Abstract:
Provided is a back sheet for a solar cell module, and more particularly, a back sheet having a novel multilayer structure substituted for an existing structure in which PVF (Tedlar) film/PET film/PVF (Tedlar) film are sequentially laminated, excellent hydrolysis resistance, and significantly excellent heat adhesion.

Description:
FIELD OF THE INVENTION 
       [0001]    The following disclosure relates to a back sheet for a solar cell module and a manufacturing method thereof, and more particularly, to a back sheet for a solar cell module having a novel laminate structure capable of substituted for an existing structure in which poly vinyl fluoride (PVF, Tedlar) film/polyethylene terephthalate (PET) film/PVF (Tedlar) film are sequentially laminated, excellent hydrolysis resistance, and significantly excellent heat adhesion, and a manufacturing method thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    A solar cell for solar power generation is made of silicon or various compounds and may generate electricity. However, since sufficient output may not be obtained from a single solar cell, each of the solar cells should be connected in series or in parallel with each other. A state in which the solar cells are connected to each other as describe above is called a ‘solar cell module’. 
         [0003]    The solar cell module is configured of glass, ethylene vinyl acetate (EVA), solar cells, EVA, and a back sheet that are sequentially laminated therein. As the back sheet, which is laminated at the lowermost position of the module to serve to block dust, impact, and moisture to protect the solar cell, a TPT (Tedlar/PET/Tedlar) type back sheet are mainly used. In addition, since a ribbon is used as a path through which current flows, as the ribbon, a material made of copper coated with silver or a tin-lead alloy is used. 
         [0004]    The back sheet for a solar cell module is a core material attached to a back surface of the solar cell module to protect cell. Since the back sheet requires properties such as durability, water resistance, water permeation resistance, and the like, the back sheet is generally manufactured by laminating a fluoride film and a polyethylene terephthalate (PET) film. 
         [0005]    The fluoride film having excellent water resistance and durability is used on both surfaces of the back sheet. Currently, a Tedlar film made of a poly vinyl fluoride resin developed by DuPont in 1961 has been mainly used, but the Tedlar film is expensive and is not sufficiently supplied, such that some companies use another film such as the PET film, or the like, instead of the Tedlar film. 
         [0006]    EVA was co-developed by NASA and DuPont as a material for a solar cell used in an artificial satellite in 1970. Currently, EVA is used as a standard sealing material for a solar cell. Japanese companies (Mitsui Chemical, Bridgestone) occupy more than 70% of the global market in the EVA field. EVA serves to seal and fill the cell in the solar cell. EVA has excellent strength, transparency, and insulating property. 
         [0007]    As the polyethylene terephthalate (PET) film, a plate plastic film having a predetermined thickness and properties is used, and the PET film has excellent strength to form a basic frame of the back sheet. The PET film has excellent physical, chemical, mechanical, and optical properties to thereby be widely used from food packages and office supplies to high-tech electric and electronic products such as a semiconductor, a display, and the like. Recently, due to excellent durability and water resistance, the use of the PET film in the back sheet for a solar cell has been increased. 
         [0008]    The glass in which a content of iron is low is used so as to serve to prevent reflection of light. 
         [0009]    According to the related art, in the TPT (Tedlar/PET/Tedlar) type back sheet, in order to laminate the Tedlar film and the PET film, a process of laminating each film using an adhesive has been required, and in order to adhere the back sheet and the EVA film, which is a sealing film, to each other, a process of adhering the back sheet and the EVA film using an adhesive such as a polyurethane adhesive, or the like, has been additionally required. Since the Tedlar film used in the existing back sheet is expensive, cost of the Tedlar film currently occupies more than 80% of a manufacturing cost of the back sheet, which is a reason for cost increase in the back sheet. 
         [0010]    Therefore, in order to decrease the manufacturing cost, research into a technology of not using the Tedlar film attached to the sealing film (EVA film) has been attempted. For example, a technology of forming an ethylene vinyl acetate adhesive layer on a polyester film by an in-line coating method in order to substitute for the Tedlar film has been disclosed in Korean Patent Laid-Open Publication No. 10-2011-0118953 (Nov. 2, 2011), a technology of forming a hot-melt adhesive layer on a polyester film by an in-line coating method in order to substitute for the Tedlar film has been disclosed in Korean Patent Laid-Open Publication No. 10-2011-0119134 (Nov. 2, 2011), and a technology of decreasing a process and cost by applying a fluoride coating composition substituting for the existing Tedlar film layer onto a polyester film by an off-line coating method to form a fluoride coating layer has been disclosed in Korean Patent Laid-Open Publication No. 10-2011-0118271 (Oct. 31, 2011). 
         [0011]    The present invention relates to a technology of further improving adhesive force than those in Korean Patent Laid-Open Publication Nos. 10-2011-0118953 and 10-2011-0119134 to form a polyethylene adhesive layer on a polyester film by an in-line coating method. According to the related art, adhesive force to the sealing ethylene vinyl acetate is lower than that of the back sheet having the fluoride layer/polyester film/fluoride layer structure. However, according to the present invention, the adhesive force may be increased 2 to 3 times than that in the related art and be similar to adhesive force between the fluoride layer in the fluoride layer/polyester film/fluoride layer and the sealing ethylene vinyl acetate. 
       DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
       [0012]    The present inventors studied in order to solve process problems and a cost increase problem caused by an adhesive application process performed through various steps and discovered that the process and cost may be decreased by forming an adhesive layer on a polyester film by an in-line coating method, thereby completing the present invention. 
         [0013]    An embodiment of the present invention is directed to providing an adhesive layer capable of having more excellent adhesion with EVA, which is a sealing material as compared to the related art, and exhibiting adhesive force equal to or more than that of the existing Tedlar film. 
       Technical Solution 
       [0014]    The present invention for achieving the above-mentioned object is as follows. 
         [0015]    In one general aspect, there is provided a back sheet for a solar cell module including a base layer made of a polyester resin and an adhesive layer laminated on one surface or both surfaces of the base layer and obtained by coating and drying a water based coating composition containing a modified polyolefin based resin containing a carboxyl group at a content of 0.01 to 10 weight %, a cross linking agent, and water. 
         [0016]    The back sheet for a solar cell module may further include a functional film selected from a fluoride film and a polyester film on one surface of the base layer on which the adhesive layer is not formed. 
         [0017]    In another general aspect, there is provided a manufacturing method of a back sheet for a solar cell module, the manufacturing method including: 
         [0018]    a) melt-extruding a polyester resin to manufacture a polyester sheet; 
         [0019]    b) stretching the polyester sheet in a machine direction; 
         [0020]    c) coating a water based coating composition containing a modified polyolefin based resin containing 0.01 to 10 weight % of carboxyl group, a cross linking agent, and water onto one surface or both surfaces of the stretched polyester film and then stretching the coated film in a transverse direction; and 
         [0021]    d) heat-setting the bi-axially stretched polyester film. 
         [0022]    The manufacturing method may further include, after step d), e) laminating a functional film selected from a fluoride film, a polyester based film, and a polyolefin based film on one surface of a base layer on which an adhesive film of the polyester film is not formed. 
       Effect of the Invention 
       [0023]    In a back sheet for a solar cell module according to the present invention, a manufacturing process may be simplified, a manufacturing cost may be decreased by removing one fluoride film layer, and adhesive force between the sealing EVA and the back sheet may be equal to or more than that between the existing fluoride film and EVA. 
     
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
       [0024]    Hereinafter, the present invention will be described in more detail. 
         [0025]    An object of the present invention is to provide a back sheet having excellent hydrolysis resistance and adhesion in a structure in which a Tedlar film/PET film/Tedlar film are laminated without using a Tedlar film adhered to a sealing material. The present invention is characterized by forming an excellent adhesive layer on a polyester film in an in-line coating method so as to have excellent adhesion between the polyester film and EVA, which is a sealing material, while decreasing a manufacturing process. 
         [0026]    In the case in which the adhesive layer is applied onto the PET film by an off-line coating method, a process is additionally increased, and a coating thickness is increased, such that a cost may also be increased, and adhesion with the polyester film may be decreased. Therefore, the present inventors discovered that when a water dispersion coating composition (emulsion) is applied during a stretching process of a manufacturing process of the PET film by the in-line coating method, the coating thickness may be thin, adhesion with the PET film may become excellent, and adhesion with the EVA resin, which is a sealing material of a solar cell module, thereby completing the present invention. 
         [0027]    In addition, in a composition for forming an ethylene vinyl acetate adhesive layer, since the EVA film, which is the sealing film, and the back sheet is heat-adhered to each other during a process of assembling the solar cell module, a modified polyolefin based resin containing a carboxyl group at a content of 0.01 to 10 weight % is used as a matrix resin forming a water dispersion composition (emulsion) so as to exhibit sufficient adhesive force at the time of heat-adhesion at a high temperature, and a melamine based cross linking agent, a oxazoline based cross linking agent, or a mixture thereof is used as a cross linking agent for improving hydrolysis resistance and durability of the resin. 
         [0028]    Further, in order to be used in an in-line process, the composition needs to be prepared as the water dispersion composition (emulsion). The present invention is characterized in that the composition having excellent water dispersion property is prepared by adding a specific wetting agent and a dispersion stabilizer to the modified polyolefin based resin containing the carboxyl group at a content of 0.01 to 10 weight %. 
         [0029]    In one general aspect of the present invention, there is provided a back sheet for a solar cell module including a base layer made of a polyester resin and an adhesive layer laminated on one side of the base layer and obtained by coating and drying a coating composition containing a modified polyolefin based resin containing a carboxyl group at a content of 0.01 to 10 weight %, a cross linking agent, and water. 
         [0030]    In one general aspect of the present invention, there is provided a back sheet for a solar cell module including a base layer made of a polyester resin and an adhesive layer laminated on both side of the base layer and obtained by coating and drying a coating composition containing a modified polyolefin based resin containing a carboxyl group at a content of 0.01 to 10 weight %, a cross linking agent, and water. 
         [0031]    In another general aspect of the present invention, there is provided a back sheet for a solar cell module including a base layer made of a polyester resin, an adhesive layer laminated on the base layer and obtained by coating and drying a coating composition containing a modified polyolefin based resin containing a carboxyl group at a content of 0.01 to 10 weight %, a cross linking agent, and water, and a functional film selected from a fluoride film, a polyester based film, and a polyolefin film and laminated on a surface opposite to a surface of the base layer on which the adhesive layer is formed. 
         [0032]    The aspects are only examples for describing the present invention in detail, but the present invention is not limited thereto. 
         [0033]    In the present invention, the adhesive layer may be a surface adhered so as to correspond to the EVA, which is a sealing material. 
         [0034]    Hereinafter, the present invention will be described in more detail. 
         [0035]    In the present invention, as the polyester film, a polyethylene terephthalate film, a polyethylene naphthalate film, or the like, may be used. A polyethylene terephthalate film having an intrinsic viscosity of 0.6 to 0.7 may be more preferably used due to excellent water resistance and hydrolysis resistance thereof. In addition, the polyester film having a thickness of 12 to 300 μm may be preferable since it is advantageous in view of production and implementation of various lamination structures. 
         [0036]    According to the present invention, during the manufacturing the polyester film as described above, the adhesive layer is formed by the in-line coating method. 
         [0037]    The adhesive layer may have a dried coating thickness of 10 to 500 nm and adhesive force of 4 to 12 kg/cm 2 . In the case in which the dried coating thickness is less than 10 nm, the adhesive force may be weak, and in the case in which the thickness is more than 500 nm, the cost and adhesion property may be increased, such that processability may be deteriorated. Further, in the case in which the adhesive force is in a range of 4 to 12 kg/cm 2 , the adhesive force equal to or more than that of the Tedlar film according to the related art may be obtained. 
         [0038]    The water dispersion composition (emulsion) for forming the adhesive layer may contain 0.1 to 20 weight % of the modified polyolefin based resin containing 0.01 to 10 weight % of the carboxyl group, 0.01 to 20 weight % of the cross linking agent, 0.01 to 40 weight % of additives, and the rest water. 
         [0039]    When the content of the carboxyl group is 0.01 to 10 weight %, the adhesive force may be excellent, and when the content is preferably 1 to 8 weight %, more preferably 3 to weight %, the most excellent adhesive force may be obtained. 
         [0040]    The case in which the modified polyolefin based resin is a modified polyethylene resin is most preferable since properties with the EVA, which is a sealing material, may be excellent. More specifically, a modified polyethylene resin having a viscosity average molecular weight of 30,000 to 50,000 and adhesive strength of 4 to 12 kgf/cm 2  may be preferably used. More specifically, as a commercialized example, SE-100 series, SE-1200 series (Unitika Corp., Japan), and the like, may be used, but the present invention is not limited thereto. 
         [0041]    A content of the modified polyolefin based resin may be preferably 0.1 to 20 weight % based on the total emulsion coating composition. In the case in which the content is less than 0.1 weight %, the adhesive force may be low, and in the case in which the content is more than 20 weight %, the cost and the adhesion property may be increased, such that the processability may be deteriorated. Therefore, when the content of the modified polyolefin based resin is in the above-mentioned range, the adhesive layer having the most excellent adhesion force may be formed. 
         [0042]    As the cross linking agent, any one of the oxazoline based cross linking agent and the melamine based cross linking agent, or a mixture thereof may be preferably used, and the content of the cross linking agent in the total composition may be preferably 0.01 to 20 weight %. 
         [0043]    The oxazoline based cross linking agent may be used to improve initial adhesive force, and the melamine based cross linking agent may be used to improve final adhesive force. 
         [0044]    As the additive, an emulsifier, a wetting agent, an inorganic particle and alcohol, or the like, may be used, and a content of the additive may be preferably 0.01 to 40 weight %. 
         [0045]    The emulsifier is used to water-disperse the modified polyolefin based resin, and a non-ionic, anionic, and cationic surfactant may be used. A content of the emulsifier may be 0.01 to 1 weight %. 
         [0046]    The wetting agent is used to allow the emulsion to be uniformly coated onto the polyester film, and one selected from polyethylene glycol, polyethylene ester, modified silicon, and the like, may be preferably used since a coating property may be improved. As a specific example, there are F0-28 (NNOP Corp., Japan), Q2-5212 (Dow Corning Corp.), and the like. It is preferable that a content of the wetting agent is 0.01 to 0.5 weight % since adhesive force is excellent. 
         [0047]    Further, as needed, particles may be added in order to improve a slip property of a coating layer using ethylene vinyl acetate emulsion. Inorganic particles, organic particles, or the like, may be added. A content of the particle may be preferably 0.01 to 5 weight %. 
         [0048]    The alcohol is used to increase a wetting property to uniformly apply the composition, and as a specific example, there is isopropyl alcohol, or the like. A content thereof may be preferably 1 to 20 weight %. 
         [0049]    In addition, as needed, other additives generally used in the art such as an UV stabilizer, anti-static agent, and the like, may be added. 
         [0050]    In the present invention, as the fluoride film, a film made of poly vinyl fluoride (PVF), poly vinyllidene difluoride (PVDF), or the like, may be used, and as commercialized examples, there are Tedlar, Kynar, and the like. The fluoride film may be laminated on the surface opposite to the surface of the polyester film on which the adhesive layer is formed by the in-line coating method, using an adhesive such as a polyurethane based adhesive, a polyester based adhesive, or the like. 
         [0051]    Next, a manufacturing method of a back sheet according to the present invention will be described in detail. 
         [0052]    The manufacturing method of a back sheet according to the present invention may include: melt-extruding a polyester resin to manufacture a sheet, uni-axially stretching the sheet, coating a coating composition emulsion, and bi-axially stretching in a transverse direction to manufacture a polyester film, and may further include laminating a fluoride film on the polyester film using an adhesive. 
         [0053]    More specifically, the manufacturing method of a polyester film included in the back sheet for a solar cell module according to the present invention includes: 
         [0054]    a) melt-extruding the polyester resin to manufacture a polyester sheet; 
         [0055]    b) stretching the polyester sheet in a machine direction; 
         [0056]    c) coating a water based coating composition containing a modified polyolefin based resin containing 0.01 to 10 weight % of carboxyl group, a cross linking agent, and water onto one surface or both surfaces of the stretched polyester film in the machine direction and then stretching the coated film in the transverse direction; and 
         [0057]    d) heat-setting the bi-axially stretched polyester film. 
         [0058]    Step a) is a process of melt-extruding the resin from a cylinder to manufacture the sheet through a T-die in order to manufacture the polyester film. 
         [0059]    Step b) is a process of bi-axially stretching the polyester sheet in order to manufacture the polyester film, and the stretching in the machine direction may be preferably performed using at least one roller. 
         [0060]    Next, in step c), an adhesive layer is formed by an in-line coating method. In this case, water dispersion emulsion may be preferably used so as to be used in the in-line coating. In this case, a configuration of the coating composition for forming the adhesive layer may be the same as described above, and preferably, the coating composition may be applied so that a dried coating thickness after stretching becomes 10 to 500 nm at the time of the application. After the adhesive layer is formed by coating the coating composition, the coated film may be stretched in the transverse direction. In this case, the stretching in the transverse direction may be performed using a tenter. 
         [0061]    Then, water used in the hot-melt adhesive layer is removed, the adhesive layer is cured, and a drying and heat-fixing process is performed in order to prevent the film from being shrunk. 
         [0062]    In addition, as needed, before coating the coating composition or coating the adhesive for adhering a functional film, corona treatment may be performed on the surface of the polyester film, which is a base layer. 
         [0063]    As the functional film, a fluoride film, a polyester based film, a polyolefin based film, or the like, may be used, but the present invention is not limited thereto. 
         [0064]    Hereinafter, Examples will be provided in order to describe the present invention in more detail. However, the present invention is not limited to the following Examples. 
         [0065]    Physical properties in the present invention were measured as follows. 
         [0066]    1) Adhesive Force 
         [0067]    Adhesive force between an EVA film, which is a sealing film, and an adhesive layer according to the present invention was evaluated. The sealing EVA film and the adhesive layer were laminated so as to contact each other and adhered to each other at 150° C. for 20 minutes at a condition of 70 g/cm 2 , then they were peeled off each other at room temperature, a peeling angle of 180 degrees, and a peeling rate of 300 mm/min, thereby evaluating the adhesive force. 
         [0068]    2) Hydrolysis Resistance 
         [0069]    After the sealing EVA film and the adhesive layer were laminated so as to contact each other and adhered to each other at 150° C. for 20 minutes at a condition of 70 g/cm 2 , the obtained film was hung on a sample hanger in an autoclave to be put into the autoclave while allowing the film not to be immersed in water, then the sample was aged at a high temperature (121), high humidity (100% RH), and pressure of 2 bar for 30 hours. 
         [0070]    Then, peeling was performed by the same method as in the adhesive force test, thereby evaluating an adhesive force maintenance ratio based on the initial adhesive force. 
         [0000]      Adhesive force maintenance ratio (%)=(adhesive force after aging/initial adhesive force)×100
 
       Example 1 
     Preparation of Coating Composition 1 
       [0071]    2 weight % of SE-1015J2 (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 4 weight % of carboxyl group and 10 weight % of oxazoline based cross linking agent (1.8 weight % of modified polyolefin based resin and 0.2 weight % of oxazoline based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 97.7 weight % of water were mixed, thereby preparing a coating composition 1. 
         [0072]    Manufacturing a Polyester Film for a Back Sheet 
         [0073]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0074]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 1 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after heat-treatment was performed in a 5-stage tenter at 235° C., the heat-treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 50 nm after stretching. 
         [0075]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 2 
     Preparation of Coating Composition 2 
       [0076]    4 weight % of SE-1015J2 (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 4 weight % of carboxyl group and 10 weight % of oxazoline based cross linking agent (3.6 weight % of modified polyolefin based resin and 0.4 weight % of oxazoline based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 95.7 weight % of water were mixed, thereby preparing a coating composition 2. 
         [0077]    Manufacturing a Polyester Film for a Back Sheet 
         [0078]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0079]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 2 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 100 nm after stretching. 
         [0080]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 3 
     Preparation of Coating Composition 3 
       [0081]    6 weight % of SE-1015J2 (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 4 weight % of carboxyl group and 10 weight % of oxazoline based cross linking agent (5.4 weight % of modified polyolefin based resin and 0.6 weight % of oxazoline based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 93.7 weight % of water were mixed, thereby preparing a coating composition 3. 
         [0082]    Manufacturing a Polyester Film for a Back Sheet 
         [0083]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0084]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 3 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 150 nm after stretching. 
         [0085]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 4 
     Preparation of Coating Composition 4 
       [0086]    8 weight % of SE-1015J2 (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 4 weight % of carboxyl group and 10 weight % of oxazoline based cross linking agent (7.2 weight % of modified polyolefin based resin and 0.8 weight % of oxazoline based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 91.7 weight % of water were mixed, thereby preparing a coating composition 4. 
         [0087]    Manufacturing a Polyester Film for a Back Sheet 
         [0088]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0089]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 4 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 200 nm after stretching. 
         [0090]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 5 
     Preparation of Coating Composition 5 
       [0091]    12 weight % of SE-1015J2 (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 4 weight % of carboxyl group and 10 weight % of oxazoline based cross linking agent (10.8 weight % of modified polyolefin based resin and 1.2 weight % of oxazoline based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 87.7 weight % of water were mixed, thereby preparing a coating composition 5. 
         [0092]    Manufacturing a Polyester Film for a Back Sheet 
         [0093]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0094]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 5 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 300 nm after stretching. 
         [0095]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 6 
     Preparation of Coating Composition 6 
       [0096]    4 weight % of SE-1015J2 (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 4 weight % of carboxyl group and 10 weight % of oxazoline based cross linking agent (3.6 weight % of modified polyolefin based resin and 0.4 weight % of oxazoline based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), 10 weight % of isopropyl alcohol, and 85.7 weight % of water were mixed, thereby preparing a coating composition 6. 
         [0097]    Manufacturing a Polyester Film for a Back Sheet 
         [0098]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0099]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 6 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 100 nm after stretching. 
         [0100]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 7 
     Preparation of Coating Composition 7 
       [0101]    2 weight % of SE-1201JS (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 3 weight % of carboxyl group, 5 weight % of oxazoline based cross linking agent, and 5 weight % of melamine based cross linking agent (1.8 weight % of modified polyolefin based resin, 0.1 weight % of oxazoline based cross linking agent, and 0.1 weight % of melamine based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 97.7 weight % of water were mixed, thereby preparing a coating composition 7. 
         [0102]    Manufacturing a Polyester Film for a Back Sheet 
         [0103]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0104]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 7 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 50 nm after stretching. 
         [0105]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 8 
     Preparation of Coating Composition 8 
       [0106]    4 weight % of SE-1201JS (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 3 weight % of carboxyl group, 5 weight % of oxazoline based cross linking agent, and 5 weight % of melamine based cross linking agent (3.6 weight % of modified polyolefin based resin, 0.2 weight % of oxazoline based cross linking agent, and 0.2 weight % of melamine based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 95.7 weight % of water were mixed, thereby preparing a coating composition 8. 
         [0107]    Manufacturing a Polyester Film for a Back Sheet 
         [0108]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0109]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 8 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 100 nm after stretching. 
         [0110]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 9 
     Preparation of Coating Composition 9 
       [0111]    6 weight % of SE-1201JS (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 3 weight % of carboxyl group, 5 weight % of oxazoline based cross linking agent, and 5 weight % of melamine based cross linking agent (5.4 weight % of modified polyolefin based resin, 0.3 weight % of oxazoline based cross linking agent, and 0.3 weight % of melamine based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 93.7 weight % of water were mixed, thereby preparing a coating composition 9. 
         [0112]    Manufacturing a Polyester Film for a Back Sheet 
         [0113]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0114]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 9 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 150 nm after stretching. 
         [0115]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 10 
     Preparation of Coating Composition 10 
       [0116]    8 weight % of SE-1201JS (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 3 weight % of carboxyl group, 5 weight % of oxazoline based cross linking agent, and 5 weight % of melamine based cross linking agent (7.2 weight % of modified polyolefin based resin, 0.4 weight % of oxazoline based cross linking agent, and 0.4 weight % of melamine based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 91.7 weight % of water were mixed, thereby preparing a coating composition 10. 
         [0117]    Manufacturing a Polyester Film for a Back Sheet 
         [0118]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0119]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 10 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 200 nm after stretching. 
         [0120]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 11 
     Preparation of Coating Composition 11 
       [0121]    12 weight % of SE-1201JS (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 3 weight % of carboxyl group, 5 weight % of oxazoline based cross linking agent, and 5 weight % of melamine based cross linking agent (10.8 weight % of modified polyolefin based resin, 0.6 weight % of oxazoline based cross linking agent, and 0.6 weight % of melamine based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 87.7 weight % of water were mixed, thereby preparing a coating composition 11. 
         [0122]    Manufacturing a Polyester Film for a Back Sheet 
         [0123]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0124]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 11 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 300 nm after stretching. 
         [0125]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Example 12 
     Preparation of Coating Composition 12 
       [0126]    4 weight % of SE-1201JS (Unitika Corp., Japan) containing 90 weight % of modified polyolefin based resin containing 3 weight % of carboxyl group, 5 weight % of oxazoline based cross linking agent, and 5 weight % of melamine based cross linking agent (3.6 weight % of modified polyolefin based resin, 0.2 weight % of oxazoline based cross linking agent, and 0.2 weight % of melamine based cross linking agent were contained in the entire coating composition), 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), 10 weight % of isopropyl alcohol, and 85.7 weight % of water were mixed, thereby preparing a coating composition 12. 
         [0127]    Manufacturing a Polyester Film for a Back Sheet 
         [0128]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0129]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 12 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 100 nm after stretching. 
         [0130]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Comparative Example 1 
     Preparation of Coating Composition 13 
       [0131]    8 weight % of SE-1010 ((Unitika Corp., Japan) made of 100 weigh % of modified polyolefin based resin containing 4 weight % of carboxyl group, 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 91.7 weight % of water were mixed, thereby preparing a coating composition 13. In this case, a cross linking agent was not used. 
         [0132]    Manufacturing a Polyester Film for a Back Sheet 
         [0133]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0134]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 13 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 200 nm after stretching. 
         [0135]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
       Comparative Example 2 
     Preparation of Coating Composition 14 
       [0136]    8 weight % of NPA-400 ((Nanux Corp., Korea) made of 90 weigh % of modified polyolefin based resin containing 20 weight % of carboxyl group and 10 weight % of cross linking agent, 0.3 weight % of wetting agent (Q2-5210, Dow Corning Corp.), and 91.7 weight % of water were mixed, thereby preparing a coating composition 14. 
         [0137]    Manufacturing a Polyester Film for a Back Sheet 
         [0138]    After a polyethylene terephthalate chip from which moisture was removed at a content of 100 ppm or less was injected into a melt-extruder and melted, the resultant was cooled and solidified by a casting drum having a surface temperature of 20° C. while being extruded through a T-die, thereby manufacturing a polyethylene terephthalate sheet having a thickness of 2000 μm. 
         [0139]    The manufactured polyethylene terephthalate sheet was stretched 3.5 times in the machine direction (MD) at 110° C. and cooled to room temperature. Next, after the coating composition 14 was coated on one side of the sheet by a bar coating method, the stretched sheet was stretched 3.5 times in a transverse direction (TD) by preheating and drying at 140° C. Then, after thermal treatment was performed in a 5-stage tenter at 235° C., the thermally treated film was relaxed by 10% in the machine and transverse directions to be heat-set at 200° C., thereby manufacturing a bi-axially stretched film including an adhesive layer formed on one side thereof and having a thickness of 250 μm. The adhesive layer had a dried coating thickness of 200 nm after stretching. 
         [0140]    A single layer of the manufactured film was used as a back sheet for a solar cell module. The physical properties of the back sheet obtained as described above were shown in the following Table 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Adhesive force 
                 Hydrolysis 
               
               
                   
                 (kg/cm) 
                 resistance 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Example 1 
                 6.1 
                 0.2 
               
               
                   
                 Example 2 
                 6.9 
                 0.4 
               
               
                   
                 Example 3 
                 8.2 
                 0.5 
               
               
                   
                 Example 4 
                 6.6 
                 0.5 
               
               
                   
                 Example 5 
                 6.5 
                 0.5 
               
               
                   
                 Example 6 
                 6.9 
                 0.4 
               
               
                   
                 Example 7 
                 6.1 
                 1.4 
               
               
                   
                 Example 8 
                 6.6 
                 1.3 
               
               
                   
                 Example 9 
                 6.4 
                 1.3 
               
               
                   
                 Example 10 
                 6.2 
                 1.4 
               
               
                   
                 Example 11 
                 6.2 
                 1.3 
               
               
                   
                 Example 12 
                 6.6 
                 1.3 
               
               
                   
                 Comparative 
                 1.3 
                 0.0 
               
               
                   
                 Example 1 
               
               
                   
                 Comparative 
                 Less than 1.0 
                 0.0 
               
               
                   
                 Example 2 
               
               
                   
                   
               
             
          
         
       
     
         [0141]    As shown in Table 1, it may be appreciated that the back sheets of Examples in which the adhesive layer according to the present invention was included had adhesive force in a range similar to that of the existing fluoride film and excellent hydrolysis resistance. 
         [0142]    However, it may be appreciated that in Comparative Example 1 in which the cross linking agent was not included, the back sheet had low adhesive force and did not have hydrolysis resistance, and in Comparative Example 2 in which the carboxyl group was included at a high content of 20 weight %, the back sheet also had low adhesive force and did not have hydrolysis resistance.