WHITE POLYESTER FILM FOR A SOLAR CELL, SEALING SHEET FOR BACK SURFACE OF SOLAR CELL USING SAME, AND SOLAR CELL MODULE

Abstract

The object of the present invention is to provide a white polyester film for a solar cell that is excellent in transfer-mark concealment properties (properties that transferred irregularities are hard to be seen) while having high whiteness, and exhibiting good light resistance and hydrolysis resistance, a sealing sheet for a back surface of a solar cell and a solar cell module using the same. The white polyester film for a solar cell of the present invention has whiteness of 50 or more, an average reflectance of 50 to 95% in a range of wavelength of 400 to 800 nm, an acid value of 1 to 50 eq/ton, and a thickness of 30 to 380 m, and the film has a multilayer structure in which a polyester resin layer containing inorganic fine particles in an amount of 10 to 38% by mass is disposed as at least one outermost layer, and a half-value width of protrusion distribution on a surface of the polyester resin layer is 200 to 1000 nm, the protrusion distribution being obtained by plotting the number density of protrusions with respect to protrusion height.

Claims

1. A white polyester film for a solar cell, wherein the film has whiteness of 50 or more, an average reflectance of 50 to 95% in a range of wavelength of 400 to 800 nm, an acid value of 1 to 50 eq/ton, and a thickness of 30 to 380 m, wherein the film has a multilayer structure in which a polyester resin layer containing inorganic fine particles in an amount of 10 to 38% by mass is disposed as at least one outermost layer, and wherein a half-value width of protrusion distribution on a surface of the polyester resin layer is 200 to 1000 nm, the protrusion distribution being obtained by plotting the number density of protrusions with respect to protrusion height.

2. The white polyester film for a solar cell according to claim 1, wherein the polyester resin layer is a layer containing two or more types of inorganic fine particles.

3. The white polyester film for a solar cell according to claim 2, wherein a difference in average particle diameter between at least two types of inorganic fine particles of the two or more types of inorganic fine particles is 1.7 m or more.

4. The white polyester film for a solar cell according to claim 1, wherein the polyester resin layer contains inorganic fine particles I having an average particle diameter of 0.1 m to 1.7 m and inorganic fine particles II having an average particle diameter of 1.8 m to 7.0 m, and a difference in average particle diameter between the inorganic fine particles I and the inorganic fine particles II is 1.7 m or more.

5. The white polyester film for a solar cell according to claim 1, wherein the polyester resin layer contains at least titanium dioxide mainly composed of a rutile type.

6. The white polyester film for a solar cell according to claim 1, wherein the polyester resin layer does not contain barium sulfate.

7. The white polyester film for a solar cell according to claim 4, wherein the polyester resin layer has a larger thickness than the average particle diameter of the inorganic fine particles II.

8. The white polyester film for a solar cell according to claim 4, wherein the inorganic fine particles I comprises titanium dioxide mainly composed of a rutile type.

9. A sealing sheet for a back surface of a solar cell using the white polyester film for a solar cell according to claim 1.

10. A solar cell module comprising the sealing sheet for a back surface of a solar cell according to claim 9, an encapsulation resin layer adjacent to the sealing sheet for a back surface of a solar cell, and a solar cell device embedded in the encapsulation resin layer.

Description

EXAMPLES

[0139] Next, the present invention will be described in detail with reference to

[0140] Examples and Comparative Examples, but the present invention is not limited thereto, and the variation and the modification of the present invention without departing the gist described above and below are all included the technical scope of the present invention. The measurement and evaluation methods employed in the present invention are described as follows.

<Average Particle Diameter of the Inorganic Fine Particles>

[0141] The average particle size of the inorganic fine particles was calculated by the method described above.

<Intrinsic Viscosity (IV) of Polyester>

[0142] The intrinsic viscosity was measured at 30 C. after the polyester was dissolved in mixture solvent of phenol/1,1,2,2-tetrachloroethane in mass ratio of 6/4. In case of the master batch and film containing inorganic fine particles, the intrinsic viscosity was measured after solid content was removed with centrifugation.

<Content of Diethylene Glycol (DEG)>

[0143] After 0.1 g of each polyester was heated and decomposed at 250 C. in 2 ml of methanol, quantitive analysis was carried out by gas chromatography to determine the content of diethylene glycol.

<Acid Value>

[0144] The acid value of a film or a raw material polyester resin was measured with the following method.

(1) Preparation of Sample

[0145] A film or a raw material polyester resin was crushed and vacuum-dried at 70 C. for 24 hours, and thereafter weight in a range of 0.200.0005 g with a balance. The mass at that time was defined as W (g). To a test tube were added 10 ml of benzyl alcohol and the weighed each sample, and the test tube was immersed in a benzyl alcohol heated to 205 C. and the sample was dissolved while being stirred with a glass rod. Samples obtained by adjusting the dissolution time to be 3 minutes, 5 minutes, and 7 minutes were denoted respectively as A, B and C. Next, new test tubes were made available and only benzyl alcohol was loaded thereto and subjected to the treatment in the same procedure and samples obtained by adjusting the dissolution time to be 3 minutes, 5 minutes, and 7 minutes were denoted respectively, as a, b, and c.

(2) Titration

[0146] Titration was carried out using a 0.04 mol/l potassium hydroxide solution (ethanol solution) with a previously known factor (NF). Phenol red was used as an indicator and the moment where the greenish yellow color was changed rose pink was defined as a finishing point and the titration amount (ml) of the potassium hydroxide solution was determined. The titration amounts of the samples A, B, and C were denoted as XA, XB, and XC (ml) and the titration amounts of the samples a, b, and c were denoted as Xa, Xb, and Xc (ml).

(3) Calculation of Acid Value

[0147] Using the titration amounts XA, XB, and XC for the respective dissolution times, the titration amount V (ml) at a dissolution time of 0 min was calculated by a least squares method. Similarly, the titration amount V0 (ml) was calculated using Xa,

[0148] Xb, and Xc. Next, the acid value (eq/ton) was calculated according to the following equation.

Acid Value (eq/ton)=[(VVO)0.04NF1000]/W

<Apparent Density of Film>

[0149] The apparent density of the film was measured according to JIS K 7222 Cellular plastics and rubbersDetermination of apparent density. For simplifying the expression, the unit of apparent density obtained was converted into g/cm.sup.3.

<Whiteness of Film>

[0150] The whiteness of the film was measured according to JIS L 1015-1981-method B using Z-1001DP manufactured by NIPPON DENSHOKU INDUSTRIES CO., LTD.

<Average Reflectance of Film>

[0151] An integrating sphere was attached to a spectrophotometer (manufactured by

[0152] SHIMADZU CORPORATION, automatic spectrophotometer UV-3150), and the spectral reflectance was measured by making corrections so that the reflectance of a standard white plate (a white standard plate ZRS-99-010-W manufactured by SphereOptics GmbH) was 100%. The spectral reflectance was measured in the range of a wavelength of 400 to 800 nm every 1 nm to obtain the mean value. The measurement was made with a non-reflecting black cardboard placed on the back side of a sample film and was made with light from the layer A side.

<Accelerated Photo Degradation Test>

[0153] In order to evaluate the light resistance, a continuous UV irradiation treatment was conducted on the side of the layer concentrically containing inorganic fine particles of the film for 100 hours at 63 C., 50% RH and irradiation intensity of 100 mW/cm.sup.2 using Eye Super UV Tester (SUV-W151 manufactured by IWASAKI ELECTRIC CO., LTD.).

<Accelerated Hydrolysis Test>

[0154] In order to evaluate the hydrolysis resistance, the HAST (Highly Accelerated temperature and humidity Stress Test), which is standardized by JIS C 60068-2-66, was performed. Specifically, the sample film was cut into 70 mm190 mm, and was placed using a jig at such distances as not to make contact with each other. Next, the treatment was performed at 121 C., 100% RH and 0.03 MPa, for 48 hours, using EHS-221 manufactured by ESPEC Corp.

<Breaking Elongation Retention>

[0155] The light resistance and hydrolysis resistance were evaluated in accordance with the breaking elongation retention. Specifically, the breaking elongation of each sample before and after the accelerated photo degradation test and the accelerated hydrolysis test described above was measured according to JIS C 2318-1997 5.3.31 (tensile strength and elongation) and the breaking elongation retention was calculated according to the following equation.

[0156] Breaking elongation retention (%)=[(breaking elongation after treatment)100]/(breaking elongation before treatment)

[0157] The light resistance was evaluated according to the following criteria, and or {circle around ()} was evaluated as excellent in weather resistance. As for elongation retention after accelerated photo degradation test, it was evaluated aswhen the value was less than 35%, when the value was 35% or more and less than 60%, and {circle around ()} when the value was 60% or more.

[0158] The hydrolysis resistance was evaluated according to the following criteria, and or {circle around ()} was evaluated as excellent in hydrolysis resistance. As for elongation retention after accelerated hydrolysis test, it was evaluated aswhen the value was less than 60%, 603 when the value was 60% or more and less than 80%, and {circle around ()} when the value was 80% or more.

<Change of Color b* Value>

[0159] The sample film was cut into 40 mm40 mm, using a standard white plate of X=94.19, Y=92.22 and Z=110.58, by a color b* value difference meter (ZE-2000 manufactured by NIPPON DENSHOKU INDUSTRIES CO.,LTD.), the color b* values of the sample film before and after the accelerated photo degradation test were measured according to JIS K 7105-1981 5.3.5(a). The change of the color b* value was obtained in accordance with the following equation.

Change of color b* value=

(Color b* value after accelerated photo degradation test)(Color b* value before accelerated photo degradation test)

[0160] In this example, when the change of the color b* value was evaluated according to the following criteria, or {circle around ()} was evaluated as excellent in appearance. As for change of color b*, it was evaluated aswhen the value was more than 12, when the value was 5 or more and 12 or less, and {circle around ()} when the value was less than 5.

[0161] Thermal shrinkage rate of longitudinal direction at 150 C. (HS150)

[0162] A sample film was cut into 10 mm250 mm such a direction that longer sides each coincided with the longer direction, the film was marked at an interval of 200 mm, and an interval A was measured under a fixed tension of 5 g. Subsequently, after the sample film was left standing in an oven in an atmosphere of 150 C. for 30 minutes under no-load, the sample film was taken out from the oven to cool to room temperature. Then, a marked interval B was determined under a fixed tension of 5 g, and a thermal shrinkage rate was determined by the following equation. The above thermal shrinkage rate was measured at points in which sample was divided equally among three in the sample film width direction, average values for 3 points were rounded off to the three decimal places and rounded to the second decimal place digit.

Thermal shrinkage rate (%)=[(AB)100]/A

<Surface Strength>

[0163] The sample film cut out into a size of 5 cm20 cm was fully adhered onto a flat plate glass using a polyester double-sided adhesive tape A so that the side of the layer concentrically containing inorganic fine particles was laid outside. An adhesive tape B (manufactured by Nichiban Co., Ltd.; Cellotape (registered trademark)) of 24 mm width was adhered onto the surface of the sample film over a length of 35 mm followed by being allowed to stand for 1 minute. After that, the adhesive tape B was peeled off at a time in a direction vertical to the glass surface, and the surface of the side of the layer concentrically containing inorganic fine particles was observed.

[0164] As a result of observation, when the sample film surface was peeled off to an extent of 50% or more of the peeled area of the adhesive tape B, it was defined as peeled off, when the frequency of peeled off was less than one half upon five or more repetitions, it was evaluated as (excellent in surface strength), and when the above was one half or more, it was evaluated as (inferior in surface strength).

<Transfer-Mark Concealment Properties>

[0165] The film cut out into a size of 100 mm100 mm and the following EVA sheet cut out into a size of 70 mm90 mm were prepared, and layered in a configuration of film/EVA sheet/glass to prepare a laminate. In the above configuration, the film was disposed such that the layer B was opposed to the EVA sheet in each Example, but only in Example 9, the film was disposed such that one surface of the layer A was opposed to the EVA sheet. Embossed sheets were put on both surfaces of the laminate thus obtained, and the laminate was heated and pressed under the conditions for preparing a sample described later to prepare a sample in which irregularities were transferred to the film.

[0166] A light source (fluorescent lamp) was placed such that the incident angle of light was 45 degrees with respect to the surface of the sample thus prepared, and the difficulty in visibility of surface irregularities was evaluated by visually observing. Evaluation criteria were as follows, and or was evaluated as excellent in transfer-mark concealment properties.

[0167] : no transfer mark observed

[0168] : transfer mark slightly observed

[0169] : transfer mark clearly observed

(Conditions for Preparing Sample)

[0170] Apparatus: Vacuum laminator (manufactured by NPC Incorporated, LM3030 type)

[0171] Embossed sheet: Vacuum laminator accessories (manufactured by NPC Incorporated, glass cross sheet)

[0172] Pressure: 1 atmospheric pressure

[0173] EVA sheet: UltraPearl (registered trademark) PV (0.4 mm) manufactured by SANVIC INC.

[0174] Laminating step: at 100 C. (for 5 minutes in vacuo and for 5 minutes with pressure in vacuo)

[0175] Curing step: thermal treatment at 150 C. (at ordinary pressure for 45 minutes)

<Measurement of Half-Value Width of Protrusion Distribution of Layer Concentrically Containing Inorganic Fine Particles (Layer A)>

[0176] The half-value width (nm) of the protrusion distribution of layer A was measured by the method described above.

<Production of Polyester Resin Pellet>

(1) Production of PET Resin Pellet I (PET-I)

[0177] The temperature of an esterification reactor was raised, and when it reached 200 C., a slurry consisting of 86.4 parts by mass of terephthalic acid and 64.4 parts by mass of ethylene glycol was charged, and as catalysts, 0.017 parts by mass of antimony trioxide and 0.16 parts by mass of triethylamine were added thereto with stirring. Subsequently, the temperature was raised with pressure, and under the conditions of a gauge pressure of 3.5 kgf/cm.sup.2 (343 kPa) and 240 C., an esterification reaction was conducted under pressure. Thereafter, the air inside of the esterification reactor was returned to atmospheric pressure, and 0.071 parts by mass of magnesium acetate tetrahydrate, then 0.014 parts by mass of trimethyl phosphate were added thereto. Furthermore, the temperature was raised up to 260 C. over 15 minutes, 0.012 parts by mass of trimethyl phosphate, then 0.0036 parts by mass of sodium acetate were added thereto. After 15 minutes, the resulting esterification reaction product was transferred to a polycondensation reactor, and the temperature was gradually raised from 260 C. to 280 C. under reduced pressure, then a polycondensation reaction was carried out at 285 C.

[0178] After completion of the polycondensation reaction, a filtration treatment was conducted using a filter made of NASLON (registered trademark) where a 95%-cut radius was 5 m, and the filtrate was extruded from a nozzle into a strand shape and cooled and solidified using a cooling water which was previously subjected to a filtering treatment (pore size: 1 m or less), and was cut into pellets. The intrinsic viscosity and acid value of the resulting PET resin pellet (PET-I) were 0.62 dl/g and 15.1 eq/ton, respectively, and neither inactive particles nor internally precipitated particles were substantially contained.

(2) Production of PET Resin Pellet II (PET-II)

[0179] The PET resin pellet I (PET-I) was previously subjected to preliminary crystallization at 160 C., then subjected to a solid-phase polymerization under a nitrogen atmosphere at a temperature of 220 C. to give a PET resin pellet II (PET-II) having an intrinsic viscosity of 0.71 dl/g and an acid value of 11 eq/ton.

<Production of SiO.SUB.2.-Containing Master Batch Pellet>

[0180] (1) Production of SiO.sub.2-Containing Master Batch Pellet I (SiO.sub.2 MB-I)

[0181] In the production of the above-mentioned PET resin pellet I, SiO.sub.2 particles having an average particle diameter of 2.7 m (value obtained according to an electron microscopic method) were added at a ratio of 20,000 ppm relative to polyester to give a SiO.sub.2-containing master batch pellet I (SiO.sub.2 MB-I). This pellet I had an intrinsic viscosity of 0.62 dl/g and an acid value of 17 eq/ton.

(2) Production of SiO.sub.2-Containing Master Batch Pellet II (SiO.sub.2 MB-II)

[0182] The SiO.sub.2-containing master batch pellet I (SiO.sub.2 MB-I) obtained in the manner as mentioned above was previously subjected to preliminary crystallization at 160 C., then subjected to a solid-phase polymerization under a nitrogen atmosphere at a temperature of 220 C. to give a SiO.sub.2-containing master batch pellet II (SiO.sub.2 MB-II) having an intrinsic viscosity of 0.71 dl/g and an acid value of 11 eq/ton. This pellet II had an intrinsic viscosity of 0.71 dl/g and an acid value of 11 eq/ton.

(3) Production of SiO.sub.2-Containing Master Batch Pellet III (SiO.sub.2 MB-III)

[0183] In the production of the above-mentioned PET resin pellet I, SiO.sub.2 particles having an average particle diameter of 6.5 m (value obtained according to an electron microscopic method) were added at a ratio of 20,000 ppm relative to polyester to obtain a SiO.sub.2-containing master batch pellet. The SiO.sub.2-containing master batch pellet thus obtained was previously subjected to preliminary crystallization at 160 C., then subjected to a solid-phase polymerization under a nitrogen atmosphere at a temperature of 220 C. to give a SiO.sub.2-containing master batch pellet III (SiO.sub.2 MB-III) as shown in Table 1. This pellet III had the same intrinsic viscosity and acid value as those of the above-mentioned pellet II.

(4) Production of SiO.sub.2-Containing Master Batch Pellet IV (SiO.sub.2 MB-IV)

[0184] In the production of the above-mentioned PET resin pellet I, SiO.sub.2 particles having an average particle diameter of 1.7 m (value obtained according to an electron microscopic method) were added at a ratio of 20,000 ppm relative to polyester to obtain a SiO.sub.2-containing master batch pellet. The SiO.sub.2-containing master batch pellet thus obtained was previously subjected to preliminary crystallization at 160 C., then subjected to a solid-phase polymerization under a nitrogen atmosphere at a temperature of 220 C. to give a SiO.sub.2-containing master batch pellet IV (SiO.sub.2 MB-IV) as shown in Table 1. This pellet IV had the same intrinsic viscosity and acid value as those of the above-mentioned pellet II.

<Production of Titanium Oxide-Containing Master Batch Pellet>

(1) Production of Titanium Oxide Fine Particles-Containing Master Batch Pellet I (TiO.SUB.2 .MB-I)

[0185] As raw materials, a mixture of 50% by mass of a PET resin pellet I (PET-I) previously dried at 120 C. under 10.sup.3 torr (about 0.133 Pa) for about 8 hours with 50% by mass of rutile-type titanium dioxide having an average particle size of 0.3 m (value obtained according to an electron microscopic method) was fed to a biaxial vent-type extruder and extruded at 275 C. with deaeration by kneading to obtain a master batch pellet I (TiO.sub.2 MB-I) containing fine particles of rutile-type titanium dioxide. This pellet had an intrinsic viscosity of 0.45 dl/g and an acid value of 42.2 eq/ton.

(2) Production of titanium oxide fine particles-containing Master Batch Pellet II (TiO.sub.2 MB-II)

[0186] Solid-phase polymerization was conducted using the pellet thus obtained with a rotary vacuum polymerization reactor under a reduced pressure of 0.5 mmHg at 220 C. to obtain a master batch pellet II (TiO.sub.2 MB-II) having an intrinsic viscosity of 0.71 dl/g and an acid value of 23.5 eq/ton.

<Production of Zeolite-Containing Master Batch Pellet>

[0187] As raw materials, a mixture of 90% by mass of a PET resin pellet I (PET-I) previously dried at 120 C. under 10.sup.3 torr (about 0.133 Pa) for 8 hours with 10% by mass of zeolite having an average particle size of 2.4 m (value obtained according to an electron microscopic method) was fed to a biaxial vent-type extruder and extruded at 275 C. with deaeration by kneading to obtain a pellet, and solid-phase polymerization was conducted using the obtained pellet with a rotary vacuum polymerization reactor under a reduced pressure of 0.5 mmHg at 220 C. to obtain a zeolite-containing master batch pellet (zeolite MB) having an intrinsic viscosity of 0.71 dl/g and an acid value of 23.5 eq/ton.

[0188] The particle diameter, intrinsic viscosity, and acid value of each of the resulting master batch pellets are shown in Table 1. In Table 1, the numerical values shown in parentheses in the row of the TiO.sub.2 MB-II and the row of the SiO.sub.2 MB-II indicate the concentrations of the inorganic fine particles I and the inorganic fine particles II in the layer A, respectively.

<Production of White Polyester Film for Solar Cell>

[0189] Films of the following Examples 1 to 9 and Comparative Examples 1 to 6 were produced as mentioned below. The raw material compositions of these films are together shown in Table 1.

Example 1

[0190] Raw materials of the layer concentrically containing fine particles (layer A) obtained by mixing 44% by mass of PET-II, 36% by mass of TiO.sub.2 MB-II and 20% by mass of SiO.sub.2 MB-II and raw materials of other layer (layer B) obtained by mixing 97% by mass of PET-II and 3% by mass of TiO.sub.2 MB-II were each charged into a separate extruder, mixed and melted at 285 C., then conjugated in a melted state so as to give layer A/layer B by using a feed-block. At that time, the rate of discharge amounts for the layer A to the layer B was controlled using a gear pump. Subsequently, the resulting substance was extruded using a T-die onto a cooling drum adjusted to 30 C. to prepare a non-stretched sheet.

[0191] The resulting non-stretched sheet was uniformly heated at 75 C. using a heating roll and subjected to roll stretching (longitudinal stretching) of 3.3 times by heating at 100 C. with a non-contacting heater. The resulting uniaxially stretched film was introduced to a tenter, subjected to a transverse stretch of 4.0 times by heating at 140 C., subjected to a heating treatment at 215 C. for 5 seconds by fixing its width and further subjected to a 4% relaxation treatment at 210 C. in a width direction to obtain a white polyester film roll for a solar cell having a thickness of 50 m.

Examples 2 to 7

[0192] The same procedure was carried out as in Example 1, except that the raw material compositions of the layer A and the layer B were changed as shown in Table 1, to obtain a white polyester film roll for a solar cell.

Example 8

[0193] The same procedure was carried out as in Example 1, except that the total thicknesses of the film was changed to 38 um, and the raw material compositions of the layer A and the layer B were changed as shown in Table 1, to obtain a white polyester film roll for a solar cell.

Example 9

[0194] The same procedure was carried out as in Example 1, except that the layer structure was changed to a configuration in which the layer A, the layer B and the layer A are laminated in this order, and the raw material compositions of the layer A and the layer B were changed as shown in Table 1, to obtain a white polyester film roll for a solar cell.

Comparative Examples 1 to 6

[0195] The same procedure was carried out as in Example 1, except that the compositions of the layer A and the layer B were changed as shown in Table 1, to obtain a white polyester film roll for a solar cell.

[0196] Characteristics of the film obtained in Examples and Comparative Examples were shown in Table 2.

TABLE-US-00001 TABLE 1 (before par- in- and after ticle trinsic acid solid-phase diam- vis- value polymeri- eter amount cosity (eq/ zation) (m) (mass %) (dl/g) ton) Example 1 Example2 Example3 Example4 Example5 layer A PET PET-I 0.62 15.1 (before) PET-II 0.71 11 44 24 44 24 40 (after) fine TiO.sub.2 MB-I 0.3 50 0.45 42.2 particles I (before) TiO.sub.2 MB-II 0.3 50 0.71 23.5 36 36 36 36 36 (after) (18) (18) (18) (18) (18) fine SiO.sub.2 MB-IV 1.7 2 0.71 11 particles II (after) zeolite MB 2.4 10 0.71 23.5 4 (after) (0.4) SiO.sub.2 MB-I 2.7 2 0.62 17 (before) SiO.sub.2 MB-II 2.7 2 0.71 11 20 40 (after) (0.4) (0.8) SiO.sub.2 MB-III 6.5 2 0.71 11 20 40 20 (after) (0.4) (0.8) (0.4) concentration of inorganic fine particles in layer A (mass %) 18.4 18.8 18.4 18.8 18.8 layer B PET PET-I(befoe) 0.62 15.1 PET-II 0.71 11 97 97 97 97 97 fine TiO.sub.2 MB-I 0.3 50 0.45 42.2 particles (before) TiO.sub.2 MB-II 0.3 50 0.71 23.5 3 3 3 3 3 (after) (1.5) (1.5) (1.5) (1.5) (1.5) concentration of inorganic fine particles in layer B (mass %) 1.5 1.5 1.5 1.5 1.5 whole thickness (m) A/B 10/40 10/40 10/40 10/40 10/40 film thickness (m) A/B/A thickness ratio of layer A (%) 20 20 20 20 20 total thicknesses of film (m) 50 50 50 50 50 total concentration of inorganic fine particles in film (mass %) 4.88 4.96 4.88 4.96 4.96 (before par- in- and after ticle trinsic acid solid-phase diam- vis- value polymeri- eter amount cosity (eq/ zation) (m) (mass %) (dl/g) ton) Example6 Example7 Example8 Example9 layer A PET PET-I 0.62 15.1 (before) PET-II 0.71 11 44 40 24 24 (after) fine TiO.sub.2 MB-I 0.3 50 0.45 42.2 particles I (before) TiO.sub.2 MB-II 0.3 50 0.71 23.5 36 36 36 36 (after) (18) (18) (18) (18) fine SiO.sub.2 MB-IV 1.7 2 0.71 11 particles II (after) zeolite MB 2.4 10 0.71 23.5 20 4 (after) (2.0) (0.4) SiO.sub.2 MB-I 2.7 2 0.62 17 (before) SiO.sub.2 MB-II 2.7 2 0.71 11 20 40 40 (after) (0.4) (0.8) (0.8) SiO.sub.2 MB-III 6.5 2 0.71 11 (after) concentration of inorganic fine particles in layer A (mass %) 20.0 18.8 18.8 18.8 layer B PET PET-I(befoe) 0.62 15.1 PET-II 0.71 11 97 97 97 97 fine TiO.sub.2 MB-I 0.3 50 0.45 42.2 particles (before) TiO.sub.2 MB-II 0.3 50 0.71 23.5 3 3 3 3 (after) (1.5) (1.5) (1.5) (1.5) concentration of inorganic fine particles in layer B (mass %) 1.5 1.5 1.5 1.5 whole thickness (m) A/B 10/40 10/40 8/30 film thickness (m) A/B/A 5/40/5 thickness ratio of layer A (%) 20 20 20 20 total thicknesses of film (m) 50 50 38 50 total concentration of inorganic fine particles in film (mass %) 5.20 4.96 4.96 4.96 (before par- in- and after ticle trinsic acid Com- Com- Com- Com- Com- Com- solid-phase diam- vis- value parative parative parative parative parative parative polymeri- eter amount cosity (eq/ Exam- Exam- Exam- Exam- Exam- Exam- zation) (m) (mass %) (dl/g) ton) ple 1 ple2 ple3 ple4 ple5 ple6 layer A PET PET-I 0.62 15.1 24 (before) PET-II 0.71 11 24 64 60 92 34 (after) fine TiO.sub.2 MB-I 0.3 50 0.45 42.2 36 particles I (before) (18) TiO.sub.2 MB-II 0.3 50 0.71 23.5 36 36 36 (after) (18) (18) (18) fine SiO.sub.2 MB-IV 1.7 2 0.71 11 40 particles II (after) (0.8) zeolite MB 2.4 10 0.71 23.5 8 30 (after) (0.8) (3.0) SiO.sub.2 MB-I 2.7 2 0.62 17 40 (before) (0.8) SiO.sub.2 MB-II 2.7 2 0.71 11 (after) SiO.sub.2 MB-III 6.5 2 0.71 11 40 (after) (0.8) concentration of inorganic fine particles in layer A (mass %) 18.8 18.0 0.8 0.8 18.8 21.0 layer B PET PET-I(befoe) 0.62 15.1 97 PET-II 0.71 11 97 97 97 97 97 fine TiO.sub.2 MB-I 0.3 50 0.45 42.2 3 particles (before) (1.5) TiO.sub.2 MB-II 0.3 50 0.71 23.5 3 3 3 3 3 (after) (1.5) (1.5) (1.5) (1.5) (1.5) concentration of inorganic fine particles in layer B (mass %) 1.5 1.5 1.5 1.5 1.5 1.5 whole thickness (m) A/B 10/40 10/40 10/40 10/40 10/40 10/40 film thickness (m) A/B/A thickness ratio of layer A (%) 20 20 20 20 20 20 total thicknesses of film (m) 50 50 50 50 50 50 total concentration of inorganic fine particles in film (mass %) 4.96 4.80 1.36 1.36 4.96 5.01

TABLE-US-00002 TABLE 2 Evaluation Example 1 Example2 Example3 Example4 Example5 Example6 Example7 Example8 Example9 Transfer-Mark Concealment Properties half-value width 228 361 405 817 920 789 727 380 383 (nm) intrinsic viscosity 0.69 0.69 0.69 0.69 0.67 0.68 0.67 0.68 0.69 (dl/g) Content of DEG 1.5 1.6 1.6 1.6 1.5 1.5 1.5 1.6 1.6 (mol %) Acid Value (eq/ton) 18 18 18 18 18 18 18 18 18 Apparent Density 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 (g/cm.sup.3) Whiteness 94 94 94 94 94 94 94 94 94 Average Reflectance 75 72 74 71 75 74 75 78 70 (%) Breaking elongation retention of after accelerated hydrolysis (%) Breaking elongation retention after accelerated photo degradation (%) Change of Color b* Value HS150(%) 2.3 2.0 2.0 1.8 1.9 1.8 1.9 2.0 2.0 Surface Strength Comparative Comparative Comparative Comparative Comparative Comparative Evaluation Example1 Example2 Example3 Example4 Example5 Example6 Transfer-Mark X X X X Concealment Properties half-value width 155 97 165 169 369 1140 (nm) intrinsic viscosity 0.69 0.68 0.68 0.68 0.48 0.68 (dl/g) Content of DEG 1.6 1.6 1.6 1.5 1.6 1.6 (mol %) Acid Value (eq/ton) 18 18 18 18 52 18 Apparent Density 1.4 1.4 1.4 1.4 1.4 1.4 (g/cm.sup.3) Whiteness 94 94 44 45 94 95 Average Reflectance 75 74 33 35 77 76 (%) Breaking elongation X retention of after accelerated hydrolysis (%) Breaking elongation X X X retention after accelerated photo degradation (%) Change of Color b* X X Value HS150(%) 2.2 2.1 2.2 2.0 2.0 2.0 Surface Strength X

INDUSTRIAL APPLICABILITY

[0197] The white polyester film for a solar cell of the present invention has good whiteness, light reflectivity and environmental durability and moreover has excellent transfer.sup.-mark concealment properties. By using the white polyester film for a solar cell of the present invention, a sealing sheet for a back surface of a solar cell and a solar cell module that are excellent in excellent environmental durability, inexpensive, and light in weight can be provided.