Polyethylene naphthalate film and preparation method thereof

Abstract

The present invention provides a polyethylene naphthalate (PEN) film and a preparation method thereof, comprising a three-step process of pre-crystallization, rapid thermal stretching, and annealing. The present invention heats a PEN cast sheet to a temperature range above the glass transition temperature and maintains the temperature for 15-60 minutes to form small-sized crystallites inside the PEN cast sheet. These crystallites can serve as nucleation points for stretch-induced crystallization, thereby improving the crystallization rate and uniformity during subsequent stretching. The crystallized PEN cast sheet is then rapidly stretched at 140-170°C at a deformation rate of 0.1-1s. ‑1 The PEN film obtained after rapid hot stretching is then annealed at a temperature between 150°C and 180°C. This helps further refine the oriented crystal structure, increase crystallinity, and effectively release internal stress, thereby improving the film's dimensional stability and heat resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane materials and preparation thereof, in particular to a polyethylene naphthalate film and a preparation method thereof. Background Art

[0002] Polyethylene naphthalate (PEN) is a high-performance polyester material. Due to its excellent mechanical strength, heat resistance, gas barrier properties and electrical insulation properties, it has broad application potential in flexible electronics, photovoltaic packaging, display substrates, high-end packaging and other fields. Compared with traditional polyethylene terephthalate (PET), PEN has a higher glass transition temperature (T g ), better heat resistance, and lower water vapor transmission rate give it an advantage in the high-end functional film market. However, the performance of PEN film is highly dependent on the preparation process, especially the control of crystallinity, orientation, and residual internal stress, which significantly affect its final mechanical and thermal properties.

[0003] Existing PEN film preparation methods typically use a melt extrusion-casting-hot stretching process, in which hot stretching-induced crystallization is a key step in improving film strength and modulus. Traditional methods mainly use uniaxial or biaxial stretching to orient the molecular chains in a specific direction and crystallize under the action of a thermal field. However, due to the high glass transition temperature (approximately 120°C) and slow crystallization rate of PEN, unoptimized stretching processes often make it difficult to achieve uniform and sufficient crystallization in a short period of time, resulting in limited mechanical properties of the film. In addition, during the traditional stretching process, if there are insufficient crystal nuclei, the molecular chains may have difficulty in rapid crystallization, forming amorphous regions, which affects the dimensional stability and heat resistance of the film.

[0004] Therefore, how to prepare high-modulus, high-strength PEN films to meet the application requirements of high-end functional materials has become an urgent problem to be solved in the current technical field. Summary of the Invention

[0005] In response to the above-mentioned problems existing in the prior art, the present invention provides a polyethylene naphthalate film and a preparation method thereof to prepare a PEN film with high crystallinity and high orientation, thereby significantly improving the mechanical properties and crystallinity of the PEN film, making it suitable for flexible electronics, photovoltaic packaging, high-end electrical insulation and other fields.

[0006] The specific content of the invention is as follows:

[0007] In a first aspect, the present invention provides a method for preparing a polyethylene naphthalate film. The method comprises:

[0008] Pre-crystallization treatment: The PEN sheet is heated to a temperature above the glass transition temperature and kept warm for 15-60 minutes to allow pre-crystallization to occur inside the PEN sheet, forming a preheated PEN film with small-sized crystallites.

[0009] Rapid thermal stretching: 0.1-1s -1 The preheated PEN film is rapidly stretched at a deformation rate to form a stretched PEN film;

[0010] Annealing treatment: performing annealing treatment on the stretched PEN film to obtain the polyethylene naphthalate film.

[0011] The heating temperature of the pre-crystallization treatment is 145-210° C., and the holding time is 30-60 min.

[0012] Optionally, the rapid stretching is performed at 140-170°C.

[0013] Optionally, the stretching ratio of the rapid stretching is 2-6 times.

[0014] Optionally, the annealing treatment is performed at a temperature of 150-180° C. and for a time of 10-30 min.

[0015] Optionally, the PEN casting sheet has a thickness of 80-300 microns.

[0016] Optionally, the intrinsic viscosity of the PEN masterbatch used to prepare the PEN flakes is 0.60-0.80 dL / g.

[0017] In a second aspect, the present invention provides a polyethylene naphthalate film, which is obtained by the preparation method described in the first aspect.

[0018] Optionally, the polyethylene naphthalate film has a modulus of 9.1-12.0 GPa and a film strength of 315-452 MPa.

[0019] Optionally, the polyethylene naphthalate film has a crystallinity of 32-42% and a grain orientation factor of 0.65-0.86.

[0020] The present invention provides a polyethylene naphthalate film, the preparation method of which includes a three-step process of "pre-crystallization treatment - rapid thermal stretching - annealing treatment". The present invention heats the PEN casting sheet to a temperature higher than the glass transition temperature (T g) and maintain the temperature range (145-210 ℃) for 15-60 minutes. During this process, small-sized microcrystals are formed inside the PEN sheet. These microcrystals can serve as nucleation points for stretch-induced crystallization, improving the crystallization rate and uniformity during subsequent stretching. The crystallized PEN sheet is then rapidly stretched in the range of 140-170 ℃, with the deformation rate controlled at 0.1-1s -1 The film is then stretched to a ratio of 2-6. This step highly orients the PEN molecular chains along the stretching direction and promotes the formation of oriented crystals, thereby improving the film's modulus and strength. Finally, the present invention anneals the PEN film obtained after rapid hot stretching at a temperature between 150°C and 180°C for 10-30 minutes. This step further refines the oriented crystal structure, increases crystallinity, and effectively relieves internal stress, thereby improving the film's dimensional stability and heat resistance.

[0021] Compared with the conventional stretch-induced crystallization method, the above-mentioned preparation method provided by the present invention has the following beneficial effects:

[0022] (1) Improving the modulus and strength of the film: Microcrystals are formed during the preheating stage, which serve as nucleation points during the stretching process, promoting the uniform growth of oriented crystals and highly oriented molecular chains, significantly improving the tensile modulus and fracture strength of the PEN film. Compared with traditional methods, the resulting PEN film exhibits higher rigidity and tensile strength, making it suitable for applications such as high-end flexible electronics and photovoltaic packaging.

[0023] (2) Optimizing crystallization behavior and improving crystallinity: In the stretching process of conventional PEN films, insufficient crystallization may lead to a decrease in the mechanical properties of the film. However, the present invention induces the formation of small-sized microcrystals during the preheating stage, combines rapid stretching to promote oriented crystallization, and further improves the crystal structure during the annealing process, thereby significantly improving the crystallinity of the PEN film and enhancing its dimensional stability, heat resistance and mechanical strength.

[0024] (3) Reduce residual stress and improve dimensional stability: The optimized annealing process (temperature 150-180℃, time 10-30 minutes) can effectively release the internal stress remaining during the stretching process, avoid warping, shrinkage or cracking of the film, and improve the stability of the film during subsequent processing and use, making it more adaptable in applications such as high-precision electronic devices and optical films.

[0025] (4) Wide range of applications and strong market competitiveness: Since the PEN film of the present invention has high modulus, high strength, high crystallinity, low residual stress and excellent heat resistance, it can be widely used in flexible circuit substrates, photovoltaic packaging films, high-end electrical insulation materials, display substrates and other fields, providing a more excellent material choice for the high-end functional film market. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 A flow chart of a polyethylene naphthalate film and a preparation method thereof provided in an embodiment of the present invention is shown;

[0028] Figure 2 shows an XRD graph of a polyethylene naphthalate film provided in an embodiment of the present invention;

[0029] Figure 3 The DSC curve of the polyethylene naphthalate film provided by the embodiment of the present invention is shown. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means a limitation on the present invention and its application or use. Based on the embodiments of the present invention, any product that is identical or similar to the present invention and is obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts falls within the scope of protection of the present invention. In addition, all other embodiments obtained by ordinary technicians in this field without carrying out creative work fall within the scope of protection of the present invention.

[0031] Where specific experimental steps or conditions are not specified in the examples, the conventional experimental steps or conditions described in the prior art in the art may be used. Reagents and other instruments used, for which the manufacturer is not specified, are commercially available conventional reagent products. Furthermore, the accompanying drawings are merely schematic illustrations of embodiments of the present invention and are not necessarily drawn to scale. Identical reference numerals in the figures denote identical or similar parts, and their repeated descriptions will be omitted. Some block diagrams shown in the accompanying drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0032] Technologies, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such technologies, methods, and equipment should be considered part of the description of the present invention.

[0033] In the description of the present invention, it should be understood that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0034] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0035] Since PEN has a high glass transition temperature (about 120°C) and a slow crystallization rate, it is often difficult to achieve uniform and sufficient crystallization in a short period of time without optimizing the stretching process, resulting in limited mechanical properties of the film. In addition, during the traditional stretching process, if there are insufficient crystal nuclei, the molecular chains may have difficulty in rapid crystallization, forming amorphous regions, thereby affecting the dimensional stability and heat resistance of the film. Traditional methods are usually accompanied by high residual internal stress after stretching, which can easily cause the film to warp, crack or shrink in size in subsequent applications, limiting its application in high-end flexible electronics, optical devices, photovoltaic packaging and other fields. Therefore, in the preparation process of the PEN film, the present invention optimizes the crystallization behavior, improves the crystallinity, and reduces the internal stress to prepare a high-modulus, high-strength PEN film to meet the application requirements of high-end functional materials; the specific implementation content is as follows:

[0036] In a first aspect, the present invention provides a method for preparing a polyethylene naphthalate film. Figure 1 FIG. 1 shows a flow chart of a method for preparing a polyethylene naphthalate film according to an embodiment of the present invention, as shown in FIG. Figure 1 As shown, the preparation method includes a three-step process of pre-crystallization treatment - rapid thermal stretching - annealing treatment, specifically including:

[0037] S1. The PEN casting sheet is heated to a temperature higher than the glass transition temperature and kept at this temperature for 15-60 minutes to allow pre-crystallization to occur inside the PEN casting sheet, thereby forming a preheated PEN film with small-sized crystallites.

[0038] In specific implementation, this embodiment uses PEN sheets with a thickness of 80-300 microns to prepare polyethylene naphthalate films. The intrinsic viscosity of the PEN masterbatch used to prepare the PEN sheets is 0.60-0.80 dL / g to ensure the processability of the material and the stability of the final product.

[0039] In specific implementation, this embodiment heats the PEN sheet to a temperature above the glass transition temperature and maintains this temperature for 15-60 minutes. The high temperature promotes the formation of small-sized crystallites within the PEN sheet. The formed small-sized crystallites serve as nucleation points during the stretching process, promoting the uniform growth of oriented crystals, resulting in highly oriented molecular chains and significantly improving the tensile modulus and fracture strength of the PEN film.

[0040] In some embodiments, the heating temperature of the pre-crystallization treatment is 145-210° C., and the holding time is 30-60 min to ensure that a sufficient number of small-sized crystallites are formed inside the PEN flake.

[0041] S2, 0.1-1s -1 The preheated PEN film is rapidly stretched at a deformation rate to form a stretched PEN film.

[0042] In specific implementation, the PEN casting sheet is pre-crystallized in step S1, and small-sized microcrystals are induced to form inside the PEN casting sheet. The PEN casting sheet is further rapidly stretched to promote oriented crystallization. In this embodiment, the pre-crystallized PEN casting sheet is rapidly stretched at 140-170°C, with a stretching ratio of 2-6 times and a deformation rate of 0.1-1 s. -1 , which makes the PEN molecular chains highly oriented along the stretching direction and promotes the formation of oriented crystals, thereby improving the modulus and strength of the film.

[0043] S3. Annealing the stretched PEN film to obtain the polyethylene naphthalate film.

[0044] In specific implementation, the PEN casting sheet is pre-crystallized in step S1, and small-sized crystallites are induced to form inside the PEN casting sheet. The PEN casting sheet is then rapidly stretched to promote oriented crystallization. In this embodiment, the pre-crystallized PEN casting sheet is rapidly stretched at 140-170°C, with a stretching ratio of 2-6 times and a deformation rate of 0.1-1s. -1 , which makes the PEN molecular chains highly oriented along the stretching direction and promotes the formation of oriented crystals, thereby improving the modulus and strength of the film.

[0045] In a second aspect, the present invention provides a polyethylene naphthalate film obtained by the preparation method described in the first aspect. The resulting polyethylene naphthalate film has a modulus of 9.1-12.0 GPa, a film strength of 315-452 MPa, a crystallinity of 32-42%, and a grain orientation factor of 0.65-0.86. This film has high application value in flexible electronics, photovoltaic packaging, display substrates, high-end packaging, and other fields.

[0046] In order to enable those skilled in the art to more clearly understand the present invention, the polyethylene naphthalate film and the preparation method thereof according to the present invention are now described in detail through the following examples.

[0047] Example 1

[0048] A PEN sheet with a thickness of 100 μm was prepared using a masterbatch with an intrinsic viscosity of 0.7 dL / g. The PEN sheet was placed in a film stretching machine and preheated at 150°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1 , the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 20 minutes to obtain a PEN film.

[0049] Example 2

[0050] A 200 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.65 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1 , the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 20 minutes to obtain a PEN film.

[0051] Example 3

[0052] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 180°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1 , the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 20 minutes to obtain a PEN film.

[0053] Example 4

[0054] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 15 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1, the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 20 minutes to obtain a PEN film.

[0055] Example 5

[0056] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 160°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1 , the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 20 minutes to obtain a PEN film.

[0057] Example 6

[0058] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 0.1 s. -1 , the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 20 minutes to obtain a PEN film.

[0059] Example 7

[0060] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1 , the stretching ratio is 4 times; finally, the stretched PEN film is annealed at 160°C for 20 minutes to obtain a PEN film.

[0061] Example 8

[0062] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1, the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 170°C for 20 minutes to obtain a PEN film.

[0063] Example 9

[0064] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 145°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1 , the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 25 minutes to obtain a PEN film.

[0065] Example 10

[0066] A 100 μm thick PEN film prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 210°C for 30 minutes to obtain a preheated PEN film with small-sized crystallites inside. The film was then hot stretched at 170°C to obtain a stretched PEN film. During the stretching process, the film deformation rate was maintained at 1 s. -1 , the stretching ratio is 5 times; finally, the stretched PEN film is annealed at 160°C for 25 minutes to obtain a PEN film.

[0067] Performance testing:

[0068] The following performance tests were performed on the PEN films obtained in Examples 1-10 above:

[0069] (1) After cutting the PEN film into dumbbell-shaped strips, its mechanical properties were tested on a tensile testing machine (GB / T1040.1-2018).

[0070] (2) Synchrotron radiation X-ray diffraction (WAXD) tests were performed on the preheated PEN film and PEN casting to obtain the crystal structure of the preheated PEN film, such as crystal form, grain orientation factor, etc. The obtained one-dimensional XRD curve is shown in Figure 2 .like Figure 2 As shown in the figure, only one amorphous peak appears in the XRD curve of the PEN cast sheet, proving that no crystals have formed. However, three obvious diffraction peaks appear in the curve of the preheated PEN film, corresponding to the (010), (100), and (110) crystal planes of the α-triclinic form, confirming the formation of crystals.

[0071] (3) Weigh 5 mg of PEN film and PEN sheet respectively, and use differential scanning calorimetry (DSC) to test the crystallinity. The obtained DSC curve is shown in Figure 3 .like Figure 3 As shown in the figure, the PEN cast sheet has obvious cold crystallization peaks and melting peaks, which indicates that its crystallinity is low. However, the PEN film has no cold crystallization peaks and only melting peaks, which indicates that the crystallinity of this sample is high.

[0072] The test results are shown in Table 1:

[0073] Table 1 Synthesis process and properties of stretched PEN films of Examples 1-10

[0074]

[0075] By comparing the performance data of the PEN films obtained in Examples 1-10, it can be seen that the embodiments of the present invention control (optimize) the heating temperature and holding time of the pre-crystallization treatment, the temperature of rapid stretching, the tensile deformation rate and stretching ratio, the annealing temperature and annealing time, to obtain high-performance PEN films with a modulus of 9.1-12.0 GPa, a strength of 315-452 MPa, a crystallinity of 32-42%, and a grain orientation factor of 0.65-0.86. This ensures that the PEN film provided by the present invention has high application value in the fields of flexible electronics, photovoltaic packaging, display substrates, high-end packaging, etc.

[0076] Comparative Example 1

[0077] A 50 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.5 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes. It was then hot stretched at 145°C with a film deformation rate of 1 s -1 , the stretching ratio is 5 times; then it is annealed at 160°C for 20 minutes to obtain a PEN film.

[0078] Comparative Example 2

[0079] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 100°C for 120 min. It was then hot stretched at 145°C with a film deformation rate of 1 s -1 , the stretching ratio is 5 times; then it is annealed at 160°C for 20 minutes to obtain a PEN film.

[0080] Comparative Example 3

[0081] A PEN film with a thickness of 100 μm and prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes. It was then hot stretched at 100°C with a film deformation rate of 1 s -1, the stretching ratio is 5 times. Under this condition, the PEN film is easy to break and cannot be formed into a film.

[0082] Comparative Example 4

[0083] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes. It was then hot stretched at 145°C with a film deformation rate of 1 s -1 , the stretching ratio is 7 times. Under this condition, PEN becomes fibrous and cannot form a film.

[0084] Comparative Example 5

[0085] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes. It was then hot stretched at 145°C with a film deformation rate of 1 s -1 , the stretching ratio is 5 times; then it is annealed at 110°C for 40 minutes to obtain a stretched PEN film.

[0086] Comparative Example 6

[0087] A 100 μm thick PEN sheet prepared from a masterbatch with an intrinsic viscosity of 0.7 dL / g was placed in a film stretching machine and preheated at 150°C for 30 minutes. It was then hot stretched at 145°C with a film deformation rate of 1 s -1 , the stretching ratio is 5 times; then it is annealed at 240°C for 40 minutes to obtain a stretched PEN film.

[0088] Performance testing:

[0089] The following performance tests were performed on the PEN films obtained in Comparative Examples 1-6 above:

[0090] (1) After cutting the PEN film into dumbbell-shaped strips, its mechanical properties were tested on a tensile testing machine (GB / T1040.1-2018).

[0091] (2) Perform X-ray diffraction (XRD) test on the PEN film to obtain the crystal structure of the PEN film, such as crystal form, grain orientation factor, etc.

[0092] (3) Weigh 5 mg of PEN film and test the crystallinity of the film using differential scanning calorimetry (DSC).

[0093] The test results are shown in Table 2:

[0094] Table 2 Synthesis process and PEN film properties of Comparative Examples 1-6

[0095]

[0096] In Comparative Example 1, the intrinsic viscosity of the masterbatch and the thickness of the PEN cast sheet film are not within the ranges provided in the embodiments of the present invention (the thickness of the PEN cast sheet film is 80-300 microns, and the intrinsic viscosity of the PEN masterbatch is 0.60-0.80 dL / g). As a result, the PEN film obtained in this comparative example has good fluidity but poor tensile properties, and cannot form highly oriented crystals. The resulting PEN film has a low modulus and poor strength.

[0097] In Comparative Example 2, the preheating temperature is 100 degrees Celsius, which is lower than the glass transition temperature, and no effective crystal nuclei can be formed, resulting in low crystallinity during the subsequent thermal stretching process. Therefore, the PEN film product has a low modulus and poor performance.

[0098] In Comparative Example 3, the stretching temperature of the preheated PEN film is 100 degrees Celsius, which is lower than the glass transition temperature. This results in the PEN casting sheet being unable to deform and break at this strain rate, and thus failing to form a film.

[0099] In Comparative Example 4, the stretching ratio was too high, resulting in film breakage.

[0100] In Comparative Example 5, the annealing temperature is too low, the crystal nucleus cannot be perfected, and the lattice defects lead to poor mechanical properties.

[0101] In Comparative Example 6, the annealing temperature was too high, which resulted in the melting and destruction of the formed crystalline structure, resulting in low modulus and poor strength of the obtained PEN film.

[0102] According to Examples 1-10 and Comparative Examples 1-6, the preparation method of the polyethylene naphthalate film disclosed in the present invention can prepare a PEN film with a modulus of 9.1-12.0 GPa, a strength of 315-452 MPa, a crystallinity of 32-42%, and a grain orientation factor of 0.65-0.86. The obtained PEN film has high modulus, high strength, high crystallinity, low residual stress and excellent heat resistance, and can be widely used in flexible circuit substrates, photovoltaic packaging films, high-end electrical insulation materials, display substrates and other fields, providing a more excellent material choice for the high-end functional film market.

[0103] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

[0104] For simplicity of description, the method embodiments are described as a series of actions. However, those skilled in the art should be aware that the present invention is not limited by the order of the actions described, as certain steps can be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also be aware that the embodiments described in this specification are preferred embodiments, and the actions and components involved are not necessarily required for the present invention.

[0105] The above is a detailed introduction to a polyethylene naphthalate film and a preparation method thereof provided by the present invention. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the ideas of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A method for preparing a polyethylene naphthalate film, characterized in that: The preparation method comprises: Pre-crystallization treatment: Heat the PEN sheet to 145-210 ° C and keep it warm for 30-60 minutes to allow pre-crystallization to occur inside the PEN sheet, forming a preheated PEN film with small-sized crystallites; Rapid thermal stretching: 0.1-1s -1 The preheated PEN film is rapidly stretched at a deformation rate to form a stretched PEN film; Annealing treatment: performing annealing treatment on the stretched PEN film to obtain the polyethylene naphthalate film; The rapid stretching is performed at 140-170°C; The stretching ratio of the rapid stretching is 2-6 times; The annealing treatment temperature is 150-180°C and the time is 10-30 min; The thickness of the PEN casting sheet is 80-300 microns; The intrinsic viscosity of the PEN masterbatch used to prepare the PEN flakes is 0.60-0.80 dL / g.

2. A polyethylene naphthalate film, characterized in that: The polyethylene naphthalate film is obtained by the preparation method according to claim 1.

3. The polyethylene naphthalate film according to claim 2, characterized in that: The polyethylene naphthalate film has a modulus of 9.1-12.0 GPa and a film strength of 315-452 MPa.

4. The polyethylene naphthalate film according to claim 2, wherein: The polyethylene naphthalate film has a crystallinity of 32-42% and a grain orientation factor of 0.65-0.86.

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