DESCRIPTION

Biaxially Oriented Polyester Film

TECHNICAL FIELD

[0001]
The present mvention relates to a biaxially oriented polyester film which can be suitably used especially as a solar battery back sheet, and also relates to a method of producing the film, and a solar battery back sheet and a solar battery using the fibn.

BACKGROUND ART [0002]

Polyester resins have been used in various uses because they have excellent mechanical properties, thermal properties, chemical resistance, electrical properties, and moldability, and are mexpensive. A biaxially oriented polyester fihn obtained by making the polyester resm into a fihn has been used as an electrical insulating material, for example, for copper-clad laminates, solar battery back sheets, adhesive tapes, flexible printed boards, membrane switches, planar heating elements, or flat cables; a magnetic recordmg material; a capacitor material; a packaging material; an automotive material; a building material; and various industrial materials, for example, for photographic use, graphic use, and thermosensitive transcription use.

[0003]
Among these uses, an electrical msulating material used, in particular, outdoors (for example, solar battery back sheets and the like), an automotive material, and a buildmg material are often used in harsh environments m terms of temperature and humidity over a long period of time, and general polyester resins can discolor to brown when exposed to UV light for a long period of time. Further, UV irradiation and hydrolysis reduce the molecular weight, promoting embrittlement to reduce the mechanical properties and the like. Therefore, there is a need for inhibition of the change in color tone due to UV light and the reduction in tensile elongation and for improvement of hydrolysis resistance. Accordingly, various studies have been made in order to inhibit the hydrolysis of polyester resins.

[0004]
For example, the technique for improving the hydrolysis resistance of a polyester resin itself by adding a polyester resin which contains a certain amoimt of alkali metal, alkaline earth metal, and phosphorus and contains internally precipitated particles due to catalyst residues (Patent Docvunent 1), an epoxy compound (Patent Document 2, Patent Document 3), or polycarbodiimide (Patent Docimient 4) had been studied. For the biaxially oriented polyester film, the improvement of hydrolysis resistance by providing a film with high FV (high intrinsic viscosity) and controlling the planar orientation had been studied (Patent Document S).

[0005]
On the other hand, for application in these uses, high fimctionalization by providing the properties other than hydrolysis resistance (m particular, UV light resistance, reflectivity, and the like) as well has been desired. Therefore, higher fimctionalization by mixing a bi- or multi-component polyester or other components has been studied (for example, Patent Documents 6, 7, 8, and 9).

PRIOR ART

DOCUMENTS PATENT DOCUMENTS

[0006]
Patent Document 1: JP 60-31526 A
Patent Document 2: JP 09-227767 A
Patent Document 3: JP 2007-302878 A
Patent Document 4: JP 11-506487 W
Patent Document 5: JP 2007-70430 A
Patent Document 6: JP 2004-223714 A
Patent Document 7: JP 2004-98442 A
Patent Document 8: JP 02-191638 A
Patent Document 9: JP 08-244188

A SUMMARY OF TITLE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0007]
However, in the case where other components (for example, UV absorbers, inorganic particles, and the like) are mixed for high fimctionalization of a polyester film, the process comprising the steps of kneading the other components with a resin once for masterpelletization and diluting the masterpellet with a polyester resin constituting the film is generally carried out. However, in general, when producing a masterpellet, thermal histories during extrusion process deteriorates a polyester resin. By adding tibe master chips thus produced, a deteriorated resin is contained in the fihn, and therefore there is a problem in that the film obtained has a reduced hydrolysis resistance though it expresses the functions of the components added (in particular, UV Ught resistance). Further, in cases where morganic particles are contained for high fimctionalization, there is a problem m that, for example, the hydrolysis resistance of the film is reduced by the influence, for example, of the water adsorbed in the inorganic particles. [0008]
Thus, an object of the present invention is to provide a biaxially oriented polyester film which has excellent hydrolysis resistance and can simultaneously achieve other properties (m particular, inhibition of the change in color tone and inhibition of the reduction m tensile elongation after UV irradiation).

MEANS FOR SOLVING TOE PROBLEMS [0009]
To solve the above-described problems, the biaxially oriented polyester fihn of the present invention has either constitution [1] or [2] below:

[1] A biaxially oriented polyester film which is a polyester fihn having a polyester layer (PI layer) containing a polyester (Al) comprising ethylene terephthalate as a main constituent, a high melting point resin (Bl) having a melting point TmBi of not less than 260°C and not more than 320°C, and inorganic particles (CI), wherein the content of the high melting point resin (Bl) in the PI layer, WBI, is not less than 2% by mass and not more than 40% by mass based on the PI layer; in the PI layer, a dispersion phase(s) composed of the high melting point resin (Bl) is/are present in the polyester (Al); and the average longitudinal length of the dispersion phase is not more than 10,000 nm (10 jun), or
[2] A biaxially oriented polyester fihn, which is a polyester fihn having a polyester layer (PI layer) containmg a polyester (Al) comprising either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main constituent, a thermoplastic resm (Dl), and inorganic particles (CI), wherein the content of the thermoplastic resin (Dl) in the PI layer, WDI, is not less than 2% by mass and not more than 40% by mass based on the PI layer; 1.5 x MWAI' / MWAI < MWDI' / MWDI is satisfied, wherem MWAI is the weight-average molecular weight of the polyester (Al); MWDI is the weight-average molecular weight of the thermoplastic resin (Dl); MWAI' is the weight-average molecular weight of the polyester (Al) after treatment at 125°C and 100% RH for 72 hr; and MWDI' is the weight-average molecular weight of the thermoplastic resin (Dl) after treatment at 125°C and 100% RH for 72 hr; and, in the PI layer, the thermoplastic resm (Dl) is present in the polyester (Al) as dispersion phases, and the number of the dispersion phases having a longitudinal length of more than 30,000 nm (30 jun) is not more than 2/3 x 10^ nm^ (2/3,000 \im).

[0010]
The solar battery back sheet of the present mvention has the following constitution:

A solar battery back sheet using the biaxially oriented polyester film according to either [1] or [2] described above.

[0011]
The solar battery of the present invention has the following constitution: A solar battery using any of the solar battery back sheets described above.

[0012]
The method of producing the biaxially oriented polyester fihn of the present invention has either constitution [3] or [4] below:

[3] A method of producing the biaxially oriented polyester fihn according described above, which is a method of producing the polyester fihn having the polyester layer (PI layer) containing the polyester (Al) comprising ethylene terephthalate as a mam component; at least one high melting point resin (B1) selected from the group consisting of resins comprising 1,4-cyclohexanedimethylene terephthalate, ethylene-2,6-naphthalenedicarboxylate, and phenylene sulfide as a mam component; and the morganic particles (CI), wherein the high melting point resin (Bl) and the inorganic particles (CI) are melt kneaded to produce a masterpellet (Ml); and the polyester (Al) and the masterpellet (Ml) are meh kneaded under conditions satisfymg any of the following equations (i) to (iv), extruded into sheet form, and then biaxially stretched;

wherem the melt viscosity of the polyester (Al) is TIA; the melt viscosity of the masterpellet (Ml) is TJMI; Tmsi is the melting point (°C) of the high melting point resin (B1); Tc is the extrusion temperature (°C) during melt film forming; and r\p^ and TiMi are the melt viscosity of the polyester (Al) and the masterpellet (Ml), respectively, at a temperature of Tc (°C) and a shear rate of 200 sec"';

[4] A method of producing the biaxially oriented polyester film described above, which is a method of producing the polyester fikn having the polyester layer (PI layer) containing the polyester (Al) comprising either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main component; the thermoplastic resin (Dl) which is any of a polyester resin containing 1,4-cyclohexylenedimethylene terephthalate units in an amount of 93 mol% or more, a polyester resin comprising ethylene-2,6-naphthalenedicarboxylate units as a main constituent, or a resin comprising phenylene sulfide as a main constituent; and the inorganic particles (CI), wherein the thermoplastic resm (Dl) and the inorganic particles (CI) are melt kneaded to produce a masteipellet (Ml); and the polyester (Al) and the masterpellet (Ml) are melt kneaded under conditions satisfying any of the following equations (i), (ii), (v), (vi), extruded into sheet form, and then biaxially stretched;

wherein the melt viscosity of the polyester (Al) is TIA; the melt viscosity of the masterpellet (Ml) is TIMI; Tmoi is the melting pomt (°C) of the thermoplastic resm (Dl); Tc is the extrusion temperature (°C) during melt fihn forming; and T^A and TiMi are the melt viscosity of the polyester (Al) and the masterpellet (Ml), respectively, at a temperature of Tc (°C) and a shear rate of 200 sec'';

T1A/T1MI>0.2 (i)
T1A/T1MI<1.0 (ii)
W^lMi > -0.183 X (Tc - Tmoi) + 2.095 (v)
TIA/TIMI < -0.08 X (Tc - Tmoi) + 2.6 (vi). [0013]

In the biaxially oriented polyester fihn according to [1] of the present invention, it is preferred tiiat, in the above-described PI layer, 70% or more of the total number of the above-described inorganic particles (CI) be present in the above-

described dispersion phases or in contact with the above-described dispersion phases. [0014]
In the biaxially oriented polyester film according to [1] of the present invention, the above-described high melting point resin (Bl) is preferably at least one resin selected from the group consisting of resins comprising 1,4-cyclohexanedimethylene terephthalate, ethylene-2,6-naphthalenedicarboxylate, and phenylene sulfide as a main component.

[0015]
The biaxially oriented polyester film according to [1] of the present invention is a laminated polyester film having the above-described polyester layer (PI) layer and a polyester layer (P2 layer) containing a polyester (A2) comprising ethylene terephthalate as a main constituent, a high melting point resin (B2) having a meltmg pomt of not less than 260°C and not more than 320°C, and inorganic particles (C2), and it is preferred that, in the P2 layer, dispersion phases composed of the high melting point resin (B2) be present in the polyester (A2); the content of the inorganic particles (C2) m the P2 layer, Wc2, be not less than 0.1% by mass and not more than 5% by mass based on the P2 layer; and tiie difference between the content of the inorganic particles (CI) in the PI layer, Wci (% by mass), and tiie content of the inorganic particles (C2) in the P2 layer, Wc2 (% by mass), Wci - Wc2, be not less than 5% by mass and not more than 25% by mass.

[0016]
In the biaxially oriented polyester film according to [2] of the present invention, the thermoplastic resin (Dl) preferably meets at least one of the requirements (a) and (b).

(a) The thermoplastic resin (Dl) has a tan 5 peak temperature at a frequency of 1.0 Hz, which is obtained by dynamic mechanical analysis, of not less than 90°C and not more than 200°C.

(b) The thermoplastic resin (Dl) has a melt viscosity at a shear rate of 200 sec"\ TIDI, within the range of 500 poise to 15,000 poise at any temperature within the range of 270°C to 320°C, and does not contain an ester bond in the molecular structure.

[0017]
The biaxially oriented polyester film according to [2] of the present invention is preferably the combination in which the polyester (Al) is a resin comprismg ethylene terephthalate as a main constituent and in which the thermoplastic resin (Dl) is a resin comprising any of 1,4-cyclohexylenedimethylene terephthalate, ethylene-2,6-n^hthalenedicarboxylate, and phenylene sulfide as a main constituent, or one in which the polyester (Al) is a resin comprising ethylene-2,6-naphthalenedicarboxylate as a main component and in which the thermoplastic resin (Dl) is selected fi"om resins comprising either 1,4-cyclohexylenedimethylene terephthalate or phenylene sulfide as a main constituent [0018]

In the biaxially oriented polyester film according to [2] of the present invention, the amount of the inorganic particles CI added is preferably not less than 0.5% by mass and not more than 30% by mass based on the PI layer.

[0019]
In the biaxially oriented polyester film accordmg to [2] of the present invention, it is preferred that, in the above-described PI layer, 70% or more of the total number of the above-described inorganic particles (CI) be present in the above-described dispersion phases or in contact with the above-described dispersion phases.

[0020]
In the biaxially oriented polyester fihn according to [2] of the present invention, the melting point of the thermoplastic resin (Dl), Tmoi, is preferably 5°C to 60°C higher than the melting point of the polyester (Al), TmAi-

[0021]

In the biaxially oriented polyester film according to [2] of the present invention, the melting point of the thermoplastic resin (Dl), Tmoi, is preferably not less than 260°C and not more than 320°C. [0022]
In the biaxially oriented polyester film according to [2] of the present invention, the number of the dispersion phases is preferably not less than 1/1,000 mn (1/1 \im) and not more than 5/1,000 nm (5/1 ^mi) when a cross section in the thickness direction of the PI layer is observed.

[0023]
In the biaxially oriented polyester fihn according to [2] of the present invention, the average longitudinal length of the dispersion phases is preferably not more than 10,000 nm (10 ^m).

[0024]
In the biaxially oriented polyester film according to [2] of the present invention, the combination of the polyester (Al) and the thermoplastic resin (Dl) preferably falls under any of (c) to (e) below.

(c) The polyester (Al) is a resin comprising ethylene terephthalate as a main constituent; the thermoplastic resin (Dl) is a resin comprising 1,4-cyclohexylene- dimethylene terephthalate as a main ccmstituent; and x > 94.5 and y x lO"' < x - 94,5 are satisfied.

[0025]

Here, x: molar fraction (mol%) of 1,4-cyclohexylenedimethylene terephthalate units, and y: average longitudinal length (imi) of the dispersion phase.

(d) The polyester (Al) is a resin comprising ethylene terephthalate as a main constituent; and the thermoplastic resin (Dl) is a resin comprising ethylene-2,6-naphthalenedicarboxylate or phenylene sulfide as a main constituent.

(e) The polyester (Al) is a resin comprising ethylene-2,6-naphthalenedicarboxylate as a main constituent; and the thermoplastic resin (Dl) is a resin comprising 1,4-cyclohexylenedimethylene terephthalate or phenylene sulfide as a main constituent.

[0026]
The biaxially oriented polyester fihn according to [2] of the present invention is a laminated polyester film having the above-described polyester layer (PI) layer and a polyester layer (P2 layer) containing a polyester (A2) comprisuig either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main constituent, a thermoplastic resin (D2), and inorganic particles (C2), and it is preferred that, in the P2 layer, dispersion phases composed of the th^moplastic resin (D2) are present in the polyester (A2); the content of the inorganic particles (C2) in the P2 layer, Wc2, is not less than 0.1% by mass and not more than 5% by mass based on the P2 layer; the difference between the content of the inorganic particles (CI) in the PI layer, Wci (% by mass), and the content of the inorganic particles (C2) in the P2 layer, Wc2 (% by mass), Wci - Wc2, is not less than 5% by mass and not more than 25% by mass; and the relationship: 1.5 x MWA2' / MWA2 < MWD2' / MWD2 is satisfied, wherein MWA2 is the weight-average molecular weight of the polyester (A2); MWD2 is the weight-average molecular weight of the thermoplastic resin (D2); MWA2' is the weight-average molecular weight of the polyester (A2) after treatment at 125°C and 100% RH for 72 hr; and MWD2' is the weight-average molecular weight of the thermoplastic resin (D2) after treatment at 125°C and 100% RHfor72hr.

[0027]
In the solar battery back sheet of the present invention, it is preferable to provide the above-described polyester film at at least one outermost side.

[0028]
In tiie solar battery back sheet of the present invention, at least one outermost layer is preferably the PI layer.

EFFECTS OF THE INVENTION

[0029]
The present invention provides a biaxially oriented polyester fihn comprising ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main component which allows a balance between high hydrolysis resistance and other properties (in particular, UV light resistance) over a long period of time. Further, the use of such a biaxially oriented polyester fihn provides a solar battery back sheet with high durability and a solar battery using the same.

BRIEF DESCRIPTION OF THE DRAWINGS [0030]
Figure 1 is a schematic cross-sectional view of the solar battery using the film of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION [0031]
The present invention will now be described in detail by way of specific examples.

[0032]
The biaxially oriented polyester fihn of the present invention is a biaxially oriented polyester fihn having a polyester layer (PI layer) contaming a polyester (Al) comprising ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a mam constituent, a high melting pomt resm (Bl) or a thermoplastic resin (Dl), and inorganic particles (CI).

[0033]
In the present invention, the polyester (Al) comprismg ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main constituent refers to a polyester resin contammg ethylene terephthalate units or ethylene-2,6-naphth&lenedicarboxylate units m an amount of 50 mol% or more.

The molar fraction of the ethylene terephthalate units or the ethylene-2,6-naphthalenedicarboxylate units in the polyester (Al) is preferably 80 mol% or more, especially preferably 100 mol% (i.e., the polyester (Al) is polyethylene terephthalate orpolyethylene-2,6-naphthalenedicarboxylate).

[0034]
Generally, polyester is composed of an acid component such as aromatic dicarboxylic acids, aliphatic cyclic dicarboxylic acids, or aliphatic dicarboxylic acids, and a diol component, but in the present invention, a resin obtamed by appropriately copolymerizing ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate with other acid component or diol component can also be used as a polyester (Al) as long as the effects of the invention are not impaired. In the present invention, the thermoplastic resin (Dl) preferably has a tan 5 peak temperature at a frequency of 1.0 Hz, v^'hich is obtained by dynamic mechanical analysis, of not less than 90°C and not more than 200°C. Tan 5 peak temperatures is determined by sheeting the thermoplastic resin (Dl) and measuring the sheet by the method described in Method for evaluating the properties (9) described below.

[0035]
Sheeting of the thermoplastic resin (Dl) is performed by the following procedure.
(1) The polyester (Al) and the thermoplastic resin (Dl) are separated from tiie biaxially oriented polyester fihn of the present invention. Separation is performed by the method described below in Method for evaluating the properties (10).

(2) The separated thermoplastic resin (Dl) is then dried until the water content therein is 20 ppm or less.

(3) The thermoplastic resin (Dl) in such an amount that it has a thickness of 100,000 rmi (100 \wa) is placed on a hot pressmg machine that is set at the melting pomt of the dried thermoplastic resin (Dl) + 20°C or, in the case of a resm having no meltiog pomt, warmed io the range from the glass transition temperature + 100°C to the glass transition temperature + 200°C.

(4) The thermoplastic resin (Dl) is then pressed at an arbitrary pressure for sheeting. Entrained bubbles and the like, if any, are expelled as required.

(5) After releasing the pressure of the press, the sheet is rapidly cooled, for example, with cold water so as not to be crystallized to obtain a thermoplastic resin (Dl) press sheet of 100,000 nm (100 jmi)-

[0036]
When the tan 5 peak temperature is not less than 90°C, breakage of the molecular cham hardly occurs because the molecular mobility under the moist-heat atmosphere of 125°C and 100% RH is lower than that of the polyester (Al), resulting in a resui having a more excellent hydrolysis resistance. On the other hand, when the tan 5 peak temperature is not more than 200°C, estrangement between the stretching temperatures of the polyester (Al) and the thermoplastic resin (Dl) during biaxial stretching is not too large, resultmg in good coelongation properties. The tan 5 peak temperature m tiiis range provides a film having more excellent hydrolysis resistance while maintaining the coelongation properties with the polyester (Al). The tan 5 peak temperature is more preferably not less than 120°C and not more than 180°C. Examples of resins having a tan 5 peak temperature of not less than 90°C and not more than 200°C include, for example, resins comprising as a main component polyethylene-2,6-naphthalenedicarboxylate, polycarbonate, 1,4-polycyclohexylenedimethylene terephthalate, polyetherimide, olefin, polyphenylene oxide, or polyether ether ketone.

[0037]
It is preferred that the thermoplastic resm (Dl) of the present invention have a melt viscosity at a shear rate of 200 sec"', TJDI, within the range of not less than 500 poise and not more than 15,000 poise at any temperature within the range of 270°C to 320°C and not contain an ester bond in the molecular structure. The PI layer in the present invention has a melt extrusion temperature during melt fihn forming of not less than 270°C and not more than 320°C because it comprises as a main constituent the polyester (A 1). On the other hand, when a resin containing no ester bonds is used as the thermoplastic resin (Dl), it is immiscible with the polyester (Al) in most cases. Therefore, from the standpoint of forming a dispersion phase in the polyester (Al) and at the same time not increasing the longitudinal length of the dispersion phase, the melt viscosity of the thermoplastic resin (Dl) at a shear rate of 200 sec'\ TiDi, is preferably not less than 500 poise and not more than 15,000 poise at any temperature within the range of 270°C to 320°C, and more preferably not less than 2,000 poise and not more than 12,000 poise. The melt viscosity can be adjusted, for example, with the degree of polymerization of the resin.

[0038]
In the resin comprising polyester as a main component, the presence of ester bonds is the main cause of hydrolysis. Therefore, by formmg dispersion phases in the polyester (Al) using a pellet mastered with inorganic particles and a resin containing no ester bonds, more excellent hydrolysis resistance can be provided wirile obtaming UV light resistance that is the effect of the addition of inorganic particles, which is preferred. Examples of resins contaming no ester bonds in the molecular structure include, for example, polyetherimide, polyphenylene sulfide, olefin, nylon, polystyrene, polyphenylene oxide, and polyether ether ketone.

[0039]
In the biaxially oriented polyester film of the present invention, the mtrinsic viscosity (IV) of the polyester (Al) is preferably not less than 0.65, more preferably not less than 0.68, still more preferably not less than 0.7, and especially preferably not less than 0.72. When the IV of the polyester (Al) is not less than 0.65, high hydrolysis resistance and high mechanical properties can be obtained. Although the
upper limit of the IV is not particularly defined, fix)m the standpoint of preventing a cost disadvantage due to a too prolonged polymerization time and facilitating the melt extrusion, it is preferably not more than 1.0, and more preferably not more than 0.9.

[0040]
In the case where the polyester (Al) is a polyester resin comprising polyethylene terephthalate as a main component, if the intrinsic viscosity (TV) is 0.65 to 0.9, the melt viscosity TIA is fi"om 2,000 poise to 5,000 poise. In tiiie case where the polyester resin (Al) is a polyester resin comprising polyethylene-2,6-naphthalene-dicarboxylate as a main component, if the intrinsic viscosity (IV) is 0.65 to 0.9, the melt viscosity T^A is fi"om 5,000 poise to 12,500 poise. Here, melt viscosity TIA is measured by the Method for evaluating the properties (2) described below.

[0041]
The biaxially oriented polyester film of the present invention contains inorganic particles (C1), The inorganic particles (C1) are used for providing the film with a function required depending on the purpose. Examples of inorganic particles (CI) windi can be suitrfjly used in the present mvention include, for example, inorganic particles laving UV absorptivity, particles havmg a large refi"active index difference fi-om the polyester (Al), particles having conductivity, and pigments. These improve, for example, UV light resistance, optical properties, antistatic properties, and the color tone.

[0042]
In the present invention, as inorganic particles, those with an average primary particle diameter of 5 nm or more are used. Particle diameter herein refers to a number average particle diameter and means the particle diameter observed in a cross section of the film. In cases where the shape is not a perfect circle, the value equivalent to that of a perfect circle of the same area is considered as a particle diameter. Number average particle diameter can be determined by the following procedure (1) to (4).

(1) First, usmg a microtome, a fihn is cut m tiie thickness direction without crushing the cross section, and a scanning electron microscope is used to obtain a magnified observation image. At this tune, cutting is carried out in the direction parallel to the film TD direction (transverse direction).
(2) Next, for each particle observed in the cross section m the image, its cross-sectional area S is determined, and its particle diameter d is determined using the following equation.

[0043]
d = 2x(S/3ty^
(3) Using the particle diameter d obtained and the number of resin particles n, Dn is
determmed by the foUowmg equation.

[0044]
Dn = 2d/n
wherein, Sd is the summation of particle diameters of the particles m an observation plane; and n is the total numbo- of IIK particles in the observation plane.

(4) The above (1) to (3) are performed at five different points, and the mean value is
defined as the number average particle diameter of the particles. The above-described evaluation is performed at an area of 2.5 x lO' nm^ (2,500 nm^) or more for each observation point.

[0045]
From the standpoint of making it easy to absorb the light in the UV range from 10 nm to 400 nm, the average primary particle diameter of the inorganic particles is preferably not less than 5 nm (0.005 \im) and not more than 5,000 nm (5 pm), more preferably not less than 10 nm (0.01 )jm) and not more than 3,000 nm (3 \im), and especially preferably not less than 15 nm (0.015 fun) and not more than
2,000 nm (2 un).

[0046]
Specific examples of inorganic particles include, for example, metals such as gold, silver, copper, platinum, palladium, rhenium, vanadium, osmium, cobalt, ux)n, zinc, ruthenium, praseodymium, chromium, nickel, aluminum, tin, zinc, titanium, tantalum, zirconium, antimony, indium, yttrium, and lanthanimi; metal oxides such as zinc oxide, titanium oxide, cesium oxide, antimony oxide, tin oxide, indium tin oxide, yttrium oxide, lanthanum oxide, zirconium oxide, aluminum oxide, and silicon oxide; metal fluorides such as lithium fluoride, magnesium fluoride, alummum fluoride, and cryolite; metal phosphates such as calcium phosphate; carbonates such as calcium carbonate; sulfates such as barixmi sulfate; and besides carbonaceous materials such as talc, kaolin, carbon, fuUerene, carbon flber, and carbon nanotube.

[0047]
In view of the fact that solar batteries are often used outdoors, when inorganic particles having UV absorptivity, for example, metal oxides such as titanium oxide, zmc oxide, and cerium oxide are used, the effect of the of invention, that is, maintaining mechanical strength over a long period of time can be prominently exerted by utilizing the UV light resistance resulting fi-om the inorganic particles. [0048]
The content of the morganic particles contained in the PI layer of the biaxially oriented polyester film of the present invention is, based on tiie PI layer, preferably not less than 0.5% by mass and not more than 30% by mass, more preferably not less than 1.0% by mass and not more than 28% by mass, and still more preferably not less than 3.0% by mass and not more than 25% by mass. The content of the inorganic particles of not less than 0.5% by mass and not more than 30% by mass provides sufficient UV light resistance, mechanical strength that is not reduced when used for a long period of time, and little change m color tone after UV irradiation. In addition, reduced mechanical strength of the fihn due to too much content of the particles will not be caused.

[0049]
In the biaxially oriented polyester film accordmg to the constitution [2] of the present invention, it is necessary to satisfy the relationship: 1.5 x MWAI' / MWAI < MWDI' / MWDI, wherem MWAI is the weight-average molecular wei^t of the polyester (Al); MWDI is the weight-average molecular weight of the thermoplastic resin (Dl); MWAI' is the weight-average molecular weight of the polyester (Al) after treatment at 125°C and 100% RH for 72 hours; and MWDI' is the weight-average molecular weight of the thermoplastic resin (Dl) after treatment at 125°C and 100% RH for 72 hours.

[0050]
MWAI, MWAI', MWDI, and MWDI' are measured as follows. First, the polyester (Al) and the thermoplastic resin (Dl) in the biaxially oriented polyester film are separated. Separation of the polyester (Al) and the thermoplastic resm (Dl) is performed by the Method for evaluating the properties (10) described below. The weight-average molecular weights measured for the polyester (Al) and the thermoplastic resm (Dl) separated are MWAI and MWDI, respectively. Next, the polyester (Al) and the thermoplastic resin (Dl) separated are treated in a pressure cooker manufactured by Tabai Espec Corporation under the conditions of a temperature of 125°C and 100% RH for 72 hr to obtam a treated sample. The weight-average molecular weights measured for the post-treatment polyester (Al) and thermoplastic resm (D1) obtained are MWAI ' and MWDI ', respectively. Weight-average molecular weight in the present invention is measured by the Method for evaluating the properties (11) described below.

[0051]
Generally, when adding inorganic particles to polyester, to disperse the inorganic particles homogeneously, they are once masterpelletized with another resin, and the masterpellet is dispersed in the polyester. When inorganic particles are added, it is mevitable that the hydrolysis of polyester will be promoted, for example, by the water inherently contained the inorganic particles. Therefore, when a resin having the same composition as that of the polyester resm (Al) is used as a resm for masterpelletization, the hydrolysis resistance of the masterpelletized polyester (Al) will necessarily be worse than the hydrolysis resistance intrinsic to the polyester (Al) because inorganic particles are contained.

[0052]
Thus, in the present invention, to prevent the reduction of hydrolysis resistance due to inorganic particles, a thermoplastic resin which has a lower rate of weight-average molecular weight decrease when comparing before and after treatment at 125°C and 100% RH for 72 hr than that of the polyester (Al) is used as a resm for masterpelletization. Specifically, a thermoplastic resin which satisfies the relationship: 1.5 x MWAI ' / MWAI < MWDI ' / MWDI is used. If a resin for masterpelletization which satisfies the above-described relationship is not used, sufficient hydrolysis resistance cannot be obtained. The relationship of the rate of weight-average molecular weight decrease when comparing before and after treatment at 125°C and 100% RH for 72 hr is preferably 1.8 x MWAI'/ MWAI < MWDI' / MWDI, and more preferably 10 x MWAI' / MWAI < MWDI' / MWDI.

[0053]
In the present invention, when the polyester (Al) comprises polyethylene terephthalate as a main component, the combination with the high melting point resin (Bl) or the thermoplastic resin (Dl) that is at least one resin selected fi-om the group consisting of resins comprising 1,4-cyclohexylenedimethylene terephthalate, ethylene-2,6-naphthalenedicarboxylate, and phenylene sulfide as a main component
is preferred. When the polyester (Al) comprises polyethylene-2,6-naphthalene-dicarboxylate as a main component, the combination with the high melting point resin (Bl) or the thermoplastic resin (Dl) that is either 1,4-cyclohexylenedimethylene terephthalate or phenylene sulfide is preferred. Among the high melting point resins (Bl) or the thermoplastic resuis (Dl) used in the present invention, polycyclohexylenedimethylene terephthalate preferably has cyclohexylenedimethylene terephthalate units composed of terephthalic acid as a dicarboxylic acid component and cyclohexylenedimethanol as a diol component m an amount of 85 mol% or more, more preferably 90 mol% or more, and especially preferably 93 mol% or more, based on the total repeatmg units of the high meltmg point resin (Bl) or the thermoplastic resin (Dl), and the upper limit value thereof is 100 mol%. When the cyclohexylenedimethylene terephthalate units contained m the high melting point resin (Bl) or the tiiermoplastic resin (Dl) is 85 mol% or more, crystallinity will not be impaired, and there is no danger of causing a decrease in the melting point. As a result, a polyester film having high heat-resistance, no possibility to cause a reduction in intrinsic viscosity (hereinafter referred to as IV reduction) during the production of a masterpellet, and excellent hydrolysis resistance can be obtained.

[0054]
In the present invention, the content of the high meltmg point resin (Bl), WBI, or the content of the thermoplastic resin (Dl), WDI, in the PI layer needs to be not less than 2% by mass and not more than 40% by mass based on the PI layer. When the content of the high melting point resin (Bl), WBI, or the content of the thermoplastic resin (Dl), WDI, is less than 2% by mass, the concentration of inorganic particles in a masterpellet becomes high during the production of the masterpellet having inorganic particles. Therefore, sufiBcient dispersion of inorganic particles cannot be obtained, and it can be impossible to allow 70% or more of the total number of the above-described inorganic particles (CI) to be present in the dispersion phase or to be present in contact with the dispersion phase (hereinafter also referred to as "allow to be present or the like in the dispersion phase"). If the concentration of inorganic particles m a masterpellet is lowered in order to prevent this, the content of inorganic particles in a polyester film is likely to be insufficient, and the effects of the present invention sometimes cannot be obtained. On the other hand, when tihe content of the high melting point resin (Bl), WBI, or the content of the thermoplastic resin (Dl), WDI, in the PI layer is more than 40% by mass, the dispersion phases become excessive, which significantly deteriorates film forming ability, and therefore the film sometimes cannot be obtained. When the content of the high melting point resin (Bl), Wai, or the content of the thermoplastic resin (Dl), WDI, is not less than 2% by mass and not more than 40% by mass, hydrolysis resistance, the effect by the addition of the particles, and film forming stability can be achieved simultaneously.

[0055]
From the standpoint that the high meltmg point resin (B1) or the thermoplastic resin (Dl) in the present invention prevents IV reduction associated wifli the extrusion process during masterpelletization with inorganic particles, the melting point of the high melting point resin (Bl), Tmsi, or the melting point of the thermoplastic resin (Dl), Tmni, is preferably 5°C to 60°C higher than the meltmg point of the polyester (Al), TmAi. When the meltmg point of the high melting point resin (Bl), Tmai, or the melting point of the thermoplastic resin (Dl), Tmoi, is higher in the range as described above than the melting point of the polyester (Al), TmAi, thermal degradation during the extrusion process in masterpelletization can be prevented.

[0056]
In the present invention, polyethylene terephthalate or polyethylene-2,6- naphthalenedicarboxylate is most preferred as the polyester (Al) because they are not only inexpensive but also have excellent mechanical properties. The melting points of these resms, though some degree of error is included depending on the process, are 255°C and 265°C in the case of polyethylene terephthalate and polyethylene-2,6-naphthalenedicarboxylate, respectively. Therefore, in the present invention, the melting point of the high melting point resin (B1), Tmsi, or the melting point of the thermoplastic resin (Dl), Tmoi, is preferably in the range of not less than 260°C and not more than 320''C. When the melting point of the high melting point resin (Bl), Tn^i, OT the meltuag pomt of the thermoplastic resin (Dl), Tmoi, is in the range of not less than 260°C and not more than 320°C, heat resistance is sufficient, and thermal degradation occurring during the extrusion process in masterpelletization is small; at the same time, there is no need to imduly mcrease the extrusion temperature during film forming.

[0057]
in the PI layer of the biaxially oriented polyester fihn according to the constitution [2] of the presoit inventicm, it is necessary that the thermoplastic resin (Dl) be present in the polyester (Al) as dispersion phases and that the number of the dispersion phases having a loagitudlnal length of more than 30,000 nm (30 pm) be not more than 2/3 x lO' nm^ (2/3,000 ta&^). In the biaxially oriented polyester fihn accordmg to the constitution [2] of the present invention, since other components such as inorganic particles are homogeneously dispersed in a polyester resin, the thermoplastic resm (Dl) is in the state of being masterpelletized with the inorganic particles. Fmther, from the standpoint of suppressing hydrolysis resistance, the inorganic particles need to have a reduced number of interfaces with the polyester (Al). Therefore, from these standpoints, the thermoplastic resin (Dl) needs to form dispersion phases in the polyester (Al). On the other hand, to exert the UV light resistance of the inorganic particles, the inorganic particles needs to be homogeneously dispersed in polyester resin (Al). Thus, in the biaxially oriented polyester film according to the constitution [2] of the present invention, the masterpelletized thermoplastic resm (Dl) is a dispersion phase having a longitudinal length of more than 30,000 nm (30 pm), the number of which is not more than 2/3 x lO' imi^ (2/3,000 \un\ in the polyester (Al). The number of the dispersion phase having a longitudinal length of more than 30,000 nm (30 \an) is preferably not more than 1/3 x lo' mn^ (1/3,000 pm^), and most preferably not more than 0.01/3 x lO' nm^ (0.01/3,000 pm^). When the number of the dispersion phases having a longitudinal length of more than 30,000 nm (30 fun) is mrae &an 2/3 x l O^ nm^ (2/3,000 ^un^), the inorganic particles are in a poorly dispersed state in the polyester (Al), whereby the resistance to change in color tone due to UV light provided by the inorganic particles becomes poor. The longitudmal length of the dispersion phases is measured by the Method for evaluating the properties (12) described below.

[0058]
In the biaxially oriented polyester fihn according to the constitution [2] of the present mvention, a preferred specific means for achieving not more than 2/3 x lo' nm'^ (2/3,000 \im^) of the dispersion phases that is composed of the thermoplastic resin (Dl) and has a longitudinal length of not less than 30,000 nm (30 am) is as follows:

(1) The inorganic particles (CI) and the thermoplastic resm (Dl) are melt kneaded in advance to obtam a masterpellet (Ml).

(2) Usmg the masterpellet (Ml) obtained in (1) and the polyester (Al), these are melt-extruded to obtam a non-oriented sheet. The upper limit of the ratio of the melt viscosity of the polyester (Al), TJA (poise), to the melt viscosity of the masterpellet (Ml), TIMI (poise), TIA/TIMI, is preferably not more than 1.0 (provided that the melt temperature is the extrusion temperature during melt film forming, Tc (°C)).

Although the lower limit of T^A/TIMI is not restricted, TIA/T|MI is preferably not less than

0.2. When the upper limit of TIA/TIMI is not less than 0.2, the difference in melt viscosity is appropriate, and not more than 30000 imi (30 pm) of a longitudinal length of the dispersion phase composed of the thermoplastic resin (Dl) is easily achieved. The melt viscosity of the masterpellet can be controlled, for example, by adjusting the molecular weight of the resin.

[0059]
The extrusion temperature during melt fihn forming, Tc (°C), is preferably set at higher than the melting point of the thermoplastic resin (Dl), Tmoi; Tmoi + 10°C or higher and Tmoi + 30°C or lower, and more preferably set at Tmoi + 15°C or higher and Tmoi + 20°C or lower. When Tc (°C) is in the preferred range described above, there is no need to increase the shear rate more than necessary when the resin is melt-extruded, and therefore the IV reduction during melt film formmg can be decreased.

[0060]
Further, considering both TIA/TIMI and the extrusion temperature during melt film forming, Tc (°C), when the thermoplastic resin (Dl) is used, (i), (ii), (v), (vi) below are preferably satisfied from the standpomt that this reduces the formation of a large dispersion phase (preferably, at least one is satisfied, more preferably, all of them are satisfied).

[0061]

T1A/T1MI>0.2 (i)
T1A/T1MI<1.0 (ii)
WnMi > -0.183 X (Tc - TmDi) + 2.095 (v)
W^Mi < -0.08 X (Tc - Tmoi) + 2.6 (vi) (3) The non-oriented sheet obtamed in (2) is biaxially stretched by known means to obtain the film of the present invention.

[0062]

In the biaxially oriented polyester film according to the constitution [1] of tHe present invention, the average longitudinal length of the dispersion phases composed of the high melting point resin (Bl) is not more than 10,000 nm (10 pm). If the average longitudinal length of the dispersion phases is more than 10,000 nm (10 pm), the dispersion phases in the polyester (Al) are large, and the inorganic particles will be in an inhomogeneously dispersed state in the polyester (Al) as mentioned above. As a result, UV light resistance provided by the inorganic particles can be poor. Therefore, when the average longitudinal length of the dispersion phase composed of the high melting point resin (Bl) is not more than 10,000 nm (10 fmi), the inorganic particles can be dispersed in the polyester (Al) more homogeneously. Preferred is not more than 6,000 nm (6 ^mi). Although the lower limit is not restricted, fi-om the standpoint of reducing the nimiber of interfaces between the inorganic particles and the polyester (Al), the average longitudinal lengtii of the dispersion phases composed of the high melting point resin (Bl) is preferably not less than 500 nm (0.5 fjm).

[0063]
In the biaxially oriented polyester film according to the constitution [1] of the present invention, a preferred means for achieving not more than 10,000 nm (10 \ua) of an average longitudinal length of the dispersion phases composed of the high melting point resin (Bl) is as follows:

(1) The inorganic particles (CI) and the high melting point resin (Bl) are melt kneaded in advance to obtain a masterpellet (Ml).
(2) Using the masterpellet (Ml) obtained in (1) and the polyester (Al), these are melt-extruded to obtain a non-oriented sheet. The ratio of the melt viscosity of the polyester (Al), TIA (poise), to the melt viscosity of the masterpellet (Ml), TIMI (poise), TIA/TIMI, is preferably not more than 1.0 (provided that the melt temperature is the extrusion temperature during melt film forming, Tc (°C)). Although the lower limit
of TIA/TIMI is not restricted, TIA/TIMI is preferably not less than 0.2. When iiA/tiMi is not less than 0.2, there is no need to increase the shear rate in order to obtam dispersion phases of not more than 10000 nm (10 \un), and there is also no danger of the IV reduction due to shear heating. The melt viscosity of the masterpellet can be controlled, for example, by adjustmg the molecular weight of the high melting point resin (Bl). The average longitudinal length of the dispersion phases composed of the high melting point resm (Bl) is the measured by 1ht Method for evaluating the properties (13) described below. [0064]
The extrusion temperature during meh film fonnmg, Tc (°C), is preferably set at higher than the melting point of the high melting point resin (Bl), Tmsi; Tmsi + 10°C and Tmai + 30°C or lower, and more preferably set at Tmsi + 15°C or higher and TmBi + 20°C or lower. When Tc (°C) is in the preferred range described above, there is no need to increase the shear rate more than necessary when the resia is extruded, and therefore the IV reduction during meh fihn forming can be decreased.

[0065]
Further, considering both T|A/T|MI and the extrusion temperature during melt film forming, Tc (°C), any of (i) to (vi) below is preferably satisfied when the high meltmg point resin (Bl) is used (preferably, at least one is satisfied, more preferably, all of them are satisfied).

[0066]
T1A/T1MI>0.2 (i)
TlA/nMi<1.0 (ii)
V^Mi > -0.16 X (Tc - Tmsi) + 2.6 (iii)
TIA/^MI < -0.08 X (Tc - Tmsi) + 2.6 (iv) (3) The non-oriented sheet obtained in (2) is biaxially stretched by known means to obtam the fihn of the present invention.
27

[0067]
Further, in the biaxially oriented polyester film according to the constitution [2] of the present invention, it is preferred from the standpoint of improving the hydrolysis resistance that the cyclohexylenedimetfaylene terephthalate units be 95 mol% or more of the total repeating units in the thermoplastic resin (Dl), and satisfying the relationships: x > 94.5 and y x 10'^ < x - 94.5 can be exemplified as the most preferred means for fully exerting the hydrolysis resistance or the effect that other components such as inorganic particles has in a polyester resin.

[0068]
Here, x represents the molar fraction (mol%) of 1,4-cyclohexylenedunethylene terephthalate units, and y represents the average longitudmal length (nm) of dispersion phases.

[0069]
In the biaxially oriented polyester film according to the constitution [2] of the present invention, the number of the dispersion phases composed of the thermoplastic resin (Dl) is preferably not less than 1/1,000 nm (1/pm) and not more than 5/\im (5/1,000 nm) per a unit of a length of 1,000 nm (1 \im) in the thickness direction of the film. More preferred is not less than 1/1,000 nm (l/jmi) and not more than 4/1,000 nm (4/^m), and most preferred is not less than 1/1,000 nm (l/Mm) and not more than 3/\im (3/1,000 nm). When the number of the dispersion phases is not less than 1/1,000 nm (l/pm) and not more than 5/1,000 nm (5/nm) per a unit of a length of 1,000 nm (1 pm) in the thickness direction, the film can fiilly exert the effect of obstructing the water entering from the film surface and has excellent hydrolysis resistance, while it will not have a reduced mechanical strength due to too large a number of the dispersion phases.

[0070]
In the biaxially oriented polyester film of the present invention, it is preferred that, in the above-described PI layer, 70% or more of the total number of the above-described inorganic particles (CI) be present in the above-described dispersion phases composed of the high melting pomt resin (Bl) or the thermoplastic resin (Dl) or in contact with the above-described dispersion phases. The upper limit is not particularly limited, and the larger the percentage of the inorganic particles (CI) present in the above-described dispersion phases or in contact with the above-described dispersion phases, the more preferred it is for hydrolysis resistance. However, when 95% or more, the inorganic particles are too locally present, and the UV light resistance effect by the addition of the inorganic particles can be poor. The percentage of the inorganic particles (CI) present in the above-described dispersion phase or in contact with the above-described dispersion phases is preferably not less than 80% and not more than 95%. In the polyester (Al), when the dispersion phases composed of the high melting point resin (Bl) or the thermoplastic resin (Dl) allows most of the inorganic particles (CI), preferably 70% or more of the total number, to be present or the like in the above-described dispersion {biases, the number of tiie inorganic particles (CI) in contact with the polyester (Al) can be reduced, which in turn allows effective inhibition of hydrolysis. That is, the presence or tire like of the inorganic particles in the above-described dispersicm phases ncrt cmly prevents the promotion of hydrolysis by the inorganic particles, particularly, highly active particles such as titanium oxide, present in the polyester (Al) but also reduces the mterfaces between the polyester (Al) and the inorganic particles (CI) to prevent local hydrolysis. This allows a balance between hydrolysis resistance and UV light resistance by the addition of the inorganic particles. Whether the inorganic particles (C1) are present in the above-described dispersion phases or in contact with the dispersion phases in the PI layer of the biaxially oriented polyester film of the present invention is determined by the Method for evaluating the properties (4) described below.

[0071]
Examples of specific means for allowing 70% or more of the total number of the inorganic particles (CI) to be present or the like m tiie dispersion phases composed of the high melting point resm (Bl) or the thermoplastic resin (Dl) include, for example, a means m which the inorganic particles (CI) and the high melting pomt resin (Bl) or the thermoplastic resin (Dl) is melt kneaded in advance to form a masterpellet (Ml); and melt film forming is performed using the masterpellet (Ml) and a pellet of the polyester (Al) as materials. The method of producing the masterpellet (Ml) will be desoibed in more detail. It is preferable to dry, if necessary, the high melting point resin (Bl) or the thermoplastic resin (Dl) used in kneading; introduce it and the inorganic particles (CI) into an extruder for heating to melt/kneading; and then cut a strand discharged fix)m a die into pieces to form a pelletized masterpellet. Although the screw of the extruder when kneading may be single, a double screw is preferably used for enhancing kneadability. By using the kneadmg method, the abundance of the inorganic particles in the dispersion phases and the number of the inorganic particles in contact with the dispersion phases can be efEiciently increased.

[0072]
In the biaxially oriented polyester film of tiie iHBsent invention, for allowing the inorganic particles (CI) to be present in the dispersion phases efficiently, it is also a preferred embodiment to select the polyester (Al) and the high melting point resin (Bl) or the thermoplastic resin (Dl) such that the ratio of the meh viscosity of the polyester (Al), IIA, to the meh viscosity of the masterpellet (Ml) obtained by kneading the high melting point resin (Bl) or the thermoplastic resin (Dl) and the inorganic particles (CI), TIMI, W^MI, is not more than 1.0. When, without using a masterpellet obtained by kneadmg in advance the morganic particles (CI) and the high melting pomt resin (B1) or the thermoplastic resm (Dl), just a mixture
composed of a pellet of the polyester (Al), a pellet of the high melting point resin (Bl) or the thermoplastic resin (Dl), and the inorganic particles (CI) is introduced into a fihn forming extruder for melt film formmg, it is difilcult to allow 70% or more of the total number of the inorganic particles (CI) to be present or the like in the dispersion phases composed of the high melting point resin (Bl) or the thermoplastic resin (Dl).

[0073]
In the biaxially oriented polyester film of the present invention, the content of the inorganic particles in the masterpellet formed by using the inorganic particles and the high meltmg point resin (Bl) or the thermoplastic resin (Dl) is preferably not less than 10% by mass and not more than 70% by mass, more preferably not less than 20% by mass and not more than 60% by mass, and most preferably not less than 40% by mass and not more than 60% by mass. When the particle concentration in the masterpellet is not less than 10% by mass and not more than 70% by mass, it is easy to allow 70% or more of the total number of the inorganic particles (CI) to be present or the like in the dispersion phases composed of the high melting point resin (Bl) or the thermoplastic resin (Dl).

[0074]
In the case where the biaxially oriented polyester film of the |»esent invention is a laminated polyester film having the above-described polyester layer (PI) layer and a polyester layer (P2 layer) containing a polyester (A2) comprising either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a mam constituent, a high melting point resin (B2) or a thermoplastic resin (D2), and inorganic particles (C2), also in the P2 layer, dispersion phases composed of the high melting pomt resin (B2) or the thermoplastic resin (D2) are preferably present in the polyester (A2). Also in the P2 layer, it is preferable to reduce the number of interfaces between the inorganic particles and the polyester (A2) from the standpoint of maintaming hydrolysis resistance. Thus, it is preferred that the high melting point resin (B2) or the thermoplastic resin (D2), as in the PI layer, also be masterpelletized m advance with the inorganic particles and present in the polyester (A2) as dispersion phases.

[0075]
From the standpoint of decreasing the reduction of hydrolysis resistance due to the inorganic particles, the content of the inorganic particles (C2) in the P2 layer, Wc2, is preferably not less than 0.1% by mass and not more than 5% by mass based on the P2 layer. In addition, the difference between the content of the inorganic particles (CI) in the PI layer, Wci (% by mass), and the content of the morganic particles (C2) in the P2 layer, Wc2 (% by mass), Wci - Wc2, is preferably not less than 5% by mass and not more than 25% by mass. This is for providing the constitution in the P2 layer focusing on improving hydrolysis resistance by decreasmg the amount of the inorganic particles added to less than that of the PI layer m contrast to the constitution in the PI layer focusing on improvmg UV light resistance by mcreasing the additional concentration of the morganic particles to thereby provide the constitution by which the balance of hydrolysis resistance and UV light resistance can be significantly achieved because each two fimctions are assigned to the two layers.

[0076]
Further, it is preferred that the relationship: 1.5 x MWA2' / MWA2 < MWIH' / MWD2 be satisfied, wherein the molecular weight of the polyester (A2) is MWA2; the molecular weight of the thermoplastic resm (D2) is MWD2; the molecular weight of the polyester (A2) after treatment at 125°C and 100% RH for 72 hours is MWA2'; and the molecular weight of the thermoplastic resin (D2) after treatment at 125°C and 100% RH for 72 hours is MWD2'. AS m the PI layer, it is preferable to masterpelletize the thermoplastic resin (D2) and the morganic particles m order to reduce the number of interfaces between the inorganic particles (C2) and the polyester (A2). Thus, when the polyester (A2) and the thermoplastic resm (D2) satisfy the relationship described above, the reduction of hydrolysis resistance due to the inorganic particles (C2) can be inhibited. The lamination ratio of the PI layer to the P2 layer is preferably 1:3 to 1:8 also from the standpoint of simultaneously achieving hydrolysis resistance and UV light resistance. [0077]
For a lamination constitution, two layers of P1/P2 layer or three layers of P2/P1/P2 is a preferred embodiment because the adhesion to the ethylene-vinyl acetate copolymer (EVA) described below can be maintained by arranging the P2 layer, which contains a small amount of inorganic particles, as an outermost layer.

[0078]
As the polyester (A2), the high melting point resin (B2) or the thermoplastic resin (D2), and the inorganic particles (C2), those of the same type as the polyester (Al), the high melting point resm (Bl) or the thermoplastic resin (Dl), and the inorganic particles (CI) mentioned above can be suitably used, respectively. Further, as a method of allowing a dispersion phase composed of the high melting point resin (B2) or the thermoplastic resin (D2) to be present m the polyester (A2), the above-mentioned method of allowing a dispersion phase composed of the high melting point resin (Bl) or the thermoplastic resin (Dl) to be present in the polyester (Al) can be suitably used,

[0079]
In the present invention, it is particularly preferred that, in the P2 layer, 70% or more of the total number of the above-described inorganic particles (C2) be present in the above-described dispersion phases or in contact with the above-described dispersion phases. Also as a method of allowing 70% or more of the total number of the above-described inorganic particles (C2), in the P2 layer, to be present or the like in the above-described dispersion phases, the above-mentioned method can be suitably used.

[0080]
In the description below, M2 refers to a masterpellet (M2) obtained by melt kneading in advance the morganic particles (C2) and the high melting point resin (B2) or the thermoplastic resin (D2); r\M2 refers to the melt viscosity of the masterpellet (M2) (poise) (provided that the melt temperature is the extrusion temperature during melt film forming, Tc (°C)); TmB2 refers to the meltmg point (°C) of the high melting point resin (B2); and TmD2 refers to the melting point (°C) of the thermoplastic resin (D2). [0081]
In the present invention, preferred is a combination of the polyester (Al) comprising as a main component either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate and the high melting pomt resin (Bl) or the thermoplastic resin (Dl) which is any of the polyester resin containing 1,4-cyclohexylenedimethylene terephthalate units in an amount of 93 mol% or more, the polyester resin comprising as a main constituent ethylene-2,6-naphthalenedicarboxylate units, or the resin comprising phenylene sulfide as a main constituent

[0082]
In the method of producing the biaxially oriented polyester film of the present invention, tiie high melting point resin (Bl) or the thermoplastic resin (Dl) and the inorganic particles (CI) are melt kneaded to produce a masterpellet (Ml); and the polyester (Al) and the masterpellet (Ml) are then melt kneaded, extruded into sheet foim, and then biaxially oriented to obtain a biaxially oriented polyester film. In the melt kneading of the polyester (Al) and the masterpellet (Ml) in a series of these processes, in the case where the high melting point resin (Bl) is used, it is preferable to employ the conditions satisfying any of (i), (ii), (iii), (iv) below; and in the case where the thermoplastic resin (Dl) is used, it is preferable to employ the conditions satisfymg any of (i), (ii), (vi), (vii) below. By employmg such conditions, the effect of improving the hydrolysis resistance and the effect of improving the UV light resistance or the like of the biaxially oriented polyester fihn finally obtained can be equalized.

[0083]
Here, the melt viscosity of the polyester (Al) is r\\; the melt viscosity of the masterpellet (Ml) is T^MI; Tmsi is the melting point (°C) of the high meltmg point resin (Bl), Tmoi is the melting point (°C) of the thermoplastic resin (Dl); Tc is the extrusion temperature (°C) during melt film forming; and T^A and TIMI are the melt viscosity of the polyester (Al) and the masterpellet (Ml), respectively, at a temperature of Tc (°C) and a shear rate of 200 sec"'.

[0084]
T1A/T1MI>0.2 (i)
T1A/T1MI<1.0 (ii)
WnMi > -0.16 X (Tc - TmBi) + 2.6 (iii)
TlA^i ^ -0-08 X (Tc - TmBi) + 2.6 (iv)
V^Mi < -0.08 X (Tc - Tmoi) + 2.6 (vi)
WTIMI > -0.16 X (Tc - Tmoi) + 2.6 (vii)

The polyester film of the present invention needs to be a biaxially orientated film. Biaxial orientation effectively forms orientationally crystallized portions, thereby fiirther enhancing the hydrolysis resistance. Biaxial orientation can be achieved by stretching a film biaxially. Examples of stretching methods which can be used include sequential biaxial stretching method (a stretching method combining one-directional stretchings, such as a method in which stretching is performed in the transverse direction after stretching in the machine direction), simultaneous biaxial stretching method (a method in which stretching is performed simultaneously in the machine du-ection and the transverse direction), or a combination thereof^ any of which can be preferably used in the present invention. In addition, stretching a film biaxially by these stretching methods provides not only improved productivity but also mechanical strength and good planarity.

[0085]
The fihn thickness of the present invention is preferably not less than 1,000 nm (1 \jm) and not more than 200,000 nm (200 ^m), more preferably not less than 3,000 nm (3 \im) and not more than 150,000 nm (150 nm), and especially preferably not less than 5,000 nm (5 \im) and not more than 100,000 nm (100 pm). The thickness of the biaxially oriented polyester fihn of the present invention of not less than 1,000 nm (1 ^un) and not more than 200,000 nm (200 pm) provides the fihn with good hydrolysis resistance and handleability as well as good planarity. In particular, when the particles having UV absorptivity are contained, the UV light resistance will not be poor because the film thickness is not too thin. On the other hand, when used as a solar battery back sheet, the total thickness of the solar battery cell will not be too thick.

[0086]
Further, the biaxially <»ie^ed polyester film of the present invention may contain other additives (for example, organic particles, a heat-resistant stabilizer, an UV absorber, a weathering stabilizer, an organic lubricant, a pigment, a dye, a filler, an antistat, a nucleatmg agent, and the like) as long as the effects of the present invention are not impaired.

[0087]
The biaxially oriented polyester fihn of the present mvention preferably has a lensile elongation retention after treatment under an atmosphere at a temperature of 125°C and a humidity of 100% RH for 48 hr of 50%, more preferably 55% or more, still more preferably 60% or more, and most preferably 70% or more. Within such a range, the hydrolysis resistance of the film becomes even better.

[0088]
Further, tensile elongation retention after irradiation treatment with metal halide lamps with an mtensity of 100 mW/cm^ (wavelength range: 295 nm to 450 rmi, peak wavelength: 365 nm) under an atmosphere at a temperature of 60°C and 50% RH for 48 hr is preferably 10% or more, more preferably 15% or more, still more preferably 25% or more, and most preferably 35% or more. When the polyester fihn of the present invention is irradiated with metal halide lamps, particularly in the case where the polyester film is laminated on the other film, the uradiation is carried put such that the side of the biaxially orientated fihn of the present invention is exposed. Within such a range, the film has good UV light resistance.

[0089]
Furthermore, since the fihn whose tensile elongation retention after treatment under an atmosphere at a temperature of 125°C and a humidity of 100% RH for 48 hr and tensile elongation retention after uradiation treatment with metal halide lamps with an intensity of 100 mW/cm xmder an atmosphere at a temperature of 60°C and 50% RH for 48 hr are both in the above-described |»eferred rai^ has an excellent hydrolysis resistance and UV light resistance, it mamt^s the mechanical strength over a long period of time also when used as a solar battery back sheet, for example.

[0090]
From the standpomt of improvmg the durability of change m color tone due to UV uradiation, Ab after irradiation treatment with metal halide lamps with an intensity of 100 mW/cm^ (wavelength range: 295 irai to 450 nm, peak wavelength: 365 nm) under an atmosphere at a temperature of 60°C and 50% RH for 48 hr is preferably not more than 10, more preferably not more than 6, and still more preferably not more than 3. Ab is measured by the Method for evaluating the properties (8) described below. When Ab is not more than 10, a film having more excellent durability of change m color tone due to UV irradiation can be obtained.

[0091]
The biaxially oriented polyester fihn of the present mvention has hydrolysis resistance and sunultaneously achieves other properties such as UV light resistance and light reflectivity. Hierefore, it can be used in such applications where long-term durability is regarded as important and suitably used particularly as a fihn for a solar battery back sheet

[0092]
A solar battery back sheet is composed, for example, of the biaxially oriented polyester fihn of the present invention, an EVA adhesive layer for improving adhesion to an ethylene-vinyl acetate copolymer (hereuiafter also referred to as EVA for short), an anchor layer for enhancing adhesion to the EVA adhesive layer, a water vapor barrier layer, a UV-absorbing layer for absorbing UV light, a light-reflecting layer for enhancing generation efficiency, a light-absorbmg layer for expressing a design, an adhesive layer for bonding each layer, and the like, and, in particular, the biaxially oriented polyester film of the present invention can be suitably used as a UV-absorbing layer, a light-reflecting layer, and a light-absorbing layer.

[0093]
By combining each layer described above with the biaxially oriented polyester fihn of the present mvention, the solar battery back sheet of the present invention is formed. In the solar battery back sheet of the present invention, all the above-mentioned layers need not be formed as an independent layer, and it is also a preferred embodiment to form the layers as a functionally integrated layer combining multiple functions. Further, when the biaxially oriented polyester fihn of the present invention ah-eady has a necessary function, other layers for unparting the function can be omitted. For example, in the case where the biaxially oriented polyester fihn of the present invention comprises a layer containing a white color pigment and bubbles and has light reflectivity, the light-reflectmg layer can be omitted; in the case where the fihn comprises a layer containmg a light absorber and has light absorbency, the absorbing layer can be omitted; and in the case where the film comprises a layer containing an UV absorber, the UV-absorbing layer can be omitted, as the case may be.

[0094]
The solar battery back sheet usmg the biaxially oriented polyester fihn of the present invention preferably has a tensile elongation retention after being left to stand imder an atmosphere at a temperature of 125°C and a humidity of 100% RH for 48 hr of 50% or more, more preferably 55% or more, still more preferably 60% or more, and most preferably 70% or more. In the biaxially oriented polyester film of the present invention, if the tensile elongation retention after being left to stand under an atmosphere at a temperature of 125°C aiid a humidity of 100% RH for 48 hr is 50% or more, for example, the deterioration due to heat and humidity hardly proceeds when a solar battery equipped with the back sheet is used for a long period of time, and even when some external impacts are applied to the solar battery (for example, when a fallmg rock hits the solar battery), the back sheet will not break.

[0095]
The solar battery back sheet usmg the biaxially oriented polyester fihn of the present invention preferably has a tensile elongation retention after uradiation with metal halide lamps with an intensity of 100 mW/cm^ (wavelength range: 295 nm to 450 nm, peak wavelength: 365 nm) under an atmosphere at a temperature of 60°C and 50% RH for 48 hr of 10% or more, more preferably 15% or more, still more preferably 25% or more, and most preferably 35% or more. When the solar battery back sheet using the biaxially oriented polyester fihn of the present uivention is irradiated with UV light, particularly m the case where the polyester film of the present invention is laminated on the other fihn, the irradiation is carried out such that the side of the biaxially orientated polyester film of the present invention is exposed to the UV light. If tiie tensile elongation retention after irradiation with metal halide lamps with an intensity of 100 mW/cm^ under an atmosphere at a temperature of 60°C and 50% RH for 48 hr is not less than 10%, for example, the deterioration due to UV light hardly proceeds when a solar battery equipped with the back sheet is used for a long period of time, and when some external impacts are applied to the solar battery (for example, when a falling rock hits the solar battery), the back sheet will not break. [0096]
For the biaxially oriented polyester fihn of the present invention to exert its effects of high hydrolysis resistance and UV light resistance when used in a solar battery back sheet, the volume percent of the film of the present invention relative to the total solar battery back sheet is preferably not less than S%, more preferably not less than 10%, still more preferably not less than 15%, and especially preferably not less than 20%.

[0097]
In addition, in the present invention, the biaxially oriented polyester film of the present invention is preferably provided at at least one outermost side of the solar battery back sheet. Fiuther, it is preferable to arrange the PI layer at at least one outermost layer of the solar battery back sheet. In such an embodiment, hydrolysis resistance and UV light resistance can be maximally exerted.

[0098]
The solar battery of the present invention is characterized by using a solar battery back sheet comprising the biaxially oriented polyester film of the present invention as a component. The solar battery back sheet comprising the biaxially oriented polyester film of the present invention as a component can provide a highly durable and thin solar battery compared to conventional solar batteries by exploiting the characteristic in that it is more excellent than conventional back sheets in hydrolysis resistance and other functions, particularly, change resistance in color tone after UV irradiation. The constitution thereof is illustrated in FIG. 1. The solar battery is constituted in such a maimer that an electric generating element coimected to a lead wire for drawing electricity (not shown m FIG. 1) is sealed with a clear transparent filler agent 2 such as an EVA resin, and a transparent substrate 4 such as glass and a solar battery back sheet 1 are laminated thereon. However, the constitution is not limited thereto, and any constitution may be used.

[0099]
An electric generating element 3, which converts the light energy of a simlight 5 into electrical energy, can be used m series or parallel coimection with any element, optionally a plurality of elements depending on the desired voltage or current, such as crystalline silicon-based elements, polycrystalline silicon-based elements, microciystalline silicon-based elements, amorphous silicon-based elements, copper indium selenide-based elements, compound semiconductor-based elements, and dye sensitizer-based elements, depending on the purpose.

[0100]
The transparent substrate 4 having translucency is arranged at an outermost layer of the solar battery, and therefore transparent materials having not only high transmittance but also high weatherability, high stain resistance, and high mechanical strength properties are used. In the solar battery of the present invention, any material can be used for the transparent substrate 4 having translucency as long as it satisfies the above-described properties, and preferred examples of the materials include glass; fluorine resins such as tetrafluoroethylene-ethylene copolymer (ETFE), polyvinyl fluoride resin (PVF), polyvinylidene fluoride resin (PVDF), polytetrafluoroethylene resin (TFE), tetrafluoroethylene-exafluoropropylene copolymer (FEP), polychlorotrifluoroethylene resin (CTFE), and polyvinylidene fluoride resin: olefin resms; acrylic resins; and mixtures thereof In the case of glass, it is more preferable to use tempered one. In the case where a translucent substrate made of a resin is used, resins obtained by uniaxially or biaxially orienting the above-described resins are also preferably used from the standpoint of mechanical strength.

[0101]
In addition, it is also preferable to subject the surface of these substrates to c(nx)na treatment, plasma treatment, ozone treatment, or adhesive treatment in order to provide adhesion to an EVA resin that serves as a sealing material agent for the electric generatmg element

[0102]
The transparent filler agent 2 for sealing the electric generatmg element is not only for the purpose of electrical insulation by coating and fixing projections and depressions on the surface of Ae electric generating element with a resin to protect the electric generating element fi'om the external environment, but it also adheres to the substrate having translucency, the back sheet, and the electric generating element. TlKTefiBC materials havmg high transparency, high weatherability, high adhesion, and high heat resistance are used. Examples thereof that are preferably used include ethylene-vinyl acetate copolymer (EVA), ethylene-methyl acrylate copolymer (EMA), ethylene-ethyl aciylate copolymer (EEA) resin, ethylene-methacryl acid copolymer (EMAA), ionomer resin, polyvinyl butyral resin, and mixtures thereof Among these resins, ethylene-vinyl acetate is more preferably used in terms of excellent balance of weatherability, adhesion, repletion, heat resistance, cold resistance, and shock resistance.

[0103]
As described above, by incorporating the solar battery back sheet using the biaxially oriented polyester fihn of the present invention into a solar battery system, a solar battery system that is highly durable and/or thin compared to conventional solar batteries can be achieved. The solar battery of the present invention can be suitably used for various applications, without limitation to outdoor applications or indoor applications, such as a photovoltaic system and a power source for small electronic parts.

[Methods for evaluating the properties]
(1) Melting point of polyester (Al), high melting point resin (Bl), and thermoplastic
resm (Dl)

In accordance with JIS K7122 (1987), the melting point of a resm, Tm, was measured using a differential scanning calorimetry robot DSC-RDC220 manufactured by Seiko Instrument Inc. and "Disk session" SSC/S200 for data analysis. Measurements were made in such a manner that 5 mg of a resin was weighed into a sample pan; the resin was heated from 25°C to 320°C at a temperature rise rate of 20°C/min as 1st RUN, held there for 5 minutes, and then rapidly cooled to 2S°C or Iowa*; and the temperature was raised agam from room temperature to 320°C at a temperature rise rate of 20°C/min as 2nd Run. The obtained peak top temperature at the oystd melting peak in the 2nd Run was taken as the melting point Tm.

(2) Melt viscosity r\A., melt viscosity TIMI. melt viscosity T\M2
Measurements were made by Shimadzu Flow Tester CFT-500A manufactured by Shimadzu Corporation using a resin dried in a vacuum oven under reduced pressure at 180°C for 3 hours or more. The amoimt of the resin is about 5 g, and the melt temperature is set at the same temperature as the extrusion temperature during film forming. Using loads of 10 N, 30 N, and 50 N (loading was started after 5 minutes from the start of sample setting), the shear rate and melt viscosity at each load were determined.
The die was of (p 1 mm and L = 10 mm.

The number of the measurements was three times for each load, and each mean value was determined. The obtained numerical data of the melt viscosity and shear rate at each load were graphed, and the value at a shear rate of 200 sec'' was determined from the graph.

(3) The nimiber of dispersion phase per a imit of a length of 1,000 nm (1 pm) m the thickness direction of film

Using a microtome, observation samples in the form of a thin film section were prepared without crushing the fihn cross section in the thickness direction. Two types of samples were prepared: an MD cross-sectional tUn film section taken parallel to the machine direction (MD) direction of the fihn and a TD cross-sectional thin fihn section taken parallel to the transverse dkection (TD) direction.

[0104]
Next, the cross-sectional thm fihn section obtained was observed usmg a transmission electron microscope (TEM) (transmission electron microscope H-7100FA manufactured by Hitachi Ltd.) to obtain an image scaled up 10,000 times. When it was difiicult to distinguish a dispersion phase in the image, the film was prestained using, for example, osmic acid or rutheniimi oxide as appropriate to carry out the observation.

[0105]
Usmg the image obtained above, the number of the dispersion phase composed of the high melting point resin (Bl) or the thermoplastic resin (Dl) per a unit of the fihn thickness of 1,000 nm (1 \ua) was determined. For five points randomly determined in the fihn, the niraiber was determmed individually, and the mean value was taken as the number of the dispersion phase per a unit of a length of 1,000 nm (1 |mi) m the thickness direction of the film. If the number of the dispersion phases per a unit of 1,000 nm (1 nm) is one or more, then a dispersion phases shall be deemed to exist.

(4) Distribution of inorganic particles
The number of all the inorganic particles in the observed image scaled up 50,000 times obtained by the same method as in the section (3) above was counted and taken as the total number N, and among the particles, the number of the particles present in the dispersion phases composed of the high melting point resin (Bl) or the thermoplastic resin (Dl) or in contact with the dispersion phases, Nb, was determined. Using each value obtained, the percentage of the particles present in the dispersion phases composed of the high melting point resin (Bl) or the thermoplastic resin (Dl) or in contact with dispersion phasse relative to the total number of the particles present in the fihn, Nb / N x 100 (%), was calculated. For five points randomly determined in the polyester layer, the percentage was determined individually, and the mean value was taken as Ihe percentage of the particles.

(5) Measurement of tensile elongation at break
According to ASTM-D882 (1999), a sample was cut out to a size of 10 mm x 200 mm, the tensile elongation at break when pulled at a chuck distance of S mm and a tensile speed of 300 mm/min was measured. For the number of the samples, n = 5, and after the measurements for both the longitudinal direction and the transverse direction of the film, the tensile elongation at break was determined as the mean value thereof

(6) Tensile elongation retention after moist-heat resistance test
A sample was cut out into the shape of a measurement strip (10 mm x 200 mm), and then treated in a pressure cooker manufactured by Tabai Espec Corporation imder conditions at a temperature of 125°C and 100% RH for 48 hr, after which the tensile elongation at break was measured according to the section (5) above. In the measurements, n = 5, and after the measurements were made for both the longitudinal direction and the transverse direction of the film, the mean value

thereof was taken as the tensile elongation at break Ei. Further, for the film before treatment, the tensile elongation at break Eo was measured according to the section (5) above, and the obtained tensile elongations at break Eo and Ei were used to calculate the tensile elongation retention by the following equation.

[0106]
Tensile elongation retention (%) = Ei / Eo x 100
If the tensile elongation retention is 50% or more, the film can be suitably used as a film for a solar battery back sheet. More preferred is 55% or more, still more preferred is 60% or more, and most preferred is 70% or more. (7) Tensile elongation retention after weathering test

A sample was cut out into the shape of a measurement strip (1cm x 20cm), and then uradiated in EYE Super UV tester, SUV-W131, manufactured by IWASAKI ELECTRIC CO., LTD. under conditions at a temperature of 60°C, a relative humidity of 60% RH, and an illuminance of 100 mW/cm^ (light source: metal halide lamps, wavelength range: 295 nm to 450 nm, peak wavelength: 365 nm) for 48 hours, after which the tensile elongation at break was measured according to the section (5) above. In the measurements, n = 5, and after the measurements were made for both the longitudinal direction and the transverse direction of the fihn, the mean value thereof was taken as the tensile elongation at break E2. Further, also for the fihn before treatment, the tensile elongation at break Eo was measured according to the section (5) above, and the tensile elongations at break Eo and E2 thus obtained were used to calculate the tensile elongation retention by the following equation.

[0107]
Tensile elongation retention (%) = Ej / Eo x 100 When the fihn is a laminated film, the side of the biaxially oriented polyester fihn of the present invention is uradiated with UV light. (8)Ab
A sample was cut out into the shape of a measurement strip (10 mm x 200 mm), and then irradiated in EYE Super UV tester, SUV-W131, manufactured by IWASAKI ELECTRIC CO., LTD. under conditions at a temperature of 60°C, a relative humidity of 60% RH, and an illuminance of 100 mW/cm^ (light source: metal halide lamps, wavelength range: 295 nm to 450 nm, peak wavelength: 365 nm) for 48 hr. The b value of the PI layer in reflectance mode was measured in accordance with JIS Z 8722 (2000) using a spectral color difference meter, model SE-2000 (manufactured by NIPPON DENSHOKU E^JDUSTORIES CO., LTD.). For the number of the samples, n = 5, and each b value was measured to calculate the mean value thereof The sample measurement diameter was 30 mm q). Taking the b value of the film before UV irradiation treatment as Ko, and the b value after the above-described treatment as K; Ab was calculated by the following equation.

[0108]
Ab = K-Ko
(9) Tan 6 peak temperature of thermoplastic resin (Dl)
The tan 5 peak temperature was determined according to JIS-K7244 (1999) using a dynamic mechanical analyzer, DMS6100, manufactured by Seiko Instruments, Inc. The tan 5 temperature dependence of the thermoplastic resin (Dl) was measured under the measurement conditions of tensile mode, a drive frequency of 1.0 Hz, a chuck distance of 5 nun, a strain amplitude of 10,000 nm (10 pm), an initial value of force amplitude of lOOmN, a temperature rise rate of 2°C/min, and a measiuement temperature range fix)m 25°C to the melting point of a resin to be measured -20°C or, in the case of a resin having no meltmg point, a temperature range of Tg + 100°C. The tan 5 peak temperature was read out from the results of this measurement.

(10) Method of separating polyester (Al) and thermoplastic resin (Dl)
An example of methods of separating the polyester (Al) and the thermoplastic resin (Dl) is that they can be separated by selectively dissolving them using a solvent that dissolves the polyester (Al) and does not dissolves the thermoplastic resin (Dl) or a solvent that does not dissolve the polyester (Al) and dissolves the thermoplastic resin (Dl) and performing, for example, filtration and centrifugation. Examples of solvents mclude, for example, chlorophenol, chloroform, hexafluoroisopropanol, l-chloronaph1halene, and a mixed solvent of parachlorophenol and chloroform. When the polyester (Al) and the high melting point resin (Bl) or the thermoplastic resin (Dl) are both soluble m the above-mentioned solvent, for example, a me&od such as changing the solubility of the resin, for example, by raising the temperature of the solvent as appropriate can be combined to separate the polyester (Al) and the high meltmg point resin (Bl) or the thermoplastic resin (Dl). (11) Method of measuring weight-average molecular weight

First, the weight-average molecular weight was determined using as a detector a refractive mdex detector, RI (model RI-8020, sensitivity: 32), manufactured by SHOWA DENKO K.K. and as a coliunn a gel permeation chromatograph, GPC (16), available jfrom TOSOH CORPORATION equipped with two TSK gel GMHHR-M (9: 7.8 mm x 300 mm, theoretical plate number: 14,000 plates) available from TOSOH CORPORATION. As a moving bed, a solvent m which the polyester (Al) and the high melting point resin (Bl) or the thermoplastic resin (Dl) are both soluble is most preferably used, and since the polyester (Al) in tile present invention is a resm comprising as a main component polyethylene terephthalate or polyethylene-2,6-naphthalenedicarboxylate, examples of solvents include, for example, chlorophenol, chloroform, hexafluoroisopropanol, and 1-chloronaphthalene. When the high meltmg point resin (B1) or the thermoplastic resin (Dl) is not soluble in the above-mentioned solvent, a solvent in which they are soluble may be used, and when they are not readily soluble, the dissolution may be promoted, for example, by raismg the temperature of the solvent as appropriate. Next, the flow rate of the moving bed was 1.0 mL/min; the column temperature was 23°C ± 2°C; and the injection volume was 0.200 mL. Here, monodisperse polystyrene (TSK standard polystyrene available from TOSOH CORPORATION) was used as a standard sample, and the relative value to the polystyrene was used. This relative value was taken as the weight-average molecular weight.

(12) Method of measuring longitudinal length of dispersion phases

(a) Observed images scaled up 500 to 5,000 times obtained by the same method as in the secticm (3) above were obtained to observe the cross section such that the total cross-sectional area of the polyester layer (PI layer) is 3 x lO' nm(3,000 pm). When 3 x 10' nm (3,000 m) is not satisfied at two points in the MD and TD cross-sectional image because the thickness of the polyester layer (PI
layer) is thin, a plurality of cross sections may be used to observe such that 3 x lO' nm (3,000 jun) is satisfied in total.

[0109]
(b) For all the dispersion phases composed of the high melting point resm (Bl) or the thermoplastic resin (Dl) contained in the image obtained, the longitudinal length is individmdly measured. Here, the two points having the longest straight- line distance therebetween in the dispersion phase is determined, and the straight-line length between the two points is employed as the longitudinal length of the
dispersion phase.

[0110]
(c) The number of the dispersion phases of more than 30,000 nm (30 jun) among all the longitudinal length obtained in the above (b) is counted.

(13) Method of measuring average longitudinal length of dispersion phase

(d) Observed images scaled up 5,000 times obtained by the same method as in the section (3) above are obtained. Observation pomts are three or more points randomly determined in the polyester layer (PI layer) (i.e., three or more images can be obtamed).

[0111]
(e) For all the dispersion phases composed of the high melting point resin (Bl) or the thermoplastic resin (Dl) contained in the image obtained, the longitudinal length is mdividually measured. Here, the two points having the longest straight- line distance therebetween in the dispersion phase is determined, and the straight-line length between the two pomts is employed as the longitudinal length of the
dispersion phase.

[0112]
(f) Also for the TD cross-sectional thin film section, the longitudinal length of each dispersion phase is individually measured by tiie same method as in (d) and (e).

[0113]
(g) All the longitudinal length obtained in the above (e) and (f) is averaged to obtain the average longitudinal length.

EXAMPLES [0114]
The present invention will now be described by way of examples. (Preparation of materials) Polyethylene terephthalate (PET) (polyester (Al))
Using 100 mol% of terephthalic acid as a dicarboxylic acid component and 100 mol% of ethylene glycol as a diol component, a polycondensation reaction was carried out using magnesium acetate, antimony trioxide, and phosphorous acid as a catalyst. Then, the polyethylene terephthalate obtained was dried at 160°C for 6 hours and crystallized, after which a solid phase polymerization at 220°C and a degree of vacuum of 0.3 Torr for 9 hours was carried out to obtain polyethylene terephthalate (PET) having a melting point of 255°C and a melt viscosity TIA of 3,500 poise.

Polycyclohexylenedimethylene terephthalate (PCHT/I) (high melting point resin
(Bl) or thermoplastic resin (Dl))
Using 95 mol% of terephthalic acid and S mol% of isophthalic acid as a dicarboxylic acid component and 100 mol% of cyclohexylenedunethanol as a diol component, a polycondensation reaction was carried out using magnesium acetate, antimony trioxide, and phosphorous acid as a catalyst, to obtam polycyclohexylenedimethylene terephthalate (PCHT/I) containing 5 mol% of isophthalic acid as a copolymer component and having a melting point of 283°C. Polycyclohexylenedimethylene terephthalate (PCHT) (high melting point resin (Bl) or thermoplastic resin (Dl))

Using 100 mol% of terephthalic acid as a dicarboxylic acid component and 100 mol% of cyclohexylenedimethanol as a diol component, a polycondensation reaction was carried out using magnesium acetate, antimony trioxide, and phosphorous acid as a catalyst to obtain polycyclohexylenedimethylene terephthalate (PCHT) having a melting point of 290°C.

Polycyclohexylenedunethylene terephthalate (PCHT/G) (high melting point resin (Bl) or thermoplastic resm (Dl))

Using 100 mol% of terephthalic acid as a dicarboxylic acid competent and 87 mol% of cyclohexylenedunethanol and 13 mol% of ethylene glycol as a diol component, a polycondensation reaction was carried out using magnesium acetate, antimony trioxide, and phosphorous acid as a catalyst to obtain polycyclohexylenedunethylene terephthalate (PCHT/G) containing as a copolymer component 13 mol% of ethylene glycol and having a melting point of 265°C. Polyethylene-2,6-naphthalenedicarboxylate (PEN) (polyester (Al), or high melting point resm (Bl) or thermoplastic resin (Dl))

To an ester interchange reactor equipped with a mixing device, a rectifying column, and a condenser, added was 100 parts by mass of dimethyl 2,6-naphthalene dicarboxylic acid, 51 parts by mass of ethylene glycol, 0.06 parts by mass of calcium acetate, and 0.025 parts by mass of antimony trioxide. The temperature was gradually raised from 180°C to 240°C, and an ester interchange reaction was carried out while continuously distilling the simultaneously generated methanol out of the reaction system. To the reactant thus obtained, 0.04 parts by mass of trimethyl phosphate ester was added, and the resultmg mixture was reacted for 5 minutes. Then, the temperature was raised to 285°C while contmuously dischargmg the ethylene glycol, and the pressure was simultaneously reduced to 0.2 mmHg to carry out a polycondensation reaction, thereby obtaining polyethylene-2,6-naphthalenedicarboxylate (PEN) havuig an intrinsic viscosity of 0.82. Polyphenylene sulfide (high melting point resin (Bl) or thermoplastic resin (Dl)) A PPS resin (M3910 available from TORAY INDUSTRIES, INC.) (PPS) was used.

3 mol% 2,6-naphthalene dicarboxylic acid copolymerized PET (PET/N) (high melting point resin (Bl) or thermoplastic resm (Dl))
Used was 3 mol% 2,6-n^hthalene dicarboxylic acid copolymerized PET (PET/N) dried at 170°C for 3 hours.

Polyethylene diphenylcarboxylate (PEDPC) (high melting point resin (Bl) or thermoplastic resm (Dl))
Polyethylene diphenylcarboxylate (PEDPC) dried at 180°C for 3 hours was used. Titanium oxide (inorganic particles C)

Rutile-type titanium oxide particles with an average particle diameter of 200 nm were used. Barium sulfate (inorganic particles C)

Bariimi sulfate with an average particle diameter of 700 nm was used.

(Examples 1 to 13,17 to 29,33 to 45,49 to 60, Comparative Examples 1 to 27) Production of biaxially stretched (biaxially oriented) polyester fihn

The high melting point resin (Bl) or the thermoplastic resin (Dl) and the morganic particles (CI) shown in Tables 1,6,11, and 16 were mixed such that the contents were as shown in Tables 1, 6,11, and 16, and the resulting mixture was melt kneaded in a vented extruder at a temperature shown below to produce a masterpellet (Ml) such that the value of TIA/^IMI was as shown in Tables 1, 6,11, and 16.

Examples 17 to 29, Examples 53 to 56, Examples 64 to 66, Comparative Example 15, Comparative Example 16, Comparative Example 23, Comparative Example 26: 280°C

Examples 1 to 13, Examples 49 to 52, Examples 61 to 63, Example 70, Example 72, Comparative Examples 1 to 6, Comparative Example 13, Comparative Example 14, Comparative Examples 19 to 22, Comparative Example 25, and Comparative Example 28:290°C Examples 73 and 74: 300°C
Examples 33 to 45, Examples 57 to 60, Examples 67 to 69, Example 71, Comparative Example 17, Comparative Example 18, Comparative Example 24, Comparative Example 27, Comparative Example 30: 310°C Comparative Examples 7 to 12: 345°C

[0115]
Then, a pellet of the polyester (Al) vacuum-dried at 180°C for 3 hours shown in Tables 1, 6,11, and 16 and a masterpellet (Ml) vacuum-dried at 180°C for 3 hours were mixed such that the contents were as shown in Tables 1,6,11, and 16, and the resulting mixture was melt kneaded at an extruder temperature during film forming shown in Tables 1, 6,11, and 16 and introduced into a T-die.

[0116]

Then, the resultant was meh extruded from tiie T-die into sheet form and brought mto close contact by electro-pinning with a drum maintained at a surface temperature of 25°C to be cooled to solidify, thereby obtaining a non-oriented monolayer fihn. Next, the non-oriented monolayer fihn was preheated with a group of rolls heated to a temperature of 80°C, stretched 3.5-fold m the machine direction (longitudinal duction) usmg a heatmg roll at a temperature of 88°C, and cooled with a group of rolls at a temperature of 25°C to obtain a imiaxially stretched film.

[0117]
The imiaxially stretched fihn obtained was guided to a preheating zone at a temperature of 90°C in a tenter with both ends held by clips, and then continuously stretched 3.8-fold in the du-ection perpendicular to the machine direction (the transverse du^ction) in a heating zone mamtamed at 100°C. Further, the film was subjected to heat treatment at 220°C for 20 seconds in a heat treatment zone in the tenter, and fiirthermore relaxed m the transverse du^ction by 4% at 220°C. Then, the fihn was uniformly and slowly cooled to obtam a biaxially oriented polyester film with a thickness of 50,000 nm (50 pm).

[0118]
The fihn obtamed was evaluated for percentage of the cases where titanium oxide particles were present or the like in the dispersion phases of the high melting point resin (Bl) or the thermoplastic resin (Dl), tensile elongation retention after moist-heat resistance test, and tensile elongation retention after weathering test. The results are shown in Tables 2,5, 7,10,12,15,17, and 20.

[0119]
As shown in Tables 5 and 10, the films of Examples 1 to 32 and Examples 49 to 56 proved to be a film havmg excellent hydrolysis resistance and UV light resistance, hi addition, they were fihns havmg excellent resistance of change m color tone due to UV irradiation, wherein, when taking the extrusion temperature during melt film forming as Tc, the melting point of the thermoplastic resin (Dl) as Tmoi, the melt viscosity of the masterpellet (Ml) composed of the thermoplastic resin (Dl) as r\m (poise), and the melt viscosity of the polyester (Al) as TIA (poise), the relationship of the ratio TIA/TIMI satisfied all of (i) to (iv).

[0120]
TlA/nMi>0.2 (i)
WnMi<1.0 (ii)
WHMI >-0.16 x(Tc-TmDi) +2.6 (iii)
VMi < -0.08 X (Tc - Tmoi) + 2.6 (iv)

As shown in Table 10, the films of Examples 33 to 48 and Examples 57 to 60 were films having excellent hydrolysis resistance and UV light resistance and were excellent especially in hydrolysis resistance because they did not contain an ester bond in the resin constituting the thermoplastic resin (Dl). As shown in Table 15, the fihns of Examples 61 to 71 proved to be films having excellent hydrolysis resistance and UV light resistance. As shown in Table 15, the films of Examples 72 to 74 were films having especially excellent hydrolysis resistance and UV light resistance, wherein the relationships: x > 94.5 and y x 10"^ < x - 94.5 were satisfied. Ifee, x represents molar fiction (mol%) of 1,4-cyclohexylenedunethylene terephthakte units, and y represents average longitudinal length (nm) of the dispersion phases.

[0121]
On the other hand, the fihns of Comparative Examples proved to be poor m the following respects.

[0122]
The fihns of Comparative Examples 1 to 6 and Comparative Example 28 were films having poor hydrolysis resistance, wherein the thermoplastic resin (Dl) did not satisfy the relationship: 1.5 x MWAI' / MWAI < MWBI' / MWBI.

[0123]
The fihns of Comparative Examples 7 to 10 were fihns having poor hydrolysis resistance, wherein the polyester (Al) caused a significant IV reduction ux the film forming process because the melting point of the high melting point resin (Bl) was over 320°C.

[0124]
The fihns of Comparative Examples 1,2, 7, 8,13,15, and 17 were fihns having poor hydrolysis resistance, wherem the content of the high melting point resin (Bl) or the thermoplastic resm (Dl) in the PI layer were less than 2% by mass.

[0125]
hi Comparative Examples 5,6,11,12,14,16, and 18, the content of the high meltmg pomt resin (Bl) or the thermoplastic resin (Dl) m the PI layer was over 40% by mass, and therefore the fihn formmg ability was significantly reduced, thereby failing to obtam a fihn.

[0126]
The fihns of Comparative Examples 19 to 21 were fihns havmg poor hydrolysis resistance, wherein the dispersion phase composed of the high melting point resm (B1) or the thomcqjlastic resin (Dl) did not exist, and the particles were dispersed in the polyest^ (Al) in large amounts.

[0127]
The fihns of Comparative Examples 22 to 27 were fihns havmg poor Ab, wherein the number of the dispersion phase havmg a longitudinal length of more than 30,000 nm (30 nm) was more than 2/3 x lo' nm (2/3,000 \tm^). Production of solar batteiy back sheet Further, to the film obtained, a biaxially oriented polyester film "Lumuror" (registered trademark) XIOS (available from TORAY INDUSTRIES, INC.) with a thickness of 75000 nm (75 )im) was laminated using an adhesive (mixture of 90 parts by mass of "TAKELAC" (registered trademark) A310 (available from Mitsui Takeda Chemical Inc.) and 10 parts by mass of "TAKENATE" (registered trademark) A3 (available from Mitsui Takeda Chemical K.K.)). Further, a gas barrier film "Barrialox" (registered trademark) VM-PET1031HGTS (available from TORAY ADVANCED FILM CO., LTD.) with a thickness of 12,000 nm (12 pm) was laminated to the side of the biaxially oriented polyester film with the above-described adhesive such that a vapor deposition layer was at the outside to produce a solar batteiy back sheet with a thickness of 188,000 nm (188 ua). The results of the evaluation of the hydrolysis resistance and weatherility of the back sheet obtained are shown in Tables 5,10, IS, and 20.

[0128]
As shown in Tables 5,10,15, and 20, the solar battery back sheet using tiie fihns of Examples proved to have high hydrolysis resistance and UV light resistance. (Examples 14 to 16,30 to 32,46 to 48)
The high melting point resin (B1) or the thermoplastic resin (Dl) and the morganic particles (CI) shown in Tables 1 and 6 were mixed such that the contents were as shown m Tables 1 and 6, melt kneaded in a vented extruder at a temperature shown below to produce a masterpellet (Ml) such tiiat the vahie of ITA/I1MI was as shown in Table 1 and Table 6.

Examples 30 to 32:280°C Examples 14 to 16:290°C Examples 46 to 48: 310°C

[0129]
The high meltmg point resin (B2) or the thermoplastic resm (D2) and the inorganic particles (C2) shown in Table 3 and Table 8 were mixed such that the contents were as shown in Table 3 and Table 8, melt kneaded in a vented extruder at a temperature shown below to produce a masterpellet (M2) such that the value of WNc was as shown in Table 3 and Table 8.

Examples 30 to 32:280°C
Examples 14 to 16:290'C
Examples 46 to 48: 310°C

[0130]
Then, as a material of the PI layer, a pellet of the polyester (Al) vacuum-dried at 180°C for 3 hours shown m Table 1 and Table 6 and a masterpellet (Ml) vacuum-dried at 180°C for 3 hours were mixed such that the contents were as shown in Table 1 and Table 6, and melt kneaded in a main extruder at a temperature shown below; as a material of the P2 layer, a pellet of the polyester (A2) vacuum-dried at 180°C for 3 hours shown in Tables 3 and 8 and a masterpellet (M2) vacuum-dried at 180°C for 3 hours were mixed such that the contents were as shown in Tables 4 and 9, and melt kneaded in a side extruder at a temperature shown below; and these materials were converged usmg a feed block, a converging device for lamination, to form a two-layer laminate composed of the PI layer/the P2 layer, and introduced into a T-die.

Examples 30 to 32:280°C Examples 14 to 16: 300°C Examples 46 to 48: 315°C

[0131]
Then, the resultant was melt extruded from the T-die into sheet form and brought into close contact by electro-piiming with a drum maintained at a surface temperature of 25°C to be cooled to solidify, thereby obtaining a non-oriented two-layer laminated film. After this, the fihn formation was carried out in the same manner as in Example 1 to obtain two-layer biaxially stretched (biaxially oriented) polyester film. The properties and the like of the polyester fihns obtained are shown in Tables 5 and 10. The fihns obtained proved to be a fihn that was excellent especially in hydrolysis resistance and UV light resistance because of the two-layer constitution in which the PI layer provided with strong UV light resistance and the P2 layer provided with strong hydrolysis resistance are sharing the functions.

[0132]
Further, usmg the film obtained, a solar battery back sheet was produced in the same manner as in Example 1 such that the PI layer of the film was at the outermost side. The properties and the like of the back sheet obtained are shown in Tables 5 and 10. It was shown that the hydrolysis resistance and the UV light resistance were excellent

[0153]
(Description of Abbreviations)
PCHT: Polycyclohexylenedimethylene terephthalate
PCHT/1: 5 mol% isophthalic acid copolymerized polycyclohexylenedimethylene
Terephthalate PCHT/G: 13 mol% ethylene glycol copolymerized polycyclohexylenedunethylene
Terephthalate PET: Polyethylene terephthalate PEN:Polyethylene-2,6-naphthalenedicarboxylate
PET/N: 3 mol% naphthalene dicarboxylic acid copolymerized polyethylene terephthalate
PPS: Polyphenylene sulfide PEDPC: Polyethylene diphenylcarboxylate Percentage of the cases where CI (C2) is present or the like m dispersion phase:

Percentage of the cases where CI (C2) is present in a dispersion phase or where CI
(C2) is in contact with the dispersion phase

INDUSTRIAL APPLICABILITY

[0154]
The biaxially oriented polyester fihn of the present invention is a polyester fihn that has an excellent balance of hydrolysis resistance and UV light resistance and is able to maintain mechanical strength even when exposed to a harsh atmosphere such as outdoor use over a long period of time, and, by exploiting these properties, it can be suitably used in applications such as electrical insulating materials such as solar battery back sheets, planar heating elements, or flat cables; capacitor materials; automotive materials; and building materials.

DESCRIPTION OF SYMBOLS

[0155]

1: Solar battery back sheet 2: Transparent filler agent 3: Electric generating element 4: Transparent substrate 5: Sunlight

CLAIMS

1. A biaxially oriented polyester film which is a polyester film having a polyester layer (PI layer) containing a polyester (Al) comprising ethylene terephthalate as a main constituent, a high melting pomt resin (Bl) having a melting point TmBi of not less than 260°C and not more than 320°C, and inorganic particles (CI), wherem the content of the high melting point resin (Bl) in the PI layer, WBI, is not less than 2% by mass and not more than 40% by mass based on the PI layer; in the PI layer, dispersion phases composed of the high melting point resm (Bl) are present in the polyester (Al); and the average longitudinal length of the dispersion phases is not more than 10,000 nm (10 pm).

2. The biaxially oriented polyester film according to claim 1, wherein, in said PI layer, 70% or more of the total number of said morganic particles (CI) are present in said dispersion phases or in contact with said dispersion phases.

3. The biaxially oriented polyester film according to claim 1 or 2, wherein said high melting point resin (Bl) is at least one resin selected from the group consistmg of resins comprising 1,4-cyclohexanedmiethylene terephthalate, ethylene-2,6-naphlhalenedicarboxylate, and phenylene sulfide as a main component

4. The biaxially oriented polyester film according to any one of claims 1 to 3, which is a laminated polyester fihn having said polyester layer (PI layer) and a polyester layer (P2 layer) containing a polyester (A2) comprising ethylene terephthalate as a main constituent, a high meltmg point resm (B2) having a melting pomt of not less than 260°C and not more than 320°C, and inorganic particles (C2), wherein, in the P2 layer, dispersion phases composed of the high melting point resin (B2) are present m the polyester (A2); the content of the inorganic particles (C2) in the P2 layer, Wc2, is not less than 0.1% by mass and not more than 5% by mass based on the P2 layer; and the difference between the content of the morganic particles (CI) in the PI layer, Wci (% by mass), and the content of the inorganic particles (C2) in the P2 layer, Wc2 (% by mass), Wci - Wca, is not less than 5% by mass and not more than 25% by mass.

5. A biaxially oriented polyester film, which is a polyester fihn having a polyester layer (PI layer) containing a polyester (Al) comprising either ethylene terephthalate or etfaylene-2,6-naphthalenedicarboxylate as a main constituent, a thermoplastic resin (Dl), and inorganic particles (CI), wherein the content of the thermoplastic resin (Dl) m the PI layer, WDI, is not less than 2% by mass and not more than 40% by mass based on the PI layer; the relationship: 1.5 x MWAI' / MWAI < MWDI' / MWDI is satisfied, wherein MWAI is the weight-average molecular weight of the polyester (Al); MWDI is the weight-average molecular weight of the thermoplastic resin (Dl); MWAI' is tiie weight-average molecular weight of the polyester (Al) after treatment at 125°C and 100% RH for 72 hr; and MWDI' is the weight-average molecular weight of the thermoplastic resin (Dl) after treatment at 125°C and 100% RH for 72 hr; and, in the PI layer, the thermoplastic resin (Dl) is present in the polyester (Al) as dispersion phases, and the number of the dispersion phase having a longitudinal length of more than 30,000 mn (30 ]an) is not more than 2/3 X 10 nm (2/3,000 pan).

6. The biaxially oriented polyestCT film according to claim 5, wherein the thermoplastic resin (Dl) meets at least e CH mare of the requirements (a) to (b):

(a) The thermoplastic resin (Dl) has a tan 5 peak temperature at a frequency of 1.0 Hz, which is obtained by dynamic mechanical analysis, of not less than 90°C and not more than 200°C; and
(b) The thermoplastic resin (Dl) has a melt viscosity at a shear rate of 200 sec"', T]DI, within the range of 500 poise to 15,000 poise at any temperature within the range of 270°C to 320°C, and does not contain an ester bond m the molecular structure.

7. The biaxially oriented polyester film according to claim 5 or 6, which is selected from either the combination in which the polyester (Al) is a resin comprising ethylene terephthalate as a main constituent and in which the thermoplastic resin (Dl) is a resin comprising any of 1,4-cyclohexylenedimethylene terephthalate, e&ylene-2,6-naphthalenedicarboxylate, and phenylene sulfide as a main constituent, or the combination in which the polyester (Al) is a resin comprismg ethylene-2,6-naphthalenedicarboxylate as a main component and in which the thermoplastic resin (Dl) is a resin comprising either 1,4-cyclohexylenedimethylene terephthalate or phenylene sulfide as a main constituent

8. The biaxially oriented polyester film according to any one of claims 5 to 7, wherein the amount of the morganic particles CI added is not less than 0.5% by mass and not more than 30% by mass based on the PI layer.

9. The biaxially oriented polyester film according to any one of claims S to 8, wherein, in said PI layer, 70% or more of the total number of said inorganic particles (CI) are present in said dispersion phases or in contact with said dispersion phases.

10. The biaxially oriented polyester from according to any one of claims 5 to 9, wherein the melting point of the thermoplastic resin (Dl), Tmoi, is 5°C to 60°C higher than the melting point of the polyester (A1), TmAi.

11. The biaxially oriented polyester film according to any one of claims 5 to 10, wherein the melting point of the Aermoplastic resin (Dl), Tmoi, is not less than 260°C and not more than 320°C.

12. The biaxially oriented polyester film according to any one of claims 5 to 11, wherein the number of the dispersion phases is not less than 1/1,000 nm (1/1 in) and not more than 5/1,000 nm (5/1 \mi) when a cross section in the thickness direction of the PI layer is observed.

13. The biaxially oriented polyester film according to any one of claims 5 to 12, wherein the average longitudinal length of the dispersion phases is not more than 10,000 nm (10 µ).

14. The biaxially oriented polyester film according to any one of claims 5 to 13, wherein the combination of the polyester (Al) and the thermoplastic resin (Dl) falls under any of (c) to (e) below:

(c) The polyester (Al) is a resin comprising ethylene terephthalate as a main constituent; the thermoplastic resin (Dl) is a resin comprising 1,4-cyclohexylenedimethylene terephthalate as a main constituent; and x > 94.5 and y x10 X - 94.5 are satisfied, wherein, x: molar fraction (mol%) of 1,4-cyclohexylenedimethylene terephthalate units, and y: average longitudinal length (nm) of the dispersion phases;

(d) The polyester (Al) is a resin comprising ethylene terephthalate as a main constituent; and the thermoplastic resin (Dl) is a resin comprising ethylene-2,6-naphthalenedicarboxylate or phenylene sulfide as a main constituent; and

(e) The polyester (Al) is a resin comprising ethylene-2,6-naphthalenedicarboxylate as a main constituent; and the thermoplastic resin (Dl) is a resin comprising 1,4-cyclohexylenedimethylene terephthalate or phenylene sulfide as a main constituent.

15. The biaxially oriented polyester film according to any one of claims 5 to 14, which is a laminated polyester filim having said polyester layer (PI layer) and a polyester layer (P2 layer) containing a polyester (A2) comprising either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main constituent, a thermoplastic resin (D2), and inorganic particles (C2), wherein, in the P2 layer, dispersion phases composed of the thermoplastic resin (D2) are present in the polyester (A2); the content of the morganic particles (C2) in the P2 layer, Wc2, is not less than 0.1% by mass and not more than 5% by mass based on the P2 layer; the difference between the content of the inorganic particles (CI) in the PI layer, Wci (% by mass), and the content of the inorganic particles (C2) in the P2 layer, Wc2 (% by mass), Wci - Wc2, is not less than 5% by mass and not more than 25% by mass; and the relationship; 1.5 x MWA2' / MWA2 < MWDJ' / MWDJ is satisfied, wherein MWA2 is the weight-average molecular weight of the polyester (A2); Mwrc is the weight-average molecular weight of the thermoplastic resia (D2); MWA2' is the weight-average molecular weight of the polyester (A2) after treatment at 125°C and 100% RH for 72 hr; and MWD2' is the weight-average molecular weight of the thermoplastic resin (D2) after treatment at 125°C and 100% RH for 72 hr.

16. A solar battery back sheet using the biaxially oriented polyester film according to claims 1 to 15.

17. The solar battery back sheet according to claim 16, wherein said biaxially oriented polyester film is provided at at least one outermost side.

18. The solar battery back sheet according to claim 16 or 17, wherein at least one outermost layer is the PI layer,

19. A solar battery using the solar battery back sheet according to any one of claims 16 to 18.

20. A method of producing the biaxially oriented polyester film according to claims 1 to 4, which is a method of producing the polyester film having the polyester layer (PI layer) containing the polyester (Al) comprising ethylene terephthalate as a main component; at least one high melting point resin (Bl) selected from the group consisting of resins comprising 1,4-cyclohexanedimethylene terephthalate, ethylene-2,6-naphthalenedicarboxylate, and phenylene sulfide as a main component; and the inorganic particles (CI), wherein the high melting point resin (Bl) and the morganic particles (CI) are melt kneaded to produce a masterpellet (Ml); and the polyester (Al) and the masterpellet (Ml) are melt kneaded under conditions satisfying any of the foUowmg equations (i) to (iv), extruded into sheet form, and then biaxially stretched;

wherein the melt viscosity of the polyester (Al) is TIA; the melt viscosity of the masterpellet (Ml) is TIMI; Tmai is the melting point (°C) of the high melting pomt resin (Bl); Tc is the extrusion temperature (°C) during melt film forming; and TIA and TlMi are the melt viscosity of the polyester (Al) and the masterpellet (Ml),

respectively, at a temperature of Tc (°C) and a shear rate of 200 sec-1;

ȠA/ȠMI≥0.2 (i)
ȠA/ȠM1≤1.0 (ii)
ȠA/M1≥ -0.16 X (Tc – TmB1) + 2.6 (iii)
ȠA/M1 ≤ -0.08 X (Tc – TmB1) + 2.6 (iv).

21. A method of producing the biaxially oriented polyester film according to claims S to IS, which is a method of producing the polyester film having the polyester layer (PI layer) containing the polyester (Al) comprising either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main component; the thermoplastic resin (Dl) which is any of a polyester resin containing the 1,4-cyclohexylenedimethylene terephthalate units in an amount of 93 mol% or more, a polyester resin comprising ethylene-2,6-naphthalenedicarboxylate units as a main constituent, or a resin comprising phenylene sulfide as a main constituent; and the inorganic particles (CI), wherein the thermoplastic resin (Dl) and the inorganic particles (CI) are melt kneaded to produce a masterpellet (Ml); and the polyester (Al) and the masterpellet (Ml) are melt kneaded under conditions satisfying any of the following equations (i), (ii), (v), (vi), extruded into sheet form, and then biaxially stretched;

wherein the melt viscosity of the polyester (Al) is TIA; the melt viscosity of the masterpellet (Ml) is TIMI; Tmoi is the melting point (°C) of the thermoplastic resin (Dl); Tc is the extrusion temperature (°C) during melt film forming; and TIA and TlMi are the melt viscosity of the polyester (Al) and the masterpellet (Ml), respectively, at a temperature of Tc (°C) and a shear rate of 200 sec"';

ȠA/ȠMI≥0.2 (i)
ȠA/ȠM1≤1.0 (ii)
ȠA /MI≥ -0.183 X (Tc – TmD1) + 2.095 (v)
ȠA /MI ≤ -0.08 X (Tc – TmD1) + 2.6 (vi).