Pet protective film for flexible electronic apparatus and manufacturing method therefor

Abstract

Disclosed in the present invention are a PET protective film for a flexible electronic apparatus and a manufacturing method therefor. In the present invention, there are two mutually opposite properties in two perpendicular directions: in one direction, the PET protective film has a relatively high HCMR, which can represent foldability, resulting in the minimization of a crease depth, while in the other direction, the PET protective film has a relatively high tensile Young's modulus, which can represent a self-supporting property and rigidness and is one of the most important properties of protective films for flexible electronic apparatuses. In addition, other required properties, such as total light transmittance, haze and heat shrinkage, of the PET protective film for a flexible electronic apparatus of the present invention are similar to or slightly improved than those in the prior art. In order to obtain an extremely anisotropic PET film, the present invention uses stretching in one direction to achieve maximum unidirectional orientation and uses an annealing process to eliminate residual strain but maintain the unidirectional orientation of molecular chains. The technique achieves good heat shrinkage properties.

Description

PET protective film for flexible electronic devices and its preparation method Technical Field

[0001] This invention relates to the field of protective film materials, and in particular to a PET protective film for flexible electronic devices and its preparation method. Background Technology

[0002] Today, we are entering an era of ubiquitous internet access where users can access information anytime, anywhere, and the digital convergence of computers, communications, information, and home appliances is rapidly developing. Therefore, displays, which act as the interface between users and electronic information devices, are becoming increasingly important.

[0003] Furthermore, with the increasing demand for image information with high resolution, high brightness, and high definition, large-screen liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs) 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 globally to enhance usability and portability.

[0005] A prime example of flexible electronic devices is the foldable smartphone. It will evolve into various shapes, such as rollable and slidable.

[0006] Other electronic devices will also evolve in the same way as tablets, laptops, and TVs. Screens will get bigger and bigger.

[0007] However, electronic devices are almost entirely made of glass substrates. The downside is that glass is not flexible and is easily broken.

[0008] Broken glass shards are extremely sharp, small, and dangerous, posing a significant risk to human safety.

[0009] 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 its functions are to be transparent so that the display can be viewed with the naked eye, to be rigid so that the device can be protected from external impacts, and to be flexible so that it can be used in flexible devices.

[0011] Many methods for creating protective layers in flexible electronic devices utilize existing materials. This is insufficient for entirely new devices.

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

[0013] Currently developed PET protective films offer good transparency and sufficient rigidity for flexible electronic devices, but lack flexibility, and repeated folding and unfolding can lead to permanent deformation.

[0014] Creases are now visible on all foldable electronic devices. This is known as "crease depth" and is a major problem with foldable smartphones. It can be explained by the permanent deformation of the PET film.

[0015] Essentially, polymers exhibit viscoelastic behavior. The permanent deformation of plastics is related to their elastic behavior. This elastic behavior can be represented by the tensile yield point of the plastic, and is independent of the tensile Young's modulus.

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

[0017] The tensile strain of foldable smartphones exceeds the yield strain of the plastic film, leading to crease depth issues. This results in permanent deformation of the plastic film, which can be observed as crease depth.

[0018] The present invention aims to enhance yield strain, thereby increasing 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 flexible electronic devices and a method for preparing the same, addressing the shortcomings of the prior art.

[0020] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: In its first aspect, the present invention provides a PET protective film for flexible electronic devices, wherein the PET protective film satisfies the following conditions:

[0021] In T g ~T g When stretched within a temperature range of +30℃, the PET protective film has an HCMR value of more than 60% in at least one direction, provided that the stretching ratio in the TD direction is 3.0 to 5.0 times that in the MD direction.

[0022] Among them, T g This indicates the glass transition temperature of the PET protective film; HCMR represents the tangential modulus ratio of the mixture, calculated using the following formula:

[0023] The chordal modulus @A% strain represents the tensile modulus when the tensile strain is A%, and the initial Young's modulus represents the tensile modulus when the tensile strain is less than 1%.

[0024] Preferably, A = 2 to 5.

[0025] Preferably, the thickness of the PET protective film is 24μm to 126μm.

[0026] Preferably, the haze of the PET protective film is less than 2.0.

[0027] Preferably, the total light transmittance of the PET protective film is higher than 89%.

[0028] Preferably, the PET protective film has a heat shrinkage rate of less than 2.0% under conditions of 150°C for 30 minutes.

[0029] Preferably, the PET protective film has an initial tensile Young's modulus greater than 4.0 GPa in at least one direction.

[0030] 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:

[0031] Unstretched cast PET sheets are preheated and then stretched 3.0 to 5.0 times in the TD direction within a temperature range of Tg to Tg+30℃. After annealing, a PET protective film for flexible electronic devices is obtained. This PET protective film satisfies the requirement of having an HCMR value of more than 60% in at least one direction.

[0032] The annealing temperature should be at least 10°C higher than the stretching temperature, and the annealing time should not exceed 60 seconds.

[0033] Preferably, the method for preparing the PET protective film for the flexible electronic device includes the following steps:

[0034] Unstretched cast PET sheets are stretched using a stretching machine, preheated, and then subjected to T... g ~T g The film is stretched 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction within a temperature range of +30℃, and then annealed to obtain a PET protective film for flexible electronic devices. The PET protective film satisfies the requirement of having an HCMR value of more than 60% in at least one direction.

[0035] Preferably, the method for preparing the PET protective film for the flexible electronic device includes the following steps:

[0036] Unstretched cast PET sheets are stretched using a stretching device. First, they are preheated at 90–100°C, and then stretched at a stretching temperature of 90–100°C and a stretching speed of 13–15 mm / s, with a stretching ratio of 1.1–2 times in the MD direction and 3.0–6.0 times in the TD direction. Finally, they are annealed at 200–220°C for 30–60 seconds to obtain a PET protective film for flexible electronic devices. This PET protective film meets the requirement of having an HCMR value of over 60% in the TD direction.

[0037] The beneficial effects of this invention are:

[0038] This invention provides a new generation of performance-enhancing protective films for next-generation flexible electronic devices. The invention achieves two opposing properties in two perpendicular directions: one direction has a high HCMR, which can indicate foldability and lead to a minimum crease depth; the other direction has a high tensile Young's modulus, which can indicate self-standing and stiffness, one of the most important properties of protective films in flexible electronic devices.

[0039] Furthermore, other desirable properties of the PET protective film for the flexible electronic device of the present invention, such as total light transmittance, haze, and thermal shrinkage, are similar to or slightly improved from those of the prior art.

[0040] To obtain an extremely anisotropic PET film, this invention achieves maximum unidirectional orientation through stretching in one direction, and eliminates residual strain through an annealing process while maintaining the unidirectional orientation of the molecular chains. This technique achieves good heat shrinkage properties. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0042] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0043] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials and reagents used in the following examples are commercially available. For examples where specific conditions are not specified, conventional conditions or conditions recommended by the manufacturer are followed. For reagents or instruments whose manufacturers are not specified, they are all commercially available products.

[0044] The most difficult but crucial obstacle is that the protective film in flexible electronic devices has two conflicting requirements: rigidity for protection and flexibility for suppleness. This invention focuses on orientation. Many flexible electronic devices require flexibility in only one direction. However, roll-to-roll polymer films have properties in two directions, referred to as MD (longitudinal direction) and TD (transverse direction). This results in rigidity in one direction and flexibility in the other. This invention emphasizes the idea of ​​controlling properties independently of orientation (extremely anisotropic films). Unidirectionally oriented PET films (UOPET) exhibit two different properties in two different directions.

[0045] Before describing the present invention, new parameters need to be defined. All mechanical properties of existing polymer films are poorly suited to this purpose.

[0046] The goal is to minimize the permanent deformation of the polymer film under a specific tensile strain. This means the degree to which the material transitions from elastic to viscous behavior under a given tensile strain. The specific tensile strain depends on the fold radius.

[0047] This does not perfectly match the initial tensile Young's modulus, nor does it perfectly match the tensile chordal modulus under strain. However, their ratio matches this property well. This is because the initial tensile Young's modulus before yield strain can represent the elastic behavior of the material, while the tensile chordal modulus after yield strain can represent the viscous behavior of the material.

[0048] This invention defines the new parameter call as "HCMR" (hybrid chordal modulus ratio), as shown below.

[0049] The chordal modulus @A% strain represents the tensile modulus when the tensile strain is A%, and the initial Young's modulus represents the tensile modulus when the tensile strain is less than 1%.

[0050] HCMR (Hybrid Chord Modulus Ratio) refers to the decrease in tensile modulus at a specific strain. It also indicates the degree to which a polymer film transitions from elastic to viscous behavior. HCMR can represent the restorative properties of flexible electronic devices (related to crease depth).

[0051] Specifically, the present invention provides a PET protective film for flexible electronic devices, the PET protective film satisfying the following:

[0052] In T g ~T g When stretched within a temperature range of +30℃, if the stretching ratio in one direction is 3.0 to 5.0 times higher than the stretching ratio in the vertical direction, the PET protective film has an HCMR value of more than 60% in at least one direction.

[0053] Among them, T g This indicates the glass transition temperature of the PET protective film; HCMR represents the tangential modulus ratio of the mixture, calculated using the following formula:

[0054] The chordal modulus @A% strain represents the tensile modulus when the tensile strain is A%, and the initial Young's modulus represents the tensile modulus when the tensile strain is less than 1%.

[0055] In a preferred embodiment, A = 2 to 5. More preferably, A = 2.5.

[0056] In a preferred embodiment, the thickness of the PET protective film is 24 μm to 126 μm.

[0057] In a preferred embodiment, the haze of the PET protective film is less than 2.0.

[0058] In a preferred embodiment, the total light transmittance of the PET protective film is higher than 89%.

[0059] In a preferred embodiment, the PET protective film exhibits a heat shrinkage rate of less than 2.0% under conditions of 150°C for 30 minutes.

[0060] In a preferred embodiment, the PET protective film has an initial tensile Young's modulus greater than 4.0 GPa in at least one direction.

[0061] The present invention also provides a method for preparing a PET protective film for a flexible electronic device as described above, the method comprising the following steps:

[0062] Unstretched cast PET sheets are preheated and then stretched 3.0 to 5.0 times in the TD direction within a temperature range of Tg to Tg+30℃. After annealing, a PET protective film for flexible electronic devices is obtained. This PET protective film satisfies the requirement of having an HCMR value of more than 60% in at least one direction.

[0063] The annealing temperature should be at least 10°C higher than the stretching temperature, and the annealing time should not exceed 60 seconds.

[0064] In a preferred embodiment, the method for preparing the PET protective film for the flexible electronic device includes the following steps:

[0065] Unstretched cast PET sheets are stretched using a stretching machine, preheated, and then subjected to T... g ~T g The film is stretched 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction within a temperature range of +30℃, and then annealed to obtain a PET protective film for flexible electronic devices. The PET protective film satisfies the requirement of having an HCMR value of more than 60% in at least one direction.

[0066] In a preferred embodiment, the method for preparing the PET protective film for the flexible electronic device includes the following steps:

[0067] Unstretched cast PET sheets are stretched using a stretching device. They are 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 30–60 seconds to obtain a PET protective film for flexible electronic devices. This PET protective film meets the requirement of having an HCMR value of over 60% in the TD direction.

[0068] This invention provides a method for stretching in one direction to obtain a unidirectional oriented PET film (UOPET). It produces an extremely anisotropic PET film whose properties vary considerably depending on the orientation.

[0069] If the PET film can be uniaxially stretched as designed in this invention and has sufficiently good protective film properties, then the chemical formulation of the PET film, as well as the polymerization method and process conditions, are not limited.

[0070] Furthermore, in order to achieve thermal stability, an annealing process can be performed within 60 seconds at a temperature 10°C higher than the stretching temperature.

[0071] To achieve higher surface hardness, an additional hard coating treatment can be applied to this PET film.

[0072] To achieve anti-blocking properties, anti-blocking agents such as TiO2 and / or SiO2 and / or CaCo3 can be added.

[0073] Depending on the needs, some functional agents may also be added, such as antistatic agents and / or anti-aging agents and / or UV blockers and / or dyeing agents, etc.

[0074] This invention does not limit the extrusion method and process conditions for manufacturing PET film. T-die extrusion or tubular extrusion are both possible.

[0075] Typically, the stretching process of PET film is carried out in two vertical directions (MD and TD). However, this invention provides a PET film stretched in only one direction to achieve extreme unidirectional orientation.

[0076] After the PET unstretched sheet is extruded, a stretching process is performed, which involves stretching the sheet in one direction by 3.0 to 5.0 times at a stretching speed of more than 10 mm / s within the range from the glass transition temperature (Tg) to 30°C above the glass transition temperature (Tg).

[0077] If the stretch ratio is too low, the molecular chains of the PET film will not be unidirectional enough, and a sufficiently good crease depth cannot be obtained.

[0078] 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 SiC (stretch-induced crystals) may be formed in PET. This will result in a high haze value, making it difficult to use as the outer layer of a device display.

[0079] If the stretching temperature is too low, the PET film cannot withstand high shear forces, resulting in a higher risk of PET film breakage and / or uneven stretching. Furthermore, many large SiC (stretch-induced crystals) may be formed in the PET. This leads to high haze values, making it difficult to apply to the outer layer of a device display.

[0080] However, if the stretching temperature is too high, the molecular chains of the PET film will not be unidirectional enough, and a sufficiently good crease depth cannot be obtained.

[0081] If the stretching speed is too slow, the molecular chains of the PET film will not be unidirectional enough, and a sufficiently good crease depth cannot be obtained.

[0082] 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.

[0083] Furthermore, the present invention provides a more advantageous technology.

[0084] To achieve higher molecular chain orientation, a two-step stretching process is sometimes required.

[0085] The first step is to release the entanglement of the molecular chains. The second step is to orient the released molecular chains.

[0086] As an example in this invention, the two-step stretching method effectively orients the molecular chains. The first step involves slight stretching in one direction, such as 1.2 times. The second step involves full stretching in the vertical direction. Subsequently, the entanglement of the molecular chains from the first step is released, and the released molecular chains can be fully oriented to the second direction in the second step. This results in optimal HCMR values ​​and crease depth performance.

[0087] If the annealing temperature is too high or the time is too long, the orientation of the PET film will be released, resulting in a decrease in HCMR value and a deterioration in crease depth performance. In addition, excessive heat treatment may produce many large TICs (thermally induced crystals) in the PET. This will result in a high haze value, making it difficult to apply to the outer layer of a device display.

[0088] However, if the annealing temperature is too low or the time is too short, the annealing effect will be insufficient to eliminate residual strain. This will result in an excessively high thermal shrinkage rate, causing problems for post-processing such as coating, punching, and lamination, making it difficult to apply to a specific layer of electronic devices.

[0089] The key idea of ​​this invention is that many flexible electronic devices only require foldability in one direction, such as minimizing crease depth, while current mechanical properties cannot represent foldability such as crease depth.

[0090] The key technology of this invention is to stretch the material at the maximum speed in one direction at the lowest temperature to achieve maximum unidirectional orientation.

[0091] It provides a high HCMR value in one direction of the extremely anisotropic PET film.

[0092] The above is the general concept of the present invention. Based on this, detailed embodiments and comparative examples are provided below to further illustrate the present invention.

[0093] Example 1

[0094] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 100°C for 40 seconds; the second stretching step involves stretching at 100°C at a stretching speed of 14.1 mm / s in only one direction (TD) by 4.5 times; and the third stretching step involves annealing at 200°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0095] Example 2

[0096] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 100°C for 40 seconds; the second stretching step involves stretching at 100°C at a stretching speed of 14.1 mm / s in only one direction (TD) by 5.5 times; and the third stretching step involves annealing at 200°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0097] Example 3

[0098] A PET protective film for flexible electronic devices is prepared by means of the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 90°C for 40 seconds; the second stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in only one direction (TD) by 4.5 times; and the third stretching step involves annealing at 200°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0099] Example 4

[0100] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 100°C for 40 seconds; the second stretching step involves stretching at 100°C at a stretching speed of 14.1 mm / s in only one direction (MD) by 1.2 times; the third stretching step involves stretching at 100°C at a stretching speed of 14.1 mm / s in the vertical direction (TD) by 4.5 times; and the fourth stretching step involves annealing at 200°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0101] Example 5

[0102] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 90°C for 40 seconds; the second stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in only one direction (MD) by 1.2 times; the third stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in the vertical direction (TD) by 4.5 times; and the fourth stretching step involves annealing at 220°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0103] Example 6

[0104] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 90°C for 40 seconds; the second stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in only one direction (MD) by 1.2 times; the third stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in the vertical direction (TD) by 5.0 times; and the fourth stretching step involves annealing at 220°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0105] Example 7

[0106] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 90°C for 40 seconds; the second stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in only one direction (TD) by 4.5 times; and the third stretching step involves annealing at 220°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0107] Comparative Example 1

[0108] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 90°C for 40 seconds; the second stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in only one direction (MD) by 3.0 times; the third stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in the vertical direction (TD) by 3.5 times; and the fourth stretching step involves annealing at 200°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0109] Comparative Example 2

[0110] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 90°C for 40 seconds; the second stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in only one direction (MD) by 3.5 times; the third stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in the vertical direction (TD) by 3.5 times; and the fourth stretching step involves annealing at 200°C for 30 seconds. A 50 μm PET film is obtained according to these processes and conditions, and its properties are shown in Table 1.

[0111] Comparative Example 3

[0112] A PET protective film for flexible electronic devices is prepared by means of the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 90°C for 40 seconds; the second stretching step involves stretching at 90°C at a stretching speed of 14.1 mm / s in only one direction (TD) by 5.5 times; and the third stretching step involves annealing at 220°C for 30 seconds. Due to film breakage, a 50 μm PET film could not be obtained according to these processes and conditions; its properties are shown in Table 1.

[0113] Comparative Example 4

[0114] A PET protective film for flexible electronic devices is prepared by the following steps: Unstretched cast PET sheets are placed into a stretching apparatus for stretching. The first stretching step involves preheating at 80°C for 40 seconds; the second stretching step involves stretching at 80°C at a stretching speed of 14.1 mm / s in only one direction (TD) by 4.5 times; and the third stretching step involves annealing at 220°C for 30 seconds. Due to film breakage, a 50 μm PET film could not be obtained according to these processes and conditions; its properties are shown in Table 1.

[0115] Table 1

[0116] Where *1: the best performance and orientation of the example; *2: the performance and orientation of the currently used conventional PET film, which means that 108.2 is the depth value of the PET protective film in existing foldable smartphones.

[0117] Table 1 confirms that the HCMR value and crease depth performance are well matched, 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, extremely low HCMR (extremely high crease depth) does not completely match this trend (Comparative Example 1 and Comparative Example 2).

[0118] HCMR represents the crease depth performance and foldability of polymer films.

[0119] The analysis methods and conditions for the values ​​in Table 1 are as follows;

[0120] (1) HCMR (Hybrid chordal modulus ratio):

[0121] For the 2R foldable smartphone, the tensile strain at the outer radius was calculated, and the result was 2.5% strain.

[0122] The foldable smartphone has an inner radius of 2mm and a PET film thickness of 0.05mm.

[0123] Tensile modulus is measured using common methods and conditions.

[0124] The initial Young's modulus value was obtained using ASTM D882.

[0125] The tensile chordal modulus at 2.5% strain is also the same.

[0126] The HCMR calculation equation is:

[0127] The chordal 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%, i.e., A = 2.5.

[0128] 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 μm PET film is used with a tensile strength of 5.0%.

[0129] (2) Crease depth:

[0130] A 2R foldable smartphone model was fabricated for this purpose. Guide lines were marked on the model, and a PET film was laminated with the OCA film commonly used in foldable smartphones. The sample was cut into 50mm x 30mm pieces according to the design orientation. The OCA side of the sample was attached to the model along the guide lines. The model was folded and placed in an 8585 testing chamber (85°C, 85% RH) for 10 days. Afterward, the model was unfolded to a precise 180°, and the crease depth in the center area was measured using a laser scanning microscope (Keyence 3D scanner, Japan).

[0131] (3) Heat shrinkage rate: measured at 150℃ for 30 minutes.

[0132] Other properties are measured using common methods and conditions.

[0133] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details.

Claims

1. A PET protective film for flexible electronic devices, characterized in that, This PET protective film meets the following requirements: In T g ~T g When stretched within a temperature range of +30℃, the PET protective film has an HCMR value of more than 60% in at least one direction, provided that the stretching ratio in the TD direction is 3.0 to 5.0 times that in the MD direction. Among them, T g This indicates the glass transition temperature of the PET protective film; HCMR represents the tangential modulus ratio of the mixture, calculated using the following formula: The chordal modulus @A% strain represents the tensile modulus when the tensile strain is A%, and the initial Young's modulus represents 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 flexible electronic devices according to claim 1, characterized in that, The thickness of the PET protective film is 24μm to 126μm.

4. The PET protective film for flexible electronic devices according to claim 1, characterized in that, The haze of this PET protective film is less than 2.

0.

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

6. The PET protective film for flexible electronic devices according to claim 1, characterized in that, The PET protective film exhibits a heat shrinkage rate of less than 2.0% under conditions of 150°C for 30 minutes.

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

8. A method for preparing a PET protective film for a flexible electronic device as described in any one of claims 1-7, characterized in that, The method includes the following steps: Unstretched cast PET sheets are preheated and then stretched 3.0 to 5.0 times in the TD direction within a temperature range of Tg to Tg+30℃. After annealing, a PET protective film for flexible electronic devices is obtained. This PET protective film satisfies the requirement of having an HCMR value of more than 60% in at least one direction. The annealing temperature should be at least 10°C higher than the stretching temperature, and the annealing time should not exceed 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 includes the following steps: Unstretched cast PET sheets are stretched using a stretching machine, preheated, and then subjected to T... g ~T g The film is stretched 1.1-2 times in the MD direction and 3.0-6.0 times in the TD direction within a temperature range of +30℃, and then annealed to obtain a PET protective film for flexible electronic devices. The PET protective film satisfies the requirement of having 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 includes the following steps: Unstretched cast PET sheets are stretched using a stretching device. First, they are preheated at 90–100°C, and then stretched at a stretching temperature of 90–100°C and a stretching speed of 13–15 mm / s, with a stretching ratio of 1.1–2 times in the MD direction and 3.0–6.0 times in the TD direction. Finally, they are annealed at 200–220°C for 30–60 seconds to obtain a PET protective film for flexible electronic devices. This PET protective film meets the requirement of having an HCMR value of over 60% in the TD direction.

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