JP2011148202A - Biaxially oriented polyethylene terephthalate film for polarizing plate release - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially oriented polyethylene terephthalate film for polarizing plate release, which is excellent in contrast property and breakage resistance and has excellent optical axis accuracy and thermal dimensional stability. <P>SOLUTION: The biaxially oriented polyethylene terephthalate film satisfies the following requirements (1)-(4): (1) a heat shrinkage percentage when heated at 150°C for 30 minutes is 2.0% or less in both longitudinal and width direction; (2) the difference between the heat shrinkage percentage in the longitudinal direction and that in the width direction when heated at 150°C for 30 minutes is 1.0% or less; (3) an MOR value measured by a microwave transmission type molecular orientation meter is 1.80 to 2.10; and (4) the variation of orientation angle in the film width direction is 3.0°-5.0° per 500 mm. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate. Specifically, the present invention relates to a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate having excellent inspectability and workability.

  A polarizing plate which is a constituent member of a liquid crystal display device is a rolled state in a state of a laminate in which an adhesive layer is provided on one surface, and a release film for protecting the polarizing plate is laminated on the adhesive layer. Wrapped around and transported or stored. A biaxially stretched polyethylene terephthalate film is widely used as a base material for such a polarizing plate release film from the viewpoint of strength function and cost. A biaxially stretched polyethylene terephthalate film having a structure in which linear polymers are oriented is a birefringent body that optically exhibits birefringence. Therefore, the biaxially stretched polyethylene terephthalate film has two optical axes perpendicular to the direction parallel to and perpendicular to the molecular orientation direction. Therefore, when laminated with the optical axis of the substrate of the release film inclined with respect to the optical axis of the polarizing plate, it exhibits transmitted light and interference color when placed under crossed Nicols, and visually inspects defects. It becomes a factor to inhibit. Accordingly, the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate is required to have excellent optical axis accuracy (Patent Document 1).

  Such a biaxially stretched polyethylene terephthalate resin film for releasing a polarizing plate is stretched in the longitudinal direction between rolls having a difference in rotational speed, and then in the width direction with the end of the film held in a tenter. Manufactured by drawing and heat setting. In this case, since the distortion of the optical axis, that is, the distortion of the orientation main axis, is larger at the end than at the center of the film due to the bowing phenomenon, only a limited product at the center can be used for this purpose.

  As a method for reducing the bowing of the film, a method of performing heat treatment after once cooling to below the glass transition temperature of the polyester after stretching in the width direction, a method of providing a nip roll after stretching in the width direction, and dividing the heat treatment chamber into a plurality of zones in stages. There have been proposed a method of raising the temperature, a method of providing a temperature distribution in the width direction to lead to the heat treatment zone, a method of increasing the draw ratio in the width direction, and the like. (Patent Documents 2 to 5)

  In the polarizing plate inspection process, a polarizing plate release film and a protective film are prepared by subjecting the film with reduced distortion of the optical axis by the above-described method to a release processing or an adhesive processing, and this is used as a polarizing plate. Laminate and visually check the quality of the polarizing plate under crossed Nicols.

JP 2002-40249 A JP 2008-246665A JP 2008-163263 A JP 2005-14545 A Japanese Patent Laid-Open No. 2004-18588

  Currently, the film proposed in the above patent document is used for polarizing plate inspection. However, the increase in the screen size of displays has progressed dramatically, and 37-inch and 42-inch large-screen displays have penetrated the market. Furthermore, due to the demand for higher definition, it has become necessary to reliably recognize foreign matter and defects at a level that has not been regarded as a problem in the past. In order to satisfy such demands for the development of large-screen displays and the improvement of inspection accuracy, it has become necessary to reduce the distortion of the optical axis over a longer width.

  On the other hand, automation of inspection is progressing from the viewpoint of productivity, and automatic attachment of a release film to a polarizing plate, identification of fine foreign matters and defects by image analysis, and the like are performed. In this case, the conventional polarizing plate release film has a low degree of contrast, and fine foreign matters and defects may be overlooked during image processing. Further, when the processing speed is improved by mechanization of the pasting work, the release film may be torn.

  Furthermore, the processing speed has been improved due to an increase in the size of the display, and in addition, the processing speed has been dramatically increased in terms of productivity. For this reason, the heat treatment temperature in post-processing such as adhesive processing is higher. This trend is expected to continue further. In order to cope with this, it was considered that a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate having excellent thermal dimensional stability at high temperatures is required. In order to obtain a film with good thermal dimensional stability, it is desirable to increase the heat setting temperature in the film manufacturing process, but when the heat setting temperature is increased, the bowing phenomenon becomes more prominent and the film has high optical axis accuracy. It becomes difficult to obtain. That is, it has been difficult to achieve both a long-width optical axis accuracy and a high degree of thermal dimensional stability.

  The purpose of the present invention is to eliminate the above-mentioned problems, have good contrast and tear resistance, have excellent optical axis accuracy and high thermal dimensional stability, and have excellent inspection properties and workability. An object of the present invention is to provide a biaxially stretched polyethylene terephthalate film.

The first invention that can solve the above-mentioned problems is a biaxially stretched polyethylene terephthalate film that is a biaxially stretched polyethylene terephthalate film that satisfies the following structural requirements (1) to (4). .
(1) Thermal contraction rate when heated at 150 ° C. for 30 minutes is 2.0% or less in both the longitudinal direction and the width direction. (2) Thermal contraction rate in the longitudinal direction and heat in the width direction when heated at 150 ° C. for 30 minutes. Difference in shrinkage is 1.0% or less (3) MOR value measured by microwave transmission type molecular orientation meter is 1.80 to 2.10
(4) The amount of change in the orientation angle in the film width direction is 3.0 ° to 5.0 ° per 500 mm.
A second invention is the biaxially stretched polyethylene terephthalate film for releasing a polarizing plate that further satisfies the following requirements (5) and (6).
(5) Total light transmittance is 85% or more. (6) Three-dimensional center plane average surface roughness SRa of the film surface is 0.020 μm or more and 0.035 μm or less. The third invention is the following requirements (7) and (8 Is a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate satisfying
(7) Perform lateral stretching at a temperature of 4.2 to 4.8 times in the range of 120 ° C to 140 ° C. (8) Perform heat fixing at a temperature of 220 to 230 ° C.

  The biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention has good contrast and tear resistance, and has excellent optical axis accuracy and thermal dimensional stability. Therefore, post-processing at a high temperature is possible, and it is suitable for high-precision inspection, particularly automatic inspection, of polarizing plates for large screen applications.

  The film of the present invention is made of a polyethylene terephthalate resin. Here, the polyethylene terephthalate-based resin contains ethylene glycol and terephthalic acid as main components. Other dicarboxylic acid components and glycol components may be copolymerized as long as the object of the present invention is not impaired. Examples of the other dicarboxylic acid components include isophthalic acid, p-β-oxyethoxybenzoic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-dicarboxybenzophenone, bis- (4-carboxyphenylethane), adipine Examples include acid, sebacic acid, 5-sodium sulfoisophthalic acid, cyclohexane-1,4-dicarboxylic acid, and the like. Examples of the other glycol component include propylene glycol, butanediol, neopentyl glycol, diethylene glycol, bisphenol A and other ethylene oxide adducts, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and the like. In addition, oxycarboxylic acid components such as p-oxybenzoic acid can also be used.

  As a polymerization method of such polyethylene terephthalate (hereinafter simply referred to as PET), a direct polymerization method in which terephthalic acid and ethylene glycol and, if necessary, other dicarboxylic acid component and diol component are directly reacted, and dimethyl terephthalate are used. Any production method such as a transesterification method in which an ester (including a methyl ester of another dicarboxylic acid as necessary) and ethylene glycol (including another diol component as necessary) are transesterified can be used. .

  The intrinsic viscosity of the polyethylene terephthalate is preferably in the range of 0.45 to 0.70. If the intrinsic viscosity is lower than 0.45, the film is easily torn, and if it is higher than 0.70, the increase in filtration pressure is increased and high-precision filtration becomes difficult.

  The three-dimensional center plane average surface roughness (SRa) on at least one surface, preferably both surfaces of the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention is preferably 0.020 μm or more and 0.035 μm or less. . In responding to automation and speeding up of the pasting work, when SRa is 0.020 μm or more, suitable workability (slidability) can be achieved. The lower limit of SRa is more preferably 0.021 μm or more, and further preferably 0.022 μm or more. On the other hand, it is preferable to provide surface irregularities from the viewpoint of workability. However, if the SRa exceeds 0.035 μm, light is diffused on the film surface due to the surface irregularities, and sufficient contrast is obtained in the polarizing plate inspection step. I can't. The upper limit of SRa is more preferably 0.032 μm or less, and further preferably 0.030 μm or less.

  In order to make SRa within the above range, it is preferable to add fine particles to the polyethylene terephthalate resin in the present invention. Thereby, workability | operativity (slidability) of a film becomes favorable and it is suitable for automation of a sticking process. Any fine particles can be selected, and examples include inorganic particles such as silica, calcium carbonate, barium sulfate, calcium sulfate, alumina, kaolinite, talc, and other organic particles. In particular, from the viewpoint of transparency and contrast, silica particles, particularly amorphous silica, having a refractive index relatively close to that of the resin component are suitable.

  On the other hand, in order to obtain high transparency and contrast, it is preferable that the polyethylene terephthalate resin constituting the film does not substantially contain particles. Here, “substantially contain no particles” means, for example, in the case of inorganic particles, when the inorganic element is quantified by fluorescent X-ray analysis, it is 50 ppm or less, preferably 10 ppm or less, particularly preferably the detection limit or less. Means content.

  As a preferred embodiment of the present invention, in order to obtain good transparency and stable workability (particularly surface friction characteristics), a film having a multilayer structure and a polyethylene terephthalate layer containing fine particles only in the surface layer is used. You can also. As such a film, polyethylene terephthalate having a multilayer structure (A / B / A) in which a surface layer (A layer) containing inert particles is laminated on both sides of a central layer (B layer) by a coextrusion method. It is preferable to use a film. The layers constituting the front and back surface layers may be the same or different, but in order to maintain the flatness of the film, the polyethylene terephthalate resin of the front and back surface layers may have the same configuration. desirable.

  The average particle size of the fine particles contained in the surface layer is preferably 1 to 10 μm, more preferably 1.5 to 7 μm, and still more preferably 2 to 5 μm. If the average particle diameter of the fine particles is 1.0 μm or more, it is preferable because the surface can be provided with a concavo-convex structure suitable for imparting easy slipperiness. On the other hand, if the average particle size of the fine particles is 10 μm or less, it is preferable because high transparency and contrast are maintained. The content of inert particles in the surface layer is desirably 0.005 to 0.1% by mass, and preferably 0.008 to 0.07%. If the content of the fine particles is 0.005% by mass or more, it is preferable because the surface layer surface can be provided with a concavo-convex structure suitable for imparting slipperiness. On the other hand, if the content of fine particles is 0.1% by mass or less, high transparency and contrast properties are maintained, which is preferable.

In addition, the measurement of the average particle diameter of said particle | grain is performed with the following method.
Take a picture of the particles with a scanning electron microscope (SEM) and at a magnification such that the size of one smallest particle is 2-5 mm, the maximum diameter of 300-500 particles (between the two most distant points) Distance) is measured, and the average value is taken as the average particle diameter.

  In the case of the (A / B / A) two-type three-layer structure containing particles in the surface layer, the thickness of the surface layer on one side is preferably 0.5 to 10 μm, and more preferably 1 to 5 μm. When the thickness of the surface layer exceeds the above range, the haze of the film may be lowered.

  The thickness of the film of the present invention is not particularly limited and is arbitrary, but is preferably 9 to 300 μm, more preferably 12 to 100 μm, and still more preferably 14 to 50 μm. If the thickness exceeds 300 μm, there is a problem in cost, retardation is increased, and visibility in crossed Nicol tends to be lowered. On the other hand, when the thickness is less than 9 μm, the mechanical properties are deteriorated and the function as a protective film cannot be performed.

  Corresponding to higher definition, the inspection accuracy of polarizing plate is improved. Therefore, it is desirable that the release film has high transparency. Therefore, the light transmittance in the biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention is preferably 85% or more, more preferably 87% or more, and further preferably 89% or more. In order to improve the inspection accuracy in the polarizing plate inspection process, the higher the light transmittance, the better, but in the polyethylene terephthalate film containing particles for slipperiness, 100% light transmittance is technically It is difficult to achieve, and the practical upper limit is 91%.

  It is desirable to obtain a high contrast in order to make the presence of foreign matter stand out in automatic inspection and to obtain more inspection accuracy. Therefore, the haze in the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention is preferably 5% or less, more preferably 4% or less, and further preferably 3% or less. More preferably, it is as follows. In order to obtain a high contrast, a lower haze is preferable, but 1% seems to be the lower limit in a polyethylene terephthalate film containing particles for slipperiness. In addition, the said haze and total light transmittance can be measured using a turbidimeter according to JIS-K7105.

  Conventionally, as in Patent Document 1, in order to improve the inspection performance by suppressing the occurrence of interference colors in the visual inspection performance under crossed Nicols, the distortion of the optical axis of the film has been reduced. However, even with such a film with improved optical axis accuracy, the contrast required for image inspection may not be obtained. Therefore, as a result of intensive studies, the present inventor has found that it is not sufficient to simply reduce the distortion of the optical axis from the mechanical axis, and the degree of orientation at the two mechanical axes is important for improving the contrast. Therefore, the MOR value in the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention is preferably 1.80 or more, more preferably 1.85 or more, and further preferably 1.90 or more. preferable. The MOR (Maximum Oriented Ratio) value is a ratio (maximum value / minimum value) between the maximum value and the minimum value of the transmission microwave intensity measured by a transmission type molecular orientation meter. The MOR value becomes smaller as the balance between the orientation difference between the longitudinal direction and the transverse direction becomes smaller, and becomes larger as the orientation strength in any one direction becomes stronger.

  The effect of the MOR value on the contrast is not well understood but is considered as follows. That is. Even when the orientation main axis coincides with the mechanical axis with high accuracy, if the orientation degree of the orientation main axis in the 90-degree direction is relatively strong, the path of the polarized light beam is blocked, resulting in a decrease in contrast. It is done. Therefore, when the MOR value is 1.80 or more, the orientation strength in the orientation main axis direction becomes strong, and the polarized light passes efficiently, which is considered to lead to improvement in contrast.

  Therefore, in order to increase the inspection accuracy, it is preferable that the MOR value is high. However, if it exceeds 2.10, the orientation strength in the direction orthogonal to the orientation main axis is weakened. To do. For this reason, problems such as film breakage and running failure are likely to occur during inspection. Therefore, the upper limit of the MOR value is 2.10 or less, preferably 2.08 or less, more preferably 2.05 or less.

  In the biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention, the amount of change in the orientation angle in the film width direction is in the range of 3.0 ° to 5.0 ° per 500 mm. If the angle exceeds 5.0 °, uniform optical axis accuracy cannot be obtained, and the inspection property is deteriorated due to the generation of interference colors, and cannot be used for high-precision inspection. The upper limit of the amount of change in the orientation angle per 500 mm in the film width direction is preferably 4.8 ° or less, and more preferably 4.5 ° or more. On the other hand, it is preferable that the amount of change in the orientation angle is low, but as described above, the orientation of the film is distorted from the center to the end due to the bowing phenomenon in film formation. Therefore, the inclination of the optical axis tends to occur along the film width direction. Therefore, in order to achieve both high thermal stability suitable for high-accuracy inspection of polarizing plates for large screen applications, the lower limit of the amount of change in orientation angle per 500 mm in the film width direction is substantially 3.0. °, preferably 3.5 °, more preferably 4.0 °.

  The biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention has high thermal dimensional stability even in post-processing at a high temperature. The heat shrinkage rate of the film of the present invention when heated at 150 ° C. for 30 minutes is 2.0% or less in both the longitudinal direction and the width direction, and more preferably 1.5% or less. When the heat shrinkage rate is 2.0% or less, high dimensional stability can be obtained even by high-temperature heat treatment at 150 ° C. or higher, which can significantly contribute to improvement of productivity. The heat shrinkage rate is preferably low, but about 0.5% is considered the lower limit from the viewpoint of production.

  Further, the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention has a difference between the heat shrinkage rate in the longitudinal direction and the heat shrinkage rate in the width direction when heated at 150 ° C. for 30 minutes, of 1.0% or less, More preferably, it is 0.8% or less, More preferably, it is 0.5% or less. The film of the present invention is excellent in thermal dimensional stability as described above, and has a heat shrinkage rate that is almost equal and balanced in the longitudinal direction and the width direction. It is extremely suitable for defect inspection with polarizing plates on screens.

  The manufacturing method in the biaxially stretched polyethylene terephthalate film for polarizing plate release of this invention is demonstrated. A representative example using polyethylene terephthalate pellets will be described in detail, but the present invention is not limited thereto.

  First, it is dried by film raw material drying or hot air drying so that the moisture content is less than 100 ppm. Next, each raw material is weighed and mixed, supplied to an extruder, and melt extruded into a sheet. Furthermore, the melted sheet is brought into close contact with a rotating metal roll (casting roll) using an electrostatic application method and cooled and solidified to obtain an unstretched PET sheet.

  Further, high-precision filtration is performed at any place where the molten resin is kept at 280 ° C. in order to remove foreign substances contained in the resin. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but in the case of a stainless steel sintered filter medium, the removal performance of aggregates and high melting point organic substances mainly composed of Si, Ti, Sb, Ge, Cu Excellent and suitable.

  When the surface layer (A layer) and the intermediate layer (B layer) are coextruded and laminated, two or more extruders are used to extrude the raw materials of each layer, and a multi-layer feed block (for example, a confluence having a rectangular confluence) The two layers are joined together using a block), extruded into a sheet from a slit die, and cooled and solidified on a casting roll to form an unstretched film. Alternatively, a multi-manifold die may be used instead of the multilayer feed block.

  Next, the unstretched film obtained by the above method is subjected to sequential biaxial stretching or simultaneous biaxial stretching, followed by heat treatment.

  As disclosed in Patent Documents 2 to 5, methods for improving the optical axis accuracy have been proposed so far. However, as described above, with the method disclosed above, it has been difficult to achieve both contrast and tear resistance, and optical axial accuracy and thermal dimensional stability. Therefore, as a result of intensive studies, the inventors of the present application have achieved a high degree of compatibility between contradictory characteristics by performing the following stretching method.

(1) Control of the draw ratio in the machine and transverse directions
In order to make the MOR value in the above range, it is preferable to control the stretching ratio in the longitudinal direction and the transverse direction.

  In order to obtain a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate in the present invention, it is preferable to perform longitudinal stretching in a range of 2.0 to 3.0 times, more preferably 2.6 to 2.9 times. It is. A longitudinal draw ratio of 2.0 times or more is preferable for achieving tear resistance, and in addition, thickness variation in the flow direction is reduced, which is preferable. Moreover, it is preferable that it is 3.0 times or less because the improvement in contrast and the amount of change in the orientation main axis become small.

  On the other hand, in order to obtain the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate in the present invention, it is desirable to stretch in the width direction within a range of 4.2 to 4.8 times. A lateral stretch ratio of 4.2 times or more is preferable because the visual inspection property is good due to the improvement in contrast. When it is 4.8 times or less, the tear resistance is maintained, and the frequency of breakage is reduced, which is preferable.

  The tear resistance of the film can be evaluated by measuring the tensile elastic modulus. Specifically, the smaller of the longitudinal elastic modulus and the transverse tensile modulus of the film is preferably 80% or more, and more preferably 100% or more. If the tensile modulus is 80% or more, the film is hardly broken even in the rework work of the release film.

(2) Control of heat setting temperature So far, heat setting processing at a relatively low temperature has been recommended in order to maintain optical axis accuracy. However, in order to achieve both axial accuracy and thermal dimensional stability, the temperature of the heat setting process is preferably 220 ° C. or higher and 230 ° C. or lower in the present invention. When the temperature of the heat setting treatment is 220 ° C. or higher, the absolute value of the heat shrinkage ratio is preferably reduced. Moreover, it is preferable that the temperature of the heat setting treatment is 230 ° C. or less because the film is hardly opaque and the frequency of breakage is reduced.

  It is effective to control the heat shrinkage rate, particularly the heat shrinkage rate in the width direction, by narrowing the guide rail of the gripping tool by the heat setting process. The temperature for the relaxation treatment can be selected in the range from the heat setting treatment temperature to the glass transition temperature Tg of the polyethylene terephthalate film, but is preferably (heat setting treatment temperature) −10 ° C. to Tg + 10 ° C. The width relaxation rate is preferably 1 to 6%. If it is less than 1%, the effect is small, and if it is 6% or less, it is preferable in terms of the flatness of the film.

  The film of the present invention is produced by the above method, but is not limited to the above specifically disclosed method as long as it is within the scope of the technical idea. In producing the film of the present invention, it is important to control the longitudinal stretching, the lateral stretching, and the heat setting with high accuracy in a very narrow range based on the above technical idea.

  Still another manufacturing method will be described. As a method of stretching the film in the biaxial direction, simultaneous biaxial stretching or the obtained unstretched sheet is stretched in the longitudinal direction by a roll or a tenter type stretching machine, and then orthogonal to the first stretching direction. The method of extending | stretching to the width direction can be mentioned. The stretching temperature in the longitudinal direction needs to be 75 to 110 ° C. When the stretching temperature in the longitudinal direction is 75 ° C. or less, the film is broken and stable production cannot be performed. Further, at 110 ° C. or higher, thickness unevenness occurs in the obtained film, which can be used as a polarizing plate release application.

  Next, the effect of this invention is demonstrated using an Example and a comparative example. First, the evaluation method of the characteristic values used in the present invention is shown below.

[Evaluation methods]

(1) Light transmittance, haze Measured according to JIS K 7105 “Testing method for optical properties of plastic” haze (cloudiness value). NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd. was used for the measuring instrument.

(2) Heat shrinkage rate Measured according to JIS C 2318-1997 5.3.4 (dimensional change). In the direction to be measured, the film is cut to a width of 10 mm and a length of 250 mm, marked at intervals of 200 mm, and the distance (A) between the marks is measured under a constant tension of 5 gf. Next, the film is placed in an oven in an atmosphere of 150 ° C., subjected to heat treatment at 150 ± 3 ° C. for 30 minutes under no load, and then the mark interval (B) is measured under a constant tension of 5 gf. The thermal shrinkage rate was calculated from the following formula.
Thermal contraction rate (%) = (A−B) / A × 100

(3) Three-dimensional center plane average surface roughness (SRa)
Using a stylus type three-dimensional surface roughness meter (SE-3AK, manufactured by Kosaka Laboratory Ltd.), the surface of the film was subjected to the conditions of a needle radius of 2 μm, a load of 30 mg, and a needle speed of 0.1 mm / sec. , Measured in a longitudinal direction of the film with a cut-off value of 0.25 mm, measured over a measurement length of 1 mm, divided into 500 points at a 2 μm pitch, and the height of each point was measured with a three-dimensional roughness analyzer (manufactured by Kosaka Laboratory Ltd. , TDA-21). The same operation was performed 150 times continuously at intervals of 2 μm in the width direction of the film, that is, over 0.3 mm in the width direction of the film, and the data was taken into the analyzer. Next, the three-dimensional average surface roughness SRa was determined using the analyzer. The unit of SRa is μm. In addition, the measurement was performed 3 times and those average values were employ | adopted.

(4) MOR value The MOR value was measured using a MOA-6004 type molecular orientation meter manufactured by Oji Scientific Instruments. The points to be measured are a total of three points: a central point in the width direction of the film and a 1/5 point on the end side connecting the central portion and both ends of the film. That is, the MOR value is measured at three points at a distance of 10%, 50%, and 90% from one end of the straight line in the width direction of the film.

(5) Molecular chain principal axis orientation angle (θ), optical principal axis tilt angle (ξ)
In the film width, the edge is 0%, and the other edge is 100%. From the region corresponding to 10% of the film width to the region corresponding to 90%, n 100 mm square film samples were cut out continuously at a pitch of 100 mm in the width direction. The square film sample was cut at a right angle with respect to either the longitudinal or width axis. For each film sample, using a MOA-6004 type molecular orientation meter manufactured by Oji Scientific Instruments Co., Ltd., the orientation angle (θi) of the molecular chain principal axis relative to the film longitudinal direction, and the machine axis direction (longitudinal direction) defined by the following formula , Or the width direction), the tilt angle (ξi) of the optical principal axis was measured.
When | θ | ≦ 45 degrees ξ = | θ |
When | θ |> 45 degrees ξ = | 90 degrees− | θ ||

(6) Amount of change in orientation angle Of the inclination angles of the optical main axis measured from the film sample, the maximum value was the maximum inclination angle (ξmax) of the optical main axis, and the minimum value was the minimum inclination angle (ξmin).
When the measurement position where the maximum inclination angle is obtained is Lmax (mm) and the measurement position where the minimum inclination angle is obtained is Lmin (mm), the amount of change in the orientation angle per 500 mm can be expressed by the following equation.
(Change in orientation angle) = (ξmax−ξmin) / (Lmax−Lmin) × 500

(7) Tear resistance The obtained film was cut into a width of 10 mm, and in accordance with JIS-K-7127 (2000), using “Tensilon Universal Testing Machine RTA-T-4M” manufactured by Orientec Co., Ltd., the initial length A tensile test was performed at 50 mm and a tensile speed of 200 mm / min. Using the first linear portion of the stress-strain curve obtained by the tensile test, the tensile elastic modulus was obtained by dividing the difference in stress between two points on the straight line by the difference in strain between the same two points. . The measurement was performed in a standard environment adjusted to a temperature of 23 ± 2 ° C. and a relative humidity of 50 ± 15% RH, and the measurements were made in the vertical and horizontal directions. For the tensile modulus in the machine direction or the transverse direction, the smaller value of 80% or more was rated as ◯, and the value less than 80% was rated as x.

(8) Thermal wrinkle determination method The following silicone coating liquid was applied to one side of the obtained film with a coating tension of 10 kg / m by a die coating method and dried in an oven at 120 ° C.
(Silicone coating solution)
Curing silicone (KS847H, Shin-Etsu Chemical) 100 parts by weight Curing agent (CAT PL-50T, Shin-Etsu Chemical) 2 parts by weight Diluent Methyl ethyl ketone / xylene / methyl isobutyl ketone 898 parts by weight -Cut from the surface, spread a length of 5 m on a flat table, reflect the light of the fluorescent lamp on the coated surface, and check for the presence of thermal wrinkles by the following evaluation method.
○: Thermal wrinkles are not seen at all and are good.
(Triangle | delta): A heat wrinkle is not seen on the whole surface, but a heat wrinkle is seen partially.
X: Thermal wrinkles can be confirmed on the entire surface.

(9) Inspectability The release film obtained in (8) was adhered to the polarizing film via an adhesive so that the width direction was parallel to the orientation axis of the polarizing film, thereby obtaining a polarizing plate. Between the white light source and the camera, two polarizing plates were placed in crossed Nicols, and a polarizing plate with a release film adhered between them was placed. A 5000-pixel line camera equipped with a 180 W Metahara transmission light as a light source and a 55 mm macro lens as a camera was used. Foreign matter inspection was performed on the inspection range (12 × 7 cm) while moving the sample at 5 m / min. The image signal from the camera was processed by an image processing apparatus, and the detected defects of 50 μm or more were measured. A similar sample was also measured for defects of 50 μm or more under crossed Nicols in the inspection range (12 × 7 cm). From the obtained results, the testability was evaluated as follows.
○: Contrast is good, and visual results and automatic inspection results are equivalent (98% of visual inspection defects)
The above is automatically detected) and the slipperiness during the automatic inspection is good. △: The visual result and the automatic inspection result are almost the same (less than 98% of the number of defects of the visual inspection is automatically detected).
X: Occurrence of interference spots

Example 1
As the polyethylene terephthalate resin (A) (PET resin (A)), a polyethylene terephthalate resin containing no inert particles and having an intrinsic viscosity of 0.62 dl / g was used. Further, as the PET resin (B), a polyethylene terephthalate resin having an intrinsic viscosity of 0.62 dl / g and containing 2000 ppm of irregular-shaped massive silica particles having an average particle diameter of 2.3 μm was used.

  50 parts by mass of the PET resin (A) and 50 parts by mass of the PET resin (B) were mixed in pellets, dried at 135 ° C. under reduced pressure (1 Torr) for 6 hours, and then supplied to the extruder 1. Further, the PET resin (A) as a raw material for the intermediate layer (B) was dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and then supplied to the extruder 2. The raw material supplied to the extruder 2 and the extruder 1 is set to a resin temperature of 280 ° C. until the melting portion, kneading portion, polymer tube, gear pump, and filter of the extruder, and 275 ° C. in the subsequent polymer tube. The layers were laminated using a layered confluence block so as to be A / B / A, and melt-extruded into a sheet form from the die. In addition, the thickness ratio of A layer and B layer was controlled using the gear pump of each layer so that it might become A / B / A = 8/84/8. In addition, a stainless steel sintered filter medium (nominal filtration accuracy: 95 μm of 10 μm particles) was used for each of the filters. The temperature of the die was controlled so that the temperature of the extruded resin was 275 ° C.

  Then, the extruded resin was cast on a cooling drum having a surface temperature of 30 ° C. and brought into close contact with the cooling drum surface using an electrostatic application method to be cooled and solidified to form an unstretched film having a thickness of 480 μm.

  The obtained unstretched sheet was heated to 78 ° C. with a roll group heated to 78 ° C., then heated to 105 ° C. with an infrared heater, and the roll group with a difference in peripheral speed was 2 in the longitudinal direction. Stretched 8 times.

  Subsequently, the obtained uniaxially stretched film was gripped with a clip and subjected to transverse stretching. The transverse stretching temperature of TD1 was 120 ° C., and the transverse stretching ratio was 4.5 times. Next, heat treatment was performed at 228 ° C. for 15 seconds, and 3% relaxation treatment was performed at 200 ° C. to obtain a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate. The obtained film properties are shown in Table 2.

  The biaxially stretched polyethylene terephthalate film obtained in this example not only has excellent thermal dimensional stability at high temperatures, but also has excellent polarizing plate inspection properties. It was a film that can be suitably used for inspection.

Example 2
It was prepared in the same manner as described in Example 1 except that the longitudinal draw ratio was changed to 2.6 times. The obtained film properties are shown in Table 2. The biaxially stretched polyethylene terephthalate film obtained in this example suppressed the change in the orientation angle and achieved a further excellent polarizing plate inspection property by having a large MOR value.

Example 3
It was prepared in the same manner as described in Example 1 except that the longitudinal draw ratio was changed to 2.9 times. The obtained film properties are shown in Table 2. The biaxially stretched polyethylene terephthalate film obtained in this example was a film that could be used suitably for high-accuracy inspection, although the orientation angle fluctuation and the MOR value were slightly deteriorated.

Examples 4-7
The same method as in Example 1 was used, except for changing the conditions described in Table 1. The obtained film properties are shown in Table 2.

Example 8
The same method as in Example 1 was used, except for changing the conditions described in Table 1. The obtained film properties are shown in Table 2. The biaxially stretched polyethylene terephthalate film obtained in this example had high haze, and the contrast at the time of inspection slightly decreased.

Example 9
The same method as in Example 1 was used, except for changing the conditions described in Table 1. The obtained film properties are shown in Table 2. The biaxially stretched polyethylene terephthalate film obtained in this example was poor in slipperiness, and micro scratches were generated during the film forming process.

Comparative Example 1
It was prepared in the same manner as described in Example 1 except that the longitudinal draw ratio was 3.4 times, the transverse draw ratio was 4.3 times, and the heat setting temperature was changed to 240 ° C. The obtained film properties are shown in Table 2.
The biaxially stretched polyethylene terephthalate film obtained by this comparative example is excellent in thermal dimensional stability at high temperatures, but has a large change in orientation angle and is inferior in polarizing plate inspection.

Comparative Examples 2-4
The same method as in Example 1 was used, except for changing the conditions described in Table 1. The obtained film properties are shown in Table 2.
The biaxially stretched polyethylene terephthalate film obtained by this comparative example is inferior in thermal dimensional stability at high temperatures, has a small MOR value, and can be used for high-precision inspection in a polarizing plate manufacturing process for large screen applications. There wasn't.

Comparative Example 5
The same method as in Example 1 was used, except for changing the conditions described in Table 1. The obtained film properties are shown in Table 2. The biaxially stretched polyethylene terephthalate film obtained in this example was easy to tear in one direction and had low tear resistance.

  The biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention can be used by being attached to a polarizing plate which is a constituent member of a large liquid crystal display device, and is also suitable for defect inspection under crossed Nicols.

Claims (3)

A biaxially stretched polyethylene terephthalate film for releasing a polarizing plate, which is a biaxially stretched polyethylene terephthalate film and satisfies the following structural requirements (1) to (4).
(1) Thermal contraction rate when heated at 150 ° C. for 30 minutes is 2.0% or less in both the longitudinal direction and the width direction. (2) Thermal contraction rate in the longitudinal direction and heat in the width direction when heated at 150 ° C. for 30 minutes. Difference in shrinkage is 1.0% or less (3) MOR value measured by microwave transmission type molecular orientation meter is 1.80 to 2.10
(4) The amount of change in the orientation angle in the film width direction is 3.0 ° to 5.0 ° per 500 mm. The biaxially stretched polyethylene terephthalate film for releasing a polarizing plate according to claim 1, which is the biaxially stretched polyethylene terephthalate film and further satisfies the following requirements (5) and (6).
(5) Total light transmittance is 85% or more (6) Three-dimensional center plane average surface roughness SRa of the film surface is 0.020 μm or more and 0.035 μm or less The method for producing a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate according to claim 1 or 2, wherein the unstretched film is stretched in the longitudinal direction and the transverse direction and heat-set, and the following requirements (7) and ( A method for producing a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate that satisfies 8).
(7) Perform lateral stretching at a temperature of 4.2 to 4.8 times in the range of 120 ° C to 140 ° C. (8) Perform heat fixing at a temperature of 220 to 230 ° C.