Polyester multilayer film and its manufacturing method

Abstract

The polyester multilayer film of the present invention comprises a base layer containing a polyester and a skin layer located on at least one side of the base layer, and by satisfying a predetermined correlation between the cyclic oligomer concentration in the film and the haze value before and after heat treatment, the polyester multilayer film suppresses the generation of oligomers in the film and their migration to the surface even in a high-temperature treatment process, thereby providing a polyester multilayer film that maintains the transparency and visibility of the film over the long term.

Description

[Technical Field]

[0001] The present invention relates to a polyester multilayer film having excellent transparency, and more particularly to an economical polyester multilayer film that maintains transparency even when used in high-temperature processes by suppressing the generation of oligomers inside the film and their migration to the surface, and a method for producing the same. [Background technology]

[0002] In general, polyester films are highly durable and have excellent physical property stability 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 a steady increase in their use as optical films for displays such as LCDs and touch panels.

[0003] However, polyester films require high temperatures of over 100°C during the manufacturing process of displays, etc., which can cause low molecular weight oligomers present inside the polyester film to leach out to the surface. The leached oligomers form crystalline foreign matter known as blooming, which not only reduces transparency but also reduces productivity due to contamination caused by scattering during the process.

[0004] In order to prevent oligomers from eluting to the surface of such polyester films, a method of reducing the oligomer content through solid-state polymerization during polymerization of the polyester film has been widely used. However, this method not only increases costs due to the complicated solid-state polymerization process, but also makes it difficult to completely block oligomers, since oligomers are continuously produced within the film at high temperatures and elute to the surface.

[0005] In addition, other methods for preventing oligomers from eluting to the surface have been reported, such as using a highly heat-resistant polymer such as polyethylene naphthalate (PEN) as in Korean Patent Registration No. 10-1733193, or using a copolymer in which a monomer such as isophthalate or cyclohexyldimethanol is used instead of terephthalic acid or ethylene glycol as in Korean Patent Registration No. 10-1842456, thereby suppressing oligomer elution. However, these methods have the problem of not only causing changes in the physical properties of the polyester, but also ultimately failing to suppress elution to the surface.

[0006] In addition, a technology has been reported that attempts to control oligomer elution by forming a laminated film on a polyester film and using a high-viscosity polymer produced by solid-phase polymerization as the skin layer. However, this is not economical as it requires expensive catalysts, and it is not possible to completely block oligomer elution.

[0007] Meanwhile, Korean Patent No. 10-1424838 aims to improve the color tone of the film by using a phosphorus compound in the resin polymerization stage, but this method requires complex manufacturing processes due to issues such as inhibiting the activity of catalysts essential for the polymerization reaction in the manufacturing method of polyester itself, which increases manufacturing costs, and has no connection whatsoever to preventing oligomer formation and migration. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been devised to solve the above-mentioned problems, and an object of the present invention is to provide a polyester multilayer film having excellent transparency, which can suppress oligomer generation within the film and migration to the surface even during high-temperature treatment processes, thereby maintaining the transparency and visibility of the film for a long period of time, and a method for manufacturing the same.

[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 object of the present invention is to provide a polyester-based film that includes a substrate layer containing a polyester and a skin layer that is located on at least one surface of the substrate layer and contains a polyester and an organic phosphorus compound, and that satisfies the following formulas 1 and 2: [Formula 1] 0.5<[C3] / 10,000-ΔH<1 [Formula 2] ΔH=Hf-Hi<0.5% In Equation 1 and Equation 2, [C3] is the ppm concentration of C3 cyclic oligomer contained in the multilayer film, Hi is the haze of the multilayer film, and Hf is the haze (unit: %) of the multilayer film after heat treatment at 150°C for 30 minutes, achieved by the polyester multilayer film.

[0011] Preferably, the polyester multilayer film satisfies the following formula 3, [Formula 3] 10,000<[C3]+[P]*20<15,000 In Equation 3, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, and [P] is the ppm concentration of phosphorus (P) contained in the skin layer.

[0012] Preferably, the C3 cyclic oligomer concentration in the skin layer may be 7,000 ppm or less.

[0013] Preferably, the organic phosphorus compound has a molecular weight of 400 to 900 g / mol.

[0014] Preferably, the organic phosphorus compound acts like a C3 cyclic oligomer to maintain an equilibrium state at high temperatures and suppresses the formation of C3 cyclic oligomers inside the film.

[0015] Preferably, the organic phosphorus compound may be at least one selected from the group consisting of triaryl phosphite and trialkyl phosphite.

[0016] Preferably, the organic phosphorus compound of the skin layer may be prepared as a masterbatch chip by compounding the organic phosphorus compound separately from the polyester resin.

[0017] Preferably, the thickness ratio of the substrate layer to the skin layer may be 6:1 to 13:1.

[0018] The above object can also be achieved by a method for producing a polyester multilayer film, the method including the steps of compounding an organic phosphorus compound separately from a polyester resin to produce masterbatch chips, blending the produced masterbatch chips with a polyester resin, adding the blend to a polyester film, and co-extruding the blend onto a polyester film to produce a polyethylene terephthalate sheet, uniaxially stretching the polyethylene terephthalate sheet in the machine direction and then cooling to room temperature to produce a uniaxially stretched polyester multilayer film, and biaxially stretching the uniaxially stretched polyester multilayer film in the transverse direction and then heat-treating it to produce a biaxially stretched polyester multilayer film.

[0019] Preferably, the polyester multilayer film can have a haze change of less than 0.5 when heat treated at 150° C. for 30 minutes. [Effects of the Invention]

[0020] The polyester multilayer film according to an embodiment of the present invention has the effect of suppressing the generation of oligomers in the film even in high-temperature processes and preventing migration to the surface.

[0021] Furthermore, the method for manufacturing a polyester multilayer film according to an embodiment of the present invention has the advantage of being highly economical since it can manufacture a polyester multilayer film at low manufacturing costs.

[0022] Furthermore, due to the above-mentioned advantages, the polyester multilayer film according to an embodiment of the present invention can maintain excellent product quality in display, semiconductor, and various other industries, and can significantly increase productivity by preventing contamination within the process due to oligomer dispersion.

[0023] 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 skilled in the art to which the present invention pertains from the following description. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a cross-sectional view of a polyester multilayer film according to an embodiment of the present invention. [Figure 2] 1 is a flow chart showing a method for manufacturing a polyester multilayer film according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention may be embodied in various different forms and is not limited to the embodiments set forth herein.

[0026] In the drawings, thicknesses are exaggerated to clearly show multiple layers and regions. Similar parts are designated by the same reference numerals throughout the specification. When a part such as a layer, film, region, or plate is described as being "on" another part, this includes not only the case where it is "directly on" the other part, but also the case where there is another part between them. Conversely, when a part is described as being "directly on" the other part, it means that there is no other part between them.

[0027] 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.

[0028] FIG. 1 is a cross-sectional view of a polyester multilayer film according to one embodiment of the present invention.

[0029] 1, a polyester multilayer film according to an embodiment of the present invention includes a substrate layer 110 containing polyester, and a skin layer 120 located on at least one side of the substrate layer 110. In the example shown in FIG. 1, a configuration in which the skin layer 120 is formed on both sides of the substrate layer 110 is shown, but this is not limited thereto, and the skin layer 120 may be formed on only one side of the substrate layer 110.

[0030] In the present invention, the skin layer 120 is located on at least one side of the substrate layer 110 and contains a polyester and an organic phosphorus compound. The inventors have tried to suppress the generation of oligomers within the film and their migration to the surface even at high temperatures, and found that this can be solved by adding a compound having a structure and size similar to that of a cyclic C3 substance, a typical oligomer, only to the skin layer during the polyester film formation process. In particular, they confirmed that an organic phosphorus compound has excellent performance among substances having a structure similar to that of a cyclic C3 substance, and thus completed the present invention.

[0031] The polyester multilayer film according to an embodiment of the present invention preferably satisfies the following mathematical formula 1 (correlation coefficient) and mathematical formula 2 (difference in haze before and after heat treatment).

[0032] [Formula 1] 0.5<[C3] / 10,000-ΔH<1 [Formula 2] ΔH=Hf-Hi<0.5% In Equation 1 and Equation 2, [C3] is the ppm concentration of C3 cyclic oligomer contained in the multilayer film, Hi is the haze of the multilayer film, and Hf is the haze (unit: %) of the multilayer film after heat treatment at 150°C for 30 minutes.

[0033] A polyester multilayer film according to one embodiment of the present invention is characterized in that the skin layer 120 contains an organic phosphorus compound, and when the relationship between the C3 cyclic oligomer contained in the multilayer film and the haze change before and after heat treatment satisfies the above-mentioned Equations 1 and 2, the technical effects of the present invention can be achieved.

[0034] The C3 cyclic oligomer concentration in the film, as shown in Equation 1, and the change in haze before and after heat treatment are generally closely related. For the film to maintain excellent optical properties, such as transparency, after heat treatment, the change in haze according to Equation 2 must be less than 0.5% (Equation 2). If the change in haze exceeds 0.5%, transparency and the optical properties of the film will deteriorate. To achieve this, one possible method would be to minimize the oligomer concentration in the film. However, this would require the use of solid-phase polymerization resin throughout the film, as described below, which is uneconomical. Therefore, in the present invention, a general liquid chip is used for the base layer 110, and a low-oligomer resin and a C3 analogue compound are used only in the skin layer 120. This results in an oligomer concentration throughout the film that is not significantly different from that when a liquid polymerization resin is used, thereby suppressing oligomer generation and migration.

[0035] In addition, when the correlation coefficient between C3 cyclic oligomer and haze change in Equation 1 is 0.5 or less, a resin with a low oligomer content must be produced and used, as described below, which is economically problematic, or otherwise the haze change is too large to ensure transparency. Also, when the value of Equation 1 is 1 or more, the oligomer content in the film is 15,000 ppm or more, which is undesirable because it reaches saturation.

[0036] 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. Due to 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 and products, reducing productivity.

[0037] Various methods have been used to prevent oligomer migration to the surface of polyester films. The most common method is to produce resins through solid-state polymerization, which minimizes the initial oligomer content and minimizes surface migration even when heated. Resins produced through solid-state polymerization have a high molecular weight and intrinsic viscosity, which is known to prevent 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 preventing 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 prevent oligomer migration. Although the above-mentioned methods are partially effective, oligomers are continuously produced under high temperature conditions such as during film production and processing, and therefore there is a limit to how much they can fundamentally prevent the elution of the produced oligomers.

[0038] In contrast, a polyester multilayer film according to an embodiment of the present invention inhibits oligomer formation and prevents the migration of the oligomers to the surface. In a polyester multilayer film according to an embodiment of the present invention, polyester molecules reach an equilibrium state with oligomers at a certain temperature. In particular, at the temperature of 300°C at which polyester films are made, the equilibrium constant reaches approximately 0.019, maintaining a stable equilibrium state. Based on this, it was found that the addition of an appropriate amount of a compound having a similar structure to a typical C3 cyclic oligomer, taking into account the equilibrium concentration, inhibits the formation of additional oligomers. In particular, when a specific amount of an organic phosphorus compound is used among the compounds having a similar structure, the equilibrium state is maintained even during high-temperature film formation processes reaching 250-300°C, significantly inhibiting oligomer formation and preventing surface elution even during long-term use at high temperatures. These results indicate that even if a low-oligomer resin and phosphorus-like substance are used only in the skin layer 120, which accounts for approximately 10% of the film, additional oligomer generation can be suppressed, thereby maintaining a stable oligomer level throughout the film and suppressing migration to the surface.

[0039] The polyester multilayer film according to an embodiment of the present invention preferably satisfies the following formula 3. That is, it is preferable that the following formula 3 is satisfied by the organic phosphorus compound contained in the skin layer 120.

[0040] [Formula 3] 10,000<[C3]+[P]*20<15,000 In the formula, [C3] is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, and [P] is the ppm concentration of phosphorus (P) contained in the skin layer.

[0041] In Equation 3, when the relationship between the [C3] concentration and the [P] concentration is 10,000 or less, the generation of oligomers cannot be sufficiently suppressed to suppress the desired increase in haze, whereas when the relationship is 15,000 or more, migration of oligomers occurs or an excessive amount of phosphorus compound is used, which causes problems such as scattering, and is therefore undesirable.

[0042] In the present invention, the C3 cyclic oligomer concentration in the skin layer 120 is preferably 7,000 ppm or less. If the concentration exceeds 7,000 ppm, some of the oligomers may migrate to the surface, which is undesirable.

[0043] For this reason, the skin layer 120 preferably has a phosphorus content derived from the organic phosphorus compound of 100 to 400 ppm. If the organic phosphorus compound content is less than 100 ppm, oligomers cannot be sufficiently suppressed, and if it exceeds 400 ppm, the excess compound may cause contamination during film production, which is uneconomical.

[0044] In the present invention, the organic phosphorus compound is preferably a trialkyl phosphite or triaryl phosphite. Specifically, it preferably includes at least one selected from the group consisting of Tri(2,4-di-tert-butylphenyl)phosphite, Bis(2,4-di-tert-butylphenyl)pentaerithritol diphosphite, Bis(2,6-di-tert-butylphenyl)pentaerythritol diphite, and Bis(2,4-dicumylphenyl)pentaerythritol diphite. While phosphates, phosphonates, and phosphinates can also be used, they lack structural flexibility and compatibility with polyesters, resulting in insufficient oligomer suppression effects.

[0045] In the present invention, the organic phosphorus compound preferably has a molecular weight of 400 g / mol to 900 g / mol. If the molecular weight of the organic phosphorus compound is outside this range, the similarity to the C3 cyclic oligomer molecule is insufficient, making it difficult to maintain equilibrium. In this way, the organic phosphorus compound contained in the skin layer 120 acts like a C3 cyclic oligomer, maintaining equilibrium at high temperatures and suppressing the formation of C3 cyclic oligomers inside the film.

[0046] In the polyester multilayer film according to one embodiment of the present invention, the thickness ratio of the substrate layer 110 to the skin layer 120 is preferably 6:1 to 13:1. If this ratio is increased, oligomer formation is suppressed but migration suppression may be insufficient, while if the ratio is decreased, migration suppression effect is good but it is not economically preferable.

[0047] FIG. 2 is a flow chart showing a method for producing a polyester multilayer film according to one embodiment of the present invention.

[0048] Referring to FIG. 2, a method for manufacturing a polyester multilayer film according to one embodiment of the present invention includes the steps of compounding an organic phosphorus compound separately from a polyester resin to prepare masterbatch chips (S101), blending the prepared masterbatch chips with a polyester resin, adding the blend to a polyester film, and co-extruding the blend onto the polyester film to prepare a polyethylene terephthalate sheet (S102), uniaxially stretching the polyethylene terephthalate sheet in the machine direction and then cooling to room temperature to prepare a uniaxially stretched polyester multilayer film (S103), and biaxially stretching the uniaxially stretched polyester multilayer film in the transverse direction and then heat-treating the biaxially stretched polyester multilayer film (S104).

[0049] First, in the step of preparing masterbatch chips (S101), an organic phosphorus compound is added through a side feeder while melt-extruding a polyester resin, and the extruded resin is cooled and dried, then cut into chips of a predetermined size. The organic phosphorus compound contained in the skin layer of the present invention is compounded separately from the polyester resin to prepare masterbatch chips, which are then blended with the polyester resin during film formation. While adding the organic phosphorus compound during the polymerization stage of resin production is possible, adding the required amount is undesirable because it inhibits the activity of the catalyst essential for polymerization, lengthening the reaction time, and potentially altering the organic phosphorus compound over a long reaction time.

[0050] Next, in step S102 of producing a polyethylene terephthalate sheet, a polyester resin is used as a base layer raw material, and a raw material obtained by blending the polyester resin with the polyester masterbatch chips produced in step S101 is used as a skin layer raw material, and these are co-extruded to produce a polyethylene terephthalate sheet. At this time, the skin layer raw material is a raw material obtained by blending the produced masterbatch and polyester resin to have a predetermined phosphorus compound content.

[0051] Next, in step S103, the polyethylene terephthalate sheet is uniaxially stretched in the machine direction and then cooled to room temperature to produce a uniaxially stretched polyester multilayer film. The polyethylene terephthalate sheet thus produced is stretched 3 to 5 times in the machine direction (MD) and then cooled to room temperature to produce a uniaxially stretched polyester multilayer film.

[0052] Next, in step S104, the uniaxially stretched polyester multilayer film is biaxially stretched in the transverse direction and then heat-treated to produce a biaxially stretched polyester multilayer film. The uniaxially stretched polyester multilayer film is stretched 3 to 5 times in the transverse direction (TD), and the biaxially stretched polyester multilayer film is heat-treated at 230 to 250°C and heat-set at 200 to 220°C to produce a biaxially stretched polyester multilayer film.

[0053] However, the stretching in the description of the present invention is not limited to biaxial stretching, and the film can be produced by non-stretching or uniaxial stretching as required.

[0054] The polyester multilayer film thus produced preferably maintains a haze change of less than 0.5% when heat treated at 150° C. for 30 minutes.

[0055] The polyester multilayer film having excellent transparency produced by the above-mentioned production method does not undergo oligomer migration at high temperatures, so its transparency is maintained as it is, and oligomer scattering during the production process is also suppressed, making it possible to maintain a clean production environment.

[0056] The present invention will be described in more detail with reference to the following examples, which are provided for illustrative purposes only and are not intended to limit the scope of the present invention.

[0057] [Manufacturing example] (1) Production of polyester resins (A, B) Starting materials were 100 parts by weight of terephthalic acid and 60 parts by weight of ethylene glycol, and magnesium acetate tetrahydrate was added as a catalyst. The reaction 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. Polyester resin A was obtained with an intrinsic viscosity of 0.61 and a C3 cyclic oligomer content of 9,990 ppm.

[0058] Next, using the polyester resin A obtained above, solid-phase polymerization was carried out at a temperature of 215°C under nitrogen conditions to obtain polyester resin B having an intrinsic viscosity of 0.81 and a C3 cyclic oligomer content of 4,000 ppm.

[0059] (2) Manufacturing masterbatch chips of organic phosphorus compounds After removing moisture from polyester resin B obtained by the polyester resin production described above, the resin was placed in a twin extruder and melt-extruded at 280°C while adding a phosphorus compound, Tri(2,4-di-tert-butylphenyl)phosphite (BASF) through a side feeder. The masterbatch contained 5 parts by weight of the phosphorus compound per 100 parts by weight of polyester. The polymer strand exiting the extruder was then cooled in water, air-dried, and cut to a predetermined size to produce masterbatch C.

[0060] (3) Polyester multilayer film manufacturing Dehydrated polyester resin A was used as the base layer (main layer) raw material. Polyester resin B and masterbatch C were blended to a predetermined phosphorus compound content and then introduced into a co-extruder. The feeder block was adjusted to adjust the main layer to skin layer ratio from 6:1 to 15:1. The extrusion was then quenched and solidified on a casting drum at a surface temperature of 20°C to produce a polyethylene terephthalate sheet with a thickness of 8,000 μm. The polyethylene terephthalate sheet was then stretched 3 to 5 times in the machine direction (MD) at 80°C and cooled to room temperature. It was then preheated and dried in a tenter, and stretched 3 to 5 times in the transverse direction (TD). The sheet was then heat-treated at 230 to 250°C in the tenter, followed by heat-setting at 200 to 220°C to produce a biaxially stretched multilayer film.

[0061] [Example] [Example 1] Polyester resin A, polyester resin B, and masterbatch C were blended to a phosphorus content of 250 ppm and used as the raw material for the skin layer. Polyester resin A was used for the substrate layer, and co-extrusion was carried out at 280°C to produce a 50 μm-thick polyester multilayer film with skin layers formed on both sides of the substrate layer. The thickness ratio of the skin layer to the substrate layer was 1:13:1. The C3 cyclic oligomer content of the produced polyester multilayer film, the change in haze before and after heat treatment at 150°C for 30 minutes according to Equation 2, and the correlation coefficient according to Equation 1 are shown in Table 1 below (the same applies hereinafter).

[0062] [Example 2] A polyester multi-layer film was produced in the same manner as in Example 1, except that the skin layer had a phosphorus content of 150 ppm.

[0063] [Example 3] A polyester multilayer film was produced in the same manner as in Example 1, except that the thickness ratio of the skin layer, the substrate layer and the skin layer was 1:6:1.

[0064] [Example 4] A polyester multi-layer film was produced in the same manner as in Example 1, except that the thickness of the polyester multi-layer film was 188 μm.

[0065] [Comparative Example] [Comparative Example 1] A polyester multilayer film was produced in the same manner as in Example 1, except that polyester resin A and masterbatch C were blended to a phosphorus content of 250 ppm and used as the raw material for the skin layer, and polyester resin A was used as the raw material for the base layer.

[0066] Comparative Example 2 A polyester multilayer film was produced in the same manner as in Example 1, except that polyester resin A and masterbatch C were blended to a phosphorus content of 250 ppm and used as the raw material for the skin layer, and polyester resin B was used as the raw material for the substrate layer.

[0067] Comparative Example 3 A polyester multilayer film was produced in the same manner as in Example 1, except that polyester resin A and polyester resin B were blended (masterbatch C was omitted) and used as raw materials for the skin layer and main layer, respectively.

[0068] Comparative Example 4 A polyester multi-layer film was produced in the same manner as in Example 1, except that triphenyl phosphate (manufactured by Green Chemical Co., Ltd.) was used as the phosphorus compound.

[0069] Comparative Example 5 A polyester multilayer film was produced in the same manner as in Example 1, except that the ratio of the skin layer to the substrate layer was 1:15:1.

[0070] Comparative Example 6 A polyester multi-layer film was produced in the same manner as in Example 1, except that the skin layer had a phosphorus content of 80 ppm.

[0071] Comparative Example 7 A polyester multi-layer film was produced in the same manner as in Example 1, except that the skin layer had a phosphorus content of 450 ppm.

[0072] The properties of the polyester multi-layer films prepared in Examples 1 to 4 and Comparative Examples 1 to 7 were evaluated according to the following experimental examples, and the results are shown in Table 1.

[0073] [Experimental Example] 1) Measurement of C3 cyclic oligomer content 50 mg of the prepared multilayer film was dissolved in a chloroform / 1,1,1,3,3-hexafluoro-2-propanol (mixing ratio: 3 / 2) mixed solution, then reprecipitated in chloroform / methanol (mixing ratio: 2 / 1), filtered through a 5 μm filter, and the solvent was removed. The resulting precipitate was then dissolved in a certain amount of DMF, and the amount of C3 cyclic oligomer was measured using HPLC (Agilent 1200 series).

[0074] 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 curve 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 (6:4), the flow rate was 1.0 ml / min, and the detector was 240 nm UV.

[0075] (2) Phosphorus (P) content measurement A 1-gram sample was freeze-pulverized and hand-pressed to create a 1-mm-thick thin film, and the phosphorus content was measured using an XRF-Minipal 4 instrument (Panalytical). A calibration curve was obtained using polyester fiber containing a quantitative amount of phosphorus pentoxide.

[0076] (3) Haze (H) measurement The haze (Hi) of the film prepared as a sample was measured using a turbidity meter NDH-5000 (manufactured by Nippon Denshoku Co., Ltd.) in accordance with ASTM-D1003.

[0077] Next, the prepared film sample was fixed on a square metal support and then placed in an oven at 150° C. for 30 minutes for high-temperature heat treatment, and the haze (Hf) after heat treatment was measured.

[0078] The difference (ΔH) in film haze before and after heat treatment was determined by subtracting the haze (Hi) before heat treatment from the haze (Hf) after heat treatment.

[0079] (4) Analysis of surface conditions using an optical microscope The prepared film sample was fixed on 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. The following classification was made based on whether blooming, caused by crystals of oligomers larger than a micron, was observed.

[0080] 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 summarized in the following Table 1. In Table 1, [C3] indicates the concentration of C3 cyclic oligomers in the entire multilayer film.

[0081] [Table 1]

[0082] As shown in Table 1, the polyester multilayer film according to the present invention satisfies the correlation coefficient (Equation 1) between the C3 cyclic oligomer concentration and the change in haze before and after heat treatment, and therefore exhibits excellent transparency and does not undergo migration of oligomers to the surface. This makes it possible to provide a polyester multilayer film that can fundamentally prevent productivity degradation due to external dispersion.

[0083] In contrast, Comparative Examples 1 to 6, in which the correlation coefficient (Equation 1) value is 0.5 or less, and especially Comparative Examples 1 to 4, in which the value of Equation 1 is extremely small, show a large change in haze before and after heat treatment, and blooming occurs over a wide area on the surface. Comparative Examples 5 and 6, in which the value of Equation 1 is larger than those of Comparative Examples 1 to 4 but is 0.5 or less, show partial blooming on the surface.

[0084] Furthermore, it can be seen that in Comparative Example 7, where the value of Equation 3 exceeds 15,000, blooming occurs partially on the surface due to the excess phosphorus compound.

[0085] Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concept of the present invention defined in the following claims also fall within the scope of the present invention.

Claims

1. a base layer containing polyester; a skin layer located on at least one surface of the base layer and containing a polyester and an organic phosphorus compound, Although the following formulas 1 and 2 are satisfied, [Formula 1] 0.5<[C3] / 10,000-ΔH<1 [Formula 2] ΔH=Hf-Hi<0.5% In Equation 1 and Equation 2, [C3] is the ppm concentration of the C3 cyclic oligomer contained in the multilayer film, Hi is the haze of the multilayer film, and Hf is the haze (unit: %) of the multilayer film after heat treatment at 150°C for 30 minutes. The skin layer of the polyester multilayer film has a phosphorus content derived from the organic phosphorus compound of 100 to 400 ppm.

2. The polyester multilayer film satisfies the following formula 3, [Formula 3] 10,000<[C3]+[P]*20<15,000 2. The polyester multilayer film according to claim 1, wherein [C3] in Equation 3 is the ppm concentration of C3 cyclic oligomers contained in the multilayer film, and [P] is the ppm concentration of phosphorus (P) contained in the skin layer(s).

3. 2. The polyester multilayer film according to claim 1, wherein the skin layer has a C3 cyclic oligomer concentration of 7,000 ppm or less.

4. 2. The polyester multi-layer film according to claim 1, wherein the organic phosphorus compound has a molecular weight of 400 to 900 g / mol.

5. 2. The polyester multilayer film according to claim 1, wherein the organic phosphorus compound acts like a C3 cyclic oligomer to maintain an equilibrium state at high temperatures and suppress the formation of C3 cyclic oligomers inside the film.

6. 2. The polyester multilayer film according to claim 1, wherein the organic phosphorus compound is at least one selected from the group consisting of triaryl phosphite and trialkyl phosphite.

7. 2. The polyester multilayer film according to claim 1, wherein the organic phosphorus compound in the skin layer is prepared by compounding the organic phosphorus compound with the polyester resin separately to prepare masterbatch chips and then adding the masterbatch chips.

8. 2. The polyester multilayer film according to claim 1, wherein the thickness ratio of the substrate layer to the skin layer is 6:1 to 13:1.

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