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 its transparency even when used in a high-temperature process 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 have excellent durability and excellent stability of physical properties over a wide temperature range from low to high temperatures. Compared to other polymer resins, they 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, 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 is steadily increasing.

[0003] However, polyester films include processes that require high temperatures of over 100°C in the manufacture of displays, etc., which causes low molecular weight oligomers inside the polyester film to dissolve onto the surface. The dissolved 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 the polyester film, a method of reducing the oligomer content through solid-state polymerization during polymerization of the polyester film has been widely used. However, the complicated solid-state polymerization process not only increases costs, but also makes it difficult to completely block oligomers because oligomers are continuously produced in the film at high temperatures and elute to the surface.

[0005] As another method for preventing oligomers from eluting to the surface, a technology has been reported in which a highly heat-resistant polymer such as polyethylene naphthalate (PEN) is used as in Korean Patent Registration No. 10-1733193, or a copolymer in which a monomer such as isophthalate or cyclohexyldimethanol is used instead of terephthalic acid or ethylene glycol is used as in Korean Patent Registration No. 10-1842456, thereby suppressing the elution of oligomers. However, this not only has problems in that it causes changes in the physical properties of the polyester, but also has problems in that it is ultimately impossible to suppress the elution to the surface.

[0006] In addition, a technology has been reported that attempts to control the elution of oligomers 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 because it requires expensive catalysts, and it is not possible to completely prevent the elution of oligomers.

[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 has the problem of increasing costs during production because it requires complex manufacturing processes due to issues such as suppressing the activity of catalysts essential for the polymerization reaction as a method of manufacturing polyester itself, and has no relation to preventing the generation of oligomers or 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 and migration to the surface even in a high-temperature treatment process, thereby maintaining the transparency and visibility of the film for a long period of time, and a manufacturing method thereof.

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

[0010] The object of the present invention is to provide a method for producing a polyester-based electrophotographic film comprising: a base layer containing polyester; and a skin layer located on at least one side of the base layer and containing polyester and an organic phosphorus compound, the method satisfying 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 may be one that 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 is 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 by compounding the organic phosphorus compound separately from the polyester resin to prepare a master batch chip and then adding the master batch chip.

[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 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 a machine direction and then cooling the resulting film to room temperature to produce a uniaxially stretched polyester multilayer film, and biaxially stretching the uniaxially stretched polyester multilayer film in a transverse direction and then heat treating the resulting biaxially stretched polyester multilayer film.

[0019] Preferably, the polyester multilayer film can exhibit a haze change of less than 0.5 when heat treated at 150° C. for 30 minutes. Effect 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 a high temperature process and preventing migration to the surface.

[0021] In addition, the method for producing a polyester multi-layer film according to an embodiment of the present invention has an advantage of being highly economical since it can produce a polyester multi-layer film at low production costs.

[0022] In addition, the polyester multilayer film according to an embodiment of the present invention can maintain excellent product quality in display, semiconductor and various other industries due to the above-mentioned advantages, and can prevent contamination in processes due to oligomer scattering, thereby significantly improving productivity.

[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 having ordinary skill in the art to which the present invention pertains from the following description. [Brief description 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. [Diagram 2] 1 is a flow chart showing a method for producing a polyester multilayer film according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings, in which: FIG. 1 is a block diagram of a semiconductor device according to an embodiment of the present invention;

[0026] In the drawings, thicknesses are exaggerated to clearly depict 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, plate, etc., is described as being "on" another part, this includes not only when it is "directly on" the other part, but also when 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 skilled 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 may 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 of FIG. 1, a configuration in which the skin layer 120 is formed on both sides of the substrate layer 110 is shown, but the present invention 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 base layer 110 and contains a polyester and an organic phosphorus compound. The inventors have tried to suppress the generation of oligomers in 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 a cyclic C3 substance, which is a representative oligomer, only to the skin layer during the polyester film formation process. In particular, the inventors have confirmed that an organic phosphorus compound has excellent performance among substances having a structure similar to a cyclic C3 substance, and have completed the present invention.

[0031] It is preferable that the polyester multilayer film according to an embodiment of the present invention 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 mathematical formulas 1 and 2, the technical effects of the present invention can be achieved.

[0034] The C3 cyclic oligomer concentration in the film shown in Formula 1 and the haze change before and after heat treatment are generally closely related, and in order for the film to maintain excellent optical properties such as transparency even after heat treatment, it is necessary that the change in haze according to Formula 2 is less than 0.5% (Formula 2). If the change in haze exceeds 0.5%, the transparency decreases and the optical properties of the film deteriorate. To achieve this, there may be a method of making the oligomer concentration in the film extremely small, but in order to do so, there is a problem that a solid-phase polymerization resin must be used in the entire film as described below, which reduces the economic efficiency. Therefore, in the present invention, a general liquid chip is used in the base layer 110, and a low oligomer resin and a C3 analogue compound are used only in the skin layer 120, so that the oligomer concentration in the entire film is not significantly different from that when a liquid polymerization resin is used, and the generation and migration of oligomers can be suppressed.

[0035] In addition, when the correlation coefficient between the C3 cyclic oligomer and the haze change in the formula 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 the haze change is too large to ensure transparency. In addition, when the value of the formula 1 is 1 or more, the oligomer content in the film is 15,000 ppm or more, which is not preferable because it reaches a saturated state.

[0036] In general, polyester films contain a certain amount of oligomers when they are made by polymerizing the raw resin. The amount of oligomers contained in polyester films varies depending on the polymerization method, but typically contains about 0.5 to 2% of typical C3 cyclic oligomers. When the polyester film is heated above its glass transition temperature, these oligomers migrate to the surface, and the oligomers that migrate to the surface are highly crystalline and exist on the surface as crystalline foreign bodies of several micrometers (μm). These oligomer crystals deteriorate the optical properties of the film, such as reducing its transparency, and may scatter during the film processing process, contaminating other objects and products and reducing productivity.

[0037] In addition, various methods are used to suppress the phenomenon of oligomer migration to the surface of polyester films. The most representative method is to produce a resin through solid-state polymerization to minimize the initial oligomer content and minimize migration to the surface even when temperature is applied. In particular, resins produced through solid-state polymerization have a large molecular weight and a high intrinsic viscosity, which is known to have the effect of suppressing oligomer migration. Another method is to use a copolymer resin containing a certain portion of a monomer such as cyclocarboxyl dimethanol or isosorbide in the polyester molecule, which relatively increases the amorphous region of the polymer, allowing it to contain more oligomers and suppress migration to the surface. Other methods include a method of using expensive germanium (Ge) or titanium (Ti) catalysts to minimize oligomer generation under high temperature conditions during melt extrusion, and a method of using a laminated film that uses a resin with a high intrinsic viscosity in the skin layer to suppress 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 the extent to which the methods can fundamentally prevent the elution of the produced oligomers.

[0038] In contrast, the polyester multilayer film according to an embodiment of the present invention suppresses oligomer formation and inhibits migration of the generated oligomers to the surface. In the polyester multilayer film according to an embodiment of the present invention, polyester molecules reach an equilibrium state with oligomers at a certain temperature, and in particular, at a temperature of 300°C at which polyester films are made, the equilibrium constant value reaches about 0.019, and a stable equilibrium state is maintained. It was found that the generation of additional oligomers can be inhibited by adding an appropriate amount of a compound having a similar structure to a representative C3 cyclic oligomer in consideration of the concentration in the equilibrium state. In particular, when an organic phosphorus compound is used in a predetermined content among the substances having a similar structure, the equilibrium state is maintained even in a high-temperature process reaching 250 to 300°C during film formation, and the generation of oligomers is remarkably inhibited, and elution to the surface does not proceed even when used for a long time at high temperature. These results indicate that even if low-oligomer resin and phosphorus analogues 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] It is preferable that the polyester multilayer film according to an embodiment of the present invention 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 the formula 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 of 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 not preferable.

[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 is preferable to include at least one selected from Tri(2,4-di-ter-butylphenyl)phosphite, Bis(2,4-di-ter-butylphenyl)pentaerithritol diphosphite, Bis(2,6-di-ter-butylphenyl)pentaerythritol diphite, and Bis(2,4-dicumylphenil)pentaerythritol diphite. In addition, phosphates, phosphonates, phosphinates, etc. can be used, but these lack structural flexibility and are not compatible with polyesters, so they do not exhibit sufficient oligomer suppression effects.

[0045] In the present invention, the organic phosphorus compound preferably has a molecular weight of 400 g / mol or more and 900 g / mol or less. If the molecular weight of the organic phosphorus compound is outside this range, the similarity to the C3 cyclic oligomer molecule is insufficient, and sufficient equilibrium maintenance effect cannot be expected. In this way, the organic phosphorus compound contained in the skin layer 120 acts like a C3 cyclic oligomer to maintain an equilibrium state at high temperatures, thereby suppressing the generation 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 these ratios are made larger, the generation of oligomers is suppressed, but the suppression of migration may be insufficient, whereas if the ratios are made smaller, the effect of suppressing migration 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 producing a polyester multilayer film according to an embodiment of the present invention includes a step of compounding an organic phosphorus compound separately from a polyester resin to produce masterbatch chips (S101), a step of blending the produced masterbatch chips with a polyester resin and then adding the blend to a polyester film and co-extruding the blend onto the polyester film to produce a polyethylene terephthalate sheet (S102), a step of uniaxially stretching the polyethylene terephthalate sheet in the machine direction and then cooling the same to room temperature to produce a uniaxially stretched polyester multilayer film (S103), and a step of biaxially stretching the uniaxially stretched polyester multilayer film in the transverse direction and then heat treating the same to produce a biaxially stretched polyester multilayer film (S104).

[0049] First, the step of preparing masterbatch chips (S101) is a step of feeding an organic phosphorus compound through a side feeder while melt-extruding polyester resin, cooling and drying the extruded resin, and cutting it into a predetermined size to prepare chips. The organic phosphorus compound contained in the skin layer of the present invention is characterized in that it is compounded separately from the polyester resin to prepare masterbatch chips, and blended with the polyester resin during film formation and then fed. In this case, the organic phosphorus compound may be fed at the polymerization stage of preparing the resin, but if the required amount is fed, there is a problem that the activity of the catalyst essential for polymerization is inhibited, lengthening the reaction time, and there is a problem that the organic phosphorus compound changes during the long reaction, which is not preferable.

[0050] Next, in the step of producing a polyethylene terephthalate sheet (S102), 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 the step (S101) is used as a skin layer raw material, which is then 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 the step (S103) of uniaxially stretching the polyethylene terephthalate sheet in the machine direction and then cooling it to room temperature to produce a uniaxially stretched polyester multilayer film, the produced polyethylene terephthalate sheet 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 the step S104 of 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, the uniaxially stretched polyester multilayer film is stretched 3 to 5 times in the transverse direction (TD), and then 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 necessary.

[0054] The polyester multi-layer 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 maintains its transparency since it does not undergo oligomer migration at high temperatures, and also suppresses oligomer scattering during the production process, thereby 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 to more specifically illustrate the present invention and are not intended to limit the scope of the present invention.

[0057] [Production Example] (1) Manufacture 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 ester exchange reaction was essentially complete. This reaction mixture was transferred to a polycondensation tank, and antimony trioxide was added to carry out a polycondensation reaction for 4 hours, yielding polyester resin A 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 under nitrogen conditions at a temperature of 215°C to obtain a 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 above-mentioned polyester resin production, the polyester resin was put into a twin extruder and melt extruded at 280°C while adding a phosphorus compound, Tri(2,4-di-tert-butylphenyl)phosphite (manufactured by BASF) through a side feeder. At this time, the master batch contains 5 parts by weight of the phosphorus compound per 100 parts by weight of polyester. Next, the polymer strand discharged from the extruder was cooled in water and air-dried, and then cut to a predetermined size to produce master batch C.

[0060] (3) Polyester multi-layer 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 and master batch C were blended to a preset phosphorus compound content and fed into a co-extruder. The feeder block was adjusted to adjust the ratio of the main layer to the skin layer to 6:1 to 15:1, and extrusion was performed, followed by quenching and solidification on a casting drum with a surface temperature of 20°C to produce a polyethylene terephthalate sheet with a thickness of 8,000 μm. 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. Thereafter, the temperature was sequentially increased in the tenter, preheated, dried, and then stretched 3 to 5 times in the transverse direction TD. Thereafter, heat treatment was performed at 230 to 250°C in the tenter, and heat setting was performed at 200 to 220°C to produce a biaxially stretched multilayer film.

[0061] [Example] [Example 1] Polyester resin A, polyester resin B and master batch C were blended to have a phosphorus content of 250 ppm and used as a raw material for the skin layer, and polyester resin A was used for the substrate layer, which was co-extruded at 280°C to produce a 50 μm-thick polyester multilayer film in which skin layers were formed on both sides of the substrate layer. The thickness ratio of the skin layer, substrate layer and skin 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 (hereinafter the same).

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

[0063] [Example 3] A polyester multi-layer film was prepared 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 master batch C were blended to give 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 substrate 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 master batch C were blended to give 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 (master batch C was excluded) and used as the raw materials for the skin layer and the 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 phosphorus content in the skin layer was adjusted to 80 ppm.

[0071] [Comparative Example 7] A polyester multi-layer film was produced in the same manner as in Example 1, except that the phosphorus content in the skin layer was adjusted to 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. 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 mixed solution of chloroform / 1,1,1,3,3-hexafluoro-2-propanol (mixing ratio: 3 / 2), then reprecipitated with chloroform / methanol (mixing ratio: 2 / 1), filtered through a 5 μm filter, and the solvent was removed. The 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] Here, the amount of oligomers 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 Polaris 5 Si 100*4.6mm, the temperature was 40℃, the mobile phase was hexane / 1,4-dioxane (6:4), the flow rate was 1.0ml / min, and the detector was 240nm UV.

[0075] (2) Phosphorus (P) content measurement After freeze-pulverizing 1 g of sample, 1 mm-thick thin specimens were prepared by hand pressing, 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 rectangular 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 in film haze (ΔH) before and after heat treatment was determined by subtracting the haze before heat treatment (Hi) from the haze after heat treatment (Hf).

[0079] (4) Analysis of surface condition using an optical microscope The prepared film sample was fixed on a rectangular metal support and placed in a 150℃ oven for 30 minutes to analyze the surface of a 2*2mm area for the high-temperature heat-treated film and the film before and after heat treatment using an optical microscope (Olympus, MX50LT-1273MH). Depending on whether blooming caused by oligomer crystals of microns or more was observed, the following was determined.

[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 haze change before and after heat treatment, and therefore not only exhibits excellent transparency but also prevents the migration of oligomers to the surface. As a result, a polyester multilayer film that can fundamentally prevent productivity loss due to external scattering can be provided.

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

[0084] In addition, in Comparative Example 7, where the value of Equation 3 exceeds 15,000, it is found that blooming phenomenon 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 belong to 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 formula 1 and formula 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.

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.

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

4. 2. The polyester multilayer 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 inhibits 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 of the skin layer is prepared by compounding the organic phosphorus compound with the polyester resin separately to prepare master batch chips and adding the master batch chips.

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

1.

9. compounding an organic phosphorus compound separately from a polyester resin to prepare a master batch chip; The masterbatch chips thus prepared are blended with a polyester resin and then co-extruded onto a polyester film to prepare a polyethylene terephthalate sheet; uniaxially stretching the polyethylene terephthalate sheet in a machine direction and then cooling the sheet to room temperature to prepare a uniaxially stretched polyester multilayer film; the uniaxially stretched polyester multi-layer film is biaxially stretched in the transverse direction and then heat-treated to produce a biaxially stretched polyester multi-layer film; A method for producing a polyester multilayer film comprising the steps of:

10. The method for producing a polyester multilayer film according to claim 9, wherein the polyester multilayer film has a haze change of less than 0.5 when heat-treated at 150° C. for 30 minutes.

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