TWI600541B - Laminated polyester film and preparation method thereof - Google Patents

Description

Laminated polyester film and method for producing the same Field of invention

The present invention relates to a laminated polyester film and a process for the preparation thereof.

Background of the invention

In general, polyester films can be used as various industrial materials such as display devices, beverage or medical containers, packaging materials, sheets, and molding due to their excellent dimensional stability, thickness uniformity, and optical transparency. car parts. First, plastic films are widely used in the device components of the most important plasma display panels (PDPs) and liquid crystal displays (LCDs) in the display industry. The polyester film is used when optical transparency is required, and the polyimide film is used when durability and heat resistance are required. In particular, the biaxially stretched polyester film has excellent dimensional stability, thickness uniformity, and optical transparency, and thus is used in many industries mentioned above.

The base film used in displays requires various characteristics such as stability in process, transparency, scratch resistance, planarity, and light transmission because they are required to satisfy optical specificity in displays.

In terms of flatness, this is one of the characteristics required for the base film, and poor flatness of the base film causes unevenness in the base film manufacturing process. The force causes a slip phenomenon, thus causing scratches on the surface of the film, and in the subsequent coating process, a partially defective coating is produced. Therefore, the value of the final product may be impaired.

Regarding scratch resistance, this is another property required for the base film. Scratches on the base film may cause black spots and become electrical defects due to uneven coating of the area, or in subsequent processes, such as The problem of uneven coating is caused during the hard coating process; therefore, scratch resistance is one of the characteristics required for the base film. Scratches on the base film can also cause optical defects, which can adversely affect the quality and yield of the final product.

The properties required for these base films are intended to increase brightness, thermal stability, manufacturing characteristics, and the like. Therefore, deterioration in transparency, scratch resistance, flatness, and total light transmittance may cause brightness reduction and reliability problems, and lower yield. This reduction in brightness, in turn, requires a stronger source of light to achieve the required amount of light, which is quite detrimental to the base film used in displays, due to the need to increase material costs and higher power consumption for greater power. Light source.

Therefore, various studies to increase the brightness of the base film are currently underway. For example, Japanese Patent No. 4288607 discloses a biaxially stretched polyester film comprising a base film and a coating, wherein the coating is a light diffusing layer comprising an adhesive and particles. Japanese Patent No. 5023741 discloses a polyester film having fine bubbles in which a subsequent coating contains a light stabilizer and an antioxidant, and is laminated on the surface of the film.

However, the above conventional techniques still have problems such as formation of voids during film stretching.

Summary of invention

SUMMARY OF THE INVENTION An object of the present invention is to provide a laminated polyester film having excellent characteristics, that is, a high penetration and low amount of oligomer leakage characteristics, and a preparation method thereof.

According to an embodiment, the present invention provides a laminated polyester film comprising: a first layer comprising a first polyester resin prepared using a first catalyst; and a second layer comprising using a first catalyst different from the first catalyst A second polyester resin prepared by a catalyst.

According to another embodiment, the present invention provides a method of preparing a laminated polyester film, the method comprising: (a) preparing a first polyester resin using a ruthenium compound as a catalyst; (b) preparing a second polyester resin using ruthenium The compound acts as a catalyst; (c) coextruding the prepared polyester resin to prepare an unstretched laminate; and (d) stretching the unstretched laminate.

According to another embodiment, the present invention provides a laminated polyester film comprising: a first outer layer comprising a first polyethylene terephthalate; an intermediate layer comprising a second polyethylene terephthalate, juxtaposed Under the first outer layer; and a second outer layer comprising a third polyethylene terephthalate and disposed under the intermediate layer, wherein the first outer layer and the second outer layer each have a thickness of 7 μm or more, and After about 3 hours at about 150 ° C heat treatment, the haze changes by 1% or less.

Detailed description of the preferred embodiment Advantage of the invention

The laminated polyester film of the present invention is formed by a suitable combination of a first polyester layer prepared by using a ruthenium compound as a catalyst and a second polyester layer prepared by using a ruthenium compound as a catalyst. It has improved brightness and low amount of oligomer leakage characteristics. Therefore, due to these characteristics, the laminated polyester film can be effectively used as a base film of ITO which is recently used for a touch panel, a base film of a silver nanowire, a base film of a metal mesh, and a substrate for a prism of an LCD backlight unit. membrane.

The laminated polyester film of the embodiment of the invention can also be used as a heat-resistant protective film to protect a conductive film having a conductive layer structure, such as ITO, silver nanowire or metal mesh, laminated on the base layer.

Best mode for implementing inventions

The laminated polyester film of one embodiment comprises at least two layers. The laminated polyester film may comprise a first layer comprising a first polyester resin prepared using a first catalyst; and a second layer comprising a second polyester resin prepared using a second catalyst different from the first catalyst . The laminated polyester film may further comprise a third layer comprising a third polyester resin prepared using the first catalyst, wherein the second layer is interposed between the first layer and the third layer.

In one embodiment, the laminated polyester film may have a structure in which the first layer and the second layer are sequentially laminated.

In an embodiment, the laminated polyester film may have a structure The first layer, the second layer and the third layer are sequentially layered.

That is, the second layer is an intermediate layer, and the first layer and the third layer are outer layers. The first layer is placed over the second layer and the third layer is placed below the second layer.

The first layer, the second layer and the third layer may be formed by coextrusion. That is, the first layer can be directly bonded to the upper surface of the second layer, and the third layer can be directly bonded to the lower surface of the second layer.

The first layer comprises a first polyester resin. In particular, the first layer contains a first polyester resin as a main component. The first layer may comprise a first polyester resin in an amount of about 85 wt% or more. More particularly, the first layer may comprise a first polyester resin in an amount of about 95% by weight or more. More particularly, the first layer may comprise a first polyester resin in an amount of about 99% by weight or more.

The second layer comprises a second polyester resin. In particular, the second layer contains a second polyester resin as a main component. The second layer may comprise a second polyester resin in an amount of about 85 wt% or more. More particularly, the second layer may comprise a second polyester resin in an amount of about 95% by weight or more. More particularly, the second layer may comprise a second polyester resin in an amount of about 99% by weight or more.

The third layer comprises a third polyester resin. In particular, the third layer contains a third polyester resin as a main component. The third layer may comprise a third polyester resin in an amount of about 85 wt% or more. More particularly, the third layer may comprise a third polyester resin in an amount of about 95% by weight or more. More particularly, the third layer may comprise a third polyester resin in an amount of about 99% by weight or more.

The first polyester resin contains a diol component and a dicarboxylic acid component. In particular, the first polyester resin can be completely composed of the diol component and the dicarboxylic acid. Composition. More specifically, the first polyester resin may comprise a diol component and a dicarboxylic acid component in an amount of about 95 mol% or more.

The first polyester resin may be formed of a first catalyst. That is, the diol component and the dicarboxylic acid component may be polymerized using a first catalyst after the ester conversion reaction to form a first polyester resin.

Specific examples of the diol component may include ethylene glycol, 1,4-cyclohexanedimethanol, 1,3-propanediol, 1,2-octanediol, 1,3-octanediol, 2,3-butyl Glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol (neopentylene glycol), 2-butyl 2-ethyl-1,3-propanediol, 2,2-diethyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 3-methyl- 1,5-pentanediol, 1,1-dimethyl-1,5-pentanediol, and mixtures thereof. The diol component may include ethylene glycol in an amount of about 80 mol% or more. In particular, the diol component may comprise ethylene glycol in an amount of about 90 mol% or more. More particularly, the diol component may comprise ethylene glycol in an amount of about 95 mol% or more. More particularly, the diol component may comprise ethylene glycol in an amount of about 99 mol% or more.

Aromatic dicarboxylic acids such as terephthalic acid, dimethyl terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, and phthalic acid; aliphatic dicarboxylic acids such as adipic acid, sebacic acid, hydrazine A diacid, a decane dicarboxylic acid; an alicyclic dicarboxylic acid; and an esterified compound thereof may be used singly or in combination of two or more kinds as a dicarboxylic acid component. In particular, the dicarboxylic acid component may comprise about 80 mol% or more of an aromatic dicarboxylic acid. More particularly, the dicarboxylic acid component may comprise 80 mol% or more of terephthalic acid. More specifically, the dicarboxylic acid component may contain 90 mol% or more of terephthalic acid. More specifically, the dicarboxyl The acid component may contain 95 mol% or more of terephthalic acid. More particularly, the dicarboxylic acid component may comprise 99 mol% or more of terephthalic acid.

The first polyester resin may be polyethylene terephthalate.

The first polyester resin has an intrinsic viscosity of from about 0.65 dl/g to about 0.75 dl/g. In particular, the first polyester resin has an intrinsic viscosity of from about 0.67 dl/g to about 0.73 dl/g. More particularly, the first polyester resin has an intrinsic viscosity of from about 0.68 dl/g to about 0.72 dl/g.

The first catalyst may be a ruthenium compound and/or a titanium compound.

Examples of the ruthenium compound may preferably include (a) amorphous ruthenium oxide, (b) finely crystalline ruthenium oxide, (c) bismuth oxide dissolved in a diol in the presence of an alkali metal, an alkaline earth metal or a compound thereof. And (d) a solution obtained by dissolving cerium oxide in water. Cerium oxide can be used as a representative lanthanide catalyst, and the amount of the lanthanide catalyst is from 10 to 10,000 ppm, preferably from 10 to 1,000 ppm, relative to the polyethylene terephthalate constituting the polyester film. When the amount of the catalyst is too low, the polycondensation reaction of the polyester resin cannot be carried out efficiently, or the color of the polyester resin may deteriorate. When the amount of the catalyst is too large, there is an economic disadvantage because there is no further catalytic efficiency, or it may become an impurity in the polyester resin.

The titanium compound may be, for example, selected from the group consisting of titanium oxide, titanium chelate, tetra-n-propyl titanate, tetraisopropyl titanate, tetra-n-butyl titanate, tetraisobutyl titanate, and titanate. At least one of the group consisting of isopropyl esters. In terms of its content, it is preferred that the titanium element content is from 1 to 50 ppm based on the weight of the polymer to control discoloration caused by side reactions.

In the first polyester resin, the first catalyst can maintain the content From about 10 ppm to about 200 ppm, especially from about 20 ppm to about 150 ppm.

The second polyester resin contains a diol component and a dicarboxylic acid component. In particular, the second polyester resin may consist entirely of a diol component and a dicarboxylic acid component. More specifically, the second polyester resin may contain a diol component and a dicarboxylic acid in an amount of about 95 mol% or more.

The second polyester resin may be formed of a second catalyst. That is, the diol component and the dicarboxylic acid component may be polymerized using the second catalyst after the ester conversion reaction to form the second polyester resin.

The second polyester resin may be polyethylene terephthalate.

The second polyester resin has an intrinsic viscosity of from about 0.55 dl/g to about 0.65 dl/g. In particular, the second polyester resin has an intrinsic viscosity of from about 0.57 dl/g to about 0.63 dl/g. More particularly, the second polyester resin has an intrinsic viscosity of from about 0.58 dl/g to about 0.62 dl/g.

The second catalyst is different from the first catalyst. In particular, the second catalyst can be a ruthenium compound. Examples of the cerium compound may include cerium oxide (Sb ₂ O ₃ ), cerium acetate (Sb(CH ₃ COO) ₃ ), and the like. In the second polyester resin, the second catalyst can be maintained at a level of from about 40 ppm to about 250 ppm, particularly from about 50 ppm to about 180 ppm.

The third polyester resin contains a diol component and a dicarboxylic acid component. In particular, the third polyester resin may consist entirely of a diol component and a dicarboxylic acid component. More particularly, the third polyester resin may comprise a diol component and a dicarboxylic acid in an amount of about 95 mol% or more.

The third polyester resin can be formed using the first catalyst. In particular, the first catalyst may be a ruthenium compound and/or a titanium compound.

That is, the diol component and the dicarboxylic acid component may be polymerized using the first catalyst after the ester conversion reaction to form a third polyester resin.

The third polyester resin may be polyethylene terephthalate. In this case, the third polyester resin may have an intrinsic viscosity of from about 0.65 dl/g to about 0.75 dl/g. In particular, the third polyester resin may have an intrinsic viscosity of from about 0.67 dl/g to about 0.73 dl/g. More particularly, the third polyester resin may have an intrinsic viscosity of from about 0.68 dl/g to about 0.72 dl/g.

The third polyester resin may be substantially the same as the first polyester resin.

The viscosity of the first polyester resin is higher than the viscosity of the second polyester resin. The difference between the viscosity of the first polyester resin and the viscosity of the second polyester resin is from about 0.05 dl/g to about 0.15 dl/g. The viscosity of the third polyester resin is higher than the viscosity of the second polyester resin. The difference between the viscosity of the third polyester resin and the viscosity of the second polyester resin is from about 0.05 dl/g to about 0.15 dl/g.

In the same direction, the refractive index of the first layer is lower than the refractive index of the second layer. The difference between the refractive index of the first layer and the refractive index of the second layer is from about 0.001 to about 0.01. In the same direction, the refractive index of the third layer is lower than the refractive index of the second layer. The difference between the refractive index of the third layer and the refractive index of the second layer is from about 0.001 to about 0.01.

The refractive index of the first layer in the lateral direction is from about 1.670 to about 1.680.

The refractive index of the second layer in the lateral direction is from about 1.671 to about 1.690.

In the lateral direction, the refractive index of the third layer is from about 1.670 to about 1.680.

The first layer has a thickness of about 7 μm or more. In particular, the first layer has a thickness of from about 7 [mu]m to about 50 [mu]m. More particularly, the first layer has a thickness of from about 10 [mu]m to about 40 [mu]m. More particularly, the first layer has a thickness of from about 15 [mu]m to about 30 [mu]m.

The thickness ratio of the first layer to the second layer is from about 1:20 to about 1:3. In particular, the thickness ratio of the first layer to the second layer is from about 1:15 to about 1:5. More specifically, the thickness ratio of the first layer to the second layer is from about 1:13 to about 1:8.

The thickness of the first layer is about 10% or more of the total thickness. In particular, the thickness of the first layer is from about 10% to about 40% of the total thickness. More particularly, the thickness of the first layer is from about 11% to about 40% of the total thickness. More particularly, the thickness of the first layer is from about 15% to about 35% of the total thickness. More particularly, the thickness of the first layer is from about 20% to about 33% of the total thickness.

The thickness of the third layer is about 7 μm or more. In particular, the third layer has a thickness of from about 7 μm to about 50 μm. More particularly, the third layer has a thickness of from about 10 [mu]m to about 40 [mu]m. More particularly, the third layer has a thickness of from about 15 [mu]m to about 30 [mu]m.

The thickness ratio of the third layer to the second layer is from about 1:20 to about 1:3. In particular, the thickness ratio of the third layer to the second layer is from about 1:15 to about 1:5. More particularly, the thickness ratio of the third layer to the second layer is from about 1:13 to about 1:8.

The thickness of the first layer is about 10% or more of the total thickness. In particular, the thickness of the third layer is from about 10% to about 40% of the total thickness. More special The thickness of the third layer is from about 11% to about 40% of the total thickness. More particularly, the thickness of the third layer is from about 15% to about 35% of the total thickness. More particularly, the thickness of the third layer is from about 20% to about 33% of the total thickness.

The total thickness of the laminated polyester film of one embodiment, especially the sum of the first layer, the second layer and the third layer is from about 70 μm to about 1,000 μm. In particular, the total thickness of the laminated polyester film of one embodiment, especially the sum of the first layer, the second layer and the third layer is from about 80 μm to about 300 μm. More specifically, the total thickness of the laminated polyester film of one embodiment, especially the sum of the first layer, the second layer and the third layer is from about 100 μm to about 300 μm. More particularly, the total thickness of the laminated polyester film of one embodiment, especially the sum of the first layer, the second layer and the third layer is from about 120 μm to about 270 μm.

The laminated polyester film of one embodiment has a penetration of from about 85% to about 95%. In particular, the laminated polyester film of one embodiment has a penetration of from about 88% to about 93%.

The laminated polyester film of one embodiment has a haze of about 2% or less. In particular, the laminated polyester film of one embodiment has a haze of about 1.5% or less. More specifically, the laminated polyester film of one embodiment has a haze of about 1.1% or less. More specifically, the laminated polyester film of one embodiment has a haze of about 0.7% or less.

The first layer and the third layer have a low amount of oligomer leakage characteristics. In addition, the first layer and the third layer can be used to prevent leakage of the oligomer contained in the second layer.

Therefore, the laminated polyester film of one embodiment has a low amount of oligomer leakage characteristics as a whole. That is, in one embodiment, by layering The amount of oligomers leaking out of the ester film is relatively small.

Therefore, the laminated polyester film of one embodiment is maintained at about 130 ° C to about 150 ° C, and the haze of the laminated polyester film of this embodiment is increased by about 2% or less after 3 minutes to about 3 hours. This type of heat treatment before comparison.

In particular, in the embodiment, the laminated polyester film is maintained at about 150 ° C for about 3 hours, and the haze of the laminated polyester film of this embodiment is increased by about 2% or less, and before such heat treatment. Compared. Namely, under the above conditions, the haze of the laminated polyester film of this example was changed by about 2% or less after heat treatment. Specifically, under the above conditions, after the heat treatment of the laminated polyester film of this example, the haze was changed by about 1.5% or less. More specifically, under the above conditions, the haze of the laminated polyester film of this example was changed by heat treatment to about 1% or less. More specifically, under the above conditions, after the heat treatment of the laminated polyester film of this example, the haze was changed by about 0.7% or less. More specifically, under the above conditions, if the laminated polyester film of one embodiment is subjected to heat treatment, the haze changes by about 0.5% or less.

The laminated polyester film of one embodiment may further comprise a primer layer as a second outer layer. The primer layer may include inorganic particles having a diameter of from about 100 nm to about 150 nm as a sliding agent. Further, the primer layer may comprise urethane-, acrylate- and polyester-based resins.

The laminated polyester film of one embodiment can be prepared according to the following method, comprising the steps of: (a) preparing a first polyester resin using a ruthenium compound as a catalyst; (b) preparing a second polyester resin using a ruthenium compound as a catalyst; (c) co-extruding the prepared polyester resin to prepare an unstretched laminate; and (d) stretching the unstretched laminate. After step (b), more include A third polyester resin was prepared using a ruthenium compound as a catalyst.

In particular, a polyester resin is first prepared. The first polyester resin is prepared by esterifying a diol with a dicarboxylic acid. In this case, the first polyester resin is prepared using the first catalyst.

The first polyester resin can be prepared by a solid phase polymerization method. For example, the first polyester resin, such as polyethylene terephthalate, having a number average molecular weight of about 20,000, is prepared by melt polymerization. The melt-polymerized first polyester resin is formed into a pellet which is crystallized at 150 to 200 ° C and then polymerized at 200 to 230 ° C.

Thereafter, the second polyester resin is prepared. The second polyester resin is prepared by esterifying a diol with a dicarboxylic acid. In this case, the second polyester resin is prepared using a second catalyst.

After the above steps, the third polyester resin can be further prepared. The third polyester resin is prepared by esterifying a diol with a dicarboxylic acid. In this case, the third polyester resin is prepared using the first catalyst.

The third polyester resin is prepared by the solid phase polymerization method described above and is substantially the same as the first polyester resin.

At least two of the first polyester resin, the second polyester resin, and the third polyester resin are melt extruded to prepare an unstretched sheet. The polyester resin is coextruded into different layers. Thus, the unstretched sheet comprises at least two layers. In particular, the unstretched sheet may have a structure in which a first layer mainly comprising a first polyester resin, a second layer mainly comprising a second polyester resin, and a third layer mainly comprising a third polyester resin Layers are layered sequentially.

Thereafter, the unstretched sheet is longitudinal (ie machine direction) and transverse The direction (i.e., the tenter direction) is stretched and thermally cured to prepare a polyester film.

Preferably, the melt extrusion is carried out at a temperature of Tm + 30 to Tm + 60 °C. If the temperature of the extruder during melt extrusion is less than Tm + 30 ° C, the extrudate will not melt enough, and its viscosity will increase, thus reducing the yield. On the other hand, if the temperature is higher than Tm + 60 ° C, the molecular weight of the resin will decrease due to thermal decomposition, and the oligomer will cause problems. Further, it is preferably cooled at 30 ° C or lower, preferably 15 to 30 ° C.

The unstretched sheet can be uniaxially or biaxially stretched.

The unstretched sheet can be stretched at a suitable draw ratio in the machine direction and the transverse direction. For example, the unstretched sheet is stretched 2.5 to 6 times in the longitudinal direction and 2.5 to 6 times in the transverse direction. In particular, the unstretched sheet is stretched 2.7 to 4 times in the longitudinal direction and 2.7 to 4 times in the transverse direction. The longitudinal and transverse systems are perpendicular to each other.

The stretching is preferably carried out at a temperature of Tg + 5 to Tg + 50 °C. Although a lower Tg will increase the stretchability, a low Tg may cause cracking. In particular, in order to prevent brittleness, it is preferred to carry out the stretching in a temperature range of Tg + 10 to Tg + 40 °C.

After the thermal curing is initiated, the film will relax in the machine direction and the cross direction, and preferably the heat curing is carried out at a temperature in the range of 120 to 260 °C.

The laminated polyester film of one embodiment may be formed using at least two polyester resins which are respectively formed using different catalysts. In particular, the layers of the laminated polyester film of one embodiment may each comprise a different polyester resin.

In this case, the first layer may comprise a first polyester resin prepared using a ruthenium compound as a catalyst, and the second layer may comprise a second polymerization An ester resin which is prepared using a ruthenium compound as a catalyst.

Therefore, the first layer and the second layer have different characteristics from each other. The first polyester resin may have a high viscosity and a low refractive index. The second polyester resin may have a low viscosity and a high refractive index.

The first layer and the second layer have different characteristics from each other and are laminated. In particular, in a laminated polyester film having three or more layers, the first polyester resin may form a skin layer, and the second polyester resin may form a core layer. That is, the second polyester resin in the laminated unstretched sheet may be interposed between the first polyester resin and the third polyester resin.

The laminated polyester film of one embodiment comprises a first outer layer comprising a first polyethylene terephthalate; an intermediate layer comprising a second polyethylene terephthalate and disposed under the first outer layer; A second outer layer comprising a third polyethylene terephthalate and disposed below the intermediate layer. In this case, the first outer layer and the second outer layer each have a thickness of 7 μm or more, and after about 3 hours at a heat treatment of about 150 ° C, the haze changes by 1%.

The first polyethylene terephthalate comprises a first catalyst, and the second polyethylene terephthalate comprises a second catalyst different from the first catalyst, the third polyethylene terephthalate comprises a different a third catalyst of the second catalyst. The first catalyst and the third catalyst may each independently be a ruthenium compound and/or a titanium compound, and the second catalyst may be a ruthenium compound.

In the laminated polyester film, the intermediate layer, the first outer layer, and the second outer layer are formed by co-extrusion.

The intermediate layer, the first outer layer, and the second outer layer have a total thickness of from about 70 μm to about 1,000 μm, particularly from about 80 μm to about 300 μm. The The sum of the thickness of the intermediate layer, the first outer layer, and the second outer layer is from about 100 μm to about 300 μm. More particularly, the intermediate layer, the first outer layer, and the second outer layer have a total thickness of from about 120 μm to about 270 μm.

The thickness of the first outer layer is 10% or more of the total thickness, particularly about 10% to 40% of the total thickness. More particularly, the first outer layer has a thickness from 11% to about 40% of the total thickness. More particularly, the thickness of the first outer layer is from about 15% to about 35% of the total thickness. More particularly, the thickness of the first outer layer is from 20% to about 33% of the total thickness.

The thickness of the second outer layer is 10% or more of the total thickness, particularly about 10% to 40% of the total thickness. More particularly, the thickness of the second outer layer is from about 11% to about 40% of the total thickness. More particularly, the thickness of the second outer layer is from about 15% to about 35% of the total thickness. More particularly, the thickness of the second outer layer is from about 20% to about 33% of the total thickness.

The laminated polyester film of one embodiment is prepared in accordance with the above, and thus has improved brightness and low amount of oligomer leakage characteristics.

Therefore, the laminated polyester film of one embodiment can be used as a base film of ITO, a base film for a prism of an LCD backlight unit, a base film of a silver nanowire, or a base film of a metal mesh.

Invention mode

Hereinafter, the present invention is explained in detail by way of examples. The following examples are intended to further illustrate the invention and not to limit its scope.

Providing polyethylene terephthalate 1 (prepared using ruthenium oxide catalyst IV: 0.7 dl/g, Tg: 82.5 ° C, Tm: 253 ° C, residual ruthenium content in the wafer: 30 ppm), and polyethylene terephthalate 2 (made by SKC, making The ruthenium oxide catalyst IV was used: 0.615 dl/g). Thereafter, the polyester film was prepared using the following procedure, and the structure of each layer is shown in Table 1.

Each of polyethylene terephthalate (PET) was melt extruded through an extruder at about 280 ° C, followed by cooling at about 20 ° C by a casting roll to prepare an unstretched sheet. In Examples 1 to 7 and Comparative Example 2, unstretched sheets were prepared using a co-extrusion process. In Comparative Example 1, polyethylene terephthalate was extruded into a single layer to prepare an unstretched sheet. The resulting unstretched sheet was immediately preheated to 60 ° C, then stretched to about 3 times in the longitudinal direction at about 110 ° C, and stretched to about 3 times in the transverse direction. Thereafter, the stretched sheet was thermally cured at about 120 ° C for about 30 seconds. A polyethylene terephthalate film was prepared in accordance therewith.

Example 1: Haze change

In order to examine the haze of the films obtained from the respective examples and comparative examples, the films of Examples 1 and 2 and Comparative Example 1 were placed in an oven at 130 ° C for about 3 minutes, and the films of Examples 3 to 7 and Comparative Example 2 were placed. The oven was heated at 150 ° C for about 3 minutes, after which the haze change was measured. The results are shown in Tables 2 and 3, respectively.

Example 2: Heat Shrinkage Change

In order to detect the heat shrinkage change of the film, the film obtained by each of the examples and the comparative examples was placed in an oven at 150 ° C for about 30 minutes, and then the heat shrinkage in the longitudinal direction and the transverse direction was measured. The results are shown in Table 2 below.

As shown in Tables 2 and 3, it was found that the three-layer laminated films of Examples 1 and 2 had a large difference in refractive index and had good heat shrinkage characteristics and moldability, compared with the single layer film of Comparative Example 1, and Examples 3 to The haze of the laminated film of 7 was changed after the heat treatment, and was smaller than the haze of the laminated film of Comparative Example 2. Therefore, it is expected that the laminated film of the present invention can be effectively used as a base film of ITO, a base film for a prism of an LCD backlight unit, a base film of a silver nanowire, or a base film of a metal mesh.

Claims (18)

A laminated polyester film comprising: a first layer comprising a first polyester resin prepared using a first catalyst; and a second layer comprising a second polyester resin prepared using a second catalyst different from the first catalyst And a third layer comprising a third polyester resin prepared using the first catalyst, wherein the second layer is interposed between the first layer and the third layer. The laminated polyester film of claim 1, wherein the first catalyst is selected from the group consisting of a ruthenium compound or a titanium compound, and the second catalyst is a ruthenium compound. The laminated polyester film of claim 1, wherein the viscosity of the first polyester resin is higher than the viscosity of the second polyester resin. The laminated polyester film of claim 1, wherein the refractive index of the first layer is lower than the refractive index of the second layer. The laminated polyester film of claim 1, wherein the first polyester resin has an intrinsic viscosity (IV) of from 0.65 to 0.75, and the second polyester resin has an intrinsic viscosity (IV) of from 0.55 to 0.65. The laminated polyester film of claim 1, wherein the third polyester resin is prepared using a ruthenium compound or a titanium compound as a catalyst. The laminated polyester film of claim 1, wherein the first polyester resin, the second polyester resin, and the third polyester resin are polyethylene terephthalate. The laminated polyester film of claim 1, wherein the thickness ratio of the first layer to the second layer is 1:20 to 1:3, and the thickness ratio of the third layer to the second layer is 1:20 to 1:3. The laminated polyester film of claim 1, wherein the first layer, the second layer and the third layer have a total thickness of from 100 μm to 300 μm. The laminated polyester film according to any one of claims 1 to 9, wherein the laminated polyester film is used as a base film of ITO, a base film for a prism of an LCD backlight unit, a base film of a silver nanowire, or a metal The base film of the net. A method of preparing a laminated polyester film, the method comprising: (a) preparing a first polyester resin using a ruthenium compound as a catalyst; (b) preparing a second polyester resin using a ruthenium compound as a catalyst; (c) co-extruding Preparing the prepared polyester resin to prepare an unstretched laminate; and (d) stretching the unstretched laminate, the method further comprising, after the step (b), preparing the catalyst using the ruthenium compound as a catalyst The third polyester resin. The method of claim 11, wherein the second polyester resin in the unstretched laminate is interposed between the first polyester resin and the third polyester resin. The method of claim 11, wherein the unstretched laminated sheet is stretched 2.5 to 6 times in the longitudinal direction and 2.5 to 6 times in the transverse direction. The method of claim 13, wherein the sheet stretched in the step (d) is thermally cured at a temperature of from 120 ° C to 260 ° C. A laminated polyester film comprising: a first outer layer comprising a first polyethylene terephthalate; an intermediate layer comprising a second polyethylene terephthalate and disposed under the first outer layer; a second outer layer comprising a third polyethylene terephthalate and disposed under the intermediate layer, wherein the first outer layer and the second outer layer each have a thickness of 7 μm or more, and about 3 at a heat treatment of about 150 ° C After an hour, the haze changes by 1% or less. The laminated polyester film of claim 15, wherein the first polyethylene terephthalate comprises a first catalyst, and the second polyethylene terephthalate comprises a second catalyst different from the first catalyst, The third polyethylene terephthalate comprises a third catalyst different from the second catalyst, and the intermediate layer, the first outer layer, and the second outer layer are formed by coextrusion. The laminated polyester film of claim 16, wherein the first outer layer has a thickness of 10% or more of the total thickness, and the second outer layer has a thickness of 10% or more of the total thickness. The laminated polyester film of claim 17, wherein the intermediate layer, the first outer layer, and the second outer layer have a total thickness of from about 70 μm to about 1,000 μm.