Biaxially oriented polyester film and method for manufacturing the same

Abstract

The present invention provides a biaxially oriented polyester film and a method for manufacturing the same, which can achieve better productivity evaluations and enhance product competitiveness. [Solution] This invention provides a biaxially oriented polyester film manufactured from a polyester resin mixture. The polyester resin mixture comprises a polyester material and a crystallization-inhibiting material. When the biaxially oriented polyester film is analyzed by differential scanning calorimetry, the difference between the observed first recrystallization temperature and the second recrystallization temperature is greater than 15°C and less than or equal to 23°C when cooled from a molten state at cooling rates of 20°C / min and 40°C / min, respectively. Furthermore, a method for manufacturing the biaxially oriented polyester film is also provided.

Description

Technical Field

[0001] The present invention relates to a biaxially stretched polyester film and a method for producing the same.

Background Art

[0002] In the production of biaxially stretched polyester films, how to improve the crystallinity is an important issue. For example, when a biaxially stretched polyester film is cooled from a molten state at cooling rates of 20°C / min and 40°C / min respectively, if the difference between the two observed recrystallization temperatures is less than 15°C, the crystallinity is too high, which makes film breakage prone to occur during the production process. In addition, environmental issues have gradually attracted more attention, and how to design a biaxially stretched polyester film with product competitiveness has become a challenge.

Summary of the Invention

Problems to be Solved by the Invention

[0003] In the production of biaxially stretched polyester films, how to improve the crystallinity is an important issue. For example, when a biaxially stretched polyester film is cooled from a molten state at cooling rates of 20°C / min and 40°C / min respectively, if the difference between the two observed recrystallization temperatures is less than 15°C, the crystallinity is too high, which makes film breakage prone to occur during the production process. In addition, environmental issues have also gradually attracted attention.

Means for Solving the Problems

[0004] The present invention provides a biaxially stretched polyester film that can obtain better productivity evaluation and further have good product competitiveness, and a method for producing the same.

[0005] The present invention provides a biaxially oriented polyester film manufactured from a polyester resin mixture. The polyester resin mixture comprises a polyester material and a crystallization inhibitor, and when the biaxially oriented polyester film is analyzed by differential scanning calorimetry and cooled from a molten state at cooling rates of 20°C / min and 40°C / min, respectively, the difference between the observed first recrystallization temperature and the second recrystallization temperature is greater than 15°C and less than or equal to 23°C.

[0006] In one embodiment of the present invention, the crystal suppressing material accounts for 2 mol% to 8 mol% of the polyester resin mixture in terms of molar percentage.

[0007] In one embodiment of the present invention, the crystal suppression material includes isophthalic acid, neopentyl glycol, diethylene glycol, 1,4-cyclohexanedimethanol, or a combination thereof.

[0008] In one embodiment of the present invention, the crystal suppression material is a combination of isophthalic acid and diethylene glycol.

[0009] In one embodiment of the present invention, the polyester material includes recycled polyester material, virgin polyester material, or a combination thereof.

[0010] In one embodiment of the present invention, the first recrystallization temperature is greater than 170°C.

[0011] In one embodiment of the present invention, the second recrystallization temperature is 150°C to 175°C.

[0012] In one embodiment of the present invention, the thickness of the biaxially oriented polyester film is greater than 10 micrometers.

[0013] The present invention provides a method for manufacturing a biaxially oriented polyester film, comprising the following steps: a step of producing an unstretched thick plate by melting and extruding one or more polyester particles, wherein the polyester particles include at least chemically recycled polyester particles, the chemically recycled polyester particles have a higher content of crystallization inhibiting material than mechanically recycled polyester particles, and the melted polyester particles are a polyester resin mixture; and a step of biaxially stretching the thick plate to form a biaxially oriented polyester film, wherein the biaxially oriented polyester film is analyzed by differential scanning calorimetry, and the difference between the observed first recrystallization temperature and the second recrystallization temperature when cooled from a molten state at cooling rates of 20°C / min and 40°C / min, respectively, is greater than 15°C and less than or equal to 23°C.

[0014] In one embodiment of the present invention, the polyester particles further include one or both of virgin polyester particles and mechanically recycled polyester particles. [Effects of the Invention]

[0015] Based on the above, the present invention introduces a crystal-inhibiting material into a polyester resin mixture, thereby improving the crystallinity of biaxially oriented polyester films through its crystal-inhibiting effect, achieving better productivity evaluations, and providing superior product competitiveness. For example, recycled materials contain many unavoidable impurities (plastics other than polyester due to uncertainty in classification, inorganic or organic impurities due to uncertainty in washing, and pigments and dyes that the recycled material itself contains due to the requirements of the original product). Therefore, when polyester films are manufactured using recycled materials directly, the crystallinity of the polyester resin mixture is greatly increased by the impurities themselves, leading to serious film breakage during stretching. In some situations, the recycled material itself may contain a small amount of crystal-inhibiting material, but the content is so small that it is still insufficient to overcome the problem. Therefore, the present invention makes it possible to maintain a balance between productivity and / or mechanical strength by introducing an additional crystal-inhibiting material when manufacturing chemically recycled polyester pellets. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic flowchart of a method for manufacturing a biaxially oriented polyester film according to one embodiment of the present invention. [Modes for carrying out the invention]

[0017] The following detailed description provides illustrative examples that reveal specific details, not for the purpose of explanation but for the purpose of providing a clear understanding of the various principles of the present invention. However, as will be apparent to those skilled in the art, the present invention can be practiced in other embodiments that benefit from this disclosure and deviate from the specific details disclosed herein. Furthermore, in order to avoid obscuring the explanation of the various principles of the present invention, descriptions of well-known apparatus, methods, and materials may be omitted.

[0018] In this specification, a range can be expressed as "approximately" from one specific number to "approximately" another specific number, or it can be expressed directly as up to one specific number and / or another specific number. When expressing such a range, another embodiment includes that specific number from that specific number to another specific number. Similarly, when expressing a value as an approximation using the antecedent "approximately," it is understood that the specific value forms another embodiment. Furthermore, it is understood whether the endpoints of each range are clearly related to or independent of other endpoints.

[0019] In this specification, non-restrictive terms (e.g., possible, can, for example, or other similar terms) mean non-essential or optional implementation, inclusion, addition, or presence.

[0020] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as that of a person of ordinary skill in the art to which this invention pertains. Furthermore, terms (as defined in commonly used dictionaries) should be interpreted as having the meaning consistent with their meaning in the relevant technical context, and should not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0021] Figure 1 is a schematic flow diagram of a method for manufacturing a biaxially oriented polyester film according to one embodiment of the present invention. Referring to Figure 1, in step S101, polyester particles are provided. The polyester particles may consist of at least recycled polyester material and may also contain virgin polyester material. Furthermore, the recycled polyester material may contain a crystal-inhibiting material. In step S102, the polyester particles are melted (also called a polyester resin mixture) and extruded to produce an unstretched thick plate. In step S103, the thick plate is biaxially stretched to form a biaxially oriented polyester film. Here, when a biaxially oriented polyester film is analyzed by differential scanning calorimetry (DSC) and cooled from a molten state at cooling rates of 20°C / min and 40°C / min, respectively, the difference between the two observed recrystallization temperatures is greater than 15°C and less than or equal to 23°C (for example, 16°C, 17°C, 18°C, 19°C, 20°C, 21°C, 22°C, 23°C, or any appropriate value greater than 15°C and less than or equal to 23°C). Therefore, the present invention introduces a crystallization-inhibiting material into a polyester resin mixture, and through its crystallization-inhibiting effect, the degree of crystallization of the biaxially oriented polyester film can be improved, resulting in better productivity evaluation and even better product competitiveness. Here, when cooled from a molten state at cooling rates of 20°C / min and 40°C / min, the observed recrystallization temperatures are the first recrystallization temperature and the second recrystallization temperature, respectively.

[0022] In some embodiments, the weight ratio of virgin polyester material relative to the polyester resin mixture is from 0% by weight to 50% by weight (for example, 0% by weight, 5% by weight, 10% by weight, 20% by weight, 45% by weight, 50% by weight, or any suitable value between 0% by weight and 50% by weight), and the weight ratio of recycled polyester material relative to the polyester resin mixture is from 50% by weight to 100% by weight (for example, 55% by weight, 60% by weight, 70% by weight, 80% by weight, 95% by weight, 100% by weight, or any suitable value between 50% by weight and 100% by weight), which meets the requirements of environmental protection. On the other hand, recycled polyester materials generally contain impurities that function as nucleating agents (especially when the content of chemically recycled polyester particles exceeds 10% by weight), therefore, the use of recycled polyester materials tends to increase the crystallinity of the thick plate before stretching, causing film breaking during the stretching process, which constitutes a bottleneck that is difficult to break through technically. However, in the present invention, by introducing a crystallization-inhibiting material, excellent productivity can be achieved while using a high content of recycled polyester material.

[0023] In some embodiments, the crystallization-inhibiting material comprises isophthalic acid (IPA, CAS No: 121-91-5), neopentyl glycol (NPG, CAS No: 126-30-7), diethylene glycol (DEG, CAS No: 111-46-6), 1,4-cyclohexanedimethanol (CHDM, CAS No: 105-08-8), or a combination thereof. Preferably, the crystallization-inhibiting material is a combination of isophthalic acid and diethylene glycol.

[0024] In some embodiments, the crystallization-inhibiting material accounts for 2 mol% to 8 mol% based on the molar percentage concentration of the polyester resin mixture (for example, 2 mol%, 3 mol%, 4 mol%, 5 mol%, 6 mol%, 7 mol%, 8 mol%, or any suitable value between 2 mol% and 8 mol%). Too little crystallization-inhibiting material may adversely affect subsequent manufacturing processes. For example, when the content is below the above lower limit, the crystallinity becomes too high, which easily causes film breakage during the stretching process; when the content exceeds the above upper limit, the crystallinity becomes too low, which easily leads to deteriorated physical properties. Therefore, within the above ratio range, the crystallinity can be controlled more reliably, and the competitiveness of the product can be further improved.

[0025] In some embodiments, the polyester resin mixture consists of a polyester material and a crystallization-inhibiting material, that is, the sum of the weights of the polyester material and the crystallization-inhibiting material is 100% by weight of the polyester resin mixture, but the present invention is not limited thereto. The polyester resin mixture may further comprise other additives. For example, the sum of the weights of the polyester material, the crystallization-inhibiting material and other additives (such as lubricants) may be 100% by weight of the polyester resin mixture.

[0026] In some embodiments, the first recrystallization temperature is higher than 170°C, preferably 170°C to 190°C, and may be higher than 175°C. The second recrystallization temperature is 150°C to 175°C, preferably 155°C to 175°C, and may be 165°C to 175°C. In the present invention, by introducing at least the crystallization-inhibiting material, good stretchability in the manufacturing process can be ensured even if the recrystallization temperature falls within the above-mentioned higher temperature range.

[0027] In some embodiments, the melting point of the biaxially stretched polyester film is 243°C to 253°C (for example, 243°C, 245°C, 247°C, 249°C, 251°C, 253°C, or any suitable value between 243°C and 253°C).

[0028] In some embodiments, the thickness of the biaxially oriented polyester film is 8 micrometers to 350 micrometers, and it is preferable that the thickness of the biaxially oriented polyester film be 10 micrometers to 350 micrometers, and may also be 50 micrometers to 350 micrometers, but the present invention is not limited thereto.

[0029] In some embodiments, the polyester resin mixture further includes a lubricant. The lubricant accounts for 0.05% to 2% by weight of the polyester resin mixture (e.g., 0.05%, 1%, 1.5%, 2%, or any suitable number between 0.05% and 2%), and the particle size is 0.05 to 5 micrometers (e.g., 0.05, 0.1, 0.3, 0.35, 0.4, 0.5, 5, or any suitable number between 0.05 and 5). The lubricant may be granular. For example, the lubricant may include silicon dioxide particles, calcium carbonate particles, barium sulfate particles, polystyrene particles, silica gel particles, acrylic particles, or a combination thereof.

[0030] In some embodiments, the biaxially oriented polyester film may have a single-layer structure, or it may have a multilayer structure design of a skin layer / base layer / skin layer, where both the base layer and the skin layer can be formed from the polyester resin mixture described above. Here, in the multilayer structure design, the thickness of the skin layer may account for 2% to 30% of the total thickness of the biaxially oriented polyester film (for example, 2%, 3%, 4%, 10%, 15%, 20%, 30%, or any appropriate number between 2% and 30%).

[0031] The specific details of each of the above steps will be explained in detail below, but these details are merely illustrative for clarity and are not intended to limit the present invention. A person with ordinary skill in the art to which the invention pertains can make appropriate adjustments and embellishments based on these details.

[0032] [Recycled polyester material]

[0033] A method for recycling polyester materials may include, for example, the collection of each type of waste polyester material. The waste polyester material can be classified according to its type, color, and / or former use. The classified waste polyester material can then be compressed and packaged. The packaged waste polyester material can then be transported to a waste treatment plant. The waste polyester material may include, for example, recycled PET bottles, but the present invention is not limited to these.

[0034] A method for recycling polyester material may further include removing objects (e.g., bottle caps, labels, and / or adhesives) from the polyester material. The waste polyester material is then physically or mechanically crushed. The crushed polyester material is then separated by an appropriate method (e.g., flotation). The crushed and separated waste polyester material is then dried to obtain processed recycled polyester material.

[0035] In one embodiment, the recycled polyester material can be further recycled from a biaxially oriented polyester film formed by the method described below (for example, by recycling the selvage portion after cutting).

[0036] In this specification, terms such as "polyester" and "polyester material" refer to any type of polyester, and in particular to aromatic polyesters, and here specifically to polyesters derived from purified terephthalic acid (PTA) and ethylene glycol (EG) (i.e., polyethylene terephthalate (PET)).

[0037] Furthermore, the polyester in this specification may be, for example, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or a combination thereof. In this embodiment, the aforementioned polyester is preferably polyethylene terephthalate, polytrimethylene terephthalate, or a combination thereof. Copolymers may also be used, which in particular refer to copolymers obtained by using two or more dicarboxylic acids and / or two or more diol components.

[0038] In some embodiments, recycled polyester materials (chemically recycled polyester particles, mechanically recycled polyester particles) obtained by the following methods can further form recycled polyester particles for subsequent thin film manufacturing processes.

[0039] [Method for producing chemically recycled polyester particles] (May include crystallization-inhibiting materials)

[0040] First, the recycled polyester material is chemically depolymerized. For example, the recycled polyester material and a depolymerization solution can be placed in a suitable depolymerization tank for chemical depolymerization. Here, the chemical depolymerization solution substantially decomposes the polyester molecules in the recycled polyester material, thereby achieving the depolymerization effect. Furthermore, it may be possible to obtain ester monomers (e.g., bis(2-Hydroxyethyl)terephthalate (BHET)) with relatively short molecular chains and / or combinations of one diacid unit and two diol units. In other words, the average molecular weight of the mixture after chemical depolymerization is substantially smaller than the average molecular weight of the recycled polyester material. The present invention does not limit the type of depolymerization solution. For example, hydrolysis can be carried out with water. Alternatively, alcoholic decomposition can be carried out with alcohols (methanol, ethanol, ethylene glycol, diethylene glycol, or mixtures thereof, etc.). In one embodiment, the depolymerization solution is preferably an alcohol. A more suitable alcoholic decomposition solution is ethylene glycol. One reason for this is that ethylene glycol can be a reactive monomer for producing virgin polyester particles (virgin PET chips).

[0041] Next, the product after the aforementioned chemical depolymerization reaction is subjected to an esterification reaction. However, it should be noted that the present invention is not limited to the complete depolymerization of all polyester materials. For example, the product after the aforementioned chemical depolymerization reaction can be transferred to a suitable esterification tank for esterification. In one embodiment, before transferring the product after the aforementioned chemical depolymerization reaction to the esterification tank, filtration can be performed through a filter to remove at least some impurities in the recycled polyester material, thereby reducing the concentration of non-polyester impurities. However, trace amounts of components with small particle sizes (e.g., materials such as metal impurities) may be present. In one embodiment, the pore size of the filter may be 1 to 10 micrometers. In some embodiments, after the aforementioned esterification reaction has proceeded for a certain period of time, other additives, such as crystallization inhibitors, lubricants, and stabilizers, can be further added to the esterification tank. In some embodiments, the chemical recycling method may further add terephthalic acid or other crystal-inhibiting monomers, such as isophthalic acid, neopentyl glycol, diethylene glycol, 1,4-cyclohexanedimethanol, or a combination thereof, the amount of which may account for 1 to 20 mol% of the polyester resin mixture to adjust the properties of the film, but the present invention is not limited thereto.

[0042] Next, the product after the esterification reaction is polymerized. For example, the product after the esterification reaction can be transferred to a suitable polymerization tank and polymerized. Finally, polyester particles are formed. For example, the polymerization reaction is carried out until the material in the tank has the corresponding intrinsic viscosity (IV). Then, the material in the tank is extruded and / or cut using a granulation method commonly used for general polymer particles to form polyester particles. In this embodiment, the intrinsic viscosity of the polyester particles formed by the chemical reprocessing step described above is typically 0.5 dL / g to 0.8 dL / g, for example, 0.6 dL / g to 0.78 dL / g.

[0043] In one embodiment, the polyester particles formed by the aforementioned chemical reprocessing step are called chemically recycled polyester chips, which have a high content of crystallization-inhibiting material. By adding chemically recycled polyester chips during film manufacturing, the degree of crystallinity of the polyester mixture during film manufacturing can be reduced, thus avoiding film breakage during the production process. Therefore, the polyester particles used to form a biaxially oriented polyester film include at least chemically recycled polyester chips.

[0044] [Method for producing mechanically recycled polyester particles]

[0045] In this embodiment, recycled polyester material can be melted and kept in a molten state. Then, the molten recycled polyester material can be filtered through a filter to remove solid impurities from the recycled polyester material. Subsequently, the filtered recycled polyester material can be extruded and granulated using an extruder (e.g., a commercially available single-screw extruder (SSE), twin-screw extruder (TSE), or other similar screw extruder, but not limited to these) to form mechanically recycled environmentally friendly polyester particles. In one embodiment, the recycled polyester material can be physically and mechanically crushed before melting to reduce the time and / or energy consumption required for melting the recycled polyester material. On the other hand, the method described above involves cutting, melting, filtering, and extruding the recycled polyester material to reshape it. In other words, the mechanically recycled environmentally friendly polyester particles basically do not undergo the process of repolymerization after depolymerization, and the manufacturing process consists only of melting, filtering, and granulation.

[0046] In this embodiment, the recycled polyester material undergoes only the processes of melting, filtering, and granulation in the aforementioned physical reprocessing step. Therefore, components originally present in the recycled polyester material (e.g., materials such as metal impurities) remain present in the mechanically recycled polyester particles. In other words, some properties of the mechanically recycled polyester particles may be identical or similar to some properties of the recycled polyester material originally used. In one embodiment, the average molecular weight of the recycled polyester material in the physical reprocessing step may not change substantially (compared to chemically recycled polyester particles). In other words, the recycled polyester material may have a relatively high viscosity (i.e., low fluidity) in the molten state. Therefore, using a filter with too small a pore size may reduce filtration efficiency. In one embodiment, the pore size of the screen mesh is preferably 10 to 100 micrometers, but the present invention is not limited thereto.

[0047] The mechanically recycled polyester particles produced by the aforementioned physical reprocessing process typically have a relatively high intrinsic viscosity. In this embodiment, the intrinsic viscosity of the mechanically recycled polyester particles may be between 0.5 dL / g and 0.8 dL / g, for example, between 0.6 dL / g and 0.78 dL / g.

[0048] In one embodiment, the polyester particles formed by the aforementioned physical reprocessing operation are also called mechanically recycled polyester chips.

[0049] [Method for producing virgin polyester particles]

[0050] The method can be the same as or similar to the method described above for producing chemically recycled polyester particles, the difference being that terephthalic acid and ethylene glycol can be directly added to the esterification tank to carry out the esterification reaction. In this embodiment, the intrinsic viscosity of the polyester particles formed by the above method may be between 0.5 dL / g and 0.8 dL / g, for example, between 0.6 dL / g and 0.78 dL / g.

[0051] In one embodiment, the polyester particles formed by the above method are also called virgin polyester chips.

[0052] [Method for manufacturing biaxially oriented polyester film]

[0053] In this embodiment, at least two types of polyester particles described above (for example, the chemically recycled polyester particles and one or both of the virgin polyester particles and the mechanically recycled polyester particles) can be used directly or mixed in appropriate ratios based on design requirements.

[0054] In one embodiment, polyester particles can be dried by heating and / or low-pressure drying. For example, polyester particles can be vacuum-dried at a temperature of approximately 120°C to 180°C for approximately 3 to 8 hours.

[0055] Then, the polyester particles (which may be, but are not limited to, dried polyester particles) are heated and melted, and then extruded. For example, the polyester particles can be melted in an extruder (for example, a commercially available single-screw extruder, but are not limited to this) at a temperature of approximately 200°C to 290°C, and the molten polyester can be extruded and molded to produce an unstretched thick plate.

[0056] Subsequently, the thick plate can be subjected to the corresponding biaxial stretching process using methods commonly used for general biaxially oriented polyester films. For example, the unstretched thick plate can first be introduced into a longitudinal stretcher and stretched longitudinally to form the corresponding thin film (i.e., a thick plate stretched longitudinally). Then, the thin film can be introduced into a transverse stretcher and stretched transversely to form a biaxially oriented polyester film (i.e., a thick plate stretched transversely).

[0057] More specifically, for example, the plate is stretched longitudinally by 2 to 6 times in the length direction (also called the MD) parallel to its transport direction at a temperature of approximately 70°C to 145°C, and then the plate is stretched longitudinally by 2 to 6 times in the width direction of the thin film (i.e., in another direction perpendicular to the length direction, also called the TD) at a temperature of approximately 90°C to 160°C. In other embodiments, the longitudinal stretching and transverse stretching are performed one after the other, but the present invention is not limited thereto. In other embodiments, the longitudinal stretching and transverse stretching can be performed simultaneously at a temperature of approximately 70°C to 160°C with a stretching ratio of 2 to 6 times. Furthermore, in some embodiments, the biaxially oriented polyester film may further include transverse and / or longitudinal pre-shrinkage.

[0058] After the above steps, the manufacturing of the biaxially oriented polyester film of this embodiment is largely complete. In one embodiment, a portion of the film (e.g., the selvage) may be cut off after transverse stretching, based on design requirements. The biaxially oriented polyester film can be stored by winding, or it can be further sold and / or used.

[0059] [Examples and Comparative Examples]

[0060] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited in any way to the following examples.

[0061] Each example and comparative example can form a corresponding biaxially oriented polyester film by the method described above. The difference lies in adjusting the ratio of polyester particles and crystal-suppressing material used, where the crystal-suppressing material is a combination of isophthalic acid and diethylene glycol.

[0062] Tests were conducted on the biaxially oriented polyester films of [Examples 1] to [Examples 8] and [Comparative Examples 1] to [Comparative Examples 2] in [Table 1]. As a result, it was found that significantly adjusting the stretching conditions made production control difficult and affected production efficiency. In the present invention, a crystal-suppressing material is introduced, and due to its crystal-suppressing effect, the degree of crystallinity of the biaxially oriented polyester film can be reliably improved without adjusting the stretching conditions, thereby achieving a better productivity evaluation and further enhancing product competitiveness. The test items were the film breakage rate of the biaxially oriented polyester film (displayed as "film breakage rate (times / 24 hours)" in [Table 1]), the recrystallization temperature of the biaxially oriented polyester film under different conditions (displayed as "recrystallization temperature Tc(20)(°C)" and "recrystallization temperature Tc(40)(°C)" in [Table 1]) and their difference (displayed as "difference value △Tc(°C)" in [Table 1]), and the physical properties of the biaxially oriented polyester film (displayed as "longitudinal breaking strength (kgf / mm²)" in [Table 1]). 2 )", "Young's modulus in the longitudinal direction (kgf / mm 2 )", "Transverse breaking strength (kgf / mm 2 ) and "Young's modulus in the transverse direction (kgf / mm²) 2 )" (displayed).

[0063] The molar percentage (mol%) concentration of the crystal-inhibiting material in the polyester resin mixture was determined by accurately weighing 1.000±0.0003g of the sample and 30±0.1ml of 0.1% triethylene glycol (TEG) / methanol solution and placing them in a steel bottle. The mixture was heated at 225°C for 2.5 hours, then cooled with cold water and allowed to stand. The lid of the steel bottle was opened, and the clear upper layer of solution was drawn into a glass bottle. After centrifuging the solution in the glass bottle, the composition of the crystal-inhibiting material in the clear upper layer was analyzed by gas chromatography (GC), and the content was calculated.

[0064] The film rupture rate is the number of film ruptures per unit time under the same stretching conditions (including the longitudinal or transverse stretching described above).

[0065] Productivity Evaluation: O: No adjustment of stretching conditions, the number of film ruptures per 24 hours was 3 or less. △: No adjustment of stretching conditions, the number of film ruptures per 24 hours was between 4 and 6, or adjustment of stretching conditions was necessary, but the number of film ruptures per 24 hours was 3 or less. ×: No adjustment of stretching conditions, the number of film ruptures per 24 hours was 7 or more, or adjustment of stretching conditions was necessary, but the number of film ruptures per 24 hours was 4 or more.

[0066] Recrystallization temperature Tc(20)(°C): After accurately weighing 8±1 mg of the sample, DSC analysis was performed. The temperature was increased from 25°C to 300°C at a heating rate of 20°C / min, and then cooled at 20°C / min to analyze the crystal peaks.

[0067] Recrystallization temperature Tc(40)(°C): After accurately weighing 8±1 mg of the sample, DSC analysis was performed. The temperature was increased from 25°C to 300°C at a heating rate of 20°C / min, and then cooled at 40°C / min to analyze the crystal peaks.

[0068] Breaking strength / Young's modulus: Tests were conducted according to ASTM D882. The test specimen dimensions were: width 15 mm, length 100 mm, tensile speed 200 mm / s.

[0069] [Table 1] JPEG2026139543000003.jpg177160

[0070] In summary, the present invention introduces a crystal-inhibiting material into a polyester resin mixture, thereby improving the degree of crystallinity of biaxially oriented polyester films through its crystal-inhibiting effect, leading to better productivity evaluations and enhanced product competitiveness. [Industrial applicability]

[0071] The biaxially oriented polyester film and its manufacturing method can be applied to the field of biaxially oriented polyester films. [Explanation of Symbols]

[0072] S101, S102, S103: Process

Claims

1. A biaxially oriented polyester film made of a polyester resin mixture, The polyester resin mixture comprises a polyester material and a crystal-inhibiting material, and when the biaxially oriented polyester film is analyzed by differential scanning calorimetry, the difference between the observed first recrystallization temperature and the second recrystallization temperature is greater than 15°C and less than or equal to 23°C when cooled from a molten state at cooling rates of 20°C / min and 40°C / min, respectively. Biaxially oriented polyester film.

2. The crystal-suppressing material comprises 2 mol% to 8 mol% of the polyester resin mixture in terms of molar percentage concentration. The biaxially oriented polyester film according to claim 1.

3. The crystal-suppressing material includes isophthalic acid, neopentyl glycol, diethylene glycol, 1,4-cyclohexanedimethanol, or a combination thereof. The biaxially oriented polyester film according to claim 1.

4. The aforementioned crystal-suppressing material is a combination of isophthalic acid and diethylene glycol. The biaxially oriented polyester film according to claim 1.

5. The aforementioned polyester material includes recycled polyester material, virgin polyester material, or a combination thereof. The biaxially oriented polyester film according to claim 1.

6. The first recrystallization temperature is above 170°C. The biaxially oriented polyester film according to claim 1.

7. The second recrystallization temperature is 150°C to 175°C. The biaxially oriented polyester film according to claim 1.

8. The thickness of the biaxially oriented polyester film is more than 10 micrometers. The biaxially oriented polyester film according to claim 1.

9. A step of providing polyester particles, wherein the polyester particles include at least chemically recycled polyester particles, and the chemically recycled polyester particles have a crystal-inhibiting material, A process for producing an unstretched thick plate by melting polyester particles and extruding them, wherein the molten polyester particles are a polyester resin mixture, and the above process, Includes, A step of forming a biaxially oriented polyester film by biaxially stretching the aforementioned thick plate, wherein the biaxially oriented polyester film is analyzed by differential scanning calorimetry, and when cooled from a molten state at cooling rates of 20°C / min and 40°C / min, respectively, the difference between the observed first recrystallization temperature and the second recrystallization temperature is greater than 15°C and less than or equal to 23°C. A method for manufacturing biaxially oriented polyester film.

10. The polyester particles further include one or both of virgin polyester particles and mechanically recycled polyester particles. A method for producing a biaxially oriented polyester film according to claim 9.

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