Polyester multilayer film with excellent transparency and preparation method thereof

Abstract

The present invention relates to a polyester multilayer film having excellent transparency and a method for preparing the same, and more particularly, to a polyester multilayer film having excellent transparency and a method for preparing the same, which not only can maintain transparency and visibility of the film in a high-temperature treatment process, but also can maintain transparency and visibility of the film by suppressing the generation of oligomers in the film and migration to a surface, and which has excellent transparency, and a method for preparing the same. Also, a polyester multilayer film having excellent transparency can be prepared at a low manufacturing cost, and productivity can be greatly improved by preventing in-process contamination caused by oligomer scattering.

Description

Technical Field

[0001] The present invention relates to a polyester multilayer film having excellent transparency and a method for producing the same. More specifically, the present invention relates to a polyester multilayer film having excellent transparency and a method for producing the same, which can suppress the generation of oligomers in the film and their migration to the surface, and can maintain the transparency and visibility of the film even in a high-temperature treatment process. Background Art

[0002] Generally, polyester films have excellent durability, excellent physical property stability in a wide temperature range from low temperature to high temperature, excellent chemical resistance compared to other polymer resins, and good mechanical strength and surface characteristics. Due to the excellent physicochemical properties as described above, polyester films are widely used in displays, semiconductors, and other industries. In particular, due to their excellent transparency and visibility, as well as excellent mechanical and electrical properties, their use as optical films for displays such as LCDs and touch panels has been continuously increasing.

[0003] However, in the process of manufacturing displays and the like, since there is a process using a high temperature of 100 °C or more for polyester films, there are the following problems: low-molecular-weight oligomers inside the polyester film dissolve out to the surface, and the dissolved oligomers form crystalline foreign substances, which is called a blooming phenomenon, resulting in a decrease in transparency and scattered contamination in the process, thereby reducing productivity.

[0004] In order to prevent the oligomers of such polyester films from dissolving out to the surface, a method of reducing the oligomer content by solid-phase polymerization is widely used when polymerizing polyester films. However, the complex solid-phase polymerization process not only increases costs, but also continuously generates oligomers in the film under high-temperature conditions and dissolves out to the surface, so there is a problem that oligomers cannot be completely blocked.

[0005] In addition, as other methods for preventing oligomers from dissolving out to the surface, technologies using high heat-resistant polymers such as polyethylene naphthalate (PEN), or suppressing the dissolution of oligomers by applying copolymers using monomers such as isophthalate or cyclohexanedimethanol instead of terephthalic acid or ethylene glycol have been reported. However, these technologies not only have the problem of changing the physical properties of the polyester itself, but also have the problem that the dissolution to the surface cannot be ultimately suppressed.

[0006] In addition, a technology of forming a laminated film on a polyester film and using a high-viscosity polymer based on solid-phase polymerization in the surface layer to control the dissolution of oligomers has been reported. However, in this case, there are problems of insufficient economy (such as using expensive catalysts, etc.) and incomplete blocking of the dissolution of oligomers. Summary of the Invention

[0007] Technical problems to be solved by the present invention

[0008] The present invention is proposed to solve the problems described above and meet the existing requirements. The object of the present invention is to provide a polyester multilayer film with excellent transparency and a method for preparing the same, which can inhibit the generation and migration of oligomers in the film to the surface and maintain the transparency and visibility of the film even in high-temperature treatment processes.

[0009] Another object of the present invention is to provide a method for preparing a polyester multilayer film with excellent transparency, which can prepare a polyester multilayer film with excellent transparency at low manufacturing cost and greatly improve productivity by preventing in-process contamination caused by the scattering of oligomers.

[0010] The above objects, other objects and advantages of the present invention will become clear from the following description of the preferred embodiments.

[0011] Technical solution

[0012] The above object is achieved by a polyester multilayer film with excellent transparency, which includes: a base layer containing a polyester resin, and a surface layer located on at least one side of the base layer and containing a polyester resin; the polyester multilayer film satisfies the following Mathematical Formula 1:

[0013] (Mathematical Formula 1)

[0014] 110 < 0.013 * [C3] – Ds * (50 / Dt) < 121

[0015] Wherein, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, Ds is the thickness (μm) of the surface layer, and Dt is the thickness (μm) of the multilayer film.

[0016] Wherein, it is characterized in that the concentration of C3 cyclic oligomers contained in the polyester multilayer film is 9000 ppm to 9700 ppm.

[0017] Preferably, it is characterized in that the concentration of C3 cyclic oligomers of the polyester resin forming the base layer is 9000 ppm to 12000 ppm, and the concentration of C3 cyclic oligomers of the polyester resin forming the surface layer is 4500 ppm to 7000 ppm.

[0018] Preferably, it is characterized in that the thickness of the surface layer is 2 μm to 10 μm.

[0019] Preferably, it is characterized in that the thickness ratio of the surface layer to the base layer is 1:6 to 15.

[0020] Preferably, it is characterized in that the inherent viscosity (IV) of the polyester resin forming the surface layer is 0.60 dl / g to 0.70 dl / g.

[0021] Preferably, it is characterized in that, based on 100 parts by weight of the polyester resin, the surface layer further contains 0.1 to 1.0 part by weight of an organic phosphorus compound.

[0022] Preferably, it is characterized in that the organic phosphorus compound is selected from at least one of triaryl phosphites and trialkyl phosphites.

[0023] Preferably, it is characterized in that during the high-temperature treatment process, the haze change amount (ΔHt) of the polyester multilayer film satisfies the following Mathematical Formula 2:

[0024] (Mathematical Formula 2) ΔHt = Hf - Hi < 0.5%

[0025] Wherein, Hi is the haze before the heat treatment, and Hf is the haze after heat treatment at a temperature of 150 °C for 30 minutes.

[0026] More preferably, it is characterized in that during the high-temperature treatment process after being treated under high-temperature and high-humidity conditions, the haze change amount (ΔHht) of the polyester multilayer film satisfies the following Mathematical Formula 3:

[0027] (Mathematical Formula 3) ΔHht = He - Hs < 1.0%

[0028] Wherein, Hs is the haze before the treatment under high-temperature and high-humidity conditions, and He is the haze after heat treatment at a temperature of 150 °C for 30 minutes of the film treated under high-temperature and high-humidity conditions of 85 °C and 85% RH for 240 hours.

[0029] In addition, the above object is achieved by a method for preparing a polyester multilayer film with excellent transparency, and the preparation method includes: the first step of co-extruding the surface layer raw material of the low-oligomer polyester resin and the base layer raw material of the high-oligomer polyester resin to prepare a polyethylene terephthalate sheet; the second step of stretching the polyethylene terephthalate sheet 3 to 5 times in the machine direction (MD), then cooling at room temperature and preparing a uniaxially stretched polyester multilayer film; the third step of stretching the uniaxially stretched polyester multilayer film 3 to 5 times in the transverse direction (TD) and preparing a biaxially stretched polyester multilayer film; and the fourth step of heat-treating the biaxially stretched polyester multilayer film at a temperature of 230 °C to 250 °C and thermally setting it at a temperature of 200 °C to 220 °C to prepare a polyester multilayer film.

[0030] Preferably, it is characterized in that the concentration of the C3 cyclic oligomer in the base layer raw material is 9000 ppm to 12000 ppm, and the concentration of the C3 cyclic oligomer in the surface layer raw material is 4500 ppm to 7000 ppm.

[0031] Preferably, it is characterized in that, based on 100 parts by weight of the polyester resin, the surface layer raw material further contains 0.1 to 1.0 parts by weight of an organic phosphorus compound, and the organic phosphorus compound is selected from at least one of triaryl phosphites and trialkyl phosphites.

[0032] Preferably, it is characterized in that, independently of the oligomer polyester resin, the surface layer raw material contains the organic phosphorus compound through a master batch chip mixed with the organic phosphorus compound.

[0033] Beneficial effects

[0034] According to the present invention, there are the following effects: the generation of oligomers in the film and the migration to the surface can be inhibited, and the transparency and visibility of the film can be maintained even in a high-temperature treatment process.

[0035] In addition, according to the present invention, a polyester multilayer film with excellent transparency can be prepared at a low manufacturing cost, so it has the effect of high economy.

[0036] Furthermore, the present invention has the following effects: through the above advantages, excellent product quality can be maintained in displays, semiconductors and various industries, and the productivity can be greatly improved by preventing in-process contamination caused by the scattering of oligomers.

[0037] However, the effects of the present invention are not limited to the effects mentioned above, and those skilled in the art can clearly understand other effects not mentioned from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a schematic cross-sectional view of a polyester multilayer film with excellent transparency according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0039] Hereinafter, the present invention will be described in detail with reference to the embodiments and drawings of the present invention. For a more specific description of the present invention, these embodiments are only presented schematically, and it is obvious to those of ordinary skill in the art that the scope of the present invention is not limited to these embodiments.

[0040] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. In case of conflict, the present specification including the definitions shall prevail. In addition, although methods and materials similar or equivalent to those described in this specification can be used in the implementation or testing of the present invention, suitable methods and materials are described in this specification.

[0041] As used in this specification, the terms "comprise", "comprising", "include", "including", "containing", "characterized by", "has", "having" or any other variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, article or apparatus including a list of elements is not necessarily limited to those elements, and may include other elements not expressly listed or inherent to such process, method, article or apparatus. Additionally, unless expressly stated otherwise, "or" means inclusive "or" rather than exclusive "or".

[0042] First, referring to Figure 1 , the Figure 1 is a schematic cross-sectional view of a polyester multilayer film with excellent transparency according to a preferred embodiment of the present invention, and a polyester multilayer film with excellent transparency according to one aspect of the present invention will be described in detail.

[0043] Referring to Figure 1 , the Figure 1 is a schematic cross-sectional view of a polyester multilayer film with excellent transparency according to an embodiment of the present invention. A polyester multilayer film with excellent transparency according to an embodiment of the present invention includes: a base layer 110 containing a polyester resin; and surface layers 120 located on both sides of the base layer. In Figure 1 an example, a structure in which surface layers 120 are formed on both sides of the base layer 110 is shown, but it is not limited thereto, and surface layer 120 may be formed only on any one side of the base layer 110.

[0044] As a result of the inventors of the present invention's efforts to suppress the generation of oligomers in the film and their migration to the surface even at high temperatures, it was confirmed that it is very important to suppress the surface migration of cyclic C3 oligomers, which are typical oligomers. For this purpose, even if a polyester resin with a low oligomer content is used as a raw material in the surface layer 120 and a common resin is used as a raw material in the base layer 110, the generation of oligomers in the film and their migration to the surface can be suppressed by providing a polyester multilayer film in which the relationship between the concentration of C3 oligomers in the entire multilayer film, the thickness of the surface layer, and the thickness of the multilayer film satisfies the following Mathematical Formula 1, thereby completing the present invention.

[0045] (Mathematical Formula 1)

[0046] 110 < 0.013 * [C3] – Ds * (50 / Dt) < 121

[0047] Wherein, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, Ds is the thickness (μm) of the surface layer, and Dt is the thickness (μm) of the multilayer film.

[0048] At this time, when the value of Mathematical Formula 1 is 110 or less, the thickness of the surface layer using the low oligomer resin increases, resulting in poor economy and a drawback of reduced productivity. When the value of Mathematical Formula 1 is 121 or more, there are problems such as a large change in haze before and after film heat treatment and a large change in haze before and after film heat treatment under high temperature and high humidity treatment, and frosting widely appears on the surface of the multilayer film.

[0049] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, the concentration (total concentration) of the C3 cyclic oligomer contained in the polyester multilayer film is preferably 9000 ppm to 9700 ppm. At this time, the reason is that when the concentration of the overall oligomer is less than 9000 ppm, the thickness of the surface layer using the low oligomer resin increases, resulting in insufficient economy. When the concentration exceeds 9700 ppm, a sufficient oligomer inhibition effect cannot be exhibited.

[0050] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, the concentration of the C3 cyclic oligomer in the polyester resin forming the base layer is preferably 9000 ppm to 12000 ppm. At this time, the reason is that when the concentration of the oligomer in the polyester resin forming the base layer is less than 9000 ppm, since the low oligomer resin also needs to be used for the base layer, the economy is insufficient. When the concentration exceeds 12000 ppm, a sufficient oligomer inhibition effect cannot be exhibited.

[0051] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, the concentration of the C3 cyclic oligomer in the polyester resin forming the surface layer is preferably 4500 ppm to 7000 ppm. At this time, the reason is that when the concentration of the oligomer in the polyester resin forming the surface layer is less than 4500 ppm, it is difficult to prepare the low oligomer resin, and high cost is required to prepare the low oligomer resin meeting this requirement, so the economy is insufficient. When the concentration exceeds 7000 ppm, a sufficient oligomer inhibition effect cannot be exhibited.

[0052] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, the thickness of the surface layer is preferably 2 μm to 10 μm. At this time, the reason is that when the thickness of the surface layer exceeds 10 μm, the thickness of the surface layer using the low oligomer resin increases, resulting in insufficient economy. When the thickness is less than 2 μm, a sufficient oligomer inhibition effect cannot be exhibited.

[0053] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, the thickness ratio of the base layer 110 corresponding to the surface layer 120 is preferably 1:6 to 15. In this case, as Figure 1As shown, when the surface layer is on both sides, it refers to the thickness of the entire surface layer. At this time, the reason is that when the thickness ratio of the base material layer 110 is less than 6, although the migration inhibition effect is good, the economy is not good. When the thickness ratio exceeds 15, the processability is not good or the migration inhibition is insufficient, so it is not preferred.

[0054] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, the intrinsic viscosity (IV) of the polyester resin forming the surface layer is preferably 0.60 dl / g to 0.70 dl / g. At this time, the reason is that when the intrinsic viscosity of the surface layer is less than 0.60 dl / g, it is difficult to ensure the low oligomer resin. Even if the low oligomer resin can be ensured, it is difficult to prepare, and high costs are required to prepare the low oligomer resin that meets this requirement, so the economy is insufficient. When the intrinsic viscosity exceeds 0.70 dl / g, it is difficult to form a film, and there is a problem of reduced processability.

[0055] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, relative to 100 parts by weight of the polyester resin, the polyester resin forming the surface layer may further contain 0.1 part by weight to 1.0 part by weight of an organic phosphorus compound.

[0056] The organic phosphorus compound acts like a C3 cyclic oligomer, can maintain an equilibrium state at high temperatures, and inhibit the generation of C3 cyclic oligomers inside the film. The organic phosphorus compound may be selected from at least one of triaryl phosphites and trialkyl phosphites.

[0057] Preferably, independently of the polyester resin, the organic phosphorus compound in the surface layer is introduced in the form of a masterbatch chip prepared by mixing the organic phosphorus compound. That is, independently of the low oligomer polyester resin, the surface layer raw material can contain the organic phosphorus compound through the masterbatch chip mixed with the organic phosphorus compound. At this time, the reason is that preparing and introducing the masterbatch chip mixed with the organic phosphorus compound is beneficial to ensuring accurate content and uniform dispersion.

[0058] During the high-temperature treatment process, the haze change amount (△Ht) of the polyester multilayer film with excellent transparency according to an embodiment of the present invention can satisfy the following mathematical formula 2:

[0059] (Mathematical formula 2) △Ht = Hf - Hi < 0.5%

[0060] Wherein, Hi is the haze before heat treatment, and Hf is the haze after heat treatment at a temperature of 150 °C for 30 minutes.

[0061] At this time, when the haze change amount (△Ht) during the high-temperature treatment process is 0.5% or more, it is not preferred because sufficient transparency cannot be ensured.

[0062] In addition, during the high-temperature treatment after being treated under high-temperature and high-humidity conditions, the haze change amount (ΔHht) of the polyester multilayer film with excellent transparency according to an embodiment of the present invention can satisfy the following Mathematical Formula 3:

[0063] (Mathematical Formula 3) ΔHht = He - Hs < 1.0%

[0064] Herein, Hs is the haze before being treated under high-temperature and high-humidity conditions, and He is the haze after heat-treating the film that has been treated under high-temperature and high-humidity conditions of 85°C and 85% RH for 240 hours at a temperature of 150°C for 30 minutes.

[0065] At this time, during the high-temperature treatment after being treated under high-temperature and high-humidity conditions, when the haze change amount (ΔHht) is 1.0% or more, it is not preferred because sufficient transparency cannot be ensured.

[0066] In the polyester multilayer film with excellent transparency according to an embodiment of the present invention, the concentration of the C3 cyclic oligomer contained in the multilayer film of Mathematical Formula 1 generally has a close relationship with the haze change before and after heat treatment. In order to maintain excellent optical properties such as transparency even after the film is heat-treated, the haze change amount (ΔHt) during the high-temperature treatment of Mathematical Formula 2 needs to be less than 0.5%. If the change in haze exceeds 0.5%, the transparency decreases, resulting in a reduction in the optical properties of the film. To achieve this purpose, although a method of making the concentration of the oligomer in the film very low can be adopted, as described below, there is a problem that a solid-phase polymerization resin needs to be used throughout the film, thus reducing the economy. Therefore, in the present invention, a common liquid chip is used in the base layer 110, and only a low-oligomer resin is used in the surface layer 120. Therefore, the oligomer concentration in the entire film does not differ significantly from that when a liquid-phase polymerization resin is used, and the result of suppressing the generation and migration of oligomers is exhibited.

[0067] In addition, in Mathematical Formula 1, when the correlation coefficient of the C3 cyclic oligomer with the surface layer thickness and the base layer thickness is 110 or less, as described below, there is a problem in terms of economy because a resin with a low oligomer content needs to be prepared and used or the thickness of the surface layer needs to be increased. In addition, when the value of Mathematical Formula 1 is 121 or more, the oligomer content in the multilayer film is high or the thickness of the surface layer is thin. Therefore, after heat treatment under conditions such as high temperature, the haze change required in Mathematical Formula 2 and Mathematical Formula 3 cannot be obtained, and thus transparency cannot be ensured, so it is not preferred.

[0068] Generally, a polyester film contains a certain amount of oligomers when the resin used as a raw material is prepared through a polymerization reaction. The amount of oligomers contained in the polyester film varies depending on the polymerization method. However, based on typical C3 cyclic oligomers, it contains about 0.5% to 2% by weight. When the polyester film is subjected to heat above its glass transition temperature, these oligomers move to the surface. The oligomers that move to the surface have high crystallinity, so the oligomers exist as crystal foreign matters with a size of several micrometers (μm) on the surface. The oligomer crystals as described above deteriorate the optical properties of the film (such as a decrease in transparency), and in the film processing process, they contaminate other objects or products due to scattering, thereby reducing productivity.

[0069] In addition, in order to suppress the phenomenon of oligomers in the polyester film migrating to the surface, various methods are being used. The most typical and commonly used method is to prepare the resin through solid-phase polymerization to minimize the initial oligomer content, and even when the temperature is increased, the migration to the surface can be minimized. In particular, the resin made through solid-phase polymerization has a high molecular weight and thus a high intrinsic viscosity, and it is known that this has the effect of suppressing the movement of oligomers. As another method, when using a copolymer resin that partially contains monomers such as cyclohexanedimethanol or isosorbide in the polyester molecule, the amorphous region of the polymer relatively increases, so it can contain more oligomers, and thus the property of suppressing movement to the surface can be utilized. In addition to this, methods such as using expensive germanium (Ge) or titanium (Ti) catalysts and using laminated films are also known. The method of using catalysts can minimize the generation of oligomers under high-temperature conditions during the melt extrusion process, and the method of using laminated films suppresses the movement of oligomers by using a resin with a high intrinsic viscosity on the surface layer. However, although these methods do have a certain effect, under high-temperature conditions such as in the film forming process or processing steps of the film, since oligomers continue to be generated, there are limitations in the process of finally suppressing the dissolution of the generated oligomers.

[0070] In contrast, the polyester multilayer film with excellent transparency according to an embodiment of the present invention can suppress the generation of oligomers and suppress the migration of the generated oligomers to the surface. When the surface layer of the polyester multilayer film with excellent transparency according to an embodiment of the present invention has an appropriate oligomer concentration and thickness, regardless of the oligomer state of the base material layer, the migration of oligomers to the surface can be sufficiently suppressed even under high-temperature conditions, so the transparency can be maintained. This result means that even if a resin in an oligomer state is used only in the surface layer 120 that accounts for about 10% of the film, the oligomer level of the entire film can be stably maintained and the migration to the surface can be suppressed.

[0071] The method for preparing a polyester multilayer film with excellent transparency according to another embodiment of the present invention may include: a first step of co-extruding a surface layer raw material of a low oligomer polyester resin and a substrate layer raw material of a high oligomer polyester resin to prepare a polyethylene terephthalate sheet; a second step of stretching the polyethylene terephthalate sheet 3 to 5 times in the machine direction (MD), then cooling at room temperature and preparing 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) and preparing a biaxially stretched polyester multilayer film; and a fourth step of heat-treating the biaxially stretched polyester multilayer film at a temperature of 230°C to 250°C and thermally setting it at a temperature of 200°C to 220°C to prepare a polyester multilayer film with excellent transparency.

[0072] In the method for preparing a polyester multilayer film with excellent transparency according to another embodiment of the present invention, the description that duplicates the content of the polyester multilayer film with excellent transparency according to one embodiment of the present invention described above will be omitted.

[0073] First, the first step is a step of co-extruding a surface layer raw material of a low oligomer polyester resin and a substrate layer raw material of a high oligomer polyester resin to prepare a polyethylene terephthalate sheet. The concentration of the C3 cyclic oligomer in the substrate layer raw material is preferably 9000 ppm to 12000 ppm, and the concentration of the C3 cyclic oligomer in the surface layer raw material is preferably 4500 ppm to 7000 ppm.

[0074] At this time, when the surface layer has an appropriate low oligomer concentration and thickness, even if the substrate layer is a specific ordinary high oligomer polyester resin, regardless of the oligomer state of the substrate layer, the raw materials of the surface layer and the substrate layer are made different so that transparency can be maintained by sufficiently suppressing the migration of oligomers to the surface even under high temperature conditions. In addition, in order to reduce the concentration of oligomers, a solid chip with a high intrinsic viscosity (IV) is used and is only used in the thin surface layer. Thus, when preparing a polyester multilayer film with excellent transparency, the use of expensive solid chips can be reduced, and thus advantages such as ensuring economy can be achieved.

[0075] In addition, relative to 100 parts by weight of the polyester resin, the surface layer raw material may further contain 0.1 part by weight to 1.0 part by weight of an organic phosphorus compound, and this organic phosphorus compound may be selected from at least one of triaryl phosphites and trialkyl phosphites. At this time, independently of the oligomer polyester resin, the surface layer raw material may contain the organic phosphorus compound through a masterbatch sheet mixed with the organic phosphorus compound.

[0076] Next, the second step is to stretch the polyethylene terephthalate sheet 3 to 5 times in the machine direction (MD), then cool it at room temperature and prepare a uniaxially stretched polyester multi-layer film. The third step is to stretch the uniaxially stretched polyester multi-layer film 3 to 5 times in the transverse direction (TD) and prepare a biaxially stretched polyester multi-layer film.

[0077] However, in the preparation method of the present invention, stretching refers to biaxial stretching. However, the method for preparing the polyester multi-layer film with excellent transparency of the present invention is not limited to biaxial stretching. If necessary, the film can be prepared without stretching or by uniaxial stretching.

[0078] Next, the fourth step is to heat-treat the biaxially stretched polyester multi-layer film at a temperature of 230°C to 250°C and thermally set it at a temperature of 200°C to 220°C to prepare a polyester multi-layer film with excellent transparency.

[0079] When the polyester multi-layer film with excellent transparency prepared by the above preparation method is heat-treated at 150°C for 30 minutes, the haze change preferably remains below 0.5%. At the same time, after being treated under the conditions of 85°C and 85% RH for 240 hours and then heat-treated at 150°C for 30 minutes, the haze change preferably remains below 1.0%. As described above, the polyester multi-layer film with excellent transparency according to an embodiment of the present invention has the following advantages: excellent transparency, no oligomer migration phenomenon even at high temperatures, so the transparency can be maintained, and in the preparation process, the scattering of oligomers can also be suppressed and a clean preparation environment can be maintained, etc.

[0080] Hereinafter, the constitution and effects of the present invention will be described in more detail through examples and comparative examples. However, these examples are used to more specifically illustrate the present invention, and the scope of the present invention is not limited to these examples.

[0081] [Examples]

[0082] [Preparation Example]

[0083] (1) Preparation of polyester resins (A and B)

[0084] Using 100 parts by weight of terephthalic acid and 60 parts by weight of ethylene glycol as starting materials, adding magnesium acetate tetrahydrate as a catalyst and placing it in a reactor, setting the initial reaction temperature at 150°C and slowly raising the reaction temperature to 230°C after 3 hours. After 4 hours, the transesterification reaction was substantially completed. Transfer the reaction mixture to a polycondensation tank and add antimony trioxide, and carry out a 4-hour polycondensation reaction to obtain polyester resin A with an inherent viscosity of 0.61 dl / g and a content of C3 cyclic oligomers of 9990 ppm.

[0085] Next, using the obtained polyester resin A, solid-phase polymerization was carried out at a temperature of 215 °C under a nitrogen atmosphere to obtain polyester resin B with an inherent viscosity of 0.70 dl / g and a C3 cyclic oligomer content of 4000 ppm.

[0086] (2) Preparation of polyester multilayer film

[0087] The moisture-removed polyester resin A was used as the raw material for the base layer (main layer). Polyester resin B was blended with resin A to achieve a preset C3 cyclic oligomer content and then fed into a co-extrusion machine. By adjusting the feeder block, the weight ratio of the base layer to the surface layer was adjusted to 6:1 to 15:1 and extrusion was carried out. It was rapidly cooled and solidified using a casting cylinder with a surface temperature of 20 °C to prepare a polyethylene terephthalate sheet with a thickness of 8000 μm.

[0088] Next, the prepared polyethylene terephthalate sheet was stretched 3 to 5 times in the machine direction (MD) at a temperature of 80 °C and then cooled to room temperature. After that, the temperature was raised successively in a tenter, and after preheating and drying, it was stretched 3 to 5 times in the transverse direction (TD). Then, heat treatment was carried out at a temperature of 230 °C to 250 °C in the tenter, and heat setting was carried out at a temperature of 200 °C to 220 °C to prepare a biaxially stretched multilayer film.

[0089] [Example 1]

[0090] Polyester resin A and polyester resin B were blended to have a C3 cyclic oligomer content of 6000 ppm and used as the raw material for the surface layer. In the base layer, co-extrusion was carried out using polyester resin A at a temperature of 280 °C to prepare a 50-μm polyester multilayer film with a 3-μm-thick surface layer formed on both sides of the base layer. The C3 cyclic oligomer content of the prepared polyester multilayer film, the correlation coefficient based on Formula 1, the haze change before and after heat treatment at 150 °C for 30 minutes based on Formula 2, and the haze change before and after heat treatment after treatment at a temperature of 85 °C and a relative humidity of 85% based on Formula 3 are respectively recorded in Table 1 below (hereinafter, the same).

[0091] [Example 2]

[0092] A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content in the surface layer was 4500 ppm.

[0093] [Example 3]

[0094] A polyester multilayer film was prepared in the same manner as in Example 1, except that the C3 cyclic oligomer content in the surface layer was 6500 ppm and the thickness of the surface layer was 4 μm.

[0095] [Example 4]

[0096] A polyester multilayer film was prepared in the same manner as in Example 1, except that the thickness of the surface layer was 6 μm.

[0097] [Example 5]

[0098] The content of C3 cyclic oligomers in the surface layer was 6500 ppm, the thickness of the surface layer was 10 μm, and the thickness of the polyester multilayer film was 100 μm. Otherwise, a polyester multilayer film was prepared in the same manner as in Example 1.

[0099] [Example 6]

[0100] The content of C3 cyclic oligomers in the surface layer was 4500 ppm, the thickness of the surface layer was 8 μm, and the thickness of the polyester multilayer film was 100 μm. Otherwise, a polyester multilayer film was prepared in the same manner as in Example 1.

[0101] [Example 7]

[0102] The content of C3 cyclic oligomers in the surface layer was 7000 ppm, the thickness of the surface layer was 8 μm, and the thickness of the polyester multilayer film was 100 μm. Otherwise, a polyester multilayer film was prepared in the same manner as in Example 1.

[0103] [Example 8]

[0104] The thickness of the surface layer was 6 μm, and the thickness of the polyester multilayer film was 100 μm. Otherwise, a polyester multilayer film was prepared in the same manner as in Example 1.

[0105] [Example 9]

[0106] The content of C3 cyclic oligomers in the surface layer was 4500 ppm, the thickness of the surface layer was 2 μm, and the thickness of the polyester multilayer film was 30 μm. Otherwise, a polyester multilayer film was prepared in the same manner as in Example 1.

[0107] [Example 10]

[0108] The content of C3 cyclic oligomers in the surface layer was 6500 ppm, and the thickness of the polyester multilayer film was 38 μm. Otherwise, a polyester multilayer film was prepared in the same manner as in Example 1.

[0109] [Example 11]

[0110] The content of C3 cyclic oligomers in the surface layer was 6500 ppm. Using a 5% organic phosphorus compound masterbatch sheet, the phosphorus content in the surface layer was made to reach 250 ppm. The thickness of the surface layer was 3 μm, and the thickness of the polyester multilayer film was 50 μm. Otherwise, a polyester multilayer film was prepared in the same manner as in Example 1.

[0111] [Comparative Example]

[0112] [Comparative Example 1]

[0113] The content of the C3 cyclic oligomer in the surface layer was 4500 ppm, and the thickness of the surface layer was 2 μm. Except for this, a polyester multilayer film was prepared in the same manner as in Example 1.

[0114] [Comparative Example 2]

[0115] The content of the C3 cyclic oligomer in the surface layer was 7000 ppm. Except for this, a polyester multilayer film was prepared in the same manner as in Example 1.

[0116] [Comparative Example 3]

[0117] The content of the C3 cyclic oligomer in the surface layer was 5000 ppm, the thickness of the surface layer was 7 μm, and the thickness of the polyester multilayer film was 50 μm. Except for this, a polyester multilayer film was prepared in the same manner as in Example 1.

[0118] [Comparative Example 4]

[0119] The content of the C3 cyclic oligomer in the surface layer was 8500 ppm, the thickness of the surface layer was 7 μm, and the thickness of the polyester multilayer film was 100 μm. Except for this, a polyester multilayer film was prepared in the same manner as in Example 1.

[0120] [Comparative Example 5]

[0121] The content of the C3 cyclic oligomer in the surface layer was 8500 ppm, the thickness of the surface layer was 6 μm, and the thickness of the polyester multilayer film was 100 μm. Except for this, a polyester multilayer film was prepared in the same manner as in Example 1.

[0122] [Comparative Example 6]

[0123] The content of the C3 cyclic oligomer in the surface layer was 7500 ppm, the thickness of the surface layer was 2 μm, and the thickness of the polyester multilayer film was 38 μm. Except for this, a polyester multilayer film was prepared in the same manner as in Example 1.

[0124] Using the polyester multilayer films of Examples 1 to 11 and Comparative Examples 1 to 6, the physical properties were measured by the following Experimental Examples, and the results are shown in Table 1.

[0125] [Experimental Example]

[0126] (1) Measurement of the content of the C3 cyclic oligomer

[0127] Dissolve 50 mg of the prepared multilayer film in 1 ml of 1,1,1,3,3,3-hexafluoro-2-propanol, then add 10 ml of chloroform and 8 ml of methanol and reprecipitate. After filtering through a 5-μm filter, remove the solvent. Next, dissolve the obtained precipitate in a certain amount of chloroform, and determine the amount of C3 cyclic oligomers using HPLC (Agilent 1200 series).

[0128] Among them, the amount of oligomers using HPLC is obtained from the ratio of the peak area of a common standard sample to the peak area of the measured sample (absolute calibration curve method). The chromatographic column used was Polaris5Si 100*4.6 mm, the temperature was 40 °C, the mobile phase used was hexane / 1,4-dioxane (6:4), the flow rate was 1.0 ml / minute, and the detector used was ultraviolet (UV) at 240 nm.

[0129] (2) Measurement of the haze difference (△Ht) before and after heat treatment

[0130] For the film prepared as a sample, according to ASTM-D1003, measure the haze of the film using a haze meter NDH-5000 (Nippon Denshoku).

[0131] First, after measuring the haze (Hi) of the film prepared as a sample, fix the prepared film sample on a quadrilateral metal bracket, then leave it standing in an oven at 150 °C for 30 minutes, and after performing high-temperature heat treatment, measure the haze (Hf) after heat treatment.

[0132] The difference in haze (ΔH) of the film before and after heat treatment is calculated by subtracting the haze (Hi) before heat treatment from the haze (Hf) after heat treatment.

[0133] (3) Measurement of the haze difference (△Hht) before and after heat treatment of the film after high-temperature and high-humidity treatment

[0134] For the film prepared as a sample, according to ASTM-D1003, measure the haze of the film using a haze meter NDH-5000 (Nippon Denshoku).

[0135] First, after measuring the haze (Hs) of the film prepared as a sample, leave the film standing at 85 °C and 85% RH for 240 hours, then leave it standing in an oven at 150 °C for 30 minutes again, and after performing high-temperature heat treatment, measure the haze (He) after heat treatment of the film after high-temperature and high-humidity treatment.

[0136] The haze difference (ΔHht) before and after heat treatment of the film treated with high temperature and high humidity is calculated by subtracting the haze (Hs) before heat treatment of the film treated with high temperature and high humidity from the haze (He) after heat treatment of the film treated with high temperature and high humidity.

[0137] (4) Measurement of surface layer thickness

[0138] For cross-section processing, the film prepared as a sample was sliced using a microtome (LEICA RM2255) and platinum-coated, and then the thickness was measured using a scanning electron microscope (SEM, HITACHI S-4800).

[0139] (5) Surface state analysis based on optical microscopy

[0140] After fixing the film prepared as a sample on a quadrilateral metal support, it was left standing in an oven at 150 °C for 30 minutes. For the film after high-temperature heat treatment and the film before heat treatment, the surface with a width of 2 * 2 mm was analyzed using an optical microscope (Olympus MX50LT-1273MH). Evaluation was carried out according to the following criteria based on whether frosting of crystals larger than microns was observed due to oligomers.

[0141] O: Not observed at all

[0142] Δ: Observed finely

[0143] X: Widely observed on the entire surface

[0144] The physical properties of the multilayer films prepared through the above-mentioned examples and comparative examples are summarized in Table 1 below. In Table 1, [C3] represents the concentration of C3 cyclic oligomers in the entire multilayer film.

[0145] Table 1

[0146]

[0147] As shown in Table 1 above, it can be confirmed that the polyester multilayer film of the present invention shows the correlation coefficient (Equation 1) of the concentration of C3 cyclic oligomers, the thickness of the surface layer, and the thickness of the multilayer film, and satisfies the haze change before and after heat treatment according to it (Equation 2) and the haze change under high temperature and high humidity conditions (Equation 3). Therefore, it not only exhibits excellent transparency but also does not cause the phenomenon of oligomer migration to the surface. Thus, a polyester multilayer film can be provided, which can prevent a reduction in productivity caused by scattering to the outside from the source. In addition, as shown in Example 11, it can be confirmed that the inhibitory effect of oligomers is further enhanced by using an organic phosphorus compound in the surface layer.

[0148] On the contrary, in Comparative Examples 1 to 2 and Comparative Examples 4 to 6 where the value of the correlation coefficient (Formula 1) is 121 or more, it can be seen that the haze change before and after the heat treatment and the haze change before and after the heat treatment of the film treated with high temperature and high humidity are large and the surface is widely frosted. In addition, as shown in Comparative Example 3, when the correlation coefficient is 110 or less, although the haze change before and after the heat treatment and the haze change before and after the heat treatment of the film treated with high temperature and high humidity are small, the surface thickness of the oligomer resin increases compared to the overall thickness, resulting in poor economic efficiency and reduced productivity, and is therefore not preferred.

[0149] As described above, in order to reduce the concentration of oligomers, the polyester multilayer film with excellent transparency of the present invention uses a solid chip with a high intrinsic viscosity (IV) and is only used in a thin surface layer. Therefore, when preparing the polyester multilayer film with excellent transparency, the use of expensive solid chips can be reduced, thereby having the advantages of ensuring economy, etc.

[0150] In this specification, although only a few examples of various embodiments made by the inventor are used for illustration, the technical concept of the present invention is not limited to or restricted thereto, and can be modified and implemented in various ways by ordinary technicians in this technical field.

Claims

1. A polyester multilayer film with excellent transparency, characterized in that it comprises: a base material layer containing a polyester resin, and a surface layer located on at least one side of the base material layer and containing a polyester resin; the polyester multilayer film satisfies the following Mathematical Formula 1: Mathematical Formula 1 110 < 0.013 * [C3] – Ds * (50 / Dt) < 121 wherein, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, Ds is the thickness of the surface layer, the unit of Ds is μm, and Dt is the thickness of the multilayer film, the unit of Dt is μm.

2. The polyester multilayer film with excellent transparency according to claim 1, characterized in that the concentration of C3 cyclic oligomers contained in the polyester multilayer film is 9000 ppm to 9700 ppm.

3. The polyester multilayer film with excellent transparency according to claim 1, characterized in that the concentration of C3 cyclic oligomers of the polyester resin forming the base material layer is 9000 ppm to 12000 ppm, and the concentration of C3 cyclic oligomers of the polyester resin forming the surface layer is 4500 ppm to 7000 ppm.

4. The polyester multilayer film with excellent transparency according to claim 1, characterized in that the thickness of the surface layer is 2 μm to 10 μm.

5. The polyester multilayer film with excellent transparency according to claim 1, characterized in that the thickness ratio of the surface layer to the base material layer is 1:6 to 15.

6. The polyester multilayer film with excellent transparency according to claim 1, characterized in that the intrinsic viscosity of the polyester resin forming the surface layer is 0.60 dl / g to 0.70 dl / g.

7. The polyester multilayer film with excellent transparency according to claim 1, characterized in that relative to 100 parts by weight of the polyester resin, the surface layer further contains 0.1 part by weight to 1.0 part by weight of an organic phosphorus compound.

8. The polyester multilayer film with excellent transparency according to claim 7, characterized in that the organic phosphorus compound is selected from at least one of triaryl phosphites and trialkyl phosphites.

9. The polyester multilayer film with excellent transparency according to claim 1, characterized in that during the high-temperature treatment process, the haze change amount △Ht of the polyester multilayer film satisfies the following Mathematical Formula 2: Mathematical Formula 2 △Ht = Hf - Hi < 0.5% wherein, Hi is the haze before heat treatment, and Hf is the haze after heat treatment at a temperature of 150 °C for 30 minutes.

10. The polyester multilayer film with excellent transparency according to claim 1, characterized in that during the high-temperature treatment process after being treated under high-temperature and high-humidity conditions, the haze change amount △Hht of the polyester multilayer film satisfies the following Mathematical Formula 3: Mathematical Formula 3 △Hht = He - Hs < 1.0% wherein, Hs is the haze before being treated under high-temperature and high-humidity conditions, and He is the haze after heat treatment at a temperature of 150 °C for 30 minutes of the film treated under high-temperature and high-humidity conditions of 85 °C and 85% RH for 240 hours.

11. A method for preparing a polyester multilayer film with excellent transparency, characterized in that it comprises: First step: Co-extrude the surface layer raw material of the low oligomer polyester resin and the substrate layer raw material of the high oligomer polyester resin to prepare a polyethylene terephthalate sheet; Second step: Stretch the polyethylene terephthalate sheet 3 to 5 times in the machine direction, then cool it at room temperature and prepare a uniaxially stretched polyester multi-layer film; Third step: Stretch the uniaxially stretched polyester multi-layer film 3 to 5 times in the transverse direction and prepare a biaxially stretched polyester multi-layer film; and Fourth step: Heat-treat the biaxially stretched polyester multi-layer film at a temperature of 230°C to 250°C and heat-set it at a temperature of 200°C to 220°C to prepare a polyester multi-layer film.

12. The method for preparing a polyester multi-layer film with excellent transparency according to claim 11, wherein the concentration of the C3 cyclic oligomer in the substrate layer raw material is 9000 ppm to 12000 ppm, the concentration of the C3 cyclic oligomer in the surface layer raw material is 4500 ppm to 7000 ppm.

13. The method for preparing a polyester multi-layer film with excellent transparency according to claim 11, wherein relative to 100 parts by weight of the polyester resin, the surface layer raw material further contains 0.1 part by weight to 1.0 part by weight of an organic phosphorus compound, the organic phosphorus compound is selected from at least one of triaryl phosphite and trialkyl phosphite.

14. The method for preparing a polyester multi-layer film with excellent transparency according to claim 11, wherein independently of the oligomer polyester resin, the surface layer raw material contains the organic phosphorus compound through a masterbatch sheet mixed with the organic phosphorus compound.

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