Void-containing polyester film and method for producing same

Abstract

A void-containing polyester is disclosed which is excellent in concealing properties, whiteness, and thermal dimensional stability. A void-containing polyester film includes an internal void-containing layer (layer A). The void-containing layer contains a polyester matrix resin and a polypropylene dispersed resin, and 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.sup.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.sup.−1 is 90 to 400 Pa.Math.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.sup.−1 is 300 to 850 Pa.Math.s (3) A melt viscosity ratio (η2/η1) of the polyester resin and the polypropylene resin at a melting temperature of 280° C. and a shear rate of 121.6 sec.sup.−1 is 1.5 to 4.5.

Claims

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.sup.2 are present per 10000 μm.sup.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.sup.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.sup.−1 is 90 to 400 Pa.Math.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.sup.−1 is 300 to 850 Pa.Math.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.sup.−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.Math.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.sup.2 are present per 10000 μm.sup.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.sup.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.sup.−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.sup.−1 is 300 to 850 Pa.Math.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.sup.−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.sup.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.sup.2 are present per 10000 μm.sup.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.sup.−1 is 90 to 400 Pa.Math.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.sup.−1 is 300 to 850 Pa.Math.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.sup.−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.sup.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.sup.2 are present per 10000 μm.sup.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.sup.−1 is 90 to 400 Pa.Math.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.sup.−1 is 300 to 850 Pa.Math.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.sup.−1 is 1.6 to 4.2.

Description

EXAMPLES

(1) Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention should not be limited to such Examples.

(2) Evaluation items in the following examples and comparative examples were measured by the following methods.

(3) (1) Melt Viscosity (η1,η2)

(4) The melt viscosities of the polyester resin and the polypropylene resin were measured under the conditions of a melting temperature of 280° C. and a shear rate of 121.6 sec.sup.−1, using Capilograph 1D (capillary length: 10 mm, capillary diameter: 1 mm) manufactured by Toyo Seiki Seisaku-sho, Ltd.

(5) (2) Dispersed Particle Diameter of Polypropylene Resin

(6) The dispersed particle diameter of the polypropylene resin in the layer A was measured by the following procedure. First, the raw material resins were melt-extruded into a sheet form from a T-shaped nozzle, closely attached to a casting drum by an electrostatic application method and cooled and solidified at 30° C. to obtain an unstretched film. A cross section of the obtained unstretched film was cut out with a microtome so as to be parallel to the longitudinal direction. Subsequently, the cross section was subjected to a platinum vapor deposition treatment at a discharge current of 40 mA and a treatment time of 30 sec using a magnetron sputtering apparatus “MSP-1S” manufactured by Vacuum Device Inc., and then observed using a scanning electron microscope “JSM-6510A” manufactured by JEOL Ltd. From the obtained image, dispersions of the polypropylene resin in the polyester resin were randomly selected at 300 points, the area of each dispersion was obtained, and the average value of diameters converted into a true circle was calculated to obtain a dispersed particle diameter.

(7) (3) Film-Forming Properties

(8) Evaluation was made as follows, based on the number of breaks when film formation was continued for 2 hours. In this example, good was evaluated as excellent in film-forming properties. good: No break fair: 2 or 3 breaks poor: Film frequently broke, and film formation was impossible
(4) Area and Number of Voids

(9) The area and number of voids in the layer A were measured by the following procedure. First, a cross section of the obtained film was cut out with a microtome so as to be perpendicular to the film surface. Subsequently, the cross section was subjected to a platinum vapor deposition treatment at a discharge current of 40 mA and a treatment time of 30 sec using a magnetron sputtering apparatus “MSP-1S” manufactured by Vacuum Device Inc., and then observed at a magnification of 500 times using a scanning electron microscope (SEM) “JSM-6510A” manufactured by JEOL Ltd. to obtain an SEM image. Next, using the image analysis software (WINROOF, manufactured by Mitani Corporation), the layer A portion in the SEM image was set within the analysis range, and all voids were extracted by automatic binarization process by discriminant analysis method. Then, the area of each void and the number of voids were calculated, and the number of voids having an area of 10 to 50 μm.sup.2 per 10000 μm.sup.2 was obtained from the following equation. For the measurement, the area of the layer A within the analysis range was set to at least 10000 μm.sup.2. Therefore, for example, when the above minimum area was not exceeded in an SEM image, SEM images were taken until the area exceeds 10000 μm.sup.2, and it was set as the analysis range.
Number of Voids of 10 to 50 μm.sup.2 (pieces/10000 μm.sup.2)=[Number of Voids of 10 to 50 μm.sup.2 (pieces)/Area of Layer A in Analysis Range (μm.sup.2)]×10.sup.4
(5) Apparent Density

(10) Four samples of films cut out into 5.0 cm square were superimposed, the thickness of the samples was measured at 10 points changing the position using a micrometer, and the average value of the total thickness of the four superimposed films was obtained to four significant figures. This average value was divided by 4 and rounded to three significant figures to obtain the average thickness per sheet (t: μm). The total weight (w: g) of the above four samples was measured to four significant figures using an automatic even balance, and the apparent density was obtained from the following equation. The apparent density was rounded to three significant figures.
Apparent Density (g/cm.sup.3)=w/(5.0×5.0×t×10.sup.−4×4)
(6) Optical Density (OD value)

(11) OD value was measured using a transmission densitometer “Ihac-T5” manufactured by Ihara Electronic Industries Co., Ltd. and converted into a value per 50 μm film thickness. The higher the OD value, the greater the concealing properties.

(12) (7) Color Tone b Value

(13) Color tone b value was measured according to JIS-Z8722 using a color difference meter “ZE6000” manufactured by Nippon Denshoku Industries Co., Ltd. and converted into a value per 50 μm film thickness. The smaller the color tone b value, the higher the whiteness is determined and the weaker the yellow taste is determined.

(14) (8) Heat Shrinkage in Longitudinal Direction and Width Direction

(15) Samples cut out into width 10 mm and length 250 mm with respect to the longitudinal direction and the width direction were prepared, and marked at 200 mm intervals, and interval (A) of the marks was measured under a constant tension of 5 gf. Next, after heat treatment of the film at 150° C. for 30 minutes under no load condition, interval (B) of the marks was measured under a constant tension of 5 gf, and the heat shrinkage was obtained from the following formula.
Heat Shrinkage (%)=(A−B)/A×100

(16) Using the raw materials shown in Table 1, films of the following examples and comparative examples were prepared.

Example 1

(17) [Production of Titanium Oxide Master Pellet (M1)]

(18) A mixture of 50% by mass of anatase titanium dioxide having an average particle size of 0.3 μm (electron microscopy) to 50% by mass of a polyethylene terephthalate resin having a melt viscosity of 200 Pa.Math.s was supplied to a vent type twin screw extruder, and kneaded to produce a titanium oxide-containing master pellet (M1).

(19) [Production of Unstretched Film]

(20) 83% by mass of a polyethylene terephthalate resin having a melt viscosity of 200 Pa.Math.s, 12% by mass of a polypropylene resin having a melt viscosity of 500 Pa.Math.s, and 5% by mass of the titanium oxide-containing master pellet (M1) were mixed and subjected to vacuum drying to prepare raw materials for a void-containing polyester layer A. On the other hand, 30% by mass of the titanium oxide-containing master pellet (M1) and 70% by mass of a polyethylene terephthalate resin having a melt viscosity of 200 Pa.Math.s were pellet-mixed and subjected to vacuum drying to prepare raw materials for an inorganic particle-containing polyester layer B. These raw materials were supplied to separate extruders and melted at 280° C. so that the void-containing polyester layer A and the inorganic particle-containing polyester layer B were laminated in the order of B/A/B, and joined by a feed block so that the thickness ratio thereof was 10/80/10, then extruded from a T-die onto a cooling drum adjusted to 30° C. to produce an unstretched film of two-kind three-layer constitution.

(21) [Production of Void-Containing Polyester Film]

(22) The unstretched film thus obtained was uniformly heated to 70° C. using a heating roll and longitudinally stretched by 3.4 times between two pairs of nip rolls having different peripheral speeds. At this time, as an auxiliary heating device of the film, an infrared heater (rated 20 W/cm) equipped with a gold reflecting film at the middle part of the nip rolls was placed so as to face the both surfaces of the film at a distance of 1 cm from the film surface, and the film was heated. The uniaxially stretched film thus obtained was introduced into a tenter, heated to 140° C., transversely stretched by 4.0 times, fixed in width, heat set at 240° C., and further relaxed 3% at 210° C. in the width direction to obtain a void-containing polyester film with a thickness of 50 μm (B/AB). The evaluation results of Example 1 are also shown in Table 1. As shown in Table 1, since the film of Example 1 used the raw material resins satisfying the requirements (1) to (3), the dispersed particle diameter of the polypropylene resin was controlled to an appropriate size, the number of large voids of 10 to 50 μm.sup.2 was also large, and the apparent density, the OD value (concealing properties), the color tone b value, the heat shrinkage [MD direction (Machine direction) and TD direction (Transverse direction)], and film-forming properties were all good.

Example 2

(23) In this example, a void-containing polyester film containing a reclaimed raw material was prepared. In detail, a reclaimed raw material was prepared by crushing and melt-extruding the ear portions obtained in the transverse stretching step in the tenter of Example 1. The addition amounts of the polyethylene terephthalate resin, the polypropylene resin, and the titanium oxide master pellet (M1) were adjusted so as to have the same composition of the layer A of the Example 1, except that the reclaimed raw material thus obtained was added to the layer Aso that 25% by mass of the layer A was the reclaimed raw material, and this was used as raw materials for the layer A. A void-containing polyester film (B/A/B) with a thickness of 50 μm was obtained, in the same manner as in Example 1 except for that. The evaluation results of Example 2 are also shown in Table 1. As in Example 1, all characteristics of the film of Example 2 were also good.

Example 3

(24) A void-containing polyester film (B/A/B) with a thickness of 50 μm containing a reclaimed raw material was obtained, in the same manner as in Example 2 except that the polyethylene terephthalate resin in the layer A was changed to the one having a melt viscosity of 132 Pa.Math.s in Example 2. The evaluation results of Example 3 are also shown in Table 1. As in Examples 1 and 2, all characteristics of the film of Example 3 were also good.

Example 4

(25) A void-containing polyester film (B/A/B) with a thickness of 50 μm using the reclaimed raw material was obtained, in the same manner as in Example 2 except that the polypropylene resin in the layer A was changed to one having a melt viscosity of 610 Pa.Math.s and the addition amount of the reclaimed raw material was 45% by mass, in Example 2. The evaluation results of Example 4 are also shown in Table 1. As in Examples 1 to 3, all characteristics of the film of Example 4 were also good.

Example 5

(26) A void-containing polyester film (B/AB) with a thickness of 50 μm containing the reclaimed raw material was obtained, in the same manner as in Example 2 except that the polyethylene terephthalate resin in the layer A was changed to one having a melt viscosity of 185 Pa.Math.s and the polypropylene resin was changed to one having a melt viscosity of 372 Pa.Math.s, in Example 2. The evaluation results of Example 5 are also shown in Table 1. As in Examples 1 to 4, all characteristics of the film of Example 5 were also good.

Example 6

(27) A void-containing polyester film (B/AB) with a thickness of 50 μm containing the reclaimed raw material was obtained, in the same manner as in Example 2 except that the polyethylene terephthalate resin in the layer A was changed to one having a melt viscosity of 310 Pa.Math.s in Example 2. The evaluation results of Example 6 are also shown in Table 1. As in Examples 1 to 5, all characteristics of the film of Example 6 were also good.

Example 7

(28) A void-containing polyester film (B/AB) with a thickness of 50 μm containing the reclaimed raw material was obtained, in the same manner as in Example 2 except that the polyethylene terephthalate resin in the layer A was changed to one having a melt viscosity of 132 Pa.Math.s and the polypropylene resin was changed to one having a melt viscosity of 553 Pa.Math.s, in Example 2. The evaluation results of Example 7 are also shown in Table 1. As in Examples 1 to 6, all characteristics of the film of Example 7 were also good.

Example 8

(29) In Example 8, a film consisting only of the layer A was prepared. In detail, a void-containing polyester film (the layer A only) with a thickness of 50 μm containing the reclaimed raw material was obtained, in the same manner as in Example 2 except that the layer B was not used, the polypropylene resin in the layer A was changed to one having a melt viscosity of 759 Pa.Math.s, and the composition ratio was changed so as to be the same as in Example 1, in Example 2. At this time, the addition amounts of the polyethylene terephthalate resin, the polypropylene resin, and the titanium oxide master pellet (M1) were adjusted so that the composition ratio was the same as the composition ratio in all layers of Example 1. The evaluation results of Example 8 are also shown in Table 1. Break sporadically occurred during TD stretching (evaluation of film-forming properties in Table 1 was fair), but when a small amount of the sample before breaking was taken and each characteristic was measured, characteristics other than film-forming properties were all good.

Comparative Example 1

(30) A void-containing polyester film (B/A/B) with a thickness of 50 μm containing the reclaimed raw material was obtained, in the same manner as in Example 2 except that the polypropylene resin in the layer A was changed to one having a melt viscosity of 200 Pa.Math.s in Example 2. The evaluation results of Comparative Example 1 are also shown in Table 1. In Comparative Example 1, since the polypropylene resin has a low melt viscosity and the raw material resins of a low melt viscosity ratio were used, the dispersed particle diameter became small, the number of voids of 10 to 50 μm.sup.2 was small, the apparent density was large, and concealing properties deteriorated.

Comparative Example 2

(31) A void-containing polyester film (B/A/B) with a thickness of 50 μm containing the reclaimed raw material was formed, in the same manner as in Example 2 except that the polyethylene terephthalate resin in the layer A was changed to one having a melt viscosity of 78 Pa.Math.s in Example 2. Break frequently occurred during TD stretching, but a small amount of the sample before breaking was taken and each physical property was measured. The evaluation results of Comparative Example 2 are also shown in Table 1. In Comparative Example 2, since the polyethylene terephthalate resin has a low melt viscosity and the raw material resins of a high melt viscosity ratio were used, the dispersed particle diameter was large, the number of voids of 10 to 50 μm.sup.2 was small, and concealing properties deteriorated.

Comparative Example 3

(32) A void-containing polyester film (B/A/B) with a thickness of 50 μm containing the reclaimed raw material was obtained, in the same manner as in Example 2 except that the polyethylene terephthalate resin in the layer A was changed to one having a melt viscosity of 420 Pa.Math.s and the polypropylene resin was changed to one having a melt viscosity of 402 Pa.Math.s, in Example 2. The evaluation results of Comparative Example 3 are also shown in Table 1. In Comparative Example 3, since the polyethylene terephthalate resin has a high melt viscosity and the raw material resins of a low melt viscosity ratio were used, the dispersed particle diameter of the polypropylene resin became small, the number of voids of 10 to 50 μm.sup.2 was small, the apparent density was large, and concealing properties deteriorated.

Comparative Example 4

(33) Comparative Example 4 is an example in which a film consisting only of the layer A was prepared and a compatibilizer was added to the layer A, as in Example 8. In detail, a void-containing polyester film with a thickness of 50 μm containing the reclaimed raw material was obtained (the layer A only), in the same manner as in Example 8 except that 1% by mass of polyethylene glycol PEG (molecular weight 4000) as a dispersant was added to the layer A in Example 8. Since break sporadically occurred during TD stretching, a small amount of the sample before breaking was taken and each characteristic was evaluated. The evaluation results of Comparative Example 4 are also shown in Table 1. In Comparative Example 4, since the compatibilizer was added, the dispersed particle diameter of the polypropylene resin became small, the apparent density was large, concealing properties deteriorated, and the color tone b value increased. In addition, since the compatibilizer was added and the melt viscosity of the polypropylene resin was high, film-forming properties decreased.

(34) TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Layer A PET η1 at 280° C. 200 200 132 200 185 310 132 (Pa .Math. s) PP η1 at 280° C. 500 500 500 610 372 500 553 (Pa .Math. s) Melt Viscosity Ratio 2.5 2.5 3.8 3.1 2.0 1.6 4.2 (η2/η1) Compatibilizer (PEG) — — — — — — — Layer B PET η1 at 280° C. 200 200 200 200 200 200 200 (Pa .Math. s) Dispersed Particle μm 8.7 8.8 11.3 10.5 8.4 7.5 12.2 Diameter Apparent Density g/cm.sup.3 1.06 1.06 1.00 1.02 1.11 1.17 0.97 OD Value 0.65 0.65 0.61 0.62 0.63 0.58 0.59 (Optical Density: Concealing Properties) Color Tone b Value 1.3 1.6 1.8 1.8 1.6 1.6 1.8 Number of Voids of numbers/10000 μm.sup.2 50 48 40 43 32 26 30 10 to 50 μm.sup.2 Heat Shrinkage % 1.5 1.5 1.4 1.5 1.5 1.5 1.5 (MD Direction) Heat Shrinkage % 0.3 0.3 0.3 0.3 0.3 0.3 0.3 (TD Direction) Film-Forming Properties good good good good good good good Comparative Comparative Comparative Comparative Example 8 Example 1 Example 2 Example 3 Example 4 Layer A PET η1 at 280° C. 200 200 78 420 200 (Pa .Math. s) PP η1 at 280° C. 759 200 500 402 759 (Pa .Math. s) Melt Viscosity Ratio 3.8 1.0 6.4 1.0 3.8 (η2/η1) Compatibilizer (PEG) — — — — 1 Layer B PET η1 at 280° C. — 200 200 200 — (Pa .Math. s) Dispersed Particle μm 11.5 6.0 15.0 6.2 3.3 Diameter Apparent Density g/cm.sup.3 0.99 1.28 0.92 1.26 1.31 OD Value 0.60 0.51 0.53 0.51 0.49 (Optical Density: Concealing Properties) Color Tone b Value 2.6 2.2 2.3 1.8 4.5 Number of Voids of numbers/10000 μm.sup.2 42 5 5 3 0 10 to 50 μm.sup.2 Heat Shrinkage % 1.5 1.4 1.3 1.4 1.5 (MD Direction) Heat Shrinkage % 0.3 0.2 0.3 0.2 0.3 (TD Direction) Film-Forming Properties fair good poor good fair

INDUSTRIAL APPLICABILITY

(35) According to the present invention, it is possible to provide a void-containing polyester film excellent in lightweight properties and cushioning properties, also having good concealing properties, whiteness, and thermal dimensional stability even when an inexpensive polypropylene resin is used as a void-generating agent.