Lower supporting film for flexible folding display screen and preparation method of lower supporting film

Abstract

The invention belongs to the technical field of film materials, and particularly relates to a lower supporting film for a flexible folding display screen and a preparation method of the lower supporting film. The supporting film sequentially comprises a protective film layer, a stainless steel base material layer, an adhesive layer and a release film layer from top to bottom; wherein the adhesive layer is prepared by coating a release film with modified organic silica gel liquid, drying and curing. The ultra-thin stainless steel is used as the base material layer, can resist external force impact and scraping, is low and stable in thermal expansion coefficient, can reduce the influence of environment temperature and humidity on the size of the supporting film, and guarantees the structural consistency of the screen module in long-term use; and through multi-component composite modification of polyurethane-thiazole-organic silicon, the peeling strength of the adhesive layer and stainless steel and the bending frequency of the supporting film are improved, and the service life of the supporting film is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of membrane material technology, specifically relating to a lower support film for flexible folding displays and its preparation method. Background Technology

[0002] OLED display technology, with its advantages of self-illumination, high contrast, and thinness, has become the core display solution for products such as foldable phones, wearable devices, and flexible automotive displays. Among them, the support film, as a key structural layer of the OLED display, plays a crucial role in supporting the flexible OLED substrate, buffering bending stress, and blocking external environmental interference. Its performance determines the durability and display stability of flexible devices.

[0003] Currently, polyethylene terephthalate (PET) or polyimide (PI) are commonly used as the substrate for the support film. A pressure-sensitive adhesive is coated on the surface of the substrate, and a release film and a protective film are then laminated to form a complete structure. However, as foldable screens upgrade towards "high-frequency folding, long lifespan, and adaptability to complex environments," the problem of mechanical performance degradation caused by folding fatigue is becoming increasingly serious.

[0004] Chinese patent application CN118853031A discloses an acrylic pressure-sensitive adhesive, a support film, and a method for preparing the same for flexible OLED support films. The support film comprises a protective film layer, a PET substrate layer, an acrylic pressure-sensitive adhesive layer, and a release layer, stacked sequentially. The acrylic pressure-sensitive adhesive formulation incorporates 5-methyl-3-vinyl-2-oxazolinone monomer and hydroxyl-containing acrylate monomers, which improves the product's adhesion, allowing it to adhere well to the polyimide film surface for support and reinforcement. However, OLED displays generate localized heat during operation, and the support film and metal hinge have significantly different coefficients of thermal expansion. This mismatch leads to significant thermal stress at the interface between the support film and hinge during temperature cycling. Repeated stress weakens the adhesion between the acrylic pressure-sensitive adhesive layer and the substrate, ultimately causing delamination, bubbles, and other failure phenomena. Summary of the Invention

[0005] In the prior art, the support film has the problem that the pressure-sensitive adhesive layer and the substrate layer are prone to delamination after long-term use; in order to solve this problem, the present invention provides a lower support film for flexible folding displays and its preparation method.

[0006] To achieve the objectives of this invention, the following technical solution is adopted: In a first aspect, the present invention provides a lower support film for a flexible folding display screen, which comprises, from top to bottom, a protective film layer, a stainless steel substrate layer, an adhesive layer and a release film layer; The adhesive layer is prepared by coating a release film with modified silicone liquid, drying and curing it. The preparation method of the modified silicone liquid includes the following steps: (1) Under anaerobic conditions, polycaprolactone polyol, 4,4-dicyclohexylmethane diisocyanate and dibutyltin dilaurate were mixed evenly and reacted once. Hydroxyethyl acrylate was added for a second reaction to obtain the PU-V system. (2) Mix 4-methyl-5-vinylthiazole, azobisisobutyronitrile and acetone evenly, add to the PU-V system and continue the reaction to obtain the PU-MVT system; (3) Add vinyl-terminated polymethyl vinyl siloxane to the PU-MVT system and mix evenly. Add crosslinking agent, photoinitiator and antioxidant. Remove acetone by rotary evaporation to obtain modified silicone liquid.

[0007] By adopting the above technical solution, the stainless steel substrate layer provides rigid support to prevent damage to the internal components of the display screen during folding, while its ductility can adapt to the deformation during folding; the adhesive layer can alleviate the interface stress concentration during folding through polyurethane-thiazole-organosilicon composite modification; the structure of the protective film layer-stainless steel substrate layer-adhesive layer-release film layer from top to bottom forms a complementary function, taking into account both support strength and folding flexibility.

[0008] By adopting the above technical solution, the modified silicone liquid achieves component synergy through a three-step reaction. The polyurethane segments formed from polyols and isocyanates provide excellent flexibility and substrate wettability, ensuring that the adhesive layer does not become brittle after repeated folding. 4,-Methyl-5-vinylthiazole (MVT) introduces thiazole rings, improving adhesion to the stainless steel substrate layer. At the same time, its vinyl groups participate in crosslinking, enhancing the cohesive force of the adhesive layer. The vinyl-terminated polymethylvinylsiloxane introduces siloxane segments, giving the adhesive layer high and low temperature resistance and weather resistance, solving the problem of traditional polyurethane adhesive layers easily softening at high temperatures. Finally, the adhesive layer is rapidly crosslinked through curing, forming a three-dimensional network structure. The silicone components are fixed in the network through covalent crosslinking, preventing silicon migration and contamination of the display screen's optical components.

[0009] Preferably, in step (1), the mass ratio of polycaprolactone polyol to 4,4-dicyclohexylmethane diisocyanate is 1:(0.096-0.1); the amount of dibutyltin dilaurate is 0.8%-1.1% of the total mass of polycaprolactone polyol and 4,4-dicyclohexylmethane diisocyanate; and the amount of hydroxyethyl acrylate is 5%-8% of the total mass of polycaprolactone polyol and 4,4-dicyclohexylmethane diisocyanate.

[0010] By adopting the above technical solution, under this ratio, the molecular chains formed by the soft segment and hard segment of polyurethane are balanced, giving the PU-V system excellent flexibility and moderate cohesion; too much dibutyltin dilaurate as a catalyst will cause the reaction to be too fast, and too little will easily lead to incomplete reaction in one step; the vinyl group introduced by hydroxyethyl acrylate can be fully copolymerized with 4-methyl-5-vinylthiazole in step (2) under this amount.

[0011] Preferably, in step (1), the temperature of the first reaction is 75-85℃ and the time of the first reaction is 2-3h; the temperature of the second reaction is 55-65℃ and the time of the second reaction is 1-2h.

[0012] By adopting the above technical solution, if the primary reaction temperature is too high, it is easy to cause the self-polymerization of 4,4-dicyclohexylmethane diisocyanate; if the temperature is too low, the reaction will be incomplete, and the residual -NCO may undergo side reactions with the thiazole ring of the subsequent 4-methyl-5-vinylthiazole; if the secondary reaction temperature is too low, the reaction rate between -OH and -NCO will be slow, and if the temperature is too high, vinyl groups are prone to undergo free radical self-polymerization, resulting in local cross-linking of the system.

[0013] Preferably, in step (2), the amount of 4-methyl-5-vinylthiazole is 5%-10% of the mass of PU-V; the amount of azobisisobutyronitrile is 1.6%-2% of the mass of 4-methyl-5-vinylthiazole.

[0014] By adopting the above technical solution, the thiazole ring in 4-methyl-5-vinylthiazole can coordinate with metal ions on the stainless steel surface. A dosage of 5%-10% can make the density of the thiazole ring moderate and improve the peel strength. If the dosage is too high, it will easily lead to the formation of a rigid layer at the interface, which will reduce the interface toughness during folding. If the dosage is too low, the content of thiazole ring and vinyl group is low, which will not effectively improve the adhesion and will be difficult to be compatible with the subsequent siloxane segments. At this dosage, azobisisobutyronitrile can be uniformly initiated and gradually copolymerized, so that the reaction proceeds smoothly.

[0015] Preferably, in step (2), the mass ratio of terminal vinyl polymethyl vinyl siloxane to polycaprolactone polyol is (0.14-0.18):1, the amount of crosslinking agent is 1%-3% of the mass of terminal vinyl polymethyl vinyl siloxane, and the amount of photoinitiator is 0.5%-1% of the mass of terminal vinyl polymethyl vinyl siloxane.

[0016] By adopting the above technical solution, the proportion of siloxane is moderate at this ratio, the vinyl groups of siloxane crosslink with the vinyl groups of the PU-MVT system, and the siloxane segments are anchored in the polyurethane network through covalent bonds, ensuring long-term stable adhesion between the adhesive layer and the stainless steel substrate; the crosslinking network formed by the crosslinking agent at this dosage is moderate, which can ensure that the siloxane segments fully participate in crosslinking; the photoinitiator can efficiently initiate and completely cure at this dosage.

[0017] Preferably, the thickness of the adhesive layer is 15-30 μm.

[0018] By adopting the above technical solutions, when the adhesive layer thickness is insufficient, the buffer space of the cross-linked network is limited, and the stress concentration factor that the adhesive layer needs to withstand during folding will increase, which is prone to local cracking or poor adhesion of the substrate. After multiple folds, the entire support film may fail. If the adhesive layer is too thick, it will cause indentation at the fold after long-term folding, and at the same time increase the overall thickness of the support film. With a thickness of 15-30um, the adhesive layer can adapt to the folding stress, while the overall flexibility of the support film is not affected.

[0019] Preferably, the stainless steel substrate layer is an ultra-thin high-strength stainless steel with a thickness of 10μm-30μm.

[0020] By adopting the above technical solutions, ultra-thin high-strength stainless steel of 10μm-30μm can provide sufficient rigidity to prevent damage to OLED devices inside the display screen when pressed or folded, and also has excellent flexibility, so that it can be folded with the screen without plastic deformation or breakage; ultra-thin high-strength stainless steel can be made into rolls, which are compatible with processes such as adhesive coating, and can meet the needs of continuous mass production of flexible displays.

[0021] Preferably, the protective film layer is a PET release film with a thickness of 50μm-75μm.

[0022] By adopting the above technical solution, PET release film with a thickness of 50-75μm can cover the actual needs of the support film in terms of protective performance, and the cost is relatively low.

[0023] Preferably, the protective film layer is an acrylic PET protective film or a polyurethane PET protective film with a thickness of 50μm-75μm.

[0024] By adopting the above technical solution, a thickness of 50μm-75μm can ensure that the protective film has sufficient support to cover the surface of the support film without wrinkles, while maintaining appropriate flexibility so that it can be rolled up with the support film and is suitable for roll-to-roll storage and transportation.

[0025] Secondly, the present invention provides a method for preparing the lower support film for the above-mentioned flexible folding display screen, comprising the following steps: The modified silicone liquid is coated onto the release film, dried to form an adhesive layer, the other side of the adhesive layer is bonded to stainless steel, pressed into shape, cured, and finally a protective film layer is applied on top of the stainless steel to obtain the lower support film for the flexible folding display screen.

[0026] By adopting the above technical solution, the modified silicone liquid is first coated onto the release film and dried. The flat surface of the release film can be used to achieve uniform coating of the adhesive layer. The adhesive layer and the stainless steel layer are pressed together to promote wetting. Finally, a protective film layer is attached to the stainless steel surface, which can avoid scratches or contamination of the stainless steel surface in the early process. The preparation method is simple and can be mass-produced.

[0027] In summary, the beneficial effects of this invention are: (1) The present invention uses ultra-thin stainless steel instead of PET / PI substrate as substrate layer, which can resist external impact and scratch, has a low and stable coefficient of thermal expansion, can reduce the influence of ambient temperature and humidity on the size of the support film, and ensure the structural consistency of the screen module for long-term use. (2) The present invention improves the peel strength between the adhesive layer and stainless steel and the number of bending cycles of the support film by multi-component composite modification of polyurethane-thiazole-organosilicon. (3) The ultra-thin high-strength stainless steel substrate of the present invention provides sufficient rigidity, the adhesive layer relieves the interface stress concentration during folding, the PET protective film layer has both high scratch resistance and self-healing properties, the release film layer ensures that the adhesive layer is not contaminated during storage and processing, and the multi-layer synergy keeps the total thickness of the support film within a relatively thin range, which is suitable for the design requirements of flexible folding screens to be thinner and lighter. The multi-layer structure has complementary functions and balances the support rigidity and flexible folding requirements. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of the lower support film for the flexible folding display screen of the present invention; Figure 2 This is an infrared spectrum of the adhesive layer in the lower support film for the flexible folding display screen of the present invention; Explanation of reference numerals in the attached figures: 1. Protective film layer; 2. Stainless steel substrate layer; 3. Adhesive layer; 4. Release film layer. Detailed Implementation

[0029] The technical solution of the present invention will be explained in detail below with reference to several representative embodiments.

[0030] Unless otherwise specified, the experimental methods used in the following examples and comparative examples are conventional methods. Unless otherwise specified, the materials and reagents used in the following examples and comparative examples are commercially available.

[0031] Preparation Example 1 The preparation method of the modified silicone liquid in this example includes the following specific steps: (1) Under nitrogen protection, 100g of polycaprolactone polyol with a molecular weight of 3000, 9.8g of 4,4-dicyclohexylmethane diisocyanate and 0.88g of dibutyltin dilaurate were mixed and stirred for 30min. The mixture was heated to 78℃ for 3h and then cooled to 65℃. 6.6g of hydroxyethyl acrylate was added for a second reaction for 1h to obtain the PU-MVT system. (2) Mix 9.38g of 4-methyl-5-vinylthiazole, 0.16g of azobisisobutyronitrile and 150g of acetone evenly, add to the PU-V system and continue to react at 70℃ for 4h to obtain the PU-MVT system; (3) Add 14g of vinyl-terminated polymethyl vinyl siloxane with a molecular weight of 5000 to the PU-MVT system and stir at 1500rpm for 30min. Add 0.28g of triallyl isocyanurate (TAIC), 0.12g of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO) and 0.16g of antioxidant 1010 and continue stirring for 30min. After removing acetone by rotary evaporation, the modified silicone liquid is obtained.

[0032] Preparation Example 2 The preparation method of the modified silicone liquid in this example includes the following specific steps: (1) Under nitrogen protection, 100g of polycaprolactone polyol with a molecular weight of 3000, 10g of 4,4-dicyclohexylmethane diisocyanate and 1.1g of dibutyltin dilaurate were mixed and stirred for 30min. The mixture was heated to 85℃ and reacted for 2h. The mixture was then cooled to 60℃ and 5.5g of hydroxyethyl acrylate was added for a second reaction for 2h to obtain the PU-MVT system. (2) Mix 11.5g of 4-methyl-5-vinylthiazole, 0.18g of azobisisobutyronitrile and 150g of acetone evenly, add to the PU-V system and continue to react at 60℃ for 4h to obtain the PU-MVT system; (3) Add 18g of vinyl polymethyl vinyl siloxane with a molecular weight of 5000 to the PU-MVT system and stir at 1500rpm for 30min. Add 0.18g TAIC, 0.1g TPO and 0.16g antioxidant 1010 and continue stirring for 30min. After removing acetone by rotary evaporation, the modified silicone liquid is obtained.

[0033] Preparation Example 3 The preparation method of the modified silicone liquid in this example includes the following specific steps: (1) Under nitrogen protection, 100g of polycaprolactone polyol with a molecular weight of 3000, 9.6g of 4,4-dicyclohexylmethane diisocyanate and 1.2g of dibutyltin dilaurate were mixed and stirred for 30min. The mixture was heated to 75℃ and reacted for 3h. The mixture was then cooled to 55℃ and 8.7g of hydroxyethyl acrylate was added for a second reaction for 2h to obtain the PU-MVT system. (2) Mix 6g of 4-methyl-5-vinylthiazole, 0.12g of azobisisobutyronitrile and 150g of acetone evenly, add to the PU-V system and continue to react at 65℃ for 3h to obtain the PU-MVT system; (3) Add 16g of vinyl-terminated polymethyl vinyl siloxane with a molecular weight of 5000 to the PU-MVT system and stir at 1500rpm for 30min. Add 0.48g of TAIC, 0.16g of TPO and 0.16g of antioxidant 1010 and continue stirring for 30min. After removing acetone by rotary evaporation, the modified silicone liquid is obtained.

[0034] Example 1 This embodiment of a flexible folding display screen lower support film includes, from top to bottom, a protective film layer of 75μm thick acrylic PET protective film, a stainless steel substrate layer of 15μm thick ultra-thin high-strength stainless steel, an adhesive layer of 20μm thick modified silicone, and a release film layer of 75μm thick PET release film.

[0035] The specific steps of the preparation method of the lower support film for a flexible folding display screen in this embodiment are as follows: The modified silicone liquid prepared in Preparation Example 1 was vacuum degassed and coated onto a PET release film. It was baked at 80°C for 1 minute to form an adhesive layer. The other side of the adhesive layer was then bonded to stainless steel, pressed into shape, cured by UV irradiation, and baked in an oven at 80°C. Finally, a protective film layer was applied to the top of the stainless steel to obtain the lower support film for the flexible folding display screen.

[0036] Figure 1 A schematic diagram of the lower support film for a flexible foldable display screen; Figure 1 In the diagram, 1 represents the protective film layer, 2 represents the stainless steel substrate layer, 3 represents the adhesive layer, and 4 represents the release film layer.

[0037] Figure 2 The infrared spectrum of the adhesive layer; Figure 2 Middle, 3392cm -1 With 1526cm -1 The peak value is the stretching vibration peak of -NH- at 2967 cm⁻¹. -1 The peak at 1746 cm⁻¹ is the stretching vibration peak of -CH-. -1 The peak of the stretching vibration at C=O is located at 1500-1600 cm⁻¹. -1The vicinity is near the skeletal vibration peak of the benzene ring / thiazole ring; 1231 cm⁻¹ -1 The peak at 1065 cm⁻¹ represents the stretching vibration of -COC-. -1 The peak at 831 cm⁻¹ is the stretching vibration peak of -Si-O-Si-. -1 The peak at this location is the stretching vibration peak of -Si-CH3-.

[0038] Example 2 This embodiment of a flexible folding display screen lower support film includes, from top to bottom, a protective film layer of 60μm thick acrylic PET protective film, a stainless steel substrate layer of 30μm thick ultra-thin high-strength stainless steel, an adhesive layer of 15μm thick modified silicone, and a release film layer of 55μm thick PET release film.

[0039] The specific steps of the preparation method of the lower support film for a flexible folding display screen in this embodiment are as follows: The modified silicone liquid prepared in Preparation Example 2 was vacuum degassed and coated onto a PET release film. It was baked at 80°C for 1 minute to form an adhesive layer. The other side of the adhesive layer was then bonded to stainless steel, pressed into shape, cured by UV irradiation, and baked in an oven at 80°C. Finally, a protective film layer was applied to the top of the stainless steel to obtain the lower support film for the flexible folding display screen.

[0040] Example 3 This embodiment of a flexible folding display screen lower support film includes, from top to bottom, a protective film layer of polyurethane-based PET protective film with a thickness of 50μm, a stainless steel substrate layer of ultra-thin high-strength stainless steel with a thickness of 10μm, an adhesive layer of modified silicone with a thickness of 30μm, and a release film layer of PET release film with a thickness of 65μm.

[0041] The specific steps of the preparation method of the lower support film for a flexible folding display screen in this embodiment are as follows: The modified silicone liquid prepared in Preparation Example 3 was vacuum degassed and coated onto a PET release film. It was baked at 80°C for 1 minute to form an adhesive layer. The other side of the adhesive layer was then bonded to stainless steel, pressed into shape, cured by UV irradiation, and baked in an oven at 80°C. Finally, a protective film layer was applied to the top of the stainless steel to obtain the lower support film for the flexible folding display screen.

[0042] Comparative Example 1 The difference from Example 1 is that this comparative example uses a PI film instead of ultra-thin high-strength stainless steel.

[0043] Comparative Example 2 The difference from Example 1 is that this comparative example uses a PI film instead of ultra-thin high-strength stainless steel, and the adhesive layer is prepared using commercially available silicone.

[0044] Comparative Example 3 The difference from Example 1 is that the modified silicone liquid in this comparative example is not modified with 4-methyl-5-vinylthiazole.

[0045] Comparative Example 4 The difference from Example 1 is that the adhesive layer in this comparative example is prepared using commercially available silicone.

[0046] Related performance tests The performance of the lower support film for the flexible folding display screens prepared in Examples 1-3 and Comparative Examples 1-4 was tested, and the test results are shown in Table 1.

[0047] Table 1 Test Results

[0048] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

Claims

1. A lower support film for a flexible folding display screen, characterized in that, The support film comprises, from top to bottom, a protective film layer, a stainless steel substrate layer, an adhesive layer, and a release film layer; The adhesive layer is prepared by coating a release film with modified silicone liquid, drying and curing it. The preparation method of the modified silicone liquid includes the following steps: (1) Under anaerobic conditions, polycaprolactone polyol, 4,4-dicyclohexylmethane diisocyanate and dibutyltin dilaurate were mixed evenly and reacted once. Hydroxyethyl acrylate was added for a second reaction to obtain the PU-V system. (2) Mix 4-methyl-5-vinylthiazole, azobisisobutyronitrile and acetone evenly, add to the PU-V system and continue the reaction to obtain the PU-MVT system; (3) Add vinyl-terminated polymethyl vinyl siloxane to the PU-MVT system and mix evenly. Add crosslinking agent, photoinitiator and antioxidant. Remove acetone by rotary evaporation to obtain modified silicone liquid.

2. The lower support film for a flexible folding display screen according to claim 1, characterized in that, In step (1), the mass ratio of polycaprolactone polyol to 4,4-dicyclohexylmethane diisocyanate is 1:(0.096-0.1); the amount of dibutyltin dilaurate is 0.8%-1.1% of the total mass of polycaprolactone polyol and 4,4-dicyclohexylmethane diisocyanate; and the amount of hydroxyethyl acrylate is 5%-8% of the total mass of polycaprolactone polyol and 4,4-dicyclohexylmethane diisocyanate.

3. The lower support film for a flexible folding display screen according to claim 1, characterized in that, In step (1), the temperature of the first reaction is 75-85℃ and the time of the first reaction is 2-3h; the temperature of the second reaction is 55-65℃ and the time of the second reaction is 1-2h.

4. The lower support film for a flexible folding display screen according to claim 1, characterized in that, In step (2), the amount of 4-methyl-5-vinylthiazole is 5%-10% of the mass of PU-V; the amount of azobisisobutyronitrile is 1.6%-2% of the mass of 4-methyl-5-vinylthiazole.

5. The lower support film for a flexible folding display screen according to claim 1, characterized in that, In step (2), the mass ratio of terminal vinyl polymethyl vinyl siloxane to polycaprolactone polyol is (0.14-0.18):1, the amount of crosslinking agent is 1%-3% of the mass of terminal vinyl polymethyl vinyl siloxane, and the amount of photoinitiator is 0.5%-1% of the mass of terminal vinyl polymethyl vinyl siloxane.

6. The lower support film for a flexible folding display screen according to claim 1, wherein the thickness of the adhesive layer is 15-30 μm.

7. The lower support film for a flexible folding display screen according to claim 1, wherein the stainless steel substrate layer is an ultra-thin high-strength stainless steel with a thickness of 10μm-30μm.

8. The lower support film for a flexible folding display screen according to claim 1, characterized in that, The protective film layer is a PET release film with a thickness of 50μm-75μm.

9. The lower support film for a flexible folding display screen according to claim 1, characterized in that, The protective film layer is an acrylic PET protective film or a polyurethane PET protective film with a thickness of 50μm-75μm.

10. A method for preparing a lower support film for a flexible folding display screen according to any one of claims 1-9, characterized in that, The steps include: The modified silicone liquid is coated onto the release film, dried to form an adhesive layer, the other side of the adhesive layer is bonded to stainless steel, pressed into shape, cured, and finally a protective film layer is applied on top of the stainless steel to obtain the lower support film for the flexible folding display screen.

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