Highly transparent polyester multilayer film and its manufacturing method

Abstract

The present invention relates to a polyester multilayer film having excellent transparency and a method for producing the same. More specifically, the present invention provides a polyester multilayer film having excellent transparency and a method for producing the same, which not only maintains the transparency and visibility of the film even during high-temperature treatment by suppressing the generation of oligomers in the film and their migration to the surface, but also enables the production of a polyester multilayer film having excellent transparency at low production costs and prevents contamination in the process due to oligomer scattering, thereby significantly increasing productivity.

Description

[Technical Field]

[0001] The present invention relates to a polyester multilayer film having excellent transparency and a method for producing the same, and more particularly to a polyester multilayer film having excellent transparency that can maintain the transparency and visibility of the film even during high-temperature treatment by suppressing the generation of oligomers within the film and their migration to the surface, and a method for producing the same. [Background technology]

[0002] In general, polyester films have excellent durability and stable physical properties across a wide temperature range, from low to high. Compared to other polymer resins, they also have excellent chemical resistance, mechanical strength, and surface properties. Due to their excellent physical and chemical properties, polyester films are widely used in displays, semiconductors, and other industrial applications. In particular, their excellent transparency and visibility, as well as their excellent mechanical and electrical properties, have led to their increasing use as optical films for displays such as LCDs and touch panels.

[0003] However, polyester films require high temperatures of 100°C or higher during the manufacturing process of displays, etc., and low-molecular-weight oligomers present inside the polyester film are eluted onto the surface, forming crystalline foreign matter known as blooming, which not only reduces transparency but also reduces productivity due to contamination caused by scattering within the process.

[0004] In order to prevent oligomers from eluting to the surface of such polyester films, a method of reducing the oligomer content by solid-state polymerization during polymerization of polyester films has been widely used. However, the solid-state polymerization process is complicated, which increases costs. Furthermore, at high temperatures, oligomers are continuously produced in the film and elute to the surface, making it difficult to completely block the oligomers.

[0005] As another method for preventing oligomers from leaching to the surface, a technique for suppressing leaching of oligomers by using a highly heat-resistant polymer such as polyethylene naphthalate (PEN) or by applying a copolymer in which a monomer such as isophthalate or cyclohexyldimethanol is used instead of terephthalic acid or ethylene glycol has been reported. However, such a technique not only has the problem of changing the physical properties of the polyester itself, but also has the problem of ultimately being unable to suppress leaching to the surface.

[0006] Furthermore, a technology has been reported in which a laminated film is formed on a polyester film and a high-viscosity polymer produced by solid-phase polymerization is used as the skin layer to control the elution of oligomers. However, this method is not economically viable because it requires the use of expensive catalysts, and it is not possible to completely prevent the elution of oligomers. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been devised to solve the above-mentioned problems and meet the conventional requirements. An object of the present invention is to provide a polyester multilayer film having excellent transparency, which can suppress the generation of oligomers within the film and their migration to the surface, thereby maintaining the transparency and visibility of the film even during high-temperature treatment, and a method for producing the same.

[0008] Another object of the present invention is to provide a method for producing a polyester multilayer film having excellent transparency, which can produce a polyester multilayer film having excellent transparency at low production costs and can significantly increase productivity by preventing contamination in the process due to oligomer scattering.

[0009] These and other objects and advantages of the present invention will become apparent from the following description of the preferred embodiment. [Means for solving the problem]

[0010] The above object is achieved by a polyester multilayer film having excellent transparency, which includes a substrate layer containing a polyester resin and a skin layer located on at least one side of the substrate layer and containing a polyester resin, and which satisfies the following mathematical formula 1:

[0011] (Equation 1) 110<0.013*[C3]-Ds*(50 / Dt)<121 Here, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, Ds is the thickness of the skin layer (μm), and Dt is the thickness of the multilayer film (μm).

[0012] Here, the concentration of C3 cyclic oligomer contained in the polyester multilayer film is characterized by being 9,000 to 9,700 ppm.

[0013] Preferably, the polyester resin forming the substrate layer has a C3 cyclic oligomer concentration of 9,000 to 12,000 ppm, and the polyester resin forming the skin layer has a C3 cyclic oligomer concentration of 4,500 to 7,000 ppm.

[0014] Preferably, the thickness of the skin layer is characterized by being between 2 and 10 μm.

[0015] Preferably, the thickness ratio of the substrate layer to the skin layer is 1:6 to 15.

[0016] Preferably, the polyester resin forming the skin layer has an intrinsic viscosity (IV) of 0.60 to 0.70 dl / g.

[0017] Preferably, the skin layer further contains 0.1 to 1.0 parts by weight of an organic phosphorus compound relative to 100 parts by weight of the polyester resin.

[0018] Preferably, the organic phosphorus compound is at least one selected from the group consisting of triaryl phosphite and trialkyl phosphite.

[0019] Preferably, the polyester multilayer film is characterized in that the haze change (ΔHt) upon high-temperature treatment satisfies the following formula 2:

[0020] (Equation 2) △Ht=Hf-Hi<0.5% Here, Hi is the haze before heat treatment, and Hf is the haze after heat treatment at 150° C. for 30 minutes.

[0021] More preferably, the polyester multilayer film is characterized in that the haze change (ΔHht) upon high-temperature treatment after treatment under high-temperature and humid conditions satisfies the following formula 3:

[0022] (Equation 3) △Hht=He-Hs<1.0% Here, Hs is the haze before treatment under high temperature and humidity conditions, and He is the haze after heat treatment at 150° C. for 30 minutes for a film that has been treated under high temperature and humidity conditions at 85° C. and 85% RH for 240 hours.

[0023] The above object can also be achieved by a method for producing a polyester multilayer film having excellent transparency, comprising: a first step of co-extruding a raw material for a skin layer made of a low-oligomer polyester resin and a raw material for a base layer made of a high-oligomer polyester resin to produce a polyethylene terephthalate sheet; a second step of stretching the polyethylene terephthalate sheet 3 to 5 times in the machine direction (MD) and then cooling at room temperature to produce a uniaxially oriented polyester multilayer film; a third step of stretching the uniaxially oriented polyester multilayer film 3 to 5 times in the transverse direction (TD) to produce a biaxially oriented polyester multilayer film; and a fourth step of heat-treating the biaxially oriented polyester multilayer film at 230 to 250°C and heat-setting it at 200 to 220°C to produce a polyester multilayer film.

[0024] Preferably, the C3 cyclic oligomer concentration in the raw material for the substrate layer is 9,000 to 12,000 ppm, and the C3 cyclic oligomer concentration in the raw material for the skin layer is 4,500 to 7,000 ppm.

[0025] Preferably, the raw material of the skin layer further comprises 0.1 to 1.0 parts by weight of an organic phosphorus compound per 100 parts by weight of the polyester resin, and the organic phosphorus compound is at least one selected from the group consisting of triaryl phosphite and trialkyl phosphite.

[0026] Preferably, the raw material of the skin layer contains an organic phosphorus compound in the form of masterbatch chips compounded with the organic phosphorus compound, in addition to the low-oligomer polyester resin. [Effects of the Invention]

[0027] According to the present invention, it is possible to suppress the generation of oligomers within the film and their migration to the surface, thereby maintaining the transparency and visibility of the film even in a high-temperature treatment process.

[0028] Furthermore, according to the present invention, a polyester multilayer film having excellent transparency can be produced at low production costs, which has the effect of being highly economical.

[0029] Furthermore, due to the above-mentioned advantages, the present invention has the effect of maintaining excellent product quality in the display, semiconductor and various other industries, preventing contamination in the process due to scattering of oligomers, and significantly improving productivity.

[0030] However, the effects of the present invention are not limited to the above effects, and other effects not mentioned will be clearly understood by those having ordinary skill in the art to which the present invention pertains from the following description. [Brief explanation of the drawings]

[0031] [Figure 1]1 is a cross-sectional view of a polyester multilayer film having excellent transparency according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] The present invention will be described in detail below with reference to the following examples and drawings. It should be obvious to those skilled in the art that these examples are merely provided for illustrative purposes in order to more specifically explain the present invention, and that the scope of the present invention is not limited by these examples.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, shall control. In addition, although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein.

[0034] As used herein, the terms "comprise," "comprising," "include," "including," "containing," "characterized by," "has," "having," or any other variation thereof, are intended to cover an exclusive inclusion. For example, a process, method, article, or apparatus that includes a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent in such process, method, article, or apparatus. Also, unless expressly stated to the contrary, "or" means an inclusive "or" and not an exclusive "or."

[0035] First, a polyester multilayer film having excellent transparency according to one aspect of the present invention will be described in detail with reference to FIG. 1, which is a cross-sectional view of the polyester multilayer film having excellent transparency according to a preferred embodiment of the present invention.

[0036] 1, which is a cross-sectional view of a polyester multilayer film having excellent transparency according to an embodiment of the present invention, the polyester multilayer film having excellent transparency according to an embodiment of the present invention includes a substrate layer 110 containing a polyester resin, and skin layers 120 disposed on both sides of the substrate layer. In the example of FIG. 1, a configuration in which skin layers 120 are disposed on both sides of substrate layer 110 is shown, but the present invention is not limited thereto, and skin layers 120 may be disposed on only one side of substrate layer 110.

[0037] The inventors of the present invention have endeavored to suppress the generation of oligomers within a film and their migration to the surface even at high temperatures. As a result, they have found that it is important to suppress the surface migration of cyclic C3 oligomers, a typical oligomer, and that to achieve this, even if a polyester resin with a low oligomer content is used as the raw material for the skin layer 120 and a general resin is used as the raw material for the base layer 110, the generation of oligomers within the film and their migration to the surface can be suppressed by providing a polyester multilayer film in which the C3 oligomer concentration of the entire multilayer film, the skin layer thickness, and the multilayer film thickness satisfy the following mathematical formula 1. This finding led to the completion of the present invention.

[0038] (Equation 1) 110<0.013*[C3]-Ds*(50 / Dt)<121 Here, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, Ds is the thickness of the skin layer (μm), and Dt is the thickness of the multilayer film (μm).

[0039] If the value of Equation 1 is 110 or less, the thickness of the skin layer using the low oligomer resin becomes thick, which is uneconomical and reduces productivity. If the value is 121 or more, the change in haze before and after heat treatment of the film and the change in haze before and after heat treatment of a film treated at high temperature and humidity become large, which causes a problem of blooming occurring over a wide area on the surface of the multilayer film.

[0040] In the polyester multilayer film having excellent transparency according to an embodiment of the present invention, the concentration (total concentration) of C3 cyclic oligomers contained in the polyester multilayer film is preferably 9,000 to 9,700 ppm. If the total oligomer concentration is less than 9,000 ppm, the skin layer using a low oligomer resin becomes thick, making it uneconomical, while if it exceeds 9,700 ppm, sufficient oligomer suppression effect cannot be achieved.

[0041] In the polyester multilayer film having excellent transparency according to an embodiment of the present invention, the C3 cyclic oligomer concentration of the polyester resin forming the substrate layer is preferably 9,000 ppm to 12,000 ppm. If the oligomer concentration of the polyester resin forming the substrate layer is less than 9,000 ppm, a low-oligomer resin must be used for the substrate layer, which is uneconomical, while if it exceeds 12,000 ppm, sufficient oligomer suppression effect cannot be achieved.

[0042] In a polyester multilayer film having excellent transparency according to an embodiment of the present invention, the C3 cyclic oligomer concentration of the polyester resin forming the skin layer is preferably 4,500 ppm to 7,000 ppm. If the oligomer concentration of the polyester resin forming the skin layer is less than 4,500 ppm, it is difficult to produce a low-oligomer resin, and producing a suitable low-oligomer resin is expensive and therefore uneconomical. If the oligomer concentration exceeds 7,000 ppm, sufficient oligomer suppression effect cannot be achieved.

[0043] In the polyester multilayer film having excellent transparency according to an embodiment of the present invention, the thickness of the skin layer is preferably 2 μm to 10 μm. If the thickness of the skin layer exceeds 10 μm, the thickness of the skin layer using a low oligomer resin becomes too thick and economical, and if the thickness is less than 2 μm, sufficient oligomer suppression effect cannot be exhibited.

[0044] In a polyester multilayer film having excellent transparency according to an embodiment of the present invention, the thickness ratio of the substrate layer 110 to the skin layer 120 is preferably 1:6 to 1:15. In this case, when skin layers are positioned on both sides as shown in Figure 1, this refers to the thickness of the entire skin layer. In this case, if the thickness ratio of the substrate layer 110 is less than 6, the migration suppression effect is good but it is not preferable from an economical standpoint, and if it exceeds 15, it is not preferable because the processability is poor or the migration suppression may be insufficient.

[0045] In a highly transparent polyester multilayer film according to an embodiment of the present invention, the intrinsic viscosity (IV) of the polyester resin forming the skin layer is preferably 0.60 to 0.70 dL / g. If the intrinsic viscosity of the skin layer is less than 0.60 dL / g, it is difficult to obtain a low-oligomer resin, and even if a low-oligomer resin can be obtained, it is difficult to manufacture. Therefore, manufacturing a suitable low-oligomer resin requires high costs, making it uneconomical. On the other hand, if the intrinsic viscosity exceeds 0.70 dL / g, it is difficult to manufacture a film, resulting in poor processability.

[0046] In the polyester multilayer film having excellent transparency according to an embodiment of the present invention, the polyester resin forming the skin layer may further contain 0.1 to 1.0 parts by weight of an organic phosphorus compound based on 100 parts by weight of the polyester resin.

[0047] The organic phosphorus compound may act like a C3 cyclic oligomer to maintain an equilibrium state at high temperatures and suppress the formation of C3 cyclic oligomers inside the film. The organic phosphorus compound may be at least one selected from a triaryl phosphite and a trialkyl phosphite.

[0048] Preferably, the organic phosphorus compound of the skin layer may be prepared by compounding masterbatch chips with an organic phosphorus compound separately from the polyester resin. That is, the raw material for the skin layer may contain the organic phosphorus compound by compounding masterbatch chips with an organic phosphorus compound separately from the low-oligomer polyester resin. In this case, preparing and compounding masterbatch chips with an organic phosphorus compound is advantageous for ensuring accurate content and uniform dispersion.

[0049] The polyester multilayer film having excellent transparency according to an embodiment of the present invention may have a haze change (ΔHt) upon high temperature treatment that satisfies the following Equation 2:

[0050] (Equation 2) △Ht=Hf-Hi<0.5% Here, Hi is the haze before heat treatment, and Hf is the haze after heat treatment at 150° C. for 30 minutes.

[0051] In this case, if the amount of change in haze (ΔHt) during high-temperature treatment is 0.5% or more, sufficient transparency cannot be ensured, which is undesirable.

[0052] In addition, the polyester multilayer film having excellent transparency according to an embodiment of the present invention may have a haze change (ΔHht) upon high temperature treatment after treatment under high temperature and humidity conditions that satisfies the following Equation 3:

[0053] (Equation 3) △Hht=He-Hs<1.0% Here, Hs is the haze before treatment under high temperature and humidity conditions, and He is the haze after heat treatment at 150° C. for 30 minutes for a film that has been treated under high temperature and humidity conditions at 85° C. and 85% RH for 240 hours.

[0054] In this case, if the haze change (ΔHht) during high-temperature treatment after treatment under high-temperature and humid conditions is 1.0% or more, sufficient transparency cannot be ensured, which is undesirable.

[0055] In a polyester multilayer film having excellent transparency according to an embodiment of the present invention, the concentration of C3 cyclic oligomers contained in the multilayer film of Equation 1 generally correlates closely with the change in haze before and after heat treatment. To maintain excellent optical properties, such as transparency, after heat treatment, the haze change (ΔHt) during high-temperature treatment according to Equation 2 must be 0.5% or less. If the haze change exceeds 0.5%, transparency and the optical properties of the film will deteriorate. To achieve this, one approach would be to minimize the oligomer concentration in the film. However, this requires the use of a solid-phase polymerization resin throughout the film, as described below, which is uneconomical. Therefore, in the present invention, a conventional liquid chip is used for the base layer 110, while a low-oligomer resin is used only for the skin layer 120. This results in a film with an oligomer concentration similar to that of a liquid-phase polymerization resin, thereby suppressing oligomer generation and migration.

[0056] Furthermore, when the correlation coefficient between the C3 cyclic oligomer and the skin layer thickness and the substrate layer thickness in Equation 1 is 110 or less, a resin with a low oligomer content must be produced and used, or the skin layer thickness must be increased, as described below, which is problematic in terms of economic efficiency. Furthermore, when the value of Equation 1 is 121 or more, the oligomer content in the multilayer film is high or the skin layer thickness is thin, making it impossible to obtain the haze change required in Equations 2 and 3 after heat treatment under high temperature conditions, and transparency cannot be ensured, which is undesirable.

[0057] Polyester films generally contain a certain amount of oligomers when they are produced by polymerizing the raw resin. The amount of oligomers contained in polyester film varies depending on the polymerization method, but typically contains approximately 0.5-2% of typical C3 cyclic oligomers. These oligomers migrate to the surface when the polyester film is heated above its glass transition temperature. Because of their high crystallinity, these oligomers exist on the surface as crystalline impurities measuring several micrometers (μm). These oligomer crystals can reduce the film's transparency and other optical properties. They can also scatter during film processing, contaminating other objects or products and reducing productivity.

[0058] Various methods have been used to prevent oligomer migration to the surface of polyester films. The most common method is to produce resins using solid-state polymerization, which minimizes the initial oligomer content and minimizes surface migration even when heated. Resins produced using solid-state polymerization have a high molecular weight and intrinsic viscosity, which is known to inhibit oligomer migration. Another method utilizes copolymer resins containing a certain amount of monomers, such as cyclocarboxyl dimethanol or isosorbide, within the polyester molecule, which increases the relative amorphous region of the polymer, allowing for greater oligomer content and thus inhibiting surface migration. Other known methods include the use of expensive germanium (Ge) or titanium (Ti) catalysts to minimize oligomer formation under high-temperature conditions during melt extrusion, and the use of laminated films that use high-intrinsic viscosity resins in the skin layer to inhibit oligomer migration. However, although the above-mentioned methods are partially effective, oligomers are continuously produced under high temperature conditions such as during film stripping and processing, and therefore there is a limit to how effectively they can prevent the elution of the produced oligomers.

[0059] In contrast, a polyester multilayer film with excellent transparency according to an embodiment of the present invention can suppress the generation of oligomers and prevent the generated oligomers from migrating to the surface. When the skin layer has an appropriate low oligomer concentration and thickness, the polyester multilayer film with excellent transparency according to an embodiment of the present invention can maintain transparency by sufficiently suppressing oligomer migration to the surface even under high temperature conditions, regardless of the oligomer state of the substrate layer. These results indicate that even if a low-oligomer resin is used only in the skin layer 120, which accounts for approximately 10% of the film, the oligomer level throughout the film can be stably maintained and migration to the surface can be suppressed.

[0060] A method for producing a polyester multilayer film having excellent transparency according to another embodiment of the present invention includes the steps of: a first step of co-extruding a low-oligomer polyester resin skin layer raw material and a high-oligomer polyester resin substrate layer raw material to produce a polyethylene terephthalate sheet; a second step of stretching the polyethylene terephthalate sheet 3 to 5 times in the machine direction (MD) and then cooling at room temperature to produce a uniaxially stretched polyester multilayer film; a third step of stretching the uniaxially stretched polyester multilayer film 3 to 5 times in the transverse direction (TD) to produce a biaxially stretched polyester film; and a fourth step of heat-treating the biaxially stretched polyester multilayer film at 230 to 250°C and heat-setting it at 200 to 220°C to produce a polyester multilayer film having excellent transparency.

[0061] In the method for manufacturing a polyester multilayer film having excellent transparency according to another embodiment of the present invention, the description overlapping with the description of the polyester multilayer film having excellent transparency according to the above-described embodiment of the present invention will be omitted.

[0062] The first step is to produce a polyethylene terephthalate sheet by co-extruding a low-oligomer polyester resin skin layer raw material and a high-oligomer polyester resin base layer raw material, and it is preferable that the C3 cyclic oligomer concentration of the base layer raw material is 9,000 to 12,000 ppm, and the C3 cyclic oligomer concentration of the skin layer raw material is 4,500 to 7,000 ppm.

[0063] In this case, if the skin layer has an appropriate low oligomer concentration and thickness, even if the substrate layer contains a normal high-oligomer polyester resin, oligomer migration to the surface can be sufficiently suppressed even under high temperature conditions regardless of the oligomer state of the substrate layer, thereby maintaining transparency. Therefore, the raw materials for the skin layer and the substrate layer are separate. Furthermore, although solid phase chips with a high intrinsic viscosity (IV) are used to reduce the oligomer concentration, they are used only in the thin skin layer, which reduces the use of expensive solid phase chips when producing a polyester multilayer film with excellent transparency, thereby ensuring economic efficiency.

[0064] The raw material for the skin layer may further include 0.1 to 1.0 parts by weight of an organic phosphorus compound per 100 parts by weight of the polyester resin, and the organic phosphorus compound may be at least one selected from the group consisting of triaryl phosphite and trialkyl phosphite. In this case, the raw material for the skin layer may include the organic phosphorus compound through masterbatch chips compounded with the low-oligomer polyester resin and the organic phosphorus compound.

[0065] Next, in the second step, the polyethylene terephthalate sheet is stretched 3 to 5 times in the machine direction MD and then cooled at room temperature to produce a uniaxially stretched polyester multilayer film, and in the third step, the uniaxially stretched polyester multilayer film is stretched 3 to 5 times in the transverse direction TD to produce a biaxially stretched polyester film.

[0066] However, although the stretching in the manufacturing method of the present invention is described as biaxial stretching, the manufacturing method of the polyester multilayer film having excellent transparency according to the present invention is not limited to biaxial stretching only, and the film can be manufactured by non-stretching or uniaxial stretching as necessary.

[0067] Next, the fourth step is a step of heat treating the biaxially stretched polyester multilayer film at 230 to 250°C and heat setting it at 200 to 220°C to produce a polyester multilayer film with excellent transparency.

[0068] The polyester multilayer film having excellent transparency manufactured by the above manufacturing method preferably maintains a haze change of 0.5% or less when heat-treated at 150°C for 30 minutes. Furthermore, the haze change preferably maintains a haze change of 1.0% or less when heat-treated at 85°C and 85% RH for 240 hours and then heat-treated at 150°C for 30 minutes. As described above, the polyester multilayer film having excellent transparency according to one embodiment of the present invention has excellent transparency and does not undergo oligomer migration even at high temperatures, so it can maintain its transparency. It also has advantages such as suppressing oligomer dispersion during the manufacturing process and maintaining a clean manufacturing environment.

[0069] The present invention will be described in more detail with reference to the following examples and comparative examples, but the scope of the present invention is not limited to these examples.

[0070] [Example] <Production example> (1) Production of polyester resins (A, B) 100 parts by weight of terephthalic acid and 60 parts by weight of ethylene glycol were used as starting materials, and magnesium acetate tetrahydrate was added as a catalyst and placed in a reactor. The reaction temperature was started at 150°C and gradually increased to 230°C after 3 hours. After 4 hours, the transesterification reaction was essentially complete. This reaction mixture was transferred to a polycondensation tank, and antimony trioxide was added and polycondensation reaction was carried out for 4 hours, yielding Polyester Resin A with an intrinsic viscosity of 0.61 dl / g and a C3 cyclic oligomer content of 9,990 ppm.

[0071] Next, the polyester resin A obtained above was subjected to solid-phase polymerization at 215°C under nitrogen conditions to obtain polyester resin B having an intrinsic viscosity of 0.70 dl / g and a C3 cyclic oligomer content of 4,000 ppm.

[0072] (2) Polyester multilayer film manufacturing The polyester resin A from which moisture had been removed was used as the base layer (main layer) raw material, and polyester resin B was blended with resin A to achieve a predetermined C3 cyclic oligomer content, and then fed into a co-extruder. The feeder block was adjusted to adjust the weight ratio of the base layer to the skin layer from 6:1 to 15:1, and the extrusion was carried out. The resulting mixture was then rapidly cooled and solidified in a casting drum with a surface temperature of 20°C to produce a polyethylene terephthalate sheet with a thickness of 8,000 μm.

[0073] Next, the produced polyethylene terephthalate sheet was stretched 3 to 5 times in the machine direction (MD) at 80°C and then cooled to room temperature. The sheet was then heated in a tenter, preheated, dried, and then stretched 3 to 5 times in the transverse direction (TD). The sheet was then heat-treated at 230 to 250°C in the tenter and heat-set at 200 to 220°C to produce a biaxially stretched multilayer film.

[0074] [Example 1] Polyester resin A and polyester resin B were blended to a C3 cyclic oligomer content of 6,000 ppm and used as the raw material for the skin layer. Polyester resin A was used for the substrate layer, and co-extrusion was performed at 280°C to produce a 50μm-thick polyester multilayer film with 3μm-thick skin layers formed on both sides of the substrate layer. The C3 cyclic oligomer content of the produced polyester multilayer film, the correlation coefficient according to Equation 1, the change in haze before and after heat treatment at 150°C for 30 minutes according to Equation 2, and the change in haze before and after heat treatment at 85°C and 85% relative humidity according to Equation 3 are each shown in Table 1 below (the same applies hereinafter).

[0075] [Example 2] A polyester multi-layer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content in the skin layer was 4,500 ppm.

[0076] [Example 3] A polyester multi-layer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 6,500 ppm and the thickness of the skin layer was 4 μm.

[0077] [Example 4] A polyester multi-layer film was prepared in the same manner as in Example 1, except that the thickness of the skin layer was 6 μm.

[0078] [Example 5] A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 6,500 ppm, the skin layer thickness was 10 μm, and the polyester multilayer film thickness was 100 μm.

[0079] [Example 6] A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 4,500 ppm, the skin layer thickness was 8 μm, and the polyester multilayer film thickness was 100 μm.

[0080] [Example 7] A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 7,000 ppm, the skin layer thickness was 8 μm, and the polyester multilayer film thickness was 100 μm.

[0081] [Example 8] A polyester multilayer film was prepared in the same manner as in Example 1, except that the thickness of the skin layer was 6 μm and the thickness of the polyester multilayer film was 100 μm.

[0082] [Example 9] A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 4,500 ppm, the skin layer thickness was 2 μm, and the polyester multilayer film thickness was 30 μm.

[0083] [Example 10] A polyester multi-layer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content in the skin layer was 6,500 ppm and the thickness of the polyester multi-layer film was 38 μm.

[0084] [Example 11] A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content in the skin layer was 6,500 ppm, a 5% masterbatch of an organic phosphorus compound was used to adjust the phosphorus content in the skin layer to 250 ppm, the skin layer thickness was 3 μm, and the polyester multilayer film thickness was 50 μm.

[0085] [Comparative Example] [Comparative Example 1] A polyester multi-layer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 4,500 ppm and the thickness of the skin layer was 2 μm.

[0086] Comparative Example 2 A polyester multi-layer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content in the skin layer was 7,000 ppm.

[0087] Comparative Example 3 A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 5,000 ppm, the skin layer thickness was 7 μm, and the polyester multilayer film thickness was 50 μm.

[0088] Comparative Example 4 A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 8,500 ppm, the skin layer thickness was 7 μm, and the polyester multilayer film thickness was 100 μm.

[0089] Comparative Example 5 A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 8,500 ppm, the skin layer thickness was 6 μm, and the polyester multilayer film thickness was 100 μm.

[0090] Comparative Example 6 A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content of the skin layer was 7,500 ppm, the skin layer thickness was 2 μm, and the polyester multilayer film thickness was 38 μm.

[0091] The polyester multi-layer films according to Examples 1 to 11 and Comparative Examples 1 to 6 were used to measure physical properties in the following experiments. The results are shown in Table 1 below.

[0092] [Experimental Example] (1) Measurement of C3 cyclic oligomer content 50 mg of the prepared multilayer film was dissolved in 1 ml of 1,1,1,3,3,3-hexafluoro-2-propanol, then reprecipitated with 10 ml of chloroform and 8 ml of methanol, filtered through a 5 μm filter, and the solvent was removed. The resulting precipitate was then dissolved in a certain amount of chloroform and the amount of C3 cyclic oligomer was measured using HPLC (Agilent 1200 series).

[0093] The amount of oligomers measured using HPLC was determined from the peak area ratio of the commonly used standard sample peak area to the measured sample peak area (absolute calibration method). The column used was a Polaris 5 Si 100*4.6 mm, the temperature was 40°C, the mobile phase was hexane / 1,4-dioxane (weight ratio: 6:4), the flow rate was 1.0 ml / min, and the detector was 240 nm UV.

[0094] (2) Measurement of haze difference before and after heat treatment (△Ht) The haze of the film was measured for a film prepared as a sample using a turbidity meter NDH-5000 (manufactured by Nippon Denshoku Co., Ltd.) in accordance with ASTM-D1003.

[0095] First, the haze (Hi) of the film prepared as a sample was measured, and then the prepared film sample was fixed to a square metal support and placed in a 150°C oven for 30 minutes for high-temperature heat treatment, after which the haze (Hf) after heat treatment was measured.

[0096] The difference in haze (ΔHt) between the film before and after heat treatment was calculated by subtracting the haze (Hi) before heat treatment from the haze (Hf) after heat treatment.

[0097] (3) Measurement of the haze difference (△Hht) before and after heat treatment of film treated with high temperature and humidity The haze of the film was measured for a sample film using a turbidity meter NDH-5000 (Nippon Denshoku Co., Ltd.) in accordance with ASTM-D1003.

[0098] First, the haze (Hs) of the film prepared as a sample was measured, and then the film was left to stand at 85°C and 85% RH for 240 hours. The film was then placed in a 150°C oven for 30 minutes for high-temperature heat treatment, and the haze (He) of the film after heat treatment at high temperature and humidity was measured.

[0099] The difference in haze (△Hht) before and after heat treatment of the film treated at high temperature and humidity was calculated by subtracting the haze (Hs) before the heat treatment from the haze (He) after the heat treatment of the film treated at high temperature and humidity.

[0100] (4) Skin thickness measurement The film prepared as a sample was subjected to cross-sectional processing using a microtome (LEICA RM2255), platinum coating was performed, and the thickness was measured using a SEM (Scanning electron microscope, HITACHI S-4800).

[0101] (5) Surface condition analysis using an optical microscope The sample film was fixed to a rectangular metal support and placed in a 150°C oven for 30 minutes. The surface of a 2 x 2 mm area was analyzed using an optical microscope (Olympus, MX50LT-1273MH) for both the high-temperature heat-treated film and the unheated film. Blooming, caused by oligomer crystals of more than a micron in size, was evaluated according to the following criteria.

[0102] O: Not observed at all Δ: Finely observed X: Widely observed over the surface The physical properties of the multilayer films prepared in the above examples and comparative examples are shown in the following Table 1. In Table 1, [C3] indicates the concentration of C3 cyclic oligomers in the entire multilayer film.

[0103] [Table 1]

[0104] As can be seen from Table 1, the polyester multilayer film of the present invention exhibits a correlation coefficient (Equation 1) between the C3 cyclic oligomer concentration, skin layer thickness, and multilayer film thickness, and satisfies the corresponding haze change before and after heat treatment (Equation 2) and the haze change under high-temperature, high-humidity conditions (Equation 3). This confirms that not only does it exhibit excellent transparency, but it also prevents oligomer migration to the surface. This provides a polyester multilayer film that essentially prevents productivity loss due to external dispersion. Furthermore, as shown in Example 11, it was confirmed that the oligomer suppression effect can be further enhanced by using an organic phosphorus compound in the skin layer.

[0105] In contrast, in Comparative Examples 1 to 2 and 4 to 6, where the correlation coefficient (Equation 1) value is 121 or more, the change in haze before and after heat treatment and the change in haze of the film treated at high temperature and humidity are large, and blooming appears over a wide area of ​​the surface. Also, when the correlation coefficient is 110 or less, as in Comparative Example 3, the change in haze before and after heat treatment and the change in haze of the film treated at high temperature and humidity are small, but the thickness of the skin layer using the low oligomer resin becomes thicker compared to the overall thickness, which is not economical and reduces productivity, and is therefore not preferred.

[0106] As described above, the polyester multilayer film having excellent transparency according to the present invention uses a solid phase chip having a high intrinsic viscosity (IV) to reduce the concentration of oligomers, but by using it only in the thin skin layer, the use of expensive solid phase chips can be reduced during the production of the polyester multilayer film having excellent transparency, thereby ensuring economic efficiency.

[0107] In this specification, only a few examples of various embodiments carried out by the inventors are described as examples, and it goes without saying that the technical ideas of the present invention are not limited or restricted to these examples, and can be modified and implemented in various ways by those skilled in the art.

Claims

1. a base layer containing a polyester resin; A polyester multilayer film having excellent transparency, comprising: a skin layer located on at least one surface of the base layer and containing a polyester resin; and satisfying the following mathematical formula 1: (Equation 1) 110<0.013*[C3]-Ds*(50 / Dt)<121 Here, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, Ds is the skin layer thickness (μm), and Dt is the multilayer film thickness (μm).

2. 2. The polyester multilayer film according to claim 1, wherein the concentration of the C3 cyclic oligomer contained in the polyester multilayer film is 9,000 to 9,700 ppm.

3. The polyester resin forming the substrate layer has a C3 cyclic oligomer concentration of 9,000 to 12,000 ppm; 2. The polyester multilayer film according to claim 1, wherein the polyester resin forming the skin layer has a C3 cyclic oligomer concentration of 4,500 to 7,000 ppm.

4. 2. The polyester multilayer film according to claim 1, wherein the thickness of the skin layer is 2 to 10 [mu]m.

5. 2. The polyester multilayer film according to claim 1, wherein the thickness ratio of the substrate layer to the skin layer is 1:6 to 1:

15.

6. 2. The polyester multilayer film according to claim 1, wherein the intrinsic viscosity (IV) of the polyester resin forming the skin layer is 0.60 to 0.70 dl / g.

7. 2. The polyester multilayer film according to claim 1, wherein the skin layer further comprises 0.1 to 1.0 parts by weight of an organic phosphorus compound based on 100 parts by weight of the polyester resin.

8. 8. The polyester multilayer film according to claim 7, wherein the organic phosphorus compound is at least one selected from the group consisting of triaryl phosphite and trialkyl phosphite.

9. 2. The polyester multilayer film according to claim 1, wherein the haze change (ΔHt) upon high-temperature treatment satisfies the following formula 2: (Equation 2) △Ht=Hf-Hi<0.5% Here, Hi is the haze before the heat treatment, and Hf is the haze after the heat treatment at 150° C. for 30 minutes.

10. 2. The polyester multilayer film according to claim 1, wherein the haze change (ΔHht) upon high-temperature treatment after treatment under high-temperature and humid conditions satisfies the following formula 3: (Equation 3) △Hht=He-Hs<1.0% Here, Hs is the haze before treatment under high temperature and humidity conditions, and He is the haze after heat treatment at 150° C. for 30 minutes for a film that has been treated under high temperature and humidity conditions at 85° C. and 85% RH for 240 hours.

11. A first step of producing a polyethylene terephthalate sheet by co-extruding a raw material for a skin layer made of a low-oligomer polyester resin and a raw material for a base layer made of a high-oligomer polyester resin; a second step of stretching the polyethylene terephthalate sheet 3 to 5 times in a machine direction (MD) and then cooling the stretched sheet at room temperature to prepare a uniaxially stretched polyester multilayer film; a third step of stretching the uniaxially stretched polyester multilayer film 3 to 5 times in the transverse direction TD to produce a biaxially stretched polyester multilayer film; a fourth step of heat-treating the biaxially stretched polyester multilayer film at 230 to 250°C and heat-setting it at 200 to 220°C to produce a polyester multilayer film.

12. the C3 cyclic oligomer concentration of the raw material for the substrate layer is 9,000 to 12,000 ppm; 12. The method for producing a polyester multilayer film having excellent transparency according to claim 11, wherein the C3 cyclic oligomer concentration in the raw material of the skin layer is 4,500 to 7,000 ppm.

13. The raw material of the skin layer further includes 0.1 to 1.0 parts by weight of an organic phosphorus compound based on 100 parts by weight of the polyester resin, The method for producing a polyester multilayer film with excellent transparency according to claim 11, wherein the organic phosphorus compound is at least one selected from the group consisting of triaryl phosphite and trialkyl phosphite.

14. 12. The method for producing a polyester multilayer film with excellent transparency according to claim 11, wherein the raw material of the skin layer contains an organic phosphorus compound in the form of masterbatch chips compounded with the organic phosphorus compound, in addition to the low-oligomer polyester resin.

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