CA1044419A

CA1044419A - Process for producing porous polymer film

Info

Publication number: CA1044419A
Application number: CA202,085A
Authority: CA
Inventors: Takashi Suzuki
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1973-06-11
Filing date: 1974-06-10
Publication date: 1978-12-19
Also published as: FR2232430B1; GB1476791A; DE2427862A1; FR2232430A1

Abstract

Abstract of the Disclosure A process for producing a porous polymer film, having many fine voids which are flat in the thickness direction, com-prises forming many fine cracks in a stretchable polymer composi-tion having a fine phase separation structure by mechanical deformation or chemical treatment, heating the thus treated polymer composition to its stretching temperature under tension, and then stretching the polymer composition at the stretching temperature. The polymer composition preferably contains a material which is fluid at the stretching temperature.

Description

1()444~9 The present invention relates to a process for producing a porous polymer film having many fine voids which are flat in the thickness direction.
The term "film" used herein is a general term for materials in the form of sheet or film and include also a part having a sheet or film form of cylindrical film, hollow yarn and various polymer mouldings.
A blend composition comprising two or more polymers, a composition comprising a block copolymer, a crystalline polymer composition in which spherulites have grown, a polymer composition containing various fillers such as various low molecular weight compounds which are low in compatibility with the polymer and inorganic and organic substances, etc. have a fine phase separation structure of about 0.01 - 100 microns in average diameter. When the compositions are deformed `~ by enforced low stretching, bending, twisting, etc. at . ~ ., i a considerably lower temperature than their melting point, fine cracks are generally formed between the s phases or in a specific phase of the phase separation structrue and reduction in transmittancy (usually ; turbidity) is brought about.
~ The term "phase separation structure" used herein means a structure in which the same or different compounds form two or more phases. Spherulite struc-~ ture consisting of a single polymer is also included 3 in the phase separation structure since a crystalline : ?
phase and an amorphous phase are considered to form separate phases. Also, the term "crack" used herein '~ .

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10~44~9 means not only true cracks but also crazing.
When cracks are formed in a polymer composition having a phase separation structure as described above and the polymer composition is forcibly stretched at a temperature considerably lower than its melting point, necking occurs and the polymer composition is converted into a porous film. Such a process is known, but this stretching step has many troubles in commercial - practice. Thus, the tension required for the stretching is at least four times that required for conventional stretching. When defects are present non-uniformly in the composition, there is a marked tendency for the stretching tension to be concentrated at the defects and to cause breaks, so that the process has little success. Conventional stretching is carried out at a temperature which is lower than the melting point of the polymer composition ~ -and higher than the temperature at which cracks are formed on deformation of the polymer composition. In this case, even if the polymer composition has a fine phase separation structure, voids are difficult to form on stretching and a non-porous film is generally obtained. A temperature at which such stretching is possible is herein referred to as a "stretching temperature".
Japanese Patent Kokai (Laid-Open) No. 16,976/73 (Applicant E. I.
d duPont de Nemours & Co., Application No. 6418/72 dated June 28, 1972, Laid-Open March 3, 1973, Inventors Richard Masayoshi ~
Ikeda and George Joseph Ostabtschenko) discloses that, in the case - -of a crystalline polyolefin composition containing a large amount ~
of fillers, stretching at the stretching temperature occasionally -gives a porous film. -~t~ An object of the present invention is to provide a process for producing a porous polymer film by stretching a - ;
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,~;, ., . , ., : . : ,, lV44419 polymer composition, which can be selected from a wide range of polymeric materials, at its stretching temperature.
According to the present invention there is provided a process for producing a porous polymer film comprising the steps of forming many fine cracks in a stretchable polymer composition having a fine phase separation structure; heating said polymer composition to a stretching temperature, which is lower than the melting point of the polymer composition and higher than the temr perature at which cracks are formed on deformation of the polymer composition, under tension so that the said fine cracks may not disappear; and enlarging said fine cracks by stretching the polymer composition to form said film at said stretching tempera-ture. .
The stretchable polymer compositon having a fine phase separation structure is exemplified by a composition having a developed spherulite structure and consisting mainly of a crystalline polyolefin such as polyethylene, isotactic polypropy-lene, isotactic polybutene-l or isotactic poly-4-methylpentene- :
' 1, a crystalline polyamide such as nylon 6, nylon 66, nylon 11 or nylon 12, a polyester such as polyethylene terephthalate, .
polybutylene terephthalate or polynaphthalene terephthalate, or a crystalline polyhalide .
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1~4419 such as polyvinylidene fluoride, polyfluorotrichloro-ethylene or polyvinylidene chloride, a composition consisting of a blend of various crystalline polymers, a composition consisting of a blend of a crystalline polymer and an amorphous polymer such as polyvinyl chloride, polystyrene, polycarbonate or polysulfon, a composition consisting of a blend of amorphous polymers, and a composition consisting of a block co-polymer having a phase separation structure. It is possible to add various fillers, additives, etc. to -the respective compositions if necessary. The porosity of the resulting porous film can be improved by the addition of a fine inorganic or organic filler. In -the case of a composition consisting mainly of a poly-olefin polymer, the gas and liquid permeability of the resulting porous film can be improved by the addition of a metal salt of a fatty acid such as zinc stearate , or aluminum stearate. In the case of a composition consisting mainly of a crystalline polymer, the hole -diameter of the resulting porous film can be reduced by the addition of a nucleus agent for the crystalline polymer.
Further, compositions obtained by dispersing various low molecular weight compounds, which are poor in compatibility with a specific polymer, in the : , specific polymer are suitable as the composition of ~
the present invention. ---The above-mentioned compositions have a - stretching temperature which is about 50 - 100C lower than their melting point and can be easily stretched 10~4419 by a compa~atively small tension of about 1 kg or less per mm2.
In this temperature range, voids are substantially not formed by stretching and the porosity of the resulting film is low even if voids are formed. At a temperature lower than the stretching temperature, a tension of several kg per mm2 is required for stretching. As described above, breaks can easily occur on stretching and many voids are formed with turbidity on stretching.
In the process of the present invention, the cracks in the composition can be formed by various methods including a method which comprises subjecting the composition to mechanical deformation at a temperature lower than its stretching temperature and a method which comprises subjecting the composition to chem-ical treatment. The former method includes, for example, bending , the composition on a small diameter roller, rolling the composi-tion between two rubber belts, mechanically beating the composi-tion, stretching the composition enforcedly between two differen-tial rollers. It is desirable that the degree of deformation is instantaneously several percent and that a permanent set of 0.02%
or more remains. In enforced stretching, the stretch ratio is :
desirably lower than such a ratio as brings about necking. The cracks are usually formed in a direction at right angles to the deformation. The deformation direction is desirably in accord with the subsequent stretching direction, but is not limited thereto. The latter chemical-treatment method is exemplified by a method which comprises giving a residual stress to the composition and then dipping the composition in a liquid which may form stress cracks. The composition usually becomes white turbid on the formation of cracks. If the length or space of . , .

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the cracks is ver~ small, the composition does nat become white turbid in some cases. In any case, the formation of cracks can be confirmed by reduction in transmittancy as long as the composition is transparent. If the cracks are locally formed, the resulting film becomes only partly amorphous.
The composition in which cracks have been formed as described above is heated to its stretching temperature and is then stretched. It is neces~ary for the composition to be maintained under tension at least at temperatures in the region of the stretching temper-ature, in order to prevent the disappearance of the cracks by the heating to the stretching temperature.
It is necessary to apply a tension of 0.1 kg/mm or more to the composition at least in one direction. The tension direction is desirably at right angles to the crack direction, but it is not limited thereto. The ten-sioning is carried out to overcome the thermal expansion of the composition on heating. The simplest method is to --;
use a pair of differential rollers. If tension is not :~ .
maintained, the cracks disappear and a porous film is --difficult to obtain by stretching. It is possible in commercial practice to effect the formation of cracks and stretching continuously with heating. Stretching at the stretching temperature of the composition is `
carried out by various known methods such as, for example, longitudinal stretching between differential rollers, lateral stretching by a tenter, simultaneous biaxial ~ .

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1~44419 stretching b~ an accelerative tenter and simultaneous polyaxial stretching by inflation. By stretching the cracks are enlarged mainly with regard to the stretching direction into holes. Stretching is desirably plural-axial stretching (biaxial or polyaxial stretching). The composition is preferably stretched to 1.5 - 10 times its original length in one direction, and the ratio of longi-tudinal and lateral stretches in biaxial stretching is preferably 1 : 5 to 5 ~ 1. These ranges usually give through holes.
It is preferable that the polymer composition used contains a material which is fluid at the stretching temperature of the composition, because a porous film .
i having a microfibril structure on its surface, good gas and liquid permeability and a pearly luster can be obtained in this case. The material is selected to be such that the temperature at which stretching is completed is ` at least about 20 C below the melting point for crystal-line polymers, the Vicat softening point for amorphous ' 20 polymers, and the melting point for low molecular weight compounds. The amount of the material is desirably 0.01 v, - 50% by weight based on the weight of the whole composi-tion. The presence of this material ensures that on ' stretching the cracks are enlarged and the surface of the composition is simultaneously coated by the fluid material. It is considered that a pearly luster is pro-duced thereby.

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~)44419 Electron microscopic observation showed that a porous film obtained according to an embodiment of the present invention had a stratified structure wherein many fine voids which are flat in the direction of film thick-` ness had been laminated.
Porous polymer films obtained according to theprocess of the present invention are useful as insulating materials impregnated with insulating oils, dielectrics for capacitors, or insulating materials for power cables.
For a better understanding of how the present ; invention can be practiced, the following examples are given by way of illustration and not by way of limitation.
` ................................................ ' . ' ' Example 1 A melt-extruded sheet of isotactic polypropylene having a thickness of 0.2 mm was bent along a steel bar of 5 mm outside diameter in liquid nitrogen to form fine ~ -cracks and was then removed into air at room temperature.
In a laboratory biaxial stretcher, the sheet was heated to 140C under tension and was then stretched to three times the original length in each direction while it was ~
~` heated to 140 - 150C. Thus, a translucent film having -.~
fine voids extending in the plane direction of the sheet ~ and crossing each other was obtained.
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Example 2 The polypropylene sheet used in Example 1 was -, annealed in the air at 165C to let crystalline particles . ~ .

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4~9 to grow and cracks were then formed with a steel bar of 2 mm outside diameter at room temperature, The treated sheet was heated to 135 C under tension, subjected to monoaxial stretching to a~out 2.0 times the original length at this temperature, and then stretched to about

3.0 times the original width in the perpendicular direc-tion at 145C. Thus, a silver porous film was obtained.
, Example 3 A composition comprising a high impact poly-styrene (butadiene content 5%) was placed on a rubber plate at room temperature and the rounded tip of a metal bar was pressed down against the composition to form cracks locally. The thus treated composition was heated i~ to 130C under tension and then subjected to biaxial ~;
stretching to 2.5 times the original length in each direction at this temperature. Thus, a white patterned ~- film was obtained.
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Example 4 A sheet (thickness 0.3 mm) of a composition obtained by melt-blending 20~ by weight of a low density polyethylene with an isotactic polypropylene was wound -round a metal bar of 5 mm diameter at room temperature to form cracks only at the surface layer on one side. The thu~ treated sheet was heated to 125C under tension and then subjected to biaxial stretching to 1.5 times the -;
original length and width at this temperature. Thus, a -.-.

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Examples 5 - 11 The compositions, crack forming conditions and stretching conditions are summarized in Table 1.
The respective compositions were in the form of a sheet of 0.1 - 1.0 mm in thickness. Cracks were formed in all cases by enforced stretching which comprises rolling the sheet between urethane rubber belts. The stretch ratio, which is (the stretched length/the original length in ~, the stretched direction) x (the stretched length/the original length in the width direction), is shown in the -`` `
table. In all the examples an opaque porous film was -obtained.

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11~)44419 . 1 Table . , _ _ Crack forming ¦ Stretching . Example Polymer Temper- Enforced Temper- Stretch ature stretch ature ratio (C) ratio (C) ,.~ .. _.
. 2: 1 (weight .
ratio) melt polyamide 100 1. 2 180 2 x 2 (Nylon 6) and (bi-polystyrene axial) ~ "
1 : 1 (weight ratio) melt blend of .
isotactic 6 and ethylene- 25 1.5 145 3 X 3 ~ vinyl acetate (mono-:~ copolymer axial) (vinyl acetate content 25 %) , .
3 10 : 1 ( weight .~ ratio) melt blend of 7 polyethylene 120 1.3 230 3 X 3 , terephthalate (bi-i~ and polyamide axial) .~ (Nylon 12) . . _ _ 10 : 1 (weight ratio) solution blend of ~. polyvinyl . - :
~ 8 chloride and 25 1.2 150 2 x 2 .~
acetate copoly- (mono- :. -.
mer (vinyl axia ) acetate content _ about 35 %) ._ _ : ::
~ : 100 : 1 (weight . .: --ratio) melt blend of iso- . . :
9 tactic poly- 25 1.1 135 3 X 3 :
. . propylene and (bi- .. . atactic poly- . axial) ~-.
propylene . .
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~ ~)44419 .'' 1 . .i 5 : 1 (wei.ght ratio) melt blend of polyacetal 60 (bli5 170 2 x 2 and low axial) .
ethylene _ 10 : 1 (weight ~
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11 blend of poly- 100 1.2 200 2 x 2 carbon~te and (bi-polystyrene axial) As is clear from the above description, a , ~ porous film can be easily obtained from a polymer com-position consisting of a wide range of polymeric materials under conventional commercial stretching conditions ac-cording to the present invention. Thus, the present ~ invention is very useful as a commercial process.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing a porous polymer film comprising the steps of forming many fine cracks in a stretchable polymer composition having a fine phase separation structure; heating said polymer composition to a stretching temperature, which is lower than the melting point of the polymer composition and higher than the temperature at which cracks are formed on deformation of the polymer composition, under tension so that the said fine cracks may not disappear; and enlarging said fine cracks by stretching the polymer composition to form said film at said stretching temperature.

2. A process according to claim 1, wherein said stretchable polymer composition contains a material which is fluid at said stretching temperature.

3. A process according to claim 1, wherein said fine cracks are formed by enforced stretching of the polymer composition at a temperature at which cracks are formed on deformation of the polymer composition.

4. A process according to claim 1, 2, or 3 wherein said stretching for enlarging said fine cracks comprises plural-axial stretching.