EP1114082A1

EP1114082A1 - Microporous polyolefin films permeable to gases and impermeable to liquids

Info

Publication number: EP1114082A1
Application number: EP99929638A
Authority: EP
Inventors: Guy Benassi; Christian Dubouchet; Michel Gillet; Guy Pinchard
Original assignee: ACE SA
Current assignee: ACE SA
Priority date: 1998-07-24
Filing date: 1999-07-22
Publication date: 2001-07-11
Also published as: BE1012087A4; AU4641499A; WO2000005295A1; US6676871B1; BR9912357A

Abstract

The invention concerns a method for producing a microporous film with strong isotropic tendency permeable to gases and impermeable to aqueous liquids, consisting of at least one layer, comprising the following successive steps: preparing a mixture consisting of a polyolefin matrix containing at least a polymer and at least a mineral and/or organic filler; extruding at least a ply by hot-casting the mixture; pre-stretching the ply with a drag roll; cooling and solidifying the pre-stretched ply, using the drag roll; drawing the solidified ply at sufficient temperature for forming the microporous film. The invention is characterised in that the cooling of the pre-stretched melting ply with the drag roll is partial and limited in controlled manner at a temperature in the temperature range required for its drawing; the drawing of the ply, brought to the temperature required for drawing, by the partial cooling is carried out by traction, at the time of its tangential separation from the drag roll, said roller acting as take-up roller for drawing. Said microporous film can be used on its own or combined with other porous supports for various applications such as corporal hygiene articles, dressings, medical articles, protective clothing, sportswear, insulating coats in the building industry, and the like.

Description

GAS PERMEABLE POLYOLEFINIC MICROPOROUS FILMS

LIQUID WATERPROOF

Field of the invention

The invention relates to flexible, flexible and breathable polyolefin microporous films, formed of at least one layer, permeable to gases and water vapors but impermeable to aqueous liquids, with high mechanical characteristics and a pleasant feel. The invention relates to a method for producing by extrusion casting of polyolefin microporous films. made up of at least one layer, films permeable to gases and water vapor, but impermeable to aqueous liquids, with high mechanical characteristics and a pleasant feel. The invention finally relates to the use of the aforementioned films alone or associated in complex form with other porous supports.

State of the art

It has long been known to produce flexible and flexible microporous films having the capacity to breathe, that is to say to be permeable to gases and to water vapor but impermeable to aqueous liquids, and to associate them with other materials such as macro-perforated films, non-woven fabrics, woven fabrics and papers, these other materials providing the mechanical characteristics and the desired feel, in particular the textile feel. The most conventional method for producing these microporous films consists in preparing a mixture formed by a matrix of thermoplastic polymers having elastic properties and a pulverulent filler, generally inorganic, in extruding by casting this mixture in the molten state , in the form of a thick web, to take up this thick web by means of a metal cylinder (the surface of which can be provided with patterns for effecting an embossing of the web) having a function of pre-stretching the molten sheet and its cooling, then stretching it at least uniaxially after having heated it to a temperature favorable for this operation, in order to achieve the desired film. During stretching (post-cooling) enabling the thick web to pass into a thin film state, the grains of the inorganic filler dispersed within the matrix of thermoplastic polymers are at least partially separated from the polymeric material by a mechanical effect, creating micro-channels inside and through the thickness of the film. The choice of the hydrophobic nature of the polymer matrix as well as that of the inorganic filler, and in particular the size of its grains, the quality of the dispersion of the filler within the polymer matrix and the choice of the stretching rate applied to the sheet for the film formation, are all factors which favor the creation of these micro-channels (whose equivalent diameter is a few microns) and which give the film its porosity to gases and its impermeability to aqueous liquids. The state of the art reveals that microporous breathable films have been the subject of numerous researches relating to:

- On the composition of the polymer matrix and the mineral fillers such as calcium carbonate, barium sulfate (US 4,472,328). adding elastomers (such as polybutadiene) to improve the stretchability; - on the film manufacturing process and in particular on an embossing operation of the sheet extruded by casting and slightly stretched, in order to reveal variations in the thickness of the film (2 μ to 3 mm) over very short distances, and also on the achievement of its association with other materials to increase the mechanical resistance and / or the quality of the touch. By way of illustration, European patent EP232060 which describes a process for the production of a porous gas-permeable film, according to the following steps, which comprises:

- The formation of a mixture of a polyolefin resin and an inorganic filler; - extrusion by casting a ribbon from the molten mixture;

- pre-stretching, embossing and cooling the molten ribbon on a metal cylinder to give it a rough configuration;

- and finally the stretch itself. uniaxial or biaxial, after heating, to form the microporous film and give a macromolecular and crystalline orientation sufficient to give it all the desired properties. According to this process:

- The polyolefin resin can comprise homopolymers such as polypropylene. low density polyethylene, linear low density polyethylene, high density polyethylene and polybutylene. copolymers such as the ethylene-propylene copolymer and the ethylene vinyl acetate copolymer, or mixtures of these polymers. Particularly preferred are polypropylene, low density polyethylene, linear low density polyethylene or high density polyethylene, or a mixture of two or more of these polymers;

- And the inorganic filler is preferably barium sulfate or calcium carbonate.

In the same spirit, another process can also be cited which was the subject of European patent No. EP283200. This other process appears very close to that described in the previous patent since it proceeds according to the same steps but is nevertheless distinguished from it:

- by the composition of the film formed which is a mixture of a single copolymer of ethylene and of alpha-olefin C to Cio and an inorganic filler (calcium carbonate); - and by the pattern of the embossing which appears to be a selection of geometric shape

(hexagonal, circular or rhombus).

In both of these methods, the molten mixture constituting the composition of the film is extruded by casting in the form of a thick and hot strip. This strip is almost simultaneously pre-stretched. cooled and embossed by means of a cylinder metallic cooling and embossing, provided with a surface pattern, reducing the thickness of the completely cooled strip to approximately 100-150 μm. As the embossed strip resulting from the embossing operation does not yet have the desired characteristics for the microporous film such as, in particular, a small thickness, a microporosity giving it the ability to breathe while being impermeable to aqueous liquids, or properties mechanical necessary for its subsequent operation, it is subjected, after being heated to the appropriate temperature (by calendering for example) to a drawing operation between two pairs of rollers (forming a drawing bench) rotating at different speeds. The first pair of rollers constitutes the calling torque which simultaneously plays the role of delivering the embossed ribbon to be stretched and possibly of helping to maintain the sheet delivered at the desired temperature for drawing.

The second pair of rollers constitutes the actual drawing torque of the embossed ribbon heated by applying to it the desired drawing rate, this rate being given by the ratio of the linear speeds delivered by the pair of drawing rollers and the torque of rollers.

Beyond the stretching, the film according to the method practiced can undergo a heat treatment, generally under tension to control its shrinkage. Thus, it is obvious that the pre-stretched molten strip entering the embossing zone undergoes a substantial and practically complete cooling there and that this embossed strip must be reheated to a sufficient temperature to be able to be correctly stretched in the appropriate stretching zone , and best transformed into a film, with the expected characteristics. However, at the moment when the strip resulting from the extrusion by casting is pre-stretched, cooled and embossed, the constituent material of the strip, which is initially in an almost isotropic state, undergoes internal physical modifications during these stages, in particular macromolecular deformations and / or a macromolecular orientation and / or an evolution of the crystallinity, which are fixed in the state, during cooling. Although the strip from the embossing-cooling is heated to best undergo the actual stretching step (between the two pairs of rollers), this heating appears to be still insufficient to completely free the material of the strip from its internal state. frozen by cooling. This is why the final stretching of the embossed strip is carried out on a material already under stress which further adds new stresses to those existing at the time of stretching. This accumulation of internal stresses is manifested by the importance of the shrinkage by heating of the film leaving the stretch, which often requires post-stretch thermal stabilization to at least partially eliminate these stresses. Consequently, the film resulting from the pre-stretching, cooling, embossing and then stretching stages according to the state of the art, cannot have a sufficiently isotropic physical state as is desirable even if it has, thanks to these stages of many desired features.

Summary of the invention This is why:

- A first object of the invention is to provide an improved process for manufacturing a microporous film permeable to gases and water vapor and impermeable to aqueous liquids, at least monolayer, based on polyolefin; - Another object of the invention is to provide a method of manufacturing a microporous film, at least monolayer having a physical state with a strong isotropic tendency at the end of the drawing of the cooled sheet, that is to say an essentially amorphous and homogeneous physical state;

- Another object of the invention is to have a microporous film, at least monolayer, having satisfactory permeability to gases and impermeability to aqueous liquids, formed by drawing at least uniaxial from at least one sheet prepared for from a mixture of at least one polyolefin polymer and at least one mineral and / or organic filler. Consequently, the invention relates both to an improved process for producing an at least monolayer microporous film, permeable to gases and impermeable to aqueous liquids and to the film itself.

According to the invention, the process for producing a microporous film with a strong isotropic tendency permeable to gases and impermeable to aqueous liquids, consisting of at least one layer, comprising the successive steps of:

- the preparation of a mixture composed of a polyolefinic matrix comprising at least one polymer and at least one mineral and / or organic filler; - Extruding a sheet comprising at least one layer by hot pouring the mixture;

- pre-stretching the web by a drive cylinder;

- cooling and solidifying the pre-stretched sheet, by means of the drive cylinder; - stretching the sheet, at the appropriate temperature to form the microporous film. is characterized in that:

- The cooling of the molten sheet pre-stretched by the drive cylinder is partial and limited in a controlled manner to a temperature lying in the temperature range necessary for its stretching.

- The stretching of the sheet brought to the necessary temperature by partial cooling is carried out by traction, at the moment when its tangential separation from said drive cylinder occurs, said cylinder playing the role of roll of appeal for the drawing. According to the invention, the microporous film, permeable to gases and impermeable to aqueous media, is characterized by an isotropic, homogeneous and partly amorphous physical state. Detailed description of the invention

Thus, according to the invention, the sheet extruded by casting in at least one layer and pre-stretched, is only partially cooled, and this in a controlled manner, by the drive cylinder, to be brought to a temperature lying in the desired temperature range for drawing said sheet, and giving it the most complete isotropy characteristics, manifested by less internal stresses, whereas according to the state of the art previously mentioned, the sheet extruded and pre -drawn is cooled without precaution and without limit, by the passage on the cooling and embossing cylinder and. therefore, must be warmed to the temperature necessary for its stretching to be able to be stretched: the stretched film resulting from the state of the art is only partially isotropic as revealed by its high shrinkage capacity.

This is why, in the method according to the invention, the isotropic characteristics of the extruded and pre-stretched sheet are not fundamentally disturbed by the partial cooling practiced under control, but they are readily available at the moment when the sheet reaches in the stretching zone to undergo this operation which is carried out without brutality, in general at a lower stretching rate than that practiced in the prior art, by providing the film obtained with all the desired physical and mechanical properties , and in particular a state of strong isotropic tendency. Thus, unlike the process of the invention and according to the state of the art, it appears that:

- the fact of bluntly cooling the pre-stretched extruded ply on the drive cylinder causes the internal characteristics obtained during pre-stretching to be fixed; - The fact of reheating said sheet for the drawing operation after it has been bluntly cooled does not make it possible to provide the film obtained with an internal state with an isotropic tendency close to that of the post-extrusion sheet. Composition used for the production of the film

The polyolefinic microporous film according to the invention, permeable to gases and water vapor, and impermeable to aqueous liquids, is composed of a polymer matrix comprising at least one polyolefinic polymer and at least one mineral and / or organic filler.

The polyolefin microporous film according to the invention can comprise a single layer called the main layer or be formed from several layers assembled together, each layer being able to be produced from a particular composition which is specific to it.

When the microporous film according to the invention is formed from a single layer, obtained by stretching an extruded sheet containing a high filler content, this stretched layer must have excellent mechanical characteristics, in particular very good resistance to traction to undergo without damage the stresses to which it is subjected in its various applications. This is why the polyolefinic matrix is essentially composed of at least one polymer meeting this requirement of excellent mechanical strength which for this reason requires a rather high density resulting "de facto" in a high melting point. However, this layer, which is the seat of certain internal tensions, generated by the drawing operation, can have certain drawbacks during subsequent operations such as:

- hot welding of the film on itself or on other materials, welding which may concern the entire surface of the film or only a few points distributed over the surface of said film; - Or application of hot-melt adhesives to create complex structures with other flat products such as non-woven fabrics, during which the internal tensions are released under the effect of heat by causing a shrinkage of the film. This is why it may be advantageous for the microporous film according to the invention to be formed from several layers, the main one having high mechanical strength (as mentioned in the case of a monolayer film) associated with at least one another so-called specific layer, the polyolefin matrix of which can be formed not only of the polymer or polymers involved in the formation of the main layer and secondary layers but also of at least one specific olefin polymer and / or copolymer by the effect provided to the layer specific container, such as:

- a low density and therefore a lower melting point, allowing for example better adhesion to the cooling cylinder, very high drawing rates, great ease of welding on itself or on other materials, easy application of hot-melt adhesives, great freedom for the creation of complex structures by association with other materials, without causing the main layer of the microporous film to shrink;

- a higher density and a higher melting point to create affinities with other associated materials (non-woven fabrics for example) by hot calendering for example;

- a change in polarity of said specific layer with respect to the main layer;

- a modification of the surface state to create anti-slip anti-block or anti-static effects, in said specific layers, in particular when they are in the external position;

- a combination of the various olefin polymers and / or copolymers to create a multi-layer film, each layer of which has its own function thanks to its own composition. In the case where the polyolefin microporous film comprises a single so-called main layer, this layer is formulated so that it provides the film produced with the essential and maximum mechanical properties mentioned above. The polyolefinic matrix. in the case of a monolayer film, comprises one or more olefin polymers and / or copolymers obtained by polymerization in presence of catalysts of the ZIEGLER-NATTA type or of the metallocene type, used alone or as a mixture, chosen from the group consisting of plastomeric polyolefins that are the copolymers of ethylene and alpha-olefins. homopolymer polyethylenes (low radical density) or copolymers of ethylene and alpha-olefins (high density linear, low density linear, very low density linear), ethylene vinyl acetate copolymers (EVA). ethylene acrylate the (EMA). ethylene ethyl acrylate (EEA), ethylene acrylic acid (EAA) and ionomers. homopolymer polypropylenes. metallocene pol> propenes. propylene-ethylene, propylene-alpha-olefin copolymers. The polyolefins (non-polar) entering into the composition of the matrix have a density which is in the range 0.865 to 0.965 g / cm ^" 'and preferably in the range from 0.900 to 0.935 g / cm ³ .

The filler of mineral origin used in the composition of the film according to the invention can be chosen from the group consisting of calcium carbonate, talc, a clay, kaolin, silica, diatomaceous earth, magnesium carbonate , barium carbonate, magnesium sulfate, barium sulfate, calcium sulfate, aluminum hydroxide magnesium hydroxide, calcium oxide, magnesium oxide zinc oxide, titanium oxide, alumina, mica, glass powder, a zeolite or the like, and preferably calcium carbonate and barium sulphate, used alone or as a mixture.

The filler of organic origin entering into the composition of the film according to the invention can be chosen for example from the group consisting of cellulose powders, polymer powders, such as polyurethane, epoxy. of ABS. polyfluorinated. polyamides, polyesters or other polymers, some of these powders being prepared by cryogenic grinding, used alone or as a mixture.

The particle size of the mineral and / or organic filler is generally chosen to be less than 40 μm and preferably between 0.5 and 10 μ. The amount of filler used in the preparation of the film composition is generally between 50 and 500 parts by weight and preferably from 80 to 400 parts by weight per 100 parts by weight of the polyolefin matrix. it being understood that outside these extreme limits, the porosity of the film can be made insufficient by default of the load and the rigidity of the film made too high by excess of said load, causing, in the latter case. a difficult stretching operation.

Before introducing it into the polyolefinic matrix. it may be advantageous to subject the mineral filler to a surface treatment using a well-known treatment agent, chosen from the group of fatty acids or their salts, silicone oils and silanes, this treatment promoting the good dispersion of the mineral filler within said matrix.

But it may also be interesting to introduce into the polyolefinic matrix. simultaneously with the mineral and / or organic filler, a dispersing agent (in small quantity) having the property of promoting the dispersion of the filler within the polyolefin matrix, of enabling the quantity of the filler in said matrix to be increased and to prevent the formation of agglomerates of filler grains in the sheet extruded by casting and then in the film obtained by stretching. Such agglomerates can be the cause of instantaneous drawbacks (tears) or later drawbacks (local macroporosities and passage of liquids) which are detrimental to the quality of the microporous film. Such an agent, used according to the invention, can correspond to the following general formula which comprises at least one free acid function:

R- (P) _λ - (Q) _r Z in which (P) represents ethylene oxide (Q) represents propylene oxide. x and y taking values such that their cumulation is included in the range 0 to 24, R is a linear or unsaturated, saturated or unsaturated alkyl radical, an aryl radical. an alk> 1- aryl radical, a saturated or unsaturated heterocycle. containing from 5 to 28 carbon atoms and preferably from 8 to 24 carbon atoms, or also a steroid radical. Said radical R can also be branched or not and / or comprise one or more functional groups of the halogen -OH type. -COOH, -COOR. -NO ₂ . -NH ₂ , -CONH ?, -CN, sulfonic, sulfuric. phosphonic, phosphoric. while Z can be one of the carboxylic functions. sulfonic. sulfuric, phosphonic. phosphoric.

In the case where Z comprises several acid functions, at least one of them is free, the others being able to be salified or esterified by means of an alcohol of formula R'-OH in which the radical R 'can be a carbon chain containing 1 to 4 carbon atoms or one of the radicals belonging to the group previously defined for R, the radical R 'being able to be identical to the radical R. Thus, agents for treating the mineral and / or organic charge and / or dispersing agents of said mineral and / or organic filler within the polyolefinic matrix contribute to obtaining the best dispersion of the filler in the polyolefinic matrix and thereby to increasing the drawing capacity of the film by further improving its fineness characteristics (reduction in final thickness, and in microporosity, more breathable film). The mixture formed by the polyolefin matrix and the filler can also receive other active agents such as lubricants, pigments, dyes, thermal stabilizers, photochemical stabilizers, flame retardants, plasticizers, antistatics. This mixture can lead, in particular, and preferably, to the production of a microporous bactericidal and acaricidal film by the introduction (into said mixture) of suitable biocidal agents.

In the case where the polyolefin microporous film has several layers, the composition of the main layer is the same as that stated in the case of a breathable monolayer microporous film. The other layer or layers called specific layers associated with the main layer are produced by means of particular compositions which differentiate them from the main layer and which can also differentiate them from one another in the case of a multi-layer film. each specific layer can have a particular function. The polyolefinic matrix of the specific layers is composed of at least one specific olefinic polymer and / or copolymer by the effect which it gives to the layer containing it, and more especially the polyolefins of very low density (0.865 to 0.915 g / cm3) and / or low melting point (60 to 100 ° C). polyolefins of higher density (from 0.920 to 0.965 g / cm ¹ ) and / or of high melting point (from 100 to 160 ° C), polyolefins with polar tendency, in particular ethylene-acetate copolymers of vinyl, ethylene methyl acrylate. ethylene ethyl acrylate. ethylene acrylic acid, ionomers. polyolefins modifying the surface condition of the layer, in particular to allow good adhesion with polar products such as cellulosic fibers (paper), polyester fibers, viscose fibers and others.

However, the polyolefinic matrix of the specific layers can also comprise the olefinic polymer (s) and / or copolymers entering said main layer. The polyolefin matrix of each specific layer consists of 5% to 100% by weight, and preferably, from 15% to 100% by weight relative to the total mass of said matrix of at least one specific olefin polymer and / or copolymer and at most 95%, and preferably, 85% of at least one olefinic polymer and / or copolymer entering into the composition of the matrix of the main layer. The layers other than the main layer may also contain at least one mineral and / or organic filler chosen from the groups mentioned above and in quantities chosen in the intervals also already mentioned in the case of the main layer. The preparation of the mixture according to the invention, comprising a polyolefinic matrix and an inorganic and / or organic filler, can be carried out according to known methods, it being understood that the means used for the application of these methods well lead to obtaining a molten mixture by casting extrusion in which the mineral and / or organic filler is correctly dispersed, that is to say free of agglomerates. One of the methods may consist in using the various constituents in a twin-screw extruder with multiple feeds and in extruding the molten mixture by casting. Another method can consist in carrying out the preparation by extrusion of granules of a masterbatch by dispersing the charge in a fraction of the polymers constituting the polyolefinic matrix, then in diluting this masterbatch in the remaining fraction of the polyolefinic matrix. or to use a masterbatch representing the essential part of the composition, and to extrude the molten mixture by casting.

Process for producing the film As soon as each mixture necessary for the production of each layer (consisting of the polyolefin matrix and the filler according to the layer to be produced) is prepared, these mixtures can be used in the molten state for the production of a breathable microporous film formed from one or more layers. According to the method of the invention for producing a microporous film comprising at least one layer, the molten mixture is hot extruded by casting through a flat die, to form a molten sheet by means of the polyolefin matrix. with a thickness between 300 and 2000 μm at the outlet of the die. The temperature of the die and of the melted sheet at the time of extrusion is between 170 ° C and 270 ° C. The thick web formed is pre-stretched by a drive cylinder, of generally metallic surface, associated with a counter-cylinder of rubberized surface for example, or with an air knife, by subjecting it to a stretching whose ratio is generally between 4 and 25, and preferably between 5 and 10 between the outlet of the extrusion die and the coming into contact of the drive cylinder. According to the invention, the web pre-stretched by the drive cylinder is partially cooled there in contact with it to the temperature necessary for its subsequent stretching, simultaneously solidified in a controlled manner. To do this, the temperature of the cylinder is adjusted so that the temperature of the sheet in contact with it is brought by controlled cooling to the temperature necessary for its drawing. that is to say brought into a range of temperatures practiced to carry out the stretching of the sheet under the best thermal conditions: the temperature given to the sheet in contact with the drive cylinder is chosen in the range of 40 ° C to 165 ° C and. preferentially. in the range of 55 ° C to 135 ° C. The surface of the web drive cylinder may be a smooth surface of the glossy or matt type.

However, to avoid the risk of the pre-stretched web sliding on the drive and cooling cylinder, the surface of said cylinder can be formed from a surface structure with a high coefficient of friction, either by the material used. for said surface, either by creating on this surface a regular or irregular, or even random, structure of the surface, such as for example a sanded, satin, dotted, or other aspect.

During the pre-stretching of the sheet, of its cooling adjusted to a determined temperature and of its partial solidification, said sheet can also undergo an embossing operation by means of said cylinder. In the latter case, the surface of the drive cylinder is provided with a regular or irregular pattern, in relief or in hollow, the most frequent shapes of which may be lines in the longitudinal direction, transverse, diagonal to the direction of advancement of the tablecloth, parallel or intersecting to form geometric figures, more or less open rafters, sinusoids, teeth, or even figures such as polygonal or circular (hexagon, rhombus, circle, for example) , or random distribution figures.

The contact time of the web with the drive cylinder which allows it to be placed at the best temperature level for drawing by its controlled cooling can be adjusted by the use of a counter cylinder placed in the tangential outlet zone of said tablecloth. This counter cylinder makes it possible to fix the length of the arc of contact of the ply with the drive cylinder, that is to say to adjust the extent of the embracing of the cylinder by the ply by imposing on it the moment of exit tangential of said cylinder. Thus, the web is best prepared to undergo stretching in the best conditions.

According to the invention, the stretching of the pre-stretched sheet. brought to the desired temperature (for said stretching) in contact with the drive cylinder by partial cooling, adjusted and controlled, is immediately carried out in a single step by traction by means of a pair of rollers at the instant when the web at adequate temperature separates tangentially from said drive cylinder.

Unlike the drawing bench conventionally used in the state of the art, which comprises two pairs of rollers rotating at different speeds, one for drawing and the other for drawing the web, the cylinder d drive of the sheet, also ensuring its pre-stretching, its partial cooling, adjusted and controlled, also plays the role of call cylinder delivering the sheet to be stretched when it has been brought to the most favorable temperature for this surgery. From the tangential outlet of the drive cylinder, the web to be stretched is taken up by a pair of rollers, placed downstream of said cylinder, which constitutes the actual drawing torque of the web delivered by applying to it the desired stretching rate. . This pair of drawing rollers, placed downstream of the drive cylinder, is brought to a temperature between 40 ° C and 165 ° C and preferably between 55 ° C and 135 ° C: this temperature can be the same or different that of the drive cylinder. The stretching rate according to the invention, given by the ratio of the linear speeds delivered by the couple of the stretching rollers and the drive cylinder (pre-stretching, cooling and drawing) of the web brought to the suitable temperature for stretching is chosen in the range from 1.5 to 10 and preferably chosen in the range from 2 to 6. But it is also possible, according to the invention, that the stretching of the web (pre-stretched). brought to the desired temperature by partial cooling, adjusted and controlled in contact with the drive cylinder, takes place in several successive stages. In this case, the first stretching step, called tangential stretching, is immediately carried out (as previously mentioned) by traction using a pair of rollers placed downstream of the drive cylinder where the web (which is placed at the appropriate stretching temperature between 40 ° C and 165 ° C) tangentially separates from said drive cylinder. The stretching of the web, in this first step, represents a fraction of the total desired stretching: this fraction is between 80% and 20% of the total stretching envisaged.

At the end of this first stretching step, the partially stretched film can undergo relaxation by passing over an appropriate cylinder (called relaxation cylinder) the temperature of which is chosen in the range from 125 ° C. to 20 ° C. At the exit of the relaxation cylinder, the relaxed film is reheated and brought to the desired temperature by passing over a reheating cylinder, for the continuation and / or completion of the stretching: the reheating temperature of the partially stretched film at during the first stage is chosen in the range of 40 ° C to 165 ° C the output of the reheating cylinder, the partially stretched film, brought to the desired temperature for further stretching, enters a zone of conventional stretching which comprises two pairs of rollers rotating at different speeds, one for drawing and the other for stretching the web: in this conventional stretching zone, the stretching of the film can be completed.

In the case where the stretching of the web is carried out in two successive stages, the stretching fraction practiced in the second stage is between 20% and 80% of the total stretching envisaged.

In the case where the stretching of the web is carried out successively in three or more stages, the stretching fraction practiced in the first stage is between 20% and 70% of the total stretching envisaged, the stretching fraction practiced in the second stage is between 15% and 70% of the total stretching envisaged and the stretching fraction practiced in the other stages, including the final stage of stretching (by sharing) is between 15% and 70 % of the total stretch envisaged.

When the number of stretching steps is greater than two, downstream of the second stretching step, the same means as previously mentioned are found, of relaxation of the film, reheating the film, stretching the film and relaxation-cooling of the film in the end, means which are repeated as many times as there are stretching steps. The complete cooling of the microporous film at the drawing outlet can be ensured by an appropriate relaxation or cooling station, formed for example of one or more cascade cylinders, providing a decreasing temperature to the temperature desired by the storage. .

Beyond the stretching zone of the web and its transformation into a microporous film. it can be practiced a heat treatment of the stretched film making it possible to release the tensions induced in the film throughout the manufacturing process. However, it is recalled however that one of the objects of the invention being to produce a microporous film (at least monolayer) having a physical state that is as isotropic as possible at the end of the stretching of the web, the internal tensions mentioned are less strong (in the film according to the invention) than those developed in films according to the state of the art: - since, according to the invention, the pre-stretched sheet is brought to its drawing temperature by the cylinder d 'training.

- whereas, according to the state of the art, the pre-stretched sheet is bluntly cooled by the drive cylinder to a temperature lower than that necessary for drawing and is subsequently reheated to the drawing temperature in order to be able to to be stretched.

The stretching method practiced according to the invention is much milder than that practiced in the prior art.

In the same way that the extruded web can undergo an embossing operation by means of the drive cylinder, the film can undergo an embossing operation in various zones of the process according to the invention.

In the case where said method comprises only one stretching step (called tangential stretching). the film can undergo an embossing operation in one and / or the other following zone (s) of said process. a) in the tangential stretching zone, using the pair of web stretching rollers, placed downstream of said drive cylinder, as an embossing means, d) in the relaxation zone of the stretched film, using at least one of the relaxation cylinders as embossing means In the case where said method comprises several stretching steps, the film can undergo an embossing operation in one and / or the other (s) following zones of said process: a) in the tangential stretching zone using, as previously expressed, the pair of rolls for drawing the web, placed downstream of said drive cylinder, as an embossing means. b) in the reheating zone which precedes the second stretching step, using the reheating cylinder as an embossing means. c) in the second stretching zone, where the stretching of the film from the tangential stretching continues. comprising two pairs of rollers rotating at different speeds, one of the torques being in demand and rotating at a speed close to the reheating cylinder, the other of the stretching rotating at a speed greater than the inrush torque of the film to stretching, using one and / or the other pair of rollers as embossing means. d) in the relaxation zone of the stretched film, using at least one of the relaxation cylinders as an embossing means. Just as the web can be embossed before stretching, the film can be embossed in the stretching, relaxation, reheating and relaxation-cooling zones, via the surface of at least one rollers of the stretching and / or drawing pairs, relaxation and / or reheating and / or relaxation-cooling cylinders: the surface of the roller (s) and / or of the cylinder (s) intended for embossing the film is provided of a regular or irregular pattern, in relief and _' or hollow, the most frequent shapes of which may be, for example, lines in the longitudinal direction, transverse, diagonal to the direction of advancement of the film, parallel or intersecting to form geometric figures, more or less open rafters, sinusoids, teeth, or even figures such as polygonal or circular (hexagon. rhombus, circle) or random distribution figures. In the case where the embossing is desired for the appearance and the final qualities of the film, this embossing can be carried out once or several times in the process according to the invention.

When the embossing is done only once, it can be done on the web before stretching by means of the drive cylinder (zone a)) or on the film in one of the zones b) or c) or d) previously mentioned.

When embossing is done several times, it can be done by combining, for example: the embossing of the web (zone a)) with the embossing of the film in at least one of zones b), c), d) , - the embossing of the single film by combining at least two of the embossing zones b), c). d).

Similarly, the microporous film according to the invention being stretched in the longitudinal direction, can also undergo beyond this first stretching another stretching, but transverse. In this case, the transverse stretching is practiced by means of known devices, with a stretching ratio generally between 1, 1 and 10 and preferably between 1.1 and 4.

Finally, at the end of these various stages, practiced within the framework of the process according to the invention, the microporous stretched film is stored continuously on a suitable support at a linear winding speed lower by 0.5% to 25% at the speed linear film from the previous step, to allow it to relax, in particular the longitudinal stretching step when the heat treatment step is not performed.

Thus, the method according to the invention leads to a reduction of the stretching efforts thanks to a stretching carried out at the most favorable temperature without having subjected it to an uncontrolled cooling followed by significant heating to satisfy the thermal conditions of the drawing.

When the process according to the invention relates to the manufacture of a single-layer microporous film, that is to say formed from the single main layer with high resistance mechanical, this film is produced by means of a single extruder feeding the flat extrusion die by casting;

However, when the process according to the invention relates to the manufacture of a multilayer microporous film, the film can be produced by means of several extruders. that is to say as many extruders as there are layer compositions, which feed a flat die provided with a coextrusion device.

The molten sheet, formed of several layers coextruded by casting, then undergoes the same steps as those mentioned for the formation of a monolayer microporous film, that is to say:

- pre-stretching by the drive cylinder:

- The partial and limited cooling of the pre-stretched sheet in a controlled manner by bringing it by means of the drive cylinder to a temperature lying in the desired temperature range for its stretching;

- the stretching by traction of the sheet at the moment when the tangential separation of said drive cylinder occurs, possibly adjusted by a downstream counter cylinder, this cylinder also playing the role of roll of roll of the sheet for drawing . To demonstrate the effect of the invention on the microporous films obtained according to the method, an orientation index of the physical state of the film or of its tendency to isotropy was established. This index is formed by the ratio of the values of the mechanical characteristics of the films, measured in the machine direction and in the transverse direction (SM / ST).

In the case of an absolutely isotropic film, this ratio is equal to 1. In the case of a film completely oriented in the machine direction, that is to say non-isotropic, this ratio tends towards infinity. When the mechanical characteristics of the films, such as the breaking load, the elongation at break, the loads at 5%. 10%. 15% and 25% of elongation are measured, the index of the film according to the invention is always between the value 1 and the value of the index of the film produced according to the state of the art. More precisely, and for the abovementioned mechanical characteristics (except for elongation at break), the index of the film according to the invention is 1.4 to 2 times lower than the index of the film according to the state of the art and tends towards the value 1, showing its capacity to be more isotropic than films according to the state of the art. The microporous film according to the invention can be used alone or combined with other porous supports, such as, for example, non-woven fabrics, woven fabrics, knits, mesh-type meshes, paper, for applications miscellaneous such as personal care articles, dressings, medical articles, protective clothing, sports clothing, insulating coatings in construction, and others.

To better understand the invention, the method is described according to the single figure, a particular case of the method with two-stage stretching which has no limiting effect on its scope.

According to this figure, the molten mixture consisting of the polyolefin matrix and the filler is used by extrusion. through the flat die (1) at the temperature proper to said mixture.

The thick web formed (2) is pre-stretched by the drive cylinder (3) with a matified metallic surface associated with a counter cylinder (4) with a rubberized surface, according to an appropriate and chosen stretch. The surface of the drive cylinder can be provided with a regular or irregular pattern, recessed or in relief, to allow the embossing of the web. The pre-stretched web (2) by the drive cylinder is partially cooled there in contact with it to the temperature necessary for the first step of a two-step stretch. The contact time of the ply (5) with the drive cylinder (3) is adjusted by the counter cylinder (6) which. according to its position, increases or decreases the length of the arc for embracing the cylinder (3) by the ply (5): the tangential exit from the ply (5) is controlled by the counter cylinder (6).

From the tangential outlet of the drive cylinder (3), the sheet to be stretched (5) is taken up by a pair of rollers (8) and (9), placed downstream of said cylinder (3) which constitutes the tangential stretching torque of the first partial stretching step, applying the desired stretching rate to it. Thus, the ply (5) is stretched according to the film (7), between the drive cylinder (3) and the pair of rollers (8) and (9). The cylindrical surface of at least one of the rollers (8) and (9) can be provided with patterns, as previously mentioned, to allow the film to be embossed.

At the end of this first stretching step, the partially stretched film (10) undergoes relaxation by passing over the cylinder (11), the temperature of which is adjusted in the interval mentioned above. At the outlet of the relaxation cylinder, the relaxed film (10) is reheated on the cylinder (12) and brought into contact with it at the desired temperature to complete its stretching in a second step. The temperature of the cylinder (12) is adjusted to a value chosen in the interval previously mentioned. In addition, the cylindrical surface of the cylinder (12) can be provided with patterns, as previously mentioned, to allow the film to be embossed when it is reheated. In the case of embossing of the relaxed and reheated film, the cylinder can be provided with a counter-cylinder with a rubberized surface (not shown).

The partially stretched film (13), brought to the desired temperature, enters a conventional stretching zone (second stretching step) which comprises two pairs of rollers rotating at different speeds, one of which is called [rollers ( 14) and (15)], the other stretching [rollers (17) and (18)]: the stretching of the film (13) ends between these two pairs of rollers according to the film (16). The cylindrical surface of at least one of the rollers (14) and (15) and / or the rollers (17) and (18) can be provided with hollow or raised patterns (as indicated above) to allow the film embossing. At the exit of the pair of rollers (17) and (18), from the second stretching step, the stretched film (19) enters a relaxation station comprising in cascade the cylinders (20), (21) and (22 ) delivering the film (23) stretched and optionally embossed, at a temperature chosen for its storage on a reel. The cylindrical surface of the cylinder (20) can be provided with patterns such as those previously indicated, to allow embossing of the film beyond the final stretching step. Thus, the embossing can be carried out either on the web, or on the film, or successively on the web and then the film, in as many steps deemed desirable for the final quality of the film.

The invention will be better understood from the examples given below, the physical properties of the films obtained having been determined according to the ASTM standards indicated.

Example 1

To allow the establishment of the orientation index of the physical state (aforementioned) of films produced according to the invention and according to the state of the art, three films were produced (reference 3 for the invention, references 1 and 2 for the state of the art) on the same flat die extrusion line of a pilot installation which included:

- a 45 mm diameter COLLI extruder (300 mm die, 0.6 mm opening);

- a drive-cooling-embossing cylinder (diameter 125 mm) and a rubber counter-cylinder (diameter 125 mm).

The composition of the three films was the same: it consisted of a mixture of a polyolefinic matrix and a CaCO ₃ charge (of diameter between 1 and 5 μm) at a rate of 1 10 parts by weight of the charge per 100 parts by weight of the matrix. The polyolefinic matrix was formed of:

- a 50/50% mixture of a homopolymer

LDPE and a copolymer of ethylene and 42.6% of alpha-olefin constituting the polyolefin part of the masterbatch marketed under the reference AMPACET 100196 by the company AMPACET EUROPE.

- ethylene-octene copolymer (plastomers) marketed under the brand name AFFINITY PL 1845 57.4% by the company DOW CHEMICAL density: 0.910 g / cm melt index: 2.5 g / 10 min (ASTM 1238)

The layers (references 1, 2 and 3) obtained by extrusion casting the mixture, then pre-stretching by means of the drive-cooling-embossing cylinder had thicknesses at the inlet of the cylinder of 80 to 85 μm.

The film (reference 3) according to the invention was partially cooled on the drive-cooling-embossing cylinder to the temperature of 60 ° C desired for stretching and immediately stretched to this temperature at a rate of 4 from the tangential outlet of said cylinder regulated by the presence of a counter cylinder.

The films (references 1 and 2) were cooled on the drive-cooling-embossing cylinder to a temperature of 30 ° C. and then, after leaving the cooling zone, were reheated to 60 ° C. for the reference film 1. 80 ° C for the reference film 2, and drawn at these temperatures at a rate of 4 on an appropriate drawing bench. All the physical properties of the films, as well as the corresponding index, have been collated in Table 1 below.

Table 1

(x) according to the state of the art (xx) according to the state of the art (xxx) according to the invention

Example 2

A three-layer polyolefin microporous film, permeable to gases and impermeable to liquids, was produced by the process of the invention by means of an industrial installation, the specific layers being at low melting point to subsequently facilitate hot assembly of the said film with a nonwoven fabric. The main (internal) layer of the finished film was 20 μm thick while the specific (external) layers each had a thickness of 2.5 μm.

For the production of such a film, the composition of the main layer was the same as that described in Example 1.

As for the two specific (external) layers of the same composition, they also consisted of a polyolefin matrix, a mineral filler and various specific agents, the mineral filler being present in the mixture at a rate of

1 10 parts by weight per 100 parts by weight of the olefinic matrix.

The polyolefin matrix of the specific layers was formed of three polymers according to the following weight percentages: - 50/50% mixture of a homopolymer

LDPE and a copolymer of ethylene and alpha-olefin, constituting the polyolefin part of the 35.8% masterbatch sold under the reference AMPACET 100196 by the company AMPACET EUROPE. - very low density polyethylene marketed under the brand AFFINITY KC 8852 by the company DOW CHEMICAL 64.2%

* density: 0.875 g / cm ^J

* fluidity index: 3 g / 10 min The mineral charge was composed of CaCO ₃ , the grains of which had a diameter between 1 μm and 5 μm.

Various agents, such as biocide (in particular acaricide). lubricant, thermal stabilizer, ... have been introduced into the matrix in the usual amounts.

Each dry mixture intended for each layer was kneaded in an extruder suitable for each layer.

The three extruders working in parallel fed a flat die fitted with a coextrusion device. The temperature of the flat die as well as of the three-layer sheet extruded by casting was around 200 ° C., and the lip of the die open between 0.3 and 2 mm.

The extruded ply, formed of the three layers, was taken up between the metallic drive cylinder and a counter-cylinder provided with a rubber coating.

The thinning of the three-layer sheet was approximately 10 times. The temperature of the metallic drive cylinder was adjusted to 65 ° C, so that the sheet was brought to this same temperature.

As in Example 1, the metallic drive cylinder acted on the molten sheet by:

- the drive function. - an effect of stabilizing the temperature of the pre-stretched sheet by requiring it to be placed at the desired temperature for the stretching step thanks to a partial and controlled cooling of the sheet extruded by casting;

- the creation of a rough configuration on the sheet, of a depth between 2 μm and 3 mm, by its embossing on the metal cylinder whose surface is provided with protruding patterns;

- By another function played which is that of the roll of call of the pre-stretched sheet and placed at an adequate temperature, delivering said sheet to stretch it, from its tangential exit. The actual stretching of the web was carried out in the longitudinal direction at the temperature of 65 ° C. with a stretching ratio of 3 to 4.

The microporous film thus obtained was not further processed: it was cut in line and then wound up for its applications.

The physical properties of the three-layer film appear in Table 2 below:

Table 2

Claims

1. Method for producing a microporous film with a strong isotropic tendency permeable to gases and impermeable to aqueous liquids, consisting of at least one layer, comprising the successive steps of: the preparation of a mixture composed of a polyolefinic matrix comprising at least one polymer and at least one mineral and / or organic filler; the extrusion of a sheet comprising at least one layer by hot casting of the mixture; pre-stretching the web by a drive cylinder; - cooling and solidifying the pre-stretched sheet, by means of the drive cylinder; the stretching of the solidified sheet, at the appropriate temperature to form the microporous film, characterized in that: - the cooling of the pre-stretched molten sheet by the drive cylinder is partial and limited in a manner adjusted to a temperature being in the temperature range necessary for its stretching, the stretching of the web brought to the necessary temperature by partial cooling is carried out by traction, at the moment when its tangential separation from said drive cylinder occurs , said cylinder acting as a take-up roll for drawing.

2. Method according to claim 1, characterized in that the tangential separation of the ply from the drive cylinder is adjusted by a counter-cylinder which fixes the length of the arc of contact of the ply with said cylinder d 'training.

3. Method according to claim 1 or 2, characterized in that when the film comprises a single layer, or main layer, the polyolefin matrix is formed of olefin polymers and / or copolymers obtained by polymerization in the presence of catalysts of the ZIEGLER-NATTA type or of the metallocene type, chosen from the group consisting of polyolefin plastomers copolymers of ethylene and alpha-olefins, polyethylene homopolymers (low radical density) or co-polymers of ethylene and alpha-olefins (linear high density, linear low density, linear very low density), ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), ethylene ethyl acrylate (EEA), ethylene acrylic acid (EAA) and ionomers , homopolymer polypropylenes, metallocene polypropylenes, propylene-ethylene, propylene-alpha-olefin copolymers.

4. Method according to any one of claims 1 to 3, characterized in that the non-polar polyolefins used in the composition of the matrix have a density in the range 0.865 g / cm 'to 0.965 g / cm ³ and preferably in the range 0.900 g / cm ³ to 0.935 g / cm ³ .

5. Method according to any one of claims 1 to 4, characterized in that the mineral filler used in the composition of the film is chosen from the group consisting of calcium carbonate, talc, a clay, kaolin, silica , diatomaceous earth, magnesium carbonate, carbonate barium, magnesium sulfate, barium sulfate, calcium sulfate, aluminum hydroxide magnesium hydroxide, calcium oxide, magnesium oxide zinc oxide, titanium, alumina, mica, glass powder, a zeolite or the like, and preferably calcium carbonate and barium sulphate, used alone or as a mixture.

6. Method according to claim 5, characterized in that the mineral filler is treated by means of a treatment agent, chosen from the group of fatty acids or their salts, silicone oils and silanes.

7. Method according to any one of claims 1 to 6, characterized in that the filler of organic origin entering into the composition of the film is chosen from the group consisting of cellulose powders, polymer powders, such as polyurethane, epoxy, ABS, polyfluorinated, polyamides, polyesters or other polymers, used alone or as a mixture.

8. Method according to any one of claims 1 to 7, characterized in that the particles of the filler have a chosen dimension of less than 40 μm and preferably between 0.5 and 10 μm.

9. Method according to any one of claims 1 to 8, characterized in that the quantity of filler entering into the composition of the film is between 50 and 500 parts by weight and preferably from 80 to 400 parts by weight per 100 parts in weight of the polyolefin matrix.

10. Method according to any one of claims 1 to 9, characterized in that a filler dispersing agent is introduced into the polyolefin matrix.

1. Method according to claim 10, characterized in that the dispersing agent is of general formula:

R- (P) _x - (Q) _y -Z which comprises at least one free acid function, in which Z is a carboxylic, sulphonic, sulfuric, phosphonic or phosphoric radical, (P) is ethylene oxide, ( Q) is propylene oxide, with 0 <(x + y) <24, R is chosen from the group formed by linear or non-linear alkyl radicals. saturated or unsaturated, aryl radicals, linear or unsaturated alkyl-aryls, saturated or unsaturated, aryl radicals, alkyl-aryls, saturated or unsaturated heterocycles, containing from 5 to 28 carbon atoms and preferably from 8 to 24 carbon atoms, or a steroid radical.

12. Method according to claim 1 1, characterized in that, when Z comprises several acid functions, at least one of them is free, the others being salified or esterified with an alcohol of formula R'-OH in which R 'is chosen from the group consisting of a carbon chain containing from 1 to 4 carbon atoms or one of the radicals belonging to the group defined for R.

13. Method according to claim 11, characterized in that R comprises at least one functional group of the halogen type, -OH, -COOH, -COOR, -NO2, - NH2, -CONH2, -CN, sulfonic, sulfuric, phosphonic, phosphoric.

14. Method according to any one of claims 11 to 13, characterized in that the radical R 'is identical to the radical R.

15. Method according to any one of claims 1 to 14, characterized in that the mixture formed by the polyolefin matrix and the filler receives other active agents such as lubricants, pigments, dyes, thermal stabilizers, photochemical stabilizers, flame retardants, plasticizers, antistatics, biocides in particular.

16. Method according to any one of claims 1 to 15, characterized in that, when the film comprises several layers, the olefinic matrix of the specific layers (other than the main layer) comprises at least one specific olefinic polymer and / or copolymer chosen from the group consisting of polyolefins of very low density (0.865 to 0.915 g / cm ³ ) and / or of low melting point (from 60 to 100 ° C), polyolefins of higher density (of 0.920 to 0.965 g / cm ³ ) and / or high melting point (from 100 to 160 ° C), polyolefins with a polar tendency, in particular copolymers of ethylene-vinyl acetate, ethylene-methyl acrylate, ethylene-acrylate ethyl, ethylene-acrylic acid, ionomers, polyolefins modifying the surface state of the layer.

17. The method of claim 16, characterized in that the polyolefin matrix of the specific layers consists of 5% to 100% by weight, and preferably, at a rate of 15% to 100% by weight relative to the total mass of said matrix, of at least one specific olefinic polymer and / or copolymer and of at most 95%, and preferably, 85% of at least one olefinic polymer and / or copolymer entering the matrix of the main layer.

18. Method according to one of claims 15 or 16, characterized in that the layers other than the main layer contain a mineral and / or organic filler chosen from the group stated for the main layer and according to the same quantitative intervals.

19. Method according to any one of claims 1 to 18, characterized in that, in the case of the manufacture of a monolayer microporous film, the molten mixture is hot extruded by casting by means of a flat die forming a molten sheet of thickness between 300 and 2000 μm at the outlet of the die.

20. Method according to any one of claims 1 to 18, characterized in that, in the case of the production of a multilayer microporous film, the molten mixtures corresponding to the various layers are extruded by at least as many extruders as there are layers of different composition, these extruders supplying a flat die provided with a coextrusion device by casting, forming a molten sheet of thickness between 300 and 2000 μm at the outlet of the die.

21. Method according to any one of claims 1 to 20, characterized in that the temperature of the molten sheet at the time of extrusion is between 170 ° C and 270 ° C.

22. Method according to any one of claims 1 to 21, characterized in that the molten sheet extruded by casting is pre-stretched by means of a drive cylinder in a ratio comprised in the interval 4 to 25 and preferably between 5 and 10.

23. Method according to any one of claims 1 to 22, characterized in that the pre-stretched web is partially cooled, in a controlled manner, by the drive cylinder, to a temperature necessary for the stretching of the tablecloth.

24. The method of claim 23, characterized in that the controlled temperature of partial cooling of the web is chosen in the range between 40 ° C and 165 ° C and preferably between 55 ° C to 135 ° C.

25. Method according to any one of claims 1 to 24, characterized in that the surface of the drive cylinder has a high coefficient of friction, smooth of the shiny or matt type.

26. Method according to any one of claims 1 to 24, characterized in that the surface of the drive cylinder is provided with a regular or irregular pattern, in relief or in hollow, in shapes which are lines in the longitudinal, transverse, diagonal to the direction of advancement of the sheet, parallel or intersecting to form geometric figures, more or less open rafters, sinusoids, teeth, polygonal or circular figures.

27. The method of claim 26, characterized in that the web, pre-stretched and cooled in a controlled manner to a determined temperature, is embossed by means of the drive cylinder.

28. Method according to any one of claims 1 to 27, characterized in that the stretching of the sheet brought to the temperature necessary for this operation, carried out by traction at the time when the tangential separation of said sheet from drive cylinder, is carried out in a single step at a temperature between 40 ° C and 165 ° C and preferably between 55 ° C and 135 ° C.

29. Method according to claim 28, characterized in that the stretching rate of the ply practiced at the outlet of the drive cylinder is chosen between 1, 5 and 10 and preferably between 2 and 6.

30. Method according to any one of claims 1 to 27, characterized in that the stretching of the sheet is carried out in several successive stages, the first stage being that carried out by traction at the instant when the separation occurs. tangential of said ply with the drive cylinder: at a temperature between 40 ° C and 165 ° C - for a stretch fraction between 80% and 20% of the total desired stretch.

31. Method according to claim 30, characterized in that, when the stretching is carried out in two stages, the second stage is carried out in a stretching zone comprising two pairs of rollers, one of call and the another drawing, at a temperature between 40 ° C and 165 ° C and for a drawing fraction between 20% and 80% of the total desired drawing.

32. Method according to claim 30, characterized in that, when the drawing is carried out in three stages, the second and third stages are carried out in separate drawing zones, each comprising two pairs of rollers, one of 'call and the other stretch, the temperature of the stretch in the three stages is between 40 ° C and 165 ° C and the stretch fraction: in the first stage is between 20% and 70% the total stretching envisaged, - in the second stage is between 15% and 70% of the total stretching envisaged, - in the third stage is between 15% and 70% of the total stretching envisaged.

33. Method according to any one of claims 30 to 32, characterized in that the partially stretched film coming from the first stretching step, is subjected to relaxation at a temperature between 125 ° C and 20 ° C, then reheating at a temperature between 40 ° C and 165 ° C, before entering the second stretching step.

34. Method according to any one of claims 30 to 33, characterized in that the stretched film is subjected to a relaxation-cooling treatment after stretching.

35. Method according to any one of claims 1 to 33, characterized in that the stretched film is subjected to at least one embossing operation in at least one of the stretching, relaxation, reheating and relaxation zones. cooling using as embossing means the surface of at least one of the rollers of the stretching and / or drawing couples, the relaxation and / or reheating and / or relaxation-cooling cylinders, said surface being with the appropriate embossing pattern.

36. Method according to any one of claims 1 to 35, characterized in that the embossing is carried out successively on the web and at least once on the film.

37. Method according to any one of claims 1 to 36, characterized in that the longitudinally stretched web is stretched transversely according to a stretching ratio between 1, 5 and 10 and preferably between

1, 5 and 4.

38. Method according to any one of claims 1 to 37, characterized in that the stretched film is stored on a suitable support, at a lower speed from 0.5 to 25% to that of the preceding stretch to allow it a certain relaxation.

39. Microporous film according to any one of claims 1 to 38, characterized in that it has an isotropic state, homogeneous, and partly amorphous.

40. Use of the polyolefin microporous film according to claim 39, alone or in combination with other porous supports, for personal care articles, dressings, medical articles, protective clothing, sportswear, insulating coatings. in the construction.