JP2007185898A - Biaxially oriented polyester film and its manufacturing process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film which is thick, has a low haze and is appropriate for the optical use and the like in which transparency and toughness is required by solving problems that the haze becomes high so that the transparency becomes defective if the thickness is increased in order to enhance the toughness of the film in a conventional method although an optical film having high transparency in addition to toughness is required recently since a display device comes to big. <P>SOLUTION: The biaxially oriented polyester film has a thickness of not less than 200 μm and not greater than 500 μm, and a haze of not greater than 2.5%. The preferred lowering crystallization temperature (Tmc) is not less than 150°C and not greater than 200°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a biaxially stretched polyester film that is effectively used for applications requiring transparency and high rigidity, such as optical films, surface protective materials, electrical insulating materials, and mold release materials, especially optical films and surface protective materials. And a manufacturing method thereof.

  Biaxially stretched polyester films are widely used for various applications because of their excellent physical properties and excellent productivity. In recent years, with the growth of the IT field, antireflection films for displays, Applications for optical films such as base plates for liquid crystal backlight diffusion plates, touch panels, and process paper for optical members such as optical disks and liquid crystal retardation plates are increasing.

  Among such optical films, display antireflection films and liquid crystal members demand high transparency because they require visibility, and TV screens are becoming larger. A high film is required. In order to increase the rigidity of the polyester film, it is effective to increase the thickness. However, because of the crystallinity of the polyester, for example, in the process of extruding molten polyester and cooling and solidifying it into a sheet, the sheet The thicker the film, the slower the cooling rate and the more crystallization proceeds, resulting in lower transparency. It is very difficult to satisfy both the haze and thickness of the polyester film and satisfy the characteristics required for optical use.

  Conventionally, polyester films having low haze have been proposed (see Patent Document 1 and Patent Document 2). However, these documents do not describe a specific method for producing a low-haze and thick film, and a thick polyester as required in recent optical applications such as the scope of the present invention. The film cannot be provided in a low haze.

In addition, a method has been proposed in which a polyester film melt-extruded as a photographic polyester film is cooled and solidified using an auxiliary cooling device on a cooling drum (see Patent Document 3). However, with this proposal, the thickness of the film is thin and the rigidity required for the current optical film cannot be obtained.
JP 2005-263853 A JP 2004-359938 A JP-A-9-204004

  In these conventional methods, it is difficult to achieve both the film thickness range and the haze range in the present invention, and the transparency and rigidity of the film required for optical applications cannot be provided. An object of the present invention is to provide a highly rigid biaxially stretched polyester film having a low haze and a large thickness.

  In order to solve this problem, the biaxially stretched polyester film of the present invention has the following configuration.

  1. A biaxially stretched polyester film, wherein the biaxially stretched polyester film has a thickness of 200 μm to 500 μm and a haze of 2.5% or less.

  2. 2. The biaxially stretched polyester film as described in 1 above, wherein the temperature-falling crystallization temperature (Tmc) is 150 ° C. or higher and 200 ° C. or lower.

  3. 3. The biaxially stretched polyester film as described in 1 or 2 above, wherein an unoriented polyester film having a thickness of 1500 μm or more and 3000 μm or less is biaxially stretched.

  4). The above-mentioned 1 to 3 characterized by biaxially stretching an unstretched polyester film having a thickness of 1500 μm or more and 3000 μm or less obtained at a film cooling rate of 5 ° C./second or more in a casting step for obtaining an unstretched polyester film. The manufacturing method of the biaxially stretched polyester film in any one of.

  5. In the casting process for obtaining an unstretched polyester film, when air is blown on the opposite side of the drum surface of the cast film to cool it, the air temperature is 7 ° C. or more and 15 ° C. or less, and the air speed is 15 m / sec or more. 5. The method for producing a biaxially stretched polyester film as described in 4 above, which is 30 m / second or less.

  According to the present invention, a highly rigid biaxially oriented polyester film having a low haze and a large thickness can be obtained, and in particular, it has a high rigidity that can cope with the transparency and large screen required for an optical film. A biaxially stretched polyester film can be obtained.

  Hereinafter, the biaxially stretched polyester film of the present invention and the production method thereof will be described in more detail.

  The biaxially stretched polyester film of the present invention is made of polyester, has a film thickness of 200 μm or more and 500 μm or less, and a haze of 2.5% or less.

  The polyesters used in the present invention are polyesters mainly composed of dibasic acid and glycol. Examples of dibasic acids include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, and cyclohexanedicarboxylic acid. And adipic acid, succinic acid, sebacic acid and dimer acid. Examples of the glycol component include ethylene glycol, propylene glycol, tetramethylene glycol, polyethylene glycol, naphthalene diol, and cyclohexane dimethanol.

  Specific examples of polyesters include, for example, polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate and poly-1,4-cyclohexanedimethylene terephthalate. Particularly, polyethylene terephthalate and polyethylene -2,6-naphthalate is particularly preferably used because it is excellent in physical properties such as mechanical strength and dimensional stability and also in productivity.

  In addition, the polyesters used in the present invention may be one kind of those listed above, a copolymer of two or more polyesters, a mixture of two or more polyesters, or the like. Various additives can be added to these polyesters depending on the purpose. For example, inert particles such as colloidal silica, alumina, calcium carbonate, organic silicone, and polydivinylbenzenesulfonic acid can be added for imparting slipperiness. Further, an antistatic agent or an antioxidant may be added. However, in order to make the haze of the polyester film within the scope of the present invention, it is desirable not to add these additives, and even if added, it is preferable to add only a very small amount of a particle having a small particle diameter.

  The thickness of the biaxially stretched polyester film of the present invention needs to be 200 μm or more and 500 μm or less, preferably 220 μm or more and 420 μm or less, more preferably 250 μm or more and 360 μm or less. When the thickness of the polyester film is less than 200 μm, the rigidity of the film is insufficient, so that the stiffness required for the optical film is insufficient, and the display cannot be enlarged. On the other hand, when the thickness of the polyester film exceeds 500 μm, it is too thick, so that it becomes difficult to handle, for example, it floats on the transport roll in the user's processing step and cannot be coated.

  The haze of the biaxially stretched polyester film of the present invention needs to be 2.5% or less, preferably 2.2% or less, and more preferably 1.8% or less. When haze exceeds 2.5%, the visibility and brightness as an optical film are deteriorated.

  The temperature-falling crystallization temperature (Tmc) of the biaxially stretched polyester film of the present invention is preferably 150 ° C. or higher and 200 ° C. or lower, more preferably 150 ° C. or higher and 190 ° C. or lower, and further preferably 160 ° C. or higher and 190 ° C. or lower. It is below ℃. By setting the temperature drop crystallization temperature (Tmc) within the above range, the biaxially stretched polyester film of the present invention having low haze and high rigidity can be suitably formed.

  Next, a method for biaxially stretching an unstretched polyester film will be described.

  As a biaxial stretching method, an unstretched polyester film is stretched in the longitudinal direction or the width direction, and then sequentially biaxially stretched in the direction perpendicular to the previous stretching direction, or once in the longitudinal direction and the width direction. A method such as simultaneous biaxial stretching is used.

  An example of the sequential biaxial stretching method is as follows. First, polyesters (thermoplastic resin) are melted using an extruder, extruded from a die having a slit-like discharge port, and cooled on a cooling roll to be amorphous. A quality unstretched polyester film is obtained (hereinafter, this process may be referred to as a casting process). At this time, in order to improve the adhesion of the unstretched polyester film to the roll, the electrostatic application casting method of applying static electricity to the unstretched polyester film using a metal wire or tape electrode is a casting process. This is a preferable means for increasing the cooling efficiency and obtaining the film haze of the present invention.

  Further, the thickness of the obtained unstretched polyester film is preferably 1500 μm or more and 3000 μm or less, more preferably 1600 μm or more and 2900 μm or less, and further preferably 1700 μm or more and 2800 μm or less. When the thickness of the unstretched polyester film is less than 1500 μm, even if the thickness of the biaxially stretched polyester film is within the range of the present invention, a satisfactory film rigidity cannot be obtained, and when the thickness exceeds the above range of 3000 μm, Since cooling cannot be performed sufficiently, crystallization of polyester proceeds and it becomes difficult to obtain a film having a haze within the range of the present invention.

  In the above casting process, the method of cooling by blowing low temperature gas or liquid on the surface opposite to the contact surface of the cooling roll is effective for increasing the cooling rate of the polyesters and suppressing crystallization. . In particular, the method of blowing low temperature air is advantageous in terms of simplicity of equipment. When blowing low-temperature air, the air temperature is preferably 7 ° C. or higher and 15 ° C. or lower, more preferably 7 ° C. or higher and 12 ° C. or lower in order to increase the cooling rate.

  Further, the speed of the air blown in the casting step is preferably 15 m / second or more and 30 m / second or less, more preferably 17 m / second or more and 28 m / second or less. If the air speed is less than the above range, the heat transfer coefficient between the air and the film will be insufficient and the cooling speed will be slow. If the air speed exceeds 30 m / sec, the film between the die and the roll will be vibrated by the air, resulting in uneven thickness. become.

  As a method of blowing air, it is known that air is uniformly blown from nozzles having slits of several mm to several tens of mm in the width direction of the film forming process and nozzles having diameters of several mm to several tens of mm arranged randomly. Can be used. In addition, passing the heat exchanger using cooling water or liquid nitrogen before supplying air to the nozzle is effective for setting the temperature of the air within the above range.

  Further, the cooling rate of the film in the casting step is preferably 5 ° C./second or more, more preferably 6 ° C./second or more, and further preferably, in order to reduce haze and obtain good transparency for optics. Is 7 ° C./second or more.

  The sheet-like unstretched polyester film obtained in this way is brought into contact with several rolls controlled in temperature as described above, or heated by radiant heat from a heater such as an infrared heater. It is heated to a temperature equal to or higher than the glass transition temperature of the kind, and stretched in the longitudinal direction using the peripheral speed difference of the preceding and following rolls. The draw ratio at this time is preferably about 2 to 8 times. Stretching may be performed in one step or stepwise in two or more steps, but the one-step stretching at a relatively low temperature is less likely to cause defects such as scratches on the film surface and adhesive marks on the roll, This is advantageous in obtaining the visibility required for optical use.

  The uniaxially stretched polyester film stretched in the longitudinal direction is once cooled and then stretched in the width direction by a stenter oven. The uniaxially stretched polyester film is gripped by the clip running on the rail in the stenter oven, heated again above the glass transition temperature of the polyester in the oven, and with the spread of the rail on which the clip runs, Stretched in the width direction. The draw ratio in the width direction is preferably about 2 to 5 times.

  The polyester film stretched biaxially in the longitudinal direction and the width direction is subsequently heat treated. The heat treatment may be performed in the same stenter oven following the stretching in the width direction, or may be performed in an oven different from the stenter oven that has been stretched in the width direction. When the polyester is polyethylene terephthalate, the heat treatment can be performed at a relatively high temperature of preferably about 180 ° C to 250 ° C. By performing the heat treatment, the dimensional stability when exposed to a high temperature during use as a subsequent processing step or final product is improved. Further, after the heat treatment, the dimensional stability can be further improved by relaxing the biaxially stretched polyester film by several% in the longitudinal direction and / or the width direction.

  Next, a method for simultaneously biaxially stretching an unstretched polyester film will be described. In the case of simultaneous biaxial stretching, as in the case of sequential biaxial stretching, a sheet-like unstretched polyester film is obtained, and this sheet-shaped unstretched polyester film is used as a power source for clip travel by using a linear motor to unstretched polyester. The film is stretched simultaneously in the longitudinal direction and the width direction by a stenter oven that can arbitrarily change the speed of the clip in the running direction of the film. Unstretched by gripping a sheet-like unstretched polyester film with a clip, heating it in the oven above the glass transition temperature of polyester, and gradually increasing the speed of the clip while gradually expanding the travel path of the clip The polyester film is stretched in the longitudinal direction and the width direction simultaneously. The polyester film biaxially stretched by such a method is subjected to heat treatment and relaxation treatment as in the case of sequential biaxial stretching.

  The biaxially stretched polyester film stretched biaxially is once wound up as an intermediate product by a wide-width winder and then cut into a necessary width and length by a slitter.

  Various coating materials can be applied to the biaxially stretched polyester film for the purpose of imparting adhesiveness to one or both sides of the film. For example, in the case of an optical polyester film, an antireflection layer, a hard coat layer, a mat layer for light diffusion, and the like can be formed on one side or both sides of a biaxially stretched polyester film.

  As the coating material, aqueous solutions or aqueous dispersions of polyester, acrylic polymer, polyamide, polyurethane and the like are preferably used. As a coating method of the coating material, methods such as a roll coater, a gravure coater, and a bar coater can be used. The coating process can be selected as needed before stretching the biaxially stretched polyester film, during the stretching process, or after performing stretching / heat treatment, but the film can be used before stretching or during the stretching process. If it is performed within the manufacturing process, the process can be simplified. Performing a treatment such as corona discharge treatment on the biaxially stretched polyester film before applying the coating material is an effective means for improving the wettability of the film surface and stabilizing the application.

  The biaxially stretched polyester film of the present invention is suitably used for optical films, surface protective materials, electrical insulating materials, release materials and the like.

[Methods for measuring physical properties and methods for evaluating effects]
The physical property measurement method and the effect evaluation method in the present invention are as follows.

(A) Haze of biaxially stretched polyester film According to JIS K 7105 (1981), total light transmittance (%) and scattered light transmittance (%) were determined for any 10 points of the biaxially stretched polyester film. The average value of these 10 points was defined as total light transmittance Tt (%) and scattered light transmittance Td (%), and haze (Td / Tt × 100) (%) was calculated.

(B) Temperature drop crystallization temperature (Tmc)
Using a differential scanning calorimeter TAC7DX, it was melted at a temperature of 300 ° C. for 5 minutes and then cooled at a cooling rate of 20 ° C./min, and the temperature of the exothermic peak accompanying crystallization was read as Tmc.

(C) Thickness of polyester film The biaxially stretched polyester film was measured and averaged at 10 points using a digital micrometer μ mate M-30 manufactured by Sony Precision Technology Co., Ltd. About the unstretched polyester film, the running film after the casting process was measured and averaged at both ends 60 mm inside from the film end surface using a thickness gauge HR-1 manufactured by Ozaki Seisakusho.

(D) Unstretched polyester film cooling rate in the casting step Using a non-contact thermometer IT2-80 manufactured by Keyence, the unstretched polyester film temperature just before the ground contact between the base and the roll at the emissivity of 0.95 is 5 in the width direction. The difference between the average temperature measured by spot measurement and the unstretched polyester film temperature immediately after leaving the cooling roll at five points in the width direction is Ta (° C.), and the cooling roll and the unstretched polyester film are in contact with each other. The time (ta) in which the cooling roll and the unstretched polyester film are in contact with each other was determined from the distance and the roll speed, and Ta / ta was defined as the cooling rate (° C./second).

(E) Air temperature and speed An anemo master anemometer Model 6112 manufactured by Kanomax was used to measure the wind speed and temperature at the tip of the air supply nozzle of the air blowing device. For all nozzles, 5 points in the width direction were measured and averaged.

(F) Rigidity of film Five points were measured and averaged by the method of JIS P8125 (1976).

  Hereinafter, specific embodiments of the present invention will be described based on examples, but the present invention is not limited to these examples.

Example 1
After the polyethylene terephthalate having an intrinsic viscosity of 0.65 substantially free of inert particles is dried at a temperature of 180 ° C. for 5 hours under a reduced pressure of 3 torr, it is put into an extruder and melted at a temperature of 280 ° C. After filtration through a filter with a filtration accuracy of 8 μm, the product was extruded from the die into a sheet. The extruded sheet was brought into close contact with a cooling roll having a surface temperature of 20 ° C. by an electrostatic application casting method. At this time, cooling air was blown onto the film on the opposite surface of the cooling roll by blowing air at a temperature of 10 ° C. and a wind speed of 21 m / sec using an air blowing device to obtain an unstretched polyethylene terephthalate film having a thickness of 2700 μm. At this time, the film cooling rate in the casting step was 6.9 ° C./second. This unstretched polyethylene terephthalate film was stretched 2.5 times at a temperature of 90 ° C. by a longitudinal stretching machine comprising a roll group, and cooled to obtain a uniaxially stretched polyethylene terephthalate film. After applying a coating liquid having the following composition on both sides of this uniaxially stretched polyethylene terephthalate film using a bar coater, the film was stretched in the width direction of 3.1 times at a temperature of 100 ° C. by a stenter oven. In the oven, heat treatment was carried out at a temperature of 228 ° C. for 20 seconds to obtain a 350 μm-thick biaxially stretched polyethylene terephthalate film. The biaxially stretched polyethylene terephthalate film had a haze of 1.2%, a Tmc of 172 ° C., and a rigidity of 12.3 mN · m.

[Prescription of coating liquid]
A coating solution prepared by adding 5 parts by weight of the following melamine-based crosslinking agent solution and 1 part by weight of colloidal silica particles having an average particle size of 0.1 μm to 100 parts by weight of the following polyester resin emulsion was used.

[Polyester resin]
A polyester copolymer emulsion obtained by copolymerizing an acid component and a diol component having the following composition.
<Acid component>
・ Terephthalic acid 50mol%
・ Isophthalic acid 40mol%
・ 10 mol% of 5-sodium sulfoisophthalic acid
<Diol component>
・ Ethylene glycol 96mol%
・ Neopentyl glycol 3 mol%
-Diethylene glycol 1 mol%.

[Melamine crosslinking agent]
A solution obtained by diluting imino group-type methylated melamine with a mixed solvent of isopropyl alcohol and water (10/90 (weight ratio)).

(Examples 2-7, Comparative Examples 1-4)
Biaxial stretching in the same manner as in Example 1 except that the thickness of the biaxially stretched polyester film, the thickness of the unstretched polyester film, the film cooling rate in the casting process, the air temperature, and the air speed were changed as shown in Table 1. A polyethylene terephthalate film was obtained. Table 1 shows the haze, Tmc, and stiffness of the biaxially stretched polyethylene terephthalate films of Examples 2 to 7 and Comparative Examples 1 to 4.

  In Comparative Example 4, high rigidity is obtained, but since the rigidity is too high, for example, it floats on the conveying roll during the user's processing process, and the coating cannot be made uniform, or the film snakes. Therefore, it cannot be said that it is a preferable characteristic.

  The biaxially stretched polyester film of the present invention is effectively used for applications such as optical films, surface protective materials, electrical insulating materials and mold release materials, especially for applications requiring transparency and high rigidity such as optical films and surface protective materials. Therefore, it is possible to meet the demand for larger screens and higher brightness of optical displays.

Claims (5)

  A biaxially stretched polyester film, wherein the biaxially stretched polyester film has a thickness of 200 μm to 500 μm and a haze of 2.5% or less.   The biaxially oriented polyester film according to claim 1, wherein the temperature-falling crystallization temperature (Tmc) is 150 ° C or higher and 200 ° C or lower.   The biaxially stretched polyester film according to claim 1 or 2, wherein an unstretched polyester film having a thickness of 1500 µm to 3000 µm is biaxially stretched.   The unoriented polyester film having a thickness of 1500 μm or more and 3000 μm or less obtained by biaxial stretching is obtained by setting the film cooling rate in the casting step for obtaining an unstretched polyester film to 5 ° C./second or more. 4. A method for producing a biaxially stretched polyester film according to any one of 3 above.   In the casting process for obtaining an unstretched polyester film, when air is blown on the opposite side of the drum surface of the cast film to cool it, the air temperature is 7 ° C. or more and 15 ° C. or less, and the air speed is 15 m / The method for producing a biaxially stretched polyester film according to claim 4, wherein it is at least 30 seconds / second.