White porous polyester film and preparation method thereof

1. A white porous polyester film comprising

a blend of a polyester-based monopolymer and a polyester-based copolymer;

a crystalline polymer resin which is not miscible with a polyester and has a heat deformation temperature of 90° C. or higher, wherein the crystalline polymer consists of a syndiotactic polystyrene;

inorganic particles;

a whitening agent; and

a stabilizing agent,

wherein the amounts of the crystalline polymer resin and the inorganic particles are in the range of 4 to 15 wt % and 15 to 30 wt %, respectively, based on the total weight of the film;

wherein the sum of the amounts of the syndiotactic polystyrene and the inorganic particles is in the range of 19 to 45 wt % based on the total weight of the film;

wherein the blend is obtained by blending: (a) a polyester-based monopolymer resin selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, and a mixture thereof; and (b) a polyester-based copolymer resin prepared from isophthalic acid, dimethyl isophthalate, or cyclohexane dimethanol as a monomer, wherein the weight ratio of (a) the polyester-based monopolymer and (b) the polyester-based copolymer is in the range of 8:2 to 2:8;

wherein the inorganic particles are selected from the group consisting of particles of barium sulfate, titanium dioxide, calcium carbonate, silica, kaoline, talc, zeolite, and a mixture thereof, each having an average particle diameter of 0.1 to 0.7 μm; and

wherein the film comprises a first population of voids having an average diameter ranging from 1 to 5 μm in the interface between the inorganic particles and the polyester resin and a second population of voids having an average diameter ranging from 10 to 20 μm in the interface between the crystalline polymer resin and the polyester resin.

2. The white porous polyester film of claim 1 , wherein the syndiotactic polystyrene resin has a mixing enthalpy value with a polyester of at least 0.2 (MJm −3 ) 1/2 .

3. The white porous polyester film of claim 1 , wherein the film comprises the blend of polyester-based monopolymer and polyester-based copolymer, the whitening agent, and the stabilizing agent in amounts of 50 to 89 wt %, 0.01 to 0.2 wt %, and 0.01 to 5 wt %, respectively, based on the total weight of the film.

4. The white porous polyester film of claim 1 , wherein the film has a whiteness index of at least 95%, a light transmittance of 5% or less, an average reflectance at a wavelength range of 400 to 700 nm of at least 95%, and a reflectance at 550 nm wavelength of at least 95%.

5. The white porous polyester film of claim 1 , wherein the whitening agent is 2,2′-(1,2-ethendiyldi-4,1-phenylene)bisbenzoxazole or 2,2-(4,4-diphenol vinyl)dibenzoxazole.

6. A method for preparing a white porous polyester film comprising:

(i) mixing a blend of polyester-based monopolymer and a polyester-based copolymer, a crystalline polymer resin which is not miscible with a polyester and has a heat deformation temperature of 90° C. or higher, inorganic particles, a whitening agent, and a stabilizing agent such that the sum of the amounts of the crystalline polymer resin and the inorganic particles is in the range of 19 to 45 wt % based on the total weight of the film,

wherein the amounts of the crystalline polymer resin and the inorganic particles are in the range of 4 to 15 wt % and 15 to 30 wt %, respectively, based on the total weight of the film;

(ii) melt-extruding the blended resin to obtain a film sheet; and

(iii) biaxially drawing the sheet in the longitudinal and transverse directions,

wherein the crystalline polymer consists of a syndiotactic polystyrene;

wherein the blend is obtained by mixing (a) a polyester-based monopolymer resin selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, and a mixture thereof; and (b) a polyester-based copolymer resin prepared from isophthalic acid, dimethyl isophthalate, or cyclohexane dimethanol as a monomer, wherein the weight ratio of (a) the polyester-based monopolymer and (b) the polyester-based copolymer is in the range of 8:2 to 2:8;

wherein the inorganic particles are selected from the group consisting of particles of barium sulfate, titanium dioxide, calcium carbonate, silica, kaoline, talc, zeolite, and a mixture thereof, each having an average particle diameter of 0.1 to 0.7 μm;

wherein the film comprises two populations of voids having respective average diameters ranging from 1 to 5 μm and from 10 to 20 μm; and

wherein the film comprises a first population of voids having an average diameter ranging from 1 to 5 μm in the interface between the inorganic particles and the polyester resin and a second population of voids having an average diameter ranging from 10 to 20 μm in the interface between the crystalline polymer resin and the polyester resin.

7. The method of claim 6 , wherein in step (b), the blended resin remains in a molten state for a period of 0.5 to 3 min.

8. The method of claim 6 , wherein in step (c), the sheet is drawn in the longitudinal and transverse directions at a draw ratio of 3 to 6, respectively.

9. The method of claim 6 , wherein in step (c), the sheet is drawn in multi steps of at least two in both the longitudinal and transverse directions, the first drawing step in either direction being conducted at a temperature higher than Tg of the polyester-based blend by 10 to 30° C. at a draw ratio of at least 1.5, before the second drawing step.