Super flexible pet protective film and preparation method thereof

Abstract

This invention provides an ultra-flexible PET protective film and its preparation method, relating to the field of flexible film technology. The film includes a flexible PET substrate layer, a UV-curable coating, an optically transparent pressure-sensitive adhesive layer, and a magnetron stress buffer layer. By setting a magnetron stress buffer layer between the flexible PET substrate layer and the optically transparent pressure-sensitive adhesive layer, the magnetron stress buffer layer comprises a magnetorheological elastomer material. It has a first elastic modulus E1 when no external magnetic field is applied, and its elastic modulus increases to a second elastic modulus E2 when subjected to an external magnetic field. In practical applications, the hinge cavity of a foldable screen phone generates a magnetic field, causing the elastic modulus of the magnetron stress buffer layer to increase from the first elastic modulus E1 to the second elastic modulus E2. This changes the local stress distribution in the bending area, reducing the peak strain experienced by each layer of the protective film during bending, thus providing better "bending protection."

Description

Technical Field

[0001] This invention relates to the field of flexible film technology, specifically to an ultra-flexible PET protective film and its preparation method. Background Technology

[0002] With the rapid development of foldable electronic devices, screen protectors need to maintain high surface hardness and high light transmittance while possessing excellent resistance to dynamic bending. Currently, commercially available flexible screen protectors are mainly divided into two categories: The first category is flexible protective films represented by thermoplastic polyurethane (TPU), which have good flexibility but low surface hardness (usually ≤2H), making them prone to nail marks and indentations, and difficult to meet daily scratch resistance requirements; the second category is composite protective films with polyethylene terephthalate (PET) as the base material, which achieve surface hardening (hardness can reach 3H-9H) by coating the PET base material with a UV-curable hardening coating.

[0003] To improve bending durability, existing technologies typically employ multi-layer stacked structures. For example, Chinese Patent Publication No. CN113913125B, entitled "An Ultra-High Definition Flexible Glass Protective Film and Its Preparation Method and Application," discloses a multi-layer flexible protective film that disperses bending stress by alternately setting PET layers, polyurethane acrylic coatings, and pressure-sensitive adhesive layers. Chinese Patent Publication No. CN120680792A, entitled "A Bending-Resistant High-Transmittance Protective Film for Foldable Screens and Its Preparation Process," adds different amounts of organic-inorganic hybrid particles to two flexible substrates to achieve gradient enhancement. Furthermore, existing research has attempted to integrate shape memory polymers (SMPs) or magnetostrictive materials into foldable displays, using external stimuli such as heat, light, and magnetism to eliminate creases generated during use.

[0004] However, the existing technology still has the following shortcomings: First, the multi-layer stacking scheme is a static passive design. The material parameters (mainly the elastic modulus) of each functional layer are fixed after the protective film is prepared. It cannot adapt to the folding state, resulting in an inherent contradiction between "flexibility" and "bending protection". Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an ultra-flexible PET protective film and its preparation method. This solves the problem that multi-layer stacking schemes are all static and passive designs, and the material parameters (mainly elastic modulus) of each functional layer are fixed after the protective film is prepared, making it impossible to adapt to the folding state. This results in an inherent contradiction between "flexibility" and "bending protection".

[0006] To achieve the above objectives, the present invention provides the following technical solution: An ultra-flexible PET protective film, comprising: Flexible PET substrate layer; A UV-curable coating is disposed on one side surface of the flexible PET substrate layer; An optically transparent pressure-sensitive adhesive layer disposed on the other side surface of the flexible PET substrate layer; And a magnetron stress buffer layer disposed between the optically transparent pressure-sensitive adhesive layer and the flexible PET substrate layer; The magnetically controlled stress buffer layer contains a magnetorheological elastomer material. The magnetically controlled stress buffer layer has a first elastic modulus E1 when there is no external magnetic field, and the elastic modulus increases to a second elastic modulus E2 when it is subjected to an external magnetic field, and E2 > E1.

[0007] Preferably, the flexible PET substrate layer is a copolymer-modified or blend-modified polyethylene terephthalate film, the thickness of the flexible PET substrate layer accounts for at least 65% of the total thickness of the ultra-flexible PET protective film, and the elastic modulus of the flexible PET substrate layer is 2,500,000 kPa-3,300,000 kPa.

[0008] Preferably, the flexible PET substrate layer is a copolymer-modified or blend-modified polyethylene terephthalate film, the thickness of the flexible PET substrate layer accounts for at least 75% of the total thickness of the ultra-flexible PET protective film, and the elastic modulus of the flexible PET substrate layer is 2,500,000 kPa-3,300,000 kPa.

[0009] Preferably, the flexible PET substrate layer is a copolymer-modified or blend-modified polyethylene terephthalate film, the thickness of the flexible PET substrate layer accounts for at least 80% of the total thickness of the ultra-flexible PET protective film, and the elastic modulus of the flexible PET substrate layer is 2,500,000 kPa-3,300,000 kPa.

[0010] Preferably, the first elastic modulus E1 is 50-100 kPa, the second elastic modulus E2 is 150-200 kPa, and E2 / E1≥2.

[0011] Preferably, the magnetorheological elastomer material comprises the following components by weight: 50-80 parts of elastic matrix material, 20-50 parts of magnetic filler, 0.5-5 parts of dispersant, and 0.5-5 parts of coupling agent.

[0012] A method for preparing an ultra-flexible PET protective film, comprising the following steps: S1. Mold a flexible PET substrate layer and coat one side of it with a magnetorheological elastomer precursor solution to form a wet film. S2. The flexible PET substrate layer coated with magnetorheological elastomer precursor solution is placed in an external magnetic field for curing, so that the magnetic filler forms a chain-like ordered structure along the direction of the magnetic field in the elastic matrix, and a magneto-controlled stress buffer layer is obtained. S3. Apply a UV-curable coating liquid to the other side of the flexible PET substrate layer, and cure it with UV light to form a UV-curable coating. S4. Coat the surface of the magnetron stress buffer layer with an optically transparent pressure-sensitive adhesive, dry and cure it to form an optically transparent pressure-sensitive adhesive layer, and then laminate it with a release film layer to obtain a semi-finished product. S5. Place the semi-finished product at 40-60℃ for 48-72 hours to obtain an ultra-flexible PET protective film.

[0013] Preferably, in step S1, the magnetorheological elastomer precursor solution is coated using narrow-slit coating, screen printing, or transfer bonding processes, so that the magnetron stress buffer layer only covers the folded position of the flexible PET substrate layer, with a thickness of 3-5 μm and a coverage width of 5-20 mm.

[0014] Preferably, in step S2, the strength of the external magnetic field is 500mT to 1T, the curing temperature is 80-120℃, the curing time is 30-120 minutes, and the direction of the external magnetic field is perpendicular to the thickness direction of the magneto-controlled stress buffer layer.

[0015] Preferably, in step S3, the UV-curable coating liquid contains polyurethane acrylate oligomer, reactive diluent monomer, photoinitiator, nano-inorganic particles and fluorine-containing anti-fingerprint additive; the UV curing irradiation energy is 300-1000 mJ / cm², and the thickness of the cured coating is 2-6 μm.

[0016] Preferably, in step S4, the optically transparent pressure-sensitive adhesive is an acrylic pressure-sensitive adhesive, and the dry adhesive thickness after coating is 10-30 μm.

[0017] Preferably, before step S1, the flexible PET substrate layer is subjected to corona treatment to make its surface tension ≥52 dyne / cm.

[0018] This invention provides an ultra-flexible PET protective film and its preparation method. It has the following beneficial effects: This invention utilizes a magnetron stress buffer layer disposed between a flexible PET substrate layer and an optically transparent pressure-sensitive adhesive layer. This layer comprises a magnetorheological elastomer material, possessing a first elastic modulus E1 in the absence of an external magnetic field, and increasing to a second elastic modulus E2 under the influence of an external magnetic field. In practical applications, the hinge cavity of a foldable phone generates a magnetic field (approximately 0.2T), causing the elastic modulus of the magnetron stress buffer layer to rise from E1 to E2. This alters the local stress distribution in the bending area, reducing the peak strain experienced by each layer of the protective film during bending, thus exhibiting better "bending" properties. "Protective"; When unfolded, the ultra-flexible PET protective film forms a gap with the hinge cavity of the foldable screen phone, reducing the magnetic field acting on the magnetron stress buffer layer to less than 1 / 10 of its original value. The magnetron stress buffer layer basically recovers to its first elastic modulus E1, reducing its impact on the optically transparent pressure-sensitive adhesive layer and maintaining optimal "flexibility". Overall, based on the design of the magnetron stress buffer layer, the ultra-flexible PET protective film exhibits superior performance during repeated folding. The number of repeated folding and opening cycles with the first crease is greater than 80,000 (crease depth less than 20μm), and the folding life is greater than 150,000 cycles. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of an ultra-flexible PET protective film proposed in this invention.

[0020] The components include: 1. Flexible PET substrate layer; 2. UV-curable coating; 3. Optically transparent pressure-sensitive adhesive layer; and 4. Magnetonized stress buffer layer. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0022] like Figure 1 As shown, this embodiment of the invention provides an ultra-flexible PET protective film for use in foldable screen mobile phones, which specifically includes: a flexible PET substrate layer 1, a UV-curable hardening coating 2, an optically transparent pressure-sensitive adhesive layer 3, and a magnetron stress buffer layer 4.

[0023] The flexible PET substrate layer 1 is the main substrate, which is a copolymer-modified or blend-modified polyethylene terephthalate film with an elastic modulus of 2,500,000 kPa-3,300,000 kPa. For example, a substrate obtained by biaxial stretching of a blend-modified polyethylene terephthalate film has a thickness of 25-50 μm, an elastic modulus of 2,700,000 kPa-3,200,000 kPa, and an elongation at break controlled at 65%-120%, balancing flexibility and stiffness. After 100,000 dynamic folding tests, the elastic modulus retention rate is not less than 85%, and the transmittance is >90%.

[0024] A UV-curable hardening coating 2 is applied to one side of the flexible PET substrate layer 1, facing the user end, providing a hardened surface layer with a hardness of approximately 7H to prevent nail marks and indentations, meeting daily scratch resistance requirements. An optically transparent pressure-sensitive adhesive layer 3 is applied to the other side of the flexible PET substrate layer 1 (the side facing away from the user end), and a magnetron stress buffer layer 4 is located between the flexible PET substrate layer 1 and the optically transparent pressure-sensitive adhesive layer 3. The optically transparent pressure-sensitive adhesive layer 3 is used to adhere the entire protective film to the surface of the foldable display panel; its thickness is 4μm to 10μm, light transmittance is ≥92%, haze is ≤1.0%, and peel force (to glass) is 1000gf / inch to 2000gf / inch. gf / inch, meeting the reliability requirements of foldable screens in different environments. The magnetic stress buffer layer 4 is directly connected to the surface of the flexible PET substrate layer 1. It is used to absorb interfacial shear stress in the folded state and provide uniform back pressure through magnetic field control in the unfolded state, thereby optimizing the stress distribution between the flexible PET substrate layer 1 and the display panel. The elastic modulus of the magnetic stress buffer layer 4 is E1 when no magnetic field is applied, and switches to E2 when a magnetic field of about 0.2T is applied, and E2 > E1.

[0025] In one preferred embodiment, the first elastic modulus E1 is 50-100 kPa, the second elastic modulus E2 is 150-200 kPa, and E2 / E1 ≥ 2.

[0026] Understandably, in practical applications, the hinge cavity of a foldable phone generates a magnetic field (around 0.2T). During folding, the ultra-flexible PET protective film approaches the hinge cavity, and the elastic modulus of the magnetron stress buffer layer 4 increases from the first elastic modulus E1 to the second elastic modulus E2, changing the local stress distribution in the bending area and reducing the peak strain experienced by each layer of the protective film during bending, thus exhibiting good "bending protection." When the ultra-flexible PET protective film is unfolded, the magnetron stress buffer layer 4 forms a gap with the hinge cavity of the foldable phone, reducing the magnetic field acting on the magnetron stress buffer layer to less than 1 / 10 of its original value. The magnetron stress buffer layer 4 essentially returns to its first elastic modulus E1, reducing its impact on the optically transparent pressure-sensitive adhesive layer and maintaining optimal "flexibility." Overall, based on the design of the magnetron stress buffer layer, the ultra-flexible PET protective film exhibits superior performance during repeated folding, with the number of repeated folding and opening cycles exceeding 80,000 (crease depth less than 20μm) and a folding life exceeding 150,000 cycles.

[0027] The applicant learned that the patent with publication number "US20190187752A1" and title "Backplate and Foldable Display Device Including the Same" describes a method of changing the flexibility of the backplate in folded and unfolded states based on the adjustment of magnetorheological fluid. The elastic modulus of the magnetorheological fluid is as low as 20 Pa without a magnetic field and as high as 350,000 Pa with a magnetic field. The flexibility of the backplate is changed by the change of the elastic modulus of the magnetorheological fluid itself. Compared with the above disclosure, this application adopts a magnetic stress buffer layer between the flexible PET substrate layer and the optically transparent pressure-sensitive adhesive layer. The magnetorheological elastomer can improve the local stress distribution between the flexible PET substrate layer and the optically transparent pressure-sensitive adhesive layer with a small change in elastic modulus, which greatly improves the performance of the ultra-flexible PET protective film.

[0028] In one preferred embodiment, the thickness of the flexible PET substrate layer is 42 μm, the thickness of the optically transparent pressure-sensitive adhesive layer is 7 μm, the thickness of the magnetron stress buffer layer is 4 μm, and the thickness of the UV-curable coating is 5 μm; the thickness of the resulting ultra-flexible PET protective film is approximately 58 μm.

[0029] In one preferred embodiment, the thickness of the flexible PET substrate layer is 45 μm, the thickness of the optically transparent pressure-sensitive adhesive layer is 7 μm, the thickness of the magnetron stress buffer layer is 5 μm, and the thickness of the UV-curable coating is 4 μm; the thickness of the resulting ultra-flexible PET protective film is approximately 61 μm.

[0030] In one preferred embodiment, the magnetorheological elastomer material comprises the following components by weight: 50-80 parts of elastic matrix material, 20-50 parts of magnetic filler, 0.5-5 parts of dispersant, and 0.5-5 parts of coupling agent; for example, 60-70 parts of elastic matrix material, 30-45 parts of magnetic filler, 1-3 parts of dispersant, and 1-3 parts of coupling agent; and for yet another example, 68 parts of elastic matrix material, 37 parts of magnetic filler, 1.6 parts of dispersant, and 1.6 parts of coupling agent. Example

[0031] This invention provides a method for preparing an ultra-flexible PET protective film, comprising: a flexible PET substrate layer, a UV-curable coating, an optically transparent pressure-sensitive adhesive layer, and a magnetron stress buffer layer, specifically including the following steps: Step S1: Preparation of flexible PET substrate layer and coating of magnetron layer A copolymer-modified flexible PET film with a thickness of 25 μm (elastic modulus 1.7 GPa, elongation at break 150%) was selected as the flexible PET substrate layer. Before coating, the substrate layer was subjected to corona treatment using a corona treatment machine (power 2.5 kW, processing speed 20 m / min) to achieve a surface tension of 54 dyne / cm.

[0032] The magnetorheological elastomer precursor solution was coated onto one side surface of the corona-treated flexible PET substrate layer through a narrow-slit coating head. The formulation of the magnetorheological elastomer precursor solution was as follows: 65 parts of polydimethylsiloxane (PDMS) matrix, 30 parts of carbonyl iron powder (average particle size 5 μm), and 5 parts of silane coupling agent (KH-550). The components were mixed in a vacuum degassing mixer at 800 rpm for 30 minutes to obtain a uniformly dispersed precursor solution.

[0033] Step S2: Magnetic field solidification to form a magnetic stress buffer layer A flexible PET substrate layer coated with a precursor solution was fed into a magnetic field curing device with an electrode spacing of 50 mm and the magnetic field direction perpendicular to the substrate surface (i.e., perpendicular to the magnetron layer thickness direction). The magnetic field strength was controlled to be stable at 800 mT. Under the action of the magnetic field, the substrate was cured in an oven at 100°C for 60 minutes, causing carbonyl iron powder to form a chain-like ordered structure in the PDMS matrix along the magnetic field direction. After curing, a magnetron stress buffer layer with a thickness of 4 μm was obtained.

[0034] Step S3: Apply UV-cured hardening coating A UV-curable coating liquid was applied to the other side of the flexible PET substrate layer (i.e., the side without the MRE layer). The formulation of the curing coating liquid was as follows (parts by weight): 55 parts of aliphatic polyurethane acrylate oligomer, 30 parts of dipentaerythritol hexaacrylate (DPHA), 3 parts of photoinitiator (184), 10 parts of nano silica (particle size 40 nm), and 2 parts of perfluoropolyether acrylate. The wet film thickness was controlled to 8 μm by slit coating. After drying with hot air at 80°C for 30 seconds, the film was UV-cured under nitrogen protection with an irradiation energy of 600 mJ / cm² to obtain a curing coating with a thickness of 4 μm.

[0035] Step S4: Coat an optically transparent pressure-sensitive adhesive layer and laminate with a release film. An optically transparent pressure-sensitive adhesive was coated onto the surface of the magnetron stress buffer layer. The adhesive formulation (by weight) consisted of 85 parts isooctyl acrylate-acrylic acid copolymer, 3 parts isocyanate curing agent, 1 part epoxy silane coupling agent, and 0.5 parts conductive polymer antistatic agent. A doctor blade coating method was used to control the dry adhesive thickness to 15 μm. After coating, the adhesive was sequentially dried and cured in three ovens at 60℃, 80℃, and 100℃ (each oven segment was 3 m long, with a linear speed of 5 m / min). Finally, a 50 μm thick fluorine release film (release force 8 gf / inch) was laminated onto the adhesive surface to obtain the semi-finished product.

[0036] Step S5: Aging treatment The semi-finished product is placed in a constant temperature oven at 50℃ for 60 hours to fully cross-link the optically transparent pressure-sensitive adhesive layer and release the internal stress of the PET substrate layer. After curing, the ultra-flexible PET protective film is obtained.

[0037] The total thickness of the ultra-flexible PET protective film prepared in Example 2 is 48 μm (flexible PET substrate layer 25 μm + magnetron stress buffer layer 4 μm + UV curing coating 4 μm + optically transparent pressure-sensitive adhesive layer 15 μm + release film 50 μm; wherein the release film is peeled off before use and is not included in the total thickness). Example

[0038] This invention provides a method for preparing an ultra-flexible PET protective film, comprising: a flexible PET substrate layer, a UV-curable coating, an optically transparent pressure-sensitive adhesive layer, and a magnetron stress buffer layer, specifically including the following steps: Step S1: Preparation of flexible PET substrate layer and coating of magnetron layer A 40 μm thick copolymer-modified flexible PET film (elastic modulus 1.7 GPa, elongation at break 120%) was selected as the flexible PET substrate layer. Before coating, the substrate layer was subjected to corona treatment using a corona treatment machine (power 2.5 kW, processing speed 20 m / min) to achieve a surface tension of 54 dyne / cm.

[0039] The magnetorheological elastomer precursor solution was coated onto one side surface of the corona-treated flexible PET substrate layer through a narrow-slit coating head. The formulation of the magnetorheological elastomer precursor solution was as follows: 65 parts of polydimethylsiloxane (PDMS) matrix, 30 parts of carbonyl iron powder (average particle size 5 μm), and 5 parts of silane coupling agent (KH-550). The components were mixed in a vacuum degassing mixer at 800 rpm for 30 minutes to obtain a uniformly dispersed precursor solution.

[0040] Step S2: Magnetic field solidification to form a magnetic stress buffer layer A flexible PET substrate layer coated with a precursor solution was fed into a magnetic field curing device with an electrode spacing of 50 mm and the magnetic field direction perpendicular to the substrate surface (i.e., perpendicular to the magnetron layer thickness direction). The magnetic field strength was controlled to be stable at 800 mT. Under the action of the magnetic field, the substrate was cured in an oven at 100°C for 60 minutes, causing carbonyl iron powder to form a chain-like ordered structure in the PDMS matrix along the magnetic field direction. After curing, a magnetron stress buffer layer with a thickness of 3 μm was obtained.

[0041] Step S3: Apply UV-cured hardening coating A UV-curable coating liquid was applied to the other side of the flexible PET substrate layer (i.e., the side without the MRE layer). The formulation of the curing coating liquid was as follows (parts by weight): 55 parts aliphatic polyurethane acrylate oligomer, 30 parts dipentaerythritol hexaacrylate (DPHA), 3 parts photoinitiator (184), 10 parts nano silica (particle size 40 nm), and 2 parts perfluoropolyether acrylate. The wet film thickness was controlled to 8 μm by slit coating. After drying with hot air at 80°C for 30 seconds, the film was UV-cured under nitrogen protection with an irradiation energy of 600 mJ / cm² to obtain a curing coating with a thickness of 2 μm.

[0042] Step S4: Coat an optically transparent pressure-sensitive adhesive layer and laminate with a release film. An optically transparent pressure-sensitive adhesive was coated onto the surface of the magnetron stress buffer layer. The adhesive formulation (parts by weight) consisted of 85 parts isooctyl acrylate-acrylic acid copolymer, 3 parts isocyanate curing agent, 1 part epoxy silane coupling agent, and 0.5 parts conductive polymer antistatic agent. A doctor blade coating method was used to control the dry adhesive thickness to 10 μm. After coating, the adhesive was sequentially dried and cured in three ovens at 60℃, 80℃, and 100℃ (each oven segment was 3 m long, with a linear speed of 5 m / min). Then, a 50 μm thick fluorine release film (release force 8 gf / inch) was laminated onto the pressure-sensitive adhesive surface to obtain the semi-finished product.

[0043] Step S5: Aging treatment The semi-finished product is placed in a constant temperature oven at 50℃ for 60 hours to fully cross-link the optically transparent pressure-sensitive adhesive layer and release the internal stress of the PET substrate layer. After curing, the ultra-flexible PET protective film is obtained.

[0044] The total thickness of the ultra-flexible PET protective film prepared in Example 3 is 55 μm (40 μm flexible PET substrate layer + 3 μm magnetron stress buffer layer + 2 μm UV curing coating + 10 μm optically transparent pressure-sensitive adhesive layer + 50 μm release film; wherein the release film is peeled off before use and is not included in the total thickness). Example

[0045] The difference from the above embodiment three is that: in step S1, the magnetorheological elastomer precursor solution is applied to one side surface of the corona-treated flexible PET substrate layer through a narrow slit coating head, and the width of the coating on the flexible PET substrate layer is controlled to be 12mm (this area must correspond to the folded area of ​​the ultra-flexible PET protective film). The remaining part is compensated by an optically transparent pressure-sensitive adhesive layer to ensure the flatness of the surface of the ultra-flexible PET protective film after molding.

[0046] The vivo X Fold series foldable phones (taking the vivo X Fold3 as an example) were used as the test platform. The vivo X Fold3 features an 8.03-inch inner screen (unfolded dimensions are 159.68 mm × 142.29 mm). The aerospace-grade floating wing hinge of this series of phones is equipped with six neodymium iron boron (NdFeB) magnets, with a minimum bending radius of approximately 2.3 mm.

[0047] The ultra-flexible PET protective films prepared in Examples 2, 3, and 4 were cut into samples that matched the size of the vivo X Fold3 inner screen (approximately 160 mm × 143 mm). After cutting, the release film layer at the bottom of the protective film was peeled off to expose the optically transparent pressure-sensitive adhesive layer. The protective films were then sequentially attached to the surface of the vivo X Fold3 inner screen, ensuring a smooth and bubble-free fit, and aligning the area where the magnetron stress buffer layer was located with the area of ​​the phone hinge magnet (especially the ultra-flexible PET protective film in Example 4, which should be properly positioned during cutting).

[0048] Test steps Step 1: Place the vivo X Fold3 phone with the protective film applied in the dynamic bending test equipment and set the bending parameters: bending radius 3 mm, bending angle from 0° to 180° repeatedly, bending frequency 30 times / minute. Because the vivo X Fold series phone hinge incorporates a neodymium iron boron permanent magnet array, it automatically generates a magnetic field during opening and closing—when folded, the magnets approach the magnetron layer of the protective film, strengthening the magnetic field; when unfolded, the magnets move away from the magnetron layer, weakening the magnetic field—achieving dynamic coupling between the magnetic field and the bending action, eliminating the need for an external magnetic field generator.

[0049] Step 2: Start the dynamic bending test and automatically record the number of bends.

[0050] Step 3: After every 1000 bends, pause the test, unfold the phone screen to a completely flat state, and observe the surface of the protective film under a D65 standard light source at a 45° angle, recording the appearance of creases; simultaneously, use a laser confocal microscope to scan and measure the crease area along the direction perpendicular to the folding axis, recording the crease depth. Step 4: When the crease depth of the protective film reaches 20μm, record the number of folds as the number of repeated folds and openings that occurred when the initial crease appeared.

[0051] Step 5: When the protective film shows hardened coating crack length ≥ 0.5 mm, interlayer peeling, light transmittance decrease of more than 5%, pencil hardness retention rate of less than 80%, or edge damage, record the number of bends as the bending life.

[0052] Step 6: Repeat the above steps 3 times for the test samples of Examples 2, 3 and 4 respectively, and take the arithmetic mean as the final test result.

[0053] Test Result Record Following the above testing steps, three samples of each of the ultra-flexible PET protective films prepared in Examples 2, 3, and 4 were tested. The arithmetic mean was taken as the final test result, and the results are recorded as follows:

[0054] Table 1 From Example 2 to Example 4, with the increase in the proportion of PET thickness and the optimization of the local coating design of the MRE layer, the bending life of the protective film increased from 128,000 cycles to 223,000 cycles, an increase of approximately 74.2%; the number of initial crease occurrences increased from 65,000 cycles to 126,000 cycles, an increase of approximately 93.8%; and the crease depth after 100,000 cycles decreased from 28 μm to 12 μm, a decrease of approximately 57.1%.

[0055] Example 4 performed best in the test: its bending life reached 223,000 cycles, exceeding the industry reliability threshold of 200,000 cycles; the crease depth after 200,000 cycles was only 15 μm, far below the industry acceptable threshold of 30-50 μm; slight cracks appeared in the hardened coating at approximately 210,000 cycles, but no complete delamination failure occurred until 223,000 cycles. The test results of this example show that when the PET thickness accounts for nearly 80% and the MRE layer adopts a local coating design, the ultra-flexible PET protective film of this invention achieves a leading level in both bending life and crease suppression for foldable screen protective films.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An ultra-flexible PET protective film, characterized in that, include: Flexible PET substrate layer (1); A UV-curable hardening coating (2) is disposed on one side surface of the flexible PET substrate layer (1); An optically transparent pressure-sensitive adhesive layer (3) is disposed on the other side surface of the flexible PET substrate layer (1). And a magnetron stress buffer layer (4) disposed between the optically transparent pressure-sensitive adhesive layer (3) and the flexible PET substrate layer (1). The magnetic stress buffer layer (4) contains a magnetorheological elastomer material. The magnetic stress buffer layer (4) has a first elastic modulus E1 when there is no external magnetic field, and the elastic modulus increases to a second elastic modulus E2 when subjected to an external magnetic field, and E2 > E1.

2. The ultra-flexible PET protective film according to claim 1, characterized in that: The flexible PET substrate layer (1) is a copolymer-modified or blend-modified polyethylene terephthalate film. The thickness of the flexible PET substrate layer accounts for at least 65% of the total thickness of the ultra-flexible PET protective film. The elastic modulus of the flexible PET substrate layer is 2,500,000 kPa to 3,300,000 kPa.

3. The ultra-flexible PET protective film according to claim 1, characterized in that: The first elastic modulus E1 is 50-100 kPa, the second elastic modulus E2 is 150-200 kPa, and E2 / E1≥2.

4. The ultra-flexible PET protective film according to claim 1, characterized in that, The magnetorheological elastomer material comprises the following components by weight: 50-80 parts of elastic matrix material, 20-50 parts of magnetic filler, 0.5-5 parts of dispersant, and 0.5-5 parts of coupling agent.

5. A method for preparing an ultra-flexible PET protective film, characterized in that, The method for preparing the ultra-flexible PET protective film according to any one of claims 1-4 comprises the following steps: S1. A flexible PET substrate layer (1) is formed, and a magnetorheological elastomer precursor solution is coated on one side of the substrate to form a wet film. S2. The flexible PET substrate layer (1) coated with magnetorheological elastomer precursor solution is placed in an external magnetic field for curing, so that the magnetic filler forms a chain-like ordered structure along the magnetic field direction in the elastic matrix, and a magnetic stress buffer layer (4) is obtained. S3. Apply a UV-curable coating liquid to the other side of the flexible PET substrate layer (1) and cure it with UV to form a UV-curable coating (2). S4. An optically transparent pressure-sensitive adhesive is coated on the surface of the magnetron stress buffer layer (4), dried and cured to form an optically transparent pressure-sensitive adhesive layer (3), and a release film layer is laminated to obtain a semi-finished product; S5. Place the semi-finished product at 40-60℃ for 48-72 hours to obtain an ultra-flexible PET protective film.

6. The method for preparing an ultra-flexible PET protective film according to claim 5, characterized in that: In step S1, the magnetorheological elastomer precursor solution is coated using narrow slit coating, screen printing or transfer bonding processes, so that the magnetron stress buffer layer (4) only covers the folded position of the flexible PET substrate layer (1), with a thickness of 3-5 μm and a coverage width of 5-20 mm.

7. The method for preparing an ultra-flexible PET protective film according to claim 5, characterized in that: In step S2, the strength of the external magnetic field is 500mT to 1T, the curing temperature is 80-120℃, the curing time is 30-120 minutes, and the direction of the external magnetic field is perpendicular to the thickness direction of the magnetic stress buffer layer (4).

8. The method for preparing an ultra-flexible PET protective film according to claim 5, characterized in that: In step S3, the UV-curable coating liquid contains polyurethane acrylate oligomers, reactive diluent monomers, photoinitiators, nano-inorganic particles, and fluorinated anti-fingerprint additives; the UV curing irradiation energy is 300-1000 mJ / cm², and the thickness of the cured coating is 2-6 μm.

9. The method for preparing an ultra-flexible PET protective film according to claim 5, characterized in that: In step S4, the optically transparent pressure-sensitive adhesive is an acrylic pressure-sensitive adhesive, and the dry adhesive thickness after coating is 10-30 μm.

10. The method for preparing an ultra-flexible PET protective film according to claim 5, characterized in that: Before step S1, the flexible PET substrate layer (1) is subjected to corona treatment to make its surface tension ≥52 dyne / cm.

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