Polyester film and preparation method thereof

Abstract

An embodiment of the present invention provides a polyester film and a method for producing the same, the polyester film comprising a poly (cyclohexylenedimethylene terephthalate) resin, the tensile strength in the mechanical direction measured at a temperature of 25 DEG C being MDTS25, the tensile strength in the mechanical direction measured after being left in a container at a temperature of 200 DEG C for 24 hours being MDTS200, and the tensile strength in the mechanical direction measured after being left in the container at a temperature of 200 DEG C for 24 hours. The tensile strength reduction rate in the mechanical direction, which is represented by {(MDTS25-MDTS200) / MDTS25} * 100%, is 43.5% or less.

Description

Technical Field

[0001] Embodiments relate to a polyester film and a method for preparing the same. Background Art

[0002] Vehicles powered by internal combustion engines can utilize engine waste heat for heating through ventilation. However, electric vehicles, such as electric vehicles, have difficulty utilizing engine waste heat for heating, resulting in reduced heating efficiency and possibly slightly reduced mileage depending on the season.

[0003] To improve the heating efficiency of these electric vehicles, various heat-radiating components and structures are installed inside, and films are applied to protect these components. Materials with low elongation, such as polyethylene terephthalate (PET) and polyimide (PI), have poor formability and adhesion when applied to the curved surfaces of heat-radiating components, making it difficult to maintain effective heat conduction and heat generation.

[0004] Therefore, there is an urgent need to develop an improved film that has excellent formability and can also meet the high reliability required for use in the harsh conditions required for electric vehicles.

[0005] The above-mentioned background technology is the technical information possessed by the inventor in deriving the present invention or the technical information mastered in the process of deriving the present invention. Therefore, it cannot be considered as the public known technology disclosed to the public before applying for the present invention.

[0006] Related prior arts include “Biaxially oriented polyester film and method for producing the same” disclosed in Korean Patent Publication No. 10-2021-0088586 and “Biaxially oriented polyester film for molding” disclosed in Korean Patent Publication No. 10-2014-0113664. Summary of the Invention

[0007] Problems to be solved by the invention

[0008] The embodiment aims to provide a polyester film which has little change in physical properties even after long-term storage under harsh conditions such as high temperature and high pressure, and has excellent formability, and is suitable for use in heat-resistant and heat-dissipating parts of electric vehicles.

[0009] Another object of the embodiment is to provide a method for preparing a polyester film, which, through a low-temperature stretching process, does not change much in physical properties under harsh conditions.

[0010] Means used to solve problems

[0011] In order to achieve the above-mentioned object, a polyester film according to an embodiment includes a polycyclohexylene dimethylene terephthalate resin.

[0012] The mechanical direction tensile strength of the above-mentioned polyester film measured at a temperature of 25°C is MDTS25, and the mechanical direction tensile strength measured after being placed in a container at a temperature of 200°C for 24 hours is MDTS200. The mechanical direction tensile strength reduction rate expressed by {(MDTS25-MDTS200) / MDTS25}×100% can be less than 43.5%.

[0013] In one embodiment, the tensile strength in the width direction of the polyester film measured at a temperature of 25°C is TDTS25, and the tensile strength measured under the above-mentioned temperature condition of 200°C is TDTS200. The reduction rate of the tensile strength in the width direction expressed by {(TDTS25-TDTS200) / TDTS25}×100% can be less than 29%.

[0014] In one embodiment, the mechanical elongation of the polyester film measured at a temperature of 25°C is MDE25, and the mechanical elongation measured under the above-mentioned temperature condition of 200°C is MDE200. The change rate of mechanical elongation expressed by (|MDE25-MDE200| / MDE25)×100% can be less than 50%.

[0015] In one embodiment, the widthwise elongation of the polyester film measured at 25°C is TDE25, and the widthwise elongation measured at the above-mentioned temperature condition of 200°C is TDE200. The widthwise elongation change rate represented by (|TDE25-TDE200| / TDE25)×100% can be less than 15%.

[0016] In one embodiment, the polyester film may have a machine direction elongation of 35% or more measured at a temperature of 200° C., and a width direction elongation of 50% or more measured at a temperature of 200° C.

[0017] In one embodiment, the polyester film can be used as a heat-resistant component film for electric vehicles.

[0018] In one embodiment, the polycyclohexylene dimethylene terephthalate resin includes repeating units derived from dicarboxylic acid compounds and repeating units derived from diol compounds.

[0019] The repeating unit derived from the dicarboxylic acid compound may include 80 mol % to 100 mol % of a terephthalic acid residue and 0 mol % to 20 mol % of an isophthalic acid residue.

[0020] The repeating unit derived from the diol compound may contain 85 mol % to 100 mol % of cyclohexanedimethanol residues.

[0021] To achieve the above-mentioned purpose, a method for preparing a polyester film according to an embodiment includes: a sheeting step of melt-extruding a film-forming composition containing a polycyclohexylene dimethanol terephthalate resin to form a sheet; and a stretching step of stretching the sheet formed in the sheeting step in a machine direction and in a width direction, and heat-setting the sheet to prepare a polyester film.

[0022] The stretching step may include, in sequence, an MD stretching step of stretching in a machine direction and a TD stretching step of stretching in a width direction.

[0023] The MD stretching step may include: a preheating process of preheating the sheet formed in the sheet forming step; and an MD stretching process of stretching the preheated sheet in a machine direction.

[0024] The temperature of the preheating process may be 80° C. to 86.5° C., the temperature of the MD stretching process may be 80° C. to 89° C., and the width direction stretching temperature of the TD stretching step may be 100° C. to 118° C.

[0025] The mechanical direction tensile strength of the above-mentioned polyester film measured at 25°C is MDTS25, and the mechanical direction tensile strength measured after being placed in a container at a temperature of 200°C for 24 hours is MDTS200. The mechanical direction tensile strength reduction rate expressed by {(MDTS25-MDTS200) / MDTS25}×100% can be less than 43.5%.

[0026] In one embodiment, the TD stretching step includes: a primary preheating process, in which the sheet stretched in the MD stretching step is preheated once; a secondary preheating process, in which the sheet that has undergone the primary preheating process is preheated twice; and a TD stretching process, in which the sheet that has undergone the secondary preheating process is stretched in the width direction.

[0027] The temperature of the first preheating process may be 90°C to 103°C, and the temperature of the second preheating process may be 90°C to 108°C.

[0028] The TD stretching step may be a step of stretching the sheet 2.5 to 3.5 times.

[0029] The MD stretching step may be a step of stretching the sheet 3.3 to 4.5 times.

[0030] Effects of the Invention

[0031] The polyester film according to the embodiment is manufactured using a unique low-temperature stretching process and thus has properties suitable for heat-resistant and heat-dissipating components of electric vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The graph shows the results of the machine direction tensile strength at 25°C (MDTS25) and the machine direction tensile strength after 200°C high temperature test (MDTS200) of the samples of Examples 1 and 2 (E1 and E2) and Comparative Examples 1 to 3 (CE1 to CE3).

[0033] Figure 2 The graph shows the results of the width direction tensile strength (TDTS25) at 25°C and the width direction tensile strength (TDTS200) after a high temperature test at 200°C for samples of Examples 1 and 2 (E1 and E2) and Comparative Examples 1 to 3 (CE1 to CE3).

[0034] Figure 3 The graph shows the machine direction elongation (MDE25) at 25°C and the machine direction elongation (MDE200) after a high temperature test at 200°C for samples of Examples 1 and 2 (E1 and E2) and Comparative Examples 1 to 3 (CE1 to CE3).

[0035] Figure 4 The graph shows the results of the widthwise elongation at 25°C (TDE25) and the widthwise elongation after a high-temperature test at 200°C (TDE200) of the samples of Examples 1 and 2 (E1 and E2) and Comparative Examples 1 to 3 (CE1 to CE3).

[0036] Figure 5 It is a graph showing the results of the machine direction tensile strength reduction rate (MDTS_R) and the width direction tensile strength reduction rate (TDTS_R) of the samples of Examples 1 and 2 (E1 and E2) and Comparative Examples 1 to 3 (CE1 to CE3).

[0037] Figure 6 It is a graph showing the results of the machine direction elongation change rate (MDTS_R) and the width direction elongation change rate (TDTS_R) of the samples of Examples 1 and 2 (E1 and E2) and Comparative Examples 1 to 3 (CE1 to CE3). DETAILED DESCRIPTION

[0038] Hereinafter, one or more implementation methods will be described in detail with reference to the accompanying drawings to facilitate implementation of the present invention by one of ordinary skill in the art. However, the embodiments may be implemented in a variety of different ways and are not limited to the embodiments described in this specification. Throughout the specification, identical or similar components are denoted by the same reference numerals.

[0039] In this specification, when it is described that a certain component “includes” a certain component, unless there is a special description to the contrary, it means that other components are also included rather than excluded.

[0040] In this specification, when it is described that one component is “connected” to another component, it includes not only a case of being “directly connected” but also a case of being “connected with another component interposed therebetween.”

[0041] In this specification, B being located on A means that B is located on A in direct contact or with another layer therebetween, and should not be construed as being limited to B being located on the surface of A in contact.

[0042] In this specification, the term "combination of..." included in the Markush-type description refers to a mixture or combination of one or more constituent elements selected from the group consisting of the constituent elements of the Markush-type description, thereby meaning that the present invention includes one or more constituent elements selected from the group consisting of the above constituent elements.

[0043] Throughout the present specification, descriptions in the form of "A and / or B" mean "A or B, or A and B".

[0044] Throughout this specification, unless otherwise specified, terms such as "first," "second," "A," "B," etc. are used to distinguish the same terms from each other.

[0045] Unless otherwise specified, the expression of a single number in this specification is interpreted as including the meaning of the single number or plural numbers explained in the context.

[0046] polyester film

[0047] In order to achieve the above-mentioned object, a polyester film according to an embodiment includes polycyclohexylene dimethylene terephthalate resin.

[0048] The machine direction tensile strength measured at 25°C is MDTS25.

[0049] The mechanical direction tensile strength measured after being placed in a container at a temperature of 200°C for 24 hours is MDTS200.

[0050] The machine direction tensile strength reduction rate, MDTS_R, is represented by {(MDTS25-MDTS200) / MDTS25}×100%.

[0051] The MDTS_R of the polyester film may be 43.5% or less.

[0052] The polycyclohexylenedimethyleneterephthalate (PCT) resin of the polyester film may be formed by copolymerizing a dicarboxylic acid compound and a diol compound, and may contain residues and repeating units derived therefrom.

[0053] The poly(cyclohexanedimethylene terephthalate) may contain 80 mol% or more, 90 mol% or more, or 100 mol% or less of terephthalic acid residues and 20 mol% or less, 10 mol% or less, 0 mol% or more, 1 mol% or more, or 2 mol% or more of isophthalic acid residues, based on 100 mol% of the total repeating units derived from dicarboxylic acid compounds.

[0054] The polycyclohexanedimethanol terephthalate may contain 70 mol% or more, 80 mol% or more, 90 mol% or more, and 100 mol% or less of cyclohexanedimethanol residues based on 100 mol% of the total repeating units derived from the diol compound.

[0055] When the repeating unit derived from the dicarboxylic acid compound contains the terephthalic acid residue and the isophthalic acid residue in the above-mentioned amounts, it may have a relatively high melting point characteristic and a low crystallinity characteristic.

[0056] In addition to the repeating units derived from the above-mentioned cyclohexanedimethanol, the repeating units derived from the above-mentioned diol compounds may include repeating units derived from the following compounds. Exemplarily, ethylene glycol, 1,3-propylene glycol, 1,2-octanediol, 1,3-octanediol, 2,3-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol (neopentyl glycol), 2-butyl-2-ethyl-1,3-propanediol, 2,2-diethyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,1-dimethyl-1,5-pentanediol, and residues thereof may be included.

[0057] The polycyclohexylene dimethylene terephthalate resin may have a weight average molecular weight (Mw) of 30,000 g / mol to 50,000 g / mol, or 30,000 g / mol to 40,000 g / mol.

[0058] A catalyst may be applied to the polycyclohexylene dimethylene terephthalate resin to improve the efficiency of the polymerization reaction.

[0059] The polycyclohexylene dimethylene terephthalate may include the above catalyst in an amount of about 0.1 ppm to about 500 ppm, or may include the above catalyst in an amount of about 0.5 ppm to about 100 ppm, based on 100 parts by weight of the polycyclohexylene dimethylene terephthalate.

[0060] The catalyst may be a titanium compound, an antimony compound, a germanium compound, an aluminum compound, or a mixture thereof. For example, the catalyst may be a titanium compound. The titanium compound may include titanium tetraisopropoxide.

[0061] During the polymerization of the polycyclohexylene dimethylene terephthalate resin, an antioxidant may be used. To inhibit thermal oxidation at the temperatures used for the esterification reaction, such antioxidants may be used as needed. However, an appropriate amount of antioxidant is generally used at this time. Adding an excessive amount of antioxidant during the polymerization process may delay the reaction and potentially reduce the intrinsic viscosity of the resulting resin. Antioxidants that affect resin polymerization are consumed during the polymerization process and can be distinguished from antioxidants added during subsequent film formation.

[0062] The above antioxidants include phenolic antioxidants, phosphorus antioxidants and sulfur antioxidants.

[0063] The antioxidant may be included in an amount of 0.01 to 1 part by weight based on 100 parts by weight of the polycyclohexylene dimethylene terephthalate.

[0064] When preparing a film from the polycyclohexyl dimethylene terephthalate resin, an electrostatic applicator may be used. Examples of the electrostatic applicator include alkali metal salts and alkaline earth metal salts, as well as magnesium compounds and calcium compounds, such as magnesium acetate and calcium acetate.

[0065] The static electricity applicator may include the metal or metal ion in an amount of about 300 ppm to about 1000 ppm based on 100 parts by weight of the polycyclohexylene dimethylene terephthalate resin.

[0066] In the polyester film, the machine direction (MD) is the longitudinal direction parallel to the direction in which the film moves during the film forming process, and the transverse direction (TD) is the direction perpendicular to the machine direction.

[0067] The polyester film has a good physical property retention rate under harsh conditions such as the pressure vessel test, and can have characteristics suitable for use as a heat-resistant and heat-dissipating film.

[0068] The machine direction tensile strength measured at 25°C is MDTS25, and the machine direction tensile strength measured at the above 200°C temperature condition is MDTS200.

[0069] The polyester film may have a machine direction tensile strength reduction rate MDTS_R (expressed as {(MDTS25 - MDTS200) / MDTS25} × 100%) of 43.5% or less. The machine direction tensile strength reduction rate may be 42.6% or less, 41.6% or less, or 30% or less. Furthermore, the machine direction tensile strength reduction rate may be 10% or greater.

[0070] The tensile strength in the width direction measured at a temperature of 25°C is TDTS25, and the tensile strength in the width direction measured at the above-mentioned temperature condition of 200°C is TDTS200.

[0071] The polyester film may have a widthwise tensile strength reduction rate TDTS_R represented by {(TDTS25 - TDTS200) / TDTS25} × 100% of 29% or less. The widthwise tensile strength reduction rate may be 27.7% or less, 21.4% or less, or 18% or less. The widthwise tensile strength reduction rate may be 5% or more.

[0072] Since the polyester film has the aforementioned tensile strength reduction rates in the machine direction and the width direction, it can maintain good durability under harsh conditions and effectively maintain the function of protecting heat-resistant and heat-dissipating components.

[0073] The MDTS25 of the polyester film can be 12 kgf / mm 2 Up to 20kgf / mm 2 、14kgf / mm 2 Up to 19kgf / mm 2 or 15kgf / mm 2 Up to 18kgf / mm 2 .

[0074] The MDTS200 of the polyester film can be 5 kgf / mm 2 Up to 15kgf / mm 2 , 6kgf / mm 2 Up to 14kgf / mm 2 or 8.6kgf / mm 2 Up to 13kgf / mm 2 .

[0075] The TDTS25 of the polyester film can be 12 kgf / mm 2 Up to 25kgf / mm 2 、15kgf / mm 2 Up to 24kgf / mm 2 or 16kgf / mm 2 Up to 23kgf / mm 2 .

[0076] The TDTS200 of the polyester film can be 8 kgf / mm 2 Up to 18kgf / mm 2 , 9kgf / mm 2 Up to 17kgf / mm 2 or 11kgf / mm 2 Up to 16kgf / mm2 .

[0077] The polyester film has the above-mentioned range of MDTS25, MDTS200, TDTS25 and / or TDTS200, so that there will be no drastic change in physical properties between room temperature and harsh conditions, and it can maintain good durability and effectively maintain the function of protecting heat-resistant and heat-dissipating components.

[0078] The machine direction elongation measured at a temperature of 25°C is MDE25, and the machine direction elongation measured at the above-mentioned temperature condition of 200°C is MDE200.

[0079] The polyester film may have a machine direction elongation change rate MDE_D (expressed as (|MDE25 - MDE200| / MDE25) × 100%) of 50% or less. The machine direction elongation change rate may be 40% or less, 35.3% or less, 20% or less, or 2.4% or less. The machine direction elongation change rate may be 0.1% or greater. The symbol "||" in the above-mentioned formula for the machine direction elongation change rate and the following formulas indicates an absolute value.

[0080] The widthwise elongation measured at 25°C is TDE25, and the widthwise elongation measured at 200°C is TDE200.

[0081] The polyester film may have a widthwise elongation variation rate (TDE_D) represented by (|TDE25 - TDE200| / TDE25) × 100% of 15% or less. The widthwise elongation variation rate may be 12% or less, 9.4% or less, or 7.2% or less. The widthwise elongation variation rate may be 0.1% or greater.

[0082] The polyester film has the above-mentioned elongation variation rates in the machine direction and the width direction, and thus can maintain good durability under severe conditions and effectively maintain the function of protecting heat-resistant and heat-dissipating components.

[0083] The MDE25 of the polyester film may be 40% to 110%, 45% to 105%, or 55% to 100%.

[0084] The MDE200 of the polyester film may be 25% to 90%, 35% to 85%, or 40% to 80%.

[0085] The TDE25 of the polyester film may be 45% to 90%, 47% to 80%, or 50% to 75%.

[0086] The TDE200 of the polyester film may be 45% to 90%, 47% to 80%, or 55% to 80%.

[0087] The polyester film has the above-mentioned ranges of MDE25, MDE200, TDE25 and / or TDE200, so there will be no drastic difference in physical properties between room temperature and harsh conditions, it can maintain good durability, and can effectively maintain the function of protecting heat-resistant and heat-dissipating components.

[0088] The elongation refers to the ratio of the length of the film stretched before breaking to the initial length, which can be calculated according to the following formula.

[0089] Elongation = {(length after stretching - initial length) / initial length} × 100%

[0090] Physical properties related to the tensile strength and elongation can be measured according to ASTM D882, and can be measured using an Instron Model 4206-0010 device as described in the experimental examples below.

[0091] The polyester film may have a thickness of 1 μm to 1000 μm or 10 μm to 500 μm.

[0092] The polyester film can be produced by a unique low-temperature stretching process in the following preparation method, thereby having properties suitable for heat-resistant and heat-dissipating components of electric vehicles.

[0093] In order to achieve the above-mentioned object, a heat radiation member for an electric vehicle according to an embodiment may include the above-mentioned polyester film.

[0094] The heat radiation member may include a carbon-based material, for example, graphite, carbon nanotube, carbon fiber, graphene, diamond, fullerene, etc.

[0095] Preparation method of polyester film

[0096] To achieve the above-mentioned object, a method for producing polyester according to an embodiment includes: a sheeting step of melt-extruding a film-forming composition containing a poly(cyclohexylene dimethylene terephthalate) resin to form a sheet; and a stretching step of stretching the sheet formed in the sheeting step in a machine direction and in a width direction and heat-setting the sheet to produce a polyester film.

[0097] The stretching step may include: an MD stretching step of stretching the sheet formed in the sheet forming step in a machine direction; and a TD stretching step of stretching the sheet stretched in the MD stretching step in a width direction and heat-setting the sheet, thereby preparing a polyester film.

[0098] The MD stretching step includes: a preheating process of preheating the sheet formed in the sheet forming step; and an MD stretching process of stretching the preheated sheet in a machine direction.

[0099] The temperature of the preheating process may be 80°C to 86.5°C.

[0100] The temperature during the MD stretching process may be 80°C to 89°C.

[0101] The width direction stretching temperature in the TD stretching step may be 100°C to 118°C.

[0102] The tensile strength in the machine direction of the polyester film measured at 25° C. is MDTS25, and the tensile strength in the machine direction of the polyester film measured under pressure vessel test conditions is MDTS200.

[0103] The pressure cooker test was conducted by placing the container in an oven at a temperature of 200° C., a pressure of 1.4 atm, and a relative humidity of 100% for 24 hours.

[0104] The polyester film has the characteristics and structure of the aforementioned polyester film. For example, the machine direction tensile strength reduction rate MDTS_R of the polyester film, represented by {(MDTS25-MDTS200) / MDTS25}×100%, may be 43.5% or less.

[0105] The composition for preparing the film may include polycyclohexene dimethanol terephthalate resin, an antioxidant, and an electrostatic applicator, and may be melt-extruded during the film preparation process. The antioxidant and electrostatic applicator are as described above and will not be described again here.

[0106] The above composition for preparing a film may be dried before melting.

[0107] The drying may be performed at a temperature of 150°C or lower. Alternatively, the drying may be performed at a temperature ranging from 70°C to 148°C.

[0108] The composition for preparing a film may be dried so that the moisture content of the composition is 100 ppm or less or 50 ppm or less relative to the total moisture content. When the drying process is performed at a temperature higher than 150° C., the resin itself may undergo an unexpected color change.

[0109] The above-mentioned composition for preparing a film may be in the form of sheets, granules, plates, etc., and may be in a form that can be easily put into a film preparation process and efficiently mixed.

[0110] The polycyclohexylene dimethylene terephthalate resin in the above composition for preparing the film can be prepared by a conventional polymerization method. For example, it can be polymerized in the presence of a catalyst containing a metal such as titanium or antimony.

[0111] As described above, the polycyclohexylene dimethylene terephthalate resin in the composition for preparing the film may be prepared by copolymerizing a dicarboxylic acid compound and a diol compound, and may include repeating units derived therefrom.

[0112] The extrusion in the sheeting step may be performed at a temperature of 230°C to 300°C, or at a temperature of 250°C to 290°C.

[0113] The preheating process in the MD stretching step can be achieved by heat-treating the sheet at a temperature of 80° C. to 86.5° C. for 10 seconds to 1 minute. Alternatively, the preheating process in the MD stretching step can be achieved by heat-treating the sheet at a temperature of 83° C. to 86° C. for the same period of time.

[0114] The MD stretching process in the MD stretching step can be achieved by stretching the preheated sheet in the machine direction by 2.5 to 3.5 times at a temperature of 80° C. to 89° C. Alternatively, the MD stretching process can be performed at a temperature range of 85° C. to 89° C., or 87° C. to 89° C., at the same ratio as above.

[0115] The MD stretching step may include heating the preheated unstretched sheet by an infrared heater positioned 30 to 200 mm above or below the preheated unstretched sheet. The surface temperature of the infrared heater may be 500 to 800°C.

[0116] In the MD stretching step, through the preheating process and the MD stretching process, the obtained film can meet the elongation and strength characteristics required under severe conditions.

[0117] The TD stretching step may include: a primary preheating process, preheating the sheet stretched in the MD stretching step; a secondary preheating process, preheating the sheet that has undergone the primary preheating process; and a TD stretching process, stretching the sheet that has undergone the secondary preheating process in the width direction.

[0118] The primary preheating process may be performed at a temperature of 90° C. to 103° C. for 10 to 60 seconds. Alternatively, the primary preheating process may be performed at a temperature of 95° C. to 103° C., or 98° C. to 102° C. for the same time as above.

[0119] The secondary preheating process may be performed at a temperature of 90° C. to 108° C. for 10 to 60 seconds. Alternatively, the secondary preheating process may be performed at a temperature of 97° C. to 105° C. or 100° C. to 105° C. for the same time as above.

[0120] The TD stretching process can be achieved by stretching the sheet that has undergone the secondary preheating process by 3.3 to 4.5 times in the width direction at a temperature of 100° C. to 118° C. Alternatively, the TD stretching process can be achieved by stretching the sheet that has undergone the secondary preheating process by 3.3 to 4.5 times in the width direction at a temperature of 105° C. to 115° C. or at a temperature of 107° C. to 113° C. at the same ratio as above in the width direction.

[0121] The heat setting in the TD stretching step may be performed at a temperature of 200° C. to 250° C. for 5 to 600 seconds. Alternatively, the heat setting in the TD stretching step may be performed for 10 to 200 seconds.

[0122] In the TD stretching step, the preheating process and the TD stretching process can enable the obtained film to meet the elongation and strength characteristics required under harsh conditions.

[0123] The film after the TD stretching step may be subjected to a predetermined relaxation treatment in the longitudinal direction and / or the width direction. The temperature of the relaxation treatment may be 150° C. to 250° C. The relaxation rate of the relaxation treatment may be 1% to 10%, or 3% to 7%.

[0124] Hereinafter, the present invention will be described in more detail through specific examples. The following examples are only examples for helping to understand the present invention, and the scope of the present invention is not limited thereto.

[0125] Example 1 - Preparation of Low-Temperature Stretched PCT Film

[0126] A monomer mixture of 100 mol% cyclohexanedimethanol (CHDM) as a diol compound, 96 mol% terephthalic acid (TPA) as a dicarboxylic acid compound, and 4 mol% isophthalic acid (IPA) was placed in a stirrer. 1 ppm of a titanium catalyst was then added based on 100 parts by weight of the mixture, and an ester exchange reaction was carried out at 275°C.

[0127] The material after the transesterification reaction was transferred to a separate reactor equipped with a vacuum device, and then polymerized at 285° C. for 160 minutes to obtain polycyclohexyl dimethylene terephthalate (PCT) resin.

[0128] The PCT resin can be processed into masterbatch chips together with antioxidants and electrostatic applicators, and then dried at 140°C. The raw materials are then placed in an extruder, extruded into sheets at a temperature of approximately 295°C, and cast onto a casting roll.

[0129] The extruded sheet was preheated at 83°C for 30 seconds and then stretched 3 times in the machine direction (MD) at 85°C. During the MD stretching, the film was further heated by infrared heaters at the upper and lower ends, each spaced 80 mm apart from the upper and lower portions of the film. The surface temperature of the upper heater was 600°C, and the surface temperature of the lower heater was 500°C. Subsequently, a primary preheating treatment was performed at 95°C for 10 seconds, a secondary preheating treatment was performed at 100°C for 30 seconds, and the sheet was stretched 3.5 times in the width direction (TD) at 110°C. The stretched sheet was then heat-set at 240°C for approximately 30 seconds and then relaxed to produce a PCT film having a thickness of 50 μm.

[0130] Example 2 - Preparation of Low-Temperature Stretched PCT Film

[0131] In the above Example 1, the extruded sheet was preheated to 85° C. and stretched 3.2 times in the machine direction to produce a PCT film.

[0132] Comparative Example 1-Preparation of PCT Film 1

[0133] In the above-mentioned Example 1, the preheating temperature before stretching in the machine direction (MD) was changed to 87°C, the temperature during stretching in the machine direction was changed to 90°C, the first preheating temperature before stretching in the width direction (TD) was changed to 105°C, the second preheating temperature was changed to 110°C, the temperature during stretching in the width direction was changed to 120°C, and the heat setting temperature was changed to 230°C, thereby preparing a PCT film.

[0134] Comparative Example 2-Preparation of PCT Film 2

[0135] In the above-mentioned Example 1, the preheating temperature before stretching in the machine direction (MD) is changed to 92°C, the temperature during stretching in the machine direction is changed to 95°C, the first preheating temperature before stretching in the width direction (TD) is changed to 115°C, the second preheating temperature is changed to 115°C, the temperature during stretching in the width direction is changed to 120°C, the surface temperature of the above-mentioned upper heater is set to 600°C, and the surface temperature of the above-mentioned lower heater is set to 500°C, and the PCT film is prepared in this way.

[0136] Comparative Example 3-Preparation of PCT Film 3

[0137] In the above-mentioned Comparative Example 2, the heating process using the above-mentioned infrared heater was omitted to prepare a PCT film.

[0138] The above examples and comparative examples are summarized in Table 1 below (temperature unit: °C).

[0139] Table 1

[0140]

[0141] Experimental Example - Determination of tensile strength and elongation before and after 200℃ high temperature test

[0142] The tensile strength and elongation of the PCT films prepared in Examples 1 and 2 and Comparative Examples 1 to 3 were measured as follows.

[0143] The films in the above-mentioned embodiments and comparative examples were cut and processed into samples of 100 mm × 15 mm. Tensile tests were performed in the machine direction (MD) and width direction (TD) using an Instron 4206-001 apparatus at room temperature, i.e., 25°C, at a speed of 50 mm / min according to ASTM D882. The tests were repeated five times in each direction to obtain the average values ​​of the machine direction tensile strength (MDTS25), width direction tensile strength (TDTS25), machine direction elongation (MDE25), and width direction elongation (TDE25).

[0144] Subsequently, each film in the above-mentioned embodiments and comparative examples was processed into samples using the same method. After standing in an oven at a temperature of 200°C for 24 hours, a tensile test was performed in the same manner as above to measure the mechanical direction tensile strength (MDTS200), width direction tensile strength (TDTS200), mechanical direction elongation (MDE200) and width direction elongation (TDE200) after the high-temperature test.

[0145] The results of the tensile strength, elongation, reduction rate and change rate of the samples before and after the high temperature test are shown in Table 2, Table 3 and Figures 1 to 5 As shown. Figures 1 to 5 In the table, E represents an example, CE represents a comparative example, MDTS_R represents a decrease in tensile strength in the machine direction, TDTS_R represents a decrease in tensile strength in the width direction, MDE_D represents a change in elongation in the machine direction, and TDE_D represents a change in elongation in the width direction.

[0146] Table 2

[0147] Classification MDTS25 MDTS200 TDTS25 TDTS200 MDE25 MDE200 TDE25 TDE200 Example 1 16 11.9 16.8 13.9 61 62 64 70 Example 2 16.1 9.4 17.1 13.4 74 48 61 65 Comparative Example 1 17 8.4 19.6 13.5 88 12 60 41 Comparative Example 2 16.3 9.0 19.8 13.1 93 24 51 34 Comparative Example 3 15.1 7.5 20.1 13.9 78 4 51 38

[0148] Tensile strength unit: kgf / mm 2 , elongation unit: %

[0149] Table 3

[0150]

[0151]

[0152] Refer to Table 2, Table 3 and Figures 1 to 4 The results show that the examples that underwent low-temperature stretching in both the machine and width directions exhibited superior performance in terms of machine direction tensile strength reduction (MDTS_R), width direction tensile strength reduction (TDTS_R), machine direction elongation change (MDE_D), and width direction elongation change (TDE_D) compared to the comparative examples that did not undergo this process. The changes in physical properties were significantly less than those of the comparative examples under long-term exposure to high temperature and pressure, demonstrating their suitability for use as heat-resistant and heat-dissipating films in applications such as electric vehicles.

[0153] The preferred embodiments of the present invention are described in detail above, but the scope of the present invention is not limited thereto. Various modifications and improvements made by ordinary technicians in the technical field of the present invention using the basic concepts of the present invention defined in the appended claims also fall within the scope of the present invention.

Claims

1. A polyester film, characterized in that Contains polycyclohexylene dimethylene terephthalate resin, The mechanical direction tensile strength measured at 25°C is MDTS25. The mechanical direction tensile strength measured after being placed in a container at 200°C for 24 hours is MDTS200. The reduction rate of tensile strength in the machine direction represented by {(MDTS25-MDTS200) / MDTS25}×100% is 43.5% or less.

2. The polyester film according to claim 1, wherein The width direction is the direction perpendicular to the above machine direction, The tensile strength in the width direction measured at 25°C is TDTS25. The tensile strength measured at the above temperature of 200°C is TDTS200. The reduction rate of tensile strength in the width direction represented by {(TDTS25-TDTS200) / TDTS25}×100% is 29% or less.

3. The polyester film according to claim 1, wherein The mechanical elongation measured at 25°C is MDE25. The mechanical elongation measured at the above temperature of 200°C is MDE200. The rate of change in machine direction elongation expressed by (|MDE25-MDE200| / MDE25)×100% is 50% or less.

4. The polyester film according to claim 1, wherein The width direction is the direction perpendicular to the above machine direction, The widthwise elongation measured at 25°C is TDE25. The widthwise elongation measured at the above-mentioned temperature of 200°C is TDE200. The change rate of elongation in the width direction represented by (|TDE25-TDE200| / TDE25)×100% is 15% or less.

5. The polyester film according to claim 1, wherein The mechanical elongation measured at the above temperature of 200°C is more than 35%. The elongation in the width direction measured under the above-mentioned temperature condition of 200° C. is 50% or more.

6. The polyester film according to claim 1, wherein The polyester film is used as a film for heat-resistant parts of electric vehicles.

7. The polyester film according to claim 1, wherein The polycyclohexylene dimethylene terephthalate resin comprises repeating units derived from dicarboxylic acid compounds and repeating units derived from diol compounds. The repeating units derived from the dicarboxylic acid compound contain 80 mol% to 100 mol% of terephthalic acid residues and 0 mol% to 20 mol% of isophthalic acid residues. The repeating units derived from the diol compound contain 85 mol % to 100 mol % of cyclohexanedimethanol residues.

8. A method for preparing a polyester film, characterized in that: include: In the sheeting step, a film-forming composition containing polycyclohexylene dimethylene terephthalate resin is melt-extruded to form a sheet. MD stretching step, stretching the sheet formed in the sheet forming step in the machine direction, and a TD stretching step of stretching the sheet stretched in the MD stretching step in the width direction and heat-setting the sheet to prepare a polyester film; The above-mentioned MD stretching step includes: Preheating process, preheating the sheet formed in the above sheeting step, and MD stretching process, stretching the sheet after the above preheating process in the machine direction; The temperature of the above preheating process is 80°C to 86.5°C; The temperature of the MD stretching process is 80°C to 89°C; The width direction stretching temperature in the TD stretching step is 100° C. to 118° C.; The mechanical direction tensile strength of the above-mentioned polyester film measured at 25°C is MDTS25, and the mechanical direction tensile strength measured after being placed in a container at a temperature of 200°C for 24 hours is MDTS200. The mechanical direction tensile strength reduction rate expressed by {(MDTS25-MDTS200) / MDTS25}×100% is less than 43.5%.

9. The method for preparing a polyester film according to claim 8, wherein: The above-mentioned TD stretching step includes: The primary preheating process is to preheat the sheet stretched in the MD stretching step once, the secondary preheating process is to preheat the sheet after the primary preheating process twice, and The TD stretching process stretches the sheet in the width direction after the secondary preheating process; the temperature of the primary preheating process is 90°C to 103°C; The temperature of the secondary preheating process is 90°C to 108°C.

10. The method for preparing a polyester film according to claim 8, wherein: In the TD stretching step, the sheet is stretched 2.5 to 3.5 times. In the MD stretching step, the sheet is stretched 3.3 to 4.5 times.

Images