WO2019158266A1

WO2019158266A1 - Separator film having improved mechanical properties

Info

Publication number: WO2019158266A1
Application number: PCT/EP2019/000041
Authority: WO
Inventors: Bertram Schmitz; Thilo Mohr
Original assignee: Treofan Germany Gmbh & Co. Kg
Priority date: 2018-02-16
Filing date: 2019-02-14
Publication date: 2019-08-22

Abstract

The invention relates to a biaxially oriented, single- or multi-layer film having at least one porous layer that consists of propylene polymer and ß-nucleating agent. The porosity is produced by transformation of ß-crystalline polypropylene during stretching of the film, the non-stretched pre-film being stretched in the longitudinal direction and the longitudinally stretched film then being stretched in the longitudinal and in the transverse direction at the same time.

Description

Separator film with improved mechanical properties

The present invention relates to a porous film and its use as a separator, and to a process for producing the film.

Modern devices require an energy source, such as batteries or rechargeable batteries, which enable a spatially independent use. For this purpose, more and more accumulators (secondary batteries) are used, which can be recharged with the help of chargers on the mains repeatedly. For example, nickel-cadmium (NiCd) batteries can achieve a lifetime of approximately 1000 charge cycles when used properly.

Batteries consist of two electrodes that dip into an electrolyte solution and a separator that separates the anode and cathode. The different battery types differ by the electrode material used, the electrolyte and the separator used. A battery separator has the task of spatially separating the cathode and anode in batteries, or negative and positive electrodes in accumulators. The separator must be a barrier which electrically insulates the two electrodes from each other to avoid internal short circuits. At the same time, however, the separator must be permeable to ions so that the electrochemical reactions in the cell can proceed.

Furthermore, electrochemical double-layer capacitors (DSC) are becoming increasingly important as a complementary source of energy, closing the gap between conventional batteries or rechargeable batteries and capacitors. By quickly absorbing and providing a high level of electrical power, they can either power an existing power source or supplement an existing generator or bypass a momentary power failure until an emergency power can be started with a time delay. The design and manufacture of the DSK are comparable to those of lithium-ion batteries. An electrochemical double-layer capacitor consists essentially of two electrodes immersed in an electrolyte solution and separated by the separator. This separator must be porous and absorb the electrolyte. It must at the same time be permeable to the electrolyte, in particular to the ions which form by dissociation of the electrolyte salt dissolved in the electrolyte. As separators therefore porous materials are selected, for example made of paper. However, it is also possible separators made of other materials, such as plastic films, felts or fabrics made of plastic or glass fibers.

Usually, to increase the capacitance, a plurality of electrode layers and separator layers are stacked alternately one over the other, for example as a planar stack or in the form of a coil. The size of the gap between the two electrodes is determined by the thickness of the separator and possibly by any existing seals. So that the electrolyte / separator combination contributes as little as possible to the internal resistance, the separator should be thin and very porous, since the thickness is linear and the porosity approximately quadratic contribute to the electrical resistance. In addition, the mechanical stresses during the winding or stacking process on the separator high mechanical demands in particular on the mechanical strength in the longitudinal direction, so that even in cell production, the separator thickness is limited down. Later in the cell thermal stability and in the given electrolyte sufficient chemical stability are required.

To improve the internal resistance, the thickness of the separator must be reduced or its porosity increased. The increase in porosity, however, at the same time leads to the reduction of mechanical stability. At high porosity and at the same time small thickness of the separator then no longer has sufficient strength and there are problems in the processing of the separators, in the production of energy storage and ultimately in their use. Ultimately comes it reduces the lifetime and security problems of the cells. In addition, rough, granular or particulate contaminated electrode surfaces easily pierce the separator, which is why a high puncture resistance is required. Separators in which the mechanical stability is optimized in one direction only, for example in monoaxially stretched films, show an excessive tendency to splice in the longitudinal direction. Although separators made of biaxially oriented porous films have the advantage that they have a higher strength in the transverse direction than monoaxially oriented films, but the achieved strength in the longitudinal direction and the puncture strengths are no longer sufficient. US Pat. No. 7,235,203 describes that a high orientation of the β-crystallites after the longitudinal stretching advantageously contributes to the formation of a high porosity. As a result, however, these porous films are not sufficiently stable in the transverse direction. For these reasons, biaxially oriented porous polypropylene films are produced today with a minimum thickness of 20 microns.

Therefore, there is a need to improve the mechanical stability of porous films having high porosity and small thicknesses and to provide a process by which thinner biaxially oriented porous films and biaxially oriented porous films having improved longitudinal strength and transverse strength can be produced.

Various methods are known by which porous polyolefin films can be produced: filler method; Cold drawing, extraction process and ß-crystallite process. These methods are distinguished by the different mechanisms by which the pores are created.

For example, can be produced by the addition of very high amounts of filler porous films. The pores are formed during stretching due to the incompatibility of the fillers with the polymer matrix. However, the large amounts of filler of up to 40% by weight required to achieve high porosity compromise mechanical strength despite high draw considerably so that these products are not used as separators.

In the so-called extraction methods, the pores are in principle produced by dissolving out a component from the polymer matrix by means of suitable solvents. Here, a variety of variants have developed, which differ in the nature of the additives and the appropriate solvents. Both organic and inorganic additives can be extracted. This extraction can be done as the last step in the production of the film or combined with a subsequent drawing. Today, the process mainly produces polyethylene separators with good mechanical properties but low thermal resistance.

An older but successful process is based on stretching the polymer matrix at temperatures well below the melting point of the polymer (cold drawing). For this purpose, the film is first extruded and then annealed to increase the crystalline content for a few hours. In the next process step, the cold stretching is carried out in the longitudinal direction at very low temperatures in order to produce a plurality of defects in the form of the smallest microcracks. This pre-stretched film with voids is then stretched in the same direction again at elevated temperatures with higher factors, enlarging the voids to pores forming a network-like structure. These films combine high porosity and good mechanical strength in the direction of their drawing, generally the longitudinal direction. However, the mechanical strength in the transverse direction remains poor, whereby the puncture resistance is poor and results in a high splice tendency in the longitudinal direction. Overall, the process is costly.

Another known process for producing porous films is based on the admixture of β-nucleating agents to polypropylene. By the ß-nucleating agent forms the polypropylene during cooling of the melt so-called ß-crystallites in high concentrations. During the subsequent longitudinal stretching, the β phase is converted into the alpha modification of the polypropylene. Since these different crystal modifications differ in their density, many microscopic defects are also created here, which are torn to pores by stretching. This process does not require any extraction step and is therefore also referred to in the art as a dry process ("dry process") and thus delimited from the wet process with extraction ("wet process"). The films produced by this process have good porosities and good mechanical strength in the longitudinal and transverse directions and a very good economy. These films are also referred to below as porous films. However, this method can not produce high-strength films that can compete with the mechanical properties of the wet-process polyethylene separators. To improve the strength in the longitudinal direction, the orientation can be increased in the longitudinal direction. However, this leads then in the subsequent transverse extent due to the splice tendency in the longitudinal direction, to lack of running safety. The orientation or stretching in the longitudinal direction to increase strength are therefore limited. Due to the inadequate mechanical strengths, when used as a separator, these β-porous films have a thickness of at least 20 μm in practice.

US Pat. No. 7,235,203 describes β-porous films with high porosities and Gurley values of less than 500 s / 100 ml, whose porosity is improved by a high orientation in the longitudinal direction. According to this teaching, the orientation in the longitudinal direction is increased by allowing a very high jump of 25 to 50% or more in the longitudinal direction. As an alternative, a second method is described according to which needle-shaped crystals are used as β-nucleating agents. As a result of these needle-shaped crystals, the β-crystallites with a preferred orientation in the longitudinal direction are already formed on cooling of the melt to the prefilm. These longitudinally oriented crystals contribute to an increased orientation, so that after the longitudinal stretching a particularly high orientation in the longitudinal direction is present. These two methods can also be combined so that either an elongated film with a very high longitudinal orientation is obtained either via the entrance or the use of acicular crystallites or by both means. After the subsequent transverse stretching of this highly longitudinally oriented film, very high porosities are achieved. However, the high longitudinal orientation, despite the final transverse stretching leads to a strong splice tendency in the longitudinal direction. However, the tendency to splice impairs the running safety of the film during the transverse stretching and during the intended processing to the separator.

WO2011 134626A1 describes a process for producing a β-porous polypropylene film in which propylene polymer and β-nucleating agent is melted in an extruder, extruded through a flat die onto a take-off roll on which the melt film cools and solidifies to form β-crystallites. This film is then stretched longitudinally and then transversely, drawing in the transverse direction at a slow stretching speed of less than 40% / sec. By slow stretching in the transverse direction higher porosity can be achieved.

All known processes for the preparation of stretched porous films have one principle in common: In a first step, defects in the polymer matrix are produced by different mechanisms. These defects are torn in a subsequent stretching step so that a pore structure is formed, which basically gives the film a gas permeability. In filler-containing films, the particles cause a tearing between polymer matrix and filler during longitudinal stretching, so that a defect is created around the particles. In the wet process, an additive is dissolved out whose original position forms a defect. In the case of the cold-stretched films, a defect is formed at the first moderate stretching of the highly crystalline films at low temperatures. In the β-porous films, the transformation of the β-crystallites into alpha crystals produces a defect by the different density of the two crystal forms, wherein ß-crystals can be generated in different ways in the first step.

The type, size and distribution of the pores and the mechanical strengths of the films depend essentially on the basic process and differ considerably. In principle, however, the generation of a defect is always necessary in a first process step, which can then form a porous network structure of interconnected pores in a next stretching step.

It is known from the films of the particle method with high filler quantities and of opaque packaging films that incompatible fillers in a polypropylene matrix in the sequential biaxial orientation form a structure with vacuoles around the particles. Basically, the formation of vacuoles in the particle process relies on the generation of microcracks at the interface between the polymer and the particulate additive during longitudinal stretching. During the subsequent transverse stretching, these fine longitudinal cracks tear open to air-filled closed cavities or to a network of pores. It has been tried several times to produce these types of film by means of a simultaneous biaxial stretching. However, it was found that no comparable results could be achieved. The porosity of the high filler content films was orders of magnitude lower than sequential stretching. Even with the packaging films where the requirements for vacuolation are lower, could not be produced by LISIM process in comparable quality. Only with selected particle shape or particle size (see WO03 / 033574) was it possible to generate a small proportion of vacuoles. In the art, it is believed that simultaneous stretching does not create enough primary imperfections around the fillers since the orientation occurs simultaneously in both directions. Therefore, an alternative foaming agent technology has been developed for this process. To date, are not simultaneously-stretched Polypropylene films with vacuole-initiating fillers are available on the market, be they opaque packaging films or porous films.

The object of the present invention was to provide a thin ß-porous film which has a high permeability and is improved in mechanical strength and thus can be used in reduced thicknesses as a separator in a variety of cell applications.

The problem underlying the invention is thus achieved by a biaxially oriented, single-layer or multi-layered film having at least one porous layer, this porous layer containing at least one propylene polymer and β-nucleating agent and the porosity of this porous layer by conversion of β-crystalline Polypropylene is produced in the stretching of the film, wherein the unstretched prefilm stretched in the longitudinal direction and then the longitudinally stretched film is stretched simultaneously in the longitudinal direction and in the transverse direction.

Surprisingly, it is then possible to simultaneously stretch a longitudinally stretched film of polypropylene and β-nucleating agents in the longitudinal direction and in the transverse direction to form a porous film. The films are characterized by a high porosity and excellent mechanical properties.

It has been found within the scope of the present invention that it is possible to simultaneously stretch a polypropylene film which contains β-nucleating agents and has already been oriented longitudinally in a separate stretching step in such a way that the porous network structure known per se is formed. The film is extruded in a conventional manner and cooled so that a high content of ß-polypropylene is formed and then first stretched in the longitudinal direction and then directly simultaneously. Surprisingly, a plurality of interconnected pores are formed during the simultaneous drawing, so that an open-pore porous film is obtained. The problem underlying the invention is thus also achieved by a process for producing a biaxially oriented, single-layer or multilayer porous polypropylene film in which propylene polymer and ß-nucleating agent is melted in an extruder and extruded through a flat die onto a take-off roll on which the melt film under Formation of β-crystallites is cooled and solidified and the unstretched prefilm is stretched in the longitudinal direction and the longitudinally stretched film is then stretched simultaneously in the longitudinal direction and in the transverse direction.

The subclaims specify preferred embodiments of the film according to the invention or the process according to the invention.

The film according to the invention comprises at least one porous layer which is composed of propylene polymers, preferably propylene homopolymers and / or propylene block copolymers, and contains β-nucleating agents. Optionally, other polyolefins may additionally be included in minor amounts, provided they do not adversely affect porosity and other essential properties. In addition, if appropriate, the porous layer additionally contains customary additives, for example stabilizers and / or neutralizing agents, in respective effective amounts.

Suitable propylene homopolymers contain from 98 to 100% by weight, preferably from 99 to 100% by weight of propylene units and have a melting point (DSC) of 150 ° C or higher, preferably 155 to 170 ° C, and generally a melt flow index of 0.5 to 10 g / 10 min, preferably 2 to 8 g / 10 min, at 230 ° C and a force of 2.16 kg (DIN 53735). Isotactic propylene homopolymers having an n-heptane soluble content of less than 15% by weight, preferably from 1 to 10% by weight, are preferred propylene homopolymers for the layer. Advantageously, isotactic propylene homopolymers having a high chain isotacticity of at least 96%, preferably 97-99% ( ¹³ C NMR, triad method) are used. These raw materials are called HIPP Polymers (high isotactic polypropylenes) or HCPP (high crystalline polypropylenes) are known in the art and are characterized by high stereoregularity of the polymer chains, higher crystallinity and higher melting point (compared to propylene polymers having a ¹³ C NMR isotacticity of 90 to <96%, which can also be used).

Propylene block copolymers generally have a melting point above 140 to 170 ° C, preferably from 145 to 165 ° C, especially 150 to 160 ° C and generally a melt range of above 120 ° C, preferably in a range of 125-140th ° C begins. The comonomer, preferably ethylene content is for example between 1 and 20 wt .-%, preferably 1 and 10 wt .-%. The melt flow index of the propylene block copolymers is generally in a range of 1 to 20 g / 10 min, preferably 1 to 10 g / 10 min.

Optionally, the porous layer may additionally contain other polyolefins, provided that they do not adversely affect the properties, in particular the porosity and the mechanical strengths. Other polyolefins include, for example, random copolymers of ethylene and propylene having an ethylene content of 20% by weight or less, random copolymers of propylene with C ₄ -C ₈ olefins having an olefin content of 20% by weight or less, terpolymers of propylene, Ethylene and butylene having an ethylene content of 10% by weight or less and having a butylene content of 15% by weight or less, or polyethylenes such as LDPE, VLDPE, and LLDPE.

In a preferred embodiment, the porous layer is composed only of propylene homopolymer and / or Propylenblockcopolmyer and ß-nucleating agent, and optionally stabilizer and neutralizing agent.

Basically, all known additives which promote the formation of β crystals of the polypropylene during cooling of a polypropylene melt are suitable as β-nucleating agents for the porous layer. Such β-nucleating agents, as also their mode of action in a polypropylene matrix, are per se known in the art and will be described in detail below.

From polypropylene, various crystalline phases are known. When a melt is cooled, the a-crystalline PP whose melting point is in the range from 155 to 170 ° C., preferably 158 to 162 ° C., is usually formed predominantly. By a certain temperature control, a small proportion of β-crystalline phase can be produced during cooling of the melt, which has a significantly lower melting point than the monoclinic a modification at 145-152 ° C., preferably 148-150 ° C. In the prior art, additives are known which lead to an increased proportion of the β-modification during cooling of the polypropylene, for example γ-quinacridones, dihydroquinacridines or calcium salts of phthalic acid.

For the purposes of the present invention, highly active β-nucleating agents are preferably used which, on cooling a propylene homopolymer polymer melt, produce a β content of 40-95%, preferably of 50-85% (DSC). The β content is determined from the DSC of the cooled propylene homopolymer melt. For example, preference is given to a two-component β-nucleation system composed of calcium carbonate and organic dicarboxylic acids, which is described in DE 3610644, to which reference is hereby expressly made. Particularly advantageous are calcium salts of dicarboxylic acids, such as calcium pimelate or calcium suberate as described in DE 4420989, to which also expressly incorporated by reference. The dicarboxamides described in EP-0557721, in particular N, N-dicyclohexyl-2,6-naphthalenedicarboxamides, are also suitable β-nucleating agents.

In addition to the β-nucleating agents, maintaining a certain temperature range and residence times at these temperatures during cooling of the melt film is important for achieving a high level of β-crystalline polypropylene in the unstretched prefilm. The cooling of the Melting film is preferably carried out at a temperature of 60 to 140 ° C, in particular 80 to 135 ° C, for example 95 to 130 or 1 10-125 ° C. Slow cooling also promotes the growth of β-crystallites, therefore, the withdrawal speed, ie the rate at which the melt film passes over the first chill roll, should be slow so that the residence times at the selected temperatures are sufficiently long. The take-off speed is preferably less than 25 m / min, in particular 1 to 20 m / min. The residence time of the melt film is generally 20 to 300 seconds; preferably 30 to 200s.

The porous layer generally contains 45 to <100% by weight, preferably 50 to 95% by weight, of propylene homopolymer and / or propylene block copolymer and 0.001 to 5% by weight, preferably 50 to 10,000 ppm of at least one β-nucleating agent on the weight of the porous layer. In the event that further polyolefins are contained in the layer, the proportion of propylene homopolymer or of the block copolymer is reduced accordingly. In general, the amount of the additional polymers in the layer will be 0 to <10% by weight, preferably 0 to 5% by weight, in particular 0.5 to 2% by weight, if these are additionally present. Likewise, said propylene homopolymer or propylene block copolymer portion is reduced when higher levels of up to 5 weight percent nucleating agent are employed. In addition, the porous layer conventional stabilizers and neutralizing agents, and optionally further additives, in the usual small amounts of less than 2 wt .-%.

In a preferred embodiment, the porous layer is composed of a mixture of propylene homopolymer and propylene block copolymer. The porous layer in these embodiments generally contains from 50 to 85% by weight, preferably from 60 to 75% by weight, of propylene homopolymers and from 15 to 50% by weight of propylene block copolymers, preferably from 25 to 40% by weight, and from 0.001 to 5 % By weight, preferably 50 to 10,000 ppm of at least one β-nucleating agent, based on the weight of the layer, and optionally the additives already mentioned, such as stabilizers and neutralizing agents. Again, it is true that further polyolefins may be present in an amount of 0 to <10 wt .-%, preferably 0 to 5 wt .-%, in particular 0.5 to 2 wt .-%, and the proportion of the propylene homopolymer or the Block copolymer is then reduced accordingly.

Particularly preferred embodiments of the porous film according to the invention contain from 50 to 10,000 ppm, preferably from 50 to 5000 ppm, in particular from 50 to 2000 ppm of calcium pimelate or calcium suberate as β-nucleating agent in the porous layer.

It has been found that by simultaneous stretching in the longitudinal and transverse directions very thin ß-porous polypropylene films with a thickness of less than 20 .mu.m can be made safe to run. Surprisingly, these thin β-porous films have very good mechanical strengths in both directions. In particular, the film exhibits an unexpectedly high longitudinal tear strength. Also, the transverse strengths were increased over sequentially stretched films. In addition, a significant increase in puncture resistance compared to sequentially stretched films is achieved. It was originally expected that, compared with a sequentially stretched film, increasing the tear strength or increasing the modulus of elasticity in the longitudinal direction would inevitably be accompanied by a substantial impairment of the corresponding values in the transverse direction. This is surprisingly not, or so little pronounced the case that the film produced by the process according to the invention despite the very small thickness of less than 20 microns in the usual processing and application processes can be used. The film withstands the mechanical stress that occurs in these processes, the mechanical strengths are surprisingly so good in both directions, so that the film can be processed trouble-free and safe to run. The porous film may be one or more layers. The thickness of the film is generally in a range of 3 to 100 μm, preferably 3 to 60 μm, in particular 3 to 50 μm. Surprisingly, film thicknesses of less than 10 .mu.m can be realized with the process according to the invention. According to the invention, these porous thin films have a thickness of 3 to <10 .mu.m, preferably a thickness of 3.5 to 8 .mu.m, in particular a thickness of 4 to 7 .mu.m.

In a multilayered embodiment, the film comprises further porous layers which are constructed as described above, wherein the composition of the different porous layer need not necessarily be identical. For multilayered embodiments, the thickness of the individual layers is generally 1 to 50 μm.

The density of the β-porous film is generally in a range of 0.1 to 0.6 g / cm ³ , preferably 0.2 to 0.5 g / cm ³ . For the use of the film as a separator, the film generally has a Gurley value of 10 to 500 s, preferably a Gurley value of 10 to 200 s. The bubble point of the film is generally not more than 350 nm, preferably in the range from 20 to 300 nm, and the mean pore diameter is generally in the range from 40 to 100 nm, preferably in the range from 50 to 80 nm.

The present invention further relates to a process for producing the biaxially oriented β-porous film. According to this method, the porous film is produced by the known flat film extrusion or coextrusion process. In this process, the procedure is such that propylene homopolymer and / or propylene block copolymer and ß-nucleating agent and optionally further polymers of the respective layer is mixed, melted in an extruder and, optionally together and simultaneously, extruded through a flat die on a take-off roll or coextruded / be on which solidifies the one- or multi-layer melt film to form the ß crystallites and cools. The cooling temperatures and cooling times are chosen so that the highest possible proportion of ß-crystalline polypropylene in the unstretched prefilm arises. In general, this temperature of the take-off roll or the take-off rolls is 60 to 140 ° C, preferably 80 to 130 ° C. The residence time at this temperature may vary and should be at least 20 to 300 seconds, preferably 30 to 100 seconds. The prefilm thus obtained generally contains a proportion of β-crystallites of 40-95%, preferably 50-85%.

This prefilm with a high proportion of β-crystalline polypropylene is subsequently stretched such that upon stretching, the β-crystallites are converted into alpha-crystalline polypropylene and a network-like, porous structure is formed. According to the invention, first of all, the longitudinal direction of the prefilm is stretched and then a simultaneous biaxial stretching in which the film is simultaneously stretched longitudinally (in the machine direction) and transversely (perpendicular to the machine direction) (LISIM or MESIM method).

For the first stretching only in the longitudinal direction, the cooled prefilm is first passed over one or more heating rollers, which heat the film to the appropriate temperature. In general, this temperature is less than 140 ° C, preferably 70 to 120 ° C. The longitudinal stretching is then generally carried out with the help of two according to the desired stretch ratio of different fast-running rollers. The longitudinal draw ratio is in a range from 1, 5: 1 to 6: 1, preferably 2: 1 to 5: 1. Optionally, to avoid too high an orientation in the longitudinal direction of the width recess in the longitudinal sections are kept low, for example by setting a comparatively narrow stretch gap. The length of the stretched gap for these embodiments is generally 3 to 100 mm, preferably 5 to 50 mm. Optionally, also fixing elements, such as spreaders contribute to a small latitude. The jump should be less than 10%, preferably 0.5-8%, in particular 1-5% for these embodiments. After the longitudinal stretching, the film is first cooled again over appropriately tempered rolls or kept at the temperature. After the longitudinal stretching takes place in the so-called Aufheizfeldern again heating to a suitable stretching temperature for the simultaneous stretching in the longitudinal and transverse directions. This biaxial stretching is carried out simultaneously according to the invention. The film is stretched simultaneously in the longitudinal direction (ie in the machine direction = MD direction) and in the transverse direction (ie perpendicular to the machine direction = TD direction). During the simultaneous stretching, a network-like porous structure is formed. Surprisingly, in the simultaneous stretching, the desired network-like porous structure is formed, whereby at the same time the molecular chains of the polymer matrix are oriented.

For the purposes of the present invention, simultaneous stretching processes include processes in which an unstretched prefilm is stretched simultaneously in the longitudinal and transverse direction by suitable devices. Such methods and devices for carrying out the methods are known in the art, for example as LISIM or as MESIM (Mechanical Simultaneous Drawing) methods. LISIM methods are described in detail in EP 1 112 167 and EP 0 785 858, to which reference is hereby expressly made. An MESIM method is described in US 2006/01 15548, to which reference is also expressly made. In a further, but not preferred embodiment, the β-porous film can also take place in a so-called diamond frame, in which a first separately prepared longitudinally stretched film is clamped, heated to the desired temperature and then by pulling the film in both directions simultaneously Longitudinal and transverse direction is stretched. (Articulated arms)

According to the LISIM ^® method, the simultaneous stretching takes place according to a continuous simultaneous stretching method. The elongated film is transported in a stretching oven with a transport system, which works according to the LISIM ^® method. The film edges are detected by so-called clips, which be driven by a linear motor. Individual clips, for example, each Drite, are equipped with permanent magnets and also serve as a secondary part of a linear motor drive. Over almost the entire circulating transport path, the primary parts of the linear motor drive are arranged parallel to the guide rail. The non-driven clips only serve to absorb film forces across the direction of travel and to reduce the sag between the stops.

After the film edges are gripped by the clips, the elongated film passes through a preheat zone in which the guide rails of the clips are substantially parallel. In this region of the stretching furnace, the elongated film is heated from the inlet temperature to the stretching temperature by a suitable heating device, for example a convection heater or IR radiator. Thereafter, the simultaneous stretching process begins by accelerating the mutually independent clip-on carriages in the direction of film travel (MD) and thus separating them, i. increase their distance from each other. In this way, the film is further stretched in length. At the same time, a transverse extension is superimposed on this process, specifically in that the guide rails diverge in the area of claw acceleration.

Thereafter, the biaxially stretched film is fixed in view of the desired mechanical film properties. Here, a heat setting at elevated temperature, in which the film is optionally slightly relaxed in the longitudinal or transverse direction in the clamped state takes place. Particularly advantageous may be the simultaneous relaxation in the longitudinal and transverse directions. Here, the speed of the clip-on wagon during or after the drawing process is reduced, which reduces their distance from each other and relaxation in the machine direction. Relaxation in the transverse direction is made possible by converging guide rails of the transport system. In a further embodiment of the invention, the drawing speeds are varied during the stretching process. This process variant enables the LISIM process. For example, the clips can be accelerated more strongly when stretching the film, ie at the beginning of the simultaneous stretching process, resulting in a fast stretching speed at the beginning of the stretching process or additionally accelerated after stretching, which after a moderate stretching anschießend to a faster stretching of the film in the frame leads. By choosing these parameters both economy and product properties can be influenced.

According to the MESIM® method, the simultaneous stretching is carried out according to a principle equivalent to the LISIM method. The longitudinally stretched film is here also transported in a stretching oven with a transport system of clips on guide rails. Here, at each edge of the film, there is a pair of rails on the opposite clips and clip-like elements are arranged and connected to each other via a scissors joint. By the scissors joint, the distance between the clips can be varied. In which the scissors joint is pulled apart, the distance of the clips to each other increases. Conversely, the distance is reduced when moving the joint. In the stretching oven, the two guide rails of the respective pair of rails (with scissor joint) are arranged converging, whereby the scissor joint is pulled apart and accelerate the clips in the direction of the film and increase their distances from each other. As a result, the film is stretched further in length. At the same time takes place by the divergent arrangement of the rail pairs at each edge of the film, a simultaneous transverse extension.

In a further variant, the simultaneous stretching can take place on a laboratory stretching frame. The stretched film is cut to the appropriate size and clamped in clips. The clips are connected via scissor joints with rails that run in the transverse and longitudinal directions. The clips hold the elongated film on its longitudinal and transverse sides. The Frame is closed after clamping the film, then the film is heated in the frame via a heater fan a certain preheating and then stretched simultaneously in both directions. For this purpose, the clips simultaneously pull apart the heated film over diverging rails. By speed and way of moving apart so different yield speeds and different stretch ratios can be realized. After the stretching operation is completed, the biaxially stretched film of the device can be removed directly or the film is still a certain time in the clamped state at a certain temperature. This can also be a relaxation.

In simultaneous drawing according to the above-described LISIM or MESIM method or on the laboratory stretching frame, the elongate film having a high content of β-crystalline polypropylene in the preheating zone or in the laboratory stretching frame is stretched to 90 to 160 ° C, preferably 110 to 150 ° C, at which the simultaneous stretching finally takes place. The stretch ratios can be chosen flexibly, so that films with different Gurley values can be achieved depending on the application. Surprisingly, the longitudinal stretching factor in the subsequent simultaneous stretching can be increased to up to 10 (1000%), in particular up to 8 (80%), and thus lies well above the technically feasible longitudinal stretching factors in a sequential drawing. Preferably, the stretch factor in the longitudinal direction is 3 (300%) to 10 (1000%), in particular 4 (400%) to 8 (800%). Despite this surprisingly high longitudinal draw, it is simultaneously possible to maintain the transverse stretching factors known per se of up to 9 (900%), in particular from 4 (400%) to 8 (800%). Preferably, the stretch factor in the transverse direction is 5 (500%) to 7 (700%), especially 5 (500%) to 6 (600%). Thus, exceptionally high surface stretch ratios can be realized. Such area stretching ratios are not approachable in a sequential stretching. It is therefore surprising that overall a very high longitudinal stretching of β-porous films by means of the combined stretching according to the invention (first longitudinal stretching and then simultaneous stretching) is possible. Thus, the sum of the longitudinal stretching factors is preferably in a range of 4 to 15, in particular in a range of 5 to 12. By combining a total of extremely high longitudinal stretching and a high transverse stretching comparatively high mechanical strengths are achieved, previously for biaxially stretched ß -porous films could not be realized. The combined stretching thus enables surprisingly high area stretching ratios, which hitherto have not been achieved for biaxially stretched β-porous films and which preferably lie in the range from 20 to 100, in particular in the range from 30 to 80, in particular in the range from 40 to 70.

Surface stretch ratio = (stretch ratio MD [%] x stretch ratio TD [%]) / 100

To achieve the desired high porosities, the simultaneous drawing is carried out with a moderate to slow stretching speed of> 0 to 40% / s, preferably in a range of 0.5 to 30% / s, in particular 1 to 15% / s. The slow simultaneous stretching leads surprisingly to a higher porosity and higher permeability while maintaining good running safety of the film. The stretching speed can in principle be varied over the speed of the method itself or over the length of the stretching frame. The faster (or slower) the production speed of the film (process speed) is, the higher (or the slower) is the stretch rate, each at a given stretch factor. Alternatively, the stretch may be increased by the same factor over a longer path, i. a longer stretching frame are performed to reduce the transverse stretching speed.

Optionally, after the simultaneous stretching, a surface of the film is corona, plasma or flame treated according to one of the known methods. Finally Optionally, a heat-setting (heat treatment) is carried out, in which the film is held at a temperature of 110 to 150 ° C, preferably at 125 to 145 ° C for about 5 to 500s, preferably for 10 to 300s. Alternatively, the film at the elevated temperature in the longitudinal or transverse direction controlled relax slightly in the clamped state. Particularly advantageous may be the simultaneous relaxation in the longitudinal and transverse directions. Here the clip-on wagons are decelerated, reducing their distance from each other. At the same time, the guide rails of the transport system are easily converged. By convergence is meant a slight collapse of the guide rails, so that the maximum width of the frame, which is given at the end of the stretching process, is greater than the width at the end of the heat-setting. The same applies, of course, for the width of the film web. The degree of convergence of the stretching frame is expressed as the convergence calculated from the maximum width of the stretching frame B _max and the final _film width B _foil according to the following formula:

The relaxation in the longitudinal direction or convergence in the transverse direction is within a range of 5 to 25%, in particular 8 to 20%. Finally, the film is wound in the usual way with a winding device.

By the above-mentioned process speeds of the drawing process is meant that speed, for example in m / min, with which the film runs during the final winding.

The process conditions in the process according to the invention for producing the porous films differ from the process conditions which are usually observed in the production of a biaxially oriented film. To achieve high porosity and permeability, both the cooling conditions when solidifying to the precursor film, and the temperatures and the factors involved in drawing are critical. First, through correspondingly slow and moderate cooling, ie at relatively high temperatures, a high proportion of ß crystallites in the pre-film can be achieved. Surprisingly, during the combined stretching, the β-crystals of the polypropylene are converted into the alpha modification in such a way that, with the final simultaneous stretching, the porous network structure is formed. The first stretch in the longitudinal direction initiates the conversion of β-crystallites into the alpha crystallites and generates a small number of defects in the form of microcracks. In the case of sequential drawing, on the other hand, these defects must initially be produced during the longitudinal stretching in sufficient numbers and in the correct form, whereby the process conditions during the longitudinal stretching can be varied only within a limited range. Only at the transverse extent, according to this prior art, the impurities are torn to pores, so that the characteristic network structure of these porous films is formed.

The novel combination process according to the invention initiates a small number of impurities during the first moderate longitudinal stretching and creates the network structure with the pores during the simultaneous stretching and allows high stretching forces to efficiently produce β-porous films with very good mechanical strengths and high porosities in small thicknesses , The β-porous film according to the invention therefore has an outstanding combination of high porosity and permeability, mechanical strength, good running safety in the production process and low splice tendency in the longitudinal direction.

The film can be used advantageously in many applications in which very high permeabilities are required or have an advantageous effect. For example, as a highly porous separator in batteries, especially in lithium batteries with high performance requirements.

To characterize the raw materials and the films, the following measuring methods were used: melt Flow Index

The melt flow index of the propylene polymers was measured according to DIN 53 735 at 2.16 kg load and 230 ° C.

melting point

The melting point in the context of the present invention is the maximum of the DSC curve. To determine the melting point, a DSC curve is recorded with a heating and cooling rate of 10 K / 1 min in the range of 20 to 200 ° C. For the determination of the melting point, as usual, the second heating curve after having been cooled at 10K / 1 min in the range of 200 to 20 ° C is evaluated. ß content of the prefilm

The determination of the β-content of the precursor film is likewise carried out by means of a DSC measurement which is carried out on the prefilmate film as follows: The precursor film is first heated to 220 ° C. in the DSC at a heating rate of 10 K / min and melted and cooled again 1 heating curve, the degree of crystallinity K _{ß DSC is determined} as the ratio of the enthalpies of fusion of the β-crystalline phase (H _ß ) to the sum of the enthalpies of fusion of ß- and a-crystalline phase.

density

The density is determined according to DIN 53 479, method A.

Bubble Point:

The bubble point was measured according to the ASTM F316. Puncture resistance and puncture elongation:

Puncture resistance and puncture elongation is determined as described in ASTM F 1306. The measured puncture resistance divided by the film thickness then gives the relative puncture resistance in N / μm

Mechanic solidity:

Tear strengths, elongation at break and modulus of elasticity are determined using a standard tensile testing machine according to DIN EN ISO 527-1 / -3. For the measurements in the longitudinal direction, test strips in the machine direction and for the measurement in the transverse direction, test strips are cut out in the transverse direction.

porosity

As porosity, the density reduction of the film over the

Density of pure polypropylene calculated as follows:

Permeability / permeability (Gurley value)

The permeability of the films was measured with the Gurley Tester 41 10, according to ASTM D 726-58. It determines the time (in seconds) that 100 cm ^{3 of} air will take to permeate through the 1 inch ² (6,452 cm ² ) film surface. The pressure difference across the film corresponds to the pressure of a water column of 12.4 cm in height. The time required then corresponds to the Gurley value.

Examples:

Example A: Preparation of the unstretched prefilm

In a twin-screw extruder, the following raw material mixture was melted at 235 ° C: Approximately 80% by weight of highly isotactic propylene homopolymer (PP) having a ¹³ C-NMR isotacticity of 97% and an n-heptane-soluble fraction of 2.5% by weight (based on 100% of PP) and a 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) and

about 20 wt .-% propylene-ethylene block copolymer having an ethylene content of 5 wt .-% based on the block copolymer and a MFI (230 ° C and 2.16 kg) of 6 g / 10min and a melting point (DSC) of 165 ° C used and 0.04 wt .-% Ca-pimelate as ß-nucleating agent.

The mixture additionally contained stabilizer and neutralizing agent in customary small amounts.

The melt was then extruded through a slot die and the hot melt film removed on a chill pick roll. To form the β-crystal modification in the prefilm a take-off roll temperature of 120 ° C was selected, the take-off speed was 2 m / min. The precursor had a β-crystallinity of 67%, which was determined by DSC.

Process parameters for producing the prefilm:

Extrusion temperature: 235 ° C

Screw speed: 300 1 / min

deduction:

Temperature take-off roll: 120 ° C,

Withdrawal speed: 2 m / min

This undrawn prefilm was used as starting film in all subsequent stretching experiments (simultaneous and sequential). example 1

The unstretched prefilm was first heated to approximately 110 ° C. by means of preheat rolls and then stretched in the longitudinal direction (MD machine direction) by a factor of 200%, then cooled to 60 ° C. by means of cooling rolls. The cooled, elongated film was run at a speed of 4 m / min into the LISIM frame and heated to 140 ° C without stretching. The heated longitudinally stretched film was then simultaneously stretched transversely and stretched 350% and wound at a speed of 14 m / min.

The following process conditions were set:

Stretching conditions in MD:

Stretching temperature: 1 10 ° C

Longitudinal stretch ratio: 200 [%]

Stretching temperature in the LISIM frame: 140 ° C

Preheating time 1 min

Longitudinal stretch ratio: 350 [%]

Transverse stretch ratio: 550 [%]

Fixation temperature 145 ° C

Fixation duration 120 sec

At the end of the stretching process, a single-layer β-porous film with a thickness of 14 μm, a porosity of 49% and a Gurley value of 345 s was obtained with excellent mechanical properties.

Example 2 The unstretched prefilm was heated to 110 ° C. as described in Example 1 and then stretched in the longitudinal direction by a factor of 250% and then cooled to 60 ° C. by means of cooling rolls before being then simultaneously drawn. The cooled, longitudinally stretched film was run at a speed of 5 m / min in the LISIM frame and preheated without stretching to 140 ° C and then simultaneously 550% transversely stretched and 350% elongated and wound at a speed of 17.5 m / min

The following process conditions were set:

Stretching conditions in MD:

Stretching temperature: 1 10 ° C

Longitudinal stretch ratio: 250 [%]

Stretching temperature in the LISIM frame: 140 ° C

Preheating time 1 min

Longitudinal stretch ratio: 350 [%]

Transverse stretch ratio: 550 [%]

Fixation temperature 145 ° C

Fixation duration 100 sec

At the end of the stretching process, a single-layer β-porous film with a thickness of 12 μm, a porosity of 45% and a Gurley value of 486 s was obtained with excellent mechanical properties.

Example 3

The unstretched prefilm was heated to 110 ° C. as described in Example 1 and then stretched in the longitudinal direction by a factor of 400% and then cooled to 60 ° C. by means of cooling rolls before being then drawn simultaneously. The cooled, longitudinally stretched film was at a speed of 8 driven in the LISIM frame and preheated without stretching to 140 ° C and then simultaneously 550% transversely stretched and 350% longitudinally stretched and wound up at a speed of 24m / min. The following process conditions were set:

Stretching conditions in MD:

Stretching temperature: 1 10 ° C

Longitudinal stretching ratio: 400 [%]

Stretching temperature in the LISIM frame: 140 ° C

Preheating time 1 min

Longitudinal stretch ratio: 300 [%]

Transverse stretch ratio: 550 [%]

Fixation temperature 145 ° C

Fixation duration 70 sec

After the end of the stretching process, a single-layered β-porous film with a thickness of 9 μm, a porosity of 43% and a Gurley value of 418 s was obtained with excellent mechanical properties.

The process parameters are summarized below in Table 1 and the film properties in Table 2. Table 1: Process parameters extension

Table 2: Separator properties:

Claims

claims

1. A biaxially oriented, single or multilayer film having at least one porous layer, said porous layer containing at least one propylene polymer and β-nucleating agent and the porosity of said porous layer is produced by converting β-crystalline polypropylene during stretching of the film, characterized that

(1) the unstretched prefilm stretched longitudinally and

(2) then the longitudinally stretched film is stretched simultaneously in the longitudinal direction and in the transverse direction.

2. A film according to claim 1, characterized in that the Gurley value is less than 1000s Gurley.

3. A film according to claim 1 or 2, characterized in that the propylene polymer is a propylene homopolymer and / or a propylene block copolymer.

4. A film according to any one of claims 1 to 3, characterized in that the nucleating agent is a calcium salt of pimelic acid and / or suberic acid or a nanoscale iron oxide.

5. A film according to any one of claims 1 to 4, characterized in that the film contains propylene homopolymer and propylene block copolymer

6. A film according to any one of claims 1 to 5, characterized in that the film contains 50 to 85 wt .-% propylene homopolymer, 15 to 50 wt .-% propylene block copolymer and 50 to 10,000 ppm ß-nucleating agent.

7. A film according to any one of claims 1 to 6, characterized in that the density of the film is in a range of 0.1 to 0.5 g / cm ³ .

8. A film according to any one of claims 1 to 7, characterized in that the film has a thickness of 3 to 25μm, preferably 5 to 10μm.

9. A film according to any one of claims 1 to 7, characterized in that the tensile strength in the longitudinal direction> 80 N / mm ² .

10. A process for producing a single-layer or multilayer ß- porous polypropylene film in which propylene polymers and ß-nucleating agent is melted in an extruder and extruded through a flat die onto a take-off roll on which the melt film cools and solidifies to form ß-crystallites and

(1) the unstretched prefilm stretched longitudinally and

1 1. A method according to claim 10, characterized in that is drawn in the simultaneous drawing with a stretching speed of less than 40% / sec.

12. The method according to claim 10, characterized in that is stretched only at a stretching speed of less than 40% / sec and then stretched at a stretching speed of more than 40% / sec.

13. Use of a film according to one of claims 1 to 9 as a separator in an energy store.

14. Use of a film produced by a method according to any one of claims 10 to 12 as a separator in an energy store.

15. Electrochemical energy store containing a film according to one of Claims 1 to 9.

16. An electrochemical energy store containing a film produced by a method according to one of claims 10 to 12.