JP5997996B2 - Biaxially stretched polyester film for polarizing plate - Google Patents

Description

  The present invention relates to a polyester film suitably used as a polarizing plate member.

  A polarizing plate used for a liquid crystal display is generally composed of a protective film / polarizing film / protective film, or a protective film / polarizing film / retardation film. The polarizing film protective film has high transparency, optics, etc. A triacetyl cellulose (hereinafter sometimes abbreviated as TAC) film has been widely used because of its directivity. However, the TAC film cannot be said to exhibit sufficient performance in terms of chemical resistance, scratch resistance, and the like, and in recent years, as the liquid crystal display becomes larger and thinner, Strength, dimensional stability, low durability, and high moisture permeability are problems.

  In consideration of the above problems, materials such as cycloolefin polymers and acrylics have been studied instead of TAC films (Patent Documents 1 and 2). There is a problem that becomes high.

  On the other hand, the examination which uses a polyester film as a substitute of a TAC film is also performed (patent document 3). Among them, the polyethylene terephthalate film has lower moisture permeability than the TAC film and is excellent in handling. Moreover, since it is a general purpose resin, there is also an advantage that the cost can be reduced. Polyethylene terephthalate film is generally subjected to treatment such as uniaxial stretching and biaxial stretching according to the required properties, and in the unstretched state, there is a merit that generation of light interference color due to molecular orientation can be suppressed. Therefore, there is a demerit that the strength is remarkably lowered. Therefore, it cannot be said that it is suitable to use for the protective film of the recent polarizing plate with a strong demand for thickness reduction.

  Although studies have been made to biaxially stretch pomopolyester for the purpose of improving dimensional stability, thermal stability, and mechanical strength while suppressing the occurrence of light interference colors (Patent Document 4), It is doubtful whether it is a level that can be used as a polarizing plate.

  In addition, as a method of suppressing the light interference color in the biaxially stretched polyester, there is a method of containing a copolymer polyester, but since the strength is reduced, there is a problem that it is easy to tear or breaks easily during cutting. is there.

  As described above, it cannot be said that a TAC alternative film that does not generate a light interference color and is excellent in cost, mechanical strength, and productivity can be provided to the market.

JP-A-6-511117 JP 2006-227090 A JP 2002-116320 A JP 2011-99089 A

  The present invention has been made in view of the above-mentioned actual situation, and the problem to be solved is that there is no generation of a light interference color when arranged in a crossed Nicols state as a polarizing plate, and polarization with excellent cost, mechanical strength, and productivity. It is providing the polyester film for board members.

  As a result of intensive studies in view of the above problems, the present inventors have found that the above problems can be easily solved by a polyester film having a specific configuration, and have completed the present invention.

That is, the gist of the present invention contains a polyester copolymer comprising at least three layers is biaxially oriented polyester film having a laminated structure of a layer other than the two outermost third component other than principal components, both the outermost The outer layer is made of homopolyester, the thickness of each outermost layer is 7.0 to 15.0% of the total layer thickness, the in-plane retardation of the film is 600 nm or less, and the length direction of the film and perpendicular thereto The polyester film is characterized in that the end tear resistance value in the direction to be is 70 N or more and the internal haze is 1.0% or less.

  According to the present invention, it is possible to provide a polyester film for a polarizing plate member that does not generate a light interference color when disposed in a crossed Nicol state as a polarizing plate, and is excellent in cost, mechanical strength, and productivity. The industrial value of is high.

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

  The polyester film of the present invention is a laminated film composed of at least three layers having two outermost layers and one or more inner layers, and both outermost layers are usually made of a homopolyester, and the inner layer contains a copolyester. It is characterized by. When all layers are homopolyesters, the retardation generally increases in proportion to the thickness of the film. Therefore, the occurrence of light interference color increases when placed in a crossed Nicols state as a polarizing plate, which is actually used. Appearance unbearable. On the other hand, there is a method of using a copolyester for all layers, but it is effective in suppressing retardation, but the dimensional stability at high temperature is lowered, and the film is easily torn and the productivity is remarkably lowered. There is.

  In the present invention, both outermost layers are preferably made of homopolyester as described above. Here, the polyester is obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol. Examples of the aromatic dicarboxylic acid include isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, and oxycarboxylic acid (for example, P-oxybenzoic acid), and the like. Examples of the glycol include ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol and the like. Representative polyesters include polyethylene terephthalate, polyethylene-2,6-naphthalenedicarboxylate (PEN), and the like. Among these, by using polyethylene terephthalate for both outermost layers, excellent mechanical strength and fracture resistance can be exhibited in the film.

  Moreover, the inner layer of the film of the present invention contains a copolyester. The copolyester is a polyester containing a third component other than the main component, and the dicarboxylic acid component includes isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid and oxy One type or two or more types selected from carboxylic acids (for example, P-oxybenzoic acid and the like) can be used. One glycol component includes one or more selected from ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, and the like. In general, a copolyester has a molecular structure that is difficult to crystallize or orient, and by using the copolyester, it becomes possible to reduce retardation that is an indicator of a light interference color.

  The total thickness of the film of the present invention is preferably 2 to 50 μm. More preferably, it is the range of 4-38 micrometers. If the thickness of the film is less than 4 μm, film formation tends to be difficult and handling tends to be difficult. Moreover, when the thickness of the film is greater than 50 μm, the total thickness of the polarizing plate is increased and the retardation tends to increase in proportion to the thickness.

  The thickness of the outermost layer of the film of the present invention is usually 0.2 μm or more as a thickness on only one side, and needs to be 7.0 to 15.0% of the total thickness of the film. If the thickness is less than 0.2 μm, the oligomer (cyclic trimer) contained in the inner layer is deposited on the film surface due to the thermal history during processing, etc., resulting in production line contamination and an increase in the amount of foreign matter on the film surface. May be seen. If the outermost layer thickness is smaller than 7.0% of the total layer thickness, the slit process frequently causes breakage due to insufficient strength, resulting in a decrease in productivity. On the other hand, since retardation becomes large at 15.0% or more, a light interference color is strongly observed when it is made into a polarizing plate and arranged in crossed Nicols. Furthermore, the amount of particles blended in the outermost layer increases and transparency is impaired.

  The end tear resistance value of the polyester film of the present invention is 70 N or more in both the longitudinal direction (MD) and the width direction (TD) of the film. When it is 70 N or less, breakage occurs frequently in the slitting process, making it difficult to collect the roll.

  In the present invention, the method for adjusting the end resistance value includes a ratio of a layer having a molecular orientation to a layer having substantially no molecular orientation, a crystallinity of a layer having a molecular orientation, or a substantial A method based on the degree of molecular orientation of the layers having no molecular orientation and the molecular weight of the polyester resin constituting each layer can be used.

  In order to prevent the occurrence of light interference color when the polyester film of the present invention is used as a polarizing plate member, the in-plane retardation of the polyester film needs to be 600 nm or less, preferably 300 nm or less. When the in-plane retardation of the film is larger than 600 nm, the interference color of light becomes strong, and a rainbow pattern is confirmed on the liquid crystal display, and the original color of the image cannot be obtained.

  The polyester film in the present invention preferably contains fine particles in order to improve workability in the film winding process or the like. Examples of the particles include silicon dioxide, alumina, calcium carbonate, kaolin, titanium oxide, and organic particles. These particles may be used alone or in combination of two or more components. Among these, porous silica particles that are aggregates of primary particles are effective in reducing internal haze because they are inexpensive and less likely to generate voids around the particles when the film is stretched. The internal haze is a physical property obtained by evaluating scattering caused by the internal structure (void or the like) of the film, and can be measured by using, for example, ethanol as a rough surface compensation solvent.

  In the present invention, the internal haze of the film in the measurement method described later needs to be 1.0% or less. When the internal haze is larger than 1.0%, transparency when the film surface is treated with a hard coat or the like is lowered. In order to set the internal haze within the above range, for example, the type of particles used, the particle size, the addition amount, the thickness of the particle-containing layer, the strengthening of the filter in the production line, the film production conditions (film stretching temperature, stretching ratio), etc. It can be achieved by appropriately combining various conditions.

  As an average particle diameter of the particle | grains mix | blended with a polyester film, it is 0.5-5.0 micrometers normally, Preferably it is the range of 0.8-4.0 micrometers. When the average particle size is less than 0.5 μm, it is necessary to increase the amount of addition in order to impart sufficient slipperiness, and as a result, transparency may be impaired. In addition, when the average particle size exceeds 5.0 μm, the degree of roughening of the film surface becomes too large, and it is difficult to uniformly apply in the vicinity of the protrusion when applying the coating agent or the adhesive layer. Or the transparency of the film may be impaired.

  In order to improve the transparency of the film, a method of blending particles only in the outermost layer is also preferably employed. In this case, the outermost layer is at least one of the front and back layers, and particles can be blended in both the front and back layers.

  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 polyester, but it can be 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. You 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.

  Conventionally known antioxidants, heat stabilizers, lubricants, antistatic agents, fluorescent brighteners, dyes, pigments, and the like can be added to the polyester film of the present invention as necessary. Depending on the use, an ultraviolet absorber, particularly a benzoxazinone-based ultraviolet absorber, may be contained. Such additives may be blended only in the inner layer or only in the outermost layer. In order to prevent precipitation on the surface, it is preferably added to the inner layer.

  Next, although the manufacturing method of the film of this invention is demonstrated concretely, as long as the summary of this invention is satisfied, it 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 of dimethyl terephthalate and ethylene glycol, transfer the product to a polymerization tank, raise the temperature while reducing the pressure, and finally under vacuum To 280 ° C. to advance the polymerization reaction to obtain polyethylene terephthalate.

  The polyester may be chipped 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 polyester obtained has an intrinsic viscosity of preferably 0.40 dl / g or more, more preferably 0.40 to 0.90 dl / g, and particularly preferably 0.50 to 0.80 dl / g. When the intrinsic viscosity is less than 0.40 dl / g, the strength of the film is insufficient, the breakage occurs frequently at the time of film production, the productivity decreases, and the amount of oligomer contained increases, so it is used for optical use. In such a case, problems such as a decrease in transparency of the film may occur. On the other hand, when the intrinsic viscosity exceeds 0.90 dl / g, in the process of melt extrusion during film production, the viscosity is too high, the polyester deteriorates due to the temperature rise due to shearing heat generation, and the color tone changes, or streaky thickness unevenness occurs. Problems such as being likely to occur may occur.

  Next, the polyester chip obtained as described above 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 the die, and rapidly cooled and solidified on the rotary cooling drum so as to have a temperature 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.

  Stretching conditions are that the unstretched sheet is stretched 2 to 6 times at 70 to 145 ° C. in the longitudinal direction to obtain a longitudinal uniaxially stretched film, and then stretched 2 to 6 times at 90 to 160 ° C. in the transverse direction. Heat treatment is preferably performed 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 the method 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., improvement of secondary workability, For the purpose of improving weather resistance and surface hardness, a coating treatment with an aqueous solution, aqueous emulsion, aqueous slurry or the like can be performed. Such a coat may be either single-sided or double-sided. The coating material is preferably an aqueous system.

In order to make it adhere to the adhesive to PVA as a polarizing plate, or in order to improve adhesiveness with a hard coat, it is preferable to provide an application layer on at least one side.
The coating layer may contain an antistatic agent, an antifoaming agent, a coating property improving agent, a thickener, an antioxidant, an ultraviolet absorber, a foaming agent, a dye, a pigment, and the like.

As a coating method of the coating agent, a reverse roll coater, a gravure coater, a rod coater, an air doctor coater, or a coating apparatus other than these can be used.
In order to improve the coating property and adhesiveness of the coating agent to the film, the film may be subjected to chemical treatment or discharge treatment before coating. Further, in order to further improve the surface characteristics, a discharge treatment may be performed after the coating layer is formed.

The thickness of the coating layer is usually in the range of 0.02 to 0.5 μm, preferably 0.03 to 0.3 μm, as the final dry thickness. When the thickness of the coating layer is less than 0.02 μm, the effect of the present invention may not be sufficiently exhibited. When the thickness of the coating layer exceeds 0.5 μm, the films tend to stick to each other, and particularly when the coated film is re-stretched to increase the strength of the film, it adheres to the roll in the process. There is a tendency to become easy.
In addition, you may coat by the method called offline coating after manufacture of a film as needed. The coating material may be either water-based and / or solvent-based.

  Examples of the present invention will be described in detail below, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. Various physical properties and characteristics are measured or defined as follows. In the examples, “%” means “% by weight”.

(1) Measurement of intrinsic viscosity of polyester 1 g of polyester was accurately weighed and dissolved in 100 mL of a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio) and measured at 30 ° C.

(2) Average particle diameter (d50)
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.

(3) Measurement of thickness of each layer The cross section of the laminated film was observed using a scanning electron microscope (SEM) -S4500 manufactured by Hitachi, Ltd., and the thickness of each layer was determined.

(4) Measurement of in-plane retardation In-plane retardation at a wavelength of 589 nm was measured using a cell gap inspection apparatus RETS-1100A manufactured by Otsuka Electronics Co., Ltd. The measurement was continuously performed at 20 mm intervals in the width direction (TD full width) of the slit roll. In-plane retardation was measured at 23 ° C. using a rotating analyzer method with an aperture diameter of 2 mm.

(5) Light interference color observation After stretching 3 times in a first bath (iodine, KI aqueous solution-30 ° C) using a polyvinyl alcohol (hereinafter abbreviated as PVA) film (manufactured by Kuraray Co., Ltd., polymerization degree 2400). In a second bath (boric acid, KI aqueous solution-55 ° C.), the total draw ratio was drawn up to 6 times to obtain a polarizer. Thereafter, using a PVA adhesive, a TAC film having a thickness of 40 μm was bonded to one side of the PVA, and a polyester film was bonded to the other side of the PVA to prepare a polarizing plate. The polarizing plate was mounted on a liquid crystal panel as a polarizing plate on the backlight unit side so that the TAC film side was on the liquid crystal side, and visibility was confirmed.
◎: Good with no light interference color ○: Light interference color is confirmed slightly, but there is no problem in use ×: The light interference color is strong and the image is different from the original color tone

(6) Measurement of end tear resistance The ease of breaking of the master roll in the slit process was determined from the end tear resistance. The measurement is based on JIS C2318-1975, and each of MD (length direction) and TD (width direction) is measured at three measurement points on the right end, center, and left end of the slit roll, and the average of three points cut out from each position. The value was defined as an end tear resistance value. In addition, 70% or more of both MD and TD is a satisfactory level of fracture resistance.

(7) Measurement of internal haze Measurement was performed using a haze meter (HZ-2) manufactured by Suga Test Instruments. The measurement was performed by inserting the film into a glass cell with two jigs hollowed in a circle with a diameter of 50 mm in the center, and filling the glass cell with ethanol as a surface compensation solvent for the film. The measurement was performed at three measurement points on the right end, center, and left end of the slit roll, and the average value of the three points cut out from each position was used as the internal haze value.

(8) Comprehensive evaluation at each position (Roll right end, center, left end) Regarding the evaluation results of items (4) to (7) at each picking position, those that are excellent as polarizing plate members are not good. X.

(9) Final evaluation at all positions (Roll right end, center, left end) Considering the productivity as a polyester film roll, the final evaluation is good only when all the evaluations at each position in (8) are ○. did.

(Polyester chip manufacturing method)
(Method for producing polyester A)
Using 100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol as starting materials, 0.09 parts by weight of magnesium acetate tetrahydrate as a catalyst is placed in the reactor, the reaction start temperature is set to 150 ° C., and the methanol is distilled off gradually. The reaction temperature was raised to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. After 0.04 part of ethyl acid phosphate was added to this reaction mixture, 0.03 part of antimony trioxide was added and a polycondensation reaction was carried out 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 0.640 due to a change in the stirring power of the reaction vessel, and the polymer was discharged under nitrogen pressure. The intrinsic viscosity of the obtained polyester A was 0.640.

(Method for producing polyester B)
In the production method of polyester A, polyester B was obtained using the same method as the production method of polyester A, except that the starting dicarboxylic acid was 78 parts by weight of dimethyl terephthalate and 22 parts by weight of dimethyl isophthalate. The obtained polyester B had an intrinsic viscosity of 0.715.

(Method for producing polyester C)
Polyester A was preliminarily crystallized at 160 ° C. and then solid-phase polymerized in a nitrogen atmosphere at a temperature of 220 ° C. to obtain polyester C having an intrinsic viscosity of 0.850.

(Method for producing polyester D)
In the polyester A production method, after adding 0.04 part of ethyl acid phosphate, 0.6 part of silica particles having an average particle diameter of 3.2 μm dispersed in ethylene glycol and 0.03 part of antimony trioxide were added. Polyester D was obtained using the same method as the production method of polyester A, except that the polycondensation reaction was stopped at the time corresponding to the intrinsic viscosity of 0.620. Obtained polyester D was intrinsic viscosity 0.620.

(Manufacture of polyester film)
The raw materials obtained by mixing the above polyesters A to D in the proportions shown in Table 1 are respectively supplied to two extruders, and the layer structure is O (outermost layer) / M (intermediate layer) / O (outermost layer) polyester. A film was prepared. First, polyester was melt-extruded at 285 ° 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 3.4 times in the longitudinal direction at 85 ° C., and then subjected to a preheating process in a tenter and 4.0 times transverse stretching at 125 ° C., followed by heat treatment at 231 ° C. to obtain a polyester film. The total thickness of the obtained film was 38.0 μm from SEM observation, and the thickness of each layer is as shown in Table 1. Various evaluation results are as shown in Tables 2, 3, and 4.

  The film of this invention can be utilized suitably as a polarizing plate member, for example.

Claims (1)

It is a biaxially oriented polyester film having a laminate structure of at least three layers, and a layer other than both outermost layers contains a copolymerized polyester containing a third component other than the main constituent component, both outermost layers are made of homopolyester, The thickness of the outermost layer is 7.0 to 15.0% of the total layer thickness, the in-plane retardation of the film is 600 nm or less, and the end resistance value in the length direction of the film and the direction perpendicular thereto is A polyester film characterized in that both are 70 N or more and the internal haze is 1.0% or less.