Biaxially stretched polyester film and method for producing the same

A method for producing a biaxially stretched polyester film, comprising:

• feeding a polyester resin base material and a polyester resin filler into a first extruder;
• feeding a polyester resin matrix material and a plurality of filler particles into a second extruder;
• using a co-extrusion method to co-extrude the polyester resin base material and the polyester resin filler that are disposed in the first extruder, and the polyester resin matrix material and the plurality of filler particles that are disposed in the second extruder, so that the polyester resin base material and the polyester resin filler are collectively formed into a polyester resin base layer, and the polyester resin matrix material and the plurality of filler particles are collectively formed into a matte layer that is disposed on a side surface of the polyester resin base layer;
• wherein a thickness of the polyester resin base layer is between 12 μm and 100 μm, and a thickness of the matte layer is between 10 μm and 80 μm;
• and
• biaxially stretching an un-stretched polyester film formed by the polyester resin base layer and the matte layer, so that a biaxially stretched polyester film is formed;
• wherein based on the total weight of the polyester resin base layer, a content of the polyester resin base material is between 99.5 wt % and 99.8 wt %, and a content of the polyester resin filler is between 0.01 wt % and 0.5 wt %;
• wherein an intrinsic viscosity of the polyester resin base material is between 0.6 dL/g and 0.8 dL/g, and an intrinsic viscosity of the high viscosity polyester resin material polyester resin filler is between 0.9 dL/g and 1.1 dL/g;
• wherein based on the total weight of the matte layer, a content of the polyester resin matrix material is between 80 wt % and 98.8 wt %, and a total content of the plurality of filler particles is between 1.2 wt % and 20 wt %;
• wherein an intrinsic viscosity of the polyester resin matrix material is between 0.6 dL/g and 0.8 dL/g, and the plurality of filler particles have an average particle size of between 0.15 μm and 10 μm;
• wherein the plurality of filler particles are further divided into:
• a plurality of first filler particles and a plurality of second filler particles;
• wherein a content of the first filler particles is between 1 wt % and 2 wt %, a first average particle size of the first filler particles is between 2.5 μm and 6 μm, a content of the second filler particles is between 0.2 wt % and 18 wt %, and a second average particle size of the second filler particles is between 1.4 μm and 2 μm;
• wherein the polyester resin base layer does not include any filler particle, and the matte layer does not include any polyester resin filler;
• wherein an outer surface of the matte layer is defined as a matte surface, and an outer surface of the polyester resin base layer is defined as a flat surface;
• wherein a surface roughness (Ra) of the matte surface is between 226 nm and 803 nm, a surface roughness (Ra) of the flat surface is between 171 nm and 321 nm, and a ratio of the surface roughness of the matte surface relative to that of the flat surface is between 1.32 and 3.06;
• wherein a gloss value of the matte surface is between 37% and 59%, a gloss value of the flat surface is between 64% and 71%, and a ratio of the gloss value of the matte surface relative to that of the flat surface is between 0.56 and 0.86;
• wherein the gloss values of the matte surface and the flat surface, and the ratio of the gloss values are with respect to G60 values as measured in accordance with JIS Z 8741;
• wherein an entire transparency of the polyester film is between 82.9% and 86.5%, and an entire haze value of the polyester film is between 60.01% and 75.21%.