Void-containing polyester film and method for producing same

1. A void-containing polyester film comprising:

an internal void-containing layer (layer A),

wherein the layer A contains a polyester matrix resin and a polypropylene dispersed resin,

wherein the layer A comprises 3 to 25% by mass of the polypropylene dispersed resin, based on a total amount of resin in the layer A, the polypropylene dispersed resin having a dispersed particle diameter of from 7.5 to 12.2 μm,

wherein 26 to 50 voids having an area of 10 to 50 μm 2 are present per 10000 μm 2 when observing a vertical cross section of the film,

wherein the layer A does not contain a compatibilizer,

wherein the layer A satisfies the following requirements (1) to (3), and

an apparent density of the film is in a range of 0.8 to 1.2 g/cm 3 ,

(1) a melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 90 to 400 Pa·s,

(2) a melt viscosity (η2) of the polypropylene resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 300 to 850 Pa·s, and

(3) a ratio η2/(η1) of the melt viscosity (η2) of the polypropylene resin to the melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 1.6 to 4.2.

2. The void-containing polyester film according to claim 1 , wherein the melt viscosity (η2) of the polypropylene resin is 700 Pa·s or less.

3. The void-containing polyester film according to claim 1 , wherein the layer A further contains 5 to 60% by mass of a reclaimed raw material of the void-containing polyester film.

4. The void-containing polyester film according to claim 1 , wherein a layer (layer B) made of a polyester resin is laminated on at least one side of the layer A.

5. The void-containing polyester film according to claim 4 , wherein the layer B contains inorganic particles, the content of the inorganic particles being from 1 to 35% by mass.

6. The void-containing polyester film according to claim 5 , wherein the inorganic particles are titanium oxide.

7. The void-containing polyester film according to claim 1 , wherein an optical density is 0.55 or more (in terms of thickness 50 μm), and a color tone b value is 4 or less.

8. The void-containing polyester film according to claim 1 , wherein heat shrinkage in a longitudinal direction and a width direction when heat-treated at 150° C. for 30 minutes are both 2.0% or less.

9. The void-containing polyester film according to claim 1 , wherein the polypropylene dispersed resin corresponds to 90% by mass or more of a total amount of non-polyester resin in the layer A.

10. A void-containing polyester film comprising:

an internal void-containing layer (layer A),

wherein the layer A contains a polyester matrix resin and a polypropylene dispersed resin which satisfies the following requirements (1) to (3),

wherein the polypropylene dispersed resin corresponds to 100% by mass of a total amount of non-polyester resin in the layer A,

wherein the layer A comprises 3 to 25% by mass of the polypropylene dispersed resin, based on a total amount of resin in the layer A, the polypropylene dispersed resin having a dispersed particle diameter of from 7.5 to 12.2 μm,

wherein 26 to 50 voids having an area of 10 to 50 μm 2 are present per 10000 μm 2 when observing a vertical cross section of the film,

wherein the layer A does not contain a compatibilizer, and

wherein an apparent density of the film is in a range of 0.8 to 1.2 g/cm 3 ,

(1) a melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 90 to 400 Pas,

(2) a melt viscosity (η2) of the polypropylene resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 300 to 850 Pa·s, and

(3) a ratio (η2/η1) of the melt viscosity (η2) of the polypropylene resin to the melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 1.6 to 4.2.

11. A method for producing a void-containing polyester film comprising an internal void-containing layer (layer A), and having an apparent density in a range of 0.8 to 1.2 g/cm 3 , comprising:

an extrusion step for forming a sheet having the layer A containing a polyester matrix resin and a polypropylene dispersed resin satisfying the following requirements (1) to (3), wherein the polypropylene dispersed resin has a dispersed particle diameter of 7.5 to 12.2 μm in average circle equivalent diameter, by melt extrusion, and

a stretching step for stretching the sheet at least in a uniaxial direction,

wherein the layer A comprises 3 to 25% by mass of the polypropylene, based on a total amount of resin in the layer A,

wherein 26 to 50 voids having an area of 10 to 50 μm 2 are present per 10000 μm 2 when observing a vertical cross section of the film,

wherein the layer A does not contain a compatibilizer:

(1) a melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 90 to 400 Pa·s,

(2) a melt viscosity (η2) of the polypropylene resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 300 to 850 Pa·s, and

(3) a ratio (η2/η1) of the melt viscosity (η2) of the polypropylene resin to the melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 1.6 to 4.2.

12. The method for producing a void-containing polyester film according to claim 11 , comprising, after the stretching step,

a heat setting step for thermally setting at a temperature of (Tm−60° C.) to Tm, when the melting point of the polyester resin is Tm (° C.).

13. The method for producing a void-containing polyester film according to claim 11 , wherein the reclaimed raw material obtained from the void-containing polyester film is returned to the extrusion step, and a proportion of the reclaimed raw material is set to 5 to 60% by mass in 100% by mass of resin raw materials of the layer A.

14. A method for producing a void-containing polyester film in which a layer (layer B) made of a polyester resin is laminated on at least one side of an internal void-containing layer (layer A), and having an apparent density in a range of 0.8 to 1.2 g/cm 3 , comprising:

an extrusion step for forming a sheet having the layer A containing a polyester matrix resin and a polypropylene dispersed resin satisfying the following requirements (1) to (3), wherein the polypropylene dispersed resin has a dispersed particle diameter of 7.5 to 12.2 μm in average circle equivalent diameter; and the layer B, by melt extrusion, and

a stretching step for stretching the sheet at least in a uniaxial direction,

wherein the layer A comprises 3 to 25% by mass of the polypropylene, based on a total amount of resin in the layer A,

wherein 26 to 50 voids having an area of 10 to 50 μm 2 are present per 10000 μm 2 when observing a vertical cross section of the film,

wherein the layer A does not contain a compatibilizer:

(1) a melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 90 to 400 Pa·s,

(2) a melt viscosity (η2) of the polypropylene resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 300 to 850 Pa·s, and

(3) a ratio (η2/η1) of the melt viscosity (η2) of the polypropylene resin to the melt viscosity (η1) of the polyester resin at a melting temperature of 280° C. and a shear rate of 121.6 sec −1 is 1.6 to 4.2.