DESCRIPTION

Polyester Film for Solar Cells, Solar Cell Back Sheet Using the Same, and Solar Cell

TECHNICAL FIELD

[0001] The present invention relates to a polyester film for solar cells, which has excellent heat resistance and hydrolysis resistance; a solar cell back sheet comprising the same; and a solar cell comprising the above-described solar cell back sheet.

BACKGROUND ART

[0002] In recent years, solar power generation which is a clean energy system has been drawing much attention as a semipermanent and pollution-free next-generation energy source, resulting in a rapid popularization of solar cells. As a film of solar cell back sheet, for example, a polyethylene-based resin, a polyester-based resin sheet and a fluorine-based film are known to be employed (see Patent Documents 1 to 3). Solar cells are often installed in an outdoor setting; therefore, it is strongly demanded that such solar cells have durability against the natural environment (weatherability, heat resistance and UV (ultraviolet radiation) resistance).

PRIOR ART DOCUMENTS PATENT DOCUMENTS

[0003]
[Patent Document 1] JPH11-261085A

[Patent Document 2] JP H 1 1-186575A

[Patent Document 3] JP 2006-270025A SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY T
HE INVENTION
[0004]
However, even the films according to Patent Documents 1 to 3 do not have sufficient heat resistance and hydrolysis resistance, preventing solar cells from being used outdoors for an extended period of time.

MEANS FOR SOLVING THE PROBLEM

[0005] Therefore, in order to solve the above-described problem, an object of the present invention is to provide a polyester film for solar cells, which has both heat resistance and hydrolysis resistance; a solar cell back sheet comprising the same; and a solar cell comprising the above-described solar cell back sheet.

[0006] That is, the present invention provides a polyester film for solar cells, which has a terminal carboxyl group concentration of not higher than 13 eq/ton and a microscopic endothermic peak temperature Tmeta (°C) determined by differential scanning calorimetry (DSC) of not higher than 220°C; a solar cell back sheet comprising the same; and a solar cell comprising the above-described solar cell back sheet.

EFFECTS OF THE INVENTION

[0007] According to the present invention, a polyester film for solar cells, which has both heat resistance and hydrolysis resistance; a solar cell back sheet comprising the same; and a solar cell comprising the above-described solar cell back sheet can be provided. Further, by using the above-described polyester film for solar cells, for example, a solar cell back sheet can have an improved durability and be made thinner as compared to conventional ones, and a solar cell can also have an improved durability and be made thinner.

BRIEF DESCRIPTION OF THE DRAWING
[0008] [Fig. 1] Fig. 1 shows one example of the measurement results of differential scanning calorimetry (DSC) carried out to determine the microscopic endothermic peak temperature of a film.

MODE FOR CARRYING OUT THE INVENTION

[0009] It is required that the film according to the present invention be a polyester film. In the present invention, from the standpoints of heat resistance and mechanical properties, it is preferred that the polyester film comprise not less than 90 mol% of an ethylene terephthalate component with respect to the ester component of polyester. As other copolymer components, a variety of dicarboxylic acids or ester-forming derivatives thereof and diols may also be copolymerized. Examples of copolymerizable dicarboxylic acid component include isophthalic acid, phthalic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 4,4'-diphenyl dicarboxylic acid, 4,4'-diphenylether dicarboxylic acid and 4,4'-dipheylsulfone dicarboxylic acid. Further, examples of copolymerizable alicyclic dicarboxylic acid component include 1,4-cyclohexane dicarboxylic acid. Still further, examples of diol component include aliphatic, . alicyclic and aromatic diols such as ethylene glycol, 1,2-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexane dimethanol, 1,3-cyclohexane dimethanol, 1,4-cyclohexane dimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol and 2,2-bis(4'-(3-hydroxyethoxyphenyl)propane. These components may be used individually or two or more thereof may be used in combination. It is preferred that the melting point of preferably used polyester be not lower than 250°C from the standpoint of heat resistance and not higher than 300°C from the standpoint of productivity. Within this range, other component(s) may be copolymerized or blended. Furthermore, in this polyester, a variety of known additives such as antioxidant, antistatic agent, nucleating agent, inorganic particle and organic particle may also be added. In particular, inorganic and organic particles impart easy lubrication to the film surface and are, therefore, effective for improving the film handling properties,

[0010] The polyester can be produced in accordance with a conventionally known polyester production method. That is, the polyester can be produced by allowing a dialkyl ester, which is use as an acid component, and a diol component to undergo a transesterification reaction and then heating the resulting reaction product under a reduced pressure to allow polycondensation thereof while removing excess diol component. Further, the polyester can also be produced by a conventionally known direct polymerization method using a dicarboxylic acid as an acid component. As a reaction catalyst therefor, for example, a conventionally known titanium compound, lithium compound, calcium compound, magnesium compound, antimony compound or germanium compound can be employed. By subjecting such polyester obtained in this manner to solid phase polymerization, its polymerization degree can be further increased and, at the same time, the terminal carboxyl group concentration can be reduced. The solid phase polymerization is carried out in a dryer at a temperature of 200°C to 250°C under a reduced pressure of not higher than 1 torr or under nitrogen gas flow.

[0011] In the present invention, in order to attain satisfactory hydrolysis resistance, it is preferred that the polyester film have an intrinsic viscosity ranging from 0.6 to 1.2 dl/g. The intrinsic viscosity of the polyester film is more preferably 0.65 to 0.80 dl/g, still more preferably 0.70 to 0.80 dl/g. In order to improve the hydrolysis resistance, it is preferred to increase the intrinsic viscosity; however, an intrinsic viscosity of greater than 1.2 dl/g may not be preferable since it requires an increased duration of solid phase polymerization at the time of polyester resin production, which leads to a considerable increase in the cost. Moreover, an intrinsic viscosity of less than 0.6 dl/g is also not preferred since it results in a considerable decrease in the heat resistance and hydrolysis resistance due to a low polymerization degree. The above-described preferred range of intrinsic viscosity can be attained by adjusting the polymerization conditions in the polyester resin production.

[0012] In the present invention, in order to improve the hydrolysis resistance, it is required that the terminal carboxyl group concentration of the polyester film be not higher than 13 eq(equivalents)/ton. The terminal carboxyl group concentration is preferably not higher than 12 eq/ton, more preferably not higher than 8 eq/ton, and most preferably not higher than 5 eq/ton. The lower limit thereof is not particularly restricted; however, it is theoretically 0 eq/ton.

[0013] In order to make the terminal carboxyl group concentration be in the above-described preferred range, it is preferred that a polyester resin having a low terminal carboxyl group concentration be employed as a polyester resin used as starting material. The terminal carboxyl group concentration of the polyester resin can be reduced by extending the duration of solid phase polymerization in the production of the polyester resin.

[0014] Further, in order to make the terminal carboxyl group concentration be in the above-described preferred range, use of a terminal-blocking agent is also a preferred embodiment. Examples of the terminal-blocking agent include carbodiimide compounds, oxazoline compounds, epoxy compounds and carbonate compounds. The terminal-blocking agent is more effective if added together with the polyester resin at the time of film formation. It is preferred to employ a carbodiimide compound and the content thereof is preferably 0.3 to 5% by weight based on the whole polyester film. Needless to say, solid phase polymerization and the terminal- blocking agent may be used at the same time.

[0015] In the film according to the present invention, in order to satisfy sufficient hydrolysis resistance, it is required that the microscopic endothermic peak temperature Tmeta (°C) determined by differential scanning caloritnetry (DSC) be not higher than 220°C. It is preferably not higher than 205°C, more preferably not higher than 195°C. The lower limit thereof is not particularly restricted; however, since the thermal shrinkage becomes considerably high at a microscopic endothermic peak temperature of lower than 150°C, it is preferably not lower than 150°C, more preferably not lower than 160°C.

[0016] The above-described preferred range of the microscopic endothermic peak temperature can be attained by changing the heat treatment temperature at the time of film formation. Although it varies depending on the film thickness and film formation rate in the film formation, it is preferred that the heat treatment temperature be not higher than 220°C. It is noted here that the method of forming the film according to the present invention and the heat treatment step thereof will be described later in detail.

[0017] Since a production process of a solar cell comprises many heat application steps, if a polyester film for the solar cell has a large thermal shrinkage, the polyester film for solar cells or a solar cell back sheet may contract during the solar cell production process, resulting in a strain in the entire solar cell unit to cause a fracture thereof. Therefore, in the film according to the present invention, a smaller thermal shrinkage is preferred. Specifically, it is preferred that the film have thermal shrinkages at 150°C in 30 minutes in the machine direction (MD) and transverse direction (TD) (may also referred to as width direction) of, respectively, not higher than 0.6%, more preferably not higher than 0.4%, still more preferably not higher than 0.2%. Furthermore, it is preferred that the thermal shrinkages be not less than -0.5%.

[0018] In the present invention, since the heat treatment temperature is preferably set low for improving the hydrolysis resistance, the thermal shrinkages may become large. Therefore, in order to make the thermal shrinkages be in the above-described preferred range, it is preferred to adopt either of the following methods (1) or (2) (Needless to say, both of the methods (1) and (2) may be carried out in combination.).
[0019] Method (1): A method comprising a heat treatment step in which a film is allowed to contract by 0.5 to 10% in each of the film MD and TD simultaneously with a heat treatment of the film

[0020] Method (2): A method comprising introducing a formed film to a separate device (for example, an oven) and subjecting the film to an off-line heat treatment. In this method, the heating temperature is preferably 150 to 220°C and the heating time is preferably 10 to 60 seconds.

[0021] In the present invention, the film has a coefficient of planar orientation of preferably not less than 0.130, more preferably not less than 0.165, still more preferably not less than 0.168, still more preferably not less than 0.170, most preferably not less than 0.174. This is because the hydrolysis resistance can be further improved. In the present invention, the coefficient of planar orientation is determined by using an Abbe refractometer and the following Equation (A). Coefficient of planar orientation = (nMD + nTD)/2 - nZD (A) In the above Equation (A), nMD represents a refractive index in the film machine direction (MD); nTD represents a refractive index in the film transverse direction (TD); and nZD represents a refractive index in the film thickness direction. The film can have a coefficient of planar orientation of the above-described preferred range by increasing the draw ratio at the time of film formation. It is preferred that the draw ratio be adjusted to 2.5 to 6.0 in both of the film machine direction (MD) and the film transverse direction (TD), respectively. In order to make the coefficient of planar orientation of the film to be not less than 0.165, it is preferred that the draw ratio be adjusted to 3.0 to 5.0 in the MD and TD, respectively. Here, the upper limit of the coefficient of planar orientation of the film is not particularly restricted; however, the film-forming stability becomes deteriorated when the draw ratio is increased to increase the coefficient of planar orientation; therefore, from the standpoint of productivity, the coefficient of planar orientation of the film is preferably not higher than 0.200, more preferably not higher than 0.185.

[0022] In the film according to the present invention, it is preferred that the microscopic endothermic peak temperature Tmeta (°C) determined by differential scanning calorimetry (DSC) and the coefficient of planar orientation B2 of the film satisfy the following Equation (B).

[0023] B2 > 0.000886 x Tmeta - 0.00286 Equation (B) By satisfying the Equation (B), the hydrolysis resistance (for example average retention rate of elongation after aging for 72 hours at 125°C, 100% RH) can be improved.

[0024] In the present invention, it is preferred that a compound which suppresses hydrolysis be added to the film. It is particularly preferred that the film contain a phosphorus compound. Accordingly, in the present invention, the film has an amount of phosphorus atom, which is determined by fluorescent X-ray analysis, of preferably not less than 200 ppm, more preferably not less than 300 ppm, still more preferably not less than 400 ppm. As the phosphorus compound, it is preferred to use at least one phosphorus compound selected from the group consisting of phosphoric acids; phosphorous acids; phosphonic acids; methyl esters, ethyl esters, phenyl esters, half esters of these acids; and other derivatives thereof. In the present invention, particularly preferred are methyl esters, ethyl esters and phenyl esters of phosphoric acids, phosphorous acids and phosphonic acids. As for a method of adding the phosphorus compound(s), it is preferred to add the phosphorus compound(s) at the time of producing a polyester material chip.

[0025] When the film is used in a solar cell back sheet, it is preferred that the solar cell back sheet be not likely to be deteriorated by sunlight. Therefore, a UV (ultraviolet radiation) absorber or a material having a property to reflect UV radiation may also be added to the film. Further, in a preferred embodiment, the average reflectance of at least one film surface in the wavelength range of 400 to 700 run is also made to be not less than 80%. It is more preferably not less than 85%, especially preferably not less than 90%. By making the film have an average reflectance in the wavelength range of 400 to 700 ran of not less than 80%, even if a solar cell using the film according to the present invention is used under direct sunlight, deterioration of the film would be limited.

[0026] Examples of a method of allowing the film to have an average reflectance in the wavelength range of 400 to 700 nm of not less than 80% include a method of incorporating inorganic particles in a film and a method of creating voids in a polyester film by adding a resin immiscible with polyester. Examples of the inorganic particles that can be suitably used in the former method include wet- process and dry-process silicas, colloidal silica, calcium carbonate, aluminum silicate, calcium phosphate, alumina, magnesium carbonate, zinc carbonate, titanium oxide, zinc oxide (zinc flower), antimony oxide, cerium oxide, zirconium oxide, tin oxide, lanthanum oxide, magnesium oxide, barium carbonate, zinc carbonate, basic lead carbonate (white lead), barium sulfate, calcium sulfate, lead sulphate, zinc sulfide, mica, mica titanium, talc, clay, kaolin, lithium fluoride and calcium fluoride. Particularly preferred are titanium dioxide and barium sulfate. The content of the inorganic particles is 5 to 55% by weight, preferably 5 to 35% by weight, based on the whole polyester film. When the content is less than the above-described range, the reflectance of the film becomes inferior, while when the content is greater than the above-described range, the film breakage or the like may become likely to occur when stretched, which results in a decrease in the productivity. If the productivity is regarded as important, it is preferred that the film according to the present invention comprise at least two polyester layers. In cases where the film comprises at least two polyester layers, one of the polyester layers preferably contains the above-described inorganic particles in an amount of 5 to 55% by weight, more preferably 5 to 35% by weight, based on the polyester layer. The content of the inorganic particles in the other polyester layer(s) is not particularly restricted; however, the smaller the content, the more improvement can be attained in the productivity.

[0027] Further, as the resin immiscible with polyester, which is used in the latter method, for example, a polyolefm resin such as polyethylene, polypropylene, polybutene or polymethylpentene, polystyrene resin, polyacrylate resin, polycarbonate resin, polyacrylonitrile resin, polyphenylene sulfide resin or fluorocarbon resin can be preferably used. These immiscible resins may be a homopolymer or copolymer, and two or more thereof may also be used in combination. Among these immiscible resins, those polyolefm resins having small surface tension such as polypropylene and polymethylpentene are preferred and the most preferred is polymethylpentene. Polymethylpentene is especially preferred because it has a large surface tension differential relative to polyester, as well as a high melting point, and therefore, characteristically has a large effect of void formation with respect to its added amount. In cases where such immiscible resin is incorporated, the amount thereof is in the range of 0.5 to 20% by weight, preferably 0.5 to 10% by weight, based on the whole polyester film. When the content is less than the above-described range, the reflectance of the film becomes inferior, while when the content is greater than the above-described range, the apparent density of the whole film becomes excessively low, so that film breakage or the like may become likely to occur during when stretched, which results in a decrease in the productivity.

[0028] In the present invention, in order to further improve the hydrolysis resistance, it is preferred that the film comprise at least two polyester layers. Especially preferably, from the standpoints of the film characteristics and cost, it is advantageous to laminate a layer having particularly superior hydrolysis resistance and a layer having an average reflectance in the wavelength range of 400 to 700 nm of not less than 80%.

[0029] The polyester film for solar cells according to the present invention has an average retention rate of elongation after aging for 48 hours at 125°C, 100% RH of preferably not less than 50%, more preferably not less than 55%, still more preferably not less than 60%, especially preferably not less than 65%, and most preferably not less than 70%. An average retention rate of elongation of less than 50% is not preferred since it results in a reduced mechanical strength of the film after a long-term use, which may lead to a breakage of the back sheet using the film when some sort of external impact is applied to the solar cell comprising the back sheet (for example, when a falling rock or the like hits the solar cell). In the polyester film for solar cells according to the present invention, by making the average retention rate of elongation be not less than 50%, the durability of the mechanical strength of the back sheet in a long-term use can be improved.

[0030] The polyester film for solar cells according to the present invention, it is preferred that the average retention rate of elongation after aging for 72 hours at 125°C, 100% RH be not less than 10%. The test for average retention rate of elongation after aging for 72 hours at 125°C, 100% RH is a more severe accelerated aging test than the test for the average retention rate of elongation after aging for 48 hours. Therefore, in those applications where a long-term durability is particularly demanded, such as solar cell applications, average retention rate of elongation after aging for 72 hours is employed as a performance index. The average retention rate of elongation after aging for 72 hours is more preferably not less than 20%, still more preferably not less than 30%, especially preferably not less than 40%, and most preferably not less than not less than 50%.

[0031] If the average retention rate of elongation after aging for 72 hours is less than 10%, the mechanical strength of the film for solar cells becomes considerably reduced after a long-term use. Therefore, such an average retention rate of elongation after aging for 72 hours is not preferred since it may result in a breakage of the back sheet when an external impact is applied to the solar cell during use (for example, when a falling rock or the like hits the solar cell).

[0032] Further, also in a solar cell back sheet comprising at least one polyester film for solar cells according to the present invention, it is preferred that the average retention rate of elongation after aging for 48 hours at 125°C, 100% RH be not less than 50%. The average retention rate of elongation determined by the above-described method is more preferably not less than 55%, still more preferably not less than 60%, especially preferably not less than 65%, and most preferably not less than 70%.

[0033] In order to make the average retention rate of elongation be in the above-described preferred range, it is preferred that the polyester film for solar cells according to the present invention have a thickness ratio of 5 to 100% based on the thickness of the whole back sheet. That is, in order to further increase the average . retention rate of elongation, it is preferred to make the polyester film for solar cells according to the present invention thicker.

[0034] Next, a concrete method of producing the polyester film according to the present invention (one example) will be described.

[0035] First, as required, a polyethylene terephthalate (PET) resin is dried under a nitrogen atmosphere or vacuum atmosphere. Then, the thus dried polyester resin is fed to a uniaxial or biaxial extruder and melt-extruded in the form of a sheet from a T-die onto a cooling drum to obtain an unstretched sheet.

[0036] Next, the thus obtained unstretched sheet is stretched, for example, by a sequential biaxial stretching method in which the unstretched film is stretched in the machine direction and then in the width direction, or it is stretched in the width direction and then in the machine direction; or by a simultaneous biaxial stretching method in which the unstretched film is almost simultaneously stretched in the machine and width directions of the film.

[0037] After the stretching, the film is subjected to a heat treatment. The heat treatment can be carried out by any conventionally known method, for example, in a tenter or heating oven or on a heated roll. This heat treatment is usually carried out at a temperature not higher than the melting point of polyester; however, in the present invention, in order to attain a Tmeta (°C) of not higher than 220°C, the heat treatment temperature is preferably not higher than 220°C, more preferably not higher than 210°C, still more preferably not higher than 200°C, and most preferably not higher than 190°C. The lower limit of the heat treatment temperature is not particularly restricted; however, since the thermal shrinkage becomes considerably high at a heat treatment temperature of lower than 150°C, it is preferably not lower than 150°C, more preferably not lower than 160°C.

[0038] In addition, the heat treatment may also be carried out while allowing the film to relax in the machine direction and/or width direction. Then, the thus heat-treated film is rolled up to obtain the film according to the present invention.

[0039] Further, a plurality of heat treatment steps can also be carried out, and in such a case, the highest heat treatment temperature in the plurality of heat treatment steps is preferably not higher than 220°C, more preferably not higher than 210°C, still more preferably not higher than 200°C, and most preferably not higher than 190°C.

[0040] [Characteristics evaluation methods] (1) Intrinsic viscosity A film was dissolved in o-chlorophenol, and from the viscosity of the thus obtained solution measured at 25°C, the intrinsic viscosity was determined by the following equation. In the above equation, nsp = (solution viscosity/solvent viscosity) -1; C is the weight of the dissolved polymer per 100 ml of solvent (1 g/100 ml in the present measurement); and K is a Huggins constant (0.343). Further, the solution viscosity and the solvent viscosity were measured by using an Ostwald viscometer.

[0041] (2) Terminal carboxyl group concentration In o-cresol, 0.5 g of a film was dissolved, and the terminal carboxyl group concentration was determined by potentiometric titration with potassium hydroxide.

[0042] (3) Microscopic endothermic peak temperature Tmeta (°C) determined by differential scanning calorimetry (DSC) The microscopic endothermic peak temperature Tmeta (°C) was measured in accordance with JIS K 7122-1987 (reference was made to JIS Handbook (1999)) by using a differential scanning calorimeter, "Robot DSC-RDC220" manufactured by SEIKO Electronics, and a disc session "SSC/5200" for data analysis. On a sample pan, 5 mg of a film was weighed, and the measurement was carried out by increasing the temperature from 25°C to 300°C at a heating rate of 20°C/min. The Tmeta (°C) was defined as the microscopic endothermic peak temperature before reaching the crystal melting peak in the thus obtained differential scanning calorimetric chart. When it was difficult to identify a microscopic endothermic peak, it was determined by making an enlargement of the vicinity of the peak at the data analysis section.

[0043] The JIS does not describe a method of reading a graph of microscopic endothermic peak; therefore, it was carried out in accordance with the following method. First, a straight line is drawn connecting the values at 135°C and 155°C to determine the area created on the endothermic side by the straight line and the graph curve. In the same manner, 17 areas created between the values at 140°C and 160°C, 145°C and 165°C, 150°C and 170°C, 155°C and 175°C, 160°C and 180°C, 165°C and 185°C, 170°C and 190°C, 175°C and 195°C, 180°C and200°C, 185°C and 205°C, 190°C and 210°C, 195°C and 215°C, 200°C and 220°C, 205°C and 225°C, 210°C and 230°C, 215°C and 235°C, and 220°C and 240°C are determined. Since the amount of endothermic heat at a microscopic peak is normally 0.2 to 5.0 J/g, only those data for areas representing an endothermic heat of not less than 0.2 J/g and not greater than 5.0 J/g is used. Among the total of 18 area data, the endothermic peak temperature within the temperature range having valid and the largest area is defined as Tmeta (°C). In case of no valid data, it is regarded as no Tmeta (°C). Fig. 1 shows a chart example.

[0044] (4) Thermal shrinkage In accordance with JIS-C2318 (2007), a sample having a width of 10 mm and marked lines separated by about 100 mm was subjected to a 30-minute heat treatment at 150°C with a load of 0.5 g. Before and after the heat treatment, the distance between the marked lines were measured using a thermal shrinkage measuring device manufactured by Techno Needs Company Ltd. (model AMM-1) and the thermal shrinkage of the sample was calculated using the following equation. Thermal shrinkage (%) = {(L0 - L)/L0} x 100 L0: Distance between the marked lines before the heat treatment L: Distance between the marked lines after the heat treatment (5) Coefficient of planar orientation Using an Abbe refractometer Type 4T manufactured by Atago Co., Ltd. and a sodium lamp as light source, the film refractive index was measured. Coefficient of planar orientation = (nMD + nTD)/2-nZD (A) In the above Equation (A), nMD represents a refractive index in the film machine direction (MD); nTD represents a refractive index in the film transverse direction (TD); and nZD represents a refractive index in the film thickness direction.

[0045] (6) Fluorescent X-ray analysis of phosphorus atom content By a fluorescent X-ray method (ZSXIOOe manufactured by Rigaku Corporation), the phosphorus atom content was measured.

[0046] (7) Average reflectance in the wavelength range of 400 to 700 nm An integrating sphere attachment device (ISR2200 manufactured by Shimadzu Corporation) is attached to a spectral photometer (UV2450 manufactured by Shimadzu Corporation) and using barium sulfate as standard plate, relative reflectance is measured taking the reflectance of the standard plate as 100%. In the wavelength range of 400 to 700 nm, relative reflectance is measured at every 0.5 nm of wavelength, and the average value of the measurements is defined as the average reflectance.

[0047] (8) Retention rate of elongation after aging for 48 hours at 125°C, 100% RH Measurements of elongation at break were carried out in accordance with ASTM-D882-97 (reference was made to ANNUAL BOOK OF ASTM STANDARDS, 1999 edition). A sample was cut out in a size of 1 cm ><20 cm and stretched at a chuck distance of 5 cm and a tensile rate of 300 mm/min to measure the elongation at break (initial). It is noted here that this measurement was carried out for five samples and the average value of the measurements was defined as the elongation at break (initial), A0.

[0048] Next, a sample was cut out in a size of 1 cm x20 cm and subjected to a 48-hour aging treatment at 125°C, 100% RH using a highly accelerated stress test system EHS-221MD manufactured by Espec Corp. Then, in accordance with ASTM-D882-97 (reference was made to ANNUAL BOOK OF ASTM STANDARDS, 1999 edition), the thus aged sample was stretched at a chuck distance of 5 cm and a tensile rate of 300 mm/min to measure the elongation at break (after aging). It is noted here that this measurement was carried out for five samples and the average value of the measurements was defined as the elongation at break (after aging), Al.

[0049] Using the thus obtained values for the elongation at break, A0 and Al, the retention rate of elongation was calculated using the following equation (1). Retention rate of elongation (%) = A1/A0 * 100 (1) In addition, the average retention rate of elongation was calculated by the following equation (2). Average retention rate of elongation (%) = (Retention rate of elongation in the MD + Retention rate of elongation in the TD)/2 (2) It is noted here that, since measurements by a highly accelerated stress test system (HAST system) PC-304R8D manufactured by Hirayama Manufacturing Corp. yield the same values as those obtained by using the highly accelerated stress test system EHS-221MD manufactured by Espec Corp., the measurements may also be carried out by using the highly accelerated stress test system (HAST system) PC-304R8D manufactured by Hirayama Manufacturing Corp.

[0050] (9) Retention rate of elongation after aging for 72 hours at 125°C, 100% RH Measurements of elongation at break were carried out in accordance with ASTM-D882-97 (reference was made to ANNUAL BOOK OF ASTM STANDARDS, 1999 edition). A sample was cut out in a size of 1 cm x20 cm and stretched at a chuck distance of 5 cm and a tensile rate of 300 mm/min to measure the
elongation at break (initial). It is noted here that this measurement was carried out for five samples and the average value of the measurements was defined as the elongation at break (initial), A2.

[0051] Next, a sample was cut out in a size of 1 cm *20 cm and subjected to a 72-hour aging treatment at 125°C, 100% RH using the highly accelerated stress test system (HAST system) PC-304R8D manufactured by Hirayama Manufacturing Corp. Then, in accordance with ASTM-D8 82-97 (1999) (reference was made to ANNUAL BOOK OF ASTM STANDARDS, 1999 edition), the thus aged sample was stretched at a chuck distance of 5 cm and a tensile rate of 300 mm/min to measure the elongation at break (after aging). It is noted here that this measurement was carried out for five samples and the average value of the measurements was defined as the elongation at break (after aging), A3.

[0052] Using the thus obtained values for the elongation at break, A2 and A3, the retention rate of elongation was calculated using the following equation (3). Retention rate of elongation (%) = A3/A2 x 100 (3) In addition, the average retention rate of elongation was calculated using the following equation (4). Average retention rate of elongation (%) = (Retention rate of elongation in the MD + Retention rate of elongation in the TD)/2 (4) EXAMPLES

[0053] The present invention will now be described by way of Examples thereof; however, the present invention is not restricted thereto.

[0054] Example 1 (Starting material: PET-1) To a mixture of 100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol, 0.08 parts by weight of calcium acetate and 0.03 parts by weight of antimony trioxide were added, and the resultant was heated to elevate the temperature by a conventional method to carry out transesterification reaction. Then, after adding 0.16 parts by weight of lithium acetate and 0.11 parts by weight of trimethyl phosphate to the transesterification reaction product, the resulting mixture is transferred to a polymerization vessel. Thereafter, the pressure of the reaction system was gradually decreased while heating to elevate the temperature to carry out polymerization in accordance with a conventional method under a reduced pressure of 1 mmHg at 290°C, thereby obtaining a polyester (polyethylene terephthalate) having an intrinsic viscosity [n] of 0.52. The thus obtained polyester was cut into a rectangular parallelepiped of 2 mm x 4 mm x 4 mm and subjected to a 20-hour heat treatment under a reduced pressure of 0.5 mmHg at 230°C using a rotary vacuum polymerization equipment to obtain a polyester having an intrinsic viscosity [n] of 0.79 and a terminal carboxyl group concentration of 10.5 eq/ton.

[0055] The starting material PET-1 obtained in the above was dried under reduced pressure for 2 hours at a temperature of 180°C and a vacuum level of 0.5 mmHg and fed to an extruder heated to 295°C. After filtering out particulate contaminants by a 50-um cut filter, the resultant was introduced to a T-die mouthpiece. Then, from the T-die mouthpiece, the resultant was extruded in the form of a sheet to obtain a molten monolayer sheet, which was then tightly cooled and solidified by electrostatic application onto a drum having a surface temperature maintained at 20°C to obtain an unstretched monolayer film. Subsequently, after being preheated with heated rolls at 85°C, the unstretched monolayer film was stretched in the machine direction (MD) using heated rolls at 90°C with a draw ratio of 3.3 and then cooled with rolls at 25°C to obtain a uniaxially stretched film (uniaxially oriented film). While being held with clips at both ends, the thus obtained uniaxially stretched film was introduced to a preheating zone at 95 °C in a tenter and then stretched continuously in a heating zone at 105°C in the direction perpendicular to the machine direction (TD) with a draw ratio of 3.6. Thereafter, continuously, the film was subjected to a 20-second heat treatment in a heat treatment zone in the tenter at a heat treatment temperature of 185°C (first heat treatment temperature) and then relaxed in the width direction (TD) at a temperature of 180°C with a relax ratio of 3%. Then, after allowing the film to uniformly cool to 25 °C, the film was rolled up to obtain a film having a thickness of 125 um. The evaluation results of the thus obtained film are shown in Table 1. The hydrolysis resistance of this film was evaluated to be good. ,

[0056] Further, a solar cell back sheet was prepared in accordance with the following method. First, as a first layer, the film according to the present invention having a thickness of 125 μ m, which was obtained in the above, is used. Then, as an adhesive layer, 90 parts by weight of "TAKELAC (registered trademark)" A310 (manufactured by Mitsui Takeda Chemicals Inc.) and "TAKENATE (registered trademark)" A3 (manufactured by Mitsui Takeda Chemicals Inc.) were applied to the surface of the first layer. Further, on top of this adhesive layer, a 12-μm thick BARRIALOX "HGTS" (a PET film with aluminum oxide vapor deposition manufactured by Toray Advanced Film Co., Ltd.) was laminated as a second layer with the vapor-deposited layer facing opposite to the first layer. Then, the same adhesive layer as the above-described one was applied on the second layer, and on top of this adhesive layer, a 50-μm thick biaxially oriented polyester film "LUMILAR (registered trademark)" E20 (manufactured by Toray Industries, Inc.) was further laminated to prepare a back sheet having a total thickness of 187 μm.

The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0057] Examples 2 to 4 Polyester films were obtained by carrying out film formation in the same manner as in Example 1, except that the film-forming conditions were changed as shown in a Table. The evaluation results of the thus obtained films are shown in a Table. The hydrolysis resistances of these films were evaluated to be good.

[0058] Then, using the thus obtained polyester films, back sheets were prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheets are shown in a Table. The hydrolysis resistances of these back sheets were evaluated to be good.

[0059] Example 5 (Starting material: PET-2) A polyester (polyethylene terephthalate) having an intrinsic viscosity [n] of 0.82 and a terminal carboxyl group concentration of 8.5 eq/ton was obtained in the same manner as the above-described method of producing the starting material PET-1, except that the heat treatment was carried out for 40 hours under a reduced pressure of 0.5 mmHg at 230°C using a rotary vacuum polymerization equipment.

[0060] A polyester film was obtained by carrying out film formation in the same manner as in Example 1, except that the above-described PET-2 was used as the starting material. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0061] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0062] Examples 6 to 8 Polyester films were obtained by carrying out film formation in the same manner as in Example 5, except that the film-forming conditions were changed as shown in Tables. The evaluation results of the thus obtained films are shown in Tables. The hydrolysis resistances of these films were evaluated to be good.

[0063] Then, using the thus obtained polyester films, back sheets were prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheets are shown in Tables. The hydrolysis resistances of these back sheets were evaluated to be good.

[0064] Example 9 (Starting material: PET-3) To 90 parts by weight of the starting material PET-1,10 parts by weight of "STABAXOL P10 0 " manufactured by Rhein Chemie Rheinau GmbH (polycarbodiimide) was added and compounded. This compounded product is defined as starting material PET-3.

[0065] A polyester film was obtained by carrying out film formation in the same manner as in Example 1, except that 90 parts by weight of the starting material PET-1 and 10 parts by weight of the starting material PET-3 (corresponding to 1 part by weight of polycarbodiimide) as the starting material. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0066] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0067] Examples 10 to 12 Polyester films were obtained by carrying out film formation in the same manner as in Example 9, except that the film-forming conditions were changed as shown in a Table. The evaluation results of the thus obtained films are shown in a Table. The hydrolysis resistances of these films were evaluated to be good.

[0068] Then, using the thus obtained polyester films, back sheets were prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheets are shown in a Table. The hydrolysis resistances of these back sheets were evaluated to be good.

[0069] Examples 13 to 14 Polyester films were obtained by carrying out film formation in the same manner as in Example 6, except that the film-forming conditions were changed as shown in a Table. The evaluation results of the thus obtained films are shown in a Table. The hydrolysis resistances of these films were evaluated to be good.

[0070] Then, using the thus obtained polyester films, back sheets were prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheets are shown in a Table. The hydrolysis resistances of these back sheets were evaluated to be good.

[0071] Example 15 (Starting material: PET-4) A polyester (polyethylene terephthalate) having an intrinsic viscosity [n] of 0.82 and a terminal carboxyl group concentration of 8.5 eq/ton was obtained in the same manner as the above-described method of producing the starting material PET-1, except that trimethyl phosphate was added in an amount of 0.13 parts by weight and that the heat treatment was carried out for 40 hours under a reduced pressure of 0.5 mmHg at 230°C using a rotary vacuum polymerization equipment.

[0072] A polyester film was obtained by carrying out film formation in the same manner as in Example 13, except that the above-described PET-4 was used as the starting material. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0073] Then, using the thus obtained polyester film, a back.sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0074] Example 16 (Starting material: PET-5) A polyester (polyethylene terephthalate) having an intrinsic viscosity [n] of 0.82 and a terminal carboxyl group concentration of 8.5 eq/ton was obtained in the same manner as the above-described method of producing the starting material PET- 1, except that trimethyl phosphate was added in an amount of 0.25 parts by weight and that the heat treatment was carried out for 40 hours under a reduced pressure of 0.5 rnmHg at 230°C using a rotary vacuum polymerization equipment.

[0075] A polyester film was obtained by carrying out film formation in the same manner as in Example 13, except that the above-described PET-5 was used as the starting material. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0076] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0077] Example 17 The starting material PET-5 was dried under reduced pressure for 2 hours at a temperature of 180°C and a vacuum level of 0.5 mmHg and fed to an extruder heated to 295°C. After filtering out particulate contaminants by a 50-μm cut filter, the resultant was introduced to a T-die mouthpiece. Then, from the T-die mouthpiece, the resultant was extruded in the form of a sheet to obtain a melt monolayer sheet, which was then tightly cooled and solidified by electrostatic application onto a drum having a surface temperature maintained at 20°C to obtain an unstretched monolayer film. Subsequently, after being preheated with heated rolls at 85°C, the unstretched monolayer film was stretched in the machine direction (MD) using heated rolls at 90°C with a draw ratio of 3.5 and then cooled with rolls at 25°C to obtain a uniaxially stretched film. While being held with clips at both ends, the thus obtained uniaxially stretched film was introduced to a preheating zone at 95 °C in a tenter and then stretched continuously in a heating zone at 105 °C in the direction perpendicular to the machine direction (TD) with a draw ratio of 4.0. Thereafter, continuously, the film was subjected to a 20-second heat treatment in a heat treatment zone in the tenter at a temperature of 205 °C (first heat treatment temperature). The thus heat-treated film was then relaxed in the width direction (TD) at a temperature of 180°C with a relax ratio of 3%, as well as in the machine direction (MD) with a relax ratio of 1.5% by shortening the distance between the tenter clips. Then, after allowing the film to uniformly cool to 25°C, the film was rolled up to obtain a polyester film. The evaluation results of the thus obtained polyester film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0078] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0079] Example 18 A polyester film was obtained by carrying out film formation in the same manner as in Example 17, except that the stretching in the machine direction (MD) by shortening the distance between the tenter clips was carried out at a relax ratio of 2.0%. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0080] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0081] Example 19 A complex film-forming apparatus comprising extruders (a) and (b) was employed.

[0082] Titanium oxide (non-surface-treated, rutile-type) in an amount of 5 parts by weight, which has an average particle size of 0.2 μm, 0.15 parts by weight of a fluorescent brightener "OB-1" (manufactured by Eastman Kodak Co.) and 94.85 parts by weight of the starting material PET-5 were mixed. The resulting mixture was dried under reduced pressure for 2 hours at a temperature of 180°C and a vacuum level of 0.5 mmHg and fed to the extruder (a) to be melt-extruded at 280°C. Then, after filtering out particulate contaminants by a 50-um cut filter, the resultant was introduced to a T-die complex mouthpiece.

[0083] In addition, the starting material PET-5 was dried under reduced pressure for 2 hours at a temperature of 180°C and a vacuum level of 0.5 mmHg and then fed to the extruder (b) heated to 295°C. After filtering out particulate contaminants by a 50-um cut filter, the resultant was introduced to the T-die complex mouthpiece.

[0084] Then, in the T-die complex mouthpiece, polymer from the extruder (a) and polymer from the extruder (b) were allowed to join in such a manner that they are laminated. The polymers were then co-extruded in the form of a sheet to obtain a molten laminate sheet. It is noted here that the extrusion amounts of both extruders were controlled to attain a composite ratio [extruder (a) layer/([extruder (a) layer + [extruder (b) layer]] of 12%.

[0085] Then, the molten laminate sheet extruded from the T-die mouthpiece in the form of a sheet was tightly cooled and solidified by electrostatic application onto a drum having a surface temperature maintained at 20°C to obtain an unstretched laminate film. Subsequently, after being preheated with heated rolls at 85°C, the unstretched laminate film was stretched in the machine direction (MD) using heated rolls at 90°C with a draw ratio of 3.5 and then cooled with rolls at 25°C to obtain a uniaxially stretched film. While being held with clips at both ends, the thus obtained uniaxially stretched film was introduced to a preheating zone at 95 °C in a tenter and then stretched continuously in a heating zone at 105°C in the direction perpendicular to the machine direction (TD) with a draw ratio of 4.0. Thereafter, continuously, the film was subjected to a 20-second heat treatment in a heat treatment zone in the tenter at a temperature of 205°C (first heat treatment temperature). The thus heat-treated film was then relaxed in the width direction (TD) at a temperature of 180°C with a relax ratio of 3%, as well as in the machine direction (MD) with a relax ratio of 1.5% by shortening the distance between the tenter clips.

[0086] Then, after allowing the film to uniformly cool to 25 °C, the film was rolled up to obtain a polyester film having a thickness of 125 urn. In the thus obtained film, the thicknesses of the layers (a) and (b) were 15 urn and 110 urn, respectively. The evaluation results of this film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0087] Further, a solar cell back sheet was prepared in accordance with the following method.

[0088] First, the bilayered laminate film obtained in the above is used as a first layer. Then, as an adhesive layer, 90 parts by weight of "TAKELAC (registered trademark)" A310 (manufactured by Mitsui Takeda Chemicals Inc.) and "TAKENATE (registered trademark)" A3 (manufactured by Mitsui Takeda Chemicals Inc.) were applied to the surface of the layer (b). On top of this adhesive layer, a 12-μm thick BARRIALOX "HGTS" (a PET film with aluminum oxide vapor deposition manufactured by Toray Advanced Film Co., Ltd.) was laminated as a second layer with the vapor-deposited layer facing opposite to the first layer.

[0089] Then, the same adhesive layer as the above-described one was applied on the second layer, and on top of this adhesive layer, a 50-μm thick biaxially oriented polyester film "LUMILAR (registered trademark)" E20 (manufactured by Toray Industries, Inc.) was further laminated to prepare a back sheet having a total thickness of 187μm. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good. In addition, since the layer (a) formed the outermost layer, the UV resistance was also good.

[0090] Example 20 A polyester film was obtained by carrying out film formation in the same manner as in Example 19, except that a mixture of 30 parts by weight of titanium oxide (non-surface-treated, rutile-type) having an average particle size of 0.2 um, 0.15 parts by weight of a fluorescent brightener "OB-1" (manufactured by Eastman Kodak Co.) and 69.85 parts by weight of the starting material PET-5 was dried under reduced pressure for 2 hours at a temperature of 180°C and a vacuum level of 0.5 mmHg and then fed to the extruder (a). The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0091] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0092] Example 21 (Starting material: PET-6) A polyester (polyethylene terephthalate) having an intrinsic viscosity [n] of 0.65 and a terminal carboxyl group concentration of 18 eq/ton was obtained in the same manner as the above-described method of producing the starting material PET-1, except that the heat treatment was carried out for 5 hours under a reduced pressure of 0.5 mmHg at 230°C using a rotary vacuum polymerization equipment.

[0093] (Starting material: PET-7) To 90 parts by weight of the starting material PET-6, 10 parts by weight of "STABAXOL P100 " manufactured by Rhein Chemie Rheinau GmbH (polycarbodiimide) was added and compounded. This compounded product is defined as starting material PET-7.

[0094] A polyester film was obtained by carrying out film formation in the same manner as in Example 6, except that 90 parts by weight of the starting material PET-6 and 10 parts by weight of the starting material PET-7 (corresponding to .1 part by weight of polycarbodiimide) as the starting material. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0095] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0096] Example 22 (Starting material: PET-8) A polyester having an intrinsic viscosity [n] of 1.2 and a terminal carboxyl group concentration of 8.0 eq/ton was obtained in the same manner as the above-described method of producing the starting material PET-1, except that the heat treatment was carried out for 100 hours under a reduced pressure of 0.5 mmHg at 230°C using a rotary vacuum polymerization equipment.

[0097] A polyester film was obtained by carrying out film formation in the same manner as in Example 6, except that the above-described PET-8 was used as the starting material. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be good.

[0098] Then, using the thus obtained polyester film, a back sheet was prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0099] Example 23 A film having a thickness of 125 urn was obtained in the same manner as in Example 6. The evaluation results of this film are shown in a Table.

[0100] Further, a solar cell back sheet was prepared in accordance with the following method.

[0101] The film having a thickness of 125 urn, which was obtained in the above, is used as a first layer. Then, as an adhesive layer, 90 parts by weight of "TAKELAC (registered trademark)" A310 (manufactured by Mitsui Takeda Chemicals Inc.) and "TAKENATE (registered trademark)" A3 (manufactured by Mitsui Takeda Chemicals Inc.) were applied to the surface of the first layer. On top of this adhesive layer, a 12-um thick BARRIALOX "HGTS" (a PET film with aluminum oxide vapor deposition manufactured by Toray Advanced Film Co., Ltd.) was laminated as a second layer with the vapor-deposited layer facing opposite to the first layer. Then, the same adhesive layer as the above-described one was applied on the second layer, and on top of this adhesive layer, a 250-μm thick biaxially oriented polyester film "LUMTLAR (registered trademark)" S10 (manufactured by Toray Industries, Inc.) was laminated as a third layer. Further, the same adhesive layer as the above-described one was applied on this third layer, and on top of this adhesive layer, a 50-μm thick biaxially oriented polyester film "LUMILAR (registered trademark)" E20 (manufactured by Toray Industries, Inc.) was laminated to prepare a back sheet having a total thickness of 437μm. The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0102] Example 24 A polyester film was obtained in the same manner as in Example 6, except that the thickness thereof was 50 μm . The evaluation results of this film are shown in a Table.

[0103] Further, a solar cell back sheet was prepared in accordance with the following method. The film having a thickness of 50 μm, which was obtained in the above, is used as a first layer. Then, as an adhesive layer, 90 parts by weight of "TAKELAC (registered trademark)" A310 (manufactured by Mitsui Takeda Chemicals Inc.) and "TAKENATE (registered trademark)" A3 (manufactured by Mitsui Takeda Chemicals Inc.) were applied to the first layer. On top of this adhesive layer, a 12-um thick BARRIALOX "HGTS" (a PET film with aluminum oxide vapor deposition manufactured by Toray Advanced Film Co., Ltd.) was laminated as a second layer with the vapor-deposited layer facing opposite to the first layer. Then, the same adhesive layer as the above-described one was applied on the second layer, and on top of this adhesive layer, a 250-μm thick biaxially oriented polyester film "LUMILAR (registered trademark)" S10 (manufactured by Toray Industries, Inc.) was laminated as a third layer. Further, the same adhesive layer as the above-described one was applied on this third layer, and on top of this adhesive layer, a 188-um thick biaxially oriented polyester film "LUMTLAR (registered trademark)" E20 (manufactured by Toray Industries, Inc.) was laminated to prepare a back sheet having a total thickness of 500 μm . The evaluation results of the thus prepared back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be good.

[0104] Examples 25 to 42 Polyester films were obtained by carrying out film formation in the same manner as in Example 5, except that the film-forming conditions were changed as shown in Tables. The evaluation results of the thus obtained films are shown in Tables. The hydrolysis resistances of these films were evaluated to be good.

[0105] Then, using the thus obtained polyester films, back sheets were prepared in the same manner as in Example 1. The evaluation results of the thus prepared back sheets are shown in Tables. The hydrolysis resistances of these back sheets were evaluated to be good.

[0106] Comparative Example 1 (Starting material: PET-9) To a mixture of 100 parts of dimethyl terephthalate and 60 parts of ethylene glycol, 0.08 parts of calcium acetate and 0.03 parts of antimony trioxide were added, and the resultant was heated to elevate the temperature by a conventional method to carry out transesterification reaction. Then, after adding 0.16 parts of lithium acetate and 0.11 parts of trimethyl phosphate to the transesterification reaction product, the resulting mixture is transferred to a polymerization vessel. Thereafter, the pressure of the reaction system was gradually decreased while heating to elevate the temperature to carry out polymerization in accordance with a conventional method under a reduced pressure of 1 mmHg at 290°C, thereby obtaining a polyester having an intrinsic viscosity [n] of 0.52. The thus obtained polyester was cut into a rectangular parallelepiped of 2 mm * 4 mm x 4 mm and subjected to an 8-hour heat treatment under a reduced pressure of 0.5 mmHg at 230°C using a rotary vacuum polymerization equipment to obtain a polyester having an intrinsic viscosity [n] of 0.74 and a terminal carboxyl group concentration of 13 eq/ton.

[0107] A film having a thickness of 125 um was obtained in the same manner as in Example 1, except that the above-described PET-9 was used as the starting material. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be inferior.

[0108] Further, a back sheet having a thickness of 187 μm was prepared in the same manner as in Example 1. The evaluation results of the thus obtained back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be inferior.

[0109] Comparative Example 2 A polyester film was obtained by carrying out film formation in the same manner as in Comparative Example 1, except that the film-forming conditions were changed as shown in a Table. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be particularly inferior.

[0110] Further, a back sheet having a thickness of 187 μm was also prepared in the same manner as in Example 1. The evaluation results of the thus obtained back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be particularly inferior.

[0111] Comparative Example 3 A polyester film was obtained by carrying out film formation in the same manner as in Example 1, except that the film-forming conditions were changed as shown in a Table. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be particularly inferior.

[0112] Further, a back sheet having a thickness of 187 μm was also prepared in the same manner as in Example 1. The evaluation results of the thus obtained back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be particularly inferior.

[0113] Comparative Example 4 A polyester film was obtained by carrying out film formation in the same manner as in Example 9, except that the film-forming conditions were changed as shown in a Table. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be particularly inferior.

[0114] Further, a back sheet having a thickness of 187 μm was also prepared in the same manner as in Example 1. The evaluation results of the thus obtained back sheet are shown in a Table. . The hydrolysis resistance of this back sheet was evaluated to be particularly inferior.

[0115] Comparative Example 5 A polyester film was obtained by carrying out film formation in the same manner as in Example 5, except that the film-forming conditions were changed as shown in a Table. The evaluation results of the thus obtained film are shown in a Table. The hydrolysis resistance of this film was evaluated to be particularly inferior.

[0116] Further, a back sheet having a thickness of 187 μm was also prepared in the same manner as in Example 1. The evaluation results of the thus obtained back sheet are shown in a Table. The hydrolysis resistance of this back sheet was evaluated to be particularly inferior.

[0117]

[0124] It is noted here that in the above tables sheet means back sheet

INDUSTRIAL APPLICABILITY

[0125] The films according to the present invention can be suitably used for solar cells comprising a back sheet.

CLAIMS

1. A polyester film for solar cells, which has a terminal carboxyl group concentration of not higher than 13 eq/ton and a microscopic endothermic peak temperature Tmeta (°C) determined by differential scanning calorimetry (DSC) of not higher than 220°C.

2. The polyester film for solar cells according to claim 1, which has thermal shrinkages at 150°C in 30 minutes in machine direction (MD) and transverse direction (TD) of not higher than 0.6%, respectively.

3. The polyester film for solar cells according to claim 1 or 2, which has a coefficient of planar orientation B2 of not less than 0.165.

4. The polyester film for solar cells according to any one of claims 1 to 3, which has a phosphorus atom content determined by fluorescent X-ray analysis of not less than 200 ppm.

5. The polyester film for solar cells according to any one of claims 1 to 4, wherein said microscopic endothermic peak temperature Tmeta (°C) is not higher than205°C.

6. The polyester film for solar cells according to any one of claims 1 to 5, which has an average reflectance of at least one surface in the wavelength range of 400 to 700 nm of not less than 80%.

7. The polyester film for solar cells according to any one of claims 1 to 6, which has an intrinsic viscosity ranging from 0.6 to 1.2 dl/g.

8. The polyester film for solar cells according to any one of claims 1 to 7, wherein said terminal carboxyl group concentration is not higher than 12 eq/ton.

9. The polyester film for solar cells according to any one of claims 1 to 8, comprising at least two polyester layers.

10. The polyester film for solar cells according to any one of claims 1 to 9, which has an average retention rate of elongation after aging for 48 hours at 125°C, 100%

RH of not less than 50%.

11. The polyester film for solar cells according to any one of claims 1 to 10, which has an average retention rate of elongation after aging for 72 hours at 125°C, 100% RH of not less than 10%.

12. The polyester film for solar cells according to any one of claims 1 to 11, wherein said Tmeta (°C) and coefficient of planar orientation B2 satisfy the following equation (B):

B2 > 0.000886 x Tmeta - 0.00286 Equation (B)

13. A method of producing said polyester film for solar cells according to any one of claims 1 to 12, said method comprising stretching an unstretched polyester film at least uniaxially and then subjecting the stretched film to a heat treatment at a temperature of not higher than 220°C.

14. A solar cell back sheet, comprising at least one polyester film for solar cells according to any one of claims 1 to 12.

15. The solar cell back sheet according to claim 14, which has an average retention rate of elongation after aging for 48 hours at 125°C, 100% RH is not less than 50%.

16. A solar cell comprising said solar cell back sheet according to claim 14 or 15.

17. A method of producing a polyester film for solar cells, which has a terminal carboxyl group concentration of not higher than 13 eq/ton, said method comprising stretching an unstretched polyester film at least uniaxially and then subjecting the stretched film to a heat treatment at a temperature of not higher than 205°C.

18. The method of producing a polyester film for solar cells according to claim 17, wherein said terminal carboxyl group concentration of said polyester film is not higher than 12 eq/ton.