PET protective film of flexible electronic device and preparation method of PET protective film

Abstract

The invention discloses a PET protective film of a flexible electronic device and a preparation method thereof, and the PET protective film obtains two opposite properties in two vertical directions: the PET protective film has high HCMR in one direction, can represent the folding performance, and minimizes the crease depth; the other direction has high tensile Young's modulus which can represent self-supporting property and hardness and is one of the most important properties of a protective film in a flexible electronic device. In addition, other required properties, such as total light transmittance, haze, thermal shrinkage and the like, of the PET protective film of the flexible electronic device are similar to or slightly improved compared with those of the prior art. In order to obtain an extreme anisotropic PET film, the invention achieves maximum unidirectional orientation through stretching in one direction, and eliminates residual strain but maintains unidirectional orientation of molecular chains through an annealing process, which technology achieves good thermal shrinkage properties.

Description

Technical Field

[0001] The invention relates to the field of protective film materials, and in particular to a PET protective film for a flexible electronic device and a preparation method thereof. Background Art

[0002] Nowadays, the era of the Internet, where users can access information anytime and anywhere, is coming, and the digital convergence of computers, communications, information, home appliances, etc. is developing rapidly. Therefore, the display that serves as an interface between users and electronic information devices has become more important.

[0003] In addition, as the demand for image information with high resolution, high brightness and high definition becomes stronger, large-screen liquid crystal displays (LCD), organic light emitting diodes (OLED), etc. are competing with each other.

[0004] In recent years, flexible displays, a type of portable next-generation display that can be twisted or bent, have attracted widespread attention. Many flexible electronic devices exist around the world to enhance usability and portability.

[0005] The representative flexible electronic device is a foldable smartphone, which will develop into various shapes, such as rollable and slidable.

[0006] Other electronic devices will also develop like tablets, laptops, and TVs. Display screens are getting bigger and bigger.

[0007] However, electronic devices are almost all made of glass substrates. Unfortunately, glass is not flexible and is easily broken.

[0008] The pieces of broken glass are very sharp, small, and hazardous, making them very dangerous to human safety.

[0009] In order to manufacture such flexible electronic devices, a novel flexible substrate is needed to replace the rigid glass substrate.

[0010] In particular, the outer layer of the device is a very important part because the functions of the outer layer are to be transparent so that the display can be observed with naked eyes, to have hardness so as to protect the device from external impact, and to have flexibility for a flexible device.

[0011] Many approaches to protective layers for flexible electronics use existing materials. For completely new devices, this is not enough.

[0012] For example, thin glass has good transparency and hardness, but insufficient flexibility and safety. As a representative polymer film, PET also has good transparency, but insufficient hardness and flexibility.

[0013] The PET protective films currently developed have good transparency and sufficient hardness for flexible electronic devices, but they are not flexible enough, and repeated folding and unfolding will cause permanent deformation.

[0014] Wrinkle marks can be observed on all foldable electronic devices today. This is called "crease depth" and is a major problem for foldable smartphones. It can be explained by the permanent deformation of the PET film.

[0015] Basically, polymers have viscoelastic behavior. The permanent deformation of a plastic is related to the elastic behavior of the plastic. The elastic behavior can be expressed by the tensile yield point of the plastic. It has nothing to do with the tensile Young's modulus.

[0016] In a foldable smartphone, the folding force of a human hand is strong enough. Therefore, the only thing to be concerned about is the yield strain, not the yield strength.

[0017] The foldable smartphone’s tensile strain exceeds the yield strain of the plastic film, leading to the crease depth problem. This causes permanent deformation of the plastic film. The permanent deformation can be observed in the form of crease depth.

[0018] The present invention aims to enhance the yield strain and thus the crease depth of flexible electronic devices. Summary of the invention

[0019] The technical problem to be solved by the present invention is to provide a PET protective film for a flexible electronic device and a preparation method thereof in view of the deficiencies in the above-mentioned prior art.

[0020] In order to solve the above technical problems, the technical solution adopted by the present invention is: In a first aspect of the present invention, a PET protective film for a flexible electronic device is provided, and the PET protective film meets the following requirements:

[0021] In T g ~T g When stretched in the temperature range of +30°C, the PET protective film has a HCMR value exceeding 60% in at least one direction when the stretching ratio in the TD direction is 3.0 to 5.0 times the stretching ratio in the MD direction;

[0022] Among them, T g It represents the glass transition temperature of the PET protective film; HCMR represents the mixed chord modulus ratio, and the calculation formula is as follows:

[0023]

[0024] The chord modulus @ A% strain indicates the tensile modulus when the tensile strain is A%, and the initial Young's modulus indicates the tensile modulus when the tensile strain is less than 1%.

[0025] Preferably, A=2-5.

[0026] Preferably, the thickness of the PET protective film is 24 μm to 126 μm.

[0027] Preferably, the haze of the PET protective film is less than 2.0.

[0028] Preferably, the total light transmittance of the PET protective film is higher than 89%.

[0029] Preferably, the thermal shrinkage of the PET protective film at 150° C. for 30 minutes is less than 2.0%.

[0030] Preferably, the PET protective film has an initial Young's modulus in tension in at least one direction higher than 4.0 GPa.

[0031] A second aspect of the present invention provides a method for preparing a PET protective film for a flexible electronic device as described above, the method comprising the following steps:

[0032] The unstretched cast PET sheet is preheated, then stretched by 3.0 to 5.0 times in the TD direction within a temperature range of Tg to Tg+30°C, and then annealed to obtain a PET protective film for a flexible electronic device, wherein the PET protective film has an HCMR value of more than 60% in at least one direction;

[0033] The annealing temperature is at least 10° C. higher than the stretching temperature, and the annealing time is no more than 60 seconds.

[0034] Preferably, the method for preparing the PET protective film of the flexible electronic device comprises the following steps:

[0035] The unstretched cast PET sheet is stretched by a stretching device, preheated, and then g ~T g The PET protective film for flexible electronic devices is obtained by stretching 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction within a temperature range of +30°C, and then annealing, wherein the PET protective film has an HCMR value of more than 60% in at least one direction.

[0036] Preferably, the method for preparing the PET protective film of the flexible electronic device comprises the following steps:

[0037] The unstretched cast PET sheet is stretched by a stretching device, first preheated at 90-100°C, then stretched 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction at a stretching temperature of 90-100°C and a stretching speed of 13-15 mm / s, and then annealed at 200-220°C for 30-60s to obtain a PET protective film for a flexible electronic device, which satisfies the HCMR value of more than 60% in the TD direction.

[0038] The beneficial effects of the present invention are:

[0039] The present invention provides a new generation of performance-enhanced protective films for new generation flexible electronic devices. The present invention obtains two opposing properties in two perpendicular directions: one direction has a higher HCMR, which can represent foldable performance and minimize the crease depth; the other direction has a higher tensile Young's modulus, which can represent self-support and hardness, which is one of the most important properties of protective films in flexible electronic devices.

[0040] In addition, other desired properties of the PET protective film for the flexible electronic device of the present invention, such as total light transmittance, haze, thermal shrinkage, etc., are also similar to or slightly improved over those of the prior art.

[0041] In order to obtain an extremely anisotropic PET film, the present invention realizes maximum unidirectional orientation by stretching in one direction, and eliminates residual strain by annealing process while maintaining unidirectional orientation of molecular chains, and this technology realizes good heat shrinkage properties. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below in conjunction with embodiments so that those skilled in the art can implement the invention with reference to the description.

[0043] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0044] The test methods used in the following examples are conventional methods unless otherwise specified. The materials and reagents used in the following examples are all commercially available unless otherwise specified. In the following examples, if no specific conditions are specified, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used, if the manufacturer is not specified, are all conventional products that can be purchased commercially.

[0045] The most difficult but most important obstacle is that there are two conflicting requirements for protective films in flexible electronics. That is: hardness is required for protection, and softness is required for flexibility. The present invention focuses on direction. Many flexible electronics only require flexibility in one direction. And roll-to-roll manufactured polymer films have properties in two directions. They are called MD (longitudinal direction) and TD (transverse direction). Make one direction hard and the other direction soft. The present invention focuses on this idea of ​​controlling properties independently by direction (extreme anisotropic films). Unidirectionally oriented PET film (UOPET) has two different properties in two different directions.

[0046] Before describing the present invention, new parameters need to be defined. All mechanical property parameters of existing polymer films are poorly matched to this purpose.

[0047] The aim is to minimize the permanent deformation of the polymer film at a specific tensile strain. This means the degree to which the material changes from elastic to viscous behavior at a specific tensile strain. The specific tensile strain depends on the fold radius.

[0048] This does not exactly match the tensile initial Young's modulus, nor does it exactly match the tensile chord modulus in strain. But their ratios match this property well. Because the tensile initial Young's modulus before the yield strain can represent the elastic behavior of the material. The tensile chord modulus after the yield strain can represent the viscous behavior of the material.

[0049] The present invention defines a new parameter called "HCMR" (Hybrid Chordal Modulus Ratio) as follows

[0050]

[0051] The chord modulus @ A% strain indicates the tensile modulus when the tensile strain is A%, and the initial Young's modulus indicates the tensile modulus when the tensile strain is less than 1%.

[0052] HCMR (Hybrid Chord Modulus Ratio) means the reduction in tensile modulus at a specific strain. It also means the degree to which a polymer film changes from elastic behavior to viscous behavior. HCMR can indicate the performance of the recovery properties of flexible electronic devices. (Related to crease depth)

[0053] Specifically, the present invention provides a PET protective film for a flexible electronic device, wherein the PET protective film satisfies:

[0054] In T g ~T gWhen stretched in a temperature range of +30°C, the PET protective film has a HCMR value of more than 60% in at least one direction when the stretching ratio in one direction is 3.0 to 5.0 times higher than the stretching ratio in the vertical direction;

[0055] Among them, T g It represents the glass transition temperature of the PET protective film; HCMR represents the mixed chord modulus ratio, and the calculation formula is as follows:

[0056]

[0057] The chord modulus @ A% strain indicates the tensile modulus when the tensile strain is A%, and the initial Young's modulus indicates the tensile modulus when the tensile strain is less than 1%.

[0058] In a preferred embodiment, A = 2 to 5. More preferably, A = 2.5.

[0059] In a preferred embodiment, the thickness of the PET protective film is 24 μm to 126 μm.

[0060] In a preferred embodiment, the haze of the PET protective film is less than 2.0.

[0061] In a preferred embodiment, the total light transmittance of the PET protective film is higher than 89%.

[0062] In a preferred embodiment, the thermal shrinkage of the PET protective film at 150° C. for 30 minutes is less than 2.0%.

[0063] In a preferred embodiment, the PET protective film has an initial Young's modulus in tension in at least one direction higher than 4.0 GPa.

[0064] The present invention also provides a method for preparing the PET protective film of the flexible electronic device as described above, the method comprising the following steps:

[0065] The unstretched cast PET sheet is preheated, then stretched by 3.0 to 5.0 times in the TD direction within a temperature range of Tg to Tg+30°C, and then annealed to obtain a PET protective film for a flexible electronic device, wherein the PET protective film has an HCMR value of more than 60% in at least one direction;

[0066] The annealing temperature is at least 10° C. higher than the stretching temperature, and the annealing time is no more than 60 seconds.

[0067] In a preferred embodiment, the method for preparing the PET protective film of the flexible electronic device comprises the following steps:

[0068] The unstretched cast PET sheet is stretched by a stretching device, preheated, and then g ~T g The PET protective film for flexible electronic devices is obtained by stretching 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction within a temperature range of +30°C, and then annealing, wherein the PET protective film has an HCMR value of more than 60% in at least one direction.

[0069] In a preferred embodiment, the method for preparing the PET protective film of the flexible electronic device comprises the following steps:

[0070] The unstretched cast PET sheet is stretched by a stretching device, first preheated at 90-100°C, then stretched 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction at 90-100°C, and then annealed at 200-220°C for 30-60s to obtain a PET protective film for a flexible electronic device, which satisfies the HCMR value of more than 60% in the TD direction.

[0071] The present invention provides a one-way stretching to obtain a unidirectional oriented PET film (UOPET). It makes an extremely anisotropic PET film, and its properties vary greatly depending on the direction.

[0072] If the PET film can be subjected to the uniaxial stretching designed by the present invention and has sufficiently good protective film properties, the chemical formula of the PET film as well as the polymerization method and process conditions are not limited.

[0073] And, in order to obtain thermal stability, an annealing process may be performed at a temperature 10° C. higher than the stretching temperature within 60 seconds.

[0074] To achieve even higher surface hardness, an additional hard coating treatment can be applied to this PET film.

[0075] In order to obtain anti-blocking properties, anti-blocking agents such as TiO2 and / or SiO2 and / or CaCo3 may also be added.

[0076] Some functional agents may also be added as needed, such as antistatic agents and / or anti-aging agents and / or UV blockers and / or dyes.

[0077] The present invention does not limit the extrusion method and process conditions of PET film production. T-die extrusion or tubular extrusion is possible.

[0078] Generally, the stretching process of PET film is carried out in two vertical directions (MD and TD). However, the present invention only provides a PET film stretched in one direction to achieve extreme unidirectional orientation.

[0079] After the PET unstretched sheet is extruded, a stretching process is performed, that is, stretching 3.0 to 5.0 times in one direction at a stretching speed of more than 10 mm / second in the range of glass transition temperature (Tg) to 30°C higher than the glass transition temperature (Tg).

[0080] If the stretching ratio is too low, the molecular chain of the PET film is not unidirectionally oriented enough, and a sufficiently good crease depth cannot be obtained.

[0081] However, if the stretch ratio is too high, the PET film cannot withstand high shear forces, and the risk of PET film breakage is high. In addition, many large SICs (stretch induced crystals) may be produced in PET. This will result in high haze values ​​and is difficult to apply to the outer layer of the device display.

[0082] If the stretching temperature is too low, the PET film cannot withstand high shear forces, and there is a high risk of PET film breakage and / or uneven stretching. In addition, many large SICs (stretch induced crystals) may be produced in PET. This will result in high haze values, making it difficult to apply to the outer layer of the device display.

[0083] However, if the stretching temperature is too high and the molecular chain of the PET film is not unidirectionally oriented enough, a sufficiently good crease depth cannot be obtained.

[0084] If the stretching speed is too slow, the unidirectional orientation of the molecular chains of the PET film is not enough, and a sufficiently good crease depth cannot be obtained.

[0085] However, if the stretching speed is too fast, the PET film cannot withstand the high shear force and the risk of the PET film breaking is high.

[0086] Furthermore, the present invention provides a more beneficial technique.

[0087] In order to obtain a higher degree of molecular chain orientation, a two-step stretching process is sometimes required.

[0088] The first step is to release the entanglement of molecular chains. The second step is to orient the released molecular chains.

[0089] As an example in the present invention, the 2-step stretching has a good effect on the orientation of the molecular chains. The first step is to stretch slightly in one direction, such as 1.2 times. The second step is to stretch completely in the vertical direction. Then, the entanglement of the molecular chains in the first step is released, and the released molecular chains can be completely oriented to the second direction in the second step. Thereby, the best HCMR value and crease depth performance are obtained.

[0090] If the annealing temperature is too high and the time is too long, the orientation of the PET film is released, resulting in a decrease in the HCMR value and poor crease depth performance. In addition, excessive heat treatment may produce many large TICs (thermally induced crystals) in PET. This will result in high haze values ​​and is difficult to apply to the outer layer of the device display.

[0091] However, if the annealing temperature is too low and the time is too short, the annealing effect is insufficient to eliminate the residual strain. This will lead to excessively high thermal shrinkage values, which will cause some problems in post-processing such as coating, punching, and lamination, making it difficult to apply to a certain layer of an electronic device.

[0092] The key idea of ​​the present invention is that many flexible electronic devices only require foldability in one direction, such as minimizing the crease depth, while current mechanical properties cannot represent foldability such as crease depth.

[0093] The key technology of the present invention is to perform maximum ratio stretching in one direction at the maximum speed at the lowest temperature to achieve maximum unidirectional orientation.

[0094] It provides high HCMR values ​​in one direction of extremely anisotropic PET films.

[0095] The above is the overall concept of the present invention, and detailed embodiments and comparative examples are provided below on the basis of the overall concept of the present invention to further illustrate the present invention.

[0096] Example 1

[0097] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 100°C for 40 seconds, the second stretching treatment step is stretching 4.5 times in only one direction (TD) at a stretching speed of 14.1 mm / s at 100°C, and the third stretching treatment step is annealing at 200°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0098] Example 2

[0099] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 100°C for 40 seconds, the second stretching treatment step is stretching 5.5 times in only one direction (TD) at a stretching speed of 14.1 mm / s at 100°C, and the third stretching treatment step is annealing at 200°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0100] Example 3

[0101] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 90°C for 40 seconds, the second stretching treatment step is stretching 4.5 times in only one direction (TD) at 90°C at a stretching speed of 14.1 mm / s, and the third stretching treatment step is annealing at 200°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0102] Example 4

[0103] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 100°C for 40 seconds, the second stretching treatment step is stretching 1.2 times in only one direction (MD) at a stretching speed of 14.1 mm / s at 100°C, the third stretching treatment step is stretching 4.5 times in a vertical direction (TD) at a stretching speed of 14.1 mm / s at 100°C, and the fourth stretching treatment step is annealing at 200°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0104] Example 5

[0105] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 90°C for 40 seconds, the second stretching treatment step is stretching 1.2 times in only one direction (MD) at 90°C at a stretching speed of 14.1 mm / second, the third stretching treatment step is stretching 4.5 times in a vertical direction (TD) at 90°C at a stretching speed of 14.1 mm / second, and the fourth stretching treatment step is annealing at 220°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0106] Example 6

[0107] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 90°C for 40 seconds, the second stretching treatment step is stretching 1.2 times in only one direction (MD) at a stretching speed of 14.1 mm / second at 90°C, the third stretching treatment step is stretching 5.0 times in a vertical direction (TD) at a stretching speed of 14.1 mm / second at 90°C, and the fourth stretching treatment step is annealing at 220°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0108] Example 7

[0109] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 90°C for 40 seconds, the second stretching treatment step is stretching 4.5 times in only one direction (TD) at 90°C at a stretching speed of 14.1 mm / s, and the third stretching treatment step is annealing at 220°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0110] Comparative Example 1

[0111] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 90°C for 40 seconds, the second stretching treatment step is stretching 3.0 times in only one direction (MD) at a stretching speed of 14.1 mm / second at 90°C, the third stretching treatment step is stretching 3.5 times in a vertical direction (TD) at a stretching speed of 14.1 mm / second at 90°C, and the fourth stretching treatment step is annealing at 200°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0112] Comparative Example 2

[0113] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 90°C for 40 seconds, the second stretching treatment step is stretching 3.5 times in only one direction (MD) at a stretching speed of 14.1 mm / second at 90°C, the third stretching treatment step is stretching 3.5 times in a vertical direction (TD) at a stretching speed of 14.1 mm / second at 90°C, and the fourth stretching treatment step is annealing at 200°C for 30 seconds. According to these processes and conditions, a 50 μm PET film is obtained, and its properties are shown in Table 1.

[0114] Comparative Example 3

[0115] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 90°C for 40 seconds, the second stretching treatment step is stretching 5.5 times in only one direction (TD) at 90°C at a stretching speed of 14.1 mm / sec, and the third stretching treatment step is annealing at 220°C for 30 seconds. Since the film is broken, a 50 μm PET film cannot be prepared according to these processes and conditions, and its properties are shown in Table 1.

[0116] Comparative Example 4

[0117] A PET protective film for a flexible electronic device, the preparation method thereof comprises the following steps: placing an unstretched cast PET sheet into a stretching device for stretching. The first stretching treatment step is preheating at 80°C for 40 seconds, the second stretching treatment step is stretching 4.5 times in only one direction (TD) at 80°C at a stretching speed of 14.1 mm / sec, and the third stretching treatment step is annealing at 220°C for 30 seconds. Since the film is broken, a 50 μm PET film cannot be prepared according to these processes and conditions, and its properties are shown in Table 1.

[0118] Table 1

[0119]

[0120] Among them, *1: the best performance and direction of the example; *2: is the performance and direction of the conventional PET film currently used, which means that 108.2 is the depth value of the PET protective film in existing foldable smartphones.

[0121] In Table 1, it can be confirmed that the HCMR value matches the crease depth performance very well, that is, within a certain range, the higher the HCMR value, the smaller the crease depth in the MD direction and the larger the crease depth in the MD direction; however, the extremely low HCMR (extremely high crease depth) does not completely match this trend (Comparative Examples 1 and 2).

[0122] HCMR indicates the crease depth performance and foldability of a polymer film.

[0123] The analysis methods and conditions for the values ​​in Table 1 are as follows;

[0124] (1) HCMR (hybrid chord modulus ratio):

[0125] For the 2R foldable smartphone, the tensile strain at the outer radius is calculated and the result is 2.5% strain.

[0126] R length Stretch inner radius 2.00 6.28 Outer Radius 2.05 6.44 2.5％

[0127] The inner radius of the foldable smartphone is 2mm and the thickness of the PET film is 0.05mm.

[0128] The tensile modulus is measured by conventional methods and conditions.

[0129] The tensile initial Young's modulus value was obtained by ASTM D882.

[0130] The tensile chord modulus at 2.5% strain value is also the same.

[0131] The HCMR calculation equation is:

[0132]

[0133] The chord modulus @ A% strain represents the tensile modulus when the tensile strain is 2.5%, and the initial Young's modulus represents the tensile modulus when the tensile strain is less than 1%, that is, A=2.5 at this time.

[0134] In addition, page A can take other values ​​within the range of 2 to 5. For example, when the inner radius R=1, a 50 um PET film is used with a stretching rate of 5.0%.

[0135] R length Stretch inner radius 1.00 3.14 Outer Radius 1.05 3.30 5％

[0136] (2) Crease depth:

[0137] A 2R foldable smartphone model was made for this purpose. Guide lines were marked on the model, and the PET film was laminated with the OCA film commonly used in foldable smartphones. The sample was cut into 50mm*30mm according to the design direction. The OCA side of the sample was attached to the model according to the guide lines. The model was folded and placed in an 8585 test chamber (85°C, 85RH% humidity) for 10 days. After that, the model was unfolded to an exact 180°, and the crease depth in the center area was measured by a laser scanning microscope (keyence 3D scanner, made in Japan).

[0138] (3) Thermal shrinkage: measured at 150°C for 30 minutes.

[0139] Other properties were measured by conventional methods and conditions.

[0140] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the implementation modes. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A PET protective film for a flexible electronic device, characterized in that: The PET protective film meets the following requirements: In T g ~T g When stretched in the temperature range of +30°C, the PET protective film has a HCMR value exceeding 60% in at least one direction when the stretching ratio in the TD direction is 3.0 to 5.0 times the stretching ratio in the MD direction; Among them, T g It represents the glass transition temperature of the PET protective film; HCMR represents the mixed chord modulus ratio, and the calculation formula is as follows: The chord modulus @ A% strain indicates the tensile modulus when the tensile strain is A%, and the initial Young's modulus indicates the tensile modulus when the tensile strain is less than 1%.

2. The PET protective film for flexible electronic devices according to claim 1, characterized in that: in, A＝2～5。 3. The PET protective film for a flexible electronic device according to claim 1, characterized in that: The thickness of the PET protective film is 24 μm to 126 μm.

4. The PET protective film for a flexible electronic device according to claim 1, characterized in that: The haze of the PET protective film is less than 2.

0.

5. The PET protective film for a flexible electronic device according to claim 1, characterized in that: The total light transmittance of the PET protective film is higher than 89%.

6. The PET protective film for a flexible electronic device according to claim 1, characterized in that: The thermal shrinkage rate of the PET protective film at 150° C. for 30 minutes is less than 2.0%.

7. The PET protective film for a flexible electronic device according to claim 1, characterized in that: The PET protective film has an initial Young's modulus in tension in at least one direction higher than 4.0 GPa.

8. A method for preparing a PET protective film for a flexible electronic device as claimed in any one of claims 1 to 7, characterized in that: The method comprises the following steps: The unstretched cast PET sheet is preheated, then stretched by 3.0 to 5.0 times in the TD direction within a temperature range of Tg to Tg+30°C, and then annealed to obtain a PET protective film for a flexible electronic device, wherein the PET protective film has an HCMR value of more than 60% in at least one direction; The annealing temperature is at least 10° C. higher than the stretching temperature, and the annealing time is no more than 60 seconds.

9. The method for preparing a PET protective film for a flexible electronic device according to claim 8, characterized in that: The method comprises the following steps: The unstretched cast PET sheet is stretched by a stretching device, preheated, and then g ~T g The PET protective film for flexible electronic devices is obtained by stretching 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction within a temperature range of +30°C, and then annealing, wherein the PET protective film has an HCMR value of more than 60% in at least one direction.

10. The method for preparing a PET protective film for a flexible electronic device according to claim 9, characterized in that: The method comprises the following steps: The unstretched cast PET sheet is stretched by a stretching device, first preheated at 90-100°C, then stretched 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction at a stretching temperature of 90-100°C and a stretching speed of 13-15 mm / s, and then annealed at 200-220°C for 30-60s to obtain a PET protective film for a flexible electronic device, which satisfies the HCMR value of more than 60% in the TD direction.

Images