Highly porous separator foil

Abstract

The invention relates to a biaxially oriented single- or multilayer porous foil, the porosity of which is generated by transformation of ss-crystalline polypropylene during orientation of the foil. The Gurley value of the foil is <250 s. The invention also relates to a process for producing the foil by using a low transverse stretching velocity for the transverse orientation process.

Claims

1. A biaxially oriented, single layer or multilayer porous film, porosity of which is generated by transformation of -crystalline polypropylene during stretching of the film, which comprises at least one porous layer, which layer contains at least one propylene polymer and -nucleating agent, wherein the film has a Gurley value of <250 s and wherein when the propylene polymer is a propylene block polymer, the propylene block polymer has a melting range that begins at over 120 C.

2. The film according to claim 1, wherein the Gurley value of the film is 10 to 200 Gurley.

3. The film according to claim 1, wherein the at least one propylene polymer is a propylene homopolymer and/or a propylene block copolymer.

4. The film according to claim 1, wherein the -nucleating agent is a calcium salt of pimelic acid and/or of suberic acid or a nanoscale iron oxide.

5. The film according to claim 1, wherein the film contains propylene homopolymer and propylene block copolymer.

6. The film according to claim 1, wherein the film contains 50 to 85% by weight propylene homopolymer, 15 to 50% by weight propylene block copolymer and 50 to 10,100 ppm -nucleating agent.

7. The film according to claim 1, wherein the density of the film is in a range from 0.1 to 0.5 g/cm.sup.3.

8. The film according to claim 1, wherein the film has a thickness from 10 to 100 m.

9. A method for producing a single layer or multilayer porous polypropylene film in which in a first longitudinal stretching process propylene polymer and -nucleating agents are melted in an extruder and extruded through a flat nozzle onto a take-off roller, on which the melt film cools down and solidifies, forming -crystallites, and this film is then stretched longitudinally, cooled and wound up, and in a second, transverse stretching process, this longitudinally stretched, wound up film is unwound, heated to a transverse stretching temperature and stretched in the transverse direction, wherein the advance speed of the longitudinal stretching process is greater or less than the advance speed of the transverse stretching process.

10. The method according to claim 9, wherein the advance speed of the transverse stretching process is selected such that the transverse stretching takes place at a stretching speed of less than 40%/sec.

11. A separator which comprises the film according to claim 1.

12. A separator which comprises the film produced according to the method according as claimed in claim 9.

13. A double layer capacitor containing the film according to claim 1.

14. A double layer capacitor containing the film produced according the method according to claim 9.

15. The film according to claim 1, wherein the film obtained in a first longitudinal stretching process propylene polymer and -nucleating agents are melted in an extruder and extruded through a flat nozzle onto a take-off roller, on which the melt film cools down and solidifies, forming -crystallites, and this film is then stretched longitudinally, cooled and wound up, and in a second, transverse stretching process, this longitudinally stretched, wound up film is unwound, heated to a transverse stretching temperature and stretched in the transverse direction, wherein the advance speed of the longitudinal stretching process is greater or less than the advance speed of the transverse stretching process.

16. The film according to claim 15, wherein the advance speed of the transverse stretching process is selected such that the transverse stretching takes place at a stretching speed of less than 40%/sec.

17. The film according to claim 1, wherein the propylene block polymer has a melting range from 125 to 140 C.

Description

EXAMPLE 1

1a: Longitudinal Stretching Process

(1) After the extrusion process, a single-layer prefilm was extruded from a flat sheet die at an extrusion temperature from 240 to 250 C. respectively. This prefilm was first drawn off and cooled on a cooling roller. Then, the prefilm was heated to the temperature for longitudinal stretching and stretched longitudinally over rollers running at different speeds. At this point, the length of the stretching gap is about 30 mm. The width of the foil was reduced by about 5% following the longitudinal stretching. Then the foil is cooled by passing it over cooling rollers. The cooled longitudinally stretched foil was then wound up.

1b: Transverse Stretching Process

(2) The wound, longitudinally stretched foil was then stretched transversely as follows. The longitudinally stretched foil is unwound and passed over rollers into the heating spring of the tenter frame, heated to the transverse stretching temperature and oriented in the transverse direction. This transverse orientation is followed by thermosetting, in which the foil is advanced in converging manner. Finally, the foil is wound up. The foil had the following composition:

(3) Approximately 80% by weight high-isotactic propylene homopolymerisate (PP) with .sup.13C-NMR isotacticity of 97% and an n-heptane soluble fraction of 2.5% by weight (relative to 100% PP) and an melting point of 165 C.; and a melt flow index of 2.5 g/10 min at 230 C. and 2.16 kg load (DIN 53 735)

(4) and

(5) approximately 20% by weight propylene-ethylene block copolymerisate with an ethylene fraction of 5% by weight relative to the block copolymer and an MFI (230 C. and 2.16 kg) of 6 g/10 min and a melting point (DSC) of 165 C. were used with

(6) 0.04% by weight Ca-pimelate as the -nucleating agent.

(7) The foil also contained standard small quantities of stabilizers and neutralizing agents in both layers.

(8) In detail, the following conditions and temperatures were selected for the production of foil:

(9) 1a: Longitudinal Stretching Process:

(10) Extrusion:

(11) Extrusion temperature 235 C.

(12) Drawing Off:

(13) Temperature of drawing-off roller: 125 C.,

(14) Drawing-off speed: 4 m/min

(15) Longitudinal Stretching:

(16) Temperature of preheating rollers: 90 C.

(17) Temperature of stretching roller: 90 C.

(18) Longitudinal stretching factor: 4.5

(19) Temperature of cooling roller: 90 C.:

(20) Dwell time on cooling roller: 40 s

(21) Advance Speed

(22) when winding up 18 m/min

(23) 1b Transverse Stretching Process

(24) Transverse Stretching:

(25) Temperature of heating fields: 135 C.

(26) Temperature of tentering fields: 135 C.

(27) Transverse stretching factor: 5

(28) Stretching speed: 7.5%/s

(29) Thermosetting:

(30) Temperature: 140 C.

(31) Convergence: 15%

(32) Dwell time in thermosetting field: 20 s

(33) The porous foil thus produced was approximately 25 m thick. The foil had a density of 0.31 g/cm.sup.3 and had a uniform white-opaque appearance with a low Gurley value of 205 s.

EXAMPLE 2

(34) A foil was produced as described in Example 1. Compared with Example 1, only the tentering speed during transverse stretching was changed. The longitudinally stretched foil was stretched in the transverse direction at a speed of 6%/s at 135 C. Otherwise, the composition of the foil was unchanged and the other process conditions were retained.

(35) The porous foil thus produced was approximately 27 m thick. The foil had a density of 0.29 g/cm.sup.3 and had a uniform white-opaque appearance and an even lower Gurley value than Example 1, 161 s.

EXAMPLE 3

(36) A foil was produced as described in Example 1. Compared with Example 1, only the tentering speed during transverse stretching was changed. The longitudinally stretched foil was stretched in the transverse direction at a speed of 4.5%/s at 135 C. Otherwise, the composition of the foil was unchanged and the other process conditions were retained.

(37) The porous foil thus produced was approximately 28 m thick. The foil had a density of 0.28 g/cm.sup.3 and had a uniform white-opaque appearance and a Gurley value of 130 s.

EXAMPLE 4

(38) A foil was produced as described in Example 1. Compared with Example 1, only the tentering speed during transverse stretching was changed. The longitudinally stretched foil was stretched in the transverse direction at a speed of 2.5%/s at 135 C. Otherwise, the composition of the foil was unchanged and the other process conditions were retained.

(39) The porous foil thus produced was approximately 29 m thick. The foil had a density of 0.26 g/cm.sup.3 and had a uniform white-opaque appearance and a Gurley value of 60 s, considerably lower than that of Example 1.

EXAMPLE 5

(40) A foil was produced as described in Example 1. Compared with Example 1, only the tentering speed during transverse stretching was changed. The longitudinally stretched foil was stretched in the transverse direction at a speed of 1%/s at 135 C. Otherwise, the composition of the foil was unchanged and the other process conditions were retained.

(41) The porous foil thus produced was approximately 30 m thick. The foil had a density of 0.25 g/cm.sup.3 and had a uniform white opaque appearance and a Gurley value of 40 s, considerably lower than that of Example 1.

COMPARISON EXAMPLE 1

(42) A foil was produced as described in Example 1. Compared with Example 1, only the length of the stretching gap during longitudinal stretching was changed. The stretching gap was opened to a length of 150 mm. The width of the longitudinally stretched foil was reduced by 12% by the neck-in ratio during longitudinal stretching.

(43) The foil thus produced had approximately the same density and Gurley value as the foil according to Example 1. However, the foil demonstrated limited running reliability to its tendency to split. During production of the foil, tearing occurred frequently in the transverse stretching phase, with the result that the foil is uneconomical to produce.

COMPARISON EXAMPLE 2

(44) A foil was produced as described in Example 1. Compared with Example 1, only the length of the stretching gap during longitudinal stretching was changed. The stretching gap was opened to a length of 300 mm. The width of the longitudinally stretched foil was reduced by 22% by the neck-in ratio during longitudinal stretching.

(45) The foil thus produced had approximately the same density and Gurley value as the foil according to Example 1. However, the foil demonstrated limited running reliability to its tendency to split. Tearing occurred frequently in the transverse stretching phase, during production of the foil.

COMPARISON EXAMPLE 3

(46) A foil was produced as described in Example 1. Compared with Example 1, only the stretching speed during transverse stretching was changed. The longitudinally stretched foil was stretched transversely at an increased speed of 50%/s at 135 C. Otherwise, the composition of the foil was unchanged and the other process conditions were retained.

(47) A foil with low porosity of 50% and a Gurley value of 1250 s was obtained.

COMPARISON EXAMPLE 4

(48) A foil was produced as described in Example 1. Compared with Example 1, only the stretching speed during transverse stretching was changed. The longitudinally stretched foil was stretched transversely at an increased speed of 100%/s at 135 C. Otherwise, the composition of the foil was unchanged and the other process conditions were retained.

(49) A foil with low porosity of 40% and an inadequate Gurley value of 2800 s was obtained.

(50) The properties of examples 1-5 and comparison examples 1-4 are summarised in the following table. This reveals that only the foils of examples 1-5, which were produced according to the method of the invention have the desired properties such as high porosity and very low Gurley value while retaining good running reliability throughout the production process.

(51) TABLE-US-00001 TABLE Neck-in Longitudinal ratio Transverse stretching Longitudinal stretching Stretching stretching speed Running Porosity Gurley gap [mm] [%] [%/s] reliability [%] [s] Ex. 1 10 5 7.5 Good 66 205 Ex. 2 10 5 6 Good 68 161 Ex. 3 10 5 4.5 Good 69 130 Ex. 4 10 5 2.5 Good 72 60 Ex. 5 10 5 1 Good 73 40 Comp. 150 12 7.5 Poor 65 203 ex. 1 Comp. 300 22 7.5 Very poor 66 190 ex. 2 Comp. 10 5 50 Moderate 50 1250 ex. 3 Comp. 10 5 100 Moderate* 40 2800 ex. 4 *higher lengthwise orientation causes more frequent tears during transverse stretching