Stretched polyester film and method for preparing the same

Abstract

The present invention relates to a stretched polyester film and a method for preparing the same. The stretched polyester film contains a polyester resin in which a diol component and a dicarboxylic acid component are polymerized, and the heat shrinkage rate at 80°C in the main shrinkage direction of the film is 40% or more, and when the film is analyzed by differential scanning calorimetry (DSC) during the first and second heat treatments, the first melting point (T m1 ) and the second melting point (T m2 ) appears. The stretched polyester film according to the present invention has excellent heat shrinkability and improves the efficiency of the recycling process for waste plastic containers.

Description

Detailed Description of the Invention

[0001] [Technical Field] The present invention relates to a stretched polyester film having heat shrinkability and controlled crystallinity for improving the efficiency of the recycling process, and a method for preparing the stretched polyester film.

[0002] [Background Art] Among polymers, polyester is used as a material in various fields due to its excellent mechanical strength, heat resistance, transparency, and gas barrier properties. In particular, heat-shrinkable films prepared using polyester resins are suitable for packaging and / or labels for plastic containers such as PET bottles because they have high heat resistance and an appropriate heat shrinkage rate.

[0003] On the other hand, although the use of plastic containers makes real life convenient, environmental problems have become extremely serious due to their unlimited disposal or excessive use. Therefore, various methods for recycling used plastic containers have been prepared.

[0004] The process of recycling waste plastic containers can be divided into a physical recycling process involving washing and pulverization and a chemical recycling process involving a depolymerization process. However, in order to carry out such a recycling process, a pretreatment operation of separating waste plastics by plastic type is required, and this operation reduces the efficiency of the recycling process. For example, in order to recycle a PET bottle with a polyester film attached as a label, an operation of separating (removing) the polyester film from the PET bottle must be performed. This is because there is a difference in crystallinity between the polyester resin contained in the polyester film and the PET contained in the PET bottle, and therefore, when a PET bottle with a polyester film attached is subjected to a recycling process, the recycling process does not function.

[0005] Therefore, in order to improve the efficiency of the recycling process, it is necessary to develop a technology that can eliminate the pre-treatment step of separating (removing) the polyester film from plastic containers such as PET bottles.

[0006] [Prior art document] [Patent] [Patent Document 1] Korean Published Patent No. 2009-0062882

[0007] [Disclosure of the Invention] [Technical issues] To solve the aforementioned problems in the prior art, the inventors conducted various studies. As a result, they discovered that a stretched polyester film that can be subjected to a recycling process without being separated (removed) from waste plastic containers can be obtained by controlling the crystallinity of a stretched polyester film prepared using polyester resin.

[0008] Therefore, the object of the present invention is to provide a stretched polyester film with controlled crystallinity and a method for preparing the same.

[0009] [Solutions to the problem] To solve the aforementioned problems, the present invention provides a stretched polyester film comprising a polyester resin in which a diol component and a dicarboxylic acid component are polymerized, wherein the film has a thermal shrinkage rate of 40% or more at 80°C in the main shrinkage direction, and, when the film is analyzed by differential scanning calorimetry (DSC) during the first and second heat treatments, the first melting point (T m1 ) and the second melting point (T m2 The present invention provides a stretched polyester film in which ) appears.

[0010] Furthermore, the present invention relates to a method for preparing a stretched polyester film, comprising polymerizing a diol component and a dicarboxylic acid component to prepare a polyester resin, preparing a stretched sheet from the polyester resin, and heat-fixing the stretched sheet, wherein the heat shrinkage rate of the film at 80°C in the main shrinkage direction is 40% or more, and when the film is analyzed by differential scanning calorimetry (DSC) during the first and second heat treatments, the first melting point (T m1 ) and the second melting point (T m2 This provides a method for the appearance of ).

[0011] [Advantageous effects of the invention] The stretched polyester film according to the present invention undergoes heat treatment under specific conditions to obtain two different melting points (T m1 and T m2 This results in the ability to possess high crystallinity and excellent thermal shrinkage properties.

[0012] Therefore, the stretched polyester film according to the present invention can be advantageously used as a heat-shrinkable film for labels on packaging and / or plastic containers (e.g., PET bottles).

[0013] In addition, the stretched polyester film according to the present invention, after heat treatment under specific conditions, will have crystallinity equivalent to that of highly crystalline plastic containers (e.g., PET bottles). In the recycling process of used waste plastic containers, it may be possible to recycle the waste plastic containers without separating the stretched polyester film (for example, it is possible to recycle waste plastic containers with the stretched polyester film that has been subjected to the first and second heat treatments still attached). As a result, the present invention can improve the efficiency of the recycling process compared to the prior art. [Brief explanation of the drawing]

[0014] [Figure 1]DSC analysis results (graph) of the drawn polyester film according to Example 2 of the present invention.

[0015] [Best Mode for Carrying Out the Invention] Hereinafter, the present invention will be described in detail. The present invention described herein is not limited to the disclosure shown below, and can be modified into various forms as long as the gist of the present invention is not changed.

[0016] In this specification, the term "comprising" is intended to specify a specific feature, region, step, process, element and / or component. The term "comprising" does not exclude the presence or addition of other features, regions, steps, processes, elements and / or components unless otherwise specified.

[0017] Throughout this specification, terms such as first, second, etc. are used to describe various components. However, the components should not be limited by these terms. These terms are used for the purpose of distinguishing one element from another.

[0018] It should be understood that all numbers and expressions regarding amounts such as components and reaction conditions used in this specification are modified by the term "about" unless otherwise indicated.

[0019] Generally, the melting point (T m ) of a polymer resin is defined as the critical point of the crystallization temperature (T c ) and is the temperature at which the crystalline portion of the polymer resin begins to melt. A polymer resin having a plurality of melting points (T m ) requires a large amount of heat (heat of fusion) for melting the crystalline portion, which can be interpreted to mean that the crystallinity of the polymer resin is maximized.

[0020] Based on the above, the drawn polyester film of the present invention containing a polyester resin as a polymer resin has a plurality of melting points (T m) has, thereby enabling it to have maximized crystallinity. In other words, the stretched polyester film according to the present invention contains a polyester resin and has two different melting points (T m1 and T m2 It possesses ), which results in high crystallinity. This will be explained in detail below.

[0021] (Stretched polyester film) The stretched polyester film according to the present invention comprises a polyester resin in which a diol component and a dicarboxylic acid component are polymerized, and when the film is analyzed by differential scanning calorimetry (DSC), the melting point (T m ) does not appear, however, when the film is analyzed by differential scanning calorimetry (DSC) during the first and second heat treatments, the first melting point (T m1 ) and the second melting point (T m2 ) appears. Specifically, the stretched polyester film according to the present invention is a stretched film prepared from a polyester resin. This stretched film can be controlled to have high crystallinity by heat treatment at a first heat treatment temperature and a subsequent heat treatment at a second heat treatment temperature. For example, the stretched polyester film according to the present invention may be a polyester film prepared by heat-treating a stretched film prepared from a polyester resin at a first heat treatment temperature, and then heat-treating it again at a second heat treatment temperature.

[0022] The first heat treatment temperature is not particularly limited, but may be 60-105°C, specifically 63-104°C, 66-103°C, 67-102°C, 68-100°C, 70-100°C, 70-98°C, 70-97°C, or 70-96°C.

[0023] In addition, the second heat treatment temperature is not particularly limited, but may be 140-220°C, specifically 141-219°C, 142-218°C, 143-217°C, 144-216°C, 145-215°C, 145-214°C, or 146-213°C.

[0024] When the first and second heat treatment temperatures are within the aforementioned ranges, the stretched polyester film according to the present invention can obtain high crystallinity and excellent heat shrinkage to the required level.

[0025] The differential scanning calorimeter (DSC) used to analyze the stretched polyester film after the first and second heat treatments may specifically be a modulated differential scanning calorimeter (modulated DSC or MDSC), and more specifically, a temperature-modulated differential scanning calorimeter (TMDSC). For analysis by differential scanning calorimetry (DSC), a heating rate condition of 10°C / min to reach 280°C from room temperature can be used.

[0026] According to the present invention, the first melting point (T) that appears during the first and second heat treatments m1 ) and the second melting point (T m2 The first melting point (T) is not particularly limited, but can be between 150 and 230°C. m1 ) and the second melting point (T m2 These may be different from each other. Specifically, the first melting point (T m1 ) is the second melting point (T m2 It can be lower than ).

[0027] That is, the first melting point (T m1 The second melting point (T) may be 150-230°C, specifically 152-225°C, 154-210°C, 156-205°C, 158-200°C, 160-198°C, 165-196°C, 170-195°C, 172-190°C, or 172-187°C. In addition, the second melting point (T) m2 The first melting point (T) may be 150-230°C, specifically 155-230°C, 160-230°C, 165-230°C, 170-230°C, 175-229°C, 180-229°C, 185-228°C, 190-228°C, or 195-228°C. m1 ) and the second melting point (T m2 When each of these is within the aforementioned range, the stretched polyester film according to the present invention can have excellent heat shrinkability, heat resistance, and durability, as well as high crystallinity to the required level.

[0028] According to the present invention, the first melting point (T m1 ) and the second melting point (T m2 ) may have a certain correlation.

[0029] Specifically, the first melting point (T m1 ) and the second melting point (T m2 ) and a difference in the range of 10-60°C (│T m1 -T m2 │) may have. More specifically, the difference (│Tm1-Tm2│) may be 10.5~58℃, 11~56℃, 11.5~54℃, 12~52℃, 12.5~50℃, 13~48℃, or 14~46℃.

[0030] Furthermore, the first melting point (T m1 ) and the second melting point (T m2 The total heat of fusion in each of these can be 3 J / g or more. Specifically, the first melting point (T m1 The first heat of fusion (H1) and the second melting point (T) in ) m2 The sum of the second heat of fusion (H2) in (H1+H2) may be 3 J / g or more, 4 J / g or more, 5 J / g or more, 7 J / g or more, 9 J / g or more, 11 J / g or more, 13 J / g or more, 15 J / g or more, 17 J / g or more, 19 J / g or more, or 20 J / g or more. More specifically, the sum (H1+H2) may be 3 to 25 J / g, 3.5 to 23 J / g, 4 to 21 J / g, 4.3 to 20 J / g, or 4.5 to 19.5 J / g.

[0031] Furthermore, the first melting point (T m1 ) and the second melting point (T m2 The ratio of the heat of fusion (H1 / H2) in each case can be 0.1 to 10. Specifically, the first melting point (T m1 The second melting point (T) of the first heat of fusion (H1) at ) m2 The ratio (H1 / H2) to the second heat of fusion (H2) in ) may be 0.1 to 10, more specifically 0.2 to 9.5, 0.3 to 9, 0.4 to 8.5, 0.5 to 8, 0.6 to 7.9, 0.7 to 7.8, or 0.75 to 7.7.

[0032] The correlation between the first melting point (Tm1) and the second melting point (Tm2) is the difference (│T m1 -T m2 When the sum (H1+H2) and ratio (H1 / H2) are within the aforementioned ranges, the stretched polyester film according to the present invention can have excellent heat shrinkage, heat resistance, and durability, as well as high crystallinity to the required level.

[0033] The stretched polyester film according to the present invention can have excellent heat shrinkage properties. Specifically, the stretched polyester film according to the present invention may have a heat shrinkage rate of 40% or more at 80°C in the main shrinkage direction (for example, when immersed in 80°C water for 10 seconds), specifically 42% or more, 44% or more, 46% or more, 48% or more, 50% or more, 52% or more, 55% or more, or 60% or more. For example, the heat shrinkage rate of the stretched polyester film according to the present invention at 80°C in the main shrinkage direction may be 40-70%, 41-65%, 42-60%, 43-60%, 44-58%, or 45-56%.

[0034] Furthermore, the stretched polyester film according to the present invention may have a heat shrinkage rate of 50% or more at 90°C in the main shrinkage direction (for example, when immersed in 90°C water for 10 seconds), specifically 51% or more, 52% or more, 54% or more, 56% or more, 60% or more, 63% or more, 65% or more, or 70% or more. For example, the heat shrinkage rate of the stretched polyester film according to the present invention at 90°C in the main shrinkage direction may be 50-80%, 51-80%, 52-80%, 53-79%, 54-79%, or 55-78%.

[0035] When the stretched polyester film according to the present invention is heated to 195°C after the first and second heat treatments, the stretched polyester film may have a deformation rate of 10% or less according to the following formula 1. Specifically, the deformation rate may be 0-10%, 0.01-9.5%, 0.01-9%, 0.03-8.8%, 0.03-8.5%, 0.05-8%, 0.05-7%, 0.08-5%, or 0.1-3%. When the deformation rate is within the above range, the recyclability of the stretched polyester film can be said to be excellent. Specifically, the temperature of 195°C is the temperature used in the recycling process of waste plastic containers (e.g., PET bottles). Since the stretched polyester film according to the present invention has high crystallinity when subjected to the first and second heat treatments, melting of the stretched polyester film according to the present invention is minimized when heated to 195°C. Therefore, even when the stretched polyester film according to the present invention is attached to a waste plastic container and the recycling process is carried out, melting with the waste plastic container does not occur, resulting in excellent recyclability.

[0036] [Formula 1] Deformation rate (%) = (F m / F T ) × 100

[0037] In Equation 1, F m F is the total number of stretched polyester film samples that were already melted when heated to 195°C (when heated to 195°C after the first and second heat treatments). T This is the total number of stretched polyester film samples before heating to 195°C (before heating to 195°C after the first and second heat treatments), and the size of the stretched polyester film samples is 1 cm in width and 1 cm in length.

[0038] The stretched polyester film according to the present invention has excellent heat shrinkability and high crystallinity, and can be advantageously used as a heat-shrinkable film for labels on packaging and / or plastic containers (e.g., PET bottles). In particular, the stretched polyester film according to the present invention, having been subjected to the first and second heat treatments, has crystallinity equivalent to or equal to that of highly crystallin plastic containers (e.g., PET bottles) (specifically, it has crystallinity equivalent to or equal to that of polyethylene terephthalate (PET), a component of PET bottles). Since melting together with the plastic container does not occur in the recycling process of used plastic containers, the recycling process of used plastic containers can be carried out without the step of separating (removing) the stretched polyester film from the used plastic containers. Therefore, the present invention can improve the efficiency of the recycling process of used plastic containers compared to the prior art.

[0039] Furthermore, the polyester resin contained in the stretched polyester film according to the present invention may be a polyester resin in which a diol component and a dicarboxylic acid component are polymerized.

[0040] According to the present invention, the diol component may be a known diol component. Specifically, the diol component may be bis(2-hydroxyethyl) terephthalate, isosorbide, neopentyl glycol, ethylene glycol, cyclohexanedimethanol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, 2-methylene-1,3-propanediol, 2-ethyl-1,3-propanediol, 2-isopropyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 1,4-butanediol, 2,3-butanediol, 3-methyl-1,5-pentanediol, 3-methyl-2,4-pentanediol It may contain at least one selected from the group consisting of ol, 1,6-hexanediol, 1,2-cyclohexanediol, 1,4-cyclohexanediol, diethylene glycol, 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate (CHDM derivative), 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol (CHDM derivative), regenerated cyclohexanedimethanol, regenerated ethylene glycol, regenerated bis(2-hydroxyethyl)terephthalate, and regenerated diethylene glycol. Preferably, considering the crystallinity, heat shrinkability, and economic efficiency of the stretched polyester film, the diol component may include two or more (specifically, three or more, four or more, or five or more) selected from the group consisting of bis(2-hydroxyethyl) terephthalate, isosorbide, ethylene glycol, cyclohexanedimethanol, neopentyl glycol, diethylene glycol, 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate, 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol, and recycled bis(2-hydroxyethyl) terephthalate.

[0041] For example, the diol component may include ethylene glycol and at least one (specifically, two or more, three or more, or four or more) selected from the group consisting of bis(2-hydroxyethyl) terephthalate, isosorbide, cyclohexanedimethanol, neopentyl glycol, diethylene glycol, 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate, 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol, and regenerated bis(2-hydroxyethyl) terephthalate as a comonomer.

[0042] When the diol component contains ethylene glycol and a comonomer, the amounts of these components are not particularly limited, but the amount of ethylene glycol may be 10-100% by weight, 15-90% by weight, 20-85% by weight, 30-85% by weight, or 50-85% by weight relative to the total weight of the diol component, and the amount of comonomer may be greater than 0-90% by weight, greater than 0-85% by weight, 1-90% by weight, 1-50% by weight, 2-50% by weight, 10-85% by weight, 15-80% by weight, 15-70% by weight, or 15-50% by weight. In particular, when diethylene glycol is used as a comonomer, the amount of diethylene glycol may be 0-50% by weight, 1-50% by weight, 2-50% by weight, 3-45% by weight, 4-40% by weight, or 5-35% by weight relative to the total weight of the diol component. Therefore, the polyester resin according to the present invention may contain diethylene glycol-derived structural units in an amount of 0-50% by weight, 1-50% by weight, 2-50% by weight, 3-45% by weight, 4-40% by weight, or 5-35% by weight relative to the total weight of the polyester resin.

[0043] According to the present invention, the dicarboxylic acid component may be a known dicarboxylic acid. Specifically, the dicarboxylic acid component may include at least one selected from the group consisting of isophthalic acid, terephthalic acid, dimethyl isophthalate, phthalic acid, dimethyl phthalate (dimethyl terephthalate), phthalic anhydride, 2,6-naphthalenedicarboxylic acid, dimethyl 2,6-naphthalenedicarboxylate, diphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 1,3-cyclohexanedicarboxylate, sebacic acid, succinic acid, isodecylsuccinic acid, maleic acid, maleic anhydride, fumaric acid, adipic acid, glutaric acid, azelaic acid, regenerated isophthalic acid, regenerated terephthalic acid, regenerated dimethyl isophthalate, and regenerated dimethyl phthalate (regenerated dimethyl terephthalate). Preferably, considering the crystallinity and heat shrinkage of the stretched polyester film, the dicarboxylic acid component may include at least one selected from the group consisting of isophthalic acid, terephthalic acid, dimethyl phthalate, and dimethyl isophthalate.

[0044] According to the present invention, the polyester resin may have an intrinsic viscosity (IV) of 0.5 to 1.2 dl / g at 35°C, specifically 0.52 to 1.15 dl / g, 0.55 to 1.13 dl / g, 0.58 to 1.1 dl / g, 0.6 to 0.9 dl / g, 0.62 to 0.88 dl / g, 0.65 to 0.85 dl / g, 0.68 to 0.83 dl / g, or 0.7 to 0.8 dl / g. If the intrinsic viscosity is within the above range, the processability of the polyester resin can be said to be ensured.

[0045] (Method for preparing stretched polyester film) The present invention provides a method for preparing the stretched polyester film described above. Specifically, the method for preparing a stretched polyester film according to the present invention includes polymerizing a diol component and a dicarboxylic acid component to prepare a polyester resin (S-1), preparing a stretched sheet from the polyester resin (S-2), and heat-fixing the stretched sheet (S-3).

[0046] [Step (S-1): Preparation of polyester resin] In step (S-1), a polyester resin (copolymer) is prepared by polymerizing the diol component and the dicarboxylic acid component. The diol component and dicarboxylic acid component used in polymerization are the same as those described above, so their explanation will be omitted.

[0047] Polymerization of diol components and dicarboxylic acid components can be carried out by conventionally known methods (e.g., liquid-phase polymerization, solid-phase polymerization, etc.). Batch reactors or continuous reactors can be used for the polymerization reaction. Specifically, the polymerization reaction may include an esterification reaction (transesterification reaction) to prepare an oligomer by reacting the diol component with the dicarboxylic acid component, and a polycondensation reaction of this oligomer.

[0048] The temperature during the esterification reaction is not particularly limited, but considering the physical properties of the polyester resin and stretched polyester film, it may be 230-270°C, 235-268°C, 240-265°C, or 240-260°C. In addition, the length of time during the esterification reaction is not particularly limited, but may be 1-24 hours, 2-22 hours, 3-20 hours, or 4-18 hours. In addition, the pressure during the esterification reaction is not particularly limited, but may be 0-5.0 kgf / cm². 2 , 0.1~4.5 kgf / cm² 2 , 0.1~4.0 kgf / cm 2 , or 0.1~3.0 kgf / cm² 2 That's fine.

[0049] The temperature during the polycondensation reaction is not particularly limited, but considering the physical properties of the polyester resin and the stretched polyester film, it may be 245-290°C, 250-285°C, 255-280°C, or 255-270°C. In addition, the length of time for carrying out the polycondensation reaction is not particularly limited, but may be 1-24 hours, 2-24 hours, 5-22 hours, or 7-20 hours.

[0050] Furthermore, additives containing at least one selected from the group consisting of catalysts, stabilizers, colorants, crystallizers, antioxidants, and branching agents can be used in the polymerization reaction.

[0051] The catalyst is not particularly limited, but may specifically be sodium and magnesium methylates; acetates, borates, fatty acid salts or carbonates of Zn, Cd, Mn, Co, Ca, and Ba; and oxides or hydrates of Mg, Pb, Mn, Ti, Zn, Sb, and Ge. For example, the catalyst may be tetraethyl titanate, acetyl tripropyl titanate, tetrapropyl titanate, tetrabutyl titanate, 2-ethylhexyl titanate, octylene glycol titanate, triethanolamine titanate, acetyl acetonate titanate, ethyl acetoacetate titanate, isostearyl titanate, titanium dioxide, germanium dioxide, germanium tetrachloride, germanium ethylene glycoside, germanium acetate, or a combination thereof.

[0052] The stabilizer is not particularly limited, but phosphorus compounds such as phosphoric acid, trimethyl phosphate, and triethyl phosphate can be used.

[0053] The colorants are not particularly limited, but organic compounds such as cobalt compounds, anthraquinone compounds, perinone compounds, azo compounds, and methine compounds (for example, cobalt acetate, cobalt propionate, Clarient's Polysynthren Blue RLS toner, and Clarient's Solvaperm Red BB toner) can be used.

[0054] The crystallizing agent is not particularly limited, but nucleating agents, UV absorbers, polyolefin resins, polyamide resins, etc., can be used.

[0055] The antioxidant is not particularly limited, but hindered phenol compounds, phosphite compounds, thioether compounds, etc., can be used.

[0056] The branching agent is not particularly limited, but trimellitic anhydride, trimethylolpropane, trimellitic anhydride, etc., can be used.

[0057] [Step (S-2): Preparation of stretched sheet] In step (S-2), a stretched sheet is prepared from the polyester resin obtained in step (S-1). The stretched sheet can be prepared by conventionally known methods. Specifically, the stretched sheet can be prepared using a melt casting step of the polyester resin, a uniaxial or biaxial stretching step, and so on.

[0058] The melt casting step of the polyester resin can be carried out using an extruder. In this case, the melt temperature is not particularly limited, but may be 180-310°C, 200-310°C, 230-310°C, 240-300°C, or 250-290°C. An unstretched sheet can be obtained through such a step, and the unstretched sheet thus obtained can be sent to the stretching step. The unstretched sheet may be preheated to a predetermined temperature (e.g., 90-120°C) before proceeding to the stretching step.

[0059] The uniaxial or biaxial stretching step may include stretching the unstretched sheet obtained through the molten casting step in the longitudinal (MD), transverse (TD), or both directions. Longitudinal stretching can be carried out at 55-180°C or 60-170°C with a stretch ratio of 1-5 times or 1.1-4.5 times. In addition, transverse stretching can be carried out at 55-180°C or 60-170°C with a stretch ratio of 1.5-6 times or 2.5-5.5 times.

[0060] [Step (S-3): Heat fixation] In step (S-3), the stretched sheet obtained in step (S-2) is heat-fixed. The temperature at which the stretched sheet is heat-fixed is not particularly limited, but may be equal to or higher than the temperature at which it was heat-fixed in the stretching step. Specifically, the heat-fixing temperature may be 60-200°C, 65-190°C, 65-180°C, or 65-170°C. When the heat-fixing temperature is within the aforementioned range, a stretched polyester film with high crystallinity and high mechanical strength can be prepared.

[0061] The stretched polyester film prepared through such a heat-setting step may have a heat shrinkage rate of 40% or more at 80°C in the main shrinkage direction, specifically 42% or more, 44% or more, 46% or more, 48% or more, 50% or more, 52% or more, 55% or more, or 60% or more. For example, the heat shrinkage rate at 80°C in the main shrinkage direction may be 40-70%, 41-65%, 42-60%, 43-60%, 44-58%, or 45-56%.

[0062] In addition, the stretched polyester film prepared through such a heat-setting step may have a heat shrinkage rate of 50% or more at 90°C in the main shrinkage direction, specifically 51% or more, 52% or more, 54% or more, 56% or more, 60% or more, 63% or more, 65% or more, or 70% or more. For example, the heat shrinkage rate at 90°C in the main shrinkage direction may be 50-80%, 51-80%, 52-80%, 53-79%, 54-79%, or 55-78%.

[0063] In the stretched polyester film prepared through the steps described above according to the present invention, when the film is analyzed by differential scanning calorimetry (DSC) as described above during the first and second heat treatments, the first melting point (T m1 ) and the second melting point (T m2 ) appears, and therefore the film can have high crystallinity along with excellent heat shrinkage, heat resistance, and durability.

[0064] The first heat treatment temperature is not particularly limited, but may be 60-105°C, specifically 63-104°C, 66-103°C, 67-102°C, 68-100°C, 70-100°C, 70-98°C, 70-97°C, or 70-96°C. For example, the first heat treatment can be carried out by immersing the stretched polyester film obtained through heat fixation in step (S-3) in hot water within the aforementioned temperature range for 5-15 seconds. Alternatively, the first heat treatment may be carried out by placing the stretched polyester film obtained through heat fixation in an oven within the aforementioned temperature range and applying the aforementioned temperature range for 5-15 seconds.

[0065] The second heat treatment temperature is not particularly limited, but may be 140-220°C, specifically 141-219°C, 142-218°C, 143-217°C, 144-216°C, 145-215°C, 145-214°C, or 146-213°C. For example, the second heat treatment may be carried out by placing the stretched polyester film subjected to the first heat treatment in an oven and applying the aforementioned temperature range for 15-45 minutes.

[0066] [Mode of the invention] The present invention will be described in more detail below with reference to embodiments. However, the examples described herein are provided for illustrative purposes only, and the present invention is not limited to these examples.

[0067] (Example 1) (1) Preparation of polyester resin (copolymer) A water-coolable 10-liter reactor equipped with a column and condenser was packed with terephthalic acid (TPA, 7,693.0 g), isosorbide (ISB, 96.7 g), ethylene glycol (EG, 2,601.3 g), 1,4-cyclohexanedimethanol (CHDM, 1,557.1 g), diethylene glycol (DEG, 1,213.7 g), Ge catalyst (GeO2, 1.0 g), Ti catalyst (1.0 g), phosphoric acid (1.5 g), blue toner (0.01 g), and red toner (0.005 g). Next, the reactor temperature was raised to 265°C, and then the reactor was operated at 265°C and a pressure of 2 kgf / cm². 2 An esterification reaction (ES) was carried out below to obtain a clear resultant.

[0068] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 270°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.77 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0069] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0070] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0071] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 70°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 160°C oven for 30 minutes.

[0072] (Example 2) (1) Preparation of polyester resin (copolymer) A water-cooled 10-liter reactor equipped with a column and condenser was packed with terephthalic acid (TPA, 7,663.2g), ethylene glycol (EG, 3,345.5g), 1,4-cyclohexanedimethanol (CHDM, 1,255.7g), neopentyl glycol (NPG, 106.7g), diethylene glycol (DEG, 978.7g), cyclohexanedimethanol derivative (CHDM derivative, 247.4g), Ti catalyst (1.0g), blue toner (0.01g), and red toner (0.01g). Next, the reactor temperature was raised to 255°C, and then the reactor was operated at 255°C and a pressure of 1 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0073] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 285°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.65 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0074] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0075] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0076] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 90°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 175°C oven for 30 minutes.

[0077] (Example 3) (1) Preparation of polyester resin (copolymer) A water-coolable 10-liter reactor equipped with a column and condenser was packed with recycled bis(2-hydroxyethyl) terephthalate (r-BHET, 1,202.9 g), terephthalic acid (TPA, 7,075.2 g), ethylene glycol (EG, 3,488.9 g), 1,4-cyclohexanedimethanol (CHDM, 1,363.9 g), diethylene glycol (DEG, 1,063.0 g), Ti catalyst (1.0 g), blue toner (0.02 g), and red toner (0.01 g). The reactor temperature was then raised to 255°C, followed by heating at 255°C and a pressure of 0.5 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0078] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 285°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.70 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0079] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0080] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0081] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 95°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 155°C oven for 30 minutes.

[0082] (Example 4) (1) Preparation of polyester resin (copolymer) A water-coolable 10-liter reactor equipped with a column and condenser was packed with terephthalic acid (TPA, 7,918.3 g), ethylene glycol (EG, 4,080.5 g), 1,4-cyclohexanedimethanol (CHDM, 1,068.5 g), diethylene glycol (DEG, 832.8 g), Ge catalyst (GeO2, 1.0 g), blue toner (0.05 g), and red toner (0.02 g). Next, the reactor temperature was raised to 260°C, and then the reactor was operated at 260°C and a pressure of 1 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0083] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 275°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.78 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0084] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0085] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0086] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 95°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 165°C oven for 30 minutes.

[0087] (Example 5) (1) Preparation of polyester resin (copolymer) A water-cooled 10-liter reactor equipped with a column and condenser was packed with dimethyl phthalate (DMT, 7,878.5 g), isosorbide (ISB, 494.9 g), ethylene glycol (EG, 4,076.7 g), 1,4-cyclohexanedimethanol (CHDM, 1,139.1 g), diethylene glycol (DEG, 887.8 g), Mn catalyst (Mn(II) acetate tetrahydrate, 1.5 g), Sb catalyst (Sb2O3, 1.8 g), and cobalt acetate (0.8 g). Next, the reactor temperature was raised to 240°C, and then the reactor was operated at 240°C and a pressure of 0.1 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0088] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 255°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.85 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0089] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0090] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0091] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 95°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 175°C oven for 30 minutes.

[0092] (Example 6) (1) Preparation of polyester resin (copolymer) A water-cooled 10-liter reactor equipped with a column and condenser was packed with terephthalic acid (TPA, 8,279.3g), ethylene glycol (EG, 4,432.3g), neopentyl glycol (NPG, 864.5g), Ge catalyst (GeO2, 1.0g), cobalt acetate (0.4g), blue toner (0.03g), and red toner (0.01g). Next, the reactor temperature was raised to 250°C, and then the reactor was operated at 250°C and a pressure of 1 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0093] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 285°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.70 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0094] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0095] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0096] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 95°C water for 10 seconds. Next, the stretched polyester film that had undergone the first heat treatment was subjected to a second heat treatment by placing it in a 160°C oven for 30 minutes.

[0097] (Example 7) (1) Preparation of polyester resin (copolymer) A water-coolable 10-liter reactor equipped with a column and condenser was packed with recycled bis(2-hydroxyethyl) terephthalate (r-BHET, 2,288.5g), terephthalic acid (TPA, 5,982.5g), ethylene glycol (EG, 739.3g), 1,4-cyclohexanedimethanol (CHDM, 1,441.6g), neopentyl glycol (NPG, 1,301.4g), diethylene glycol (DEG, 1,123.6g), Ge catalyst (GeO2, 1.0g), blue toner (0.03g), and red toner (0.01g). Next, the reactor temperature was raised to 265°C, and then the reactor was operated at 265°C and a pressure of 2 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0098] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 270°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.78 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0099] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0100] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0101] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 100°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 210°C oven for 30 minutes.

[0102] (Comparative Example 1) (1) Preparation of polyester resin (copolymer) A water-coolable 10-liter reactor equipped with a column and condenser was packed with regenerated bis(2-hydroxyethyl) terephthalate (r-BHET, 2,256.8 g), terephthalic acid (TPA, 5,899.6 g), ethylene glycol (EG, 824.5 g), 1,4-cyclohexanedimethanol (CHDM, 2,487.8 g), diethylene glycol (DEG, 1,939.0 g), Ge catalyst (GeO2, 1.0 g), blue toner (0.01 g), and red toner (0.01 g). Next, the reactor temperature was raised to 273°C, and then the reactor was operated at 273°C and a pressure of 0.5 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0103] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 285°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.80 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0104] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0105] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0106] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 110°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 225°C oven for 30 minutes.

[0107] (Comparative Example 2) (1) Preparation of polyester resin (copolymer) A water-coolable 10-liter reactor equipped with a column and condenser was packed with recycled bis(2-hydroxyethyl) terephthalate (r-BHET, 2,509.9 g), terephthalic acid (TPA, 6,561.3 g), ethylene glycol (EG, 3,281.5 g), 1,4-cyclohexanedimethanol (CHDM, 790.5 g), diethylene glycol (DEG, 616.1 g), Ti catalyst (1.0 g), blue toner (0.01 g), and red toner (0.01 g). The reactor temperature was then raised to 263°C, followed by heating at 263°C and a pressure of 1 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0108] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 275°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.70 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0109] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0110] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0111] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 80°C water for 10 seconds. Next, the stretched polyester film subjected to the first heat treatment was subjected to a second heat treatment by placing it in a 170°C oven for 30 minutes.

[0112] (Comparative Example 3) (1) Preparation of polyester resin (copolymer) A water-coolable 10-liter reactor equipped with a column and condenser was packed with terephthalic acid (TPA, 7,918.3 g), ethylene glycol (EG, 4,080.5 g), 1,4-cyclohexanedimethanol (CHDM, 1,068.5 g), diethylene glycol (DEG, 832.8 g), Ge catalyst (GeO2, 1.0 g), blue toner (0.05 g), and red toner (0.02 g). Next, the reactor temperature was raised to 260°C, and then the reactor was operated at 260°C and a pressure of 1 kgf / cm². 2 The esterification reaction (ES) was carried out below, yielding a clear result product.

[0113] Next, the resulting product was transferred to a polycondensation reactor, and a polycondensation reaction (PA) was carried out at 275°C, during which the pressure in the polycondensation reactor was maintained at a pressure lower than atmospheric pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.78 dl / g, the mixture was discharged from the polycondensation reactor to form strands, which were then solidified with a cooling liquid and granulated to have an average weight of approximately 12-14 mg to prepare polyester resin (copolymer) chips.

[0114] (2) Preparation of stretched polyester sheet Polyester resin chips were fed into an extruder, melt-extruded at a temperature of 180-310°C, and then cast at a temperature of 20-70°C to prepare an unstretched polyester sheet. Subsequently, this unstretched film was heated to 75-90°C and stretched five times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0115] (3) Heat fixation A stretched polyester sheet was heat-set at 60-200°C to prepare a stretched polyester film with a thickness of 50 μm.

[0116] Subsequently, in order to control the crystallinity of the stretched polyester film, the stretched polyester film was subjected to a first heat treatment by immersing it in 65°C water for 10 seconds.

[0117] (Test Example 1) The stretched polyester films heat-treated in Examples 1-7 and Comparative Examples 1-3 were analyzed by differential scanning calorimetry (DSC) to determine their melting points (T m We determined whether or not the following occurred. The results are shown in Table 1 and Figure 1 below. DSC analysis of the stretched polyester film was performed as follows.

[0118] DSC analyzer: A Mettler Toledo DSC1 model was used.

[0119] Sample preparation: Approximately 6-10 mg of stretched polyester film subjected to the first or first and second heat treatments was taken and placed in an aluminum pan.

[0120] Scanning conditions: The sample was heated from room temperature to 280°C at a rate of 10°C, followed by annealing at 280°C for 3 minutes to obtain the DSC curve.

[0121] Determination of the melting point: In the obtained DSC curve, the temperature at which an endothermic peak appeared during the heating procedure was defined as the melting point. Subsequently, based on the obtained results, the melting point (T m1 , T m2 ),│T m1 -T m2 │, H1+H2, and H1 / H2 were calculated. Here, │T m1 -T m2 The │ symbol indicates that the value has been rounded to two decimal places.

[0122] (Test Example 2) In Examples 1-7 and Comparative Examples 1-3, the thermal shrinkage rate of each stretched polyester film before heat treatment (stretched polyester film before the first or first and second heat treatments) was evaluated as follows. The results are shown in Table 1 below.

[0123] Sample preparation: Stretched polyester film was cut into 5cm x 5cm pieces and stored at room temperature (20°C).

[0124] Thermal shrinkage at 80°C: A stretched polyester film sample was immersed in 80°C water for 10 seconds, and the change in length in the transverse direction (TD), which is the primary shrinkage direction, was calculated using the following equation 2.

[0125] Thermal shrinkage at 90°C: A stretched polyester film sample was immersed in 90°C water for 10 seconds, and the change in length in the transverse direction (TD), which is the primary shrinkage direction, was calculated using the following equation 2.

[0126] [Formula 2] Thermal contraction rate (%) = {(F S1 -F S2 ) / F S1}×100

[0127] In Equation 2, F S1 This is the transverse (TD) length of the stretched polyester film sample before immersion in hot water, and F S2 This is the transverse (TD) length of the stretched polyester film sample after immersion in hot water.

[0128] (Test Example 3) In Examples 1-7 and Comparative Examples 1-3, the recyclability of each heat-treated stretched polyester film was evaluated as follows. The results are shown in Table 1 below.

[0129] Sample preparation: Stretched polyester films subjected to the first or first and second heat treatments were cut into 1cm x 1cm pieces and stored at room temperature (20°C).

[0130] Test conditions: 100 stretched polyester film samples were placed on a Teflon-coated plate and heated in an oven at 195°C (the temperature of the PET recycling process) for 30 minutes. The deformation rate of the stretched polyester film was then calculated using Equation 1 below. A deformation rate of less than 10% was considered to indicate excellent recyclability. If a stretched polyester film sample melted and adhered to the plate, making it impossible to remove, that sample was categorized as a case of melted stretched film sample in Equation 1.

[0131] [Formula 1] Deformation rate (%) = (F m / F T ) × 100

[0132] In Equation 1, F m This is the total number of molten stretched polyester film samples when heated to 195°C, and F T This is the total number of stretched polyester film samples before heating to 195°C.

[0133] ◎: Deformation rate is 0-5% ○: Deformation rate is greater than 5% to 10% ×: Deformation rate is greater than 10%

[0134] [Table 1]

[0135] Referring to Table 1, the stretched polyester films of Examples 1 to 7 each have two melting points (T m1 and T m2 ) has (see Figure 1), and │T m1 -T m2 Since the values ​​of │, H1+H2, and H1 / H2 were controlled within the scope of the present invention, it had a high thermal shrinkage rate at each temperature and reliable regeneration capability.

[0136] In contrast, the stretched polyester film of Comparative Example 1, which had only one melting point, had significantly lower recyclability. In the stretched polyester film of Comparative Example 2, the thermal shrinkage rate was low when a small amount of comonomer (e.g., diethylene glycol) was used. Furthermore, in the stretched polyester film of Comparative Example 3, the recyclability was significantly lower when only the first heat treatment (one time) was performed.

Claims

1. A stretched polyester film comprising a polyester resin in which a diol component and a dicarboxylic acid component are polymerized, wherein the heat shrinkage rate of the film at 80°C in the main shrinkage direction is 40% or more, and when the film is analyzed by differential scanning calorimetry (DSC) during the first and second heat treatments, the first melting point (T ｍ１ ) and the second melting point (T ｍ２ A stretched polyester film in which ) appears.

2. First melting point (T ｍ１ The first melting point (T) is 150-230°C, and the second melting point (T) ｍ２ The temperature is 150-230°C, and the first melting point (T ｍ１ ) and the second melting point (T ｍ２ The stretched polyester film according to claim 1, wherein the two are different from each other.

3. The first melting point (T ｍ１ ), and the difference (│T ｍ２ │) within the range of 10 to 60 °C between the first melting point (T ｍ１ ) and the second melting point (T ｍ２ ), the stretched polyester film according to claim 1.

4. First melting point (T ｍ１ The first heat of fusion (H) in ) １ ) and the second melting point (T ｍ２ The second heat of fusion (H) in ) ２ ) and the sum (H １ +H ２ The stretched polyester film according to claim 1, wherein the concentration is 3 J / g or more.

5. First melting point (T ｍ１ The first heat of fusion (H) in ) １ The second melting point (T ｍ２ The second heat of fusion (H) in ) ２ ) Ratio (H １ / H ２ The stretched polyester film according to claim 1, wherein the ratio is 0.1 to 10.

6. The stretched polyester film according to claim 1, wherein the first heat treatment temperature is 60 to 105°C.

7. The stretched polyester film according to claim 1, wherein the second heat treatment temperature is 140 to 220°C.

8. The diol components are bis(2-hydroxyethyl) terephthalate, isosorbide, neopentyl glycol, ethylene glycol, cyclohexanedimethanol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, 2-methylene-1,3-propanediol, 2-ethyl-1,3-propanediol, 2-isopropyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 1,4-butanediol, 2,3-butanediol, 3-methyl-1,5-pentanediol, and 3-methyl-2,4-pentanediol. The stretched polyester film according to claim 1, comprising at least one selected from the group consisting of 1,6-hexanediol, 1,2-cyclohexanediol, 1,4-cyclohexanediol, diethylene glycol, 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate, 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol, regenerated cyclohexanedimethanol, regenerated ethylene glycol, regenerated bis(2-hydroxyethyl)terephthalate, and regenerated diethylene glycol.

9. The stretched polyester film according to claim 1, wherein the dicarboxylic acid component comprises at least one selected from the group consisting of isophthalic acid, terephthalic acid, dimethyl isophthalate, phthalic acid, dimethyl phthalate, phthalic anhydride, 2,6-naphthalenedicarboxylic acid, dimethyl 2,6-naphthalenedicarboxylate, diphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 1,3-cyclohexanedicarboxylate, sebacic acid, succinic acid, isodecyl succinic acid, maleic acid, maleic anhydride, fumaric acid, adipic acid, glutaric acid, azelaic acid, regenerated isophthalic acid, regenerated terephthalic acid, regenerated dimethyl isophthalate, and regenerated dimethyl phthalate.

10. The drawn polyester film according to Claim 1, having a heat shrinkage rate of 50% or more at 90 °C in the main shrinkage direction.

11. The drawn polyester film according to Claim 1, wherein the polyester resin has an intrinsic viscosity (IV) of 0.5 dl / g to 1.2 dl / g.

12. When the film is heated to 195 °C after the first heat treatment and the second heat treatment, the film has a deformation rate of 10% or less according to the following Formula 1: 【Formula 1】 Deformation rate (%) = (F ｍ / F Ｔ ) × 100 (In Equation 1, F ｍ This is the total number of molten stretched polyester film samples when heated to 195°C, and F Ｔ This is the total number of stretched polyester film samples before heating to 195°C, and the size of the stretched polyester film samples is 1 cm in width and 1 cm in length. The drawn polyester film according to Claim 1.

13. A method for preparing a drawn polyester film, comprising: a step of preparing a polyester resin by polymerizing a diol component and a dicarboxylic acid component; a step of preparing a drawn sheet from the polyester resin; and a step of heat-fixing the drawn sheet. The thermal shrinkage rate of the film at 80°C in the main shrinkage direction is 40% or more, and when the film is analyzed by differential scanning calorimetry (DSC) during the first and second heat treatments, the first melting point (T ｍ１ ) and the second melting point (T ｍ２ The method by which ) appears.

14. The method for preparing a drawn polyester film according to Claim 13, wherein the first heat treatment temperature is 60 to 105 °C.

15. The method for preparing a drawn polyester film according to Claim 13, wherein the second heat treatment temperature is 140 to 220 °C.

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