JP2013235219A - Laminated polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated polyester film which reduces deflection in a chassis of a reflection sheet and is excellent in brightness unevenness.SOLUTION: There is provided a laminated polyester film in which a polyester layer (B) is laminated on at least one side of a polyester layer (A) containing cavities inside. If the center of gravity of a shape formed by removing the cavity part is defined as the area center and the thickness of the whole film is defined as 1 in an arbitrary cross-section in a thickness direction of the film, a distance X from the area center to a nearer film surface is 0.35 or more and 0.48 or less and the rigidity at a film bending angle at 15° measured by a taper-type rigidity tester satisfies the following expression: (1) 0.15≤ΔS≤0.70 and (2)ΔS≤-8X+4, provided ΔS: the difference in rigidity between the front side and the back side ΔS=SB-SA)/((SA+SB)/2) and SB: the rigidity (mN m) when bending to the B layer side and the rigidity (mN m) when bending to the A layer side.

Description

  The present invention relates to a laminated polyester film suitable for use as a light reflecting plate. More specifically, the present invention relates to a polyester film containing a cavity inside the film, excellent in film forming properties and reflection characteristics, and having excellent brightness unevenness by reducing deflection, and includes a backlight device and a lamp reflector for image display. The reflective sheet, the reflective sheet for lighting fixtures, the reflective sheet for lighting signs, the back reflective sheet for solar cells, etc., and the laminate that can be suitably used for the reflective plate of the backlight device for image display in particular. It relates to a polyester film.

  White polyester film containing cavities in applications such as reflectors and reflectors for surface light source devices, back reflector sheets for lighting signs, back reflector sheets for solar cells, etc., in flat image display systems used for liquid crystal displays, etc. 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 thick film is used because rigidity is required for a larger screen, and reflection sheets having various thicknesses from 100 μm to 500 μm are used according to the screen size. However, increasing the thickness of the reflective sheet is not preferable from the viewpoint of reducing the thickness of the liquid crystal display as well as increasing the cost, and there is a demand for increasing the rigidity of the reflective sheet without depending on the increase in thickness.

  On the other hand, in recent years, a method using an LED light source with low power consumption and high output has been used as a light source of a liquid crystal display. In addition to a method of arranging a light source on the back surface of a conventional backlight device, the light source has a side surface. A system that is advantageous for thinning is adopted. Correspondingly, the LED backlight housing has a concave and convex shape with a height of 10 mm or less for improving the strength of the housing, storing electrical wiring and the board, and heat dissipation at the edge of the housing near the LED light source. There may be a groove processing. In the method in which the LED light source is arranged on the side surface, the reflection sheet is installed on the above-described uneven housing, and the light guide plate is overlaid on the reflection sheet. The reflective sheet is pressed against the uneven portion of the housing. For this reason, the conventional reflective sheet has a problem that the reflective sheet is bent along the recess of the casing and uneven brightness occurs, and the demand for higher rigidity of the reflective sheet is becoming increasingly strong.

  However, since the cavity-containing polyester film used for the reflective sheet usually contains a cavity, its rigidity is inferior to that of a normal polyester film that does not contain a cavity. In the receiving paper application in which the void-containing polyester film is used as in the case of the reflection sheet, there is a problem that problems such as deterioration in handling property and generation of creases occur 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) are known.

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

  However, the thickness range for receiving paper is generally 25 to 75 μm, whereas the thickness range of the void-containing polyester film for reflecting sheet is as thick as 100 μm to 500 μm. Increasing the specific gravity of the void-containing polyester film leads to higher costs, and there is a problem that the reflective sheet is increased in weight and the workability (handling property) is deteriorated. Moreover, there is a concern that the reflectivity may decrease 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 problems, and reduces voids in the casing of the reflective sheet regardless of the increase in specific gravity and thickness, thereby providing a void-containing polyester film that is excellent in luminance unevenness and maintains high reflection characteristics. The purpose is to provide.

The laminated polyester film of the present invention that solves the above problems is a laminated polyester film in which a polyester layer (B) is laminated on at least one side of a polyester layer (A) containing a cavity therein,
In any cross section in the thickness direction of the film, the center of area of the shape formed by excluding the cavity from the shape P defined as follows is the center of area, and the thickness of the entire film is close to the center of the area when the thickness is 1. The distance X to the other film surface is 0.35 or more and 0.48 or less,
A laminated polyester film characterized in that the rigidity of a film at a bending angle of 15 degrees by a Taber stiffness tester satisfies the following formula. Here, the shape P is a rectangle, and one set of the two sides facing each other is perpendicular to the film thickness direction, the length is 60 μm, and the other set of sides is parallel to the film thickness direction. The length is the thickness T (μm) of the entire film.
(1) 0.15 ≦ ΔS ≦ 0.70
(2) ΔS ≦ −8X + 4
However, ΔS: Rigidity difference ΔS = (SB-SA) / ((SA + SB) / 2), SB: Stiffness when bent to B layer side (mN · m), SA: A layer side The degree of rigidity when bent (mN · m).

  According to the present invention, it is possible to obtain a laminated polyester film that reduces the deflection of the reflection sheet in the casing, has excellent luminance unevenness, and has excellent film forming properties and reflection characteristics at a low cost without increasing the specific gravity. .

It is sectional drawing of the backlight of this invention.

  The inventors of the present invention have made extensive studies on a laminated polyester film that solves the above-described problem, that is, has excellent brightness unevenness by reducing the deflection of the reflection sheet in the casing without increasing the specific gravity, and has excellent film forming properties and reflection characteristics. As a result, the inventors have found and found that a polyester film that selectively strengthens only the rigidity when bent to the support layer side can solve such problems all at once.

  The polyester film of the present invention needs to have a difference in front and back. In most cases, the reflection sheet is bent when the reflection sheet is bent toward the case. Therefore, the reflection sheet is required to have high rigidity when bent toward the case. It is preferable to provide a film that has a difference in rigidity between the front and back and selectively strengthens only the rigidity when bent to the housing side. Because there is no need to increase the film thickness, increase the specific gravity, and increase the thickness of the support layer unlike the conventional technology with no difference in rigidity, the film can achieve a low specific gravity. It is preferable because the amount of cavities serving as a reflection interface can be increased and high reflectance can be achieved. Moreover, the film excellent also in workability | operativity (handling property) at the time of a process can be obtained.

  The laminated polyester film of the present invention is a laminated polyester film in which a polyester layer (B) is laminated on at least one side of a polyester layer (A) containing a cavity inside. The A layer refers to the polyester layer (A), and the B layer refers to the polyester layer (B).

  In the present invention, the A layer contains cavities inside. By containing a large number of cavities inside, the reflectance can be increased while suppressing scattering loss by utilizing the difference in refractive index between the polyester resin and the cavities. As a method of making the layer A contain voids and exhibiting high reflectivity and concealment, (1) adding a foaming agent to the polyester and foaming it by heating during extrusion or film formation, or foaming by chemical decomposition (2) a method of adding a gas or a vaporizable substance during extrusion of the polyester, (3) adding a thermoplastic resin (incompatible resin) incompatible with the polyester to the polyester, (4) A method of adding a large amount of bubble-forming inorganic fine particles in place of the incompatible resin, and the like.

  In the present invention, from the viewpoints of film forming properties, ease of adjusting the amount of cavities contained therein, ease of forming cavities of a finer and uniform size, and the like (3) The use of an incompatible resin and the method (4) using inorganic fine particles are preferably used, and the use of the incompatible resin (3) is most preferable from the viewpoint of light weight. In this method, the more the content of the resin and / or inorganic particles incompatible with the polyester, the higher the stretching ratio in the biaxial stretching process, the more the interface that reflects light is generated inside the polyester. It is possible to exhibit high reflectivity and concealment.

  The incompatible resin may be a homopolymer or a copolymer. Polyolefin resin such as polyethylene, polypropylene, polybutene, polymethylpentene, cyclic polyolefin resin, polystyrene resin, polyacrylate resin, polycarbonate Resins, polyacrylonitrile resins, polyphenylene sulfide resins, fluororesins and the like are 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.

  Examples of the inorganic particles include titanium oxide, barium sulfate, calcium carbonate, silicon dioxide and the like, and barium sulfate is particularly preferably used.

  In the present invention, various additives may be added to the A layer as long as the effects of the present invention are not impaired. For example, inorganic fine particles such as calcium carbonate, titanium dioxide, zinc oxide, zirconium oxide, zinc sulfide, basic lead carbonate (lead white), barium sulfate, and alumina are used to adjust glossiness, whiteness, and light resistance. It may be added. These additives represented by inorganic particles can be used alone or in combination.

  The following are mentioned as a component which the polyester resin used for the laminated polyester film of this invention comprises. Examples of the dicarboxylic acid component include terephthalic acid, isophthalic acid, 5-sodium sulfoisophthalic acid, phthalic acid, diphenic acid and ester derivatives thereof for aromatic dicarboxylic acids, and adipic acid, for aliphatic dicarboxylic acids. Examples of sebacic acid, dodecadioic acid, eicoic acid, dimer acid and ester derivatives thereof include 1,4-cyclohexanedicarboxylic acid and ester derivatives thereof for alicyclic dicarboxylic acids, and trimellitic acid, pyrophosphate for polyfunctional acids. A merit acid and its ester derivative are mentioned as a representative example. Examples of the diol component include ethylene glycol, propanediol, butanediol, neopentyl glycol, pentanediol, hexanediol, octanediol, decanediol, cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyethylene glycol, and tetramethylene glycol. Typical examples include polyethers such as polyethylene glycol and polytetramethylene glycol. Considering the mechanical strength, heat resistance, production cost, etc. of the polyester film to be produced, it is preferable that the polyester resin in the present invention has polyethylene terephthalate as a basic structure. The basic constitution in this case means that 50% by weight or more of polyethylene terephthalate is contained with respect to the contained polyester resin.

In these polyester resins, various additives such as a fluorescent brightening agent, a crosslinking agent, a heat stabilizer, an oxidation stabilizer, an ultraviolet absorber, an organic lubricant, an inorganic lubricant, and the like within the range not impairing the effects of the present invention. Fine particles, fillers, light-proofing agents, antistatic agents, nucleating agents, dyes, dispersing agents, coupling agents, and the like may be added.
In the laminated polyester film of the present invention, the rigidity of the film at a bending angle of 15 degrees by a Taber stiffness tester needs to satisfy the following formula.
(1) ΔS = (SB-SA) / ((SA + SB) / 2)
(2) 0.15 ≦ ΔS ≦ 0.70
(3) ΔS ≦ −8X + 4
However, ΔS: rigidity difference, SB: rigidity when bent to the B layer side (mN · m), SA: rigidity when bent to the A layer side (mN · m), X: film thickness In any cross section in the direction, the film closer to the center of the area when the center of area is the center of gravity of the shape formed by removing the cavity from the shape P defined as follows, and the thickness of the entire film is 1. The distance to the surface. However, the shape P is a rectangle, and one set of the two sides facing each other is perpendicular to the film thickness direction, the length is 60 μm, and the other set of sides is parallel to the film thickness direction. The length is the thickness T (μm) of the entire film.
ΔS needs to be 0.15 or more and 0.70 or less. An increase in ΔS means that the rigidity when bent to the B layer side which is the support layer is selectively increased, and when used as a reflective sheet, the B layer side is directed to the housing side. This is preferable because the bending in the housing can be reduced. When it is used as a reflection sheet, if ΔS is less than 0.15, it is not preferable because bending within the housing is likely to occur and luminance unevenness occurs. Further, in order to prevent bending, the reflectance is impaired when the cavity of the A layer is reduced or the thickness of the B layer is increased as in the method performed in the prior art (see Patent Document 4). In addition, the increase in specific gravity is not preferable because the cost increases. From the viewpoint of luminance unevenness and reflectance, ΔS is more preferably 0.30 or more, further preferably 0.35 or more, and most preferably 0.50 or more. On the other hand, if ΔS exceeds 0.70 or does not satisfy the formula (2), the rigidity when bent to the A layer side which is the reflective surface becomes too low, and the film-forming property deteriorates, or the backlight This is not preferable because when a sheet is processed for incorporation, wrinkles are generated and luminance unevenness occurs. ΔS is more preferably 0.65 or less. ΔS can be adjusted to the above range by adjusting X and the curl value.

  In the laminated polyester film of the present invention, when the film thickness is 1, the distance X from the center of the area excluding the cavity portion of the cross section in the film thickness direction to the surface of the film closer to 0.35 or more and 0.48. Must be: When X is less than 0.35, the cavity occupation ratio of the A layer is too high, and the film forming property is deteriorated, which is not preferable. Further, when X exceeds 0.48, the difference between the front and back of the rigidity becomes small, that is, ΔS becomes small and falls below 0.15, which is not preferable. When X exceeds 0.46, ΔS is less than 0.30, so X is more preferably 0.46 or less, and most preferably 0.44 or less.

  Furthermore, it is preferable that the center of the area (hereinafter referred to as the area center) excluding the hollow portion of the cross section in the film thickness direction of the laminated polyester film of the present invention is present in the A layer. When the center of area does not exist in the A layer, the B layer becomes too thick, which is not preferable because of poor reflectance.

  A method of setting X in the above range is not particularly limited, but a method of making the layer configuration asymmetric in the thickness direction is preferable. Among them, a two-layer configuration of A layer / B layer is preferable, but a three-layer configuration such as B layer / A layer / C layer or B layer / A layer / C layer, or a configuration of four layers or more. May be. In the case of three or more layers, the layer disposed on the surface closer to the center of the area was the B layer, and the layer disposed on the opposite surface was the C layer.

  Moreover, X can be adjusted by adjusting the cavity occupation rate of each layer and the thickness of each layer in the cross section when cut perpendicularly to the surface of each film. In the laminated polyester film of the present invention, it is preferable from the viewpoint of uneven brightness that the B layer does not substantially contain bubbles. The phrase “substantially containing no bubbles” means a layer state in which the cavity occupation ratio is less than 5%. In the laminated polyester film of the present invention, since the reflection characteristics are expressed by the cavity of the A layer, the higher the cavity occupation ratio of the A layer, the more preferable in terms of reflectance. The cavity occupation ratio of the A layer is 20% or more. Preferably, it is 32% or more. On the other hand, when the cavity occupancy of the A layer becomes too high, it is preferable in terms of reflectivity, but the mechanical strength of the A layer decreases and the film forming property deteriorates. It is preferable that it is 60% or less. In the laminated polyester film of the present invention, the cavity occupation ratio of the A layer is preferably higher than the cavity occupation ratio of the B layer, and X can be reduced by increasing the difference between the cavity occupation ratios of the A layer and the B layer. . In order to set X in the above range, the difference in the cavity occupancy between the A layer and the B layer is preferably 15% or more, and more preferably 32% or more. On the other hand, if the difference in the cavity occupancy between the A layer and the B layer becomes too large, the mechanical strength decreases and the film forming property deteriorates. From the viewpoint of film forming property, the difference in the cavity occupancy between the A layer and the B layer is It is preferable that it is 50% or less.

  Next, the thickness of the B layer will be described. In the laminated polyester film of the present invention, as described above, it is preferable that the cavity occupation ratio of the A layer is higher than the cavity occupation ratio of the B layer. In this case, X can be reduced by increasing the thickness of the B layer. However, if the thickness of the B layer exceeds 35% with respect to the total thickness of the film, it becomes difficult for the center of the area to be located in the A layer, resulting in poor reflectivity. Therefore, from the point that X can be in the above range and the reflectance, the thickness of the B layer is preferably 10 to 50%, more preferably 10 to 35%, and most preferably 15 with respect to the thickness of the entire film. ~ 35%.

In the present invention, the total thickness of the laminated polyester film is preferably 100 μm or more and 500 μm or less, and more preferably 150 μm or more and 350 μm or less. The total thickness of the laminated polyester film is preferably 100 μm or more from the viewpoint of reflectivity, and the thicker the thickness, the higher the rigidity and the less likely to bend in the housing. The upper limit is not particularly limited, but is preferably 500 μm or less, more preferably 350 μm or less from the viewpoint of reflectance, workability, and cost. If the thickness exceeds 500 μm, an increase in reflectivity cannot be expected even if the thickness is increased beyond this, and workability (handling properties) deteriorates due to the increase in weight when working as a single wafer for incorporation into a backlight.

In the present invention, the overall apparent density of the laminated polyester film is preferably 0.5 g / cm ³ or more and 0.9 g / cm ³ or less. By setting the apparent density to 0.5 g / cm ³ or more, the rigidity is increased, and it is possible to suppress the unevenness in brightness caused by the bending of the reflection sheet due to the unevenness of the casing and the bending during processing and assembly. Is also preferable. On the other hand, from the viewpoint of reflectance, it is preferable that there are many cavities, that is, the apparent density is low, and the apparent density is preferably 0.9 g / cm ³ or less. An apparent density of 0.9 g / cm ³ or less is preferable from the viewpoint of cost. Further, when used as a reflective sheet for a large screen backlight having a diagonal size of 101.6 cm or more, the reflective sheet is high. Since it does not increase in weight, it is also preferable from the viewpoint of workability (handling property) when working as a single sheet for incorporation into a backlight.

  The curl value of the laminated polyester film of the present invention is preferably from 0.25 mm to 0.80 mm, more preferably from 0.30 mm to 0.70 mm. Here, the curl value is a curl value when the film is cut into a rectangle with a width of 38 mm and a length of 70 mm, and is measured with the direction convex toward the B layer as a positive value. By curling so as to be convex toward the B layer side, ΔS can be increased, and when used as a reflective sheet, the B layer side is directed toward the housing side, so that bending within the housing is unlikely to occur. Therefore, the occurrence of uneven brightness can be suppressed, and the curl value is preferably a positive value from the viewpoint of film forming properties. As the curl value increases, ΔS can be increased. In order to set ΔS to 0.30 or more, the curl value is preferably set to 0.25 mm or more. In order to set ΔS to a value of −8X + 4 or less, or 0.70 or less, the curl value is preferably 0.80 mm or less.

  The relative reflectance at a wavelength of 560 nm of at least one surface of the laminated polyester film of the present invention is preferably 97.0% or more when used as a reflective sheet. The relative reflectance is more preferably 98.5% or more, further preferably 99.0% or more, and most preferably 99.5% or more. The reflectance can be increased by increasing the cavity occupancy of the A layer or by reducing the B layer within a range where X does not deviate from the above range.

  Next, the manufacturing method of the laminated polyester film of this invention is demonstrated. Here, as a method for causing the layer A to contain cavities, a thermoplastic resin (incompatible resin) that is incompatible with the polyester is added to the above-mentioned (3) polyester, and then uniaxially or biaxially stretched. A method for generating fine cavities, (4) a method for adding a large amount of bubble-forming inorganic fine particles in place of the incompatible resin, and the like will be described. However, the method is not limited to this example.

As a manufacturing method of the polyester film of this invention, it is preferable that it is a process which includes the following processes in the order.
[Step 1] A step of melting a polyester resin with an extruder.
[Step 2] A step of extruding a molten resin from an extruder to form an unstretched film.
[Step 3] A step of stretching the unstretched film in at least one direction.
[Step 1] [Step 2] [Step 3] will be described in detail below.

  First, in a composite film forming apparatus having an extruder (A) and an extruder (B), first, in order to form an A layer, polyester pellets having a melting point of 230 to 280 ° C. are sufficiently vacuum dried. In order to develop voids, it is preferable to add 10 to 50% by weight of non-amorphous resin or bubble-forming inorganic particles to this dry raw material. Next, this dry raw material is supplied to an extruder (A) heated to a temperature of 240 to 320 ° C., filtered through a 10 to 50 μm cut filter after melt extrusion, and then introduced into a T-die composite die. .

  On the other hand, in order to form B layer, the vacuum-dried polyester pellet is fully vacuum-dried. This dry raw material includes fluorescent brighteners, crosslinking agents, heat stabilizers, oxidation stabilizers, UV absorbers, organic lubricants, inorganic fine particles, fillers, light stabilizers, antistatic agents, nucleating agents, dyes, You may add a dispersing agent, a coupling agent, etc. Next, this dry raw material is supplied to an extruder (A) heated to a temperature of 240 to 320 ° C., filtered through a 10 to 50 μm cut filter after melt extrusion, and then introduced into a T-die composite die. .

  The polymer of the extruder (A) and the polymer of the extruder (B) are introduced into a T-die composite die, and are laminated so as to be A / B or A / B / A using a feed block device. To obtain a melt-laminated sheet. This melt-laminated sheet is cooled and solidified by a casting drum cooled to 10 to 60 ° C. to become an unstretched film. At this time, if the cooling capacity is different between the cooling roll surface and the opposite surface, the structure such as crystallinity is different between the front and back of the film, and the curl value fluctuates. Therefore, it is preferable to cool the film surface opposite to the cooling roll surface by blowing a low-temperature gas or liquid, and the method of blowing low-temperature air is particularly advantageous in terms of simplicity of equipment. When blowing low-temperature air, in order to increase the cooling rate, the air temperature is preferably 7 ° C. or higher and 15 ° C. or lower, and the air velocity is preferably 15 m / second or higher and 30 m / second or lower. . If the air speed is less than the above range, the heat transfer coefficient between the air and the film will be insufficient and the cooling speed will be slow. If the air speed exceeds 30 m / sec, the film between the die and the roll will be vibrated by the air, resulting in uneven thickness. become. As a method for blowing air, a known apparatus that blows air uniformly from nozzles in which slits and holes are randomly arranged can be used. In addition, passing the heat exchanger using cooling water or liquid nitrogen before supplying air to the nozzle is effective for setting the temperature of the air within the above range.

Next, this unstretched film is heated above the glass transition temperature (Tg) of the polyester by roll heating, infrared heating or the like, and stretched in the longitudinal direction (hereinafter referred to as the longitudinal direction) to obtain a longitudinally stretched film. This stretching is preferably performed by utilizing the difference in peripheral speed between two or more rolls. Moreover, it is preferable that the temperature of the both surfaces of the film in extending | stretching satisfy | fills the following conditions.
5 ≦ T1-T2 ≦ 30
T1: The temperature of the film surface on the side of the A layer, T2: The temperature of the film surface of the side of the B layer side is differentiated between the front and back surfaces of the film, thereby causing distortion in the structure on the front and back of the film, resulting in a curl value. Can be adjusted. When T1-T2 is lower than the lower limit, the curl value becomes small, which is not preferable. Further, when T1-T2 is less than 0 ° C., the film-forming property in the subsequent film-forming process is deteriorated due to the deterioration in flatness in the longitudinal stretching process. This is not preferable because uneven brightness occurs. On the other hand, if the value is higher than the upper limit, the curl value exceeds 0.80 mm, and when the sheet is processed for incorporation into a backlight, wrinkles are generated, resulting in uneven brightness and crystallization of polyester progresses. It is not preferable because the film-forming stability after the longitudinal stretching is deteriorated due to the deterioration of the flatness or the flatness.

  The ratio of longitudinal stretching is preferably 2.2 to 4.0 times, more preferably 2.3 to 3.9 times, although it depends on the required characteristics of the application. If it is less than 2.2 times, the reflection characteristics are inferior, the thickness unevenness of the film is deteriorated and a good film cannot be obtained, and if it exceeds 4.0 times, breakage tends to occur during film formation, which is not preferable. The film after longitudinal stretching is subsequently subjected to stretching in the direction orthogonal to the longitudinal direction (hereinafter referred to as the transverse direction), heat setting, and thermal relaxation to form a biaxially oriented film. While running. The transverse stretching process starts from a temperature higher than the glass transition temperature (Tg) of the polyester. And it is performed while raising the temperature to (5 to 70) ° C. higher than Tg. Although the temperature rise in the transverse stretching process may be continuous or stepwise (sequential), the temperature is usually raised sequentially. For example, the transverse stretching zone of the tenter is divided into a plurality along the film running direction, and the temperature is raised by flowing a heating medium having a predetermined temperature for each zone. The transverse stretching ratio is preferably 2.5 to 4.5 times, more preferably 2.8 to 3.9 times, although it depends on the required characteristics of the application. If it is less than 2.5 times, the thickness unevenness of the film is deteriorated and a good film cannot be obtained, and if it exceeds 4.5 times, breakage tends to occur during film formation.

  In this way, by stretching the obtained unstretched film in at least one direction, it is possible to develop cavities with the polyester or non-phase-like resin or bubble-forming inorganic particles as nuclei. Here, the case of stretching by the sequential biaxial stretching method has been described in detail as an example, but the laminated polyester film of the present invention may be stretched by either the sequential biaxial stretching method or the simultaneous biaxial stretching method.

  In order to complete the crystal orientation of the obtained biaxially stretched film and to impart flatness and dimensional stability, it was then heat-set in a tenter at a temperature of 120 to 220 ° C. for 1 to 30 seconds, and uniform After slow cooling, cool to room temperature and wind up. During the heat treatment step, a relaxation treatment of 3 to 12% may be performed in the horizontal direction or the vertical direction as necessary.

  When the obtained laminated film is wound in the shape of a roll and wound on a film roll, it is preferably wound so that the B layer side is on the outer surface side of the roll. By winding so that the B layer side becomes the outer surface of the roll, the laminated polyester film used by being unwound from the roll is preferable because it has appropriate curl on the A layer side and ΔS is improved.

  The laminated polyester film of the present invention obtained in this way and the laminated polyester film unwound from the polyester film roll can maintain the reflectivity and film-forming property without increasing the specific gravity, while maintaining the brightness of the liquid crystal backlight. Since unevenness is reduced, it can be suitably used as a reflection sheet for a surface light source of a liquid crystal screen edge light and a direct light, and a reflector. In particular, it can be suitably used in a monitor backlight device in which the diagonal dimension of the screen in which the LED light source is arranged on the side surface is 50.8 cm or more.

  In addition, the present invention relates to an edge light type backlight for a liquid crystal display using the above-described laminated polyester film, and from the viewpoint of uneven brightness, the B layer is preferably arranged toward the housing side, and the edge for the liquid crystal display A light-type backlight is preferred. Hereinafter, embodiments of a backlight according to the present invention will be described with reference to the drawings. In addition, although embodiment of this invention is described based on drawing below, those drawings are provided for illustration and this invention is not limited to those drawings.

FIG. 1 is a schematic sectional view showing the configuration of an embodiment of a backlight according to the present invention.
In the backlight of this embodiment, a plurality of point light sources 4 are arranged at one end of a light guide plate 1 made of a light-transmitting material, and a reflector 2 is provided on the side opposite to the light emitting surface side of the light guide plate 1. The reflector 2 is placed on the housing 3.

  Here, the housing 3 is configured to have irregularities with a height of 5 to 10 mm on the surface facing the reflector 2. Moreover, the laminated polyester film mentioned above is used for the reflecting plate 2. About the point light source 4, the structure made into any arrangement | positioning of the one side edge part of the light-guide plate 1, a both-sides edge part, and a four side edge part may be sufficient. The light guide plate 1 is not a feature of the present invention, and a detailed description thereof will be omitted.

  In the backlight of this embodiment, since the laminated polyester film described above is used for the reflector 2, even if the reflector 2 is placed on the uneven housing 3, the heat generated from the light source when the light source is turned on. It is possible to suppress the bending of the reflecting plate 2 at the groove portion. Therefore, a backlight with little luminance unevenness can be obtained.

  Moreover, in the backlight of this embodiment, since the housing 3 has unevenness with a height of 5 to 20 mm on the surface facing the reflector 2, the rigidity of the housing itself is increased. Further, the concave portion serves as a vent hole to release heat inside the backlight to the outside, and an abnormal increase in the temperature of the backlight can be suppressed. Furthermore, a wiring board can be accommodated on the back side of the convex portion. In addition, the height of the unevenness was measured by using another scale for the distance of the recess from the scale placed on the convex surface. When the height of the unevenness is less than 5 mm, it is difficult to store the wiring board. When the height is more than 20 mm, it is difficult to make the backlight thin, which is not preferable.

In the present invention, the size of the backlight is preferably such that the diagonal dimension of the screen is 66 cm or more and 177.8 cm or less. Here, the size of the backlight refers to the length of the diagonal line of the backlight. Backlights within the above range are less susceptible to bending due to the weight of the film itself constituting the reflector. In addition, a brightness non-uniformity prevention effect is sufficiently obtained with a backlight exceeding the above range.
[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 using a microtome without crushing in the thickness direction, and a section sample was obtained.
The cross section of the section sample was imaged at a magnification of 2000 using a scanning electron microscope (FE-SEM) S-2100A manufactured by Hitachi, Ltd., and the thickness of each layer was calculated by measuring the laminated thickness.

(2) Cavity occupancy As shown in (1) above, from the region of width 60 (μm) in the film surface direction of the cross-sectional photograph taken with a scanning electron microscope, only the cavity portion is traced with an oil pen on the OHP transparent film. Then, using the image analyzer, the occupied area (A) of the cavity of the A layer painted with the oil-based pen was calculated. Using the obtained occupied area (A) and the area of the width A (μm) in the film surface direction of the A layer, the value calculated by the following formula is defined as the cavity occupation ratio. Similarly, for the B layer, the area occupied by the cavity of the B layer (B) is calculated, the area of the width of the B layer in the film surface direction 60 (μm) is used, and the value calculated by the following formula is the cavity occupation ratio. And

Cavity occupancy in layer A = (A) / area of film layer direction width 60 (μm) × 100 (%)
Cavity occupancy in layer B = (B) / area of width 60 (μm) in the film surface direction of layer B × 100 (%).

(3) Apparent density The film was cut into a 100 × 100 mm square, and a thickness of 10 points was measured with a dial gauge (No. 2109-10, manufactured by Mitoyo Seisakusho) attached with a 10 mm diameter probe (No. 7002). Then, the average thickness d (μm) is calculated. Further, this film is weighed with a direct balance, and the weight w (g) is read up to a unit of 10 ⁻⁴ g. The value calculated by the following formula is the apparent density.
Apparent density = w / d × 100 (g / cm ³ ).

(4) Rigidity The rigidity was measured at a bending angle of 15 ° according to JIS P-8125-2000, and a taper stiffness tester TELEDYNE MODEL150-D (NORTH Tonowanda, New York USA) was used. The stiffness when bent to the B layer side and the stiffness when bent to the A layer side were measured at three points, and the average values were taken as SB and SA, respectively.

(5) Center of area The center of area is a set of sides perpendicular to the film thickness direction when the cross section of the film cut perpendicularly to the film surface is observed with a scanning electron microscope as in (1) above. The center of gravity of the shape formed by removing the cavity from the rectangular shape P formed by another set of sides (length T (μm)) parallel to the film thickness direction (length 60 μm), and X is the film This is the distance from the center of the area to the closest film surface when the total thickness is 1. That is, X is always 0.5 or less. The calculation method is as follows. First, the thickness of each layer, the area I of the portion excluding the cavity in the cross section in the film thickness direction from the cavity occupancy, and the cavity in one of the regions divided in the thickness direction at the center of the area are excluded. X was calculated by calculating the area II of the part and solving the equation I ÷ 2 = II. In addition, when obtaining the area I and the area II, the length in the film surface direction may be calculated with an arbitrary value. However, in the calculation method described below, the area per 60 μm in the surface direction is defined as the area I. , Calculating area II. Here, TA: A layer thickness (μm), TB: B layer thickness (μm), TC: C layer thickness (μm), T: Total film thickness (μm), YA: A layer occupied by cavity Rate (%), YB: cavity occupancy of layer B (%), YC: cavity occupancy of layer C (%).

(I) In the case of a two-layer configuration of A / B layers A layer having a high cavity occupation ratio was calculated as the A layer. Since the cavity occupancy is higher in the A layer than in the B layer, the area center exists at a position close to the surface of the B layer. Therefore, the following calculation was performed on the assumption that the distance from the B layer surface to the area center is X. . The area of the portion excluding the cavity in the cross section in the film thickness direction is calculated by the following equation.
I (μm ² ) = 60 × (TA × (1−YA / 100) + TB × (1−YB / 100))
On the other hand, assuming that the area of the portion excluding the cavity in the region on the B layer side that is divided into two in the thickness direction at the center of the area has the center of the area in the A layer
II (μm ² ) = 60 × ((T × X−TB) × (1−YA / 100) + TB × (1−YB))
Is calculated. Since II is half of I, X was obtained from I ÷ 2 = II. When the area center was present in the A layer, that is, when the value of X obtained was larger than the value obtained from the formula (TB ÷ T), this value was taken as X.

On the other hand, when the area center exists in the B layer, that is, when the obtained value of X is smaller than the value obtained from the formula (TB ÷ T), the B layer side divided into two in the thickness direction at the area center The area of the part excluding the cavity in the area of
II ′ (μm ² ) = 60 × T × X × (1-YB / 100)
Is calculated. Since II ′ is half of I, X was calculated from I ÷ 2 = II ′.

(Ii) In the case of a three-layer configuration of B / A / C layers In the present invention, in the case of a configuration of three or more layers, the layer disposed on the surface closer to the area center is the B layer, and the opposite surface The arranged layer is a C layer, but if one of the surfaces close to the center of the area is unknown, first, temporarily determine that one surface is the B layer, and reach the center of the area from the film surface on the B layer side. X is calculated by performing the calculation described below, where X is the distance. When the obtained X value was 0.5 or less, the temporarily determined B layer was determined as the B layer, and the obtained X value was used. On the other hand, when the obtained value of X exceeds 0.5, the provisionally determined layer B is wrong, so the layer disposed on the surface opposite to the provisionally determined layer B is B The layer was determined to be X, and the value of X ′ obtained by X ′ = 1−X was used as X.

The area of the part except the cavity part in the whole film is calculated by the following formula.
I (μm ² ) = 60 × (TA × (1−YA / 100) + TB × (1−YB / 100) + TC × (1−YC / 100))
On the other hand, assuming that the area of the portion excluding the cavity in the region on the B layer side that is divided into two in the thickness direction at the center of the area has the center of the area in the A layer
II (μm ² ) = 60 × ((T × X−TB) × (1−YA / 100) + TB × (1−YB))
Is calculated. Since II is half of I, X was obtained from I ÷ 2 = II. When the area center exists in the A layer, that is, when the obtained value of X is larger than the value obtained from the equation (TB ÷ T) and smaller than the value obtained from the equation ((TB + TA) ÷ T). This value was taken as X.

On the other hand, when the obtained area center exists in the B layer or the C layer, that is, the obtained value of X is smaller than the value obtained from the formula (TB ÷ T) or from the formula ((TB + TA) ÷ T). When larger than the obtained value, assuming that the area of the portion excluding the cavity in the region on the B layer side divided into two in the thickness direction at the center of the area has the area center in the B layer,
II ′ (μm ² ) = 60 × T × X × (1-YB / 100)
Is calculated. Since II ′ is half of I, X was obtained from I ÷ 2 = II ′. When the area center was present in the A layer, that is, when the obtained value of X was smaller than the value obtained from the formula (TB ÷ T), this value was taken as X.

On the other hand, when the obtained area center exists in the C layer, that is, when the obtained value of X is larger than the value obtained from the equation ((TB + TA) ÷ T), the area center is divided into two in the thickness direction. The area of the portion excluding the cavity in the region on the B layer side,
II ′ ′ (μm ² ) = 60 × T × (1-X) × (1-YC / 100)
Is calculated. Since II ′ ′ is half of I, X was calculated from I ÷ 2 = II ′ ′.

(Iii) In the case of a multi-layer structure of 4 layers or more In the same way as in the case of 2 layers and 3 layers, first, the thickness of each layer, the area I of the portion excluding the cavity in the entire film, the thickness at the center of the area The area II of the part excluding the cavity in one of the two regions divided in the direction was calculated, and X was calculated by solving the equation I ÷ 2 = II.

  Here, the calculation method of X has been described on the assumption that the cavity occupancy of each layer is uniform in the thickness direction. However, when the cavity occupancy is non-uniform in the thickness direction, the above ( From the cross-sectional photograph taken with a scanning electron microscope as in 1), only the cavity is traced on the OHP transparent film with an oil pen, and then the center of mass of the portion painted with the oil pen using an image analyzer is used. The layer located on the film surface closer to the mass center of gravity obtained by the analysis is defined as layer B, and the thickness of the entire film is assumed to be 1 when the thickness of the entire film is 1. The distance to was X.

(6) Curl value The film was cut into a rectangle with a width of 38 mm and a length of 70 mm, and placed on a flat surface so that the B layer was on the lower surface. The rising of the four corners of the film at this time was measured, and the average value of the four corners was taken as the curl value (mm). At this time, when the four corners were not lifted, the film was turned over and the film was placed on a flat surface so that the B layer was on the upper surface, and the lift of the four corners was measured, and the average value of the four corners was taken as the curl value. At this time, the curl value when the four corners of the film were lifted when the B layer was the lower surface was a positive value, and the curl value when the film four corners were lifted when the B layer was the upper surface was a negative value.

  In addition, when the B layer was used as the lower surface and the upper surface, when the four corners of the film did not rise in any case, the curl value was set to zero.

(7) Surface temperature of film during stretching The temperature of the film surface during stretching was measured at an emissivity of 0.95 using a Keyence non-contact thermometer IT2-80. The measurement was performed at the center in the width direction of the film and at three points at both ends. The average value was defined as the film temperature, the temperature of the film surface on the A layer side was T1, and the temperature of the film surface on the B layer side was T2.

(8) Relative reflectance When a spectrophotometer (Shimadzu Corporation UV2450) is equipped with an integrating sphere attachment device (ISR2200, Shimadzu Corporation), barium sulfate is used as a standard plate, and the standard plate is used as 100%. The relative reflectance at a wavelength of 550 nm was measured.
A: 99.5% or more ○: 99% or more and less than 99.5% Δ: 98.5% or more and less than 99% Δ: less than 98.5% ×: less than 97.0%.

(9) Brightness unevenness
A backlight having a diagonal size of 116.8 cm in the C7000 series manufactured by Samsung Corp. (patterned light guide plate having a thickness of 5 mm, a plurality of LED light sources arranged on the left and right sides of the light guide plate, An edge light type backlight for a liquid crystal display in which a reflecting plate disposed on the side opposite to the light emitting surface side of the light guide plate and a casing on which the reflecting plate is placed are disposed on the reflecting plate side of the casing. Has four concave portions that are arranged substantially parallel to each other and connected from the upper side portion to the lower side portion, and convex portions (height 7 mm) partitioned by the concave portions and the upper and lower sides of the housing, and the rear surface of the convex portion of the housing. The edge light type backlight for the liquid crystal display in which the electronic board is stored on the side.) The reflective film laminated inside was changed to a predetermined film sample and turned on. In addition, the film sample was installed so that the B layer side might be arranged toward the housing | casing side at this time. In this state, after waiting for 1 hour to stabilize the light source, an object that can be visually recognized as uneven luminance was observed under a 500 lx illumination environment or a dark environment. Furthermore, after overlapping two diffusion plates RM803 (manufactured by Sumitomo Chemical Co., Ltd., thickness 2 mm) / diffusion sheet GM3 (manufactured by Kimoto Co., Ltd.), it is also visually observed in a 500 lx illumination environment or in a dark environment. The presence or absence and the degree of luminance unevenness were observed, luminance unevenness was determined according to the following criteria, x was rejected, and Δ or more was determined to be acceptable. Note that the luminance unevenness referred to here is due to bending, wrinkling, or poor flatness of the reflection sheet.
A: Excellent (even if the diffusion plate / diffusion sheet is overlapped or not overlapped, luminance unevenness cannot be seen in a 500 lx illumination environment or in a dark environment)
○: Good (Without diffusion plate / diffusion sheet, luminance unevenness is not visible in a 500 lx lighting environment, but in a dark environment, uneven luminance is visible. (There is no brightness unevenness in both lighting and dark environments.)
△ ○: Yes (Without diffuser / diffusion sheet, brightness unevenness can be seen both in a 500 lx lighting environment and in a dark environment. However, if a diffusion plate / diffusion sheet is stacked, both in a 500 lx lighting environment and in a dark environment Uneven brightness is not visible.)
△: Inferior (Without diffusion plate / diffusion sheet, luminance unevenness can be seen both in a 500 lx lighting environment and in a dark environment. When a diffusion plate / diffusion sheet is stacked, luminance unevenness cannot be seen in a 500 lx lighting environment. However, uneven brightness is visible in a dark environment.)
×: very inferior
(Even if the diffusion plate / diffusion sheet is stacked, luminance unevenness can be seen both in a 500 lx illumination environment and in a dark environment.)

(10) Film formation stability It was evaluated according to the following criteria whether the film could be formed stably. X was rejected, and Δ or more was determined to be acceptable.
A: A film can be stably formed for 36 hours or more.
○: A film can be stably formed for 24 hours or more.
Δ: Film can be stably formed for 12 hours or more and less than 24 hours.
X: Breakage occurs within 12 hours, and stable film formation is not possible.

  The present invention will be described based on examples.

<Examples 1-4, 5, 6, 8-22>
Using terephthalic acid as the acid component and ethylene glycol as the glycol component, adding to the polyester pellets that can obtain antimony trioxide (polymerization catalyst) to 300 ppm in terms of antimony atoms, performing a polycondensation reaction, limiting viscosity Polyethylene terephthalate pellets (PET) having 0.63 dl / g and carboxyl end group amount of 40 equivalents / ton were obtained.
The raw material for the A layer and the raw material for the B layer shown in Table 1 or 2 are respectively supplied to two extruders of an extruder (A) and an extruder (B) heated to 280 ° C. Using a two-layer feed block apparatus in which the layer and the B layer are A layer / B layer, they were introduced into a T-die composite die, laminated, and coextruded into a sheet to obtain a melt-extruded sheet. Abbreviations in Table 1 have the following meanings.
・ PMP: Polymethylpentene (“TPX” manufactured by Mitsui Chemicals, Inc.)
COC: Cyclic olefin copolymer (copolymer resin of ethylene and norbornene) (“TOPAS” manufactured by Polyplastics)
BaSO ₄ : Barium sulfate (average particle size 0.5 μm)
Further, this sheet was cooled and solidified with a cooling drum having a surface temperature of 20 ° C. to obtain an unstretched film. At this time, the opposite side of the cooling drum was cooled by blowing cold air of 10 ° C. at a wind speed of 20 m / sec. Next, this unstretched film is led to a roll group heated to 85 ° C., and stretched 3.4 times in the longitudinal direction (longitudinal direction) using the peripheral speed difference of two or more rolls. Cooled down. In addition, the output was adjusted by the radiation heater installed in each of the A layer side and the B layer side so that the A layer side film temperature TA and the B layer side film temperature TB during stretching would be the temperatures shown in Table 1. While stretching, the film being stretched was heated. Subsequently, the film was stretched at a magnification of 3.6 in a direction perpendicular to the longitudinal direction (lateral direction) in an atmosphere heated to 120 ° C. while being held at both ends of the longitudinally stretched film with clips. Then, heat setting was performed at 190 ° C. in a tenter, lateral width was placed, and the mixture was cooled to room temperature to obtain a cavity-containing biaxially stretched film. A sample of 10 mm × 10 mm was arbitrarily cut from the obtained film, and the film was cut without crushing in the thickness direction using a microtome to obtain a slice sample. The cross section of the section sample was observed with SEM, and X was calculated from the obtained image. In addition, observation was performed by 2000 times and the range of 60 micrometers was observed as the length of the surface direction of a film. The other physical properties of the obtained X and film were as shown in Table 3 or Table 4.

<Example 5>
The raw materials described in Table 1 are respectively supplied to two extruders of an extruder (A) and an extruder (B) heated to 280 ° C. so that the A layer and the B layer become A layer / B layer. Using a two-layer feed block device, it was introduced into a T-die composite die, laminated, and coextruded into a sheet to obtain a melt-extruded sheet. Further, this sheet was cooled and solidified with a cooling drum having a surface temperature of 20 ° C. to obtain an unstretched film. At this time, cold air was not blown to the opposite surface side of the cooling drum. Thereafter, except for taking the conditions shown in Table 1, longitudinal stretching, lateral stretching, heat setting and lateral width insertion were performed in the same manner as in Example 1, and cooled to room temperature to obtain a void-containing biaxially stretched film. Table 3 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1.

<Example 7>
The raw material for layer B shown in Table 1 is supplied to the extruder (B) heated to 280 ° C., and the raw material for layer A shown in Table 1 is supplied to the extruder (A). It was used, introduced into a T-die composite die, laminated and coextruded into a sheet to obtain a melt-extruded sheet. At this time, the polymer of the extruder (B) is at the center, the polymer of the extruder (A) is on both sides, and the pressure difference between the front and back layers in the composite pipe is different, so that the front and back surface thicknesses are different. A laminated sheet composed of B layer / A layer / B layer (C layer) was coextruded to obtain a molten laminated sheet. Thereafter, a white laminated polyester film was obtained in the same manner as in Example 1 except that the conditions shown in Table 1 were taken. Table 3 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1.

<Comparative Examples 1-4>
The raw material for A layer and the raw material for B layer shown in Table 2 are respectively supplied to two extruders of an extruder (A) and an extruder (B) heated to 280 ° C., and B layer / A layer / B layer (C layer) was introduced into a T-die composite die using a three-layer feed block device and laminated and coextruded into a sheet to obtain a melt-extruded sheet. Thereafter, a white laminated polyester film was obtained in the same manner as in Example 1 except that the conditions shown in Table 1 were taken. Table 4 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1. X exceeded 0.48, and the difference ΔS in rigidity was less than 0.15, which was very inferior in luminance unevenness.

<Comparative Examples 5 and 6>
A void-containing film was produced in the same manner as in Example 1 except that the raw materials and conditions described in Table 2 were taken. Table 4 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1. The curl value exceeded 0.80 mm, the stiffness difference ΔS exceeded the value of −8X + 4, breakage occurred, and stable film formation was not possible, resulting in poor film formation stability. In addition, the obtained film was very inferior in luminance unevenness because it easily folds and wrinkles when working as a sheet for incorporation into a backlight.

<Comparative Example 7>
A void-containing film was produced in the same manner as in Example 1 except that the raw materials and conditions described in Table 2 were taken. Table 4 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1. The curl value exceeded 0.80 mm, the stiffness difference ΔS exceeded the value of −8X + 4, breakage occurred, and stable film formation was not possible, resulting in poor film formation stability. Further, the obtained film was easy to bend and wrinkled when working as a single sheet for incorporation into a backlight, but brightness unevenness occurred, but it was at a usable level.

<Comparative Example 8>
A void-containing film was produced in the same manner as in Example 1 except that the raw materials and conditions described in Table 2 were taken. Table 4 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1. The curl value exceeded 0.80 mm, ΔS was as high as 0.73, and exceeded the value of −8X + 4. Breaking occurs, stable film formation is not possible, and film formation stability is inferior, and the obtained film tends to be wrinkled when working as a single sheet for incorporation into a backlight, resulting in uneven brightness. It was inferior.

<Comparative Example 9>
A void-containing film was produced in the same manner as in Example 1 except that the raw materials and conditions described in Table 2 were taken. Table 4 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1. The curl value exceeded 0.80 mm, ΔS was as high as 0.81, breakage occurred, stable film formation was not possible, and film formation stability was inferior. In addition, the obtained film was inferior in luminance unevenness because it easily folds and wrinkles when working as a sheet for incorporation into a backlight.

<Comparative Example 10>
A void-containing film was produced in the same manner as in Example 1 except that the raw materials and conditions described in Table 2 were taken. Table 4 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1. The curl value was −0.70 mm, curling with the A layer side convex, and the difference in stiffness ΔS was less than 0.15. The flatness of the film after longitudinal stretching deteriorated, breakage occurred, and stable film formation was not possible, resulting in poor film formation stability.

<Comparative Examples 11 and 12>
A void-containing film was produced in the same manner as in Example 1 except that the raw materials and conditions described in Table 2 were taken. Table 4 shows the physical properties of the obtained film. Since the cavity occupation ratio of the A layer was too high, X was less than 0.35, and the film forming property was inferior.

<Comparative Example 13>
A void-containing film was produced in the same manner as in Example 1 except that the raw materials and conditions described in Table 2 were taken. Table 4 shows X and other physical properties calculated from the obtained film in the same manner as in Example 1. Since the cavity occupation rate of the A layer was too low, X exceeded 0.48, and the reflectance was very poor.

1 Light Guide Plate 2 Reflector Plate 3 Housing 4 Light Source

Claims (10)

A laminated polyester film in which a polyester layer (B) is laminated on at least one side of a polyester layer (A) containing a cavity inside,
In any cross section in the thickness direction of the film, the center of area of the shape formed by excluding the cavity from the shape P defined as follows is the center of area, and the thickness of the entire film is close to the center of the area when the thickness is 1. The distance X to the other film surface is 0.35 or more and 0.48 or less,
A laminated polyester film characterized in that the rigidity of a film at a bending angle of 15 degrees by a Taber stiffness tester satisfies the following formula. Here, the shape P is a rectangle, and one set of the two sides facing each other is perpendicular to the film thickness direction, the length is 60 μm, and the other set of sides is parallel to the film thickness direction. The length is the thickness T (μm) of the entire film.
(1) 0.15 ≦ ΔS ≦ 0.70
(2) ΔS ≦ −8X + 4
However, ΔS: Rigidity difference ΔS = (SB-SA) / ((SA + SB) / 2), SB: Stiffness when bent to B layer side (mN · m), SA: A layer side The degree of rigidity when bent (mN · m). The laminated polyester film according to claim 1, wherein the center of the area is present in the A layer. The laminated polyester film according to claim 1 or 2, wherein the difference in front and back ΔS of the rigidity is 0.30 or more and 0.70 or less. The curl value when the film is cut into a rectangle having a width of 38 mm and a length of 70 mm is 0.25 to 0.80 mm or less, with the direction convex to the B layer side being positive. Laminated polyester film. The laminated polyester film according to claim 4, wherein the curl value is 0.30 or more and 0.80 or less. The laminated polyester film according to any one of claims 1 to 5, wherein a relative reflectance at a wavelength of 550 nm of at least one surface is 97.0% or more. The manufacturing method of the laminated polyester film in any one of Claims 1-6 which contain the following processes in the order.
[Step 1] A step of melting a polyester resin with an extruder.
[Step 2] A step of extruding a molten resin from an extruder to form an unstretched film.
[Step 3] A step of stretching an unstretched film in at least one direction, wherein the temperatures of both surfaces of the stretched film satisfy the following conditions.
5 ≦ T1-T2 ≦ 30
T1: Temperature (° C.) of the film surface on the layer A side.
T2: Temperature (° C.) of the film surface on the layer B side. [Step 2] is a step of extruding the molten resin into a sheet shape and cooling it on a cooling roll to obtain an unstretched film, and the step of cooling by blowing air on the film surface opposite to the cooling roll surface. The method for producing a laminated polyester film according to claim 7, comprising: An edge light type backlight for a liquid crystal display comprising the casing and the laminated polyester film according to any one of claims 1 to 6, wherein the layer B side of the laminated polyester film is arranged toward the casing side. Edge-lit backlight for liquid crystal displays. A manufacturing method of an edge light type backlight for a liquid crystal display using a casing and a laminated polyester film produced by using the production method according to claim 7, wherein the laminated polyester film is B. A manufacturing method of an edge light type backlight for a liquid crystal display in which a layer side is arranged toward a housing side.

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