Back sheet for solar cell module and method for manufacturing the same

1. A polyester white film for back sheet for a solar cell module,

said polyester white film having a whiteness index of 100 or higher, a reflectance of 90% or higher at 550 nm, a machine direction (MD) thermal shrinkage of 2.0% or lower at 150° C. after 30 minutes, a color difference (ΔE) of 2.0 or less after irradiation of QUV at 1.23 W/m 2 ×340 nm×60° C.×99 hours, and an MD elongation retention of 50% or higher at 121° C. and RH 100% after 50 hours,

wherein the polyester white film comprises a matrix resin made of a polyester resin, a rutile type titanium dioxide, a benzoxazole based fluorescence whitening agent, an oxazoline based thickener, and an oleic acid;

wherein the rutile type titanium dioxide is contained in a content of 10-20 wt % and the benzoxazole based fluorescence whitening agent is contained in a content of 100-900 ppm, both based on the total weight of the film; and

wherein the oxazoline based thickener is contained in a content of 50-700 ppm based on, the total weight of the film, and the oleic acid is contained in a content of 40-400 ppm based on the total weight of the film.

2. The polyester white film of claim 1 , wherein the rutile type titanium dioxide has an average particle size of 0.1-0.5 μm.

3. The polyester white film of claim 1 , wherein the polyester resin is polyethylene terephthalate.

4. A back sheet for a solar cell module, wherein the back sheet is made of the polyester white film of claim 1 , or comprises one or more layers of the polyester white film of claim 1 .

5. A method for manufacturing a polyester white film for back sheet for a solar cell module, the method comprising:

a) preparing a first master batch chip by mixing a polyester resin powder obtained by pulverizing a first polyester resin chip, a rutile type titanium dioxide, a benzoxazole-based fluorescence whitening agent, an oxazoline based thickener, and an oleic acid, followed by melt-extrusion at an extruder temperature of 240-265° C.,

wherein the oxazoline based thickener is added in a content of 50-700 ppm based on the total weight of the film, and the oleic acid is added in a content of 40-400 ppm based on the total weight of the film;

b) preparing an unstretched sheet by feeding the first master batch chip and a second polyester resin chip into an extruder, followed by melt-extrusion at a temperature of 280-320° C.; and

c) preparing a film by uniaxially or biaxially stretching the unstretched sheet,

wherein in the step b), the first master batch chip comprises 10-20 wt % of the rutile type titanium dioxide and 100-900 ppm of the benzoxazole-based fluorescence whitening agent, both based on the total weight of the film,

wherein the polyester white film has a whiteness index of 100 or higher, a reflectance of 90% or higher at 550 nm, a machine direction (MD) thermal shrinkage of 2.0% or lower at 150° C. after 30 minutes, a color difference (ΔE) of 2.0 or less after irradiation of QUV at 1.23 W/m 2 ×340 nm×60° C.×99 hours, and an MD elongation retention of 50% or higher at 121° C. and RH 100% after 50 hours.

6. The method of claim 5 , wherein in the step a), at the time of preparing the first master batch chip, a twin screw extruder with a vacuum vent is used while the extruder temperature is 240-265° C. and the retention time of polymer within the extruder is 1 minute or less, and a 300-500 mesh screen change filter at the front end of the twin screw extruder is used to implement uniform mixing.