CH644789A5 - MEMBRANE WITH POROESE SURFACE. - Google Patents

Description

The invention relates to a membrane in the form of a flat film, tubular film or a hollow thread made of synthetic polymers, processes for their production and their use, in particular as a filter or membrane carrier for separation purposes.

Membranes in the form of flat foils, tubular foils or hollow fibers have long been known. A number of methods are available to the person skilled in the art to produce such membranes from a wide variety of polymers. Hollow threads can be used in the manufacture of textile products; but they are also used in filtration, ultrafiltration, microfiltration, dialysis, reverse osmosis, etc.

If membranes in the form of flat films, tubular films or hollow fibers are used in separating devices, the permeability and the selectivity of the membranes are crucial, because on the one hand the membranes should retain certain substances, on the other hand other substances, e.g. let the solvent through a solution as quickly as possible.

Films with a microporous or porous structure have been known for a long time. Among other things, DE-OS 2 737 745 describes processes for producing films which have a porous or microporous structure. The structures described there are undoubtedly valuable products that are used in a whole range of areas.

In the reworking of the teachings described there, however, it was found that the films described there generally have a closed surface. In particular, it is not possible to obtain foils that have pores on both sides that are noticeable as openings directly on the surface. There is therefore still a need for improved production methods, in particular for those processes which lead to porous films which are notable for good permeability and high selectivity and which also have open pores on both surfaces and at the same time have a smooth, flat surface structure highlight.

A process for producing permeability-selective hollow fibers is described, for example, in DE-AS 1 494 579, in which an intimate mixture of a thermoplastic polymer is melt-spun with a plasticizer and the plasticizer is then extracted from the hollow fibers obtained. It is among other things required that the plasticizer be easily and substantially completely removable from the spun hollow fiber. Often, however, this requires a relatively long treatment of the thread during extraction; nor is it always possible to completely remove the plasticizer. In addition, the hollow fibers produced by this process are relatively low

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Permeability. Finally, it is not possible to vary the proportions of plasticizer and polymer within wide ranges; if the plasticizer content is high, there is no longer any thread formation; if the plasticizer content is too low, the permeability is too low. There is also the risk that when the plasticizers are mixed, they are not sufficiently distributed in the thermoplastic polymer, so that agglomerations form which, when washed out, lead to holes or oversized pores, which preclude the usability of the hollow fibers for many purposes.

Another process for the production of hollow fibers is described in DE-AS 2 346 Ol 1, in which a solution of a copolymer of acrylonitrile is spun into an aqueous solution of mineral salts. It is necessary that coagulation liquid is also injected into the inside for coagulation inside. The process described there is relatively complicated; moreover, it is difficult to obtain hollow fibers with constant properties.

US Pat. No. 3,674,628 describes a process in which a solution of a fiber-forming polymer is first spun, then the outer and optionally also the inner zone is subjected to gel formation and then or both zones are coagulated or simultaneously. This creates a hollow fiber that has a skin-like structure inside and out. The process according to this US patent is also relatively complicated, and the permeability of the hollow fibers obtained leaves something to be desired.

Although numerous membranes in the form of flat films, tubular films or hollow filaments are already known, there is still a need for improved membranes, in particular those which are simple to produce and which can be worked with a simple spinning mass without complicated spinning baths. There is also a need for improved membranes that are porous and that, among other things. characterized by good permeability with high selectivity.

It is an object of the invention to provide porous membranes which are distinguished from the known membranes by a favorable open surface, in which both the outer and the inner wall have an open pore structure which is nevertheless smooth. The object of the invention is also to provide membranes that are used both in the textile, technical and medical sectors such. B. can be used in separation processes and are particularly suitable as a filter, microfilter, membrane support and as a carrier substrate for certain substances.

This object is achieved by a membrane according to claim 1. Particularly advantageous embodiments of the membrane are described in claims 2 and 3.

The membranes can be used as filters, in particular as filters for microfiltration.

The membranes can also serve as membrane supports, i.e. a layer is applied to the membrane, which also acts as a membrane, possibly with other permeabilities and other selectivities.

The membranes can also be used as a carrier substrate, i.e. the membranes can be impregnated with certain substances that are to be released at a later time. They can also be used as oxygenators.

It is therefore also an object of the invention to provide a process in which polymers can be easily converted into an extrudable spinning mass and which at the same time allows the extruded material to be extruded and solidified without having to work with complicated spinning techniques or spinning baths must become. It is also an object of the invention to provide a method which, only by varying the method parameters, makes it possible to obtain membranes with adjustable porosity, permeability or permeability.

This object is achieved by a method according to claim 4. Advantageous embodiments of this method are specified in claims 5 to 17.

To carry out the process according to the invention and for the production of the membranes according to the invention, conventional, in particular fiber-forming, macromolecular substances, in particular synthetic polymers, can be used, which, for. B. can be obtained by polymerization, poly addition or polycondensation, provided that the polymer is meltable, d. H. can pass into the liquid state without decomposition and forms a binary system with a liquid which is inert to it, which has a range of complete miscibility in the liquid state and also has a region with a mixture gap in the liquid state.

Such systems have a phase diagram for the liquid state, of the type described, for example, in the Textbook of Physical Chemistry by S. Glasstone, Macmillian and Co. Ltd., St. Martin's Street, London, 1953 on page 724 for the system aniline hexane is reproduced. In this diagram there is complete miscibility for the two components above the curved curve. Below the curve, two liquid phases are in equilibrium with each other.

For the feasibility of the invention, it is not absolutely necessary that in the 2-phase range the two components each have considerable solubility in relation to the other component, as is the case in the diagram mentioned above. In many cases it is sufficient if there is marginal solubility in the liquid 2-phase area. It is essential, however, that the two components in the liquid state still form two liquid phases next to one another. In this respect, the systems which can be used according to the invention differ from those systems in which the dissolved polymer precipitates directly as a solid substance when the temperature is lowered without first having to pass through the liquid state during cooling.

In the context of the invention, customary meltable polymers can be used, such as the polymers obtained by polymerization, polyethylene, polypropylene, polyvinyl chloride, polyacrylates, polycaprolactam and corresponding copolymers, etc. Polycondensation polymers such as poly-ethylene terephthalate, polybutylene terephthalate, polyamide-6.6, polyphenylene oxide and polyaddition polymers such as polyurethanes and polyureas.

In principle, all those liquids which form a binary system of the type mentioned above with the polymer in the liquid state are suitable as inert liquid in the context of the invention. Inert to the polymer means that the liquid does not cause the polymer to degrade considerably or react with the polymer itself within a short period of time.

Even if the state diagram of the aniline / hexane system mentioned above reproduces the conditions for a binary mixture which in and of itself consists of only two essentially pure, uniform substances, the term binary system is not intended to be strict within the scope of the invention on mixtures of only two pure units

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Chen substances are used. The average person skilled in the art knows that a polymer substance consists of a multitude. number of molecules of different molecular weights is composed, therefore such polymers with a corresponding molecular weight distribution are to be regarded as a component within the scope of the invention, the same also applies to mixed polymers. Under certain circumstances, even polymer mixtures can behave as a single component, form a single-phase mixture with an inert solvent, and separate into two liquid phases below the critical temperature. However, preferably only one polymer is used.

The liquid also does not necessarily have to be completely pure and to represent a completely uniform substance. So it often does no harm if smaller amounts of impurities, possibly also proportions of homologous compounds, as they are caused by large-scale production, are present.

To carry out the process in practice, a homogeneous mixture is produced from the two components at the required temperatures. This can be done in such a way that the inert liquid is mixed with the comminuted polymer and heated to appropriate temperatures, with appropriate mixing being ensured.

Another suitable method is that the two components are brought to the required temperature separately and the two components are only continuously mixed in the desired proportions shortly before the extrusion. This mixing can take place in a pin mixer, which is expediently arranged between the metering pumps for the individual components and the spinning pump. A subsequent homogenization can be recommended.

In many cases it is advisable to deaerate the homogeneous mixture by applying a suitable vacuum before extruding.

The ratio of polymer to inert liquid in the dope can be varied within wide limits. By adjusting the ratio of polymer to inert liquid, the pore volume inside and also the surface structure and the number of open pores on the surfaces of the membrane obtained can be controlled to a large extent. Suitable membranes can thus be obtained for a wide variety of uses.

In general, it is sufficient if the temperature of the homogeneous mixture before extrusion is only a few degrees above the critical temperature or above the segregation temperature according to the particular composition.

However, by increasing the difference between the temperature of the homogeneous mixture to be extruded and the separation temperature, interesting effects can also be achieved with regard to the structure of the membrane obtained.

The homogeneous spinning mass is then extruded into a bath which contains the inert liquid of the extruded component mixture and has a temperature below the separation temperature. The bath preferably consists entirely or for the most part of the inert liquid which is also present in the extruded mixture.

The temperature of the bath is below the segregation temperature of the binary mixture used, i.e. below the temperature above which the two components are completely homogeneously miscible. The temperature of the bath is preferably at least 100 ° C. below the separation temperature of the mixture used.

The temperature can already be so low that you are already moving in the area in which a solid phase occurs, in accordance with the state diagram applicable to the binary system.

If the temperature of the bath is so high that you are still in the liquid 2-phase range, it is necessary to immediately solidify the resulting membrane structure, which can be done by reducing the temperature accordingly after a certain distance within the bath.

It is important that the extruded mixture is still single-phase before it enters the bath, i.e. that there is essentially no separation in two phases.

It has proven to be advantageous, especially when

if the membrane is to be produced in the form of a tubular film or a hollow thread, if in certain cases the bath is preceded by a spinning tube which is also filled with the bath liquid and which is immersed in the spinning bath. The spinning tube can have a conventional spinning funnel at its inlet opening, and the tube can be curved at its lower end in order to facilitate the removal of the membrane by the bath.

The spinning tube can be filled via a level vessel surrounding the spinning tube by overflow into the spinning tube; To completely fill and maintain the level in the spinning tube, it is necessary to supply this level vessel with more bath liquid from a main reservoir than is flowing through the spinning tube; the excess amount of bath can be returned to the main reservoir through a second overflow on the level vessel. The main reservoir and level vessel can be thermostatted.

After it has left the spinning bath, the membrane can be washed out with an appropriate extractant. A number of solvents such as acetone, cyclohexane, ethanol and the like are required for extraction. as well as mixtures of such liquids.

In some cases it is not necessary to wash out the membrane, in particular if the inert liquid used itself is intended to impart certain additional properties to the thread for later use or to perform a function itself. For example, liquids can be used that have an antistatic effect or act as a lubricant.

It has proven to be expedient that for a number of applications between the exit surface of the extrusion tool, i.e. an air gap is maintained between the exit surface of a corresponding nozzle, for example, and the surface of the bath. By varying the air gap, it is possible to influence the structure of the membrane obtained, in particular its surface.

It has been found that by lengthening the air gap, the number of open pores in the surface can be reduced and can be increased by shortening the same; the diameter of the pores also decreases with an increasing air gap.

The air gap can be heated, or to a temperature above the separation temperature of the extruded mixture.

In general, the air gap is at least about 1 mm wide and can be up to about 10 cm long depending on the working conditions. It is important that there is no or at least no noticeable segregation in two liquid phases in the air gap before entering the bath.As mentioned, this can be controlled by shortening the distance or by heating, but it is also possible to increase the exit speed counter premature segregation at the nozzle.

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In a special embodiment of the method according to the invention, however, the homogeneous mixture is extruded directly into the bath, with open pores having a maximum diameter being formed on the surface.

The membranes obtained can be used well as filters. They can mainly be used in microfiltration and ultrafiltration. The membranes are particularly suitable for the medical field, where they are used e.g. B. because of their selectivity to separate bacteria, in the filtration of blood z. B. can be used to separate blood platelets. They are also very suitable as oxygenators, where oxygen flows through the inside of the membrane, while blood flows around the outside.

For a number of purposes, the membranes can also be used as membrane supports for other membrane layers. Because of their excellent smooth surface structure with open pores, they can be coated exceptionally well with a firmly adhering thin layer of a material functioning as a membrane, which is often done by coating or spraying with appropriate film-forming solutions. Because of its excellent surface properties, the resulting membrane layer adheres very well to the membranes according to the invention. The coating solution can be applied very evenly as a thin skin without the solution penetrating or even dripping into the interior of the hollow thread, so that very effective membrane combinations can be produced for a wide variety of applications.

Due to their special surface structure and the structure inside, the membranes are also ideal as a substrate for certain substances. It is also possible to introduce the active substance into the inner continuous cavity of a hollow fiber membrane.

In this way, bodies in the form of flat foils, tubular foils or hollow filaments or their sections can be obtained, which slowly release the active substance absorbed again. The membranes according to the invention can also be used for the adsorption of substances.

According to the invention, films of conventional dimensions are accessible, e.g. B. a thickness of 20 microns to a few millimeters. The films can be in the form of flat or tubular films. The foils can also be used as insulating material, e.g. be used for thermal insulation and sound insulation.

According to the invention, hollow fibers are accessible within a wide range of dimensions. Outward diameters of up to several millimeters can be achieved. The wall thicknesses can also be varied to a large extent. B. are between 20 microns and about 1 to 2 millimeters. •

The pores in the membranes according to the invention can have a wide variety of shapes. So they can be round or elongated and are connected to each other, sometimes. through small connecting cavities, partly by going straight into each other. Even with membranes obtained from mixtures containing only about 30% polymer, the polymer can still be the matrix in which the individual pores are distributed and form more or less discrete but interconnected cavities. Conversely, structures can also arise in which the cavities form the matrix, similarly to nonwovens, and the polymer substance is arranged in a quasi-fibril-like manner. The transitions between these two structures are fluid and come partly. mixed before; the structural shapes can also be influenced by further process parameters, such as take-off speed, cooling speed, and warping below the nozzle.

The membranes according to the invention are characterized above all by a high permeability to gases such as. Nitrogen or air. The permeability can be indicated by the so-called permeability coefficient K, as described in the book Flow of Fluids through Porous Materials by R.E. Collins, published by Reinhold Publishing Corp., New York 1961, page 10. K is defined as

Q-t |

K =

A (AP / h)

where Q is the volume flow in the time unit (e.g. m3 / s), t | the viscosity of the flowing medium (Pa • s), A is the mean area through which the gas exits, A P is the pressure difference (Pa) and h is the wall thickness.

The permeability coefficient of the membranes according to the invention is at least 10 • 10 "12 cm2, preferably at least 22 • 10" 12 cm2, values over 100 • 10 "12 cm2 can be achieved.

In the case of membranes in the form of flat films or tubular films, the coefficient was measured using nitrogen, which is pressed under pressure against a film fastened in a flange. The air escaping through the foils is measured with the help of a flow meter.

The following values were found for films according to the invention, which were produced from a mixture of 30% by weight of polypropylene and 70% by weight of NN-bis (2'-hydroxyethyl) hexadecylamine and an air gap between the nozzle and the bath will:

K (10 ~ 12 cm2) air gap

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27 10

With hollow fiber membranes, the measurement was carried out as follows:

31 cm long hollow threads are embedded in two 5 cm long PVC hoses with the help of a hardenable polyurethane compound. After the polyurethane has cured, a PVC tube is cut, and the exposed openings are connected to a nitrogen bottle via a feed, the end of the other tube is sealed with a stopper. The air emerging through the threads is measured with the help of a flow meter.

The following values could be found for hollow membrane filaments according to the invention, which were produced from a mixture of 30% by weight of polypropylene and 70% by weight of NN-bis (2-hydroxyethyl) hexadecylamine and compliance with an air gap between the nozzle and the bath :

K (10 "12 cm2) air gap (mm)

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22 20

The hollow membrane filaments according to the invention can also be used as insulating means.

A suitable device for producing the hollow membrane filaments according to the invention is explained in more detail in FIG. 1 is a thermostattable container, from which the inert liquid is metered into the mixer 8 via a double-piston pump 3 and a further heater 4. The heater 2 is used for preheating. From the shredded container 5, an extruder 6 and a gear pump 7 feed polypropylene into the mixer 8, of which via a gear

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Pump 9 is fed a hollow fiber nozzle 10, which is supplied with the required amount of nitrogen via a rotary knife 17. The emerging mass passes through an air gap into a spinning tube 12 provided with a spinning funnel 11, which is supplied with inert liquid from the main reservoir 14 via a level vessel 13. The spinning tube has a curvature at its lower end, the threads are guided to the winding 16 after leaving the bath 15.

The invention is illustrated by the following examples:

example 1

Polypropylene with a melt index of 1.5 g / 10 min. Is melted in an extruder at heating temperatures of 260 to 280 ° C. and metered in via a gear pump in a highly effective pin mixer.

At the same time, NN-bis- (2-hydroxyethyl) -hexadecyl-amine is preheated to 135 C with a double-piston pump, using a continuous-flow heater, and also metered into the mixer using a separate line.

The mixing ratio of polypropylene: amine is 30:70. The mixer speed is set to 400 rpm.

After passage of the mixer, the two substances that have become homogeneous are passed through a gear pipe with a quantity of 15 g / min. into a hollow thread nozzle with a clear 0 of 2000 (im and a free annular gap of 400 (im. By adding 41 / h of nitrogen into the gas capillary of the nozzle, the formation of the hollow thread is achieved.

The emerging molten thread dips into the spinning funnel filled with amine as a coagulation bath after a free falling distance of 3 mm, flows with the coagulant through the subsequent spinning tube with a length of 8 mm 0 and 400 mm and after passing through a subsequent spinning bath of 1 m length 7 m / min. wound on a winding unit.

The hollow fiber obtained is extracted with alcohol and freed from amine.

The hollow fiber has a 0 of 2200 µm and a lumen of 1400 µm.

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Example 2

Polypropylene chips are melted in an extruder and metered into a pin mixer via a gear pump.

15 At the same time, liquid NN-bis (2-hydroxyethyl) hexadecylamine at a temperature of 40 ° C is pumped from a heatable storage container into an electrically heated heater by means of a double piston pump and from there at a temperature of about 150 ° C into the mixer -20 tet. A pen mixer is used as the mixer.

After the two components have been homogenized, the melt is pressed through a slit nozzle by means of a measuring pump and extruded into a bath which consists of pure NN-bis (2-hydroxyethyl) hexadecylamine and which has a temperature of 50 ° C.

After passing the flat film through the bath, which has a length of 50 cm, the film obtained is extracted with ethanol and dried. A film with excellent membrane properties and a particularly good surface structure is obtained.

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Claims (22)

644 789 1. membrane in the form of a flat film, tubular film or a hollow thread made of synthetic polymers, characterized by 10 to 90 percent by volume of interconnected pores and a smooth, open-pore surface, the proportion of openings in the area being 10 to 90%. 2. Membrane according to claim 1, characterized by an apparent density of 10 to 90% of the true density of the polymer used. 2nd PATENT CLAIMS 3. Membrane according to one of claims 1 to 2, characterized by a permeability coefficient of at least 10- 10 "12 cm2. 4. A method for producing the membrane in the form of a flat film, tubular film or a hollow thread according to claim I, characterized in that a homogeneous mixture containing two components, one component being a meltable synthetic polymer and the other component being a polymer is an inert liquid, and both components form a binary system which, in the liquid state of matter, has an area of complete miscibility and an area with a miscibility gap, extrudes at a temperature above the separation temperature into a bath which contains the inert liquid of the extruded component mixture and a temperature below has the separation temperature, and solidifies the membrane formed in the form of a flat film, tubular film or a hollow thread. 5. The method according to claim 4, characterized in that the membrane formed is washed after solidification with a solvent. 6. The method according to claim 5, characterized in that acetone is used for washing out. 7. The method according to any one of claims 4 to 6, characterized in that an air gap is maintained between the exit surface of the extrusion tool and the surface of the bath. 8. The method according to claim 7, characterized in that the air gap is heated. 9. The method according to any one of claims 4 to 6, characterized in that the homogeneous mixture is extruded directly into the bath. 10. The method according to any one of claims 4 to 8, characterized in that the temperature of the bath is at least 100 C below the separation temperature of the binary mixture used. 11. The method according to any one of claims 4 to 9, characterized in that the homogeneous mixture is first extruded into a spinning tube upstream of the bath and filled with the bath liquid. 12. The method according to any one of claims 4 to 11, characterized in that polypropylene is used as the polymer. 13. The method according to any one of claims 4 to 12, characterized in that the inert liquid used is N, N-bis (2-hydroxyethyl) hexadecylamine. 14. The method according to any one of claims 4 to 13, characterized in that one uses a temperature-graded bath. 15. The method according to any one of claims 4 to 14, characterized in that one uses a homogeneous mixture of 10 to 90 wt .-% polymer and 90 to 10 wt .-% inert liquid. 16. The method according to any one of claims 4 to 15, characterized in that the two components are continuously mixed before extrusion. 17. The method according to claim 16, characterized in that one uses a pin mixer for mixing. 18. Use of the membrane according to claim 1 as a filter. 19. Use according to claim 18 as a filter for the microfiltration. 20. Use of the membrane according to claim 1 as a carrier for other membrane layers. 21. Use of the membrane according to claim I for the slow release of active ingredients. 22. Use of the membrane according to claim 1 as Oxy-generator membrane.