Recycled polyester resin and film, and methods for producing them.

Abstract

This polyester resin has its cyclic trimer and diethylene glycol content adjusted to a specific range, resulting in excellent heat resistance and processability. It can also improve the strength, elongation, and hydrolysis resistance of polyester films produced from this resin.

Description

Detailed description of the invention

[0001] [Technical field] The embodiments relate to recycled polyester resin, films, and methods for preparing them.

[0002] [Background technology] Due to its excellent mechanical strength, heat resistance, transparency, and gas barrier properties, polyester is widely used as a material for beverage filling containers, packaging films, audio and video films, and other applications. Furthermore, polyester is widely manufactured worldwide as an industrial material such as medical fibers and tire cords. In particular, polyester sheets or plates have good transparency and excellent mechanical strength, and are therefore widely used as raw materials for cases, boxes, partitions, shelves, panels, packaging materials, building materials, and interior and exterior materials.

[0003] As a result, plastic waste, including polyester, is generated globally every year at an unmanageable level. In recent years, regulations and plans for the recycling of waste plastic resources, including waste polyester, have been formulated in countries around the world. For example, there are attempts to use recycled resin at a certain percentage or higher in packaging materials used in various fields. Physical and chemical methods are used to recycle waste polyester, but physical recycling methods cannot guarantee purity and, as a result, are not widely used.

[0004] In chemical recycling, the ester bonds in waste polyester are cleaved and then depolymerized. Reactions such as glycolysis, hydrolysis, methanolysis, and aminolysis are used. Among these, glycolysis involves adding glycols such as ethylene glycol or diethylene glycol at high temperatures to decompose the waste polyester. The reaction product mainly contains bis(2-hydroxyethyl) terephthalate (BHET). After crystallization or purification, bis(2-hydroxyethyl) terephthalate can be used as a raw material for preparing unsaturated polyesters or ester polyols.

[0005] [Prior art document] [Patent] [Patent Document 1] Korean Patent No. 1386683 [Patent Document 2] U.S. Patent No. 7,211,193

[0006] [Overview of the prefecture] [Problems the invention aims to solve] Generally, the depolymerization products of waste polyester resins contain large amounts of oligomers, such as dimers and trimers, in addition to bis(2-hydroxyethyl) terephthalate (BHET). The presence of by-reactants, such as diethylene glycol (DEG), formed at high depolymerization temperatures, is unavoidable. In particular, oligomers are always present in a constant amount at equilibrium concentrations with the polymer during the preparation process, making it difficult to control and reduce their content. These substances cause a decrease in the appearance, performance, and processability of the final product.

[0007] Therefore, the inventors controlled the recycled raw materials and polycondensation method to produce recycled polyester resins in which the content of cyclic trimers and diethylene glycol was adjusted to a specific range. It was confirmed that the heat resistance, processability, and mechanical properties of the polyester films produced from these resins were improved.

[0008] Therefore, the object of the present invention is to provide polyester films having the above compositions, polyester films containing them and having excellent properties, and methods for preparing them.

[0009] [Means for solving the problem] The present invention provides a polyester resin containing recycled bis(2-hydroxyethyl) terephthalate obtained by depolymerization of waste polyester, wherein the peak area fraction of the cyclic trimer measured by high-performance liquid chromatography (HPLC) is 3,000 ppm or less, and the diethylene glycol (DEG) content measured by gas chromatography (GC) is 1.0% by weight or less.

[0010] Furthermore, the present invention provides a polyester film containing this polyester resin.

[0011] Furthermore, the present invention provides a method for preparing a polyester film, comprising the steps of: (1) depolymerizing waste polyester to prepare recycled bis(2-hydroxyethyl) terephthalate; (2) mixing the recycled bis(2-hydroxyethyl) terephthalate with a solvent to prepare a solution of recycled bis(2-hydroxyethyl) terephthalate; (3) using the recycled bis(2-hydroxyethyl) terephthalate solution to prepare a polyester resin via a polycondensation reaction; and (4) melt-extruding the polyester resin to stretch the extruded product, wherein the polyester resin has a peak area fraction of cyclic trimers measured by high-performance liquid chromatography (HPLC) of 3,000 ppm or less, and a diethylene glycol (DEG) content measured by gas chromatography (GC) of 1.0% by weight or less.

[0012] [Advantageous effects of the invention] In the polyester resin according to the present invention, the content of cyclic trimers and diethylene glycol is adjusted to a specific range. As a result, it is possible to improve the heat resistance, processability, and mechanical properties of the polyester film produced from these. Furthermore, in the polyester resin according to the present invention, the concentration of carboxyl terminal groups is adjusted to a specific range to improve the hydrolysis resistance of the final product.

[0013] Therefore, the film containing the polyester resin of the present invention exhibits excellent properties in terms of strength, elongation, and elongation retention under high temperature and high humidity conditions.

[0014] Therefore, the polyester resins and films of the present invention can exhibit quality and performance equivalent to unused products, even though they are products regenerated through chemical recycling.

[0015] [Modes for carrying out the invention] In this specification, the terms referring to each component are used to distinguish them from each other and are not intended to limit the scope of the embodiments. Further, in this specification, unless otherwise specified in the context, singular expressions are construed to include plural ones as well.

[0016] In this specification, terms such as first, second, etc. are used to describe various components. However, the components should not be limited by the terms. This term is used for the purpose of distinguishing one element from another.

[0017] In this specification, the term "comprising" is intended to specify a particular property, region, step, method, element, and / or component. Unless otherwise stated to the contrary, it does not exclude the presence or addition of any other property, region, step, method, element, and / or component.

[0018] The molecular weight of the compounds or polymers described in this specification, such as the number average molecular weight or the weight average molecular weight, is, as is well known, the relative mass based on carbon 12. Although their units are not described, it will be understood that, if necessary, they are the molar mass (g / mole) of those numerical values.

[0019] In the numerical ranges that limit the size, physical properties, etc. of the components described in this specification, when the numerical ranges limited only by the upper limit and the numerical ranges limited only by the lower limit are separately exemplified, it will be understood that the numerical ranges obtained by combining these upper and lower limits are also included in the exemplified ranges.

[0020] The polyester resin according to one embodiment contains recycled bis(2-hydroxyethyl) terephthalate obtained by depolymerizing waste polyester. In this resin, the peak area fraction of the cyclic trimer measured by high performance liquid chromatography (HPLC) is 3,000 ppm or less, and the content of diethylene glycol (DEG) measured by gas chromatography (GC) is 1.0 wt% or less.

[0021] The present invention will be described in more detail below.

[0022] Regenerated bis(2-hydroxyethyl) terephthalate The polyester resin according to the present invention contains bis(2-hydroxyethyl) terephthalate obtained by the depolymerization of waste polyester.

[0023] Bis(2-hydroxyethyl) terephthalate is an ester of two ethylene glycol molecules and one terephthalic acid molecule. For example, bis(2-hydroxyethyl) terephthalate is a compound formed as an intermediate in methods for preparing polyesters such as polyethylene terephthalate (PET) through polymerization of ethylene glycol with terephthalic acid or its ester.

[0024] The bis(2-hydroxyethyl) terephthalate (BHET) used as a polymerization raw material for polyester resins according to the present invention is obtained from waste polyester having repeating units of ethylene glycol and terephthalic acid, such as polyethylene terephthalate (PET) or glycol-modified polyethylene terephthalate (PETG). For example, it can be obtained by well-known depolymerization methods such as glycolysis, hydrolysis, and methanolysis.

[0025] In this specification, bis(2-hydroxyethyl) terephthalate (BHET) obtained by the depolymerization of the waste polyester described above will be referred to as "recycled bis(2-hydroxyethyl) terephthalate (recycled BHET)," or abbreviated as r-BHET or rBHET, but it should be understood that it is different from pure BHET compounds.

[0026] Recycled BHET may contain reagents or solvents used in various chemical steps during the depolymerization of waste polyester, or by-products formed by side reactions with them. These impurities may remain in trace amounts even after several purification cycles. Therefore, recycled BHET generally contains trace amounts of organic and inorganic impurities in addition to the main component, BHET. For this reason, recycled BHET can be considered a single composition containing two or more components, i.e., a BHET composition. Recycled BHET can be used as a polymerization raw material for producing polyester resins.

[0027] Specifically, recycled BHET may contain, in addition to BHET as the main component, BHET analogs such as monohydroxyethyl terephthalic acid (MHET), BHET dimers, BHET trimers, by-products such as diethylene glycol esters, inorganic components such as metal ions, and trace amounts of other organic components such as residual solvent components.

[0028] In this invention, regenerated BHET is used in which the content of such heterogeneous organic components is adjusted to a certain range. The content of each component in the regenerated BHET can be determined by measuring the peak area fraction (%) of the total peak area in the spectrum obtained using high-performance liquid chromatography (HPLC).

[0029] Specifically, when the recycled bis(2-hydroxyethyl) terephthalate (BHET) used as a raw material in the present invention is measured by high-performance liquid chromatography (HPLC), the peak area fraction of BHET is 96% or higher. More specifically, the peak area fraction of BHET measured by HPLC may be 96.5% or higher, 97% or higher, 97.5% or higher, or 98% or higher.

[0030] Furthermore, the recycled bis(2-hydroxyethyl) terephthalate may have a total peak area fraction of organic impurities measured by HPLC of less than 5%, specifically less than 4%, less than 3%, less than 2%, less than 1%, or less than 0.7%.

[0031] In particular, the polyester resin according to the present invention includes recycled BHET in which the content of diethylene glycol ester (DEG ester) has been adjusted to a certain level or lower. For example, when recycled bis(2-hydroxyethyl) terephthalate is measured by HPLC, the total peak area fraction of diethylene glycol ester compounds may be less than 2%. Specifically, the total peak area fraction of diethylene glycol ester compounds may be less than 1.5%, less than 1%, less than 0.8%, or less than 0.7%.

[0032] As an example, a diethylene glycol ester compound may be a condensate between an aromatic dicarboxylic acid such as terephthalic acid and diethylene glycol. As another example, a diethylene glycol ester compound may be a condensate between an aromatic dicarboxylic acid such as terephthalic acid and a glycol (e.g., ethylene glycol) in addition to diethylene glycol.

[0033] According to one embodiment, recycled BHET may contain 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate (CAS number 65133-69-9) as the first diethylene glycol ester. According to another embodiment, recycled BHET may contain bis[2-(2-hydroxyethoxy)ethyl]benzene-1,4-dicarboxylate (CAS number 26850-76-0) as the second diethylene glycol ester. When the polyester resin is prepared from recycled BHET in which the content of the first and second diethylene glycol esters is adjusted to below a certain level, the quality of the polyester resin is hardly impaired compared to unused resin, even though it is a polyester resin recycled by chemical recycling.

[0034] According to one embodiment, when regenerated bis(2-hydroxyethyl) terephthalate (BHET) is measured by high-performance liquid chromatography (HPLC), the peak area fraction of 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate (first diethylene glycol ester) is 2.5% or less. Specifically, the peak area fraction of 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate measured by HPLC may be 2.0% or less, 1.5% or less, 1.0% or less, or 0.5% or less.

[0035] According to another embodiment, when regenerated bis(2-hydroxyethyl) terephthalate (BHET) is measured by high-performance liquid chromatography (HPLC), the peak area fraction of bis[2-(2-hydroxyethoxy)ethyl]benzene-1,4-dicarboxylate (second diethylene glycol ester) is 0.5% or less. Specifically, the peak area fraction of bis[2-(2-hydroxyethoxy)ethyl]benzene-1,4-dicarboxylate measured by HPLC may be 0.2% or less, more specifically, 0.15% or less, 0.1% or less, or 0.05% or less.

[0036] Furthermore, when regenerated bis(2-hydroxyethyl) terephthalate is measured by high-performance liquid chromatography, the total peak area fraction of the oligomer may be 3% or less.

[0037] Specifically, when regenerated bis(2-hydroxyethyl) terephthalate is measured by HPLC, the peak area fraction of the BHET dimer may be less than 3%, less than 2%, less than 1%, or less than 0.7%. Furthermore, the peak area fraction of the BHET trimer measured by HPLC for bis(2-hydroxyethyl) terephthalate produced by the above method may be less than 1%, less than 0.5%, less than 0.3%, less than 0.1%, or 0%.

[0038] Furthermore, recycled bis(2-hydroxyethyl) terephthalate may contain impurities having a structure similar to that of bis(2-hydroxyethyl) terephthalate. For example, it may contain at least one selected from the group consisting of monohydroxyethyl terephthalic acid (MHET), bis(2-hydroxypropyl) terephthalate, and monohydroxyethyl ethoxyterephthalic acid. When measured by HPLC, the peak area fraction of impurities having a structure similar to that of bis(2-hydroxyethyl) terephthalate may be less than 3%, less than 2%, less than 1%, or less than 0.5%.

[0039] Furthermore, the total content of residual solvent (e.g., ethylene glycol) in the recycled bis(2-hydroxyethyl) terephthalate may be less than 1% by weight, based on the weight ratio detected by gas chromatography analysis. Specifically, the total content of oligomeric material may be less than 0.5% by weight, less than 0.3% by weight, less than 0.2% by weight, less than 0.1% by weight, or less than 0.9% by weight.

[0040] Furthermore, recycled bis(2-hydroxyethyl) terephthalate may have a yellow index (YID) of 3.0 or less when measured by a spectrophotometer in a 25% by weight solution. Specifically, the yellow index may be 2.5 or less, 2.0 or less, 1.5 or less, or 1.0 or less.

[0041] Furthermore, recycled bis(2-hydroxyethyl) terephthalate may have a total inorganic substance content of less than 5 ppm, as measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, the total inorganic substance content may be less than 3 ppm, less than 1 ppm, or nearly 0 ppm.

[0042] Polyester resin The polyester resin of the present invention is a polyester resin recycled by chemical recycling of waste polyester.

[0043] Specifically, since the polyester resin of the present invention is polymerized using recycled BHET, the polymer chain contains repeating units derived from recycled BHET.

[0044] The recycled BHET content in the polyester resin of the present invention may be 1% by weight or more, 5% by weight or more, 10% by weight or more, 30% by weight or more, 50% by weight or more, 70% by weight or more, or 90% by weight or more. Furthermore, the recycled BHET content may be 100% by weight or less, 99% by weight or less, 80% by weight or less, 60% by weight or less, 40% by weight or less, or 20% by weight or less.

[0045] For example, recycled bis(2-hydroxyethyl) terephthalate can be used in amounts ranging from 10% to 99% by weight, based on the weight of the polyester resin.

[0046] On the other hand, since bis(2-hydroxyethyl) terephthalate has a structure in which two ethylene glycol molecules and one terephthalic acid molecule are bonded together, the polyester resin of the present invention may essentially contain repeating units derived from ethylene glycol and terephthalic acid.

[0047] As described above, the polyester resin of the present invention contains diacid components and glycol components as monomers that constitute them. Furthermore, the polyester resin of the present invention may further contain additional diacid components and additional glycol components for the polymerization of polyester.

[0048] In the polyester resin of the present invention, the diacid component may be a dicarboxylic acid or a derivative thereof, and the glycol component may be a diol.

[0049] In particular, dicarboxylic acids include terephthalic acid, and the physical properties of polyester resins, such as heat resistance, chemical resistance, and weather resistance, can be improved by terephthalic acid. For example, terephthalic acid may be used in an amount of 5 mol% to 100 mol% based on the total number of moles of dicarboxylic acid. Furthermore, the terephthalic acid component can be formed from alkyl terephthalate esters such as dimethyl terephthalate.

[0050] Furthermore, the diol contains ethylene glycol, which can contribute to improving the transparency and impact resistance of the polyester resin. For example, ethylene glycol may be used in an amount of 5 mol% to 100 mol% based on the total number of moles of the diol.

[0051] According to one embodiment, the polyester resin of the present invention may be a copolymer resin containing two or more dicarboxylic acid components and / or two or more diol components.

[0052] Specifically, the dicarboxylic acid component may further include aromatic dicarboxylic acid components, aliphatic dicarboxylic acid components, or mixtures thereof, other than terephthalic acid. Dicarboxylic acids other than terephthalic acid may be used in amounts of 1 mol% to 30 mol% based on the total weight of the dicarboxylic acid components.

[0053] The aromatic dicarboxylic acid component may be an aromatic dicarboxylic acid having 8 to 20 carbon atoms, preferably 8 to 14 carbon atoms, or a mixture thereof. Examples of aromatic dicarboxylic acids include, but are not limited to, isophthalic acid, naphthalenedicarboxylic acids such as 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, 4,4'-stilbenedicarboxylic acid, 2,5-franzicarboxylic acid, and 2,5-thiophenedicarboxylic acid.

[0054] The aliphatic dicarboxylic acid component may be an aliphatic dicarboxylic acid having 4 to 20 carbon atoms, preferably 4 to 12 carbon atoms, or a mixture thereof. Examples of aliphatic dicarboxylic acids include, but are not limited to, linear, branched, or cyclic aliphatic dicarboxylic acid components, such as cyclohexanedicarboxylic acids including 1,4-cyclohexanedicarboxylic acid and 1,3-cyclohexanedicarboxylic acid, phthalic acid, sebacic acid, succinic acid, isodecylsuccinic acid, maleic acid, fumaric acid, adipic acid, glutaric acid, azelaic acid, etc.

[0055] Furthermore, the diol component may further contain comonomers other than ethylene glycol. The comonomer may include, for example, at least one selected from the group consisting of cyclohexanedimethanol, cyclohexanedimethanol derivatives, isosorbide, and diethylene glycol.

[0056] Cyclohexanedimethanol (e.g., 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, and 1,4-cyclohexanedimethanol) can contribute to improving the transparency and impact resistance of the manufactured polyester resin. For example, cyclohexanedimethanol may be used in amounts of 5 mol% to 90 mol%, based on the total number of moles of the diol. Cyclohexanedimethanol derivatives may be 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate or 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol. Cyclohexanedimethanol derivatives may be used in amounts of 0.1 mol% to 25 mol%, based on the total number of moles of the diol.

[0057] Isosorbide can improve the processability of the final polyester resin. While the transparency and impact resistance of the polyester resin are improved by the diol components of cyclohexanedimethanol and ethylene glycol, the shear fluidization properties should be improved and the crystallization rate should be slowed in terms of processability; however, it is difficult to achieve this effect using only cyclohexanedimethanol and ethylene glycol. Therefore, when isosorbide is used as the diol component, the shear fluidization properties are improved and the crystallization rate is slowed while maintaining transparency and impact resistance, thereby improving the processability of the manufactured polyester resin. Preferably, isosorbide may be used in an amount of 0.1 mol% to 50 mol% based on the total number of moles of the diol.

[0058] As a specific example, the polyester resin contains a diacid component and a glycol component, the diacid component may include at least one selected from the group consisting of terephthalic acid, isophthalic acid, dimethyl isophthalate, phthalic acid, dimethyl phthalate, phthalic anhydride, 2,6-naphthalenedicarboxylic acid, dimethyl 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, dimethyl 1,4-cyclohexanedicarboxylic acid, dimethyl 1,3-cyclohexanedicarboxylic acid, sebacic acid, succinic acid, isodecyl succinic acid, maleic acid, maleic anhydride, fumaric acid, adipic acid, glutaric acid, and azelaic acid, and the glycol component may include isosorbide, ethylene glycol, 1,2-propanediol, 1,3-propanediol It may contain at least one selected from the group consisting of cindiol, 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, 1,6-hexanediol, 1,2-cyclohexanediol, 1,4-cyclohexanediol, diethylene glycol, 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate, and 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol.

[0059] In the polyester resin according to the present invention, the content of cyclic trimer and diethylene glycol is adjusted to a specific range. As a result, it is possible to improve the heat resistance, processability, and mechanical properties of the polyester film produced from these.

[0060] The content of cyclic trimers in the polyester resin according to the present invention can be measured using liquid chromatography or the like. Specifically, the content of cyclic trimers can be calculated by measuring the fraction (ppm) of the peak area of ​​cyclic trimers relative to the total peak area in the spectrum obtained using high-performance liquid chromatography (HPLC).

[0061] For example, in the polyester resin according to the present invention, the peak area fraction of the cyclic trimer measured by high-performance liquid chromatography (HPLC) may be 9,000 ppm or less, 7,000 ppm or less, or 5,000 ppm or less.

[0062] According to one embodiment, in the polyester resin according to the present invention, the peak area fraction of the cyclic trimer measured by high-performance liquid chromatography (HPLC) may be 3,000 ppm or less. Within this range, the processability of the polyester resin and the film prepared therefrom can be further improved, and the heat resistance and mechanical properties can be excellent.

[0063] For example, the peak area fraction of the cyclic trimer measured by HPLC may be 3,000 ppm or less, 2,900 ppm or less, 2,800 ppm or less, 2,700 ppm or less, or 2,600 ppm or less. On the other hand, the lower limit of the peak area fraction of the cyclic trimer is not particularly limited, but may be, for example, 0 ppm or more, 1 ppm or more, 10 ppm or more, 100 ppm or more, or 1,000 ppm or more.

[0064] Cyclic trimers may be produced in chemical recycling methods used to prepare polyester resins from waste polyester. This can lead to a decrease in the quality of the final product.

[0065] The cyclic trimer may be a cyclic trimer consisting of any one of the repeating units that make up the polyester resin.

[0066] For example, the repeating units of a cyclic trimer may include an aliphatic alkylene group and an aromatic dicarboxylate group.

[0067] As a specific example, a cyclic trimer may be, for instance, an ethylene terephthalate cyclic trimer.

[0068] As a more specific example, cyclic trimers may include ethylene terephthalate cyclic trimers.

[0069] Such cyclic trimers readily separate or precipitate on the surface in methods for preparing polyester resins or films, causing problems in the method or degrading the appearance or performance of the final product. However, such problems can be solved by adjusting their content to the above-mentioned levels.

[0070] Furthermore, in the polyester resin of the present invention, for example, the concentration of diethylene glycol measured by gas chromatography may be 2.5% by weight or less, 1.5% by weight or less, 1.2% by weight or less, 1.0% by weight or less, 0.9% by weight or less, or 0.8% by weight or less.

[0071] According to one embodiment, the diethylene glycol content of the polyester resin, as measured by gas chromatography, may be 1.0% by weight or less. Within this content range, the melting point of the polyester resin, as well as the heat resistance, mechanical properties, and processability of the film prepared therefrom, can be improved.

[0072] On the other hand, the lower limit of the diethylene glycol content is not particularly limited, but may be, for example, 0% by weight or more, 0.01% by weight or more, 0.1% by weight or more, or 0.3% by weight or more.

[0073] In the polyester resin according to the present invention, by adjusting the concentration of carboxyl-terminated groups (-COOH) to a specific range, the hydrolysis resistance of the polyester resin and the film prepared therefrom can be improved, and the heat resistance and mechanical properties can be made excellent.

[0074] For example, when a polyester resin is dissolved in benzyl alcohol at 180°C and titrated using a 0.1N sodium hydroxide-benzyl alcohol solution, the concentration (E) of the carboxyl terminal group calculated by the following formula (1) may be 40 equivalents / ton or less, 30 equivalents / ton or less, or 20 equivalents / ton or less.

[0075] As a specific example, when a polyester resin is dissolved in benzyl alcohol at 180°C and titrated using a 0.1N sodium hydroxide-benzyl alcohol solution, the concentration (E) of the carboxyl terminal group calculated by the following formula (1) may be 20 equivalents / ton or less.

[0076]

number

[0077] In the formula, S is the volume (L) of 0.1N sodium hydroxide-benzyl alcohol solution consumed in the titration of the polyester resin, B is the volume (L) of 0.1N sodium hydroxide-benzyl alcohol solution consumed in the blank titration, N is the concentration of the sodium hydroxide-benzyl alcohol solution, which is 0.1N, W is the weight (kg) of the polyester resin used in the titration, and f is the factor of the 0.1N sodium hydroxide-benzyl alcohol solution, which is 0.95 to 0.99.

[0078] Specifically, the concentration of the carboxyl terminal group may be 20 equivalents / ton or less, 19 equivalents / ton or less, 18 equivalents / ton or less, 16 equivalents / ton or less, or 15 equivalents / ton or less. On the other hand, the lower limit of the concentration of the carboxyl terminal group is not particularly limited, but may be, for example, 0 equivalents / ton or more, 1 equivalent / ton or more, 3 equivalents / ton or more, 5 equivalents / ton or more, or 10 equivalents / ton or more.

[0079] Furthermore, the polyester resin of the present invention may have a melting point (Tm) of, for example, 240°C or higher, 245°C or higher, 250°C or higher, or 255°C or higher when measured by differential scanning calorimetry (DSC), and may also have a melting point of 270°C or lower, 265°C or lower, or 260°C or lower. Specifically, the polyester resin may have a melting point of 255°C or higher when measured by differential scanning calorimetry (DSC). The melting point may be measured, for example, by a method that includes placing the polyester resin in a differential scanning calorimeter (DSC) and raising the temperature from room temperature to 280°C at a constant rate.

[0080] The intrinsic viscosity of the polyester resin according to the present invention at 35°C may be 0.5 dl / g or more, 0.6 dl / g or more, or 0.7 dl / g or more, and may also be 1.2 dl / g or less, 1.1 dl / g or less, 1.0 dl / g or less, or 0.9 dl / g or less. For example, the polyester resin may have an intrinsic viscosity of 0.6 dl / g to 1.2 dl / g at 35°C. The intrinsic viscosity can be calculated, for example, by dissolving the polyester resin in a solvent such as orthochlorophenol and determining the specific viscosity using an Ubbelohde viscometer or the like.

[0081] polyester film The present invention provides a polyester film containing the above-mentioned polyester resin.

[0082] The polyester film is obtained from a polyester resin containing recycled bis(2-hydroxyethyl) terephthalate obtained by the depolymerization of waste polyester. In this resin, the peak area fraction of the cyclic trimer, as measured by high-performance liquid chromatography (HPLC), is 3,000 ppm or less, and the diethylene glycol (DEG) content, as measured by gas chromatography (GC), is 1.0% by weight or less.

[0083] Since the polyester film of the present invention is prepared from the polyester resin described above, it has excellent processability, mechanical strength, hydrolysis resistance, and heat resistance.

[0084] As an example, in the step of melt-extruding a film from a polyester resin, the pressure applied to a 10 μm filter can be 100 kg / cm 2 or less, 90 kg / cm 2 or less, 80 kg / cm 2 or less, 70 kg / cm 2 or less, or 60 kg / cm 2 or less. Further, in the step of stretching the polyester film, the average number of film breaks during 24 hours can be less than 1, specifically, 0.

[0085] The polyester film can have a tensile strength in the machine direction (MD), for example, of 10 kgf / mm 2 or more, 13 kgf / mm 2 or more, 14 kgf / mm 2 or more, 15 kgf / mm 2 or more, 17 kgf / mm 2 or more, or 19 kgf / mm 2 or more. Further, the tensile strength in the machine direction (MD) can be, for example, 40 kgf / mm 2 or less, 30 kgf / mm 2 or less, 25 kgf / mm 2 or less, or 20 kgf / mm 2 or less.

[0086] The polyester film can have a tensile strength in the transverse direction (TD), for example, of 20 kgf / mm 2 or more, 22 kgf / mm 2 or more, 23 kgf / mm 2 or more, 24 kgf / mm 2 or more, or 25 kgf / mm 2 or more. Further, the tensile strength in the transverse direction (TD) can be, for example, 80 kgf / mm 2 or less, 70 kgf / mm 2 or less, or 60 kgf / mm 2 or less.

[0087] A polyester film may have, for example, a longitudinal (MD) elongation of 100% or more, 115% or more, 120% or more, 125% or more, 130% or more, or 135% or more. Furthermore, a polyester film may have, for example, a longitudinal (MD) elongation of 180% or less, 170% or less, 160% or less, or 150% or less.

[0088] The polyester film may have, for example, a transverse (TD) elongation of 60% or more, 70% or more, 75% or more, or 80% or more. Furthermore, the polyester film may have, for example, a transverse (TD) elongation of 120% or less, 110% or less, or 100% or less.

[0089] Tensile strength and elongation can be measured, for example, by cutting the film into 1.5 cm wide strips, mounting them longitudinally on clips spaced 5 cm apart, and testing them at 25°C with a tensile speed of 200 mm / min according to ASTM D882.

[0090] As a specific example, when a polyester film was cut into 1.5 cm wide strips, mounted longitudinally on clips placed at 5 cm intervals, and tested according to ASTM D882 at 25°C with a tensile speed of 200 mm / min, the tensile strength in the longitudinal direction (MD) was 15 kgf / mm². 2 The above is possible, and the growth in the vertical direction (MD) can be 125% or more.

[0091] Furthermore, the polyester film may have an elongation retention rate (R) of 30% or more, 35% or more, 40% or more, 45% or more, or 50% or more, calculated by the following formula (2).

[0092] As a specific example, polyester film may have an elongation retention rate (R) of 40% or more, calculated by the following formula (2).

[0093]

number

[0094] In the formula, S0 is the initial elongation (%) of the polyester film sample, and S1 is the elongation (%) measured after being left for 96 hours at 121°C, 100% RH, and 2 atmospheres. The elongation is measured by cutting the polyester film into 1.5 cm wide strips, mounting them longitudinally on clips spaced 5 cm apart, and testing them at 25°C and a tensile speed of 200 mm / min according to ASTM D882.

[0095] On the other hand, the upper limit of the elongation retention rate (R) is not particularly limited. It could be, for example, 100% or less, 80% or less, 70% or less, or 60% or less.

[0096] A method for preparing a polyester film according to one embodiment of the present invention comprises the steps of: (1) depolymerizing waste polyester to prepare recycled bis(2-hydroxyethyl) terephthalate; (2) mixing the recycled bis(2-hydroxyethyl) terephthalate with a solvent to prepare a solution of recycled bis(2-hydroxyethyl) terephthalate; (3) using the recycled bis(2-hydroxyethyl) terephthalate solution to prepare a polyester resin via a polycondensation reaction; and (4) melt-extruding the polyester resin to stretch the extruded product, wherein the peak area fraction of the cyclic trimer measured by high-performance liquid chromatography (HPLC) is 3,000 ppm or less, and the diethylene glycol (DEG) content measured by gas chromatography (GC) is 1.0% by weight or less.

[0097] The polycondensation reaction in step (3) may include at least one of (3a) a polycondensation reaction under a pressure of 200 mmHg to 600 mmHg (first polycondensation reaction) and (3b) a polycondensation reaction under a pressure of less than 200 mmHg (second polycondensation reaction).

[0098] In a particular embodiment, the polycondensation reaction of step (3) may include a step of preparing an oligomer by subjecting a (3a) regenerated bis(2-hydroxyethyl) terephthalate solution to a first polycondensation reaction under a pressure of 200 mmHg to 600 mmHg. Furthermore, the polycondensation reaction may further include a step of preparing a polyester resin by subjecting the (3b) oligomer to a second polycondensation under a pressure of less than 200 mmHg after step (3a).

[0099] A method for preparing a polyester resin according to one embodiment of the present invention comprises the steps of: (1) depolymerizing waste polyester to prepare recycled bis(2-hydroxyethyl) terephthalate; (2) mixing the recycled bis(2-hydroxyethyl) terephthalate with a solvent to prepare a solution of recycled bis(2-hydroxyethyl) terephthalate; (3a) subjecting the recycled bis(2-hydroxyethyl) terephthalate solution to a first polycondensation reaction under a pressure of 200 mmHg to 600 mmHg to prepare an oligomer; and (3b) subjecting the oligomer to a second polycondensation under a pressure of less than 200 mmHg to prepare a polyester resin, wherein the peak area fraction of the cyclic trimer measured by high-performance liquid chromatography (HPLC) is 3,000 ppm or less, and the diethylene glycol (DEG) content measured by gas chromatography (GC) is 1.0% by weight or less.

[0100] Oligomers such as cyclic trimers are always present in a constant amount at equilibrium concentrations with the polymer during the preparation process, making it difficult to control and reduce their content. However, in a preferred embodiment of the method for preparing recycled polyester resin, recycled bis(2-hydroxyethyl) terephthalate is used as the main monomer (e.g., 100% monomer). Therefore, no transesterification reaction is performed, and the polycondensation reaction is carried out separately under a first low vacuum and a second high vacuum. As a result, the content of cyclic trimers can be reduced to a level significantly lower than in the prior art.

[0101] The following describes in detail each step of the method for preparing the polyester resin and film according to the present invention.

[0102] Preparation of waste polyester The waste polyester used as a raw material in this invention can be obtained from polyester material products that have been discarded after use.

[0103] For example, waste polyester can be obtained from products such as beverage bottles, cloth, film, cases, boxes, partitions, shelves, protective panels, packaging materials, building materials, and interior and exterior materials made from various polyester materials that have been discarded after being used by consumers.

[0104] The waste polyester material may be pre-treated before being subjected to the depolymerization step.

[0105] First, any other plastics, metals, and impurities mixed into the waste polyester material are removed, and then, if necessary, the material is washed and sorted according to its detailed characteristics such as color.

[0106] The waste polyester material sorted in this way is placed in a pulverizer and crushed into small flakes. The resulting waste polyester flakes may be sieved using a mesh to separate them into particles smaller than the desired size. The mesh size may be, for example, 4 mm or less, 3 mm or less, or 2 mm or less.

[0107] The sieved flakes can be washed, dried with hot air, and then subjected to the depolymerization step.

[0108] On the other hand, the waste polyester subjected to the glycolysis reaction may have a controlled particle size. For example, the waste polyester may be pulverized by the pretreatment step described above and be in the form of flakes.

[0109] Specifically, the particle size of the waste polyester may be 4 mm or less, 3 mm or less, 2 mm or less, or 1 mm or less. Within the above particle size range, glycolysis can be carried out under relatively low temperature conditions. For example, the temperature conditions for the first glycolysis reaction may be adjusted to 195°C or less, 190°C or less, 185°C or less, or 180°C, and the temperature conditions for the subsequent second glycolysis reaction may be adjusted to 160°C or less or 150°C or less. Furthermore, within the above particle size range, glycolysis can be carried out in a relatively short time. For example, the time for the first and second glycolysis reactions may be 3 hours or less, 2 hours or less, or 1 hour or less from the time the appropriate temperature is reached.

[0110] Furthermore, waste polyester may have a fine structure such as fibers. For example, waste polyester may be waste fibers or fibrous materials such as discarded banners.

[0111] As a specific example, the waste polyester may have a particulate form or a fibrous form with a particle size of 4 mm or less.

[0112] The fibers may include at least one of monofilament yarns and multifilament yarns. The diameter of the monofilament yarn may be, for example, 0.05 denier to 100 denier, corresponding to about 0.001 mm to 0.1 mm. Specifically, the monofilament yarn may have a diameter of 0.05 denier to 7 denier, or 7 denier to 100 denier. The diameter of the multifilament yarn may be, for example, 1 denier to 10,000 denier, corresponding to about 0.01 mm to 1 mm. Specifically, the multifilament yarn may have a diameter of 0.01 denier to 0.2 denier, or 0.2 denier to 1 denier.

[0113] By adjusting the particle size or diameter of waste polyester to a specific range before depolymerization, solvation can be promoted even under relatively low temperature and short reaction time conditions.

[0114] In particular, the invention involves a two-step glycolysis reaction (i.e., a first glycolysis reaction and a second glycolysis reaction). The first glycolysis reaction promotes solvation, allowing the second glycolysis reaction to carry out the transesterification of the waste polyester under lower temperatures and shorter reaction times. Therefore, the concentration of diethylene glycol (DEG), which is naturally formed at typical glycolysis temperatures, can be significantly reduced, and the content of diethylene glycol ester compounds (DEG esters) in the final prepared bis(2-hydroxyethyl) terephthalate can be significantly reduced.

[0115] The diethylene glycol ester compounds present in bis(2-hydroxyethyl) terephthalate act as disruptors of the regularity of the final polymer during subsequent polymerization of polyesters and polyester copolymers, thereby degrading the heat resistance properties of the final polymer, such as melting point (Tm) and glass transition temperature (Tg).

[0116] However, polyester resins and products manufactured using bis(2-hydroxyethyl) terephthalate obtained by the depolymerization method of the present invention can be polymerized into polymers without unnecessary structural defects, as is the case when using unused, non-recycled raw materials. Preparation of recycled bis(2-hydroxyethyl) terephthalate First, waste polyester is depolymerized to prepare recycled bis(2-hydroxyethyl) terephthalate.

[0117] According to one embodiment, recycled bis(2-hydroxyethyl) terephthalate can be prepared by a method comprising: (1a) subjecting waste polyester to depolymerization by a first glycolysis reaction at a temperature of 180°C to 200°C to obtain a first reactant; (1b) subjecting the first reactant to depolymerization by a second glycolysis reaction at a temperature of 150°C to 170°C to obtain a second reactant; (1c) subjecting the second reactant to ion exchange with an ion exchange resin to obtain a third reactant; (1d) removing unreacted glycol from the third reactant by distillation at a temperature of 150°C or lower to obtain a fourth reactant; and (1e) subjecting the fourth reactant to distillation to obtain crude bis(2-hydroxyethyl) terephthalate.

[0118] According to this embodiment, since the waste polyester is subjected to a pretreatment of pulverization and a multi-step depolymerization reaction at a low temperature, the content of glycol dimers (diethylene glycol) formed during the depolymerization reaction can be significantly reduced. This has the advantage of improving the purity of the bis(2-hydroxyethyl) terephthalate obtained at the end, and minimizing the side reaction structures in the subsequent repolymerization to polyester.

[0119] According to one embodiment, the depolymerization includes subjecting waste polyester to a first glycolysis reaction at a high temperature (180-200°C) to obtain a first reactant, and subjecting the first reactant to a second glycolysis reaction at a low temperature (150-170°C) to obtain a second reactant.

[0120] As is well known, glycolysis is a chemical reaction in which polymer chains, etc., are broken down by glycols. Glycols may include, for example, at least one selected from the group consisting of ethylene glycol, propylene glycol, and diethylene glycol.

[0121] A catalyst may be used in the glycolysis reaction. The catalyst may be a metal catalyst, such as a metal salt catalyst or a metal organocatalyst. Specifically, the catalyst may be a metal acetate, carbonate, oxide, or hydroxide, and the metal may be an alkali metal, alkaline earth metal, or transition metal.

[0122] As a specific example, catalysts include metal acetates or their anhydrous or hydride forms. More specifically, they may be at least one selected from the group consisting of zinc acetate, sodium acetate, cobalt acetate, manganese acetate, or their hydrates or anhydrous forms.

[0123] The total weight of the glycol added may be 1, 2, or 3 times or more the weight of the waste polyester resin, and may also be 7, 5, or 4 times or less. For example, the weight of the glycol added may be 1 to 7 times, specifically 2 to 5 times, or more specifically 3 to 4 times, the weight of the waste polyester resin.

[0124] Furthermore, the weight of the catalyst to be added may be 0.01 parts by weight or more, 0.1 parts by weight or more, 0.2 parts by weight or more, or 0.3 parts by weight or more per 100 parts by weight of waste polyester resin, and may also be 5 parts by weight or less, 1 part by weight or less, 0.7 parts by weight or less, 0.5 parts by weight or less, or 0.4 parts by weight or less. For example, the weight of the catalyst to be added may be 0.1 parts by weight to 1 part by weight, specifically 0.2 parts by weight to 0.7 parts by weight per 100 parts by weight of waste polyester resin. More specifically, the catalyst may be used in an amount of 0.2 parts by weight to 0.4 parts by weight per 100 parts by weight of waste polyester.

[0125] The temperature during the first glycolysis reaction may be 170°C or higher, 180°C or higher, or 190°C or higher, and may be 205°C or lower, 200°C or lower, 195°C or lower, or 190°C or lower. For example, the temperature during the first glycolysis reaction may be 180°C to 200°C, specifically 180°C to 195°C, and more specifically 180°C to 190°C.

[0126] Furthermore, the temperature during the second glycolysis reaction may be 140°C or higher, 150°C or higher, or 160°C or higher, and may be 170°C or lower, or 160°C or lower. For example, the temperature during the second glycolysis reaction may be 150°C to 170°C, specifically 150°C to 160°C, and more specifically 150°C to 155°C.

[0127] The time required for the first and second glycolysis reactions may be 1 hour or more, 2 hours or more, or 4 hours or less, or 3 hours or less, from the time the appropriate temperature is reached. For example, the time required for the first and second glycolysis reactions may be 1 to 4 hours, specifically 1 to 3 hours, or more specifically 1 to 2 hours, from the time the appropriate temperature is reached.

[0128] For example, the first glycolysis reaction may be carried out at a temperature of 180°C to 190°C for 1 to 3 hours. Furthermore, the second glycolysis reaction may be carried out at a temperature of 150°C to 160°C for 1 to 3 hours.

[0129] For example, the first glycolysis reaction may be carried out in the presence of a zinc acetate anhydride catalyst. Specifically, the first glycolysis reaction may be carried out in the presence of a zinc acetate anhydride catalyst at a temperature of 180°C to 200°C for 1 to 3 hours. The zinc acetate anhydride catalyst may be used in an amount of 0.2 to 0.4 parts by weight per 100 parts by weight of waste polyester. Furthermore, the second glycolysis reaction may be carried out at a temperature of 140°C to 160°C for 1 to 3 hours by adding ethylene glycol without adding the catalyst.

[0130] The second reaction product obtained by depolymerization is then cooled and can be used in the next step.

[0131] The cooling temperature may be, for example, 150°C or lower, 140°C or lower, 130°C or lower, 120°C or lower, 110°C or lower, or 100°C or lower, and may also be 50°C or higher, 60°C or higher, 70°C or higher, 80°C or higher, or 90°C or higher.

[0132] For example, cooling may be performed by a vacuum flash process. Specifically, the temperature of the second reactant can be lowered by applying a vacuum in a vacuum flash process to evaporate ethylene glycol.

[0133] For example, the second reactant may be further subjected to a step of cooling to below 120°C by vacuum flashing before the subsequent step. More specifically, the temperature of the second reactant may be lowered to below 110°C or 100°C by the vacuum flashing process.

[0134] The pressure conditions for the depressurization flash process may be, for example, 200 Torr or less, 100 Torr or less, or 50 Torr or less, specifically 10 Torr to 200 Torr, 10 Torr to 100 Torr, or 10 Torr to 50 Torr.

[0135] Subsequently, insoluble impurities can be removed from the cooled second reactant by filtration. Specifically, before the ion exchange in step (1c), the second reactant may be cooled to below 120°C, and a filter aid may be added to filter the second reactant. As a result, fine particles and insoluble organic matter present in the second reactant can be removed by solid-liquid separation.

[0136] Known components such as diatomaceous earth, perlite, and asbestos powder can be used as filter aids. For example, 0.1 to 2.0 parts by weight of a filter aid may be added to 100 parts by weight of the second reactant.

[0137] Since bis(2-hydroxyethyl) terephthalate (BHET) or oligomers obtained by the depolymerization reaction exist as solids at room temperature, it is difficult to separate impurities at room temperature. Therefore, it is preferable to separate impurities at a temperature of 90°C to 150°C, more specifically 110°C to 150°C. Furthermore, if the above temperature range is maintained, the fluidity is good, which may facilitate the removal of insoluble impurities.

[0138] Various methods and devices can be used to remove insoluble impurities by solid-liquid separation. For example, devices such as pressure filters, centrifuges, filter presses, and belt presses may be used. However, the method is not limited to these, as long as any method capable of separating impurities is used.

[0139] The second reaction product, which has been depolymerized, cooled, and filtered, is subjected to ion exchange using an ion exchange resin to obtain a third reaction product.

[0140] When the second reactant is subjected to ion exchange, ionic impurities present in the second reactant, specifically catalysts and metallic contaminants, can be removed.

[0141] As is well known, ion exchange resins refer to resins or polymers that function as a medium for ion exchange. Ion exchange resins may include cation exchange resins, anion exchange resins, amphoteric ion exchange resins, chelate resins, and the like.

[0142] Cation exchange resins may include strongly acidic cation exchange resins having sulfonic acid groups (-SO3H) and weakly acidic cation exchange resins having carboxyl groups (-COOH). Anion exchange resins may include strongly basic anion exchange resins in the form of quaternary ammonium salts and weakly basic anion exchange resins having primary to tertiary amino groups.

[0143] As a specific example, the ion exchange resin may include at least one selected from the group consisting of strongly acidic cation exchange resins, weakly acidic cation exchange resins, and chelate resins.

[0144] According to one embodiment, ion exchange is performed by adding an ion exchange resin to the first reactant.

[0145] The weight of the ion exchange resin added may be 1, 3, or 5 times or more the weight of the catalyst added in the depolymerization reaction, and may also be 20, 15, 10, or 8 times or less. For example, the weight of the ion exchange resin added may be 1 to 20 times, specifically 3 to 15 times, or more specifically 5 to 8 times, the weight of the catalyst added in the depolymerization reaction.

[0146] Furthermore, the weight of the ion exchange resin to be added may be 1 part by weight or more, 3 parts by weight or more, or 5 parts by weight or more, per 100 parts by weight of the waste polyester resin used in the depolymerization reaction, and may also be 50 parts by weight or less, 20 parts by weight or less, 15 parts by weight or less, 10 parts by weight or less, or 7 parts by weight or less.

[0147] For example, the ion exchange resin may be used in an amount of 1 to 20 parts by weight per 100 parts by weight of waste polyester.

[0148] According to another embodiment, ion exchange is performed using a column containing an ion exchange resin.

[0149] Specifically, the column can be packed with ion exchange resin particles, and ion exchange can be performed while passing a second reactant through the column.

[0150] The particle size of the ion exchange resin particles may be, for example, 0.3 mm to 1.5 mm, or more specifically, 0.6 mm to 0.9 mm.

[0151] The ion exchange temperature may be, for example, 140°C or lower, 130°C or lower, 120°C or lower, 110°C or lower, or 50°C or higher, 60°C or higher, 70°C or higher, 80°C or higher, or 90°C or higher.

[0152] The unreacted glycol is removed from the third reactant by distillation to obtain the fourth reactant.

[0153] Even after filtration in the previous step, unreacted glycol remains in the depolymerization product; therefore, it is necessary to remove the unreacted glycol from the reactants before proceeding to the next step.

[0154] Furthermore, in order to carry out the depolymerization method economically, it is necessary to perform a step to recover unreacted glycol. Specifically, glycols that were used in the depolymerization process beforehand and remain without participating in the glycolysis reaction, such as ethylene glycol, propylene glycol, and diethylene glycol, can be recovered and reused.

[0155] Distillation to remove unreacted glycol can be carried out, for example, by vacuum distillation. A glass distillation apparatus or a rotary evaporator may be used for this purpose.

[0156] Since vacuum distillation to remove unreacted glycol is performed at temperatures below 150°C, the purity of BHET can be increased by further reducing the formation of diethylene glycol and impurities derived from diethylene glycol. For example, vacuum distillation to remove unreacted glycol may be performed at temperatures below 150°C, below 130°C, or below 120°C, and above 80°C, above 90°C, above 100°C, or above 110°C. Specifically, the temperature during distillation to remove unreacted glycol may be between 80°C and 190°C, or between 90°C and 150°C. As a more specific example, distillation to remove unreacted glycol may be performed at temperatures between 100°C and 130°C.

[0157] The pressure during distillation to remove unreacted glycol may be, for example, 0.1 Torr to 200 Torr, or more specifically, 0.5 Torr to 30 Torr.

[0158] The fourth reaction product, from which unreacted glycol has been removed, is subjected to distillation to obtain crude bis(2-hydroxyethyl)terephthalate.

[0159] Various methods may be used for distillation, but a distillation method that separates the mixture into a thin film can be used to increase the surface area in contact with the heat source.

[0160] For example, distillation can be carried out by thin-film evaporation, downward-flowing film evaporation, or short-pass evaporation. For this purpose, a thin-film evaporator, a downward-flowing film evaporator, and a short-pass evaporator can be used, respectively.

[0161] As a specific example, distillation to obtain bis(2-hydroxyethyl) terephthalate may be carried out by thin-film evaporation. Specifically, the mixture supplied to the evaporator of the thin-film evaporator forms a thin film on the inner wall of the evaporator by a wiper rotor. Then, distillation is carried out under appropriate temperature conditions by heating. Furthermore, a cooler for recovering the evaporated product may be provided inside the thin-film evaporator.

[0162] Thin-film evaporation may also be performed by short-pass evaporation. Such short-pass and thin-film evaporation have short residence times and allow for reduced-pressure distillation under high vacuum, making it possible to separate high-boiling-point or high-molecular-weight substances that are difficult to separate by other distillation methods, while minimizing thermal changes to the reactants. Furthermore, lowering the pressure inside the thin-film evaporator lowers the vapor pressure of the substance, which has the advantage of allowing evaporation to be performed at a temperature lower than the original boiling point.

[0163] As a specific example, the fourth reactant is supplied to a short pass and a thin-film evaporator, and a wiper for forming a thin film is rotated at 300 rpm or more. As a result, the evaporated and non-evaporated substances can be separated from each other.

[0164] The internal thin film temperature at the top of the thin film evaporator during thin film evaporation may be, for example, 100°C or higher, 110°C or higher, 120°C or higher, or 125°C or higher, and may be 250°C or lower, 200°C or lower, 150°C or lower, or 135°C or lower, specifically 150°C to 250°C, 190°C to 250°C, or 180°C to 220°C.

[0165] Furthermore, the internal pressure at the top of the thin-film evaporator during thin-film evaporation may be, for example, 0.005 Torr to 5.0 Torr, specifically 0.05 Torr to 5.0 Torr, 0.05 Torr to 1.5 Torr, or 0.05 Torr to 1 Torr. More specifically, distillation to obtain crude bis(2-hydroxyethyl) terephthalate may be carried out by thin-film evaporation under a pressure of 0.05 Torr to 0.4 Torr.

[0166] Crude bis(2-hydroxyethyl) terephthalate is subjected to an adsorption-crystallization step to provide high-purity, high-quality bis(2-hydroxyethyl) terephthalate.

[0167] For example, adsorption-crystallization may be carried out by adding an adsorbent using water as a solvent, followed by filtration and crystallization.

[0168] Various solvents may be used for adsorption and crystallization, but preferably a solvent capable of dissolving bis(2-hydroxyethyl) terephthalate is used. Specifically, to obtain the final reaction product, water is added as a solvent to crude bis(2-hydroxyethyl) terephthalate dissolved by heating, an adsorbent is added to this, and the solution obtained by filtration is subjected to cooling, crystallization, and final filtration. As a result, high-purity bis(2-hydroxyethyl) terephthalate can be obtained. Water may be added in an amount of 100 to 500 parts by weight, specifically 200 to 400 parts by weight, or more specifically 300 to 350 parts by weight, per 100 parts by weight of crude bis(2-hydroxyethyl) terephthalate.

[0169] Furthermore, the dissolution temperature may be 50°C to 95°C, specifically 60°C to 85°C, or more specifically 70°C to 75°C.

[0170] The added adsorbent can act to adsorb and remove other impurities. The adsorbent may be added in an amount of 0.1 to 3 parts by weight per 100 parts by weight of crude bis(2-hydroxyethyl) terephthalate. The type and form of the adsorbent are not particularly limited. For example, activated carbon may be used.

[0171] Bis(2-hydroxyethyl) terephthalate, i.e., the regenerated BHET finally obtained by the above steps, is of high purity and contains organic impurities, particularly diethylene glycol and by-products derived from diethylene glycol (such as DEG esters), or other oligomers and inorganic substances, at levels below a certain threshold, and has excellent color quality. Preparation of a solution of recycled bis(2-hydroxyethyl) terephthalate The regenerated bis(2-hydroxyethyl) terephthalate prepared above is mixed with a solvent to prepare a solution of regenerated bis(2-hydroxyethyl) terephthalate.

[0172] Examples of solvents used in the preparation of a solution of regenerated BHET include water, ethylene glycol, methanol, and ethanol. Specifically, the solvent may contain at least one of water and ethylene glycol. More specifically, the solvent may contain both water and ethylene glycol, in which case the mixed weight ratio of water and ethylene glycol may be 1:5 or 1:2.

[0173] The temperature at which the regenerated BHET is mixed with the solvent (dissolution temperature) may be, for example, 60°C or higher, 65°C or higher, 70°C or higher, or 75°C or higher, and may also be 197°C or lower, 180°C or lower, 165°C or lower, 140°C or lower, 120°C or lower, 100°C or lower, 90°C or lower, or 80°C or lower.

[0174] According to one embodiment, the temperature at which the regenerated BHET is mixed with water may be 60°C to 100°C. According to another embodiment, the temperature at which the regenerated BHET is mixed with ethylene glycol may be 60°C to 197°C. According to one embodiment, the temperature at which the regenerated BHET is mixed with a mixed solvent of water and ethylene glycol may be 60°C to 197°C. Within this preferred temperature range, the regenerated BHET dissolves sufficiently, which is more advantageous in preventing the problem of reduced purity due to the formation of diethylene glycol esters caused by thermal decomposition at high temperatures.

[0175] For example, the concentration of the regenerated BHET solution may be 25% by weight or more, 35% by weight or more, 50% by weight or more, 65% by weight or more, or 75% by weight or more, and may be 99% by weight or less, 95% by weight or less, 90% by weight or less, 85% by weight or less, or 80% by weight or less. Specifically, the concentration of the regenerated BHET solution may be 25% by weight to 99% by weight, more specifically 50% by weight to 95% by weight. Within the above preferred range, it may be even more advantageous to have excellent reaction efficiency and induce a uniform polymerization reaction. The concentration of the regenerated BHET solution can be calculated as a percentage of the weight of regenerated BHET based on the total weight of the solution (i.e., the total weight of regenerated BHET and solvent).

[0176] Preparation of polyester resin The recycled bis(2-hydroxyethyl) terephthalate solution prepared above is used to polymerize polyester resin.

[0177] The polymerization of the polyester resin of the present invention comprises preparing a low molecular weight oligomer by performing a polycondensation reaction (first polycondensation reaction) under low vacuum, and preparing a polyester resin by subjecting this oligomer to a polycondensation reaction (second polycondensation reaction) under high vacuum.

[0178] The first and second polycondensation reactions can be carried out under reduced pressure, allowing the solvent contained in the regenerated bis(2-hydroxyethyl) terephthalate solution and by-products of the polycondensation reaction (such as glycols) to be discharged from the system.

[0179] The pressure during the first polycondensation reaction may be, for example, 700 mmHg or less, 600 mmHg or less, 500 mmHg or less, 400 mmHg or less, 350 mmHg or less, 300 mmHg or less, or 250 mmHg or less, or 160 mmHg or more, 180 mmHg or more, 200 mmHg or more, 220 mmHg or more, or 240 mmHg or more. According to one embodiment, the pressure during the first polycondensation reaction is 200 mmHg to 600 mmHg. Within the above preferred range, it may be even more advantageous to sufficiently remove by-products of the polycondensation reaction under low vacuum while maintaining the vacuum level during the polycondensation reaction.

[0180] Furthermore, the temperature during the first polycondensation reaction may be, for example, 100°C or higher, 130°C or higher, 160°C or higher, 180°C or higher, or 200°C or higher, and may also be 300°C or lower, 280°C or lower, 250°C or lower, or 230°C or lower. Specifically, the first polycondensation reaction may be carried out at a temperature of 180°C to 250°C and a pressure of 200 mmHg to 400 mmHg.

[0181] Furthermore, the first polycondensation reaction may be carried out until the number-average molecular weight of the low molecular weight oligomer reaches an appropriate level. The time required for the first polycondensation reaction is not particularly limited, but may be, for example, 30 minutes or more, 1 hour or more, 2 hours or more, or 3 hours or more, or 15 hours or less, 10 hours or less, 5 hours or less, or 4 hours or less. Specifically, it may be between 1 and 5 hours.

[0182] The pressure during the second polycondensation reaction may be, for example, less than 200 mmHg, 150 mmHg or less, 100 mmHg or less, 50 mmHg or less, 10 mmHg or less, or 1 mmHg or less, or 0.001 mmHg or more, 0.01 mmHg or more, 0.1 mmHg or more, or 0.5 mmHg or more. According to one embodiment, the pressure during the second polycondensation reaction is less than 200 mmHg. Within the above preferred range, it may be even more advantageous to sufficiently remove by-products of the polycondensation reaction while maintaining the vacuum level during the polycondensation reaction.

[0183] Furthermore, the temperature during the second polycondensation reaction may be, for example, 230°C or higher, 240°C or higher, 250°C or higher, or 260°C or higher, and may be 300°C or lower, 290°C or lower, 280°C or lower, or 270°C or lower. Specifically, the second polycondensation reaction may be carried out at a temperature of 250°C to 300°C and a pressure of 0.01 mmHg to 150 mmHg. Within the above preferred range, it may be even more advantageous to sufficiently remove by-products of the polycondensation reaction while maintaining the vacuum level during the polycondensation reaction, thereby suppressing yellowing of the final resin.

[0184] Furthermore, the second polycondensation reaction may be carried out until the number-average molecular weight of the polyester resin reaches an appropriate level. The time required for the second polycondensation reaction is not particularly limited, but may be, for example, 30 minutes or more, 1 hour or more, 2 hours or more, or 5 hours or more, or 60 hours or less, 48 ​​hours or less, 24 hours or less, or 15 hours or less. Specifically, it may be between 1 and 24 hours.

[0185] The method for preparing the polyester resin of the present invention may further include steps commonly used in the art, in addition to the steps described above.

[0186] As an example, a method for preparing a polyester resin may further include the step of molding the polyester resin to form pellets after a polycondensation reaction.

[0187] As another example, a method for preparing a polyester resin may further include a step of subjecting the polyester resin to solid-phase polymerization after a polycondensation reaction. The temperature during solid-phase polymerization may be, for example, 180°C or higher, 190°C or higher, 200°C or higher, or 205°C or higher, and may be 260°C or lower, 240°C or lower, 220°C or lower, or 215°C or lower. Specifically, solid-phase polymerization may be carried out at a temperature of 200°C to 220°C. Furthermore, the pressure during solid-phase polymerization may be, for example, 10.0 Torr or lower, 5.0 Torr or lower, 2.0 Torr or lower, or 1.0 Torr or lower, and may be 0.01 Torr or higher, 0.1 Torr or higher, 0.2 Torr or higher, or 0.5 Torr or higher. Specifically, it may be 0.2 Torr to 2.0 Torr. Furthermore, solid-phase polymerization may be carried out in an inert gas atmosphere such as nitrogen.

[0188] The polyester resin according to the present invention can be prepared as a copolymerized polyester resin by further adding an additional diacid component to recycled bis(2-hydroxyethyl) terephthalate. The additional diacid component may be a dicarboxylic acid or a derivative thereof. The dicarboxylic acid may include at least one selected from terephthalic acid and isophthalic acid. For example, a dicarboxylic acid or a derivative thereof may be further added during the first polycondensation reaction.

[0189] Furthermore, the polycondensation reaction may be carried out in the presence of a polycondensation catalyst. The polycondensation catalyst may be selected from the group consisting of titanium-based compounds, germanium-based compounds, antimony-based compounds, and aluminum-based compounds. The amount of polycondensation catalyst used is preferably 0.1 ppm to 500 ppm, based on the amount of metal elements relative to the weight of the final polyester resin. The amount used may vary depending on the desired color and the stabilizers and colorants used, as it affects the color of the final polyester resin.

[0190] In addition to the polycondensation catalyst, stabilizers, colorants, crystallizers, antioxidants, branching agents, etc., may be used. The timing of adding these additives is not particularly limited and they may be added at any point during the polyester resin preparation step.

[0191] As stabilizers, phosphorus compounds, such as phosphoric acid, trimethyl phosphate, triethyl phosphate, and triethylphosphonoacetate, can generally be used. The amount of stabilizer added can be 10 to 200 ppm relative to the weight of the polyester resin, based on the amount of the element. Furthermore, as colorants added to improve the color of the polyester resin, common colorants such as cobalt acetate and cobalt propionate can be exemplified. The amount of colorant added can be 10 to 200 ppm relative to the weight of the polyester resin, based on the amount of cobalt element. If necessary, organic colorants such as anthraquinone compounds, perinone compounds, azo compounds, and methine compounds may be used. Commercial toners such as Clarient's Polysynthren Blue RLS or Clarient's Solvaperm Red BB may be used. The amount of organic compound colorant added can be adjusted to 0 to 50 ppm, based on the weight of the polyester resin. Examples of crystallizing agents include crystal nucleating agents, ultraviolet absorbers, polyolefin resins, polyamide resins, etc. Examples of antioxidants include hindered phenol antioxidants, phosphite antioxidants, thioether antioxidants, or mixtures thereof. Examples of branching agents include conventional branching agents having three or more functional groups, such as trimellitic anhydride, trimethylolpropane, trimellitic acid, or mixtures thereof.

[0192] Preparation of polyester film Subsequently, the polyester resin is melt-extruded and stretched to prepare a polyester film. In one embodiment, the polyester film can be prepared by biaxial stretching of the polyester resin. Specifically, the polyester film can be prepared by melt-extruding the polyester resin to produce an unstretched polyester film containing a resin layer formed from the polyester resin; and then biaxially stretching the unstretched polyester film at a temperature above the glass transition temperature of the polyester resin.

[0193] In the step of producing an unstretched polyester film, the thermal decomposition of the polymer can be minimized by melt-extruding the polyester resin. Specifically, the step of producing an unstretched polyester film can be carried out at a temperature of 265°C to 300°C. If the melt-extrusion temperature is lower than 265°C, the polymer may not melt. If the melt-extrusion temperature exceeds 300°C, the thermal decomposition of the polymer increases, and the film may be damaged or broken during stretch molding, making it difficult to achieve the desired physical properties. The unstretched polyester film can be cooled to an appropriate temperature. The unstretched polyester film can then be stretched at a temperature above the glass transition temperature of the polyester resin.

[0194] The step of stretching the unstretched polyester film can be performed at a temperature of 80°C to 200°C, thereby enabling the unstretched polyester film to be stretched to a high elongation ratio. Specifically, with respect to biaxial stretching, the unstretched polyester film can be stretched 2 to 6 times, specifically 2 to 5 times, in the longitudinal direction and 2 to 6 times, specifically 2 to 5 times, in the transverse direction.

[0195] After biaxial stretching of the unstretched polyester film, a heat setting step may be performed to provide dimensional stability to the biaxially oriented polyester film thus obtained. The heat setting step may be performed at a temperature of 160°C to 240°C for 1 to 60 seconds.

[0196] [Modes of the Invention] The present invention will be described in more detail below with reference to embodiments. However, these examples are provided for illustrative purposes only, and the present invention is not limited thereto.

[0197] Example 1 Step 1: Preparation of regenerated bis(2-hydroxyethyl) terephthalate 2,000 g of waste polyester resin, pulverized to a particle size of 4 mm or less, 4,000 g of ethylene glycol, and 7.0 g of anhydrous zinc acetate were added to a first reactor made of stainless steel (SUS). The temperature inside the reactor was raised to 180°C, and depolymerization (first glycolysis reaction) was carried out for 2 hours. The resulting reactant (first reactant) was transferred to a second reactor and cooled to 150°C. Another 2,000 g of ethylene glycol was added, and depolymerization (second glycolysis reaction) was carried out for 2 hours while maintaining the reactor temperature at 150°C.

[0198] The resulting reaction product (second reaction product) was cooled to 120°C by vacuum flashing, 16 g of a filter aid was added, and then solid-liquid separation was performed by pressurized filtration. The separated liquid reaction product was passed through a column packed with ion exchange resin (Bonlite BC107(H)) to remove ionic impurities, and a mixture containing bis(2-hydroxyethyl) terephthalate and ethylene glycol (third reaction product) was obtained.

[0199] This mixture (third reactant) was transferred to a 10-liter distillation apparatus and vacuum distillation was performed at 130°C to recover unreacted ethylene glycol. The reactant from which ethylene glycol had been removed (fourth reactant) was evaporated in a thin film evaporator (VTA VKL70-4S) at 220°C and 0.08 Torr to obtain 1,040 g of product from which dimers and oligomers had been removed.

[0200] Subsequently, for adsorption and crystallization, 1,040 g of the above product and 3,120 g of distilled water were placed in a 20-liter glass reactor and dissolved at a temperature of 70°C. Then, 5.2 g of activated carbon was added, and the mixture was stirred for 30 minutes and filtered. The filtrate was cooled to room temperature to crystallize, filtered, and dried in a vacuum oven. As a result, 1,980 g of the final product containing regenerated bis(2-hydroxyethyl) terephthalate was obtained.

[0201] Step 2: Preparation of a solution of regenerated bis(2-hydroxyethyl) terephthalate 1,980 g of recycled bis(2-hydroxyethyl) terephthalate (r-BHET), 312 g of water, and 483 g of ethylene glycol (EG) were uniformly mixed at 70°C to prepare an r-BHET solution (concentration: 71.4% by weight).

[0202] Step 3: Polycondensation reaction under low vacuum In a 7-liter reactor capable of reacting under vacuum, 2,775 g of the regenerated bis(2-hydroxyethyl) terephthalate (r-BHET) solution prepared above, 0.8 g of antimony trioxide as catalyst, 0.6 g of triethyl phosphate as stabilizer, and 0.4 g of cobalt acetate as colorant were added. The reactor temperature was raised to 190°C over 2 hours. Once the temperature reached 190°C, the reactor pressure was reduced from standard pressure to 200 Torr (absolute pressure: 200 mmHg) over 30 minutes. While maintaining the reactor pressure at 200 Torr (absolute pressure: 200 mmHg), the polycondensation reaction was carried out under low vacuum for 1 hour.

[0203] Step 4: Polycondensation reaction under high vacuum The reactor pressure was reduced from 200 Torr (absolute pressure: 200 mmHg) to 5 Torr (absolute pressure: 5 mmHg) over 30 minutes. Simultaneously, the reactor temperature was raised to 280°C over 1 hour, and the polycondensation reaction was carried out under high vacuum while maintaining the reactor pressure at 1 Torr (absolute pressure: 1 mmHg) or less. At the start of the polycondensation reaction under high vacuum, the stirring speed may be set high. As the polycondensation reaction under high vacuum progresses, if the viscosity of the reactants increases and the stirring force weakens, or if the temperature of the reactants exceeds the set temperature, the stirring speed may be adjusted appropriately accordingly. The polycondensation reaction under high vacuum was carried out until the intrinsic viscosity (IV) of the mixture (molten material) in the reactor reached 0.64 dl / g. Once the intrinsic viscosity of the mixture in the reactor reached the desired level, the mixture was then discharged from the reactor to form strands, solidified with a coolant, and then granulated to an average weight of approximately 12-14 mg to prepare the polyester resin.

[0204] Example 2 In Step 2, 1,980 g of recycled bis(2-hydroxyethyl) terephthalate (r-BHET) and 312 g of water were uniformly mixed at 70°C to prepare an r-BHET solution (concentration: 86.4% by weight). The polyester resin was prepared in the same manner as in Example 1.

[0205] Example 3 The polyester resin was prepared using the same procedure as in Example 1, except that the polycondensation reaction was not carried out under low vacuum in Step 3, while the polycondensation reaction was carried out under high vacuum in Step 4.

[0206] Example 4 In Step 1, the polyester resin was prepared using the same procedure as in Example 1, except that the first glycolysis reaction was carried out at 180°C for 1 hour.

[0207] Example 5 In Step 1, the polyester resin was prepared using the same procedure as in Example 1, except that 2,000 g of waste fiber was used as the waste polyester raw material.

[0208] Example 6 In Step 1, the polyester resin was prepared using the same procedure as in Example 1, except that 2,000g of discarded banner material was used as the waste polyester raw material.

[0209] Example 7 In Step 1, the polyester resin was prepared using the same procedure as in Example 1, except that adsorption and crystallization were not performed after thin film evaporation.

[0210] Comparative Example 1 2,000 g of waste polyester resin, 8,000 g of ethylene glycol, and 7.0 g of anhydrous zinc acetate were added to a stainless steel (SUS) reactor. The temperature inside the reactor was raised to 196°C, and depolymerization (glycolysis) was carried out over 4 hours. The resulting reaction product was cooled to 30°C, and crystallization of bis(2-hydroxyethyl) terephthalate was carried out over 2 hours. The obtained slurry of bis(2-hydroxyethyl) terephthalate and ethylene glycol was subjected to solid-liquid separation using a centrifuge. The bis(2-hydroxyethyl) terephthalate obtained by centrifugation was washed twice with sufficient distilled water, and the residual solvent was removed in an oven to obtain approximately 2,020 g of the final product containing bis(2-hydroxyethyl) terephthalate.

[0211] Using the bis(2-hydroxyethyl) terephthalate obtained in this manner, a polyester resin was prepared in the same manner as in Example 1.

[0212] Comparative Example 2 In Step 1, the depolymerization (glycolysis) was carried out at 210°C, but otherwise the polyester resin was prepared using the same procedure as in Comparative Example 1.

[0213] Comparative Example 3 In Step 1, a polyester resin was prepared using the same procedure as in Comparative Example 1, except that depolymerization (glycolysis) was performed at 196°C and adsorption-crystallization was not carried out.

[0214] Comparative Example 4 In a 5 kg batch reactor, terephthalic acid (2,161.9 g), ethylene glycol (1,050 g), antimony trioxide (0.8 g) as a catalyst, triethyl phosphate (0.6 g) as a stabilizer, and cobalt acetate (0.4 g) as a coloring agent were added. After adding the raw materials, nitrogen was then injected into the reactor to raise the pressure to 2.0 kgf / cm² above the standard pressure. 2 The mixture was then pressurized to a high degree. Next, the reactor temperature was raised to 250°C, and the esterification reaction was carried out at 250°C while visually observing the mixture in the reactor until it became clear. Upon completion of the esterification reaction, the nitrogen in the pressurized reactor was released to the outside, reducing the pressure in the reactor to standard pressure, and then the mixture in the reactor was transferred to a 7-liter reactor capable of reacting under vacuum.

[0215] Next, the reactor pressure was reduced from standard pressure to 5 Torr (absolute pressure: 5 mmHg) over 30 minutes. Simultaneously, the reactor temperature was raised to 280°C, and the polycondensation reaction was carried out for 2 hours while maintaining the reactor pressure at 0.5–1 Torr. The polycondensation reaction was continued until the intrinsic viscosity (IV) of the mixture (molten material) in the reactor reached 0.64 dl / g. Once the intrinsic viscosity reached the desired level, the mixture was then discharged from the reactor to form strands, solidified with a coolant, and then granulated to an average weight of approximately 12–14 mg.

[0216] Test Example 1: Evaluation of Regenerated Bis(2-hydroxyethyl) Terephthalate Analysis of 1A.r-BHET components - HPLC The components of recycled bis(2-hydroxyethyl) terephthalate (BHET) were analyzed by high-performance liquid chromatography (HPLC).

[0217] Approximately 0.01 g of the sample was diluted in approximately 20 ml of methanol, and then measured by HPLC. Model: Waters e2695 Column: C18 (4.6 × 250 mm), 5 μm UV detector: 242nm Injection volume: 10μl Eluents (gradient): A: H2O + H3PO4, B: Acetonitrile Measured components: Bis(2-hydroxyethyl) terephthalate (BHET), mono(2-hydroxyethyl) terephthalate (MHET), 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl] terephthalate (DEG ester 1), bis[2-(2-hydroxyethoxy)ethyl]benzene-1,4-dicarboxylate (DEG ester 2), dimer, trimer, others

[0218] 2B.Residual solvent-GC The residual ethylene glycol (EG) content in recycled bis(2-hydroxyethyl) terephthalate (BHET) was measured by gas chromatography (GC).

[0219] Approximately 0.1 g of the sample was diluted in approximately 10 ml of CHCl3, filtered through a 0.45 μm filter, and then measured by GC. Model: Agilent 7890B Column: DB-624 (30m x 0.25mm x 1.4μm) Oven temperature: 60℃ (2 min) - 10℃ / min - 200℃ (0 min) - 20℃ / min - 260℃ (5 min) Injection temperature: 250℃ Detector temperature: 250℃ Flow rate: 1.5ml / min (N2), split ratio: 1 / 50

[0220] 1C. Melting temperature, glass transition temperature, and crystallization temperature - DSC Differential scanning calorimetry (DSC, Q20 model, TA instrument) was used. Each sample was placed in an aluminum pan and heated to 280°C at 10°C / min, maintained at 280°C for 5 minutes, and then cooled to 30°C at -300°C / min. Subsequently, the glass transition temperature (Tg) and melting temperature (Tm) were determined from the heat flow rate obtained when the temperature was raised to 280°C at 10°C / min. Then, the temperature at the peak of the exothermic curve when the temperature was maintained at 280°C for 5 minutes and then cooled to 30°C at -10°C / min was defined as the cooling crystallization temperature (Tmc).

[0221] 1D.Inorganic substances-ICP-AES Approximately 0.3 g of each sample was pretreated with ultrasound and diluted with ultrapure water. Inorganic components were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES, 5100, Agilent) (detection limit: 5 ppm).

[0222] Test Example 2: Evaluation of Polyester Resin 2A. Melting point (Tm) - DSC The melting point of each polyester resin was measured using DSC in the same manner as described in 1C above.

[0223] 2B. Intrinsic viscosity (IV) Each polyester resin was dissolved in orthochlorophenol (OCP) at a concentration of 1.2 g / dl at 150°C to obtain a solution, and its intrinsic viscosity was measured using an Ubbelohde viscometer. Specifically, the temperature of the viscosity tube was maintained at 35°C, and the time required for the solvent to pass through a specific part of the viscosity tube (flow time) and the time required for the solution to pass through and obtain a specific viscosity were used to calculate the intrinsic viscosity.

[0224] Content of 2C.DEG - GC Each polyester resin was finely ground using a pulverizer, 2 g of the resin was subjected to aminolysis with hydrazine hydrate, and then the diethylene glycol (DEG) content (by weight) was measured by gas chromatography (GC).

[0225] 2D. Concentration of carboxyl-terminal groups The resin sample and benzyl alcohol as a solvent were placed in a glass container of a predetermined size and heated to 180°C to dissolve. A small amount of phenol red was added to the dissolved sample solution as an indicator, and the color change of the solution was observed while adding a small amount of dilute sodium hydroxide solution. The concentration (E) of the -COOH terminal group was calculated from the amount of sample added, the concentration of the sodium hydroxide solution, and the amount added using the following formula (unit: equivalents / ton).

[0226]

number

[0227] In the formula, S is the volume (L) of 0.1N sodium hydroxide-benzyl alcohol solution consumed in the titration of the polyester resin, B is the volume (L) of 0.1N sodium hydroxide-benzyl alcohol solution consumed in the blank titration, N is the concentration of the sodium hydroxide-benzyl alcohol solution, which is 0.1N, W is the weight (kg) of the polyester resin used in the titration, and f is the factor of the 0.1N sodium hydroxide-benzyl alcohol solution, which is 0.95 to 0.99, specifically 0.97.

[0228] 2E. Cyclic trimer - HPLC A polyester resin was finely ground using a pulverizer to obtain a sample. 0.1 g of the sample was dissolved in 1 mL of 1,1,1,3,3,3-hexafluoro-2-propanol / chloroform = 2 / 3 (v / v). 3 mL of methanol was added, and the mixture was vortexed to reprecipitation. The solution was then filtered through a 0.2 μm filter. The content of the cyclic trimer was measured using high-performance liquid chromatography (HPLC).

[0229] Test Example 3: Evaluation of Polyester Film 3A. Preparation of polyester film Each polyester resin prepared in the examples and comparative examples was dried in a nitrogen atmosphere at 150°C for 6 hours to adjust the moisture content to 50 ppm or less.

[0230] A dry polyester resin was melt-extruded into a sheet form through a die at a temperature of 280°C, then rapidly cooled and solidified to prepare an unstretched film. Subsequently, the unstretched film was primary stretched 3.6 times in the longitudinal direction (MD) at 100°C, and then secondary stretched 3.6 times in the transverse direction (TD) at 120°C. The stretched film was heat-set at 230°C for 10 seconds to impart dimensional stability, thereby preparing a polyester film with a thickness of 50 μm.

[0231] 3B. Tensile strength and elongation Film samples were cut into strips of at least 5 cm in length and 1.5 cm in width in both the longitudinal (MD) and transverse (TD) directions. These strips were mounted longitudinally on clips spaced 5 cm apart in a universal testing machine (UTM 5566A, Instron), and stretched at a tensile speed of 200 mm / min at 25°C according to ASTM D882 until fracture occurred, obtaining stress-strain curves.

[0232] The strength at the point of fracture of the film sample was defined as the tensile strength, and the length stretched was defined as the elongation.

[0233] 3C. Elongation retention rate (%) - MD, PCT conditions Each prepared film was left for 96 hours under pressurized cooker test conditions of 121°C, 100% relative humidity, and 2 atmospheres, and then its elongation in the longitudinal direction (MD) was measured. The method for measuring elongation was the same as in section 3B above. The elongation retention rate (R) was calculated from the measured values ​​using the following formula 2.

[0234]

number

[0235] In the formula, S0 is the initial elongation (%) of the polyester film sample, and S1 is the elongation (%) measured after the film sample has been left at 121°C, 100% RH, and 2 atmospheres for 96 hours.

[0236] 3D Machinability Evaluation In the process of melt-extruding polyester resin, a 10 μm filter was used, and the filter pressure applied during the process of removing foreign matter and unmelted gel-like resin was measured. The number of film breaks was measured over 24 hours during the stretching process of the polyester film, and the processability was evaluated according to the following evaluation criteria. ○ (Very good): 80kg / cm 2 The following filter pressures and failures occurred: 0 △ (Good): 80-150 kg / cm 2 Filter pressure failure occurred 1 × (defective):150kg / cm2 The following filter pressures and failures occur (2 or more times): The results of the test examples are shown in the table below.

[0237] [Table 1]

[0238] [Table 2]

[0239] As can be seen from the table above, in the recycled polyester resins of Examples 1 to 7, the cyclic trimer content was 3,000 ppm or less, the diethylene glycol (DEG) content and the concentration of carboxyl terminal groups were adjusted to a certain level or lower, and the melting point was high. When prepared into films, the processability was excellent under high temperature and high humidity conditions, with superior strength, elongation, and elongation retention.

[0240] In contrast, the recycled polyester resins of Comparative Examples 1-3 had content of cyclic trimers, diethylene glycol, and carboxyl-terminated groups exceeding the desirable range, resulting in low melting points. When prepared as films, their processability was poor under high temperature and high humidity conditions, with low strength, elongation, and elongation retention. Furthermore, Examples 1-7 showed equivalent or better properties compared to the unused polyester resin and its film from Comparative Example 4, which was not subjected to the recycling process.

Claims

1. A polyester resin containing recycled bis(2-hydroxyethyl) terephthalate obtained by depolymerization of waste polyester, wherein the peak area fraction of the cyclic trimer measured by high-performance liquid chromatography (HPLC) is 3,000 ppm or less, and the diethylene glycol (DEG) content measured by gas chromatography (GC) is 1.0% by weight or less.

2. The polyester resin according to claim 1, having a melting point of 255°C or higher when measured by differential scanning calorimetry (DSC).

3. The polyester resin according to claim 1, having an intrinsic viscosity (IV) of 0.6 dl / g to 1.2 dl / g at 35°C.

4. The polyester resin according to claim 1, wherein the recycled bis(2-hydroxyethyl) terephthalate has a total oligomer peak area fraction of 3% or less when measured by high-performance liquid chromatography.

5. A polyester film comprising the polyester resin described in any one of claims 1 to 4.

6. When cut into 1.5 cm wide strips and mounted vertically on clips spaced 5 cm apart, and tested according to ASTM D882 at 25°C with a tensile speed of 200 mm / min, the tensile strength in the longitudinal direction (MD) was 15 kgf / mm². ２ The polyester film according to claim 5, wherein the elongation in the longitudinal direction (MD) is 125% or more.

7. The following equation (2): [Math 1] (In the formula, S0 is the initial elongation (%) of the polyester film sample, and S1 is the elongation (%) measured after being left at 121°C, 100% R.H., and 2 atmospheres for 96 hours.) The polyester film according to claim 5, having an elongation retention rate (R) of 40% or more, calculated by the method, wherein the elongation is measured by cutting the polyester film into 1.5 cm wide strips, mounting them longitudinally on clips placed at 5 cm intervals, and testing them at a tensile speed of 200 mm / min at 25°C according to ASTM D882.

8. A method for preparing a polyester film, (1) A step of depolymerizing waste polyester to prepare recycled bis(2-hydroxyethyl) terephthalate, (2) The step of preparing a solution of regenerated bis(2-hydroxyethyl) terephthalate by mixing the regenerated bis(2-hydroxyethyl) terephthalate with a solvent, (3) A step of preparing a polyester resin via a polycondensation reaction using the recycled bis(2-hydroxyethyl) terephthalate solution, (4) The step of melt-extruding the polyester resin and stretching the extruded product. Includes, In the aforementioned polyester resin, the peak area fraction of cyclic trimers measured by high-performance liquid chromatography (HPLC) is 3,000 ppm or less, and the diethylene glycol (DEG) content measured by gas chromatography (GC) is 1.0% by weight or less. method.

9. The method for preparing a polyester resin according to claim 8, wherein the polycondensation reaction comprises the step of (3a) subjecting the regenerated bis(2-hydroxyethyl) terephthalate solution to a first polycondensation reaction under a pressure of 200 mmHg to 600 mmHg to prepare an oligomer.

10. The method for preparing a polyester resin according to claim 9, wherein the polycondensation reaction includes, after step (3a), step (3b) of subjecting the oligomer to a second polycondensation under a pressure of less than 200 mmHg to prepare a polyester resin.

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