Stretched polyester film and preparation method thereof

Abstract

The present invention relates to a stretched polyester film and to 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, in which the film has a thermal shrinkage rate of 40% or more in the main shrinkage direction at 80 DEG C, and exhibits a first melting temperature (Tm1) and a second melting temperature (Tm2) when the film is analyzed by differential scanning calorimetry (DSC) after a first heat treatment and a second heat treatment. The stretched polyester film according to the present invention has excellent heat-shrinkable properties, and improves the efficiency of a waste plastic container recovery process.

Description

Technical Field

[0001] The present invention relates to a stretched polyester film having heat shrinkability and controllable crystallinity to improve the efficiency of a recycling process, and to a method for preparing the stretched polyester film. Background Art

[0002] Polyester among polymers 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 have high heat resistance and a suitable heat shrinkage rate; therefore, they are suitable for packaging and / or labeling of plastic containers such as PET bottles.

[0003] Meanwhile, the use of plastic containers makes practical life convenient, however, environmental problems due to arbitrary disposal or excessive use thereof become very serious. Therefore, various methods for recycling waste plastic containers after use are being prepared.

[0004] The process of recycling waste plastic containers can be divided into a physical recycling process through washing and pulverization and a chemical recycling process through a depolymerization process. However, in order to carry out such a recycling process, a pre-treatment operation of separating waste plastics according to plastic types is required, which reduces the efficiency of the recycling process. For example, in order to recycle a PET bottle attached with a polyester film as a label, an operation of separating (removing) the polyester film from the PET bottle must be accompanied. This is attributed to the difference in crystallinity between the polyester resin contained in the polyester film and the PET contained in the PET bottle, which makes the recycling process infeasible when the recycling process is carried out on the PET bottle attached with the polyester film.

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

[0006] [Prior art literature]

[0007] [Patent Document]

[0008] (Patent Document 1) Korean Patent Publication No. 2009-0062882. Summary of the invention

[0009] Technical issues

[0010] In order to solve the above problems in the prior art, the present inventors have conducted various studies. As a result, it has been found that by controlling the crystallinity of a stretched polyester film prepared using a polyester resin, a stretched polyester film capable of being recycled without being separated (removed) from a waste plastic container is obtained.

[0011] Therefore, an object of the present invention is to provide a stretched polyester film having controllable crystallinity and a method for preparing the same.

[0012] Technical Solution

[0013] In order to solve the above problems, the present invention provides a stretched polyester film, which comprises a polyester resin in which a diol component and a dicarboxylic acid component are polymerized, wherein the film has a heat shrinkage rate of 40% or more in a main shrinkage direction at 80° C., and after a first heat treatment and a second heat treatment, when the film is analyzed by differential scanning calorimetry (DSC), a first melting temperature (T m1 ) and the second melting temperature (T m2 ).

[0014] In addition, the present invention provides a method for preparing a stretched polyester film, the method 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-setting the stretched sheet, wherein the film has a heat shrinkage rate of 40% or more in a main shrinkage direction at 80° C., and after a first heat treatment and a second heat treatment, when the film is analyzed by differential scanning calorimetry (DSC), a first melting temperature (T m1 ) and the second melting temperature (T m2 ).

[0015] Beneficial Effects

[0016] The stretched polyester film according to the present invention has two different melting temperatures (T m1 and T m2 ), so that it can have high crystallinity and excellent heat shrinkability.

[0017] Therefore, the stretched polyester film according to the present invention can be advantageously used as a heat-shrinkable film for packaging and / or labels of plastic containers such as PET bottles.

[0018] In addition, the stretched polyester film according to the present invention has a crystallinity similar to that of a highly crystalline plastic container (e.g., a PET bottle) by heat treatment under specific conditions. In the recycling process of the waste plastic container after use, the recycling process of the waste plastic container without separating the stretched polyester film may be possible (e.g., the waste plastic container attached with the stretched polyester film that has been subjected to the first heat treatment and the second heat treatment may be put into the recycling process). Therefore, compared with the prior art, the present invention can improve the efficiency of the recycling process. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 : is the result of DSC analysis of the stretched polyester film according to Example 2 of the present invention (Figure). DETAILED DESCRIPTION

[0020] Hereinafter, the present invention will be described in detail. The present invention is not limited to the disclosure given below herein, but may be modified into various forms as long as the gist of the present invention is not changed.

[0021] In this specification, the term "comprising" is intended to specify specific features, regions, steps, processes, elements and / or components. Unless there is a specific description to the contrary, it does not exclude the existence or addition of any other features, regions, steps, processes, elements and / or components.

[0022] Throughout this specification, the terms first, second, etc. are used to describe various components. However, the components should not be limited by the terms. The terms are used to distinguish one element from another element.

[0023] Unless otherwise indicated, all numbers and expressions relating to amounts of components, reaction conditions, and so forth used herein are to be understood as modified by the term "about."

[0024] Generally, the melting temperature (T m ) is defined as the crystallization temperature (T c ) (which is the temperature at which the crystalline part of the polymer resin begins to melt). m ) requires a large amount of heat (heat of fusion) to melt the crystalline portion, which can be interpreted as meaning that the crystallinity of the polymer resin is maximized.

[0025] Based on the above, the stretched polyester film of the present invention comprising a polyester resin (which is a polymer resin) has a plurality of melting temperatures (T m ), so that it can have a maximized crystallinity. That is, the stretched polyester film according to the present invention comprises a polyester resin and has two different melting temperatures (T m1 and T m2 ), so that it has high crystallinity. Hereinafter, it will be described in detail.

[0026] Stretch polyester film

[0027] 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, wherein when the film is analyzed by differential scanning calorimetry (DSC), no melting temperature (T m ), and after the first heat treatment and the second heat treatment, when the film is analyzed by differential scanning calorimetry (DSC), a first melting temperature (T m1 ) and the second melting temperature (T m2). Specifically, the stretched polyester film according to the present invention is a stretched film prepared from a polyester resin. It can be controlled to have a high crystallinity by heat treatment at a first heat treatment temperature and then heat treatment at a second heat treatment temperature. For example, the stretched polyester film according to the present invention can 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.

[0028] The first heat treatment temperature is not particularly limited, but it can be 60°C to 105°C, specifically, 63°C to 104°C, 66°C to 103°C, 67°C to 102°C, 68°C to 100°C, 70°C to 100°C, 70°C to 98°C, 70°C to 97°C, or 70°C to 96°C.

[0029] In addition, the second heat treatment temperature is not particularly limited, but it can be 140°C to 220°C, specifically, 141°C to 219°C, 142°C to 218°C, 143°C to 217°C, 144°C to 216°C, 145°C to 215°C, 145°C to 214°C, or 146°C to 213°C.

[0030] When the first heat treatment temperature and the second heat treatment temperature are each within the above range, the stretched polyester film according to the present invention may have high crystallinity to a desired level and excellent heat shrinkability.

[0031] The differential scanning calorimeter (DSC) used to analyze the stretched polyester film after the first heat treatment and the second heat treatment may be specifically a modulated differential scanning calorimeter (modulated DSC or MDSC), more specifically a temperature modulated differential scanning calorimeter (TMDSC). For analysis by differential scanning calorimetry (DSC), a heating rate condition of 10°C / min from room temperature to 280°C may be adopted.

[0032] According to the present invention, the first melting temperature (T m1 ) and the second melting temperature (T m2 ) are not particularly limited, but they may each be 150° C. to 230° C. Here, the first melting temperature (T m1 ) and the second melting temperature (T m2 ) may be different from each other. Specifically, the first melting temperature (T m1 ) can be lower than the second melting temperature (T m2 ).

[0033] That is, the first melting temperature (T m1) may be 150°C to 230°C, specifically, 152°C to 225°C, 154°C to 210°C, 156°C to 205°C, 158°C to 200°C, 160°C to 198°C, 165°C to 196°C, 170°C to 195°C, 172°C to 190°C, or 172°C to 187°C. In addition, the second melting temperature (T m2 ) may be 150°C to 230°C, specifically, 155°C to 230°C, 160°C to 230°C, 165°C to 230°C, 170°C to 230°C, 175°C to 229°C, 180°C to 229°C, 185°C to 228°C, 190°C to 228°C, or 195°C to 228°C. When the first melting temperature (T m1 ) and the second melting temperature (T m2 ) are each within the above ranges, the stretched polyester film according to the present invention can have a high crystallinity reaching a desired level, as well as excellent heat shrinkability, heat resistance and durability.

[0034] According to the present invention, the first melting temperature (T m1 ) and the second melting temperature (T m2 ) may have a certain correlation.

[0035] Specifically, the first melting temperature (T m1 ) and the second melting temperature (T m2 ) m1 –T m2 │) can be in the range of 10°C to 60°C. More specifically, the difference (│T m1 –T m2 │) can be 10.5℃ to 58℃, 11℃ to 56℃, 11.5℃ to 54℃, 12℃ to 52℃, 12.5℃ to 50℃, 13℃ to 48℃, or 14℃ to 46℃.

[0036] In addition, at the corresponding first melting temperature (T m1 ) and the second melting temperature (T m2 The sum of the heat of fusion at the first melting temperature (T m1 ) at the first melting heat (H1) and at the second melting temperature (T m2) under the second heat of fusion (H2) of the sum (H1+H2) can be 3J / g or more, 4J / g or more, 5J / g or more, 7J / g or more, 9J / g or more, 11J / g or more, 13J / g or more, 15J / g or more, 17J / g or more, 19J / g or more, or 20J / g or more. More specifically, the sum (H1+H2) can be 3J / g to 25J / g, 3.5J / g to 23J / g, 4J / g to 21J / g, 4.3J / g to 20J / g, or 4.5J / g to 19.5J / g.

[0037] In addition, at the corresponding first melting temperature (T m1 ) and the second melting temperature (T m2 ) can be 0.1 to 10. Specifically, at the first melting temperature (T m1 The first melting heat (H1) at the second melting temperature (T m2 ) can 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.

[0038] When the first melting temperature (T m1 ) and the second melting temperature (T m2 ) between the correlation (│T m1 –T m2 │), the total (H1+H2) and the ratio (H1 / H2) are each within the above ranges, according to the stretched polyester film of the present invention can have a high crystallinity reaching a desired level, as well as excellent heat shrinkability, heat resistance and durability.

[0039] The stretched polyester film according to the present invention may have excellent heat shrinkability. Specifically, the heat shrinkage rate of the stretched polyester film according to the present invention in the main shrinkage direction at 80° C. (for example, when immersed in hot water at 80° C. for 10 seconds) may be 40% or more, 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 in the main shrinkage direction at 80° C. may be 40% to 70%, 41% to 65%, 42% to 60%, 43% to 60%, 44% to 58%, or 45% to 56%.

[0040] In addition, the heat shrinkage rate of the stretched polyester film according to the present invention in the main shrinkage direction at 90° C. (for example, when immersed in hot water at 90° C. for 10 seconds) may be 50% or more, 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 in the main shrinkage direction at 90° C. may be 50% to 80%, 51% to 80%, 52% to 80%, 53% to 79%, 54% to 79%, or 55% to 78%.

[0041] When the stretched polyester film according to the present invention is heated to 195°C after the first heat treatment and the second heat treatment, it may have a deformation rate of 10% or less according to the following formula 1. Specifically, the deformation rate may be 0% to 10%, 0.01% to 9.5%, 0.01% to 9%, 0.03% to 8.8%, 0.03% to 8.5%, 0.05% to 8%, 0.05% to 7%, 0.08% to 5%, or 0.1% to 3%. When the deformation rate is within the above range, the recyclability of the stretched polyester film may be excellent. Specifically, a temperature of 195°C is a temperature employed in the recycling process of waste plastic containers (e.g., PET bottles). The stretched polyester film according to the present invention has a high degree of crystallinity because it has been subjected to the first heat treatment and the second heat treatment, so that its melting is minimized when heated to 195°C. Therefore, even if the recycling process is carried out in a state where it is attached to the waste plastic container, fusion with the waste plastic container does not occur, resulting in excellent recyclability.

[0042] [Formula 1]

[0043] Deformation rate (%) = (F m / F T )×100

[0044] In formula 1, F m F is the total number of stretched polyester film samples that melt when heated to 195°C (when heated to 195°C after the first heat treatment and the second heat treatment), T is the total number of stretched polyester film samples before being heated to 195° C. (before being heated to 195° C. after the first heat treatment and the second heat treatment), and the size of the stretched polyester film samples is 1 cm in width and 1 cm in length.

[0045] 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 packaging and / or labels for plastic containers (e.g., PET bottles). In particular, the stretched polyester film according to the present invention has been subjected to a first heat treatment and a second heat treatment to have a crystallinity similar to or equal to that of a plastic container (e.g., PET bottle) having a high crystallinity (specifically, it has a crystallinity similar to or equal to that of polyethylene terephthalate (PET) as a component of a PET bottle). Since fusion with the plastic container does not occur during the recycling process of the waste plastic container after use, the recycling process of the waste plastic container can be carried out without the step of separating (removing) the stretched polyester film from the waste plastic container. Therefore, compared with the prior art, the present invention can improve the efficiency of the waste plastic container recycling process.

[0046] Meanwhile, 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.

[0047] According to the present invention, the diol component may be a known diol component. Specifically, the diol component may include at least one selected from the group consisting of bis(2-hydroxyethyl)terephthalate, isosorbide, neopentyl glycol, ethylene glycol, cyclohexanedimethanol, 1,2-propylene glycol, 1,3-propylene glycol, 2-methyl-1,3-propylene glycol, 2-methylene-1,3-propylene glycol, 2-ethyl-1,3-propylene glycol, 2-isopropyl-1,3-propylene glycol, 2,2-dimethyl-1,3-propylene glycol, 1,4-butylene glycol, 2,3-butylene glycol, 3-methyl- 1,5-pentanediol, 3-methyl-2,4-pentanediol, 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), recovered cyclohexanedimethanol, recovered ethylene glycol, recovered bis(2-hydroxyethyl)terephthalate, and recovered diethylene glycol. Preferably, when considering the crystallinity, heat shrinkability and economic efficiency of the stretched polyester film, the diol component may include two or more (specifically, 3 or more, 4 or more, or 5 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.

[0048] For example, the diol component may contain ethylene glycol and at least one (specifically, 2 or more, 3 or more, or 4 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 recycled bis(2-hydroxyethyl)terephthalate as comonomers.

[0049] If the diol component includes ethylene glycol and a comonomer, the amounts of these components are not particularly limited, but the amount of ethylene glycol may be 10 wt % to less than 100 wt %, 15 wt % to 90 wt %, 20 wt % to 85 wt %, 30 wt % to 85 wt %, or 50 wt % to 85 wt %, and the amount of the comonomer may be greater than 0 wt % to 90 wt %, greater than 0 wt % to 85 wt %, 1 wt % to 90 wt %, 1 wt % to 50 wt %, 2 wt % to 50 wt %, 10 wt % to 85 wt %, 15 wt % to 80 wt %, 15 wt % to 70 wt %, or 15 wt % to 50 wt %, based on the total weight of the diol component. In particular, when diethylene glycol is used as a comonomer, the amount of diethylene glycol may be 0 wt % to 50 wt %, 1 wt % to 50 wt %, 2 wt % to 50 wt %, 3 wt % to 45 wt %, 4 wt % to 40 wt %, or 5 wt % to 35 wt %, based on the total weight of the diol component. Therefore, the polyester resin according to the present invention may include a structural unit derived from diethylene glycol in an amount of 0 wt % to 50 wt %, 1 wt % to 50 wt %, 2 wt % to 50 wt %, 3 wt % to 45 wt %, 4 wt % to 40 wt %, or 5 wt % to 35 wt %, based on the total weight of the polyester resin.

[0050] According to the present invention, the dicarboxylic acid component can be a well-known dicarboxylic acid. Specifically, the dicarboxylic acid component can include at least one selected from the following: isophthalic acid, terephthalic acid, dimethyl isophthalate, phthalic acid, dimethyl phthalate (dimethyl terephthalate), phthalic anhydride, 2,6-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid dimethyl ester, diphenyl dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid dimethyl ester, 1,3-cyclohexane dicarboxylic acid dimethyl ester, sebacic acid, succinic acid, isodecyl succinic acid, maleic acid, maleic anhydride, fumaric acid, adipic acid, glutaric acid, azelaic acid, recycled isophthalic acid, recycled terephthalic acid, recycled dimethyl isophthalate and recycled dimethyl phthalate (recycled dimethyl terephthalate). Preferably, when the crystallinity and heat shrinkability of the stretched polyester film are considered, the dicarboxylic acid component may include at least one selected from the group consisting of isophthalic acid, terephthalic acid, dimethyl phthalate, and dimethyl isophthalate.

[0051] According to the present invention, the intrinsic viscosity (IV) of the polyester resin at 35° C. may be 0.5 to 1.2 dl / g, 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. When the intrinsic viscosity is within the above range, the processability of the polyester resin can be ensured.

[0052] Method for preparing stretched polyester film

[0053] The present invention provides a method for preparing the above-mentioned stretched polyester film. Specifically, the method for preparing the stretched polyester film according to the present invention comprises: 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-setting the stretched sheet (S-3).

[0054] Step (S-1): Preparation of polyester resin

[0055] In step (S-1), a diol component and a dicarboxylic acid component are polymerized to prepare a polyester resin (copolymer). Since the diol component and the dicarboxylic acid component used for the polymerization are the same as described above, the description thereof is omitted.

[0056] The polymerization of the diol component and the dicarboxylic acid component can be carried out by conventional known methods (e.g., liquid phase polymerization, solid phase polymerization, etc.). A batch reactor or a continuous reactor can be used for the polymerization reaction. Specifically, the polymerization reaction may include an esterification reaction (transesterification reaction) of preparing an oligomer by reacting the diol component with the dicarboxylic acid component and a polycondensation reaction of the oligomer.

[0057] The temperature for conducting the esterification reaction is not particularly limited, but when considering the physical properties of the polyester resin and the stretched polyester film, it may be 230° C. to 270° C., 235° C. to 268° C., 240° C. to 265° C., or 240° C. to 260° C. In addition, the time period for conducting the esterification reaction is not particularly limited, but it may be 1 hour to 24 hours, 2 hours to 22 hours, 3 hours to 20 hours, or 4 hours to 18 hours. In addition, the pressure for conducting the esterification reaction is not particularly limited, but it may be 0 kgf / cm 2 Up to 5.0kgf / cm 2 、0.1kgf / cm 2 Up to 4.5kgf / cm 2 、0.1kgf / cm 2 Up to 4.0kgf / cm 2 , or 0.1kgf / cm 2 Up to 3.0kgf / cm 2 .

[0058] The temperature for conducting the polycondensation reaction is not particularly limited, but when considering the physical properties of the polyester resin and the stretched polyester film, it may be 245° C. to 290° C., 250° C. to 285° C., 255° C. to 280° C., or 255° C. to 270° C. In addition, the time period for conducting the polycondensation reaction is not particularly limited, but it may be 1 hour to 24 hours, 2 hours to 24 hours, 5 hours to 22 hours, or 7 hours to 20 hours.

[0059] Meanwhile, an additive may be used in the polymerization reaction, the additive comprising at least one selected from a catalyst, a stabilizer, a colorant, a crystallizing agent, an antioxidant, and a branching agent.

[0060] The catalyst is not particularly limited, but it can be specifically sodium methoxide and magnesium methoxide; acetate, borate, fatty acid salt or carbonate of Zn, Cd, Mn, Co, Ca, Ba, and oxide or hydrate of Mg, Pb, Mn, Ti, Zn, Sb and Ge. For example, the catalyst can be tetraethyl titanate, acetyl tripropyl titanate, tetrapropyl titanate, tetrabutyl titanate, 2-ethylhexyl titanate, octanediol titanate, triethanolamine titanate, acetylacetonate titanate, ethyl acetoacetate titanate, isostearyl titanate, titanium dioxide, germanium dioxide, germanium tetrachloride, ethylene glycol germanium, germanium acetate or a combination thereof.

[0061] The stabilizer is not particularly limited, but phosphorus-based compounds such as phosphoric acid, trimethyl phosphate, and triethyl phosphate may be used.

[0062] The colorant is not particularly limited, but organic compounds such as cobalt-based compounds, anthraquinone-based compounds, perinone-based compounds, azo-based compounds, and methine-based compounds (e.g., cobalt acetate, cobalt propionate, Clarient's Polysynthren Blue RLS toner, and Clarient's Solvaperm Red BB toner) can be used.

[0063] The crystallizing agent is not particularly limited, but a crystal nucleating agent, an ultraviolet absorber, a polyolefin resin, a polyamide resin, or the like can be used.

[0064] The antioxidant is not particularly limited, but hindered phenol-based compounds, phosphite-based compounds, thioether-based compounds, and the like may be used.

[0065] The branching agent is not particularly limited, but trimellitic anhydride, trimethylolpropane, trimellitic acid, etc. can be used.

[0066] Step (S-2): Preparation of stretched sheet

[0067] In step (S-2), a stretched sheet is prepared from the polyester resin obtained in step (S-1). The preparation of the stretched sheet can be carried out by conventional known methods. Specifically, the stretched sheet can be prepared by melting and casting steps of the polyester resin, uniaxial or biaxial stretching steps, etc.

[0068] The melting and casting steps of the polyester resin can be carried out by an extruder. In such a case, the melting temperature is not particularly limited, but it can be 180°C to 310°C, 200°C to 310°C, 230°C to 310°C, 240°C to 300°C or 250°C to 290°C. An unstretched sheet can be obtained by such a step, and the unstretched sheet thus obtained can be transferred to a stretching step. Before the stretching step is performed, the unstretched sheet can be preheated to a predetermined temperature (e.g., 90°C to 120°C).

[0069] The uniaxial or biaxial stretching step may include a step of stretching the unstretched sheet obtained by the melting and casting step in the longitudinal direction (MD), the transverse direction (TD), or both directions. The stretching in the longitudinal direction may be performed at 55°C to 180°C or 60°C to 170°C with a stretching ratio of 1 to 5 times or 1.1 to 4.5 times. In addition, the stretching in the transverse direction may be performed at 55°C to 180°C or 60°C to 170°C with a stretching ratio of 1.5 to 6 times or 2.5 to 5.5 times.

[0070] Step (S-3): Heat setting

[0071] In step (S-3), the stretched sheet obtained in step (S-2) is heat-set. The temperature at which the stretched sheet is heat-set is not particularly limited, but it may be similar to or higher than the temperature in the stretching step. Specifically, the heat-setting temperature may be 60°C to 200°C, 65°C to 190°C, 65°C to 180°C, or 65°C to 170°C. When the heat-setting temperature is within the above range, a stretched polyester film having high crystallinity and high mechanical strength may be prepared.

[0072] The stretched polyester film prepared by such a heat setting step may have a heat shrinkage rate of 40% or more in the main shrinkage direction at 80° C., 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 in the main shrinkage direction at 80° C. may be 40% to 70%, 41% to 65%, 42% to 60%, 43% to 60%, 44% to 58%, or 45% to 56%.

[0073] In addition, the stretched polyester film prepared by such a heat setting step may have a heat shrinkage rate in the main shrinkage direction at 90° C. of 50% or more, 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 in the main shrinkage direction at 90° C. may be 50% to 80%, 51% to 80%, 52% to 80%, 53% to 79%, 54% to 79%, or 55% to 78%.

[0074] In the stretched polyester film prepared by the above steps according to the present invention, after the first heat treatment and the second heat treatment, when analyzed by differential scanning calorimetry (DSC) as described above, the first melting temperature (T m1 ) and the second melting temperature (T m2 ), so that it can have high crystallinity, as well as excellent heat shrinkability, heat resistance and durability.

[0075] The first heat treatment temperature is not particularly limited, but it may be 60°C to 105°C, specifically, 63°C to 104°C, 66°C to 103°C, 67°C to 102°C, 68°C to 100°C, 70°C to 100°C, 70°C to 98°C, 70°C to 97°C, or 70°C to 96°C. For example, the first heat treatment may be performed by immersing the stretched polyester film obtained by heat setting in step (S-3) in hot water having the above temperature range for 5 seconds to 15 seconds. In addition, the first heat treatment may be performed by placing the stretched polyester film obtained by heat setting in an oven having the above temperature range and applying the above temperature range to perform heat treatment for 5 seconds to 15 seconds.

[0076] The second heat treatment temperature is not particularly limited, but it may be 140° C. to 220° C., specifically, 141° C. to 219° C., 142° C. to 218° C., 143° C. to 217° C., 144° C. to 216° C., 145° C. to 215° C., 145° C. to 214° C., or 146° C. to 213° C. For example, the second heat treatment may be performed by placing the stretched polyester film subjected to the first heat treatment in an oven and applying the above temperature range to perform heat treatment for 15 minutes to 45 minutes.

[0077] Embodiments of the invention

[0078] Hereinafter, the present invention will be described in more detail with reference to embodiments. However, these embodiments are provided only for illustrative purposes, and the present invention is not limited thereto.

[0079] [Example 1]

[0080] (1) Preparation of polyester resin (copolymer)

[0081] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged 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, after the temperature of the reactor was increased to 265° C., the reaction mixture was heated at 265° C. under 2 kgf / cm 2 The esterification reaction (ES) was carried out under a pressure of 100 ℃ to obtain a transparent resultant.

[0082] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 270° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.77 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0083] (2) Preparation of stretched polyester sheets

[0084] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0085] (3) Heat setting

[0086] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0087] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 70°C for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 160°C for 30 minutes to perform a second heat treatment.

[0088] [Example 2]

[0089] (1) Preparation of polyester resin (copolymer)

[0090] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged with terephthalic acid (TPA, 7,663.2 g), ethylene glycol (EG, 3,345.5 g), 1,4-cyclohexanedimethanol (CHDM, 1,255.7 g), neopentyl glycol (NPG, 106.7 g), diethylene glycol (DEG, 978.7 g), cyclohexanedimethanol derivative (CHDM derivative, 247.4 g), Ti catalyst (1.0 g), blue toner (0.01 g) and red toner (0.01 g). Next, after the temperature of the reactor was increased to 255° C., the reaction mixture was stirred at 255° C. under a pressure of 1 kgf / cm 2 The esterification reaction was carried out under pressure to obtain a transparent resultant.

[0091] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 285° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.65 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0092] (2) Preparation of stretched polyester sheets

[0093] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0094] (3) Heat setting

[0095] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0096] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 90° C. for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 175° C. for 30 minutes to perform a second heat treatment.

[0097] [Example 3]

[0098] (1) Preparation of polyester resin (copolymer)

[0099] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged with recovered 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). Next, after the temperature of the reactor was increased to 255° C., the reaction mixture was stirred at 255° C. under a pressure of 0.5 kgf / cm 2 The esterification reaction (ES) was carried out under pressure of 1% to obtain a transparent product.

[0100] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 285° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.70 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0101] (2) Preparation of stretched polyester sheets

[0102] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0103] (3) Heat setting

[0104] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0105] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 95°C for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 155°C for 30 minutes to perform a second heat treatment.

[0106] [Example 4]

[0107] (1) Preparation of polyester resin (copolymer)

[0108] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged 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, after the temperature of the reactor was raised to 260° C., a reaction mixture was stirred at 260° C. under a pressure of 1 kgf / cm 2 The esterification reaction (ES) was carried out under pressure of 1% to obtain a transparent product.

[0109] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 275° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.78 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0110] (2) Preparation of stretched polyester sheets

[0111] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0112] (3) Heat setting

[0113] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0114] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 95°C for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 165°C for 30 minutes to perform a second heat treatment.

[0115] [Example 5]

[0116] (1) Preparation of polyester resin (copolymer)

[0117] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged 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) acetic acid tetrahydrate, 1.5 g), Sb catalyst (Sb2O3, 1.8 g) and cobalt acetate (0.8 g). Next, after the temperature of the reactor was raised to 240°C, the mixture was stirred at 240°C under a pressure of 0.1 kgf / cm 2 The esterification reaction (ES) was carried out under pressure of 1% to obtain a transparent product.

[0118] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 255° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.85 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0119] (2) Preparation of stretched polyester sheets

[0120] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0121] (3) Heat setting

[0122] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0123] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 95°C for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 175°C for 30 minutes to perform a second heat treatment.

[0124] [Example 6]

[0125] (1) Preparation of polyester resin (copolymer)

[0126] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged with terephthalic acid (TPA, 8,279.3 g), ethylene glycol (EG, 4,432.3 g), neopentyl glycol (NPG, 864.5 g), a Ge catalyst (GeO2, 1.0 g), cobalt acetate (0.4 g), a blue toner (0.03 g) and a red toner (0.01 g). Next, after the temperature of the reactor was raised to 250° C., a reaction mixture was stirred at 250° C. under a pressure of 1 kgf / cm 2 The esterification reaction (ES) was carried out under pressure of 1% to obtain a transparent product.

[0127] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 285° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.70 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0128] (2) Preparation of stretched polyester sheets

[0129] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0130] (3) Heat setting

[0131] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0132] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 95°C for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 160°C for 30 minutes to perform a second heat treatment.

[0133] [Example 7]

[0134] (1) Preparation of polyester resin (copolymer)

[0135] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged with recovered bis(2-hydroxyethyl)terephthalate (r-BHET, 2,288.5 g), terephthalic acid (TPA, 5,982.5 g), ethylene glycol (EG, 739.3 g), 1,4-cyclohexanedimethanol (CHDM, 1,441.6 g), neopentyl glycol (NPG, 1,301.4 g), diethylene glycol (DEG, 1,123.6 g), Ge catalyst (GeO2, 1.0 g), blue toner (0.03 g) and red toner (0.01 g). Next, after the temperature of the reactor was increased to 265° C., the reaction mixture was stirred at 265° C. under 2 kgf / cm 2 The esterification reaction (ES) was carried out under a pressure of 100 ℃ to obtain a transparent resultant.

[0136] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 270° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.78 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0137] (2) Preparation of stretched polyester sheets

[0138] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0139] (3) Heat setting

[0140] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0141] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 100°C for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 210°C for 30 minutes to perform a second heat treatment.

[0142] [Comparative Example 1]

[0143] (1) Preparation of polyester resin (copolymer)

[0144] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged with recovered 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, after the temperature of the reactor was raised to 273° C., the reaction mixture was stirred at 273° C. under a pressure of 0.5 kgf / cm 2 The esterification reaction (ES) was carried out under pressure of 1% to obtain a transparent product.

[0145] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 285° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.80 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0146] (2) Preparation of stretched polyester sheets

[0147] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0148] (3) Heat setting

[0149] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0150] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 110° C. for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 225° C. for 30 minutes to perform a second heat treatment.

[0151] [Comparative Example 2]

[0152] (1) Preparation of polyester resin (copolymer)

[0153] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged with recovered 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). Next, after the temperature of the reactor was raised to 263° C., the reaction mixture was stirred at 263° C. under a pressure of 1 kgf / cm 2 The esterification reaction (ES) was carried out under pressure of 1% to obtain a transparent product.

[0154] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 275° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.70 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0155] (2) Preparation of stretched polyester sheets

[0156] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0157] (3) Heat setting

[0158] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0159] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 80°C for 10 seconds to control the crystallinity of the stretched polyester film. Next, the stretched polyester film subjected to the first heat treatment was stored in an oven at 170°C for 30 minutes to perform a second heat treatment.

[0160] [Comparative Example 3]

[0161] (1) Preparation of polyester resin (copolymer)

[0162] A 10-liter reactor equipped with a tower and a condenser that can be cooled with water was charged 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, after the temperature of the reactor was raised to 260° C., a reaction mixture was stirred at 260° C. under a pressure of 1 kgf / cm 2 The esterification reaction (ES) was carried out under pressure of 1% to obtain a transparent product.

[0163] Subsequently, the resultant was transferred to a polycondensation reactor, and then a polycondensation reaction (PA) was performed at 275° C. while the pressure of the polycondensation reactor was maintained at a pressure lower than normal pressure. When the intrinsic viscosity (IV) of the mixture in the polycondensation reactor reached 0.78 dl / g, the mixture was then discharged to the outside of the polycondensation reactor to form a strand, solidified with a cooling liquid, and then granulated to have an average weight of about 12 mg to 14 mg to prepare polyester resin (copolymer) chips.

[0164] (2) Preparation of stretched polyester sheets

[0165] The polyester resin chips were fed to an extruder, melt-extruded at a temperature of 180° C. to 310° C., and cast at a temperature of 20° C. to 70° C. to prepare an unstretched polyester sheet. Subsequently, the unstretched film was heated to 75° C. to 90° C. and stretched 5 times in the transverse direction (TD) to prepare a stretched polyester sheet.

[0166] (3) Heat setting

[0167] The stretched polyester sheet was heat-set at 60° C. to 200° C. to prepare a stretched polyester film having a thickness of 50 μm.

[0168] Thereafter, the stretched polyester film was subjected to a first heat treatment by immersing the stretched polyester film in hot water at 65° C. for 10 seconds to control the crystallinity of the stretched polyester film.

[0169] [Test Example 1]

[0170] The stretched polyester films heat-treated in Examples 1 to 7 and Comparative Examples 1 to 3 were each analyzed by differential scanning calorimetry (DSC) to determine whether a melting temperature (T m The results are shown in Table 1 and Figure 1 The DSC analysis of the stretched polyester film was performed as follows.

[0171] DSC analysis apparatus: DSC Model 1 manufactured by Mettler Toledo was used.

[0172] Sample preparation: About 6 mg to 10 mg of the stretched polyester film subjected to the first heat treatment or the first heat treatment and the second heat treatment was taken and placed in an aluminum pan.

[0173] Scanning conditions: it was heated from room temperature to 280°C at a rate of 10°C, and then annealed at 280°C for 3 minutes to obtain a DSC curve.

[0174] Determination of melting temperature: The temperature showing an endothermic peak during the temperature increase process in the obtained DSC curve is defined as the melting temperature. Thereafter, the melting temperature (T m1 , T m2),│T m1 –T m2 │, H1+H2 and H1 / H2. Here, │T m1 –T m2 │Round to the second decimal place.

[0175] [Test Example 2]

[0176] In Examples 1 to 7 and Comparative Examples 1 to 3, the heat shrinkage of each of the stretched polyester films before heat treatment (stretched polyester films before the first heat treatment or before the first and second heat treatments) was evaluated as follows. The results are shown in Table 1 below.

[0177] Sample preparation: The stretched polyester films were each cut into a size of 5 cm×5 cm and stored at room temperature (20° C.).

[0178] Heat shrinkage at 80° C.: A stretched polyester film sample was immersed in hot water at 80° C. for 10 seconds, and a length change in the transverse direction (TD) as a main shrinkage direction was calculated by the following Formula 2.

[0179] Heat shrinkage at 90° C.: A stretched polyester film sample was immersed in hot water at 90° C. for 10 seconds, and a length change in the transverse direction (TD) as a main shrinkage direction was calculated by the following Formula 2.

[0180] [Formula 2]

[0181] Thermal shrinkage (%) = {(F S1 –F S2 ) / F S1}×100

[0182] In formula 2, F S1 : The length of the polyester film sample stretched in the transverse direction (TD) before immersing in hot water, and F S2 : The length of the polyester film sample in the transverse direction (TD) stretched after immersing in hot water.

[0183] [Test Example 3]

[0184] In Examples 1 to 7 and Comparative Examples 1 to 3, the recyclability of each of the heat-treated stretched polyester films was evaluated as follows. The results are shown in Table 1 below.

[0185] Sample preparation: The stretched polyester films subjected to the first heat treatment or the first heat treatment and the second heat treatment were each cut into a size of 1 cm×1 cm and stored at room temperature (20° C.).

[0186] Test conditions: 100 polyester stretch film samples were placed on a Teflon-coated plate and heated in an oven at 195°C (temperature of the PET recycling process) for 30 minutes. Then, the deformation rate of the stretched polyester film was calculated according to the following formula 1. When the deformation rate is less than 10%, the recyclability is evaluated as excellent. When the stretched polyester film sample melts and adheres to the plate without detaching from it, it falls into the case where the stretched film sample melts in formula 1.

[0187] [Formula 1]

[0188] Deformation rate (%) = (F m / F T )×100

[0189] In formula 1, F m is the total number of stretched polyester film samples that melt when heated to 195°C, F T is the total number of stretched polyester film samples before being heated to 195°C.

[0190] ◎: Deformation rate 0% to 5%

[0191] ○: Deformation rate is greater than 5% to 10%

[0192] ×: Deformation rate is greater than 10%

[0193] [Table 1]

[0194]

[0195] Referring to Table 1, each of them has two melting temperatures (T m1 and T m2 )(See Figure 1 ) At the same time│T m1 –T m2 The stretched polyester films of Examples 1 to 7 in which H1, H1+H2 and H1 / H2 were controlled within the range of the present invention had high heat shrinkage at each temperature and ensured recyclability.

[0196] In contrast, the stretched polyester film of Comparative Example 1 having one melting temperature has significantly poor recyclability. The stretched polyester film of Comparative Example 2 using a small amount of comonomer (e.g., diethylene glycol) has a low thermal shrinkage. In addition, the stretched polyester film of Comparative Example 3 that was subjected to only the first heat treatment (once) has significantly poor recyclability.

Claims

1. 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 heat shrinkage rate of 40% or more in a main shrinkage direction at 80° C., and after a first heat treatment and a second heat treatment, when the film is analyzed by differential scanning calorimetry (DSC), a first melting temperature (T m1 ) and the second melting temperature (T m2 ).

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

3. The stretched polyester film according to claim 1, wherein the first melting temperature (T m1 ) and the second melting temperature (T m2 ) m1 -T m2 │)In the range of 10℃ to 60℃.

4. The stretched polyester film according to claim 1, wherein at the first melting temperature (T m1 ) at the first melting heat (H1) and at the second melting temperature (T m2 ) is 3 J / g or more.

5. The stretched polyester film according to claim 1, wherein at the first melting temperature (T m1 ) and the first heat of fusion (H1) at the second melting temperature (T m2 ) in a ratio (H1 / H2) of the second heat of fusion (H2) of 0.1 to 10. The stretched polyester film according to claim 1 , wherein the first heat treatment temperature is 60° C. to 105° C. The stretched polyester film according to claim 1 , wherein the second heat treatment temperature is 140° C. to 220° C.

8. The stretched polyester film according to claim 1, wherein the diol component comprises at least one selected from the group consisting of bis(2-hydroxyethyl)terephthalate, isosorbide, neopentyl glycol, ethylene glycol, cyclohexanedimethanol, 1,2-propylene glycol, 1,3-propylene glycol, 2-methyl-1,3-propylene glycol, 2-methylene-1,3-propylene glycol, 2-ethyl-1,3-propylene glycol, 2-isopropyl-1,3-propylene glycol, 2,2-dimethyl-1,3-propylene glycol, 1,4- Butanediol, 2,3-butanediol, 3-methyl-1,5-pentanediol, 3-methyl-2,4-pentanediol, 1,6-hexanediol, 1,2-cyclohexanediol, 1,4-cyclohexanediol, diethylene glycol, 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate, 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol, recovered cyclohexanedimethanol, recovered ethylene glycol, recovered bis(2-hydroxyethyl)terephthalate, and recovered 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-naphthalene dicarboxylic acid, dimethyl 2,6-naphthalene dicarboxylate, diphenyl dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid, dimethyl 1,4-cyclohexane dicarboxylate, dimethyl 1,3-cyclohexane dicarboxylate, sebacic acid, succinic acid, isodecyl succinic acid, maleic acid, maleic anhydride, fumaric acid, adipic acid, glutaric acid, azelaic acid, recycled isophthalic acid, recycled terephthalic acid, recycled dimethyl isophthalate, and recycled dimethyl phthalate. 10 . The stretched polyester film according to claim 1 , which has a heat shrinkage rate of 50% or more in a main shrinkage direction at 90° C. 11 . The stretched polyester film according to claim 1 , wherein the intrinsic viscosity (IV) of the polyester resin is 0.5 to 1.2 dl / g.

12. The stretched polyester film according to claim 1, wherein 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 formula 1, F m is the total number of stretched polyester film samples that melt when heated to 195°C, F T is the total number of stretched polyester film samples before being heated to 195° C., and the size of the stretched polyester film samples is 1 cm in width and 1 cm in length.

13. A method for preparing a stretched polyester film, the method comprising: polymerizing a diol component and a dicarboxylic acid component to prepare a polyester resin; preparing a stretched sheet from the polyester resin; as well as heat setting the stretched sheet, wherein the film has a heat shrinkage rate of 40% or more in the main shrinkage direction at 80° C., and after the first heat treatment and the second heat treatment, when the film is analyzed by differential scanning calorimetry (DSC), a first melting temperature (T m1 ) and the second melting temperature (T m2 ). 14 . The method for producing a stretched polyester film according to claim 13 , wherein the first heat treatment temperature is 60° C. to 105° C. 15 . The method for preparing a stretched polyester film according to claim 13 , wherein the second heat treatment temperature is 140° C. to 220° C.

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