DE2833493A1 - Polypropylene membranes with porous surface - are used for filters, microfilters, membrane supports and as oxygenation agents - Google Patents

Abstract

Mfr. of membranes of a thermoplastic polymer contg. 10-90 vol. % pores in connection with each other comprises (A) extruding a homogeneous liq. mixt. consisting of at least 10-90 wt.% of a fusible polymer (I) and 90-10 wt.% of a pore-forming liq. (II) which is inert w.r.t. (I); (B) choosing the type and amt. of the inert liq. (II) such that (II) forms a binary system with (I) which in the liq. state has a region of complete miscibility and a region of incomplete miscibility of the components and which demixes into two phases at a temp. (the demixing temp.) below the extrusion temp.; (C) leading the extruded mass into a bath in which the bath liquor is below the demixing temp. and cooling such that the (II) and (I) demix into two co-existing liq. phases before the polymer (I) solidifies and (D) opt. washing (II) out of the solidified membrane with a solvent, such that the bath contains the same inert solvent, e.g. N,N-bis-(2-hydroxyethyl)hexadecylamine, as the extruded mixt. and the membrane, esp. formed from polypropylene and extruded in the form of flat film, tubular film or a hollow fibre, has a smooth surface with open pores occupying 10-90 % of the surface area.

Description

Hollow filaments

The invention relates to porous hollow fibers and processes for their production and their use, in particular as filters or membrane supports for separation purposes.

Hollow fibers have been known for a long time. The specialist has one Range of processes available to make hollow fibers from a wide variety of polymers to manufacture. Hollow fibers can be used in the manufacture of textile products will; But they are also used in filtration, ultrafiltration, microfiltration, Dialysis, reverse osmosis, etc.

If hollow fibers are used in cutting devices, it is crucial on the permeability and selectivity of the hollow fibers, because they act as filters or membrane functioning hollow fibers should on the one hand hold back certain substances, on the other hand allow other substances, e.g. the solvent in a solution, to pass through as quickly as possible.

A method for making permeability selective hollow fibers is described for example in DE-AS 14 94 579, in which an intimate mixture a thermoplastic polymer is melt spun with a plasticizer and the plasticizer is then extracted from the hollow fibers obtained. Included Among other things, it is required that the plasticizer from the hollow spun fiber is light and is substantially completely removable. Often, however, one is proportionate to this long treatment of the thread required when extracting; nor is it always possible to completely remove the plasticizer. In addition, they prove Hollow fibers produced using this process have a relatively low permeability on. After all, it is not possible to change the proportions of plasticizer and polymer within wider ranges to vary; if there is a high proportion of plasticizer, there is no thread formation Instead, if the plasticizer content is too low, the permeability is too low achieved. There is also the risk that the plasticizers will not go down when they are mixed sufficiently distributed in the thermoplastic polymer so that agglomerations are formed, which lead to holes or oversized pores when washed out, which impair the usability exclude hollow fibers for many purposes.

Another method for producing hollow fibers is described in DE-AS 23 46 011 described, in which a solution of a copolymer of acrylonitrile is spun into an aqueous solution of mineral salts. It is necessary to that coagulating liquid is also injected into the interior for coagulation will. The procedure described there is relatively complicated; Furthermore it is difficult to obtain hollow fibers with constant properties.

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

Although already numerous methods of making hollow shutters are known, there is still a need for improved manufacturing methods, especially after those in which the spinning mass to be extruded is simple can produce and work without complicated spinning baths. It also exists a need for improved hollow filaments that are porous and which, among other things, extend through are characterized by good permeability and high selectivity at the same time.

The object of the invention is therefore to provide a method to provide, in the polymer in a simple manner in an extrudable spinning mass can be reshaped and at the same time extruding and solidifying of the extruded material without the need for complicated spinning techniques or Spinning baths must be worked. Another object of the invention is to provide a method to make available that only by varying the process parameters it enables threads with adjustable porosity, permeability or permeability to win. Another object of the invention is to provide porous hollow fibers to ask, which is opposite to the known threads through a favorable open surface characterized in which both the outer and the inner wall of the hollow fiber one with open pores, but still has a flat structure. The object of the invention is also to provide hollow fibers that are both in the textile, technical and medical sectors, e.g. in separation processes Can be used and in particular as a filter, microfilter, membrane carrier and suitable as a carrier substrate for certain substances.

This object is achieved by a method for making porous Hollow fibers dissolved, which is characterized in that there is a homogeneous mixture of at least two components, one component being a meltable polymer and the other component is a liquid inert to the polymer, and Both components form a binary system, which in the liquid state of aggregation is one Has an area of complete miscibility and an area with a miscibility gap a temperature above the separation temperature extruded into a bath that contains the inert liquid of the extruded component mixture and a temperature below the demixing temperature, and the hollow filament structure formed solidifies.

The hollow fiber structure formed can after solidification with a Solvents are washed out, acetone in particular being suitable.

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

It is also possible to add the homogeneous mixture directly to the bath extrude.

In a particular embodiment of the method according to the invention a temperature-graded bath is used. Here can the bathroom off consist of one or more parts that have a temperature gradient such as that the temperature from the beginning of the spinning bath to the exit end is continuous decreases.

It is also possible to use two or more separate baths, each of which has a different temperature.

It has proven to be beneficial if the bath has a temperature which is at least 1000C lower than the demixing temperature of the binary used Composition.

The homogeneous mixture can also first be poured into a spinning tube filled with the bath liquid are extruded.

According to the invention, homogeneous mixtures of 10 to 90 percent by weight can Polymer and 90 to 10 weight percent inert liquid are extruded.

The preferred polymer is polypropylene and the inert liquid NN-bis (2-hydroxyethyl) hexadecylamine is used.

The two components, namely the molten zone, are expedient Polymer and the inert liquid preferably continuously prior to extrusion mixed, it being advantageous if the mixing is not carried out until immediately before the extrusion takes place. The mixture can be homogenized before extrusion. To the A pen mixer is particularly suitable for mixing.

The hollow fibers according to the invention are characterized in that they have 10 to 90 volume percent intercommunicating pores and a flat surface with open pores own, the The proportion of open pores in the area is from 10 to 90%. The apparent The density of the hollow fibers is approximately between 10 and 90% of the true density of the one used Polymers; the permeability coefficient of the hollow fibers is at least 10 ~ 10 1210-12 cm2.

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

The hollow fibers can also serve as a membrane carrier, i.e. that on the Hollow fibers a layer is applied, which itself acts as a membrane.

The hollow fibers can also be used as a carrier substrate, i. The hollow fibers can be filled with certain substances at a later date should be given, soaked. They can also be used as oxygenators will.

To carry out the method according to the invention and for the production of the hollow fibers according to the invention can be conventional, in particular fiber-forming macromolecular substances, especially synthetic polymers, are used e.g.

obtained by polymerization, polyaddition or polycondensation The prerequisite is that the polymer is meltable, i.e. in the liquid Physical state can pass without decomposition and with an inert to him Liquid forms a binary system, which in the liquid state of aggregation forms an area has complete miscibility and also still has a liquid state of aggregation Possess area with miscibility gap.

Such systems have a phase diagram for the liquid state in the way, for example, in the Textbook of Physical Chemistry by S. Glasstone, Macmillian and Co. Ltd., St. Martins's Street, London 1953 Page 724 for the system aniline liexan is reproduced. This diagram is complete miscibility is given for the two components above the curved curve. Below the curve, two liquid phases are in equilibrium with one another.

It is not absolutely necessary for the invention to be carried out that in the 2-phase range the two components each have a considerable solubility compared to the other components, like the one in the diagram above the case is. In many cases it is sufficient if there is marginal solubility in the liquid 2-phase area is available. It is essential, however, that the two components are in the liquid state still form two liquid phases side by side. In this respect, they differ Systems that can be used in the present invention include such systems which the dissolved polymer directly becomes more solid when the temperature is lowered Substance precipitates without first changing its liquid state during cooling to go through.

Conventional meltable polymers can be used in the context of the invention like those obtained by polymerisation, polymers, polyethylene, polypropylene, Polyvinyl chloride, polyacrylates, polycaprolactam and corresponding copolymers, among others; Polycondensation polymers such as polyethylene terephthalate, polybutylene terephthalate, Polyamide-6.6, polyphenylene oxide and polyaddition polymers such as polyurethanes and polyureas.

In principle, they are suitable as inert liquids within the scope of the invention all those liquids that are binary with the polymer in the liquid state Form a system of the type mentioned above. Inert to the polymer means that the liquid does not have a significant amount in a short period of time Degradation of the polymer causes or reacts with the polymer itself.

Even if the above-mentioned state diagram of the aniline / exan system reproduces the ratios for a binary mixture that in and of itself only consists of consists of two essentially pure, uniform substances, so should in the context of the invention, the term binary system does not apply strictly to mixtures of only two pure uniform substances are used. The average specialist knows that a polymer substance is made up of a multitude of different molecules Molecular weight is composed, therefore, such polymers are with a corresponding molecular weight distribution in the context of the invention as a component the same applies to copolymers.

Under certain circumstances, even polymer blends like a uniform component behave, a single-phase mixture with an inert solvent form and separate into two liquid phases below the critical temperature.

However, only one polymer is preferably used.

The liquid also does not necessarily have to be completely pure and to represent a completely uniform substance.

So it often does not harm, even if small amounts of impurities, possibly also proportions of homologous compounds, such as those produced by large-scale production are conditional, are present.

For the practical implementation of the process, the two components are used a homogeneous mixture is produced at the required temperatures. This can done in such a way that the inert liquid with the crushed polymer mixes and heated to appropriate temperatures, taking for an appropriate Mixing is ensured.

Another suitable method is that one of the two components brings separately to the required temperature and the two Components in the desired proportions only continuously shortly before extrusion mixed together. This mixing can take place in a pen mixer, the appropriate between the metering pumps for the individual components and the spinning pump is arranged. A subsequent homogenization can be recommended.

It is often advisable to create a homogeneous mixture by creating a appropriate vacuum before extrusion.

The ratio of polymer to inert liquid in the spinning mass can be varied within wide limits.

By adjusting the ratio of polymer to inert liquid can to a large extent the pore volume inside and also the surface structure how the number of open pores on the surfaces of the hollow thread obtained is controlled will. This means that they can be used for a wide variety of purposes Win hollow fibers.

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

By increasing the difference between the temperature of the homogeneous However, the mixture to be extruded and the demixing temperature can also be used achieve interesting effects in terms of the structure of the threads obtained.

The homogeneous spinning mass is then extruded into a bath that the contains inert liquid of the extruded component mixture and a temperature below the demixing temperature. Preferably the bath is complete or for the most part from the inert liquid, which is also in the extruded Mixture is present. The temperature of the bath is below the demixing temperature of the binary mixture used, i.e. below the temperature above which the both components can be mixed completely homogeneously with one another. Preferably lies the temperature of the bath at least 1000C below the demixing temperature of the used mixture.

The temperature can also already be so low that one is accordingly the state diagram applicable to the binary system has already moved in the area in which a solid phase occurs.

Is the temperature of the bath so high that one is still in the liquid Moving the 2-phase area, it is necessary to immediately reconstruct the thread structure that is created to solidify, which can be done by following a certain distance decreases the temperature accordingly within the bath.

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

It has been found beneficial when using the bath in certain cases a spinning tube is connected upstream, which is also filled with the bath liquid and that dips into the spinning bath. The spinning tube can be at its inlet opening have a conventional spinning funnel, at its lower end the tube can be curved to make it easier to pull the thread through the bath.

The spinning tube can be filled via a level vessel surrounding the spinning tube take place through overflow into the spinning tube; for complete filling and maintaining the level in the spinning tube, it is necessary to use this level vessel more Supplying bath liquid from a main reservoir than draining through the spinning tube; the excess amount of bath can be poured into the tank through a second overflow Main reservoir to be returned. The main reservoir and level vessel can be thermostated will.

After it has come out of the spinning bath, the thread can with a appropriate extractants are washed out.

A number of solvents are available for extraction such as Acetone, cyclohexane, ethanol and the like. as well as mixtures of such liquids are suitable.

In some cases, there is no need to wash out the thread, especially if the inert liquid used is itself the thread for the give certain additional properties to later areas of application or even themselves to exercise a function. For example, liquids can be used which have an antistatic effect on the thread or which act as lubricants works.

It has been found to be useful for a number of applications between the exit surface of the extrusion tool, i.e. the exit surface for example a corresponding hollow fiber nozzle and the surface of the bath Air gap is maintained. By varying the air gap it is possible to change the structure of the hollow fiber obtained, in particular to influence its surface.

It has been found that by lengthening the air gap, the number the open pores in the surface are reduced and increased by shortening it can be; the diameter of the pores also decreases as the air gap increases.

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

Generally, the air gap is at least about 1 mm wide and can Assume a length of up to 10 cm, depending on the working conditions. Important is, that in the air gap before entry into the bath none, or at least none noticeable Segregation occurs in two liquid phases, this can as said by the brevity the distance or can be controlled by heating, but it is also possible by Increase the exit speed at the nozzle to counteract premature segregation.

In a special embodiment of the method according to the invention however, the homogeneous mixture is extruded directly into the bath, with the Surface open pores with maximum diameter arise.

The hollow fibers obtained can be used particularly well as filters. They can mainly be used in microfiltration. Particularly suitable are the threads for the medical field, where they e.g. because of their selectivity for the separation of bacteria, for the filtration of blood e.g. for the separation of Platelets can be used. They are also very suitable as oxygenators, where oxygen flows through the inside of the hollow filaments while the outside of blood is washed around.

The hollow fibers can also be used as membrane supports for a number of purposes be used. Due to their excellent even surface structure with open Pores can be thinned extremely well with a firmly adhering thin one Cover a layer of a material that acts as a membrane 0 what often by coating or spraying with appropriate film-forming solutions happens. Because of its excellent surface properties, the The resulting membrane layer on the hollow threads is not only very good, but also the Coating solution can be applied without it penetrating or even dripping through the solution gets inside the hollow thread, apply it very evenly as a thin skin, so that very effective membranes are made for a wide variety of applications can be.

Due to their special surface structure and the structure in the Inside the hollow fibers, they are also ideally suited as a substrate for certain Substances. So the threads can be soaked with antistatic agents that or it can already be used as an inert liquid during the spinning process only at a later point in time, after the thread has been produced, by treating e.g. the agent can be introduced into the thread structure by soaking. It is also possible, to bring the active substance into the inner continuous cavity of the thread.

In this way, bodies with long-term effects can be obtained, which slowly release the absorbed active ingredient.

Conversely, the hollow fibers can also be used to adsorb substances.

In accordance with the invention, hollow filaments are within a wide range of dimensions accessible. Outside diameters of up to several millimeters can be achieved, the wall thicknesses can also be varied to a large extent and can, for example, be between 20 Microns and about 1 to 2 millimeters.

The pores in the hollow fibers according to the invention can be very diverse Have shapes. So they can be round or elongated and stand together in connection, partly through small connecting cavities, partly because they are directly pass each other. Even with hollow fibers made from mixtures with a content of only about 30% polymer has been obtained, the polymer can still be the matrix, in which the individual pores are distributed and still more or less discrete, but form interconnected cavities. Conversely, structures can also arise in which the cavities form the matrix and the polymer substance, similar to nonwovens is arranged quasi-fibril-like. The transitions of these two structures are fluent and sometimes mixed; the structural forms can also be further Process parameters such as take-off speed, cooling speed, warpage can be influenced below the nozzle.

The hollow fibers according to the invention are characterized above all by a high permeability to gases such as nitrogen or air. The permeability can be given by the so-called permeability coefficient K, as he in the book Flow of Fluids through PorousMaterials by R.E. Collins appeared at Reinhold Publishing Corp., New York 1961, page 10 will be discussed in more detail. K is defined as K = A (4P./h>, where Q is the volume flow in the time unit (e.g. m3 / s), X the viscosity of the flowing medium (Pa e 1), A the mean area through which the gas escapes, AP is the pressure difference (Pa) and h is the wall thickness of the filaments.

The permeability coefficient of the hollow fibers according to the invention is at least 10 ~ 10 1210-12 cm2, preferably at least 22 ~ 10 12io # 12 cm2, values over 100 ~ 1 o 12 cm2 can be achieved.

The coefficient was measured in the following way: 31 cm long Hollow fibers are made into two 5 cm long with the help of a hardenable polyurethane mass PVC tubing embedded.

After the polyurethane has hardened, a PVC hose is cut, and the exposed openings are via a feed with a nitrogen bottle connected, the end of the other hose is tightly closed with a stopper. The air escaping through the threads is measured with the aid of a flow meter.

In the case of hollow fibers according to the invention, which consist of a mixture of 30 percent by weight Polypropylene and 70 wt.% NN-bis (2-hydroxyethyl) hexadecylamine and compliance with one The following values were found for the air gap between the nozzle and the bath be: K (10 cm2) air gap to the 99 3-22 20 The hollow fibers can also be used as an insulating material and are used in textile fields of application.

A suitable device for producing the hollow fibers according to the invention is explained in more detail in FIG.

1 is a thermostattable container from which the inert liquid metered into the mixer 8 via a double piston pump 3 and a further heater 4 will. The heater 2 is used for preheating. From the schnitzel container 5 passes over an extruder 6 and a gear pump 7 polypropylene into the mixer 8, from which a hollow fiber nozzle 10 is fed via a gear pump 9, which via a rotameter 17 is supplied with the required amount of nitrogen. The exiting Mass passes through an air gap into a spinning funnel 11 provided with a spinning funnel Spinning tube 12, via a level vessel 13 from the main reservoir 14 with inert liquid is supplied. The spinning tube has a curvature at its lower end that After leaving the bath 15, threads are directed to the winding 16.

The invention is explained in more detail by the following example: Example In an extruder at heating temperatures of 260-0 280 C polypropylene is one Melt index 1.5 g / 10 min and melted via a gear pump in one good effective pen mixer dosed.

At the same time, NN-bis (hydroxyethyl> hexadecylamine by means of a Double piston pump, preheated to 1350C via a flow heater, by means of a 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 passing through the mixer, the two substances that have become homogeneous become through a gear pump at a rate of 15 g / min into a hollow fiber nozzle with a clear ~ of 2000 # around and a free annular gap of 400 pressed. By adding 4 l / h nitrogen into the gas capillary of the nozzle, the formation of the hollow filament is achieved.

The emerging molten thread dips after a free fall path of 3 mm into the spinning funnel filled with amine as a precipitation bath, flows with the Precipitant through the subsequent spinning tube of 8 mm ~ and 400 mm in length and is after passing through a subsequent spinning bath of 1 m length at 7 m / min on one Winding unit wound up.

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

The hollow filament has a ~ of 2200 / an and a lumen of 1400 / um.

Claims (22)

Claims 1. A method for producing porous hollow fibers, characterized in that a homogeneous mixture of at least two components, wherein one component is a fusible polymer and the other component liquid is inert to the polymer and both components are binary Form system, which in the liquid state of aggregation an area of complete miscibility and has a miscibility region at a temperature above that Separation temperature extruded into a bath containing the inert liquid of the extruded Components mixture contains and a temperature below the demixing temperature possesses, and solidifies the hollow filament structure formed. 2. The method according to claim 1, characterized in that the formed hollow filament structure washes out with a solvent after solidification. 3. The method according to claim 2, characterized in that one for Wash out acetone used. 4. The method according to claim 1 to 3, characterized in that one between the exit surface of the extrusion tool and the surface of the bath Maintains the air gap. 5. The method according to claim 4, characterized in that the air gap is heated. 6. The method according to claims 1 to 3, characterized in that that the homogeneous mixture is extruded directly into the bath. 7. The method according to claims 1 to 6, characterized in that that the temperature of the bath is at least 1000C below the separation temperature of the binary mixture used. 8. The method according to claims 1 to 7, characterized in that that the homogeneous mixture is first placed in an upstream bath with the bath liquid Filled spinning tube extruded. 9. The method according to claims 1 to 8, characterized in that that the polymer used is polypropylene. 10. The method according to claims 1 to 9, characterized in that that the inert liquid used is NN-bis (2-hydroxyethyl) hexadecylamine. 11. The method according to claims 1 to 10, characterized in that that a temperature-graded bath is used. 12. The method according to claims 1 to 11, characterized in that that a homogeneous mixture of 10 to 90 percent by weight polymer and 90 to 10 Weight percent inert liquid used. 13. The method according to claims 1 to 12, characterized in that that the two components are continuously mixed prior to extrusion. 14. The method according to claim 13, characterized in that for Mix used a pen mixer. 15. Hohifädan made of synthetic polymers, characterized by 10 up to 90 percent by volume of interconnected pores and a flat, surface having open pores, the proportion of openings in the area 10 to 90%. 16. Hollow fibers according to claim 15, characterized by an apparent Density from 10 to 90% of the true density of the polymer used. 17. Hollow fibers according to claims 15 and 16, characterized by a permeability coefficient of at least 10. 1o ~ 12 cm2. 18. Use of the hollow fibers according to claims 1 to 17 as a filter 19. Use of the hollow fibers according to claims f to 18 as a filter for microfiltration. 20. Use of the hollow fibers according to claims 1 to 17 as a membrane carrier. 21. Use of the hollow fibers according to claims 1 to 17 as a carrier substrate. 22. Use of the hollow thread according to claims 1 to 17 as oxygenators.