Polyester-based oriented films, methods of making same, and articles formed therefrom

Abstract

The invention relates to a polyester oriented film, a preparation method thereof and a product formed by the polyester oriented film. The polyester-based oriented film contains a copolymerized polyester resin obtained by copolymerization of a diol component and a dicarboxylic acid component, and has a tear index of less than 2.5. The polyester-based oriented film according to the present invention can be used as a heat-shrinkable film for packaging and / or labels of plastic containers. In particular, the polyester-based oriented film exhibits excellent thermal shrinkage, tear index, and tear performance, and thus does not deform when applied to a container, and can be uniformly and cleanly removed along a removal line when separated from the container. Therefore, the polyester-based oriented film can realize high-quality products such as packages and labels, and can improve recovery processing efficiency.

Description

Technical Field

[0001] The present invention relates to a stretch polyester film having heat shrinkage properties and excellent removability when separated from a container, a method for preparing the film, and an article formed from the film. Background Technology

[0002] In recent years, with the rapid increase in the use of plastic packaging containers, relevant international environmental regulations have also been strengthened. Consequently, the industry's interest in sustainable packaging has grown significantly. Furthermore, for example, when recyclable waste is disposed of separately, it is necessary to remove labels from beverage containers. Therefore, there is a need to develop label resins and films that are easy to remove from containers and have high shrinkage properties.

[0003] However, polystyrene and oriented polystyrene (OPS), which are widely used as heat-shrinkable film label materials, suffer from insufficient heat resistance and heat-shrinkability. To address these issues, efforts have been made to develop heat-shrinkable films using polyester resins.

[0004] However, generally speaking, heat-shrinkable polyester films containing polyester resin have high tear strength in the machine direction but low tear strength in the direction perpendicular to the machine direction. This leads to a problem that, when removed from the container after being used as a label, the film cannot be cleanly removed along the intended removal line.

[0005] Therefore, there is a need to develop a polyester film that has excellent shrinkage properties in terms of shrinkage stress and shrinkage rate, and is easy to remove from containers, in order to improve recycling efficiency.

[0006] [Existing Technical Documents]

[0007] [Patent Literature]

[0008] (Patent Document 1) Korean Patent Publication No. 2002-0062838. Summary of the Invention

[0009] Technical issues

[0010] To address the aforementioned problems in the prior art, the inventors conducted various studies. The results showed that by controlling the tensile strength and tear index of stretched polyester films prepared from polyester resin, they can be effectively used as heat-shrinkable films for packaging and / or labeling plastic containers; in particular, a stretched polyester film with excellent heat shrinkage rate, tear index, and tear resistance can be obtained, which does not deform when applied to containers and can be easily removed when separated from containers.

[0011] Therefore, one object of the present invention is to provide a stretched polyester film that has excellent shrinkage properties due to an optimized tear index and excellent removability when separated from a container, a method for preparing the film, and an article formed from the film.

[0012] Solution to the problem

[0013] To achieve the above objectives, the present invention provides a stretchable polyester film comprising a polyester resin copolymerized from a diol component and a dicarboxylic acid component, and having a tear index of less than 2.5, as expressed by Equation 1 below: [Equation 1]

[0014] In equation 1, TS MD It is the tear strength (N / mm) of a stretched polyester film sample in the machine direction (MD), and it is a unitless value. TS TD It is the tear strength (N / mm) of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD), and it is a unitless value; DT is the stretching temperature (°C) of polyester film, and it is a unitless value. Tg is the glass transition temperature (°C) of polyester resin, and it is a unitless value.

[0015] In addition, the present invention also provides an article formed from a stretched polyester film.

[0016] Beneficial effects of the invention

[0017] The stretched polyester film according to the present invention meets a specific tear index range, thus exhibiting excellent heat shrinkage properties, does not cause deformation when applied to containers, and can be easily removed from containers upon separation.

[0018] Therefore, the stretched polyester film according to the invention can be advantageously used as a heat-shrinkable film for packaging and / or labeling plastic containers.

[0019] Furthermore, the stretched polyester film according to the present invention, due to its excellent tear properties, can be removed uniformly and neatly along the removal line when separated from the container. Therefore, it not only ensures the excellent quality of products such as packaging materials and labels, but also improves recycling efficiency.

[0020] Best Implementation of the Invention

[0021] The present invention will now be described in detail. The present invention is not limited to the contents disclosed below; various modifications can be made without altering the essence of the invention.

[0022] In this specification, the term "comprising" is intended to specify a particular feature, area, step, process, element, and / or component. Unless otherwise expressly stated, the presence or addition of any other feature, area, step, process, element, and / or component is not excluded.

[0023] Unless otherwise stated, all numbers and expressions used in this document relating to component quantities, reaction conditions, etc., should be understood to be modified by the term "approximately".

[0024] In this specification, the term "residue" refers to a specific part or unit derived from a particular compound and included in the chemical reaction product when the particular compound participates in a chemical reaction. Specifically, "residue" of a dicarboxylic acid component or "residue" of a diol component refers to the portion of the copolyester resin formed by esterification or polycondensation that originates from the dicarboxylic acid component or the diol component, respectively.

[0025] Generally speaking, the tear strength of stretched polyester film in the longitudinal direction (MD) (hereinafter referred to as MD tear strength) is greater than the tear strength in the direction perpendicular to the longitudinal direction (TD) (hereinafter referred to as TD tear strength).

[0026] When such stretched polyester film is used as a label on a container and then separated for recycling, the lower its longitudinal tear strength, the easier it is to tear, which makes it easier to remove from the container.

[0027] Furthermore, when stretched polyester film is used as a label on a container and then separated for recycling, the separation pattern may vary depending on the TD tear strength, the ratio of MD tear strength to TD tear strength, and the MD tear strength itself. If the MD tear strength is too high, the label may be difficult to separate from the container. If the TD tear strength is too low, the label may deviate from the removal line when separated from the container, for example, by tearing, and may not tear smoothly. In particular, if the MD tear strength is too high, resulting in an excessively large ratio of MD tear strength to TD tear strength, it may be difficult to obtain good quality when stretched polyester film is applied to container labels.

[0028] In other words, when appropriate MD tear strength, TD tear strength, and the ratio of MD tear strength to MD tear strength are achieved, the label can separate uniformly. That is, when torn or deviated from the removal line, the label can be neatly removed along the removal line without sliding laterally. Therefore, it not only achieves excellent quality for products such as packaging materials and labels, but also improves recycling efficiency.

[0029] Therefore, in order to achieve the above effects, it is very important to control not only the MD tear strength of the stretched polyester film, but also the TD tear strength and the ratio of these tear strengths (tear ratio).

[0030] Specifically, the tear index, calculated using MD tear strength, TD tear strength, and the ratio of these tear strengths, can not only indicate the ease of removal from plastic containers, but also serve as an important indicator of the quality of packaging materials and labels, as well as the efficiency of recycling.

[0031] The tear index may vary depending on factors such as the type of resin contained in the stretched polyester film, the type of monomers constituting the resin, the physical properties of the resin, the stretching temperature, and the physical properties of the stretched polyester film prepared from the resin.

[0032] In this invention, the tear index of the stretched polyester film is controlled by various adjustments to the aforementioned conditions. With this controlled tear index, the stretched polyester film according to the invention can be advantageously used as a heat-shrinkable film for packaging materials and / or labels of plastic containers (e.g., PET bottles). Furthermore, the stretched polyester film of the invention is technically significant because it does not cause deformation when applied to containers and is easily separated from the plastic containers during recycling, thereby significantly improving recycling efficiency.

[0033] This will be described in detail below.

[0034] Stretched polyester films

[0035] According to one embodiment, the stretched polyester film comprises a polyester resin copolymerized from a diol component and a dicarboxylic acid component, and has a tear index of less than 2.5, expressed by the following Equation 1:

[0036] [Equation 1]

[0037] In equation 1, TS MD It is the tear strength (N / mm) of a stretched polyester film sample in the machine direction (MD), and it is a unitless value. TS TD It is the tear strength (N / mm) of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD), and it is a unitless value; DT is the stretching temperature (°C) of polyester film, and it is a unitless value. Tg is the glass transition temperature (°C) of polyester resin, and it is a unitless value.

[0038] Because stretched polyester films have excellent shrinkage properties and meet the tear index requirement of less than 2.5 according to Equation 1 above, they can be easily removed from containers, thereby improving recycling efficiency.

[0039] Specifically, the tear index represented by Equation 1 above indicates the ease and uniformity of removal when a stretched polyester film is applied to a container label and then separated. For example, when the tear index according to Equation 1 meets the above-mentioned range, the label can be easily and neatly torn along the direction to be removed without deviating from the removal line; therefore, the ease and uniformity of removal are excellent. The tear index may vary depending on the resin's TD tear strength, MD tear strength, stretching temperature, and glass transition temperature.

[0040] In other words, the tear index is a value obtained by dividing the tear ratio by the difference between the stretching temperature and the glass transition temperature of the resin (DT–Tg), where the tear ratio represents the ratio of the MD tear strength to the square of the TD tear strength of the stretched polyester film (TS). MD / (TS TD ) 2 A low tear index facilitates achieving the desired optimal tear strength and allows for easy, uniform, and neat removal (tearing) from the container, thus explaining excellent recycling efficiency.

[0041] In this specification, a stretched polyester film can be a film obtained by extruding polyester resin into a sheet (forming an unstretched sheet) and then stretching the sheet.

[0042] Specifically, stretched polyester films can be prepared by extrusion (or melt extrusion) of polyester resin or a mixed resin containing polyester resin, stretching, and / or heat setting. The specific processing methods for stretched polyester films are described below.

[0043] The tear index of stretched polyester films can be less than 2.5, specifically 2.4 or less, 2.3 or less, 2.2 or less, 2.1 or less, 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, 1.6 or less, or 1.5 or less. For example, the tear index of stretched polyester films can be greater than 0 to less than 2.5, 0.01 to 2.4, 0.01 to 2.0, 0.01 to 1.9, 0.01 to 1.8, 0.01 to 1.5, 0.05 to 2.4, 0.05 to 2.0, 0.05 to 1.9, 0.05 to 1.8, 0.05 to 1.5, 0.1 to 2.4, 0.1 to 2.0, 0.1 to 1.9, 0.1 to 1.8, or 0.1 to 1.5.

[0044] When the tear index of the stretched polyester film meets the above-mentioned range, suitable TD tear strength and MD tear strength, as well as a tear ratio within the optimal range for the intended use, can be obtained. Furthermore, when stretched polyester film is used for container labels, it exhibits excellent heat-shrink properties, allowing the label to be easily removed from the container and neatly removed along the guide removal line, thereby improving recycling efficiency.

[0045] Furthermore, in order to achieve the desired effect of this invention, TS is controlled. MD TS TD It is important to consider the ratio of the tear index and its proportion, while also meeting the range of the tear index.

[0046] In Equation 1, TS MD It is the tear strength (N / mm) of a stretched polyester film sample in the longitudinal direction (MD). It can be 50 N / mm or less, 48 ​​N / mm or less, 46 N / mm or less, or 45 N / mm or less, or it can be 1 to 50 N / mm, 3 to 50 N / mm, 3 to 46 N / mm, 3 to 45 N / mm, 5 to 50 N / mm, 5 to 46 N / mm, 5 to 45 N / mm, 7 to 50 N / mm, 7 to 46 N / mm, 7 to 45 N / mm, 10 to 50 N / mm, 10 to 46 N / mm, 10 to 45 N / mm, 10 to 35 N / mm, 10 to 33 N / mm, or 10 to 32 N / mm.

[0047] When TS MD When the above conditions are met, the label can be easily removed when separated from the container, thereby improving recycling efficiency. If TS MD Outside of the aforementioned scope, when the film is applied to the label of a container, the label may not be easily removed from the container. In particular, if TS MD If the label is removed from the container beyond the specified range, it may become difficult to remove the label, which could reduce recycling efficiency.

[0048] The test was conducted according to ASTM D1922 standard: The prepared copolyester resin was extruded into a sheet, stretched by 500% in the direction perpendicular to the longitudinal direction, and then tested using a Leading Instruments TGT-01 tear tester at a weight (g) 16 times the thickness (m) of the stretched polyester film. The obtained values ​​were then converted according to the thickness of the stretched polyester film.

[0049] In Equation 1, TS TDThis refers to the tear strength (N / mm) of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD). It can be 5.0 N / mm or less, 4.5 N / mm or less, 4.0 N / mm or less, 3.5 N / mm or less, 3.0 N / mm or less, or 2.5 N / mm or less, or it can be 0.5 to 5.0 N / mm, 0.5 to 4.5 N / mm, 1.0 to 5.0 N / mm, 1.0 to 4.5 N / mm, 1.1 to 5.0 N / mm, or 1.1 to 4.5 N / mm. 1.1 to 2.0 N / mm, 1.2 to 5.0 N / mm, 1.2 to 4.5 N / mm, 1.2 to 4.0 N / mm, 1.2 to 3.5 N / mm, 1.2 to 3.0 N / mm, 1.2 to 2.5 N / mm, 1.2 to 2.3 N / mm, 1.2 to 2.0 N / mm, 1.2 to 1.8 N / mm, 1.2 to 1.6 N / mm, or 1.2 to 1.5 N / mm.

[0050] When TS TD When the above conditions are met, the label can be easily removed from the container and neatly removed along the guided removal line, thereby improving recycling processing efficiency. If TS TD Outside of the aforementioned range, when the film is applied to the label of a container, the label may be too easily torn, or may deviate from the guide removal line when torn, making its removal difficult.

[0051] The test was conducted according to ASTM D1922 standard: The prepared copolyester resin was extruded into a sheet, stretched by 500% in the longitudinal (MD) direction, and then tested using a Leading Instruments TGT-01 tear tester at a weight (g) eight times the thickness (m) of the stretched polyester film. The obtained values ​​were then converted according to the thickness of the stretched polyester film.

[0052] Furthermore, in Equation 1, TS MD / (TS TD ) 2This represents the tear ratio. It can be 1 to 30, 1 to 29, 1 to 28, 1 to 26, 1 to 25, 1 to 22, 1 to 20, 1 to 18, 1 to 17, 1 to 15, 1 to 13, 1 to 12, 3 to 30, 3 to 29, 3 to 28, 3 to 26, 3 to 25, 3 to 22, 3 to 20, 3 to 18, 3 to 17, 3 to 15, 3 to 13, 3 to 12, 5 to 30, 5 to 29, 5 to 28, 5 to 26, 5 to 25, 5 to 22, 5 to 20, 5 to 18, 5 to 17, 5 to 15, 5 to 13, 5 to 12, 8 to 30. 8 to 29, 8 to 28, 8 to 26, 8 to 25, 8 to 22, 8 to 20, 8 to 18, 8 to 17, 8 to 15, 8 to 13, 8 to 12, 10 to 30, 10 to 29, 10 to 28, 10 to 26, 10 to 25, 10 to 22, 10 to 20, 10 to 18, 10 to 17, 10 to 15, 12 to 30, 12 to 28, 12 to 25, 15 to 30, 15 to 28, 15 to 25, 16 to 30, 16 to 28, 16 to 25, 18 to 30, 18 to 28 or 20 to 30.

[0053] When TS MD / (TS TD ) 2 When the above range is met, the tear ratio is excellent; therefore, when the label is separated from the container, it can be easily removed; and when torn, it can be neatly removed along the removal line without deviating from the guide removal line.

[0054] Furthermore, in Equation 1, DT–Tg, which is the difference between the elongation temperature (DT) and the glass transition temperature (Tg) of the resin, can be 5°C to 20°C, 6°C to 20°C, 8°C to 20°C, 8°C to 19°C, 8°C to 18°C, or 8°C to 17°C. When DT–Tg meets the above ranges, it may be more conducive to achieving the desired effect.

[0055] Here, DT is the stretching temperature (°C) of the polyester film. DT can be a temperature 5°C or higher, 6°C or higher, 8°C or higher, or 10°C or higher than the glass transition temperature (Tg) of the polyester resin. For example, DT can be a temperature 5°C to 20°C higher than the glass transition temperature (Tg) of the polyester resin. Specifically, DT can be 50°C to 180°C, 60°C to 170°C, 60°C to 150°C, 60°C to 120°C, 60°C to 100°C, 70°C to 120°C, 70°C to 100°C, or 80°C to 95°C. When DT meets the above ranges, it may be more beneficial to achieve the desired effect.

[0056] Tg is the glass transition temperature (°C) of polyester resin. It can range from 45°C to 170°C, 55°C to 150°C, 60°C to 100°C, 62°C to 90°C, 63°C to 85°C, 63°C to 82°C, 63°C to 80°C, 68°C to 79°C, 69°C to 78°C, 70°C to 78°C, or 71°C to 78°C. If the glass transition temperature of the polyester resin is too low, it may crack during the heat treatment process for processing and preparing stretched films. If the temperature is too high, sufficient stretching for label application may not be achieved.

[0057] When a resin sample is heated at a rate of 10 kJ / min in the range of 25°C to 150°C, Tg can be measured by differential scanning calorimetry (DSC).

[0058] Meanwhile, the shrinkage stress of the stretched polyester film at 85°C can be 12.0 MPa or less, and it can be 2.0 MPa to 11.0 MPa, 2.5 MPa to 10 MPa, 3 MPa to 10 MPa, 3.5 MPa to 10 MPa, 3.5 MPa to 9.5 MPa, or 3.5 MPa to 8.0 MPa. When the shrinkage stress of the stretched polyester film meets the above ranges, it exhibits excellent adhesion when applied to container labels and can minimize container deformation problems, such as defects or distortions caused by uneven shrinkage.

[0059] Meanwhile, the thermal shrinkage rate (TTS) of stretched polyester films in the direction perpendicular to the longitudinal direction (TD) is shown in Equation 2-1 below. 85 It can be 40% or higher: [Equation 2-1]

[0060] In equation 2-1, TL 25 It is the length of a stretched polyester film sample along the direction perpendicular to the longitudinal direction (TD) at 25°C. TL 85 It is the length of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD) after immersing it in hot water at 85°C for 10 seconds.

[0061] Thermal shrinkage rate (TTS) of stretched polyester film in the direction perpendicular to the longitudinal direction (TD). 85 The percentage can be 40% or higher, 50% or higher, or 60% or higher, or it can be 40% to 85%, 40% to 80%, 40% to 75%, 50% to 90%, 50% to 80%, 50% to 75%, 60% to 90%, 60% to 80%, or 60% to 75%. The heat shrinkage rate (TTS) of the stretched polyester film in the direction perpendicular to the longitudinal direction (TD) is also considered. 85When the above range is met, it exhibits excellent adhesion when applied to container labels and can minimize container deformation problems, such as defects or distortions caused by uneven shrinkage.

[0062] Meanwhile, the thermal shrinkage rate (MTS) of stretched polyester films in the longitudinal direction (MD) is shown in Equation 2-2 below. 85 It can be -20% to 15%. [Equation 2-2]

[0063] In equation 2-2, ML 25 It is the longitudinal length of a stretched polyester film sample at 25°C. ML 85 It is the length along the longitudinal direction (MD) of a stretched polyester film sample after immersing it in hot water at 85°C for 10 seconds.

[0064] Specifically, the thermal shrinkage rate (MTS) of stretched polyester films in the longitudinal direction (MD) 85 The range can be -20% to 15%, -20% to 10%, -15% to 15%, -15% to 10%, -13% to 10%, or -12% to 5%.

[0065] When the thermal shrinkage rate (MTS) of a stretched polyester film in the longitudinal direction (MD) is... 85 When the above range is met, the shrinkage in the longitudinal (MD) direction is small when it is applied to the label of the container. This minimizes the occurrence of wrinkles and can minimize container deformation problems, such as twisting due to uneven shrinkage.

[0066] Meanwhile, according to one embodiment, the stretched polyester film may include 100% by weight of copolyester resin.

[0067] Furthermore, stretched polyester films may further comprise different types of crystalline resins. Specifically, stretched polyester films may comprise a mixture of copolyester resins and different types of crystalline resins. For example, different types of crystalline resins may include polyethylene terephthalate resins, but are not limited thereto.

[0068] When the stretched polyester film includes a blended resin, the copolyester resin may be 5 to 95% by weight, 10 to 95% by weight, 20 to 95% by weight, 30 to 90% by weight, 40 to 80% by weight, 50 to 90% by weight, 60 to 90% by weight, or 70 to 80% by weight, based on the total weight of the resin in the stretched polyester film.

[0069] Furthermore, different types of crystalline resins may include polyethylene terephthalate resin, which may be 1% to 10% by weight, 5% to 35% by weight, 10% to 50% by weight, 10% to 40% by weight, or 20% to 30% by weight of the total resin content in the stretched polyester film. When the content of polyethylene terephthalate resin meets the above ranges, the stretched polyester film can enhance thermal properties and further improve crystallinity and ease of removal from containers, which helps ensure excellent recyclability.

[0070] When stretched polyester films contain blended resins, the weight ratio of the copolyester resin to different types of crystalline resins can be 50:50 to 90:10, 60:40 to 90:10, 70:30 to 90:10, 50:50 to 80:20, 50:50 to 75:25, 60:40 to 80:20, 60:40 to 75:25, or 60:40 to 70:30.

[0071] Polyethylene terephthalate resin can refer to a resin containing a polyethylene terephthalate structure, wherein terephthalic acid (TPA) or dimethyl terephthalic acid (DMT) is polymerized with 90% or more, 95% or more, 97% or more, or 98% or more of ethylene glycol (EG).

[0072] In addition, the polyethylene terephthalate resin may be recycled polyethylene terephthalate resin (PCR-PET), or may include a resin containing structural units derived from terephthalic acid, wherein the terephthalic acid comprises at least one selected from the group consisting of recycled terephthalic acid, recycled dimethyl terephthalate, recycled bis(2-hydroxyethyl) terephthalic acid, and recycled hydroxyethyl terephthalic acid.

[0073] In addition, polyethylene terephthalate resins may include resins containing ethylene glycol structural units derived from recycled materials.

[0074] Stretched polyester films can have single-layer or multi-layer structures.

[0075] For example, the thickness of stretchable polyester films can be 10 μm to 250 μm, 10 μm to 200 μm, 10 μm to 150 μm, 10 μm to 100 μm, 20 μm to 80 μm, 30 μm to 70 μm, 35 μm to 65 μm, 35 μm to 55 μm, 40 μm to 60 μm, or 35 μm to 50 μm.

[0076] Copolyester resin

[0077] According to one embodiment, the copolyester resin contained in the stretched polyester film can be a polyester resin copolymerized from a diol component and a dicarboxylic acid component.

[0078] According to one implementation, the tear index of a stretched polyester film may vary depending on the type of monomers in the copolyester resin that constitutes the stretched polyester film, its physical properties, etc.

[0079] Specifically, a copolyester resin polymerized from dicarboxylic acid or its derivatives and a diol can have a structure in which the acid moiety derived from the dicarboxylic acid or its derivatives and the diol moiety derived from ethylene glycol or its derivatives repeat.

[0080] In this specification, the acid moiety and the diol moiety refer to the residues remaining after the polymerization of a dicarboxylic acid or its derivative with a diol, upon removal of hydrogen, hydroxyl, or alkoxy groups. Although dicarboxylic acids or their derivatives and diols will be described in more detail below, for example, dicarboxylic acids or their derivatives may primarily be terephthalic acid or its derivatives, and diols may include ethylene glycol and further include its derivatives.

[0081] Copolyester resins can contain linear diol monomer residues and cyclic monomer residues.

[0082] The cyclic monomer residues may be derived from dicarboxylic acids or their derivatives, or diols other than ethylene glycol. Dicarboxylic acids or their derivatives, and diols other than ethylene glycol are not limited, provided they are used in copolyester resins and are monomers having an aliphatic or aromatic ring structure in their molecular structure. For example, they may be at least one selected from the group consisting of 1,4-cyclohexanediethanol, 1,4-cyclohexanedicarboxylic acid, and terephthalic acid or its derivatives. Terephthalic acid or its derivatives may be at least one selected from the group consisting of terephthalic acid, dimethyl terephthalate, hydroxyethyl terephthalate, recycled terephthalic acid, recycled dimethyl terephthalate, and recycled hydroxyethyl terephthalate.

[0083] Based on 100 mol% of all residues, the content of cyclic monomer residues can be 50 mol% to 80 mol%, 50 mol% to 55 mol%, 55 mol% to 65 mol%, or 65 mol% to 75 mol%.

[0084] Linear diol monomer residues can be derived from diols. There are no restrictions on the diol, as long as it is used in a copolyester resin and has a linear molecular structure. For example, it can be derived from at least one selected from the group consisting of ethylene glycol, diethylene glycol, neopentyl glycol, recycled ethylene glycol, recycled diethylene glycol, and recycled neopentyl glycol.

[0085] The content of linear glycol monomer residues other than ethylene glycol is a variable. In this case, based on 100 mol% of all residues, the content of linear glycol monomer residues other than ethylene glycol can be 25 mol% or less. Specifically, based on 100 mol% of all residues, the content of linear glycol monomer residues other than ethylene glycol can be 0.5 mol% or more, 1 mol% or more, 1.5 mol% or more, 2.5 mol% or more, 3.0 mol% or more, 3.5 mol% or more, 4.0 mol% or more, 4.5 mol% or more, 5.0 mol% or more, or 5.5 mol% or more, and 20 mol% or less, 18 mol% or less, 15 mol% or less, or 14 mol% or less.

[0086] The diols in copolyester resins can include ethylene glycol. Without ethylene glycol, the longitudinal (MD) tear strength becomes too high; therefore, when used as a label on a container and then separated, it may not tear easily and the shrinkage stress may be too high.

[0087] According to one embodiment, the diol component comprises ethylene glycol and may include at least one selected from the group consisting of diethylene glycol, neopentyl glycol, cyclohexanediethanol, recycled ethylene glycol, recycled diethylene glycol, recycled neopentyl glycol, and recycled cyclohexanediethanol.

[0088] In addition, stretched polyester films may contain 0.1 to 20 mol% of diethylene glycol comonomers. For example, they may contain 0.5 mol% or higher, 1 mol% or higher, 1.5 mol% or higher, 2.5 mol% or higher, 3.0 mol% or higher, 3.5 mol% or higher, 4.0 mol% or higher, 4.5 mol% or higher, 5.0 mol% or higher, or 5.5 mol% or higher, and 20 mol% or lower, 18 mol% or lower, 15 mol% or lower, or 14 mol% or lower of diethylene glycol comonomers.

[0089] Furthermore, the stretched polyester film may include at least one comonomer selected from the group consisting of neopentyl glycol and diethylene glycol, in an amount of 2.5 mol% or higher. For example, the stretched polyester film may include at least one comonomer selected from the group consisting of neopentyl glycol and diethylene glycol, in an amount of 3 mol% or higher, 5 mol% or higher, or 8 mol% or higher, or 2.5 mol% to 30 mol%, 2.5 mol% to 25 mol%, or 3 mol% to 20 mol%.

[0090] When the content of comonomers meets the above range, it is more conducive to obtaining the required tear index and tensile strength, while having excellent heat shrinkage properties in the main shrinkage direction, and improving the ease of removal from the container.

[0091] Furthermore, when both ethylene glycol and comonomers are used within the aforementioned range, it is more conducive to achieving the desired effect, and the heat shrinkage rate is easier to control. Thus, when the film is applied to a container, it can more effectively prevent wrinkles or deformation and further improve the ease of removal from the container.

[0092] Meanwhile, the specific components and their contents contained in the stretched polyester film are as follows.

[0093] Preparation method of stretched polyester film

[0094] According to one embodiment, a method for preparing the above-described stretched polyester film is provided.

[0095] Specifically, according to one embodiment, the method for preparing a stretched polyester film includes: copolymerizing a diol and a dicarboxylic acid to prepare a copolyester resin (S-1); and preparing a stretched polyester film from the polyester resin (S-2), wherein the film has a tear index of less than 2.5 as expressed by Equation 1 above.

[0096] This will be described in detail below.

[0097] Step (S-1): Preparation of polyester resin

[0098] In step (S-1), the diol component and the dicarboxylic acid component are copolymerized to prepare a copolyester resin (polymer).

[0099] Copolyester resins can be prepared by esterification or transesterification of dicarboxylic acids or their derivatives with diols; and by polycondensation of the products of esterification or transesterification.

[0100] Catalysts can be used in esterification or transesterification reactions. Catalysts can be methylates of sodium and magnesium; acetates, borates, fatty acid salts, or carbonates of Zn, Cd, Mn, Co, Ca, and Ba; metallic Mg; and oxides of Ti, Pb, Zn, Sb, and Ge.

[0101] Esterification or transesterification reactions can be carried out in batch, semi-continuous, or continuous processes. Although each feedstock can be fed individually, it is preferred to feed it as a mixed slurry of diol and dicarboxylic acid or their derivatives.

[0102] Before the esterification or transesterification reaction begins, polycondensation catalysts, stabilizers, colorants, crystallizers, antioxidants, branching agents, etc., can be added to the slurry; or they can be added to the product after the reaction is complete.

[0103] However, there are no particular restrictions on the timing of adding these additives; they can be added at any time during the preparation of the copolyester resin. One or more common titanium, germanium, antimony, aluminum, and tin compounds can be selected as polycondensation catalysts. Examples of effective titanium-based catalysts include tetraethyl titanate, tripropyl acetylacetonate, tetrapropyl titanate, tetrabutyl titanate, polybutyl titanate, 2-ethylhexyl titanate, octyl glycol titanate, lactate titanate, triethanolamine titanate, acetylacetone titanate, ethyl acetoacetate titanate, isostearyl titanate, titanium dioxide, copolymers of titanium dioxide and silica, and copolymers of titanium dioxide and zirconium dioxide. Furthermore, examples of effective germanium-based catalysts include germanium dioxide and copolymers using germanium dioxide. Phosphorus-based compounds, such as phosphoric acid, trimethyl phosphate, and triethyl phosphate, are commonly used as stabilizers. Depending on the phosphorus content, the amount added can be 10 to 200 ppm by weight of the final polymer (copolyester resin). If the added stabilizer content is less than 10 ppm, the stabilizing effect is not significant, and the polymer color may turn yellow. If the concentration exceeds 200 ppm, it may be impossible to obtain a polymer with the desired high degree of polymerization.

[0104] In addition, common colorants, such as cobalt acetate and cobalt propionate, can be used as example added colorants to enhance the color of the polymer. Depending on the cobalt content, the amount added can be 10 to 200 ppm by weight of the final polymer (copolyester resin). If necessary, anthraquinone compounds, perinone-based compounds, azo compounds, methyst compounds, etc., can be used as organic colorants. Commercially available toners, such as Clariant's Polysynthren Blue RLS or Clariant's Solvaperm Red BB, can be used. The amount of added organic colorant can be adjusted from 0 to 50 ppm by weight of the final polymer. If the amount of colorant exceeds the above range, the yellowing of the copolyester resin may not be adequately covered, or its physical properties may deteriorate.

[0105] Examples of crystallizing agents include nucleating agents, ultraviolet absorbers, polyolefin resins, and polyamide resins. Examples of antioxidants include hindered phenolic antioxidants, phosphite antioxidants, thioether antioxidants, or mixtures thereof.

[0106] Esterification can be carried out at temperatures of 200°C to 300°C or 230°C to 280°C, and at concentrations of 0 to 10.0 kgf / cm³. 2 (0 to 7,355.6 mmHg), 0 to 5.0 kgf / cm 2 (0 to 3,677.8 mmHg), or 0.1 to 3.0 kgf / cm²2 The transesterification reaction can be carried out at pressures ranging from 73.6 to 2,206.7 mmHg. Furthermore, the transesterification reaction can be performed at temperatures ranging from 150°C to 270°C or 180°C to 260°C, and at pressures ranging from 0 to 5.0 kgf / cm³. 2 (0 to 3,677.8 mmHg), or 0.1 to 3.0 kgf / cm² 2 The pressure was tested at (73.6 to 2,206.7 mmHg). The pressure outside the parentheses refers to gauge pressure (unit: kgf / cm²). 2 The pressure in parentheses refers to absolute pressure (unit: mmHg).

[0107] If the reaction temperature and pressure exceed the above range, there is a concern that the performance of the copolyester resin may deteriorate. The reaction time (average residence time) is typically 1 to 24 hours or 2 to 8 hours, depending on the reaction temperature, pressure, and the molar ratio of the diol to the dicarboxylic acid or its derivatives used.

[0108] The products obtained through esterification or transesterification can be used to prepare copolyester resins with higher degrees of polymerization through polycondensation. Generally, polycondensation is carried out at temperatures of 150°C to 300°C, 200°C to 290°C, or 260°C to 290°C, and under reduced pressures of 400 to 0.01 mmHg, 100 to 0.05 mmHg, or 10 to 0.1 mmHg. Here, pressure refers to absolute pressure. The purpose of the reduced pressure of 400-0.01 mmHg is to remove unreacted substances and ethylene glycol, a byproduct of the polycondensation reaction. Therefore, if the reduced pressure exceeds the above range, there is concern that the removal of byproducts and unreacted substances may be insufficient. Furthermore, if the temperature of the polycondensation reaction exceeds the above range, there is concern that the properties of the copolyester resin may degrade. The polycondensation reaction can proceed for the desired time until the desired intrinsic viscosity is reached, for example, 1 hour to 24 hours.

[0109] To reduce the content of unreacted substances remaining in the copolyester resin, the reaction can be intentionally maintained in a vacuum for an extended period at the end of the esterification or transesterification reaction, or at the beginning of the polycondensation reaction, i.e., when the resin viscosity is not high enough to remove unreacted raw materials from the system. As the resin viscosity increases, the raw materials remaining in the reactor become more difficult to remove. For example, before carrying out the polycondensation reaction, the reaction products obtained from the esterification or transesterification reaction can be allowed to stand under reduced pressure of about 400 to 1 mmHg or about 200 to 3 mmHg for 0.2 to 3 hours to effectively remove unreacted substances remaining in the copolyester resin. In this case, the temperature of the product can be controlled to be equal to, or between, the temperature of the esterification or transesterification reaction and the temperature of the polycondensation reaction.

[0110] The intrinsic viscosity of the polymer is suitably between 0.30 and 1.0 dl / g during polycondensation. If the intrinsic viscosity is less than 0.30 dl / g, the reaction rate in the solid-state reaction decreases significantly. If the intrinsic viscosity exceeds 1.0 dl / g, the viscosity of the melt will increase during melt polymerization, which increases the likelihood of polymer discoloration due to shear stress between the stirrer and the reactor, and also increases the production of byproducts such as acetaldehyde.

[0111] If necessary, the copolyester resin according to one embodiment may undergo a further solid-state reaction after polycondensation to obtain a higher degree of polymerization.

[0112] Specifically, the polymer obtained through polycondensation is discharged from the reactor and granulated. Granulation methods can include wire cutting, where the polymer is extruded into a wire, cured in a coolant, and then cut with a cutter; or underwater cutting, where the die is immersed in the coolant, the polymer is extruded directly into the coolant, and then cut with a cutter. Generally, in wire cutting, the coolant temperature must be kept low to ensure proper curing of the wire bundle, thus preventing problems during cutting. In underwater cutting, it is preferable to maintain the coolant temperature at a level suitable for the polymer to ensure uniform polymer shape. However, for crystalline polymers, the coolant temperature can be intentionally kept higher to induce crystallization during extrusion.

[0113] Unreacted water-soluble raw materials can be removed from granular polymers by washing them with water. Smaller particle sizes result in a larger surface area relative to particle weight; therefore, smaller particle sizes are more advantageous. To achieve this, granules can be prepared with an average weight of approximately 15 milligrams or less. For example, granular polymers can be washed by immersing them in water at the polymer's glass transition temperature or approximately 50°C to 100°C for 5 minutes to 10 hours.

[0114] Dicarboxylic acid or its derivatives

[0115] The dicarboxylic acid component refers to the main monomer that, together with the diol component, constitutes the copolyester resin.

[0116] In this specification, "dicarboxylic acid or its derivatives" means at least one compound selected from dicarboxylic acids and their derivatives. Furthermore, "derivatives of dicarboxylic acids" refers to alkyl esters of dicarboxylic acids (lower alkyl esters having 1-4 carbon atoms, such as monomethyl esters, monoethyl esters, dimethyl esters, diethyl esters, or dibutyl esters) and anhydrides of dicarboxylic acids. Therefore, for example, terephthalic acid or its derivatives are collectively referred to as compounds capable of reacting with diols to form terephthaloyl groups, such as terephthalic acid; monoalkyl or dialkyl esters of terephthalic acid; and terephthalic anhydrides.

[0117] Specifically, in this specification, the residues of the dicarboxylic acid component may include at least one residue selected from terephthalic acid, dimethyl terephthalate, hydroxyethyl terephthalate, and derivatives thereof. More specifically, terephthalic acid or its derivatives may be at least one selected from the group consisting of terephthalic acid, dimethyl terephthalate, hydroxyethyl terephthalate, recycled terephthalic acid, recycled dimethyl terephthalate, and recycled hydroxyethyl terephthalate.

[0118] Furthermore, the copolyester resin may contain an acid moiety derived from a comonomer other than terephthalic acid or its derivatives. Specifically, the comonomer may be at least one selected from aromatic dicarboxylic acids or their derivatives having 8 to 14 carbon atoms and aliphatic dicarboxylic acids or their derivatives having 4 to 12 carbon atoms. Examples of aromatic dicarboxylic acids or their derivatives having 8 to 14 carbon atoms include isophthalic acid, naphthalenedicarboxylic acid (e.g., 2,6-naphthalenedicarboxylic acid), diphenyl dicarboxylic acid, 4,4'-stilbene dicarboxylic acid, 2,5-furandicarboxylic acid, 2,5-thiophene dicarboxylic acid, etc., but specific examples of aromatic dicarboxylic acids are not limited thereto. Examples of aliphatic dicarboxylic acids or their derivatives containing 8 to 14 carbon atoms include straight-chain, branched, or cyclic aliphatic dicarboxylic acid components, such as cyclohexanedicarboxylic acids (e.g., 1,4-cyclohexanedicarboxylic acid and 1,3-cyclohexanedicarboxylic acid), phthalic acid, sebacic acid, succinic acid, isodecylsuccinic acid, maleic acid, fumaric acid, adipic acid, glutaric acid, azelaic acid, etc., but specific examples of aliphatic dicarboxylic acids are not limited thereto. Specifically, in addition to terephthalic acid or its derivatives, the comonomer may be at least one selected from the group consisting of isophthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, azelaic acid, succinic acid, and 1,4-cyclohexanedicarboxylic acid.

[0119] In addition, based on 100 mol% of all dicarboxylic acids or their derivatives, the dicarboxylic acids or their derivatives may include 70 mol% or more of terephthalic acid or its derivatives, and at least one of the following groups selected from the group consisting of aromatic dicarboxylic acids or their derivatives having 8 to 14 carbon atoms and aliphatic dicarboxylic acids or their derivatives having 4 to 12 carbon atoms, in a content of 30 mol% or less.

[0120] Diol component

[0121] The diol component in this specification refers to the main monomer that, together with the aforementioned dicarboxylic acid component, constitutes the copolyester resin. Specifically, in addition to ethylene glycol, the diol component may also contain diethylene glycol, neopentyl glycol, or cyclohexanediol as comonomers. Furthermore, recycled ethylene glycol, recycled diethylene glycol, or recycled neopentyl glycol may be used as monomers in the diol component.

[0122] Ethylene glycol can be a component that helps improve the transparency and impact resistance of copolyester resins. Preferably, based on 100 mol% of all residues, the content of ethylene glycol residues can be 25 to 90 mol%, 25 to 80 mol%, 25 to 70 mol%, 30 to 70 mol%, 40 to 80 mol%, or 50 to 80 mol%.

[0123] Diethylene glycol can be a component that helps regulate the shrinkage properties of copolyester resins. Preferably, based on 100 mol% of all residues, the content of diethylene glycol can be 1.0 mol% or more, 1.5 mol% or more, 2.0 mol% or more, 2.5 mol% or more, 3.0 mol% or more, 3.5 mol% or more, 4.0 mol% or more, 4.5 mol% or more, 5.0 mol% or more, or 5.5 mol% or more; and 20 mol% or less, 18 mol% or less, 15 mol% or less, or 14 mol% or less.

[0124] Cyclohexanediethanol (e.g., 1,2-cyclohexanediethanol, 1,3-cyclohexanediethanol, and 1,4-cyclohexanediethanol) can be a component that helps improve the transparency and impact resistance of the copolyester resin. Preferably, the content of cyclohexanediethanol residues can be 5 to 35 mol%, 6 to 33 mol%, or 7 to 30 mol% based on the total residues of 100 mol% of the copolyester resin.

[0125] Step (S-2): Preparation of stretched polyester film

[0126] In step (S-2), a stretched polyester film is prepared using the polyester resin obtained in step (S-1).

[0127] According to one embodiment, the stretched polyester film may comprise 100% by weight of copolyester resin.

[0128] Furthermore, stretched polyester films may further comprise different types of crystalline resins. Specifically, stretched polyester films may comprise a blend of copolyester resin and different types of crystalline resins. The different types of crystalline resins contained in the blend and their amounts are as described above.

[0129] Meanwhile, the preparation of stretched polyester films can be carried out using conventionally known methods. Specifically, stretched films can be prepared through extrusion steps involving polyester resin, stretching steps, and / or heat setting steps.

[0130] Specifically, the preparation of stretched polyester films can be achieved by extruding (melt extrusion) polyester resin or a mixture of polyester resin and different types of crystalline resin to prepare unstretched sheets; then stretching the unstretched sheets.

[0131] The extrusion (melt extrusion) step of polyester resin or a mixture of polyester resin and different types of crystalline resin can be carried out using an extruder. In this case, there are no particular limitations on the melt extrusion temperature, which can be 180°C to 310°C, 200°C to 310°C, 230°C to 310°C, 240°C to 300°C, or 250°C to 290°C. This step yields an unstretched sheet, which can then be transferred to a stretching step. Before the stretching step, the unstretched sheet can be preheated to a predetermined temperature (e.g., 50°C to 120°C).

[0132] The stretching step can be performed by uniaxial stretching of the unstretched sheet obtained from the extrusion (melt extrusion) step in the longitudinal direction (MD) or perpendicular to the longitudinal direction (TD); or by biaxial stretching of the sheet in both the longitudinal direction (MD) and perpendicular to the longitudinal direction (TD).

[0133] Longitudinal (MD) stretching can be performed at a stretch ratio of 1 to 5 times (100% to 500%) or 1.1 to 4.5 times (110% to 450%) at temperatures ranging from 55°C to 180°C or from 60°C to 170°C. Furthermore, perpendicular-to-longitudinal (TD) stretching can be performed at a stretch ratio of 1.5 to 6 times (150% to 600%) or 2.5 to 5.5 times (250% to 550%) at temperatures ranging from 55°C to 180°C or from 60°C to 170°C.

[0134] The stretched polyester films prepared by the above process meet a certain tear index range, and the tear index can be controlled by various methods.

[0135] Specifically, the tear index of a stretched polyester film may be adjusted by factors such as the type of resin contained in the stretched polyester film, the type and proportion of monomers constituting the resin, the physical properties of the resin, stretching conditions (stretching temperature, stretch ratio, etc.), and the physical properties of the stretched polyester film prepared from the resin.

[0136] Furthermore, because the tear index, after this adjustment, has a specific range, it can be advantageously used for heat-shrinkable films in packaging materials for plastic containers (e.g., PET bottles) and / or labels. Additionally, the plastic containers and labels can be easily separated during recycling, significantly improving recycling efficiency.

[0137] Products

[0138] According to one embodiment, an article formed from a stretched polyester film is provided.

[0139] Specifically, the article is made of a stretched polyester film comprising a polyester resin copolymerized from a diol component and a dicarboxylic acid component, and has a tear index of less than 2.5 as expressed by Equation 1 above.

[0140] The product can be obtained by molding a resin containing polyester resin through molding methods such as extrusion and injection. It can be a film (or sheet) or component used in the automotive, power and electronics industries.

[0141] In addition, the article may include labels or caps for various containers (such as plastic) or packaging.

[0142] Preferred embodiments of the present invention are given below to better understand the invention. However, the following examples are only for aiding understanding of the invention, and the scope of the invention is not limited thereto. Detailed Implementation

[0143] Preparation of stretched polyester films

[0144] Example 1

[0145] (1) Preparation of copolyester resin

[0146] Step (1): Esterification reaction

[0147] Terephthalic acid (TPA, 2,350 g), ethylene glycol (EG, 970 g), 1,4-cyclohexanediethanol (CHDM, 330 g), and diethylene glycol (DEG, 220 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0148] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0149] Step (2): Polycondensation reaction

[0150] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out at a reactor pressure of 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted accordingly if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.70 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0151] (2) Preparation of stretched polyester films

[0152] Copolyester resin is fed into an extruder and melt-extruded at a temperature of 180°C to 310°C to prepare unstretched polyester sheets. Subsequently, the unstretched sheets are heated to a temperature 5°C to 20°C higher than the glass transition temperature (Tg) of the polyester resin and stretched only in the transverse direction (TD) at a stretch ratio of 5 times. Then, they are heat-set to produce stretched polyester films with a thickness of 50 μm.

[0153] Example 2

[0154] (1) Preparation of copolyester resin

[0155] Step (1): Esterification reaction

[0156] Terephthalic acid (TPA, 305 g), 1,4-cyclohexanediethanol (CHDM, 900 g), recovered dihydroxyethyl terephthalate (r-BHET, 4200 g), and diethylene glycol (DEG, 10 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0157] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0158] Step (2): Polycondensation reaction

[0159] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted accordingly if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.68 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0160] (2) Preparation of stretched polyester films

[0161] Stretched polyester films were prepared using the same process as in Example 1.

[0162] Example 3

[0163] (1) Preparation of copolyester resin

[0164] Step (1): Esterification reaction

[0165] Terephthalic acid (TPA, 3,000 g), ethylene glycol (EG, 1,170 g), 1,4-cyclohexanediethanol (CHDM, 700 g), and diethylene glycol (DEG, 190 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0166] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0167] Step (2): Polycondensation reaction

[0168] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted appropriately if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.72 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0169] (2) Preparation of stretched polyester films

[0170] Stretched polyester films were prepared using the same process as in Example 1.

[0171] Example 4

[0172] (1) Preparation of copolyester resin

[0173] Step (1): Esterification reaction

[0174] Terephthalic acid (TPA, 2,350 g), ethylene glycol (EG, 960 g), 1,4-cyclohexanediethanol (CHDM, 430 g) and diethylene glycol (DEG, 140 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0175] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0176] Step (2): Polycondensation reaction

[0177] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted accordingly if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.70 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0178] (2) Preparation of stretched polyester films

[0179] Stretched polyester films were prepared using the same process as in Example 1.

[0180] Example 5

[0181] (1) Preparation of copolyester resin

[0182] Step (1): Esterification reaction

[0183] Terephthalic acid (TPA, 3,300 g), ethylene glycol (EG, 1,300 g), 1,4-cyclohexanediethanol (CHDM, 430 g), 1,4-cyclohexanedicarboxylic acid (CHDA, 300 g), and diethylene glycol (DEG, 200 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0184] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0185] Step (2): Polycondensation reaction

[0186] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted appropriately if the stirring force weakens due to increased reactant viscosity or reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.75 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0187] (2) Preparation of stretched polyester films

[0188] Stretched polyester films were prepared using the same process as in Example 1.

[0189] Example 6

[0190] (1) Preparation of copolyester resin

[0191] Step (1): Esterification reaction

[0192] Terephthalic acid (TPA, 2,300 g), isophthalic acid (IPA, 30 g), ethylene glycol (EG, 960 g), 1,4-cyclohexanediethanol (CHDM, 420 g) and diethylene glycol (DEG, 150 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0193] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0194] Step (2): Polycondensation reaction

[0195] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted appropriately if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.72 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0196] (2) Preparation of stretched polyester films

[0197] Stretched polyester films were prepared using the same process as in Example 1. Except as shown in Table 1 below, the resin composition used was a mixture of the copolyester resin prepared in step (1) and recycled PET resin at a weight ratio of 70:30.

[0198] Example 7

[0199] (1) Preparation of copolyester resin

[0200] Step (1): Esterification reaction

[0201] Terephthalic acid (TPA, 3,000 g), ethylene glycol (EG, 1,200 g), recycled 1,4-cyclohexanediethanol (630 g), and diethylene glycol (DEG, 240 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0202] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0203] Step (2): Polycondensation reaction

[0204] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted accordingly if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.70 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0205] (2) Preparation of stretched polyester films

[0206] Stretched polyester films were prepared using the same process as in Example 1.

[0207] Example 8

[0208] (1) Preparation of copolyester resin

[0209] Step (1): Esterification reaction

[0210] Terephthalic acid (TPA, 2,300 g), ethylene glycol (EG, 930 g), 1,4-cyclohexanediethanol (CHDM, 490 g) and diethylene glycol (DEG, 130 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0211] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0212] Step (2): Polycondensation reaction

[0213] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted accordingly if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.70 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0214] (2) Preparation of stretched polyester films

[0215] Stretched polyester films were prepared using the same process as in Example 1. Except as shown in Table 1 below, the resin composition used was a mixture of the copolyester resin prepared in step (1) and recycled PET resin at a weight ratio of 75:25.

[0216] Example 9

[0217] (1) Preparation of copolyester resin

[0218] Step (1): Esterification reaction

[0219] Terephthalic acid (TPA, 2,250 g), ethylene glycol (EG, 800 g), 1,4-cyclohexanediethanol (CHDM, 650 g), neopentyl glycol (NPG, 30 g), and diethylene glycol (DEG, 140 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0220] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0221] Step (2): Polycondensation reaction

[0222] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted appropriately if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.72 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0223] (2) Preparation of stretched polyester films

[0224] Stretched polyester films were prepared using the same process as in Example 1.

[0225] Comparative Example 1

[0226] (1) Preparation of copolyester resin

[0227] Step (1): Esterification reaction

[0228] Terephthalic acid (TPA, 3,300 g), ethylene glycol (EG, 1,650 g), neopentyl glycol (NPG, 360 g), and diethylene glycol (DEG, 230 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0229] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0230] Step (2): Polycondensation reaction

[0231] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted accordingly if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.70 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0232] (2) Preparation of stretched polyester films

[0233] The stretched polyester film was prepared using the same process as in Example 1. Except as shown in Table 2 below, the copolyester resin prepared in step (1) was used.

[0234] Comparative Example 2

[0235] (1) Preparation of copolyester resin

[0236] Step (1): Esterification reaction

[0237] Terephthalic acid (TPA, 2,050 g), ethylene glycol (EG, 550 g), 1,4-cyclohexanediethanol (CHDM, 1,060 g), and diethylene glycol (DEG, 100 g) were added to a 10-liter reactor equipped with a water-cooled column and condenser.

[0238] Subsequently, nitrogen gas was injected into the reactor to increase the reactor pressure by 1.0 kgf / cm² above atmospheric pressure. 2 Then, the reactor temperature was raised to 220°C over 90 minutes and maintained at 220°C for 2 hours, followed by a second raising of the temperature to 260°C over the next 2 hours. Esterification was then carried out at 260°C while the mixture in the reactor was visually observed until it became transparent. During this process, byproducts were discharged through a column and condenser. After esterification was complete, nitrogen gas in the pressurized reactor was released to the outside, reducing the pressure inside the reactor to atmospheric pressure. The mixture was then transferred to a 7-liter reactor capable of reacting under vacuum.

[0239] Step (2): Polycondensation reaction

[0240] Within 30 minutes, the pressure in the 7-liter reactor is reduced from atmospheric pressure to 5 Torr (absolute pressure: 5 mmHg). Simultaneously, within one hour, the reactor temperature is raised to 280°C, and the polycondensation reaction is carried out while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or lower. At the start of the polycondensation reaction, the stirring speed can be set relatively high. As the polycondensation reaction proceeds, the stirring speed can be adjusted accordingly if the stirring force weakens due to increased reactant viscosity or the reactant temperature exceeding the set temperature. The polycondensation reaction continues until the intrinsic viscosity (IV) of the mixture (melt) in the reactor reaches 0.70 dl / g. When the intrinsic viscosity of the mixture in the reactor reaches the desired level, the polycondensation mixture is discharged outside the reactor to form a filament, which is then solidified with a coolant and granulated to an average weight of approximately 12-14 mg to prepare a copolyester resin (polymer).

[0241] (2) Preparation of stretched polyester films

[0242] Stretched polyester films were prepared using the same process as in Example 1.

[0243] Comparative Example 3

[0244] Commercially available oriented polystyrene film was used in Comparative Example 3.

[0245] [Evaluation Example]

[0246] The physical properties of the copolyester resins or polystyrene resins of Examples 1 to 9 and Comparative Examples 1 to 3 were measured using the following methods, and the results are shown in the table below.

[0247] Evaluation Example 1: Intrinsic Viscosity

[0248] At 150°C, each prepared copolyester resin was dissolved in a 0.12% o-chlorophenol (OCP) solution, and the intrinsic viscosity of the solution was measured using an Ubbelohde viscometer in a 35°C constant-temperature water bath. Specifically, the time (t0) required for the solvent to pass through a specific inner section of the viscometer tube (outflow time) and the time (t) required for the solution to pass through that inner section were measured. Subsequently, the values ​​of t0 and t were substituted into Equation 3 to calculate the specific viscosity. The calculated specific viscosity value was then substituted into Equation 4 to calculate the intrinsic viscosity.

[0249] [Equation 3]

[0250] [Equation 4]

[0251] Evaluation Example 2: Glass Transition Temperature (Tg)

[0252] The glass transition temperature was determined using differential scanning calorimetry (DSC). A Mettler Toledo DSC1 instrument was used as the measuring device.

[0253] The glass transition temperature of each copolyester resin prepared was measured when the resin sample was heated at a rate of 10 k / min in the range of 25°C to 150°C.

[0254] Evaluation Example 3: Shrinkage Stress

[0255] The shrinkage stress (unit: MPa) of each copolyester resin was determined by fixing both ends of the film in a direction perpendicular to the longitudinal direction (TD) after shrinkage began under hot air conditions at 85°C, and measuring the final shrinkage stress (TD) at 30°C.

[0256] Evaluation Example 4: Tear Strength

[0257] Longitudinal (TD) tear strength (TS) MD )

[0258] TS MD This is the tear strength (N / mm) of a stretched polyester film sample in the longitudinal direction (MD). It is determined according to ASTM D1922: the prepared copolyester resin is extruded into a sheet, stretched by 500% perpendicular to the longitudinal direction, and then tested using a Leading Instruments TGT-01 tear tester at a weight (g) 16 times the thickness (m) of the stretched polyester film sample. The obtained value is then converted according to the thickness of the stretched polyester film sample.

[0259] Tear strength (TS) in the direction perpendicular to the longitudinal direction (TD) TD )

[0260] TS TD This is the tear strength (N / mm) of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD). It is determined according to ASTM D1922: the prepared copolyester resin is extruded into a sheet, stretched 500% parallel to the longitudinal direction, and then tested using a Leading Instruments TGT-01 tear tester at a weight (g) eight times the thickness (m) of the stretched polyester film sample. The obtained value is then converted according to the thickness of the stretched polyester film sample.

[0261] Evaluation Example 5: Tear Index

[0262] The tear strength and glass transition temperature measured in the above evaluation example are used in Equation 1 below to calculate the tear index.

[0263] [Equation 1]

[0264] In Equation 1, TS MD TS is the tear strength (N / mm) of a stretched polyester film sample in the machine direction (MD), and it is a unitless value; TD DT is the tear strength (N / mm) of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD), and it is a unitless value; DT is the stretching temperature (°C) of the stretched polyester film, and it is a unitless value; Tg is the glass transition temperature (°C) of the polyester resin, and it is a unitless value.

[0265] Evaluation Example 6: Thermal Shrinkage Rate and Maximum Shrinkage Rate

[0266] The stretched polyester film samples obtained in Examples 1 to 9 and Comparative Examples 1 to 3 were immersed in warm water at 85°C for 10 seconds for heat shrinkage. Then, the length of the stretched polyester film sample in the direction perpendicular to the longitudinal direction (MD) and the length in the longitudinal direction (MD) were measured. These were used in Equations 2-1 and 2-2 below to calculate the heat shrinkage rate (TTS) in the direction perpendicular to the longitudinal direction (TD). 85 ) and longitudinal (MD) thermal shrinkage rate (MTS) 85 The results are shown in Tables 3 and 4 below.

[0267] Sample preparation: The stretched polyester film was cut into 4cm×4cm pieces and stored at room temperature (25℃).

[0268] [Equation 2-1]

[0269] In equation 2-1, TL 25 It is the length of a stretched polyester film sample along the direction perpendicular to the longitudinal direction (TD) at 25°C. TL 85 It is the length of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD) after immersing it in hot water at 85°C for 10 seconds.

[0270] [Equation 2-2]

[0271] In equation 2-2, ML 25 It is the length of the stretched polyester film sample along the longitudinal direction (TD) at 25°C. ML 85It is the length along the longitudinal direction (MD) of a stretched polyester film sample after immersing it in hot water at 85°C for 10 seconds.

[0272] [Table 1]

[0273] [Table 2]

[0274] [Table 3]

[0275] [Table 4]

[0276] As can be seen from Tables 3 and 4 above, the tear index of the stretched polyester films in Examples 1 to 9 is controlled below 2.5, all of which have certain shrinkage characteristics, so that the containers will not deform, while having an appropriate tear strength range.

[0277] In contrast, the tear indexes of the stretched polyester films in Comparative Examples 1 and 2 both exceeded 2.5, indicating either excessively high tear strength or excessively low heat shrinkage in the longitudinal direction (TD) at 85°C. Specifically, the tear index of the stretched polyester film in Comparative Example 1 was 2.7. When stretched polyester films are used for labeling containers, they are not easily torn, resulting in poor product quality in packaging materials and labels, leading to low practicality.

[0278] Meanwhile, the stretched film using oriented polystyrene (OPS) resin in Comparative Example 3 exhibits a low thermal shrinkage rate perpendicular to the longitudinal direction (TD) at 85°C. Therefore, when applied to complex-shaped packaging applications, the uniformity of thermal shrinkage may be very low. Its thermal resistance is also low, thus its usability at high temperatures may be significantly lower than that of the stretched polyester films in the embodiments of this invention.

Claims

1. A stretchable polyester film comprising a polyester resin copolymerized from a diol component and a dicarboxylic acid component, and having a tear index of less than 2.5 as expressed by Equation 1 below: [Equation 1]: ， in, In equation 1, TS MD It is the tear strength (N / mm) of a stretched polyester film sample in the longitudinal direction (MD), and it is a unitless value. TS TD It is the tear strength (N / mm) of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD), and it is a unitless value; DT is the stretching temperature (°C) of polyester film, and it is a unitless value. Tg is the glass transition temperature (°C) of polyester resin, and it is a unitless value.

2. The stretched polyester film as described in claim 1, wherein, In Equation 1, TS MD / (TS TD ) 2 TS is 1 to 30. MD 50 N / mm or less, and TS TD It is 5.0 N / mm or less.

3. The stretched polyester film as described in claim 1, wherein, In Equation 1, DT–Tg ranges from 5°C to 20°C.

4. The stretched polyester film as claimed in claim 1, wherein the heat shrinkage rate (TTS) in the direction perpendicular to the longitudinal direction (TD) is shown by the following equation 2-1. 85 ) is 40% or higher: [Equation 2-1]: ， in, In equation 2-1, TL 25 It is the length of a stretched polyester film sample along the direction perpendicular to the longitudinal direction (TD) at 25°C. TL 85 It is the length of a stretched polyester film sample in the direction perpendicular to the longitudinal direction (TD) after immersing it in hot water at 85°C for 10 seconds.

5. The stretched polyester film as claimed in claim 1, wherein the thermal shrinkage rate (MTS) in the longitudinal direction (MD) is shown by the following equation 2-2. 85 The range is -20% to 15%. [Equation 2-2]: ， in, In equation 2-2, ML 25 It is the longitudinal length of a stretched polyester film sample at 25°C. ML 85 It is the length along the longitudinal direction (MD) of a stretched polyester film sample after immersing it in hot water at 85°C for 10 seconds.

6. The stretch polyester film as described in claim 1, wherein the shrinkage stress at 85°C is 12.0 MPa or less.

7. The stretched polyester film as described in claim 1, wherein, The diol component comprises ethylene glycol and at least one comonomer selected from the group consisting of diethylene glycol, neopentyl glycol, cyclohexanediethanol, recycled ethylene glycol, recycled diethylene glycol, recycled neopentyl glycol, and recycled cyclohexanediethanol.

8. The stretched polyester film as described in claim 1, wherein, The dicarboxylic acid component comprises at least one selected from the group consisting of terephthalic acid, dimethyl terephthalate, hydroxyethyl terephthalate, recovered terephthalic acid, recovered dimethyl terephthalate, and recovered hydroxyethyl terephthalate.

9. The stretched polyester film of claim 1, further comprising different types of crystalline resins.

10. The stretched polyester film as described in claim 9, wherein, The different types of crystalline resins comprise 10% to 50% by weight of polyethylene terephthalate resin based on the total weight of the resin in the stretched polyester film.

11. The stretched polyester film as claimed in claim 10, wherein, The polyethylene terephthalate resin is recycled polyethylene terephthalate resin (PCR-PET), or includes a resin containing structural units derived from terephthalic acid, wherein the terephthalic acid comprises at least one selected from the group consisting of recycled terephthalic acid, recycled dimethyl terephthalate, recycled bis(2-hydroxyethyl) terephthalic acid, and recycled hydroxyethyl terephthalic acid.

12. An article formed from the stretched polyester film of claim 1.

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