DE3329578A1 - Pore-containing polyvinylidene fluoride mouldings - Google Patents

Abstract

An improved process is described for the production of pore-containing mouldings, in particular membranes, from polyvinylidene fluoride (PVDF). To this end, a single-phase liquid mixture of PVDF and one or more mixture components is forced through a nozzle into a cooling device. Cooling of the mixture is accompanied first by separation into two liquid phases, and then by solidification. The cooling device contains a cooling liquid whose mean velocity in the device is significantly lower than the velocity at which the PVDF mixture emerges from the nozzle. The liquid level is kept constant at the PVDF mixture inlet and outlet. The PVDF mixture passes through the device from top to bottom, at least until the point at which solidification begins, and only then is it diverted in order to pass through a second zone from bottom to top. The process steps enable the mechanical load on the PVDF mixture to be kept very low until it starts to solidify. This enables even relatively low-viscosity mixtures to be processed. The pore-containing PVDF mouldings are suitable for microfiltration, in particular even of chemically aggressive media, and for transmembrane distillation.

Description

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Polyvinylidene fluoride standard bodies with pores

A k ζ ο GmbH Wuppertal

The invention relates to a method for producing polyvinylidene fluoride (PVDF) molded bodies having pores, which is improved over known methods. It also relates to PVDF moldings that can be produced by this process, e.g. in the form of hollow threads, tubes or foils.

Polyvinylidene fluoride (PVDF) is an interesting material because of its chemical nature. Especially his low chemical reactivity as well as its hydrophobic properties make it a valuable raw material for the most diverse areas of application. Weatherproof paints and coatings are an example here. It therefore made sense to try to produce PVDF moldings containing pores, e.g. in the form of membranes. These membranes were expected to be suitable for the Use a wide variety of separation processes.

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In particular, they should be able to be used to advantage where other polymers, due to their chemical nature, i.e. their instability against some organic substances, their sensitivity to hydrolysis in acidic or alkaline Medium or their susceptibility to oxidation, cannot be used or can only be used to a limited extent.

There are therefore not only known processes for the production of membranes from polymers, but also PVDF membranes of various embodiments. These are prepared by _methods% can also be used for the preparation of membranes from other polymers in the same or slightly modified form. For example, US Pat. No. 4,238,571 describes a process for producing a porous material, for example from PVDF, in which the solvent is evaporated starting from a solution of the polymer in order to obtain a preliminary stage of porous polymer which is then subjected to a subsequent reaction. European application 40 670 specifies a process for the production of porous PVDF in which a membrane is produced from a solution of the polymer in a special mixture by pouring the solution onto a solid base and then bringing it into contact with a coagulating medium . European application 37 836 describes porous hollow filaments made of PVDF, which are produced by extruding a solution of the polymer and then coagulating it with a coagulating solution.

In addition to these processes for producing porous PVDF membranes, there are also special processes for producing

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Membranes or porous structures known from other polymers. For example, DE-OS 27 37 745 describes a method for the production of microporous polymer structures, in which of a homogeneous solution is assumed, which first passes through a liquid two-phase system before cooling Solidification takes place. Compared to other methods, this results in advantages in the design of the pore system achieved.

From DE-OS 28 33 493 a process for the production of porous hollow fibers emerges which allows the pore system to influence in a targeted manner. Here, too, the segregation of a polymer solution into a liquid two-phase system is used Use made before solidification occurs in a cooling bath. Finally, DE-OS 30 26 718 describes a Process for the production of porous polymer hollow fibers, in which a homogeneous polymer solution when cooled in a The bath first forms two liquid phases and the solidification in the bath is carried out under special conditions.

The processes described for the production of porous polymer structures such as membranes, in addition to some However, there are also advantages, sometimes serious, disadvantages. This is the case in those cases where precipitation or Solidification of the polymer takes place without passing through a liquid two-phase system, not at all or only to a limited extent possible to specifically influence the pore system with regard to pore size and characteristics.

Manufacturing process in which the polymer solution segregates into two liquid phases before solidification

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takes place at which the solidification temperature is at or below room temperature, however, have the disadvantage afflicted that to form the porous polymer structure a washing or extraction process is necessary. The speed at which the pore system forms is dependent on this on the rate of diffusion of the extractant through the gradually solidifying outer layers into the interior. The process speed and the thickness of the membrane to be produced are thereby subject to strong Restrictions. In addition, the dimensional stability during the manufacture of the products, especially in the case of Hollow fibers, often much to be desired. The manufacturing processes in which a liquid two-phase system goes through and in which the polymer solidifies above room temperature, have clear advantages over this on. They are well suited for the production of porous structures from different polymers and lead to good dimensional stability of the products. However, it has been found that even with these methods certain difficulties can arise when porous molded articles are produced from polymer compositions that are sensitive to mechanical stress during processing. This susceptibility to mechanical stress is particularly evident in the case of low-viscosity polymer mixtures.

Also according to the favorable processes in which a mixture of a homogeneous liquid polymer composition is used Cooling first passes through a liquid two-phase area and then solidifies above room temperature, it is

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difficult to process these mixtures into pore-containing moldings if the viscosity of the to be processed Mixing before its extrusion through a nozzle below a certain range, namely below about 15Pas lies. Especially in the case of PVDF compositions however, the viscosities are often lower than in the case of liquid mixtures containing other polymers. Furthermore It is often desirable to process mixtures of relatively low viscosity in order to achieve special configurations of the To achieve pore system.

The object of the present invention was therefore to provide an improved process for the production of pores having PVDF moldings, in particular PVDF membranes in the form of To provide hollow fibers, tubes or foils. The process should have good dimensional stability of the products guarantee during their production and thus also the processing of low-viscosity PVDF mixtures, e.g. up to down to about 2 Pa · s, enable products with constant and deliberately variable properties without difficulty. One of the main requirements of the process to be developed was therefore the mechanical stress on the polymer composition at least up to the point at which the polymer begins to solidify, since the composition only after the start of solidification does it gain a certain stability against mechanical stress.

The object is achieved by a method which has the following steps:

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a) Production of a single-phase liquid mixture of PVDF and one or more inert to PVDF Mixture component (s), wherein at least one mixture component at the temperature of the preparation of the mixture represents a solvent for PVDF and the mixture in the liquid state has a temperature range above room temperature complete miscibility, above room temperature a temperature range with a miscibility gap and a solidification temperature above room temperature

b) Conveying the single-phase liquid mixture at a temperature above the separation temperature a nozzle from top to bottom into a cooling device, which is filled with a cooling liquid, in which PVDF at the temperature of the cooling liquid not or dissolves only insignificantly and which has a temperature below the solidification temperature of the mixture, wherein the level of the cooling liquid both at the point of entry of the mixture into the cooling liquid and is kept constant at the exit point for the solid shaped body formed and wherein the middle Speed of the cooling liquid in the device is adjusted so that it, measured in the direction of movement of the polymer mixture, in the area between the point of entry of the mixture and the point where it begins Solidification is significantly less than the speed at which the mixture exits the nozzle exit

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c) Passing the mixture from the point of entry into the cooling liquid at least to the point of incipient solidification from top to bottom through a channel-shaped zone surrounded by a wall

d) deflection of the formed body and subsequent guidance of the shaped body from below up in a second zone

e) removing the shaped body from the cooling liquid.

These process steps make it possible to produce molded bodies with pores / e.g. PVDF membranes in the form of To produce hollow fibers, tubes or foils, which are characterized by good dimensional stability. It can besides Membranes but also porous PVDF threads can be produced by the process. Success through the process steps it, as explained in more detail below, the mechanical load that the PVDF mixture up to the point in time the incipient solidification, i.e. incipient dimensional stability, is exposed to be kept lower than with known methods. This makes it possible to use even relatively low-viscosity mixtures to form PVDF moldings to process, which is not or only partially possible according to previously known methods. After this Process according to the invention, products of constant and reproducible quality can be produced, while in the case of production according to previously known methods, especially in the case of processing

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low viscosity polymer blends, uncontrollable Quality fluctuations occur, which often lead to reduce the high mechanical stress on the not yet solidified or shape-stabilized polymer mixtures permit. The inventive method allows the Pore size and characteristics can be reproduced over a wide range by varying process parameters specifically set.

The mixture used to produce the PVDF molded body having pores is made of PVDF and one or more Compound component (s) which are inert towards PVDF are produced. In this context, inert is understood to mean that the mixture components do not chemically alter the PVDF. At least one of the mixture components must be at the temperature of the Make the mixture a solvent for PVDF so that a single phase liquid mixture is obtained. The type and amount of the mixture component (s) must also be selected so that the mixture can cool down first, as a result of segregation, passes through a temperature range in which two liquid phases coexist occur and only then solidify to form a solid shaped body. One of the two after segregation The phases formed represent a PVDF-depleted liquid phase composed of mixture component (s), the other a liquid phase depleted in mixture component (s) and enriched with PVDF. The latter leads to further Cooling to the PVDF molding. Both the temperature at which segregation occurs and the solidification temperature must be above room temperature

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to ensure that without additional work steps such as cooling to below room temperature or extraction of the solvent of the molded body is obtained on cooling to room temperature. Preferably the solidification temperature is above 50 ° C. It is possible, in addition to a solvent, in addition to use other mixture components such as nonsolvents, pigments, thickeners and surfactants, provided that the o.a. conditions are met. The addition of a non-solvent for PVDF in particular usually has advantages. The addition of a nonsolvent does not cause any significant change in the solidification temperature, increased but, depending on the type and amount of nonsolvent, the demixing temperature. This will set the temperature range enlarged, in which two liquid phases are present next to each other and thus the scope for a targeted one Variation of the pore system. In addition, the targeted addition of non-solvents in selected types and quantities leads to that more leeway is gained as to the type and amount of solvent. The choice of some solvents either in terms of type or quantity, without the addition of nonsolvents, the fact that mixtures, which only contain these solvents are not capable of forming liquid two-phase systems.

The method according to the invention allows, as mentioned above, to process low-viscosity mixtures. This option is specially designed for the production of Pore-containing molded bodies made of PVDF are important, since single-phase liquid mixtures containing PVDF, often lower viscosities at the temperature of the

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Show promotion through the nozzle than solutions of other polymers. The by the inventive method Given ^ greater latitude in terms of viscosity compared to known methods leads to greater latitude in the composition of the mixture, what PVDF concentration, Type and amount of the solvent and possibly other components of the mixture. This increased margin in turn leads to more possibilities in the design of the pore system.

The viscosity of the mixture used at the temperature at which it is conveyed through the nozzle can have values accept down to 2 Pa · s. The viscosity is preferably between 5 and 35 Pa * s. This means that the weight fraction of PVDF in the mixture about 10 - 90% and the weight fraction of solvent about 90 - 10% can be. However, the PVDF content is preferably between about 15 and 35% by weight.

The PVDF used to make the blend can be of commercial grade. The molecular weight however, it must be at least so high that the polymer is capable of thread formation. Solef, for example, is suitable 1012 (manufacturer: Solvay, Brussels). As a solvent for making the mixture are in principle all substances are suitable which result in a mixture which meets the requirements set out in claim 1 Fulfills. However, substances of low toxicity which are liquid at room temperature are particularly preferred, the a boiling point well above the demixing temperature of the PVDF mixture produced with them

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own. Glycerol triacetate, glycerol diacetate, 2- <2-butoxy-ethoxy-) ethyl acetate have proven to be particularly advantageous or mixtures thereof have been proven. The glycerol diacetate can be the 1.2 or 1.3 isomer or a mixture of both be. In the event that a non-solver is added, any non-solver can also be used Substance or any mixture of substances can be selected, provided that the requirements set out in claim 1 are met are fulfilled. As particularly advantageous, especially in connection with the preferred ones mentioned Solvents, di-n-octyl adipate and castor oil or mixtures thereof have been found.

The homogeneous liquid mixture is conveyed through a nozzle for further processing according to known methods and then enters a cooling device which contains a cooling liquid. The nozzle is depending on the one you want End product e.g. designed as a hollow filament nozzle, a nozzle for the production of hoses or a slot nozzle. To the mechanical It is particularly important to keep the stress on the mixture low before the point at which it begins to solidify in the case of the production of hollow fibers, it may be advantageous not to add one to the inner lumen by metering in, as is usual Gas, but to form an internal filling liquid. The internal filling liquid can be metered in in the nozzle or at the point where the mixture exits the nozzle. The internal liquid must be a liquid chosen that do not dissolve PVDF at the temperature of the conveyance through the nozzle. Preferred examples are glycerin or a liquid that was used in the manufacture of the PVDF mixture. The advantage

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of working with an internal filling liquid instead of the widely used gases is that a liquid in the subsequent cooling to lower volume work because of their lower coefficient of thermal expansion and thus leads to increased dimensional stability of the PVDF molded body during its formation. In addition, offers the use a liquid instead of a gas has the advantage / that the specific weight of the mixture leaving the nozzle can be set in a targeted manner in broader areas. This will make it possible to increase the speed during the subsequent approximately free fall of the mixture or the speed at which the mixture enters the cooling liquid to vary in a targeted manner up to the point of beginning solidification. In the event that the internal volume is due to a gas is generated, nitrogen is preferred.

The PVDF mixture emerging from the nozzle enters a cooling device one that is filled with a cooling liquid. Between the nozzle and the entry point of the mixture into the There may be an air gap for cooling liquid. This makes it easier to maintain the required temperature constancy at the nozzle achieved than when the nozzle is in contact with the (colder) cooling liquid stands. Since it is essential for the process, the mechanical stress on the mixture at least up to To keep the time of the onset of solidification as short as possible, the nozzle is perpendicular or almost perpendicular above the The entry point of the mixture into the cooling liquid is attached so that the mixture exits the nozzle enters the cooling liquid in approximately free fall. The mixture is cooled in the cooling device and solidification takes place to form the shaped body. the

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For this purpose the device is equipped with a cooling liquid filled. Cooling liquid can be added continuously during the process, whereby the direction with which this flows through the device, the direction of movement of the PVDF mixture or the formed Shaped body can be rectified or opposite. In this case, a continuous metering has the added liquid has a constant temperature which is below the solidification temperature of the mixture. The entry point for the cooling liquid into the device is in the case of simultaneous movement close to the point of entry of the mixture into the cooling liquid. At the exit point of the formed body In this case there is an overflow device at which the cooling liquid leaves the device. In the case of opposite directions of movement, the entry and exit points of the cooling liquid are appropriate reversed. However, it is also possible to work under stationary conditions, i.e. there is no continuous Dosing of cooling liquid takes place, but only the losses are compensated that arise as a result, that the formed body takes cooling liquid with it. Here too, of course, the cooling liquid must be used have a temperature in the device below the solidification temperature of the mixture. In this case it is expedient to maintain constant temperature conditions in the device, an external To carry out thermostatting of the device. The necessary Maintaining constant temperature conditions takes place depending on the process variant either by the flowing cooling liquid or by external thermostatting or both together or by itself temperature equilibrium during the process.

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In the case of large dimensions of the device, external thermostatting is recommended. Under constant temperature conditions is not understood in this context that the temperature of the cooling liquid at each point of the device has the same value - which is not at all due to the higher temperature supply of the polymer mixture is possible - but that at each point of the device the temperature prevailing there during the process does not change or changes only marginally. However, there may be a temperature gradient over the length of the device. For the invention it is of essential importance that the polymer mixture at least up to the point in time Solidification, i.e. subject to the lowest possible mechanical stress until the shape stabilization begins is. It is therefore necessary to ensure that the Mixing by the cooling liquid no acceleration - This applies to the case of the same direction of movement of the mixture and the cooling liquid - as well as no excessive braking - This applies to the opposite direction of movement. Therefore the mean speed must be measured in the direction of movement of the polymer mixture with which the cooling liquid flows through the device, at least in the zone between the point of entry of the mixture and its beginning solidification be significantly lower than the speed, with which the mixture exits the nozzle. Below a significantly slower speed is one to understand at least 25% slower speed. In the event that the cooling liquid is contrary to the Direction of movement of the mixture flows through the device,

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this value of at least 25% is of course always met, since the speed of the cooling liquid in this case has a negative sign. Thus, there is a wide margin for the speed of the cooling liquid. This is on the one hand for the same direction of movement by the above limit of 75% of the speed of the polymer mixture limited. In the case of the opposite direction of movement, this speed may of course not be arbitrary assume high values, but finds its limit where there is too high a relative speed between the polymer mixture and cooling liquid is achieved. This limit value depends on the respective process parameters and is by few Experiments easy to identify. The speed for the opposite direction of movement finds its there Limit where deformation or cracking of the not yet stabilized polymer mixture occurs. A rule of thumb for that The limit of the speed of the cooling liquid in the case of opposite direction of movement is a value of approx 5 · \ / v, ', where V ^ is the speed of the polymer mixture in m / min at nozzle outlet means. One would be the medium speed set the cooling liquid to the same value as the mixture when it exits the nozzle has, because of the velocity distribution in flowing liquids, the cooling liquid would have in the immediate Approach the mixture to a higher value than the above-mentioned nozzle exit speed and the mixture would accelerate.

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The speed of the cooling liquid, which is crucial Significance in the zone between the point of entry of the polymer mixture into the cooling liquid and the point at which it begins The solidification of the mixture must therefore be controlled. This is made possible by having the level of Keeps cooling liquid constant both at the point of entry of the polymer mixture into the cooling liquid and at the point of exit. For this reason, the Exit point an overflow device. In addition to the apparatus Measure, the speed is controlled by adding cooling liquid accordingly. in the In the case of a stationary bath, i.e. in the case where the speed of the cooling liquid has the value zero, means Dosing, of course, only compensates for losses.

A particular advantage of the process is, in addition to the possibility of To process low-viscosity polymer mixtures, in that because of the constant level of the cooling liquid and the low flow rate of the cooling liquid is given a wide leeway in terms of viscosity and Temperature of the cooling liquid and thus the type of cooling liquid regards. In known methods, this margin is much more limited because of the associated mechanical stress on the not yet stabilized polymer mixture.

In the normal case, the level of the cooling liquid is at the Entry and exit points not only kept constant, but the level is also constant at both points same high. However, there may also be a difference in altitude between the two.

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A preferred embodiment of the cooling device consists of a U-shaped bent tube, as shown in Figure 1, but embodiments as shown in Figure 2 can also be used. Figure 1 shows a
U-shaped bent device with the following components:

1. Hollow filament spinning nozzle

2. Cooling liquid metering and temperature control attachment

3. Stabilization zone of the hollow fiber

4. Cleaning drain for coolant

5. overflow cup with level control device for
Coolant

6. Degree of deduction

7. Cooling liquid supply from the thermostat

8. Coolant drain to the thermostat

Description of figure 1 :

In order to create a good possibility of observation during the process, the U-tube is mostly made of glass.

The tube, which is essentially composed of five parts, has a diameter of approx. 4 cm and one leg length from approx.

a) Coolant metering and temperature control attachment

In order to avoid the supply of the PVDF mixture as quickly as possible Compensating for the temperature change caused is in In this embodiment the entry point for the cooling liquid near the entry point of the hollow fiber.

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The temperature control jacket supports this function. At the same time, this outer jacket and the metering openings take care of it at the upper end of the pipe for a distribution of the liquid flows and allow a possible gentle treatment of the unstable hollow

thread. The entrance with a surface of approx. 5 cm The opening of the cooling tube ensures, on the one hand, sufficient leeway for thin and thick hollow filaments or hoses and, on the other hand, reduces a greater unsteadiness of the surface of the cooling bath.

b) Inlet leg of the U-tube (1st zone):

In this area, the hollow filament passes through the stage of separation into two liquids when it cools Phases and solidification. Depending on the recipe, nozzle temperature or cooling conditions, the beginning Solidification or stabilization can be observed in the first or second third of the pipe. Man recognizes this from the fact that the initially transparent thread becomes increasingly milky and begins at this point Solidification retains a certain final opacity.

c) deflection of the thread

The stabilized thread can be deflected without being deformed. In contrast to other embodiments the cooling device is no deflection wheel or no roller installed here. The one after the 1. Zone downwardly directed pipe continuation allows convenient handling when piecing. The sinking one Hollow thread collects here and can be pulled out of the outlet limb (2nd zone) by means of wire be placed on the trigger wheel.

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d) outlet legs

Here the thread reaches the desired final temperature.

e) Outlet cup with overflow device

This device is made of metal. The height-adjustable middle section determines the overflow height of the Cooling medium and thus also the normally level height of the cooling liquid under the spinneret. In this way, an air gap between the nozzle and the cooling liquid can be set in a simple manner.

Components of the device shown in Figure 2:

1. nozzle

2. Cooling liquid metering

with cooling pipe, inlet cup and overflow collecting tray

3. cooling tube (transparent)

4. Idler pulley

5. Cooling box with window

6. Overflow device with level control (adjustable: level with inlet or slight level difference)

7. Trigger wheel

For the invention it is essential that the PVDF mixture between the time it leaves the nozzle and the When solidification begins, there is as little mechanical stress as possible, be it by tension or by Shear forces. In particular, the mixture in this area must not be highly mechanical due to the cooling liquid

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can be influenced, e.g. by acceleration. In addition to the above-mentioned control of the speed of the cooling liquid this is achieved in that the mixture is in a first zone, which is from the entry point of the mixture reaches into the cooling liquid to the point of beginning solidification, is guided from top to bottom. These Zone represents a relatively narrow channel surrounded by walls, e.g. in the form of a cylindrical tube, the diameter of which is significantly shorter than its length. The point of beginning solidification of the mixture, up to which the mechanical Stress must be kept as low as possible, can be determined in a simple manner. This is done through Observation of the changes which the mixture undergoes after exiting the nozzle. First takes place as a result Cooling takes place the segregation into two liquid phases. The formation of two phases is accompanied by an increase in viscosity in advance. With the formation of two phases, the mixture begins to become cloudy, which continues until it begins to solidify reinforced. Since the solidification begins on the outer layers and only then progresses inward, it decreases the visually perceptible cloudiness only increases between the time of separation and the onset of solidification and then no longer changes. The point of beginning solidification is therefore the point at which there is no increase the clouding is more noticeable. It can be determined easily and fairly accurately.

Only after the point of beginning solidification does the shaped body have a certain stability, the something allows greater mechanical stress. Therefore, the deflection device may only be located after this point; it

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however, it is not necessary that it is placed immediately after this point. After the redirection, the partially or completely formed solid shaped body a second zone of the cooling liquid, in which he from bottom to is performed above. Redirection and guidance from bottom to top are necessary in order to be able to use a device in which kept both the entry point and the exit point of the cooling liquid at a constant liquid level can be. This second zone, in which the molding is guided from bottom to top, does not have to be like the first Zone channel-shaped but can be a zone of wider dimensions e.g. in the form of a tub, as shown in Figure 2. However, it must not coincide with the first zone, i.e. the shaped body must not be in the channel-shaped first zone between the entry point and the point of incipient solidification are guided from bottom to top. The second zone can, however, also be designed as a channel-shaped zone, as is the case, for example, when as a device a U-shaped bent tube as shown in Figure 1 is used.

It is particularly advantageous if the cooling liquid at the point where the PVDF mixture enters it, a slightly, i.e. up to 20%, lower specific Own weight than the mixture. This prevents the mixture from sagging too quickly or too strong deceleration due to large differences in density. This slight difference in specific weight can be achieved in addition to the choice of a certain cooling liquid, that the specific weight of the

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Polymer mixture changed, e.g. through the mentioned use of an inner liquid for hollow fibers or tubes.

In principle, any liquids can be used as the cooling liquid can be used in which PVDF does not dissolve significantly at the temperature of the cooling liquid and which do not cause any chemical change in the polymer mixture.

In addition to other liquids, it has water, which may be a surfactant to reduce surface tension contains, proved to be suitable.

The formed body can be withdrawn from the cooling liquid by known methods, wherein Care must be taken that the peeling does not place any strong mechanical stress on the mixture between The nozzle exit and the start of solidification is caused.

In addition, it can optionally be advantageous to remove the shaped body from the at the same speed Remove the cooling liquid that the mixture possesses when it exits the nozzle.

By the method described, in particular by the low mechanical stress until the onset of solidification, PVDF moldings with pores are obtained, whose dimensional stability is good and of consistently good quality. Yarn breaks and quality

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Fluctuations such as uncontrollable imperfections in the form of pore size and characteristics deviating from the target, are kept to a minimum.

Although it may be desirable in certain cases, entrapped in the pores of the resulting PVDF molded body Substances such as solvents and nonsolvents, not to be washed out, will normally be the formed body washed. This can be done by extraction, which is continuously related to the production of the molded body or which is carried out discontinuously. After the extraction, the shaped body is dried.

The moldings obtained have pores on each of their surfaces, i.e. also the inner surfaces of hollow fibers or tubes have pore openings. Due to the wide range of options available regarding the type and amount of Solvent, type and amount of nonsolvent, type, amount and throughput of the cooling liquid and the temperature control is given during the entire process The pore size and characteristics of the PVDF molding can be adjusted in a targeted and reproducible manner over a wide range. So it is possible to have mean pore sizes of about 0.1 μ. to get about 5 μ. The total pore volume leaves can also influence and lie within a wide range, e.g. via the weight ratio PVDF / mixture components approximately between 10 and 90%. Different process parameters can vary depending on the setting Characteristics of the pore system can be obtained. Depending on

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Type and amount of the solvent and, if applicable, of the non-solvent, type, amount and throughput of the cooling liquid as well as depending on the temperature control in the cooling device, can in principle two different types of Pore structures are obtained, between which transitions are possible:

a) a pore system with essentially spherical cavities, which are separated from each other by porous partitions. The pores of the partition walls have a smaller mean diameter than the spherical cavities.

b) a three-dimensional network of pores, which only through narrow intermediate webs, but not through partition walls are separated.

In addition to these two possibilities for designing the pore system, it can also be influenced in another way will. This can be done above all by varying the viscosity of the polymer mixture and the type and temperature the cooling liquid obtained either an isotropic or anisotropic pore system. With an anisotropic Pore system have pore size and / or structure a gradient in the direction from the surface into the Interior of the molded body. An anisotropic pore system is obtained above all when the pores are relatively low in viscosity Polymer blends are processed.

The PVDF moldings produced according to the invention can be used in many ways, e.g. for microfiltration

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of aqueous solutions and of organic solvents. In many cases, PVDF membranes can be used here, because they show an extraordinary resistance to oxidative attack, resistant to an unusual large numbers of organic solvents are and their mechanical data only at temperatures close the melting point drop significantly. In the case of the filtration of aqueous solutions, PVDF membranes can also be used there find where other polymers cannot because of their susceptibility in the strongly acidic or strongly alkaline pH range can be used, e.g. for the filtration of hypochlorite solutions.

PVDF hollow membranes with anisotropic structure, in which the If the pore size decreases from the inside out, the use of a prefilter for microfiltration can be superfluous do; Pre-filtration is often used to trap large particles that cause membrane coverage and thus lead to a rapid decline in the river. These large particles are in the anisotropic membrane structures but retained in the large, inner pores without reducing the filtering surface. The so-called The "dirt capacity" of the anisotropic PVDF membrane is accordingly greater than in the case of isotropic structures.

The PVDF membranes produced according to the invention are also suitable for transmembrane distillation. In the Transmembrane distillation, in which the short diffusion path (membrane wall thickness) is used to evaporate aqueous Solutions is used on one side

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the heated, concentrated solution on the membrane, on the other hand "cooling water". Migrates through the difference in steam Water as a gas from the hot to the cold side of the membrane. Here, the temperature resistance, the low surface tension of the PVDF, which prevents the breakthrough of aqueous solution, and the oxidative ones required for cleaning cycles Advantageous use is made of persistence.

The invention is illustrated by the following examples:

example 1

A mixture of 20 parts by weight of polyvinylidene fluoride - PVDF Solef 1012 (LV = 2.68 measured in dimethylformamide) (Solvay, Belgium) and 80 parts by weight of a solvent mixture consisting of 37.5% glycerol triacetate (solvent) and 62.5% Witamol 320 (non-solvent dioctyl adipate - Dynamit Nobel). The polymer granulate and the solvent mixture were ^{brought to approx. 215 °} C. with vigorous stirring and in a nitrogen atmosphere. The granulate was swollen at approx. 145 ^° C. and a homogeneous, low-viscosity solution was formed as the temperature increased.

Part of the solution thus produced was extruded at about 220 ° C. through a hollow thread nozzle of a spinning machine into the U-shaped tube shown in FIG. 1 at a speed of 15 m / min. Distilled glycerine was used as an inner filling to create the lumen of the thread. After passing an air gap of approx. 1 cm, the thread entered the approx. 2 m long U-shaped glass tube charged ^{with water at approx. 25 ° C.} After entering the cooling medium, the thread slowly sank into

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the lower part of the tube and was pulled out of the outlet leg by means of a wire and onto a pull-off wheel placed. During the process, the water flowed through at an average speed of 1 m / min Direction as the polymer blend the device. It could be clearly observed how the thin liquid Polymer solution after a short residence time in the water at the beginning Phase separation became milky and finally stabilized on solidification, so that without deformation could be deflected and withdrawn continuously at a speed of 20 m / min.

After extraction of the liquid components using isopropanol at 50 ° C, the thread was vacuumed at about 50 ° C dried.

Properties of the hollow fiber membrane obtained:

Outside diameter: 1.24 mm
Inner lumen: 0.88 mm

max.pore size: 0.58 μm

2 transmembrane flow (isopropanol) in ml / cm · min at

0.1 bar: 0.95

The hollow filament was used to measure the maximum pore size immersed in ethanol and charged with nitrogen from the inside. The pressure at which the ethanol from the Walls of the hollow thread was displaced by nitrogen and the first gas bubbles could be seen on the outside.

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The maximum pore size is calculated from the value found ("blow point")

_d
Max.

Max,

where d = maximum pore diameter, P = blowing point C ^bar H

To determine the Isopropanolflusses the hollow fiber was charged from the inside with tempered at 35 ⁰ C isopropanol, and the flow rate measured at 0.1 bar.

Microscopic examinations showed pore structure with openings on the inner and outer walls.

An essentially spherical pore-shaped structure as specified in claim 30 resulted.

Example 2

There were added 30 parts by weight PVDF (Solef 1012) and 70 parts of 2- (.2-butoxy-ethoxy) · ethyl acetate at a temperature of about 155 ⁰ C - 160 ⁰ C in a homogeneous solution of medium viscosity transferred. At about 120 ⁰ C - 130 ° C, incipient sources could be observed of the granules.

The PVDF solution, the solidification temperature was about HO ⁰ C, was analogously to Example 1 is spun at about 150 ° C and water in a bath of glycerine / (1: 1) cooled from 35 ⁰ C. The extracted and dried hollow thread had the following characteristics:

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Outer diameter: 1.20 mm, inner volume: 0.80 mm

max.pore size: 0.94 μm

Transmembrane flow (isopropanol) at 0.1 bar in ml / cm min:

A pore system resulted which essentially consisted of a three-dimensional network of pores, such as stated in claim 31.

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Claims (36)

Annex to the submission dated August 2, 1984 A3GW32060 Claim i Process according to patent application P 33 27 638.2 for the production of molded bodies having pores by extruding a homogeneous, single-phase, liquid mixture of one or more polymers and one or more mixing partners that are liquid at the temperature of the preparation of the mixture, the mixture being above room temperature in the liquid State has a range of complete miscibility and a miscibility gap and has a solidification range above room temperature / in a cooling device containing cooling liquid and peeling off the formed body, the polymer / mixture partner mixture at a temperature above the miscibility gap with an average linear speed V ₁ through a The nozzle conveys from top to bottom into a cooling liquid which does not dissolve the polymer or dissolves it only insignificantly at the cooling temperature and which has a temperature below the solidification point, and the extruded mixture is removed from the inlet place in the cooling liquid at least up to the point of incipient solidification through a channel-shaped zone surrounded by a wall and in this channel-shaped zone the mean linear velocity V _{2 of} the cooling liquid is kept lower than v., the shaped body is deflected after the beginning of solidification of the polymer and passes from bottom to top through a second zone, from which cooling liquid is drawn off, and the level of the cooling liquid is kept constant both at the point of entry of the mixture into the cooling liquid and the level of the cooling liquid at the point of exit of the shaped body from the cooling liquid, thereby characterized in that the polymer used is polyvinylidene fluoride (PVDF) and the speed V _{2 is} significantly lower than v ... - 2 - A3GW32O6O 2. The method according to claim 1, characterized / that the PVDF molded body produced is a membrane in the form of a hollow thread, tube or film « 3. The method according to claim 1 or 2, characterized in that that the shaped body is washed following process step e) until all components except PVDF have been completely or almost completely removed, and that it is then dried. L J - 3 - A3GW32O6O 4. The method according to one or more of claims 1 to 3, characterized in that in the case of the production of Hollow fibers or tubes, the inner lumen is created by introducing an inner filling liquid that is used in the The temperature at which the inner lumen is generated is not a solvent for PVDF. 5. The method according to claim 4, characterized in that the inner filling liquid is glycerine or a liquid, which was used as a mixture component in the preparation of the mixture. 6. The method according to one or more of claims 1 to 3, characterized in that, in the case of the production of hollow fibers or tubes, the inner lumen is inserted by means of insertion generated by gaseous nitrogen. 7. The method according to one or more of claims 1 to 6, characterized in that the solidification temperature of the mixture above is from about 50 ⁰ C. 8. The method according to one or more of claims 1 to 7, characterized in that the cooling device is a U-shaped bent tube. 9. The method according to one or more of claims 1 to 8, characterized in that the liquid level of the cooling liquid at the exit point for the formed molded body is kept at the same level as at the point of entry of the mixture into the Cooling liquid. L J A3GW32O6O 10. The method according to one or more of claims 1 to 9, characterized in that the cooling liquid is close to the point of entry of the mixture into the cooling liquid the device is continuously metered so that it runs concurrently with the direction the movement of the mixture flows through the device and at an overflow device at the outlet the shaped body leaves the device. 11. The method according to one or more of claims 1 to 9, characterized in that the cooling liquid is continuously metered in close to the exit point of the formed body of the device, so that it flows through the device in the movement of the mixture in the opposite direction and the device at an "overflow facility" near the Entry point of the mixture into the cooling liquid leaves. 12. The method according to one or more of claims 1 to 9, characterized in that there is no continuous metering of cooling liquid, but rather only losses are compensated for by the entrainment of cooling liquid through the shaped body develop. 13. The method according to one or more of claims 1 to 12, characterized in that the cooling device is thermostated by an external device. - 5 - A3GW32O6O 14. The method according to one or more of claims 1 to 13, characterized in that between the nozzle and the entry point of the mixture in the cooling liquid there is an air gap. 15. The method according to one or more of claims 1 to 14, characterized in that the cooling liquid a slightly lower specific weight at the point of entry of the mixture into the cooling liquid than the mixture at this point possesses. 16. The method according to one or more of claims 1 to 14, characterized in that the cooling liquid Water, optionally with an addition of surfactant, is used. 17. The method according to one or more of claims 1 to 16, characterized in that the formed PVDF molded body is withdrawn from the cooling liquid at the rate at which the mixture emerges from the nozzle. 18. The method according to one or more of claims 1 to 17, characterized in that in the preparation of the mixture as a solvent for PVDF or one several of the compounds glycerol triacetate, glycerol diacetate and 2- (2-butoxyethoxy) ethyl acetate are used will or will be. - 6 - A3GW32O6O 19. The method according to one or more of claims 1 to 18, characterized in that when producing the Mixture is used in addition to at least one solvent at least one mixture component, which is a nonsolvent at the temperature of preparation of the mixture and its delivery through the nozzle for PVDF is. 20. The method according to claim 19, characterized in that the nonsolvent is di-n-octyl adipate or castor oil or a mixture thereof is used. 21. The method according to one or more of claims 1 to 20, characterized in that the proportion of PVDF in the Mixture is 10-90% by weight. 22. The method according to claim 21, characterized in that that the proportion of PVDF is 15-35% by weight. 23. The method according to one or more of claims 1 to 22, characterized in that the viscosity of the Mixture at the beginning of process step b) of claim 1 has a value of 2 Pa "s or higher. 24. The method according to claim 23, characterized in that the viscosity has a value between 5 and 35 Pa · s having. 25. The method according to one or more of claims 1 to 24, characterized in that the pore size and characteristics as well as the total pore volume of the - 7 - A3GW32O6O formed shaped body specifically adjusts by targeted variation of one or more of the following parameters: a) Type and amount of PVDF used b) Type and amount of solvent c) Type and amount of the nonsolvent d) Temperature of the mixture when conveyed through the nozzle e) Type and quantity and throughput of the cooling liquid f) Temperature in the cooling device g) Speed of the mixture or of the shaped body formed in the cooling device h) nozzle geometry i) Flow rate through the nozzle 26. Device for performing the method according to one or more of claims 1 to 25, characterized in that that it contains a cooling device with the following components: a) an inlet opening for cooling liquid b) an overflow device on which the cooling liquid can escape from the device c) an inlet and an outlet opening for the PVDF mixture or the molded body formed d) a channel-shaped zone surrounded by a wall, which adjoins the entry point of the PVDF mixture and in which the mixture goes from above can be guided below e) a deflection device which is located below the channel-shaped zone L J - 8 - A3GW32O6O £) a second zone following the deflection device in which the molded body formed from bottom to top can be performed g) if necessary, an external thermostating device. 27. Polyvinylidene fluoride (PVDF) molded bodies attached to each of their Have surfaces and pores in the interior, producible by the following steps a) Preparation of a single-phase liquid mixture of PVDF and one or more compared to PVDF inert mixture component (η), at least one mixture component at the temperature of the Making the mixture represents a solvent for PVDF and the mixture is in the liquid state a temperature range of complete miscibility above room temperature, above room temperature a temperature range with a miscibility gap and a Has solidification temperature above room temperature. b) Conveying the single-phase liquid mixture at a temperature above the separation temperature through a nozzle from top to bottom into a cooling device that is filled with a cooling liquid in which PVDF does not dissolve or dissolves only insignificantly at the temperature of the cooling liquid and which has a temperature below the solidification temperature of the mixture, the level the cooling liquid both at the point of entry of the mixture into the cooling liquid and at the - 9 - A3GW32O6O The exit point for the solid shaped body formed is kept constant and the average speed the cooling liquid in the device is set so that it, measured in the direction of movement the polymer mixture, in the area between the entry point of the mixture and the Imagine the beginning of solidification is significantly slower than the speed with which the mixture is emerges from the nozzle. c) Passing the mixture from the point of entry into the cooling liquid at least to the point of incipient solidification from top to bottom through a channel-shaped zone surrounded by a wall. d) deflection of the formed body and subsequent guidance of the shaped body from bottom to top in a second zone. e) removing the shaped body from the cooling liquid. 28. PVDF molded body, producible according to claim 27, wherein the molded bodies are membranes in the form of hollow fibers, tubes or foils. 29. PVDF molded body, producible according to claim 27 or 28, wherein the shaped bodies are washed following step e) of claim 26 until all of the constituents except for PVDF are completely or almost completely removed, and they are then dried. _ 10 - A3GW32O6O 30. PVDF moldings, producible according to one or more of claims 27 to 29, characterized in that the cavities formed by pores in the interior of the molded body Have spherical shape or approximately spherical shape and that the spherical cavities by partition walls are separated from each other, which pores with a smaller mean diameter than that of the spherical Have cavities. 31. PVDF molded body, producible according to one or more of claims 27 to 29, characterized in that a three-dimensional network structure of pores is present in the interior of the molded body, which only through narrow Intermediate webs are separated from each other. 32. PVDF molded body, producible according to one or more of claims 27 to 31, characterized in that an anisotropic pore system is present. 33. Use of the PVDF molded bodies produced according to one or more of the preceding claims for Microfiltration of strongly acidic or strongly alkaline aqueous solutions. 34. Use of the PVDF molded body produced according to one or more of the preceding claims for Microfiltration of oxidizing media 35. Use of the PVDF molded body produced according to one or more of the preceding claims for Microfiltration of aqueous hypochlorite solutions. - 11 - A3GW32O6O 36. Use of the PVDF molded bodies produced according to one or more of claims 1 to 32 for transmembrane distillation .