NO153879B - MANUFACTURING A MEMBRANE WITH POROES SURFACE. - Google Patents

Description

The present invention relates to the production of a membrane in the form of flat foil, hose foil or a hollow wire of synthetic polymer and its use as a filter, preferably for microfiltration.

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

If membranes in the form of flat foils, hose foils or hollow wires are used in separation processes, it largely depends on the permeability and selectivity of the membrane, because the membranes must on the one hand hold

bake certain substances and on the other hand as quickly as possible let through e.g. the solvent in a solution.

Foils with microporous or porous structure has already been known for a long time. Thus it is described e.g. in DE-OS 2.737. 74 5 a method for producing foils which exhibit a porous or microporous structure. The structures described there are undoubtedly valuable products that find application in a whole range of areas.

When following up the teaching described there, however, it has been shown that the foils described generally have a closed surface. In particular, it is not possible to obtain foils which on both sides exhibit pores which make themselves noticeable on the surface as openings. There is therefore still a need for improved methods, especially for such methods which lead to porous foils, which are distinguished by good permeability at the same time high selectivity and which, in addition to this, have open pores on both surfaces and at the same time exhibit a smooth and uniform surface structure.

A method for producing permeability-selective hollow fibers is described, for example, in DE-AS 1,494,579 in which a thorough mixture of a thermoplastic polymer with a plasticizer is melt-spun, after which the plasticizer is extracted from the hollow fiber obtained. Thereby, it is necessary, among other things, that the plasticizer can be easily and so to speak completely removed from the spun hollow fiber. Often, however, for this purpose, a relatively long treatment of the thread during extraction is necessary, nor is it always possible to completely remove the plasticizer. In addition to this, hollow fibers produced by this method exhibit a relatively low permeability. Finally, it is also not possible to vary the proportion of plasticizer and polymer within wide areas, with a high proportion of plasticizer no thread formation is achieved, and with too low a plasticizer content too low permeability is achieved. Furthermore, there is a danger that the plasticizer is not sufficiently distributed in the thermoplastic polymer during mixing so that agglomerates form which, when washed out, lead to holes and oversized pores, which preclude the use of the hollow fiber for many purposes.

A further method for producing hollow fibers is described in DE-AS 2,346,011, in which a solution of a copolymer of acrylonitrile is spun in an aqueous solution of mineral salts. Thereby, it is necessary that in order to achieve coagulation in the interior, coagulation liquid is also injected into the interior. The method described there is relatively complicated, and in addition to this, it is difficult to obtain hollow fibers with constant properties.

US-PS 3,674,628 describes a method there

a solution of a fibre-forming polymer is first spun, after which the outer and possibly also the inner zone under-

a gel formation is thrown and subsequently or simultaneously both zones are coagulated. This results in a hollow fiber that has a skin-like structure inside and out. The method according to this US patent is also relatively complicated and the permeability of the hollow fiber obtained leaves much to be desired.

bake to wish.

Although numerous membranes in the form of flat foils, hose foils or hollow wires are already known, there is still a need for improved membranes, especially for those that can be produced easily and that can be processed with a simple spinning mass without complicated spinning baths. There is also a need for improved membranes which are porous and which, among other things, are characterized by good permeability with simultaneously high selectivity.

A task for the invention is therefore to produce a method by which polymers can be easily transformed into extrudable spinning masses and at the same time allow extrusion and stiffening of the extruded material without having to work with complicated spinning techniques or spinning bath. A further task for the invention is to offer a method in which, only by varying the process parameters, it is possible to produce membranes with adjustable porosity, permeability or permeability.

This task is solved according to the invention by a method for producing a membrane in the form of a flat foil, hose foil or a hollow wire of synthetic polymers with 10 to 90% by volume of interconnected pores, an apparent density of 10 to 90% of the true density of the polymer used, a surface that exhibits smooth,

open pores, whereby the proportion of openings in the surface is 10 to 90% and a permeability coefficient of at least 10 x 10~<12 >cm 2 , and this method is characterized by extruding a homogeneous mixture of at least two components, whereby one component is a fusible component and the other component is a liquid inert to the polymer, whereby the mixture contains 10 to 90% by weight of polymer and 90 to 10% by weight of inert liquid, and both components form an inert system which in the liquid aggregate state exhibits an area of full miscibility and a region of mixing holes, at a temperature above the mixing temperature of a bath containing the inert

liquid to the extruded component mixture and has a temperature below the separation temperature, and fixes the formed membrane in the form of a flat foil, hose foil or hollow wire.

The invention also relates to the use of a membrane as described above as a filter, preferably as a filter for microfiltration.

The formed membrane structure can be washed after solidification

out with a solvent, whereby acetone in particular is suitable.

It is advantageous if an air gap is maintained between the exit surface of the extrusion tool and the bath surface. This air gap can be heated.

It is also possible to extrude the homogeneous mixture immediately into the bath.

In a particular embodiment of the method according to the invention, a bath with different temperature steps is used. Thereby, the bath can consist of one or more parts which exhibit a temperature gradient in such a way that the temperature from the beginning of the spinning bath to the exit from there, continuously drops.

It is also possible to use two or more separate bathrooms

which exhibit different temperatures.

It has proven to be beneficial when the bath has a temperature that is at least 100°C below the separation temperature of the binary composition used. Especially with hose foils and hollow wires, the homogeneous mixture can also first be extruded in a spinning tube placed in front of the bath and filled with bath liquid.

According to the invention, homogeneous mixtures of 10-90% by weight polymer and 90-10% by weight inert liquid can be extruded.

Polypropylene is preferably used as polymer, and as inert liquid, N,N-bis-(2-hydroxyethyl)-hexadecylamine.

The two components, namely the molten polymer and the inert liquid, are suitably mixed continuously before the extrusion, whereby it is advantageous that the mixing only takes place immediately before the extrusion. The mixture can also be homogenised before extrusion. For the mixture, you can especially use a stick mixer.

For carrying out the method according to the invention

for the production of the membranes according to the invention, one can use in and of themselves ordinary, particularly fibre-forming macromolecular substances, above all synthetic polymers such as e.g. obtained by polymerization, polyaddition or polycondensation, provided

that the polymer is fusible, i.e. that it can pass into the liquid aggregate state without splitting and form an inert system with a liquid inert to the polymer which in the liquid aggregate state exhibits an area of full miscibility and also in the liquid aggregate state exhibits an area with mixing holes.

Such systems exhibit the liquid state

a phase diagram of the type that e.g. is reproduced in "Physical Chemistry" by S. Glasstone, Macmillian and Co. Ltd., St. Martin's Street, London, 1953, on page 724 for the system anilinehexane.

In this diagram, the two components above the curved curve are completely miscible, while below the curve there are two liquid phases in equilibrium with each other.

For the practicability of the invention, it is not absolutely necessary that the two components in the 2-phase region exhibit any significant solubility in relation to the other component, as is the case in the diagram indicated above. It is often sufficient that a marginal solubility is present in the liquid 2-phase region. It is essential, however, that the two components in the liquid state form two liquid phases next to each other. So far, the systems that can be used according to the invention differ from such systems where the dissolved polymer, upon a reduction of the temperature, directly falls out as a solid substance without first passing through the liquid aggregate state upon cooling.

Within the scope of the invention, you can use common fusible polymers such as the polymers obtained by polymerisation, namely polyethylene, polypropylene, polyvinyl chloride, polyacrylate, polycaprolactam and corresponding copolymers, further polycondensation polymers such as polyethylene terephthalate, polybutylene terephthalate, polyamide-6,6-polyphenylene oxide and further polyaddition polymers such as polyurethane and polyureas.

In principle, within the scope of the invention, all liquids which form a binary system of the above-mentioned type with the polymer in a liquid state can be used as an inert liquid. Inert towards the polymer means that the liquid does not cause a significant degradation of the polymer even within a short period of time,

respectively itself reacts with the polymer.

Although the above-mentioned state diagram for the system aniline/hexane reproduces the conditions for a binary mixture which in and of itself only consists of two essentially pure uniform substances, the term binary system within the scope of the invention shall not be strictly applied to mixtures of only two pure uniform substances. The average person knows that a polymer of a number of molecules exhibits different molecular weights, therefore the kind of polymers with a corresponding molecular weight distribution within the scope of the invention are to be considered a component, the same also applies to mixed polymers. Under special circumstances, even polymer mixtures can behave as uniform components, form a single-phase mixture with an inert solvent and separate into two liquid phases below the critical temperature. Preferably, however, only one polymer is used.

Nor does the liquid need to be completely pure and represent a completely uniform substance. There is therefore often no shortage if smaller amounts of contaminants, possibly also proportions of homologous compounds as this is conditioned by large-scale manufacturing, are present.

For the practical implementation of the method, a homogeneous mixture is prepared from the two components and at the required temperature. This can be done by mixing the inert liquid with the crushed polymer and heating to the corresponding temperature, whereby a corresponding thorough mixing is ensured.

A further suitable method is separately brought

the two components to the required temperature and then only shortly before extrusion to continuously mix the two components in the desired quantity ratio with each other. This mixing can take place in a pin mixer which is suitably arranged between the dosing pumps for the individual components and the spinning pump. A subsequent homogenization may be recommended.

It is often recommended to deaerate the homogeneous mixture by applying a suitable negative pressure before extrusion.

The ratio between polymer and inert liquid in the spinning mass can be varied within wide limits. By means of setting the polymer:inert liquid ratio, the pore volume in the interior and also the surface structure as well as the number of open pores in the surfaces of the resulting membrane can be largely controlled. In this way, suitable membranes can be produced for a wide variety of applications.

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 melting temperature corresponding to the composition in question.

By increasing the difference between the temperature in the homogeneous mixture to be extruded and the separation temperature, however, interesting effects can be achieved with regard to the structure of the resulting membrane.

The homogeneous spinning mass is then extruded in a bath which contains the inert liquid in the extruded component mixture, and which has a temperature below the separation temperature. Preferably, the bath consists entirely or at least mostly of the inert liquid which is also present in the extruded mixture.

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

The temperature can also already be so low that, corresponding to the state diagram for the binary system in question, one moves in the area where a solid phase occurs.

If the temperature in the bath is so high that you are still moving in the liquid 2-phase area, it is necessary to allow the standing membrane structure to harden as quickly as possible, which can be done by walking in the bath for a certain length of time , reduces the temperature accordingly.

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

It has proven advantageous, especially when the membrane is to be produced in the form of a hose foil or a hollow wire, when in special cases a spinning tube is arranged in front of the bath which is also filled with bath liquid and which dips into the spinning bath. The spinning bath can have a normal spinning funnel in the inlet opening, and at the lower end the tube can be curved to facilitate pulling off the membrane through the bath.

The spinning tube can be filled via a level container

which surrounds the spinning tube when it overflows into the spinning tube, to completely fill and maintain the level in the spinning tube it is necessary to add more bath liquid from a main reservoir to the level container than what flows out, the excess bath quantity can be fed back to the main reservoir via a second overflow. The main reservoir and level container can be controlled using a thermostat.

After it has been removed from the spinning bath, the membrane can be washed with a corresponding extraction agent. For extraction, a number of solvents such as e.g. acetone, cyclohexane, ethanol and others as well as mixtures thereof, suitable.

In some cases, it is not necessary to wash out the membrane, especially when the inert liquid used itself gives the thread special additional properties in connection with the later area of application, or is itself to perform a function. Thus, one can e.g. use liquids that have a certain antistatic effect or that act as lubricants.

It has proven appropriate for a number of application cases to have an air gap between the outlet surface of the extrusion tool, i.e. the outlet surface e.g. for a corresponding nozzle, and the bath surface. By varying the air gap, it is possible to influence the structure of the resulting membrane, especially the surface.

It has been found that by extending the air gap, the number of open pores in the surface is reduced, and by shortening the opening, the number is increased and the cross-section of the pores with growing air gap decreases.

The air gap can be heated, e.g. to a temperature

above the separation temperature of the extruded mixture.

In general, the air gap is at least approx. 1 mm wide and, depending on the working conditions, can have a length of up to approx. 10 cm. It is important, however, that there is no or at least no perceptible separation of two liquid phases in the air gap before entering the bath. As mentioned above, this can be controlled by regulating the path or by heating, but it is also possible to counteract shilling by increasing the outlet speed from the nozzle.

In a particular embodiment of the method according to the invention, however, the homogeneous mixture is extruded directly into the bath, whereby open pores with maximum cross-section are formed on the surface.

The membranes obtained can be excellently used as filters. They can above all be used for microfiltration and ultrafiltration, the membranes are particularly suitable for medical areas where they e.g. can be used when separating bacteria, when filtering blood, e.g. in the separation of platelets, due to its selectivity. They are also particularly suitable as oxygenators where oxygen flows in the interior of the membrane, while the exterior is in contact with blood.

The membranes can also be used for a number of purposes as membrane carriers for other membrane layers. Because of the outstanding smooth surface structure with open pores, the membrane can be particularly well equipped with a firmly adherent thin layer of a substance acting as a membrane, which often happens when coating or spraying with corresponding film-forming solutions. Because of its excellent surface properties, the resulting membrane layer adheres very well to the membrane according to the invention. The application solution can be applied very regularly as a thin skin, without penetration or dripping of the solution into the interior of the hollow wire, so that very effective membrane combinations can be produced for the most diverse application areas.

Due to the special surface structure and the structure in the interior, the membranes are also excellently suitable as a substrate for special substances. It is also possible to introduce an active substance into the internal continuous cavity of a hollow wire membrane.

In this way, you can obtain bodies in the form of flat foils, snake foils or hollow wires, respectively. section thereof, which slowly emits absorbed active substance. The membranes produced according to the invention can also be used for the adsorption of substances.

According to the invention, foils of ordinary dimensions have been made available, i.e. a thickness of 20 ym to a few mm. The foils can be available as flat or tube foils. The foils can also be used as insulation materials, e.g. for heat insulation or sound dampening.

According to the invention, hollow wires within large dimensional ranges have also been made available. In this way, outer diameters of up to several mm can be achieved, and the wall thickness can be varied within wide limits and can e.g. lie between 20 ym and up to 1-2 mm.

The pores in the membranes produced according to the invention can have a wide variety of shapes. Thus, they can be round or elongated and connected to each other, e.g. via small connecting cavities, in part they can go directly into each other. Even with membranes obtained from mixtures with an in-

hold only approx. 30% polymer, the polymer can still be the matrix in which the individual pores are distributed and exhibit more or less separate but interconnected cavities. Conversely, structures can also arise where the cavities form the matrix in the same way as in flor, and the polymer substance is arranged quasi-fibril-like. The transitions between these different structures are fluid and occur partly in a mixture, furthermore, the structure shapes can also be influenced by further process parameters such as extraction speed, cooling speed,

stretch under the nozzle etc.

The membranes produced according to the invention are distinguished 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 more detail in "Flow of Fluids through Porous Materials" by R.E. Collins, Reinhold Publishing Corp., New York 1961, page 10. K is defined as:

K <=>A(AP/h) where °- is the volume flow per time unit (e.g. m<3>/sec.),

n is the viscosity of the flowing medium (Pascal . sec.), A is the average surface through which the gas exits, AP is the pressure

the difference (Pascal) and h is the wall thickness.

The permeability coefficient for the membranes according to the invention is at least 10.10 cm and preferably at least 22.10 cm, but values of over 100.10 cm can also be achieved.

In the case of membranes in the form of flat foils or hose foils, the coefficient is measured using nitrogen which is pressed under pressure against a foil fixed in a flange. Using a flow meter, the air passing through the foil is measured.

With foils produced according to the invention" which consist of a mixture of 30% by weight polypropylene and 70% by weight N,N-bis-(2-hydroxyethyl)-hexadecylamine and by maintaining an air gap between nozzle and bath, one could achieve following values:

In the case of hollow wire membranes, the measurement took place in the following way:

31 cm long hollow wires were embedded in 50 50 cm long PVC hoses using curable polyurethane compound. After the polyurethane had cured, a PVC hose was cut out, and the exposed opening was connected to a nitrogen bottle via a supply line, and the end of the other hose was sealed with a stopper. With the help of a flow meter, the amount of air that came out of the wire could be measured.

With raembran hollow wires produced according to the invention and consisting of a mixture of 30% by weight polypropylene and 70% by weight N,N-bis-(2-hydroxyethyl)-hexadecylamine and by maintaining an air gap between nozzle and bath, the following was achieved values:

The membrane hollow wires can also be used as insulating materials.

A suitable device for producing the membrane hollow wires is described in more detail in fig. 1. 1 is a thermostatically controlled container from which the inert liquid via a double piston pump 3 and a further heating device 4 is dosed to the mixer 8. The heating device 2 serves for preheating. From a cutting container 5 via an extruder 6 and a gear pump 7, polypropylene comes to the mixer 8, from which a hollow wire nozzle 10 is fed via a gear pump 9, while sufficient nitrogen is supplied via a Rotameter 17. The exiting quantity moves over an air gap and down into a spinner tube 12 equipped with a spinning funnel 11, the spinning tube being fed with inert liquid via a level container 13 from a main reservoir 14. The spinning tube has a curvature at its lower end, and the thread is led after it has left the bath 15 to be wound on a spool 16.

The invention shall be described in more detail by the following examples.

Example 1

In an extruder at a heating temperature of 260-280°C, polypropylene is melted with a melting index of 1.5 g/10 min and fed from there by means of a gear pump to a well-functioning stick mixer.

At the same time, N,N-bis-(2-hydroxyethyl)-hexadecylamine is dosed to the mixer via a special supply line by means of a double piston pump after preheating by means of a continuous heater.

The mixing ratio between polypropylene and amine is 30:70. The mixer rotation speed was set to

400 rpm.

After running through the mixer, the homogeneous mixture was fed to a hollow wire nozzle in a quantity of 15 g/min, the hollow wire nozzle having a light opening of 2000 um and a free annular gap of 400 um. By adding 4 l/hour of nitrogen to the gas capillary in the nozzle, formation of the hollow wire was achieved.

The emerging melt-flowing wire, after a free fall of 3 mm, descended into the spinning funnel filled with amine as a precipitation bath, moved together with the precipitation agent through the then following spinning tube with a diameter of 8 mm and a length of 400 mm and, after passing through a subsequent spinning bath with a length of 1 m wound up on a bobbin unit at a speed of 7 m/min.

The hollow wire obtained was extracted with alcohol and freed from amine.

The hollow wire had a diameter of 2200 ym and a lumen of 1400 ym.

Example 2

Polypropylene shavings were melted in an extruder and dosed to a stick mixer via a gear pump.

At the same time, liquid N,N-bis-(2-hydroxyethyl)-hexadecylamine with a temperature of 40°C was poured from a heatable storage container via a double piston pump into an electrically heated heater and from there at a temperature of approx. 150°C fed to the mixer. A stick mixer was used as a mixer.

After homogenization of the components, the melt was pressed through a slit die by means of a metering pump and extruded in a bath consisting of pure N,N-bis-(2-hydroxyethyl)-hexadecylamine and exhibiting a temperature of 50°C.

After the flat foil had run through the bath which had

a length of 50 cm, the film obtained was extracted with ethanol and dried. A foil with outstanding membrane properties and a particularly good surface structure was obtained.

Claims (14)

1. Process for the production of a membrane in the form of flat foil, hose foil or a hollow wire of synthetic polymers with 10 to 90% by volume of interconnected pores, an apparent density of 10 to 90% of the true density of the polymer used , a surface that exhibits smooth, open pores, whereby the proportion of openings in the surface amounts to 10 to 90% and a permeability coefficient of -12 2 at least 10x10 cm, characterized by extruding a homogeneous mixture of at least two components, whereby one component is a meltable component and the other component is a liquid inert to the polymer, whereby the mixture contains 10 to 90% by weight of polymer and 90 to 10 % by weight inert liquid, and both components form an inert system which in the liquid aggregate state exhibits a region of full miscibility and a region of mixing holes, at a temperature above the mixing temperature of a bath containing the inert liquid of the extruded component mixture and having a temperature below the boiling temperature, and fixes the formed membrane in the form of a flat foil, hose foil or hollow wire. 2. Method according to claim 1, characterized in that the formed membrane is washed out after fixation with a solvent. 3. Method according to claim 2, characterized in that acetone is used for washing out. 4. Method according to claims 1 to 3, characterized in that an air gap is maintained between the outlet surface of the extruder tool and the bath surface. 5. Method according to claim 4, characterized in that the air gap is heated. 6. Method according to claims 1 to 3, characterized in that the homogeneous mixture is extruded immediately into the bath. 7. Method according to claims 1 to 5, characterized in that the temperature in the bath is kept at least 100°C below the separation temperature for the binary mixture used. 8. Method according to claims 1 to 6, characterized in that the homogeneous mixture is first extruded into a spinner tube filled with bath liquid which is located upstream of the bath. 9. Method according to claims 1 to 8, characterized in that polypropylene is used as polymer. 10. Method according to claims 1 to 9, characterized in that N,N-bis-(2-hydroxyethylene)-hexadecylamine is used as inert liquid. 11. Method according to claims 1 to 10, characterized in that a bath with temperature steps is used. 12. Method according to claims 1 to 11, characterized in that the two components are mixed continuously before extrusion. 13. Method according to claim 12, characterized in that a stick mixer is used for the mixture. 14. Use of a membrane in the form of a flat foil, hose foil or hollow wire, produced according to claim 1, as a filter, preferably as a filter for microfiltration.