JP2015030232A - White polyester film and light reflection plate using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a void-containing polyester film with which crease during handling of a reflection film and crease during peeling of a tape are reduced, and which maintains high reflection characteristics.SOLUTION: A white polyester film has a film constitution of A-layer/B-layer or A-layer/B-layer/A-layer, and satisfies requirements mentioned below, wherein the B-layer has voids. (1) The B-layer contains a resin incompatible with polyester, inorganic particles, and air bubbles; (2) the content of the resin incompatible with polyester is 3-10 mass%; (3) the content of the inorganic particles is 10-30 mass%; (4) the specific gravity GA of the A-layer and the specific gravity GA of the B-layer satisfy following formula: 0≤|GA-GB|≤0.5; (5) the rigidity of the white polyester film is less than 1.0 mN; (6) the total thickness of the white polyester film is less than 150 μm; and (7) when a folding test using a core rod having a diameter of 6 mm is performed in such a state that the A-layer is set to be inside, in a bending test according to JISK-5600-5-1, the formation of crease in the A-layer is not found.

Description

  The present invention relates to a white polyester film suitable for use as a light reflecting plate, a light reflecting plate using the same, and a backlight for a liquid crystal display.

White polyester film containing cavities in applications such as reflectors and reflectors for surface light source devices, back reflector films for lighting signs, back reflector films for solar cells, etc., in flat image display systems used in liquid crystal displays and the like However, it is widely used because of its uniform and high brightness, dimensional stability and low cost. As a method to achieve high brightness, polyester film contains many inorganic particles such as barium sulfate, and uses light reflection at the interface between the polyester resin and the particles and the hollow interface of the fine cavities generated using the particles as nuclei. (Refer to Patent Document 1), a method of utilizing light reflection at the cavity interface of fine cavities generated by mixing an incompatible resin with polyester and using an incompatible resin as a nucleus (Patent Document 2) Inorganic contained in the polyester film, such as a method utilizing light reflection at the interface of the cavity formed inside by impregnating the polyester film with an inert gas in a pressure vessel (see Patent Document 3) A method using a difference in refractive index between particles and polyester resin and a difference in refractive index between fine voids and polyester resin is widely used.

In recent years, liquid crystal displays have been diversified, and the screen size of liquid crystal displays has a wide range of applications from portable mobile applications to personal computers and large screen TV applications. In the reflection sheet, a smaller reflection film is used as the screen is smaller, and reflection sheets having various thicknesses ranging from 80 μm to 500 μm are used in accordance with the screen size.
Especially in portable mobile applications and small applications of less than 20 inches, such as personal computers, the case of the backlight manufacturing process prevents the reflective film placed on the back surface from being displaced due to vibrations when carrying it in a car or on foot. There is a step of attaching a reflective film to a double-sided tape attached to a metal chassis. There is a problem that the thinner the reflective film, the lower the rigidity, and the tendency to crease when handling the reflective film. Moreover, when a sticking position shifted | deviated, although a reflective film was once peeled from a double-sided tape, the wrinkle generate | occur | produced in that case, and the subject that a defective article was generated as a result occurred.

  Usually, the void-containing polyester film used for the reflective sheet contains voids inside, and therefore has a problem that it is less rigid and more likely to bend and wrinkle than a normal polyester film that does not contain voids. there were. In the receiving paper application in which the void-containing polyester film is used in the same manner as the reflection sheet, there are problems such as deterioration in handling property and generation of wrinkles due to a decrease in rigidity. As a means for increasing the rigidity of the void-containing polyester film in receiving paper applications, a method of increasing the specific gravity of the film (see Patent Document 4) or a method of increasing the thickness of the support layer laminated on both sides of the void-containing layer (patent) Documents 4 and 5) and a method for increasing the overall thickness of the film (see Patent Document 6) are known.

JP 2003-160682 A Japanese Patent Publication No. 8-16175 JP 2001-166295 A JP 2002-105232 A JP 2001-121665 A JP 2012-179816 A

  However, while the thickness range for receiving paper is generally 25 to 75 μm, the thickness range of the void-containing polyester film for reflective sheet is as thick as 80 μm to 500 μm, and the specific gravity of the void-containing polyester film is increased. There are concerns about problems leading to higher costs and a decrease in reflectivity due to a decrease in the cavity interface. On the other hand, the method of securing rigidity by increasing the thickness of the support layer also leads to an increase in specific gravity, and the ratio of the reflection layer to the total thickness decreases, so the problem that the reflectivity decreases as the support layer increases. there were.

  The present invention solves the above-mentioned problems, and provides a void-containing polyester film that reduces creases during handling of the reflective film and creases during peeling of the tape and maintains high reflection characteristics, regardless of the increase in specific gravity and thickness. The purpose is to provide.

  The white polyester film of the present invention that solves the above problems comprises a film configuration of A layer / B layer or A layer / B layer / A layer composed of polyester, and B layer is a white polyester film containing cavities. The white polyester film is a white polyester film for a light reflecting plate characterized by satisfying all of the following requirements (1) to (7).

(1) The B layer contains a resin and inorganic particles that are incompatible with the polyester, and has air bubbles. (2) The content of the resin incompatible with the polyester in the B layer is based on the mass of the B layer. 3% by mass or more and less than 10% by mass (3) The content of inorganic particles contained in the B layer is 10% by mass or more and 30% by mass or less with respect to the mass of the B layer. The density GB of the GA and B layers satisfies the following formula.

0 ≦ | GA-GB | ≦ 0.5
(5) The rigidity of the white polyester film is less than 1.0 mN. (6) The total thickness of the white polyester film is less than 150 μm. (7) The diameter in the bending test specified in JIS K5600-5-1. A white polyester film characterized in that when a test is performed using a 6 mm core rod to turn the A layer inside, the A layer is not folded and wrinkled.
Moreover, the object of the present invention is achieved by a light reflecting film using the white polyester film.

  According to the present invention, it is possible to reduce creases during handling of the reflective film and creases during peeling of the tape regardless of the increase in specific gravity and thickness of the white polyester film for light reflector. The present invention can be suitably used particularly in a backlight device for a portable use and a small use of less than 20 inches such as a personal computer.

It is a schematic sectional drawing which shows the liquid crystal screen of the sidelight system incorporating the white polyester film of this invention.

  The white polyester film of the present invention comprises an A layer / B layer or an A layer / B layer / A layer film structure composed of polyester, and the B layer is a white polyester film containing cavities, the white polyester film A white polyester film for a light reflector, wherein the film satisfies all the following requirements (1) to (7).

(1) The B layer contains a resin and inorganic particles that are incompatible with the polyester, and has air bubbles. (2) The content of the resin incompatible with the polyester in the B layer is based on the mass of the B layer. 3% by mass or more and less than 10% by mass (3) The content of inorganic particles contained in the B layer is 10% by mass or more and 30% by mass or less with respect to the mass of the B layer. The density GB of the GA and B layers satisfies the following formula.

0 ≦ | GA-GB | ≦ 0.5
(5) The rigidity of the white polyester film is less than 1.0 mN. (6) The total thickness of the white polyester film is less than 150 μm. (7) The diameter in the bending test specified in JIS K5600-5-1. The white polyester film is characterized in that when a test is performed using a 6 mm core rod to turn the A layer inside, the A layer is not folded and wrinkled.
The present invention is required to have a film configuration of A layer / B layer or A layer / B layer / A layer. In order to achieve a high reflectivity, the B layer needs to be a layer containing fine bubbles. Further, the A layer is necessary for increasing the reflectance improvement effect, ensuring film-forming stability, and achieving excellent handling properties.

[A layer]
In the present invention, the A layer is a surface layer. In order to provide the role of preventing light leakage to the back surface and the role of a support layer that stabilizes the film formation, the A layer preferably contains inorganic particles in the polyester and has a role of scattering light. The light scattering property of the A layer can be adjusted mainly by controlling the surface roughness. As another method, for example, a method of adding particles having different refractive indexes to a polyester resin can be mentioned. Here, the kind of inorganic fine particles to be contained in the A layer is not particularly limited, but preferably has a Mohs hardness of 3.0 or more, for example, calcium carbonate, titanium dioxide, zinc oxide, silicon dioxide, zinc sulfide. , Barium sulfate, alumina, talc and the like. These inorganic particles can be used alone or in combination depending on the necessity of imparting surface functions such as glossiness adjustment, whiteness adjustment, and light resistance.

In this invention, preferable content of the inorganic particle contained in A layer is 0.01 to 25 mass% with respect to A layer mass. It is preferable when the apparent density of the A layer is 0.7 g / cm ³ or more and 1.2 g / cm ³ or less. Moreover, when content of an inorganic particle exceeds 25 mass%, there exists a problem that a film tends to cohesively break by A layer at the time of tape peeling. Here, cohesive failure is a phenomenon in which when the tape is peeled off after the tape is applied to the A layer, the film peels off in the A layer because the specific gravity of the A layer is low, and the peeled film adheres to the tape surface. It is. On the other hand, if it is less than 0.01% by mass, the scattered light may be insufficient and sufficient reflection performance may not be obtained.

  In the present invention, the resin constituting the A layer and the B layer is polyester. In particular, polyethylene terephthalate and polyethylene naphthalate are preferable.

  In addition, various known additives such as an antioxidant and an antistatic agent may be added to the polyester.

[B layer]
The B layer is whitened by containing fine bubbles inside the film. The B layer is a layer satisfying the following. (1) A layer containing a resin and inorganic particles that are incompatible with polyester, and having bubbles. (2) The content of the resin incompatible with the polyester in the B layer is 3% by mass or more and less than 10% by mass with respect to the mass of the B layer. (3) Content of the inorganic particle contained in B layer is 10 to 30 mass% with respect to the mass of B layer. Formation of fine bubbles can be achieved by finely dispersing a polymer incompatible with polyester in a film base material such as polyester and stretching it (for example, biaxial stretching).

[Incompatible resin (resin incompatible with polyester)]
In the present invention, the B layer needs to contain a resin incompatible with the polyester. As a resin incompatible with polyester (hereinafter sometimes abbreviated as incompatible resin), it may be a homopolymer or a copolymer, such as polyethylene, polypropylene, polybutene, polymethylpentene, etc. A polyolefin resin, a cyclic polyolefin resin, a polystyrene resin, a polyacrylate resin, a polycarbonate resin, a polyacrylonitrile resin, a polyphenylene sulfide resin, a fluorine resin, or the like is preferably used. Two or more of these may be used in combination. In particular, a resin having a large difference in critical surface tension from polyester and being hardly deformed by heat treatment after stretching is preferable, and specifically, a polyolefin-based resin is preferable. Examples of the polyolefin resin include polyolefin resins such as polyethylene, polypropylene, polybutene, and polymethylpentene, cyclic polyolefin resins, and copolymers thereof. Among these, a copolymer of ethylene and bicycloalkene, which is a cyclic olefin copolymer, is particularly preferable.

  By containing the incompatible resin, a cavity having the incompatible resin as a nucleus is formed at the time of stretching, and light reflection occurs at the cavity interface.

In this invention, preferable content of the incompatible resin contained in B layer is 3 mass% or more and 10 mass% or less. Since the number of cavities increases by increasing the content of the incompatible resin, the density decreases. By setting it as the above content, the apparent density of the B layer can be 0.7 g / cm ³ or more and 1.2 g / cm ³ or less. On the other hand, when the content of the incompatible resin is 10% by mass or more, the apparent density is lowered, and creases and wrinkles are easily generated when the reflective film is handled. On the other hand, if it is less than 3% by mass, the apparent density increases and no creases occur, but the reflection performance decreases.

  In the present invention, the incompatible resin to be contained in the B layer is dispersed in a matrix composed of a polyester resin with a number average particle size of 0.4 μm or more and 3.0 μm or less. It is preferable for obtaining film strength, and more preferably in the range of 0.5 μm or more and 1.5 μm or less. The number average particle size referred to here is a cross section of the film in the width direction (TD), and the B layer portion of the cross section is observed using a scanning electron microscope (FE-SEM) S-2100A manufactured by Hitachi, Ltd. It is the average value of the diameter when the area of 100 particles is calculated and converted into a perfect circle.

In this invention, preferable content of the inorganic particle contained in B layer is 10 to 30 mass% with respect to B layer mass. By setting this range, the apparent density of the B layer can be 0.7 g / cm ³ or more and 1.2 g / cm ³ or less. On the other hand, if the content of the inorganic particles is less than 10% by mass, the apparent density is lowered, and the crease during handling of the reflective film is likely to occur. On the other hand, if it exceeds 30% by mass, the apparent density increases and no creases occur, but film formation may be difficult.
Examples of the inorganic particles contained in the B layer include calcium carbonate, titanium dioxide, zinc oxide, zirconium oxide, zinc sulfide, basic lead carbonate (lead white), and barium sulfate. Among these, 400 to 700 nm. Calcium carbonate, barium sulfate, titanium dioxide and the like, which absorb less in the visible light region, are preferable from the viewpoints of reflection characteristics, concealability, production cost, and the like. In the present invention, barium sulfate and titanium dioxide are the most preferable from the viewpoints of rollability of the film, long-term film-forming stability, and improved reflection characteristics. As the particle size of the inorganic particles, it is preferable to use particles having a number average particle size of 0.1 μm or more and 3.0 μm or less in order to realize excellent reflectivity and concealability.

[Copolyester]
In the present invention, it is preferable to use a copolyester for the B layer. Even a composition containing an inorganic substance at a high concentration in the B layer can be stably formed, and has a role as a dispersant for the incompatible resin in the B layer.

  Copolyester has a main dicarboxylic acid component of terephthalic acid, a main glycol component of ethylene glycol, and a copolymer component of aromatic carboxylic acid or aliphatic carboxylic acid such as isophthalic acid or naphthalene dicarboxylic acid, and tetra A polyester containing at least one selected from the group consisting of aliphatic diols such as methylene glycol, cyclohexanedimethanol, polyethylene glycol, and polytetramethylene glycol.

  The content of the copolyester contained in the B layer is 1% by mass or more and 20% by mass or less based on the mass of the B layer. If it is less than 1% by mass, film formation may be difficult, and if it exceeds 20% by mass, thermal dimensional stability may be lacking.

  The ratio of the copolymerization component in the copolymerized polyester is preferably 1 to 30 mol%, more preferably 3 to 25 mol% per total dicarboxylic acid component or total diol component. If it is less than 1 mol%, the dispersibility of the incompatible resin is lowered, the reflection performance is lowered, and the stretching stress is increased when inorganic particles are contained, so that film formation may not be possible. On the other hand, if it exceeds 30 mol%, thermal dimensional stability may be lacking or film formation may be difficult.

[Rigidity]
In the present invention, the white polyester film needs to have a rigidity of less than 1.0 mN. A film having a rigidity exceeding 1.0 mN is easy to handle and is not easily broken. By making the total thickness of the white polyester film less than 150 μm and the apparent density of the A layer and the B layer being 0.7 or more and 1.2 or less, the rigidity can be made less than 1.0 mN.

[Apparent density]
In the present invention, the relationship between the apparent density GA of the A layer and the apparent density GB of the B layer is 0 ≦ | GA−GB | ≦ 0.5. When GA-GB exceeds 0.5, the internal density is low and the rigidity is low, and when the film is bent, the interface between the incompatible resin, which is the core of the cavity, and PET tends to buckle, and finally the A layer An external force that bends and acts, causing wrinkles. In addition, when the film is bent, the force propagating from the surface of the film is concentrated between the A layer and the B layer, so that wrinkles are generated. Moreover, when GA-GB is less than -0.5, the internal density is high, the rigidity is high, and the B layer is less likely to buckle and bend and wrinkle when the film is bent. Moreover, there exists a problem that a film is easy to cohesive failure with A layer at the time of tape peeling.

  In the present invention, the apparent density of the A layer and the B layer is preferably from 0.7 to 1.2. If the apparent density of the A layer is less than 0.7, the film is likely to cohesively break in the A layer when the tape is peeled off. On the other hand, if the film exceeds 1.2, the B layer is likely to be folded and wrinkled. There's a problem.

  If the apparent density of the B layer is less than 0.7, the B layer is likely to buckle when the film is bent. Therefore, even if the apparent density of the A layer is 0.7 or more and 1.2 or less, creases are likely to occur. On the other hand, when the apparent density of the B layer exceeds 1.2, there is a problem that the specific gravity is increased and the cost is increased and the reflectance is decreased.

[Bending test]
In order to prevent generation of creases during handling of the reflective film and creases during peeling of the tape, in the present invention, in the bending test specified in JIS K5600-5-1, the A layer is formed using a core rod having a diameter of 6 mm. When folded inward, it is necessary that the A layer does not crease.
In the above bending test, in order to prevent the A layer from being folded and wrinkled, the total thickness of the white polyester film is set to less than 150 μm, and the relationship between the apparent density GA of the A layer and the apparent density GB of the B layer is 0 ≦ | GA− This can be achieved by GB | ≦ 0.5.

[Total thickness]
In particular, in the present invention, the white polyester film for a light reflector is preferably used in a portable device or a backlight device having a small size of less than 20 inches such as a personal computer. .

[Production method]
Next, although the manufacturing method of the white polyester film of this invention is demonstrated, it is not limited to this example. Cyclic olefin as incompatible resin, polyethylene glycol, polybutylene terephthalate and polytetramethylene glycol copolymer as low specific gravity agent are mixed with polyethylene terephthalate, and then mixed and dried to a temperature of 270 to 300 ° C. To the extruder B heated to Polyethylene terephthalate containing barium sulfate as inorganic particles is supplied to Extruder A by a conventional method, and A layer / B layer / A layer is formed so that the polymer of Extruder B layer is on both surface layers in the T-die 3 layer die. A three-layer construction sheet is obtained.

  This melted sheet is closely cooled and solidified by electrostatic force on a drum cooled to a drum surface temperature of 10 to 60 ° C., and the unstretched film is led to a group of rolls heated to 80 to 120 ° C. in the longitudinal direction. The film is stretched 2.0 to 5.0 times longitudinally and cooled with a roll group of 20 to 50 ° C. Subsequently, the film is stretched in the direction perpendicular to the longitudinal direction in an atmosphere heated to 90 to 140 ° C. while being guided to a tenter while holding both ends of the longitudinally stretched film with clips. The stretching ratio is 2.5 to 4.5 times in the longitudinal and lateral directions, and the area ratio (longitudinal stretching ratio x lateral stretching ratio) is preferably 9 to 16 times. If the area magnification is less than 9 times, the whiteness of the resulting film becomes poor. Conversely, if it exceeds 16 times, the film tends to be broken during stretching and the film forming property tends to be poor. In order to give the flatness and dimensional stability of the biaxially stretched film in this way, heat setting at 150 to 230 ° C. in a tenter, uniform cooling, cooling to room temperature and winding up A polyester film is obtained.

  The white polyester film for reflecting a liquid crystal display according to the present invention thus obtained has a high reflectance because fine bubbles are formed inside the film, and is used as a reflector for liquid crystal displays of side light type and direct light type. In this case, high brightness can be obtained.

[Measurement of physical properties and evaluation method of effects]
The physical property value evaluation method and the effect evaluation method of the present invention are as follows.

(1) Film thickness / layer thickness The thickness of the film was measured according to JIS C2151-2006.

The film was cut in the thickness direction using a microtome to obtain a section sample.
The cross section of the slice sample was imaged at a magnification of 3,000 times using a field emission scanning electron microscope (FE-SEM) S-800 manufactured by Hitachi, Ltd., and the thickness of each layer was calculated by measuring the stack thickness.

(2) Apparent density After peeling a film into each layer, it cuts out to a 100x100 square mm, measures thickness of 10 points with what attached the dial gauge, and calculates the average value d (micrometer) of thickness. Further, each layer is weighed with a direct balance, and the weight w (g) is read to a unit of 10 ⁻⁴ g. At this time, the apparent density of the A layer and the B layer was determined by the following equation.

Apparent density (g / cm ³ ) = w / d × 100.

(3) Rigidity The rigidity was measured at a bending angle of 15 ° according to JIS P-8125-2000, and a taper type rigidity tester TELEDYNE MODEL150-D (NORTH Tonowanda, manufactured by New York USA) was used.

(4) Bending test A bending test specified in JIS K5600-5-1 is performed in the longitudinal direction of the test piece, in which a test piece having a sample size of 100 mm x 50 mm is folded using a core rod having a diameter of 6 mm with the A layer inside. This was done, and the bent portion was visually judged.
A: No wrinkle in the A layer and withstanding bending F: A wrinkle in the A layer.

(5) Tape peeling test (cohesive failure)
A white film is sampled into a rectangle with a width of 20 mm and a length of 100 mm, and the double-sided tape (Nitto Denko) with a width of 20 mm and a length of 100 mm so that the film and the mirror-finished stainless steel plate (SUS304) are aligned. Affixed with double-sided tape No. 500). In the case of two layers, the layers are bonded so as to be A layer / B layer / double-sided tape / SUS304, and in the case of three layers, they are bonded so as to be A layer / B layer / A layer / double-sided tape / SUS304. Next, the edge of the white film is peeled off and lifted, and peeled at 135 ° from the stainless steel (peeling speed 50 mm / sec) in the longitudinal direction to completely peel off. Cohesive failure was visually evaluated.
(Cohesive failure is a phenomenon in which when the tape is peeled off, the specific gravity of the A layer is low, so the film peels off in the A layer and the peeled film adheres to the tape surface.)
A: The peeled film is not attached to the double-sided tape. F: The peeled film is attached to the double-sided tape.

(6) Tape peeling test (folded wrinkles)
The crease of the film surface after peeling of (5) was visually judged from the A layer side. A and B are acceptable.
A: Generation of wrinkles is 0/10 mm ²
B: Generation of wrinkles 1 to 3/10 mm ²
F: Generation of wrinkles is 4 or more / 10 mm ² .

(7) Relative luminance (side light type luminance)
As shown in FIG. 1, the reflection film of the 4-lamp backlight of the liquid crystal monitor (750B) manufactured by Samson Co., Ltd. was changed to the reflection film produced in each example and comparative example, and measurement was performed. For luminance measurement, a household power supply 100V is used, and voltage is applied by switching the ON / OFF switch. It waited for 1 hour until the brightness of the cold cathode tube became uniform and constant. Thereafter, the luminance was measured with a luminance meter (BM-7fast manufactured by topcon) at a measurement distance of 500 mm. The number of measurements is 3, and the average value is calculated. For brightness evaluation, Toray Industries, Inc. # 250E6SL was used as a reference sample (100%),
The evaluation results were as follows.
A: 103% or more B: 102% or more and less than 103% C: 101% or more and less than 102% F: Less than 101%.

(8) Film formation stability The grade of whether or not the film could be stably formed was determined according to the following criteria.

  A: The film can be stably formed for 24 hours or more.

  B: The film can be stably formed for 12 hours or more and less than 24 hours.

  F: Breakage occurs within 12 hours, and stable film formation cannot be performed.

  The present invention will be described based on examples.

[Example 1]
Polyethylene glycol having a molecular weight of 4000 and a color tone of polyethylene terephthalate after polymerization (measured by JIS K7105-1981, stimulus value direct reading method) of L value 62.8, b value 0.5, haze 0.2% Using terephthalate, 83.3 parts by mass of polyethylene terephthalate, 1.4 parts by mass of (PBT / PTMG) copolymer of polybutylene terephthalate and polytetramethylene glycol (trade name: Hytrel manufactured by Toray DuPont), ethylene glycol In contrast, 2.3 parts by mass of copolymerized polyethylene terephthalate (33 mol% CHDM copolymerized PET) obtained by copolymerizing 33 mol% of 1,4-cyclohexanedimethanol, 3 parts by mass of poly (5-methyl) norbornene, and 10 wt% of rutile-type titanium oxide. 3 parts at 180 ° C. After drying for a period of time, it was supplied to the extruder B heated to 270 to 300 ° C. (B layer).

  On the other hand, 72 parts by mass of polyethylene terephthalate chips, 20 parts by mass of barium sulfate polyethylene terephthalate master having a number average particle size of 1.4 μm (containing 60% by mass of barium sulfate with respect to the total amount of master chips), and isophthalic acid to polyethylene terephthalate 8 parts by mass of 18 mol% copolymerized (PET / I) was vacuum dried at 180 ° C. for 3 hours, and then supplied to Extruder A heated to 280 ° C. (A layer). The layers were laminated through a laminating apparatus so as to be B layer / A layer (4 μm / 137 μm / 4 μm), and formed into a sheet form from a T die. Further, the unstretched film obtained by cooling and solidifying this film with a cooling drum having a surface temperature of 25 ° C. was led to a roll group heated to 85 to 98 ° C., longitudinally stretched 3.4 times in the longitudinal direction, and cooled with a roll group at 21 ° C. . Subsequently, the film was stretched by 3.6 times in a direction perpendicular to the longitudinal direction in an atmosphere heated to 120 ° C. while being guided to a tenter while holding both ends of the longitudinally stretched film with clips. Thereafter, the film was heat-set at 200 ° C. in a tenter, uniformly cooled, cooled to room temperature, and a biaxially stretched laminated film was obtained. The physical properties of the light reflecting substrate are shown in Table 1.

[Examples 2 to 22]
A white polyester film was obtained in the same manner as in Example 1, except that the raw material composition of the A layer and B layer, the thickness of the A layer, and the total film thickness were changed as described in Tables 1 to 3. In all examples, the bending test, cohesive failure, crease, and luminance were good.

[Comparative Examples 1-6]
A film having a thickness of 145 μm was obtained in the same manner as in Example 1 except that the laminated structure and the raw material composition were changed as described in Table 4. Although there was film-forming stability and high luminance was obtained even in relative luminance, since the content of the incompatible resin in the B layer exceeded 10% by mass, bending test and bending wrinkle were insufficient.

[Comparative Example 7]
A film having a thickness of 145 μm was obtained in the same manner as in Example 1 except that the laminated structure and the raw material composition were changed as described in Table 4. Although high luminance was obtained even in relative luminance, the film formation stability was insufficient because the content of the inorganic particles in the B layer exceeded 30% by mass.

[Comparative Example 8]
A film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the laminated structure and the raw material composition were changed as described in Table 4. Although high luminance was obtained even in relative luminance, the bending test was insufficient because the thickness was 150 μm or more.

Here, the abbreviations in Tables 1 to 4 represent the following contents. That is,
PET: Polyethylene terephthalate,
PET / I: Polyethylene terephthalate copolymerized with 18 mol% isophthalic acid,
PET / CHDM: polyethylene-1,4-cyclohexylenedimethylene terephthalate (polyethylene terephthalate copolymer obtained by copolymerizing 33 mol% of 1,4-cyclohexanedimethanol with respect to ethylene glycol),
PBT / PTMG: Polyester ether elastomabutylene / poly (alkylene ether) phthalate (copolymer having 30 mol% of alkylene glycol with respect to butylene terephthalate) (trade name: Hytrel manufactured by Toray DuPont)
It is.

  The present invention relates to a white polyester film suitable for use as a light reflecting plate, a light reflecting plate using the same, and a backlight for a liquid crystal display. In particular, an object is to provide a void-containing polyester film that reduces creases during handling of the reflective film and creases during peeling of the tape and maintains high reflection characteristics regardless of the increase in specific gravity and thickness. .

1: White polyester film for light reflection 2: Light guide plate 3: Prism sheet 4: Cold cathode tube 5: CCD camera 6: Image analysis device (eye scale)

Claims (4)

The A layer / B layer or the A layer / B layer / A layer composed of polyester comprises a white polyester film containing voids, and the white polyester film has the following (1) A white polyester film for a light reflector characterized by satisfying all the requirements of (7).
(1) The B layer contains a resin and inorganic particles that are incompatible with the polyester, and has air bubbles. (2) The content of the resin incompatible with the polyester in the B layer is based on the mass of the B layer. 3% by mass or more and less than 10% by mass (3) The content of the inorganic particles contained in the B layer is 10% by mass or more and 30% by mass or less with respect to the mass of the B layer. Density GA and apparent density GB of layer B satisfy the following formula
0 ≦ | GA-GB | ≦ 0.5
(5) The rigidity of the white polyester film is less than 1.0 mN. (6) The total thickness of the white polyester film is less than 150 μm. (7) The diameter in the bending test specified in JIS K5600-5-1. When a test is performed using a 6mm core rod with the A layer facing inward, the A layer should not be folded and wrinkled. The white polyester film for a light reflecting plate according to claim 1, wherein the apparent density of the A layer and the B layer is 0.7 or more and 1.2 or less. The white polyester film for a light reflecting plate according to claim 1 or 2, wherein the inorganic particles of the B layer are titanium dioxide or barium sulfate. The light reflection board using the white polyester film for light reflection boards in any one of Claims 1-3.

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