JP4830274B2 - White polyester film - Google Patents

Description

The present invention relates to a white polyester film. More specifically, the present invention relates to a white polyester film having a laminated structure composed of at least an inner layer and a surface layer and excellent in whiteness, surface gloss, and the like.

Various white polyester films containing white inorganic fine particles such as calcium carbonate, titanium oxide, and barium sulfate in polyester have been proposed for various uses such as magnetic recording cards, photographic paper, and receiving paper. Widely used. Further, such white polyester film has fine voids formed around and in the vicinity of white inorganic fine particles, and the light scattering effect of the voids improves whiteness and concealment, and the film itself. It is also known that the specific gravity can be reduced (see, for example, Patent Document 1 and Patent Document 2). Moreover, even if not only inorganic particles but also a thermoplastic resin incompatible with polyester is added, fine voids are formed around the incompatible resin, and a low specific gravity white film is obtained (for example, patents). Reference 3).

Furthermore, a method has also been proposed in which a layer having such voids is formed into a multilayer structure to improve optical characteristics such as whiteness and hiding properties (for example, Patent Document 4).
JP 11-277700 A JP 2000-37835 A JP-A-2-26739 JP 2004-50479 A

However, in conventional white polyester films, it is effective to have voids in the interior from the aspect of characteristics, so the content of white inorganic fine particles and incompatible thermoplastic resin is increased and the void formation rate is increased. As a result, there was a problem that the film was easily broken during stretching, and the productivity was remarkably lowered. Furthermore, the void itself becomes too large, and there is a problem that the whiteness and the like are likely to be lowered as the light scattering effect is reduced. In addition, the multi-layer structure improves the light reflectivity of the film and has the effect of reducing tearing during film production, but it tends to cause uneven lamination in the width direction and longitudinal direction. In addition, it is difficult to adjust the surface roughness of the outermost layer, and if the amount of particles is small, the glossiness becomes too high and brightness unevenness tends to occur. There is a problem that the particles are scraped off and the particles fall off to cause defects.

In order to solve the above problems, the present invention has the following configuration. That is, it is a three-layer laminated polyester film in which the surface layer (B) is provided on both sides of the basic layer (A), the basic layer (A) contains 51 to 60 % by weight of inorganic particles, and the basic layer The polyester of the layer (A) is composed of copolymer polyester or two or more polyesters, at least one of melting peaks is present at 190 to 245 ° C., the reflectance is 97% or more, and the glossiness of the surface layer is 50%. A white polyester film for a surface light source reflecting member that is biaxially stretched and is characterized by the following.

  According to the present invention, it is possible to provide a white polyester film having high light scattering properties and excellent whiteness and glossiness with high production efficiency.

That is, the white polyester film of the present invention is a three-layer polyester film in which the surface layer (B) is provided on both sides of the basic layer (A) , and the basic layer (A) contains 51 to 60 weights of inorganic particles. %, And the polyester of the basic layer (A) is composed of a copolymerized polyester or two or more kinds of polyesters, at least one of melting peaks is present at 190 to 245 ° C., the reflectance is 97% or more, and the surface A white polyester film for a surface light source reflecting member that is biaxially stretched, characterized in that the glossiness of the layer is 50% or less.

  Here, the polyester in the present invention is a general term for polymer compounds in which the main bonds in the main chain are ester bonds, and can usually be obtained by polycondensation reaction of a dicarboxylic acid component and a glycol component. . Here, examples of the dicarboxylic acid component include aromatic dicarboxylic acids such as terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, 5-sodiumsulfonedicarboxylic acid, and phthalic acid. , Oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, fumaric acid and other aliphatic dicarboxylic acids, cyclohexanedicarboxylic acid and other alicyclic dicarboxylic acids, and paraoxybenzoic acid and other oxycarboxylic acids be able to. Examples of the glycol component include aliphatic glycols such as ethylene glycol, propanediol, butanediol, pentanediol, hexanediol and neopentyl glycol, polyoxyalkylene glycols such as diethylene glycol, polyethylene glycol and polypropylene glycol, and cyclohexanedimethanol. And aromatic glycols such as bisphenol A and bisphenol S. In particular, polyethylene terephthalate and polyethylene naphthalate are preferable.

  Of course, these polyesters may be homopolyesters or copolyesters. Examples of copolymer components include diol components such as diethylene glycol, neopentyl glycol, cyclohexane dimethanol, and polyalkylene glycol, and adipic acid. Dicarboxylic acid components such as sebacic acid, phthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and 5-sodium sulfoisophthalic acid can be used.

   Further, in this polyester, if necessary, appropriate additives within the range in which the effects of the present invention are not impaired, for example, heat stabilizer, oxidation stabilizer, anti-glare stabilizer, ultraviolet absorber, dye, dispersion An agent, a coupling agent, etc. may be blended.

  In the present invention, the surface layer (B) is provided for the basic layer (A). In this case, each layer may be the same polyester composition or a different polyester composition. In particular, the composition in which each layer is different, for example, the basic layer (A) is a copolyester, or a mixture of two or more polyesters, and the surface layer (B) is made of a homopolyester, reduces film breakage and thickness. This is preferable in terms of manufacturing stability such as unevenness reduction. In the case of a copolymerized polyester, for example, a copolymer component such as isophthalic acid or cyclohexanedimethanol can be used. Examples of combinations of two or more polyesters include polyethylene terephthalate and polybutylene terephthalate, or polyethylene terephthalate and polycyclohexylene dimethylene terephthalate.

  The intrinsic viscosity of the polymer of the white polyester film of the present invention is preferably 0.65 dl / g or more, more preferably 0.66 to 0.75 dl / g. When the intrinsic viscosity is low, the dispersibility of the inorganic particles is poor, and tearing may easily occur in the film production process, and the strength of the completed film may be easily lowered. If the intrinsic viscosity is too high, melt extrusion may become unstable, and it is desirable to use a material having an appropriate range.

  There may be a plurality of melting temperature peaks of the white polyester film of the present invention. When measured with a differential scanning calorimeter (DSC), it is preferable that at least one of the melting peaks exists at 190 to 245 ° C. 200-240 degreeC is more preferable. This is because the stretchability can be maintained well while containing a large amount of inorganic particles. For example, a plurality of melting peaks may appear when two or more kinds of polyesters of the basic layer (A) are mixed, or when the polymers of the basic layer (A) and the surface layer (B) are different. From the viewpoint of productivity and performance stability, it is preferable that at least one of the peaks falls within the above range. These are achieved, for example, by adjusting the melting point of the polymer by copolymerization or by partially transesterifying the mixed polyester in an extruder.

  Examples of the inorganic particles used in the present invention include zinc carbonate, calcium carbonate, magnesium carbonate, titanium oxide, magnesium oxide, zinc oxide, silica, talc, kaolin, lithium fluoride, calcium fluoride, barium sulfate, zinc sulfide, and alumina. , Calcium phosphate, mica, etc. At least one selected from these may be used. In the present invention, inorganic particles such as calcium carbonate, titanium oxide, and barium sulfate are particularly used from the viewpoint of whiteness, reflectance, and void formation. More preferably it is used.

  The inorganic particles may be in the form of a porous or hollow porous material, and are further subjected to a surface treatment for further promoting the formation of voids and a surface treatment for improving the dispersibility of the resin. May be used.

  The average particle diameter of the inorganic particles is preferably 0.05 to 5 μm, more preferably 0.1 to 3 μm. If the average particle size is out of the above range, it may be difficult to achieve uniform dispersion or the smoothness of the film surface may be deteriorated.

  Further, it is important that the content of the inorganic particles is 50 to 70% by weight in the basic layer (A) layer. Preferably it is 50 to 65% by weight, more preferably 50 to 60% by weight. When the inorganic particles exceed 70% by weight, the aggregation of the particles is extremely likely to occur, so that the productivity is lowered, and the film becomes brittle and the handleability and workability are lowered. On the other hand, when the addition amount is less than 50% by weight, the void formation rate is low, and it may be difficult to improve characteristics such as whiteness and hiding properties of the film.

  The inorganic particle content of the surface layer (B) is not particularly limited, but is preferably 0.1 to 20% by weight, more preferably 1 to 15% by weight. The amount of added particles affects the surface properties of the film, but if the amount of particles is small, the gloss becomes high and the slipperiness deteriorates. In some cases, the film may be easily torn during manufacture, or particles may be scraped off during processing. Therefore, the content of the particles may be adjusted according to the target characteristics. In particular, the above range is preferable for use as a surface light source reflecting member.

  In addition, the same kind or property of the inorganic particles may be used in the surface layer part and the inner layer part, or those different from each other may be used. It is also preferable to add two or more kinds of particles. For example, barium sulfate is added to the surface layer portion, and a mixed system of barium sulfate and titanium oxide is added to the inner layer portion. Further, particles having different particle size distributions may be mixed and used. In the present invention, in addition to the above-mentioned inorganic particles added to each layer, fine particles precipitated by the reaction of a catalyst residue and a phosphorus compound in a polyester polycondensation reaction system can be used in combination. As such precipitated fine particles, for example, those composed of calcium, lithium and phosphorus compounds, or those composed of calcium, magnesium and phosphorus compounds can be used. The content of these particles in the polyester is preferably 0.05 to 1% by weight.

  The apparent specific gravity of the white polyester film of the present invention is preferably 0.5 to 1.35, more preferably 0.7 to 1.3, and still more preferably 0.75 to 1.25. If the specific gravity is less than 0.5, the film strength tends to be low, so that the film productivity and the handleability are lowered. On the other hand, if the specific gravity exceeds 1.35, the reflection characteristics tend to be insufficient.

  In the present invention, a fluorescent whitening agent may be added to the surface layer portion and / or the inner layer portion in order to give the white polyester film of the present invention a high-grade image color with clearer whiteness and bluishness. desirable.

  In the present invention, the fluorescent whitening agent absorbs ultraviolet rays in sunlight or artificial light, retains the function of radiating the ultraviolet rays into violet to blue visible rays, and the lightness of the polymer substance by the fluorescence action. It is a compound that promotes whiteness without lowering. Examples of fluorescent brighteners include “Ubitec” (Ciba-Geigy), “OB-1” (Eastman), “TBO” (Sumitomo Seika Co., Ltd.), and “Kaycoal” (Nippon Soda Co., Ltd.). "Kayalite" (Nippon Kayaku Co., Ltd.), "Lyukopua" EGM (Client Japan Co., Ltd.), etc. can be used. The optical brightener is not particularly limited, and may be used alone or in combination of two or more in some cases. In the present invention, it is desirable to select those that are particularly excellent in heat resistance, have good compatibility with the above-described polyester, can be uniformly dispersed, have little coloration, and do not adversely affect the resin.

  The content of the fluorescent brightening agent in each layer is preferably 0.005 to 1% by weight, and more preferably 0.05 to 0.5% by weight. When the content is lower than the above range, it is difficult to obtain a sufficient whitening effect, and when the content exceeds the above range, uniform dispersibility and whiteness are liable to decrease.

The glossiness of the surface of the white polyester film of the present invention is 50 % or less , more preferably 15 to 50%. When the glossiness exceeds 50 %, for example, when used for a reflector of a surface light source, luminance unevenness is not preferable. The method for adjusting the reflectance is not particularly limited, but it can be adjusted by the amount of inorganic particles added in the surface layer (B), the thickness of the surface layer, stretching conditions, heat treatment conditions, and the like.

  In the present invention, the thickness of the white polyester film is not particularly limited, but is preferably in the range of about 10 to 500 μm, preferably about 20 to 300 μm because it is excellent in practical handling as a base film. Moreover, the thickness of the surface layer to laminate | stack is preferable 1-50 micrometers, and what is in the range of 5-30 micrometers is more preferable from the point of the light scattering property, whiteness, and concealment property.

  Moreover, it is preferable that the optical density of the white polyester film of this invention is 0.5 or more. More preferably, it is 0.8 or more and 1.5 or less. If the optical density is less than 0.6, the film is less concealed and the back side can be seen through, which is generally not preferable. .

  In the white polyester film of the present invention, the whiteness is preferably 70% or more. More preferably, it is 80% or more and 110% or less. If the whiteness is less than 70%, the whiteness is insufficient depending on the application.

  In addition, the white polyester film of the present invention preferably has a color tone b value determined by a color difference meter in the range of -7 to +1, preferably -5 to +0.5, since the whiteness becomes clearer. When the color tone b value is outside the above range, the apparent whiteness is insufficient. For example, when the image receiving substrate is used, the sharpness of the color tone tends to be lowered.

  Next, the manufacturing method of the white polyester film of this invention is demonstrated. In a composite film-forming apparatus having an extruder (A) and an extruder (B), an extruder heated at 270 to 300 ° C. after mixing polyester chips and polyester and inorganic particle master chips and sufficiently drying in vacuum Supply to (A). In addition, in order to laminate the surface layer (B), polyester chips and inorganic particle master chips are mixed and sufficiently vacuum-dried and supplied to the extruder (B), and the extruder ( The polymer of B) is laminated so as to be on the surface layer (one side) or both surface layers (both sides) of the polymer of the extruder (A), and is molded into a sheet shape to obtain a molten laminated sheet.

  The melted laminated sheet is closely cooled and solidified by static electricity on a drum cooled to a surface temperature of 10 to 60 ° C. to produce an unstretched laminated film. The unstretched laminated film is led to a roll group heated to 80 to 120 ° C., stretched 2 to 5 times in the longitudinal direction (longitudinal direction, that is, the traveling direction of the film), and cooled with a roll group of 20 to 30 ° C. .

  Subsequently, the both ends of the laminated film stretched in the longitudinal direction are guided to a tenter while being held by clips, and are stretched in the direction perpendicular to the longitudinal direction in an atmosphere heated to 90 to 140 ° C.

  The stretching ratio is 2 to 5 times in the longitudinal and lateral directions, and the area ratio (longitudinal stretching ratio x lateral stretching ratio) is preferably 6 to 20 times. When the area magnification is within such a preferable range, the obtained film has sufficient whiteness and transmission density, while being hardly broken during stretching.

  In order to give the flatness and dimensional stability of the biaxially stretched film thus obtained, heat setting at 150 to 230 ° C. in a stenter, uniform cooling, and cooling to room temperature, winding up, white polyester film Is made.

  When the simultaneous biaxial stretching method is employed, it can be said that this is a more preferable method because stretching can be facilitated because the degree of voids is within an appropriate range and the orientation in the longitudinal and transverse directions and the heat shrinkage rate balance can be easily obtained.

  Since the white polyester film of the present invention thus obtained is particularly excellent in reflection characteristics and concealment, the surface light source using the film for a reflector or reflector as a surface light source reflecting member makes the liquid crystal screen bright and uniform. In addition, the image on the screen can be highly clear and very easy to see. In addition, the liquid crystal screen can be illuminated very efficiently without transmission loss of light from the illumination light source.

First, various measurement methods will be described.
(1) Apparent specific gravity A sample sample obtained by cutting a film into a size of 50 mm × 60 mm was used in a JIS K-7112 (1999) using a high-precision electronic hydrometer SD-120L (manufactured by Mirage Trading Co., Ltd.). Measured according to Method A (substitution method in water). The measurement was performed under conditions of a temperature of 23 ° C. and a relative humidity of 65%.

(2) Intrinsic viscosity Polyester was dissolved in orthochlorophenol and measured at 25 ° C using an Ostwald viscometer. In addition, when collecting a sample from a laminated film, the sample was collected by scraping each layer individually.

(3) Glossiness Using a digital variable glossiness meter UGV-5B (manufactured by Suga Test Instruments Co., Ltd.), the gloss was measured according to JIS Z-8741 (1997) from the skin layer side of the light reflecting film. The measurement conditions were an incident angle = 60 ° and a light receiving angle = 60 °.

(4) Melting peak temperature About 5 mg of polyester resin or film was measured with a differential scanning calorimeter (Seiko Denshi Kogyo RDC220 type) at a heating rate of 20 ° C./min to determine the endothermic peak temperature.

(5) Thickness of polyester composite layer The thickness of each polyester layer is taken from a cross-sectional photograph taken by magnifying the cross section of the film 2000 times using a scanning electron microscope S-2100A type (manufactured by Hitachi, Ltd.). The length in the direction was measured, and the thickness of each layer was determined by calculating backward from the magnification. In measurement, a total of five cross-sectional photographs arbitrarily selected from different measurement visual fields were used, and the average value was calculated.

(6) Reflectance 560 nm reflectivity with φ60 integrating sphere 130-063 (manufactured by Hitachi, Ltd.) and 10 ° inclined spacer attached to spectrophotometric diameter U-3410 (manufactured by Hitachi, Ltd.) Was determined for both surfaces of the film, and the maximum value was defined as the reflectance of the film. As the standard white plate, the one attached to U-3410 (manufactured by Hitachi, Ltd.) was used.

(7) Whiteness According to JIS-L-1015 (1999), when the reflectance at wavelengths of 450 nm and 550 nm is set to B% and G% using Shimadzu Corporation UV-2600, whiteness (%) = 4B-3G.

(8) Optical density The optical density was measured using an optical densitometer TR927 manufactured by Macbeth.

(9) Melting peak temperature About 5 mg of the polyester resin or film was measured with a differential scanning calorimeter (DSC, model RDC220 manufactured by Seiko Denshi Kogyo Co., Ltd.) at a heating rate of 20 ° C./min, and the endothermic peak temperature was measured.

(Manufacture of polyester)
The polyester resin used for film formation was prepared as follows.

(Polyester A)
0.08 parts by weight of magnesium acetate and 0.022 parts by weight of antimony trioxide were added to dimethyl terephthalate to a mixture of 100 parts by weight of dimethyl terephthalate and 67 parts by weight of ethylene glycol, and a transesterification reaction was carried out by a conventional method. Next, 0.019 part by weight of 85% phosphoric acid aqueous solution is added, and the temperature is gradually raised and reduced. Finally, the temperature is raised and reduced to 290 ° C. and 0.5 mmHg, and the intrinsic viscosity becomes 0.65 dl / g. The polycondensation reaction was carried out until it was discharged into a strand, cooled and cut to obtain a polyethylene terephthalate resin.

(Polyester B)
Add 0.08 parts by weight of magnesium acetate and 0.022 parts by weight of antimony trioxide to dimethyl terephthalate to a mixture of 88 parts by weight of dimethyl terephthalate, 12 parts by weight of dimethyl isophthalate and 67 parts by weight of ethylene glycol. A transesterification reaction was performed. Next, ethylene glycol slurry of agglomerated silica particles having an average secondary particle size of 2.6 μm and 0.019 parts by weight of 85% aqueous phosphoric acid solution were added so that the particle concentration would be 0.06% by weight. The temperature is reduced, and finally the temperature is increased to 290 ° C. and 0.5 mmHg, the pressure is reduced, and the polycondensation reaction is performed until the intrinsic viscosity becomes 0.69 dl / g, and then the strand is discharged, cooled, and cut. A polyethylene terephthalate resin copolymerized with 12 mol% of isophthalic acid was obtained.

(Polyester C)
Polyester B is cut into a 3 mm diameter cube, and solid phase polymerization is performed using a rotary vacuum polymerization apparatus under a reduced pressure of 1 hPa at 225 ° C. until the intrinsic viscosity becomes 0.75 dl / g to obtain polyester C. It was.

(Particle Master D)
Precipitated barium sulfate particles having an average particle diameter of 1.0 μm and polyester C were each dried at 170 ° C. for 3 hours, and fed to a twin-screw extruder so that the barium sulfate concentration in the polyester was 60% by weight. After melt-kneading and extruding, it was cooled and solidified to obtain a barium sulfate master chip.

(Particle Master E)
An anatase type titanium oxide having an average particle size of 0.3 μm and polyester C were melt kneaded in the same manner as the particle master D to obtain a 50% by weight master chip of titanium oxide.

[Example 1]
Polyester A and particle master D are mixed so that the barium sulfate particles in the polyester are adjusted to 10% by weight, vacuum dried at 180 ° C. for 5 hours, and then supplied to the extruder (B). Melted at ℃. On the other hand, polyester C and particle master D were mixed to adjust the barium sulfate particles to 51% by weight, vacuum-dried at 160 ° C. for 5 hours, and then supplied to the extruder (A). To obtain a laminated melt sheet in which the polyester layer (B) is laminated on both surface layers of the polyester layer (A) in the T-die composite die.

  The laminated melt sheet was closely cooled and solidified by an electrostatic application method on a cooling drum maintained at a surface temperature of 20 ° C. to obtain an unstretched laminated film. Subsequently, the unstretched laminated film was stretched 3.0 times in the longitudinal direction using a roll group heated to 91 ° C. according to a conventional method, and cooled with a roll group at 20 ° C. Further, the stretched laminated film was guided to a stenter and stretched 3.1 times in the transverse direction in an atmosphere heated to 120 ° C. Then, after heat setting at 190 ° C. for 5 seconds in a stenter, it was gradually cooled to 140 ° C., 110 ° C., and 80 ° C. and wound up. The polyester layer (A) had a thickness of 120 μm A white laminated polyester film having a thickness of 15 μm on one side and a total thickness of 150 μm was obtained.

[Example 2]
The polyester B layer was prepared in the same manner as in Example 1. The polyester A layer was prepared by mixing polyester C and particle master D so that the barium sulfate particles were 55% by weight to obtain a laminated melt sheet. The laminated melt sheet was closely cooled and solidified by an electrostatic application method on a cooling drum maintained at a surface temperature of 20 ° C. to obtain an unstretched laminated film. Subsequently, the unstretched laminated film was stretched 2.9 times in the longitudinal direction using a roll group heated to 91 ° C. according to a conventional method, and cooled with a roll group at 20 ° C. Further, the stretched laminated film was guided to a stenter and stretched 3.0 times in the transverse direction in an atmosphere heated to 120 ° C. Then, after heat setting at 190 ° C. for 5 seconds in a stenter, it was gradually cooled to 140 ° C., 110 ° C., and 80 ° C. and wound up. The polyester layer (A) had a thickness of 120 μm and the polyester layer (B). A white laminated polyester film having a thickness of 20 μm on one side and a total thickness of 160 μm was obtained.

[Example 3]
The polyester B layer is prepared by mixing polyester A and particle master D so that the barium sulfate particles are 7% by weight. The polyester A layer is prepared by mixing polyester C, particle master D and particle master E with barium sulfate particles. Was adjusted to 50 wt% and titanium oxide particles to 10 wt% to obtain a laminated melt sheet. The laminated melt sheet was closely cooled and solidified by an electrostatic application method on a cooling drum maintained at a surface temperature of 20 ° C. to obtain an unstretched laminated film. Subsequently, the unstretched laminated film was stretched 2.9 times in the longitudinal direction using a roll group heated to 91 ° C. according to a conventional method, and cooled with a roll group at 20 ° C. Further, the stretched laminated film was guided to a stenter and stretched 2.9 times in the transverse direction in an atmosphere heated to 120 ° C. Then, after heat setting at 180 ° C. for 5 seconds in a stenter, it was gradually cooled to 140 ° C., 110 ° C. and 80 ° C. and wound up. The polyester layer (A) had a thickness of 120 μm A white laminated polyester film having a thickness of 30 μm on one side and a total thickness of 180 μm was obtained.

[Comparative Example 1]
The polyester B layer was prepared in the same manner as in Example 1, and the polyester A layer was prepared by mixing polyester C and particle master D so that the barium sulfate particles were 40% by weight to obtain a laminated melt sheet. The laminated melt sheet was closely cooled and solidified by an electrostatic application method on a cooling drum maintained at a surface temperature of 20 ° C. to obtain an unstretched laminated film. Subsequently, the unstretched laminated film was stretched 3.0 times in the longitudinal direction using a roll group heated to 91 ° C. according to a conventional method, and cooled with a roll group at 20 ° C. Further, the stretched laminated film is guided to a stenter and 3.3 in the transverse direction in an atmosphere heated to 120 ° C. The film was stretched twice. Then, after heat setting at 190 ° C. for 5 seconds in a stenter, it was gradually cooled to 140 ° C., 110 ° C., and 80 ° C. and wound up. The polyester layer (A) had a thickness of 120 μm and the polyester layer (B). A white laminated polyester film having a thickness of 15 μm on one side and a total thickness of 150 μm was obtained.

[Comparative Example 2]
Polyester B layer is prepared by mixing polyester A and particle master D so that the barium sulfate particles are 1% by weight. For polyester A layer, polyester C and particle master D are mixed and barium sulfate particles are 55% by weight. It adjusted so that it might become, and the lamination melt sheet was obtained. The laminated melt sheet was closely cooled and solidified by an electrostatic application method on a cooling drum maintained at a surface temperature of 20 ° C. to obtain an unstretched laminated film. Subsequently, the unstretched laminated film was stretched 2.9 times in the longitudinal direction using a roll group heated to 91 ° C. according to a conventional method, and cooled with a roll group at 20 ° C. Further, the stretched laminated film was guided to a stenter and stretched 3.0 times in the transverse direction in an atmosphere heated to 120 ° C. Then, after heat setting at 190 ° C. for 5 seconds in a stenter, it was gradually cooled to 140 ° C., 110 ° C., and 80 ° C. and wound up. The polyester layer (A) had a thickness of 120 μm and the polyester layer (B). A white laminated polyester film having a thickness of 15 μm on one side and a total thickness of 150 μm was obtained.

[Comparative Example 3]
Polyester B layer is prepared by mixing polyester A and particle master D so that the barium sulfate particles are 1% by weight. For polyester A layer, polyester C, particle master D and particle master E are mixed to obtain barium sulfate particles. Was adjusted to 55 wt% and titanium oxide particles to 20 wt% to obtain a laminated melt sheet. The laminated melt sheet was closely cooled and solidified by an electrostatic application method on a cooling drum maintained at a surface temperature of 20 ° C. to obtain an unstretched laminated film. Subsequently, the unstretched laminated film was stretched 2.8 times in the longitudinal direction using a roll group heated to 91 ° C. according to a conventional method, and cooled with a roll group at 20 ° C. Further, the stretched laminated film was guided to a stenter and stretched in the transverse direction, but the film was torn and film formation was difficult.

  The films of Examples 1 to 3 have good optical characteristics such as reflectance, glossiness, and whiteness, and are suitable as, for example, a film for a surface light source reflector. On the other hand, Comparative Example 1 has low reflectance and whiteness, and Comparative Example 2 has high glossiness. Therefore, any film has insufficient characteristics as a surface light source reflector.

  Although the use of the white polyester film of the present invention is not particularly limited, it is preferably used as a plate-like material incorporated in a surface light source for light reflection. Specifically, it is preferably used for a reflection plate of an edge type backlight for a liquid crystal screen, a reflection plate of a surface light source of a direct type backlight, a lamp reflector around a cold cathode tube, and the like.

Claims (5)

A three-layer polyester film in which a surface layer (B) is provided on both sides of a basic layer (A) ,
The basic layer (A) contains 51 to 60 % by weight of inorganic particles, and the polyester of the basic layer (A) is composed of a copolymer polyester or two or more polyesters, and at least one of melting peaks is 190 to 245 ° C. Exist in the
A white polyester film for a surface light source reflecting member, which is biaxially stretched, having a reflectance of 97% or more and a glossiness of a surface layer of 50% or less. A white polyester film for a surface light source reflecting member, wherein the apparent specific gravity of the polyester film according to claim 1 is 0.5 to 1.35. The white polyester film for a surface light source reflecting member according to claim 1 or 2, wherein the basic layer (A) contains at least two kinds of inorganic particles. The white polyester film for a surface light source reflecting member according to any one of claims 1 to 3 , wherein the intrinsic viscosity of the polymer of the polyester film is 0.65 dl / g or more. The white polyester film for a surface light source reflecting member according to any one of claims 1 to 4, wherein the surface layer (B layer) contains 7 to 20% by weight of inorganic particles.