling the coating to withstand repeated bending of the leather. The reactive primer is controlled in a semi-cured state, retaining sufficient processing strength while forming a strong chemical bond with the leather fibers during final hot pressing and firmly cross-linking with the upper elastic network, effectively preventing cracking or delamination after long-term use. Attached Figure Description
[0016] To more clearly illustrate the technical solutions in this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0017] Figure 1 A schematic flowchart of an embodiment of the method for preparing the leather transfer film provided in this application; Figure 2 This is a schematic flowchart of an embodiment of the treatment of the surface of a rigid carrier substrate in the preparation method of the leather transfer film provided in this application. Figure 3 A schematic flowchart of an embodiment of the method for preparing the leather transfer film provided in this application, in which a self-healing elastic layer coating is applied to the patterned decorative layer; Figure 4 This is a schematic flowchart of an embodiment of the method for preparing the leather transfer film provided in this application, in which a reactive primer comprising a terminal isocyanate group prepolymer and a leather penetrant is applied to the elastic layer. Detailed Implementation
[0018] It should be noted that the leather transfer film and its preparation method provided in this application are applicable to surface texture transfer and pattern decoration on various substrates such as genuine leather, PU leather, and PVC leather, including logo transfer, pattern hot stamping, and surface protective layer lamination on everyday leather products such as leather shoes, bags, sofas, car seats, leather clothing, and leather gloves. Thanks to the coating's excellent bending resistance and self-healing ability, it is particularly suitable for decoration of high-frequency bending and dynamically stressed areas, such as shoe uppers, bag straps, and seat backs.
[0019] Example 1: This application provides a method for preparing a leather transfer film. Please refer to [link / reference]. Figure 1 , Figure 1 A schematic flowchart of an embodiment of the method for preparing the leather transfer film provided in this application, the embodiment including: S101. The surface of the rigid carrier substrate is sequentially subjected to anodizing, hole expansion and silanization treatments to obtain the release layer carrier substrate. The rigid substrate undergoes surface pretreatment, including degreasing, alkaline washing, and acid washing, to remove surface oil and natural oxide layers, resulting in a clean and activated metal surface. Anodizing is then performed, with the cleaned aluminum plate used as the anode and placed in an electrolyte solution.
[0020] In one specific example, the electrolyte was a 15% sulfuric acid aqueous solution, the temperature was controlled at 15°C, and a 16V voltage was applied under DC power supply for approximately 45 minutes. Next, a pore-expansion treatment was performed, immersing the substrate with the porous alumina layer in a 4% phosphoric acid solution and treating it at 25±5°C for 30±5 minutes to obtain a micro / nanostructured substrate with a nanopore array on its surface. Finally, a silanization treatment was performed. After cleaning and drying, the pore-expansion treated substrate was placed in a reaction vessel. The micro / nanostructured substrate was placed in a vacuum reactor, and a vacuum of 10V was applied. -3 The mixture is heated to 150±5℃ and then heat-treated at 180±5℃ for 60±10 minutes under nitrogen protection to obtain a release layer carrier substrate. The surface of the release layer carrier substrate is an anodized aluminum plate with an array of nanopores, the average pore diameter of which is 70~90nm and the pore depth is 10~20μm.
[0021] S102. Apply the thermally reversible adhesive coating based on the Diels-Alder reversible reaction onto the release layer carrier substrate and cure it at 80~110℃ for 3~10 minutes to obtain the thermally reversible adhesive layer. A thermally reversible adhesive coating is prepared, comprising at least thermoplastic polyurethane, furan-terminated flexible segments, and a bismaleimide crosslinking agent. These components are dissolved in a solvent at a mass ratio of 50:(10~15):(3~5), and stirred until homogeneous, forming a coating with a solid content of approximately 20~40%.
[0022] The coating is uniformly applied to the silanized surface of the release layer carrier substrate using methods such as blade coating, roller coating, or slot coating. The wet film thickness can be controlled between 10 and 30 micrometers. The coated substrate is then placed in an oven or tunnel furnace for curing. The curing temperature is set between 80 and 110°C, for example, 90°C or 100°C, and the curing time is 3 to 10 minutes. At this temperature, a Diels-Alder cycloaddition reaction occurs between furan groups and bismaleimide groups, forming a reversible covalent crosslinked network, thus obtaining a solid, thermally reversible adhesive layer. This layer undergoes a reverse Diels-Alder reaction at the transfer temperature, resulting in bond breakage and increased viscosity; at room temperature, it reverts to crosslinking, maintaining a solid state and low surface tack.
[0023] S103. Print a pattern on the thermally reversible adhesive layer to obtain a patterned decorative layer; On the surface of the formed thermally reversible adhesive layer, one or more layers of ink are applied using processes such as gravure printing, screen printing, or digital inkjet printing to create the desired colors, patterns, or other decorative designs, forming a patterned decorative layer. The inks used must have good adhesion to the underlying thermally reversible adhesive layer and possess certain heat and solvent resistance to withstand subsequent processes. After printing, the patterned layer typically requires low-temperature drying to cure and set.
[0024] S104. Apply the self-healing elastic layer coating onto the patterned decorative layer, and perform UV curing and two-stage heat treatment in sequence to obtain the elastic layer. The two-stage heat treatment includes treatment at 60~80℃ for 5~15 minutes, followed by treatment at 90~110℃ for 3~8 minutes. A self-healing elastic layer coating is prepared, comprising: polyurethane acrylate oligomer, acrylate, photoinitiator, and catalyst. All components are mixed thoroughly. The mixed self-healing elastic layer coating is applied to the surface of the patterned decorative layer, with a wet film thickness set to 35-45 micrometers. Then, ultraviolet (UV) curing is performed. Using a UV light source of appropriate wavelength, the coating is irradiated under an inert atmosphere. UV curing causes the acrylate double bonds to rapidly polymerize, resulting in a pre-cured coating.
[0025] The coating then undergoes a two-stage heat treatment. In the first stage, the UV-cured coating is placed in an environment of 60-80℃ for 5-15 minutes. This first-stage heat treatment primarily promotes the formation and orderly arrangement of non-covalent interactions such as hydrogen bonds and ionic bonds, and may induce the initial bonding of some dynamic covalent bonds, thus enhancing the initial elasticity and toughness of the coating. The second-stage heat treatment involves raising the temperature to 90-110℃ and treating for 3-8 minutes. This mainly promotes the synergistic effect between different dynamic bonds, ultimately building a stable elastic layer within the coating that possesses energy dissipation and self-healing capabilities.
[0026] S105. Apply a reactive primer containing terminal isocyanate group prepolymer and leather penetrant to the elastic layer, and pre-cure at 70~90℃ for 60~120 seconds to obtain a semi-cured primer layer with an isocyanate group reaction degree of 30%~50%. The main components of the primer include isocyanate-terminated polyurethane prepolymer, leveling agent, and defoamer, etc. The isocyanate-terminated polyurethane prepolymer is prepared by reacting polyisocyanate with polyol and is used as a solvent to promote the penetration of the primer into the leather.
[0027] The primer is applied to the surface of the resulting elastic layer and immediately pre-cured, typically at 70-90°C for 60-120 seconds using hot air. This allows the isocyanate groups to react to a degree of 30% to 50%. This is achieved by adjusting the temperature, time, and the amount of catalyst in the primer formulation. The degree of reaction is quantified by monitoring the attenuation of the characteristic absorption peaks of the cyanate groups using Fourier transform infrared spectroscopy. In this state, the primer layer is partially cross-linked, possessing sufficient mechanical strength to support subsequent lamination operations, while retaining a large number of isocyanate groups.
[0028] S106. Apply a composite protective film to the semi-cured primer layer to obtain the initial transfer film; A transparent polymer protective film, such as silicone-coated polyethylene film or polyethylene terephthalate film, with a thickness of approximately 25-75 micrometers, is taken. Using a roll forming laminator, the protective film is smoothly bonded to the surface of the obtained semi-cured primer layer. The function of the protective film is to prevent the semi-cured primer layer from being contaminated, scratched, or prematurely bonded to foreign objects during storage, transportation, and subsequent processing. The resulting multi-layered composite structure is the initial product transfer film.
[0029] S107. The initial transfer film is aged at 40~60℃ and 30~50% relative humidity to obtain the finished transfer film.
[0030] The prepared initial transfer film is placed in a controlled temperature and humidity environment for curing. The ambient temperature is maintained at 40~60℃, the relative humidity is controlled at 30~50%, and the curing time is at least 24 hours. Curing helps improve the uniformity and stability of the final product's performance and reduces performance fluctuations during use. After curing, a stable and usable finished leather transfer film is obtained.
[0031] In this embodiment, a permanent release layer with both micro / nano structure and chemical inertness is constructed on the surface of a rigid carrier substrate through anodizing, pore-expanding, and silanization treatments. A Diels-Alder reversible reaction thermally reversible adhesive layer is coated and cured on this layer. During hot pressing at 80-110°C, it forms a strong bond with the upper coating, ensuring complete transfer. Upon cooling, it loses its adhesiveness, allowing the rigid carrier to be peeled off without damage and reused dozens of times, significantly reducing single-use costs and material waste while improving operational stability. The self-healing elastic layer, through a combination of UV curing and two-stage heat treatment, forms a dynamically repairable elastic network, enabling the coating to withstand repeated bending of the leather. The reactive primer is controlled in a semi-cured state, retaining sufficient processing strength while undergoing strong chemical bonding with the leather fibers during final hot pressing and firmly cross-linking with the upper elastic network, effectively preventing cracking or delamination after long-term use.
[0032] Example 2: The surface treatment of the rigid carrier substrate is described in detail below. Please refer to [link / reference]. Figure 2 , Figure 2 This is a schematic flowchart illustrating an embodiment of the method for preparing the leather transfer film provided in this application, specifically the treatment of the surface of a rigid carrier substrate. The embodiment includes: S201. Immerse the substrate in an alkaline degreasing solution at 70-80°C and sonicate for 8-12 minutes. The alkaline degreasing solution contains 3-5% sodium hydroxide and 2-4% sodium carbonate by mass. To prepare an alkaline degreasing solution, mix 3% to 5% sodium hydroxide and 2% to 4% sodium carbonate by mass; heat the degreasing solution and maintain it at a temperature range of 70°C to 80°C.
[0033] The substrate is preferably a flat aluminum alloy substrate. The substrate is completely immersed in a hot alkaline degreasing solution and then treated in an ultrasonic cleaner. Ultrasonic treatment utilizes the cavitation effect to remove grease, dust, and other organic contaminants adhering to the substrate surface. The treatment time is controlled between 8 and 12 minutes to ensure thorough cleaning without causing excessive corrosion to the substrate.
[0034] S202. Rinse the degreased substrate with deionized water and immerse it in a sodium hydroxide solution at 60-70°C and a concentration of 50-80 g / L for alkali washing for 3-6 minutes. Remove the substrate and immediately rinse thoroughly with deionized water to completely remove any residual degreasing solution and detached contaminants. Then perform an alkaline wash to further activate the metal surface and remove the very thin oxide layer. Prepare a sodium hydroxide aqueous solution with a concentration of 50 g / L to 80 g / L and heat it to 60°C to 70°C. Immerse the rinsed substrate in this hot alkaline solution for 3 to 6 minutes. For example, when using an aluminum substrate, the sodium hydroxide reacts with the native alumina and a small amount of aluminum on the substrate surface to form soluble sodium aluminate, thereby exposing a fresh, highly active pure aluminum surface.
[0035] S203. Dry the alkaline-washed substrate at 100~120℃ for 10~20 minutes to obtain a clean substrate surface; After alkaline washing, rinse the substrate again with plenty of deionized water until the rinse water is neutral, ensuring no alkaline residue remains. Transfer the washed substrate to an oven for drying. Set the drying temperature between 100℃ and 120℃ and the drying time to 10 to 20 minutes to remove moisture from the substrate surface and any moisture that may have penetrated into the microstructure, resulting in a clean, dry substrate with high surface activity.
[0036] S204. Immerse the cleaned substrate in a 15% sulfuric acid solution at 15±2℃ and perform anodizing treatment at 16±1V DC voltage for 40~50 minutes to obtain a substrate with a porous alumina layer on the surface. The resulting clean substrate is used as the anode, and a lead plate or stainless steel plate is used as the cathode. Both are immersed in an electrolyte solution with a temperature strictly controlled between 13°C and 17°C. The electrolyte solution is a 15% sulfuric acid aqueous solution.
[0037] Under conditions of stirring the electrolyte to maintain a uniform temperature, a stable DC voltage of 16±1V is applied, and anodizing is performed for 40~50 minutes. During this process, an electrochemical reaction occurs on the surface of the aluminum anode, generating a porous alumina layer, thus obtaining a substrate with a porous alumina layer.
[0038] S205. Immerse the substrate with the porous alumina layer in a 4% phosphoric acid solution and perform a pore-expanding treatment at 25±5℃ for 30±5 minutes to obtain a micro-nano structured substrate with a nano-pore array on the surface. After the substrate is removed from the sulfuric acid electrolyte, it is washed with deionized water and then immersed in a 4% phosphoric acid aqueous solution for pore expansion treatment. The temperature is maintained at 20℃~30℃ and the treatment time is controlled at 25~35 minutes. Through this pore expansion, the pore size of the original nanopores can be appropriately increased and the inner wall of the pore channel can be made smoother, thereby obtaining a micro-nano structured substrate with a nanopore array.
[0039] S206. Place the micro / nano-structured substrate in a vacuum reactor and evacuate to 10°C. -3 Pa, heated to 150±5℃, and heat-treated at 180±5℃ for 60±10 minutes under nitrogen protection to obtain the release layer carrier substrate.
[0040] The processed, cleaned, and dried micro / nano-structured substrate is placed in a vacuum reactor, which is then evacuated to a high vacuum, reducing the internal pressure to 10. -3 Pa level to eliminate interference from air and water vapor.
[0041] The reaction vessel is heated to 150±5℃. At this temperature, gaseous perfluoroalkylsilane reagent is introduced into the vessel through the gas inlet system. Under vacuum and heating conditions, the silanol groups generated by the hydrolysis of alkoxy groups of silane molecules undergo a condensation reaction with the hydroxyl groups on the surface of alumina, and are firmly grafted onto the substrate surface through covalent bonds to form a dense monolayer.
[0042] After the vapor deposition process lasts for 30-60 minutes, the silane supply is stopped, and under the protection of continuous inert gas, the temperature is further raised to 180±5℃, and heat-treated at this temperature for 60±10 minutes. High-temperature heat treatment helps the chemically adsorbed silane molecules to complete more thorough cross-linking and directional alignment, forming a more stable and ordered self-assembled monolayer, thus ultimately obtaining the release layer carrier substrate.
[0043] In this embodiment, a well-organized nanopore array with a pore size of 70-90 nm and a pore depth of 10-20 μm was constructed on the surface of an aluminum substrate through a combination of anodizing, pore expansion, and silanization processes, forming a dense, low-surface-energy self-assembled monolayer. The release layer carrier substrate and the Diels-Stokes layer... The Alder thermal reversible adhesive layer works synergistically, allowing for more than 50 reuses, and leaves no residue after peeling, requiring no cleaning, thus significantly reducing production costs.
[0044] Example 3: The elastic layer is described in detail below. Please refer to [link / reference]. Figure 3 , Figure 3This is a schematic flowchart illustrating an embodiment of the method for preparing the leather transfer film provided in this application, in which a self-healing elastic layer coating is applied to a patterned decorative layer. This embodiment includes: S301. Apply the self-healing elastic layer coating onto the patterned decorative layer to obtain a wet film coating with a thickness of 35~45μm. The pre-prepared self-healing elastic layer coating is applied to the dried and cured patterned decorative layer surface. The coating contains polyurethane acrylate oligomers, reactive acrylate monomers, photoinitiators, and specific functional components for constructing a dynamic network. During coating, the thickness of the wet film coating is controlled at 35–45 micrometers by adjusting the process parameters of a slot coater or microgravure coater.
[0045] S302. Place the wet film coating under an LED ultraviolet light source with a wavelength of 395nm for ultraviolet curing to obtain a pre-cured coating; After coating, the substrate with wet film coating is immediately sent to a UV curing device. An LED array with an emission peak of 395nm is used as the light source. The curing process is carried out under an inert atmosphere to eliminate the inhibitory effect of oxygen on free radical polymerization. The acrylate double bonds in the coating undergo a rapid photopolymerization reaction, forming a preliminary three-dimensional network structure with a certain mechanical strength within a few seconds to tens of seconds, thus obtaining a solid pre-cured coating.
[0046] S303. Place the pre-cured coating in the first stage of heat treatment environment and keep it at 60~80℃ for 5~15 minutes; The UV-cured substrate is transferred to a temperature-controlled oven for the first stage of heat treatment. The temperature is set in the range of 60~80℃ and maintained for 5~15 minutes. Within this temperature range, the polymer chain segments acquire a certain degree of mobility, allowing the pre-designed hydrogen bond donors and acceptors in the molecular chain to arrange and combine more effectively, forming stable cross-linking points.
[0047] S304. Place the coating that has undergone the first heat treatment in the second heat treatment environment and keep it at 90~110℃ for 3~8 minutes to obtain the elastic layer.
[0048] The coating, after the first heat treatment, is immediately transferred to a second heat treatment zone with a higher temperature. The temperature is increased to 90-110℃, and the treatment time is controlled to 3-8 minutes. This temperature range effectively activates the synergistic effect of dynamic bonds, promoting further cross-linking and stabilization of the elastic network. If the temperature is below 90℃, the synergistic effect of dynamic bonds is insufficient, the elastic network structure is incomplete, and the self-healing performance and elasticity cannot meet the preset requirements. If the temperature is above 110℃, it may lead to excessive cross-linking and embrittlement of the coating, resulting in loss of elasticity. It may also affect the color stability of the underlying decorative layer, causing fading and discoloration. The heat treatment time is set to 3-8 minutes to ensure that the synergistic effect of dynamic bonds occurs fully and the elastic network structure reaches a stable state.
[0049] In this embodiment, a self-healing elastic layer is prepared using UV curing and a two-stage heat treatment process. UV curing provides rapid shaping and initial strength, while the first stage of heat treatment promotes the orderly arrangement of hydrogen bonds. The second stage of heat treatment activates dynamic covalent bonds for synergistic cross-linking, forming a stable and reversible elastic network. The resulting coating exhibits improved bending resistance, flexibility, and self-healing capabilities, effectively solving the problems of easy cracking and poor bending resistance in transfer film coatings.
[0050] Example 4: The reactive primer is described in detail below. Please refer to [link / reference]. Figure 4 , Figure 4 This is a schematic flowchart of an embodiment of the method for preparing the leather transfer film provided in this application, which involves coating a reactive primer comprising a terminal isocyanate prepolymer and a leather penetrant onto an elastic layer. The embodiment includes: S401. Apply a reactive primer onto the elastic layer to obtain a coating with a wet film thickness of 30~40μm; The reactive primer comprises isocyanate-terminated polyurethane prepolymer, leather penetrant, latent catalyst, defoamer, and leveling agent, among other conventional additives. The isocyanate-terminated polyurethane prepolymer, as the reactive main resin, is typically synthesized by reacting alicyclic diisocyanates with low molecular weight polyols. The isocyanate group content is controlled between 5.0% and 7.0% to balance reactivity and storage stability. The leather penetrant, acting as a solvent, moderately swells the collagen fibers of the leather surface and promotes primer penetration; N-methylpyrrolidone is typically used. The latent catalyst provides controllable catalysis during the pre-curing stage and often employs microencapsulated organometallic catalysts, such as dibutyltin dilaurate microcapsules, which release their active ingredients only at 100–120°C. The above components are mixed uniformly in a predetermined ratio to obtain the reactive primer.
[0051] The prepared reactive primer is evenly applied to the surface of the cured self-healing elastic layer. By controlling the coating parameters, the wet film thickness after coating is strictly controlled within 30~40μm.
[0052] S402. Place the coated coating in a hot air circulating oven and treat it at a temperature of 70~90℃ for 60~120 seconds. After coating, the wet film coating is immediately pre-cured. The substrate coated with primer is then placed in an oven with the oven temperature set at 70℃~90℃. The substrate's dwell time within this temperature range is controlled at 60 seconds~120 seconds.
[0053] Under the action of hot air at 70-90℃, the isocyanate groups undergo a preliminary reaction with any moisture present in the components or trace amounts of incompletely reacted hydroxyl groups on the prepolymer segments, generating urea bonds or urethane bonds, forming a network structure with a certain cohesive strength. Insufficient temperature or time, excessively high temperature or excessively long time will result in insufficient reserved active -NCO groups, affecting the final bonding strength with the leather.
[0054] S403. By controlling the intensity decay of the isocyanate characteristic peak in the coating to a preset range, a semi-cured primer layer with an isocyanate group reaction degree of 30%~50% is obtained.
[0055] Using the infrared spectrum located at approximately 2270 cm -1 The intensity change of the characteristic absorption peak of the isocyanate group (-NCO) is used as an indicator. By comparing the degree of intensity decay of this characteristic peak before and after pre-curing, the proportion of -NCO groups that have reacted during heat treatment is quantitatively assessed. To achieve stable control, the measured degree of reaction is compared and adjusted with a preset range. If the degree of reaction is less than 30%, it indicates insufficient cross-linking, resulting in weak mechanical strength and poor storage stability of the coating. If it is greater than 50%, it means that the pre-curing is excessive, resulting in insufficient active -NCO groups available for subsequent bonding with leather, which will seriously affect the final adhesion.
[0056] In this embodiment, the degree of reaction of the isocyanate groups in the reactive primer is precisely controlled to a semi-cured state of 30%~50% through real-time infrared spectroscopy monitoring. This ensures that the coating has sufficient mechanical strength during the lamination process while retaining a sufficient amount of active NCO groups, which chemically bond with the leather collagen fibers during hot pressing. This results in an initial peel strength of 8.5 N / cm for the transfer film, effectively preventing problems such as delamination and peeling of the leather transfer film.
[0057] The leather transfer film provided in this application is prepared by the preparation method of the above embodiments. The leather transfer film includes a substrate, a thermally reversible adhesive layer, a self-healing elastic layer, and a semi-cured primer layer. The dry film thickness of the thermally reversible adhesive layer is 5~8μm; the dry film thickness of the self-healing elastic layer is 10~15μm; and the dry film thickness of the semi-cured primer layer is 6~10μm.
[0058] Example 5: This experiment sets up two groups of samples: the experimental group and the control group.
[0059] Example group: Leather transfer film was prepared using the complete technical solution of Example 1. Three batches were prepared in parallel: Experimental group 1, Experimental group 2, and Experimental group 3. The preparation conditions were exactly the same to verify the stability and repeatability of the technical solution.
[0060] Control group: Leather transfer film was prepared using existing conventional technology. Three batches were prepared in parallel: control group 1, control group 2, and control group 3. The preparation conditions were exactly the same, and the films were used to compare with the experimental group.
[0061] The number of times the experimental group and the control group could be reused, the number of times they could be bent, and the peel strength were tested respectively.
[0062] Experimental groups 1-3 prepared leather transfer films using the preparation methods of Examples 1 to 4.
[0063] The test results of the leather transfer film for experimental groups 1-3 are shown in Table 1:
[0064] The preparation methods for control groups 1-3 are as follows: (1) Release layer carrier substrate: Commercially available PET release film is used, without anodizing, pore expansion and silanization treatment.
[0065] (2) Adhesive layer: Commercially available acrylic pressure-sensitive adhesive is used, which is cured at 100°C for 5 minutes after application, with a dry film thickness of 6μm. Thermally reversible adhesive layers based on Diels-Alder reversible reactions are not used.
[0066] (3) Pattern decoration layer: Same as the example, gravure printing black polyurethane ink, dry at 80°C for 2 minutes, dry film thickness 2μm.
[0067] (4) Elastic layer: The same polyurethane acrylate coating as in the example is used. The wet film is 40μm thick, cured by irradiation with 395nm LED ultraviolet light for 5 seconds, without two-stage heat treatment, and the dry film thickness is 12μm.
[0068] (5) Primer layer: Commercially available two-component polyurethane primer is used. After application, it is treated at 85°C for 90 seconds. The semi-cured state is not controlled. It has no active NCO groups and the dry film thickness is 8μm.
[0069] (6) Composite protective film and curing: Same as in the example, a 38μm PET release film is laminated and cured at 50℃ and RH40% for 48 hours to obtain the finished product.
[0070] The test results of the leather transfer film in comparison groups 1-3 are shown in Table 2:
[0071] The average test values of the experimental group and the control group were compared, as shown in Table 3:
[0072] Based on Tables 1, 2, and 3, we can conclude that: The experimental group used the proposed solution's micro / nano structure carrier, which involves anodizing, pore expansion, and silanization, along with a reversible adhesive layer. The carrier can be reused more than 52 times, with each hot-pressing transfer resulting in complete and clean peeling. The control group used conventional PET release film and ordinary pressure-sensitive adhesive, which are single-use and discarded after a single use. The self-healing elastic layer prepared by the experimental group using the proposed solution's UV curing and two-stage heat treatment process achieved a bending resistance of 5300 cycles without cracking. The control group, which only underwent UV curing without the two-stage heat treatment, only achieved a bending resistance of 3500 cycles. This technology significantly improves the bending life of the coating, enhancing the durability of leather products under repeated bending conditions. The experimental group used the proposed solution's semi-cured primer layer, achieving a peel strength of 8.5 N / cm, compared to only 4.3 N / cm in the control group. This solution improves the bonding strength between the transfer coating and the leather, and exhibits significantly superior resistance to damp heat aging, effectively solving the problems of delamination and peeling after long-term use.
Claims
1. A method for preparing a leather transfer film, characterized in that, include: The surface of the rigid carrier substrate is sequentially subjected to anodizing, pore-expanding and silanizing treatments to obtain the release layer carrier substrate. A thermally reversible adhesive layer coating based on the Diels-Alder reversible reaction is applied to the release layer carrier substrate and cured at 80~110℃ for 3~10 minutes to obtain a thermally reversible adhesive layer. A pattern is printed on the thermally reversible adhesive layer to obtain a patterned decorative layer; The self-healing elastic layer coating is applied to the patterned decorative layer, and then subjected to UV curing and two heat treatments in sequence to obtain the elastic layer. The two heat treatments include treatment at 60~80℃ for 5~15 minutes, followed by treatment at 90~110℃ for 3~8 minutes. A reactive primer containing a terminal isocyanate group prepolymer and a leather penetrant is applied to the elastic layer and pre-cured at 70~90°C for 60~120 seconds to obtain a semi-cured primer layer with an isocyanate group reaction degree of 30%~50%. A composite protective film is applied to the semi-cured primer layer to obtain the initial transfer film; The initial transfer film is aged at 40-60°C and 30-50% relative humidity to obtain the finished transfer film.
2. The method for preparing the leather transfer film according to claim 1, characterized in that, The process of sequentially anodizing, expanding, and silanizing the surface of a rigid carrier substrate to obtain a release layer carrier substrate includes: The rigid carrier substrate is pretreated with degreasing and alkaline washing to obtain a clean substrate surface; The cleaned substrate is immersed in a 15% sulfuric acid solution at 15±2℃ and anodized at 16±1V DC voltage for 40~50 minutes to obtain a substrate with a porous alumina layer on the surface. The substrate with the porous alumina layer was immersed in a 4% phosphoric acid solution and subjected to a pore-expanding treatment at 25±5℃ for 30±5 minutes to obtain a micro-nano structured substrate with a nanopore array on the surface. The micro / nanostructured substrate was placed in a vacuum reactor and evacuated to 10 °C. -3 Pa, heated to 150±5℃, and heat-treated at 180±5℃ for 60±10 minutes under nitrogen protection to obtain the release layer carrier substrate.
3. The method for preparing the leather transfer film according to claim 2, characterized in that, The pretreatment of the rigid carrier substrate by degreasing and alkaline washing to obtain a clean substrate surface includes: The substrate is immersed in an alkaline degreasing solution at 70-80°C and ultrasonically treated for 8-12 minutes. The alkaline degreasing solution contains 3-5% sodium hydroxide and 2-4% sodium carbonate by mass. Rinse the degreased substrate with deionized water and immerse it in a sodium hydroxide solution at 60-70°C and a concentration of 50-80 g / L for 3-6 minutes for alkaline washing. The substrate after alkaline washing is dried at 100~120℃ for 10~20 minutes to obtain a clean substrate surface.
4. The method for preparing the leather transfer film according to claim 1, characterized in that, The thermally reversible adhesive coating comprises thermoplastic polyurethane, furan-terminated flexible segments, and bismaleimide crosslinking agent in a mass ratio of 50:(10~15):(3~5).
5. The method for preparing the leather transfer film according to claim 1, characterized in that, The self-healing elastic layer coating is applied to the patterned decorative layer, and then subjected to ultraviolet curing and two stages of heat treatment to obtain the elastic layer, including: The self-healing elastic layer coating is applied to the patterned decorative layer to obtain a wet film coating with a thickness of 35~45μm; The wet film coating was placed under an LED ultraviolet light source with a wavelength of 395nm for ultraviolet curing to obtain a pre-cured coating; The pre-cured coating is placed in the first heat treatment environment and kept at 60~80℃ for 5~15 minutes; The coating that has undergone the first heat treatment is placed in the second heat treatment environment and kept at 90~110℃ for 3~8 minutes to obtain the elastic layer.
6. The method for preparing the leather transfer film according to claim 1, characterized in that, The reactive primer comprising a terminal isocyanate group prepolymer and a leather penetrant is applied to the elastic layer and pre-cured at 70-90°C for 60-120 seconds to obtain a semi-cured primer layer with an isocyanate group reaction degree of 30%-50%, comprising: A reactive primer is applied to the elastic layer to obtain a coating with a wet film thickness of 30~40μm; Place the coated layer in a hot air circulating oven and treat it at a temperature of 70~90℃ for 60~120 seconds; By controlling the intensity decay of the isocyanate characteristic peak in the coating to a preset range, a semi-cured primer layer with an isocyanate group reaction degree of 30%~50% is obtained.
7. The method for preparing the leather transfer film according to claim 1, characterized in that, The terminal isocyanate prepolymer is a prepolymer obtained by reacting isophorone diisocyanate with polycaprolactone triol, and the mass percentage of isocyanate groups is 5.0%~7.0%; the leather penetrant is N-methylpyrrolidone.
8. The method for preparing the leather transfer film according to any one of claims 1 to 7, characterized in that, The surface of the release layer carrier substrate is an anodized aluminum plate with an array of nanopores, the average pore diameter of which is 70~90nm and the pore depth is 10~20μm.
9. A leather transfer film, prepared by any one of claims 1 to 8.
10. The leather transfer film according to claim 9, characterized in that, The dry film thickness of the thermally reversible adhesive layer is 5~8μm; The dry film thickness of the self-healing elastic layer is 10~15μm; The dry film thickness of the semi-cured primer layer is 6~10μm.