JP2011246533A - Polyester film for release film of polarizing plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film suitable for a base film for a release film capable of achieving the high-degree accuracy of inspection of a polarizing plate in a state of being laminated with the release film when carrying out the inspection of the presence or absence of defects such as distortion and unevenness, and foreign matter by visual inspection by using a crossed nicols method.SOLUTION: The polyester film for the release film of the polarizing plate has a surface roughness (Ra) of the film of 18-33 nm, an image clarity value of ≥89%, and a haze of the film of ≤8% after heat treatment at 180°C for 5 min.

Description

  The present invention is a polyester film that is an important property in films for liquid crystal display applications, etc., is excellent in optical properties, and can obtain a high degree of accuracy in defect detection in film inspection, particularly a release film for polarizing plates It is related with the polyester film used suitably for.

  Polyester films represented by polyethylene terephthalate and polyethylene naphthalate have excellent mechanical strength, dimensional stability, flatness, heat resistance, chemical resistance, optical properties, etc. It is used in applications. However, as its uses diversify, the processing conditions and usage conditions of the film diversify, and when used as a release film for polarizing plates, the particle component in the release film becomes a bright spot when inspecting foreign matter, There are problems such as a decrease in inspection accuracy.

  In recent years, with the rapid spread of mobile phones and personal computers, the demand for liquid crystal displays (LCDs) that are thinner, lighter, consume less power, and have higher image quality than the conventional display CRT is growing significantly. There is also a remarkable growth in the technology for increasing the screen size of LCDs. As an example of increasing the screen size of LCD, recently, LCD is used for large TV applications of 30 inches or more. An LCD with a large screen is often a bright LCD with a large screen by increasing the luminance of a backlight incorporated in the LCD or incorporating a film for improving the luminance into a liquid crystal unit.

  In such a so-called high-brightness type LCD, a small bright spot existing in the display is often a problem, and in a component such as a polarizing plate, a retardation plate or a retardation polarizing plate incorporated in the display, As a result, there is a problem of a foreign material having a minute size that has not been a problem in conventional low-brightness LCDs. For this reason, while preventing the entry of foreign matter in the manufacturing process, it is also important to improve the inspection accuracy so that even if foreign matter is mixed, it can be recognized as a defect.

  Conventionally, the particles in the polyester film are usually used to ensure the slipperiness and winding characteristics of the film, and if the appropriate particle size and blending amount are not satisfied, the desired slipperiness cannot be ensured. As a result, the winding characteristics deteriorate, and as a result, the productivity deteriorates. However, when the particle size and blending amount are within the normally used range, as described above, when used as a release film for polarizing plates, the particles become bright spots in the foreign substance inspection process, which hinders inspection. Is a problem.

  For example, as a defect inspection of a polarizing plate, a visual inspection by a crossed Nicols method is generally used. Further, for example, for a polarizing plate used for a large TV application of 40 inches or more, an inspection by an automatic particle inspection device using the crossed Nicols method Is also being implemented. According to this crossed Nicol method, two polarizing plates are put in a quenching state with their orientation principal axes orthogonal to each other, and if there are foreign matters or defects, they appear as bright spots, so a visual defect inspection or a line sensor camera, etc. Automatic defect inspection can be performed. Here, the pressure-sensitive adhesive layer is usually provided on the polarizing plate, and a polyester film provided with a release layer is used as the release film for that purpose. When a product having such a configuration is inspected, a crossed Nicols inspection is performed in a state where a release polyester film is sandwiched between two polarizing plates. In general, when a release polyester film is used for this, foreign matter and defects are difficult to see in the crossed Nicols inspection, and it may be easy to miss them.

  Regarding these, when a polyester film is sandwiched between two polarizing plates, an inspection property is improved when the retardation value is within a certain range (see Patent Document 1). Even if these are used at a level where quality is required, accuracy may be insufficient when inspection is performed to find defects reliably.

  Further, when inspecting defects such as distortion and unevenness of the polarizing plate, a visual reflection inspection under a fluorescent lamp may be performed. In this case, in order to inspect the polarizing plate provided thereunder through the release film, if the release film is extremely white, defects of the polarizing plate are difficult to see and it is easy to miss them. May occur. In this regard, a technique has been disclosed in which the inspection property is improved when the surface roughness (Ra) and the image clarity value of the film are within a certain range (see Patent Document 2), but higher quality is required. Even if these are used at a certain level, accuracy may be insufficient.

JP 2000-338327 A JP 2009-161569 A

  This invention is made | formed in view of the said situation, The solution subject is to provide the polyester film for mold release films which can implement | achieve a high precision in the test | inspection by the cross Nicol method of a polarizing plate.

  As a result of intensive studies in view of the above problems, the present inventor can provide a polyester film that is particularly suitable for a polyester film for a release film, without impairing excellent film properties, according to a polyester film having a specific configuration. As a result, the present invention has been completed.

  That is, the gist of the present invention is that the surface roughness (Ra) of the film is 18 to 33 nm, the image clarity value is 89% or more, and the haze of the film after heat treatment at 180 ° C. for 5 minutes is 8% or less. It exists in the polyester film for polarizing plate release films characterized by the above-mentioned.

  According to the present invention, it is possible to provide a polyester film that can be suitably used for a release film such as a polarizing plate and a retardation plate, the film has as few bright spots as possible, and can improve the accuracy of foreign matter inspection, The industrial value of the present invention is high.

  The polyester film referred to in the present invention is a film which is melt-extruded from an extrusion die, and is subjected to stretching and heat treatment as necessary after cooling a molten polyester sheet extruded by a so-called extrusion method.

The polyester constituting the film of the present invention is obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol. Examples of the aromatic dicarboxylic acid include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of the aliphatic glycol include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. Representative polyesters include polyethylene terephthalate (PET), polyethylene-2,6-naphthalenedicarboxylate (PEN), and the like. The polyester used may be a homopolyester or a copolyester. In the case of a copolyester, it may be a copolymer containing 30 mol% or less of the third component.
The dicarboxylic acid component of the copolymerized polyester is selected from isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid and oxycarboxylic acid (for example, P-oxybenzoic acid). The glycol component includes one or more selected from ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, and the like.

  The polyester used in the present invention has a weather resistance agent, a light resistance agent, an antistatic agent, a lubricant, a light-shielding agent, an antioxidant, a fluorescent whitening agent, a matting agent, and a heat stability, as long as the gist of the present invention is not impaired. You may mix | blend colorants, such as an agent, dye, or a pigment.

  Examples of the particles to be blended in the film include silicon oxide, alumina, calcium carbonate, kaolin, titanium oxide, and crosslinked polymer fine powder as described in JP-B-59-5216. These particles may be used alone or in combination of two or more components. The blending amount of these particles is usually 1% by weight or less, preferably 0.01 to 1% by weight, more preferably 0.02 to 0.6% by weight, based on the polyester constituting the film. When the content of the particles is small, the film surface cannot be appropriately roughened, and in the film production process, there is a tendency that the surface is easily scratched or the winding properties are inferior. Further, when the content of the particles exceeds 1% by weight, the degree of roughening of the film surface becomes too large, and the transparency may be impaired.

  The average particle diameter of the particles contained in the polyester film is usually 0.02 to 5 μm, preferably 0.1 to 3 μm, and more preferably 0.2 to 1.8 μm. When the particle size is less than 00.2 μm, the film surface may not be appropriately roughened, and the winding characteristics in the film production process tend to be inferior. When the particle diameter exceeds 5 μm, when used as a polarizing plate release film, it may become a bright spot and hinder foreign matter inspection.

  On the other hand, in order to improve the transparency of the film, when a laminated film of two or more layers is used, a method of blending particles only in the surface layer is also preferably employed. The surface layer in this case is at least one of the front and back layers, and of course, particles can be blended in both the front and back layers. The blending amount of the particles in the case of such a laminated film is preferably 0.01 to 1% by weight, more preferably 0.02 to 0.6% by weight, based on the polyester constituting the surface layer.

  The particle size distribution of the particles used is preferably sharp. Specifically, those having a particle size distribution value of 1.0 to 2.0, which is an index representing the sharpness of the particle size distribution, are preferable. Here, the particle size distribution value is the particle size distribution value d25 / d75 (d25 and d75 are calculated by calculating the accumulated accumulation of the particle group from the large particle side, and the particle size corresponding to 25% and 75% of the total volume (μm Is a value defined by When the particle size distribution value exceeds 2.0, coarse particles become luminescent spots, which may similarly hinder foreign matter inspection.

  In the present invention, the image clarity value of the film in the measurement method described later needs to be 89% or more. When the image clarity value of the film is less than 89%, or when used as a polarizing plate release film, the image is distorted during defect inspection of the polarizing plate by transmitted light, which hinders visual inspection and automatic inspection. .

  Further, the surface roughness (Ra) of the film needs to be 18 to 33 nm. When the film Ra exceeds 33 nm, the flatness of the surface is impaired and the film becomes whitish, which hinders inspection. On the contrary, when Ra of the film is less than 18 nm, the film surface becomes extremely flat, and the winding characteristics in the film manufacturing process are inferior.

  The haze of the film after heat treatment at 180 ° C. for 5 minutes needs to be 8% or less, preferably 3 to 5%. A heat treatment process may be required in the process of manufacturing a polarizing plate. At that time, if the film haze when heat-treated exceeds 8%, the field of view becomes clouded during the transmitted light inspection, thereby hindering the inspection.

  In the present invention, the method of blending the particles with the polyester is not particularly limited, and a known method can be adopted. For example, it can be added at any stage for producing the polyester, but it is preferably added as a slurry dispersed in ethylene glycol or the like at the stage of esterification or before the start of the polycondensation reaction after completion of the transesterification reaction. The condensation reaction may proceed. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a method of blending dried particles and a polyester raw material using a kneading extruder Etc.

  The polyester to be used may be converted into chips after melt polymerization, and further subjected to solid phase polymerization as necessary under heating under reduced pressure or in an inert gas stream such as nitrogen. The intrinsic viscosity of the obtained polyester is preferably 0.40 dl / g or more, and more preferably 0.40 to 0.90 dl / g.

  In the present invention, it may be a film having a structure obtained by coextrusion laminating a polyester having a low oligomer content on the surface of at least one side of a layer composed of a polyester having a normal oligomer content. In the polyester film for release film obtained in the present invention, the effect of preventing bright spots due to the precipitated oligomer is obtained, which is particularly preferable.

  The polyester film in the present invention preferably has a variation in orientation angle of 8 degrees / 40 mm or less, more preferably 6 degrees / 40 mm or less, in the measurement method described in the examples. When the variation in the orientation angle exceeds 8 degrees / 40 mm, the transmitted light intensity varies depending on the position of the polarizing plate when inspecting the polarizing plate, which is an undesirable obstacle to stable inspection of the polarizing plate.

  The polyester film in the present invention preferably has a refractive index (nβ) in the direction perpendicular to the film in-plane direction of 1.6400 or less with respect to the main orientation axis. There is a tendency for the angle fluctuation to increase, which not only becomes an obstacle to stable inspection of the polarizing plate, but also the formation of voids in the particles becomes prominent, making it easier to see as a bright spot during polarizing plate inspection, hindering inspection. It is not preferable.

  The total thickness of the film of the present invention is not particularly limited as long as it can be formed as a film, but is usually in the range of 4 to 100 μm, preferably 9 to 50 μm.

  Next, although the manufacturing method of the film of this invention is demonstrated concretely, as long as the summary of this invention is satisfied, this invention is not specifically limited to the following illustrations.

  First, the preferable example of the manufacturing method of polyester used by this invention is demonstrated. Here, an example in which polyethylene terephthalate is used as the polyester is shown, but the production conditions differ depending on the polyester used. According to a conventional method, esterification from terephthalic acid and ethylene glycol, or transesterification reaction between dimethyl terephthalate and ethylene glycol, the product is transferred to a polymerization tank, the temperature is increased while reducing pressure, and finally vacuum is applied. Under heating to 280 ° C., the polymerization reaction proceeds to obtain a polyester.

  Next, for example, the polyester chip obtained as described above and dried by a known method is supplied to a melt-extrusion apparatus and heated to a temperature equal to or higher than the melting point of each polymer to melt. Next, the molten polymer is extruded from a die and rapidly cooled and solidified on a rotary cooling drum so that the temperature is equal to or lower than the glass transition temperature to obtain a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed. In the present invention, the sheet thus obtained is stretched biaxially to form a film. Specifically describing the stretching conditions, the unstretched sheet is preferably stretched 2 to 6 times at 70 to 145 ° C. in the longitudinal direction to form a longitudinal uniaxially stretched film, and then 2 to 90 to 160 ° C. in the lateral direction. It is preferable to perform ~ 6 times stretching and heat treatment at 150 to 240 ° C for 1 to 600 seconds. Further, at this time, a method of relaxing 0.1 to 20% in the longitudinal direction and / or the transverse direction in the maximum temperature zone of the heat treatment and / or the cooling zone at the heat treatment outlet is preferable. Further, it is possible to add re-longitudinal stretching and re-lateral stretching as necessary. Furthermore, it is also possible to perform simultaneous biaxial stretching of the unstretched sheet so that the area magnification is 10 to 40 times.

  The polyester film of the present invention can be subjected to so-called in-line coating for treating the film surface during the stretching step as long as the effects of the present invention are not impaired. Although it is not limited to the following, for example, after the first stage of stretching is completed, before the second stage of stretching, improvement of antistatic property, slipperiness, adhesion, etc., secondary workability improvement, weather resistance, etc. In order to improve the property and surface hardness, a coating treatment with an aqueous solution, an aqueous emulsion, an aqueous slurry, or the like can be performed. Various coatings may be performed by offline coating after film production. Such a coat may be either single-sided or double-sided. The coating material may be either water-based and / or solvent-based for off-line coating, but is preferably water-based for in-line coating.

  When a release layer is installed on the polyester film of the present invention, the material constituting the release layer is not particularly limited as long as it has releasability, and a type mainly composed of a curable silicone resin may be used. Alternatively, a modified silicone type obtained by graft polymerization with an organic resin such as a urethane resin, an epoxy resin, or an alkyd resin may be used. Among these, when a curable silicone resin is used as a main component, good releasability is sufficient.

  Types of curable silicone resins include solvent addition type, solvent condensation type, solvent ultraviolet curable type, solventless addition type, solventless condensation type, solventless ultraviolet curable type, solventless electron beam curable type, etc. But it can also be used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. In the examples and comparative examples, “parts” means “parts by weight”. The measuring method used in the present invention is as follows.

(1) Measurement of Intrinsic Viscosity of Polyester 1 g of polyester was precisely weighed, 100 ml of a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio) was added and dissolved, and measurement was performed at 30 ° C.

(2) Average particle size (d50) and particle size uniformity (d25 / d75)
The average particle size d50 was defined as the particle size having an integrated volume fraction of 50% in an equivalent spherical distribution measured using a centrifugal sedimentation type particle size distribution analyzer (SA-CP3 type) manufactured by Shimadzu Corporation. In addition, the ratio d25 / d75 of the diameter of the point having a weight fraction of 25% and the diameter of the point having a weight fraction of 75%, integrated from the large particle side, was used as the particle size distribution value.

(3) Measurement of variation in orientation angle in the film In the width direction of the polyester film, from the center position toward the both ends in the width direction, cut out the samples at the positions of 40 mm and the both ends, respectively. Using an automatic birefringence meter (KOBRA-21ADH) manufactured by the company, the change in orientation angle for every 40 mm in the film width direction was determined. When calculating the variation in the orientation angle including the positions at the extreme ends, if the sample is longer than 225 mm, the variation in the orientation angle every 40 mm is calculated by proportional calculation. Subsequently, the film was cut out in the longitudinal direction of the film, and samples were cut out from a total of 5 positions every 40 mm (including both ends) in the longitudinal direction from the center in the film width direction, and the orientation angle was determined. Thus, the variation of the orientation angle for every 40 mm in the width direction and the longitudinal direction was obtained, and the maximum variation value was defined as the variation of the orientation angle of the film. Further, in the measurement, it is important to make the reference axis of the orientation angle the same in all samples, and the reference axis can be arbitrarily determined.

(4) Measurement of image clarity value In accordance with JIS-K7105, the image clarity value of the film was measured by the transmission method with the image clarity measuring device ICM-1 manufactured by Suga Test Instruments Co., Ltd. The value is read from an optical comb of 0.125 mm.

(5) Measurement of Ra It measured according to JIS B0601-1994 using the surface roughness measuring machine SE3500 type | mold by Kosaka Laboratory. The measurement length was 2.5 mm.

(6) Measurement of film haze The haze of the film was measured with an integrating sphere turbidimeter NDH-20D manufactured by Nippon Denshoku Industries Co., Ltd. according to JIS-K7105. Moreover, the haze after heat processing measured the haze of the film heat-processed for 5 minutes in 180 degreeC atmosphere in the tensionless state.

(7) Visual inspection property under reflected light 100 parts of curable silicone resin (“KS-779H” manufactured by Shin-Etsu Chemical) using the obtained polyester film, curing agent (“CAT-PL-8” manufactured by Shin-Etsu Chemical) A release agent comprising 1 part, 2200 parts of methyl ethyl ketone (MEK) / toluene mixed solvent system was applied so that the coating amount was 0.1 g / mm ² , and dried at 170 ° C. for 10 seconds. Then, the release film is adhered to the polarizing film through an adhesive so that the width direction of the release film is parallel to the orientation axis of the polarizing film. Were visually observed, and visual inspection under reflected light was evaluated according to the following criteria. In the measurement, an A4 size sample was cut out.
<Criteria for visual inspection under reflected light>
(Good inspection) ◎>○>△>×> XX (Inspection failure)
Among the above criteria, those above Δ are levels that can be used without any problem in actual use.

(8) Foreign substance recognition under crossed Nicols 100 parts of curable silicone resin (“KS-779H” manufactured by Shin-Etsu Chemical), 1 part of curing agent (“CAT-PL-8” manufactured by Shin-Etsu Chemical), methyl ethyl ketone (MEK) / After applying a release agent comprising 2200 parts of a toluene mixed solvent system to one side of a polyester so that the coating amount is 0.1 g / mm ² and drying at 170 ° C. for 10 seconds to obtain a release film The release film was adhered to the polarizing film through a known acrylic pressure-sensitive adhesive so that the width direction of the release film was parallel to the orientation axis of the polarizing film, thereby producing a polarizing plate with the release film. Here, when the polarizing plate was prepared, black metal powder (foreign matter) having a size of 50 μm or more was mixed between the pressure-sensitive adhesive and the polarizing film so as to be 50 / m ² . A polarizing plate for inspection is superimposed on the polarizing plate release film mixed with the foreign matter thus obtained so that the orientation axis is orthogonal to the width direction of the release film, and white light is irradiated from the polarizing plate side. Then, it was visually observed from the polarizing plate for inspection, and whether or not a foreign substance mixed between the pressure-sensitive adhesive and the polarizing film could be found was evaluated according to the following classification. In addition, in the case of a measurement, it evaluated using the center part of the obtained film.
<Foreign substance recognition classification criteria>
(Good foreign body recognition) ◎ ＞ ○ ＞ △ ＞ × (foreign body poor recognition)
Among the above criteria, those above ○ are levels that can be used without any problem in actual use.

<Manufacture of polyester (A)>
Starting from 100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol, magnesium acetate tetrahydrate is added as a catalyst to the reactor, the reaction start temperature is 150 ° C., and the reaction temperature is gradually increased as methanol is distilled off. The temperature was raised to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. Ethyl acid phosphate was added to the reaction mixture, which was then transferred to a polycondensation tank, and 0.04 part of antimony trioxide was added to carry out a polycondensation reaction for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.63 due to a change in stirring power of the reaction tank, and the polymer was discharged under nitrogen pressure to obtain a polyester chip (A). The intrinsic viscosity of this polyester was 0.63.

<Manufacture of polyester (B)>
In the production method of polyester (A), after adding ethyl acid phosphate, the content of ethylene glycol slurry of divinylbenzene / styrene copolymer crosslinked particles having an average particle size of 1.0 μm is 0.6% by weight with respect to polyester. A polyester (B) was obtained using the same method as the method for producing the polyester (A) except that the polyester (A) was added. The obtained polyester (B) had an intrinsic viscosity of 0.63.

<Manufacture of polyester (C)>
In the method for producing polyester (B), the additive particles are synthetic calcium carbonate particles having an average particle diameter of 1.0 μm, and the content with respect to the polyester is 2.0% by weight. A polyester (C) was obtained using the same method. The obtained polyester (C) had an intrinsic viscosity of 0.63.

<Manufacture of polyester (D)>
In the method for producing polyester (B), the additive particles are the same as the method for producing polyester (B), except that the additive particles are silica particles having an average particle diameter of 3.4 μm and the content with respect to the polyester is 0.6% by weight. Polyester (D) was obtained using the method. The obtained polyester (D) had an intrinsic viscosity of 0.63.

Examples 1-6 and Comparative Examples 1-2:
A raw material for the outermost layer (surface layer) and intermediate layer obtained by mixing the polyester (A) chip and the polyester (B), (C), and (D) chips in the proportions shown in Tables 1 and 2 below. Each was supplied to two extruders, melted and extruded at 280 ° C., and then cooled and solidified on a cooling roll having a surface temperature set to 30 ° C. using an electrostatic application adhesion method to obtain an unstretched sheet. . Next, the film was stretched 3.2 times in the longitudinal direction at 85 ° C., then subjected to a preheating step in a tenter and subjected to transverse stretching of 5.7 times at 100 ° C., followed by heat treatment at 230 ° C. for 10 seconds to obtain a width. Each 250 mm polyester film was obtained. The total thickness of the obtained film was 38 μm, and the thickness of each layer was 4 μm / 32 μm / 4 μm. In addition, since the film of Comparative Example 1 had an extremely flat surface shape and deteriorated slipperiness, when the film after stretching and heat treatment was wound into a roll, the film could not be wound well, and the film Scratches occurred on the entire surface, and the product was not realized. The film of Comparative Example 2 had many scratches, and the testability could not be determined.

The film of the present invention can be suitably used as a polyether film for a release film that can realize a high degree of accuracy in various inspection methods for polarizing plates.

Claims (1)

The film surface roughness (Ra) is 18 to 33 nm, the image clarity value is 89% or more, and the haze of the film after heat treatment at 180 ° C. for 5 minutes is 8% or less. Polyester film for mold film.